Self-Healing Polymer-Based Systems [1 ed.] 0128184507, 9780128184509

Table of contents :
Cover
Copyright
Contents
List of Contributors
1 Self-healing polymeric systems—fundamentals, state of art, and challenges
1.1 Introduction
1.1.1 Extrinsic self-healing in polymeric systems
1.1.2 Intrinsic self-healing in polymeric systems
1.2 Role of nanofillers in self-healing polymeric systems
1.3 Key developments in the field of self-healing polymeric systems
1.4 Challenges for fabricating self-healing materials based on polymeric systems
1.5 Conclusions
References
2 Types of chemistries involved in self-healing polymeric systems
2.1 Introduction: chemical aspects in self-healing process
2.1.1 Extrinsic and intrinsic self-healing
2.2 Key requirements of self-healing process
2.3 Dynamic covalent network in self-healing
2.4 Thermoreversible Diels–Alder and retro Diels–Alder chemistry
2.5 Photoinduced self-healing: [2+2] cycloaddition
2.5.1 [4+4] Cycloaddition reactions
2.6 Chemical transformations involved in self-healing
2.6.1 Thiol-ene click chemistry
2.6.2 Dynamic exchange of disulfide bonds
2.6.3 Dynamic chemistry of selenium
2.7 Reversible covalent reaction involved in self-healing
2.7.1 Dynamic reversible boronate ester bond
2.7.2 Dynamic reversibility of hindered urea bond
2.7.3 Dynamically reversible alkoxyamines fission/radical recombination
2.7.4 Reversible dynamic covalent Schiff-base (imine) linkage-based self-healing chemistry
2.7.5 Reversible covalent acylhydrazone bond in self-healing chemistry
2.8 Chemical transformations through involved reaction in self-healing
2.8.1 Exchangeable hydrazide Michael adduct linkages
2.8.2 Dynamic siloxane bond exchange
2.8.3 Dynamic covalent exchange network in polyesters
2.8.4 Self-healing based on exchangeable reactions involving hypervalent iodine
2.9 Supramolecular noncovalent interaction
2.9.1 Hydrogen-bonding-based self-healing
2.9.2 Self-healing involved through electrostatic interactions
2.9.2.1 Ionomeric or ionic mechanism
2.9.2.2 Self-healing through ionic salts
2.9.2.3 Magnesium ions (Mg2+)-based self-healing mechanism
2.9.2.4 Calcium ions (Ca2+)-based self-healing mechanism
2.9.2.5 Frustrated Lewis pair polymers as responsive self-healing gels
2.9.3 Self-healing based on van der Waals force of attraction
2.9.4 Reversible metallosupramolecular polymer
2.9.5 Host–guest interactions
2.9.6 π-Interactions based self-healing polymer
2.9.7 Hydrophobic interactions
2.9.8 Interpenetrating polymer network for self-healing
2.10 Chemistries involved in microcapsule-based self-healing polymeric system
2.10.1 Microcapsule mediated ring-opening metathesis polymerization
2.10.2 Azide-alkyne click chemistry
2.10.3 Controlled radical polymerization in microcapsule system
2.10.3.1 Other microcapsule-embedded systems
2.10.4 Vascular-based self-healing system
2.10.4.1 Macrovascular system
2.10.4.2 Microvascular-based self-healing
2.11 Conclusions
References
3 Self-healing polymers: from general basics to mechanistic aspects
3.1 Introduction
3.2 General mechanism of self-healing polymers
3.3 Concepts for the design of self-healing polymers
3.4 Extrinsic self-healing polymers
3.5 Intrinsic self-healing polymers
3.6 Other mechanistic aspects
3.7 Conclusions
References
4 Shape memory-assisted self-healing polymer systems
4.1 Introduction
4.2 Shape memory and self-healing mechanisms
4.2.1 Shape memory mechanism
4.2.2 Intrinsic self-healing of polymers
4.2.2.1 Covalent self-healing
4.2.2.2 Noncovalent self-healing
4.3 Shape memory-assisted self-healing
4.3.1 Thermoplastic polymers
4.3.2 Elastomers
4.3.3 Thermoset polymers
4.3.4 Extrinsic shape memory-assisted self-healing
4.4 Applications
4.5 Conclusions
References
5 Characterization of self-healing polymeric materials
5.1 Introduction
5.2 Methods for evaluating self-healing behavior of the polymeric composites
5.2.1 Qualitative methods
5.2.1.1 Visualization techniques
5.2.1.2 Acoustical microscopy
5.2.1.3 X-ray microtomography
5.2.1.4 Evaluation of self-healing reaction heat
5.2.2 Quantitative methods
5.2.2.1 Tensile testing
5.2.2.2 Bending testing
5.2.2.3 Tapered double cantilever beam
5.2.2.4 Ballistic impact
5.2.2.5 Dynamic mechanical thermal analyses
5.3 Methods for evaluating self-healing behavior of the polymeric coatings
5.3.1 Qualitative methods
5.3.1.1 Visual inspection and optical microscopy
5.3.1.2 Scanning electron microscopy
5.3.1.3 Confocal microscopy
5.3.1.4 Scanning electrochemical microscopy
5.3.1.5 Scanning vibrating electrode technique
5.3.2 Quantitative methods
5.3.2.1 Healing of hydrophobicity
5.3.2.2 Atomic force microscopy
5.3.2.3 Tribological properties
5.3.2.4 Corrosion assessment tests
5.3.2.4.1 Potentiostatic and potentiodynamic techniques
5.3.2.4.2 Electrochemical impedance spectroscopy
5.4 Summary and outlook
References
6 Role of nanoparticles in self-healing of polymeric systems
6.1 Introduction
6.2 Self-healing polymer using metal nanoparticles
6.3 Self-healing polymer using inorganic nanoparticles
6.4 Self-healing polymer using organic nanoparticles
6.4.1 Self-healing by shape-memory organic nanoparticles
6.4.2 Self-healing by organic micro- or nanocapsules
6.4.3 Evaluation of self-healing capability
6.4.3.1 Crack growth method
6.4.3.2 Beam on elastic foundation method
6.4.3.3 Direct tensile tests
6.4.4 Introduction of a real application of self-healing nanocapsules
6.5 Further advice
References
7 Self-healing biomaterials based on polymeric systems
7.1 Introduction
7.2 Self-healing biomaterials in tissue engineering
7.2.1 Self-healing hydrogels
7.2.1.1 Mechanism
7.2.1.1.1 Noncovalent bonding
7.2.1.1.1.1 Electrostatic interaction
7.2.1.1.1.2 Hydrogen bonding
7.2.1.1.1.3 Host–guest interaction
7.2.1.1.1.4 Metal–ligand coordination
7.2.1.1.1.5 Hydrophobic interactions
7.2.1.1.1.6 Crystallization
7.2.1.1.2 Dynamic covalent bonding
7.2.1.1.2.1 Diels–Alder reaction
7.2.1.1.2.2 Thiol–disulfide exchange
7.2.1.1.2.3 Imine bonds
7.2.1.1.2.4 Acylhydrazone bonds
7.2.1.1.2.5 Oxime bonds
7.2.1.2 The evaluation of self-healing efficiency
7.2.1.3 Applications in tissue engineering
7.2.1.3.1 Tissue adhesives
7.2.1.3.2 Cell scaffolds
7.2.2 Self-healing films
7.2.2.1 Mechanism
7.2.2.2 Applications in tissue engineering
7.2.2.2.1 Cell coculture
7.2.2.2.2 Biosensor
7.3 Self-healing biomaterials in drug/gene delivery systems
7.3.1 Injectable hydrogels
7.3.2 Particles and capsules
7.4 Self-healing functional surfaces
7.4.1 Antibacterial and antifouling surfaces
7.4.2 Surface-mediated drug delivery
7.4.3 Challenges
7.5 The characterization of self-healing
7.6 New opportunities and challenges
7.6.1 3D printing
7.6.2 Wound dressing
7.6.3 Electronic skin
References
8 Self-healing Diels–Alder engineered thermosets
8.1 Fundamentals of self-healing
8.2 Types of self-healing systems
8.3 Diels–Alder reaction
8.3.1 Kinetics and thermodynamics of Diels–Alder reaction
8.3.2 Diels–Alder reaction of furan/maleimide
8.4 Diels–Alder-based healable thermosets
8.4.1 Applications of Diels–Alder-based self-healing networks
8.4.1.1 Healable Diels–Alder-based hydrogels
8.4.1.2 Diels–Alder-based healable rubbers
8.4.1.3 Diels–Alder-based healable polymer composites
8.4.1.3.1 Healing of Diels–Alder-based polymer composites by nonthermal methods
8.4.1.3.2 Self-healing Diels–Alder-based nanocomposites
8.4.1.3.2.1 Graphene-based Diels–Alder nanocomposites
8.4.1.3.4.2 Carbon nanotube-based Diels–Alder nanocomposites
8.4.1.3.4.3 Silver nanowires-based Diels–Alder nanocomposites
8.4.1.4 Healable Diels–Alder-based polymer coatings
8.4.1.5 Diels–Alder-based healable polymer adhesives
8.4.1.6 Diels–Alder-based self-healing actuators and robots
8.5 Summary and outlook
References
9 Self-healing polymeric coatings containing microcapsules filled with active materials
9.1 Introduction
9.2 Requirements for designing a self-healing coating
9.3 Microcapsule-based self-healing systems
9.4 Microcapsule preparation methods
9.5 Materials selection for core and shell components of microcapsules
9.6 Limitations and shortcomings of microcapsule-embedded coatings
9.7 Summary
References
10 Capsule-based self-healing polymers and composites
10.1 Introduction
10.2 Capsule synthesis and characterization
10.3 Self-healing polymers and composites
10.4 Self-healing coatings
10.5 Conclusions and future trends
References
11 Ionomers as self-healing materials
11.1 Introduction
11.2 Materials, chemistry, and fundamentals
11.3 The self-healing mechanisms
11.4 Activation methods
11.5 Applications
11.6 Summary
References
12 Self-healing materials utilizing supramolecular interactions
12.1 Intrinsic self-healing systems
12.1.1 Supramolecular bonds
12.1.2 Hydrogen bonding
12.1.3 Metal coordination
12.1.4 Host–guest interactions
12.1.5 Dynamic covalent chemistry
12.2 Main-chain supramolecular polymers
12.2.1 Supramolecular polymerizations
12.2.2 Isodesmic supramolecular polymerization
12.2.3 Ring-chain supramolecular polymerization
12.2.4 Cooperative supramolecular polymerization
12.2.5 Directional noncovalent interactions
12.3 Self-healing materials driven by metal coordination
12.3.1 Early development
12.3.2 Network formation and self-healing
12.3.3 Ligand systems
12.3.4 Naturally occuring and bioinspired self-healing systems
12.3.5 Photoresponsive systems
12.3.6 Self-assembly: directional bonding approaches
12.3.6.1 Metal-organic framework
12.4 Self-healing mediated by electrostatic interactions
12.4.1 Electrostatic self-assembly
12.4.2 Bulk ionomer self-healing materials
12.4.3 Self-healing poly(ionic liquids)
12.5 Host–guest interactions in self-healing materials
12.5.1 Introduction to host–guest interactions
12.5.2 Mechanism of self-healing based on host–guest interactions
12.5.3 Design of self-healing, host–guest materials
12.5.4 Self-healing materials utilizing cyclodextrin
12.5.5 Self-healing materials utilizing crown ethers
12.5.6 Self-healing materials utilizing cucurbiturils
12.5.7 Enhancing mechanical properties in host–guest self-healing materials
12.6 Dynamic covalent self-healing materials
12.6.1 Introduction to dynamic covalent bonds
12.6.2 Self-healing from dynamic condensation reactions
12.6.2.1 Acylhydrazone bonds
12.6.2.2 Boronate ester bonds
12.6.2.3 Imine and enamine bonds
12.6.3 Reversible cycloaddition reactions
12.6.3.1 Reversible Diels–Alder
12.6.4 DCB exchange through chemical or catalytic stimuli
12.6.5 Photo-induced dynamic covalent self-healing
12.7 Hydrogen bonding in self-healing systems
12.7.1 Hydrogen bonding
12.7.2 Self-complementary hydrogen bonding in self-healing materials
12.7.3 Self-healing polymers utilizing weak hydrogen bonding
12.7.4 Combination of hydrogen bonding and dynamic covalent bonds
12.8 Conclusions and future outlook
References
13 Self-healing hydrogels
13.1 Introduction
13.1.1 Gel and hydrogel
13.1.2 Hydrosol and hydrogel
13.2 Self-healing
13.3 Self-healing and its characterization
13.3.1 Extrinsic self-healing
13.3.2 Intrinsic self-healing
13.4 Chemistry involved in intrinsic self-healing
13.4.1 Reversible covalent bonds
13.4.1.1 Reversible cycloaddition reactions
13.4.1.2 Exchange reactions
13.4.1.3 Stable free radical-mediated reshuffle reactions
13.4.1.4 Heterocyclic compounds and carbohydrates in self-healing polyurethanes
13.4.2 Supramolecular chemistry
13.4.2.1 Hydrogen bonds
13.4.2.2 π–π Stacking interactions
13.4.2.3 Metal–ligand interaction
13.4.2.4 Ionic interaction
13.4.2.5 Host–guest interaction
13.5 Self-healing process
13.5.1 Autonomic self-healing hydrogels
13.5.2 Nonautonomic self-healing hydrogels
13.6 Classification of self-healing hydrogels
13.6.1 Inorganic-based self-healing hydrogels
13.6.2 Polymer-based self-healing hydrogels
13.6.3 Nanocomposite-based self-healing hydrogels
13.7 Mechanism of self-healing of hydrogels
13.7.1 Physically (diffusion) self-healing mechanism
13.7.2 Chemically self-healing mechanism
13.8 Factors impact on self-healing mechanism
13.8.1 Separation time
13.8.2 Self-healing time
13.8.3 Temperature
13.8.4 Chain length
13.8.5 The content of nanomaterials
13.9 Sacrificial bonds
13.10 Nature and mechanisms of sacrificial bonds
13.10.1 Sacrificial bonds in biological materials
13.10.2 Constitutive theories of sacrificial bonding systems
13.11 Inspired sacrificial bonds in artificial polymeric materials
13.11.1 Sacrificial covalent bonds
13.11.2 Sacrificial noncovalent bonds
13.12 Sacrificial bonds in hydrogels
13.12.1 Sacrificial ionic bonds
13.12.2 Sacrificial hydrogen bonds
13.12.3 Sacrificial metal–ligand coordination bonds
13.12.4 Sacrificial hydrophobic interactions
13.12.5 Sacrificial host–guest complexes
13.13 Metal–ligand polymer hydrogels
13.13.1 Cross-linked hydrogels via metal coordination
13.13.2 Covalently cross-linked hydrogels
13.13.3 Hybrid cross-linked hydrogels
13.14 Self-healing gels mechanism based on constitutional dynamic chemistry
13.15 Natural polymer-based hydrogels
13.15.1 Alginate
13.15.2 Agarose-based self-healing hydrogels
13.15.3 Chitosan
13.15.4 Cellulose
13.15.5 Hydroxyethyl cellulose
13.15.6 Dextrin-based self-healing hydrogels
13.15.7 Guar gum-based self-healing hydrogels
13.15.8 Gelatin-based self-healing hydrogels
13.15.9 Glycogen-based self-healing hydrogels
13.15.10 Hyaluronic acid-based hydrogels
13.15.11 Xanthan-gum-based self-healing hydrogels
13.16 Recent development in miscellaneous application fields
13.16.1 Superabsorbent hybrid hydrogels
13.16.2 Conductive polymer hydrogels
13.16.3 Polysaccharide-based natural hydrogels
13.16.4 Protein-based hydrogels
13.16.5 Hydrogels for energy applications
References
14 A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers
14.1 Introduction
14.2 Finite deformation kinematics: elastic, plastic, damage, and healing in polymers
14.3 Plastic deformation in polymers
14.4 Continuum damage and healing mechanics
14.4.1 Scalar damage-healing variables for isotropic problems
14.4.2 Anisotropic damage-healing problems
14.5 Physically consistent evolution laws for the damage and healing processes
14.5.1 Thermodynamic consistent damage and healing model
14.5.2 Mechanisms-based phenomenological healing models
14.6 Concluding remarks
References
15 Self-healing fiber-reinforced polymer composites for their potential structural applications
15.1 Introduction
15.2 Scope of self-healing in fiber-reinforced polymer composites
15.3 Extrinsic self-healing approaches for fiber-reinforced polymer composites
15.3.1 Microcapsule-based self-healing
15.3.2 Hollow fiber-based self-healing
15.3.3 Microvascular-based self-healing
15.4 Intrinsic self-healing approach for fiber-reinforced polymer composites
15.5 Thermoreversible healing of FRP
15.6 Assessment of self-healing efficiency for fiber-reinforced polymer composites
15.7 Conclusions
References
16 Self-healing polymeric coating for corrosion inhibition and fatigue repair
16.1 Background of self-healing and corrosion inhibition
16.2 Self-healing and corrosion inhibitor materials
16.2.1 Self-healing materials
16.2.1.1 Single catalyst with microcapsules
16.2.1.2 Dual capsule-based system
16.2.2 Corrosion inhibitors
16.2.2.1 Types of corrosion inhibitors
16.2.2.2 Anodic corrosion inhibitors
16.2.2.3 Nitrites
16.2.2.4 Molybdates
16.2.2.5 Cathodic corrosion inhibitor
16.2.2.6 Organic inhibitors
16.3 Case study for self-healing material and corrosion inhibitors
16.3.1 Green synthesis of self-healing corrosion-inhibiting coating using neem oil as self-healing and corrosion inhibitor
16.3.2 Synthesis of nanocapsules using ultrasound and conventional method
16.4 Polymer capsules-based self-healing coating for corrosion inhibition
16.4.1 Self-healing capsules based on polymeric materials
16.5 Nanocontainer-based self-healing approach for corrosion inhibition
16.6 Clay-based self-healing materials for corrosion inhibition
16.6.1 Commercial applications and future prospectus
Conclusions
References
17 Applications of self-healing polymeric systems
17.1 Introduction
17.2 Application in wound healing
17.3 Application in tissue engineering
17.4 Application in three-dimensional printing
17.5 Application in drug delivery
17.6 Application in anticorrosion coating
17.7 Application in electronic application
17.8 Application in aerospace applications
17.9 Conclusions
References
Index
Back Cover

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Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-818450-9 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Edward Payne Editorial Project Manager: Charlotte Rowley Production Project Manager: Sojan P. Pazhayattil Cover Designer: Victoria Pearson Typeset by MPS Limited, Chennai, India

Contents List of contributors ix

3. Self-healing polymers: from general basics to mechanistic aspects 75

1. Self-healing polymeric systems— fundamentals, state of art, and challenges 1

Martin D. Hager and Stefan Zechel

3.1 Introduction 75 3.2 General mechanism of self-healing polymers 76 3.3 Concepts for the design of self-healing polymers 79 3.4 Extrinsic self-healing polymers 79 3.5 Intrinsic self-healing polymers 82 3.6 Other mechanistic aspects 89 3.7 Conclusions 90 References 91

Anu Surendran and Sabu Thomas

1.1 Introduction 1 1.2 Role of nanofillers in self-healing polymeric systems 6 1.3 Key developments in the field of self-healing polymeric systems 8 1.4 Challenges for fabricating self-healing materials based on polymeric systems 11 1.5 Conclusions 13 References 13

4. Shape memory-assisted self-healing polymer systems 95

2. Types of chemistries involved in self-healing polymeric systems 17

Wenjing Wu, James Ekeocha, Christopher Ellingford, Sreeni Narayana Kurup and Chaoying Wan

Anil K. Padhan and Debaprasad Mandal

4.1 Introduction 95 4.2 Shape memory and self-healing mechanisms 96 4.3 Shape memory-assisted self-healing 4.4 Applications 116 4.5 Conclusions 118 References 118

2.1 Introduction: chemical aspects in self-healing process 17 2.2 Key requirements of self-healing process 18 2.3 Dynamic covalent network in self-healing 20 2.4 Thermoreversible Diels Alder and retro Diels Alder chemistry 20 2.5 Photoinduced self-healing: [2 1 2] cycloaddition 25 2.6 Chemical transformations involved in self-healing 26 2.7 Reversible covalent reaction involved in self-healing 29 2.8 Chemical transformations through involved reaction in self-healing 36 2.9 Supramolecular noncovalent interaction 39 2.10 Chemistries involved in microcapsule-based self-healing polymeric system 60 2.11 Conclusions 64 References 65

107

5. Characterization of self-healing polymeric materials 123 Saied Nouri Khorasani and Rasoul Esmaeely Neisiany

5.1 Introduction 123 5.2 Methods for evaluating self-healing behavior of the polymeric composites 124 5.3 Methods for evaluating self-healing behavior of the polymeric coatings 129 5.4 Summary and outlook 135 References 136

v

vi

Contents

6. Role of nanoparticles in self-healing of polymeric systems 141 Junfeng Su

6.1 Introduction 141 6.2 Self-healing polymer using metal nanoparticles 143 6.3 Self-healing polymer using inorganic nanoparticles 144 6.4 Self-healing polymer using organic nanoparticles 148 6.5 Further advice 162 References 163

10. Capsule-based self-healing polymers and composites 259 Maria Kosarli, Dimitrios Bekas, Kyriaki Tsirka and Alkiviadis S. Paipetis

7. Self-healing biomaterials based on polymeric systems 167 Baolin Guo and Rui Yu

7.1 Introduction 167 7.2 Self-healing biomaterials in tissue engineering 168 7.3 Self-healing biomaterials in drug/gene delivery systems 188 7.4 Self-healing functional surfaces 191 7.5 The characterization of self-healing 194 7.6 New opportunities and challenges 195 Acknowledgments 200 References 200

8. Self-healing Diels Alder engineered thermosets 209 Zeinab Karami, Mohsen Zolghadr and Mohammad Jalal Zohuriaan-Mehr

8.1 Fundamentals of self-healing 209 8.2 Types of self-healing systems 210 8.3 Diels Alder reaction 210 8.4 Diels Alder-based healable thermosets 8.5 Summary and outlook 225 References 227

9.4 Microcapsule preparation methods 245 9.5 Materials selection for core and shell components of microcapsules 248 9.6 Limitations and shortcomings of microcapsule-embedded coatings 252 9.7 Summary 253 References 254

10.1 Introduction 259 10.2 Capsule synthesis and characterization 260 10.3 Self-healing polymers and composites 267 10.4 Self-healing coatings 273 10.5 Conclusions and future trends 274 References 275

11. Ionomers as self-healing materials 279 S. Mojtaba Mirabedini and Farhad Alizadegan

11.1 Introduction 279 11.2 Materials, chemistry, and fundamentals 11.3 The self-healing mechanisms 283 11.4 Activation methods 284 11.5 Applications 284 11.6 Summary 288 References 288

280

12. Self-healing materials utilizing supramolecular interactions 293 James F. Reuther, Randall A. Scanga, Ali Shahrokhina and Priyanka Biswas

214

9. Self-healing polymeric coatings containing microcapsules filled with active materials 235 S. Mojtaba Mirabedini and Farhad Alizadegan

9.1 Introduction 235 9.2 Requirements for designing a self-healing coating 236 9.3 Microcapsule-based self-healing systems 237

12.1 Intrinsic self-healing systems 293 12.2 Main-chain supramolecular polymers 297 12.3 Self-healing materials driven by metal coordination 305 12.4 Self-healing mediated by electrostatic interactions 322 12.5 Host guest interactions in self-healing materials 332 12.6 Dynamic covalent self-healing materials 340 12.7 Hydrogen bonding in self-healing systems 351 12.8 Conclusions and future outlook 356 References 356

vii

Contents

13. Self-healing hydrogels 369 Imtiaz Hussain and Guodong Fu

13.1 13.2 13.3 13.4

Introduction 369 Self-healing 371 Self-healing and its characterization 371 Chemistry involved in intrinsic self-healing 372 13.5 Self-healing process 378 13.6 Classification of self-healing hydrogels 379 13.7 Mechanism of self-healing of hydrogels 381 13.8 Factors impact on self-healing mechanism 384 13.9 Sacrificial bonds 385 13.10 Nature and mechanisms of sacrificial bonds 385 13.11 Inspired sacrificial bonds in artificial polymeric materials 386 13.12 Sacrificial bonds in hydrogels 387 13.13 Metal ligand polymer hydrogels 390 13.14 Self-healing gels mechanism based on constitutional dynamic chemistry 392 13.15 Natural polymer-based hydrogels 392 13.16 Recent development in miscellaneous application fields 408 References 413

14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers 425 Amir Shojaei and Guoqiang Li

14.1 Introduction 425 14.2 Finite deformation kinematics: elastic, plastic, damage, and healing in polymers 429 14.3 Plastic deformation in polymers 431 14.4 Continuum damage and healing mechanics 434 14.5 Physically consistent evolution laws for the damage and healing processes 442 14.6 Concluding remarks 450 References 451

15. Self-healing fiber-reinforced polymer composites for their potential structural applications 455 Nazrul Islam Khan and Sudipta Halder

15.1 Introduction

455

15.2 Scope of self-healing in fiber-reinforced polymer composites 456 15.3 Extrinsic self-healing approaches for fiber-reinforced polymer composites 460 15.4 Intrinsic self-healing approach for fiber-reinforced polymer composites 463 15.5 Thermoreversible healing of FRP 465 15.6 Assessment of self-healing efficiency for fiber-reinforced polymer composites 466 15.7 Conclusions 468 Acknowledgments 468 References 468

16. Self-healing polymeric coating for corrosion inhibition and fatigue repair 473 Vikas S. Hakke, Uday D. bagale, Shirish H. Sonawane, Dipak Pinjari, S. Manigandan and Shriram Sonawane

16.1 Background of self-healing and corrosion inhibition 473 16.2 Self-healing and corrosion inhibitor materials 474 16.3 Case study for self-healing material and corrosion inhibitors 478 16.4 Polymer capsules-based self-healing coating for corrosion inhibition 484 16.5 Nanocontainer-based self-healing approach for corrosion inhibition 487 16.6 Clay-based self-healing materials for corrosion inhibition 489 Conclusions 490 References 490

17. Applications of self-healing polymeric systems 495 Jomon Joy, Elssa George, S. Anas and Sabu Thomas

17.1 Introduction 495 17.2 Application in wound healing 496 17.3 Application in tissue engineering 498 17.4 Application in three-dimensional printing 499 17.5 Application in drug delivery 500 17.6 Application in anticorrosion coating 501 17.7 Application in electronic application 503 17.8 Application in aerospace applications 506 17.9 Conclusions 508 References 508

Index 515

List of Contributors Farhad Alizadegan Iran Polymer Petrochemical Institute, Tehran, Iran

Germany; Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Jena, Germany

and

S. Anas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India; Advanced Molecular Materials Research Centre, Mahatma Gandhi University, Kottayam, India

Vikas S. Hakke Department of Chemical Engineering, National Institute of Technology, Warangal, India Sudipta Halder Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar, India; Department of Civil, Construction and Environmental Engineering, The University of Alabama Engineering, Tuscaloosa, AL, United States

Uday D. bagale Department of Chemical Engineering, National Institute of Technology, Warangal, India Dimitrios Bekas Structural Integrity and Health Monitoring Group, Department of Aeronautics, Imperial College London, South Kensington Campus, London, United Kingdom

Imtiaz Hussain School of Chemistry and Chemical Engineering Southeast University, Nanjing, P.R. China; College of Science, Nanjing Forestry University, Nanjing, P.R. China

Priyanka Biswas Department of Chemistry, University of Massachusetts Lowell, MA, United States

Jomon Joy Mahatma India

James Ekeocha International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom

School of Chemical Sciences, Gandhi University, Kottayam,

Zeinab Karami Biobased Monomers and Polymers Division, Adhesive & Resin Department, Iran Polymer and Petrochemical Institute, Tehran, Iran

Christopher Ellingford International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom

Nazrul Islam Khan Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar, India; Department of Mechanical Engineering, GMRIT, Srikakulam, India

Guodong Fu School of Chemistry and Chemical Engineering Southeast University, Nanjing, P.R. China Elssa George School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

Saied Nouri Khorasani Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran

Baolin Guo Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, P.R. China

Maria Kosarli Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece

Martin D. Hager Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Jena,

Sreeni Narayana Kurup International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom

ix

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List of Contributors

Guoqiang Li Department of Engineering, Louisiana State Baton Rouge, LA, United States

Mechanical University,

Debaprasad Mandal Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, India S.

Manigandan Department of Chemical Engineering, Indian Institute of Technology, Ropar, India

S. Mojtaba Mirabedini Iran Polymer and Petrochemical Institute, Tehran, Iran Rasoul Esmaeely Neisiany Department of Materials and Polymer Engineering, Faculty of Engineering, Hakim Sabzevari University, Sabzevar, Iran Anil K. Padhan Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, India Alkiviadis S. Paipetis Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece Dipak Pinjari National Center for Nanoscience and Nanotechnology, University of Mumbai, Mumbai, India James F. Reuther Department of Chemistry, University of Massachusetts Lowell, MA, United States Randall A. Scanga Department of Chemistry, University of Massachusetts Lowell, MA, United States Ali Shahrokhina Department of Chemistry, University of Massachusetts Lowell, MA, United States Amir Shojaei Varian Medical Systems, Palo Alto, CA, United States; Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA, United States Shirish H. Sonawane Department of Chemical Engineering, National Institute of Technology, Warangal, India Shriram Sonawane Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India

Junfeng Su Department of Polymer Science, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin, P.R. China Anu Surendran International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India; International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Kyriaki Tsirka Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece Chaoying Wan International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom Wenjing Wu International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom; Aerospace Research Institute of Materials & Processing Technology, Beijing, P.R. China Rui Yu Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, P.R. China Stefan Zechel Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Jena, Germany; Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Jena, Germany Mohammad Jalal Zohuriaan-Mehr Biobased Monomers and Polymers Division, Adhesive & Resin Department, Iran Polymer and Petrochemical Institute, Tehran, Iran Mohsen Zolghadr School of Chemistry, University of Tehran, Tehran, Iran

C H A P T E R

1 Self-healing polymeric systems— fundamentals, state of art, and challenges Anu Surendran1 and Sabu Thomas1,2 1

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India 2School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

1.1 Introduction The potential applications of the polymeric systems are fast advancing in the recent years in various structural applications such as aerospace, defense, and construction industries. The damage triggered by various factors such as mechanical, thermal, and chemical factors has serious implications on the structural integrity, performance, and life span of the material. A visible failure could be easily detected, whereas structural level microcracks remained undetected. A surge in the present understanding of the microstructure and failure mechanism has effected in exploring strategies for addressing the fatigue response of the material [1]. The prerequisite for self-healing is that damage triggers self-healing by generating a mobile phase which covers the damage zone by either physical or chemical interactions. One of the milestones was the development of smart materials where the “damage could automate a healing response in the material” [2]. The damage repair costs are higher, time consuming, and sometimes difficult to monitor if it occurs in the microstructure level. Self-healing occurs by either autonomic or nonautonomic based on the type of response to damage. Autonomic response does not require any external stimuli; damage itself initiates the healing process. Nonautonomic healing requires an external stimulus such as light or heat for initiation of self-healing process. Another way to express the class of selfhealing materials is as “extrinsic” and “intrinsic” self-healing materials. Extrinsic selfhealing implies the encapsulation of micro- or nanocapsules into the material during the initial fabrication resulting in the healing. The healing action is triggered by the rupture of

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00001-5

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© 2020 Elsevier Inc. All rights reserved.

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

these capsules in the path of cracks, causing the release of healing agents onto the crack site. Intrinsic self-healing does not require any encapsulation of healing agents. It rather occurs by the physical/chemical interactions established between the crack interfaces which impart the self-healing action. The ability to functionalization has paved the way for inducing self-healing property for polymeric systems. Also the inherent potential to accommodate healing agents in the apparently larger volume of macromolecular chain network also facilitates the easiness to induce self-healing property. Such self-healing materials find potential applications in automobile, civil, and aerospace applications. Self-healing materials manages to reduce the damage repair cost and economic burden enhancing the life time and material reliability. The demands for such smarter materials are booming and hence researchers show tremendous interest in fabricating and designing materials with self-healing property.

1.1.1 Extrinsic self-healing in polymeric systems Extrinsic self-healing has been facilitated to polymeric materials for recovering the original properties of the materials at reasonable cost after damage. Extrinsic self-healing implies on three approaches based on micro/nanocapsules embedment, hollow fiber embedment, and microvascular system. In microencapsulation technique, healing agent is embedded or phase separated within the matrix so that healing occurs without external intervention. A catalyst is also incorporated into the matrix. The crack ruptures the micro/ nanocapsules causing the release of healing agent into the matrix. The released healing agent traverses in the matrix through the capillary action, which come into contact with the catalyst causing polymerization and further clears the damage. The disadvantage of this technique is that it causes limited healing action due to the small amount of healing agent. Therefore multiple healing actions are not possible with the micro/nanocapsules. Bond and coworkers demonstrated self-healing utilizing the hollow glass fibers which contains healing agent [3
5]. Hollow glass fiber approach encapsulates more healing agent and also could reinforce the matrix and mostly preferred than micro encapsulated selfhealing approach. Bond et al. [4] observed apparent restoration of compressive strength in epoxy resin-bonded hollow glass fiber. Fibers with large diameter and increased hollow fraction have incremental effect to the strength determined under axial compressive loading. About 97% of the mechanical strength was restored after investigation of impact properties followed by four-point bend flexural testing [3]. Fig. 1.1 [6] represents the schematic representation of self-healing via hollow fibers. Fig. 1.2 represents the representation of self-healing by micro/nanocapsules. Microvascular system mimics the biological vascular system in plants and animals with a continuous supply of healing agent through a centralized network. The crack-induced delivery of healants to the material furnishes multiple healing abilities and restores the properties [7]. The continuous delivery of healing agent in the three-dimensional microvascular systems opened up new avenues for repeatable healing in structural components. Of the three autonomic healing systems, microvascular healing systems often have highest efficiency. Self-healing using microvascular interpenetrating networks was fabricated in epoxy resins via dual ink deposition and vertical ink writing [9]. Healing efficiency of 50% was

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1.1 Introduction

One-part resin Polymer matrix Hollow fiber Resin system

FIGURE 1.1 Schematic representation of selfhealing by hollow fibers [6]. Source: Reprinted from S. Bleay, C. Loader, V. Hawyes, L. Humberstone, P. Curtis, A smart repair system for polymer matrix composites. Compos. Part A Appl. Sci. Manuf. 32 (2001) 1767
1776. doi:10.1016/ S1359-835X(01)00020-3, Copyright (2001), with permission from Elsevier.

Hardener system Hollow fiber

Resin system Micro encapsulated hardener Hollow fiber

retained even after 30 consecutive healing cycles. Nancy et al. [10] developed two sets of independent vascular networks; one comprising of resin part and other comprising of amine curing agent was embedded in the polymer substrate coating. The healing components got wicked under capillary action in the damage site and close the crack due to the reaction of the resin and curing agent. Coaxial electrospinning techniques were utilized for incorporation of linseed oil, a self-healing agent in graphene oxide (GO)-reinforced polyacrylonitrile (PAN) shells [11]. GO decorated PAN fibers were incorporated into PU coatings. When crack forms, linseed oil will be released and will react with oxygen and gets solidified covering the crack. Moreover, GO had improved the thermal stability of the material.

1.1.2 Intrinsic self-healing in polymeric systems This is a class of nonautonomic healing system which requires external stimuli for selfhealing. This involves healing process via bond rupture and bond reformation and could be operated over multiple times. Chemical reactions and molecular interactions which can be activated by heat, light, electrical energy, and magnetism are examples for such systems. Intrinsic self-healing is based on the presence of particular reversible chemical bonds. Since, intrinsic reversibility of these chemical bonds enables multiple healing responses at the same location. Thermally reversible Diels
Alder (DA) reactions are most widely used for fabricating thermally reversible self-healing systems. DA reaction represents the class of [4 1 2] cycloaddition reaction which occurs between a diene and a dienophile. Alkenes and alkynes attached to electron withdrawing groups are mostly used as a dienophile to bring about the reaction with a diene. Furan
maleimide chemistry is mostly widely used in thermally reversible self-healing polymeric systems. Peterson et al. [12] utilized DA click chemistry for developing a reversibly cross-link gel as self-healing site in traditional epoxy-amine reaction. The healing could be repeated for about five cycles and 21% of the composite

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

FIGURE 1.2 Schematic representation of micro/nanocapsule-embedded self-healing systems [8]. Source: Reprinted from M. Samadzadeh, S.H. Boura, M. Peikari, S.M. Kasiriha, A. Ashrafi, A review on self-healing coatings based on micro/nanocapsules. Prog. Org. Coat. 68 (2010) 159
164. doi:10.1016/J.PORGCOAT.2010.01.006, Copyright (2010), with permission from Elsevier.

strength was recovered after the first healing cycle. Bowman et al. [13] observed interconversions of furan and maleimide and observed that reversible conversions occurred at 74% at 85 C to 24% at 155 C, in a cross-linked polymeric system. Park et al. [14] fabricated a novel self-mendable bis-maleimide tetrafuran (2MEP4F), based on DA reaction chemistry. Multiple self-healing after electrical resistive heating and shape memory effects were observed for these functional composites. 2MEP4F polymers restored molecular structure and retained the similar or slightly improved fracture resistance properties by thermally reversible DA and retro-DA chemistry [15]. Anthracene
maleimide DA system were also studied by many researchers for fabricating thermally reversible self-healing systems [16,17]. Poly(ethylene terephthalate) copolymers containing anthracene structural units are modified by DA reactions with maleimides was found to be thermally reversible at 250 C [16]. Syrett et al. [18] reported the synthesis of novel well-defined linear and star methyl methacrylate polymers bearing anthracene
maleimide DA adducts within their macromolecular backbone exhibiting selfhealing properties. An evaluation of their ability to cleave on heating to 200 C and to reform on slow cooling back to ambient temperature was observed. H1NMR spectra revealed two new signals at 6.49 and 5.3 ppm, not previously seen in the spectra of the DA polymers were observed after the retro-DA process. The reformed product formed was a mixture of endo and exo isomeric DA linkers. Thermally stable self-healing polymer based on DA reaction between anthracene and maleimide were fabricated by Yoshie et al. [19]. Here, self-healing was accomplished at room temperature by DA addition reaction and retro DA reverse reaction was induced by mechanical stress. Wang et al. [20] reported

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1.1 Introduction

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the self-healing of polyurethane elastomer based on DA reaction between furan and bismaleimide end capped groups and obtained an efficiency of 81% after tensile break recovery. Thermoreversible reactions at the alkoxyamine junction also serve as a method to induce self-healing properties and restoration of cracks in the polymeric chain. The dynamic equilibrium of C
ON bond in alkoxyamine incorporated epoxy cured with diethylenetriamine enabled restoration of impact properties which was studied by Rong et al. [21]. The reversibility of C
ON bond breakage and reformation was also enhanced by incorporation of Si
O linkages in the epoxy polymeric chains. Stiff polyurethane polymer with self-healing property utilizing alkoxyamine chemistry was reported by Zhang et al. [22]. These materials exhibited repeatable self-healing property at room temperature and exhibited impact fracture recovery at room temperature. The polymeric systems which exhibit self-healing through reversible hydrogen bonding interaction had been fabricated successfully by many researchers [23
26]. Selfhealing rubber by reversible hydrogen bonds was fabricated at room temperature which exhibited little creep on load by Liebler et al. [27]. Guadagno et al. [28] reported selfhealing in epoxy-MWCNT nanocomposites via reversible hydrogen bonding. MWCNTs were covalently functionalized with hydrogen bonding moieties, such as barbiturate and thymine, and incorporated into rubber-modified epoxy network established reversible hydrogen bonded MWCNT bridges across the matrix. Here, they have achieved characteristic self-healing efficiencies ranging for more than 50%. Hydrogels functionalized with poly(styrene-acrylic acid) core
shell nanoparticles bearing carboxyl groups having self-healing property was fabricated by Han et al. [29]. Enhancing mechanical properties together with self-healing property is a challenge in the case of elastomers. Huang et al. [30] fabricated a novel self-healing elastomer with high tensile stress (2.6 MPa), high toughness (B14.7 MJ m23), high stretchability (B1700%), and excellent self-healing ability (90%). Elastomers with self-healing property are desirable for fabrication of flexible electronics. The elastomer was synthesized by a one-pot polycondensation reaction between bis(3-aminopropyl)-terminated poly(dimethylsiloxane) (PDMS) and 2,40 -tolylene diisocyanate. The prepared elastomer was finally coordinated with Al(III) ions. Both hydrogen bonds and coordination bonds were responsible for inducing self-healing property together with enhanced mechanical property. Finally they were successful in demonstrating a flexible electrode with their fabricated PDMS
TDI
Al elastomer film (Fig. 1.3). Self-healing systems managed by π
π stacking interactions have also gained attention recently. Supramolecular polymeric systems based on π
π stacking interactions were used for imparting thermoreversible healing behavior. Self-healing of supramolecular polymeric systems based on chain folding of π-electron-poor receptor sites of polydiimide sites and π-electron-rich chain ends of the polysiloxane were reported by Greenland et al. [31]. Supramolecular polymeric systems based on blend of a pyrenyl-tweezer-ended polyamide intercalating with a chain-folding polyimide showed self-healing property and enhanced toughness were reported by Colquhoun et al. [32]. Another work done by the same group [33] reported both hydrogen bonding and aromatic π 2 π stacking between the π-electrondeficient diimide groups and the π-electron-rich pyrenyl units which accounted for selfhealing property of supramolecular polymeric systems.

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

FIGURE 1.3 (A) SEM images of the Au film electrode. (B) Schematic demonstration of the stretching process involving the flexible electrode. (C) Stress
strain curves of the polymer films for PDMS
TDI
Al-3 (black), original (red), and healing electrode (blue). (D) Photographs of a notched strain electrode on relaxed and stretching station. (E) Change of relative resistance of the electrode under different strains. (F) Durability test of the strain electrode under a repeated stretching and release of 40%. (G) Photographs of the electrode on relaxed and stretching station. (H) Photographs of the healing process for the electrode with an LED in series with a self-healing electrode. PDMS, Polydimethylsiloxane. Source: Reprinted with permission from X. Wu, J. Wang, J. Huang, S. Yang, Robust, stretchable, and self-healable supramolecular elastomers synergistically cross-linked by hydrogen bonds and coordination bonds. ACS Appl. Mater. Interfaces. 11 (2019) 7387
7396. doi:10.1021/acsami.8b20303, Copyright (2019) American Chemical Society.

1.2 Role of nanofillers in self-healing polymeric systems The nanofillers aid in improving the self-healing efficiency and mechanical properties in polymeric systems. It will provide better scratch resistance and thermal stability. Nanofillers even catalyzes the self-healing action. Nanofillers such as graphene, carbon nanotubes, nanosilica, and nanocellulose were usually used in self-healing polymeric

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1.2 Role of nanofillers in self-healing polymeric systems

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systems. Synergestic performance of the material for various functional applications often compromises with the mechanistic performance. Therefore imparting self-healing functions improves the overall performance and life time of the material. The ease of functionalization of the nanofillers also could control the polymer
nanofiller interfacial movement, interaction chemistries for healing action, mechanical properties enhancement and stiffness reinforcement. Usually the capsule-based healing system need to compromise with the tensile properties. Wang et al. [34] demonstrated self-healing properties of rubber
MWCNT nanocomposites based on DA bonding. MWCNT acted as both a healant and reinforcing agent in furfuryl-grafted styrene-butadiene rubber (SBR). The furfurylgrafted SBR and furfuryl-terminated MWCNT were reacted with bifunctional maleimide to form DA adduct. The nanocomposites exhibited thermal healing and higher healing efficiency was observed for nanocomposites which were subjected to longer healing time and higher healing temperature. Han et al. [35] developed a conducting polymer hydrogels based on a viscoelastic polyvinyl alcohol (PVA)
borax gel matrix and nanostructured cellulose nanofibers
polypyrrole (CNFs
PPy) complexes. CNF acted as a biotemplate and PPy imparted conductive property to the hydrogels. CNF
PPy complexes tangle with the PVA through hydrogen bonding and also form reversible cross-linking with the borate ions anchored on the PVA chains. The reversible cross-linking renders the self-healing property. Kongparakul et al. [36] developed a self-healing anticorrosive coating based on epoxy nanosilica composites. Epoxy resins embedded with 3 wt.% (3-glycidoxypropyl)trimethoxysilane-modified nanosilica and 10 wt.% perfluorooctyl triethoxysilane (POTS) microcapsules delivered the best anticorrosive properties with the corrosion rate of 0.09 mm year21 and oxygen permeability about 0.14 barrer. The addition of modified nanosilica and selfhealing agent apparently increased the length of the diffusion pathways and thereby decreased the oxygen permeability of the coating. Rana and coworkers [37] reported a graphene-based self-healing system where the nanofillers had compensated the reduced tensile strength due to the embedded microcapsules. Apart from this the nanofiller had also acted as a catalyst for self-healing reaction in epoxy-based systems. Guadagno et al. [28] developed a novel epoxy nanocomposites exhibiting self-healing properties by reversible hydrogen bonding. The functionalization of MWCNT by hydrogen bonding groups, such as barbiturate and thymine, had effected in inducing self-healing property. Kong et al. [38] demonstrated EMI shielding of Fe3O4-loaded multiwalled carbon nanotubes/polyazomethine (PAM) nanocomposites. These materials exhibited excellent healing efficiency of 95% imparted by dynamic imine bonds. Apart from the EMI shielding effectiveness, these nanocomposites could be degraded on dipping in an acidic solution for 30 min at room temperature and the degraded products could again be used for the synthesis of the nanocomposites. Yu et al. [39] developed a hydrogel having superior mechanical and self-healing property. Hydrogels exhibiting both mechanical strength and self healability are rare. They prepared a novel GO/poly(acryloyl-6-aminocaproic acid) composite hydrogels exhibiting self-healing property in response to pH stimulus. The dispersion of nanofillers still remains a challenge for researchers. The role of nanofillers in self-healing polymeric systems will be dealt more in the following chapters.

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

1.3 Key developments in the field of self-healing polymeric systems Research on self-healing polymer-based materials was quite active from the early 1900s. With the advent of innovative techniques, key understanding in the field of material science has been increased. Self-healing of polymeric systems could be induced either autonomously or nonautonomously. Apart from inducing self-healing properties without compromising other properties of the materials is a challenge for practical applications. Self-healing materials are extensively used as structural components such as coatings, biomedical applications, prosthetics, sensing applications, medical implants, electrical applications, and membrane separation. One of the most important applications for the self-healing polymeric materials was used for corrosion protection. Sun et al. [40] developed a superhydrophobic coating obtained by layer-by-layer assembly of poly(allylamine hydrochloride) and sulfonated poly(ether ketone). Shchukin et al. [41] also demonstrated layer-by-layer assembled polymeric coatings embedded with corrosion inhibitor encapsulated in nanocontainer for corrosion protection. The cracks induced pH changes which released the corrosion inhibitor benzotriazole. Park and Braun [42] utilized electrospinning technique for polysiloxane-based healant encapsulation in an acrylate matrix. Here, healing agents were encapsulated into a bead on string morphology and electrospun onto a polymeric substrate. The advantage of this technique underlies in the ability to control the size of microcapsules and thereby the release of healing agents. Polymeric electrolytes could be used as alternatives to liquid electrolytes for solving the issue of electrolyte leakage and inflammability. The self-mendable polymer electrolyte will be able to address the issues related to short circuits and service life of the battery due to crack formation. Xue et al. [43] had fabricated a flexible, highly stretchable polymer electrolyte having self-mending property via quadruple hydrogen bonding of ureidopyrimidinone (UPy) moieties in polyethylene backbone. The polyethylene glycol (PEG) side chains impart high ionic conductivity. Upy moieties impart physical cross-links via hydrogen bonding resulted in highly stretchable and flexible electrolyte. A polymeric electrolyte with self-healing property was developed for developing high performance lithium ion battery using SiO2. UPy-functionalized SiO2 (SiO2-UPy) facilitates the uniform dispersion of SiO2 in the polymer matrix, which is essential for fast conduction of Li ions [44]. Here, the self-healing property is induced by formation of supramolecular network between the polymer matrix via UPy on SiO2 and polymer matrix. Quadruple hydrogen bonding is responsible for self-healing property. The power transmission cables require the insulating dielectric polymers which needed to be protected from electric treeing. Wang et al. [45] used PEG-functionalized iron oxide/polypropylene composites which exhibited healing property after damaged by electric treeing along with restoration of insulating properties also. Polyelectrolytes based on poly(acrylic acid) (PAA) cross-linked by hydrogen bonding having silica particles exhibit the same capacitance even after 20 cycles of healing [46]. Pan et al. [47] reported a hydrogel electrolyte based on sodium alginate cross-linked by dynamic catechol
borate ester bonding. The most important advantage of this electrolyte is that the healability and capacitance properties were maintained both at room temperature and low temperature.

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1.3 Key developments in the field of self-healing polymeric systems

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Park and coworkers [48] had developed a stretchable and self-healing energy storage device based on nickel flakes, eutectic gallium indium particles and carboxylated polyurethane. It has excellent healing ability and retains 100% stretchability after healing and shows about 75% electrical healing efficiency. The thermally driven self-healing at 80 C is due to the reassociation of hydrogen bonds anchored by carboxyl group on the PU surface. Fig. 1.4 represents the self-healing action of the developed device. Multifunctional materials which exhibit both mechanical self-healing and a functional property which make it suitable for robust applications are a challenge. Self-healing materials exhibiting

FIGURE 1.4 Self-healing action of energy storage device based on PU; (A) photos of the device in original, broken, and healed, (B) GCD profile, (C) capacity retention at various breaking/healing cycles, (D) GCD profiles and capacity retention at different strains of the device with different strains after breaking/healing, and (E) device under 0%, 50%, and 100% strain after breaking/healing [48]. GCD, Galvanostatic charge discharge.

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

sensing properties and flexibility could be used for soft robotics, prosthetics and other applications. But integrating both functions is a challenge for the scientists. Coe et al. [49] fabricated a material by layer-by-layer assembly of copper-clad polyimide sheets, polyimide sheets and an ultraviolet (UV)-curable epoxy. The UV curable resin acted as a structural adhesive and self-healing component. Polysiloxane due to its stretchability was used widely for flexible electronic devices. Jiang et al. [50] had demonstrated a pressure sensor on polysiloxane cross-linked with DA hydrogen bonds. The incorporation of graphene nanosheets imparted high mechanical and electrical properties. The fabricated sensor undergoes a solid
liquid
solid transformation during self-healing process. The functionalization of PDMS with furan and maleimide and further polymerization process resulted in a DA product. The thermal reversibility of DA bond breaking and formation resulted in self-healing of the polysiloxane elastomer. Zhang et al. [51] proposed a novel self-healing design based on polyurethane/silver nanowires for strain sensing applications. The selfhealing was induced in sunlight for the material causing the exchange of the included disulfide bonds. The advantage of sunlight driven healing is that the material could be used for applications related to human body such as human
machine interactions, smart prosthetics, and wearable health monitoring devices. Xu et al. [52] developed a ternary polymer composite comprising of polyaniline, PAA, and phytic acid showing high stretchability (500%), excellent electrical conductivity (0.12 S cm21), and self-healing properties. The self-healing is furnished by the hydrogen bonds and the composites is highly strain and pressure sensitive and hence could be utilized as strain and pressure sensor. Zhang et al. [53] developed a novel self-healing strain sensor based on commercially available elastomers through a supramolecular assembly. The supramolecular hierarchical nanostructure of carbon nanotubes was constructed by activated cellulose nanocrystals through thermally reversible hydrogen bonds. The supramolecular assembly of polyethyleimine with carboxyl-functionalized nitrile rubber was showing self-healing and excellent electrical and mechanical properties. Environmental pollution caused by either human interference/other factors may pollute the water bodies and may pose serious threat to health of living beings. Membrane separation of waste water is an emerging technique to meet the challenges regarding pollution. Membrane separation is feasible only when the material could be used for long term. The cost issues for fabrication of membranes are quite high. Lu et al. [53] fabricated a multiwalled carbon nanotube film and polydivinylbenzene which imparts superior hydrophobicity. The coating of POTS (1H, 1H, 2H, 2H-perfluorooctyl triethoxysilane) imparted selfhealing ability for this film. The self-healing of the polymer-coated MWCNT film was tested by etching the POTS layer to O2 plasma. This significantly reduced the fluorine content on the film surface, but after 15 min the content of fluorine was restored. They also observed the deterioration of hydrophobicity just after the O2 plasma treatment and the restoration of hydrophobicity after 15 min. The mechanism of self-healing is based on the hydrolysis of POTS in the presence of water and subsequent cross-linking to form a polysilicone compound causing restoration of hydrophobicity. The membrane showed superior ability to separate water-in-oil emulsions and oil/water mixtures. A facile self-healing and super wettable nanofibrous poly(ethylenimine)-poly(acrylic acid)/hyaluronic acid (bPEI-PAA/HA, named PPH) membrane was fabricated by electrospinning and layer-bylayer approach by Lu and coworkers [54]. A pure PAN membrane was fabricated by

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1.4 Challenges for fabricating self-healing materials based on polymeric systems

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electrospinning technique, followed by dip-coating of a layer-by-layer assembly of bPEI-PAA/HA. The self-healing ability was immediately evidenced after 30 s in the presence of water. The self-healing ability is brought about by the hydrogen bonding. When a crack occurs and kept in contact with water, the polymers have a high ability to swell and fill the gap caused by the damage. Polymeric membranes are widely used in redox flow batteries for separation of charge carriers in redox active electrolytes. Xu et al. [55] synthesized a block copolymer from vinylbenzyl chloride and 2-((4-vinylbenzyloxy) methyl) furan by RAFT polymerization which could be used as anion exchange membranes. The self-healing property was incorporated by temperature controlled DA reaction which healed the crack at 150 C. The material delivered a stable cycling performance over 100 consecutive charge/discharge cycles, with a Coulombic efficiency of more than 97% and an energy efficiency of B79%. Qin and coworkers [56] had fabricated an anisotropic, self-healing hydrogels with multiresponsive actuating ability. The metal nanostructure assemblies could provide dynamic interactions with the polymers which could be utilized for self-healing applications. They proposed thiolate-silver (Ag) linkages from silver nanoparticles for fabrication of tough and self-healing hydrogels. A water soluble disulfide ligand N,N-bis(acryloyl)cystamine (BACA) is used for modifying silver nanoparticles. Ag@BACA nanocomposites were obtained by mixing in aqueous medium at room temperature, resulting in the cleavage of S-S bond. Under the UV radiation, the solvophobic effect of silver nanoparticles caused the Ag@BACA nanocomposites intergrate into 2D lamellar assemblies. The polymerization of Ag@BACA with polyacrylamide resulted in the formation of Ag/PAM lamellae under UV radiation. After 30 min of UV radiation exposure, SNPP hydrogels were formed via photothermal polymerization. The large amount of reversible RS-Ag sites could be regulated in the presence of light and pH stimulus. The photothermal effect of Ag nanoparticles rendered the self-healing property under NIR laser. The affinity of Ag to Lewis proton in strong acids rendered the self-healing under the effect of pH. Fig. 1.5 illustrates the self-healing performance of SNPP hydrogels. The unique anisotropic structure made SNPP gels to exhibit in-plane and out-of-plane bending actuations. Similarly gold-thiolate interactions were also used as a self-healing motif with remarkable mechanical properties in nanocomposite hydrogels [57].

1.4 Challenges for fabricating self-healing materials based on polymeric systems There are a lot of challenges related with the fabrication of self-healing polymeric systems. The reliability of the material is determined by the efficiency to fully undergoing healing and regaining the original properties. There is another important parameter which affects the self-healing polymeric systems; regarding the continuity of healing life-cycle of polymeric systems. There is apparently another important parameter which should be taken care; that is, on the localized response of self-healing action. Intrinsic self-healing had tackled the problem to some extent. For the effective performance of the smart materials, these factors should be taken into account. Research should be focused in this aspect. So, more works should be done exploring the novel self-healing chemistries, repeated and localized supply of healing agents in extrinsic healing, restoration of strength, stiffness,

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1. Self-healing polymeric systems—fundamentals, state of art, and challenges

FIGURE 1.5 Self-healing performance of SNPP hydrogels. (A) Illustration of self-healing performance under laser and pH, (B) time
temperature plot of SNPP hydrogels at different powers of laser output, (C) stress
strain curves of SNPP hydrogels for different times of NIR laser healing, (D) UV
vis spectra of Ag@BACA recorded to monitor the binding behavior between BACA and silver NPs in different pH solutions, and (E) stress
strain curves of the healed SNPP hydrogel piece through a pH mediated way [56]. BACA, N,N-bis(Acryloyl)cystamine; UV, ultraviolet.

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toughness, and overall performance of the material. The stability of the materials, performance, and self-healing action should not be affected at ambient conditions. So research should be based on these challenges.

1.5 Conclusions Self-healing in polymeric systems has attracted attention in the recent years. Selfhealing systems mimics the biological systems, of the ability of the cellular systems to heal by itself. Due to wide applicability of polymeric systems in various applications, methods to improve its structural integrity and life time are important. This chapter deals with the fundamentals of healing in polymeric systems and the different types of healing systems used in polymeric systems. Usually extrinsic healing systems is accomplished by three approaches based on micro/nanocapsules embedment, hollow fiber embedment, and microvascular system. It relies on the fact that no external stimuli are required for initiation of self-healing action. Although, these systems have limited action since the healing action ceases once the catalysts are consumed. Intrinsic healing systems require an external stimulus, which is based on the reversible chemical interactions. The advent of nanotechnology has aided in increasing the self-healing action together with improving the stiffness of the material. Apparently multiresponsive smart polymers were fabricated which will self-heal based on the specific chemical interactions in response to different stimuli. More insights into the chemistries responsible for self-healing action need to be explored. This will improve the life time and performance of soft materials which could be utilized in a wide range of applications such as coatings, polymeric membranes for water purification, smart prosthetics, and health monitoring devices.

References [1] J.L. Mercy, S. Prakash, 8
Investigation of Damage Processes of a Microencapsulated Self-Healing Mechanism in Glass Fiber-Reinforced Polymers, Elsevier, 2019. Available from: https:doi.org/10.1016/B978-008-102289-4.00008-4. [2] P. Du, X. Wang, Reversible cross-linking polymer-based self-healing materials, Recent. Adv. Smart SelfHealing Polym. Compos (2015) 159
179. Available from: https://doi.org/10.1016/B978-1-78242-280-8.00006-6. [3] J.W.C. Pang, I.P. Bond, ‘Bleeding composites’—damage detection and self-repair using a biomimetic approach, Compos. Part A Appl. Sci. Manuf 36 (2005) 183
188. Available from: https://doi.org/10.1016/J.COMPOSITESA. 2004.06.016. [4] M. Hucker, I. Bond, S. Bleay, S. Haq, Investigation into the behaviour of hollow glass fibre bundles under compressive loading, Compos. Part A Appl. Sci. Manuf 34 (2003) 1045
1052. Available from: https://doi. org/10.1016/S1359-835X(03)00238-0. [5] R.S. Trask, I.P. Bond, Biomimetic self-healing of advanced composite structures using hollow glass fibres, Smart Mater. Struct 15 (2006) 704
710. Available from: https://doi.org/10.1088/0964-1726/15/3/005. [6] S. Bleay, C. Loader, V. Hawyes, L. Humberstone, P. Curtis, A smart repair system for polymer matrix composites, Compos. Part A Appl. Sci. Manuf 32 (2001) 1767
1776. Available from: https://doi.org/10.1016/S1359835X(01)00020-3. [7] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater 6 (2007) 581
585. Available from: https://doi.org/10.1038/nmat1934. [8] M. Samadzadeh, S.H. Boura, M. Peikari, S.M. Kasiriha, A. Ashrafi, A review on self-healing coatings based on micro/nanocapsules, Prog. Org. Coat 68 (2010) 159
164. Available from: https://doi.org/10.1016/J.PORGCOAT. 2010.01.006.

Self-Healing Polymer-Based Systems

14

1. Self-healing polymeric systems—fundamentals, state of art, and challenges

[9] C.J. Hansen, W. Wu, K.S. Toohey, N.R. Sottos, S.R. White, J.A. Lewis, Self-healing materials with interpenetrating microvascular networks, Adv. Mater 21 (2009) 4143
4147. Available from: https://doi.org/10.1002/ adma.200900588. [10] K.S. Toohey, C.J. Hansen, J.A. Lewis, S.R. White, N.R. Sottos, Delivery of two-part self-healing chemistry via microvascular networks, Adv. Funct. Mater 19 (2009) 1399
1405. Available from: https://doi.org/10.1002/ adfm.200801824. [11] J. Li, Y. Hu, H. Qiu, G. Yang, S. Zheng, J. Yang, Coaxial electrospun fibres with graphene oxide/PAN shells for self-healing waterborne polyurethane coatings, Prog. Org. Coat 131 (2019) 227
231. Available from: https://doi.org/10.1016/j.porgcoat.2019.02.033. [12] A.M. Peterson, R.E. Jensen, G.R. Palmese, Reversibly cross-linked polymer gels as healing agents for epoxy 2 amine thermosets, ACS Appl. Mater. Interfaces 1 (2009) 992
995. Available from: https://doi.org/ 10.1021/am900104w. [13] B.J. Adzima, H.A. Aguirre, C.J. Kloxin, T.F. Scott, C.N. Bowman, Rheological and chemical analysis of reverse gelation in a covalently cross-linked Diels 2 Alder polymer network, Macromolecules 41 (2008) 9112
9117. Available from: https://doi.org/10.1021/ma801863d. [14] J.S. Park, T. Darlington, A.F. Starr, K. Takahashi, J. Riendeau, H. Thomas Hahn, Multiple healing effect of thermally activated self-healing composites based on Diels
Alder reaction, Compos. Sci. Technol 70 (2010) 2154
2159. Available from: https://doi.org/10.1016/J.COMPSCITECH.2010.08.017. [15] T.A. Plaisted, S. Nemat-Nasser, Quantitative evaluation of fracture, healing and re-healing of a reversibly cross-linked polymer, Acta Mater 55 (2007) 5684
5696. Available from: https://doi.org/10.1016/J. ACTAMAT.2007.06.019. [16] J.R. Jones, C.L. Liotta, D.M. Collard, D.A. Schiraldi, Cross-linking and modification of poly(ethylene terephthalate-co-2,6-anthracenedicarboxylate) by Diels-Alder reactions with maleimides, Macromolecules 32 (1999) 5786
5792. Available from: https://doi.org/10.1021/ma990638z. [17] R.M. Kriegel, K.L. Saliba, G. Jones, D.A. Schiraldi, D.M. Collard, The rapid chain extension of anthracenefunctional polyesters by the Diels-Alder reaction with bismaleimides, Macro mol. Chem. Phys 206 (2005) 1479
1487. Available from: https://doi.org/10.1002/macp.200500063. [18] J.A. Syrett, G. Mantovani, W.R.S. Barton, D. Price, D.M. Haddleton, Self-healing polymers prepared via living radical polymerisation, Polym. Chem 1 (2010) 102. Available from: https://doi.org/10.1039/b9py00316a. [19] N. Yoshie, S. Saito, N. Oya, A thermally-stable self-mending polymer networked by Diels
Alder cycloaddition, Polym. (Guildf.) 52 (2011) 6074
6079. Available from: https://doi.org/10.1016/J.POLYMER.2011.11.007. [20] P. Du, X. Liu, Z. Zheng, X. Wang, T. Joncheray, Y. Zhang, Synthesis and characterization of linear selfhealing polyurethane based on thermally reversible Diels
Alder reaction, RSC Adv 3 (2013) 15475. Available from: https://doi.org/10.1039/c3ra42278j. [21] C. Yuan, M.Q. Zhang, M.Z. Rong, Application of alkoxyamine in self-healing of epoxy, J. Mater. Chem. A 2 (2014) 6558
6566. Available from: https://doi.org/10.1039/c4ta00130c. [22] Z.P. Zhang, M.Z. Rong, M.Q. Zhang, C. Yuan, Alkoxyamine with reduced homolysis temperature and its application in repeated autonomous self-healing of stiff polymers, Polym. Chem 4 (2013) 4648. Available from: https://doi.org/10.1039/c3py00679d. [23] R.F.M. Lange, M. Van Gurp, E.W. Meijer, Hydrogen-bonded supramolecular polymer networks, J. Polym. Sci. Part A Polym. Chem. 37 (1999) 3657
3670. doi:10.1002/(SICI)1099-0518(19991001)37:19 , 3657::AIDPOLA1 . 3.0.CO;2-6. [24] K. Chino, M. Ashiura, Themoreversible cross-linking rubber using supramolecular hydrogen-bonding networks, Macromolecules 34 (2001) 9201
9204. Available from: https://doi.org/10.1021/ma011253v. [25] C.-C. Peng, V. Abetz, A simple pathway toward quantitative modification of polybutadiene: a new approach to thermoreversible cross-linking rubber comprising supramolecular hydrogen-bonding networks, Macromolecules 38 (2005) 5575
5580. Available from: https://doi.org/10.1021/ma050419f. [26] E. Kolomiets, E. Buhler, S.J. Candau, J.-M. Lehn, Structure and properties of supramolecular polymers generated from heterocomplementary monomers linked through sextuple hydrogen-bonding arrays, Macromolecules 39 (2006) 1173
1181. Available from: https://doi.org/10.1021/ma0523522. [27] P. Cordier, F. Tournilhac, C. Soulie´-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977
980. Available from: https://doi.org/10.1038/nature06669.

Self-Healing Polymer-Based Systems

References

15

[28] L. Guadagno, L. Vertuccio, C. Naddeo, E. Calabrese, G. Barra, M. Raimondo, et al., Self-healing epoxy nanocomposites via reversible hydrogen bonding, Compos. Part B Eng 157 (2019) 1
13. Available from: https:// doi.org/10.1016/J.COMPOSITESB.2018.08.082. [29] J. Chen, R. An, L. Han, X. Wang, Y. Zhang, L. Shi, et al., Tough hydrophobic association hydrogels with selfhealing and reforming capabilities achieved by polymeric core-shell nanoparticles, Mater. Sci. Eng. C 99 (2019) 460
467. Available from: https://doi.org/10.1016/J.MSEC.2019.02.005. [30] X. Wu, J. Wang, J. Huang, S. Yang, Robust, stretchable, and self-healable supramolecular elastomers synergistically cross-linked by hydrogen bonds and coordination bonds, ACS Appl. Mater. Interfaces 11 (2019) 7387
7396. Available from: https://doi.org/10.1021/acsami.8b20303. [31] S. Burattini, H.M. Colquhoun, B.W. Greenland, W. Hayes, A novel self-healing supramolecular polymer system, Faraday Discuss 143 (2009) 251. Available from: https://doi.org/10.1039/b900859d. [32] S. Burattini, B.W. Greenland, W. Hayes, M.E. Mackay, S.J. Rowan, H.M. Colquhoun, et al., Polymer based on tweezer-type π 2 π stacking interactions: molecular design for healability and enhanced toughness, Chem. Mater 23 (2011) 6
8. Available from: https://doi.org/10.1021/cm102963k. [33] S. Burattini, B.W. Greenland, D.H. Merino, W. Weng, J. Seppala, H.M. Colquhoun, et al., A healable supramolecular polymer blend based on aromatic π 2 π stacking and hydrogen-bonding interactions, J. Am. Chem. Soc 132 (2010) 12051
12058. Available from: https://doi.org/10.1021/ja104446r. [34] X. Kuang, G. Liu, X. Dong, D. Wang, Enhancement of mechanical and self-healing performance in multiwall carbon nanotube/rubber composites via Diels-Alder bonding, Macromol. Mater. Eng 301 (2016) 535
541. Available from: https://doi.org/10.1002/mame.201500425. [35] Q. Ding, X. Xu, Y. Yue, C. Mei, C. Huang, S. Jiang, et al., Nanocellulose-mediated electroconductive selfhealing hydrogels with high strength, plasticity, viscoelasticity, stretchability, and biocompatibility toward multifunctional applications, ACS Appl. Mater. Interfaces 10 (2018) 27987
28002. Available from: https:// doi.org/10.1021/acsami.8b09656. [36] S. Kongparakul, S. Kornprasert, P. Suriya, D. Le, C. Samart, N. Chantarasiri, et al., Self-healing hybrid nanocomposite anticorrosive coating from epoxy/modified nanosilica/perfluorooctyl triethoxysilane, Prog. Org. Coat 104 (2017) 173
179. Available from: https://doi.org/10.1016/j.porgcoat.2016.12.020. [37] S. Rana, D. Do¨hler, A.S. Nia, M. Nasir, M. Beiner, W.H. Binder, “Click”-triggered self-healing graphene nanocomposites, Macromol. Rapid Commun 37 (2016) 1715
1722. Available from: https://doi.org/10.1002/ marc.201600466. [38] X. Dai, Y. Du, J. Yang, D. Wang, J. Gu, Y. Li, et al., Recoverable and self-healing electromagnetic wave absorbing nanocomposites, Compos. Sci. Technol 174 (2019) 27
32. Available from: https://doi.org/ 10.1016/j.compscitech.2019.02.018. [39] H.-P. Cong, P. Wang, S.-H. Yu, Stretchable and self-healing graphene oxide
polymer composite hydrogels: a dual-network design, Chem. Mater 25 (2013) 3357
3362. Available from: https://doi.org/10.1021/ cm401919c. [40] Y. Li, L. Li, J. Sun, Bioinspired self-healing superhydrophobic, Coat. Angew. Chem. Int. Ed 49 (2010) 6129
6133. Available from: https://doi.org/10.1002/anie.201001258. [41] D.G. Shchukin, M. Zheludkevich, K. Yasakau, S. Lamaka, M.G.S. Ferreira, H. Mo¨hwald, Layer-by-layer assembled nanocontainers for self-healing corrosion protection, Adv. Mater 18 (2006) 1672
1678. Available from: https://doi.org/10.1002/adma.200502053. [42] J.-H. Park, P.V. Braun, Coaxial electrospinning of self-healing coatings, Adv. Mater 22 (2010) 496
499. Available from: https://doi.org/10.1002/adma.200902465. [43] B. Zhou, D. He, J. Hu, Y. Ye, H. Peng, X. Zhou, et al., A flexible, self-healing and highly stretchable polymer electrolyte via quadruple hydrogen bonding for lithium-ion batteries, J. Mater. Chem. A 6 (2018) 11725
11733. Available from: https://doi.org/10.1039/C8TA01907J. [44] B. Zhou, Y.H. Jo, R. Wang, D. He, X. Zhou, X. Xie, et al., Self-healing composite polymer electrolyte formed via supramolecular networks for high-performance lithium-ion batteries, J. Mater. Chem. A (2019). Available from: https://doi.org/10.1039/C9TA01214A. [45] Y. Yang, J. He, Q. Li, L. Gao, J. Hu, R. Zeng, et al., Self-healing of electrical damage in polymers using superparamagnetic nanoparticles, Nat. Nanotechnol 14 (2019) 151
155. Available from: https://doi.org/10.1038/ s41565-018-0327-4.

Self-Healing Polymer-Based Systems

16

1. Self-healing polymeric systems—fundamentals, state of art, and challenges

[46] Y. Huang, M. Zhong, Y. Huang, M. Zhu, Z. Pei, Z. Wang, et al., A self-healable and highly stretchable supercapacitor based on a dual crosslinked polyelectrolyte, Nat. Commun 6 (2015) 10310. Available from: https:// doi.org/10.1038/ncomms10310. [47] F. Tao, L. Qin, Z. Wang, Q. Pan, Self-healable and cold-resistant supercapacitor based on a multifunctional hydrogel electrolyte, ACS Appl. Mater. Interfaces 9 (2017) 15541
15548. Available from: https://doi.org/ 10.1021/acsami.7b03223. [48] S. Park, G. Thangavel, K. Parida, S. Li, P.S. Lee, A stretchable and self-healing energy storage device based on mechanically and electrically restorative liquid-metal particles and carboxylated polyurethane composites, Adv. Mater 31 (2019) 1805536. Available from: https://doi.org/10.1002/adma.201805536. [49] J.A. Carlson, J.M. English, D.J. Coe, A flexible, self-healing sensor skin, Smart Mater. Struct. 15 (2006) N129
N135. Available from: https://doi.org/10.1088/0964-1726/15/5/N05. [50] L. Zhao, B. Jiang, Y. Huang, Self-healable polysiloxane/graphene nanocomposite and its application in pressure sensor, J. Mater. Sci 54 (2019) 5472
5483. Available from: https://doi.org/10.1007/s10853-018-03233-6. [51] Y.X. Song, W.M. Xu, M.Z. Rong, M.Q. Zhang, A sunlight self-healable transparent strain sensor with high sensitivity and durability based on a silver nanowire/polyurethane composite film, J. Mater. Chem. A 7 (2019) 2315
2325. Available from: https://doi.org/10.1039/C8TA11435H. [52] T. Wang, Y. Zhang, Q. Liu, W. Cheng, X. Wang, L. Pan, et al., Highly stretchable, and solution processable conductive polymer composite for ultrasensitive strain and pressure sensing, Adv. Funct. Mater 28 (2018) 1705551. Available from: https://doi.org/10.1002/adfm.201705551. [53] X. Liu, C. Lu, X. Wu, X. Zhang, Self-healing strain sensors based on nanostructured supramolecular conductive elastomers, J. Mater. Chem. A 5 (2017) 9824
9832. Available from: https://doi.org/10.1039/ C7TA02416A. [54] Y. Cai, D. Chen, N. Li, Q. Xu, H. Li, J. He, et al., Self-healing and superwettable nanofibrous membranes for efficient separation of oil-in-water emulsions, J. Mater. Chem. A 7 (2019) 1629
1637. Available from: https://doi.org/10.1039/C8TA10254F. [55] J. Hou, Y. Liu, Y. Liu, L. Wu, Z. Yang, T. Xu, Self-healing anion exchange membrane for pH 7 redox flow batteries, Chem. Eng. Sci 201 (2019) 167
174. Available from: https://doi.org/10.1016/J.CES.2019.02.033. [56] H. Qin, T. Zhang, N. Li, H.-P. Cong, S.-H. Yu, Anisotropic and self-healing hydrogels with multi-responsive actuating capability, Nat. Commun. 10 (2019) 2202. Available from: https://doi.org/10.1038/s41467-01910243-8. [57] H. Qin, T. Zhang, H.-N. Li, H.-P. Cong, M. Antonietti, S.-H. Yu, Dynamic Au-thiolate interaction induced rapid self-healing nanocomposite hydrogels with remarkable mechanical behaviors, Chemistry 3 (2017) 691
705. Available from: https://doi.org/10.1016/J.CHEMPR.2017.07.017.

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C H A P T E R

2 Types of chemistries involved in self-healing polymeric systems Anil K. Padhan and Debaprasad Mandal Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, India

2.1 Introduction: chemical aspects in self-healing process Polymer materials with self-healing ability to repair the damages or cracks from the aging and structural failure due to thermal, electrical, mechanical, and weather aggression increase the service life, safety index, and sustainability [1
3]. Introducing the self-healing properties in polymeric systems and the chemistry of self-healing process for the regeneration of polymer network structure is governed by the chemistry of the different functional groups either by reversible physical interactions or by chemical reactions of these functional groups. From the chemistry point of view, the self-healing reaction will be governed by thermodynamics and/or reaction kinetics. But how the chemical thermodynamics and/ or reaction kinetics of self-healing properties are synchronized with the remodeling of physical network at bulk is intriguing. In general, the choice of self-healings functionality (chemistry) depends on many factors such as easy incorporation in polymer, compatibility or suitability with monomer, tolerances of functional groups, reactivity of functional groups, and reaction conditions as well as reaction conversions. The nature of reactions is also important such as faster kinetics versus reversibility, use of catalysts or external stimuli for construction of the self-healing polymeric materials. Also whether healing components are chemically attached to a polymer backbone or physically dispersed in a polymer matrix as well as the intrinsic properties of the polymer such as physical/chemical compositions and concentrations, density, elasticity, and specific energy will determine their healing ability. For example, the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD) was utilized in microcapsule-based self-healing polymeric materials for the first time by White et al. [4]. Later on other microcapsules chemistry was developed such as azide-alkyne click chemistry, epoxy-amine cross-linking, and hydrosilylation reactions [5]. However, these reactions are irreversible in nature and hence the healing process can be observed for a single time. The reversible reactions under certain conditions are

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2. Types of chemistries involved in self-healing polymeric systems

important for repeating the self-healing process multiple times via formation and reformation of dynamic covalent bonds such as thermoreversible Diels
Alder (DA) reactions and photoreversible disulfides metathesis reactions [1]. Therefore it is also important how easily the self-healing process can be controlled through external stimuli such as temperature, water or moisture, salts, pH, light, mechanical stress, electric, or magnetic fields. Unlike dynamic covalent bonds, dynamic noncovalent supramolecular interactions are reversible in nature but relatively weak when compared with covalent bonds. Hence dynamic noncovalent bonds with supramolecular interactions such as hydrogen bonding and ionic interaction are capable of healing the damages for multiple cycles without any external stimuli. Therefore several strategies and chemistry are developed toward designing of self-healing polymers. In this chapter, various types of chemistries involved in self-healing polymeric systems are discussed. The strategic development of self-healing chemistry with various polymeric materials is useful for fabricating materials for several applications such as biomaterials, bioelectronics, sensors, actuators and coating, paints technologies, electronics, or energy devices such as membranes, 3D/4D printing, tissue engineering, and soft robotics skin and analyzing their potential for realworld high-performance applications [1,6,7].

2.1.1 Extrinsic and intrinsic self-healing The purpose of any self-healing process is to regenerate the cross-linked network from the physical interactions or chemical reactions of various functional groups. The self-healing processes can be classified into intrinsic or extrinsic (with or without add-on components), but the polymer will not repair by themselves unless it is their intrinsic property. Extrinsic self-healing process comprises of external healing agents such as monomers, catalysts, and cross-linkers in the form of capsules, fibers, or vascular. These reactive healing agents are embedded within the polymeric matrix [3,5,6]. When crack or damage occurs, the healing agents are released from the ruptured containers and flow into the cracks to heal the materials by polymerization or through bond formation from chemical reactions. In the extrinsic process, the healing efficiencies can be achieved over 100% even when the damage is large. However, it works only for a single time, whereas the intrinsic self-healing is repeatable and the action can be achievable for multiple times. Intrinsic self-healing materials are designed in such a way that in the below the critical limit of damage, the damaged part can be rejoined by either chemical cross-linking through reversible dynamic covalent bond formation or physical cross-linking via noncovalent supramolecular interactions [1,5,8].

2.2 Key requirements of self-healing process Designing the self-healing polymeric materials requires understanding physical interactions, the chemistry, mechanism of healing process, and the necessary conditions along with key factors required for the self-healing process. Fig. 2.1 shows the general

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2.2 Key requirements of self-healing process

19

FIGURE 2.1 A damage
repair cycle in polymeric materials involved in self-healing process. Source: Reproduced with permission from W.H. Binder, Self-Healing Polymers: From Principles to Applications, John Wiley & Sons, 2013, Copyright 2013, Wiley-VCH.

approach of self-healing process involved physically and chemically. The interdiffusion of chains between the polymeric networks occurs which is essential for any polymeric damages recovery [3]. However, for any self-healing process the key requirement is the regeneration of the cross-linked network [1,6,7]. For example, crack healing in thermoplastic polymers (e.g., amorphous, semicrystalline, block copolymers, and fiberreinforced composites) is possible due to molecular diffusion across the interfaces when two pieces of the same polymers are brought into contact above its glass transition temperature (Tg). Diffusion can be possible when low Tg or viscous liquid type intermediate is generated in situ process [9]. However, chemical functionality is the key requirement to rejoin the crack or damaged network structure via the formation of covalent bonds or supramolecular interaction or both. This polymer network structures are either covalent bonding or physical cross-linking to achieve healing efficiency and regain original physical and mechanical properties. In natural rubber (NR) the self-healing is a natural process which is controlled by the diffusion process and is termed as tack [10]. The self-healing phenomenon is primarily the contribution from the tackiness property of NR. Tack is the ability of two unvulcanized elastomeric materials to resist separation after bringing their surfaces into contact for a short time under light pressure. The tack is called autohesive when an identical chemical composition of unvulcanized elastomeric pieces is taken, and adhesive tack is termed when the two dissimilar compositions unvulcanized elastomeric materials [11]. Apart from van der Waals interactions, the tackiness depends on an intimate contact which allows the interdiffusion of macromolecular segments across the interface.

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2. Types of chemistries involved in self-healing polymeric systems

Diffusion processes control the mechanism of autohesive tack of elastomers and strength of tack. So the diffusion and the autohesive tack are influenced by many factors such as pressure, temperature, contact time, surface roughness at the interface, and the polymer chain structure [11,12]. The high tack of NR is attributed not only to the strain-induced crystallization in the debonding step but also to its high accessible large free volume for interdiffusion of polymer chain segments across the interface during bond formation. Physical diffusion is one of the key for self-healing process. Without diffusion, practically many of the reactions in self-healing chemistry do not give useful bonding. Polymers consist of microcavities or interchain free volume. These free volumes are accessible for the small individual polymer chain segments, which can migrate as kinetic units. Such migration requires relatively little activation energy than a whole macromolecule [11,13]. Thus designing of self-healing polymer materials need tuning of interplays between chemical reactions and physical network remodeling and interdiffusion of polymer chain segments for the development of damage
repair.

2.3 Dynamic covalent network in self-healing Two main strategies exist for the synthesis of self-healing materials: (1) the impregnation of catalysts in the form of microcapsules that are activated during the fracture and form new polymer network and (2) the dynamic reversible bond formation in the polymer matrix either in the form of cross-linking network or as (hetero-) bifunctional building blocks (i.e., monomers, oligomers, etc.). Dynamic covalent chemistry offers an appealing prospect in the construction of selfhealing polymeric materials via reversible covalent bonds. Constitution or distribution of the products and their properties can be tuned by adjusting the thermodynamic control of these reactions involving such a reversible covalent bond. Furthermore strong covalent bonds are more stable than noncovalent bonding but these polymers are smart enough like other supramolecular entities due to reversible nature in response to external stimuli. The nature of dynamic reversible bond can be covalent, such as the C
C bond formation in DA reaction, disulfide and diselenide bridging, imine bonds, acylhydrazones, reversible alkoxyamine, reversible bulky urea and reversible boronate ester bonds, among others, or noncovalent interactions, including hydrogen bonding, ionic interaction, metal
ligand coordination, hydrophobic interaction, π
π stacking, host
guest interaction, and van der Waals interaction. The nature of these cross-linking bonds used in combination with the physicochemical properties of the polymers determines, in part, the stimulus
responsiveness of the materials and their thermodynamic parameters [14].

2.4 Thermoreversible Diels
Alder and retro Diels
Alder chemistry DA reaction is a thermoreversible [4 1 2] cycloaddition of a diene and a dienophile. One of the most studied reversible self-healing polymer systems is based on thermally triggered DA reaction [2,15]. The reversibility of DA and retro-DA (rDA) chemistry

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2.4 Thermoreversible Diels
Alder and retro Diels
Alder chemistry

21

FIGURE 2.2 Self-healing via dynamic covalent of thermoreversible DA reaction. DA, Diels
Alder.

provides promising opportunities for the design and synthetic strategies toward the development of high-performance self-healing polymers as shown in Fig. 2.2. DA reaction is one of the most dominant synthetic strategies for the development of thermally healable material as it is synthetically accessible and versatile which also allows the suitability with various monomer synthons with the capacity of recyclability. Owing to its thermoreversibility and relatively strong covalent bond formation, the introduction of DA cycloaddition ability in the polymer matrix is an excellent opportunity for introducing intrinsic selfrepairing properties. DA and rDA reactions are promising to achieve self-healing ability for multiple times only by changing the temperature without any catalysts or healing agents. Functionalization of polymer chains with the furan and maleimide for DA reaction is a convenient approach to prepare multifunctional compounds for the preparation of DA adduct-based self-healable cross-linked networks. Other diene-dienophile pairs, such as anthracene
maleimide and cyclopentadiene-dicyclopentadiene (CP-DCPD), have also been utilized for the development of thermally induced self-healing materials [16]. Wudl et al. constructed a novel two-component remendable polymeric material with self-healing capability comprising of a 3-arm maleimide (3M) synthon and a 4-arm furan (4F) capped precursor as shown in Fig. 2.3 [17]. This DA adduct shows a 30% debonding extent via rDA reaction at B150 C for 15 min. The disconnected furan and maleimide moieties have enough mobility to heal the cracks of the polymeric material through network reconnection efficiency of about 50%. More importantly the 3M
4F network has a superior tensile strength of 68 MPa compare to epoxy resins (27
88 MPa). Furthermore the self-healing efficiency increased to B80% when a relatively high mobility cross-linked polymer matrix was used from a low-melting furan and maleimide derivatives as alternative monomers. The advantages of using liquid monomers in the construction of self-healing polymers are obvious. For example, a mixture of liquid monomers at room temperature ensures the easy processing of the polymer system in a solvent-free manner or in a solution with lowboiling-point solvents such as acetone. Liu and Hsieh designed multifunctional furan (TF) and maleimide (TMI) compounds for self-healing polymers [19]. The liquid mixture of furan (TF) and maleimide (TMI) monomers at room temperature ensures the processing in a solvent-free condition or in a low-boiling solvent solution such as acetone which can be easily removed. However, the relatively low chain mobility restricts the polymer chain motion to heal the cracks, which can be improved by reducing the chain rigidity of the polymer precursors. The DA and rDA chemistry for the self-healing process was utilized

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2. Types of chemistries involved in self-healing polymeric systems

FIGURE 2.3 Thermally remendable self-healing cross-linked polymer via DA reactions [17,18]. DA, Diels
Alder. Source: Reproduced with permission from S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev. 39 (2010) 1973
1985, The Royal Society of Chemistry.

in various acrylates, biomass-derived furan functional units, polyurethanes, polysiloxane, epoxy resin, and fluoropolymers. Several furan-modified polymer precursors are reported toward DA-based self-healing chemistry. In this context, Singha et al. developed DA-based self-healing chemistry introducing furan functional unit by atom transfer radical copolymerization (ATRP) of furfuryl methacrylate (FMA) with methyl methacrylate (MMA) and a triblock copolymer of poly(2ethylhexyl acrylate) and butyl methacrylate (BMA) (PFMA-co-PBMA) by reversible addition-fragmentation chain-transfer (RAFT) polymerization [20,21]. Furthermore a welldefined block copolymer of 4,40 -vinylphenyl-N,N-bis(4-tert-butylphenyl)benzenamine with furfuryl isocyanate was prepared by anionic block copolymerization by the formation of thermoreversible network via DA reaction [22]. Nasresfahani and Zelisko introduced furan moieties in polyhedral oligomeric silsesquioxane which exhibit temperaturecontrolled self-healing behavior [23]. Biomass-derived compounds enriched with furan functionality were also reported for cross-linking with maleimide units to relay the selfhealing properties. Yoshie et al. prepared biobased copolymers, poly(2,5-furan dimethylene succinate-co-butylene succinate) from the polycondensation of 2,5-bis(hydroxymethyl) furan derived from biomass, 1,4-butanediol, and succinic acid [24]. These copolymers were cross-linked with bismaleimide to form the polymer network by reversible DA reaction which shows room temperature healing ability, and the healing efficiency was enhanced by solvent or heat. Self-healing chemistry has also been utilized in various polymeric materials, for example, in conventional thermosets such as epoxy and unsaturated polyester resins. The development of self-healing epoxy resins is still attractive as epoxy resins are one of the most used polymers in microelectronics, construction, and coatings. The preparation of

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2.4 Thermoreversible Diels
Alder and retro Diels
Alder chemistry

23

self-healing epoxy resins has been reported by Palmese’s group [25,26]. To impart self-healing properties in conventional epoxy resins, a reversible cross-linked gel of furan
maleimide DA adduct has been introduced. The gel was crushed into powders of B660 mm particle size and added to an epoxy-amine resin system as a healing agent. During crack formation in the sample, heating at 90 C causes the gel phases to liquefy via retro-DA reaction which fill-up the crack and heal the crack via DA reaction to generate the gel structure. However, the healing efficiency is relatively low (B21%) due to poor mobility of the healing agent in a highly cross-linked epoxy matrix. A modified system with a solution of maleimide healing agents in solvents was used to lower the viscosity for better delivery of the healing agents to the crack site. High healing efficiency of about 73% healing was achieved from the DA reaction between furan groups of the epoxy resin and the added maleimide healing agent. The high self-healing efficiency of the epoxy resins warrants its suitability in the preparation of self-healing glass-fiber-reinforced composites, as epoxy-based composites are widely used in microelectronic and aerospace industries. The as-prepared epoxy composite materials were shown to have a moderate healing efficiency of B48%. The diffusion and reaction phenomena of this self-healing epoxy system have been well studied to provide more insight toward the design and preparation of such smart materials. The possible modification of the system would be an encapsulation of the maleimide healing agent in carriers and the development of high-performance selfhealing polymer coatings. On the other hand, Tian et al. reported a tetrafunctional epoxy compound-containing furan
bismaleimide adduct linkages [27]. The epoxy monomercontaining furan ring was synthesized and cured in the presence of bismaleimide for the direct incorporation of furan
bismaleimide adduct in conventional epoxy in the formulation of the epoxy resins which provides better accessibility in practical applications. Fluoropolymers continue to be the focus on extensive research due to their remarkable properties enabling them toward a wide range of applications in many high-tech applications, such as aeronautics, aerospace, and electronics, in composite materials, films, and paints. Fluoropolymers are specialized unique materials exhibiting superior properties such as high resistance to harsh weather conditions and excellent inertness to a wide range of chemical environments due to their exceptionally strong C
F bonds, low polarizability, and weak intermolecular van der Waals interactions [28,29]. Furthermore due to the presence of perfluoroalkyl chains, fluoropolymers are highly hydrophobic and oleophobic along with low surface energy. Hence, a combination of both fluorous and self-healing functionality would be important and fascinating for future high-tech real-world applications. Recently we have shown the self-healing of a partially fluorinated cross-linked copolymer by utilizing the thermoreversible DA chemistry. The fluorous self-healing cross-linked polymer was prepared by copolymers (PHFBA-co-PFMA) of perfluorobutyl acrylate (HFBA) and furfurylmethacrylate (FMA) and cross-linked with bismalemide (BM) by DA reaction as presented in Fig. 2.4 [30]. The self-healing property of cross-linked polymer (PHFBA-co-PFMA)/BM was shown in microscopic damages (Fig. 2.5A) and thermally recovered fluorous cross-linked copolymer (Fig. 2.5B). The schematic mechanism of reversible DA chemistry-based self-healing process of cross-linked (PHFBA-co-PFMA)/BM polymer is shown in Fig. 2.5C(a). The cut on sample resulting damage is observed as shown in Fig. 2.5C(b). On heating above 120 C for 18 h the furan and maleimide disconnect via rDA mechanism and the displaced structure started rearranging as described in Fig. 2.5C(c).

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2. Types of chemistries involved in self-healing polymeric systems

FIGURE 2.4 Thermoreversible self-healing fluorous cross-linked copolymer via DA and rDA chemistry [30]. DA, Diels
Alder; rDA, retro-DA.

FIGURE 2.5 Microscopic image of (PHFBA-co-PFMA)/BM sample (A) damaged polymer, (B) self-healed polymer, and (C) schematic representation of self-healing process, (a) original structure, (b) damaged/structurally displaced sample, and (c) structurally rearrangement by rDA of the sample at 120 C and again reformation of the cross-linked polymer through DA reaction as shown in (a) [30]. BM, Bismaleimide; DA, Diels
Alder; rDA, retro-DA.

Again, maintaining the temperature at 60 C for 12 h the furan and maleimide group reconnects the structure via DA mechanism and recovers damages to the original shape as shown in Fig. 2.5C(a). Wang and Zuilhof reported self-healing superhydrophobic fluoropolymer brushes as highly protein-repellent coatings [31,32]. Superhydrophobic coatings based on sulfonated poly(ether ketone) (SPEEK) with self-healing performances were studied by Li et al. [33]. In addition, sustainable, self-healing superhydrophobic, and superoleophobic surfaces from fluorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fluorinated alkyl silane were developed [34]. Zhou et al. produced robust, selfhealable superamphiphobic fabrics via a two-step coating of fluoro-containing polymer, fluoroalkyl silane, and modified silica nanoparticles [35]. Ameduri et al. have recently reported a functional fluorinated polymer from a furan-bearing alpha-fluoro acrylate monomer via DA chemistry. Poly(α-fluoro acrylate) materials are relevant for optical fibers where more fluorine atoms are desired with a less hydrogen content to allow less signal

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FIGURE 2.6 Monomeric systems utilized in self-healings polymeric systems involved via DA chemistry. (A) [37], (B) [19], (C) [38], (D) [23], (E) [22], (F) [39], (G) [20], (H) [36], and (I) [40]. DA, Diels
Alder.

absorptions [36]. Since optical fibers cross the ocean’s bed to supply signals from one continent to another, self-healing cores and claddings of such high-tech materials can be of some interest. Various monomers utilized in self-healing polymeric systems involving DA chemistry are shown in Fig. 2.6.

2.5 Photoinduced self-healing: [2 1 2] cycloaddition Photomediated reversible [2 1 2] cycloaddition chemistry is one of the prevailing selfhealing strategies with great commercial importance because the stimulation can occur at ambient temperature and the ability to heal the damage repeatedly. Moreover, these photoreactions are inexpensive, easy to handle, and exposure can be limited to target which enables three-dimensional spatial control of the healing reaction as well as the ability to remotely trigger a process on and off. Compounds with olefin moiety such as cinnamic acid, thymine, coumarin, and anthracene undergo [2 1 2] cyclodimerization on irradiation with UV light of wavelength λ . 300 nm to form cyclobutane adduct [1]. Kim et al. have shown photochemical [2 1 2] cyclodimerization of cinnamoyl group in a cross-linked 1,1,1-tris-(cinnamoyloxymethyl)ethane for the reversible self-healing to form cyclobutane structure. During crack formation and propagation, the cyclobutane reverse to the original cinnamoyl structure which undergoes recycloaddition of cinnamoyl groups during the healing as illustrated in Fig. 2.7A [41]. Polyisobutylene (PIB) is a gasimpermeable synthetic rubber used in lubricants, sealants, adhesive, and cling films. A smart coating of self-healing polymer for photovoltaic devices was developed with lowmolecular-weight coumarin-functionalized triarm star PIB with excellent oxygen and moisture barrier properties. UV light (λmax 5 365 nm)-induced reversible photodimerization of

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2. Types of chemistries involved in self-healing polymeric systems

FIGURE 2.7 Self-healing via covalent bond reformation utilizing (A) [2 1 2] cycloaddition [41], (B) photo radical recombination [42], and (C) [4 1 4] cycloaddition [43].

the coumarin moiety was used for self-healing in the cross-linked elastomeric film with simultaneous monitoring of the photodimerization/photoscission by UV
vis spectroscopy, and the healing process was studied by atomic force microscopy [44]. Scott et al. developed a poly(ethylene glycol)-based polymer gel with homolytically cleavable hexaarylbiimidazole (HABI) functional group pendent, which shows rapid selfhealing [42]. As shown in Fig. 2.7B these HABI cleaves homolytically under UV or visible light to yield lower reactive lophyl radical and spontaneously recombine without any side reactions in the absence of light. The mechanical properties were rapidly recovered within 1
3 min on exposure of light, for the gels swollen in the presence of organic solvents. The recombination kinetics of backbone-borne HABI photolysis and lophyl radical is influenced by the swelling solvent gels, for example, swollen gel exhibits higher radical concentrations in 1,1,2-trichloroethane (TCE) than in acetonitrile or water under equivalent irradiation conditions, possibly due to the hydrophobic HABI afford more rapid HABI cleavage and slower radical recombination in TCE than in water.

2.5.1 [4 1 4] Cycloaddition reactions Photomediated [4 1 4] cycloaddition reactions also offer reversible polymerization of anthracene derivatives via four-membered ring opening and closing at 254 and 366 nm, respectively, as shown in Fig. 2.7C. Inducing a monochromatic radiation source such as a laser at the damaged area can selectively activate the healing process, but the side reactions due to changes in the local temperature can adversely affect the overall process [43].

2.6 Chemical transformations involved in self-healing 2.6.1 Thiol-ene click chemistry A general thiol-ene reaction is presented in Fig. 2.8A. Radical thiol-ene reactions are class of click reactions commonly performed either thermally using radical initiators or

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27

FIGURE 2.8 Sulfur-based dynamic covalent self-healing chemistry (A) thiol-ene click chemistry [45], (B) disulfide exchange [46], and (C) TTC reshuffling reactions [47]. TTC, trithiocarbonates.

photochemically with or without photoinitiators. Furthermore the initiation of thiol-ene reactions was extended sonochemically at room temperature by Skinner et al. [48]. Thiolbased addition reactions including the thiol-Michael, thiol-ene, and thiol-yne reactions are extensively utilized in polymer chemistry due to their facile implementation and compatibility with commonly used functional groups in polymerization. Thiol-Michael reaction has been used as a powerful tool in polymer chemistry such as postpolymerization modification, ligation, dendrimer synthesis, degradable hydrogel formation, synthesis of peptide
polymer conjugates, surface and particle modification, and block copolymer synthesis. However, the dynamic properties toward self-healing can be accessed only either at elevated temperatures or by variation in pH. Konkolewicz et al. reported a self-healing polymer of an acrylate-based cross-linker and thiol-maleimide through dynamic thiol-Michael chemistry at 90 C [49,50]. The thiol-ene Michael type polyaddition reactions are used in PEG gels with inexpensive commercially available components: polyethylene glycol diacrylate and dithiothreitol by Theato et al. [51]. Furthermore the reversible thiol-ene Michael chemistry in trithiol and bis-benzylcyanoacetamide derivative was utilized in a polymer network toward self-healing above 60 C by Kuhl et al. [52]. The healing mechanism via reversible opening of the thiol-ene adducts was confirmed from the temperature-dependent Raman spectroscopy. Thiol-ene Michael cross-linked polymers offer dynamic reversibility in the presence of thermal stimuli, whereas in the absence of heat a very good long-term stability was observed against the mechanical deformation.

2.6.2 Dynamic exchange of disulfide bonds Reversible bonding and debonding chemistry of dynamic disulfide bonds (S
S bond) in response to light or heat has been utilized for the design of self-healing polymeric material. A general scheme of disulfide exchangeable reaction is presented in Fig. 2.8B. Klumperman and coworkers cured an epoxy polymer-containing disulfide group with

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a tetrafunctional thiol and the resulting material was able to fully restore its tensile strain at 60 C due to the renewal of cross-links across damaged surfaces from the reactions of disulfide groups [53]. However, the dominant mechanism is not due to the exchange between disulfide
disulfide bonds but the exchange between thiol
disulfide. Generally thermosets polymeric materials hardened permanently and it is difficult to introduce intrinsic self-healing via dynamic bond exchange. In this context, Lafont et al. introduced intrinsic self-healing in thermoset epoxy resin via dynamic exchange of disulfide chemistry [54]. Rekondo et al. replaced the aliphatic disulfide with an aromatic disulfide to obtain a cross-linked polyurea-urethane and the resulting thermoset elastomer capable of selfhealing at room temperature, with 97% healing efficiency (51% of which was due to the contribution of hydrogen bonding). However, aromatic disulfide exchange is associated with relatively low healing efficiencies, oxygen-sensitivity of thiol
disulfide exchange, and the requirement of heating [55]. The disulfide metathesis of low-molecular-weight compounds at room temperature has been extensively studied. Caraballo et al. used alkyl and aryl phosphines as catalysts to accelerate the disulfide metathesis of model aliphatic and aromatic disulfides at ambient temperature [56]. Compared with triphenylphosphine, tricyclohexylphosphine exhibited better performance but poor oxidation resistance. Elastomers are useful in various applications such as seals, bladders, and tires. Although elastomers should withstand large deflections with little or no permanent deformation, they also fail due to fractures and fatigue processes. The deformation-induced damage is cumulative which may lead to catastrophic failure. Self-healing property enhances the long-term durability of a structure by effectively removing any local micro, meso, or, sometimes, even macro damage. Conventional vulcanized natural or synthetic rubbers of excellent and stable mechanical properties derive from the creation of a stable covalently bonded three-dimensional molecular network but they do not have such self-healing capability. A self-healable sulfur vulcanized NR was reported using the common ingredients in a traditional NR formulation. When this material is not fully cured then the di- and polysulfide bonds naturally present in the covalently cross-linked rubber contribute to the healing process and the full recovery of mechanical properties was obtained at moderate temperature. However, the amount of sulfur must be tailored to reach the optimum of the cross-linking density and the disulfide/polysulfide ratio for a compromise between mechanical performance and healing capability [57]. The healing efficiency depends on the postcuring storage time and the actual healing time. This selfhealing chemistry of di-/polysulfide bridges and other reversible moieties may be very useful for the design and preparation of polymers with long-term durability. However, the photocyclization reactions of anthracene, coumarin, and cinnamate derivatives have limited relative response rates since only a single cross-link is broken reversibly for each absorbed photon and requires irradiation at UV wavelength to realize both the forward and the reverse reactions. On the contrary, photomediated metathesis reactions of disulfides, thiuram disulfides, allyl sulfides, and trithiocarbonates and functional groups are effective in covalently cross-linked polymer networks which can undergo a cascade network rearrangement, by multiple reversible bond breaking and reforming reactions for each absorbed photon. Nevertheless, this approach also offers sluggish healing rates due to limited segmental diffusion. Amamoto et al. developed a repeatable photoinduced selfhealing of covalently cross-linked polymers through the reshuffling of trithiocarbonate

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29

FIGURE 2.9 (A) Schematic representations of dynamic covalent diselenide chemistry for self-healing and (B) self-healing diselenide chemistry used in polyurethane-based polymeric system [60].

units [47]. Here, the repeatable healing was accomplished due to the reversible reshuffling of trithiocarbonate in the presence of light as given in Fig. 2.8C.

2.6.3 Dynamic chemistry of selenium Disulfide (S
S) dynamic covalent bond is widely used in self-healing materials. But considering the lower bond energy of several Se
X bonds (Se
Se, Se
N) than S
S, recently the dynamic properties of selenium-related covalent bonds were also explored. Low-bond energies of C
Se and Se
Se (C
Se, 244 kJ mol21; Se
Se, 172 kJ mol21, vs C
S, 272 kJ mol21; S
S, 240 kJ mol21) make the monoselenide and diselenide easier to be oxidized or reduced [58,59]. The diselenide bonds have been applied as sensitive redox responsive groups mimicking the glutathione peroxidase. In the human body, selenoproteins prevent cell damage from free radicals. The general representations of self-healing involved through Se
Se dynamic covalent bonds are illustrated in Fig. 2.9A. The dynamic chemistry of selenium has been realized in a series of diselenide bond-containing polyurethane elastomers, one of the examples is given in Fig. 2.9B [60,61]. These materials possess visible light-induced self-healing via dynamic Se
Se bond within 24 h under a table lamp which was shown to have accelerated healing by a blue laser of 0.4 W and 357 nm.

2.7 Reversible covalent reaction involved in self-healing 2.7.1 Dynamic reversible boronate ester bond Boronic esters are another typical example of dynamic covalent bonds and have great potential with rapid and efficient healing capability. Self-healable hydrogels based on the dynamically reversible boronate or borate ester bonds are usually prepared by reacting boronic acid or boric acid with diol compounds or polymer-containing hydroxyl group such as poly(vinyl alcohol). Although the covalent B
O bond of boronate esters is highly stable, their formation is reversible under certain conditions such as pH or the action of

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2. Types of chemistries involved in self-healing polymeric systems

certain external stimuli. The inherent acidity of the boronic acid is enhanced when diols react with boronic acid to form cyclic boronate ester (five-, six-, or seven-membered rings) in aqueous media. Boronate esters are readily formed by the condensation of boronic acids and 1,2-/1,3-diols, and the process is favored when solution pH is near or higher than the pKa of boronic acid. The dynamic covalent functionality of boronate esters with structure directing potential has been utilized to develop a variety of self-organizing systems including macrocycles, cages, capsules, and polymers [62]. A general scheme of reversible dynamic boronate ester bond formation with diols is illustrated in Fig. 2.10. Boronate ester dynamic covalent bond has been studied in various applications such as dynamic polymer assemblies and self-healing hydrogels and stimuli-responsive drug delivery for cancer therapy, contact lenses, and biomedical applications such as substitutes for skin, tendons, cartilages, 3D/4D printing, and tissue engineering [63]. Zhao et al. developed a one-pot synthesis of autonomously self-healable elastomeric hydrogels based on the dynamically reversible boronate ester from a copolymer of N,N-dimethyl acrylamide (DMAAm) and 2-hydroxyethyl acrylate (HEA) (8:2 wt./wt.) in the presence of boric acid. Resulting random copolymer in aqueous solution at pH 5 9 forms a solid hydrogel of tensile strength .0.5 MPa at 30% water content, which can autonomously self-heal with B100% efficiency within 48 h at room temperature [64]. Polymers such as PVA and pHEA-containing hydroxyl groups on the same side of a plane of backbone chains allow a succession of diols to form multiple borate ester bonds. 11BNMR is the best tool for monitoring the dynamic of boronate ester bond formation. For example, pHEA and H3BO3 in D2O at pH 5 3, in a 4:1 molar ratio of n(
OH):n(H3BO3), the resonance signals of boron appear at 17.0
19.3 ppm, due to the existence of free boron as uncomplexed boric acid form. Increasing the pH to 9 by addition of NaOH, the peak at 17.0
19.3 ppm disappeared and a new peak at B5.6 ppm appears which corresponds to the 4:1 complexation of boronate ester. There are numbers of self-healing hydrogels that are stimulus independent; however, self-healing hydrogels are often plagued with problems of longer healing time and stimuli dependency [65]. Even, the specific repair environment may create problems for the utilization of self-healing hydrogels, for example, in a wet environment or underwater healing. It is still challenging to prepare an ultrafast self-healing hydrogel (without external stimuli) applied in a wet environment and underwater. Guo et al. developed an ultrafast self-healing hydrogel of double network (DN) hydrogel consisting of a hydrogen bond-associated agarose gel as one network and the dynamic borate bond-associated polyvinyl alcohol (PVA) gel (agarose/PVA DN hydrogel) as another network [66]. The dynamic borate bond exhibits extremely fast self-healing ability (B100% recovery of initial strength and elongation in 10 s) at room temperature without external stimulus. These hydrogels also exhibit an outstanding underwater self-healing performance. The dynamic PVA-borate network provides the

FIGURE 2.10

Schematic representation of reversible dynamic boronate ester bond formation with diols.

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31

self-healing performance and the agarose network provides an additional platform to bear stress and firm the structure, thus improving strength, toughness, and stability. These materials with unique reversible network structures are recycled which has significant scope toward the smart self-healing devices and materials for biomedical application and have a great potential in soft robotics and dynamic surface coating. However, in most of reports, boronic ester hydrogels are formed at pH . 8, which impeded their use in physiological conditions. Recently Chen et al. have shown zwitterionic copolymers with benzoxaborole and catechol pendant groups [67]. Lower pKa value (7.2) of benzoxaborole allows efficient gelation from benzoxaborole
catechol complexation in PBS buffer (pH 7.4) which exhibits excellent self-healing performance. Moreover, the resulting hydrogels show dual pH/sugar responses due to the dynamic nature of boronate ester which allowed them toward further investigation for 3D cell encapsulation.

2.7.2 Dynamic reversibility of hindered urea bond In general, reversible dynamic covalent bonds require either catalyst or modification of conditions to facilitate the bond reversion and to induce the dynamic property in a bulk material [15,68,69]. Cheng’s group designed a few hindered urea bonds by substituting a bulky group on the nitrogen for the synthesis of polyureas and poly(urethane-urea)s, which show catalyst-free dynamic property and autonomous healing ability at low temperature [68,70]. As shown in Fig. 2.11, sterically hindered urea bond having a bulky group attached to the nitrogen atom can reversibly dissociate into isocyanate and amine. The isocyanate intermediate is stable enough at low temperature but reacts with the amine to reform the hindered urea bond, exhibiting the reversible formation of dynamic covalent bond chemistry. The urea bond undergoes reversible bonding and debonding, which imparts bond reorganization and self-repair capabilities after polymer chain breakage under external forces. The bulky reversible urea bond offers repetitive self-healing capability under ambient conditions without stimuli or catalyst. Integration of these tunable dynamic bond and self-healing properties is beneficial over conventional polyurea and poly(urethane-urea) polymers for widely used materials in coating, fiber, adhesive and plastics industries. However, the free isocyanates intermediate produced during reversible bonding and debonding may react with atmospheric moisture to produce primary amine through unstable carbamic acids. This highly reactive primary amines generated at the interface will react to form irreversible crosslinking. Cheong et al. reported a similar dynamic urea bond bearing self-healing polymer with bulky pendants. A copolymer of 2-(tert-butylamino)ethyl methacrylate (tBAEMA), methyl methacrylate (MMA), and n-butyl acrylate (BA), in a molar ratio of 1:10:10 was cross-linked with 1,6-diisocyanatohexane (HDI) or isophorone diisocyanate (IPDI) for the fabrication of self-healing polymers [71]. The reversible bonding
debonding between the tBAEMA polymer backbone and the isocyanate units facilitates rapid and repeatable self-healing performance. Most importantly these self-healing polymers exhibit no significant swelling under the water and the healing performance is unaffected underwater. Water-adaptability achieved since the free isocyanates dissociate from the dynamic equilibrium between tBAEMA and the isocyanate which converted to amine on contact with water for a long time.

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FIGURE 2.11 (A) Reversible association and dissociation of hinder amide bonds in polyurea or poly(urethane-urea) (with a bulky substituent at nitrogen) to corresponding isocyanate and amine and (B) examples of dynamic hindered urea bond-based cross-linked self-healable poly(urethane-urea). Source: Reproduced with permission from H. Ying, Y. Zhang, J. Cheng, Dynamic urea bond for the design of reversible and self-healing polymers, Nat. Commun. 5 (2014) 3218, Copyright 2014, Springer Nature.

2.7.3 Dynamically reversible alkoxyamines fission/radical recombination Another novel healing chemistry for the construction of reversible cross-linked structure is based on dynamic reversible (C
ON bonds) covalent bond chemistry of alkoxyamines via nitroxide-mediated radical. The healing processes are accomplished by dynamically reversible C
ON bond, which combines the bond breakage and reconnects in one step via the formation of a free radical intermediate. As illustrated in Fig. 2.12, the C
ON bond in an alkoxyamine moiety homolytically cleaves on heating and produces transient carbon-centered and persistent nitroxide radicals in equal amounts with very high-frequency factor (e.g., B2.4 3 1014 s21 in solution). The carbon-centered radicals would be rapidly trapped by the nitroxide radicals. Hence, the C
ON bonds in alkoxyamines frequently cleave under certain homolysis temperature but immediately recombine. The recombination of radicals involves crossover or exchange reaction between radicals that belong to different alkoxyamine moieties before the chain breakage [73]. The electron-withdrawing groups on the nitroxyl fragment (R1R2NO) play a major role on the C
ON bond homolysis rate in alkoxyamines (R1R2NO-R) [74]. Furthermore the dynamic equilibrium nature of nitroxide-mediated living radical polymerization has also been utilized for the synthesis of well-defined and complex molecular

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33

FIGURE 2.12 Self-healing via dynamic reversibility of alkoxyamines fission/radical recombination reactions [72].

architectures. For example, a reversible sol
gel process was reported from linear poly (methacrylic ester)s containing alkoxyamine side-chain units. On heating, the cross-linked gel in solvent (10 wt.% anisole solution of polymer) was formed through radical exchange reactions of alkoxyamine moieties, releasing alkoxyamine molecules. The retro-reaction, that is, decross-linking occurred in presence of an excess amount of alkoxyamine molecules. However, this dynamic process would be limited in a hard solid polymer matrix due to the restricted diffusion and molecular chain mobility. So, Yuan et al. developed a novel thermoreversible crack remendable solid polymer matrix incorporating alkoxyamine as cross-linker into solid polymer that cleaves and reconnects without any byproduct [72]. Because of synchronous scission of alkoxyamine moieties into radicals and their subsequent recombination, the cross-linked networks do not completely dissociate during the reaction, and the polymer preserves its integrity and mechanical properties efficiently even after the repair of the damage. Epoxy resins are widely applied in surface coatings, structural adhesives, printed circuit boards, insulation materials for electronic devices, fiber-reinforced composites, advanced composites, and most common thermosetting polymer used in aircraft structures because of its high mechanical properties, excellent chemical resistance, thermal stability, low shrinkage on cure, ease of handling and processing, etc. The development of self-healing ability and the possibility of imparting intrinsic crack remendability of epoxy polymers have added an excellent opportunity for a high growth rate of epoxy resins in recent years. Self-healing studies mostly limited to the extrinsic type due to their thermosetting nature and poor dynamic bond reversibility. The concept of dynamic equilibrium of thermally reversible reaction of the C
ON bond in alkoxyamine is applied in an epoxy monomer by Rong group which is compounded with traditional bisphenol-A diglycidyl ether and cured with diethylenetriamine as shown in Fig. 2.13 [75]. Due to the steric hindrance of tertiary butyl group in the diol, the onset of the scission of C
ON bonds and radical recombination occurred at a lower temperature.

2.7.4 Reversible dynamic covalent Schiff-base (imine) linkage-based self-healing chemistry Dynamic reversibility of Schiff-base reactions is widely used as potential pH-responsive linkers in polymer. With proper design, incorporation of Schiff-base structures with other

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FIGURE 2.13 Example of alkoxyamine-based self-healing chemistry containing epoxy system. Source: Reproduced with permission from C. Yuan, M.Q. Zhang, M.Z. Rong, Application of alkoxyamine in self-healing of epoxy, J. Mater. Chem. A 2 (2014) 6558
6566, The Royal Society of Chemistry. FIGURE 2.14 Dynamic Schiffbase (imine) linkages between aminefunctionalized glycol chitosan and telechelic difunctional aldehydes group-bearing PEG. PEG, Poly(ethylene glycol). Source: Reproduced with permission from T.C. Tseng, L. Tao, F.Y. Hsieh, Y. Wei, I.M. Chiu, S.H. Hsu, An injectable, self-healing hydrogel to repair the central nervous system, Adv. Mater. 27 (2015) 3518
3524, Copyright 2015, Wiley-VCH.

reversible covalent bonds or supramolecular interactions has been utilized in the selfhealing chemistry to form assemblies and gels, providing various functions and applications. The chemistry of dynamic Schiff-base (imine) linkages has been utilized in various ways to achieve self-healing polymer network. For example, Tseng et al. demonstrated a dynamic Schiff-base hydrogel of a chitosan derivative prepared by combining with telechelic dibenzaldehyde-terminated poly(ethylene glycol) (PEG) which exhibits self-healing within 2 h at 20 C. Furthermore the chitosan was replaced with glycol chitosan to improve the solubility under physiological conditions as depicted in Fig. 2.14 [76]. These hydrogels are injectable, autonomous self-healable in 12 h at 20 C, and act as a promising transporter by encapsulation of neural stem cells for repairing deficits of the central nervous system [77]. Ding et al. illustrated self-healable hydrogels using the chemistry of reversible imine linkages by self-cross-linked acrylamide-modified chitin and alginate dialdehyde (ADA) which shows complete recovery and regain the conformity of shape within 3 h at neutral or basic pH at 20 C [78]. However, no healing was observed at pH 5 due to the irreversible imine linkages.

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2.7.5 Reversible covalent acylhydrazone bond in self-healing chemistry Acylhydrazone-based polymeric materials are readily constructed from the condensation of hydrazides and carbonyl functional end groups. The acylhydrazone structural motif is used in different fields due to its facile synthesis, high stability, and unique structural properties. The polymer network of acylhydrazone covalent bonds is stable under normal conditions but shows reversible dynamic exchange behavior depending on the pH and responds to the self-healing by just controlling the pH. By adjusting the acidity of the system, chemical gel reveals the reversible sol
gel phase transitions. Acylhydrazone chemistry is also important to work with biomolecules such as polysaccharides, alginates, and chitosan as these molecules offer the functionalization of aldehydes or hydrazine derivatives. The reversible acylhydrazone chemistry is largely utilized in the construction of polymer gels for the biological applications mostly in peptide linker which helps in self-healing ability by adjusting the pH. Skene and Lehn illustrated the polyacylhyrazone by simple condensation of dihydrazide with a dicarbonyl compound and the network shows the healing performance due to dynamic reversibility of acylhydrazone bond [79]. As shown in Fig. 2.15, a novel reversible polymer gel based on a cross-linked network of acylhydrazone bonds with dynamic covalent

FIGURE 2.15 A covalently cross-linked polymer gel based on dynamically reversible acylhydrazone bond. Source: Adapted with permission from G. Deng, C. Tang, F. Li, H. Jiang, Y. Chen, Covalent cross-linked polymer gels with reversible sol 2 gel transition and self-healing properties, Macromolecules 43 (2010) 1191
1194, 2010 American Chemical Society.

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chemistry is generated. Deng et al. show a self-healable three-dimensional polymeric network by condensation of acylhydrazines at the two ends of poly(ethylene oxide) (PEO) with aldehyde groups in tris[(4- formylphenoxy)methyl]ethane [80]. Furthermore the chemistry of reversible covalent bond in acylhydrazones was extended by Jian et al. for the self-healing of alginate-derived hydrogels constructed from a dialdehyde terminated PEG and adipic dihydrazide-modified alginates [81].

2.8 Chemical transformations through involved reaction in self-healing 2.8.1 Exchangeable hydrazide Michael adduct linkages Dynamic covalent network possessing reversible covalent linkages has significant advantages in the material properties along with the healing process. Reversible covalent linkage with exchangeable groups instead of exchangeable bonds is another class of reversible dynamic covalent network formation utilized in the self-healing chemistry. Aza-Michael addition is a versatile reaction for the formation of dynamic covalent linkage. Aza-Michael adduct formation is facile, but the retro-Michael addition requires catalysts and high temperature. Aza-Michael additions can be made catalyst free, efficient, and reversible by using a highly activated α,β-unsaturated double bonds with electron-withdrawing groups in the geminal position as Michael acceptors. For example, Michael adduct of the donor hydrazide
NH
NH2 in Fig. 2.16 shows the dynamic reversibility in catalyst-free condition under room temperature and helps in self-healing process. A dynamic Michael addition system was developed by Ojha group based on carbonyl hydrazide donor (e.g., polyacryloyl hydrazide) and activated α,β-unsaturated double bonds as acceptor, where both adduct formation and the exchange of donors in the adduct occur readily under catalyst-free and ambient condition. The self-healed polymeric materials retain up to B23.8 MPa tensile strength and B2.5 GPa Young’s modulus [82]. Furthermore the toughness, elasticity, healing ability, adhesive, and strain sensing properties in polyacryloyl hydrazide were also improved by incorporating into ionic hydrogels. Due to the simplicity of this exchangeable hydrazide dynamic

FIGURE 2.16

Self-healing involved through exchangeable aza-Michael adduct linkages [82].

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FIGURE 2.17 Self-healing via covalent bond reformation using siloxane chain exchange reactions [84].

covalent network and wide availability of various precursors, this chemistry can be utilized for the development of various prospective materials with self-healing ability.

2.8.2 Dynamic siloxane bond exchange Chain exchange reactions in siloxanes are another class of reversible dynamic covalent network chemistry utilized in the self-healing process. Silicone-based materials are well known for their ability to restructure under certain conditions [83]. Zheng et al. prepared a self-healing exchangble siloxane network utilizing ring-opening copolymerization of octamethylcyclotetrasiloxane and bis(heptamethylcyclotetrasiloxanyl)ethane where these crosslinked copolymers containing ethylene bridges and active silanolate end groups exhibit remodeling of siloxane exchange. Fig. 2.17 represents the siloxane chain exchange reaction under ambient conditions [84].

2.8.3 Dynamic covalent exchange network in polyesters Polyesters are versatile material with a wide range of possible mechanical and chemical properties and easily accessible synthetically via condensation or ring-opening polymerization [85,86]. Brønsted acid-catalyzed dynamic covalent exchange of polyester-alcohol bonds in self-healing reversible network was demonstrated by Alaniz et al. A general scheme of Brønsted acid-promoted dynamic exchange reaction in polyester is shown in Fig. 2.18A. The covalent network comprising main-chain esters and pendant alcohols as a model for a low Tg vitrimer system was considered with Brønsted acid as shown in Fig. 2.18B and C. Relaxation time, activation energies, and Arrhenius prefactors were correlated with Brønsted acid pKa. Strong protic acid induces facile network relaxation at 25 C in the order of 104
105 s, which is significantly faster than Lewis acid-promoted exchange which functions only above 100 C. Activation energies span between 49 and 67 kJ mol21 and increase as pKa decreases [87].

2.8.4 Self-healing based on exchangeable reactions involving hypervalent iodine Hypervalent iodine compounds are widely used in organic synthesis as selective oxidants and in various electrophilic and radical reactions, including as initiators for cationic and/or radical polymerizations. Recently Vaish and Tsarevsky have utilized the dynamic Self-Healing Polymer-Based Systems

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FIGURE 2.18 (A) Brønsted acid-promoted dynamic exchange reactions in polyester-based self-healing chemistry. (B) ROP synthesis of polyester vitrimer and (C) Brønsted acid catalysts used to promote ROP and exchange reactions via transesterification. pKa values are referenced in H2O. Source: Adapted with permission from J.L. Self, N.D. Dolinski, M.S. Zayas, J. Read de Alaniz, C.M. Bates, Brønsted-acid-catalyzed exchange in polyester dynamic covalent networks, ACS Macro Lett. 7 (2018) 817
821, 2018 American Chemical Society.

FIGURE 2.19 Self-healing based on exchange reactions involving hypervalent iodine [88]. Source: Reproduced with permission from A. Vaish, N.V. Tsarevsky, Hypervalent iodine-based dynamic and self-healing network polymers, Polym. Chem. 10 (2019) 3943
3950, The Royal Society of Chemistry.

ligand-exchange reaction of hypervalent iodine for the self-healing chemistry as shown in Fig. 2.19 [88]. Linear polymers with multiple carboxylate groups as pendant (copolymers of styrene and acrylic acid) take part in ligand-exchange reactions with (diacetoxyiodo) benzene and the reaction yields polymer network with (diacyloxy)iodoarene type crosslinks. The free carboxylate groups in the polymers chain involved in dynamic ligandexchange reactions with the hypervalent iodine (III) center at the cross-links and provide self-healing properties to the resultant polymer. Self-Healing Polymer-Based Systems

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2.9 Supramolecular noncovalent interaction Supramolecular interactions are weaker, noncovalent interactions, and reversible in nature. The weak forces under supramolecular interactions are hydrogen bonding, electrostatic interactions, metal coordination, hydrophobic interactions, host
guest interaction, van der Waals forces, and π
π interaction [1,8]. These noncovalent supramolecular interactions assist in molecular self-assembly and have capability to form well-defined supramolecular structures through reorganization of molecules. Owing to the reversible nature, supramolecular interactions have a great importance for the construction of self-healing materials. These chemistries of supramolecular interaction have potential in polymer science to determining polymer bulk properties and in the design of new polymeric materials along with self-healing properties [3]. However, these interactions always rely on the cooperative effect of many weak interactions, lack directionality which generates a small network of microphase separated structure or gelation. The prerequisite for a reversible linear supramolecular polymer alternative to the dynamic covalent bond is to have strong, reversible, and highly directional interactions [89].

2.9.1 Hydrogen-bonding-based self-healing Hydrogen bonding is one of the most popular noncovalent interactions for achieving supramolecular polymers. Weak hydrogen bond (H-bond) interaction is much weaker than dynamic covalent and ionic cross-links. Hydrogen bonds between neutral organic molecules are very weak noncovalent interactions, however, due to their directionality, reversibility, and affinity, cumulatively a significant mechanical strength can be attained. Widely explored hydrogen bond motifs include 2-ureido-4 pyrimidinone (Upy) and secondary amide groups, urea/thiourea moieties, and ester/carboxylic acids groups. These groups facilitate for the formation of reversible supramolecular cross-linking networks through their intermolecular hydrogen bond. The self-healable supramolecular materials are developed by introducing hydrogen-bonding motifs as donor and acceptor into the polymer as pendant chains, arms, or at the chain ends. On physical damages, the H-bond networks are disrupted but again reformed because of their unique reversibility. The main challenge in this approach is to find the right balance between the association constant and reversibility in the system. The higher the association constant, the lesser is the reversibility and vice versa. But a lower association constant often leads to smaller assemblies and poor mechanical properties. For example, Upy end-functionalized poly(ethylene glycol) (PEG) has been developed as an injectable hydrogel which exhibits a complete self-recovery from alternate step-strain deformations (γ 5 1 or 100) at 37 C. The self-complementary quadruple hydrogen-bonding functionality of Upy units is used for the development of self-healing hydrogels. The self-healing process was accomplished by the action of reversible complementary hydrogen bonds of quadrupole donor
donor
acceptor
acceptor. A schematic presentation of the reversible quadruple hydrogen-bonding interaction in polymer responsible for self-healing chemistry is illustrated in Fig. 2.20. Meijer and coworkers extensively worked on Upy assembly using quadruple H-bond interactions with polysiloxanes, polyethers, polyesters, etc. [93]. Even a spin-off company at the Technical University of Eindhoven, SupraPolix BV, has started commercialization.

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FIGURE 2.20 Reversible quadruple H-bond interaction in UPy polymers responsible for self-healing. Upy, 2-Ureido-4 pyrimidinone. Source: Reproduced with permission from R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, et al., Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding, Science 278 (1997) 1601
1604 [90]; D. Zhu, Q. Ye, X. Lu, Q. Lu, Self-healing polymers with PEG oligomer side chains based on multiple H-bonding and adhesion properties, Polym. Chem. 6 (2015) 5086
5092 [91]; J. Cui, A. Campo, Multivalent H-bonds for self-healing hydrogels, Chem. Commun. 48 (2012) 9302
9304 [92], The Royal Society of Chemistry.

As discussed, the supramolecular interactions rely on cumulative and cooperative factors, therefore many hybrid systems are developed from the dual network of H-bonding in combination with various other noncovalent interactions such as hydrophobic interactions, salt bridges, hydrogen bonding, and ionic interaction to form hierarchical structures. Hybrid systems are mostly developed by using a supramolecular architecture with traditional polymers. Leibler et al. have reported a hybrid system that brings together supramolecular assembly to develop self-healing rubbers using fatty diacids and triacids from renewable resources [94]. Initially the fatty acids were condensed with an excess of diethylene triamine and then reacted with urea to obtain a material of rubber-like properties with selfhealing capability at room temperature. Reinforcements from supramolecular forces such as hydrogen bonds are extensively explored in polymers. The dynamic nature of hydrogen bonds allows to control various material properties as well as the self-healing chemistry by tweaking the extent of hydrogen bonding, such as viscosity modulation, adhesion, self-assembly, or even by altering the external conditions such as temperature, humidity, or solvent. Hydrogen bonds also serve as dynamic cross-link for material strength and limit the chain mobility. For instance, hydrogen bonds play a crucial role in the organization of the secondary and tertiary structures of proteins and nucleic acids. In polymer, the secondary amide groups are utilized in polyamides to induce hydrogen bond formation, determining material properties and functions. However, secondary amide groups result in flexible chains that can switch easily between conformations due to the large bond angles. Additionally hydrogen bonds in secondary amide-containing polymers such as nylon-6 promote polymer chain alignment into well-ordered structures, which in turn conceals chain flexibility [8].

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A dual network of hydrogen bonding and π-stacking is a well-known route for the construction or fabrication of hydrogels and elastomers. The seminal work of Leibler et al. showed the direction where the supramolecular assemblies were utilized through hydrogen bonding by introducing fatty acids and urea moieties in the selfhealing of rubber [94]. Hayes and coworkers reported a supramolecular combined two polymer precursors, polyimide-rich repeating units and urea-linked aromatic residues endcapped with pyrenyl [18]. In the bulk polymer naphthalenediimide segment provides H-bonding with urea moieties and also forms strong π
π stacks with the aromatic residues to produce an extremely high viscous material with a relaxation time of 1 3 1010 s (c. 300 years) at 30 C [95]. The interplay between the π-stacking and the hydrogen bonding controls the mechanical property of the polymer and exhibits satisfactory healing properties at .90 C. Furthermore the polymer matrix was doped with pyrene-coated gold nanoparticles (AuNPs) to form a film with enhanced mechanical properties [96]. The hybrid network of aromatic interactions and hydrogen bonding was also demonstrated by Zeng et al. in a triblock copolymer of a PEO as the middle block endcapped at both ends by a thermoresponsive copolymer of N-isopropylacrylamide-(N3,4-dihydroxyphenethyl acrylamide), synthesized by RAFT polymerization. The triblock copolymer exhibited intriguing gelation behavior with excellent recovery properties due to the simultaneous hydrogen bonding and aromatic interaction. In vitro cell adhesion studies were also performed to show excellent cell repellent capacity due to the PEO segment. Liu et al. reported an acrylamide (AAm)-based gel composed of N-acryloyl glycinamide (NAGA) and 2-acrylamide 2-methylpropanesulfonic acid, with stable hydrogen bonds that shows self-healing properties at a relatively lower temperature (c. 60 C) [97]. In various systems the H-bond facilitates spontaneous self-healing at ambient temperature without external energy or catalyst. The strength of quadruple hydrogen bonding in 2-ureido-4-pyrimidinone (UPy) dimers is at the intermediate between the strength of covalent bond and most of the other noncovalent bonds. Since the first report of Meijer the UPy dimers have been widely used as robust, selective, and directional hydrogen-bond in various reversible supramolecular networks of polymer [93]. Recently Zeng and coworkers reported a self-healing polybutyl acrylate (PBA)-UPy polymer and studied its surface interactions and adhesion properties [98]. Guan and coworkers developed an autonomous self-healing thermoplastic elastomer with enhanced stiffness and toughness by using UPy units as a hydrogen-bonding network in dimerization UPy end-functionalized polystyrene-b-poly(n-butyl acrylate) (PS-b-PBA) AB diblock copolymers [99]. Ma et al. used Upy unit modified dextran hydrogels to demonstrate a rapid self-healing and self-integration properties. Furthermore a hierarchical hydrogen-bonding moiety of urethane, urea, and 2-ureido-4[1H]-pyrimidinone in the polymer backbone was used by Song et al. to build a robust elastomer with high tensile strength (44 MPa) and super toughness (345 MJ m23) which shows 90% healing efficiency [100]. Yanagisawa et al. reported a self-repairable polymeric material with high tensile strength up to 40
45 MPa via slip motions of polymer chains in poly(ether-thiourea) and poly(alkylene-thiourea) through exchange of H-bonding pairs [101]. Various H-boning interactions involved in self-healing polymeric systems are illustrated in Fig. 2.21.

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FIGURE 2.21 Various H-bonding interactions involved in self-healing polymeric systems [1,89]. (A and B) [102], (C) [103], (D) [104], and (E) [105].

2.9.2 Self-healing involved through electrostatic interactions 2.9.2.1 Ionomeric or ionic mechanism Electrostatic interaction is another dynamic noncovalent interaction where the strength depends on the charge and size of the ionic groups involved and often used as ionomer or ionic polymers in the supramolecular polymer structure. Ionomers are a special class of polymeric materials that contain a hydrocarbon backbone with ionic salts as pendent, for example, carboxylic acid pendents neutralized fully or partially to form salt. The ionic content of ionomers varies over a wide range, but in general it is up to 20 mol.% to avoid selfaggregation. Ionomers usually involve the electrostatic interactions between polymeric anions (carboxylates and sulfonates) and metal ions of Group 1A/2A or transitional metal cations. Surlyn and Nucrels are two different grades of commercialized products which are derived from ethylene-methacrylic acid (EMAA) random copolymer with 15 wt.% (5.4 mol.%) acid content which has been neutralized with sodium ion [3,16,106
108]. Introduction of a small ionic content causes a drastic improvement in polymer properties, such as tensile strength, tear resistance, impact strength, and abrasion resistance. These ionic polymers are unique and their aggregates have profound mechanical and physical properties. Since ionomers are not thermosetting material, they can be processed like thermoplastics and are especially useful in ballistics and other commercial applications such as food packaging, membrane separation, roofing materials, automobile parts, golf ball covers, coatings, and so on. Besides these applications, the reversibility of ionic bonds makes them suitable for designing self-healing polymeric systems [106,109]. Ionomers self-repair macroscopic and energetic damage events as in ballistic puncture automatically and instantaneously without manual intervention with a unique self-healing mechanism. According to Eisenberg and Rinaudo, as given in Fig. 2.22, the healing of

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FIGURE 2.22 Schematic illustrations of Eisenberg
Hird
Moore ionic model of ionomer. Source: Adapted with permission from A. Eisenberg, M. Rinaudo, Polyelectrolytes and ionomers, Polym. Bull. 24 (1990) 671, Copyright 1990, Springer Nature.

projectile puncture is due to the polar ionic groups that tend to aggregate as a result of electrostatic interactions, despite the opposing elastic forces of the chain [106]. The physical cross-linking formed due to the presence of ionic groups and their interactions are mostly reversible in nature (Fig. 2.22) [106]. More importantly to note that the selfhealing phenomenon in the Surlyn-type EMAA materials is not a small crack but a circular hole of several millimeters in diameter. The interdiffusion of polymer surfaces due to ionic interactions of the aggregates is not sufficient for the large-scale motion required to bring the surfaces back together. The healing process was accomplished by the transfer of sufficient energy to the polymer on impact, heating the material above its order
disorder transition resulting in the disordering of micellar aggregates. During the postpuncture period, the ionic aggregates have potential to reorder and cover the damaged hole. The self-healing in ionomers has been elaborated extensively by Ghosh [109]. Hence, we restrict our discussion only on the recent developments of new and emerging chemistry of ionic polymers. The self-healing chemistry via ionic interactions is emerging in different areas of polymer due to their high reversibility, rapid and efficient performance even at room temperature, and works multiple times without any catalyst or external stimuli. Polymeric materials of poly(ionic liquids)-bearing multiple cations and anions show excellent self-healing properties through the intrinsic ionic interaction under the ambient condition without any external stimulus or catalysts. In addition, these ionic polymers show good mechanical and physical properties, conductivity, and ability to form ionogel or hydrogels and have a wide range of applications such as polyelectrolytes, membranes, transparent electronic materials, medical devices, capacitors, and batteries. This selfhealing chemistry utilizes reversible ionic cross-linking between polymeric anionic and cationic species such as metal ions, small molecules, or macromolecules. The polymer

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FIGURE 2.23 Self-healing of ionic mechanism in ionic polymer contains both cationic and anionic segments [110].

containing both cationic and anionic polymeric segments in the self-healing of ionic interactions is illustrated in Fig. 2.23. The construction of layer-by-layer (LbL)-deposited film by using polymers with multiple cations and anions is an emerging area in materials science, biomimicking, and supramolecular chemistry where mainly the ionic mechanism governs the self-healing process. The LbL-deposited polyelectrolytes conducting polymers, hydrogels, hybrid organic
inorganic materials, and healing agents show great potential in designing of self-healing materials [111]. The self-healing capability of these noncovalently bonded LbL-deposited multilayers relies on high mobility of the multilayer components in response to external stimuli such as humidity, temperature, pH, ionic strength, and light which can heal the defects and restore the structure. The self-healing in LbL multilayers is based on the mobility of components among the multilayers which are sensitive to external stimuli. The number of mobile chains in the multilayer is a key factor in the degree of mobility of the polyelectrolyte multilayers, whereas pH and ionic strength are two external stimuli that can affect the water content in the multilayers, thus, mobility of the polyelectrolyte chains. Smart polyelectrolyte multilayers offer a broad range of applications in the field of nonlinear optics, light emission, sensing, separation, bioadhesion, biocatalytic activity, drug delivery, etc. The LbL self-healing materials may be used as building blocks for nanoarchitecture of “intelligent” surfaces [111]. Polymers bearing multiple ionic units (cation and/or anion or both) in polymer segments have emerged from many fascinating approaches for room temperature rapid and repeatable self-healing with desired physical and mechanical properties. Yang et al. reported a composite of tough thermoplastic hybrid hydrogel-embedded Au nanoparticles which are conductive and exhibit electrical and mechanical stimuli-responsive self-healing properties. This self-healable hybrid hydrogel was made by mixing poly(ionic liquids) 1-methyl-3-(4-vinylbenzyl) imidazolium chloride (P(VBIm-Cl)) and poly(sodium p-styrenesulfonate) (P(NaSS)) followed by poly(vinyl alcohol) (PVA). Furthermore Au nanoparticle was incorporated by in situ reduction [112]. The self-healing process occurs from dual contributions of intrinsic ionic interactions and weak, reversible metal
ligand coordination in the flexible polyelectrolyte chains. The mechanical properties and conductivity of this hybrid thermoplastic and tough hydrogel were repaired simultaneously, repeatedly, and rapidly. A hybrid hydrogel was developed by Wan et al. based on electrostatic coassembly of polyoxometalates and ABA triblock copolymers poly(2-(2-guanidinoethoxy)ethyl methacrylate)-b-poly(ethylene oxide)-b-poly(2-(2-guanidinoethoxy)-ethyl methacrylate) which exhibit excellent luminescence and self-healing performance [113].

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Commercial rubber which is exclusively used in manufacturing automobile tires is not reused or recycled due to wear or irreparable damage. Automobile tires are one of the largest volume solid wastes and a threat to a sustainable environment. Integrations of selfhealing properties in these commercial rubbers will be useful in terms of long durability and safety index. Heinrich et al. described a simple approach to convert commercially widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary self-healing ability by the transformation of the bromobutyl group of BIIR into (ionic) imidazolium bromide results in the formation of ionic associates that exhibit reversible physical cross-linking ability [114]. The reversibility of ionic association facilitates the healing process from stress-induced rearrangement or in response to temperature, thereby enable healing of a fully cut sample to retain its original properties. In addition to self-healing properties, the ionic-modified BIIR shows superior mechanical properties, such as the elastic modulus, tensile strength, ductility, and hysteresis loss, when compared with conventionally sulfur-cured BIIR. This simple and easy approach to prepare a self-healable commercial rubber with properties offers unique development opportunities in the field of highly engineered materials, such as tires, for which safety, performance, and longer fatigue life are major concerns. Similarly the self-healing properties of brominated poly (isobutylene-co-isoprene) rubber (BIIR) were explored by ionically modified with various alkylimidazoles such as 1-methylimidazole, 1-butylimidazole, 1-hexylimidazole, 1-nonylimidazole, and 1-(6-chlorohexyl)-1H-imidazole [115]. Chen et al. make the selfhealing vulcanized NR by introducing ionic cross-links into NR via controlled vulcanization. The self-healing of NR was accomplished by typical ionic interaction of Zn21 with two 2OOC
C(CH3)CH2 of zinc dimethacrylate (ZDMA) which was introduced through grafting-polymerization of ZDMA with rubber [116]. The autonomous and complete healing is limited to polymers such as hydrogels and soft elastomers containing highly dynamic bonds with high molecular mobility. However, high molecular mobility generally opposes with the mechanical strength of polymer materials. Moreover, hydrogels based on intrinsic ionic interaction and H-bonding show superiorities due to their fast and autonomous self-healing ability without any stimulus or energy and involves a simple preparation method. Conventional self-healing polymer hydrogels are considered to be weak and brittle (low toughness) with ,10 J m22 fracture energy and tensile strengths of less than 1 MPa, when compared with 1000 J m22 for cartilage and 10,000 J m22 for NRs [89,117,118]. But many applications often required a good mechanical property. Despite recent progress in the design of self-healing polymer materials, the self-healing hydrogels generally possess either rapid self-healing properties or mechanically robust but not both simultaneously. Therefore the development of mechanically robust materials with autonomous self-healing on the time scale of seconds is of great demand. Hydrogels with improved mechanical properties were given efforts and certain synthetic gels have reached fracture energies of 100
1000 J m22. Gong group developed self-healable polyampholytes hydrogels from sodium p-styrenesulfonate (NaSS) and 3-(methacryloylamino)propyl trimethylammonium chloride (MPTC) with high toughness of 4000 J m22 [119]. There are only a few reports with high tensile strength ionic polymer. The toughness of 27 MJ m23 and fracture energy of  12 kJ m22 was reported as the highest intrinsically tough materials among self-healing polymers [120]. Mechanically robust material with

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autonomous and rapid self-healing on the time scale of seconds is highly required. Recently we have developed an ionically cross-linked polymeric materials with multiple physical and mechanical properties such as flexible, transparent, flame resistance, conducting nature, high strength, super tough, high fracture energy associated with hydrogen bond-containing protonated polyvinyl imidazolium cation [PVIMH]n1 and polyphosphate anion [PPA]n2 [110]. This cross-linked hydrogel self-heals completely, autonomously, repeatedly, and rapidly within the time scale of seconds under ambient conditions without any external stimuli. Furthermore the tensile strength was enhanced by the incorporation of metal ions (Ca21) within the polymer matrix to 39 MPa from 30 MPa of base hydrogel with a superior toughness 41 MJ m23 and fracture energy 4700 J m22 with B90% healing efficiency. Ca21 ion incorporation is significant, since the metal ions in particular CaPO4 like a ceramic suffer from poor interfacial-bonding interaction which leads to decrease in mechanical and the self-healing performances. This metal ion on polymer composites on self-healing and mechanical properties may be a potential candidate in tissue engineering and soft skins; bone constitutes analogous alike. Fig. 2.24 shows the various polymer ionic units used in intrinsic self-healing polymeric systems. 2.9.2.2 Self-healing through ionic salts Ionic salts, such as NaCl, can have a strong effect on the interaction strength between polycation and polyanion in polyelectrolytes therefore control the diffusion of polyelectrolyte chains within polymer matrix and multilayers. The ions of the salt break the cross-links within the polyelectrolyte through charge screening resulted in high mobility of polyelectrolyte chains. For example, the self-healing in the complex of bulk-branched poly(ethylene imine) and poly(acrylic acid) (BPEI/PAA) was enhanced by applying 1 M NaCl treatment followed by exposure to water [125]. Gillies et al. developed an ionically cross-linked network of poly(acrylic acid) (PAA) and poly(triethyl(4-vinylbenzyl)phosphonium chloride) (P-Et-P) which shows the self-healing in the presence of 0.1 M NaCl [124]. The ionically cross-linked network of PAA and P-Et-P in 0.1 M NaCl swelled up to B200% of the initial mass, however, it still retained the structural integrity for at least 2 months. Another polyelectrolyte system consists of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) presented by Schaaf et al. where the healing process and tensile strain are more efficient in the presence of 2.5 M NaCl when compared with 1 M NaCl [122]. 2.9.2.3 Magnesium ions (Mg21)-based self-healing mechanism Magnesium ions (Mg21) has a good binding affinity toward the bisphosphonate- and oxygen-containing ligands with reversible binding nature, due to which Mg21 has significant scope in self-healing chemistry. Moreover, Mg-ion has a great role in regulating diverse biological processes such as enzyme regulation in diverse biochemical reactions in our body systems, including protein synthesis, muscle and nerve functions, blood pressure regulation, oxidative phosphorylation, and even in the synthesis of DNA and RNA. Hence, the Mg-ion-loaded polymeric materials with self-healing properties have great importance for the development of biomaterials. Ossipiv group has generated a pHresponsive self-healing hydrogel network from natural polysaccharides and drug-loaded nanoparticles via Mg21
bisphosphonate ligand interactions [126]. This self-healable

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FIGURE 2.24 Various polymer ionic units used in intrinsic self-healing polymeric systems [1,89]. (A) [114,115], (B) [119], (C) [121], (D) [122], (E and F) [106,107,123], (G and H) [112], (I) [124], (J) [114], and (K and L) [110].

hydrogel was demonstrated as an injectable drug that disassembled in a tumor-specific environment, providing localized uptake of the nanoparticles. 2.9.2.4 Calcium ions (Ca21)-based self-healing mechanism Similar to Mg21, calcium ion (Ca21) also has a good binding affinity toward oxygen donor ligands such as bisphosphonates or phosphates and calcium ions contribute a vital role in living organisms and cells like in signal transduction, neurotransmitter, muscle cell contraction, fertilization, enzymes regulation, and blood-clotting cascade. Also calcium is a key constituent bone entity derived from hydroxyapatite. Hence, the development of

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polymeric materials with self-healing properties based on Ca-ion is more interesting. Kouwar et al. reported a star-shaped poly(ethylene glycol) (PEG) chain functionalized with terminal alendronate which forms hydrogel after mixing in calcium solutions by creating a transient network of reversible cross-links with Ca-ion [127]. The self-healing performance was demonstrated by cutting the hydrogel and then bringing back together to allow healing without any visible interface. According to the oscillatory rheology study, the hydrogels were recovered between 70% and 100% of the original storage and loss modulus after rupture. The self-healing process involved the intrinsically connected calcium bisphosphonate complexation equilibrium. Utilizing similar chemistry, Suo et al. developed hydrogel based on calcium alginates by forming ionically and covalently crosslinked networks [117]. This hydrogel exhibits high fracture energy of 9 kJ m22 and stretchable beyond 20 times their initial length. The self-healing and high toughness of the gels are governed by the synergistic mechanisms of crack bridging by the covalently crosslinked network and hysteresis by unzipping the network of ionic cross-links. 2.9.2.5 Frustrated Lewis pair polymers as responsive self-healing gels A highly reactive frustrated Lewis pair (FLP) is formed when the strong bonds between bulky Lewis acids and bases cannot react due to steric hindrance. The suppressed reactivity makes these reagents transformative in small molecule activations and metal-free catalysis. However, the use of FLP as a platform for developing materials chemistry is unexplored. Shaver et al. reported that macromolecular FLPs of linear copolymers functionalized with sterically hindered either Lewis bases or Lewis acids as pendant groups and structural motifs are given in Fig. 2.25 [128]. These functionalized copolymers were prepared by a controlled radical copolymerization of styrene with boron or phosphorus-containing monomers. Boron- and phosphorus-functionalized polystyrenes do not react as the steric bulk prevents the favorable Lewis acid
base interaction, whereas the addition of small molecules (diethyl azodicarboxylate) cross-link rapidly with the reactive polymer chains to form supramolecular gel network. The resulting gel is dynamic, heat responsive, and self-healable.

2.9.3 Self-healing based on van der Waals force of attraction Unlike ionic or covalent bonds, van der Waals forces of attraction are comparatively weak and therefore more susceptible to disruption and rapidly disappear with distance. FIGURE 2.25 Structural motifs of sterically hindered Lewis bases and/or Lewis acids as pendant groups used for self-healing chemistry [128].

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FIGURE 2.26 Self-healing chemistry driven by weak dipole interaction of van der Waals forces in PMMAco-PBA [129]. PMMA-co-PBA, Poly(methylmethyacrylates-co-n-butylacrylates).

Compare to van der Waals interactions, hydrogen-bonding interactions are up to 20 times stronger. But when they work together in polymer aligned in the same direction, the cumulative force becomes powerful enough to help in self-healing process. The use of van der Waals forces in self-healing polymer is interesting because van der Waals forces are ubiquitous and more universal as opposed to supramolecular and dynamic covalent interactions. Urban et al. developed polymers using common monomers such as methyl methacrylate(MMA) and butyl acrylates (BA) that self-heals via weak van der Waals force of attractions as the mechanism given in Fig. 2.26 [129]. It has the ability to repair scratches and small cuts without external intervention, and the material could be used to make longer-lived coatings and hard plastic items.

2.9.4 Reversible metallosupramolecular polymer Metallogels or hydrogels are generally obtained by self-assembly of metal
ligand interactions by forming supramolecular networks [8]. Metal coordination supramolecular polymers represent hybrid materials that combine both the viscoelastic properties of polymers and the physical properties (optical, electronic, mechanical, or magnetic) of inorganic components. Self-healing properties in supramolecular metallogels have been extensively studied by means of various interactions including metal coordination, electrostatic interactions, molecular diffusion or entanglement, association, and dynamic reversible chemical bonds. Metal
ligand interaction utilizes polymeric ligand coordination with metal ions to form a supramolecular cross-linked polymer network. These interactions can be disrupted physically, thermally, or on UV irradiation and subsequent restoration of such interaction makes the network dynamic and healable. A general schematic representation of self-healing reversible metal coordination bond is presented in Fig. 2.27. The binding motif of metal and polymeric ligands can vary from a highly dynamic bond to a strong covalent-like bonding. The kinetics and thermodynamics of metal binding of coordination complexes are tuned by choosing different metal
ligand pairs which help

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FIGURE 2.27 Illustrations of healing reversible metal binding [8].

self-

in self-healing process. For example, phosphines, carbenes, amines, pyridines, ethers, ketones, imines, and nitriles can form complexes of different coordination strength with different metal ions such as Fe, Co, Ni, and Zn, whereas the different bonding motifs and coordination number can lead to the formation of supramolecular linear chains, starshaped, highly branched, or dendritic moieties of cross-link networks [8]. Controlling the stoichiometry between ligands and metal ions is critical in metallogels. Typically the addition of metal salts to a solution results in viscosity increase to reach a maximum, but excess metal ion may again reduce the degree of polymerization [8]. Fig. 2.28 shows the various metal
ligand coordination motif involved in self-healing polymeric materials. The stability of metal
ligand coordination can be tuned by choosing proper pair of metal ion and ligand, like binding ability in pyridine-base ligands increase as the number of donor atoms increase in the order: pyridine, bipyridine, to terpyridine [137]. Furthermore interactions of terpyridine with metal ions vary from strong for Fe21, medium for Co21 and Ni21 to weak for Zn21. An optically healable supramolecular polymer network based on poly (ethylene-co-butylene) core with 2,6-bis(10 -methylbenzimidazolyl)pyridine (Mebip) ligand at the terminal cross-linked via Zn(II)-bis(terpyridine) coordination forms a phaseseparated lamellar morphology [138]. On UV light exposure, the metal
ligand complex is electronically excited and the absorbed energy is converted into heat (  200 C) and results in a temporary disruption of the metal coordination from the ligand which associated with a reversible decrease in molecular mass and viscosity of the polymer. The dissociated mobile macromonomers reform the metal
ligand coordination and the original morphology which facilitates the repair of mechanical damage. While Fe(II)
terpyridine complex with poly(alkyl methacrylate) produces aggregation of ionic clusters resulted in phase separation which required elevated temperature (100 C) for the healing of damages [139]. Since the strong Fe(II)-bis(terpyridine) coordination bonds remain stable at the healing temperature, the self-repair was attributed to reversible formation and dissociation of ionic clusters. Polymer with diamidepyridine ligands cross-linked via coordination with (FeCl3 6H2O) or terbium trifluoromethanesulfonate (Tb(OTf)3) was shown to recover the tensile strength (up to 12.7 MPa) and strain (up to 1000%) after 24 h of healing at 60 C. But the same polymer cross-linked with Zn(OTf)2 was not able to self-heal due to strong coordination bond [140]. The reversible nature of Fe31 coordination with catechol-functionalized chitosan responds the rapid self-recovery of the mechanical strength (storage modulus) in 100 s during continuous step-strain relaxation measurements at RT (20 C) [141].



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FIGURE 2.28 Various metal
ligand coordination motifs involved in self-healings polymeric systems [8,130]. (A) [131], (B
D) [132], (E
H) [133], (I) [134], (J) [135], and (K) [136].

Also the metal
ligand bond strength has significant implications on dynamic mechanical properties of supramolecular elastomers. When 2,6-pyridinedicarboxamide ligands incorporated along with poly(dimethylsiloxane) (PDMS) backbone form complex with Fe (III), a highly stretchable (4500% 6 20% of strain), elastomeric materials were obtained with room temperature self-heal ability. This was attributed to the bonding energy strength ranging from strong to weak in Fe(III)-N(pyridyl), Fe(III)-N(amido), and Fe(III)-O (amido) [132]. The weaker bonds are responsible for energy dissipation on stretching and on-demand self-healing, whereas the metal ions with stronger interactions still maintain their location near the ligands thus allowing rapid bond reformation.

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Furthermore PDMS copolymerized with 2,20 -bipyridine-5,50 -dicarboxylic amide was used for metal coordination sites as cross-linker in a self-healing dielectric elastomers [142]. These PDMS elastomers cross-linked with FeCl2 and ZnCl2 salts were integrated into organic field effect transistors as gate dielectrics. The kinetically labile Zn21 coordination with bipyridine provides fast self-healing ability of the elastomer at ambient conditions. Fully stretchable transistors with FeCl2-PDMS dielectrics were fabricated and demonstrated for ideal transfer characteristics. The gate leakage current remained low even after 1000 cycles at 100% strain. High mechanical robustness and stable electrical performance are suitable for applications in stretchable electronics. Liu et al. reported a selfhealable gel by the coordination of Zn21 with three kinds of nucleotides (AMP, GMP, and CMP), at neutral pH [143]. Banerjee et al. explored multiresponsive metallohydrogel with shape-persistent, selfstanding, load-bearing, and self-healing properties based on L-valine amino acid-derived low-molecular-weight ligand complexed with Zn(II) [144]. The same group developed another self-healable and proton conductive supramolecular metallogel by simple mixing stock solutions of copper and oxalic acid dihydrate at room temperature [145]. The selfhealed copper-oxalate metallogel exhibits similar nanofibrillar morphology, thermal stability, conductivity, and rheological properties as the freshly prepared sample. A self-healing conductive composites from this metallohydrogel were also fabricated by mixing with conductive carbonaceous materials. Wang et al. fabricated a highly stretchable poly(vinyl alcohol) (PVA) hybrid hydrogel with excellent autonomous self-healing capacity, by introducing Cu(II) ions and fibrous sepiolite (Sep) into PVA solution [146]. The coordination complex of Cu(II) with
OH group of PVA provides the primary network of these hybrid hydrogels, while the hydrogen bonding between the
OH of PVA and the silanol groups (Si
OH) on the Sep surface further stabilizes the network. The dynamic nature of the metal
ligand coordination and hydrogen bonding provides the hydrogel with autonomous self-healing properties.

2.9.5 Host
guest interactions Host
guest associations can be driven by various noncovalent interactions such as H-bonding, π
π, van der Waals, and hydrophobic interactions. Due to their high selectivity and reversibility, a large number of natural and synthetic host
guest complexes have been exploited to develop controllable association/dissociation toward sensing, surface recognition, drug delivery, and stimuli-responsive hydrogels. Specific host
guest interactions can be reversibly disassembled in response to stimuli such as redox potential, pH, and temperature and explored in the design of self-healing polymeric materials in the form of supramolecular hydrogels, organogels, and nanofibers [147]. Fig. 2.29 shows the schematic illustrations of reversible host
guest interactions in self-healing polymer. Typically host and guest molecules are attached as pendents with the polymer backbones. Host
guest complexes are formed initially in solution followed by free radical polymerization, otherwise the host and guest polymers synthesized separately and then combined them to induce host
guest complex formation. Commonly used macrocyclic host molecules include cyclodextrins (CDs), cucurbit[n]urils (CBs), crown ethers,

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FIGURE 2.29 Illustrations of self-healing via host
guest interactions [148].

calixarenes (CAs), and pillar[n]arenes. Structural components of host molecules are integrated into the polymer to facilitate selective interactions with targeted guest molecules, ranging from small molecules to biomacromolecules, or synthetic polymers. Self-healing efficiency depends on the dynamics of the complexation and the design of a suitable supramolecular network of geometry and energy. The host
guest chemistry of cyclodextrin-modified polymers are of great interest in self-healing due to their solubility in water, low cost, biocompatible, multiple stimuli responses, and favorable dissociation/ association dynamics [8]. Izzet et al. developed a cyclodextrin -based self-healing material from the host
guest chemistry of Dawson-type organic
inorganic hybrid polyoxometalate, K7[α2-P2W17O61Sn(C6H4I)], as a potential guest for the cyclodextrin cavity [149]. Copolymer of β-Cyclodextrin (host unit)-functionalized poly(acrylamide) and hydrophobic monomer N-adamantane-1-yl-acrylamide (guest) formed a hydrogel (β-CD-Ad) which shows instantaneous and autonomous self-healing with complete restoration of the initial adhesion strength after 24 h at RT (20 C). Replacing the hydrophobic guest by n-butyl acrylate and host to α-cyclodextrin (α-CD-nBu), the hydrogels lead to decrease in healing performance with 74% efficiency (in terms of adhesion strength) [142,150]. Multiple stimuli-responsive CD-based self-healing polymers have been explored using guest polymers-containing ferrocene, n-butyl acrylate, N-adamantane-1-yl-acrylamide, N-vinyl imidazole, and adamantine. Complexation thermodynamics are influenced by structural and electronic features of the cavity [8]. For example, reversible CD/ferrocene host
guest interactions are regulated by redox reaction which can undergo sol/gel conversion and exhibit controlled volume expansion and contraction [151]. Fe3O4 nanoparticle-doped hydrogels exhibit rapid self-healing by placing under an external magnetic field to induce flow [152]. Tough hydrogels were reported from the reversible host
guest interaction of CD and adamantine with different polymers backbones, and their toughness depends on the number of host
guest moieties in the polymer and on the nature of the polymer backbone [153]. Hydrogels with pAAM backbone show the highest toughness due to the abundance of amide groups capable of H-bonding among polyacrylamide (pAAM), poly(N,N-dimethyl acrylamide) (pDMAAm), poly(N-isopropyl acrylamide) (pNIPAAm), poly(2-hydroxymethyl acrylamide) (pHMAAm), and poly(2hydroxyethyl acrylate) (pHEA) [151,154]. Also polymers with crown ether host are often used for the preparation of self-healing supramolecular gels. Organogel from poly(methyl methacrylate) (PMMA) with pendent dibenzo-24-crown-8 (DB24C8) groups as host and bisammonium salt as guest shows

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rapid self-healing within 10 s [155]. Two types of chemistries are involved for this rapid healing process: (1) selective host
guest exchange and (2) nonselective electrostatic and H-bonding interactions between the ammonium salt and DB24C8 moieties. Even, complex hierarchical supramolecular networks capable of self-healing were constructed by combing directional host
guest and metal
ligand complexations. Linear supramolecular polymers were assembled from the host
guest interaction of homoditopic building blocks of bis(benzo-21-crown-7) and bis(dialkylammonium salt) containing two 1,2,3-triazole groups. Furthermore 1,2,3-triazole acts as ligand for the coordination with [PdCl2(PhCN)2] complex to form a hierarchical supramolecular cross-linked network [156]. This network structure allows multiple stimuli (metallo-, thermo-, pH-, and cation-) induced selective dissociation/association either at the backbone or at the junction points. Similarly macrocyclic cucurbit[8]uril (CB[8]) works as a host which reversibly binds two different guest molecules viologen and naphthoxy at the same time and show self-healing performance [157]. Also large varieties of available interactions in host
guest supramolecular network make them very attractive for self-healing and other applications. But it is important to know how to take advantage of these varieties of interactions and their network structural features such as selective recognition, rapid diffusion of small molecules, the coexistence of different association strengths or nonspecific secondary interactions to facilitate the optimum self-healing, and other dynamic properties. Even within these challenges, there are numerous opportunities for the development of guest
host-based self-healing network like, how the spatial orientation of guest components facilitates the formation of strong and multiple host
guest bonds or how to properly design advanced host
guest interactions for the development of larger cavities that could host multiple guests with directional bonds and much stronger bonds to incorporate in the spatially restricted polymer environment to facilitate the multiple dynamic host
guest bonds and so the self-healing.

2.9.6 π-Interactions based self-healing polymer The noncovalent interactions involving π-system occurs between an electron-rich π-system with an electron-deficient molecular species, cations, another π system, and even anions. The dynamic nature of π-interactions is utilized in self-healing chemistry. As illustrated in Fig. 2.30 the π
π interaction in a polymeric system containing diimide moieties as π-electron-deficient center and π-electron-rich pyrenyl units is reversible in nature and used for self-healing chemistry. Along with π
π stacking the self-assembly and chainfolding facilitate the self-healing process due to the presence of bulky diimide-pyrenyl groups. In this context, Burattini et al. reported complementary π
π stacking between π-electron-deficient polydiimide chain and telechelic polyurethane with pyrenyl end group which exhibits the self-healing properties [95,158]. Furthermore the same group developed tweezer-type bis-pyrenyl end groups and naphthalene-diimide chains with enhanced mechanical and self-healing properties [159]. Similar π
π interactions are utilized in polymeric systems with multiple aromatic units by introducing cellulose nanocrystals and AuNPs to enhance the mechanical properties along with self-healing capability [96,160,161]. Combination of π
π stacking and intermolecular H-bonding was used for the

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FIGURE 2.30 Self-healing via π
π interactions [8].

efficient self-healing in nitrobenzoxadiazole (NBD)-containing cholesterol, polybutadienecontaining urethane and urea groups as a spacer [95,162]. Feng et al. developed a stretchable and self-healing polymer based on cyclometalated platinum(II) complex with 6-phenyl-2,20 bipyridylligand into a polydimethylsiloxane (PDMS) backbone using the combined effect of Pt Pt and π
π interactions which show much better strength bond strength of 40 kcal mol21 compared with individual Pt(II) Pt(II) (,10 kcal mol21) or π
π interactions [163].





2.9.7 Hydrophobic interactions Hydrophobic interaction is the affinity of nonpolar substances to interact with each other and aggregates in a polar solution (usually water). Commonly long-chain hydrocarbons, silicones, perfluoroalkyl chain, and many nonpolar groups are regarded as hydrophobic and generally do not mix with water. However, the hydrophilic group or head attached in these molecules or polymers prevents phase separation through strong hydrogen bonds with water molecules and this driving force creates self-assembly. In a solution, compact selfassembly of hydrophobic associations is easily formed and quickly reform even after disturbing the associations. Copolymerization of hydrophilic and hydrophobic monomers is commonly used by the micelle copolymerization method to allow the incorporation of stable hydrophobic domains into hydrogels without disrupting the hydrophilic nature [8]. Self-healing processes with hydrophobic interactions originated mainly from the rearrangement of the hydrophobic associations of micelles. This strategy has been used in a self-healing hydrogel from the copolymerzation of octyl phenol polyethoxy ether acrylate hydrophobic monomers and a hydrophilic acrylamide in an aqueous solution containing sodium dodecyl sulfate (SDS) [164]. The association between hydrophobic copolymer and SDS form micelles to give “hydrophobic association gel” with excellent mechanical properties and transparency along with self-healing capability. A cylindrical gel sample was cut, and the halves were placed back together (submerged in water at RT, 20 C) in a closed plastic pipe, which shows B70% healing efficiency (w.r.t. tensile strain at break) after 3 days. The

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SDS of the dissociated micelles initiates the self-healing by the formation of a monolayer membrane at the cut interface, however, the hydrophobic monomers would then interact with SDS to reform micelles. The self-healing, mechanical, and remoldable properties are governed by the dissociation and reassociation through the formation of micellar structures. Okay and coworkers developed autonomous self-healing hydrogels by reversible crosslinking using similar chemistry [165]. Hydrogels were prepared via the micelle copolymerization of hydrophobic monomer [stearyl methacrylate (C18) and dodecyl acrylate (C22)] with hydrophilic monomer acrylamide (AAm) in an aqueous solution containing SDS/ NaCl as given in Fig. 2.31A. Self-healing was demonstrated by cutting the hydrogel in two halves and those halves placed back together for 10 min at 24 C. Also these hydrogels exhibited 100% healing efficiency w.r.t. their mechanical properties after cyclic compression deformations. The addition of NaCl salt leads to the progress of worm like micelles and assisted the inclusion of hydrophobic dodecyl alkyl chain (C22) into the hydrophobic poly(acrylamide) network. The role of SDS in subsequent self-healing and recovery of mechanical properties is very important which controls the reversible association by weakening the hydrophobic interaction. Underwater self-healing is one of the challenging tasks. For underwater intrinsic selfhealing, control of the surface approach, wetting, and diffusion is very important. Since hydrophilic polymers swell in water and lose their load-bearing capacity, they cannot be considered for underwater performance, but the target polymer should be amphiphilic and possess a certain degree of hydrophilicity to facilitate the dispersion and collision of macromolecules on the crack surface. Rong et al. developed an amphiphilic hyperbranched polyurethane-mercaptosuccinic acid-acrylonitrile butadiene styrene (HBPUMSA-ABS) with many hydrophilic and hydrophobic terminal groups, which are capable of self-healing and recycling capacity in acidic water toward strength restoration through

FIGURE 2.31 Micelle formation from copolymerization of hydrophobic dodecyl acrylate (C22), (A) with hydrophilic monomer AAm toward self-healing through hydrophobic interactions and (B) hydrophobic segments in hyperbranched polyurethane-mercaptosucccinic acid acrylonitrile styrene butadiene. AAm, Acrylamide. Source: (A) Adapted with permission from D.C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Tough and self-healing hydrogels formed via hydrophobic interactions, Macromolecules 44 (2011) 4997
5005, 2011 American Chemical Society. (B) Adapted with permission from N.N. Xia, X.M. Xiong, M.Z. Rong, M.Q. Zhang, F. Kong, Self-healing of polymer in acidic water toward strength restoration through the synergistic effect of hydrophilic and hydrophobic interactions, ACS Appl. Mater. Interfaces 9 (2017) 37300
37309, 2017 American Chemical Society.

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the synergistic effect of hydrophilic and hydrophobic interactions (Fig. 2.30B) [166]. The hydrophilic groups of a damaged surface will be protonated in acidic water and form hydrogen-bonding interaction at the interface to close the crack. Furthermore hydrophobic groups gradually start interacting across the interface due to the contact with a cracked surface, thus reinforce the rebonded portion. The amphiphilic nature of this polymer provides both thermodynamic and kinetic requirement for autonomous restoration under water, which supposed to be unfavored and inhibits the adhesion between hydrophobic polymeric materials, actually turns into beneficial for crack healing. This responsive underwater self-healing polymer design will be helpful for the utilization in various applications. Manna and Lynn reported self-healing and recovery of damage of superhydrophobic coatings by treatment with water to the crushed polymer film for the micro- and nanoscale topographic features [167]. These coatings can be crushed and healed multiple times using the water-assisted approach. Furthermore these porous coatings are also able to withstand severe physical manipulations (e.g., deep scratches and repeated abrasion with sandpaper) without compromising the hydrophobicity.

2.9.8 Interpenetrating polymer network for self-healing Chemical cross-linking increases the mechanical strength of a hydrogel but limits the mobility and healing ability. Whereas physical cross-linking (sacrificial bonds) network in the hydrogel increases the self-healing efficiency as well as mechanical properties. Introduction of more than one type of healing chemistry in a single polymeric network will help to achieve multiple healing cycle with rapid and increased healing efficiency where one of the healing chemistry works in response to stimuli and the other one remains silent or both work at the same time through the interpenetrating network (IPN) chemistry. Fig. 2.32 shows various examples involved in self-healing through IPN chemistry. Zheng et al. demonstrated a hybrid physically
chemically two interpenetrating cross-linked networks of agar-polyacrylamide (agar-PAAm) hydrogels, comprising of a hydrogen-bonding agar network and a covalently cross-linked PAAm network which shows fast recovery with 65% healing efficiency in 10 min. These polymeric hydrogels exhibit superior mechanical properties such as high strength (failure compressive stress of 38 MPa), tensile stress of 1.0 MPa, toughness of 9 MJ m23 and fracture energies of 102
103 J m22, and excellent extensibility (1500%
2000%) which also allow free-shapeability [170]. Also Gong et al. demonstrated a hydrogel of physically cross-linked interpenetrating polymer network of hydrogen-bonding and hydrophobic interactions consisting of a hybrid poly(acrylamide) network and amphiphilic triblock copolymer of poly (butyl methacrylate) (PBMA) and poly(methacrylic acid) (PMAA), PBMA-b-PMAA-b-PBMA. The hydrogen-bonding network contributed from poly(acrylamide) and one sacrificial hydrophobic interactions from butylacrylate makes the hydrogel dynamic and healable [171]. Li et al. developed a double IPN, both physically and chemically cross-linked hydrogel based on poly(ethylene glycol)-poly(vinyl alcohol) (PEG-PVA), which shows shape-memory and selfhealing with recovery efficiency of 68% w.r.t. tensile strength after 48 h at RT (20 C) [172]. Multistep synthesis of starting functional polymer precursors and the expensive stock materials such as tetra-polyethylene glycol are the factors limiting the broader use of IPN in the development of self-healing hydrogels. As shown in Fig. 2.32A, Yang et al. described

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FIGURE 2.32 Examples of IPN chemistry involved in self-healing combine chemistry of (A) ionic-H-bonding [112], (B) H-bonding disulfides metathesis reactions [55], (C) thiol-ene click and borax-diol chemistry [51], (D) DA-H-bonding [168], (E) DA-disulfides metathesis
ionic interactions [169], and (F and G) ionic and hydrogen bonding [110]. DA, Diels
Alder; IPN, interpenetrating network.

self-healable conductive hybrid hydrogels with a dual network of polyionic liquids (1-methyl-3-(4-vinylbenzyl) imidazolium chloride (P(VBIm-Cl))), poly(sodium p-styrenesulfonate) (P(NaSS)), and poly(vinyl alcohol) (PVA) and incorporated in situ-reduced Au nanoparticles where both ionic and hydrogen bondings are working as an IPN [112]. Odriozola et al. demonstrated a dual network of disulfide metathesis and H-bonding. The IPN utilizes bis(4-aminophenyl) disulfide as aromatic disulfide metathesis dynamic covalent bond

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chemistry and a dynamic H-bond cross-linker for the design of self-healing poly(urea-urethane) elastomeric self-healing material as given in Fig. 2.31B [51]. Theato et al. reported a facile one-pot synthesis of PEG-based self-healing hydrogels from inexpensive commercially available polyethylene glycol diacrylate and dithiothreitol. Borax was used as an efficient catalyst for rapid gelation (in between 40 s and 2 min) under the ambient condition at room temperature. For the first time, borax was used as the catalyst for a thiol-ene Michael-type polyaddition of PEG gels where borax induces the formation of two classes of bonds, covalent thioether and boronate ester bonds at the same time as IPN network as presented in Fig. 2.32C [51]. The storage modulus of the synthesized PEG gel (with 87.5% water) reached up to 104 Pa and exhibits rapid self-healing without external stimuli due to the dynamic boronate ester linkages. Furthermore this transparent hydrogel is pH and thermoresponsive. This multidirectional impact of borax in hydrogels with intriguing properties has potential application in gel sealant, biosensors, or regenerative medicines. Scha¨fer and Kickelbick described a combination of DA cross-linking and hydrogen bonding as a double reversible network for the improvement of self-healing in hybrid materials as given in Fig. 2.32D [168]. Singha et al. illustrated a self-healable antifouling zwitterionic hydrogel based on synergistic phototriggered dynamic disulfide metathesis reaction and ionic interaction along with thermoreversible DA chemistry as an IPN as given in Fig. 2.32E [169]. Recently we have demonstrated the IPN of supramolecular hydrogen bonding and ionic interactions high strength, tough, and rapidly self-healing hydrogels from polyvinyl imidazolium cation [PVIMH]n1 and polyphosphate anion [PPA]n2 as given in Fig. 2.32F [110]. In the presence of ionic interactions it is difficult to distinguish other supramolecular interactions and the cross-linking through IPN structure for bulk polymer components. The fast damage repair was ascribed as the molecular motions arising from the interionic attraction between polymeric cation [PVIMH]1 and [PPA]2 anion and simultaneous hydrogen-bonding network between the N
H bond of [PVIMH]1 and phosphates. Both ionic and hydrogen-bonding interactions work simultaneously and continuously as a dual interpenetrating cross-linking network for instant and reversible healing processes as given in Fig. 2.33. Furthermore metal ion (Ca21) was incorporated within the polymer

FIGURE 2.33 Illustrations of mechanism of IPN structure involved in self-healing Phenomenon [110]. IPN, Interpenetrating network.

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matrix to understand the effect on self-healing and mechanical properties which enhance the tensile strength 39 from 30 MPa of base hydrogel with a superior toughness 41 MJ m23 and fracture energy 4700 J m22 with B90% of healing efficiency.

2.10 Chemistries involved in microcapsule-based self-healing polymeric system 2.10.1 Microcapsule mediated ring-opening metathesis polymerization The first successful demonstration of a fully autonomic healing mechanism was developed by White et al. based on microcapsules-embedded dicyclopentadiene (DCPD) monomer and Grubbs-I catalyst in a thermosetting polymers matrix to initiate a ROMP as shown in Fig. 2.34 [4]. In this extrinsic healing process, a crack propagates through the polymer matrix and intersects the microcapsules and rupture. The monomer contained in the capsule then wicks onto the crack, encounters a catalyst particle, and a polymerization reaction is initiated. The resulting polymer bonds the crack faces back together, restoring much of the original material strength. This approach has proven effective at healing cracks in thermosetting materials generated by quasistatic fracture and fatigue. The healing chemistry utilized in this material was based on the polymerization of dicyclopentadiene (DCPD) with Grubbs’ catalyst. Grubbs’ catalyst initiates a ROMP of the DCPD monomer as shown in Fig. 2.34C. Although Grubbs’ catalyst is relatively stable at ambient conditions, exposure to amines, such as those present in the curing agents of epoxies, can deactivate the catalyst. To improve the stability of the catalyst Rule et al. encapsulated the catalyst units in wax [174]. Healing was demonstrated using wax-protected catalyst at concentrations an order-ofmagnitude lower than present in the original White et al. system.

FIGURE 2.34 (A) Microcapsule-based self-healing process [4] and (B) mechanism of ROMP by dicyclopentadiene using Grubbs’ catalyst [173]. ROMP, Ring-opening metathesis polymerization.

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2.10.2 Azide-alkyne click chemistry In recent years, Cu(I)-catalyzed azide-alkyne “click” chemistry has emerged as a promising synthetic approach in organic chemistry, polymer science, materials science, and biocompatible reactions. These azide-alkyne 1,3-dipolar cycloadditions are useful for binding two molecular building blocks together as modular units in facile, selective, water-tolerant, and under mild conditions with quantitative-yield having little or no byproducts. This chemistry of copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) click reaction for the cross-linking and connecting the building blocks under ambient conditions has been significantly utilized in self-healing polymeric materials. A general azide-alkyne click chemistry-based self-healing polymeric system is presented in Fig. 2.35. Ideally the healing process occurs at low temperatures, independent of the substrate(s) used, by forming strong and stable networks, binding to the newly generated crack interfaces to restore the original properties of polymeric material. The simplicity of the reaction as well as the chemical diversity of substrates has led the CuAAC to an enormous potential and highly attractive in self-healing system [175,176]. As azide-alkyne click cycloaddition is extrinsic self-healing process encapsulation strategies are required to preserve the click components separate by microencapsulation. Various designs of molecules with varying molecular weight, architecture, and the number and density of functional groups utilized for azide-alkyne CuAAC-based click chemistry used for selfhealing process are illustrated in Fig. 2.36. The azide-alkyne click chemistries are extensively explored by Binder et al. Toward the self-healing polymeric materials using this concept a liquid, azido-telechelic three-arm star poly(isobutylene) (Mn 5 3900 g mol21) along with trivalentalkynes were encapsulated into micron-sized capsules and embedded into a polymer-matrix (high-molecular-weight poly (isobutylene), Mn 5 250,000 g mol21) using (CuIBr(PPh3)3) as a catalyst in phenolformaldehyde microcapsules [177]. Furthermore the low-molecular-weight alkynes and three-arm star polyisobutylene azides were encapsulated in urea/formaldehyde phenolformaldehyde microcapsules in the presence of Cu(I) species and were triggered to heal cracks when microcapsules were ruptured [178]. Tang et al. show the self-healing hyperbranched poly(arolytriazole)s of 1,4-disubstituted bis(aroylacetylene)s and 1,2,3-triazoles

FIGURE 2.35 Illustrations of azide-alkyne click chemistry used for the self-healings in polymeric systems.

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FIGURE 2.36 Few multivalent azides and alkynes used in microcapsule-based self-healing polymeric systems [175].

using the same azide-alkyne chemistry [179]. Overall there is excellent interplay-based azide-alkyne click reactions for making building block polymeric materials autonomous as well as damage-triggered self-healing via balanced cross-linking chemistries.

2.10.3 Controlled radical polymerization in microcapsule system Often, ATRP and RAFT polymerization are utilized in microcapsule-based self-healing polymeric materials, as ATRP is a versatile and powerful controlled radical polymerization

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(CRP) technique. (Co)polymers with well-defined molecular microstructure and narrow molecular weight distribution can be synthesized by ATRP process which also allows tolerance toward various functional groups. Microcapsules of glycidyl methacrylate in melamine-formaldehyde shells were dispersed in PMMA-Br matrix (poly(methyl methacrylate) (PMMA) with terminal Br). Glycidyl methacrylate (GMA) is released when microcapsules are ruptured, and react with PMMA-Br via ATRP to extend the polymer chain. Such self-healing by chain extension via ATRP was also used in ATRP macroinitiators mediated with Cu species at ambient temperatures to form PMMA-b-PGMA diblock copolymers [180]. Similarly RAFT polymerization chemistry was utilized in melamineformaldehyde microcapsules containing GMA and polystyrene as a micro-RAFT, which on rupture allows the polymerization of GMA with polystyrene macroinitiator under RAFT condition, and respond the recovery [181]. 2.10.3.1 Other microcapsule-embedded systems In addition to the use of ROMP described earlier, a number of unique chemistries have been explored on microcapsule-embedded system as illustrated in Fig. 2.37 for a few typical examples. Tin-catalyzed hydrosilylation has been explored for the selfhealing in PDMS-based materials. Polyurethane microcapsules containing di-n-butyl tin dilaurate catalyst was used with siloxane-based healing agents phase separated in vinyl ether matrix. Capsules are broken during damage to release the tin catalyst, which catalyzes the polyaddition of hydroxyl-terminated PDMS with polydiethoxysiloxane [182]. Sun et al. developed a clinically applicable self-healing dental composite made from contemporary dental components along with a healing powder (strontium fluoroaluminosilicate particles) and silica microcapsules containing healing liquid (aqueous polyacrylic acids solution) [183]. Epoxy-amine curing chemistry is particularly attractive for the self-healing composites due to their usage in fabrications of epoxy materials. Epoxy-amine curing serves as a healing chemistry and the heal region would be chemically or mechanically compatible with host matrix processing the same chemistry. However, preparations of capsules containing liquids amine are tedious when compared with encapsulation of epoxy. Epoxy-amine and epoxy/mercaptan curing chemistry were used in microcapsule-based self-healing polymeric systems at room temperature [3,184].

FIGURE 2.37 (A) Epoxy-amine/mercaptan chemistry and (B) hydrosilylation reaction used in microcapsule embedded in polymer for the development of extrinsic self-healing materials.

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2.10.4 Vascular-based self-healing system The microcapsule-based healing chemistries are also utilized in vascular networks. The advantage of vascular-based system is that it provides the self-healing ability for a larger damage volume and multiple self-healing cycles. The healing agent along with a hardener is kept in external reservoirs which are delivered to damage sites through the embedded vascular networks. Although microvascular-based composite materials have the ability to heal microcracks only, the macrovascular system can deliver larger content to the damage sites, thus able to heal macrocracks. 2.10.4.1 Macrovascular system The vascular networks are filled with a liquid healing agent along with hardener after all the manufacturing processes are completed. Crack-induced delivery of the healing agent to the damage site via the vascular network leads to the repair process. Mostly the incorporation of vascular networks has minimum influence on the initial static mechanical properties of the host structure. Trask and coworkers presented vascular networks within a fiber-reinforced polymer composite laminate [185]. The vascules were fabricated by a “lost-wax”-type process using low melting temperature wire (0.25 mm diameter, Sn 60%, Pb 40%) located at interfaces between plies to create parallel channels within laminar and the technique was applied to the carbon-fiber-reinforced composites and glass-fiberreinforced composites [186]. 2.10.4.2 Microvascular-based self-healing The microvascular systems are better for the repeatable self-healing performance on minor damages. The 3D microvascular network embedded in polymer substrates via selfassembly and filled with DCPD as a liquid healing agent. Solid Grubbs’ catalyst particles were incorporated within the coating. On minor crack propagation of the liquid reagent wicked into the crack due to capillary forces through the interpenetrating microvascular network. The ROMP of DCPD in the coating layer leads to the healing of the epidermal layer of coating within 12 h at room temperature [187,188].

2.11 Conclusions In summary, this chapter highlights several exciting developments of the chemistry involved in the designs and synthetic strategies of high-performance self-healing polymeric materials and composites toward their potential applications. During the selfhealing processes, the dynamic covalent bonds are strong enough but reversible in nature in the presence of external stimuli such as heat, light, and variations of pH and creates a permanent network after recovery of damages with multiples time healing capabilities. Hence, the dynamic covalent bonds are good options for rigid material required for applications or even thermoplastic elastomer. The dynamic supramolecular interactions are also reversible in nature and offer room temperature multiple rapid self-healing with mostly free from external stimulus, whereas microcapsule-based self-healing process is fast but reactions are irreversible and work for single time only. The self-healing through electrostatic

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interactions, particularly in poly(ionic liquids)-bearing multiple cations and anions, are emerging due to rapid, room temperature, multiple, and high healing capability without any external stimuli or catalyst along with good mechanical property. The hydrogen-bonding and hydrophobic interactions-based healing chemistry are more important in the biological system and the interactions can also be seen in synthetic materials such as polyurethanebased polymeric systems. Beyond a strong interest in both academic and commercial points of view, the approaches toward several emerging new types of self-healing technology have been developed over the past decade. The rapid development and rich chemistry for incorporating self-healing capabilities in polymeric materials can now effectively address numerous damage mechanisms at molecular and structural levels. More importantly along with selfhealing ability different other excellent properties included in these advanced polymer materials such as self-assembly along with self-healing, self-bending, conductive material with self-healing, underwater adhesion with self-healing, Self-healable supercapacitor, self-healing perovskite film, self-healable organic FETs are emerging smart materials. Certainly there are areas to still need improvements, despite the current progress in the design of self-healing polymeric materials. Biological systems use multiple healing mechanisms simultaneously, like “patch and repair” which rely on fast forming patches to seal and then slow regeneration of the final repair tissue. In contrast to these mechanisms, most of the self-healing processes are a single step. The repair mechanisms for bone, tendons, and skin are also based on a multimechanistic approach, involving initial inflammatory responses in combination with the regeneration of the damaged material. Similarly the tough challenges demand the introduction of multiple healing chemistry in a polymeric network to control and increase the healing efficiency in a polymeric material where one of the healing chemistries fail but others will work or all would work simultaneously. Current research on self-healing materials leads to a range of new commercial applications, and new concepts in self-healing chemistry covering the properties other than mechanical is emerging. Also developing polymeric materials with rapid and multiple cycles of self-healing, at the same time improving the physical and mechanical strength are challenging. One of the possible ways is to introduce more than one type of healing chemistry in a single polymeric network where one type of healing chemistry works in response to damage and other one remains silent or both works simultaneously through the IPN chemistry.

References [1] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (2013) 7446
7467. [2] Y.-L. Liu, T.-W. Chuo, Self-healing polymers based on thermally reversible Diels
Alder chemistry, Polym. Chem. 4 (2013) 2194
2205. [3] D.Y. Wu, S. Meure, D. Solomon, Self-healing polymeric materials: a review of recent developments, Prog. Polym. Sci. 33 (2008) 479
522. [4] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794
797. [5] S.Y. An, D. Arunbabu, S.M. Noh, Y.K. Song, J.K. Oh, Recent strategies to develop self-healable crosslinked polymeric networks, Chem. Commun. 51 (2015) 13058
13070.

Self-Healing Polymer-Based Systems

66

2. Types of chemistries involved in self-healing polymeric systems

[6] M.D. Hager, P. Greil, C. Leyens, S. van der Zwaag, U.S. Schubert, Self-Healing Materials, Adv. Mater. 22 (2010) 5424
5430. [7] D. Mills, Self-healing materials: fundamentals, design strategies, and applications, Corros. Eng. Sci. Technol. 44 (2009) 163. [8] Y. Yang, M.W. Urban, Self-healing of polymers via supramolecular chemistry, Adv. Mater. Interfaces 5 (2018) 1800384. [9] W.H. Binder, Self-Healing Polymers: From Principles to Applications, John Wiley & Sons, 2013. [10] O.K.F. Bussemaker, Tack in rubber, Rubber Chem. Technol. 37 (1964) 1178
1189. [11] K.D. Kumar, G.C. Basak, A.K. Bhowmick, Adhesion between unvulcanized elastomers: a critical review, Prog. Adhes. Adhes. 3 (2018) 185
252. [12] M. Hernandez, A. Grande, S. Van Der Zwaag, S. Garcia, Monitoring network and interfacial healing processes by broadband dielectric spectroscopy: a case study on natural rubber, ACS Appl. Mater. Interfaces 8 (2016) 10647
10656. [13] C. Gay, L. Leibler, Theory of tackiness, Phys. Rev. Lett. 82 (1999) 936
939. [14] N. Roy, B. Bruchmann, J.-M. Lehn, DYNAMERS: dynamic polymers as self-healing materials, Chem. Soc. Rev. 44 (2015) 3786
3807. [15] A.J.R. Amaral, G. Pasparakis, Stimuli responsive self-healing polymers: gels, elastomers and membranes, Polym. Chem. 8 (2017) 6464
6484. [16] H. Colquhoun, B. Klumperman, Self-healing polymers, Polym. Chem. 4 (2013) 4832
4833. [17] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698
1702. [18] S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev. 39 (2010) 1973
1985. [19] Y.-L. Liu, C.-Y. Hsieh, Crosslinked epoxy materials exhibiting thermal remendablility and removability from multifunctional maleimide and furan compounds, J. Polym. Sci. Part. A: Polym. Chem. 44 (2006) 905
913. [20] A.A. Kavitha, N.K. Singha, “Click chemistry” in tailor-made polymethacrylates bearing reactive furfuryl functionality: a new class of self-healing polymeric material, ACS Appl. Mater. Interfaces 1 (2009) 1427
1436. [21] N.B. Pramanik, G.B. Nando, N.K. Singha, Self-healing polymeric gel via RAFT polymerization and Diels
Alder click chemistry, Polymer 69 (2015) 349
356. [22] B.-G. Kang, N.B. Pramanik, N.K. Singha, J.-S. Lee, J. Mays, Precise synthesis of thermoreversible block copolymers containing reactive furfuryl groups via living anionic polymerization: the countercation effect on block copolymerization behavior, Polym. Chem. 6 (2015) 6732
6738. [23] A. Nasresfahani, P.M. Zelisko, Synthesis of a self-healing siloxane-based elastomer cross-linked via a furanmodified polyhedral oligomeric silsesquioxane: investigation of a thermally reversible silicon-based crosslink, Polym. Chem 8 (2017) 2942
2952. [24] T. Ikezaki, R. Matsuoka, K. Hatanaka, N. Yoshie, Biobased poly(2,5-furandimethylene succinate-co-butylene succinate) crosslinked by reversible Diels
Alder reaction, J. Polym. Sci. Part. A: Polym. Chem. 52 (2014) 216
222. [25] A.M. Peterson, R.E. Jensen, G.R. Palmese, Reversibly cross-linked polymer gels as healing agents for epoxy 2 amine thermosets, ACS Appl. Mater. Interfaces 1 (2009) 992
995. [26] A.M. Peterson, R.E. Jensen, G.R. Palmese, Room-temperature healing of a thermosetting polymer using the Diels 2 Alder reaction, ACS Appl. Mater. Interfaces 2 (2010) 1141
1149. [27] Q. Tian, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, A thermally remendable epoxy resin, J. Mater. Chem. 19 (2009) 1289
1296. [28] S.N. Chavan, A.K. Padhan, D. Mandal, Self-assembly of fluorous amphiphilic copolymers with ionogels and surface switchable wettability, Polym. Chem. 9 (2018) 2258
2270. [29] D.W. Smith, S.T. Iacono, S.S. Iyer, Handbook of Fluoropolymer Science and Technology, John Wiley & Sons, 2014. [30] A.K. Padhan, D. Mandal, Thermo-reversible self-healing in a fluorous crosslinked copolymer, Polym. Chem. 9 (2018) 3248
3261. [31] Z. Wang, H. Zuilhof, Self-healing fluoropolymer brushes as highly polymer-repellent coatings, J. Mater. Chem. A 4 (2016) 2408
2412.

Self-Healing Polymer-Based Systems

References

67

[32] Z. Wang, H. Zuilhof, Self-healing superhydrophobic fluoropolymer brushes as highly protein-repellent coatings, Langmuir 32 (2016) 6310
6318. [33] Y. Li, L. Li, J. Sun, Bioinspired self-healing superhydrophobic coatings, Angew. Chem. 122 (2010) 6265
6269. [34] H. Wang, Y. Xue, J. Ding, L. Feng, X. Wang, T. Lin, Durable, self-healing superhydrophobic and superoleophobic surfaces from fluorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fluorinated alkyl silane, Angew. Chem. (Int. Ed. Engl.) 50 (2011) 11433
11436. [35] H. Zhou, H. Wang, H. Niu, A. Gestos, T. Lin, Robust, self-healing superamphiphobic fabrics prepared by two-step coating of fluoro-containing polymer, fluoroalkyl silane, and modified silica nanoparticles, Adv. Funct. Mater. 23 (2013) 1664
1670. [36] S. Banerjee, B.V. Tawade, B. Ame´duri, Functional fluorinated polymer materials and preliminary self-healing behavior, Polym. Chem. 10 (2019) 1993
1997. [37] N. Yoshie, M. Watanabe, H. Araki, K. Ishida, Thermo-responsive mending of polymers crosslinked by thermally reversible covalent bond: polymers from bisfuranic terminated poly(ethylene adipate) and trismaleimide, Polym. Degrad. Stab. 95 (2010) 826
829. [38] C. Zeng, H. Seino, J. Ren, K. Hatanaka, N. Yoshie, Bio-based furan polymers with self-healing ability, Macromolecules 46 (2013) 1794
1802. [39] K. Adachi, A.K. Achimuthu, Y. Chujo, Synthesis of organic 2 inorganic polymer hybrids controlled by Diels 2 Alder reaction, Macromolecules 37 (2004) 9793
9797. [40] A. Gandini, The furan/maleimide Diels
Alder reaction: a versatile click
unclick tool in macromolecular synthesis, Prog. Polym. Sci. 38 (2013) 1
29. [41] A. Hyder Ali, K.S.V. Srinivasan, Photoresponsive functionalized vinyl cinnamate polymers: synthesis and characterization, Polym. Int. 43 (1997) 310
316. [42] D. Ahn, S.R. Zavada, T.F. Scott, Rapid, photomediated healing of hexaarylbiimidazole-based covalently cross-linked gels, Chem. Mater. 29 (2017) 7023
7031. [43] P. Froimowicz, H. Frey, K. Landfester, Towards the generation of self-healing materials by means of a reversible photo-induced approach, Macromol. Rapid Commun. 32 (2011) 468
473. [44] S. Banerjee, R. Tripathy, D. Cozzens, T. Nagy, S. Keki, M. Zsuga, et al., Photoinduced smart, self-healing polymer sealant for photovoltaics, ACS Appl. Mater. Interfaces 7 (2015) 2064
2072. [45] A.B. Lowe, Thiol
ene “click” reactions and recent applications in polymer and materials synthesis: a first update, Polym. Chem. 5 (2014) 4820
4870. [46] J. Canadell, H. Goossens, B. Klumperman, Self-healing materials based on disulfide links, Macromolecules 44 (2011) 2536
2541. [47] Y. Amamoto, J. Kamada, H. Otsuka, A. Takahara, K. Matyjaszewski, Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units, Angew. Chem. Int. Ed. 50 (2011) 1660
1663. [48] E.K. Skinner, F.M. Whiffin, G.J. Price, Room temperature sonochemical initiation of thiol-ene reactions, Chem. Commun. (Cambridge, Engl.) 48 (2012) 6800
6802. [49] B. Zhang, Z.A. Digby, J.A. Flum, P. Chakma, J.M. Saul, J.L. Sparks, et al., Dynamic thiol
michael chemistry for thermoresponsive rehealable and malleable networks, Macromolecules 49 (2016) 6871
6878. [50] P. Chakma, L.H. Rodrigues Possarle, Z.A. Digby, B. Zhang, J.L. Sparks, D. Konkolewicz, Dual stimuli responsive self-healing and malleable materials based on dynamic thiol-Michael chemistry, Polym. Chem. 8 (2017) 6534
6543. [51] L. He, D. Szopinski, Y. Wu, G.A. Luinstra, P. Theato, Toward self-healing hydrogels using one-pot thiol
ene click and borax-diol chemistry, ACS Macro Lett. 4 (2015) 673
678. [52] N. Kuhl, R. Geitner, R.K. Bose, S. Bode, B. Dietzek, M. Schmitt, et al., Self-healing polymer networks based on reversible Michael addition reactions, Macromol. Chem. Phys. 217 (2016) 2541
2550. [53] M. Pepels, I. Filot, B. Klumperman, H. Goossens, Self-healing systems based on disulfide
thiol exchange reactions, Polym. Chem. 4 (2013) 4955
4965. [54] U. Lafont, H. van Zeijl, S. van der Zwaag, Influence of cross-linkers on the cohesive and adhesive selfhealing ability of polysulfide-based thermosets, ACS Appl. Mater. Interfaces 4 (2012) 6280
6288. [55] A. Rekondo, R. Martin, A. Ruiz de Luzuriaga, G. Caban˜ero, H.J. Grande, I. Odriozola, Catalyst-free roomtemperature self-healing elastomers based on aromatic disulfide metathesis, Mater. Horiz. 1 (2014) 237
240.

Self-Healing Polymer-Based Systems

68

2. Types of chemistries involved in self-healing polymeric systems

[56] R. Caraballo, M. Rahm, P. Vongvilai, T. Brinck, O. Ramstro¨m, Phosphine-catalyzed disulfide metathesis, Chem. Commun. (2008) 6603
6605. [57] M. Herna´ndez, A.M. Grande, W. Dierkes, J. Bijleveld, S. van der Zwaag, S.J. Garcı´a, Turning vulcanized natural rubber into a self-healing polymer: effect of the disulfide/polysulfide ratio, ACS Sustain. Chem. Eng. 4 (2016) 5776
5784. [58] N.K. Kildahl, Bond energy data summarized, J. Chem. Educ. 72 (1995) 423. [59] N. Ma, Y. Li, H. Xu, Z. Wang, X. Zhang, Dual redox responsive assemblies formed from diselenide block copolymers, J. Am. Chem. Soc. 132 (2010) 442
443. [60] S. Ji, W. Cao, Y. Yu, H. Xu, Visible-light-induced self-healing diselenide-containing polyurethane elastomer, Adv. Mater. 27 (2015) 7740
7745. [61] S. Ji, J. Xia, H. Xu, Dynamic chemistry of selenium: Se
N and Se
Se dynamic covalent bonds in polymeric systems, ACS Macro Lett. 5 (2016) 78
82. [62] S.D. Bull, M.G. Davidson, J.M.H. van den Elsen, J.S. Fossey, A.T.A. Jenkins, Y.-B. Jiang, et al., Exploiting the reversible covalent bonding of boronic acids: recognition, sensing, and assembly, Acc. Chem. Res. 46 (2013) 312
326. [63] W.L.A. Brooks, B.S. Sumerlin, Synthesis and Applications of Boronic Acid-Containing Polymers: From Materials to Medicine, Chem. Rev. 116 (2016) 1375
1397. [64] Y. Chen, W. Qian, R. Chen, H. Zhang, X. Li, D. Shi, et al., One-pot preparation of autonomously self-healable elastomeric hydrogel from boric acid and random copolymer bearing hydroxyl groups, ACS Macro Lett. 6 (2017) 1129
1133. [65] O.R. Cromwell, J. Chung, Z. Guan, Malleable and self-healing covalent polymer networks through tunable dynamic boronic ester bonds, J. Am. Chem. Soc. 137 (2015) 6492
6495. [66] W.-P. Chen, D.-Z. Hao, W.-J. Hao, X.-L. Guo, L. Jiang, Hydrogel with ultrafast self-healing property both in air and underwater, ACS Appl. Mater. Interfaces 10 (2018) 1258
1265. [67] Y. Chen, D. Diaz-Dussan, D. Wu, W. Wang, Y.-Y. Peng, A.B. Asha, et al., Bioinspired self-healing hydrogel based on benzoxaborole-catechol dynamic covalent chemistry for 3D cell encapsulation, ACS Macro Lett. 7 (2018) 904
908. [68] H. Ying, Y. Zhang, J. Cheng, Dynamic urea bond for the design of reversible and self-healing polymers, Nat. Commun. 5 (2014) 3218. [69] Y. Zhang, H. Ying, K.R. Hart, Y. Wu, A.J. Hsu, A.M. Coppola, et al., Malleable and recyclable poly (urea-urethane) thermosets bearing hindered urea bonds, Adv. Mater. 28 (2016) 7646
7651. [70] H. Ying, J. Cheng, Hydrolyzable polyureas bearing hindered urea bonds, J. Am. Chem. Soc. 136 (2014) 16974
16977. [71] J.I. Park, A. Choe, M.P. Kim, H. Ko, T.H. Lee, S.M. Noh, et al., Water-adaptive and repeatable self-healing polymers bearing bulky urea bonds, Polym. Chem. 9 (2018) 11
19. [72] C. Yuan, M.Z. Rong, M.Q. Zhang, Z.P. Zhang, Y.C. Yuan, Self-healing of polymers via synchronous covalent bond fission/radical recombination, Chem. Mater. 23 (2011) 5076
5081. [73] H. Otsuka, K. Aotani, Y. Higaki, A. Takahara, Polymer scrambling: macromolecular radical crossover reaction between the main chains of alkoxyamine-based dynamic covalent polymers, J. Am. Chem. Soc. 125 (2003) 4064
4065. [74] P. Nkolo, G. Audran, R. Bikanga, P. Bremond, S.R.A. Marque, V. Roubaud, C-ON bond homolysis of alkoxyamines: when too high polarity is detrimental, Org. Biomol. Chem. 15 (2017) 6167
6176. [75] C. Yuan, M.Q. Zhang, M.Z. Rong, Application of alkoxyamine in self-healing of epoxy, J. Mater. Chem. A 2 (2014) 6558
6566. [76] T.C. Tseng, L. Tao, F.Y. Hsieh, Y. Wei, I.M. Chiu, S.H. Hsu, An injectable, self-healing hydrogel to repair the central nervous system, Adv. Mater. 27 (2015) 3518
3524. [77] B. Yang, Y. Zhang, X. Zhang, L. Tao, S. Li, Y. Wei, Facilely prepared inexpensive and biocompatible selfhealing hydrogel: a new injectable cell therapy carrier, Polym. Chem. 3 (2012) 3235
3238. [78] F. Ding, S. Wu, S. Wang, Y. Xiong, Y. Li, B. Li, et al., A dynamic and self-crosslinked polysaccharide hydrogel with autonomous self-healing ability, Soft Matter 11 (2015) 3971
3976. [79] W.G. Skene, J.-M.P. Lehn, Dynamers: polyacylhydrazone reversible covalent polymers, component exchange, and constitutional diversity, Proc. Natl Acad. Sci. U. S. A. 101 (2004) 8270
8275.

Self-Healing Polymer-Based Systems

References

69

[80] G. Deng, C. Tang, F. Li, H. Jiang, Y. Chen, Covalent cross-linked polymer gels with reversible sol 2 gel transition and self-healing properties, Macromolecules 43 (2010) 1191
1194. [81] L. Qiao, C. Liu, C. Liu, L. Yang, M. Zhang, W. Liu, et al., Self-healing alginate hydrogel based on dynamic acylhydrazone and multiple hydrogen bonds, J. Mater. Sci. 54 (2019) 8814
8828. [82] S. Debnath, R.R. Ujjwal, U. Ojha, Self-healable and recyclable dynamic covalent networks based on room temperature exchangeable hydrazide michael adduct linkages, Macromolecules 51 (2018) 9961
9973. [83] S.W. Kantor, W.T. Grubb, R.C. Osthoff, The mechanism of the acid- and base-catalyzed equilibration of siloxanes, J. Am. Chem. Soc. 76 (1954) 5190
5197. [84] P. Zheng, T.J. McCarthy, A surprise from 1954: siloxane equilibration is a simple, robust, and obvious polymer self-healing mechanism, J. Am. Chem. Soc. 134 (2012) 2024
2027. [85] B.D. Mather, K. Viswanathan, K.M. Miller, T.E. Long, Michael addition reactions in macromolecular design for emerging technologies, Prog. Polym. Sci. 31 (2006) 487
531. [86] N.E. Kamber, W. Jeong, R.M. Waymouth, R.C. Pratt, B.G.G. Lohmeijer, J.L. Hedrick, Organocatalytic ringopening polymerization, Chem. Rev. 107 (2007) 5813
5840. [87] J.L. Self, N.D. Dolinski, M.S. Zayas, J. Read de Alaniz, C.M. Bates, Brønsted-acid-catalyzed exchange in polyester dynamic covalent networks, ACS Macro Lett. 7 (2018) 817
821. [88] A. Vaish, N.V. Tsarevsky, Hypervalent iodine-based dynamic and self-healing network polymers, Polym. Chem. 10 (2019) 3943
3950. [89] D.L. Taylor, M. in het Panhuis, Self-healing hydrogels, Adv. Mater. 28 (2016) 9060
9093. [90] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, et al., Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding, Science 278 (1997) 1601
1604. [91] D. Zhu, Q. Ye, X. Lu, Q. Lu, Self-healing polymers with PEG oligomer side chains based on multiple H-bonding and adhesion properties, Polym. Chem. 6 (2015) 5086
5092. [92] J. Cui, A. Campo, Multivalent H-bonds for self-healing hydrogels, Chem. Commun. 48 (2012) 9302
9304. [93] G.B.W.L. Ligthart, H. Ohkawa, R.P. Sijbesma, E.W. Meijer, Complementary quadruple hydrogen bonding in supramolecular copolymers, J. Am. Chem. Soc. 127 (2005) 810
811. [94] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977
980. [95] S. Burattini, B.W. Greenland, D.H. Merino, W. Weng, J. Seppala, H.M. Colquhoun, et al., A healable supramolecular polymer blend based on aromatic π 2 π stacking and hydrogen-bonding interactions, J. Am. Chem. Soc. 132 (2010) 12051
12058. [96] R. Vaiyapuri, B.W. Greenland, H.M. Colquhoun, J.M. Elliott, W. Hayes, Molecular recognition between functionalized gold nanoparticles and healable, supramolecular polymer blends
a route to property enhancement, Polym. Chem. 4 (2013) 4902
4909. [97] Q. Wu, J. Wei, B. Xu, X. Liu, H. Wang, W. Wang, et al., A robust, highly stretchable supramolecular polymer conductive hydrogel with self-healability and thermo-processability, Sci. Rep. 7 (2017) 41566. [98] A. Faghihnejad, K.E. Feldman, J. Yu, M.V. Tirrell, J.N. Israelachvili, C.J. Hawker, et al., Adhesion and surface interactions of a self-healing polymer with multiple hydrogen-bonding groups, Adv. Funct. Mater. 24 (2014) 2322
2333. [99] J. Hentschel, A.M. Kushner, J. Ziller, Z. Guan, Self-healing supramolecular block copolymers, Angew. Chem. Int. Ed. 51 (2012) 10561
10565. [100] Y. Song, Y. Liu, T. Qi, G.L. Li, Towards dynamic but supertough healable polymers through biomimetic hierarchical hydrogen-bonding interactions, Angew. Chem. Int. Ed. 57 (2018) 13838
13842. [101] Y. Yanagisawa, Y. Nan, K. Okuro, T. Aida, Mechanically robust, readily repairable polymers via tailored noncovalent cross-linking, Science 359 (2018) 72
76. [102] A. Phadke, C. Zhang, B. Arman, C.-C. Hsu, R.A. Mashelkar, A.K. Lele, et al., Rapid self-healing hydrogels, Proc. Natl Acad. Sci. 109 (2012) 4383
4388. [103] C. Hilger, R. Stadler, L. Liane, Multiphase thermoplastic elastomers by combination of covalent and association chain structures: 2. Small-strain dynamic mechanical properties, Polymer 31 (1990) 818
823. [104] F. Herbst, K. Schro¨ter, I. Gunkel, S. Gro¨ger, T. Thurn-Albrecht, J. Balbach, et al., Aggregation and chain dynamics in supramolecular polymers by dynamic rheology: cluster formation and self-aggregation, Macromolecules 43 (2010) 10006
10016.

Self-Healing Polymer-Based Systems

70

2. Types of chemistries involved in self-healing polymeric systems

[105] M. Weck, Side-chain functionalized supramolecular polymers, Polym. Int. 56 (2007) 453
460 [106] A. Eisenberg, M. Rinaudo, Polyelectrolytes and ionomers, Polym. Bull. 24 (1990) 671. [107] S.J. Kalista, T.C. Ward, Z. Oyetunji, Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture, Mech. Adv. Mater. Struct. 14 (2007) 391
397. [108] S.J. Kalista Jr., T.C. Ward, Thermal characteristics of the self-healing response in poly(ethylene-comethacrylic acid) copolymers, J. R. Soci. Interface 4 (2007) 405
411. [109] S.K. Ghosh, Self-Healing Materials: Fundamentals, Design Strategies, and Applications, Wiley Online Library, 2009. [110] A.K. Padhan, D. Mandal, Rapid self-healing and superior toughness in ionically crosslinked polymer hydrogels. Unpublished, 2019. [111] E.V. Skorb, D.V. Andreeva, Layer-by-layer approaches for formation of smart self-healing materials, Polym. Chem. 4 (2013) 4834
4845. [112] X. He, C. Zhang, M. Wang, Y. Zhang, L. Liu, W. Yang, An electrically and mechanically autonomic selfhealing hybrid hydrogel with tough and thermoplastic properties, ACS Appl. Mater. Interfaces 9 (2017) 11134
11143. [113] H. Wei, S. Du, Y. Liu, H. Zhao, C. Chen, Z. Li, et al., Tunable, luminescent, and self-healing hybrid hydrogels of polyoxometalates and triblock copolymers based on electrostatic assembly, Chem. Commun. 50 (2014) 1447
1450. [114] A. Das, A. Sallat, F. Bo¨hme, M. Suckow, D. Basu, S. Wießner, et al., Ionic modification turns commercial rubber into a self-healing material, ACS Appl. Mater. Interfaces 7 (2015) 20623
20630. [115] M. Suckow, A. Mordvinkin, M. Roy, N.K. Singha, G. Heinrich, B. Voit, et al., Tuning the properties and self-healing behavior of ionically modified poly(isobutylene-co-isoprene) rubber, Macromolecules 51 (2018) 468
479. [116] C. Xu, L. Cao, B. Lin, X. Liang, Y. Chen, Design of self-healing supramolecular rubbers by introducing ionic cross-links into natural rubber via a controlled vulcanization, ACS Appl. Mater. Interfaces 8 (2016) 17728
17737. [117] J.Y. Sun, X. Zhao, W.R. Illeperuma, O. Chaudhuri, K.H. Oh, D.J. Mooney, et al., Highly stretchable and tough hydrogels, Nature 489 (2012) 133
136. [118] J. Kang, D. Son, G.-J.N. Wang, Y. Liu, J. Lopez, Y. Kim, et al., Tough and water-insensitive self-healing elastomer for robust electronic skin, Adv. Mater. 30 (2018) 1706846. [119] T.L. Sun, T. Kurokawa, S. Kuroda, A.B. Ihsan, T. Akasaki, K. Sato, et al., Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity, Nat. Mater. 12 (2013) 932. [120] S.-M. Kim, H. Jeon, S.-H. Shin, S.-A. Park, J. Jegal, S.Y. Hwang, et al., Superior toughness and fast selfhealing at room temperature engineered by transparent elastomers, Adv. Mater. 30 (2018) 1705145. [121] Y. Huang, P.G. Lawrence, Y. Lapitsky, Self-assembly of stiff, adhesive and self-healing gels from common polyelectrolytes, Langmuir 30 (2014) 7771
7777. [122] A. Reisch, E. Roger, T. Phoeung, C. Antheaume, C. Orthlieb, F. Boulmedais, et al., On the benefits of rubbing salt in the cut: self-healing of saloplastic PAA/PAH compact polyelectrolyte complexes, Adv. Mater. 26 (2014) 2547
2551. [123] D.Y. Zhang, A.J. Turberfield, B. Yurke, E. Winfree, Engineering entropy-driven reactions and networks catalyzed by DNA, Science 318 (2007) 1121
1125. [124] T.J. Cuthbert, J.J. Jadischke, J.R. de Bruyn, P.J. Ragogna, E.R. Gillies, Self-healing polyphosphonium ionic networks, Macromolecules 50 (2017) 5253
5260. [125] H. Zhang, C. Wang, G. Zhu, N.S. Zacharia, Self-healing of bulk polyelectrolyte complex material as a function of pH and salt, ACS Appl. Mater. Interfaces 8 (2016) 26258
26265. [126] L. Shi, Y. Han, J. Hilborn, D. Ossipov, “Smart” drug loaded nanoparticle delivery from a self-healing hydrogel enabled by dynamic magnesium
biopolymer chemistry, Chem. Commun. 52 (2016) 11151
11154. [127] P.M. Lopez-Perez, R.M.P. da Silva, I. Strehin, P.H.J. Kouwer, S.C.G. Leeuwenburgh, P.B. Messersmith, Selfhealing hydrogels formed by complexation between calcium ions and bisphosphonate-functionalized starshaped polymers, Macromolecules 50 (2017) 8698
8706. [128] M. Wang, F. Nudelman, R.R. Matthes, M.P. Shaver, Frustrated Lewis pair polymers as responsive selfhealing gels, J. Am. Chem. Soc. 139 (2017) 14232
14236.

Self-Healing Polymer-Based Systems

References

71

[129] M.W. Urban, D. Davydovich, Y. Yang, T. Demir, Y. Zhang, L. Casabianca, Key-and-lock commodity selfhealing copolymers, Science 362 (2018) 220
225. [130] F. Herbst, D. Do¨hler, P. Michael, W.H. Binder, Self-healing polymers via supramolecular forces, Macromol. Rapid Commun. 34 (2013) 203
220. [131] M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, An overview of N-heterocyclic carbenes, Nature 510 (2014) 485. [132] C.-H. Li, C. Wang, C. Keplinger, J.-L. Zuo, L. Jin, Y. Sun, et al., A highly stretchable autonomous selfhealing elastomer, Nat. Chem. 8 (2016) 618. [133] D. Mozhdehi, S. Ayala, O.R. Cromwell, Z. Guan, Self-healing multiphase polymers via dynamic metal
ligand interactions, J. Am. Chem. Soc. 136 (2014) 16128
16131. [134] Z. Wang, M.W. Urban, Facile UV-healable polyethylenimine
copper (C2H5N
Cu) supramolecular polymer networks, Polym. Chem. 4 (2013) 4897
4901. [135] K.A. Williams, A.J. Boydston, C.W. Bielawski, Towards electrically conductive, self-healing materials, J. R. Soc. Interface 4 (2007) 359
362. [136] G. Hong, H. Zhang, Y. Lin, Y. Chen, Y. Xu, W. Weng, et al., Mechanoresponsive healable metallosupramolecular polymers, Macromolecules 46 (2013) 8649
8656. [137] S. Bode, M. Enke, R.K. Bose, F.H. Schacher, S.J. Garcia, S. van der Zwaag, et al., Correlation between scratch healing and rheological behavior for terpyridine complex based metallopolymers, J. Mater. Chem. A 3 (2015) 22145
22153. [138] M. Burnworth, L. Tang, J.R. Kumpfer, A.J. Duncan, F.L. Beyer, G.L. Fiore, et al., Optically healable supramolecular polymers, Nature 472 (2011) 334. [139] S. Bode, L. Zedler, F.H. Schacher, B. Dietzek, M. Schmitt, J. Popp, et al., Self-healing polymer coatings based on crosslinked metallosupramolecular copolymers, Adv. Mater. 25 (2013) 1634
1638. [140] Z. Wang, C. Xie, C. Yu, G. Fei, Z. Wang, H. Xia, A facile strategy for self-healing polyurethanes containing multiple metal-ligand bonds, Macromol. Rapid Commun. 39 (2018) e1700678. [141] N. Holten-Andersen, M.J. Harrington, H. Birkedal, B.P. Lee, P.B. Messersmith, K.Y. Lee, et al., pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli, Proc. Natl. Acad. Sci. U. S. A. 108 (2011) 2651
2655. [142] Y.-L. Rao, A. Chortos, R. Pfattner, F. Lissel, Y.-C. Chiu, V. Feig, et al., Stretchable self-healing polymeric dielectrics cross-linked through metal
ligand coordination, J. Am. Chem. Soc. 138 (2016) 6020
6027. [143] H. Liang, Z. Zhang, Q. Yuan, J. Liu, Self-healing metal-coordinated hydrogels using nucleotide ligands, Chem. Commun. 51 (2015) 15196
15199. [144] S. Saha, J. Bachl, T. Kundu, D. Dı´az Dı´az, R. Banerjee, Amino acid-based multiresponsive low-molecular weight metallohydrogels with load-bearing and rapid self-healing abilities, Chem. Commun. 50 (2014) 3004
3006. [145] T. Feldner, M. Ha¨ring, S. Saha, J. Esquena, R. Banerjee, D.D. Dı´az, Supramolecular metallogel that imparts self-healing properties to other gel networks, Chem. Mater. 28 (2016) 3210
3217. [146] Y. Hui, Z.-B. Wen, F. Pilate, H. Xie, C.-J. Fan, L. Du, et al., A facile strategy to fabricate highly-stretchable self-healing poly(vinyl alcohol) hybrid hydrogels based on metal
ligand interactions and hydrogen bonding, Polym. Chem. 7 (2016) 7269
7277. [147] X. Yang, H. Yu, L. Wang, R. Tong, M. Akram, Y. Chen, et al., Self-healing polymer materials constructed by macrocycle-based host
guest interactions, Soft Matter 11 (2015) 1242
1252. [148] T. Kakuta, Y. Takashima, M. Nakahata, M. Otsubo, H. Yamaguchi, A. Harada, Preorganized hydrogel: selfhealing properties of supramolecular hydrogels formed by polymerization of host
guest-monomers that contain cyclodextrins and hydrophobic guest groups, Adv. Mater. 25 (2013) 2849
2853. [149] G. Izzet, M. Me´nand, B. Matt, S. Renaudineau, L.-M. Chamoreau, M. Sollogoub, et al., Cyclodextrin-induced auto-healing of hybrid polyoxometalates, Angew. Chem. Int. Ed. 51 (2012) 487
490. [150] K. Miyamae, M. Nakahata, Y. Takashima, A. Harada, Self-healing, expansion
contraction, and shapememory properties of a preorganized supramolecular hydrogel through host
guest interactions, Angew. Chem. Int. Ed. 54 (2015) 8984
8987. [151] L. Peng, H. Zhang, A. Feng, M. Huo, Z. Wang, J. Hu, et al., Electrochemical redox responsive supramolecular self-healing hydrogels based on host
guest interaction, Polym. Chem. 6 (2015) 3652
3659.

Self-Healing Polymer-Based Systems

72

2. Types of chemistries involved in self-healing polymeric systems

[152] C. Yu, C.-F. Wang, S. Chen, Robust self-healing host
guest gels from magnetocaloric radical polymerization, Adv. Funct. Mater. 24 (2014) 1235
1242. [153] A.B. Ihsan, T.L. Sun, T. Kurokawa, S.N. Karobi, T. Nakajima, T. Nonoyama, et al., Self-healing behaviors of tough polyampholyte hydrogels, Macromolecules 49 (2016) 4245
4252. [154] X. Ma, Y. Zhao, Biomedical applications of supramolecular systems based on host
guest interactions, Chem. Rev. 115 (2014) 7794
7839. [155] M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, et al., Self-healing supramolecular gels formed by crown ether based host
guest interactions, Angew. Chemie 124 (2012) 7117
7121. [156] X. Yan, D. Xu, J. Chen, M. Zhang, B. Hu, Y. Yu, et al., A self-healing supramolecular polymer gel with stimuli-responsiveness constructed by crown ether based molecular recognition, Polym. Chem. 4 (2013) 3312
3322. [157] E.A. Appel, F. Biedermann, U. Rauwald, S.T. Jones, J.M. Zayed, O.A. Scherman, Supramolecular crosslinked networks via host 2 guest complexation with cucurbit[8]uril, J. Am. Chem. Soc. 132 (2010) 14251
14260. [158] S. Burattini, H.M. Colquhoun, B.W. Greenland, W. Hayes, A novel self-healing supramolecular polymer system, Faraday Discuss. 143 (2009) 251
264. [159] S. Burattini, B.W. Greenland, W. Hayes, M.E. Mackay, S.J. Rowan, H.M. Colquhoun, A supramolecular polymer based on tweezer-type π 2 π stacking interactions: molecular design for healability and enhanced toughness, Chem. Mater. 23 (2011) 6
8. [160] L.R. Hart, J.H. Hunter, N.A. Nguyen, J.L. Harries, B.W. Greenland, M.E. Mackay, et al., Multivalency in healable supramolecular polymers: the effect of supramolecular cross-link density on the mechanical properties and healing of non-covalent polymer networks, Polym. Chem. 5 (2014) 3680
3688. [161] J. Fox, J.J. Wie, B.W. Greenland, S. Burattini, W. Hayes, H.M. Colquhoun, et al., High-strength, healable, supramolecular polymer nanocomposites, J. Am. Chem. Soc. 134 (2012) 5362
5368. [162] Z. Xu, J. Peng, N. Yan, H. Yu, S. Zhang, K. Liu, et al., Simple design but marvelous performances: molecular gels of superior strength and self-healing properties, Soft Matter 9 (2013) 1091
1099. [163] J.-F. Mei, X.-Y. Jia, J.-C. Lai, Y. Sun, C.-H. Li, J.-H. Wu, et al., A Highly stretchable and autonomous selfhealing polymer based on combination of Pt Pt and π
π interactions, Macromol. Rapid Commun. 37 (2016) 1667
1675. [164] G. Jiang, C. Liu, X. Liu, G. Zhang, M. Yang, Q. Chen, et al., Self-healing mechanism and mechanical behavior of hydrophobic association hydrogels with high mechanical strength, J. Macromol. Sci. A 47 (2010) 335
342. [165] D.C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Tough and self-healing hydrogels formed via hydrophobic interactions, Macromolecules 44 (2011) 4997
5005. [166] N.N. Xia, X.M. Xiong, M.Z. Rong, M.Q. Zhang, F. Kong, Self-healing of polymer in acidic water toward strength restoration through the synergistic effect of hydrophilic and hydrophobic interactions, ACS Appl. Mater. Interfaces 9 (2017) 37300
37309. [167] U. Manna, D. M. Lynn, Restoration of Superhydrophobicity in Crushed Polymer Films by Treatment with Water: Self-Healing and Recovery of Damaged Topographic Features Aided by an Unlikely Source, Adv. Mater. 25 (2013) 5104
5108. https://doi.org/10.1002/adma.201302217. [168] S. Scha¨fer, G. Kickelbick, Double reversible networks: improvement of self-healing in hybrid materials via combination of Diels
Alder cross-linking and hydrogen bonds, Macromolecules 51 (2018) 6099
6110. [169] S.L. Banerjee, K. Bhattacharya, S. Samanta, N.K. Singha, Self-healable antifouling zwitterionic hydrogel based on synergistic phototriggered dynamic disulfide metathesis reaction and ionic interaction, ACS Appl. Mater. Interfaces 10 (2018) 27391
27406. [170] Q. Chen, L. Zhu, C. Zhao, Q. Wang, J. Zheng, A robust, one-pot synthesis of highly mechanical and recoverable double network hydrogels using thermoreversible sol-gel polysaccharide, Adv. Mater. 25 (2013) 4171
4176. [171] H.J. Zhang, T.L. Sun, A.K. Zhang, Y. Ikura, T. Nakajima, T. Nonoyama, et al., Tough physical doublenetwork hydrogels based on amphiphilic triblock copolymers, Adv. Mater. 28 (2016) 4884
4890. [172] G. Li, H. Zhang, D. Fortin, H. Xia, Y. Zhao, Poly(vinyl alcohol)
poly(ethylene glycol) double-network hydrogel: a general approach to shape memory and self-healing functionalities, Langmuir 31 (2015) 11709
11716.



Self-Healing Polymer-Based Systems

References

73

[173] M.S. Sanford, J.A. Love, R.H. Grubbs, Mechanism and activity of ruthenium olefin metathesis catalysts, J. Am. Chem. Soc. 123 (2001) 6543
6554. [174] J.D. Rule, E.N. Brown, N.R. Sottos, S.R. White, J.S. Moore, Wax-protected catalyst microspheres for efficient self-healing materials, Adv. Mater. 17 (2005) 205
208. [175] D. Do¨hler, P. Michael, W.H. Binder, CuAAC-based click chemistry in self-healing polymers, Acc. Chem. Res. 50 (2017) 2610
2620. [176] Y. Shi, X. Cao, H. Gao, The use of azide
alkyne click chemistry in recent syntheses and applications of polytriazole-based nanostructured polymers, Nanoscale 8 (2016) 4864
4881. [177] M. Gragert, M. Schunack, W. H. Binder, Azide/Alkyne-“Click”-Reactions of Encapsulated Reagents: Toward Self-Healing Materials, Macromol. Rapid Commun. 32 (2011) 419
425. [178] M. Schunack, M. Gragert, D. Do¨hler, P. Michael, W. H. Binder, Low-Temperature Cu(I)-Catalyzed “Click” Reactions for Self-Healing Polymers, Macromol. Chem. Phys. 213 (2012) 205
214. [179] Q. Wei, J. Wang, X. Shen, X.A. Zhang, J.Z. Sun, A. Qin, et al., Self-healing hyperbranched poly(aroyltriazole)s, Sci. Rep. 3 (2013) 1093. [180] H.P. Wang, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, Self-healing of thermoplastics via living polymerization, Macromolecules 43 (2010) 595
598. [181] L. Yao, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, Self-healing linear polymers based on RAFT polymerization, Polymer 52 (2011) 3137
3145. [182] S.H. Cho, S.R. White, P.V. Braun, Room-temperature polydimethylsiloxane-based self-healing polymers, Chem. Mater. 24 (2012) 4209
4214. [183] G. Huyang, A.E. Debertin, J. Sun, Design and development of self-healing dental composites, Mater. Des. 94 (2016) 295
302. [184] S. Meure, D.Y. Wu, S. Furman, Polyethylene-co-methacrylic acid healing agents for mendable epoxy resins, Acta Mater. 57 (2009) 4312
4320. [185] R.S. Trask, I.P. Bond, Bioinspired engineering study of plantae vascules for self-healing composite structures, J. R. Soc. Interface 7 (2010) 921
931. [186] B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, Self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (2010) 179
211. [187] D. Therriault, S.R. White, J.A. Lewis, Chaotic mixing in three-dimensional microvascular networks fabricated by direct-write assembly, Nat. Mater. 2 (2003) 265
271. [188] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (2007) 581
585.

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C H A P T E R

3 Self-healing polymers: from general basics to mechanistic aspects Martin D. Hager1,2 and Stefan Zechel1,2 1

Laboratory for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Jena, Germany 2Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Jena, Germany

3.1 Introduction One aspect of synthetic polymeric materials (also known as plastics in everyday life) is that these materials are not made for eternity. Sooner or later, the materials will fail due to damage—for instance, plastics get brittle and crack under stress. As another example, it should be mentioned that coatings (based on polymers) are scratched during usage (e.g., a car driving too close to the hedge). Consequently damage will shorten the life time/use time of the materials and the objects made of these materials [1]. In safety-relevant applications this fact is circumvented by the regular exchange of the corresponding components independently of their current state to prevent a failure of critical components. Furthermore the classical engineering approach is devoted to the “damage prevention” [2]. The stronger the material in the beginning, the longer the life time/use time of the material. Damage should be prevented by “better” and “stronger” materials [1]. However, also the strongest polymer will reach its limit within a finite time, resulting inevitably in the failure of this material. In contrast, self-healing polymers are designed according to the principle of “damage management,” that is, the material can deal with the occurring damage [2]. These polymers feature the ability to restore their original functionality (particularly mechanical properties) after a damage event. Considering man-made materials, this ability is rather unusual and appears partly as science fiction. However, if we think about nature, self-healing/regeneration is quite familiar, even obvious for us. Bones are not the strongest “construction material” compared to synthetic materials; however, a broken leg can heal (within some weeks) in contrast to a cracked steel beam [3]. Cuts in the finger are

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00003-9

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sealed immediately, and the skin underneath is healing afterwards [4]. Besides the macroscopic damages, healing also occurs on the molecular level in our body—namely, the healing of DNA secures the stored genetic information [5]. One last example to be mentioned is the fascinating animal called axolotl. This amphibian can regenerate even whole limbs, which were detached from the body [6]. Not surprisingly nature serves as a role model for the design of self-healing polymers [7]. Concepts successfully implemented in living organisms can be (partially) transferred into synthetic materials if the intrinsic properties of the materials are considered [8]. Some of the design concepts discussed below appear familiar considering natural examples (microvascular networks and bleeding). However, there is one serious difference: healing in nature is almost exclusively found in “living material”/tissue [7]. There are only limited examples for the healing of non-living biologic materials—for instance mussel byssus threads [9,10].

3.2 General mechanism of self-healing polymers Before a general mechanism of self-healing polymers can be considered, one should have a closer look to the preceding step, that is, the damage event leads to the damage of the material (equivalent to the partial loss of the material’s properties). The polymeric material can be damaged in many different ways, leading to a large variety of different damage scenarios, for example, scratches, cracks, and abrasion (see Fig. 3.1). The specific nature of the damage will also have implications in the following healing step. Depending on the nature of the polymer, the stress strain behavior can be different, which will influence the development of the damage [11]. For instance thermosets (i.e., often highly crosslinked polymers) are brittle and feature excellent mechanical properties such as high E-moduli (reaching the GPa region). If a small crack occurs, this damage will lead upon increasing stress to the failure of the materials, which is on the molecular level synonymous to the breaking of covalent bonds within the material (see Fig. 3.2). Thermoplastic polymers can also feature this behavior, in particular, if these materials feature a high glass transition temperature (Tg) or if the material is exposed to low temperatures (compared to the Tg-value). At higher temperatures, these materials will show a transition to viscoelastic behavior. The ductile deformation of the material is not always leading immediately to a cleavage of covalent bonds. Preceding to this process, the FIGURE 3.1 General aspects for self-healing level.

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polymers—macroscopic

3.2 General mechanism of self-healing polymers

77

FIGURE 3.2 General aspects for self-healing polymers—molecular level.

morphology of semicrystalline polymers is changed after the amorphous regions have been stretched. A breaking of the large crystalline domains occurs, which is leading to further deformation of the polymer. This behavior is the typical deformation of a plastic polymer under stress [12,13]. The deformation at low strains is still reversible. If the strain is going beyond the yield point, plastic deformation (i.e., nonreversible deformation) occurs [14]. This behavior has also implications for the self-healing process. If the deformation is still (partially) reversible, a potential driving force for the closure of the scratch/crack is available (comparable to a cocked spring). In contrast, purely plastic deformation leads to a lack of such a driving force within the material. The reflow/transport of the material has to be triggered and promoted, respectively, by other factors—for instance, by pressing of the cut surfaces together. The third class, elastomers, shows in principle comparable behavior. In particular, these materials often show a more pronounced viscoelastic behavior, leading to more plastic deformation. On the other hand, these materials are softer providing an increased mobility. The nature of the damage is also important considering a potential healing mechanism. Typically cracks within the polymeric materials or cut specimen are tested [15]. Furthermore glassy polymers often feature crazing under stress, that is, small microdamages occur, and bridges of the materials still remain [16]. Although the above-described mechanical properties can be observed during classical stress strain tests, similar effects can be observed in scratch tests. The scratch-induced damage can lead to different modes of failures (see Fig. 3.3 for a scratch map summarizing the different damages modes) [18]. Depending on the material and the temperature, different damage patterns (e.g., fish-scale damage) can occur. Moreover the material can also be removed completely by scratching, in particular if high loads are applied. Consequently this material cannot contribute to the healing process and is not available to close this scratch; consequently material is missing. Polymer composites, for example, fiber-reinforced polymers, are important construction materials—for instance, for aerospace applications. These materials can/will also be damaged during application. Although the reinforcement often does not fail, the matrix material, the polymer, is damaged [19,20]. Depending on the kind of damage, for example, delamination between the layers, different healing strategies have to be applied to restore the original properties.

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Weak ductile Weak brittle Strong brittle

Substrate material type

Strong ductile

(A)

Parabolic crack or fish scale

Mar

Mar

Mar

Fish scale

Material removal

Material removal

Fish scale mixed with crack/void/craze

Material removal

Material removal

Parabolic crack

Mar

Scratch normal load (B)

Mar or ploughing

Whitening

First critical point

Second critical point

2N

Mar or ploughing

Cutting 50N

First critical point

Whitening

To whitening from mar

Second critical point To cutting from whitening

FIGURE 3.3 (A) Scratch map of polymer: the kind of the damage depends on the applied load and the material type. (B) Depiction of the different damage modes for PMMA [17].

Considering the damage, one important aspect is the extent of the damage. The complete collapse/catastrophic failure of the material is the most impressive case; however, within the context, self-healing polymers are out of reach. Typically microdamage (damage in the order of μm) can be healed. The prevention of the accumulation of small damages resulting in larger cracks and, finally, in the breaking of the material, will lead to longer lifetimes. Damages in the order of “cm” will be most challenging for implementing a self-healing strategy, in particular if the material is removed. Notably considering cracks/scratches, the most important aspect, which is limiting the self-healing, is the crack width (as well as depth) and not the length of the damage. Summarizing the overall influence of the damage on the self-healing mechanism and the healing process, there are several major aspects: the type of the damage (e.g., cut, scratch,

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and crack), dimensions of the damage (i.e., volume, crack length, and width), the nature of the material (e.g., crosslinked polymer network, thermoplast, and elastomer), generation of new “bonds” to regain the mechanical properties and the (re)flow of material. Based on these findings, there are some considerations for the self-healing mechanism. Generally mobility is required for a successful healing [21]. This concept was formulated as a general basic principle for all self-healing materials, that is, valid for all material classes. First, a mobile phase must be generated, which can close the damage. Subsequently the material has to be immobilized again, and, consequently, the material’s properties are restored. Beyond this general principle, there are additional considerations for polymers. Starting on the molecular level, reversibility is also one major aspect for self-healing polymers. This property is the most important aspect for intrinsic self-healing polymers (see Fig. 3.2). The reversibility can be utilized to regain the destroyed covalent bonds and to promote the required mobility. Moreover, the reversible activation of covalent bonds plays also an important role in the field of mechanochemistry [22]. On the macroscopic level, the above mentioned mobility is an important parameter, providing a reflow of the material. Extrinsic self-healing materials provide this mobility by liquid healing agents, which are encapsulated in the polymer matrix and are released upon damage.

3.3 Concepts for the design of self-healing polymers The above-described basic principles for the self-healing mechanism have been utilized for the design of different self-healing polymers. Generally two different strategies can be distinguished. Extrinsic self-healing polymers are based on encapsulated healing agents (nano and microcaspsules) within a polymer matrix. This matrix alone does not feature the ability for self-healing. Only the addition of the healing agent provides the healing ability (see Section 3.4)—this concept is mainly manifested on the macroscopic level. In contrast, intrinsic self-healing polymers feature the ability for self-healing without the requirement of additional healing agents (see Section 3.5). This ability is mainly based on the molecular reversibility of selected (reversible) bonds within material.

3.4 Extrinsic self-healing polymers In 2001 the groups of White, Sottos, and Moore described the prime example of a selfhealing material [23]. Microcapsules filled with dicylopentadiene were dispersed within an epoxy resin. Upon damage the liquid monomer is released and polymerized subsequently with the help of a catalyst (Grubb’s catalyst), which is also dispersed in the polymer matrix. Consequently the damage itself triggers the healing process. The solid poly(dicyclopentadiene) closes the crack, and the mechanical properties are restored. Considering the basic principles of the mechanism, the crack is counteracted directly by the release of the liquid healing agent. These processes occur on the macroscopic level. The molecular structure can nearly be neglected within certain assumptions (e.g., compatibility of the healing agent and

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the polymer matrix). Notably the healed polymer does not necessarily feature the original chemical composition [e.g., epoxy resin is healed by the poly(dicyclopentadiene)], which can also result in slightly different properties (e.g., improved mechanical performance). Starting from encapsulated dicyclopentadiene as the first example, a manifold of different chemistries has been developed. These chemistries can be divided into the main classes: (1) encapsulated monomer plus catalyst; (2) encapsulated monomer A and encapsulated monomer B; and (3) liquid oligomers. Furthermore, also pure solvents have been utilized, which enable the formation of new entanglements (see also intrinsic polymers) [24]. The fast formation of the new polymer is very important to achieve a fast healing. Therefore the useful chemistries are limited (for a review summarizing the best 15 chemistries see Ref. 25) to highly reactive ones [25]. Besides the capsule content, the shell of the capsules is also important. The capsule has to be labile enough that the damage within the material leads to a rupture of the shell [26]. However, if the shell is too labile, processing without destroying the capsules prematurely will be very difficult. Exemplarily microcapsules are currently not really suitable for rubbers because the capsules will not survive the typical processing steps. Therefore the capsule material was mostly based on polyurethanes and/or polyureas or poly(urea-formaldehyde) since this class of material enables a sufficient stability during preparation, and a damage-induced breaking is obtained[27,28]. Furthermore phenol formaldehyde (PF) resin is a novel option as a capsule material enabling a higher stability, in particular considering elevated temperature [29]. The size of the capsules determines, on the one hand, their potential applications. For instance in coatings, the capsules should be smaller than the thickness of the coating. On the other hand, the size determines the volume of the healing agent. Generally if the loading of the capsules and their size is too small, the size of the healable area is limited due to the limited amount of healing agent [30]. Furthermore the healing efficiency will be reduced with smaller capsules [31]. It is important to note that microcapsules are only capable for a one-time healing. If all healing agent is released, a second crack at the previously damage area cannot be healed due to the missing healing agent, which was released during the previous healing step. Consequently the microcapsule concept was developed further. Microvascular networks have been utilized, which contain the healing agent(s) [32]. These networks are capable to deliver the required healing agent. After a repeated damage, the microvascular network can still deliver the required healing agent, allowing multiple damage-healing-cycles. There are different possibilities to fabricate these microvascular networks; for instance, hollow glass tubes can be applied or sacrificial fibers are embedded within the matrix [33,34]. These fibers are removed subsequently, leaving behind the hollow channels for the healing agent(s). The orientation of the vascular networks is important to provide the required healing agent (also depending on the damage mode) [33,35]. 3D printed channels within a polymer matrix revealed a strong dependency of the achievable healing efficiency depending on the orientation of the channels and the channel volume [36]. Interestingly the encapsulation approaches offer additional opportunities. For instance microcapsules for self-healing coatings were also utilized to encapsulate additional corrosion inhibitors, which are released upon damage and prevent the corrosion of the underlying metal substrate [37,38].

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Fig. 3.4 summarizes the advantages and disadvantages of both the microcapsules and the microvascular networks [38]. The main differences are the speed of the healing process and the differences in the fabrication of both materials. An interesting approach is a hybrid system of both, which combines the advantages of both strategies. Table 3.1 summarizes the important mechanistic aspects of the extrinsic self-healing materials and their benefits, as well as the limitations of these strategies. The main mechanistic aspects are related to the macroscopic level. The embedded healing agents, which can be utilized for a large variety of polymer matrixes, are released upon damage and close the crack or restore the mechanical properties. Due to the encapsulation of the healing agents, the damage itself will trigger the healing process. Mostly a relatively fast and efficient healing can be achieved. The mechanical properties can be fully restored. Notably also healing efficiencies above 100% have been achieved if the released healing agents form stronger polymers compared to the matrix polymer. The utilization of capsules limits the number of healing cycles; however, microvascular networks allow a multiple healing. The latter approach is the only approach for the healing of very large damages because a sufficient amount of healing agent can be delivered to the network [39]. With the help of a two-stage process (i.e., first gelation then polymerization), the large damage zone can be filled without dripping of the healing agent, and premature polymerization blocks the total recovery of the damage. Thus healing efficiencies up to 100% can be obtained by this design principle [40]. Although many matrixes can be utilized, the stability of the capsules also limits their applicability. For instance most of the elastomer processing techniques will lead to the rupture of the capsules during processing. In view of the required high reactive compounds/monomers within the capsules, the long-term stability of the encapsulation is also questionable.

FIGURE 3.4 Comparison of the microcapsule approach and the microvascular network approach and the hybrid system [38].

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TABLE 3.1 Overview over the mechanistic aspects of extrinsic self-healing materials and their benefits and limitations. Mechanistic aspects

Benefits

Limitations

• Macroscopic level: release of healing agent • Scratch/crack filling • Polymerization of liquid healing agents • Restoration of mechanical properties not of the original material

• Autonomic and fast self-healing • Can be utilized for nearly every polymer matrix • Multiple healing possible (microvascular networks) • Full recovery of mechanical properties possible • Large-scale damage can be healed

• One-time healing (microcapsules) • Capsules have to survive processing • Microvascular network has to be integrated • Long-term stability of encapsulation

3.5 Intrinsic self-healing polymers Intrinsic self-healing polymers feature the ability for healing without the need for additional healing agents [1]. These materials are mainly based on reversible interactions. Three main systems will be considered in the following in view of the general mechanism: self-healing polyurethane coatings and crack healing by physical interactions and reversible polymers/polymer networks. The self-healing polyurethane coatings are one of the few examples of commercially available self-healing materials [41]. In contrast to classical PU clear coats, these coatings feature a different structure [42]. The network has a lower Tg, which enables a reflow of the material, for example, induced by standing of the car in the sunlight. By this process, small scratches can be removed completely [43]. However, the healing ability lasts around 3 years. After that time, the material is not able to heal the scratches anymore. Different polymeric materials can also show crack healing enabled by purely physical interactions. Considering a cracked polymer, Wool and Connor defined five stages of the healing process (see Fig. 3.5), which occur if the damaged surfaces are brought in contact to: 1. 2. 3. 4. 5.

surface rearrangement, surface approach, wetting, diffusion, and randomization [44,45].

It is noteworthy that these stages have been defined for physical healing. However, these stages will also be important for reversible polymer networks. The damage is leading to fresh surfaces, in which further processes (depending on time, temperature, and environment) can occur. As depicted in Fig. 3.1, the morphology of the material might be changed. Furthermore the material is brought out of equilibrium leading to diffusion processes. For instance polymer chains can diffuse back into the bulk, leading to a changed composition of the surface (and potentially availability of chain ends and—if present—functional groups) [45,46]. Due to different structures of polymeric backbones and the substituents, there might be surface segregation [47]. For instance

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FIGURE 3.5 The five stages of crack healing represented by two polymer coils: (A) rearrangement, (B) surface approach, (C) wetting, (D) diffusion to a distance X, and (E) diffusion to an equilibrium distance X B, and randomization [44].

self-replenishing surfaces utilize this concept on purpose by the introduction of fluorine containing substituents, which will orientate toward the surface [48]. Additionally, low molar mass polymer chains can diffuse preferentially to the surface—as it was shown for silicones [49]. On the one hand, very mobile healing agents are present; on the other hand, the physical healing depends on the formation of new entanglements. These only form for polymers above the entanglement length. The occurring processes of the surface rearrangement often influence the healing efficiency over time. Often a reorganization of the surface (toward the equilibrium state) will lower the achievable healing efficiency. The second stage is the surface approach, mostly achieved by bringing the damaged surfaces together (e.g., by pressing). Without the contact, the crack cannot be closed. Subsequently both surfaces have to wet each other. In the previously discussed extrinsic self-healing materials, the liquid healing agent has to wet the polymer surface. This is one of the crucial steps for the overall process. In polymer composites, the wetting of the fibers

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will be important for the restoration of the mechanical properties [45]. In contrast, in bulk polymers the wetting is being far from the maximum healing. In the subsequent step, the diffusion becomes more important. The polymer chains can diffuse into the other material (reptation) [45]. Due to the molecular process, the diffusion is limited to a small area (i.e., 3 nm, which is equivalent to the radius of gyration of a polymer above the entanglement length) [50]. After randomization the material is healed. Due to the molecular diffusion process, the healing is time dependent. Additionally the molar mass has a significant influence. Generally the higher the molar mass, the higher healing efficiencies can potentially be reached [51] (if the healing time is sufficient) due to the newly formed entanglements. To conclude, also in (physical) crack healing, the mobility and the formation of new “connections” between the surfaces of the damaged material are essential. Commonly these processes are attributed for the healing of polymers above the glass transition temperature, which will enable the required mobility and diffusion of polymer chains. For instance different glassy polymers have been studied [52]. Scratches in the size below 0.5 μm could successfully be healed. Interestingly these processes can also occur below the glass transition temperature; [53] this fact is on the first glance counterintuitive. For instance polystyrene showed interface healing well below (43 K) the glass transition temperature [54]. Latter parameter is typically the value of the bulk polymer (Tg,bulk). However, the important parameter is the glass transition temperature of the surface (Tg,surface). This value is lower compared to the bulk material [53,55]. Additionally there are enough chain ends in the present surface layer, which can lead to new connections after diffusing into the other material surface [53]. Overall, healing by physical interactions is possible; however, the processes can be rather slow, and only minor damages can be healed. Furthermore the formation of new entanglements (within the limited area) is often also limiting the healing efficiency. Consequently reversible polymers/polymer networks [56] have been utilized as selfhealing materials [57 61]. The main difference to the above-described approach is that these polymers feature reversible interactions (e.g., covalent bonds and supramolecular interactions), which can be reversibly opened and closed again (Fig. 3.6). As a consequence, the opening of this reversible bond can eventually form low molar mass compounds, which can flow in the damaged area, and the restoration of the mechanical properties does not only rely on the formation of new entanglements; the reformed bonds will also contribute to the restoration of the mechanical properties. Therefore unentangled networks can also be healed [62]. Typically two different types of these intrinsic self-healing polymers can be considered: (1) crosslinked linear polymer chains and (2) polymer networks formed by (multi-)functional monomers. Although the latter approach can lead to a kind of a depolymerization during the opening of the reversible bonds, the first approach is based on the reversible decrosslinking of the linear polymer chains. Often the second approach features a higher crosslinking density. The thermal (and mechanical) behavior of these networks is characterized by (if present) the glass transition temperature of the polymer (matrix) and the temperature, at which the reversible interaction is opening. This opening process can be based on addition and condensation reactions (see Fig. 3.6) [57]. Furthermore exchange reactions between the functional crosslinkers can also occur without opening of the bonds (see Fig. 3.6) [57]. This reversible behavior is also utilized in vitrimers, despite the

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FIGURE 3.6 Schematic representation of the possible mechanisms for covalent-based intrinsic self-healing materials [58].

reversible exchange the crosslinking density stays constant [63]. Considering the mechanistic aspects of these materials, one of the most important parameters is the binding strength of the reversible interaction. Depending on the system utilized, the strength varies from reversible covalent interactions [57 59] (and strong supramolecular bonds) down to weaker supramolecular interactions [60,61]. This binding strength determines the temperature (as one of the main triggers used) that has to be reached to activate the reversibility of the bond. Although some of the supramolecular systems (in particular hydrogen bonds) are already reversible at room temperature, covalent bonds typically require elevated temperatures (e.g., 80  C 120  C for the Diels Alder reaction) for the activation [64]. Due to the reversibility there is also equilibrium between the opened and the closed state depending on the temperature (or other external stimuli). This equilibrium influences the mobility of the material. Looking more into the details of these systems, the chain length of the polymeric materials influences the healing time [65]. Higher molar masses lead to longer healing times. Additionally the mobility of the chains is important. One parameter that provides information for the mobility is the Rouse friction coefficient [65]. As discussed above, the equilibrium of the reversible interaction influences the healing as well. Based on these parameters (see Table 3.2), which include the equilibrium (forward and reverse reaction rate), molecular parameters such as the chain length, and the mobility (Rouse friction coefficient), Wang et al. modeled the healing times and compared these with experimental results [65]. In Fig. 3.7 two different systems are compared. The first reversible polymer network is based on reversible covalent bonds. Diarylbibenzofuranones (DABBF) features an equilibrium based on the dissociation to the radical species (also at room temperature) [66]. The second system is based on supramolecular interactions, namely,

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TABLE 3.2 Parameters for the modeling of the healing times [65]. Parameter 21

Definition

DABBF-system

Hydrogen bonds (Leibler)

Forward reaction rate

27

2 3 10

2 3 1027

21 kr0 i (s )

Reverse reaction rate

4 3 1024

4 3 1025

Δx (m)

Distance along the energy landscape

3.8 3 1029

9.4 3 10210

b (m)

Kuhn segment length

5.2 3 10210

5.2 3 10210

nl

Minimum chain length

50

20

nm

Maximum chain length

1500

160

na

Average chain length

690

64

δ

Chain length distribution width

0.2

0.2

Chain alteration parameter

0

f0 ki

(s )

α 21

ζ (N m )

Rouse friction coefficient

0.9 24

2.3 3 10

1 3 1022

DABBF, Diarylbenzofuranone.

hydrogen bonds. The supramolecular rubber described by Leibler et al. shows also fast healing at RT [67]. Although the covalent systems requires several hours for the restoration of the mechanical properties, the supramolecular rubbers are already successfully healed after 2 h. The equilibrium for the supramolecular system is compared to the covalent one more shifted toward the open form (K 5 200 vs 2000). Additionally the supramolecular material is based on smaller oligomers. The increasing average chain length, in case of the DABBF polymer, has an enormous influence on the healing time. The mechanical properties are considered as ratio of the restored strength and the original strength. Although he DABBF polymers feature only a strength of several 10 kPa the supramolecular rubber has a strength of several MPa. The above-described examples illustrate the dependence of the healing kinetics and the molecular parameters. Notably fast healing was often only achieved in case of weaker interactions and polymers. Exemplarily the hydrogen based polymer described by Leibler and his group was healed within minutes [67]. In contrast, polymers based on strong dynamic covalent bonds, for example, reversible ureas, require days until a complete healing can be observed [68,69]. However, a detailed study regarding the healing time would start with a quantitative analysis of the damage and the healing process. For this purpose, new studies focused on the analysis of the scratch diameters. On the one hand the scratch area (2D) [70,71] and the scratch volume (3D) [72,73] are analyzed to visualize the ongoing healing progress. If these methods are applied for all polymers, a correlation between the structure and the healing can be obtained in a quantitative manner. Although the above discussion is only considering the relative ratio of the healed and original properties, Fig. 3.8 illustrates the dependence of the healing efficiency and the required temperatures from the mechanical properties [43,58]. The healing efficiency shows an L-shape. High healing efficiencies were only achieved for weaker materials,

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FIGURE 3.7 Comparison of the healing times for two reversible polymer systems: DABBF—left and supramolecular rubbers—right [65]. DABBF, Diarylbenzofuranone.

which show E-moduli around a few MPa. In contrast, in materials showing E-moduli in the range of GPa, only lower healing efficiencies could be achieved. In Table 3.3 the important mechanistic aspects of intrinsic self-healing materials, their benefits, and the limitations are summarized. In contrast to extrinsic self-healing polymers, the most important aspects can be found on the molecular level: reversible bonds/interactions, which are present within the materials. The reversible moieties might be reversible already at room temperature or they can be activated by heat (or light). Due to the opening of the reversible moieties, which is represented in the most cases of crosslinkers, mobility can be induced, which will allow a reflow of the materials and a closing of cracks. Mostly a relatively fast healing process can be achieved, in particular if highly mobile systems are utilized. The healing efficiency strongly correlates with the healing time. Theoretically (i.e., without any side reactions and degradation) intrinsic self-healing polymers can heal endlessly. The reversible opening and closing does not consume healing agent (like in the case

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FIGURE 3.8 Schematic representation of the healing efficiency as a function of the E-modulus of different self-healing polymers (Diels Alder based, [74] ureas, [75] metallopolymers, [76,77] hydrogen bonds, [67,78,79] imines, [80] π π-interactions, [81] ureas bulk, and scratch healing [68], extrinsic [82]). Furthermore the healing conditions are depicted using different colors and shapes of the individual points [58].

TABLE 3.3 Overview over the mechanistic aspects of intrinsic self-healing materials and their benefits as well as limitations. Mechanistic aspects

Benefits

Limitations

• Molecular level: reversible bonds within the material • Reversibility induces mobility • Possible reflow due to increased mobility

• Relatively fast self-healing possible • Multiple healing possible • Full recovery of mechanical properties possible • Possible design of recyclable polymers

• Often trigger (e.g., temperature) required • High mechanical strength still limits achievable healing efficiencies • Reversible interactions within the materials might lead to creep • Reversible bonds might be prone to degradation • Design of new monomers required

of capsules or vascular networks). An interesting aspect of the reversible systems is that they are in principle suitable candidates for the design of recyclable polymer networks, in contrast to classical elastomers or duromers, which are irreversibly crosslinked. Nevertheless, some limitations have also to be mentioned. Although extrinsic self-healing can be employed to different polymer matrixes, intrinsic systems require the synthesis of novel monomers (and/or polymers). Moreover, some functional moieties for the reversible interactions might interfere with the reactive groups of the monomers (e.g., epoxies). The reversibility might also lead to creep, that is, the polymers adapt to constant stress and “flow.” Some reversible interactions utilized might be prone to degradation limiting the

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possible healing cycles. Importantly, the achievable healing efficiency of hart materials is still limited [83] and often triggers like heat or light are required. Noteworthy, this fact does not necessarily limit the applicability of these materials.

3.6 Other mechanistic aspects The above detailed general considerations are mostly valid for “classical” self-healing material, that is, the restoration of mechanical damages/mechanical properties. Functional self-healing materials, [84,85] that is, beyond mechanical properties, might require different strategies, mechanisms for the restoration of other properties, and functionalities. However, these systems may also take a loan at the classical systems. For instance liquid conductive healing agents were encapsulated in microcapsules. Upon damage, this liquid healing agent can restore the conductivity of broken conductor tracks [86]. Moreover the reversibility of π-conjugated imine-containing polymers was utilized for the restoration of optical properties—comparable to the healing of the above-described intrinsic self-healing polymers [87]. Furthermore self-healing anticorrosion coatings can also feature different operations in the healing process [88]. For instance anticorrosion agents can be released, which prevent the corrosion of the underlying metal—not necessarily healing the broken polymer coating. Overall, it turns out that the healing of diverse functionalities is getting more complex, which can require different strategies, which can also beyond the classical systems. Finally one interesting approach, which assists the healing process, will be discussed. The so-called shape-memory assisted self-healing (SMASH) utilizes shape-memory polymers (SMP), which can promote the healing process [89]. SMP can restore their original shape if they are triggered by temperature in their temporary form [90]. Consequently SMPs were utilized to close cracks within the polymer (see Fig. 3.9). If the SMPs transform FIGURE 3.9 Schematic representation of the basic principle of SMASH based on the Diels Alder reaction. SMASH, Shape-memory assisted self-healing. Reprinted with permission from G. Rivero, L.-T.T. Nguyen, X.K.D. Hillewaere, F.E. Du Prez, One-pot thermo-remendable shape memory polyurethanes, Macromolecules 47 (6) (2014) 2010 2018 [91]. Copyright 2019 American Chemical Society.

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into their permanent shape they can press together crack planes, which will promote the overall healing process. This process can be based on reversible interactions like the Diels Alder reaction [91]. Furthermore the shape-memory process can also be combined with capsule-based approaches [92].

3.7 Conclusions Different design principles and approaches were chosen to obtain self-healing polymers, which are capable to restore their properties after a damage event. Generally there are two main classes of self-healing polymers. Extrinsic and intrinsic self-healing polymers can be distinguished. Concerning the basic mechanism, both principles differ significantly. Although intrinsic ones depend on reversible processes occurring in the material on the molecular scale, extrinsic self-healing polymers require the embedding of additional healing agents into the matrix. Despite their differences, both approaches still feature similarities in their general mechanism. First, damage can only be healed if the damage mode corresponds to the capabilities of the material. For instance, if the damage is too large, an efficient healing or even healing at all cannot be guaranteed. Thus the material must be designed in such a fashion that the potential damage can be restored efficiently. Second, mobility is a common theme for all healing processes. Although this mobility is generated on the macroscopic level for extrinsic self-healing polymers, intrinsic self-healing polymers feature the mobility based on processes on the molecular level. However, macroscopic processes such as plastic/elastic deformation cannot be neglected for intrinsic selfhealing, and they will also influence the healing process. This processes feature the reversible bonds on the molecular level and a material flow on the macroscopic level. Extrinsic healing can be considered as a rather robust process, which can be utilized for many different polymer matrixes. Different healing agents and chemistries can be applied for the design of such systems, resulting in a very broad potential field of application since these factors influence the healing efficiency and healing time. On the other hand, a large variety of different intrinsic self-healing polymers was investigated in recent years. However, the knowledge of the detailed interplay of all parameters/molecular properties and their leverage on the healing process itself is still rather poor. The most important factor considered is still the reversibility of the corresponding covalent bonds and supramolecular interactions, respectively. The additional factors deriving from the polymer structure, molar mass, etc., are still not fully understood. Here is, however, the key for the design of the next generation of self-healing polymers. Thus a correlation of the molecular processes with the resulting change of the macroscopic properties such as material flow is crucial for the future understanding of healing. A first approach was already described focusing on a correlation of the reversible covalent bond (i.e., thiol-ene) and the material flow using coherent antiStokes Raman scattering (CARS) spectroscopy [93]. Nevertheless, a complete understanding is not obtained at the moment, and further investigations are required. Finally the healing of functions using polymers is a completely new topic in literature and will require more tailor-made approaches [84]. Thus the generation of mobility will not be the only key factor anymore. Furthermore, it is important that the function of the

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References

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material (e.g., conductivity, absorption, and emission) is still possible after healing. Consequently there are more restrictions resulting in the demand of new advanced healing strategies going beyond pure intrinsic and extrinsic healing. However, these investigations are still in their infancies, and future developments are required [94].

References [1] S.J. Garcia, Effect of polymer architecture on the intrinsic self-healing character of polymers, Eur. Polym. J. 53 (2014) 118 125. [2] S. van der Zwaag, An introduction to material design principles: damage prevention versus damage management, in: S. van der Zwaag (Ed.), Self Healing Materials: An Alternative Approach to 20 Centuries of Materials Science, Springer Netherlands, Dordrecht, 2007, pp. 1 18. [3] I.H. Kalfas, Principles of bone healing, J. Neurosurg. 10 (4) (2001) 1 4. [4] P. Martin, Wound healing - aiming for perfect skin regeneration, Science 276 (5309) (1997) 75 81. ¨ nsal-Kac¸maz, S. Linn, Molecular mechanisms of mammalian DNA repair [5] A. Sancar, L.A. Lindsey-Boltz, K. U and the DNA damage checkpoints, Annu. Rev. Biochem. 73 (1) (2004) 39 85. [6] M. Kragl, D. Knapp, E. Nacu, S. Khattak, M. Maden, H.H. Epperlein, et al., Cells keep a memory of their tissue origin during axolotl limb regeneration, Nature 460 (2009) 60. [7] M.J. Harrington, O. Speck, T. Speck, S. Wagner, R. Weinkamer, Biological archetypes for self-healing materials, in: M.D. Hager, S. van der Zwaag, U.S. Schubert (Eds.), Self-Healing Materials, Springer International Publishing, Cham, 2016, pp. 307 344. [8] S. van der Zwaag, N.H. van Dijk, H.M. Jonkers, S.D. Mookhoek, W.G. Sloof, Self-healing behaviour in manmade engineering materials: bioinspired but taking into account their intrinsic character, Philos. Trans. R. Soc. A 367 (1894) (2009) 1689 1704. [9] M.J. Harrington, H.S. Gupta, P. Fratzl, J.H. Waite, Collagen insulated from tensile damage by domains that unfold reversibly: in situ X-ray investigation of mechanical yield and damage repair in the mussel byssus, J. Struct. Biol. 167 (1) (2009) 47 54. [10] S. Zechel, M.D. Hager, T. Priemel, M.J. Harrington, Healing through histidine: bioinspired pathways to selfhealing polymers via imidazole metal coordination, Biomimetics 4 (1) (2019) 20. [11] R. Dhanasekaran, S. Sreenatha Reddy, A. Sai Kumar, Application of self-healing polymers to overcome impact, fatigue and erosion damages, Mater. Today: Proc 5 (10) (2018) 21373 21377. Part 1. [12] L. Laiarinandrasana, T.F. Morgeneyer, H. Proudhon, C. Regrain, Damage of semicrystalline polyamide 6 assessed by 3D X-ray tomography: from microstructural evolution to constitutive modeling, J. Polym. Sci. Part B: Polym. Phys. 48 (13) (2010) 1516 1525. [13] F. Detrez, S. Cantournet, R. Seguela, Plasticity/damage coupling in semi-crystalline polymers prior to yielding: Micromechanisms and damage law identification, Polymer 52 (9) (2011) 1998 2008. [14] P.B. Bowden, J.A. Jukes, The plastic flow of isotropic polymers, J. Mater. Sci. 7 (1) (1972) 52 63. [15] S. Bode, M. Enke, M. Hernandez, R.K. Bose, A.M. Grande, S. van der Zwaag, et al., Characterization of selfhealing polymers: from macroscopic healing tests to the molecular mechanism, in: M.D. Hager, S. van der Zwaag, U.S. Schubert (Eds.), Self-Healing Materials, Springer International Publishing, Cham, 2016, pp. 113 142. [16] J. Rottler, Fracture in glassy polymers: a molecular modeling perspective, J. Phys.: Cond. Matter 21 (46) (2009). 463101. [17] J. An, B.-H. Kang, B.-H. Choi, H.-J. Kim, Observation and evaluation of scratch characteristics of injectionmolded poly(methyl methacrylate) toughened by acrylic rubbers, Tribol. Int. 77 (2014) 32 42. [18] H. Jiang, R. Browning, H.-J. Sue, Understanding of scratch-induced damage mechanisms in polymers, Polymer 50 (16) (2009) 4056 4065. [19] P.J. Schilling, B.R. Karedla, A.K. Tatiparthi, M.A. Verges, P.D. Herrington, X-ray computed microtomography of internal damage in fiber reinforced polymer matrix composites, Compos. Sci. Technol. 65 (14) (2005) 2071 2078. [20] M. Giordano, A. Calabro, C. Esposito, A. D’Amore, L. Nicolais, An acoustic-emission characterization of the failure modes in polymer-composite materials, Compos. Sci. Technol. 58 (12) (1998) 1923 1928.

Self-Healing Polymer-Based Systems

92

3. Self-healing polymers: from general basics to mechanistic aspects

[21] M.D. Hager, P. Greil, C. Leyens, S. van der Zwaag, U.S. Schubert, Self-healing materials, Adv. Mater. 22 (47) (2010) 5424 5430. [22] J. Li, C. Nagamani, J.S. Moore, Polymer mechanochemistry: from destructive to productive, Acc. Chem. Res. 48 (8) (2015) 2181 2190. [23] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794 797. [24] M.M. Caruso, D.A. Delafuente, V. Ho, N.R. Sottos, J.S. Moore, S.R. White, Solvent-promoted self-healing epoxy materials, Macromolecules 40 (25) (2007) 8830 8832. [25] X.K.D. Hillewaere, F.E. Du Prez, Fifteen chemistries for autonomous external self-healing polymers and composites, Prog. Polym. Sci. 49 50 (2015) 121 153. [26] D.Y. Zhu, M.Z. Rong, M.Q. Zhang, Self-healing polymeric materials based on microencapsulated healing agents: from design to preparation, Prog. Polym. Sci. 49 50 (2015) 175 220. [27] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene, J. Microencapsul. 20 (6) (2003) 719 730. [28] W. Wang, W. Li, W. Fan, X. Zhang, L. Song, C. Xiong, et al., Accelerated self-healing performance of magnetic gradient coating, Chem. Eng. J. 332 (2018) 658 670. [29] T. Sun, X. Shen, C. Peng, H. Fan, M. Liu, Z. Wu, A novel strategy for the synthesis of self-healing capsule and its application, Compos. Sci. Technol. 171 (2019) 13 20. [30] J.D. Rule, N.R. Sottos, S.R. White, Effect of microcapsule size on the performance of self-healing polymers, Polymer 48 (12) (2007) 3520 3529. [31] M. Kosarli, D.G. Bekas, K. Tsirka, D. Baltzis, D. Vaimakis-Tsogkas, S. Orfanidis, et al., Microcapsule-based self-healing materials: healing efficiency and toughness reduction vs. capsule size, Compos. B: Eng. 171 (2019) 78 86. [32] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (2007) 581. [33] S.C. Olugebefola, A.M. Arago´n, C.J. Hansen, A.R. Hamilton, B.D. Kozola, W. Wu, et al., Polymer microvascular network composites, J. Compos. Mater. 44 (22) (2010) 2587 2603. [34] M.-U. Saeed, Z. Chen, B. Li, Manufacturing strategies for microvascular polymeric composites: a review, Compos. A: Appl. Sci. Manuf. 78 (2015) 327 340. [35] J.F. Patrick, K.R. Hart, B.P. Krull, C.E. Diesendruck, J.S. Moore, S.R. White, et al., Continuous self-healing life cycle in vascularized structural composites, Adv. Mater. 26 (25) (2014) 4302 4308. [36] G. Postiglione, M. Alberini, S. Leigh, M. Levi, S. Turri, Effect of 3D-printed microvascular network design on the self-healing behavior of cross-linked polymers, ACS Appl. Mater. Interfaces 9 (16) (2017) 14371 14378. [37] F. Zhang, P. Ju, M. Pan, D. Zhang, Y. Huang, G. Li, et al., Self-healing mechanisms in smart protective coatings: a review, Corros. Sci. 144 (2018) 74 88. [38] S. An, M.W. Lee, A.L. Yarin, S.S. Yoon, A review on corrosion-protective extrinsic self-healing: comparison of microcapsule-based systems and those based on core-shell vascular networks, Chem. Eng. J. 344 (2018) 206 220. [39] S.R. White, J.S. Moore, N.R. Sottos, B.P. Krull, W.A. Santa Cruz, R.C.R. Gergely, Restoration of large damage volumes in polymers, Science 344 (6184) (2014) 620 623. [40] R.C.R. Gergely, W.A. Santa Cruz, B.P. Krull, E.L. Pruitt, J. Wang, N.R. Sottos, et al., Restoration of impact damage in polymers via a hybrid microcapsule microvascular self-healing system, Adv. Funct. Mater. 28 (2) (2018) 1704197. [41] ,http://www.ruehl-ag.de/index.php?id 5 160. (accessed 01.06.2019). [42] R.A.T.M. van Benthem, W. Ming, G. de With, Self healing polymer coatings, in: S. van der Zwaag (Ed.), Self Healing Materials: An Alternative Approach to 20 Centuries of Materials Science, Springer Netherlands, Dordrecht, 2007, pp. 139 159. [43] S. Zechel, M.D. Hager, U.S. Schubert, Self-healing polymers: from biological systems to highly functional polymers, in: M.A. Jafar Mazumder, H. Sheardown, A. Al-Ahmed (Eds.), Functional Polymers, Springer International Publishing, Cham, 2018, pp. 1 53. [44] R.P. Wool, K.M. O’Connor, A theory crack healing in polymers, J. Appl. Phys. 52 (10) (1981) 5953 5963. [45] R.P. Wool, Self-healing materials: a review, Soft Matter 4 (3) (2008) 400 418. [46] R.P. Wool, K.M. O’Connor, Time dependence of crack healing, J. Polym. Sci. Polym. Lett. Ed 20 (1) (1982) 7 16.

Self-Healing Polymer-Based Systems

References

93

[47] J.T. Koberstein, Molecular design of functional polymer surfaces, J. Polym. Sci. Part B: Polym Phys. 42 (16) (2004) 2942 2956. [48] T. Diki´c, W. Ming, R.A.T.M. van Benthem, A.C.C. Esteves, G. de With, Self-replenishing surfaces, Adv. Mater. 24 (27) (2012) 3701 3704. [49] H. Homma, T. Kuroyagi, K. Izumi, C.L. Mirley, J. Ronzello, S.A. Boggs, Diffusion of low molecular weight siloxane from bulk to surface, IEEE Trans. Dielectr. Electr. Insul. 6 (3) (1999) 370 375. [50] H.H. Kausch, Die Rolle der Kettenverschlaufungen bei der Ausheilung von Bru¨chen, Coll. Polym. Sci. 259 (9) (1981) 917 925. [51] Y.H. Kim, R.P. Wool, A theory of healing at a polymer-polymer interface, Macromolecules 16 (7) (1983) 1115 1120. [52] K. Jud, H.H. Kausch, J.G. Williams, Fracture mechanics studies of crack healing and welding of polymers, J. Mater. Sci. 16 (1) (1981) 204 210. [53] Y.M. Boiko, On the molecular mechanism of self-healing of glassy polymers, Coll. Polym. Sci. 294 (7) (2016) 1237 1242. [54] Y.M. Boiko, A. Bach, J. Lyngaae-Jørgensen, Self-bonding in an amorphous polymer below the glass transition: a T-peel test investigation, J. Polym. Sci. Part B: Polym. Phys. 42 (10) (2004) 1861 1867. [55] H. Morita, K. Tanaka, T. Kajiyama, T. Nishi, M. Doi, Study of the glass transition temperature of polymer surface by coarse-grained molecular dynamics simulation, Macromolecules 39 (18) (2006) 6233 6237. [56] G.A. Parada, X. Zhao, Ideal reversible polymer networks, Soft Matter 14 (25) (2018) 5186 5196. [57] N. Kuhl, S. Bode, R.K. Bose, J. Vitz, A. Seifert, S. Hoeppener, et al., Acylhydrazones as reversible covalent crosslinkers for self-healing polymers, Adv. Funct. Mater. 25 (22) (2015) 3295 3301. [58] J. Dahlke, S. Zechel, M.D. Hager, U.S. Schubert, How to design a self-healing polymer: general concepts of dynamic covalent bonds and their application for intrinsic healable materials, Adv. Mater. Interfaces 5 (17) (2018) 1800051. [59] S.Y. An, D. Arunbabu, S.M. Noh, Y.K. Song, J.K. Oh, Recent strategies to develop self-healable crosslinked polymeric networks, Chem. Commun. 51 (66) (2015) 13058 13070. [60] A. Campanella, D. Do¨hler, W.H. Binder, Self-healing in supramolecular polymers, Macromol. Rapid Commun. 39 (17) (2018) 1700739. [61] M. Enke, D. Do¨hler, S. Bode, W.H. Binder, M.D. Hager, U.S. Schubert, Intrinsic self-healing polymers based on supramolecular interactions: state of the art and future directions, in: M.D. Hager, S. van der Zwaag, U.S. Schubert (Eds.), Self-Healing Materials, Springer International Publishing, Cham, 2016, pp. 59 112. [62] E.B. Stukalin, L.-H. Cai, N.A. Kumar, L. Leibler, M. Rubinstein, Self-healing of unentangled polymer networks with reversible bonds, Macromolecules 46 (18) (2013) 7525 7541. [63] W. Denissen, J.M. Winne, F.E. Du Prez, Vitrimers: permanent organic networks with glass-like fluidity, Chem. Sci. 7 (1) (2016) 30 38. [64] Y.-L. Liu, T.-W. Chuo, Self-healing polymers based on thermally reversible Diels Alder chemistry, Polym. Chem. 4 (7) (2013) 2194 2205. [65] K. Yu, A. Xin, Q. Wang, Mechanics of self-healing polymer networks crosslinked by dynamic bonds, J. Mech. Phys. Solids 121 (2018) 409 431. [66] K. Imato, M. Nishihara, T. Kanehara, Y. Amamoto, A. Takahara, H. Otsuka, Self-healing of chemical gels cross-linked by diarylbibenzofuranone-based trigger-free dynamic covalent bonds at room temperature, Angew. Chem. Int. Ed. 51 (5) (2012) 1138 1142. [67] P. Cordier, F. Tournilhac, C. Soulie´-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977. [68] S. Zechel, R. Geitner, M. Abend, M. Siegmann, M. Enke, N. Kuhl, et al., Intrinsic self-healing polymers with a high E-modulus based on dynamic reversible urea bonds, NPG Asia Mater. 9 (2017) e420. [69] M. Abend, S. Zechel, U.S. Schubert, M.D. Hager, Detailed analysis of the influencing parameters on the selfhealing behavior of dynamic urea-crosslinked poly(methacrylate)s, Molecules 24 (19) (2019) 3597. [70] R.K. Bose, N. Hohlbein, S.J. Garcia, A.M. Schmidt, S. van der Zwaag, Connecting supramolecular bond lifetime and network mobility for scratch healing in poly(butyl acrylate) ionomers containing sodium, zinc and cobalt, Phys. Chem. Chem. Phys 17 (3) (2015) 1697 1704. [71] R.K. Bose, M. Enke, A.M. Grande, S. Zechel, F.H. Schacher, M.D. Hager, et al., Contributions of hard and soft blocks in the self-healing of metal-ligand-containing block copolymers, Eur. Polym. J. 93 (2017) 417 427.

Self-Healing Polymer-Based Systems

94

3. Self-healing polymers: from general basics to mechanistic aspects

[72] J. Ahner, D. Pretzel, M. Enke, R. Geitner, S. Zechel, J. Popp, et al., Conjugated oligomers as fluorescence marker for the determination of the self-healing efficiency in mussel-inspired polymers, Chem. Mater. 30 (8) (2018) 2791 2799. [73] M. Abend, C. Kunz, S. Stumpf, S. Gra¨f, S. Zechel, F.A. Mu¨ller, et al., Femtosecond laser-induced scratch ablation as an efficient new method to evaluate the self-healing behavior of supramolecular polymers, J. Mater. Chem. A 7 (5) (2019) 2148 2155. [74] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (5560) (2002) 1698 1702. [75] H. Ying, Y. Zhang, J. Cheng, Dynamic urea bond for the design of reversible and self-healing polymers, Nat. Commun. 5 (2014) 3218. [76] M. Burnworth, L. Tang, J.R. Kumpfer, A.J. Duncan, F.L. Beyer, G.L. Fiore, et al., Optically healable supramolecular polymers, Nature 472 (2011) 334. [77] S. Coulibaly, A. Roulin, S. Balog, M.V. Biyani, E.J. Foster, S.J. Rowan, et al., Reinforcement of optically healable supramolecular polymers with cellulose nanocrystals, Macromolecules 47 (1) (2014) 152 160. [78] Y. Chen, A.M. Kushner, G.A. Williams, Z. Guan, Multiphase design of autonomic self-healing thermoplastic elastomers, Nat. Chem. 4 (2012) 467. [79] F. Tournilhac, P. Cordier, D. Montarnal, C. Soulie´-Ziakovic, L. Leibler, Self-healing supramolecular networks, Macromol. Symp. 291 292 (1) (2010) 84 88. [80] H. Li, J. Bai, Z. Shi, J. Yin, Environmental friendly polymers based on schiff-base reaction with self-healing, remolding and degradable ability, Polymer 85 (2016) 106 113. [81] S. Burattini, B.W. Greenland, D.H. Merino, W. Weng, J. Seppala, H.M. Colquhoun, et al., A healable supramolecular polymer blend based on aromatic π 2 π stacking and hydrogen-bonding interactions, J. Am. Chem. Soc. 132 (34) (2010) 12051 12058. [82] Y.C. Yuan, M.Z. Rong, M.Q. Zhang, J. Chen, G.C. Yang, X.M. Li, Self-healing polymeric materials using rpoxy/mercaptan as the healant, Macromolecules 41 (14) (2008) 5197 5202. [83] R. Hoogenboom, Hard autonomous self-healing supramolecular materials—a contradiction in terms? Angew. Chem. Int. Ed. 51 (48) (2012) 11942 11944. [84] J. Ahner, S. Bode, M. Micheel, B. Dietzek, M.D. Hager, Self-healing functional polymeric materials, in: M.D. Hager, S. van der Zwaag, U.S. Schubert (Eds.), Self-Healing Materials, Springer International Publishing, Cham, 2016, pp. 247 283. [85] D. Chen, D. Wang, Y. Yang, Q. Huang, S. Zhu, Z. Zheng, Self-healing materials for next-generation energy harvesting and storage devices, Adv. Energ. Mater. 7 (23) (2017). 1700890. [86] B.J. Blaiszik, S.L.B. Kramer, M.E. Grady, D.A. McIlroy, J.S. Moore, N.R. Sottos, et al., Autonomic restoration of electrical conductivity, Adv. Mater. 24 (3) (2012) 398 401. [87] J. Ahner, M. Micheel, R. Geitner, M. Schmitt, J. Popp, B. Dietzek, et al., Self-healing functional polymers: optical property recovery of conjugated polymer films by uncatalyzed imine metathesis, Macromolecules 50 (10) (2017) 3789 3795. [88] D. Crespy, K. Landfester, J. Fickert, M. Rohwerder, Self-healing for anticorrosion based on encapsulated healing agents, in: M.D. Hager, S. van der Zwaag, U.S. Schubert (Eds.), Self-Healing Materials, Springer International Publishing, Cham, 2016, pp. 219 245. [89] X. Luo, P.T. Mather, Shape memory assisted self-healing coating, ACS Macro Lett. 2 (2) (2013) 152 156. [90] M.D. Hager, S. Bode, C. Weber, U.S. Schubert, Shape memory polymers: past, present and future developments, Prog. Polym. Sci. 49 50 (2015) 3 33. [91] G. Rivero, L.-T.T. Nguyen, X.K.D. Hillewaere, F.E. Du Prez, One-pot thermo-remendable shape memory polyurethanes, Macromolecules 47 (6) (2014) 2010 2018. [92] W. Fan, Y. Zhang, W. Li, W. Wang, X. Zhao, L. Song, Multi-level self-healing ability of shape memory polyurethane coating with microcapsules by induction heating, Chem. Eng. J. 368 (2019) 1033 1044. [93] R. Geitner, F.-B. Legesse, N. Kuhl, T.W. Bocklitz, S. Zechel, J. Vitz, et al., Do you get what you see? Understanding molecular self-healing, Chem. Eur. J. 24 (10) (2018) 2493 2502. [94] M.D. Bartlett, M.D. Dickey, C. Majidi, Self-healing materials for soft-matter machines and electronics, NPG Asia Mater. 11 (1) (2019) 21.

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C H A P T E R

4 Shape memory-assisted self-healing polymer systems Wenjing Wu1,2, James Ekeocha1, Christopher Ellingford1, Sreeni Narayana Kurup1 and Chaoying Wan1 1

International Institute for Nanocomposites Manufacturing, University of Warwick, Coventry, United Kingdom 2Aerospace Research Institute of Materials & Processing Technology, Beijing, P.R. China

4.1 Introduction Smart polymers are materials that can exhibit a response to external stimuli and adapt their physical properties accordingly. One example is shape memory polymers (SMPs), which can return to their original shape from a temporarily deformed state by responding to an external stimulus such as temperature, light, electrical, electromagnetic fields, and chemical environments. One important application of SMPs is electrical cable protection due to their unique advantages in offering exceptional electrical insulation, anticorrosion, mechanical protection, and strain relief. The shape memory effect (SME) can be utilized to assist the self-healing process of polymer systems. The healing of a crack typically consists of five stages: surface rearrangement, surface approach, wetting, diffusion, and randomization [1]. In most cases, the broken surfaces need to be in contact, close, and heal [2]. SMPs are capable of remembering a permanent shape, fixing into one or more temporary shapes, and recovering to the permanent shape upon one or more external stimuli. This shape memory behavior can be used to close cracks, allowing the two polymer surfaces to diffuse and heal. The SME can be induced intrinsically or extrinsically. Intrinsic SMPs can provide an omnidirectional recovery force, whereas extrinsic SMPs are created through additives such as shape memory fibers or alloy wires [3,4]. Extrinsic shape memory can only provide a unidirectional recover force and is not effective when the direction of crack propagation is not perpendicular to the additives [2].

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In this chapter, we will introduce the shape memory mechanism of polymers, followed by the self-healing of polymers with a focus on the intrinsic self-healing mechanisms. The recent development of shape memory-assisted self-healing (SMASH) of polymers and composites are also overviewed, and finally the perspective and future directions are proposed.

4.2 Shape memory and self-healing mechanisms 4.2.1 Shape memory mechanism The SME is a phenomenon involving a change in the physical state of a polymer. It was first proposed in 1941 and used in cross-linked polyethylene for making heat-shrinkable tubes and films [5]. When the polymer is deformed under an external force under certain conditions, the temporary deformed shape is “frozen” while the permanent shape is memorized. On exposure to a suitable stimulus, such as temperature, light, electrical or magnetic fields, pH, or solvent [6], a conformation transition is induced and the polymer recovers the memorized permanent shape. A molecular model of thermally induced shape memory is illustrated in Fig. 4.1 [7]. As an entropy driven transformation, the conformation of a polymer changes with the movement of internal molecular unit motion. Therefore the glass transition temperature (Tg) plays a significant role in the shape memory of polymer materials. At temperatures above the Tg, polymer chain segments possess good mobility, enabling changes in conformation to occur. Below the Tg, chain segment motion is restricted due to

FIGURE 4.1 Molecular model of thermally induced dual SMPs. The black dots are netpoints, the red lines are the molecular chains of high mobility above the transition temperature [7]. SMPs, shape memory polymers. Source: Copyright 2015, Elsevier.

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limited free volume and this enables the fixing of the temporary shape in a shape memory cycle. Similarly for a polymer system with crystalline domains, the melting temperature (Tm) is significant for shape memory and results in a two-step process for shape recovery [8]. On heating to above the Tg ( . shape fixing temperature), shape recovery is only partial, and heating to over the Tm is required for full shape recovery to be observed. In the process of shape memory, there are two competing stages: the chain deformation under external stress, and the chain contraction as an entropic response. To promote the SME, a dual segment polymer system is commonly suggested. One segment should be elastic to provide a great recovery ability which is often chemically or physically cross-linked or contains intermolecular interactions induced supramolecular interactions. The second segment should introduce the fixing effect of the temporary conformation by a change in stiffness to freeze the temporary shape in place. This is usually achieved through an amorphous/semicrystalline phase transition or intermolecular interactions. A typical shape memory cycle includes two processes, fixing deformation and shape recovery, which takes place over three stages: (1) Programming load—under an external stimulus, a sample is deformed to the required maximum programming strain of εm; (2) Fixing deformation—after removal of the prescribed condition, the sample is unloaded and the residual strain is representative of the fixed shape, defined as ε1. This ends the programming process and finishes the deformation but due to the relaxation of polymer chains upon removal of the external stimulus, ε1 is lower than εm; (3) The recovery process—the sample is exposed to the external stimulus for shape recovery and the final remaining strain is ε2. One of two important measures of SME is the shape fixity ratio (Rr 5 ε1/εm), describing the ability of the sample to maintain the temporary shape [9]. When Rf 5 1, the sample remains fully in the temporary shape. The deformation modes cover tension, compression, bending, and torsion. The second is the shape recovery ratio (Rr), which evaluates the ability to recover its permanent shape, and is defined as Rr 5 (ε12ε2)/ε1. It is the numerical recovery of the permanent shape when the material is in an unconstrained state, although the percentage value of Rr and Rf is commonly reported in the literature. When Rr 5 1, the permanent shape is fully restored. Both Rf and Rr are external stimulus and time dependent parameters. An excellent SMP should exhibit high Rf and Rr values, and fast recovery. Shape memory cycles are often programmed in thermomechanical tests performed under stress- or strain-controlled conditions. With the application of force, stress occurs under constrained conditions, and the shape memory properties are evaluated and presented in 2D graphs with multiple Y-axes representing stress, strain, and temperature plotting against time on the X-axis (Fig. 4.2A) [11]. Additionally the data is also presented in 3D graphs with temperature, stress, and strain on the X-, Y-, and Z-axes (Fig. 4.2B), respectively. Generally SMPs do not experience a reversible recovery, and the externally-applied stress is required for every cycle. This is known as one-way SME, where once the stimulus is removed, the temporary shape is retained. The second type is the two-way SME, which allows repeated transformation between the temporary shape and the permanent shape under different stimuli. To achieve excellent two-way shape memory performance, the reversible internal stress can be improved by the introduction of elastomeric networks, higher Tm crystalline phases, rubbery domains, hydrogen bonding, and uneven stress

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FIGURE 4.2 Shape memory cycle test: (A) 2D diagram [10] and (B) 3D diagram. One-way shape memory cycles [11]: (i) cooling and (ii) removing the stress (iii) yield a temporary “fixed” strain that can be recovered to the original strain upon heating (iv). Source: Copyright 2010, Springer Nature and copyright 2008, Royal Society of Chemistry.

relaxation [12]. Therefore the two-way shape memory performance has been shown in hydrogels (thermally reversible order/disorder transition) [13,14], liquid crystalline elastomers (thermally reversible nematic/isotropic transition) [5,15,16], and laminated polymers (stacked layers of cross-linked crystalline polymers or cross-linked crystalline polymer/ elastomer) [12]. The exploration of polymers exhibiting a triple and multiple shape SME is currently an active field of research. The shape memory of polymers is related to its viscoelastic behavior. Under different external conditions, the dominant viscous or elastic behavior of the polymer promotes the transformation. Among modeling approaches proposed in recent research [5,14], three modeling approaches are mainly used to investigate the polymers on the micro-, meso-, and macro-scales. The microscale modeling mainly looks at microstructure properties such as cross-linking, chain mobility, and the entanglement of polymer chains, which can be calculated by means of molecular dynamics or quantum chemical methods. This model is mostly applied to epoxies and demonstrates a good correlation between theoretical and experimentally observed shape memory behavior. The macroscale modeling, usually related to phase transition-based viscoelastic behavior [17], describes the rate-dependent behavior of a polymer using one-dimensional (1D) or three-dimensional (3D) rheological models. Meso-scale modeling approaches are between micro- and macro-scale models. They are usually detailed enough considering the structural heterogeneity of materials, such as the model calculations based on the concept of describing the active-frozen transitions [18
21], favoring to predict local stresses and strain distributions to understand damage and failure processes, but not capable of describing the materials’ properties at a molecular level. Prediction, analysis, and modeling of the stress and strain of materials are the main focus of this approach. Furthermore, based on thermodynamic theory, some models are proposed to interpret SMASH behavior of linear and cross-linked polycaprolactone (PCL) blends [22].

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An interpolated network model was proposed to simulate the cross-linked network and linear polymer system, respectively. The shape memory property of the chemically crosslinked network was captured by a viscoelastic model considering the glass and rubbery transformation of the material, while the mechanical properties of the linear polymer system were modeled by a plastic model. The theoretical and experimental results showed good agreement, indicating that the SMASH system has the capacity of healing the region of damage while also recovering the elastic and plastic properties. The intrinsic self-healing of polymer systems is an autonomous thermodynamic and kinetic process, and can be viewed as chain conformational changes from a non-Gaussian to equilibrated Gaussian state in which each chain escapes from a tube cavity within a given time and at a microscopic scale [6]. After the closure of the crack, the mechanisms of diffusion and randomization take place and healing occurs. The SME facilitates selfclosure of the crack in the polymer system, and assists to heal the damage at the structural-length scale.

4.2.2 Intrinsic self-healing of polymers Intrinsic self-healing polymeric materials utilize chemical bonds that have the ability to reform the structures after damage has occurred. The “intrinsic” nature of the healing process results from the reversible dynamic covalent bonds or noncovalent interactions. The specific molecular structures and performance of the self-healing polymers occur via a temporary increase in mobility, leading to a reflow of the material in the damaged area. This is often stimulated by temperature, light, or catalyst among others [23]. From predominant molecular mechanisms involved in the healing processes, intrinsic healing can be achieved through three modes: (1) dynamic covalent chemistry—the reversible forming and breaking of covalent bonds, (2) thermoreversible physical interactions, or (3) supramolecular chemistry—imparting the capability of self-assembly or self-organization using highly directional and reversible noncovalent interactions that dictate the overall mechanical properties of a material [24]. Investigations of intrinsic self-healing polymer systems concern the chemistry of the covalent or noncovalent interactions, which are reversible at low temperatures [23]. The efficiency of healing is defined as the extent of the ability of a system to recover its original material properties, such as tensile strength or elongation at break, and is usually quoted as a percentage [25]. This section aims to overview the recent developments surrounding the covalent and noncovalent natures of self-healing polymers. 4.2.2.1 Covalent self-healing Reversible covalent bonds can be incorporated either into the polymer backbones or as pendant groups of the polymer. The commonly studied reversible covalent bonds are reversible cycloaddition reactions, exchange reactions and stable free radical-mediated reshuffling reactions [26]. The reversible covalent reactions generally require external stimuli such as heating or the addition of a catalyst to initiate the response. The reversible “4 1 2” cycloaddition Diels
Alder (DA) reaction is the most prominent reaction, whereby an electron-rich diene and an electron-poor dienophile react to form a stable cyclohexene adduct. DA bonds are widely used due to their outstanding properties,

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such as aqueous promotion [27] and structure controllability [28]. Several compounds can be used to build up such reversible systems, most notably furan and maleimide derivatives (Fig. 4.3A). For instance, Du et al. synthesized a linear polyurethane (PU) via a DA reaction between a PU prepolymer end-capped with furan groups and bismaleimide [29]. The introduction of DA bonds enabled the thermal reversibility of the PU and subsequently exhibited self-healing properties after thermal treated at 120 C for approximately 300 s, at which point the crack was no longer visible over multiple healing cycles. The healing efficiency was 80% for the first healing cycle and 66% for a second, with respect to the maximum tensile strength. Further work utilizes a multiple-furan monomer with 1,10(methylenedi-4,1-phenylene)bismaleimide [30]. The DA initiated self-healing could heal a damaged sample in 5 min with a healing efficiency of 92.4%. More recently a cross-linking polymer resin FGE-T5000-BDM, consisting of furfuryl glycidyl ether (FGE), polyether amine (T5000) and 4,40 -diphenylmethane bismaleimide (BDM) [31] was synthesized. After treatment at 80 C for 12 h, the DA healing efficiency reached 88%. The sample temperature was above its glass transition temperature (Tg), and so existed in a high-elastic state where the molecular chains have high mobility. At 80 C, the product could be partly decomposed to the monomer segments which contained a furan ring and an imide. Broken DA bonds within the sample are resynthesized by the flow of these monomer segments in the high-elastic state, which helps the product achieve self-healing as a result through DA reactions between the sample segments. However, the high temperature required to self-heal DA bonds means that DA reactions are limited in some applications, such as biomedical applications. A second approach is to introduce exchange reactions to incorporate reversible covalent bonds into a polymer matrix to promote intrinsic self-healing. Disulfide groups are often introduced into polymer systems due to their ability to undergo reversible exchange reactions, such as disulfide exchange [32] and thiol-disulfide exchange [33] (Fig. 4.3B). Zhang et al. synthesized PCL networks with disulfide bonds through a thiol-ene “click” reaction, which led to functional properties including self-healing, shape memory, reprocessability and degradability [34]. The blend of the disulfide monomer and branched PCL cocured with the mixed thiols to prepare the PCL networks. These networks repaired mechanical damages at a pronounced healing efficiency of 92%, in relation to the yield strength, after 1 h of heating at 60 C. The heat thermally stimulated the system to overcome the bond dissociation energy of the disulfide bonds, forming radicals, which attack adjacently and result in the reformation of new disulfide bonds. The formation of reversible disulfide bonds initiated the chain exchange in the cross-linked polymer at moderate temperatures. Stress relaxation testing showed the increased chain mobility by a decrease in the stress relaxation time with increasing disulfide content. Ling et al. developed a series of self-healing linear PUs with disulfide linkages as the grafting point [35]. The effects of different ratios of soft/ hard segments on the mechanical properties and healing efficiencies were investigated and the results showed that the increase in soft segment content significantly improved the breaking strength and elongation at break of the polymer, but reduced the healing efficiency and Young’s modulus. The series of PUs differed in disulfide bond content which formed the hard segments. The specimens were cut and pushed together with the application of gentle pressure prior to microwave oven heating at 100 C for 10 min without exoteric forces. The average healing efficiencies estimated from the experiments were 74%,

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FIGURE 4.3 Detailing the schematics of (A) Diels
Alder reaction, (B) disulfide exchange reaction, and (C) reshuffling reactions with examples.

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80%, and 91% for PUs with 25%, 30%, and 33% disulfide bond content, respectively. The mechanism proposed here is similar to the previous example with the exception that the microwave oven thermally stimulated the system to reach the bond dissociation energy of the disulfide bonds. This process takes place quickly as the PU of 33% disulfide bond content could achieve 91% healing efficiency in just 10 min. Advantages of the microwave system over conventional thermal techniques include faster healing times and the ability to induce uniform heating externally and internally of an entire sample. However, the microwave approach is not suitable for biomedical applications, such as artificial skin and/or muscles. Another type of exchange reactions come in the form of vitrimer chemistry, whereby an organic network of covalently bound chains can change its topology through exchange reactions such as transesterification [36]. This produced malleable, reparable, recyclable, and insoluble epoxy networks which could replace conventional nonrecyclable epoxy materials which are destined for landfill or incineration once no longer required [37]. Liu et al. adapted vitrimer chemistry to impart stress relaxation and self-healing abilities, via dynamic transesterification, to a bio-based triepoxy (TEP) [38]. TEP was synthesized and cured with an anhydride monomer in the presence of a zinc catalyst which produced a cured material with a Tg of 187 C. They used stress relaxation tests to confirm that the dynamic transesterification reactions in the cured TEP could be activated at elevated temperature provided there was a zinc catalyst. A temperature of 220 C was used for both self-healing and stress relaxation tests because it was below the thermal degradation temperature and was above the glass transition temperature. To test the self-healing, sample films were scratched to form a surface crack and placed between two metal plates to provide compression stress. Using an optical microscope, the authors monitored the variation of the surface scratches and found that the crack width of the cured TEP decreased by 70% in 5 min. The crack width decreased even further when the test was repeated with an increased catalyst loading. The thermally activated transesterification reactions were responsible for the self-healing ability as the reaction between the hydroxyl groups and ester bonds result in the rearrangement of the cross-linked network (Fig. 4.4). This work provided a method to overcome the difficulty of imparting a self-healing ability into a high Tg material and has a moderately fast healing time of 5 min. The cured TEP would be useful for high temperature applications as a result of its temperature resistant nature. In addition, the material is bio-based and recyclable and therefore has a more limited negative environmental impact compared to nonrecyclable epoxy resins. Light-stimulated self-healing utilizes stable free radical-mediated reshuffling reactions to achieve self-healing (Fig. 4.3C). This method of self-healing is advantageous as it can occur at room temperature, is easy to handle and exposure can be limited to targeted areas if necessary. This would be particularly beneficial for building and construction-type applications in cement and concrete formulations. Amamoto et al. showed that polymer systems based on trithiocarbonate (TTC) were photoresponsive and could undergo repeatable self-healing resulting from dynamic covalent reshuffling of the TTC units [39]. Homolysis of C
S bonds was triggered by ultraviolet (UV) radiation and resulted in exchange reactions catalyzed by radicals. Carbon radicals react with other thiyl radicals in TTC groups by degenerative exchange. Thus the reshuffling reaction of TTC by UV light stimulation showed dynamic

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FIGURE 4.4 Detailing the schematics of the curing reaction between TEP and anhydride, accompanied with the illustration of the transesterification reaction in the cross-linked network structure. TEP, Triepoxy.

covalent properties. This is illustrated in Fig. 4.3C. A polymer cross-linked with TTC units, synthesized by copolymerization of n-butyl acrylate and a TTC cross-linker, showed full recombination after 24 h of UV light irradiation. Another recent example of light-stimulated self-healing via reshuffling reactions comes from a synthesis by Dong et al. where a dibromide terminated polystyrene polymer was cross-linked with poly(4-vinyl pyridine) by reversible addition-fragmentation chain transfer polymerization using a chain transfer agent containing a TTC moiety [40]. Pieces of ionic cross-linked polymer hydrogel were exposed to UV radiation for 1 h. The damages gradually disappeared visually with irradiation time, indicating that the samples had undergone light-stimulated macroscopic self-healing. Amamoto et al. changed the TTC group for thiuram disulfide (TDS) groups for the reshuffling reaction [41]. This allowed visible light to trigger the homolysis of S
S bonds for selfhealing instead of UV light. Two possible mechanisms were proposed for the reshuffling of the TDS units (Fig. 4.5). One possibility was through a radical transfer reaction, where generated S-based radicals react with TDS units to form a new unit, generating another S-based radical in a cascade transfer process. The other possibility was through a radical crossover reaction. The TDS units were incorporated into a low Tg PU and exposed to visible light. After 12 h of light exposure, the healing efficiencies for the samples was 90% for the strain at break. Self-healing was not observed in copolymers with no TDS present. Visible light inducing equipment would be cheaper and easier to use than UV radiating equipment. Visible light is also a safer alternative for biological applications and has a faster healing efficiency. 4.2.2.2 Noncovalent self-healing Inspired by the supramolecular system, noncovalent interactions are also utilized for intrinsic self-healing of polymers. Noncovalent interactions include: (1) hydrogen bonding, (2) ionic interactions, (3) π
π stacking interactions, (4) hydrophobic interactions, and (5) host
guest interactions. Hydrogen bonding is the most commonly used strategy where a hydrogen atom is connected to a strongly electronegative atom and is in the vicinity of another strongly electronegative atom with alone pair of electrons, creating a dipole
dipole attractive force [42]. Sufficient chain mobility in a damaged polymer can bring the dipoles together. Self-healing can take place at room temperature over short

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FIGURE 4.5 Showing the proposed mechanisms of reshuffling TDS units via (A) radical transfer and (B) radical crossover reactions. TDS, Thiuram disulfide.

timescale due to the formation of hydrogen bonds as soon as the dipoles come into near contact [43]. Increasing the number of dipoles present in a system generally improves the self-healing. Zhou et al. developed a polyelectrolyte using a ureidopyrimidinone (UPy) containing monomer with poly(ethylene glycol) methyl ether methacrylate for use with high-performance lithium-ion batteries [44]. UPy units form dimers through four hydrogen bonding groups to introduce self-healing in the polymer (Figs. 4.5 and 4.6). A self-healing ionic liquid gel was derived from poly(vinyl alcohol) (PVA), poly(acrylic acid), and poly(acrylamide) on the basis of dynamic noncovalent hydrogen bonds and ionic bonds [45]. Intramolecular and intermolecular hydrogen bonding served as the reversible cross-link that broke and reformed at deformation with energy change. The ion gel, 1-butyl-3-methylimidazolium hydrogen sulfate (BmimHSO4) was kept in air at room temperature for 12 h with no other external stimuli and demonstrated a 90% strain recovery. The self-healing performance of the BmimHSO4 ion gels was attributed to the balance between the ionic coordination interactions and hydrogen bonds. The ionic bonds between Bmim 1 of the ionic liquid and OH of PVA served as special dynamic junction points whereas the triple network system had an abundancy of hydrogen bonding. The work provided a new method which used ionic liquids instead of water to prepare gels with high mechanical strength and self-healing ability, suitable for high-loading applications. Supramolecular interactions such as π
π stacking and metal
ligand coordination bonds can form dynamic bonds that are selective and undergo reversible breaking and reformation [42]. Mei et al. developed a self-healing polymer based on a combination of platinum
platinum (Pt. . .Pt) and π
π interactions [46]. A cyclometalated platinum(II) complex

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FIGURE 4.6 Showing schematic illustrations of (A) the synthesis of the copolymer and (B) the formation of the self-healing polyelectrolyte and the self-healing mechanism between the cut surfaces. Source: Reproduced with permission from B. Zhou et al., A flexible, self-healing and highly stretchable polymer electrolyte via quadruple hydrogen bonding for lithium-ion batteries, J. Mater. Chem. A 6(25) (2018) 11725
11733., Copyright 2018, Royal Society of Chemistry.

(Pt
L) (L 5 6-phenyl-2,20 -bipyridyl chloride) was incorporated into a polydimethylsiloxane (PDMS) backbone. The molecular interactions between platinum complexes were strong enough to cross-link linear PDMS chains into an elastic film. Without external stimuli, a damaged film could recover its tensile strength after 12 h at room temperature. Interestingly this remained true even after a film was kept separate for 24 h prior to contact, showing that the material was not sensitive to surface aging unlike materials which self-heal via hydrogen bonding. The combination of Pt. . .Pt and π
π interactions in PDMS
Pt
L were strong enough to sustain stretching up to 20 times the original length, unlike PDMS-L which was mainly formed of hydrogen bonds and absent of Pt complexation. These weaker interactions still allowed self-healing at room temperature to take place. These kind of materials could be applied in the fields such as protective coatings, sealing agents, electronic skins, and artificial muscles. Hydrophobic interactions are especially suitable to form hydrogels with intrinsic selfhealing ability. These hydrogels are based on water-soluble polymers possessing insoluble end groups, side chains, or monomers to form physical cross-links [47]. Hydrophobic interactions differ from all other noncovalent liquid phase interactions as they do not require direct attractive intermolecular interactions between the interacting species, but are rather driven by water molecule tendencies to retain their own water
water hydrogen bonds, causing nonpolar entities to arrange to minimize the surface area contact of themselves with water [48]. Xia et al. took advantage of this concept to design a selfhealing hyperbranched amphiphilic polymer in a low-pH aqueous environment, possessing many hydrophilic and hydrophobic end groups [49]. They synthesized a bulk hyperbranched polyurethane (HBPU) containing hydrophilic mercaptosuccinic acid (MSA) and

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FIGURE 4.7 Illustrating the image of HBPU
MSA
ABS. HBPU
MSA
ABS, Hyperbranched polyurethane
mercaptosuccinic acid
acrylonitrile
butadiene
styrene copolymer. Source: Reproduced with permission from N.N. Xia, et al., Self-healing of polymer in acidic water toward strength restoration through the synergistic effect of hydrophilic and hydrophobic interactions, ACS Appl. Mater. Interfaces 9(42) (2017) 37300
37309., Copyright 2017, American Chemical Society.

hydrophobic acrylonitrile
butadiene
styrene copolymer (ABS) as the end groups (Fig. 4.7), [39]. The ABS and MSA segment interactions create reversible cross-linkages, contributing to the strength and self-healing of the polymer and the eventual rearrangement of the hyperbranched polymer at the cracked interface result in rebinding of the cracks (Fig. 4.8). The split samples were left at room temperature for 24 h in an acidic environment. HBPU
MSA
ABS self-healed with a healing efficiency of 87% with respect to the elongation at break values and boasted tensile strengths and strains of about 2 MPa and 2000%, respectively. These are indicative of elastomeric behavior in water and makes it suitable for applications such as sealing elements, flexible hose pipes, and lining. Furthermore, the healing efficiency decreased with increasing pH of the healing environment. This is because MSA increases hydrogen bonding through protonation under acidic conditions. Previous works by these authors presented polymers operating in neutral and alkali water [50,51] for self-healing in aqueous media across the entire range of pH values. Host
guest interactions are based on selective inclusion complexations between macrocyclic hosts, such as cyclodextrins and smaller guest molecules [52] and utilize noncovalent interactions including van der Waals forces, charge-transfer interactions, ion
dipole interactions, and hydrophobic interactions among others [53]. Self-healing hydrogels utilizing inclusion complexations can be designed by mixing host and guest-equipped polymers or by copolymerizing vinyl group-bearing preformed host
guest inclusion complexes with comonomers [54,55]. Deng et al. designed multifunctional hydrogels with good mechanical properties and self-healing ability by copolymerizing acryloyl-β-cyclodextrin

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FIGURE 4.8 Detailing the proposed self-healing mechanism of HPBU
MAS
ABS in acidic water. HBPU
MSA
ABS, Hyperbranched polyurethane
mercaptosuccinic acid
acrylonitrile
butadiene
styrene copolymer. Source: Reproduced with permission from N.N. Xia, et al., Self-healing of polymer in acidic water toward strength restoration through the synergistic effect of hydrophilic and hydrophobic interactions, ACS Appl. Mater. Interfaces 9(42) (2017) 37300
37309., Copyright 2017, American Chemical Society.

(AC-β-CD), N-isopropylacrylamide (NIPAM), and [poly-(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) (PF127)]/carbon nanotubes (CNTs) [56]. The CNTs acted as a physical cross-linker between AC-β-CD and NIPAM. The obtained hydrogels exhibited high conductivity, self-healing flexibility, and elastic mechanical properties with rapid stimuli
responsive properties to both temperature and near-infrared light. During selfhealing, the boundaries between hydrogel pieces became unobservable (Fig. 4.9). The authors proved that the host
guest interactions were the dominant driving force for selfhealing and the hydrogen bonding between β-CD and NIPAM were weak. On addition of a competitive reagent adamantanamine, the host
guest interaction between β-CD and NIPAM was interrupted. The cut surface of the hydrogels could not heal, demonstrating that the host
guest interaction was vital for self-healing.

4.3 Shape memory-assisted self-healing 4.3.1 Thermoplastic polymers In SMASH polymer systems, the SME can autonomously bring the crack surfaces back together, therefore promote the molecular diffusion and interaction upon external stimuli.

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FIGURE 4.9 Illustrating the host
guest interactions of AC
β
CD and NIPAM. AC-β-CD, Acryloyl-β-cyclodextrin; NIPAM, N-isopropylacrylamide. Source: Reproduced with permission from Z. Deng, et al., Multifunctional stimuli
responsive hydrogels with self-healing, high conductivity, and rapid recovery through host
guest interactions, Chem. Mater. 30(5) (2018) 1729
1742., Copyright 2018, American Chemical Society.

The reversible elasticity of the shape-memory effect is the basis of its self-healing [57], and thus thermoplastic polymers are relatively commonplace to form SMASH polymer systems. For example, PCL contains reactive ester groups in the polymer backbone, which are often modified with functional groups for self-healing. PCL is a typical semicrystalline linear polymer with a Tg of 260 C and a Tm of 60 C, and the Tm can be used as a heatrelated shape memory “switch.” To introduce SME into PCL, it is common to partially cross-link PCL, or blend it with a miscible polymer with good elastic recovery properties. In the case of linear PCL (l-PCL)/cross-linked PCL (n-PCL) blends [58,59] (Fig. 4.10A), the physical state of the blends can be tuned by the ratios of the two polymer components. n-PCL was used for elastic recovery and l-PCL provided the plastic deformation and transition from plasticity to elasticity. The temporary shape was achieved at room temperature and fixed below the crystalline temperature (Tc), while shape recovery was triggered above Tm. The Rf increased from 74.1% to 80.9% as l-PCL content increased from 20% to 70%, whereas Rr decreased from 92.7% to 69.1% as l-PCL content increased from 50% to 80%. During thermal healing, the diffusion and entanglement of the polymer chains was achieved by the l-PCL chains. This demonstrates that reversible plasticity shape memory [60] was achieved for the semicrystalline thermoset/thermoplastic SMASH systems (Fig. 4.10B). When l-PCL content was above 25 wt.%, the prepared films nearly achieved full healing (Fig. 4.10C). This was because a higher l-PCL content led to a higher energy dissipation under stretching, and the n-PCL network remained intact. At lower l-PCL content, some of the n-PCL network was broken resulting in poor SMASH [22]. By taking the l-PCL network as a viscoelastic model and the n-PCL network as a viscoplastic model, a thermodynamic model based on the viscoelastic properties of the polymer and the monomer diffusion which drove self-healing was developed by Mao et al. [22]. The theoretical model and experimental results confirmed the self-healing process as: (1) stretching the polymers to a state with a cracked surface at room temperature, (2) unload the polymer and the cracked surface is fixed, (3) reheat the polymer to assist the recovery of the deformation, and (4) the cracked surface is self-healed after a period of time. Shape memory PCL was also used to improve the self-healing performance of other polymers. Cyclic and linear PCL with two hydroxyl groups present on each chain-end were introduced into cross-linked PU as the soft segment [9]. Due to the incorporated dynamic covalent bonds between furan and maleimide (as seen in reversible DA reactions), the two type of PCL-derived PUs showed good self-healing properties. However, cyclic PCL introduced more dense networks to PU than linear PCL due to topology effects

Self-Healing Polymer-Based Systems

FIGURE 4.10 (A) l-PCL/n-PCL blends, (B) reversible plasticity shape memory cycle of l-PCL0/n-PCL100 (weight ratio, black) and l-PCL50/ n-PCL50 (blue) compositions where each sample was stretched to a strain of 200% at room temperature and recovered at 80 C. Strain versus temperature curve (red) and stress versus strain curve (green) are also shown for the l-PCL50/n-PCL50 composition; and (C) dependence of self-healing efficiency on l-PCL wt.% [58]. l-PCL, Linear PCL; PCL, polycaprolactone. Source: Copyright 2011, American Chemical Society.

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(Fig. 4.11A and B). Therefore the PUs made from cyclic PCL had a higher Rf (B95%) than those from linear PCL (B80%), and lower Rr (B70%) than those from linear PCL (B80%). The Rf of PCL-based PU was also related to the cross-linking of PU (the hard segment content), with an Rf value of 99.5% for the PU without PCL (Fig. 4.11C and D). Similarly when PCL was composed with poly(vinyl acetate) [61], the composites exhibited both selfhealing and shape memory functionalities. PU is another widely reported SMASH polymer. Modification is required to introduce reversible bonds into the polymer backbone to achieve intrinsic self-healing ability. Wan and Chen [62] reported a waterborne PU showing a significant SMASH effect triggered by a chain exchange reaction of disulfide bonds, as shown in Fig. 4.12. The healing efficiency of the tensile strength of the scratched PU film was up to 90.5% after 10 min at 65 C. A proposal of a two-step SMASH PU based on a combination of reversible disulfide bonds and SME is shown in Fig. 4.13 [2]. The crystalline soft segment could be used to control the active phase below Tg, leading to a higher Rf and a greater entropic deformation for the SME. From tensile testing, the SME could promote healing efficiency and reduce healing time significantly. Meanwhile, a longer polymer chain length in the soft segment led to higher chain mobility and a lower modulus, thus leading to a high healing efficiency of 97% after 1 h at 80 C. To test an experimental SMASH device, PU with PCL soft segments was used for a stretchable SMASH electrode [63]. The recovery ratio and healing efficiency were related to the prestretching percentage of the electrode and the recovery ratio decreased as the prestretching percentage increased above 20%, indicating that irreversible plastic deformation, permanent damage to the conductive silver network and localized movement of the silver nanowires occurred. However, a blue shift of the dominant peak in Raman spectra showed that more strain energy was stored in the polymer when a greater prestretching was applied. The healing efficiency for conductivity increased from 13.4% to 60% as the prestretching percentage varied from 0% to 20%. Fan et al. designed a two-way shape memory system from a polymer blend of PU/SBS for repeatable self-healing of large cracks [12]. In this SMASH system, the PU/SBS blend became a two-way SMP system due to the synchronous fission/radical recombination of C
ON bonds introduced into the main chains of PU. In the presence of multiwalled CNT, the system demonstrated a self-regulating Joule heating under an external electrical field and was able to undergo autonomic healing. Similarly a multiple healing effect was achieved by Joule heating in SMASH bismaleimide tetrafuran/carbon fiber composites [64], and large damage could be healed without external pressure with a healing efficiency of 90%. A dual response SMASH liquid crystalline polyester was prepared by combining the photoresponsive physical cross-linking from π
π interactions of azobenzene groups and the chemical cross-linking from propanetricarboxylic acid. The polyester exhibited an Rf  98% and Rr  98.6% and was dependant on the physical and chemical cross-linking present. Chemical cross-linking weakened the physical cross-linking and influenced the photoresponsive performance. With the presence of a small amount of chemical crosslinking, the polyester showed a tensile strength healing efficiency of 71.6% and a strain at break healing efficiency of 57.0%. Polyolefins are a typical thermoplastic polymer without functional groups in the skeleton and usually possess good SME because of great chain motion ability and a clear

Self-Healing Polymer-Based Systems

FIGURE 4.11 Schematic representation of the network difference between cyclic and linear PCL-derived PUs: (A) PU based on l-PCL-2OH; (B) PU based on c-PCL-2OH; (C) one-way shape-memory cycle for cyclic-50%; (D) shape memory experiment for cyclic-50% and linear-50% showing five cycles [9]. l-PCL, Linear PCL; PCL, polycaprolactone; PU, polyurethane. Source: Copyright 2016, Royal Society of Chemistry.

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FIGURE 4.12 The schematic of self-healing for the synthesized waterborne polyurethane films [62]. Source: Copyright 2016, Springer Nature.

FIGURE 4.13

Shape memory-assisted self-healing process based on disulfide bond and shape memory of PU [2]. PU, Polyurethane. Source: Copyright 2018, Springer Nature.

viscoelastic transition. Therefore polyolefins require the introduction of reversible reactive groups to contribute to the self-healing. For example, reversible ionic cross-linking bonds were introduced into polypropylene (PP) by grafting of maleic anhydride followed by melt compounding with zinc oxide. The polymer had high Rf (B80%) and Rr values (B100%) at a strain of 400% under heat-programming conditions (160 C). After selfhealing, the notched PP was fully repaired and recovered its Young’s modulus.

4.3.2 Elastomers Elastomers possess a high molecular weight, complicated chain entanglement, and great chain mobility. Different from thermoplastic polymers, with an obvious Tm as the “switch” and plastic deformation, elastomers usually have excellent elastic characteristics and typically a two-phase structure (hard segment and soft segment, or cross-linked phase and uncross-linked phase), contributing to a strong recovery ability. They behave similarly to

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the chemoresponsive SMPs illustrated by Huang et al. [65]. Originally the elastic and transition segments are tangled together. On heating, the transition segment becomes soft, and deformation resistance is mainly from the elastic segment. After subsequent cooling, the transition segment regains its stiffness and, thus effectively freezes the mobility of the elastic segment. The elastic energy in the deformed elastic segment is stored, while the deformed shape is largely maintained. When it is heated again, the softening of the transition segment releases the constraint on the elastic segment and promotes the recovery of the material back to its original shape. This completes a typical full cycle of the heatresponsive shape memory behavior. Based on this mechanism, a simple approach is to blend elastomers with thermoplastic polymers to possibly realize the SMASH, such as PDMS/ethylene vinyl acetate copolymer (EVA) [66,67]. The blends (namely, silicone/melting glue hybrid) had a more elastic behavior than PDMS, while EVA was harder and appeared to be elastic
plastic in nature with some large residual strain after unloading. The blends showed a shape memory ability and became softer and more elastic with the increase in EVA content. With increasing EVA content from 0% to 40%, the Rf value was increased from 0% to 81% and the Rr value (100%) was unchanged. Here, EVA acted as both the transition and the thermoresponsive components. With assisted Joule heating of a NiTi shape memory alloy spring, the bonding stress from the adhesion feature of EVA led to a strength healing efficiency of 80%. 3D-printed semiinterpenetrating polymer network (semi-IPN) elastomer composites formed by urethane diacrylate and linear semicrystalline PCL exhibited high-strain shape memory and SMASH (Fig. 4.14). The healing efficiency of the fracture strain after treated at 80 C for 20 min was lower than 30%, however, the fracture energy increased from 0.65 MJ/m3 (notched sample) to 1.32 MJ/m3 (healed sample) after healing. The healing ability of the semi-IPN systems was attributed to the diffusion/entanglement of PCL chains and the hydrogen-bond between urethane. The 3D-printed elastomer has the potential in applications of soft robotics, flexible electronics, and biomedical devices [68].

4.3.3 Thermoset polymers Reported SMASH thermoset polymers are focused on epoxy resins, PUs, and some polyesters. Owing to their structural versatility, epoxy-based polymers (epoxy resins, epoxy composites, and epoxy coatings) have gained great attention both in engineering and scientific fields. The progress in shape memory epoxy [14] and self-healing epoxy coatings [69] have been successively overviewed recently. Different from thermoplastic polymers or elastomers (unconstrained chain motion), thermoset polymers mostly have very high cross-linking density and the molecular chains are severely constrained. Thus there is not enough strain deformation and elastic recovery in the thermosets to realize shape memory, so the self-healing process is restricted. Most examples of SMASH thermosets include blending of shape memory thermosetting polymers or shape memory component with small amounts of thermoplastics or healing agents, where the former forces the crack to close via SME, and the latter repairs the damage by diffusion, entanglements and/or reversible reactions, or interactions. For example, using a thermoresponsive epoxy as the polymer matrix and carnauba wax microparticles

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FIGURE 4.14

(A) Direct ink write-based 3D printer equipped with heating elements prints each layer of the filament followed by shining UV light (50 mW cm22) to cure the resin. (B) Structure evolution of the semi-IPN elastomer during printing at 70 C and cooling down after printing [68]. IPN, Interpenetrating polymer network; UV, ultraviolet.

as the healing agent, a SMASH composite coating [70] exhibited a triggerable two-step healing mechanism, consisting of shape memory driven crack closure from B40 to B5 μm at 65 C and subsequent full crack sealing by molten wax at 90 C. Thermoset PUs are also commonly studied for SMASH. The semi-crystallized PCL chains are applied as the shape memory switching segments and thermoreversible DA covalent bonds are used to promote the self-healing. Furan-based thermoset PUs [71] exhibited shape memory favored crack closure at above Tm of PCL, and the healing was completed by the regenerated free furan/maleimide functional groups (Fig. 4.15). Nguyen et al. [72] developed SMASH urethane 2 thiourethane networks using both bismaleimidic and bisfuranic PCL chains performed shape memory using a Tm temperature switch while bis-/trismaleimidic and trisfuranic monomers displayed a Tg switch for initiating shape recovery behavior. The healing efficiency was strongly dependent on the shape recovery ability and the mechanical properties could be recovered 70% 2 80%. Additionally a crosslinked bio-based thermoset polyester was produced by Gazzotti et al. [73] combining a “one-pot” approach and 1,3-ioxolan-4-one chemistry. The polyester had polylactide-like chains connected with aromatic 2 aliphatic segments. Free hydroxyl and carboxyl groups provided SMASH capability through transesterification exchange reactions at high temperatures.

4.3.4 Extrinsic shape memory-assisted self-healing The addition of an extrinsic shape memory component can also assistant self-healing. In thermoplastic PU, a near-infrared light responsive shape memory polydopamine was

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FIGURE 4.15

Schematic depiction of the Diels 2 Alder-based shape memory-assisted selfhealing process in a polyurethane material based on PCL and furan 2 maleimide chemistry [71]. PCL, Polycaprolactone. Source: Copyright 2014, American Chemical Society.

introduced containing internal DA bonds [74]. Due to the excellent photothermal effect of polydopamine, the SMASH can be achieved at a very low polydopamine content (0.01 wt. %) in a very short time (60 s), and the healing efficiency was up to 96% with 1 wt.% polydopamine. The polydopamine showed high Rf and Rr value of 99% at a given strain of 50%. In addition, to assisting crack closure, the SME played an important role in improving the self-healing efficiency, with B96% and B76% observed for stretched and unstretched samples, respectively. Inspired by vascular networks of tissue, a hygroscopic poly(ester amide) elastomer was investigated as a SMASH system by using water as the stimulus to induce the glassy 2 rubbery transition [75]. By stacking multiple elastomer films, an elastomer microfluidic network was produced with a reduced diffusion length, thereby accelerating the actuation of the stimuli 2 responsive system. The recovery times were 4.2 and 8.0 h in the perfused and nonperfused cases, respectively. In this system, the recovery (Rr 5 91%) was not complete, attributed to dimensional swelling. A 3D-printed elastomer composite was prepared by dissolving PCL into a acrylate elastomer to form a semi-IPN [68]. The elastomer provided the recovery stress and the PCL crystals enabled the fixing of the temporary shape. During strain-controlled testing (200% stretching strain), the Rf was measured to be 99% and Rr was 90%. The healing efficiency of the fracture strain was relatively low (,30%), but the fracture energy was increased by 100% after healing. The healing ability of the system was proposed to be mainly contributed by the PCL entanglement instead of the hydrogen bonding from the urethane groups. In the case of thermosetting epoxy systems, the introduction of shape memory fibers and healing agents has shown complete healing of epoxy-based coatings [69]. In addition, to a semi-IPNs structure using a thermoplastic polymer introduces switchable shape memory and healing effect while the cross-linked thermoset contributes to fixing of the permanent shape. Embedding shape memory alloy wires into self-healing epoxy with manually injected or microencapsulated healing agent [76,77] gave a maximum self-healing

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FIGURE 4.16 Shape memory-assisted self-healing polymer systems with additional shape memory fibers [78]. Source: Copyright 2013, Elsevier.

efficiency of over 80% (determined by interlaminar fracture toughness). Shape memory PU fibers were strain hardened by a cold-drawn process followed by acrylic conformal coating [78]. The fibers possessed Rf of about 66% and recovery stress of about 25 MPa. The PU fibers were added into the epoxy matrix to act as the shape memory component and the linear PCL was added as the thermoplastic healing agent. As shown in Fig. 4.16, complete fracture of the specimen caused some fibers to pull out, and the fibers bridging over the crack were not fractured due to the high ductility. These enabled the closure of the crack to microscale distances at 80 C. Simultaneously the thermoplastic healing agent was able to flow and diffused in the narrow crack. After cooling to room temperature, the thermoplastic film was hardened and completed the crack healing. Similarly when electrospun PCL fibers were applied in shape memory epoxy matrix [79], the SME brought the cracks back and the melting PCL repaired the cracks at a temperature above Tg.

4.4 Applications SMASH is a method of prestoring and controlled releasing of strain energy to improve the self-healing process [63]. Intrinsic self-healing of polymers are designed with inherent abilities to repair the material with no healing agents or catalysts. Applications of shape memory and self-healing polymers are separately studied and discussed in number of review papers. An example of the real world application of shape memory technology is in heat-shrinkable products. Heat-shrinkable shape memory products are extensively used in industries for electrical insulation, environmental protection, strain relief, sealing, mechanical protection, wire bundling, identification, repair, and splicing. Mechanical and functional properties of composites with shape memory characteristics can be used to develop smart devices such power generators [61,80] and also used in

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space applications as light actuates, space saving structural parts, hinges and expandable or deployable structures such as solar arrays, reflector antennas, and morphing wings. Applications of SMPs in biomedical fields include the repair of cardiac valves and tissues, clot removal in endovascular stroke treatment and biodegradable SMP vascular stents to replace metal stents. Other potential applications of SMPs are smart drug delivery systems, biomedical sensors, and biomimetic actuators [81]. Surface coatings are commonly used for corrosion protection. However, a small crack on the coating exposes the metal surface to aggressive environments and starts to corrode with disastrous effects. One advantage of a self-healing coating is the reduction in the repair required from corrosion cause by mechanical damage. The self-healing capability of the coating allow improvements in the product lifetime by closing defects and thereby stopping the corrosion attack. Examples of commercially available self-healing surface coating products are “Scratch Shield,” a self-healing clear coating material developed by Nissan Motor Co. Ltd. and two component PU clear coatings from Bayer Materials Science [82]. These self-healing coating materials contain high-elastic resin or dense polymer networks with flexible linkage and the efficiency of healing depends on the surrounding temperature or the influence of heat from the sun [83]. Fiber-reinforced plastic composites (FRP) are widely used for aircraft fuselages and aerostructures. Latest civil aircrafts such as the Boeing 787 and Airbus A350 uses FRP composites for weight saving. Applications of self-healing FRP composites in aviation industries are currently being demonstrated as a promising solution to recover cracking or damages in composite structures [84]. High-performance polymers with improved physical characteristics such as thermal stability, fire resistance, and fluid resistance are commonly used in aerospace applications. The use of self-healing capability on highperformance wiring and insulation systems in aerospace applications are currently being studied at NASA Kennedy space center [85]. SMASH mechanisms of polymer systems can have higher healing efficiency and the healing can be achieved faster compared to other mechanisms that require external forces or stimuli [86]. One of the demonstrated technologies using SMASH mechanisms features the dispersion of shape memory fibers in a polymeric matrix and these fibers pull the crack surface closer when activated by an external stimulus. The position of these fibers are important for healing efficiency. Another SMASH mechanism is a phase separated morphology with fibers randomly dispersed in a shape memory matrix. The self-healing is achieved by heating the damaged coating closer to the melting point of the fiber and the glass transition temperature of the matrix. The shape recovery of the matrix brings the crack surface closer and the thermoplastic fibers melt and rebond the crack [79]. Compared to self-healing in polymers, SMASH strategies can heal larger cracks or voids. Extensive research was carried out and a large number of papers have been published in the last decade related to SMASH process in polymers. Producing commercialized SMASH products for real world applications are still limited. This is because the challenges associated with maintaining a good balance of self-healing property and the functional requirements of the end product under different conditions such as temperature, pressure, humidity, light, and in a vacuum. Current research activities should overcome these challenges, and in the future we can expect to see more real life applications of SMASH polymers.

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4.5 Conclusions SMPs have become a recognized facet in everyday life since their first market introduction in the 1960s. Alongside this, the idea of self-healing polymers introduces opportunities to reuse and repair devices merely by pressing them together and the application of an external stimulus. However, the combination of shape memory and self-healing to form SMASH polymers introduces the potential for a material to repair itself without any force required and has so far introduced materials which respond to heat, light, electromagnetic fields, and changes in the chemical environment. Shape memory and self-healing are observed in elastomers, thermoplastic and thermoset polymers and introduced either intrinsically or extrinsically. Intrinsic shape memory is introduced through the physical or chemical cross-linking of polymer chains, making shape memory not directional-dependent. Extrinsic shape memory is introduced into polymers through the addition of additives in the form of fibers and wires. The downside to this approach is that shape memory is directional-dependent and can only take place in alignment direction of the additives. Self-healing can be introduced into polymers via covalent and noncovalent dynamic bonds. SMASH polymers have demonstrated both excellent self-healing and shape memory, with shape fixity ratios, shape recovery ratios and self-healing efficiencies documented at over 90% and are achieved in either a matter of minutes or hours. This has led to many exciting applications for these polymers including repairing cardiac valves and the removal of blood clots. The potential for utilization in self-healing anticorrosive coatings could revolutionize the lifetimes of products, from tinned goods to the protection of vulnerable components in industrial applications. However, the commercial use of SMASH polymers is currently limited due to challenges in the maintenance of self-healing efficiency versus the environments in which the polymer must be able to operate. Further work to address this will likely see an increase in the use of SMASH polymers in more demanding environments.

References [1] R.P. Wool, K.M. O’Connor, A theory crack healing in polymers, J. Appl. Phys. 52 (10) (1981) 5953
5963. [2] Y. Xu, D. Chen, Shape memory-assisted self-healing polyurethane inspired by a suture technique, J. Mater. Sci. 53 (14) (2018) 10582
10592. [3] G. Li, H. Meng, J. Hu, Healable thermoset polymer composite embedded with stimuli-responsive fibres, J. R. Soc. Interface 9 (77) (2012) 3279
3287. [4] V. Srivastava, M. Gupta, Approach to self healing in metal matrix composites: a review, Mater. Today: Proc. 5 (9) (2018) 19703
19713. Part 3. [5] J. Hu, et al., Recent advances in shape
memory polymers: structure, mechanism, functionality, modeling and applications, Prog. Polym. Sci. 37 (12) (2012) 1720
1763. [6] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (17) (2013) 7446
7467. [7] Q. Zhao, H.J. Qi, T. Xie, Recent progress in shape memory polymer: new behavior, enabling materials, and mechanistic understanding, Prog. Polym. Sci. 49
50 (2015) 79
120. [8] X.L. Wu, et al., Two-step shape recovery in heating-responsive shape memory polytetrafluoroethylene and its thermally assisted self-healing, Smart Mater. Struct. 22 (12) (2013) 1
12. [9] W. Chen, et al., Shape-memory and self-healing polyurethanes based on cyclic poly(E-caprolactone), Polym. Chem. 7 (44) (2016) 6789
6797.

Self-Healing Polymer-Based Systems

References

119

[10] T. Xie, Tunable polymer multi-shape memory effect, Nature 464 (2010) 267. [11] J. Kunzelman, et al., Shape memory polymers with built-in threshold temperature sensors, J. Mater. Chem. 18 (10) (2008) 1082
1086. [12] L.F. Fan, et al., Repeated intrinsic self-healing of wider cracks in polymer via dynamic reversible covalent bonding molecularly combined with a two-way shape memory effect, ACS Appl. Mater. Interfaces 10 (44) (2018) 38538
38546. [13] C. Liu, H. Qin, P. Mather, Review of progress in shape-memory polymers, J. Mater. Chem. 17 (2007). [14] J. Karger-Kocsis, S. Ke´ki, Review of progress in shape memory epoxies and their composites, Polymers 10 (1) (2017). [15] H. Meng, G. Li, A review of stimuli-responsive shape memory polymer composites, Polymer 54 (9) (2013) 2199
2221. [16] M. Harper, et al., Various shape memory effects of stimuli-responsive shape memory polymers, Smart Mater. Struct. 22 (9) (2013) 093001. [17] Y. Li, Y. He, Z. Liu, A viscoelastic constitutive model for shape memory polymers based on multiplicative decompositions of the deformation gradient, Int. J. Plast. 91 (2017) 300
317. [18] Y. Liu, et al., Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling, Int. J. Plast. 22 (2) (2006) 279
313. [19] Y. Li, Z. Liu, A novel constitutive model of shape memory polymers combining phase transition and viscoelasticity, Polymer 143 (2018) 298
308. [20] Y. Li, J. Hu, Z. Liu, A constitutive model of shape memory polymers based on glass transition and the concept of frozen strain release rate, Int. J. Solids Struct. 124 (2017) 252
263. [21] H. Tobushi, et al., Thermomechanical constitutive model of shape memory polymer, Mech. Mater. 33 (10) (2001) 545
554. [22] Mao, Y., Yang, M., Hou, S. A thermodynamic analysis of the shape memory assisted self healing polymers, in: 21st International Conference on Composite Materials, Xi’an, China, August 20
25, 2017. [23] S.J. Garcia, H.R. Fischer, Self-healing polymer systems: properties, synthesis and applications, in: M.R. Aguilar, J. San Roma´n (Eds.), Smart Polymers and Their Applications, Woodhead Publishing, 2014, pp. 271
298. [24] M. Zhang, M. Rong, Design and synthesis of self-healing polymers, Sci. China Chem. 55 (5) (2012) 648
676. [25] L. Guadagno, et al., Healing efficiency and dynamic mechanical properties of self-healing epoxy systems, Smart Mater. Struct. 23 (2014) 045001. [26] N.I. Khan, et al., A review on Diels
Alder based self-healing polymer composites, IOP Conf. Series Mater. Sci. Eng. 377 (2018). 012007. [27] T.R. Furlani, J. Gao, Hydrophobic and hydrogen-bonding effects on the rate of Diels 2 Alder reactions in aqueous solution, J. Org. Chem. 61 (16) (1996) 5492
5497. [28] S. Itsuno, Chiral polymer synthesis by means of repeated asymmetric reaction, Prog. Polym. Sci 30 (2005) 540
558. [29] P. Du, et al., Synthesis and characterization of linear self-healing polyurethane based on thermally reversible Diels
Alder reaction, RSC Adv. 3 (35) (2013) 15475
15482. [30] P. Du, et al., Diels
Alder-based crosslinked self-healing polyurethane/urea from polymeric methylene diphenyl diisocyanate, J. Appl. Polym. Sci. 131 (9) (2014). [31] X. Wang, et al., Preparation and properties of self-healing polyether amines based on Diels
Alder reversible covalent bonds, High. Perform. Polym. 31 (1) (2018) 51
62. [32] J. Canadell, H. Goossens, B. Klumperman, Self-healing materials based on disulfide links, Macromolecules 44 (8) (2011) 2536
2541. [33] M. Pepels, et al., Self-healing systems based on disulfide
thiol exchange reactions, Polym. Chem. 4 (18) (2013) 4955
4965. [34] L. Zhang, et al., Self-healing polycaprolactone networks through thermo-induced reversible disulfide bond formation, Macromol. Rapid Commun. 39 (20) (2018) 1800121. [35] L. Ling, et al., Self-healing and shape memory linear polyurethane based on disulfide linkages with excellent mechanical property, Macromol. Res. 26 (4) (2018) 365
373. [36] W. Denissen, J.M. Winne, F.E. Du Prez, Vitrimers: permanent organic networks with glass-like fluidity, Chem. Sci. 7 (1) (2016) 30
38.

Self-Healing Polymer-Based Systems

120

4. Shape memory-assisted self-healing polymer systems

[37] D. Montarnal, et al., Silica-like malleable materials from permanent organic networks, Science 334 (6058) (2011) 965. [38] T. Liu, et al., A self-healable high glass transition temperature bioepoxy material based on vitrimer chemistry, Macromolecules 51 (15) (2018) 5577
5585. [39] Y. Amamoto, et al., Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units, Angew. Chem. Int. Ed. 50 (7) (2011) 1660
1663. [40] P. Dong, et al., Synthesis of new ionic crosslinked polymer hydrogel combining polystyrene and poly(4-vinyl pyridine) and its self-healing through a reshuffling reaction of the trithiocarbonate moiety under irradiation of ultraviolet light, Polym. Int. 67 (7) (2018) 868
873. [41] Y. Amamoto, et al., Self-healing of covalently cross-linked polymers by reshuffling thiuram disulfide moieties in air under visible light, Adv. Mater. 24 (29) (2012) 3975
3980. [42] M. den Brabander, S. Garcı´a, S. van der Zwaag, Routes to make natural rubber heal: a review au - herna´ndez santana, Marianella, Polym. Rev. (2018) 1
25. [43] P. Cordier, et al., Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977. [44] B. Zhou, et al., A flexible, self-healing and highly stretchable polymer electrolyte via quadruple hydrogen bonding for lithium-ion batteries, J. Mater. Chem. A 6 (25) (2018) 11725
11733. [45] M. Zhu, et al., Long-lasting sustainable self-healing ion gel with triple-network by trigger-free dynamic hydrogen bonds and ion bonds, ACS Sustain. Chem. Eng. 6 (12) (2018) 17087
17098. [46] J.-F. Mei, et al., A highly stretchable and autonomous self-healing polymer based on combination of Pt. . .Pt and π
π interactions, Macromol. Rapid Commun. 37 (20) (2016) 1667
1675. [47] L. Voorhaar, R. Hoogenboom, Supramolecular polymer networks: hydrogels and bulk materials, Chem. Soc. Rev. 45 (14) (2016) 4013
4031. [48] S. Otto, J.B.F.N. Engberts, Hydrophobic interactions and chemical reactivity, Org. Biomol. Chem. 1 (16) (2003) 2809
2820. [49] N.N. Xia, et al., Self-healing of polymer in acidic water toward strength restoration through the synergistic effect of hydrophilic and hydrophobic interactions, ACS Appl. Mater. Interfaces 9 (42) (2017) 37300
37309. [50] N.N. Xia, et al., A seawater triggered dynamic coordinate bond and its application for underwater selfhealing and reclaiming of lipophilic polymer, Chem. Sci. 7 (4) (2016) 2736
2742. [51] N.N. Xia, M.Z. Rong, M.Q. Zhang, Stabilization of catechol
boronic ester bonds for underwater self-healing and recycling of lipophilic bulk polymer in wider pH range, J. Mater. Chem. A 4 (37) (2016) 14122
14131. [52] S. Strandman, X. Zhu, Self-healing supramolecular hydrogels based on reversible physical interactions, Gels 2 (2016) 16. [53] S. Liu, W. Gong, X. Yang, Self-healing supramolecular polymers via host-guest interactions, Curr. Org. Chem. 18 (15) (2014) 2010
2015. [54] A. Harada, Y. Takashima, M. Nakahata, Supramolecular polymeric materials via cyclodextrin
guest interactions, Acc. Chem. Res. 47 (7) (2014) 2128
2140. [55] X. Yang, et al., Self-healing polymer materials constructed by macrocycle-based host
guest interactions, Soft Matter 11 (7) (2015) 1242
1252. [56] Z. Deng, et al., Multifunctional stimuli-responsive hydrogels with self-healing, high conductivity, and rapid recovery through host
guest interactions, Chem. Mater. 30 (5) (2018) 1729
1742. [57] X. Xiao, T. Xie, Y.-T. Cheng, Self-healable graphene polymer composites, J. Mater. Chem. 20 (17) (2010) 3508
3514. [58] E.D. Rodriguez, X. Luo, P.T. Mather, Linear/network poly(ε-caprolactone) blends exhibiting shape memory assisted self-healing (SMASH), ACS Appl. Mater. Interfaces 3 (2) (2011) 152
161. [59] Rodriguez, E.D., X. Luo, and P.T. Matherb. Shape memory miscible blends for thermal mending, in: Poceedings of SPIE, San Diego, 2009. [60] X. Zhang, Z. Tang, B. Guo, Reversible plasticity shape memory polymers: key factors and applications, J. Polym. Sci. Part B: Polym. Phys. 54 (14) (2016) 1295
1299. [61] H. Birjandi Nejad, J.M. Robertson, P.T. Mather, Interwoven polymer composites via dual-electrospinning with shape memory and self-healing properties, MRS Commun. 5 (2) (2015) 211
221. [62] T. Wan, D. Chen, Synthesis and properties of self-healing waterborne polyurethanes containing disulfide bonds in the main chain, J. Mater. Sci. 52 (1) (2017) 197
207.

Self-Healing Polymer-Based Systems

References

121

[63] H. Luo, et al., Shape memory-enhanced electrical self-healing of stretchable electrodes, Appl. Sci. (Switz.) 8 (3) (2018) 392
398. [64] J.S. Park, et al., Multiple healing effect of thermally activated self-healing composites based on Diels
Alder reaction, Compos. Sci. Technol. 70 (15) (2010) 2154
2159. [65] W.M. Huang, et al., Response to “Comment on Water-driven programmable polyurethane shape memory polymer: demonstration and mechanism, Appl. Phys. Lett. 97 (2010) 056102. [66] H. Lu, et al., Rubber-like polymeric shape memory hybrids with repeatable heat-assisted, self-healing, and joule heating functions, in: Guoqiang Li, Harper Meng (Eds.), Recent Advances in Smart Self-healing Polymers and Composites, Woodhead Publishing, 2015, pp. 263
292. [67] C.C. Wang, et al., Repeated instant self-healing shape memory composites, J. Mater. Eng. Perform. 21 (12) (2012) 2663
2669. [68] X. Kuang, et al., 3D printing of highly stretchable, shape-memory, and self-healing elastomer toward novel 4D printing, ACS Appl. Mater. Interfaces 10 (8) (2018) 7381
7388. [69] H. Pulikkalparambil, S. Siengchin, J. Parameswaranpillai, Corrosion protective self-healing epoxy resin coatings based on inhibitor and polymeric healing agents encapsulated in organic and inorganic micro and nanocontainers, Nano-Structures Nano-Objects 16 (2018) 381
395. [70] L. Wang, et al., Shape memory composite (SMC) self-healing coatings for corrosion protection, Prog. Org. Coat. 97 (2016) 261
268. [71] G. Rivero, et al., One-pot thermo-remendable shape memory polyurethanes, Macromolecules 47 (6) (2014) 2010
2018. [72] L.T.T. Nguyen, et al., Healable shape memory (thio)urethane thermosets, Polym. Chem. 6 (16) (2015) 3143
3154. [73] S. Gazzotti, et al., One-pot synthesis of sustainable high-performance thermoset by exploiting eugenol functionalized 1,3-dioxolan-4-one, ACS Sustain. Chem. Eng. 6 (11) (2018) 15201
15211. [74] L. Yang, et al., Diels
Alder dynamic crosslinked polyurethane/polydopamine composites with NIR triggered self-healing function, Polym. Chem. 9 (16) (2018) 2166
2172. [75] A. Balasubramanian, R. Morhard, C.J. Bettinger, Shape-memory microfluidics, Adv. Funct. Mater. 23 (38) (2013) 4832
4839. [76] E.L. Kirkby, et al., Embedded shape-memory alloy wires for improved performance of self-healing polymers, Adv. Funct. Mater. 18 (15) (2008) 2253
2260. [77] E.L. Kirkby, et al., Performance of self-healing epoxy with microencapsulated healing agent and shape memory alloy wires, Polymer 50 (23) (2009) 5533
5538. [78] G. Li, P. Zhang, A self-healing particulate composite reinforced with strain hardened short shape memory polymer fibers, Polymer 54 (18) (2013) 5075
5086. [79] X. Luo, P.T. Mather, Shape memory assisted self-healing coating, ACS Macro Lett. 2 (2) (2013) 152
156. [80] W. Xu, et al., Healable and shape-memory dual functional polymers for reliable and multipurpose mechanical energy harvesting devices, J. Mater. Chem. A 7 (27) (2019) 16267
16276. [81] A. Kikuchi, T. Okano, Pulsatile drug release control using hydrogels, Adv. Drug Deliv. Rev. 54 (1) (2002) 53
77. [82] S.K. Ghosh, Self-Healing Materials: Fundamentals, Design Strategies, and Applications, John Wiley & Sons, 2009, pp. 23
25. [83] H. Stoyanov, et al., Dielectric properties and electric breakdown strength of a subpercolative composite of carbon black in thermoplastic copolymer, Appl. Phys. Lett. 94 (23) (2009) 232905. [84] R. Das, C. Melchior, K. Karumbaiah, Self-healing composites for aerospace applications, Advanced Composite Materials for Aerospace Engineering, Elsevier, 2016, pp. 333
364. [85] S. T. Jolley, et al., Developing flexible, high performance polymers with self-healing capabilities. 2011. [86] A. Lutz, et al., A shape-recovery polymer coating for the corrosion protection of metallic surfaces, ACS Appl. Mater. interfaces 7 (1) (2014) 175
183.

Self-Healing Polymer-Based Systems

C H A P T E R

5 Characterization of self-healing polymeric materials Saied Nouri Khorasani1 and Rasoul Esmaeely Neisiany2 1

Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran 2 Department of Materials and Polymer Engineering, Faculty of Engineering, Hakim Sabzevari University, Sabzevar, Iran

5.1 Introduction As described in other chapters, self-healing polymeric materials, inspiring from biological systems in nature, have been attracted much research interest, considering these materials as a promising candidate for a new generation of smart materials. It is expected that these materials offer extended service life, boosted durability and reliability, and significantly reduced the cost of repairs and maintenances [1]. Therefore these materials can be employed with a greater confidence factor even in structural applications such as in the structural elements of the aircraft and spacecraft [1,2]. Efficient self-healing polymers, polymeric composites, and polymeric coatings are crucial to employ in many engineering applications due to their light specific weight, excellent processability, as well as chemical stability [2,3]. The pioneering research on the development of self-healing polymers was introduced by White et al. in 2001 [4]. Their results confirmed that encapsulated dicyclopentadiene (DCPD) could release from ruptured microcapsules and after ring-opening polymerization to poly(dicyclopentadiene), by dispersed Grubbs’ catalyst, autonomously healed the epoxy matrix [4]. They used mechanical, chemical, and morphological testing methods to evaluate the self-healing performance of the fabricated polymeric composite. Subsequently during the past 17 years myriad innovative strategies have been proposed to enable self-healing in materials such as the release of healing agents, reversible cross-links, nanoparticles, and shape memory effect with the first technique proving to be the most well studied [1,3]. Since the actual healing mechanisms and self-healing strategies, as well as recent developments in polymeric materials are presented and explained in detail in the other chapters of this book, they are not considered in the following chapter. The present chapter aims to list and summarizes the most employed testing method for the damage evaluation in the

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00005-2

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polymeric composites and coatings. It should be mentioned that each specific test method is suitable for a specific evaluation. For example, the previous researches confirmed that the scratch-based healing evaluations are useful for self-healing study in the polymeric coatings. In contrast, these types of tests are not suitable for the evaluation of the mechanical properties restoration in the polymeric composites. To determine the self-healing ability of the mechanical properties in brittle polymers, the double cantilever beam and/or tensile tests have been satisfactorily employed. In contrast, for the ductile polymers and elastomers, the evaluation of the fracture behavior has been more recommended to monitor the healing behaviors [5]. This chapter summarizes and lists the characterization techniques of the self-healing performances of the polymeric composites and coatings with both qualitative and quantitative approaches to help a better understanding of the self-healing mechanism, leading to the development of these types of smart materials.

5.2 Methods for evaluating self-healing behavior of the polymeric composites 5.2.1 Qualitative methods 5.2.1.1 Visualization techniques Both types of optical and scanning electron microscopy (SEM) have been widely used for visual-based self-healing investigations. However, the optical microscopy mostly has been employed during the self-healing evaluation of thin films and coating, particularly when the intrinsic approaches were designed [6
8]. For the new generation of SEMs apparatus, field-emission scanning electron microscopy (FE-SEM), the resolution is high enough even on the scale of 1 nm. Therefore the healing phenomena even for small features can be followed by FE-SEM [9]. Furthermore transmission electron microscopy (TEM) can be employed if a higher resolution is required. TEM micrographs provide the investigation at a lower temperature and minimize the heating effects. Fig. 5.1 shows the FE-SEM images of releasing healing agent from ruptured nanofibers in a carbon/epoxy composite. 5.2.1.2 Acoustical microscopy Although the optical and electron microscopy have been widely utilized in the visual investigation of the self-healing process in the previous researches, however, these methods showed major limitations. Polymers generally absorb the photons and electrons of optical and electron microscopy and consequently leading to impossible visualization of occurred damages in the polymeric composites and coatings. On the other hand, the acoustic waves 5
400 MHz are usually transmitted within the polymeric materials, making them suitable for recognition of delamination in the polymeric composites and coatings. This technique has been widely used in medical imaging known as ultrasound. In medical applications, the frequency of a sound wave is rather low to minimize the damages for organs. However, polymers are less sensitive to acoustic waves so the higher frequencies can be applied [9]. Although the technique is capable to monitor the in situ healing phenomena and offers great potential, this technique has been used less for

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FIGURE 5.1 FE-SEM images of released healing agent from ruptured nanofibers (A) poly(styrene-co-acrylonitrile) [10] and (B) poly(methyl methacrylate) [11] shells incorporated in carbon/epoxy composites. Source: Reprinted with permission from Elsevier.

evaluating self-healing in the polymeric materials [12]. This technique was also applied for the assessment of self-healing in the concrete [13]. 5.2.1.3 X-ray microtomography This testing method was used in the efficient penetrating X-rays within the materials to provide 2D absorption contrast images. The technique provides images at different viewing angles and reconstructs the internal 3D structure of the material. Therefore the technique supplies the recognition of internal damages and monitoring the changes during the healing process [9]. A new generation of X-ray apparatus offers a resolution of approximately 2 μm3 and was utilized for polymeric materials with a thickness of about 10 mm. McCombe et al. used X-ray micro-computed tomography (μCT) for characterization and self-healing evaluations in a 16 plies carbon fiber reinforced plastic [14]. The images confirmed that the method is suitable for recognition of delamination and the self-healing process in the laminated composites. 5.2.1.4 Evaluation of self-healing reaction heat The reaction between healing agents is mostly exothermic in nature to provide a negative enthalpy of the self-healing reaction. Therefore the self-healing process can be well evaluated by monitoring the heat of self-healing reaction by differential scanning calorimetry (DSC) [15
17]. The curing studies were performed by DSC, in which the enthalpy changes during a chemical reaction are measured with varying temperatures. The recorded peaks are indicative of the exothermic curing temperature of the healing agents and provide evidence that the healing agent components are available in the system for the curing reaction. Li et al. [15] showed that heat of reaction evaluation of the released healing agent from ruptured capsules fairly confirmed the reaction between the encapsulated epoxy resin and amine-based curing agent after microcapsules rupture. The exothermic peak centered at 125 C is related to the reaction of the healing agent within the epoxy matrix. Research by Neisiany et al. first showed the reaction between the released healing

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agent (epoxy and amine-based curing agent) from ruptured nanofibers by DSC [16] via a series of dynamic tests. They also carried out isothermal DSC tests to determine the healing reaction duration to completion and calculated the fractional conversion between the released healing agents through the time in several temperatures. In another work, Neisiany et al. used dynamic DSC tests to confirm the healing reaction between epoxy resin and amine-based curing agent in a carbon/epoxy composite host (Fig. 5.2) [10]. In another work, Ahangaran and colleagues incorporated microcapsules containing lowviscosity epoxy resin and pentaerythritol tetrakis in an epoxy matrix. They also showed that an exothermic peak at 53 C is attributed to the healing reaction between epoxy and a mercaptan-based curing agent [17]. It can be concluded that DSC test is a useful technique to confirm the occurrence of the healing reaction. Furthermore using this technique can help a better understanding and prediction of the healing reaction kinetics. Knowing the healing reaction kinetics would open a path for self-healing material to be more suitable in the scalable real application.

5.2.2 Quantitative methods 5.2.2.1 Tensile testing Tensile tests have been extensively used for the evaluation of mechanical properties of the materials. The test provides some material’s properties including tensile strength, elongation, and Young’s modulus as a function of strain rate, time, and temperature [18]. The test coupons must be in rectangular or dog-bone shape. This test is mostly carried out according to ASTM D638 and ASTM D3039 for polymeric materials and polymeric composites. The method could also be employed for the evaluation of self-healing efficiency. The pristine and healed samples are assessed under the same testing conditions. The results of

FIGURE 5.2 Dynamic DSC thermograms of (a) carbon/epoxy composite without self-healing system (b) and (c) carbon/ epoxy composites containing core
shell nanofibers immediately after three-point bending tests. Source: Adapted with permission from R.E. Neisiany, J.K.Y. Lee, S.N. Khorasani, S. Ramakrishna, Towards the development of self-healing carbon/epoxy composites with improved potential provided by efficient encapsulation of healing agents in core
shell nanofibers. Polym. Test. 62 (2017), 79
87.

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load
displacement curves have been employed for the healing efficiency determination. The healing efficiency is usually calculated from Eq. (5.1) [11]. %Healing efficiency 5

Property valuehealed 3 100 Property valueintial

(5.1)

where the properties can be tensile strength, tensile modulus, and elongation obtained from tensile properties. Although the tensile testing offers a fast method for quantitative evaluation of the selfhealing process and healing efficiency, this method is not an ideal technique for self-healing investigations. Although this testing method has been used for extrinsic self-healing [19
21], this technique is more suitable for intrinsic self-healing materials [22
25]. The most reported parameter is the maximum load at failure point of the samples. This value is mostly used for calculation of self-healing efficiency. Due to facility and rapidity of this test, this method can be easily utilized for evaluation of self-healing reaction progress during the time at a desired temperature [26
29]. Fig. 5.3 shows the stress
strain curves of original and healed a furan-based polymer with self-healing capability after 1, 5, and 10 days of healing [30]. 5.2.2.2 Bending testing Bending test is another fast technique that can be employed for evaluation of flexural mechanical properties including bending strength, bending strain, and bending modulus for both polymeric materials [18] and polymeric composites [31]. This method is mostly carried out according to ASTM D790 [32,33] for three-point bending tests and ASTM D6272 [34] and ASTM D7264 [35] for four-point bending test. The bending tests have been extensively used for evaluating self-healing in polymeric materials [8] and composites [36
39]. Similar to the tensile test, the healing efficiency can be determined from Eq. (5.1), while the properties are assumed to be bending strength, bending modulus, and bending strain at breakage. In contrast to the tensile method, this technique is more suitable for the evaluation of self-healing performance in extrinsic strategies.

FIGURE 5.3 Stress
strain curves of the original and healed polymer. Source: Reprinted with permission from C. Zeng, H. Seino, J. Ren, K. Hatanaka, N. Yoshie, Bio-based furan polymers with self-healing ability, Macromolecules 46 (2013) 1794
1802.

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5.2.2.3 Tapered double cantilever beam Tapered double cantilever beam (TDCB) has been employed as the most useful method for quantitative determination of self-healing in polymeric materials and composites with a brittle nature. The healing efficiency in this test is also determined from Eq. (5.1), while the property is fracture toughness (KIC) [4,40
43]. However, the previous researches showed that the load
displacement curves are very sensitive to the geometry and the applied load is even for small changes [44]. This test was mostly used for evaluation of self-healing in epoxy-based composites. Fig. 5.4 demonstrates the specimen geometry for TDCB test, and the crack length of conventional and self-healing epoxy versus fatigue cycle [45]. 5.2.2.4 Ballistic impact Self-healing performance in the polymeric materials after impact damages have been attracted attention during these years. This behavior was mostly reported for the soft polymeric materials when the intrinsic self-healing approach was designed for the materials [46
48]. No national or international standard has been reported for this test for the selfhealing evaluations. However, this test was carried out in two modes of bullet penetration and quasistatic mode in a desire control temperature via ballistic impacts. 5.2.2.5 Dynamic mechanical thermal analyses Dynamic mechanical thermal analysis (DMTA) or dynamic mechanical analysis (DMA) was carried out for evaluation of self-healing performance via monitoring the storage modulus and tan δ of the samples before and after damages [49
51]. The flexibility of this testing method provides the assessment of self-healing in the wide range of temperatures, frequencies, strain rates, and mode of applied loads (i.e., tension, compression, and shear). This test can be employed for both polymers and polymeric composites from soft to brittle nature. Furthermore rheological testing also carried out to assess the self-healing of elastomeric and soft polymeric materials [52
54]. The rheological method was mostly reported FIGURE 5.4 Typical dimensions of the TDCB specimen and the crack length of conventional and self-healing epoxy versus fatigue cycle. TDCB, Tapered double cantilever beam. Source: Reprinted with permission from Y.C. Yuan, X.J. Ye, M.Z. Rong, M.Q. Zhang, G.C. Yang, J.Q. Zhao, Self-Healing epoxy composite with heat-resistant healant, ACS Appl. Mater. Interfaces 3 (2011), 4487
4495.

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for evaluation of self-healing in intrinsic self-healing approaches while the DMTA test can be used for both intrinsic and extrinsic self-healing strategies.

5.3 Methods for evaluating self-healing behavior of the polymeric coatings In this section, the testing techniques for evaluation of self-healing ability in the polymeric coatings are listed and discussed.

5.3.1 Qualitative methods 5.3.1.1 Visual inspection and optical microscopy Generally to evaluate the coating performance by the salt spray test, visual inspection is utilized. Numerous previous researches are benefiting salt spray test and visual inspection techniques to determine the self-healing ability of the coatings [55
59]. Salt spray test is a valuable test to assess the self-healing performance of the coatings which is mostly conducted according to ASTM B117. The image of both control and microcapsule incorporated coatings is presented in Fig. 5.5 after exposure to salt spraying for 1 and 40 weeks. It can be observed that the severity of corrosion was considerably less in the self-healing coating in comparison to the control coating. In some cases, employing the optical microscopy has led to higher resolution investigation. Thus various kinds of optical microscopy such as fluorescence microscopy, resolution near-field scanning optical microscopy, and conventional reflection and transmission optical microscopes can be exploited to assess the self-healing behavior [61,62]. 5.3.1.2 Scanning electron microscopy To assess the self-healing capability of the coatings qualitatively, environmental SEM, FE-SEM, SEM, field-emission gun SEM, and other scanning electron microscopes have been used. In the case of materials sensitive to temperature as well as higher resolution, different types of transmission electron microscopes can be employed providing minimal heating side effects [63]. Fig. 5.6 presents the SEM images of control and self-healing coating after scratched and exposure to salt spray [60]. 5.3.1.3 Confocal microscopy Confocal microscopy, which is also known as confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is classified as an optical imaging method. This method provides an increase in optical resolution and contrast using a spatial pinhole for blocking the out-of-focus light during the image formation [64]. The high resolution of this method allows for a better understanding of the self-healing mechanism during the healing process. Therefore this technique has been recently attracted great attention for self-healing evaluations especially for coatings [65
67]. Fig. 5.7 shows using CLSM images of the healing process of an epoxy-based coating before damage and healing [66].

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FIGURE 5.5 Corrosion in the control coating (A), (D) and coating containing microcapsules (B), (C), (E), and (F) after scratching and exposure to salt spray. Source: Reprinted with permission from E. Koh, S. Lee, J. Shin, Y.-W. Kim, Renewable polyurethane microcapsules with isosorbide derivatives for self-healing anticorrosion coatings, Ind. Eng. Chem. Res. 52 (2013), 15541
15548 [60].

5.3.1.4 Scanning electrochemical microscopy This test is established on the reactions in an electrolyte dropped movable polarized ultramicroelectrode. This technique allows for monitoring the electrochemical reactions during the healing process and received great attention during recent years [68
71]. Gonza´lez-Garcı´a et al. utilized the scanning electrochemical microscopy (SECM) to examine the in situ cathodic activity in the two different kinds of the self-healing coatings coated on the aluminum alloy substrate [69]. The obtained results verified that the SECM

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FIGURE 5.6 SEM micrographs of the scratched panels after 40 weeks of scratched and salt spray test (A) control coating, and (B) self-healing coating. SEM, Scanning electron microscopy. Source: Reprinted with permission from E. Koh, S. Lee, J. Shin, Y.-W. Kim, Renewable polyurethane microcapsules with isosorbide derivatives for self-healing anticorrosion coatings, Ind. Eng. Chem. Res. 52 (2013), 15541
15548.

FIGURE 5.7 CLSM images of an epoxy-based coating: (A) the pristine coating, (B) the scratched coating; and (C) the healed coating after artificial scratched. CLSM, Confocal laser scanning microscopy. Source: Adapted with permission from H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, et al., Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (2017), 2355
2364.

is a helpful instrument in order to monitor and evaluate the self-healing events in the coatings [72]. Fig. 5.8 presents the SECM maps of an epoxy coating containing filled microcapsules with hexamethylene diisocyanate trimer as a healing agent. It can be discerned that after 48 h of creation of scratch and immersion in NaCl solution, the coating is completely healed by releasing the healing agent and healing reaction [71]. 5.3.1.5 Scanning vibrating electrode technique The scanning vibrating electrode technique (SVET) offers in situ recording of anodic and cathodic distribution spots at the surface on how the healing process can be investigated via reduction or suppression of the redox activity. Mainly the SVET method has been used for assessment of the self-healing efficiency of the coatings particularly in the

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FIGURE 5.8 SECM maps of an epoxy-based coating containing microcapsules. (A) Scratched coating, and (B) the healed coating after 48 h and immersion in NaCl solution. SECM, Scanning electrochemical microscopy. Source: Reprinted with permission from W. Wang, L. Xu, H. Sun, X. Li, S. Zhao, W. Zhang, Spatial resolution comparison of AC-SECM with SECM and their characterization of self-healing performance of hexamethylene diisocyanate trimer microcapsule coatings, J. Mater. Chem. A 3 (2015), 5599
5607.

case of utilizing corrosion inhibitors [73
75]. Fig. 5.9 illustrates the SVET 3D current density maps of the coating by the sol
gel method containing mesoporous silica loaded with various modified inhibitors. The analysis demonstrated the constant individual positive peak which is attributed to the anodic determined pitting spot. The systems presented high and low anodic current densities, instead of the control coating after 1, 6, and 12 h of dipping in 0.1M NaCl solution [77].

5.3.2 Quantitative methods In the previous section, the quantitative methods for evaluation of self-healing in the polymeric coating were introduced. The characteristic techniques for determination of selfhealing efficiency have been vital. The techniques for quantitative investigation of healing in the coating will be pinpointed in the following sections, while the Eq. (5.1) is used for calculation of healing efficiency in the polymeric coating too. 5.3.2.1 Healing of hydrophobicity In general, the hydrophilicity/hydrophobicity property of the coating’s surface can be accessed via the water contact angle measurements. Hence, the changes in the amount of water contact angle, before damage and after healing, can be used to evaluate the selfhealing efficiency of the coating. There are some parameters such as time, temperature and so on affecting the contact angle measurements which subsequently influence the healing process [78
80]. Researches confirm that the self-healing evaluations via hydrophobicity assessment have been mostly utilized when the dual action of self-healing and superhydrophobicity were desired [66,80].

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FIGURE 5.9 SVET 3D maps of the aluminum coated with the epoxy coating containing several inhibitor systems. (A) control coating (free of inhibitor), (B
D) coatings containing various inhibitors with different amounts of organic content and pore size. SVET, Scanning vibrating electrode technique. Source: Reprinted with permission from Z. Zheng, M. Schenderlein, X. Huang, N.J. Brownbill, F. Blanc, D. Shchukin, Influence of functionalization of nanocontainers on self-healing anticorrosive coatings, ACS Appl. Mater. Interfaces 7 (2015), 22756
22766 [76].

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5.3.2.2 Atomic force microscopy Another helpful technique to evaluate the healing efficiency of the coating is the atomic force microscopy (AFM). This characterization technique has been broadly employed for the examination of the self-healing behavior in the nanoscale [6,65,81]. Investigation of the surface by AFM provides the morphology of the surface during the self-healing process, and the obtained results of surface roughness can be used for calculation of self-healing efficiency. Fig. 5.10 shows the AFM images of acrylic-based polymer with intrinsic selfhealing ability with time. It can be observed that the polymer showed an acceptable ability for self-healing the crack during the time. Furthermore the AFM showed promising potential for monitoring the healing process in thin films and coatings [7]. 5.3.2.3 Tribological properties Tribological surface properties are helpful techniques to evaluate the self-healing capability of the coatings specifically the coefficients of friction [82]. To explain the evaluation of the friction coefficients affords adequate indication in the friction recovery property. Thus various tribometers such as pin on disc are probably utilized before damage and after the healing process to measure the friction coefficients of the coatings [83]. The friction evaluations are especially useful, while both abilities of self-healing and selflubrication are the desire to design for a polymeric-based coating [84]. For this purpose, several researches have been carried out while the vegetable oil-based healing agents were employed as dual functions of self-healing and self-lubricating agents [85,86]. 5.3.2.4 Corrosion assessment tests The substantial reason to employ coatings is the protection function of the metallic substrates from direct contact with environmental conditions, particularly corrosive components providing active or/and passive protection [87
89]. Whenever the protective coating fails, the corrosion may be observed in the coated metallic structure. Hence, all the FIGURE 5.10 AFM images of the self-healing process in an acrylic-based film during the time. AFM, Atomic force microscopy. Source: Reprinted with permission from J.A. Yoon, J. Kamada, K. Koynov, J. Mohin, R. Nicolay¨, Y. Zhang, et al., Self-healing polymer films based on thiol
disulfide exchange reactions and self-healing kinetics measured using atomic force microscopy, Macromolecules 45 (2012), 142
149.

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conventional techniques utilized to assess the corrosion resistance of the coatings can be employed to evaluate the self-healing behavior and the coating’s healing efficiency [90]. Subsequently some techniques are described in the next subsections. 5.3.2.4.1 Potentiostatic and potentiodynamic techniques

Potentiostatic [63,91,92], as well as potentiodynamic [93
95] techniques, have been applied to determine the healing behavior of the designed self-healing systems. To make an analogy between the potentiostatic and the potentiodynamic tests, the potentiostatic test works at a fixed potential whereas the potentiodynamic measures the current density at the open circuit potential (OCP). It should be mentioned that potentiodynamic polarization test results provide the calculation of the healing efficiency quantitatively. Eq. (5.2) offers a formula to calculate the healing efficiency from the results of potentiodynamic polarization test. %IEp 5

ðI Corr0 2 I Corr Þ 3 100 I Corr0

(5.2)

In the above equation, IEp, ICorr0, and ICorr stand for the healing efficiency, the corrosion current density before damage, and corrosion current after healing of the coating, respectively. 5.3.2.4.2 Electrochemical impedance spectroscopy

Local corrosion damages in the coatings are mostly investigated via electrochemical impedance spectroscopy (EIS) tests. This technique assesses the amount of coating impedance (Z) versus the time. The impedance indicates the resistant of a circuit to alternating current (AC) flow when it was dipped in the different electrolyte media [93,94]. When a sinusoidal current is applied, the EIS method offers a low voltage difference, and therefore this technique is considered as a nondestructive technique. The evaluation of healing behavior is prepared by considering the total impedance on the spoiled parts. According to the mentioned points, the EIS test has been widely exploited in numerous self-healing researches [95
98].

5.4 Summary and outlook The current chapter lists and explains several available testing methods to monitor damages and healing processes in the polymeric composites and polymeric coatings in both qualitative and quantitative manner. It should be noted that no specific standard method has been reported for the universal evaluation and characterization of all selfhealing polymeric materials. However, it is clear that in the undamaged material characterization using a single technique will not provide a deep understanding of the healing phenomena. Therefore several testing methods to be employed and combined to yield trustworthy results and to recognize the healing process mechanism. It can be concluded that if the test for undamaged materials carried out according to the standard tests (i.e., ASTM and ISO) the results for healing will be more trustable. Another significant point

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that should be considered is using a suitable test based on the nature of the materials. For example, some testing methods are suitable for soft polymeric materials while some of the other tests are suitable for brittle ones. Therefore due to great attention to the development of self-healing materials, it is strongly suggested to some experts to prepare and publish the relevant standards for characterization of self-healing polymeric materials of both composites and coatings.

References [1] D. Do¨hler, P. Michael, W. Binder, Principles of self-healing polymers, in: W.H. Binder (Ed.), Self-Healing Polymers: From Principles to Applications;, John Wiley & Sons, Inc, Weinheim, 2013, pp. 5
60. [2] K.T. Hafshejani, S.N. Khorasani, M. Jahadi, M.S. Hafshejani, R.E. Neisiany, Improving mechanical and thermal properties of high-density polyethylene/wood flour nanocomposites, J. Therm. Anal. Calorim. 137 (2019) 175
183. [3] R.E. Neisiany, S.N. Khorasani, J.K.Y. Lee, M. Naeimirad, S. Ramakrishna, Interfacial toughening of carbon/ epoxy composite by incorporating styrene acrylonitrile nanofibers, Theor. Appl. Fract. Mech. 95 (2018) 242
247. [4] S.R. White, N. Sottos, P. Geubelle, J. Moore, M.R. Kessler, S. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794
797. [5] S. Bode, M. Enke, M. Hernandez, R.K. Bose, A.M. Grande, S. van der Zwaag, et al., Characterization of selfhealing polymers: from macroscopic healing tests to the molecular mechanism, Self-Healing Materials, Springer, Switzerland, 2015, pp. 113
142. [6] A.B. South, L.A. Lyon, Autonomic self-healing of hydrogel thin films, Angew. Chem. Int. Ed. 49 (2010) 767
771. [7] J.A. Yoon, J. Kamada, K. Koynov, J. Mohin, R. Nicolay¨, Y. Zhang, et al., Self-healing polymer films based on thiol
disulfide exchange reactions and self-healing kinetics measured using atomic force microscopy, Macromolecules 45 (2012) 142
149. [8] N.I. Khan, S. Halder, J. Wang, Diels-Alder based epoxy matrix and interfacial healing of bismaleimide grafted gnp infused hybrid nanocomposites, Polym. Test. 74 (2019) 138
151. [9] U.L. Ranjita, K. Bose, J.M. Vega, S.J. Garcia, S. van der Zwaag, Methods to Monitor and quantify (self-) healing in polymers and polymer systems, in: W.H. Binder (Ed.), Self-Healing Polymers: From Principles to Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2013, pp. 337
360. [10] R.E. Neisiany, J.K.Y. Lee, S.N. Khorasani, S. Ramakrishna, Towards the development of self-healing carbon/ epoxy composites with improved potential provided by efficient encapsulation of healing agents in coreshell nanofibers, Polym. Test. 62 (2017) 79
87. [11] R. Esmaeely Neisiany, J.K.Y. Lee, S. Nouri Khorasani, R. Bagheri, S. Ramakrishna, Facile strategy toward fabrication of highly responsive self-healing carbon/epoxy composites via incorporation of healing agents encapsulated in poly(methylmethacrylate) nanofiber shell, J. Ind. Eng. Chem. 59 (2018) 456
466. [12] J.S.M. Zanjani, B. Saner Okan, C. Yilmaz, Y. Menceloglu, M. Yildiz, Monitoring the interface and bulk selfhealing capability of tri-axial electrospun fibers in glass fiber reinforced epoxy composites, Compos. Part. A Appl. Sci. Manuf. 99 (2017) 221
232. [13] C.-W. In, R.B. Holland, J.-Y. Kim, K.E. Kurtis, L.F. Kahn, L.J. Jacobs, Monitoring and evaluation of selfhealing in concrete using diffuse ultrasound, NDT E Int. 57 (2013) 36
44. [14] G.P. McCombe, J. Rouse, R.S. Trask, P.J. Withers, I.P. Bond, X-ray damage characterisation in self-healing fibre reinforced polymers, Compos. Part. A Appl. Sci. Manuf. 43 (2012) 613
620. [15] Q. Li, N.H. Kim, D. Hui, J.H. Lee, Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy, Compos. Part. B Eng. 55 (2013) 79
85. [16] R.E. Neisiany, S.N. Khorasani, J. Kong Yoong Lee, S. Ramakrishna, Encapsulation of epoxy and amine curing agent in PAN nanofibers by coaxial electrospinning for self-healing purposes, RSC Adv. 6 (2016) 70056
70063.

Self-Healing Polymer-Based Systems

References

137

[17] F. Ahangaran, M. Hayaty, A.H. Navarchian, Y. Pei, F. Picchioni, Development of self-healing epoxy composites via incorporation of microencapsulated epoxy and mercaptan in poly(methyl methacrylate) shell, Polym. Test. 73 (2019) 395
403. [18] M. Naeimirad, A. Zadhoush, R.E. Neisiany, Fabrication and characterization of silicon carbide/epoxy nanocomposite using silicon carbide nanowhisker and nanoparticle reinforcements, J. Compos. Mater. 50 (2016) 435
446. [19] Y. Fang, C.-F. Wang, Z.-H. Zhang, H. Shao, S. Chen, Robust self-healing hydrogels assisted by cross-linked nanofiber networks, Sci. Rep. 3 (2013) 2811. [20] M.W. Lee, S. An, H.S. Jo, S.S. Yoon, A.L. Yarin, Self-healing nanofiber-reinforced polymer composites. 1. tensile testing and recovery of mechanical properties, ACS Appl. Mater. interfaces 7 (2015) 19546
19554. [21] M.W. Lee, S. An, Y.-I. Kim, S.S. Yoon, A.L. Yarin, Self-healing three-dimensional bulk materials based on core
shell nanofibers, Chem. Eng. J. 334 (2018) 1093
1100. [22] F. Maes, D. Montarnal, S. Cantournet, F. Tournilhac, L. Corte´, L. Leibler, Activation and deactivation of selfhealing in supramolecular rubbers, Soft Matter 8 (2012) 1681
1687. [23] X.-H. Wang, F. Song, D. Qian, Y.-D. He, W.-C. Nie, X.-L. Wang, et al., Strong and tough fully physically crosslinked double network hydrogels with tunable mechanics and high self-healing performance, Chem. Eng. J. 349 (2018) 588
594. [24] Q. Zhang, S. Niu, L. Wang, J. Lopez, S. Chen, Y. Cai, et al., An elastic autonomous self-healing capacitive sensor based on a dynamic dual crosslinked chemical system, Adv. Mater. 30 (2018) 1801435. [25] C. Xu, W. Zhan, X. Tang, F. Mo, L. Fu, B. Lin, Self-healing chitosan/vanillin hydrogels based on schiff-base bond/hydrogen bond hybrid linkages, Polym. Test. 66 (2018) 155
163. [26] S. Chen, N. Mahmood, M. Beiner, W.H. Binder, Self-healing materials from V- and H-shaped supramolecular architectures, Angew. Chem. 127 (2015) 10326
10330. [27] N. Garcı´a-Huete, W. Post, J.M. Laza, J.L. Vilas, L.M. Leo´n, S.J. Garcı´a, Effect of the blend ratio on the shape memory and self-healing behaviour of ionomer-polycyclooctene crosslinked polymer blends, Eur. Polym. J. 98 (2018) 154
161. [28] A.C. Schu¨ssele, F. Nu¨bling, Y. Thomann, O. Carstensen, G. Bauer, T. Speck, et al., Self-healing rubbers based on NBR blends with hyperbranched polyethylenimines, Macromol. Mater. Eng. 297 (2012) 411
419. [29] X.Q. Feng, G.Z. Zhang, Q.M. Bai, H.Y. Jiang, B. Xu, H.J. Li, High strength self-healing magnetic elastomers with shape memory effect, Macromol. Mater. Eng. 301 (2016) 125
132. [30] C. Zeng, H. Seino, J. Ren, K. Hatanaka, N. Yoshie, Bio-based furan polymers with self-healing ability, Macromolecules 46 (2013) 1794
1802. [31] A. Sharifi, S.N. Khorasani, S. Borhani, R.E. Neisiany, Alumina reinforced nanofibers used for exceeding improvement in mechanical properties of the laminated carbon/epoxy composite, Theor. Appl. Fract. Mech. 96 (2018) 193
201. [32] R.E. Neisiany, S.N. Khorasani, M. Naeimirad, J.K.Y. Lee, S. Ramakrishna, Improving mechanical properties of carbon/epoxy composite by incorporating functionalized electrospun polyacrylonitrile nanofibers, Macromol. Mater. Eng. 302 (2017) 1600551. [33] R.E. Neisiany, J.K.Y. Lee, S.N. Khorasani, S. Ramakrishna, Self-healing and interfacially toughened carbon fibre-epoxy composites based on electrospun core
shell nanofibres, J. Appl. Polym. Sci. 134 (2017) 44956. [34] G. Williams, R. Trask, I. Bond, A self-healing carbon fibre reinforced polymer for aerospace applications, Compos. Part. A Appl. Sci. Manuf. 38 (2007) 1525
1532. [35] R. Malkin, M. Yasaee, R.S. Trask, I.P. Bond, Bio-inspired laminate design exhibiting pseudo-ductile (graceful) failure during flexural loading, Compos. Part. A Appl. Sci. Manuf. 54 (2013) 107
116. [36] J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791
1799. [37] E.T. Thostenson, T.-W. Chou, Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self healing, Adv. Mater. 18 (2006) 2837
2841. [38] X.F. Wu, A. Rahman, Z. Zhou, D.D. Pelot, S. Sinha-Ray, B. Chen, et al., Electrospinning core-shell nanofibers for interfacial toughening and self-healing of carbon-fiber/epoxy composites, J. Appl. Polym. Sci. 129 (2013) 1383
1393. [39] B.E. Wertzberger, J.T. Steere, R.M. Pfeifer, M.A. Nensel, M.A. Latta, S.M. Gross, Physical characterization of a self-healing dental restorative material, J. Appl. Polym. Sci. 118 (2010) 428
434.

Self-Healing Polymer-Based Systems

138

5. Characterization of self-healing polymeric materials

[40] E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (2002) 372
379. [41] G. Szebe´nyi, T. Cziga´ny, B. Vermes, X.J. Ye, M.Z. Rong, M.Q. Zhang, Acoustic emission study of the TDCB test of microcapsules filled self-healing polymer, Polym. Test. 54 (2016) 134
138. [42] R.J. Lemmens, Q. Dai, D.D. Meng, Side-groove influenced parameters for determining fracture toughness of self-healing composites using a tapered double cantilever beam specimen, Theor. Appl. Fract. Mech. 74 (2014) 23
29. [43] L. Guadagno, L. Vertuccio, C. Naddeo, E. Calabrese, G. Barra, M. Raimondo, et al., Self-healing epoxy nanocomposites via reversible hydrogen bonding, Compos. Part. B Eng. 157 (2019) 1
13. [44] D. Garoz Go´mez, F.A. Gilabert, E. Tsangouri, D. Van Hemelrijck, X.K.D. Hillewaere, F.E. Du Prez, et al., Indepth numerical analysis of the tdcb specimen for characterization of self-healing polymers, Int. J. Solids Struct. 64
65 (2015) 145
154. [45] Y.C. Yuan, X.J. Ye, M.Z. Rong, M.Q. Zhang, G.C. Yang, J.Q. Zhao, Self-Healing epoxy composite with heatresistant healant, ACS Appl. Mater. Interfaces 3 (2011) 4487
4495. [46] R.J. Varley, S. van der Zwaag, Development of a quasi-static test method to investigate the origin of selfhealing in ionomers under ballistic conditions, Polym. Test. 27 (2008) 11
19. [47] R.J. Varley, S. van der Zwaag, Towards an understanding of thermally activated self-healing of an ionomer system during ballistic penetration, Acta Mater. 56 (2008) 5737
5750. [48] S.J. Kalista, J.R. Pflug, R.J. Varley, Effect of ionic content on ballistic self-healing in emaa copolymers and ionomers, Polym. Chem. 4 (2013) 4910
4926. [49] Y. Zhang, A.A. Broekhuis, F. Picchioni, Thermally self-healing polymeric materials: the next step to recycling thermoset polymers? Macromolecules 42 (2009) 1906
1912. [50] G. Liberata, R. Marialuigia, N. Carlo, L. Pasquale, M. Annaluisa, H.B. Wolfgang, Healing efficiency and dynamic mechanical properties of self-healing epoxy systems, Smart Mater. Struct. 23 (2014) 045001. [51] Z. Wei, J.H. Yang, J. Zhou, F. Xu, M. Zrı´nyi, P.H. Dussault, et al., Self-healing gels based on constitutional dynamic chemistry and their potential applications, Chem. Soc. Rev. 43 (2014) 8114
8131. [52] P. Cordier, F. Tournilhac, C. Soulie´-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977. [53] F. Liu, F. Li, G. Deng, Y. Chen, B. Zhang, J. Zhang, et al., Rheological images of dynamic covalent polymer networks and mechanisms behind mechanical and self-healing properties, Macromolecules 45 (2012) 1636
1645. [54] X. Yu, X. Cao, L. Chen, H. Lan, B. Liu, T. Yi, Thixotropic and self-healing triggered reversible rheology switching in a peptide-based organogel with a cross-linked nano-ring pattern, Soft Matter 8 (2012) 3329
3334. [55] S.H. Cho, S.R. White, P.V. Braun, Self-healing polymer coatings, Adv. Mater. 21 (2009) 645
649. [56] A.M. Atta, H.A. Al-Hodan, R.S.A. Hameed, A.O. Ezzat, Preparation of green cardanol-based epoxy and hardener as primer coatings for petroleum and gas steel in marine environment, Prog. Org. Coat. 111 (2017) 283
293. [57] A.M. Atta, R.S.A. Hameed, H.A. Al-Lohedan, A.O. Ezzat, A.I. Hashem, Magnetite doped cuprous oxide nanoparticles as modifier for epoxy organic coating, Prog. Org. Coat. 112 (2017) 295
303. [58] F. Safaei, S.N. Khorasani, H. Rahnama, R.E. Neisiany, M.S. Koochaki, Single microcapsules containing epoxy healing agent used for development in the fabrication of cost efficient self-healing epoxy coating, Prog. Org. Coat. 114 (2018) 40
46. [59] A.M. Atta, A. El-Faham, H.A. Al-Lohedan, Z.A. Al Othman, M.M.S. Abdullah, A.O. Ezzat, Modified triazine decorated with Fe3O4 and Ag/Ag2O nanoparticles for self-healing of steel epoxy coatings in seawater, Prog. Org. Coat. 121 (2018) 247
262. [60] E. Koh, S. Lee, J. Shin, Y.-W. Kim, Renewable polyurethane microcapsules with isosorbide derivatives for self-healing anticorrosion coatings, Ind. Eng. Chem. Res. 52 (2013) 15541
15548. [61] C. Suryanarayana, K.C. Rao, D. Kumar, Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings, Prog. Org. Coat. 63 (2008) 72
78. [62] J.H. Park, P.V. Braun, Coaxial electrospinning of self-healing coatings, Adv. Mater. 22 (2010) 496
499. [63] C.S. Hyoun, S.R. White, P.V. Braun, Self-healing polymer coatings, Adv. Mater. 21 (2009) 645
649. [64] J. Pawley, Handbook of Biological Confocal Microscopy, Springer Science & Business Media, New York, 2010.

Self-Healing Polymer-Based Systems

References

139

[65] X. Wang, F. Liu, X. Zheng, J. Sun, Water-enabled self-healing of polyelectrolyte multilayer coatings, Angew. Chem. 123 (2011) 11580
11583. [66] H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, et al., Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (2017) 2355
2364. [67] Z. Zhao, Y. Liu, K. Zhang, S. Zhuo, R. Fang, J. Zhang, et al., Biphasic synergistic gel materials with switchable mechanics and self-healing capacity, Angew. Chem. Int. Ed. 56 (2017) 13464
13469. [68] Y. Gonza´lez-Garcı´a, J.M.C. Mol, T. Muselle, I. De Graeve, G. Van Assche, G. Scheltjens, et al., SECM study of defect repair in self-healing polymer coatings on metals, Electrochem. Commun. 13 (2011) 169
173. [69] Y. Gonza´lez-Garcı´a, S.J. Garcı´a, A.E. Hughes, J.M.C. Mol, A combined redox-competition and negativefeedback SECM study of self-healing anticorrosive coatings, Electrochem. Commun. 13 (2011) 1094
1097. [70] Z. Wei, J.H. Yang, X.J. Du, F. Xu, M. Zrinyi, Y. Osada, et al., Dextran-based self-healing hydrogels formed by reversible diels
alder reaction under physiological conditions, Macromol. Rapid Commun. 34 (2013) 1464
1470. [71] W. Wang, L. Xu, H. Sun, X. Li, S. Zhao, W. Zhang, Spatial resolution comparison of AC-SECM with SECM and their characterization of self-healing performance of hexamethylene diisocyanate trimer microcapsule coatings, J. Mater. Chem. A 3 (2015) 5599
5607. [72] Y. Huang, L. Deng, P. Ju, L. Huang, H. Qian, D. Zhang, et al., Triple-action self-healing protective coatings based on shape memory polymers containing dual-function microspheres, ACS Appl. Mater. Interfaces 10 (2018) 23369
23379. [73] A. Latnikova, D. Grigoriev, M. Schenderlein, H. Mohwald, D. Shchukin, A New approach towards “active” self-healing coatings: exploitation of microgels, Soft Matter 8 (2012) 10837
10844. [74] M.M. Caruso, B.J. Blaiszik, H. Jin, S.R. Schelkopf, D.S. Stradley, N.R. Sottos, et al., Robust, double-walled microcapsules for self-healing polymeric materials, ACS Appl. Mater. Interfaces 2 (2010) 1195
1199. [75] D. Polcari, P. Dauphin-Ducharme, J. Mauzeroll, Scanning electrochemical microscopy: a comprehensive review of experimental parameters from 1989 to 2015, Chem. Rev. 116 (2016) 13234
13278. [76] Z. Zheng, M. Schenderlein, X. Huang, N.J. Brownbill, F. Blanc, D. Shchukin, Influence of functionalization of nanocontainers on self-healing anticorrosive coatings, ACS Appl. Mater. Interfaces 7 (2015) 22756
22766. [77] D. Borisova, H. Mo¨hwald, D.G. Shchukin, Influence of embedded nanocontainers on the efficiency of active anticorrosive coatings for aluminum alloys part I: influence of nanocontainer concentration, ACS Appl. Mater. Interfaces 4 (2012) 2931
2939. [78] L. Yang, L. Long, S. Junqi, Bioinspired self-healing superhydrophobic coatings, Angew. Chem. Int. Ed. 49 (2010) 6129
6133. [79] C. Ding, Y. Liu, M. Wang, T. Wang, J. Fu, Self-healing, superhydrophobic coating based on mechanized silica nanoparticles for reliable protection of magnesium alloys, J. Mater. Chem. A 4 (2016) 8041
8052. [80] D. Zhang, L. Wang, H. Qian, X. Li, Superhydrophobic surfaces for corrosion protection: a review of recent progresses and future directions, J. Coat. Technol. Res. 13 (2016) 11
29. [81] J. Hentschel, A.M. Kushner, J. Ziller, Z. Guan, Self-healing supramolecular block copolymers, Angew. Chem. Int. Ed. 51 (2012) 10561
10565. [82] E. Hsiao, D. Kim, S.H. Kim, Effects of ionic side groups attached to polydimethylsiloxanes on lubrication of silicon oxide surfaces, Langmuir 25 (2009) 9814
9823. [83] M. Nosonovsky, B. Bhushan, Surface self-organization: from wear to self-healing in biological and technical surfaces, Appl. Surf. Sci. 256 (2010) 3982
3987. [84] S. Ataei, S.N. Khorasani, R.E. Neisiany, Biofriendly vegetable oil healing agents used for developing selfhealing coatings: a review, Prog. Org. Coat. 129 (2019) 77
95. [85] Q.B. Guo, K.T. Lau, B.F. Zheng, M.Z. Rong, M.Q. Zhang, Imparting ultra-low friction and wear rate to epoxy by the incorporation of microencapsulated lubricant? Macromol. Mater. Eng. 294 (2009) 20
24. [86] H. Li, Q. Wang, M. Li, Y. Cui, Y. Zhu, B. Wang, et al., Preparation of high thermal stability polysulfone microcapsules containing lubricant oil and its tribological properties of epoxy composites, J. Microencapsul. 33 (2016) 286
291. [87] M. Vakili Azghandi, A. Davoodi, G.A. Farzi, A. Kosari, Water-base acrylic terpolymer as a corrosion inhibitor for SAE1018 in simulated sour petroleum solution in stagnant and hydrodynamic conditions, Corros. Sci. 64 (2012) 44
54. [88] A.H. Navarchian, M. Joulazadeh, F. Karimi, Investigation of corrosion protection performance of epoxy coatings modified by polyaniline/clay nanocomposites on steel surfaces, Prog. Org. Coat. 77 (2014) 347
353.

Self-Healing Polymer-Based Systems

140

5. Characterization of self-healing polymeric materials

[89] A. Davoodi, S. Honarbakhsh, G.A. Farzi, Evaluation of corrosion resistance of polypyrrole/functionalized multi-walled carbon nanotubes composite coatings on 60Cu
40Zn brass alloy, Prog. Org. Coat. 88 (2015) 106
115. [90] S. Ataei, S.N. Khorasani, R. Torkaman, R.E. Neisiany, M.S. Koochaki, Self-healing performance of an epoxy coating containing microencapsulated alkyd resin based on coconut oil, Prog. Org. Coat. 120 (2018) 160
166. [91] M.W. Lee, S. An, C. Lee, M. Liou, A.L. Yarin, S.S. Yoon, Hybrid self-healing matrix using core
shell nanofibers and capsuleless microdroplets, ACS Appl. Mater. interfaces 6 (2014) 10461
10468. [92] S. An, M. Liou, K.Y. Song, H.S. Jo, M.W. Lee, S.S. Al-Deyab, et al., Highly flexible transparent self-healing composite based on electrospun core
shell nanofibers produced by coaxial electrospinning for anti-corrosion and electrical insulation, Nanoscale 7 (2015) 17778
17785. [93] H. Wang, Q. Zhou, Evaluation and failure analysis of linseed oil encapsulated self-healing anticorrosive coating, Prog. Org. Coat. 118 (2018) 108
115. [94] C. Zhang, H. Wang, Q. Zhou, Preparation and characterization of microcapsules based self-healing coatings containing epoxy ester as healing agent, Prog. Org. Coat. 125 (2018) 403
410. [95] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, P. Cecı´lio, M.G.S. Ferreira, TiOx selfassembled networks prepared by templating approach as nanostructured reservoirs for self-healing anticorrosion pre-treatments, Electrochem. Commun. 8 (2006) 421
428. [96] S.J. Garcı´a, H.R. Fischer, P.A. White, J. Mardel, Y. Gonza´lez-Garcı´a, J.M.C. Mol, et al., Self-healing anticorrosive organic coating based on an encapsulated water reactive silyl ester: synthesis and proof of concept, Prog. Org. Coat. 70 (2011) 142
149. [97] S.H. Boura, M. Peikari, A. Ashrafi, M. Samadzadeh, Self-healing ability and adhesion strength of capsule embedded coatings—micro and nano sized capsules containing linseed oil, Prog. Org. Coat. 75 (2012) 292
300. [98] M. Hasanzadeh, M. Shahidi, M. Kazemipour, Application of EIS and EN techniques to investigate the selfhealing ability of coatings based on microcapsules filled with linseed oil and CeO2 Nanoparticles, Prog. Org. Coat. 80 (2015) 106
119.

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C H A P T E R

6 Role of nanoparticles in self-healing of polymeric systems Junfeng Su Department of Polymer Science, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin, P.R. China

6.1 Introduction With the progress of science and technology, various kinds of composite materials are widely used. Due to the influence of thermal, mechanical, and chemical factors, microcracks, and local damage inevitably occur in use [1]. It is a very important problem to heal the damage. Due to the limitations of technology, microdamage such as matrix microcracks may not be detected. Failure to heal in time will not only affect the normal service performance and shorten the service life but also cause macrocracks and brittle fracture, which will eventually be scrapped [2]. In the late 1980s with the development and progress of materials technology and large-scale integrated circuit technology, the US military first put forward the idea and concept of intelligent materials and structures, and then launched large-scale research [3]. Nowadays, the development is very rapid and has been paid more and more attention. Self-healing technology can be used to improve material reliability and extend the service life, so composite materials with self-healing capability become an important research content of smart materials [4]. At present, intelligent selfhealing materials with self-diagnosis and self-healing function have been widely used in polymers, composites, metal materials, cement materials, asphalt materials, etc. [5]. Self-healing material is a new kind of material that can be partially or completely heal damage inflicted on them [6]. According to the healing mechanism, self-healing materials can be divided into two categories, namely, extrinsic and intrinsic [7]. The extrinsic selfhealing polymer materials accomplish self-healing with the aid of self-healing agents incorporated in microcapsules [8] or hollow fibers [9]. Self-healing agents are released when microcapsules or hollow fibers are attacked by microcracks, and then reactions involving self-healing take place, and the crack surfaces are bonded together to achieve self-healing. This method was performed easily and efficiently, but the self-healing process

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cannot be repeated, and the candidates of self-healing agents are limited [8]. Another approach is to provide energy to the material through heating, lighting and other ways. The energy makes materials to form crystallization structure, film structure or cross-linked structure with a healing capability [10]. Based on these two mechanisms, self-healing techniques have been applied in concrete, metal and polymer materials [11]. Extrinsic selfhealing is reversible. Although the preparation of self-healing polymers is complex, the self-healing process is repeatable. Therefore the durability of polymer materials can be improved. Macromolecular self-healing materials are the most studied and the most diverse materials at present. Since the polymer material itself is based on interatomic covalent bonds and hydrogen bonds, which can be controlled by chemical reactions, this provides more “convenient” conditions for self-healing [12]. In the process of use and the surrounding environment, the material will inevitably produce local aging and damage. These aging and damage are often hidden inside the material, which is difficult to find. Once the material is damaged, the mechanical properties will decrease. Electrical damage is caused in microelectronic polymer materials. Repair of damage used in structural materials is undoubtedly an important issue. Therefore the early detection and repair of microcracks are a very practical problem. The delamination that can be found by the naked eye or the macroscopic crack caused by the impact is not difficult to find and can be repaired by hand. Nondestructive testing such as ultrasound and radiography is a common technique for observing internal damage [13]. However, due to the limitations of these techniques, and the cracking of the polymer often appears in the depth of the body, microscopic damage such as microcracking of the matrix is difficult to find. If these damaged parts cannot be repaired in time, it will not only affect the normal use performance of structural members and shorten the service life but also may cause macroscopic cracks to break and cause major accidents. The polymer-based selfhealing composite is a polymer-based composite that can self-diagnose under external force and cure cracks or damage to some extent [14]. However, the healing ability of the polymer itself is limited and takes a long time. When the temperature is lower or the crack is greater than the self-healing threshold, the polymer itself is difficult to achieve selfhealing. Therefore it is important to study the biomimetic repair of polymer material selfhealing, active and automatic detection and repair of damaged parts, and the application of polymer materials in structural components and high-tech fields [15]. At present, the main research methods are microcapsule method, liquid core fiber method, capillary network method, thermoreversible cross-linking reaction repair method, and weak interaction repair method. The self-healing of nanomaterials is an important method. The growing field of nanotechnology is the study of matter at an incredibly small scale, generally between 1 and 100 nm. Nanoscale materials add strength to polymers while making them lightweight, and they make fabrics water- and stain-resistant [16]. Some pharmaceutical products have been reformulated with nanosized particles to improve their performance. Nanotechnology could bring about the next wave of innovation in science and engineering, and the possibilities are endless. The next generation of nanomaterials will be stronger, lighter, and more durable than the materials used today in buildings, bridges, airplanes, and automobiles [17]. To date, nanotechnology also holds great promise for developing revolutionary tools to help create a more efficient self-healing approach using NPs. NPs can provide self-healing solutions for polymers. In this chapter, we focus

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this minireview on NP-based self-healing. Based on the above analysis, the aim of this chapter introduced the metal, inorganic, and organic NPs applied in self-healing polymers. At the same time, the mechanism of self-healing polymer using nanocapsules is explained systematically. Interestingly we introduced a real application of nanocapsules in bituminous pavement. Furthermore advises are given to guide the application of NPs in selfhealing application.

6.2 Self-healing polymer using metal nanoparticles The self-healing mechanism of self-healing polymers using metal NPs is to fill polymers with metal NPs. When metal NPs and organic groups form reversible metal ligands and coordination, microcracks can repair themselves without external stimulation [18]. If coordination effect cannot be formed between polymer and metal NPs, external stimulation is needed. When the polymer cracks, it is heated by different energy sources, such as ultraviolet or infrared radiation and conventional heating (Fig. 6.1). Metal NPs that diffuse to the crack area (the smaller the size, the better the diffusion effect) generate special local heat in the desired area [19]. The polymer itself has some self-healing ability. In this dynamic system, heat accelerates repair. Lee et al. [20] explained the process of nanoparticles repairing multilayer composite materials through computer simulation of the micromechanical properties of materials. The simulation results showed that nanoparticles could repair the mechanical properties of the damaged area to 75% 100% of the initial properties. He et al. [18] proposed a simple method to prepare double reticular structure hydrogel by polyvinyl alcohol (PVA) and in situ reduction of gold NPs composite polyelectrolyte gel. This is a combination of chemical and physical cross-linking. The hydrogel has a variety of properties (such as electrical conductivity, good mechanical properties, and self-healing). The self-healing ability is caused by the hydrogen bond between hydroxyl groups and the coordination effect between gold NPs and hydroxyl groups. It can self heal without any external stimulation, and can achieve multiple recoveries, with almost no loss of electrical conductivity during the recovery process. Through a simple process, Altuna et al. [19] successfully FIGURE 6.1 Mechanism of metal nanoparticles used in polymer self-healing.

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incorporated water-soluble gold NPs into a biobased polymer matrix by cross-linking epoxidized soybean oil (ESO) and citric acid (CA) solution. Gold NPs have high extinction coefficient and thermal efficiency. By irradiating the material with a green laser, these gold NP aggregates effectively generate heat as a nanometer heating source, enabling remote activation of self-healing functions in cross-linked polymers. This method makes it possible to remotely control material repair without removing damaged materials, increasing the service life of materials and utilizing resources more efficiently. As a form of external aid self-healing, NP self-healing has been widely used in polymers, composites, metal materials, cement materials, asphalt materials, etc. The development and research of metal NPs have become the focus of scientific research. The use of magnetic or metallic NPs acting as nanoscopic heat source is currently explored mainly for biomedical applications e.g., drug delivery and tumor therapy. When combined with mechanically soft materials such as elastomers, new possibilities are open for the manipulation of these composites using external fields.

6.3 Self-healing polymer using inorganic nanoparticles Inorganic NPs can be added in the polymer to enhance the mechanical property [21]. To regulate the interface between NPs and polymers, NPs always have a molecular surface modification to own functional groups [22]. Because of the large specific surface area, NPs in polymer, the composites may show special performances in mechanical property, thermal property, and barrier property. Lots of work has reported on the inorganic/polymer composites in the field of material science and engineering [23]. Normally NPs can be used to stop crack propagation, and they also can play an auxiliary role in the self-healing process. It has been reported that NPs possessed an inherent potential to improve self-healing of polymers since NPs can diffuse faster than the larger particles [24]. When modified with NPs, microcracks of polymers may heal faster during rest periods because during the cracking period, the NPs migrated preferentially to the microcrack surface, and in the rest periods, molecular random movements occurred, and the micro cracks healed [25]. Due to its spherical shape, higher density, and very tiny size, the NPs possess an inherent potential to promote the ability of polymers self-healing. It was hypothesized that adding NPs to bitumen would enhance surface approach and molecular randomization and accelerate the diffusion, which may generally lead to faster healing [26]. Wool [27] described microcrack self-healing mechanism in terms of five steps: (1) surface rearrangement (e.g., NPs in the bulk could preferentially migrate into nanocracks); (2) surface approach (i.e., filling gap with healing fluid); (3) wetting (i.e., to form a wet interface); (4) diffusion (i.e., healing agent flow); and (5) randomization. Among these mechanisms, surface rearrangement due to localization of NPs within the bulk of the matrix and diffusion may contribute to the self-healing of NPs modified binders. Fig. 6.2 illustrates the self-healing mechanism of polymer matrix using NPs. Nanosilica particles due to their higher density compared to bitumen (2.33 g cm23 compared to 1.02 g cm23) and their very tiny size act like spherical metal balls in bitumen mortar matrix and facilitate bitumen mortar flow under the gravity force. The microcracks are filled with bitumen mortar flow (in the stage of surface

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FIGURE 6.2 Illustration the self-healing mechanism of polymer matrix using nanoparticles.

rearrangement and surface approach). Then because of very tiny size of nanosilica particles, faster molecular randomization movements occur, which is the cause of microcracks becoming healed. Although the effect of nanosilica modification on the performance of asphalt binders has been widely studied, less attention has been paid to the investigation of the behavior of nanosilica particles during microcrack formation and their healing process [21]. Nanosilica is a metal oxide which is widely produced and cost effective [23]. Moreover, owing to its very tiny size, spherical shape, and higher particle density (2.33 g cm23) in comparison to bitumen (1.02 g cm23), it could be hypothesized that nanosilica provides an inherent potential to improve binder diffusion (flow) into microcracks, promotes molecular randomization movements in microcrack self-healing process, and may preferentially migrate into microcracks. Zazari et al. [28] investigated the storage stability of nanosilica modified bitumen. The modified bitumen contained 2%, 4%, and 6% nanosilica. The difference between the softening points of top and bottom of the alumina tube after remaining in an oven at 163 C for 48 h was measured. They concluded that nanophase separation tendency is not a major issue for nanosilica modified bitumen. In addition, Ghasemi and Dehsheikh [29] evaluated storage stability of SBS/nanosilica modified binders (which contained 0%, 0.5%, 1%, 1.5%, and 2% nanosilica with 0% and 5% SBS) with the abovementioned method. Considering very tiny size and high specific area, it seems that nanosilica particles bond to different functional groups of asphalt binders, and this bonding prohibits NPs from segregation. Thus it is necessary to carry out more experiments to investigate the effect of nanosilica on HMA self-healing mechanism and the process by which healing capability of hot mix asphalt may be promoted. The main objective of this study is to investigate the effect of nanosilica with HMA self-healing index and its

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mechanism. In addition, the effects of aggregate type, bitumen content, and air voids on healing index (HI) were investigated using Taguchi design of experiment (DOE). Another role of self-healing process is to use inorganic NPs as nanocontainers. The concept of such type of the coatings combines the classic passive component of the coatingmatrix (layer) and active component nanocontainers with controlled permeability of the shell and loaded with corrosion inhibitors or other active agents [30]. This allows one to joint one coating classical passive functionalities such as barrier, color together with active ones responsive to both internal and external impacts such as cracks, local pH, electrochemical potential, temperature, and humidity. Use of nanocontainers as a coating pigment will provide a breakthrough in the current coating technologies making the coatings: (1) autonomic, self-repairing of the cracks and thus enhancing corrosion protection of the metal substrate, (2) more durable with increased maintenance time, and (3) multifunctional with additional active functionalities such as sensor, and antifouling. There are two types of ion-exchange release of the entrapped inhibitor from nanoclays: the less studied cation-exchange release and the more developed anion-exchange release. Both of them have a similar mechanism of the uptake and release of the corrosion inhibitors loaded between the layers. The only difference is in the triggers initiating the release. Fig. 6.3 shows the schematic presentation of the layered montmorillonite nanoclay structure. If the nanoclay layers are charged negatively, only positively charged inhibitor can be uploaded in the interlayer galleries to compensate the negative charge. The same mechanism is for the positively charged nanoclay layers—only negatively charged corrosion inhibitor can be uploaded [30]. The release for nanoclay-based nanocontainers is triggered by ion-exchange in the presence of H1, OH2, Cl2, and other corrosion products or aggressive ions. However, the release of the inhibitor in cation-exchanger clays can only be triggered by metal cations available in the surrounding environment, which may not be directly associated with corrosion processes, leading to the uncontrolled release of the inhibitor. Bentonite is a cation-exchanger and one of the rare examples of nanocontainers for self-healing coatings, consisting of stacks of negatively charged aluminosilicate sheets between which inhibiting cations such as Ca21 and Ce31 were intercalated. Ce-loaded bentonites dispersed in polyester resin layers and applied to galvanized steel substrates

FIGURE 6.3 Schematic presentation of the layered montmorillonite nanoclay structure.

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6.3 Self-healing polymer using inorganic nanoparticles

display active protection avoiding coating decomposition. The protective performance of Na-montmorillonite (Na-MMT) intercalated with zinc cations (Zn-MMT), benzoimidazole (BIAMMT), and the mixture of these two inhibitors (Zn-MMT 1 BIAMMT) is described as an example of another cation-exchanging natural clay material for self-healing anticorrosion coatings. The highest corrosion resistance and the lowest adhesion loss were observed when the mixture of Zn-MMT and BIAMMT clay nanocontainers was used. The crack filling behavior of the transparent, multilayer coating consisting of a polysiloxane film on top of a thin montmorillonite interlayer was monitored using confocal and SEM microscopy. Beyond the 2 h healing period, further expansion of the clay layers resulted in restoration of the barrier properties of the coating. Another nanoparticle as nanocontainers is halloysite (Al2Si2O5(OH)4 nH2O), which is a clay material that can be mined from deposits as a raw mineral (price range 6 10 USD kg21) [30]. Halloysite is a layered aluminosilicate chemically similar to kaolin which has a hollow tubular structure in the submicrometer range. Fig. 6.4 shows a typical TEM image of halloysite nanotubes. Size of halloysite tubules varies from 500 to 1000 nm in length and from 15 to100 nm in inner diameter depending on the deposit. Inner halloysite lumen can provide additional space and increased loading capacity for corrosion inhibitors up to 20 wt.%. Additional selective etching of the alumina inside halloysite lumen with sulfuric acid can increase lumen capacity for corrosion inhibitor loading up to 60 wt.%. This loading capacity is close to the loading capacity of polymer nanocontainers. Besides corrosion inhibitors, halloysite can be a cargo for biocides and exploited in antifouling coatings. The typical procedure of the loading of halloysite nanotubes is as follows. Halloysites are mixed with a solvent possessing high solubility for the desired corrosion inhibitor and low temperature boiling point. Then the vial containing the solution is placed in a desiccator under vacuum, which deaerates the halloysite lumen. Due to the rapid evaporation of the solvent, the inhibitor concentration increases thus improving the loading efficiency. Halloysite clay nanotubes loaded with corrosion inhibitors benzotriazole,




FIGURE 6.4 TEM image of halloysite nanotubes.

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mercaptobenzimidazole, and mercaptobenzothiazole were used as additives in selfhealing composite paint coating of copper. The release rate of benzotriazole was controlled by the formation of metal-benzotriazole stoppers of tube endings. This method provided high halloysite loading efficiency and a long release time which give additional possibilities for process optimization. While the presence of heterogeneities at macromolecular scales plays a significant role in facilitating segmental mobility and rearrangements during repair, the question is whether anisotropic rigid components are capable of generating self-healing in composites. In tuitively, rigid NPs, nanotubes, or fibers will not contribute to self-repair, unless they have inherently built-in self-healing components. For surface functionalization to be effective, polymer matrix should consist of “matching” reactive groups, which are illustrated in column C. Thus when B and C react, interfacial bonding in column D will facilitate covalent and noncovalent interfacial interactions. One can envision that reforming bonding at the fiber matrix interface may facilitate repairs when the interface undergoes partial damage repair cycle, but over-all mechanical integrity of a composite is retained. When a load is removed, the reformed interface will return to equilibrium conditions. However, if interfacial regions between polymer matrix and reinforcing rigid entities exhibit the ability of bond cleavage and reformation, interfacial regions may serve that purpose. The majority of NPs, nanorods/nanowires, nanotubes, nanofibers, and graphene sheets can be surface modified to generate amine, carboxylic acid, hydroxyl, UPy, thiol, furfuryl, cinnamoyl, anthracene, or pyrenyl groups, which can react with their counterparts in the matrix [31]. Thus the nature of modifications will be determined by the polymer motif to form reversible bonds with a polymer matrix. Consequently interfacial chemistry will be critical in achieving reversible self-healing.

6.4 Self-healing polymer using organic nanoparticles 6.4.1 Self-healing by shape-memory organic nanoparticles Physical aspects of self-healing are driven not only by chemical and morphological features of polymer or composite networks but also by the modes of damage in a given environment. Earlier concepts of self-healing came out from two physical observations [32]: (1) microcrack formation upon mechanical loading, followed by the cover to the original elastic properties; several recent studies demonstrated the dynamics of reversible bonds during self-healing by monitoring the decay of mechanical properties after multicycle load and rest. (2) Self-healing upon projectile puncture, as observed in poly(ethyleneco-methacrylic acid) copolymers (EMMA) [33]. In view of these earlier observations, dynamics of mechanical damage and dissipation of thermal energy during damage were considered in the primary sources of repairs as shown in Fig. 6.5. For example, the energy impact during the projectile melted the polymer at the damage zone, allowing interdiffusion to self-heal. Several models were established at that time to explain the underlying physics governing this behavior summarized in recent literature [34]. Initial studies on thermoplastic polymers suggested that self-healing of a crack involves five stages: segmental surface rearrangements, surface approach, wetting, diffusion, and

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FIGURE 6.5

Schematic illustration of two types of physical processes involved in self-healing: (A) the presence of shape memory components, (B) elastic forces of polymers force the closure of damage, and (C) an interfacial flow causes damages repair.

randomization [35]. In reality different damages will occur in different materials resulting in complex damage morphologies. Surface rearrangements and surface approach to fill the voids are critical to diffusion or elastic recovery as demonstrated in several studies [36]. The question how surface topography variations will impact materials recoveries is not well understood. Furthermore wetting, defined as a formation of the polymer polymer or polymer liquid interfaces prior to diffusion, was considered as prerequisite to regain mechanical properties in the repaired area via chain entanglements [37].

6.4.2 Self-healing by organic micro- or nanocapsules Microencapsulation is a type of composite material with core-shell structure. By microencapsulation, the physical properties of the material can be improved, the stability of the material can be improved, the odor can be shielded, the toxicity of the material can be reduced, the release can be controlled, the pollution of toxic substances to the environment can be reduced, and incompatible compounds can be isolated. Microcapsule technology has been widely used in carbon-free carbon paper, medicine, pesticides, cosmetics, food, and other fields. According to the need for practical application, the material and structure of capsule core and shell can be reasonably selected, which can endure microcapsules with different physical and chemical characteristics. White et al. [38] repored a testing method of self-healing behavior based on microcapsule technology: in resin matrix material, catalyst and coated with scattered throughout the repair agent microcapsules, when material damage cracks, microcapsule cracking propagation of crack, coated within the microcapsules of repair fluid release, and buried in catalyst after contact of polymerization reaction, substrate material will crack bond, so as to prevent crack expanding further. At the same time, White et al. [38] proposed three stages of microencapsulated self-repairing materials as shown in Fig. 6.6: (1) in sensing stage, that is, the crack growth in the matrix forces the self-repairing microcapsule to crack; (2) in the transportation stage, the microcapsule releases the covered repair agent into the crack; (3) in the execution stage, that is, the repair agent contacts with the preburied catalyst in the matrix, and the biochemical reaction occurs under the action of the catalyst, so as to cross-link the curing and repairing the cracks. This theory has been widely accepted, and a large number of microencapsulated self-healing materials have been designed. After years of development, many kinds of microcapsule self-healing systems have been formed; this can be divided into single microcapsule system and double microcapsule system according to the different

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FIGURE 6.6 Illustration of self-healing process of microcapsules including four steps: (A) microcrack generation, (B) microcapsules broken, (C) capillarity and the diffusion behaviors of healing agent or cross-link of healing agent, and (D) microcrack closure [39].

structures. For the reaction repair type of microcapsule, it can be divided into reactant/catalytic agent, epoxy resin/curing agent, solvent self-repair type, and click chemical type. Microcapsules were used as microcontainers to wrap the repairing agent and added to the material as self-healing components. There are two ways to release the repairing agent in self-healing microcapsules. In the first case, when the material is subjected to external force and produces microcracks, the stress concentration at the crack tip triggers the microcapsule fracture, causing the repairing agent to flow out to the damaged position, and then plays the role of material self-healing. Su et al. [39] synthesized microcapsules with self-repairing function in asphalt to delay the aging of asphalt. The microcapsule should have good thermal stability and mechanical properties, and the cross-linking reaction in asphalt should be determined by testing its surface properties. Microcapsules for asphalt self-repair were synthesized by using melamine formaldehyde resin modified with methanol as the wall material and a sticky, aromatic oil as the repair agent. The experimental results show that the microcapsule has good repair performance in asphalt under 200 C. Chemical bonds are formed between the wall material and the asphalt, and the microcapsules have excellent flexibility when no cracks occur. The second method is the continuous and slow release of repairing agent in the microcapsule, namely, controlled release. This method can meet the long-term self-healing effect of the material and extend the service life of the material. With the development of microcapsule technology, the size of microcapsules can be less than 1 μm and can be between 1 and 1000 nm. Nanocapsules are new microcapsules with manometer size. Compared to traditional microcapsules, nanocapsules have a small particle size and are easy to disperse and suspend in water to form a colloidal solution. The dispersion, targeting, and sustained release of nanocapsules is more obvious. Nanocapsules are a hot research topic in recent years and have broad application prospects. With the advance of microencapsulation technology, there are many technologies to prepare nanoencapsulation, including emulsion polymerization, interfacial

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polymerization, drying bath, layer upon layer self-assembly and electrostatic spray. Amiri and Rahimi [40] studied the effect of nanocapsules with corrosion inhibitor as core material and cyclodextrin as shell material on self-healing anticorrosive properties of coatings. The results show that the cyclodextrin cavity can store corrosion inhibitors and release them to the damaged barrier coating area, effectively repair the cracks, and have good corrosion resistance. Abbaspoor et al. [41] prepared polyurethane self-healing coating-containing microcapsules and nanocapsules by emulsion-solvent evaporation method. The results show that compared to microcapsules, nanocapsules can significantly improve the corrosion resistance of self-healing coatings. In addition, healing properties of coated nanocapsules were better than those of microcapsules. Sadrabadi et al. [42] prepared epoxy microcapsules by in situ polymerization and amine microcapsules by vacuum infiltration of diethylenetriamine. Both capsules were embedded in an epoxy resin matrix. As cracks develop and begin to grow in the coating, microcapsules near the cracks break and release their contents. Due to the curing reaction between the released curing agent (epoxy resin and amine), the healing of the crack site is completed.

6.4.3 Evaluation of self-healing capability 6.4.3.1 Crack growth method In the process of using polymer materials, aging will occur, and some microcracks will occur. The existence of microcracks reduces the service life of polymer materials. Due to the self-healing properties of polymer materials, microcracks smaller than a certain scale will heal automatically, and their modulus and strength will be restored. It takes a long time to make the polymer material to repair itself. Sometimes the polymer material cannot repair itself when the crack is too big, and the temperature is not enough. Therefore asphalt self-repair can be achieved by changing the structure of polymer materials. At present there are many ways for polymer materials to repair themselves, such as heating by mixing polymer materials with metal NPs and adding microcapsules to polymer materials. Different methods have different effects on the self-healing ability of polymer materials. Self-healing ability was evaluated by comparison. An area of nonconical length is added to the end of the sample as a crack inhibition area to prevent the sample from completely breaking into two pieces under the condition of unstable crack growth. When the crack length is less than 105 mm, the additional length does not affect the behavior of the sample. Three different types of specimens were made; one set of reference samples, one set of self-activated samples, and three sets of self-healing samples. Reference and self-activated samples were introduced in the evaluation and used as experimental controls. Healing in these specimens involves some form of manual intervention, either by injecting a healing agent (self-activated specimen) or catalyzing and injecting a healing agent (reference specimen). For self-healing specimens, microcapsules and catalysts are embedded directly into the matrix material, which automatically heals without manual injection. One group of self-healing specimens healed at a higher temperature to measure the effect of temperature on healing efficiency. The self-healing ability of the samples was evaluated by comparing the healing time of the materials.

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6.4.3.2 Beam on elastic foundation method Besides the excellent performance and suitable for applying conditions of microcapsules, we still need to prove the microcapsules really have recovery action on the selfhealing ability of aged bitumen. To date contributions have been made to identify and model the self-healing phenomenon of bitumen by means of different approaches and different levels of complexity ranging from molecular level till pavement level [43]. It is well known that self-healing of bitumen is a complex process, which is greatly dependent on the rest time between two load pulses, temperature, crack phase, and material type. The healing phenomenon of bituminous materials is composed of viscoelastic healing and viscous healing [44]. Viscoelastic healing is related to the delayed elastic recovery behavior of bituminous materials, while the viscous healing is related to the viscosity of bituminous materials. It was also observed that viscoelastic healing happen much faster than viscous healing [45]. Based on this knowledge, a method named beam on elastic foundation (BOEF) is an ideal method to research the cracking and healing behaviors of bitumen [46]. The key issue of the BOEF is using a notched bitumen beam glued on a low modulus rubber foundation, and a symmetric monotonic load was applied with loading unloading healing reloading cycles. The rubber foundation was used to avoid permanent deformation and to ensure a controllable healing process. Asphalt beam was glued on the rubber foundation to simulate full contact and full friction. When no glue was used, partial slip could occur between the beam and the rubber. The rubber elastic foundation can absorb most of the deformation, which helps for healing investigations and eliminating the influence of permanent deformation. In addition the elastic foundation can help to close the crack during unloading thus to support the healing process of the bitumen. Su [47] also notified that BOEF method is a practical approach for investigation of the recovery of the strength and the recovery of the crack opening displacement. An improved test setup called the Beam on Elastic Foundation (BOEF) was used in this study to evaluate the crack propagation and self-healing behaviors of pure bitumen. Fig. 6.7 illustrates the BOEF sketch map with design size to explain its working principle. FIGURE 6.7 Illustration of BOEF mechanical testing setup of self-healing bitumen with a V-shape notch fabricated by silicon mold and two-end aluminum blocks in the beam glued on a rubber foundation [47]. BOEF, Beam on elastic foundation.

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The testing beam was constituted by two rectangular aluminum blocks and the middle bitumen. The bitumen had a V-shape notch fabricated by silicon mold under temperature of 0 C. The two-end aluminum blocks in the beam were then glued on a rubber foundation to simulate full contact and full friction. Two rolling devices were under both aluminum blocks. The hard would support was used to connect the rubber (hardness of 40 shore, Poisson’s ratio of 0.5) and steal base by glue. The load with a speed was applied in the middle of the bitumen through an iron stick perpendicular to the beam. Then a crack was generated through the notch. The load displacement curves were recorded. To easily observe the crack, white one side of the bitumen was daubed with white paint. When the load was moved away, the crack in bitumen was closed because of the elastic foundation rubber. The beam was placed under a temperature for 24 h. After the healing period, the beam was reloaded at a temperature of 0 C under the same conditions. Fig. 6.8 shows a monotonic BOEF test with loading unloading procedure to analyze the self-healing properties of bitumen. Within the setup BOEF test, a symmetric monotonic load was applied with a loading unloading healing reloading cycle. The rubber foundation was used to avoid permanent deformation and to ensure a controllable healing process. The test was done at 0 C. A crack was generated under the pressure. When the load level had returned to 0, the external load was removed. Then the bitumen beam was putted in 0 C atmosphere for 24 h for a self-healing process. After healing, the bitumen beam was again suffered a controlled reloading at 0 C to generate a crack. The elastic foundation absorbs most of the deformation, which helps for healing investigations and eliminating the influence of permanent deformation; After a loading unloading cycle, the elastic foundation will help to close the crack during unloading thus to support the healing process of the asphalt mixes. Hence the BOEF setup is chosen for cracking and healing investigation of asphalt mixes in this study. 6.4.3.3 Direct tensile tests Tensile fracturing is one of the main failure behaviors of bitumen as films between mineral aggregates. Therefore the effect of tensile stress on bitumen is essential to understand its self-healing properties. We may be able to find a suitable tensile fracture method to prove that microcapsules provide the recovery action for multiself-healing of aged FIGURE 6.8 BOEF test procedures including monotonic primary loading, unloading, and reloading [47]. BOEF, Beam on elastic foundation.

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bitumen. The healing phenomenon of bituminous materials occurs by viscoelastic and viscous healing [48]. Moreover bitumen properties are highly temperature- and time dependent. Hence bitumen self-healing is a complex process, which is dependent on the rest time between two load pulses, the temperature, crack phase, and material type. Normally self-healing is evaluated by fracture tests at various times. Several mechanical methods have been reported to measure bitumen self-healing, including the discontinuous fatigue test with various rest times/load periods, the fatigue healing refatigue test, the intrinsic two-piece healing test, and the fracture-involved healing test [49]. Research has focused mainly on self-healing behavior during repetitive loads. For example, Hammoum [50] established a repeated fracture test method to investigate the self-healing of pure bitumen using two hemispheric protuberances, which can simulate aggregates in the asphalt mixture. A controlled tensile loading was applied with a displacement speed of 12.5 μm min21. After healing, another load was applied. The bitumen could almost recover its original properties from an analysis of the loading reloading curves. Poulikakos [51] investigated brittle and ductile regimes through the tensile behavior of two types of viscoelastic bituminous films confined between mineral aggregates or steel as adherents. Uniaxial specimens were fabricated using a prototype setup that allows for the construction of microscale thin films and a visualization of failure phenomena. To reduce limitations of time-consuming and operational complexity, Qiu [52] reported a simple self-healing test procedure, in which a fast displacement speed loading was applied first to produce a flat open crack with 100 200 μm width. Healing in this case is believed to be viscosity driven, and consists of two steps, namely, crack closure and strength gain. This testing procedure was simple and effective to evaluate and compare self-healing of bituminous materials. However, these methods were not all focused on investigating the repetitive capability of bitumen. In addition, no knowledge can be applied to establish the feasibility of microcapsules that contain rejuvenator to recover aged bitumen. • Sample preparation: the testing samples were prepared in a preheated silicone rubber mould. Fig. 6.9 shows the size of the tension test sample. The geometry of the bitumen can ensure the stress concentration in the middle of the bitumen sample. Each sample was covered with another flat preheated silicone rubber to ensure the bitumen owing the same shape on both sides. After demoulded, the samples were conditioned in a chamber for 24 h at 0 C.

FIGURE 6.9 Illustration of tension test sample size [47].

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• Fracture and healing: samples were tested by a high-low temperature tension test machine (FR-103G, Farui Tech Co. Ltd., Shanghai; 6 2.5% RH, 6 1 C) using a displacement speed of 100 mm min21 at 0 C. The middle was fractured under the direct tension. The two parts of one broken sample was immediately reputted into the module to ensure the two parts fitted the crack very well. Each sample in module was heated for 24 h under various temperatures of 10 C, 20 C, and 30 C. • Rehealing: after a healing process, the sample was reconditioned in a chamber for 24 h at 0 C. Then the sample was demoulded and repeated the fracture process with the displacement speed of 100 mm min21 at 0 C. The above refracture and rehealing rounds were repetitively tested, and the data were recorded automatically. Fig. 6.3 illustrates the relationship between the time and fracture loads. The properties of virgin bitumen, aged bitumen, and rejuvenated bitumen were tested including penetration (ASTM D5), softening point (ASTM D36) and viscosity (ASTM D4402). Five samples with the same state were mixed homogenously to test the properties. For example, to test the penetration property of aged bitumen after two tensile test cycles, five samples (after two tensile test cycles) were heated and mixture to form one big sample. Then the penetration was tested under 25 C. Evaluation of the self-healing capability: the self-healing capability of bitumen was calculated by the penetration value of divided to the last-time penetration value for the same sample, SH 5

σi 3 100% σi11

(6.1)

where SH is the self-healing percentage demonstrating the self-healing capability, σi11 is the penetration (softening point and viscosity) value, the σi is the last-time penetration (softening point and viscosity) value, and i is the fracture times as 1, 2, 3, and 4 in this study. Bitumen sample-mixing with microcapsules therefore induces multiself-healing behavior as indicated by direct tensile tests. Morphological analysis confirms that the rejuvenator is released and penetrates into the aged bitumen. A mechanism hypothesis based on microstructure was given, which would help us to understand the self-healing principle more clearly especially the microstructure of the broken bitumen surface. A schematic is used to exhibit the microstructural details of the self-healing bitumen during tensile tests. Fig. 6.10A and B shows the original states of the tensile test sample. Arrows indicate the tensile strength directions. With increase in tensile strength, the sample experiences the first tensile failure cycle as shown in Fig. 6.10C E. Given that the aged bitumen softening point is high, tensile failure produces a direct fracture with low elongation at the break. This result has been proven in the morphological analysis. Microcapsules have torn on the surface of the fracture section, and these results in a loss of strength, structure, and tightness (Fig. 6.10F). The encapsulated oily rejuvenator is released and adheres to the surface. Then the two broken pieces of the sample are reinserted into the module, while ensuring that the two parts fit the crack very well. Each sample in the module is heated at a certain temperature for 24 h as shown in Fig. 6.10G. The crack closes completely, which does not imply the total recovery of refracture strength. Su [53] pointed out that once the crack is closed, bituminous samples may still contain

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FIGURE 6.10 Illustration of self-healing of bitumen during tension test, (A) tension test sample, (B) (E) the first-cycle of tension fracture, (F) the broken microcapsules on the interface of crack, (G) the healing process of bitumen sample, and (H) (J) the second-cycle of tension fracture, rejuvenator penetration into aged bitumen.

microcracks, and air bubbles inside the sample that are difficult to detect. During the second tensile fracture cycle, the tensile process is significantly different from the first tensile fracture cycle. Because of rejuvenator penetration into aged bitumen, oily agents can reconstitute binder chemical composition and consist of lubricating and extender oils that contain a high proportion of maltene constituents. The bitumen sample is softer as shown in Fig. 6.10H and G. This increase in bitumen soft point also results from the increase in elongation at break. At the second tensile fracture (Fig. 6.10J), the bitumen shows elastic behavior. The most important property in terms of elastomer mechanical behavior is that it has a three-dimensional network with physical chain entanglement or cross-links and interpenetration of molecules, which provides strength and elasticity. Poulikakos [54] reported on the failure phenomena of a viscoelastic thin film of bitumen under direct tensile tests, which confirms that flow in bitumen is dominated by viscous effects, although capillary forces do play a role at later stages of the experiment with negligible inertia and gravitational effects. This conclusion agrees with the phenomena observed in our tests, and that rejuvenator enhances the penetration and capillary nature of bitumen significantly. Bituminous material is a viscoelastic polymeric material with a time- and temperaturedependent behavior. A direct tensile strength analysis confirmed that higher temperature improves the healing speed. With the help of rejuvenator-containing microcapsules, the microcapsule/bitumen sample has a multiself-healing ability. When its strength is lower than a limit point, the aged bitumen can no longer recover, and its self-healing ability disappears. During the first cycle, aged bitumen recovers with the help of microcapsules that contain rejuvenator. Although the bitumen cannot return to its original state, the rejuvenator softens the aged bitumen. Microcapsules therefore only partly heal the bitumen. With

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increasing number of self-healing cycles, the extent of recovery decreases, which implies a loss of self-healing ability. The recovery time also increases with increasing number of self-healing cycles, which indicates that microcracks require more healing time with increasing aging degree of bitumen. Besides temperature, time also has a significant effect on healing ability. After an immediate reloading, the bitumen sample usually cannot return to the same point of the original loading state, but returns to a point with lower strength [55]. In this study, all samples had sufficient rest time before subsequent loading. Molecules in the interface therefore have sufficient time to regulate states or reactions and the rejuvenator has sufficient time to move (capillarity and diffusion). An analysis of the recovery properties shows that crack healing can be considered to be a viscosity-driven process. But crack closure does not imply total bitumen strength recovery. Improved healing may be achieved with a longer healing time.

6.4.4 Introduction of a real application of self-healing nanocapsules Asphalt is a widely used composite material for pavement. Because of the effect of natural environment, loading, and other factors, asphalt becomes brittle and microcracks will generate. The aging problem of bitumen will damage the asphalt original properties, especially its self-healing capability. This capability of self-healing is associated with temperature, healing time, and aging degree of itself. Comparing to the deterioration process, the self-healing capability of bitumen is not enough to repair the damage as surface raveling and reflective cracking, which are caused by aging. Therefore many researchers have paid attention to enhance the self-healing capability of bitumen. Several methods have been used including rejuvenation addition, polymer blend, heating induction, and NPs mixture [56]. It has been found that rejuvenator addition is one of the most effective methods. However, oily rejuvenators nearly could not penetrate into the asphalt pavement more than 2 cm [57]. Interestingly encapsulation rejuvenator mixed in bitumen is an alternative method not only to promote repair capacity but also overcome the disadvantages of oily rejuvenator [58]. When microcracks encounter capsules in propagation, capsules triggers to rapture, and the rejuvenator fills the cracks under the help of capillarity (Fig.6.11); at the same time, the released rejuvenator reconstitutes the asphalt binder’s chemical composition caused by aging [57]. Bitumen is a binder material of aggregates, which has a melting point of 180 C. It can be imaged that self-healing microcapsules need excellent thermal stability and mechanical properties to keep their integrity in asphalt. It was showed that the size of microcapsules, the shell thickness, and the core/shell ratio are main factors determining the stability of microcapsules [59]. Inorganic/organic composite shell structure can be formed to improve the thermal and mechanical properties of microcapsules [60]. For example, microcapsules had been fabricated with a nanoCaCO3/polymer shell structure [61]. The size of microcapsules was not greatly affected by the structure of microcapsules with nanoinorganic/organic shells. On the contrary, it was varied that the shell thickness increased due to the addition of nanoCaCO3. Moreover microcapsule shells could resist a higher temperature and protect microencapsulated rejuvenator. It was noted that the addition of inorganic particles enhanced interaction between the asphalt binders and the

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FIGURE 6.11 Illustration of microcapsules containing rejuvenator dispersing in asphalt binder.

microcapsules [62]. The self-healing mechanism of asphalt using microcapsules was investigated by mechanical tests; self-healing process contained four steps: the crack generation, microcapsules broken, rejuvenator release, and rejuvenator capillarity-diffusion [63]. Oily rejuvenator flowed along microcracks with the help of capillarity and diffused into the aging bitumen [57]. Although many efforts have been carried out to investigate microcapsules containing rejuvenator in bitumen, the states of microcapsules in asphalt binders have not been explored. Asphalt is composed of bitumen, aggregate particles, and air voids. Self-healing process in asphalt normally occurs in the binders. After several years of use, bituminous material loses part of its viscoelastic capability. At the same time microcracks occur and develop at the interface between binders and aggregates. Microcapsules in asphalt binders are expected to keep ideal states in a self-healing process. In view of the above, the purpose of this work was to analyze the morphology, distribution, thermal stability of microcapsules in asphalt binders. In addition, microcracks were generated to determine the trigger rapture state of microcapsules in asphalt binders. Based on these observations, it can be deduced a conclusion about the application possibility of the self-healing microcapsules in asphalt binders. In this study self-healing microcapsules were prepared by in situ polymerization method using MMF shell. Hydrolyzed SMA was used as an amphiphilic polymeric surfactant. SMA was hydrolyzed by NaOH and absorbed at the interface of oily droplets. Rejuvenator droplets owned strong electron negative, which reduced the oil/water interfacial tension [19]. Rejuvenator droplets were formed by high-speed stirring. The oil droplets absorbed MMF prepolymer to balance the charge. The coacervation polymers were crosslinked and then formed shells under the effects of acid and heat. Fig. 6.12A and B show ESEM surface morphologies of dried microcapsules. They have a mean size of 20 50 μm. The microcapsules keep a regular global shape. Their compact shells are smooth with little adherent. Core-shell structure can be recognized form the break microcapsules in Fig. 6.12C and D. The appearance of the core-shell structure indicates that the oily rejuvenator has been microencapsulated by polymeric shell material. These microcapsules have a mean size about 20 μm. Larger microcapsules may be more likely to break or crack. Polymer shells cannot maintain integrity under an ultimate

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FIGURE 6.12 ESEM morphologies of microcapsules containing rejuvenator (A), (B) microcapsules containing rejuvenator with mean size of 20 and 50 μm, (C) shell structure of a break microcapsule, and (D) a microcapsule with a crack on shell [64].

mechanical strength or thermal stimulation. Therefore it is important that the microcapsules keep regularity and have an appropriate mean size and shell thickness. For example, the mean size of self-healing microcapsules in asphalt needs to be less than 100 μm avoiding squeeze rapture [64]. It has been found that the shell thickness can be regulated by controlling the amount of shell material [57]. Based on the previous conclusions, all microcapsules in this study had the same core/shell ratio of 2:1 to simplify the complexity. Fig. 6.13 shows a picture of asphalt samples mixing with various contents of microcapsules. The diameter of these samples is 100 mm, and the height is 67 mm. The addition ratios of microcapsules are calculated by the content of microcapsules accounting for bitumen. The practicality of microcapsule self-healing technology can be detected by preparing asphalt concrete samples. Obviously the self-healing degree of asphalt is influenced by the content of microcapsules. In previous work, it had been reported that microcapsules could survive and in melting bitumen under 180 C 200 C. The result confirms that the microcapsules have satisfactory properties of interface stability, thermal stability, and mechanical stability, which meet the needs of application in bitumen. At present it is essential to investigate the states of microcapsules in asphalt binders. Fig. 6.14A shows a ESEM morphology of asphalt composed of aggregate and bitumen. The fine aggregates and bitumen are mixed well together. Asphalt binder is pointed by an arrow. Comparing to the total mass of asphalt, it can be concluded that the bitumen is used rarely, and it only exists in the aggregates gap. Therefore it can be imaged that the microcapsules will not shapely increase the cost of asphalt pavement comparing to the traditional repairing methods of pavement. Fig. 6.14B displays the binder mixed with microcapsules. From the point of view of the morphologies of asphalt binder, microcapsules do not affect the performance of asphalt. Microcapsules have a good compatibility with bitumen so that interface debonding does not appear between

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FIGURE 6.13

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A photography of asphalt samples (MB-3, MB-5, and MB-7) mixing with various contents of

microcapsules.

FIGURE 6.14 Microstructure morphologies of microcapsules in asphalt sample (MB-7) at room temperature state, (A) ESEM morphology of asphalt with aggregate and asphalt binders, (B) ESEM morphology of microcapsules dispersing in bitumen, and (C) a fluorescence microscope morphology of microcapsules in asphalt binders [64].

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microcapsules and asphalt binders. Fig. 6.14C shows the states of microcapsules in binder. The surface of microcapsules was completely adhered by asphalt binders. It can be obviously seen a fusion of microcapsules and bitumen. Stability of microcapsules performed very well, and rupture was not produced at the shell materials. The microcapsules were not damaged by agitation in the molten asphalt. On the other hand, it is an essential issue to verify the integrity of microcapsules in asphalt binders under a high temperature. A piece of asphalt was peeled from an asphalt sample and heated to melt state. Then melting bitumen without aggregates was spread on a microscope slide. Fig. 6.15A C shows the optical morphologies of microcapsules in melting bitumen from asphalt samples (MB-3, MB-5, and MB-7) under a high temperature of 180 C. As the arrows pointing, the microcapsules keep an intact globe shape without rapture. Even the in MB-7 under 200 C, as Fig. 6.15D shows, the microcapsules still retain their integrity without premature rapture. It indicates that the microcapsules can resist high temperature and strong squeeze during the asphalt samples formation process. It is in agreement with the previous conclusions based on microcapsules in pure bitumen [65]. The world’s first self-healing asphalt pavement using microcapsules containing rejuvenator has been built in Tianjin of China with a 50 m length (Fig. 6.16A) and has completed a 3-year test from Jan 2014 to Jan 2017. The feasibility of this method is proved by continuous observation and testing, which provides a large amount of data support for future application. This commercial product is supplied by Tianjin Sinogo Tech Co., Ltd. It is a yellow powder product with low price, which now meets the requirements of large-scale production (Fig. 6.16B).

FIGURE 6.15

Optical morphologies of microcapsules in melting bitumen peeled from asphalt samples (MB-3, MB-5, and MB-7) under high temperature, (A) MB-3 under 180 C, (B) MB-5 under 180 C, and (C) and (D) MB-7 under 180 C and 200 C.

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FIGURE 6.16 A real application of self-healing nanocapsules: (A) the world’s first self-healing asphalt pavement using microcapsules containing rejuvenator in China (Tianjin) and (B) 10 kg self-healing microcapsules.

6.5 Further advice Materials’ abilities to mimic self-healing of living organisms are of great significance in prolonging lifespan and efficient utilization of resources and energy. Driven by the technological needs, several successful approaches have been explored over the last decade. Incorporating responsive components, such as dynamic covalent bonds and supramolecular chemistry into polymers with intentionally introduced heterogeneities offer highly effective strategies in the design of materials of the 21st century. Heat, electromagnetic radiation, environment changes, or the presence of atmospheric CO2 and H2O offer external stimuli to generate favorable thermodynamics and kinetics conditions for repair. While the majority of the studies focused on the chemistries of self-healing, physical aspects are of the equal significance. Numerous examples are discussed in this review point towards the significant contributions of heterogeneities and their distribution on self-repair of polymer materials. Combining hard and soft segments into one polymer network, or reinforcing with nanoobjects containing desirable interfacial chemistry will pave the path to a new generation of materials with life-like attributes. Notably one reaction usually will not be sufficient to facilitate network repairs, just like not only one single chemical bond will cleave during mechanical damage. It is the combination of orchestrated physico-chemical events that facilitate unique self-healing properties. These inherent properties make selfhealing a challenging task. Although chemical imaging may provide useful molecular level information regarding chemical processes involved in network repairs, understanding of mechanical properties along with morphological information on small scales are also critical. Clearly there is a need for combining chemical analysis with localized mechanical testing to advance our knowledge in this critical field. Future studies should take advantage of the concept of heterogeneities in achieving rapid stimuli-responsiveness of rigid materials as well as multilevel chemical interactions achieved by synchronized chain movements and repair of multiple type chemical entities within one material.

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References [1] B. Thomas, N. Farzin, S.C. Veer, Development and implementation of a multi-scale model for matrix microcracking prediction in composite structures subjected to low velocity impact, Compos. Part B- Eng. 168 (2019) 140 151. [2] N. De Belie, E. Gruyaert, A. Al-Tabbaa, P. Antonaci, C. Baera, D. Bajare, et al., A review of self-healing concrete for damage management of structures, Adv. Mater. Interfaces 5 (2018) 1800074. [3] M. Shang, G.M. Lin, Y.T. Sun, Y.F. Wang, Research situation and applications of intelligent materials. AerAdv. Eng, Res. 8 (2015) 1136 1139. [4] Li Peng, Zou Tian, Liu Yuan, Y.H. Wang, Research on self-healing composite materials, Aer-Adv. Eng. Res. 8 (2015) 427 430. [5] W. Min, J. Bjorn, G. Mette, A review: self-healing in cementitious materials and engineered cementitious composite as a self-healing material, Constr. Build. Mater. 28 (2012) 571 583. [6] M.D. Hager, P. Greil, C. Leyens, S. van der Zwaag, U.S. Schubert, Self-healing materials, Adv. Mater. 22 (2010) 5424 5430. [7] O.S. Kuponu, V. Kadirkamanathan, B. hattacharya, S.A. Pope, Using feedback control to actively regulate the healing rate of a self-healing process subjected to low cycle dynamic stress, Smart Mater. Struct. 25 (2016) 055028. [8] H.Z. Qi, Y.H. Zhao, K.Y. Zhu, X.Y. Yuan, Research progresses in self-healing polymer materials, Prog. Chem. 23 (2011) 2560 2567. [9] P. Li, G.Z. Liu, J.Y. Huang, Research progress of self-healing intelligent composite materials, IOP Conf. Series Mater. Sci. Eng. 292 (2018) 012081. [10] J. Gallego, M.A. del Val, V. Contreras, A. Paez, Heating asphalt mixtures with microwaves to promote selfhealing, Constr. Build. Mater. 42 (2013) 1 4. [11] N.P.B. Tan, L.H. Keung, W.H. Choi, W.C. Lam, H.N. Leung, Silica-based self-healing microcapsules for selfrepair in concrete, J. Appl. Polym. Sci. 133 (2016) 43090. [12] N. Kuhl, S. Bode, M.D. Hager, U.S. Schubert, Self-healing polymers based on reversible covalent bonds, SelfHealing Mater. 273 (2016) 1 58. [13] I.N. Prassianakis, The non-destructive testing method of ultrasounds, an excellent tool for solving fracture mechanics problems, Int. J. Mater. Prod. Technol. 26 (2006) 71 88. [14] S.R. White, B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, Self-healing polymers and composites, Am. Sci. 99 (2011) 392 399. [15] D.Y. Wu, S. Meure, D. Solomon, Self-healing polymeric materials: a review of recent developments, Prog. Polym. Sci. 33 (2008) 479 522. [16] I. Harsini, M.M. Sadiq, P. Soroushian, A.M. Balachandra, Polymer nanocomposites processed via selfassembly through introduction of nanoparticles, nanosheets, and nanofibers, J. Mater. Sci. 52 (2017) 1969 1980. [17] V. Mangematin, S. Walsh, The future of nanotechnologies, Technovation 32 (2012) 157 160. [18] X.Y. He, C.Y. Zhang, M. Wang, Y.L. Zhang, L.Q. Liu, W. Yang, An electrically and mechanically autonomic self-healing hybrid hydrogel with tough and thermoplastic properties, ACS Appl. Mater. Interfaces 9 (2017) 11134 11143. [19] F.I. Altuna, J. Antonacci, G.F. Arenas, V. Pettarin, C.E. Hoppe, R.J.J. Williams, Photothermal triggering of selfhealing processes applied to the reparation of bio-based polymer networks, Mater. Res. Express 3 (2016) 045003. [20] J.Y. Lee, G.A. Buxton, A.C. Balazs, Using nanoparticles to create self-healing composites, J. Chem. Phys. 121 (2004) 5531 5540. [21] M.A. Ganjei, E. Aflaki, Application of nano-silica and styrene-butadiene-styrene to improve asphalt mixture self healing, Int. J. Pavement Eng. 20 (2019) 89 99. [22] Q.C. Li, D.G. Barret, P.B. Messersmith, N. Holten-Andersen, Controlling hydrogel mechanics via bio-inspired polymer-nanoparticle bond dynamics, ACS Nano 10 (2016) 1317 1324. [23] B.K. Deka, T.K. Maji, Effect of silica nanopowder on the properties of wood flour/polymer composite, Polym. Eng. Sci. 52 (2012) 1516 1523. [24] G.X. Xu, Z.H. Huang, P.Y. Chen, T.Q. Cui, X.H. Zhang, B. Miao, et al., Optimal reactivity and improved selfhealing capability of structurally dynamic polymers grafted on janus nanoparticles governed by chain stiffness and spatial organization, Small 13 (2017) 1603155.

Self-Healing Polymer-Based Systems

164

6. Role of nanoparticles in self-healing of polymeric systems

[25] S. Kongparakul, S. Kornprasert, P. Suriya, D. Le, C. Samart, N. Chantarasiri, et al., Self-healing hybrid nanocomposite anticorrosive coating from epoxy/modified nanosilica/perfluorooctyl triethoxysilane, Prog. Org. Coat. 104 (2017) 173 179. [26] A.G. Mohamad, E. Aflaki, Application of nano silica to improve self-healing of asphalt mixes, J. Cent. South. Univ. 24 (2017) 5. [27] R.P. Wool, Self-healing materials: a review, Soft Matter 4 (2008) 400 418. [28] H. Nazari, K. Naderi, F.M. Nejad, Improving aging resistance and fatigue performance of asphalt binders using inorganic nanoparticles, Constr. Build. Mater. 170 (2018) 591 602. [29] H.G. Dehsheikh, S. Ghasemi-Kahrizsangi, The influence of silica nanoparticles addition on the physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites, Ceram. Int. 43 (2017) 16780 16786. [30] E. Shchukina, H. Wang, D.G. Shchukin, Nanocontainer-based self-healing coatings: current progress and future perspectives, Chem. Commun. 55 (2019) 3859 3867. [31] Y. Yang, X.C. Ding, M.W. Urban, Chemical and physical aspects of self-healing materials, Prog. Polym. Sci. 49 50 (2015) 34 59. [32] D.Q. Sun, G.Q. Sun, X.Y. Zhu, A. Guarin, B. Li, Z.W. Dai, et al., A comprehensive review on self-healing of asphalt materials: mechanism, model, characterization and enhancement, Adv. Colloid Interface Sci. 256 (2018) 65 93. [33] S.J. Kalista, Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture, Mech. Adv. Mater. Struct. 14 (2007) 391 397. [34] J. Nji, G.Q. Li, Damage healing ability of a shape-memory-polymer-based particulate composite with small thermoplastic contents, Smart Mater. Struct. 21 (2012) 025011. [35] Y.L. Chen, A.M. Kushner, G.A. Williams, Z.B. Guan, Multiphase design of autonomic self-healing thermoplastic elastomers, Nat. Chem. 4 (2012) 467 472. [36] F.Y. Ding, H.B. Li, Y.M. Du, X.W. Shi, Recent advances in chitosan-based self-healing materials, Res. Chem. Intermed. 44 (2018) 4827 4840. [37] C. Kim, H. Ejima, N. Yoshie, Polymers with autonomous self-healing ability and remarkable reprocessability under ambient humidity conditions, J. Mater. Chem. A 6 (2018) 19643 19652. [38] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794 797. [39] J.F. Su, E. Schlangen, Y.Y. Wang, Investigation the self-healing mechanism of aged bitumen using microcapsules containing rejuvenator, Constr. Build. Mater. 85 (2015) 49 56. [40] S. Amiri, A. Rahimi, Anticorrosion behavior of cyclodextrins/inhibitor nanocapsule-based self-healing coatings, J. Coat. Technol. Res. 13 (2016) 1095 1102. [41] S. Abbaspoor, A. Ashrafi, R. Abolfarsi, Development of self-healing coatings based on ethyl cellulose micro/ nano-capsules, Surf. Eng. 35 (2019) 273 280. [42] T.E. Sadrabadi, S.R. Allahkaram, N. Towhidi, Preparation and property investigation of epoxy/amine micro/nanocapsule based self-healing coatings, Int. Polym. Process. 33 (2018) 721 730. [43] E. Santagata, O. Baglieri, L. Tsantilis, D. Dalmazzo, Evaluation of self healing properties of bituminous binders taking into account steric hardening effects, Constr. Build. Mater. 41 (2013) 60 67. [44] P. Ayar, F. Moreno-Navarro, M.C. Rubio-Gamez, The healing capability of asphalt pavements: a state of the art review, J. Clean. Prod. 113 (2016) 28 40. [45] J. Qiu, A.A.A. Molenaar, M.F.C. van de Ven, S.P. Wu, J.Y. Yu, Investigation of self healing behaviour of asphalt mixes using setup, Mater. Struct. 45 (2012) 777 791. [46] J.F. Su, Y.Y. Wang, N.X. Han, P. Yang, S. Han, Experimental investigation and mechanism analysis of novel multi-self-healing behaviors of bitumen using microcapsules containing rejuvenator, Constr. Build. Mater. 106 (2016) 317 329. [47] J.F. Su, J. Qiu, E. Schlangen, Y.Y. Wang, Experimental investigation of self-healing behavior of bitumen/ microcapsule composites by a modified method, Mater. Struct. 48 (2015) 4067 4076. [48] F. Moreno-Navarro, M. Sol-Sanchez, M.C. Rubio-Gamez, Exploring the recovery of fatigue damage in bituminous mixtures: the role of healing, Road Mater. Pavement Des. 16 (2015) 75 89. [49] G. Mazzoni, A. Stimilli, F. Canestrari, Self-healing capability and thixotropy of bituminous mastics, Int. J. Fatigue 92 (2016) 8 17. [50] F. Hammoum, E. Chailleux, H.N. Nguyen, A. Erhlacher, J.M. Piau, D. Bodin, Experimental and numerical analysis of crack initiation and growth in thin film of bitumen, Road Mater. Pavement Des. 10 (2009) 39 61.

Self-Healing Polymer-Based Systems

References

165

[51] L.D. Poulikakos, A.C. Falchetto, D. Wang, L. Porot, B. Hofko, Impact of asphalt aging temperature on chemo-mechanics, RSC Adv. 9 (2019) 11602 11613. [52] J. Qiu, M. van de Ven, S. Wu, J. Yu, A. Molenaar, Evaluating self healing capability of bituminous mastics, Exp. Mech. 52 (2012) 1163 1171. [53] J.F. Su, P. Yang, Y.Y. Wang, S. Han, N.X. Han, W. Li, Investigation of the self-healing behaviors of microcapsules/bitumen composites by a repetitive direct tension test, Materials 9 (2016) 600. [54] L.D. Poulikakos, M.K. Tiwari, M.N. Partl, Analysis of failure mechanism of bitumen films, Fuel 106 (2013) 437 447. [55] R.K. Abu Al-Rub, M.K. Darabi, S.M. Kim, D.N. Little, C.J. Glover, Mechanistic-based constitutive modeling of oxidative aging in aging-susceptible materials and its effect on the damage potential of asphalt concrete, Constr. Build. Mater. 41 (2013) 439 454. [56] J. Qiu, M.F.C. van de Ven, S. Wu, J. Yu, A.A.A. Molenaar, Investigating the self healing capability of bituminous binders, Road. Mater. Pavement Des. 10 (2009) 81 94. [57] J.F. Su, E. Schlangen, Y.Y. Wang, Investigation the self-healing mechanism of aged bitumen using microcapsules containing rejuvenator, Constr. Build. Mater. 85 (2015) 49 56. [58] J.F. Su, Y.Y. Wang, P. Yang, S. Han, N.X. Han, W. Li, Evaluating and modeling the internal diffusion behaviors of microencapsulated rejuvenator in aged bitumen by ftir-atr tests, Materials 9 (2016) 932. [59] L.Y. Lv, E. Schlangen, Z.X. Yang, F. Xing, Micromechanical properties of a new polymeric microcapsule for self-healing cementitious materials, Materials 9 (2016) 1025. [60] X.Y. Wang, Y.D. Guo, J.F. Su, X.L. Zhang, Y.Y. Wang, Y.Q. Tan, Fabrication and characterization of novel electrothermal self-healing microcapsules with graphene/polymer hybrid shells for bituminous material, Nanomaterials 8 (2018) 419. [61] P. Yang, S. Han, J.F. Su, Y.Y. Wang, X.L. Zhang, N.X. Han, et al., Design of self-healing microcapsules containing bituminous rejuvenator with nano-CaCo3/organic composite shell: mechanical properties, thermal stability, and compactability, Polym. Compos. 39 (2018) E1441 E1451. [62] X.L. Zhang, Y.D. Guo, J.F. Su, S. Han, Y.Y. Wang, Y.Q. Tan, Investigating the electrothermal self-healing bituminous composite material using microcapsules containing rejuvenator with graphene/organic hybrid structure shells, Constr. Build. Mater. 187 (2018) 1158 1176. [63] D.Q. Sun, Q. Pang, X.Y. Zhu, Y. Tian, T. Lu, Y. Yang, Enhanced self-healing process of sustainable asphalt materials containing microcapsules, ACS Sustain. Chem. Eng. 5 (2017) 9881 9893. [64] Y.Y. Wang, J.F. Su, E. Schlangen, N.X. Han, S. Han, W. Li, Fabrication and characterization of self-healing microcapsules containing bituminous rejuvenator by a nano-inorganic/organic hybrid method, Constr. Build. Mater. 121 (2016) 471 482. [65] J.F. Su, X.L. Zhang, Y.D. Guo, X.F. Wang, F.L. Li, Y. Fang, et al., Experimental observation of the vascular self-healing hollow fibers containing rejuvenator states in bitumen, Constr. Build. Mater. 201 (2019) 715 727.

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C H A P T E R

7 Self-healing biomaterials based on polymeric systems Baolin Guo and Rui Yu Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, P.R. China

7.1 Introduction For centuries, human strives for a longer and healthier life that has encouraged numerous scientists, engineers, and doctors to invent new technology, materials, and medicines to establish a strong defend against the harsh environment and fatal diseases. Biomaterials has been studied and developed for this strive in the past 50 years [1]. Biomaterials acting as scaffolds, matrices, or constructs have been applied in various bioactive areas such as tissue engineering, regenerative medicine, and drug/cell delivery systems and have made great benefits in human safety [1
4]. In all living creatures, self-healing is a common but outstanding property, that enables the creatures to cure, to restore to their original state or health when they are impacted by any conflict or disease [5,6]. Realizing the great significance and advantages of self-healing process, scientists have made great efforts to endow biomaterials with the capability to heal, thus it will further not only expand the lifetime and durability of these biomaterials and promotes their function [7], but also give rise to new opportunities in life science. For example, the self-healing injectable hydrogels with suitable mechanical properties have been widely utilized as scaffolds for constructing implants which would facilitate minimal invasive treatments [8], or tissue engineering as biocompatible inks used in 3D printing [9,10]. Up to now, plenty self-healing biomaterials based on polymeric systems, ceramics, or metals composite have been fabricated and even some of them have been successfully applied in clinical use [7]. Self-healing ceramic or composite biomaterials are almost based on microcapsule embedment approach and also cataloged as extrinsic self-healing biomaterials for that they could not self-heal intrinsically [5,11]. Generally the healing process is triggered by the rupture of the capsules caused by mechanical damage and the release of healing liquid into the cracks of matrix which contain monomer, finally a polymerization

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reaction would take place in the cracks to complete the healing process [12]. Consequently the applications of these ceramic and composite biomaterials are limited within dental or orthopedic implant for their nature of extrinsic self-healing and specific mechanical properties. In contrast, self-healing polymeric systems have more comprehensive applications due to their vast diversity of structure, property, bioactivity, and ease of handling. Therefore compared with the self-healing ceramic or composite biomaterials, self-healing polymeric systems have more potential in developing multifunctional self-healing biomaterials for a more complex and diversified area including tissue engineering, delivery systems, and functional surfaces [13
16]. Therefore this chapter focuses on self-healing biomaterials based on polymeric systems in terms of their applications with the illustration of corresponding mechanisms. Despite the booming emergence of self-healing biomaterials based on polymeric systems, this area still needs continuous development, such as, there needs more specific standards to evaluate the self-healing efficiency and the vast majority of products are still far away from clinical requirement. So next, the shortages of this study and the perspective of its future development are presented. Moreover, the polymeric biomaterials with self-healing ability render new opportunities and benefits for traditional biomedical materials such as wound dressing, electronic skin, and other newly emerging technology, for example, 3D printing. The outlook of self-healing biomaterials based on polymeric systems is also outlined.

7.2 Self-healing biomaterials in tissue engineering Tissue engineering is an interdisciplinary field combining cell biology, biomaterials engineering, and medicine, it aims to restore or enhance tissue functions, and even replace the tissues or organs once they have been infected or damaged [17,18]. Generally one whole basic routine of tissue engineering involves three fundamental elements, scaffolds, cells, and signals. Traditionally a successful polymeric scaffold should be with the properties as high porosity, biocompatibility, and biodegradability, moreover the ability to promote cell adhesion and suitable mechanical properties [19,20]. To this end, synthetic and natural polymers have been largely utilized in fabricating scaffolds for their diverse properties and bioactivity. Natural polymers such as chitosan, fibrin, silk fibroin, and alginate are selected for their biocompatibility and biodegradability [21]. Synthetic polymers such as poly(L-lactic acid) (PLA), poly(glycolic acid) (PGA), poly(lactic-co-glycolic) acid (PLGA), and polycaprolactone (PCL) are widely adapted for their economic benefits and easy modification [22]. The combination of natural and synthetic polymers is also an effective method to fabricate scaffolds with excellent bioactivity and mechanical properties [23,24]. Aside from the infliction generated by disease, human tissues, and organs face with high intensity physiological environment during their whole work time. Similar as the original human tissues and organs could be damaged and form mechanical failure during the long-term usage, these implanted scaffolds would suffer the harsh high intensity environment and inevitable damage. Meanwhile, due to the lack of facile method and technology to detect and repair damage, there is an urgent demand for these polymeric scaffolds to have the ability of self-healing both in mechanical and structural integrity. Therefore no matter where the locations of the scaffolds are, one prerequisite of the polymeric scaffolds

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is that, they should bear the mechanical stress of human body. Generally the mechanical strength of the polymeric scaffolds need to be stronger in orders of magnitude than their corresponding tissues or organs should bear. However, cyclic loading would lead to mechanical failure of these polymeric scaffolds [25]. Meanwhile, different from the simple environment in vitro, these scaffolds are scoured by body fluids, for example, blood and tissue fluid in our whole body, gastric juice or saliva in one particular organ or tissue. Sometimes, the environment would also be affected by viruses or cancer cells, proteins, and enzymes. Inevitably these implants would be rubbed, pulled, or pushed by surrounding tissues, for example, the cardiac valves. All these complex environments in vivo provide favorable conditions for the formed damages to propagate. Besides, the mechanical failures of these polymeric scaffold which mainly generated from fatigue fracture and wear are depended on the locations and their corresponding loading patterns [26]. In the first review article about self-healing biomaterials published in 2011, Brochu et al. suggested that, self-healing biomaterials could be divided into three generations [7]. In the zeroth generation, self-healing materials retard failure forming process; while the selfhealing materials of first generation can halt and fill damage without recovering to its original state and shape. Commonly these first generation self-healing biomaterials are ceramic or polymeric composite biomaterials, and exhibit extrinsic self-healing for the needs of extra addition of healing agent and expected to be applied in dentistry and orthopedics [27]. The second generation self-healing materials can actually repair themselves and restore to their original states which indicates intrinsic self-healing. In consideration of the lack of methods and technologies to detect and repair the damage for scaffolds in vivo, it prefers the second generation self-healing biomaterials in practical use. Moreover, these intrinsic polymeric self-healing biomaterials also open up avenues for minimal invasive treatment in tissue engineering. Herein, this second generation self-healing biomaterials with intrinsic self-healing capability are mainly discussed in this section. So far, various types of self-healing biomaterials based on polymeric systems have been widely developed, such as hydrogels, films, particles, and capsules, classified by their structure types.

7.2.1 Self-healing hydrogels Hydrogels are three-dimensional interconnected networks with the ability of high content water-uptake, and tunable physical and chemical properties. Compared with other types of biomaterials as films or particles, hydrogels are excellent candidates for tissue engineering, on account for their mimic and reproducible features of the natural extracellular matrix environments [28
31]. Hydrogels with self-healing capability would have longer lifetime and prominent performance when acting as scaffolds [32]. Compared with traditional hydrogels cross-linked by covalent bonds, self-healing hydrogels are more likely to have tunable mechanical properties mimicking natural tissues, which renders them more advantages in tissue engineering [33]. Meanwhile, the shear-thinning self-healing hydrogels which are also named as injectable self-healing hydrogels could be injected through a syringe toward the specific sites of human body within minimal surgical incision and easily formed into the irregular shape to fill the specific void of tissue. These injectable self-healing hydrogels have made great benefits for both surgeons and patients

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for the realization of minimal invasive treatment [9]. Moreover, it has successfully proved that injectable self-healing hydrogels could be employed as bioink for the newly emerging technology 3D printing to fabricate more complicated construction [9,10,34]. 7.2.1.1 Mechanism 7.2.1.1.1 Noncovalent bonding

Hydrogels that cross-linked by weak, reversible noncovalent bonding are generally named as supramolecular hydrogels [13,35]. The eminent characteristics of supramolecular hydrogels are stimuli
responsiveness and self-healing. Supramolecular hydrogels with stimuli
responsiveness have great potential in delivery systems and biosensors for their environmental sensitiveness toward light, pH, thermal, or strength [36,37]. This section focuses on supramolecular hydrogels with self-healing ability and the typical types of noncovalent bonding are discussed. 7.2.1.1.1.1 Electrostatic interaction Electrostatic interaction is formed between the positively charged nuclei and the negatively charged electron of two molecules and ubiquitous in nature [38]. It is a promising method to designing self-healing hydrogels based on electrostatic interactions. Because of their strong strength and satisfactory reversibility, hydrogels based on electrostatic interactions can be both with tough mechanical properties and self-healing ability. Diverse systems have been developed based on electrostatic interaction from natural polymers, synthetic polymers, or their combinations. Tondera et al. reported a self-healing hydrogel system based on the combination of a negatively charged natural polymer and a synthetic polymer modified with peptide sequences [39]. As the type of peptide-conjugated PEG and oligosaccharides would largely affect the mechanical properties of the hydrogel, dextran sulfate, and peptide-conjugated PEG (KA5-star PEG) had been finally selected to fabricate hydrogel to ensure the optimal injectability. The main purpose of this chapter was to fabricate self-healing injectable hydrogel avoiding complex invasive operation. The hydrogel had been proved to be injectable and biocompatible, while the self-healing process was rapid and could complete in 20 min for the full recovery to the initial elastic modulus (1.5 h gelation). With the presence of peptide containing (lysine/arginine
alanine) motif, cell adhesive peptide can be conjugated to the hydrogel and finally promote the hydrogel
tissue interaction. Among diverse self-healing hydrogels based on electrostatic interactions, polyampholyte is a typical polymeric system and has been intensively selected [40,41]. The polymer architecture depends on the relative location, and can act as random copolymer, block copolymer, and nonlinear star-shaped polymer resulting in different mechanical properties. The polyampholyte-based structural biomaterials were pioneered by Sun et al. [42], as shown in Fig. 7.1. They utilized a pair of ionic monomer which are sodium pstyrenesulphonate (NaSS) and 3-(methacryloylamino)propyl-trimethylammonium chloride to form random polymers, and obtained the hydrogels from a concentrated solution of oppositely charged monomers with charge ratio near 1:1. The hydrogels were strong and tough, showing tensile stress of 0.1
2 MPa, fracture strain of 150%
1500%, and work extension at fracture of 0.1
7 MJ m23. They also had very high fatigue resistance for fully self-recovery ability. For the self-healing efficiency, it can reach to B99% with less

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FIGURE 7.1 Schematics of self-healing polyampholytes hydrogel based on electrostatic interactions. (A) The schematic illustration of the proposed mechanism for stretchable polyampholytes hydrogel. (B) The chemical structure of ionic monomers of the polyampholyte [42].

hydrophobic system in the hydrogel because the strong bonds would weaken the selfhealing process. Young’s modulus and damping ability of these hydrogel can be tuned by the type of ions, and the hydrogels also demonstrated excellent biocompatibility and antibiofouling properties. As the mechanical properties can be tuned by the types of the charged monomers and the polymer architecture, the author anticipated this system would be widely broadened, and these self-healing polyampholytes-based hydrogels have high potential in structural biomaterials. Long et al. focused on fabricating self-healing polyampholyte hydrogels endowing these hydrogels with electrical conductivity [43]. In their work, the 3-sulfopropyl methacrylate potassium salt monomers and imidazolium-based ionic liquid monomers containing urea groups acted as negatively and positively charged motifs, respectively, thus electrostatic interactions between imidazolium and sulfonate and hydrogen bonding between urea groups worked synergistically forming a self-healing hydrogel. They found that the dialysis is a key procedure to improve the mechanical strength of these hydrogels, for the remove of partial counter ions. Hydrogen bonding and electrostatic interactions could both attribute to improving the mechanical strength, toughness, and self-healing efficiency of the hydrogels. The hydrogels with the treatment of three days dialysis exhibited a healing efficiency at  91%, along with the nearly full recovery of their electrical properties even after bending for 500 times, showing their potential in wearable electronics and electrical devices.

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7.2.1.1.1.2 Hydrogen bonding Hydrogen bonding is a partial electrostatic attraction between a hydrogen atom which is directly bonded to a more electronegative atom such as nitrogen, oxygen, or fluorine and another atom in vicinity bearing a lone pair of electrons. Hydrogel bonding is ubiquitous in almost every aspect of life and nature, such as molecular recognition, protein folding, protein-ligand interactions, and catalysis. Hydrogen bonding is one of the most effective methods to design self-healing hydrogels. One characteristic accomplishment was presented by Ameya and coworkers. They reported self-healing hydrogels formulated from acryloyl-6-aminocaproic acid (A6ACA) precursors containing dangling side chains with a carboxyl group [44]. The selection of A6ACA precursors based on the acknowledgment of their optimal balance of hydrophobic and hydrophilic interactions which renders its side chain to bond with the dangling carboxyl group under an acid environment with a restriction of structure control of dangling group. At low pH, the largely protonated dangling carboxyl groups would form hydrogen bonding with other carboxyl groups or amide group at the backbone, and the whole selfhealing process of a crack or two hydrogels completed within seconds. At high pH, the hydrogen bonding was faded for the carboxyl groups at backbone were deprotonated generating electrostatic repulsion. In other words, this kind of self-healing hydrogels only exhibit self-healing at low pH, which offers them unique applications in acidic environment, such as gastric tissue engineering. Compared with the hydrogen bonding form carboxyl groups and amide groups, 2ureido-4-pyrimidone (UPy) has been more intensively exploited for its multivalent hydrogen bonding in favor of forming tough, rapid self-healing hydrogels. The UPy motifs can dimerize via quadruple hydrogen bonding in chloroform and widely utilized in fabricating supramolecular polymers [45]. However, its applications in designing self-healing hydrogels were limited by the influence of water intervening their properties. Until 2012, the UPy motifs-based self-healing hydrogels were pioneered by Meijer and coworkers [46]. As shown in Fig. 7.2, traditional hydrophobically modified poly(ethylene glycols) (PEGs) were selected as the main hydrophilic backbone, then end-functionalized with UPy motifs. In aqueous environment, these hydrophobic alkyl spacers would form as a pocket shielding UPy motifs from water, thus assemble into spherical micelle or fiber which depends

FIGURE 7.2 Illustration of self-healing hydrogels based on UPy-modified PEG hydrogelators. (A) Cryotransmission electron microscope image of one specific hydrogelator. Scale bar represents 100 nm. (B) Schematic representation of hydrogen bonding in UPy dimers and different forms as single chains, micelles and nanofibers of UPy-modified PEG hydrogelators in aqueous solution [46]. PEG, Poly(ethylene glycols); UPy, 2-ureido-4pyrimidone.

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on average molecular weight of the PEG and the length of alkyl spacer, and the process was confirmed by atomic force microscope. On cooling or concentrating, there formed soft hydrogels with self-healing and shear-thinning ability. The mechanical properties and selfhealing efficiency could be tuned by the structure and concentration. Since then, the selfhealing hydrogels used on UPy motifs have been increasingly studied [47]. Conventionally natural polymers modified with UPy motifs are an effective method to realize the applications of UPy-based self-healing hydrogels in tissue engineering. Hou and coworkers reported self-healing hydrogels formed by UPy motif-modified dextran (DEX), and confirmed that hydrogels encapsulated chondrocytes and bone marrow stem cells could be healed into an integrity and implanted subcutaneously in a nude mouse for cartilage-bone tissue engineering [48]. The mechanical properties and self-healing ability could be simply adjusted by the concentration of UPy content. Besides, some ingenious methods of UPy motif-based self-healing hydrogels have been reported. Insu Jeon and coworkers has made prominent contribution in this area [49]. Considering the tendency for UPy motifs forming dimmers dramatically decreased in hydrophilic environments, it is necessary to provide hydrophobic microenvironments for UPy motifs. Distinguished from the above-mentioned mechanism of hydrophobic alkyl spacers modified polymer backbone, sodium dodecyl sulfate (SDS) micelle was the key reagent to provide the hydrophobic microphases. The polymerization reaction would take place with the addition of NaCl, acrylamide, and UPy-based cross-linker, thus forming the self-healing hydrogel with remarkable mechanical properties. Furthermore, the self-healing process was so rapid and with high efficiency, a qualitative experiment showed a whole self-healing process completed for less than 30 s. As the hydrophobic microphases provided by micelles, the evaporation of water would change the morphology of these micelles leading to the disassembly of the UPy motif-based cross-linker. Therefore it was reasonable that the self-healing ability of hydrogels depends on whether the hydrogels were placed in dry environment or in high humidity. 7.2.1.1.1.3 Host
guest interaction Macrocyclic host
guest interaction as one typical type of noncovalent interactions bases on directional coordination between two compounds, and has been widely studied and applied in self-healing hydrogels. Generally the forming host
guest complex is induced by hydrophobic interactions, and π
π stacking. So far, various types of macrocyclic compounds which act as “host” molecule have been synthesized, such as crown ethers, cucurbit [n]urils, calxi [n]arenes, pillararene, catenanes, and cyclodextrin (CD) [50]. As considering the requirement of biocompatibility for tissue engineering, CD is an ideal candidate and has been approved safety by US FDA and widely applied in drug delivery, food, and pharmaceutical and chemistry industry. CD is a family of oligosaccharides, consisting of 6, 7, 8 glucose subunits, named as α-CD, β-CD, γ-CD, respectively [51]. Owning a hydrophobic interior and a hydrophilic exterior, CD is soluble in water and offers a hydrophobic cavity to form a complex with other hydrophobic “guest” molecules. The general strategy of these self-healing hydrogels is to incorporate CDs and “guest” onto various polymers. As the formed host
guest complex act as crosslinker, the mechanical properties of hydrogels partially depended on the choice of host and guest compounds.

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Harada et al. firstly reported the self-assembly of α-CD with polyethylene glycol (PEG), poly(tetrahydrofuran), or poly(propylene glycol), thus forming the liner pseudopolyrotaxane (PPR) [52]. Then this PPR assembly has been intensively utilized to design hydrogels with stimuli
responsiveness and self-healing ability. For instance, Liao et al. investigated a photoresponsive PPR hydrogel with the addition of azobenzene compounds [53]. They have demonstrated that adding azobenzene into the hydrogel and along with UV-Vis irradiation could effectively control the reversible sol-to-gel transition. While Pradal et al. have demonstrated the self-healing ability of PPR hydrogels based on Pluronic and α-CD, of which the self-healing efficiency reached nearly 100% after 2 h [54]. β-CD is also a widely used macrocyclic host compound in self-healing hydrogels for their large hydrophobic cavity, and amantadine and ferrocene are frequently selected as the corresponding guest molecules. Recently Deng et al. presented a self-healing multifunctional soft hydrogel based on the relative weaker host
guest interaction between β-CD and isopropylacrylamide (NIPAM) [55]. In their system, NIPAM both served as guest compound and monomer, the acrylate modified β-CD was then copolymerized with NIPAM forming a self-healing hydrogel. Meanwhile, the author added carbon nanotubes (CNTs) as another physical cross-linker and conducting substrate, and polypyrrole (PPY) as a highly conductive agent into the hydrogels to endow them high conductivity. The conductive self-healing hydrogel poly(NIPAM-co-β-CD)/CNT/PPY also exhibited stimuli
responsiveness toward near infrared (NIR) light and temperature. The soft conductive hydrogels had been successfully applied in tissue engineering as pressure-dependent sensors, and human motion sensors. 7.2.1.1.1.4 Metal
ligand coordination The interaction between metallic cations and ligands is a common interaction in all living creatures and offers plenty opportunities in designing self-healing hydrogels. However, their applications in tissue engineering are limited by high pH condition or extra addition of organic solvent [56]. Herein, the metal
ligand coordination-based system working at physiological environment is of great value. Ossipov group reported an ideal self-healing hydrogel based on metal
ligand assembly, and applied the hydrogel in bone regeneration [57]. As shown in Fig. 7.3, the natural polymer silk fibroin could be coated with calcium phosphate (CaP@mSF) by biomineralization method, then on UV irradiation, the mixture of CaP@mSF with acrylated and bisphosphonate (BP) modified hyaluronic acid (HA) formed a robust dually crosslinked self-healing hydrogel (Am-HA-BP CaP@mSF). Compared to the control sample hydrogel HA-BP CaP@mSF without dual cross-linker, the introduction of acrylate groups in HA could improve the mechanical properties and stability under physiological conditions. The Am-HA-BP CaP@mSF hydrogel had proved to be shear-thinning, and with 100% self-healing efficiency.







7.2.1.1.1.5 Hydrophobic interactions Hydrophobic interactions as hydrophobic exclusion, lead to the aggregation of the hydrophobic substances in water matrix and are ubiquitous in regulating biological systems, such as protein and DNA. Notably the hydrophobic interactions in host
guest systems are not included in this section. Herein, the polymers fabricated with hydrophobic monomers are the typical systems discussed in designing self-healing hydrogel based on hydrophobic interactions.

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175 FIGURE 7.3 Schematic illustration of in situ assembly of self-healing hydrogel based on metal
bisphosphonate coordination under physiologic conditions. [57].

Micellar copolymerization is a typical strategy in forming hydrophobic interactionsbased self-healing hydrogel. Tuncaboylu et al. presented typical works on this strategy. In the pioneer work, they presented a hydrogel based on acrylamide copolymerized with hydrophobic alkyl methacrylate (C18 or C22) with surfactant SDS providing micelles and NaCl as electrolyte [58]. Enough amount of NaCl enabled suitable micelle size and solubility of C18 or C22, thus leading strong hydrophobic associations which endows high toughness of the hydrogels. In C18 hydrogels, the self-healing efficiency was about 100% in the quantitative experiments. The length of alkyl groups had an obvious influence of the mechanical properties. C18 hydrogels break at elongation ratio of 3600% while C22 hydrogel of 1300%
1700%. Later, they studied the effect of surfactant toward the mechanical properties and self-healing efficiency of the hydrogel [59]. With the knowledge of hydrophobic substance is soluble in water assisted by enough addition of surfactant (SDS) and an electrolyte (NaCl), they developed a self-healing hydrogel by micellar copolymerization with acrylamide and stearyl methacrylate (C18) in SDS
NaCl solution. Compared the mechanical properties and self-healing behavior of the hydrogel with or without SDS, they confirmed that the existence of surfactant is a crucial element in hydrophobic interactionbased self-healing hydrogel. The hydrogels showed to be tough and fracture strain 1700%, while the hydrogels without surfactant showed fracture strain about 700% and larger ultimate strength at order of magnitude. Besides, without surfactant, the hydrogels showed no self-healing ability. Actually all the four elements in micellar copolymerization: hydrophobic group, hydrophilic group, surfactant, and electrolyte are crucial and have plenty combinations. For instance, in the self-healing hydrogel developed by Zhang et al., acrylic acid and stearyl methacrylate were utilized as hydrophobic group and hydrophilic group, ferric chloride was the electrolyte, while cetyltrimethylammonium bromide or sodium dodecyl

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benzene sulfonate was chosen to act as the surfactant [60]. Besides the concentration of ferric ions affected the mechanical, rheological, and swelling properties of the hydrogels, the electrostatic interactions between ferric ion and carboxylic acid groups endowed the hydrogels with excellent strength and self-healing ability along with hydrophobic interaction. 7.2.1.1.1.6 Crystallization Generally crystallization process in polymers, would generate a more stable and ordered hydrophobic domain with noncovalent bonding. This process takes place along with the specific regular-ordered structural conformations, such as α-helices of poly(L-leucine) and β-sheets of poly(L-valine), leading to the formation of micelles, vesicles, or 3D hydrogels. The crystallization process has been applied in developing self-healing hydrogels for a long period. The amphiphilic diblock copolypeptide hydrogels (DCHs) are synthetic materials owning tunable composition, structure, and properties. The DCHs possessing self-healing and shear-thinning, and biocompatibility, have been widely applied as biomaterials. Sofroniew group studied the biocompatibility and application of DCHs in the central nervous system [61]. The polypeptide backbone in DCH hydrogel could be easily tuned by the ratio of hydrophobic to hydrophilic amino acid, and modified with functional peptide sequence thus endowing them with more diverse properties, such as cell adhesion, and molecular signaling. Therefore it is anticipated to be an effective method to fabricate self-healing hydrogel based on DCHs. Deming group presented great work in this area. In their early work [62], upon adjusting the hydrophobic domain content to be 20 mol.%, the DCHs in which the amphiphilic copolypeptide consisting of lysine as hydrophilic residue and leucine or valine as hydrophobic residue could form at a much lower concentration of 0.4 wt. %, compared with other DCHs at 5
10 wt.%. Afterwards, his group attempted to endow the DCHs with excellent self-healing ability. As shown in Fig. 7.4, they designed a DCH with the addition of polyelectrolyte, anticipating the formation of polyion complexation along with the conformation-directed self-assemble of amphiphilic diblock polypeptide [63]. In the aggregate of hydrophobic domains, the polyion complexes (PIC) formed by ionic segments. So such a DCH possessing PIC was named as DCHPIC, and the DCHPIC showed excellent self-healing ability, with full recovery of the elasticity within several minutes and stiffness within 10 s. As the DCHPIC could be injected through a needle and show cell-compatibility, this new class of DCHPIC had great potential in designing selfhealing hydrogels.

7.2.1.1.2 Dynamic covalent bonding

Dynamic covalent bonding can reversibly assemble and disassemble simultaneously and continuously with a rapid equilibrium. Therefore hydrogels based on dynamic covalent bonding are ought to be self-healing, and tougher than those supramolecular hydrogels. As, the equilibrium of assemble and disassemble in dynamic covalent bonding also depend on the external environment, such as pH condition, temperature, redox, this kind of hydrogels is of stimuli
responsiveness. Because of the self-healing ability and tough mechanical properties compared with supramolecular hydrogels, these hydrogel fabricating from dynamic covalent bonding have been intensively studied.

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FIGURE 7.4 Schematic illustration of self-healing amphiphilic diblock copolypeptide hydrogel formed by conformation-directed crystallization [63]. 7.2.1.1.2.1 Diels
Alder reaction Diels
Alder reaction belong to “click chemistry,” is cycloaddition of unsaturated species between a conjugated diene and dienophile. The catalyst-free and high atom efficiency are of great advantages in fabricating self-healing hydrogel for tissue engineering [64]. Moreover, it has been confirmed that the gelation of systems based on Diels
Alder reaction took place under PBS buffer (pH 7.4) or physiological condition, and the hydrogels kept stable under physiological condition [65,66]. Wei et al. presented self-healing hydrogels fabricated by fluvene-modified dextran and dichloromaleic acid-modified poly(ethylene glycol) [67]. The gelation time depended on the molar ration of fulvene to dichloromaleic acid groups. And the self-healing process of a scratch in the hydrogel completed at 37 C after 7 h. Another self-healing hydrogel based on Diels
Alder reaction was fabricated by furfural modified cellulose nanocrystal crosslinked with dimaleimide poly(ethylene glycol) [68], while the cellulose nanocrystal reinforced the mechanical properties of the hydrogels. The gelation time and mechanical properties were affected by the substitution degree of furyl and molar ratio of furyl to maleimide. This hydrogel showed excellent self-healing and antifatigue properties in the cyclic tensile and compressive loading
unloading tests. 7.2.1.1.2.2 Thiol
disulfide exchange Thiol
disulfide exchange is a common process in biology as in protein folding, and have been widely incorporated in polymers for its reversibility and environmentally friendly. So far, thiol
disulfide exchange has exhibited

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its potential in fabricating self-healing hydrogel. For instance, a self-healing hydrogel was formed by the mixing of thiol functionalized triblock copolymer P127 with dithiolande modified PEG [69]. The hydrogel showed thermo-responsiveness. When the mixture was injected through a needle, the hydrogel rapidly formed in situ. The hydrogel showed rapid self-healing ability under physiological condition. As the thiol
disulfide could be intervened by oxidation of thiol, Au1 could be added as a protect agent. Dupin et al. represented such a work and proved that the addition of Au1 was no cytotoxic, and had no influence to the reversible exchange reaction and the self-healing ability [70]. Besides, Au1 could play an important role in tuning the mechanical properties of the hydrogel. 7.2.1.1.2.3 Imine bonds Imine bond, also named as Schiff base, is formed by the reaction between amine and aldehyde groups. The reversible nature of imine bonds renders its potential in fabricating self-healing systems. Moreover, the only byproduct of the dynamic Schiff crosslinking reaction is water, and it has been proved that the cytotoxicity of the imine bond-based hydrogel is minimal. Zhao et al. reported polysaccharide-based hydrogels cross-linked from the imine bond of the amino groups on N-carboxyethyl chitosan and the aldehyde groups on oxidized sodium alginate (OSA) [71]. The hydrogels can be injected into human body and their stiffness have been proved to be as similar as nature brain tissue thus enabling the application in the treatment of neurological lesions. Compared with aliphatic imine bond, aromatic imine bond has higher stability. So in another work from Wei group, benzaldehyde groups modified poly(ethylene glycol) (PEG) was chosen to react with chitosan to form a self-healing hydrogel [72]. Through tuning the mass content and CHO/NH2 ratio, one most tough hydrogel exhibited the highest G0 value (B20 kPa), which is much higher than the hydrogel based on aliphatic imine bond above mentioned. In their system, the gelation completed within 1 min, and an artificial punched hole could gradually close up after 2 h. Both the qualitative and quantitative measurement confirmed the self-healing behavior of the aromatic imine bond-based hydrogel. Besides, the hydrogel has stimuli
responsiveness, to pH, amino acids, and vitamin B6 derivate, and enzymatic degradation, which all endow them with wide applications in biological fields. Dopamine (DA)-based self-healing hydrogels have been widely fabricated by metal
catechol coordination. However, the high pH condition and restricted self-healing ability caused by oxidation of catechol group have limited their applications in tissue engineering. Ma group reported a polydopamine (PDA)-based self-healing hydrogel via a facile approach under metal-free condition [73]. This self-healing hydrogel was simply fabricated by oxidizing the mixture of gelatin and DA with NaIO4 under physiological condition. The imine bond between gelatin and PDA enabled the hydrogel with fast selfhealing ability. These injectable hydrogels could immediately self-heal into one integrity and exhibited fast recovery after 80% compression, simple remoldability and good adhesiveness. 7.2.1.1.2.4 Acylhydrazone bonds Acylhydrazone bond is formed by the reaction between hydrazine and aldehyde groups, belong to imine bond. It is reversible and stable nature under physiological conditions render great potential for fabricating self-healing hydrogels which could be applied in biological system. It is stable nature under physiological

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conditions is induced by the phenomenon that the reassembly only works under acidic conditions (pH 4.0
6.0) which indicates [74] the self-healing process only proceed under acidic conditions. To address this issue, a dually cross-linked hydrogel was designed with dynamic acylhydrazone bond and disulfide bond and extra addition of catalytic aniline accelerating the reassemble of acylhydrazone [75]. Thus the hydrogel exhibited self-healing under acidic (pH 3 and 6), basic (pH 9), and neutral conditions, as shown in Fig. 7.5. 7.2.1.1.2.5 Oxime bonds Oxime bond is formed by the reaction between hydroxylamine and aldehydes or ketones groups, belong to imine bond as well. This kind of dynamic covalent bonds is of high formation efficiency, environmentally friendly as water of only byproduct, take place under mild conditions. Besides, compared with imine bonds and hydrazone bonds, oxime bond is of higher hydrolytic stability [76]. The biocompatibility of oxime bond-based hydrogels has already been demonstrated with cell encapsulation and injection into myocardial tissue [77,78]. Thus the dynamic oxime bond is expected to fabricating self-healing hydrogels. For instance, Sumerlin et al. presented a oxime-contained self-healing hydrogel with the

FIGURE 7.5

Schematic presentation (A) and photo images (B) of a dynamic hydrogel cross-linked by acylhydrazone bond and thiol
disulfide exchange with self-healing ability under acidic, basic, and neutral conditions [75].

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backbone polymer formed by conventional radical polymerization of N, N-dimethyl acrylamide (DMA) and diacetone acrylamide (DAA), and then cross-linked with difunctional alkoxyamines O, O-1,3-propanediylbishydroxylamine dihydro-chloride [79]. The autonomously self-healing process of two cut pieces of hydrogel completed after 2 h, and the healed hydrogel could bear tensile force by stretching with tweezers perpendicularly to the cut and their own weight. 7.2.1.2 The evaluation of self-healing efficiency Self-healing describes the recovery process of materials that restore to their original state after deformation or fracture, including the integrity of structure and mechanical properties. The qualitative and quantitative experimental methods have been established to evaluate the self-healing of hydrogels. Qualitative methods are the visual observations of the integrity of a reunited hydrogel. Generally the cut pieces of a hydrogel are placed in contact without any external power supply and then these cut pieces would emerge into a reunited hydrogel. Next, the self-healed hydrogel need further visual verifications, whether they could be suspended against their own gravity, or if they could bear stretching and bending. These qualitative tests can be observed by naked eyes or along with microscopic imaging techniques, such as confocal microscopy, scanning electron microscopy (SEM), and fluorescence microscope. Quantitative methods are of more complexity. The cyclic compression and tensile tests with specific recovery periods are conducted to evaluate the recovery degree of a particular mechanical property compared with the original state. The alternate step strain test demonstrates the rheological recovery behavior of a reunited hydrogel. Besides, healing time is a crucial element for self-healing efficiency, for the relationship between the kinetics of the healing process and the mechanical crack growth are of great importance. In practice, the healing time for full recovery would be recorded and the recovery degree during certain times. 7.2.1.3 Applications in tissue engineering 7.2.1.3.1 Tissue adhesives

Tissue adhesives acting as glues or patches toward diverse tissues wound have been developed with comprehensive functions as hemostatic, adhesiveness, or sealability. The commonly used tissue adhesives include synthetic polymers such as polycyanoacrylate, poly(ethylene glycol), polyurethanes, and natural polysaccharide as chitosan, dextran, chondroitin sulfate, and protein as fibrin, gelatin, and albumin [80]. Besides, the structure of tissue adhesives plays a crucial role in wound healing. Among various types of tissue adhesives as rubber, electrospun nanofiber, membrane, foam, sponge, hydrogels have prominent advantages [81]. The high-water-content nature provides benefit for a moist wound closure and would promote the oxygen and nutrition exchange and absorb exudates. Moreover, their modulable nature renders them a wider range of applications in various human soft tissues. Endowing the hydrogels of self-healing ability would largely promote their lifetime and performance; especially reinforce their usability and durability under physiological conditions, constant motion, and external stimuli. The pilot study was

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conducted by Shyni group [44]. The self-healing hydrogen bond-based hydrogels named as A6ACA applied in gastric tissue adhesive. The hydrogel adhesiveness is strong enough to bear its weight, and successfully prevent the leakage of gastric acids for stomach perforations. The adhesion strategy employed by some maritime creatures has encouraged the development of adhesives. Under oxidizing or alkaline condition, the abundant free catechol groups would be oxidized to ortho-quinone, then triggers the crosslinking reaction of adhesive proteins. Thus the mussel-inspired self-healing hydrogel containing free catechol hydroxyl group is an excellent candidate for tissue adhesives. Early in 2002 a family of injectable citrate-based mussel-inspired hydrogels were fabricated, and successfully used as tissue adhesives [82], exhibiting stronger wet tissue strength than traditional adhesives at 2.5
8.0 folds. However, at that time, the self-healing ability was not considered in the worker’s targets. Recently a PDA
polyacrylamide hydrogel was fabricated with π
π stacking and hydrogen bonds of PDA chains, and demonstrated super stretchability, high toughness, cell affinity, and excellent stimuli-free self-healing ability [83], as shown in Fig. 7.6. The hydrogel was also confirmed with strong tissue adhesiveness toward human skin for bearing a weight of 63.94 g, and kept integrity during multiple times of adhesion
strip cycles from human skin, and continuous motion. Liu group reported a selfhealing hydrogel cross-linked by imine bond and hydrogen bonding between DA-grafted OSA and polyacrylamide chains. The abundant free catechol groups in DA-enabled the hydrogel with superior cell affinity and tissue adhesiveness [84]. The adhesive strength of

FIGURE 7.6 (A) Synthetic process and schematic structure the PDA
PAM hydrogel. (B) Photo (1) without DA prepolymerization, the hydrogel could not form. Photo (2) after DA prepolymerization, the hydrogel was cured. Photo (3) the hydrogel firmly adhered on the author’s arm bearing its own weight [83]. DA, Dopamine; PDA, polydopamine.

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this hydrogel toward porcine skin was 6.5 kPa, and the high adhesive strength kept the same as the original state after several adhesive
strip cycles. The self-healing hydrogels bearing reactive groups as aldehyde groups also have tissue adhesiveness by forming covalent bonds with human tissues. Chen group reported a dually cross-linked hydrogel, of which the dynamic Diels
Alder reactions and acyldrazone bonds enabling self-healing ability [85]. The tissue adhesiveness ascribed to the presence of aldehyde in the hydrogel and further reaction with amine groups in cartilage tissue. In the push out test of adhesive strength of tissue-engineered hydrogel, the adhesive strength of hydrogel was 10.3 6 0.7 kPa, dramatically higher than hydrogel without aldehyde groups. 7.2.1.3.2 Cell scaffolds

Scaffold acts as a bioactive platform for the cells to adhere and proliferate, and is finally transplanted into human body at the desired place. Therefore designing and fabricating scaffold with suitable properties is of great importance in tissue engineering. Hydrogelbased scaffolds have been extensively studied, not only for their similarity to the natural extracellular matrix environments, but also for their simplicity in tailoring their mechanical properties to specific soft tissue of human body. Hydrogels are of great potential in cell delivery for their three-dimensional networks and similarity to the extracellular matrices of tissues both in structure and mechanical properties, while enabling homogeneous encapsulation of cells under physiological conditions [1,28,31]. Cell therapy is a promising strategy for the treatment of tissue disorders. Various types of cells have been successfully delivered by injectable hydrogels into specific lesion sites with minimal invasive operation, such as stem cells, islet cells, skeletal myoblasts, vascular cells, and endothelial progenitor cells [86
88]. Compared with traditional injectable hydrogel, injectable hydrogels with self-healing ability would keep structural and functional integrity, thus facilitating the minimal invasive treatment and reducing the loss of cell during the injection without any external interventions. Many self-healing, injectable hydrogels have been fabricated as cell carriers. Gerecht group fabricated a gelatin/oxidized dextran injectable hydrogel with self-healing ability [89]. Then the hydrogels encapsulated with endothelia colony forming cells was demonstrated that they can undergo vasculogenesis in vivo, and promote cellular morphogenesis. Our group designed a self-healing conductive injectable hydrogel which was physically crosslinked by imine bond from chitosan
graft aniline tetramer and dibenzaldehyde-terminated poly(ethylene glycol), while the incorporating of aniline tetramer endowed the hydrogel with conductivity [90]. The C2C12 myoblast and H9c2 cardiac cells were encapsulated into the hydrogel and the features of the hydrogels including cell delivery profiles, conductivity similar to that of cardiac tissue, injectability, tissue adhesiveness all indicated their potential in cell therapy for cardiac tissue. Self-healing hydrogels have been widely applied in cartilage and bone tissue engineering. Cartilage bear extreme weighty load for protecting bones at the joints, but without self-healing capability. The damaged cartilage could only be replaced by well cell-cultured implant. Hou et al. fabricated a dextran-UPy hydrogel with self-healing and shearthinning property [48]. Two pieces of hydrogel immediately merged into one integrity once they were hold together. Different from the common application of self-healing

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hydrogel, the self-healing property of this system was utilized in designing a two- or multi-tissue construct. Chondrocytes for cartilage formation and bone morphogenetic protein 2 for bone regeneration were encapsulated into the hydrogels and successfully generated the cartilage-bone tissue complex. A silk fibroin-based hydrogel with self-healing was physically cross-linked with metal
ligand coordination by Ca21 and BP groups [57]. The hydrogel supported stem cell proliferation in vitro, and histological observation confirmed the new bone formation. In tissue engineering of central nervous system, the injectable, self-healing hydrogels are of great importance. The requirement of biomaterials applied in central nervous system should meet extremely strict requirements, for neural stem cell differentiate into neurons in soft materials (  0.1
1 kPa) and into glial cells in slightly stiffer materials (  7
10 kPa). The pilot work was present by Hsu group [91]. The self-healing hydrogel was cross-linked by imine bonds between telechelic difunctional poly(ethylene glycol) and glycol chitosan. Compared with control sample (alginate gel), neurosphere-like progenitors grew faster in the self-healing hydrogel. And in functional assay for the central nervous system rescue in zebrafish, this self-healing hydrogel exhibited prominent positive effects on nerve regeneration (81% recovery).

7.2.2 Self-healing films Thin films have important applications in tissue engineering. On one hand, before biocompatible polymers being fabricated into specific three-dimensional scaffolds for cell culture, it is necessary to evaluate their properties. Thin films are frequently used as the model to study the structural and mechanical properties of these polymers, along with their cell reactivity and tissue response. On other hand, many thin films exhibit their influence of pattern dimensions on cell alignment and orientation depending on both the type of polymer backbone and the modified groups. With facile post-treatment, thin films could be assembled into porous scaffolds. Moreover, the surface functions of well-established scaffolds could be easily changed or modified with thin films, such as reactivity, conductivity, and corrosion properties [92]. The applications of most traditional polymers are limited in tissue engineering for easily forming fractures and lack of stimuli
responsiveness. Incorporating dynamic noncovalent bond and supramolecular interaction into polymer backbone is the general strategy for fabricating self-healing polymers, and further forming self-healing thin films. 7.2.2.1 Mechanism Ureido-pyrimidinone (UPy) is a classic reversible motif used in fabricating dynamic systems, so as prevalent as in self-healing elastomers. various natural and synthetic polymers have already been modified with UPy motif. Wu et al. presented a novel self-healing supramolecular bioelastomer with shape memory [93]. Poly(glycerol sebacate) (PGS) was chosen for its biodegradability, and soft tissue-like mechanical properties. The UPy motif was grafted on PGS via the reaction between isocyanate from UPy and hydroxyl group from PGS. As there was abundant amount of hydroxyl group on PGS backbone, the UPy content could be easily tuned, while it has been proved that UPy content plays a crucial

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role in tuning the mechanical properties, including self-healing, shape memory, strength and stiffness. The PGS-graft-UPy (PGS-U) bioelastomer formed film by solvent casting exhibited outstanding self-healing ability, for two films fully merged into one whole film after placing for 1 h at 55 C. Zhibin Guan group firstly succeeded in fabricating an UPymodified self-healing block copolymer [94]. Such a hard/soft microphases-separated elastomer owned stiffness and toughness of thermoplastic elastomer and self-healing capability of reversible materials, the self-healed film by solvent casing with the recovery of tensile strength ( . 90%). The polymer systems employing furan and maleimide groups have self-healing capability by the Diels
Alder reaction. In 2002 Fred Wudl group firstly reported a highly crosslinked self-healing polymer named as 3M4F which was fabricated by two monomers containing four furan moieties and three maleimide [95], as shown in Fig. 7.7. As determined by solid-state nuclear magnetic resonance spectroscopy, only at temperatures higher than 120 C, retro Diels
Alder reaction takes place (B30%). After cooling, the disconnected bonds would reconnect. The thermally remendable process depended largely on the temperature, while the self-healing efficiency reached about 41% at 120 C and about 50% at 150 C within 2 h. Afterwards, numerous self-healing films have been fabricated toward diverse applications. For instance, Pei group developed a healable conductor, in which the polymer matrix based on reversible Diels
Alder reaction [96]. Besides, the transparent nature of this polymer attributed to their potential application in selfhealing transparent conductor. The self-healing conductor was fabricated by this transparent self-healing film and Ag nanoparticles. Self-healing tests were performed at 110 C, and the self-healing efficiency of conductivity was as high as 97% after 5 min, whilst as 86% of mechanical strength after 1 h. Self-healing films could also be fabricated by layer-by-layer technology based on imine bond. Liqin Ge group designed a transparent film with self-healing capability based on imine bond reacted by chitosan and dialdehyde group functionalized poly(ethylene glycol) [97]. The self-healing efficiency was calculated to be the recovery of the hardness of c. 98%

FIGURE 7.7 (A) Schematic presentation of self-healing polymer 3M4F based on Diels
Alder reaction. (B) SEM image of a healed film, the left side is the as-healed surface and the right side is the scraped surface [95]. SEM, Scanning electron microscope.

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and Young’s moduli of c. 81% after healing for 20 min. Notably the cyclic damage/healing process had slightly influence on transparency of the films. Dynamic metal
ligand coordination is of importance in fabricating self-healing polymers. For instance, an extraordinary work containing three types of Fe(III)-ligand coordination was presented by Bao group [98]. As shown in Fig. 7.8, 2,6-pyridinedicarboxamide group on poly(dimethylsiloxane) polymer chain would coordinate to Fe(III) forming a strong pyridyl
iron coordination (Fe
Npyridyl bond), and two weaker carboxamido
iron coordination (Fe
Namido bond and Fe
Oamido bond). The polymer exhibited high stretchability, about 45 times to the original length and exceptional self-healing ability. At room temperature, the cut films healed with about 90% healing efficiency, even at 220 C, the self-healing efficiency would reach at 68% after 72 h. In addition to their high stretchability and self-healing, the polymer film had been applied in artificial muscles. 7.2.2.2 Applications in tissue engineering 7.2.2.2.1 Cell coculture

Polymers such as chitosan, silk fibroin, polystyrene, polydimethylsiloxane (PDMS), PCL, and polyurethane have frequently casted into films for cell culture in tissue

Structure (A) and schematic illustration (B) of the [Fe(Hpdca)2]1 moiety undergoing reversible rupture and reconstruction during tensile stretching (using a force F) of the films. (C) Optical microscope images of damaged and healed samples. (D) Optical images of the healed film before and after stretching [98].

FIGURE 7.8

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engineering [99]. However, compared with comprehensive applications of self-healing hydrogels in tissue engineering, self-healing films do not own large potential in cell carriers for its two dimensional structure. Nevertheless, these self-healing films have great potential in interface tissue engineering. For that, they could realize cell coculture easily with the combination of hard and soft self-healing films, engineering interface tissues including ligament-bone, tendon-bone, and cartilage-bone. Ligament-cartilage-bone is a so complicated field which requires combinations of spatially organized soft and hard tissues, and corresponding types of cells [100,101]. The conventional biomaterials are monophasic and homogeneous and could not meet the requirement of gradients in biomaterials, while self-healing films with different stiffness can be employed to mimic interface tissues. Wu et al reported a series of self-healing PGS-U films and applied them in interface tissue engineering [93]. As shown in Fig. 7.9, PGS-3U, PGS-4U and PGS-5U with gradient growth in stiffness were synthesized and then seeded with L929 fibroblasts, chondrocytes and MC3T3 osteoblasts, respectively. Then, the cell-laden films were put together in specific sequence, and merged into one composite based on the self-healing ability. The cell proliferation and migration behaviors between each two adjacent films in this composite template confirmed that such a composite system could be applied in cell coculture systems. As the stiffness of PGS-U films could be tuned by UPy content, other cell coculture systems can be achieved by combination of different PGS-U films and different cells to mimic specific interface tissues. 7.2.2.2.2 Biosensor

In tissue engineering, the changes in cells, and tissue properties are important information for tissue fabrication. Meanwhile, electronic biosensors for detecting physical movement and mechanical deformation are great potential in biomedical engineering, such as cardiac health care, electronic skin and wearable devices. Biosensor can be utilized in various fields, for instance, monitoring temperature, media pH, humidity, oxygen saturation, glucose concentration, genetic manipulation, and cell attachment [102
104]. The self-healing polymer films utilized in fabricating biosensors would significantly promote their performance. Lu et al. used a self-healing nanostructured conductive substrate and a self-healing elastomer layer to construct a hierarchically structured sensor [105]. As shown in Fig. 7.10, both the elastomer layer and the nanostructured substrate were cross-linked by metal
ligand coordination. The self-healing sensor showed excellent electrical self-healing performance, and detected human motion after cyclic damaging/ healing process, like finger/neck bending, and touching. Notably the self-healing sensor could withstand harsh treatment, such as thousands of times bending, washing, without any influence on sensitivity and stability. Bao group has reported several self-healing biosensors based on dynamic polymeric systems [106,107]. For instance, in their pioneering work, a hydrogen bonding-based polymer was mixed with nickel particles thus forming a self-healing polymer composite [106]. The electrical self-healing efficiency could reach as high as 98% with only 15 s resting time. This composite film was then fabricated onto on 50-mm-thick PET substrate as a self-healing electronic sensor skin, whilst the electronic sensor was proved to be a tactile and flexion sensor. All these conductive polymer systems exhibited prominent

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FIGURE 7.9 Cell coculture application by incorporating different cell-laden PGS-U supramolecular films into a coculture system via modular approach. (A) Schematic illustration of the preparation of a multiple cell types coculture system. (B) L929 fibroblast (green), chondrocytes (red), and MC3T3 osteoblast (green) were seeded on PGS-3U, PGS-4U and PGS-5U films, respectively. Scale bar: 500 μm. (C) PGS-3U with fibroblast (left side), PGS-4U with chondrocytes (middle) and PGS-5U with osteoblast (right side) were welded very well via self-healing. Scale bar: 300 μm [93].

stretchability, self-healing ability, and pressure- and motion-sensitive properties, and have been successfully constructed as electronic sensor skin. In addition, a self-healing elastomer poly(2-hydroxyethyl methacrylate) based on the host
guest interactions between CD and adamantine was developed by Ding group [108]. This self-healing elastomer have been utilized to construct a biosensor with single-walled CNT and this biosensor showed human proximity and humidity sensitivity. A multifunctional self-healing film consisting of MoS2 nanosheets, β-CD-modified poly(ethylenimine) and adamantine-modified poly(acrylic acid) (PAA) was developed by lay-by-layer selfassembly technique and has demonstrated its application in heavy metal ion sensing [109]. In another report of self-healing biosensor, dynamic thiol
disulfide exchange-based self-

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FIGURE 7.10 (A) Schematic illustration of the sensors during cutting/healing process. Representative current signals and pictures (inset) of self-healed sensors during detecting finger bending (B) finger touching (C) neck bending (D), and neck shaking (E) [105].

healing polymer polyurethane was mixed with Ag nanoparticles, and this biosensor has been utilized for pressure sensing [110].

7.3 Self-healing biomaterials in drug/gene delivery systems Compared with conventional oral drug delivery of poor targeting, and higher side effects, controlled drug delivery systems have been intensively studied aiming at optimized drug-loading/releasing profiles, site-specific delivery and low toxicity [111]. Gene delivery introduces foreign genetic materials into target cells to stimulate tissue regeneration and disease treatment via viral and nonviral vector. Compared with no-viral vector, viral vectors have higher potential in clinical for gene transfection and gene expression. However, the viral vector will inevitably cause immune response [112]. Therefore the suitable carriers for gene therapy are of great importance, not only for local delivery and full therapeutic potential, but also for avoiding immune response [113]. So far, with the advantages of self-healing biomaterials, many self-healing systems for controlled drug/ gene delivery systems have been established [32].

7.3.1 Injectable hydrogels Hydrogels with water-swollen network and similar extracellular matrix environment have long been utilized in drug/gene delivery systems owing to tunable degradation and mechanical properties, high-loading content, and controlled release by external stimuli

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(magnetic fields, ultrasound, thermal, or light) [114]. Injectable hydrogels are frequently applied in practical for minimal invasive treatment [113
116]. Compared with traditional injectable hydrogels, self-healing hydrogel can self-assemble into an integrity at specific site and prevent any loss which can cause side effects, thus have more potential in delivery system [117
122]. Besides, in some well-designed self-healing hydrogels, the hydrophobic microphases provide platforms for poor water-soluble drugs. The general methods include: introducing molecules containing hydrophobic cavity which could form complex with hydrophobic drugs, incorporating hydrophobic segments into the polymer chain, and fabricating hybrid hydrogel containing micelles or nanoparticles [123
125]. E. W. Meijer group piloted in UPY-based self-healing polymeric systems for drug delivery. They reported a UPy-PEG self-healing hydrogel for drug delivery, the release of model protein can be tuned by the content of hydrophobic alkyl spacer and the hydrophilic PEG chain, and the supramolecular polymer weight percentage [46]. Various self-healing injectable hydrogels have been fabricated for delivery systems in terms of their stimuli
responsiveness. For instance, a phenylborate
catechol complexation-based PEG hydrogel in which drug release was controlled by pH, glucose, and DA [126]; a hydrogen bonding-based chitosan/polyvinyl alcohol hydrogel in which controllable release at pH 5 5.0 for antitumor treatment [127]; a azobenzene-containing hydrogen bonding-based hydrogel in which dsDNA and anticancer drug doxorubicin release controlled by light; a collagen-gold hybrid hydrogel based on metal
ligand coordination and electrostatic interaction in which antitumor drug released was controlled by photothermal or light [128]; an imine bond-based PEG/chitosan hydrogel applied in antitumor treatment in which drug release was controlled by magnetic fields [117], and an injectable alginate hydrogel based on ionic crosslinking in which drug release was controlled by ultrasound-triggered disruption [129]. In some other cases, hydrogel provides a matrix for drug-loaded particles or micelles. Dmitri Ossipov reported a drug-loaded nanoparticle (MgSiO3 NPs) and BP-modified hyaluronic acid (HA-BP) hybrid self-healing injectable hydrogel [119]. As shown in Fig. 7.11, the MgSiO3 nanoparticles act both as drug carrier and cross-linker in the hydrogel. In acidic condition, the BP groups would be protonated, subsequently the coordination between BP groups and Mg21 broke thus inducing the release of drug-loaded nanoparticles. With various cargo molecules or nanoparticles, the facile design of drug vehicles could be widely applied in controlled biomedicine. Micelles owning large hydrophobic cavity demonstrate their advantages in drug vehicles for hydrophobic drugs [130]. Curcumin, as an antioxidant, antiinflammatory, and antitumor promoter, has excellent bioeffects. However, curcumin’s pharmacological potential is limited by its extremely poor solubility [131]. Jin et al. reported a self-healing injectable QCS/PF hydrogel prepared by quaternized chitosan and benzaldehydeterminated Pluronic F127 [132]. The dynamic imine bonds between aldehyde groups on PF127 and amine groups on chitosan endow the hydrogel with self-healing capability. In the hydrogel, the amphiphilic copolymer PF127 assembled into micelles in water providing hydrophobic spaces for curcumin. Even though, in this report, curcumin only played its antioxidant ability when the hydrogel was utilized as a wound dressing. This kind of drug-loading micelles encapsulated hydrogel method have confirmed its viability and great potential in delivery system.

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FIGURE 7.11 (A) Formation of the composite hydrogel via dynamic and reversible interaction between MgSiO3 NPs and HA-BP biopolymer chains upon mixing of the aqueous components. (B) Cumulative release of MgSiO3 NPs from the HA-BP MgSiO3 composite hydrogel in PBS at different pH values (5.0 and 7.4). (C) Viability of MCF-7 cells incubated with the released medium from the HA-BP MgSiO3@Dox hydrogel (blue bars) and from the drug-free HA-BP MgSiO3 hydrogel (red bars) at pH 5.0 and different time points; the statistical difference is shown as **p , 0.01 [119].







7.3.2 Particles and capsules Polymeric micrometer-to-millimeter scale depots as particles and capsules have been used in controlled drug and gene delivery for lower side effect and site-specific targeting [133,134]. Schwendeman group have focused on fabricating self-healing micro-carriers based on biocompatible copolymer of lactic and glycolic acids (PLGA). In their pilot report, the self-healing PLGA microspheres were synthesized by a standard emulsion method with sugar leaching forming porous network, while the diameters of pores ranging from 30 to 3000 nm [135]. Different from the current organic-solventneeded microencapsulation methods for protein/peptide drug within PLGA microspheres of inevitable drug damage, poor encapsulation efficiency, and high initial burst release, this self-healing microencapsulation system provided an organic solventfree conditions for drug loading. In detail, the protein drugs entered into open polymer

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FIGURE 7.12 Scanning electron microscope images of PLGA microspheres before and after self-healing microencapsulation [135]. PLGA, Poly(lactic-co-glycolic) acid.

pores when the temperature was below than Tg value, then self-healing process of these polymer pores were triggered by temperature higher than Tg value, as shown in Fig. 7.12. Owning to self-healing ability, the drug encapsulation within PLGA microspheres completed without any organic solvent, which significantly contributed to higher protein stability, higher encapsulation efficiency and long-term slow release under physiological conditions. In their subsequent work [136], the self-healing process of PLGA microspheres was confirmed to be reproducible, and porosity value of 60%
70% proved to work for high protein loading level by passive encapsulation, while porosity, pore size, depth, and hydrophobicity all had influences on self-healing kinetics of pores in PLGA films. Besides, there is a promising self-healing biomaterial but have not been sufficiently explored for its complicacy. Vault particle is a natural protein-based particle and have been demonstrated as a new generation of delivery nanodevices for unique hollow barrelshaped uniform structure [137]. AidaLlauro´ et al. have studied the mechanical stability and striking self-healing capabilities of the vault particles and anticipated their great value as nanocontainers in delivery systems [138].

7.4 Self-healing functional surfaces Functional surfaces for materials have been intensified developed not only for protection against extreme environmental conditions but also for multiple functions [92,139
142]. Biomaterial surfaces have directly interactions toward human body. However, in some cases, these interactions should be avoided, for instance, protein adsorption [143], and infection [142,144]. To address these problems, biocompatible functional surfaces need be employed as coating on biomaterial surfaces. Moreover, as the biomaterial surfaces could be easily damaged when fully exposed to external harsh environment, the lifetime of these biomaterials would be significantly extended with the protection of self-healing functional surfaces [145].

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7.4.1 Antibacterial and antifouling surfaces Protein adsorption induced by biomaterial implants and drug/gene delivery devices is a primary reason for infection. To inhibit protein adsorption, antifouling surfaces is one facile method that have been widely employed, including hydrophilic polymers, poly(ethylene glycol) (PEG), polysaccharide, zwitterionic polymers, hydrophobic polymers, nitricoxide, peptide, and peptoids [143]. The self-healing antifouling surfaces have recently arisen for long-term antifouling and reproducibility. Besides, as antibacterial surfaces could be employed for antifouling surfaces, therefore the antibacterial surfaces would also be discussed in this section. Sergiy Minko group designed a longer durable antifouling coating, with PEG brushes grafted both on the surface and inside the host of poly(2-vinylpyridine) film network via 3D polymer grafting method [146], as shown in Fig. 7.13. The PEG brushes would maintain its antifouling property when the poly(2-vinylpyridine) film was damaged or degraded. The self-healing process was supposed to the replacement of the lost polymer chain by the prestored PEG polymers. Zuilhof group reported that hydrophobic fluoropolymer brush-based surface has selfhealing capability by rearrangement of the polymer segments during heating, and could bear damage
repair cycles for almost 12 times, along with the thickness of the brush decreasing from 75 to 43 nm [147]. As it has been well established that hydrophobic polymers have antifouling property, subsequently Zuilhof group fabricated a self-healing surface based on superhydrophobic [poly(2-perfluorooctylethyl methacrylate) (PMAF17)] brushes with strengthened antifouling ability and mechanical stability [148]. Zuilhof group also presented an antifouling surface based on zwitterionic polymer [149]. The self-healing capability was based on the ionic bonds between protonated ammonium group and

FIGURE 7.13 Schematic illustration of self-healing antifouling PEO surface via 3D polymer grafting. The self-healing aspect of the antifouling property is due to the rearrangement of internally grafted polymers to the interface (marked as dark blue chains) [146]. PEO, Poly(ethylene oxide).

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sulfonic acid. After damaging, the damaged ionic bonds would rebuild with the addition of water. The strong reversible electrostatic interaction endowed this antifouling surface with rapid self-healing for full recovery after 1 min immersion in water. Wooley group synthesized an antifouling polymer HBFP
PEG based on hyperbranched fluorinated polymer (HBFP) and poly(ethylene glycol) (PEG) with dynamic crosslinking by Diels
Alder reactions [150]. This self-healing antifouling surface exhibited not only similar antifouling behavior as traditional HBFP
PEG, but also excellent selfhealing ability, that small abrasions fully disappeared within 30 min at 60 C. In another polyelectrolyte-based self-healing film bPEI-PEG/HA presented by Sun group was also proved with antifouling ability for PEG-rich surface [151]. The layer-bylayer assembly was utilized to fabricating film with high mobility because of the benefit of the soft nature for antifouling ability. The electrostatic interactions and hydrogen bonding in bPEI-PEG and HA polyelectrolytes enabled this film with self-healing ability. Self-healing hydrogels could also be fabricated for antifouling surfaces. Zeng group provided a mussel-inspired hydrogen bonding-based self-healing hydrogel with antifouling capability, while the antifouling capability attributed to PEO [152]. Wang group fabricated a self-healing hydrogel based on thiol
disulfide exchange [153], while all three monomers as poly(ethylene glycol)methyl ether methacrylate, N-hydroxyethyl acrylamide (HEAA), and 2-(methacryloyloxy)ethyl trimethylammonium chloride endowed this hydrogel with antifouling and antibacterial properties. The most exceptional design in this hydrogel is the surface-initiated sulfonic acid photopolymerization, which makes the hydrogel could be easily modified onto biomaterials via a facile “grafting-through” approach.

7.4.2 Surface-mediated drug delivery Over the last decades, surface-mediated drug delivery has emerged as a newly developed method for delivery system [154]. Because surfaces in such systems could both strengthen surface functions and act as drug carriers. Thus self-healing surfaces for controlled drug delivery have been more intensively concerned. Xia-chao Chen et al. had made contribution in this area. An ingenious designed self-healing surface coated on glass substrates was fabricated for two drugs codelivery system [155]. The self-healing polyelectrolyte multilayer film (PEM) was formed by alternately deposited and partially ionized poly(theylenimine) and PAA via layer-by-layer assembly, subsequently the film was treated with acid immersion and freeze-drying forming a microporous structure. The electrostatic interactions enabled the hydrogel with self-healing ability, and this two drug codelivery system was proved to be well-controlled. Next, they successfully applied this microporous PEM film in hydrophobic drug delivery based on hydrophobic nanodomains within the microporous PEM films [156]. As shown in Fig. 7.14, upon exposure to 100% relative humidity environment, the self-healing process of the micropores took place with nearly 100% efficiency. The drug-loaded PEM films was successfully applied on the surface of a complicated-shaped metal stent and exhibited healed appearance after exposure to saturated humidity.

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FIGURE 7.14 (A) Fabrication of a dynamic PEM film with self-healing microporous structures and integration of a hydrophobic drug into the film. (B) Scanning electron microscope cross-sectional images of the porous PEM film exposed to 100% relative humidity for various times. Scale bar: 10 μm [156]. PEM, Polyelectrolyte multilayer.

7.4.3 Challenges Functional surfaces that have been extensively explored including the types of polymers, the structure of the film, the fabrication methods, the target functions and corresponding applications. However, functional surfaces with self-healing capability have not been comprehensively studied in spite of their advantages. To date, besides the above-mentioned antifouling/antibacterial surfaces and surfaced
mediated drug delivery, self-healing functional surfaces with antimicrobial ability [144], superoleophobic ability [157] have also been developed. Therefore we look forward to more diverse self-healing functional surfaces, such as antithrombogenic coatings, anticorrosion protection, and electrical devices. However, the development of self-healing surfaces still faces with extreme difficulty. The self-healing process of thin films are commonly more challenging than that of hydrogels, for the lower mobility of polymer chains restricted by substrates, and fewer contact areas [151].

7.5 The characterization of self-healing Self-healing ability is of great significance for biomaterials, not only prolong their lifespan and promote their performance, but also give rise to new opportunities in biology and biomedicine. Generally the characterization of self-healing is mainly the evaluation of Self-Healing Polymer-Based Systems

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self-healing efficiency for biomaterials. In Section 2.2.2, we have discussed the qualitative and quantitate measurements for self-healing hydrogels. Tensile testing is the common method for self-healing films. Yet, there is no explicit standard method for self-healing efficiency of particles, capsules, or porous structures. Notably before the quantities tests are performed, there needs a specific resting time for the hydrogel or film to merge at a motionless condition, while the resting time depend on the nature of the reversible system related to self-healing kinetics. However, in real practical system, there are scarcely motionless conditions. Not to mention, low self-healing kinetics may be not positive enough against the continuous propagation of cracks or deformations. In addition, the optimal self-healing process for multifunctional biomaterials refers to the recovery of structural, mechanical and functional integrity. Indeed, the evaluation of self-healing ability should be performed under specific external environment according to the biomaterials and the application locations. For instance, the self-healing electronic devices should not only be tested for the self-healing efficiency in mechanical properties but also in conductivity; both the mechanical integrity and antifouling ability need to be measured for antifouling surfaces. So far, all the measurement of self-healing efficiency of biomaterials are performed in vitro. Nevertheless, the physiological environment is totally distinct from the conditions in vitro. Irregular motions, body fluids, enzymes, and cell activities would all affect the self-healing process. Sometimes, for instance, even the humidity would have an influence on the self-healing process in vitro. The mimicking of biological environment in vitro or in situ monitoring and measuring the self-healing behavior need to be developed. Generally the failure of polymeric biomaterials is generated from numerous cyclic loading, and the cyclic loading patterns varies according to the specific location, as illustrated in Section 2.1. In most literatures, cyclic loading
unloading test is employed to evaluate the self-recovery and antifatigue properties of the self-healing biomaterials. However, these cyclic loading
unloading tests are always set to be hundreds times at most, that is far from the real conditions. In addition, the loading patterns that could mimic complex situation have never been taken into account for the evaluation of self-healing biomaterials.

7.6 New opportunities and challenges During the last decade, self-healing biomaterials have been intensively developed to meet the demand of longer lifespan and higher performance, and the applications of selfhealing biomaterials are versatile, including scaffolds in tissue engineering, carriers for delivery systems, and functional surfaces. Moreover, as the emergence of manufacturing techniques such as 3D printing and more stringent requirements of multifunctional biomedical materials, the applications of self-healing biomaterials have been expanded. In return, self-healing biomaterials have facilitated the development of manufacturing techniques and multifunctional biomedical materials.

7.6.1 3D printing Scaffolds which encapsulated with cells, growth factors, and other bioactive molecules acting as the replacement is the general principle in tissue engineering. 3D printing, as an Self-Healing Polymer-Based Systems

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additive manufacturing technique, aims to fabricate a three-dimensional object, and have been proved of great value in tissue engineering, for fabricating patient-specific scaffold with precise geometrical control [158,159]. Besides, compared with traditional postcell seeding process, the combination of biomaterials with cells could be utilized as bioink for 3D printing scaffolds, thus forming homogeneous cell distribution with high seeding efficiency. Moreover, the multi-channel assembly in 3D printing facilitates the modular approach of fabricating tissue complexity in a single process. Biocompatible injectable hydrogels which meet the requirements as flow under modest pressures, gel quickly, and maintain sufficient integrity after build up, have exhibited great potential as bioinks in 3D printing, especially for their similarity to extracellular matrix environments and tunable mechanical properties [160
162]. However, these injectable traditional-based hydrogel bioink for 3D printing still have limitations in printability, stability, cell adhesion, resolution, and printing efficiency and pre or postprinting crosslinking. Self-healing hydrogel could make contribution to the development of 3D printing, and expand the application of 3D printing. For instance, as shown in Fig. 7.15, Burdick group have fabricated an injectable self-healing hydrogel based on host
guest interactions, and proved that this self-healing bioink could be used to print multiple materials and complex structure at high resolution, for the dynamic supramolecular interactions both in the support and ink gels enabling directly printing continuously in 3D space [34]. Besides, this system has benefit for the formation of free-standing structures containing open channels. FIGURE 7.15 Schematic illustration of the extrusion of a supramolecular ink (red) into a supramolecular support gel (green), where (A) the undisturbed network and (B) receives the printed ink [34].

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Another supramolecular hydrogel based on adamantine (Ad) or β-CD-modified HA with shear-thinning and self-healing have realized a high resolution 3D printing and printing in any direction [163]. This self-healing hydrogel based on host
guest interactions was used as bioink for 3D printing could form a long-term stable structure with greater than 16 layers, which could not be realized by a traditional hydrogel bioink. Besides, the self-healing hydrogel-based bioinks in 3D printing have been fabricated by different polymer chains, dynamic crosslinkers, and additional active reagents. For instance, a conductive, self-healing hydrogel based on electroactive PPY modified chitosan (DCh-PPy) and PPA and have used this hydrogel as bioink for a 3D printed wearable sensor for body motion sensing [164]. Nevertheless, compared with traditional injectable hydrogels that has formed as a majority group in 3D printing, there are very few reports about self-healing biomaterials in 3D printing that have realized print at any specified direction or point with high resolution or formed bulk materials with more complex structure [165
168]. Self-healing hydrogel-based bioink is still in its infancy in spite of the great advantages.

7.6.2 Wound dressing Self-healing wound dressing is a newly emerging application of self-healing biomaterials. Traditional wound dressing only meeting the requirements of wound closure and infection prevention are far from demand. Therefore various wound dressings with ability of promoting healing efficiency, tissue adhesiveness, and hemostasis, and antibacterial have been developed. Nevertheless, the demand of self-healing wound dressing is pressing for the irregular wound sites, active body motion and unexpected external stimuli. Since then, self-healing wound dressing has aroused widely attention of scholars. Notably injectable self-healing hydrogels own more potentials than other wound dressing materials including rubbers, foam, films, electrospun nanofibers, for maintaining moist environment, absorbance of wound exudates, and especially shear-thinning for in situ gelation within any irregular wound sites [169,170]. Zhao et al. reported a selfhealing hydrogel wound dressing based on dynamic Schiff base [81]. This self-healing hydrogel wound dressing is multifunctional. The backbone polymer polyaniline quaternized chitosan contributed to inherent antibacterial activity, electroactivity, and good free radical scavenging capacity, which is all benefit for promoting wound healing. The self-healing hydrogel indeed exhibited more excellent performance in full-thickness skin wound healing compared with commercial wound dressing. Next, they attempted to fabricate more suitable wound dressing for specific positions. As considering the environment of skin wound on joints where bearing continuous motion, wound dressings for joints should own self-healing ability, adhesiveness, extensibility and compressibility. Therefore a self-healing injectable hydrogel based on micelles hybrid hydrogel composite was fabricated for joints skin wound [132]. The hydrogel was formed by quaternized chitosan and benzaldehyde-terminated Pluronic F127 with inherent antibacterial and hemostatic activity with suitable stretchable and compressive properties. The selfhealing capability and tissue adhesiveness ensured their application as joints skin wounds. Notably the PF127 micelles provided hydrophobic cavities that allow

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encapsulation of hydrophobic drugs. Curcumin could be trapped into the hydrogel thus benefiting wound healing efficiency. Cell affinity and tissue adhesiveness are crucial for self-healing wound dressing. In above systems, the aldehyde groups in dynamic Schiff base reacted with amino group from tissue generating tissue adhesiveness, and the hydrogel attached on porcine skin could bear 100 g load, and the optimal adhesion strength to porcine was 6 kPa. Another self-healing wound dressing was fabricated from DA-grafted OSA and polyacrylamide [84], in which abundant catechol groups endowed the hydrogel with excellent cell affinity and tissue adhesiveness, withstanding 60 g loads once being attached to human skin, the adhesion strength to porcine was 6.5 kPa. Exceptionally a PDA nanoparticles hybrid hydrogel wound dressing exhibited excellent tissue adhesiveness [171], while the tissue adhesion strength to porcine skin was as high as 90 kPa. Traditional wound dressings, as gauze, woven sponge, bandages, still dominate in clinical application [172,173]. However, traditional wound dressing, lack of inherent adhesiveness, antibacterial ability, hemostasis, and suitable mechanical properties, could not meet all the requirements in clinic. Multifunctional wound dressings with self-healing ability have significance in health care and will be widely developed in the future.

7.6.3 Electronic skin Human skin is the largest sensory organ of our body, and act as the physical barrier while perceiving information from surrounding environment, such as temperature, humidity, pressure, and proximity changes. The electronic skin which mimicking natural human skin has been intensively studied for their great potential in robots, personal health care, prosthetics, and communication devices [174
176]. Generally the electronic skin must be biocompatible, along with the ability of touch and pressure sensitivity, stretchability, flexibility, and self-healing capability. In the early development stage of electronic skin, highly stretchable electronics with different sensing functions were of great interests from scholars [177
180]. The study for self-healing electronic skin was pioneered by Bao group. Since 2012, Bao et al. reported the first repeatable, room temperature self-healing electronic sensor skins. The striving for self-healing capability has been the most striking objective in constructing electronic skin. The self-healing capability both in mechanical and electrical properties is necessary and has great advantages. The various well-established self-healing biomaterials based on polymeric system have great benefit for the development of self-healing electronic skin. Notably some conductive polymeric systems based on polyelectrolytes and in which the healing process is triggered by water are not suitable for electronic skin [181
183]. To date, there have been several extraordinary works in this field. In the abovementioned first work on self-healing electronic sensor skin, Bao group fabricated a polymer composite film consisted of a hydrogen bonding-based polymer and micro-nickel particles [106]. As the nickel particles were modified with a thin oxide layer, these nickel microparticles could be well dispersed in the supramolecular polymer network. The optimal electrical conductivity adjusted by nickel microparticles content was 40 s cm21.

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The self-healing process was super rapid, and the conductivity of healed composite could restore to 90% compared with its initial state with 15 s, while full recovery for mechanical properties. Moreover, the composite exhibited excellent flexion and tactile pressure sensitivities, as shown in Fig. 7.16. However, the tensile strength of this polymer composite was extremely lower than that of natural human skin and the pressure sensitivity relied on external stimuli. Aiming to design an electronic skin with high stretchability and selfactivated pressure-sensing, Li et al. reported an unusual 3D self-healing polymeric film containing reduced graphene oxide foams and poly(N, N-DMA)-poly(vinyl alcohol) [184]. The fabricated film demonstrated perfect electrical healing process which immediately completed at the moment of the cut pieces were hold together, and higher strain and tensile strength than the value of human skin, and high pressure sensitivity with any other external power supply. Because of the specific viscoelasticity, moisture, and water sensitivity for hydrogen bonding or metal
ligand coordination-based self-healing materials, the applications of

FIGURE 7.16 Application of the self-healing electronic sensor skin. (A) Self-healing flexion and tactile sensor circuit schematic and mounting on a fully articulated wooden mannequin. (B) Flexion sensor circuit demonstration: LED “eyes” light up after the elbow is bent. The intensity increases with increasing elbow flexion. (C) Tactile sensor circuit demonstration. LED intensity responds as a function of increasing tactile pressure [106].

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these materials in electronics have been limited. Bao group recently made great breakthrough in this area. They reported a metal
ligand coordination-based self-healing dielectric elastomer which composed of 2,20 -bipyridine-5,50 -dicaoxylic amide incorporated polydimethylsiloxane and Fe21 or Zn21 [185]. The self-healing process for Zn21 cross-linked polymers hybrid elastomer took place at room temperature with an optimal self-healing efficiency about 76%, while at 90 C for Fe21 cross-linked polymers with full recovery, and all these elastomers have been proved suitable for potential applications in wearable sensors and electronic skins. Another outstanding work about electronic skin from Bao group based on hydrogen bonding was also proved to be with high stretchability, toughness and underwater self-healability [107]. The polymers had two types of hydrogen bonding, that the strong one (cooperative hydrogen bonding) was formed by 4, 40 -methylenebis(phenyl urea) (MPU unit), and the weak one (anticooperative hydrogen bonding) was formed by bisurea units (IU unit). The polymers have demonstrated water-insensitiveness, and exceptional mechanical properties and realized the fabrication of self-healing electronic skin devices. Until very recently, a rehealable electronic skin based on polyimine substrate and Ag nanoparticles with multisensing capabilities has been developed by Xiao group [186]. This electronic skin demonstrated full recyclability and excellent malleability and exhibited multisensing capabilities, including pressure, flow, temperature, and humidity sensing. However, the healing process need the addition of rehealing agent and heat pressing. Despite self-healing capability is one of the most important features of electronic skin, the reports about electronic skin with self-healing capability are still very few [183]. As can be expected, self-healing electronic skin will be intensively studied hereafter, and the well-established self-healing biomaterials and well-studied self-healing mechanisms would provide great benefit for the development of electronic skin.

Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number: 51673155) and State Key Laboratory for Mechanical Behavior of Materials.

References [1] F.J. O’Brien, Biomaterials & scaffolds for tissue engineering, Mater. Today 14 (2011) 88
95. [2] R. Langer, Biomaterials in drug delivery and tissue engineering: one laboratory’s experience, Acc. Chem. Res. 33 (2000) 94
101. [3] A.S. Mao, D.J. Mooney, Regenerative medicine: current therapies and future directions, Proc. Natl Acad. Sci. 112 (2015) 14452
14459. [4] R.P. Keatch, A.M. Schor, J.B. Vorstius, S.L. Schor, Biomaterials in regenerative medicine: engineering to recapitulate the natural, Curr. Opin. Biotechnol. 23 (2012) 579
582. [5] C.E. Diesendruck, N.R. Sottos, J.S. Moore, S.R. White, Biomimetic self-healing, Angew. Chem. Int. Ed. Engl. 54 (2015) 10428
10447. [6] J.C. Cremaldi, B. Bhushan, Bioinspired self-healing materials: lessons from nature, Beilstein J. Nanotechnol. 9 (2018) 907
935. [7] A.B. Brochu, S.L. Craig, W.M. Reichert, Self-healing biomaterials, J. Biomed. Mater. Res. A 96 (2011) 492
506. [8] C.B. Rodell, M.E. Lee, H. Wang, S. Takebayashi, T. Takayama, T. Kawamura, et al., Injectable shear-thinning hydrogels for minimally invasive delivery to infarcted myocardium to limit left ventricular remodeling, Circ. Cardiovasc. Interv. 9 (2016).

Self-Healing Polymer-Based Systems

References

201

[9] C. Loebel, C.B. Rodell, M.H. Chen, J.A. Burdick, Shear-thinning and self-healing hydrogels as injectable therapeutics and for 3D-printing, Nat. Protoc. 12 (2017) 1521
1541. [10] G. Potjewyd, S. Moxon, T. Wang, M. Domingos, N.M. Hooper, Tissue engineering 3D neurovascular units: a biomaterials and bioprinting perspective, Trends Biotechnol. 36 (2018) 457
472. [11] M.D. Hager, P. Greil, C. Leyens, S. van der Zwaag, U.S. Schubert, Self-healing materials, Adv. Mater. 22 (2010) 5424
5430. [12] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794
797. [13] M.J. Webber, E.A. Appel, E.W. Meijer, R. Langer, Supramolecular biomaterials, Nat. Mater. 15 (2016) 13
26. [14] T.C. Mauldin, M.R. Kessler, Self-healing polymers and composites, Int. Mater. Rev. 55 (2013) 317
346. [15] L. Saunders, P.X. Ma, Self-healing supramolecular hydrogels for tissue engineering applications, Macromol. Biosci. (2018) e1800313. [16] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (2013) 7446
7467. [17] D. Howard, L.D. Buttery, K.M. Shakesheff, S.J. Roberts, Tissue engineering: strategies, stem cells and scaffolds, J. Anat. 213 (2008) 66
72. [18] R. Langer, J. Vacanti, Tissue engineering, Science 260 (1993) 920
926. [19] T. Weigel, G. Schinkel, A. Lendlein, Design and preparation of polymeric scaffolds for tissue engineering, Expert. Rev. Med. Dev. 3 (2006) 835
851. [20] B. Dhandayuthapani, Y. Yoshida, T. Maekawa, D.S. Kumar, Polymeric scaffolds in tissue engineering application: a review, Int. J. Polym. Sci. 2011 (2011). [21] M. Jafari, Z. Paknejad, M.R. Rad, S.R. Motamedian, M.J. Eghbal, N. Nadjmi, et al., Polymeric scaffolds in tissue engineering: a literature review, J. Biomed. Mater. Res. B Appl. Biomater. 105 (2017) 431
459. [22] B. Guo, P.X. Ma, Synthetic biodegradable functional polymers for tissue engineering: a brief review, Sci. China Chem. 57 (2014) 490
500. [23] M.S. Kim, J.H. Kim, B.H. Min, H.J. Chun, D.K. Han, H.B. Lee, Polymeric scaffolds for regenerative medicine, Polym. Rev. 51 (2011) 23
52. [24] G. Ratheesh, J.R. Venugopal, A. Chinappan, H. Ezhilarasu, A. Sadiq, S. Ramakrishna, 3D fabrication of polymeric scaffolds for regenerative therapy, ACS Biomater. Sci. Eng. 3 (2017) 1175
1194. ´ [25] M. Bartkowiak-Jowsa, R. Be˛dzinski, A. Kozłowska, J. Filipiak, C. Pezowicz, Mechanical, rheological, fatigue, and degradation behavior of PLLA, PGLA and PDGLA as materials for vascular implants, Meccanica 48 (2013) 721
731. [26] S.H. Teoh, Fatigue of biomaterials: a review, Int. J. Fatigue 22 (2000) 825
837. [27] A.S. Gladman, A.D. Celestine, N.R. Sottos, S.R. White, Autonomic healing of acrylic bone cement, Adv. Healthc. Mater. 4 (2015) 202
207. [28] J.L. Drury, D.J. Mooney, Hydrogels for tissue engineering: scaffold design variables and applications, Biomaterials 24 (2003) 4337
4351. [29] I.M. El-Sherbiny, M.H. Yacoub, Hydrogel scaffolds for tissue engineering: progress and challenges, Glob. Cardiol. Sci. Pract. 2013 (2013) 316
342. [30] N. Annabi, A. Tamayol, J.A. Uquillas, M. Akbari, L.E. Bertassoni, C. Cha, et al., 25th anniversary article: rational design and applications of hydrogels in regenerative medicine, Adv. Mater. 26 (2014) 85
124. [31] D. Seliktar, Designing cell-compatible hydrogels for biomedical applications, Science 336 (2012) 1124
1128. [32] W. Yuan, C. Wang, Self-healing hydrogels as biomedical scaffolds for cell, gene and drug delivery, Res. Rev. J. Mater. Sci. 05 (2017) 23
35. [33] H. Wang, S.C. Heilshorn, Adaptable Hydrogel Networks with Reversible Linkages for Tissue Engineering, Adv. Mater. 27 (2015) 3717
3736. [34] C.B. Highley, C.B. Rodell, J.A. Burdick, Direct 3D printing of shear-thinning hydrogels into self-healing hydrogels, Adv. Mater. 27 (2015) 5075
5079. [35] E.A. Appel, J. del Barrio, X.J. Loh, O.A. Scherman, Supramolecular polymeric hydrogels, Chem. Soc. Rev. 41 (2012) 6195
6214. [36] N. Sood, A. Bhardwaj, S. Mehta, A. Mehta, Stimuli
responsive hydrogels in drug delivery and tissue engineering, Drug. Deliv. 23 (2016) 758
780. [37] M.C. Koetting, J.T. Peters, S.D. Steichen, N.A. Peppas, Stimulus
responsive hydrogels: theory, modern advances, and applications, Mater. Sci. Eng. R. Rep. 93 (2015) 1
49.

Self-Healing Polymer-Based Systems

202

7. Self-healing biomaterials based on polymeric systems

[38] H.-X. Zhou, X. Pang, Electrostatic interactions in protein structure, folding, binding, and condensation, Chem. Rev. 118 (2018) 1691
1741. [39] C. Tondera, R. Wieduwild, E. Ro¨der, C. Werner, Y. Zhang, J. Pietzsch, In vivo examination of an injectable hydrogel system crosslinked by peptide
oligosaccharide interaction in immunocompetent nude mice, Adv. Funct. Mater. 27 (2017) 1605189. [40] T.L. Sun, F. Luo, T. Kurokawa, S.N. Karobi, T. Nakajima, J.P. Gong, Molecular structure of self-healing polyampholyte hydrogels analyzed from tensile behaviors, Soft Matter 11 (2015) 9355
9366. [41] A.B. Ihsan, T.L. Sun, T. Kurokawa, S.N. Karobi, T. Nakajima, T. Nonoyama, et al., Self-healing behaviors of tough polyampholyte hydrogels, Macromolecules 49 (2016) 4245
4252. [42] T.L. Sun, T. Kurokawa, S. Kuroda, A.B. Ihsan, T. Akasaki, K. Sato, et al., Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity, Nat. Mater. 12 (2013) 932. [43] T. Long, Y. Li, X. Fang, J. Sun, Salt-mediated polyampholyte hydrogels with high mechanical strength, excellent self-healing property, and satisfactory electrical conductivity, Adv. Funct. Mater. 28 (2018) 1804416. [44] A. Phadke, C. Zhang, B. Arman, C.-C. Hsu, R.A. Mashelkar, A.K. Lele, et al., Rapid self-healing hydrogels, Proc. Natl Acad. Sci. 109 (2012) 4383
4388. [45] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, et al., Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding, Science 278 (1997) 1601
1604. [46] P.Y.W. Dankers, T.M. Hermans, T.W. Baughman, Y. Kamikawa, R.E. Kieltyka, M.M.C. Bastings, et al., Hierarchical formation of supramolecular transient networks in water: a modular injectable delivery system, Adv. Mater. 24 (2012) 2703
2709. [47] J. Cui, Ad Campo, Multivalent H-bonds for self-healing hydrogels, Chem. Commun. 48 (2012) 9302
9304. [48] S. Hou, X. Wang, S. Park, X. Jin, P.X. Ma, Rapid self-integrating, injectable hydrogel for tissue complex regeneration, Adv. Healthc. Mater. 4 (2015) 1491
1495. 1423. [49] I. Jeon, J. Cui, W.R.K. Illeperuma, J. Aizenberg, J.J. Vlassak, Extremely stretchable and fast self-healing hydrogels, Adv. Mater. 28 (2016) 4678
4683. [50] J. Wu, Y.-W. Yang, New opportunities in synthetic macrocyclic arenes, Chem. Commun. (2018). [51] G. Crini, Review: a history of cyclodextrins, Chem. Rev. 114 (2014) 10940
10975. [52] R. Liu, A. Harada, T. Takata, Solvent-free synthesis of unmodified cyclodextrin-based pseudopolyrotaxane and polyrotaxane by grinding, Polym. J. 39 (2006) 21. [53] X. Liao, G. Chen, X. Liu, W. Chen, F. Chen, M. Jiang, Photoresponsive pseudopolyrotaxane hydrogels based on competition of host
guest interactions, Angew. Chem. Int. Ed. 49 (2010) 4409
4413. [54] C. Pradal, K.S. Jack, L. Grøndahl, J.J. Cooper-White, Gelation kinetics and viscoelastic properties of pluronic and α-cyclodextrin-based pseudopolyrotaxane hydrogels, Biomacromolecules 14 (2013) 3780
3792. [55] Z. Deng, Y. Guo, X. Zhao, P.X. Ma, B. Guo, Multifunctional stimuli-responsive hydrogels with selfhealing, high conductivity, and rapid recovery through host
guest interactions, Chem. Mater. 30 (2018) 1729
1742. [56] N. Holten-Andersen, M.J. Harrington, H. Birkedal, B.P. Lee, P.B. Messersmith, K.Y.C. Lee, et al., pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli, Proc. Natl Acad. Sci. 108 (2011) 2651
2655. [57] L. Shi, F. Wang, W. Zhu, Z. Xu, S. Fuchs, J. Hilborn, et al., Self-healing silk fibroin-based hydrogel for bone regeneration: dynamic metal-ligand self-assembly approach, Adv. Funct. Mater. 27 (2017) 1700591. [58] D.C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Tough and self-healing hydrogels formed via hydrophobic interactions, Macromolecules 44 (2011) 4997
5005. [59] D.C. Tuncaboylu, M. Sahin, A. Argun, W. Oppermann, O. Okay, Dynamics and large strain behavior of selfhealing hydrogels with and without surfactants, Macromolecules 45 (2012) 1991
2000. [60] Y. Zhang, C. Hu, X. Xiang, Y. Diao, B. Li, L. Shi, et al., Self-healable, tough and highly stretchable hydrophobic association/ionic dual physically cross-linked hydrogels, RSC Adv. 7 (2017) 12063
12073. [61] C.-Y. Yang, B. Song, Y. Ao, A.P. Nowak, R.B. Abelowitz, R.A. Korsak, et al., Biocompatibility of amphiphilic diblock copolypeptide hydrogels in the central nervous system, Biomaterials 30 (2009) 2881
2898. [62] A.P. Nowak, V. Breedveld, L. Pakstis, B. Ozbas, D.J. Pine, D. Pochan, et al., Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles, Nature 417 (2002) 424.

Self-Healing Polymer-Based Systems

References

203

[63] Y. Sun, A.L. Wollenberg, T.M. O’Shea, Y. Cui, Z.H. Zhou, M.V. Sofroniew, et al., Conformation-directed formation of self-healing diblock copolypeptide hydrogels via polyion complexation, J. Am. Chem. Soc. 139 (2017) 15114
15121. [64] H. Nandivada, X. Jiang, J. Lahann, Click chemistry: versatility and control in the hands of materials scientists, Adv. Mater. 19 (2007) 2197
2208. [65] K.C. Koehler, K.S. Anseth, C.N. Bowman, Diels
Alder mediated controlled release from a poly(ethylene glycol) based hydrogel, Biomacromolecules 14 (2013) 538
547. [66] K.C. Koehler, D.L. Alge, K.S. Anseth, C.N. Bowman, A Diels
Alder modulated approach to control and sustain the release of dexamethasone and induce osteogenic differentiation of human mesenchymal stem cells, Biomaterials 34 (2013) 4150
4158. [67] Z. Wei, J.H. Yang, X.J. Du, F. Xu, M. Zrinyi, Y. Osada, et al., Dextran-based self-healing hydrogels formed by reversible Diels
Alder reaction under physiological conditions, Macromol. Rapid Commun. 34 (2013) 1464
1470. [68] C. Shao, M. Wang, H. Chang, F. Xu, J. Yang, A self-healing cellulose nanocrystal-poly(ethylene glycol) nanocomposite hydrogel via Diels
Alder click reaction, ACS Sustain. Chem. Eng. 5 (2017) 6167
6174. [69] H. Yu, Y. Wang, H. Yang, K. Peng, X. Zhang, Injectable self-healing hydrogels formed via thiol/disulfide exchange of thiol functionalized F127 and dithiolane modified PEG, J. Mater. Chem. B 5 (2017) 4121
4127. [70] P. Casuso, I. Odriozola, A. Pe´rez-San Vicente, I. Loinaz, G. Caban˜ero, H.-J. Grande, et al., Injectable and selfhealing dynamic hydrogels based on metal(i)-thiolate/disulfide exchange as biomaterials with tunable mechanical properties, Biomacromolecules 16 (2015) 3552
3561. [71] Z. Wei, J.H. Yang, Z.Q. Liu, F. Xu, J.X. Zhou, M. Zrı´nyi, et al., Novel biocompatible polysaccharide-based self-healing hydrogel, Adv. Funct. Mater. 25 (2015) 1352
1359. [72] Y. Zhang, L. Tao, S. Li, Y. Wei, Synthesis of multiresponsive and dynamic chitosan-based hydrogels for controlled release of bioactive molecules, Biomacromolecules 12 (2011) 2894
2901. [73] X. Zhao, M. Zhang, B. Guo, P.X. Ma, Mussel-inspired injectable supramolecular and covalent bond crosslinked hydrogels with rapid self-healing and recovery properties via a facile approach under metal-free conditions, J. Mater. Chem. B 4 (2016) 6644
6651. [74] R. Chang, X. Wang, X. Li, H. An, J. Qin, Self-activated healable hydrogels with reversible temperature responsiveness, ACS Appl. Mater. Interfaces 8 (2016) 25544
25551. [75] G. Deng, F. Li, H. Yu, F. Liu, C. Liu, W. Sun, et al., Dynamic hydrogels with an environmental adaptive selfhealing ability and dual responsive sol
gel transitions, ACS Macro Lett. 1 (2012) 275
279. [76] J. Kalia, R.T. Raines, Hydrolytic stability of hydrazones and oximes, Angew. Chem. Int. Ed. 47 (2008) 7523
7526. [77] G.N. Grover, J. Lam, T.H. Nguyen, T. Segura, H.D. Maynard, Biocompatible hydrogels by oxime click chemistry, Biomacromolecules 13 (2012) 3013
3017. [78] G.N. Grover, R.L. Braden, K.L. Christman, Oxime cross-linked injectable hydrogels for catheter delivery, Adv. Mater. 25 (2013) 2937
2942. [79] S. Mukherjee, M.R. Hill, B.S. Sumerlin, Self-healing hydrogels containing reversible oxime crosslinks, Soft Matter 11 (2015) 6152
6161. [80] P.J.M. Bouten, M. Zonjee, J. Bender, S.T.K. Yauw, H. van Goor, J.C.M. van Hest, et al., The chemistry of tissue adhesive materials, Prog. Polym. Sci. 39 (2014) 1375
1405. [81] X. Zhao, H. Wu, B. Guo, R. Dong, Y. Qiu, P.X. Ma, Antibacterial anti-oxidant electroactive injectable hydrogel as self-healing wound dressing with hemostasis and adhesiveness for cutaneous wound healing, Biomaterials 122 (2017) 34
47. [82] M. Mehdizadeh, H. Weng, D. Gyawali, L. Tang, J. Yang, Injectable citrate-based mussel-inspired tissue bioadhesives with high wet strength for sutureless wound closure, Biomaterials 33 (2012) 7972
7983. [83] L. Han, L. Yan, K. Wang, L. Fang, H. Zhang, Y. Tang, et al., Tough, self-healable and tissue-adhesive hydrogel with tunable multifunctionality, NPG Asia Mater. 9 (2017) e372. [84] T. Chen, Y. Chen, H.U. Rehman, Z. Chen, Z. Yang, M. Wang, et al., Ultratough, self-healing, and tissueadhesive hydrogel for wound dressing, ACS Appl. Mater. Interfaces 10 (2018) 33523
33531. [85] F. Yu, X. Cao, J. Du, G. Wang, X. Chen, Multifunctional hydrogel with good structure integrity, self-healing, and tissue-adhesive property formed by combining diels-alder click reaction and acylhydrazone bond, ACS Appl. Mater. Interfaces 7 (2015) 24023
24031.

Self-Healing Polymer-Based Systems

204

7. Self-healing biomaterials based on polymeric systems

[86] B. Yang, Y. Zhang, X. Zhang, L. Tao, S. Li, Y. Wei, Facilely prepared inexpensive and biocompatible selfhealing hydrogel: a new injectable cell therapy carrier, Polym. Chem. 3 (2012) 3235. [87] R. Cancedda, B. Dozin, P. Giannoni, R. Quarto, Tissue engineering and cell therapy of cartilage and bone, Matrix Biol. 22 (2003) 81
91. [88] H. Cui, Y. Liu, Y. Cheng, Z. Zhang, P. Zhang, X. Chen, et al., In vitro study of electroactive tetraanilinecontaining thermosensitive hydrogels for cardiac tissue engineering, Biomacromolecules 15 (2014) 1115
1123. [89] Z. Wei, S. Gerecht, A self-healing hydrogel as an injectable instructive carrier for cellular morphogenesis, Biomaterials 185 (2018) 86
96. [90] R. Dong, X. Zhao, B. Guo, P.X. Ma, Self-healing conductive injectable hydrogels with antibacterial activity as cell delivery carrier for cardiac cell therapy, ACS Appl. Mater. Interfaces 8 (2016) 17138
17150. [91] T.C. Tseng, L. Tao, F.Y. Hsieh, Y. Wei, I.M. Chiu, S.H. Hsu, An injectable, self-healing hydrogel to repair the central nervous system, Adv. Mater. 27 (2015) 3518
3524. [92] S. Bose, S.F. Robertson, A. Bandyopadhyay, Surface modification of biomaterials and biomedical devices using additive manufacturing, Acta Biomater. 66 (2018) 6
22. [93] Y. Wu, L. Wang, X. Zhao, S. Hou, B. Guo, P.X. Ma, Self-healing supramolecular bioelastomers with shape memory property as a multifunctional platform for biomedical applications via modular assembly, Biomaterials 104 (2016) 18
31. [94] A.M. Kushner, J.D. Vossler, G.A. Williams, Z. Guan, A biomimetic modular polymer with tough and adaptive properties, J. Am. Chem. Soc. 131 (2009) 8766
8768. [95] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698
1702. [96] C. Gong, J. Liang, W. Hu, X. Niu, S. Ma, H.T. Hahn, et al., A healable, semitransparent silver nanowirepolymer composite conductor, Adv. Mater. 25 (2013) 4186
4191. [97] J. Ren, Y. Zhu, H. Xuan, X. Liu, Z. Lou, L. Ge, Highly transparent and self-healing films based on the dynamic Schiff base linkage, RSC Adv. 6 (2016) 115247
115251. [98] C. Wang, C. Keplinger, J.L. Zuo, L. Jin, Y. Sun, P. Zheng, et al., A highly stretchable autonomous selfhealing elastomer, Nat. Chem. 8 (2016) 618
624. [99] M. Mozafari, A. Ramedani, Y.N. Zhang, D.K. Mills, Thin films for tissue engineering applications, in: H.J. Griesser (Ed.), Thin Film Coatings for Biomaterials and Biomedical Applications; 2016, pp. 167
195, https://www.sciencedirect.com/science/article/pii/B9781782424536000080, doi: https://doi.org/10.1016/ B978-1-78242-453-6.00008-0. [100] A. Seidi, M. Ramalingam, I. Elloumi-Hannachi, S. Ostrovidov, A. Khademhosseini, Gradient biomaterials for soft-to-hard interface tissue engineering, Acta Biomater. 7 (2011) 1441
1451. [101] R.I. Sharma, J.G. Snedeker, Biochemical and biomechanical gradients for directed bone marrow stromal cell differentiation toward tendon and bone, Biomaterials 31 (2010) 7695
7704. [102] S. Ajami, F. Teimouri, Features and application of wearable biosensors in medical care, J. Res. Med. Sci. 20 (2015) 1208
1215. [103] S. Patel, R. Nanda, S. Sahoo, E. Mohapatra, Biosensors in health care: the milestones achieved in their development towards lab-on-chip-analysis, Biochem. Res. Int. 2016 (2016) 3130469. [104] S.P. Nichols, A. Koh, W.L. Storm, J.H. Shin, M.H. Schoenfisch, Biocompatible materials for continuous glucose monitoring devices, Chem. Rev. 113 (2013) 2528
2549. [105] Y. Han, X. Wu, X. Zhang, C. Lu, Self-healing, highly sensitive electronic sensors enabled by metal
ligand coordination and hierarchical structure design, ACS Appl. Mater. Interfaces 9 (2017) 20106
20114. [106] B.C.K. Tee, C. Wang, R. Allen, Z. Bao, An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications, Nat. Nanotechnol. 7 (2012) 825. [107] J. Kang, D. Son, G.-J.N. Wang, Y. Liu, J. Lopez, Y. Kim, et al., Tough and water-insensitive self-healing elastomer for robust electronic skin, Adv. Mater. 30 (2018) 1706846. [108] K. Guo, D.L. Zhang, X.M. Zhang, J. Zhang, L.S. Ding, B.J. Li, et al., Conductive elastomers with autonomic self-healing properties, Angew. Chem. Int. Ed. Engl. 54 (2015) 12127
12133. [109] H. Xuan, W. Dai, Y. Zhu, J. Ren, J. Zhang, L. Ge, Self-healing, antibacterial and sensing nanoparticle coating and its excellent optical applications, Sens. Actuators B: Chem. 257 (2018) 1110
1117.

Self-Healing Polymer-Based Systems

References

205

[110] T.P. Huynh, H. Haick, Self-healing, fully functional, and multiparametric flexible sensing platform, Adv. Mater. 28 (2016) 138
143. [111] K. Park, Controlled drug delivery systems: past forward and future back, J. Control. Rel. 190 (2014) 3
8. [112] J. Li, X.J. Loh, Cyclodextrin-based supramolecular architectures: syntheses, structures, and applications for drug and gene delivery, Adv. Drug. Deliv. Rev. 60 (2008) 1000
1017. [113] A. Hasan, A. Khattab, M.A. Islam, K.A. Hweij, J. Zeitouny, R. Waters, et al., Injectable hydrogels for cardiac tissue repair after myocardial infarction, Adv. Sci. (Weinh.) 2 (2015) 1500122. [114] J. Li, D.J. Mooney, Designing hydrogels for controlled drug delivery, Nat. Rev. Mater. 1 (2016). [115] L. Yu, J. Ding, Injectable hydrogels as unique biomedical materials, Chem. Soc. Rev. 37 (2008) 1473
1481. [116] C.L. Hastings, E.T. Roche, E. Ruiz-Hernandez, K. Schenke-Layland, C.J. Walsh, G.P. Duffy, Drug and cell delivery for cardiac regeneration, Adv. Drug. Delivery Rev. 84 (2015) 85
106. [117] W. Xie, Q. Gao, Z. Guo, D. Wang, F. Gao, X. Wang, et al., Injectable and self-healing thermosensitive magnetic hydrogel for asynchronous control release of doxorubicin and docetaxel to treat triple-negative breast cancer, ACS Appl. Mater. Interfaces 9 (2017) 33660
33673. [118] F.Y. Hsieh, T.C. Tseng, S.H. Hsu, Self-healing hydrogel for tissue repair in the central nervous system, Neural Regen. Res. 10 (2015) 1922
1923. [119] L. Shi, Y. Han, J. Hilborn, D. Ossipov, “Smart” drug loaded nanoparticle delivery from a self-healing hydrogel enabled by dynamic magnesium-biopolymer chemistry, Chem. Commun. (Camb.) 52 (2016) 11151
11154. [120] M.M.C. Bastings, S. Koudstaal, R.E. Kieltyka, Y. Nakano, A.C.H. Pape, D.A.M. Feyen, et al., A fast pHswitchable and self-healing supramolecular hydrogel carrier for guided, local catheter injection in the infarcted myocardium, Adv. Healthc. Mater. 3 (2014) 70
78. [121] D.L. Taylor, M. In Het Panhuis, Self-healing hydrogels, Adv. Mater. 28 (2016) 9060
9093. [122] J. Qu, X. Zhao, P.X. Ma, B. Guo, pH-responsive self-healing injectable hydrogel based on N-carboxyethyl chitosan for hepatocellular carcinoma therapy, Acta Biomater. 58 (2017) 168
180. [123] M. McKenzie, D. Betts, A. Suh, K. Bui, L.D. Kim, H. Cho, Hydrogel-based drug delivery systems for poorly water-soluble drugs, Molecules 20 (2015) 20397
20408. [124] E. Larraneta, S. Stewart, M. Ervine, R. Al-Kasasbeh, R.F. Donnelly, Hydrogels for hydrophobic drug delivery. Classification, synthesis and applications, J. Funct. Biomater. 9 (2018). [125] U. Kedar, P. Phutane, S. Shidhaye, V. Kadam, Advances in polymeric micelles for drug delivery and tumor targeting, Nanomed. Nanotechnol. Biol. Med. 6 (2010) 714
729. [126] M. Shan, C. Gong, B. Li, G. Wu, A pH, glucose, and dopamine triple-responsive, self-healable adhesive hydrogel formed by phenylborate
catechol complexation, Polym. Chem. 8 (2017) 2997
3005. [127] G. Chang, Y. Chen, Y. Li, S. Li, F. Huang, Y. Shen, et al., Self-healable hydrogel on tumor cell as drug delivery system for localized and effective therapy, Carbohydr. Polym. 122 (2015) 336
342. [128] R. Xing, K. Liu, T. Jiao, N. Zhang, K. Ma, R. Zhang, et al., An injectable self-assembling collagen
gold hybrid hydrogel for combinatorial antitumor photothermal/photodynamic therapy, Adv. Mater. 28 (2016) 3669
3676. [129] N. Huebsch, C.J. Kearney, X. Zhao, J. Kim, C.A. Cezar, et al., Ultrasound-triggered disruption and selfhealing of reversibly cross-linked hydrogels for drug delivery and enhanced chemotherapy, Proc. Natl Acad. Sci. 111 (2014) 9762
9767. [130] L. Bromberg, Polymeric micelles in oral chemotherapy, J. Controlled Rel. 128 (2008) 99
112. [131] J.J. Pillai, A.K.T. Thulasidasan, R.J. Anto, D.N. Chithralekha, A. Narayanan, G.S.V. Kumar, Folic acid conjugated cross-linked acrylic polymer (FA-CLAP) hydrogel for site specific delivery of hydrophobic drugs to cancer cells, J. Nanobiotechnology 12 (2014) 25. [132] J. Qu, X. Zhao, Y. Liang, T. Zhang, P.X. Ma, B. Guo, Antibacterial adhesive injectable hydrogels with rapid self-healing, extensibility and compressibility as wound dressing for joints skin wound healing, Biomaterials 183 (2018) 185
199. [133] A.Z. Wilczewska, K. Niemirowicz, K.H. Markiewicz, H. Car, Nanoparticles as drug delivery systems, Pharmacol. Rep. 64 (2012) 1020
1037. [134] Q. Zhao, B. Li, pH-controlled drug loading and release from biodegradable microcapsules, Nanomed. Nanotechnol. Biol. Med. 4 (2008) 302
310.

Self-Healing Polymer-Based Systems

206

7. Self-healing biomaterials based on polymeric systems

[135] S.E. Reinhold, K.G. Desai, L. Zhang, K.F. Olsen, S.P. Schwendeman, Self-healing microencapsulation of biomacromolecules without organic solvents, Angew. Chem. Int. Ed. Engl. 51 (2012) 10800
10803. [136] S.E. Reinhold, S.P. Schwendeman, Effect of polymer porosity on aqueous self-healing encapsulation of proteins in PLGA microspheres, Macromol. Biosci. 13 (2013) 1700
1710. [137] A. Casan˜as, P. Guerra, I. Fita, N. Verdaguer, Vault particles: a new generation of delivery nanodevices, Curr. Opin. Biotechnol. 23 (2012) 972
977. [138] A. Llauro´, P. Guerra, N. Irigoyen, J.F. Rodrı´guez, N. Verdaguer, P.J. de Pablo, Mechanical stability and reversible fracture of vault particles, Biophysical J. 106 (2014) 687
695. [139] S. Balta, C. Aydogan, B. Demir, C. Geyik, M. Ciftci, E. Guler, et al., Functional surfaces constructed with hyperbranched copolymers as optical imaging and electrochemical cell sensing platforms, Macromol. Chem. Phys. 219 (2018) 1700433. [140] M.F. Montemor, Functional and smart coatings for corrosion protection: a review of recent advances, Surf. Coat. Technol. 258 (2014) 17
37. [141] B. Yu, C. Wang, Y.M. Ju, L. West, J. Harmon, Y. Moussy, et al., Use of hydrogel coating to improve the performance of implanted glucose sensors, Biosens. Bioelectron. 23 (2008) 1278
1284. [142] D. Campoccia, L. Montanaro, C.R. Arciola, A review of the biomaterials technologies for infection-resistant surfaces, Biomaterials 34 (2013) 8533
8554. [143] V.B. Damodaran, N.S. Murthy, Bio-inspired strategies for designing antifouling biomaterials, Biomater. Res. 20 (2016) 18. [144] L.J. Bastarrachea, J.M. Goddard, Self-healing antimicrobial polymer coating with efficacy in the presence of organic matter, Appl. Surf. Sci. 378 (2016) 479
488. [145] C. Esteves, Self-healing functional surfaces, Adv. Mater. Interfaces 5 (2018) 1800293. [146] H. Kuroki, I. Tokarev, D. Nykypanchuk, E. Zhulina, S. Minko, Stimuli-responsive materials with selfhealing antifouling surface via 3D polymer grafting, Adv. Funct. Mater. 23 (2013) 4593
4600. [147] Z. Wang, H. Zuilhof, Self-healing fluoropolymer brushes as highly polymer-repellent coatings, J. Mater. Chem. A 4 (2016) 2408
2412. [148] Z. Wang, H. Zuilhof, Self-healing superhydrophobic fluoropolymer brushes as highly protein-repellent coatings, Langmuir 32 (2016) 6310
6318. [149] Z. Wang, E. van Andel, S.P. Pujari, H. Feng, J.A. Dijksman, M.M.J. Smulders, et al., Water-repairable zwitterionic polymer coatings for anti-biofouling surfaces, J. Mater. Chem. B 5 (2017) 6728
6733. [150] P.M. Imbesi, C. Fidge, J.E. Raymond, S.I. Caue¨t, K.L. Wooley, Model Diels
Alder studies for the creation of amphiphilic cross-linked networks as healable, antibiofouling coatings, ACS Macro Lett. 1 (2012) 473
477. [151] D. Chen, M. Wu, B. Li, K. Ren, Z. Cheng, J. Ji, et al., Layer-by-layer-assembled healable antifouling films, Adv. Mater. 27 (2015) 5882
5888. [152] L. Li, B. Yan, J. Yang, L. Chen, H. Zeng, Novel mussel-inspired injectable self-healing hydrogel with antibiofouling property, Adv. Mater. 27 (2015) 1294
1299. [153] W.J. Yang, X. Tao, T. Zhao, L. Weng, E.-T. Kang, L. Wang, Antifouling and antibacterial hydrogel coatings with self-healing properties based on a dynamic disulfide exchange reaction, Polym. Chem. 6 (2015) 7027
7035. [154] A.N. Zelikin, Drug releasing polymer thin films: new era of surface-mediated drug delivery, ACS Nano 4 (2010) 2494
2509. [155] X.C. Chen, K.F. Ren, W.X. Lei, J.H. Zhang, M.C. Martins, M.A. Barbosa, et al., Self-healing spongy coating for drug “Cocktail” delivery, ACS Appl. Mater. Interfaces 8 (2016) 4309
4313. [156] X. Chen, K. Ren, J. Zhang, D. Li, E. Zhao, Z. Zhao, et al., Humidity-triggered self-healing of microporous polyelectrolyte multilayer coatings for hydrophobic drug delivery, Adv. Funct. Mater. 25 (2015) 7470
7477. [157] K. Chen, S. Zhou, L. Wu, Self-healing underwater superoleophobic and antibiofouling coatings based on the assembly of hierarchical microgel spheres, ACS Nano 10 (2016) 1386
1394. [158] S.V. Murphy, A. Atala, 3D bioprinting of tissues and organs, Nat. Biotechnol. 32 (2014) 773
785. [159] D.B. Kolesky, R.L. Truby, A.S. Gladman, T.A. Busbee, K.A. Homan, J.A. Lewis, 3D bioprinting of vascularized, heterogeneous cell-laden tissue constructs, Adv. Mater. 26 (2014) 3124
3130. [160] M. Guvendiren, J. Molde, R.M. Soares, J. Kohn, Designing biomaterials for 3D printing, ACS Biomater. Sci. Eng. 2 (2016) 1679
1693. [161] J. Radhakrishnan, A. Subramanian, U.M. Krishnan, S. Sethuraman, Injectable and 3D bioprinted polysaccharide hydrogels: from cartilage to osteochondral tissue engineering, Biomacromolecules 18 (2017) 1
26.

Self-Healing Polymer-Based Systems

References

207

[162] P. Kondiah, Y. Choonara, P. Kondiah, T. Marimuthu, P. Kumar, L. du Toit, et al., A review of injectable polymeric hydrogel systems for application in bone tissue engineering, Molecules 21 (2016) 1580. [163] L. Ouyang, C.B. Highley, C.B. Rodell, W. Sun, J.A. Burdick, 3D printing of shear-thinning hyaluronic acid hydrogels with secondary cross-linking, ACS Biomater. Sci. & Eng. 2 (2016) 1743
1751. [164] M.A. Darabi, A. Khosrozadeh, R. Mbeleck, Y. Liu, Q. Chang, J. Jiang, et al., Skin-inspired multifunctional autonomic-intrinsic conductive self-healing hydrogels with pressure sensitivity, stretchability, and 3D printability, Adv. Mater. 29 (2017). [165] S. Liu, L. Li, Ultrastretchable and self-healing double-network hydrogel for 3D printing and strain sensor, ACS Appl. Mater. Interfaces 9 (2017) 26429
26437. [166] A.A. Pawar, G. Saada, I. Cooperstein, L. Larush, J.A. Jackman, S.R. Tabaei, et al., High-performance 3D printing of hydrogels by water-dispersible photoinitiator nanoparticles, Sci. Adv. 2 (2016) e1501381. [167] G. Postiglione, M. Alberini, S. Leigh, M. Levi, S. Turri, Effect of 3D-printed microvascular network design on the self-healing behavior of cross-linked polymers, ACS Appl. Mater. Interfaces 9 (2017) 14371
14378. [168] W. Wu, A. DeConinck, J.A. Lewis, Omnidirectional printing of 3d microvascular networks, Adv. Mater. 23 (2011) H178
H183. [169] Z. Zhang, X. Wang, Y. Wang, J. Hao, Rapid-forming and self-healing agarose-based hydrogels for tissue adhesives and potential wound dressings, Biomacromolecules 19 (2018) 980
988. [170] H. Liu, C. Wang, C. Li, Y. Qin, Z. Wang, F. Yang, et al., A functional chitosan-based hydrogel as a wound dressing and drug delivery system in the treatment of wound healing, RSC Adv. 8 (2018) 7533
7549. [171] L. Han, Y. Zhang, X. Lu, K. Wang, Z. Wang, H. Zhang, Polydopamine nanoparticles modulating stimuliresponsive PNIPAM hydrogels with cell/tissue adhesiveness, ACS Appl. Mater. Interfaces 8 (2016) 29088
29100. [172] P.R. F., B.P. J., Traditional therapies for skin wound healing, Adv. Wound Care 5 (2016) 208
229. [173] M. Farokhi, F. Mottaghitalab, Y. Fatahi, A. Khademhosseini, D.L. Kaplan, Overview of silk fibroin use in wound dressings, Trends Biotechnol. 36 (2018) 907
922. [174] S. Bauer, S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, R. Schwodiauer, 25th anniversary article: a soft future: from robots and sensor skin to energy harvesters, Adv. Mater. 26 (2014) 149
161. [175] M.L. Hammock, A. Chortos, B.C. Tee, J.B. Tok, Z. Bao, 25th anniversary article: the evolution of electronic skin (e-skin): a brief history, design considerations, and recent progress, Adv. Mater. 25 (2013) 5997
6038. [176] R.C. Webb, A.P. Bonifas, A. Behnaz, Y. Zhang, K.J. Yu, H. Cheng, et al., Ultrathin conformal devices for precise and continuous thermal characterization of human skin, Nat. Mater. 12 (2013) 938. [177] M. Park, Y.J. Park, X. Chen, Y.K. Park, M.S. Kim, J.H. Ahn, MoS2-based tactile sensor for electronic skin applications, Adv. Mater. 28 (2016) 2556
2562. [178] H.H. Chou, A. Nguyen, A. Chortos, J.W. To, C. Lu, J. Mei, et al., A chameleon-inspired stretchable electronic skin with interactive colour changing controlled by tactile sensing, Nat. Commun. 6 (2015) 8011. [179] S.C. Mannsfeld, B.C. Tee, R.M. Stoltenberg, C.V. Chen, S. Barman, B.V. Muir, et al., Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers, Nat. Mater. 9 (2010) 859
864. [180] H. Guo, C. Lan, Z. Zhou, P. Sun, D. Wei, C. Li, Transparent, flexible, and stretchable WS2 based humidity sensors for electronic skin, Nanoscale 9 (2017) 6246
6253. [181] Y. Li, S. Chen, M. Wu, J. Sun, Polyelectrolyte multilayers impart healability to highly electrically conductive films, Adv. Mater. 24 (2012) 4578
4582. [182] J.C. Lai, J.F. Mei, X.Y. Jia, C.H. Li, X.Z. You, Z. Bao, A stiff and healable polymer based on dynamiccovalent boroxine bonds, Adv. Mater. 28 (2016) 8277
8282. [183] G. Cai, J. Wang, K. Qian, J. Chen, S. Li, P.S. Lee, Extremely stretchable strain sensors based on conductive self-healing dynamic cross-links hydrogels for human-motion detection, Adv. Sci. (Weinh.) 4 (2017) 1600190. [184] C. Hou, T. Huang, H. Wang, H. Yu, Q. Zhang, Y. Li, A strong and stretchable self-healing film with selfactivated pressure sensitivity for potential artificial skin applications, Sci. Rep. 3 (2013) 3138. [185] Y.L. Rao, A. Chortos, R. Pfattner, F. Lissel, Y.C. Chiu, V. Feig, et al., Stretchable self-healing polymeric dielectrics cross-linked through metal-ligand coordination, J. Am. Chem. Soc. 138 (2016) 6020
6027. [186] Z. Zou, C. Zhu, Y. Li, X. Lei, W. Zhang, J. Xiao, Rehealable, fully recyclable, and malleable electronic skin enabled by dynamic covalent thermoset nanocomposite, Sci. Adv. 4 (2018) eaaq0508.

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C H A P T E R

8 Self-healing Diels Alder engineered thermosets Zeinab Karami1, Mohsen Zolghadr2 and Mohammad Jalal Zohuriaan-Mehr1 1

Biobased Monomers and Polymers Division, Adhesive & Resin Department, Iran Polymer and Petrochemical Institute, Tehran, Iran 2School of Chemistry, University of Tehran, Tehran, Iran

8.1 Fundamentals of self-healing In recent years, inspired by nature, many attempts are made to incorporate self-healing property into polymeric materials. Polymers extensively used in many fields in which continuous exposure of these polymers to environmental stresses including, radiation damage, impact and thermal decomposition, mechanical abrasion, chemical attack—alone or in combination—can result in degradation of the material’s physical properties [1]. So, microscopic or macroscopic damage or fracture within polymers are inevitable and finally may result in the failure of the polymeric materials. Improvement in materials durability is of great interest and attracted many researchers’ attention to design self-healing materials. Materials with self-healing ability not only prolong the lifetime of the product, but also they increase the products usage reliability. Moreover, this property decreases waste production and squandering of resources [2]. Self-healing process in materials is based on two steps: (1) first, molecular segments near or at wounded part physically flow, and (2) second, dissociated bond rejoined together after mechanical damage. These steps may take place continuously depending upon the interplay between thermodynamics and kinetics of the steps [3]. Through healing different properties can be recovered such as thermal [4], mechanical [5], electromechanical [6], corrosion protection [7], magnetic, and electrical properties [8].

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00008-8

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8.2 Types of self-healing systems In 1981, the first healing process was proposed by Jud et al. which was based on the welding of some compatible polymer blends (poly(methyl methacrylate) and styrene acrylonitrile) above their glass transition temperatures (Tg) (1 15 C . Tg). In temperature above Tg, physical links between molecular coils have been established across interfaces by interdiffusion which resulted to restore mechanical properties [9]. The mechanism of healing in the polymer can be classified into two categories: (1) extrinsic self-healing polymers and (2) intrinsic self-healing polymers [10]. Extrinsic category requires a healing agent and is based on the storage of healing agent in discrete reservoirs and embedding them throughout the polymer matrix. The healing agent can be added in different forms like particulate, fiber, vessel, and planar. The most studied methods are including capsule-based [11 15] and vessel-based systems [16 20]. Self-healing process in the intrinsic category does not require any healing agent and relies on the molecular interactions in the polymer structure such as chemical, physical and supramolecular interactions, and the creation of dynamic bonds. The interactions are including of bond formation [21,22], host guest interaction [23,24], metal ligand coordination [25,26], hydrogen bonding [27 29], ionomer formation [30 32], disulfide exchange [33 35], and Diels Alder (DA) reaction [36 39]. Intrinsic healing can be performed with and without the external stimuli including temperature, electric or magnetic field, moisture, light, radiation, pH, etc. [40,41]. Among the systems, “vitrimers” are a group of polymer networks with intrinsic thermally triggered healability due to their dynamic exchangeable covalent bonds. They are introduced first L. Leibler in 2011 [42]. In such unique networks, dynamic reactions (e.g., trans-esterification) occur through associative intermediates remain insoluble and exhibit gradual changes in viscosity as a function of temperature. Such materials have the remarkable property that they can be thermally processed in a liquid state without losing of cross-linking and network integrity. This feature renders the materials processable like vitreous glass, not requiring precise temperature control. Vitrimers, which form a bridge between thermosets and thermoplastics, can in principle be easily (re)processed, recycled, and repaired in a similar way to glass and metals [43]. Strategies to sustainable and continually recyclable crosslinked polymers have been recently reviewed with an emphasis on the DA and vitrimer chemistries [44].

8.3 Diels Alder reaction DA reaction is one of the most important and most studied reactions in the organic chemistry field and among the several dynamic bonds, DA reaction is the most studied one in the last decades. DA reaction as a [4 1 2] cycloaddition reaction firstly described in 1928 by professors Diels and Alder [45]. DA reaction exhibits promising features including forming strong covalent bonds, no emission of low molecular weight condensation products, simple control of reaction by temperature and versatility [46]. Two components of this reaction include diene (two conjugated π-bonds) and dienophile (at least one π-bond).

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Within the reaction, two new C C σ bonds are formed at the expense of two C C π bonds [47]. In DA reaction, all atoms of reactants are also present within the product, so this reaction is a self-contained dynamic reaction [48]. Also the mechanism of the reaction is concerted meaning bonds association/dissociation occurs simultaneously. DA reaction gives the so-called adduct in generally two exo and endo stereoisomers and the product is usually a mixture of two products [49]. As shown in Fig. 8.1A, the exo product substituent is pointed away from the double bond. To bring the energy levels of the molecular orbitals of diene and dienophile closer together (Fig. 8.1B) and thus their effective interaction, the diene should preferably be active (having electron donor groups), and the dienophile has to be relatively inactive (having electron withdrawing groups). Otherwise, the reaction does not occur unless under harsh conditions (e.g., high pressure and temperature) and/ or using proper catalysts.

8.3.1 Kinetics and thermodynamics of Diels Alder reaction The ratio of the two products of DA reaction strongly depends on the reaction conditions (kinetic or thermodynamic control). As typically shown for a furan maleimide DA reaction in Fig. 8.2, the endo-product is usually the major product due to the kinetic control at a lower temperature, for example, room temperature. The higher amount of endo-product can be attributed to the lower energy of the endo-transition state compared to that of the exo-transition state. But, at higher temperatures and after long reaction times, the chemical equilibrium can assert itself and the thermodynamically more stable exo isomer is formed. The exo product is more stable by virtue of a lower degree of steric congestion, while the endo-product is favored by orbital overlap in the transition state [50]. For instance, the effect of kinetic or thermodynamic control was observed in Zolghadr et al. study [51]. They synthesized a linear polymer based on the reaction of a furanfunctionalized resin and bismaleimide (BMI) through DA reaction. Fig. 8.3 shows repeated differential scanning calorimetry (DSC) cycle experiments on the linear polymer. At the

FIGURE 8.1 A schematic representative of (A) furan maleimide Diels Alder reaction, (B) molecular orbital interaction between furan (as diene) and maleimide (as dienophile).

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FIGURE 8.2 Energy levels of exo and endo stereoisomeric products and their corresponding transition states for a typical DA reaction of furan bismaleimide (graphics inspired by ref. [50]). DA, Diels Alder.

FIGURE 8.3 Repeated DSC thermograms of a thermoreversible polymer prepared via DA reaction of a difurfurylated compound and a bismaleimide [51]. DA, Diels Alder; DSC, differential scanning calorimetry. Source: Reproduced with the permission from John Wiley & Sons, Inc.

temperature above 110 C, two broad endothermic peaks ranging from 110 C to 140 C have been attributed to the DA cycloadduct dissociation which can be seen in the first cycle. In the second and third cycles, the second peak at higher temperature related to the thermodynamic isomer (exo stereoisomer) has not been observed. The disappearance of the exo peak was attributed to the relatively short time period of DSC (5 C min21) which only the kinetic product (endo stereoisomer) could be formed.

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DA reaction is reversible and the equilibrium state is governed intensely by the temperature. The equilibrium can be displaced toward the reactants by heating through retro-DA (rDA) reaction [52,53]. DA reaction is exothermic and rDA reaction is endothermic. So, based on the Le Chatelier rule, the state of equilibrium can be changed by changing the temperature. Moreover, according to the Gibbs free energy (ΔG), the effect of temperature on the equilibrium state can be defined. In a spontaneous reaction, ΔG is negative (Eq. 8.1): ΔG 5 ΔH

TΔS

(8.1)

where ΔH is the enthalpy of reaction and ΔS is the entropy of reaction. DA reaction is exothermic which leading to a negative ΔH and also, the entropy is negative (two molecules of diene and dienophile produce one molecule of DA adduct). Therefore in ambient condition, negative ΔG favoring the DA reaction. But with increasing the temperature, 2 TΔS term can dominate the positive ΔH term, leading to a negative ΔG. So, by increasing the temperature, the reverse reaction can be occurred. Besides the effect of temperature, through rDA reaction, the entropy demonstrates an increase (one molecule of DA adduct produces two molecules of diene and dienophile) which resulted in a higher 2 TΔS term [54]. In DA polymerization, the effect of entropy is much more evident. Because a large polymer chain or network (containing many DA moieties) reverts to many primary reactants.

8.3.2 Diels Alder reaction of furan/maleimide Among many pairs of diene/dienophile for DA reaction, furan/maleimide is one of the best thanks to their electronic nature and high reactivity [39]. A comprehensive review was published in 2013 to emphasis on this pair as a versatile click unlick tool in macromolecular architecture [55]. Maleimide molecule has conjugated carbonyl to π-bond in its structure which acts as π-acceptor leading to lower LUMO energy (lower gap between the higher occupied molecular orbital, HOMO, and the lower unoccupied molecular orbital, LUMO) (Fig. 8.1B). On the other hand, furan cycle due to its structure and aromaticity has higher stability and reactivity compared to other dienes. Through DA reaction, its aromaticity is disrupted. But through rDA reaction, furan cycle retrieves its aromaticity, so it requires less energy compared to other dienes. In the temperature range of furan/maleimide rDA reaction, most of the polymers are thermostable and there is no degradation in the polymer structure. Hence, polymerization/depolymerization can take place over and over with very negligible degradation [56,57]. DA reaction between this pair due to its features including relatively mild condition, fast kinetics, no catalyst needed, and no by-product gained much interest in the last decade. Besides the mild condition reaction, it carries out in aqueous media, solution, bulk, and molten state [36 39]. The DA reaction can be an effective approach to develop the reversible polymer thermosets by three strategies including (1) multifunctional monomer polymerization with furan or maleimide moieties, (2) cross-linking of linear thermoplastic bearing furan or

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maleimide pending groups and preparing reversible thermosets, and (3) polymerization of multifunctional monomers bearing furan maleimide cycloadduct [58].

8.4 Diels Alder-based healable thermosets Crosslinked polymer networks based on thermoreversible DA cycloadducts between furans and maleimides were first described in US patent [59] and other reprocessable polymer networks based on this linkage have been reported subsequently. Preparation of DA thermoreversible networks have been the broadly studied systems in the field of healing materials and the DA/rDA reactions are worthy candidates for developing industrial products [60]. The concept of DA-based thermoreversible thermosets was presented by Wudl et al. [5,61]. Later, other researchers and teams including Bowman group and polymer group of Vrije Universiteit Brussel worked on using the benefits of DA and rDA reaction to prepare self-healing thermosets. The diene or dienophile moieties can be a part of polymer chains or their cross-linkers. For instance, Li et al. [62] produced an epoxy-novalac resin having pendant furan moieties on the backbone. Afterward, a BMI cross-linker was used as a cross-linker for the polymer chains through DA reaction. A crack on the surface of the polymer was treated by rDA reaction and to make the cracked sample uniform, it was recross-linked through DA reaction. In some researches, BMI building blocks were used as cross-linkers for epoxy-amine system [diglycidyl ether of bisphenol A (DGEBA)-based epoxy with furfurylamine] [63] or the mixture of a furfurylated DGEBA and furan resin [64]. The thermosets successfully were able to show thermoreversible cycles in both microscopic and macroscopic scales. A biobased epoxy resin having furan functionalities is reported by Picchioni et al. [65]. Their study deals with the synthesis of fully epoxidized compounds from methyl oleate, methyl linoleate, and jatropha oil. The epoxidized products have been reacted with furfuryl amine and then a thermoreversible network has been obtained after DA reaction with maleimide compound. The DA network has presented fully self-healing properties according to thermomechanical investigations. In some thermoreversible DA networks, the polymer structure or the cross-linker can be included of DA adducts. For instance, a diamine cross-linker having DA functionalities was synthesized to cross-link a DGEBA epoxy resin by Bai et al. [66]. The resultant thermoset had shown its potential to heal a scratch after applying rDA reaction conditions. Recently by using nonexpensive sustainable starting materials, researchers of the BIOBASED division of Iran Polymer and Petrochemical Institute have reported several thermally self-healing polymer networks with intrinsic healability (Figs. 8.4 and 8.5). A nonisocyanate polyurethane (NIPU) network was presented by Karami et al. [67] using a cyclocarbonated epoxy resin bearing DA adducts which were crosslinked by diethylenetriamine (Fig. 8.4A). The obtained thermoset showed a great recycleability, weldability, and thermohealing properties. Through another approach [64], they achieved partially biobased thermosetting alloys with healability, weldability, and recycleability (Fig. 8.4B). The hybrid BMI-crosslinked system composing of poly(furfuryl alcohol) bioresin and furfurylated epoxy resin (FER) blends showed effective healing as evidently shown in Fig. 8.5.

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FIGURE 8.4 Intrinsically healable networks recently prepared from nonexpensive sustainable materials [51,64,67]. (A) Epoxy or NIPU networks, (B) polyFA bioresin-FER thermosetting alloys, and (C) semi-IPNs from conventional epoxy resin (DGEBA) and the biomonomer FA. DGEBA, Diglycidyl ether of bisphenol A; IPNs, interpenetrated polymer networks; NIPU, nonisocyanate polyurethane.

FIGURE 8.5 Intrinsic thermohealability of a scratched film surface of epoxy thermoset hybrid composed of poly(furfuryl alcohol) bioresin-furfurylated epoxy resin cured by BMI [64]. BMI, Bismaleimide. Source: Reproduced with the permission from John Wiley & Sons, Inc.

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In another strategy under solvent-free conditions, the same research group feasibly achieved semi-interpenetrated polymer networks (semi-IPNs) with various compositions (Fig. 8.4C). Thus a FER BMI DA polymerization and a DDM-curing of epoxy resin were simultaneously conducted according to a properly designed time temperature program [51]. The healing efficiency was reported to be B80% based on the flexural strength testing results. They claimed their approach could cover a wide range of healing thermosets, so that the tuning of desired Tg of the healable semi-IPNs could be possible by choosing of a proper ratio of FER BMI linear polyadduct/DGEBA DDM network. Finally, it should be pointed out that prior to self-healing practical investigation, the occurrence of DA and rDA reactions can often be confirmed by a series of analysis particularly Fourier transform infrared spectroscopies (and detection of disappearance or appearance of peaks related to furan rings or the ether bridge of DA adducts) [68,69], nuclear magnetic resonance (NMR) spectroscopy [68,70 72], and DSC thermal analysis (and appearance of transition relegated to rDA reaction) [70,71].

8.4.1 Applications of Diels Alder-based self-healing networks Polymer materials can be used in a variety of applications and defiantly for all applications which self-healing or recyclability is necessary the DA chemistry can be a promotor and proper device. Hence when the main component of hydrogels, (nano)composites, coatings, adhesives, and robots comes to the ability to create self-healing networks, the obtained products are potential to recover their structure and properties intrinsically. These self-healing networks can be used in tissue engineering [73], electronics [74], automobiles [75], strain sensors [76], conductive adhesives [77], automotive coatings [78], clear coats, actuators, and robots [79 81]. In the next subsections, the use of typical types of self-healing polymer networks will be discussed. 8.4.1.1 Healable Diels Alder-based hydrogels Besides the use of noncovalent interactions between molecules, oligomers, or polymer chains to prepare reversible hydrogel networks, they can be obtained over the reversible and dynamic covalent bonds such as reversible DA reaction. Most of the studies on the use of DA chemistry for preparation of hydrogels are focused on their applications for cell culture scaffolds [82,83] or carriers for drugs [72,84,85]. In these applications especially for drug delivery, healing properties are not vital, so, the selfhealing of hydrogels was rarely investigated by researchers. However, sometimes biological hydrogels require acceptable inherent self-heal ability in addition to acceptable mechanical properties. In this respect, chitosan was modified by furfural through formation of a covalent bond between the free amino groups of chitosan and the aldehyde groups of furfural. The furfural-modified chitosan was changed to Catechol-modified furfural chitosan by covalent attachment of free amino groups of modified chitosan and aldehyde groups of 3,4-dihydroxy benzaldehyde. The pendant furan groups had the ability to participate in DA reaction with a BMI and the catechol was able to have coordination effects besides the Fe31, that is, a double cross-linking were applied

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to create the network. The obtained hydrogel showed worthy thermodynamic and selfhealing properties. For investigation of self-healing, the hydrogel sample was cut into halves. The two parts were pressed together without any stimulus. After 3 h, the hydrogel was healed probably due to the dynamic cleavage and regeneration of the coordination bonds. Hence, the DA chemistry was not responsible for the healability and it is a type of cross-linking mechanism leading to improve the mechanical properties [73]. A nanocomposite hydrogel having toughness, high resilience, and self-healing properties were produced from flexible polymer chains of poly(ethylene glycol) (PEG) and the encapsulated rod-like cellulose nanocrystals (CNCs). The CNCs were furyl modified and the maleimide functionalities were introduced to the PEG to create thermally reversible covalent DA reaction. The self-recovery and self-healing of nanocomposite hydrogels were evaluated by cyclic loading unloading and tension tests, respectively, and an excellent self-healing efficiency of 78% was observed [86]. The self-healing also was proved by macroscopic test and evaluation of mechanical properties. For this purpose, a sample was split into two pieces and then the surfaces of the pieces were taken in contact according to Fig. 8.6. After applying the rDA and DA reaction conditions on the samples, the tensile FIGURE 8.6 DA selfhealing of a split sample by taking the surfaces in contact flowed by inducing rDA and DA reaction. DA, Diels Alder; rDA, retro-DA. Source: Copyright 2017. reproduced with the permission from the American Chemical Society.

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test proved the merging of two samples into one hydrogel sample as no splitting occurred at the connection line [86]. During the healing process of a DA-based hydrogel, the rDA temperature leads to cleavage of DA bonds and the consequent DA temperature causes the fusion of hydrogel pieces into their original shape and recovery of the initial properties due to the reconnection of DA bonds [3]. 8.4.1.2 Diels Alder-based healable rubbers In addition, to the ordinary applications of elastomers, one of the most modern and functional applications of them is the flexible composites for electronic applications and structural adhesives. [74]. After cross-linking via conventional methods, the elastomers endow acceptable properties while they suffer from the impossibility of recycling and selfrepairing [87]. So, using novel strategies to manufacture reversible rubbers seems to be crucial [88]. Utilizing the DA adducts in the formulation of rubbers is an effective and valuable strategy to induce self-healability and many researchers have dedicated their efforts to this issue. A furan-functionalized ethylene polyethylene rubber and a furan-functionalized polyketone were crosslinked trough DA reaction to create a reversible elastomer. Based on the mechanical investigations, after several healing cycles, the storage modulus showed no significant differences and also the mechanical properties were still unchanged [89]. A sustainable bioelastomer having significant properties including self-healing was prepared from renewable plant oil-based polymers containing furan groups. After the formation of the dynamic DA covalent bonds via DA reaction of furan functionalities and a BMI, the obtained elastomer could be completely recovered by rDA and reoccurrence of DA reaction [90]. Random copolymerization and terpolymerization of furfuryl glycidylether (FGE) and epichlorohydrin and ethylene glycol were carried out to produce the new generation of furan-functionalized elastomers. The obtained polyepichlorohydrin elastomers were reacted with a maleimide compound to give reversibility to the elastomers. The DA elastomers were remolded through the rDA reaction and reformed upon the DA reaction conditions. Based on the investigation of self-healing properties, no significant loss of the performance for the healed elastomers was observed [91]. 8.4.1.3 Diels Alder-based healable polymer composites Significant interest has been growing for inducing the self-healing ability to polymer composites, due to the environmental and economic issues [92]. Generally the fractures and cracks happen in the matrix or at the interface of the matrix and reinforcing phase. However, in a polymer composite, the matrix can contribute in the self-healing process and the reinforcing phase including (nano)particles, nanotubes, or (nano)fibers do not normally take part in self-healing process unless after applying some special treatments [93]. Organizing DA and rDA reaction conditions for a polymer composite donates selfhealability to the fabricated composites [94]. The performance of a polymer composite material is significantly dependent on the matrix-reinforcing interphase as it bears the stress which transfers between the matrix and the reinforcing phase. Due to the formation of not strong interphase that cannot tolerate the stresses, especially due to the mismatching of reinforcing and matrix mechanical

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properties, the stress concentration will happen in the interphase region. This phenomenon will propagate the crack formation and finally occurring the failures [95]. As crack formation is inevitable, preparing a healable interface seems to be necessary as a newer strategy of having high-performance polymer composites. This goal can be gained by preparing DA and rDA reaction conditions for both matrix and interphase [93]. Making an accurate balance between the self-healing and thermomechanical properties is essential for self-healing composites. It is necessary to keep the samples in their solid state at temperatures below the rDA temperatures. The healing process consists of two steps, (1) putting the defected sample at rDA temperature to develop the molecular diffusions to fill the crack and (2) annealing of the sample at DA reaction temperature for reconnection of deboned DA moieties [96]. Among all of the polymer composites, glass fiber-reinforced composites are extensively used for various applications, such as electronics, aviation, and automobile application [75], therefore, giving the healability to this category of materials seems to be defiantly necessary. There are different functionalities on the surface of glass fibers to make effective connections with the matrix which can provide the possibility of different kinds of surface treatments for glass fibers. A composite containing maleimide-treated glass fibers was prepared by Martone et al. [96] having self-healability induced by its epoxy-furan matrix. In particular, the glass fibers were activated by trialkoxysilane through an acidic treatment followed by an aminolysation. The primary amine groups of the aminosilane functionality were reacted with BMI. Fig. 8.7 represents the different steps of glass fiber treatments. The treated glass fibers were then reacted with the furan-functionalized epoxy resin. The simultaneous self-healing of interphase and the bulk was investigated. The overall recovery mechanism was examined utilizing microdroplet pull out test and spectroscopic analysis. The broken sample showed recoverability in the first cycle of healing and applying other cycles caused deterioration in the mechanical strengths. Another variety of highly applicable and qualified composites are carbon fiber reinforced composites. A valuable effort to induce self-healability to a carbon fiber composite is reported by Zhang et al. [97]. In their study, the interphase of the carbon fiberreinforced composite material was amended with thermally reversible DA adducts to create a self-healable composite material. The carbon fibers were equipped by maleimide groups in a two-step reaction and the epoxy matrix was modified with FGE as a furan

FIGURE 8.7 The DA reaction of a BMI-functionalized glass-fiber (graphics inspired by Ref. [96]). BMI, Bismaleimide; DA, Diels Alder.

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donor for DA and rDA reactions. The effect of FGE on the structural and thermomechanical properties was evaluated using a microdeboning test. In this test, the required force for deboning of the interface and the fracture energy of the interface were analyzed. The results showed that 20 and 30 wt.% of FGE leads to a high interfacial self-healing efficiency and also preserve noteworthy interfacial mechanical properties. Cellulose fibers due to the natural abundance and special properties have been used in composites as well. Cellulose fibers modified by an amphiphilic ammonium salt were used as reinforcement for healable DA thermosetting polyurethane containing pendant furan rings. After reaction of furan functionalities and BMI, the obtained composite showed a potential to being mendable and recyclable. However, in this case, the reinforcement did not participate in DA reaction [98]. 8.4.1.3.1 Healing of Diels Alder-based polymer composites by nonthermal methods

In addition, to the conventional heating method for DA and rDA reaction, in some cases, DA-based polymer self-healing composites as smart and advanced polymeric materials can be stimulated with ultrasounds [95,99], microwaves [100,101], electricity [69], light, and magnetic field [93]. Sometimes, the traditional thermal method can be destructive during different cycles of healing, but in the case of other mentioned methods, the negative impacts on the structures can be decreased. For most of the DA chemistry-based self-healing materials long healing procedure is needed in the thermal method which sometimes leads having a healing efficiency less than expected [102]. Therefore developing the new rapid and more efficient methods seem necessary [103]. 8.4.1.3.2 Self-healing Diels Alder-based nanocomposites

Like fiber-reinforced composites, for nanocomposites, producing a strong interface using surface treatments of nanoparticles leads to greater mechanical properties and if the reactive groups of the matrix phase and/or reinforcement have the ability to form dynamics bond through DA reaction, the self-repairing ability is given to the interphase [93]. 8.4.1.3.2.1 Graphene-based Diels Alder nanocomposites Graphene structures have been mentioned as good candidates for preparation of strain sensors, but they suffer from the lack of stretchability which is the prerequisite of wearable electronics and self-healability and high cost. A self-healing and conductive stretchable (up to 200%) composite based on 3D graphene structure were reported by Li et al. [76]. Furfurylamine was used as a modifier for the graphite powder and polyurethane. The DA reaction occurred between the furan functionalities of the PU, modified graphite and the bismaleimids. The obtained composites displayed a great healability under heating and microwave. The authors’ claimed that this composite can be a good candidate for applications such as sensors to detect human motion, e-skin, and human machine interfaces [76]. In another study, graphene nanoplate was used as reinforcement of healable epoxy composites. The epoxy resin was preliminarily reacted with furfury L-amine and subsequently was crosslinked through the DA reaction of furan functionalities as diene and graphene nanoplates as dienophile in the interface of the reinforcement and the matrix as it is presented in Fig. 8.8. Healability of the system was observed by healing of a scratch on the composite surface and it was capable of recycling at 160 C for 0.5 h under mold press [104].

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FIGURE 8.8 DA and rDA reaction of furfurylated epoxy and graphene nanoplates (graphics inspired by ref. [104]). DA, Diels Alder; rDA, retro-DA.

A partially biobased nanocomposite, based on DA chemistry was reported by Wu et al. [94]. In their study, a composite made of DA polyurethane covalently connected to the functionalized graphene nanosheets were formulated. The obtained flexible composite exhibited excellent mechanical properties and infrared (IR) laser self-healing potentials. The IR light can be absorbed and changed into heat energy by the reduced graphene oxide which leads to an increase of the local temperature. After 1 min of IR laser exposure, the mechanical properties including Young’s modulus, break strength, and elongation at break was retained to 100%, 96%, and 97%, respectively, for the healed sample. Also the visual analysis by scanning electron microscope (SEM) confirmed the healing [94]. 8.4.1.3.4.2 Carbon nanotube-based Diels Alder nanocomposites In another research, a healable conductive furan-functionalized polyketone-based thermoset having multi-walled carbon nanotube (MWCNT) was prepared. The incorporation of 5 wt.% of MWCNTs leads to an improvement of mechanical, thermal, and electrical properties of the material. According to X-ray photoelectron spectroscopy, as MWCNTs have diene/dienophile characters, they are able to interact covalently with the furan functionalities of the matrix through DA reaction. This reaction resulted in the improvement of interfacial adhesion of components. On the other hand, the presence of BMI can prepare the condition of another DA reaction between furan groups of the polymer and maleimide groups of BMI. The remolded composite showed the same thermal, mechanical, and electrical properties as the pristine ones [77]. A dual crosslinked MWCNT-based self-healing composite was prepared by Handique et al [105]. A furfuryl grafted epoxy and furfuryl modified MWCNTs were reacted with BMI and anhydride curing agent simultaneously to form a dual crosslinked epoxy composite with two types of intermonomer linkages, that is, reversible and nonreversible covalent bonds, respectively. The obtained epoxy composite showed thermal remendability as a result of consecutive rDA and DA reactions. The thermoremendability of the matrix was investigated for four cycles resulted in acceptable healing efficiency of 79.82%, 67.91%, and 51.80% for the first, second, and third healing tests [105]. The complete healing was not obtained maybe due to the presence of nonreversible linkages and restriction of chain motion.

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A mixture of polycaprolactone poly(furfuryl glycidylether) copolymer (PCLF) and iron oxide nanoparticles-decorated MWCNT (IONPs-MWCNTs) were reacted with BMI through multiple DA reaction to form a self-healing magnetic hybrid system. The resultant system exhibited multifunctional properties such as super-paramagnetic activities and self-healing. In this case besides the potential of pendant furan groups of PCLF to participate in DA reaction with maleimide groups, the network could be expanded by the reaction of MWCNT and maleimide. Self-healablity of the mentioned composite was evaluated by the healing of a cracked sample. The best temperature to obtain a complete healing was 130 C 140 C which resulted in transforming the DA crosslinked network to a plastic state due to the facile polymer chain mobility [106] and diffusion of them into the crack [107]. 8.4.1.3.4.3 Silver nanowires-based Diels Alder nanocomposites Regarding the self-healing conductive screens, two reports have been presented by Pei et al. [108,109]. For these capacitive touch screen sensors, silver nanowires (AgNWs) as reinforcement and a DAbased polymer network were used to prepare a transparent conductor with acceptable mechanical properties [109]. After being at 80oC for 30 s, the resulted compo% due to the occurrence of effisites were able to recover the functions and heal the cracks, cient rDA reaction [108]. The defects were healed quickly and efficiently, as a consequence of DA structural reformation of the polymer network. In fact, the structure reformation leads to reassembling of the silver nanowires to form an integrated network. The healing mechanism consisted of two steps, that is, DA reaction of the polymer matrix and the reformation of the AgNW network, based on SEM microscopy. To investigate the healing efficiency, the conductivity of the recovered composite was examined which was 97% of the original. Similarly, healed samples recovered 86% of the mechanical strength of the original ones [109]. For this selfhealing composite, the healing process could be performed repeatedly without any significant reduction in surface conductivity at the same site [109].

8.4.1.4 Healable Diels Alder-based polymer coatings The corrosion of metals imposes a high cost on the different industries. An effective method to avoid corrosion is applying an organic barrier such as polymeric coatings, on the metal substrates. However, most polymer coatings are susceptible to damages from environmental degradation and conventional repairing methods of coatings are high cost and not effective enough. Therefore the production of coatings which are intrinsically able to self-heal the defects and damages is a serious requirement of coating industries [110]. Here, incorporation of reversible DA adducts is a good method for providing self-healing coatings [111] and is studied in some researches. For an automotive scratch-healing clear coating, a thermally DA-based reversible polyurethane network (DA-PU) was used to heal the scratches via DA/rDA reactions. The thermomechanical and healing properties were compared to those of a commercial counterpart. Based on the optical and atomic force microscopic (AFM) analysis, the healing efficiency and scratch resistance of DA-PU polymer network were observed to be greater than the commercial clear coat [78].

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A thermohealable polymer network composed of more than 83% diethyleneitaconate as a sustainable source has been produced. After the copolymerization reaction of diethylitaconate with furfuryl methacrylate, the DA reaction has led to forming a network upon the addition of BMI. The potential of this network to use in a coating formulation has been investigated. The coating has shown a mendability and even recyclability during the coating lifetime by use of heat-induced treatments, that is, rDA and DA reactions [112]. In another research tri- and difunctional furan resins were synthesized to produce a thermoreversible coating after DA reaction with a bifunctional BMI at 50 C. In this case, the addition of a suitable plasticizer to the coating formulation, resulted in complete healing of a scratch at 130 C, while the mechanical strength has dropped to half [113]. A coating made of polyurethane containing DA adducts was synthesized by Singha et al. [114]. The structural part containing furan functionalities was obtained by the reaction of furfuryl amine and FGE. Afterward, polyurethane was produced by the reaction of furan-functionalized part and isocyanate. The PU possessing furan moieties reacted with maleimide functionalized POSS through DA reaction. The self-healing behavior of the obtained coating was evaluated by a tensile test and the optical microscopy of a crack healing. The results showed an acceptable repairing efficiency. A self-healing epoxy-amine network based on the DA reaction between furan and maleimide was studied by Brancart et al. [115]. Nanosized cracks on the coating surface before and after were recorded by AFM as it is displayed in Fig. 8.9. During the healing process, the scratches depth was observed by AFM at different temperatures. A complete healing has occurred after 10 min at 80  C. 8.4.1.5 Diels Alder-based healable polymer adhesives The application of thermoreversible DA chemistry for polymer adhesive has been rarely investigated by researches. Perhaps the reason for the rarity of the studies on the DAbased self-healing adhesives is the difficulty of reparability investigations for this application or sometimes no need for healing. In this application, it is important to consider the possibility of removing the adhesive at the rDA condition, or the possibility of recycling and reusing of them. That is the reason why Turkenburg et al. [116] investigated the rheological studies of a reversible adhesive and Gou et al. [117] investigated the remolding behavior of a recycled DA-based adhesive. Among the rare studies, DA reaction has been occurred between dithienylfuran and maleimide material to make a reversible and self-healable adhesive. A thermally remendable polymer has been synthesized by the DA reaction between dithienylfuran and maleimide monomers to generate a photoresponsive diarylethene. According to the authors claim, UV light (312 nm) and visible light ( . 435 nm) “gate” the reversibility of the DA reaction and turn the self-healing properties of the polymer “off” and “on,” respectively. After exposure to UV light, the strength of the polymer as an adhesive is enhanced, and visible light weakens the adhesive [44,118]. In another study, a dicyclocarbonate DA adduct was synthesized and polymerized to give a thermoresponsive NIPU adhesive. A DA adduct was obtained by DA reaction between a BMI and cyclocarbonated FGE. The dicyclocarbonate adduct was reacted with a diamine. In this work rDA reaction temperature was chosen by DSC and 1H NMR showed that after 120 min at 100 C, 85% of the adducts are deprotected [119]. It can be observed

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FIGURE 8.9 AFM images of nanodefects (A) before healing, (B) after 5 min at 70 C, (C) after 3 min at 80 C, and (D) after 10 min at 80 C [115]. AFM, atomic force microscope. Source: Reproduced with the permission from SAGE Publications.

that however, the paper title is “remendable thermosetting polymers for isocyanate-free adhesives: a preliminary study,” but no special adhesion test which could approve the mendability of this DA adhesive has been done and it seems that the aim of authors to choose the title is to show the potential of such networks being used in remendable adhesive applications. One of the fewest studies that examined the self-healing of a carbon tube/epoxy adhesive attained by DA adducts is reported by Guo et al. [120]. By choosing the different ratio of furan to maleimide groups the glass transition temperature and mechanical properties of MWCNTs/epoxy adhesives were different. The researchers used lap shear test which is a common test method for adhesives for study self-healing properties of MWCNTs/epoxy. The adhesives displayed improved mechanical properties and outstanding self-healability under rDA and DA reactions. The selection of the optimum ratio of furan/maleimide could facilitate the molecule mobility which is the requirement of effective healing. A single-component self-healing polyurethanes were prepared by the reaction of diols bearing furfuryl and maleimide moieties. The DA reaction of diols occurred in the presence of diisocyanate and polypropylene glycol. The healability of the network was investigated by efficient healing of microscratches and connection of two rectangular-shaped

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films under slight pressure at 130 C for 30 min and 50 C for 12 h. A network was formed through the chemical connotation of interfacial DA moieties which could recover the mechanical properties under the test conditions. The sample was submitted to a load of 250 g without any failure. Hence, the samples showed an appropriate adhesive to be used in adhesive polyurethane [121]. 8.4.1.6 Diels Alder-based self-healing actuators and robots Nowadays, robotic applications are growing rapidly and in the meantime implementation of pneumatic artificial muscles constructed out of flexible materials is of special importance [79]. Soft pneumatic actuators are produced by inspiration from the inherent softness and other principles of embodied organisms. These structures are generally vulnerable to the defects produced by sharp objects in unstructured environments, which limits the numbers of application cycles. Pneumatic artificial muscles made of self-healing materials were investigated recently. The use of DA chemistry provides the appropriate conditions for healing in microscopic and macroscopic scale defects over a mild heat process. In all of the studies on the topic, large realistic cuts were applied on the membrane and a complete self-healability was confirmed. This self-healing was repeatable and the mechanical properties of the muscle were recovered [79 81]. In all of the researches, a novel shaping process was used to prepare the 3D structure, that is, shaping over folding and self-healing from a DA polymer network [80]. Three prototypes, that is, a robotic hand, a gripper, and a membrane for artificial muscles were designed through finite element modeling for these studies. Creation of a defect on the inflated actuator crust which is under gas pressure leads to forming a hole, and dropping the pressure. Therefore the health of the actuator can be detected by the pressure required to control a special position. A damaged actuator can be repaired while the system is offline, for example, at night. If the fracture surfaces are in contact together again the self-healing is probable [80,81]. In a work by Terryn et al. after the self-healing process, the defect of the actuator was fully healed, while a small scar was left on the surface as a result of microscopic misalignments. Also the mechanical properties were recovered. Indeed, the failure of the actuator did not happen in the scar location. This healing was repeatable and the isometric contraction force was monitored after each cycle and compared to the undamaged sample. Only a slight difference was observed between the first, second, and third cycles [81].

8.5 Summary and outlook In recent years many researchers have attempted to synthesize and fabricate materials capable to restore their original properties after damage. Most of the thermosetting polymers have problems from the viewpoint of sustainability and lifetime due to the lack of healability after crack or damage formation. The researchers are underway to provide materials which can be repaired autonomously without external stimuli at the ambient conditions or at least with the aid of available, inexpensive, remote controllable, and manageable stimuli. Ultimately the goal of all

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these approaches is to prolong the lifetime of the systems by using self-healable materials that have proper mechanical properties, insensitive to environmental factors (light, humidity, temperature, oxygen, etc.) with repeatable and high efficiency of healing. It is preferable to use materials that are inexpensive, easily accessible, and processable, particularly derived from sustainable resources and scalable synthetic procedures. Among the effective healing and recycling/reprocessing approaches, chemistry of vitrimers and DA systems have the most attractiveness in both basic and applied viewpoints. For DA systems, although several diene dienophile pairs have been explored, furan/maleimide linkages have been most studied due to their ease of access and attractive equilibrium thermodynamics. These smart materials can be a part of the composites, rubbers, coatings or adhesives, and robots. Some of these DA networks have presented fully selfhealing properties according to thermomechanical investigations. They have the potential to heal a scratch after applying rDA reaction conditions while recovering their structure and properties. Both the DA and rDA reactions are highly functional group tolerant, enabling DA adducts to serve as reversible cross-links for a wide variety of polymers. Besides, the reversion of DA cycloadducts under relatively mild conditions without catalyst or solvent, along with the amenability to renewable substrates and polymers, indicate great potential for dynamic networks based on DA cycloadducts. Therefore DA rDA approach is one of the most attractive and effective methods in the field of self-healing materials due to their improved mechanical properties compared to the other intrinsic healing methods. The tunability and selectivity of reversible DA cycloadduct formation impart many benefits for its use in reprocessing crosslinked polymers. The kinetics and thermodynamics of the DA reaction can be tuned by the selected diene and dieneophile, and the steric bulk and regiochemistry of their substituents. The furan/maleimide linkages are commonly studied due to easy cycloadduct formation, because their dissociation occurs within temperature ranges that are compatible with many polymer backbones. Still, the breadth of available dienes/dienophiles and substitution patterns enables tuning of the solubility of monomers, oxidative and hydrolytic stability of resins, and temperatures required for cross-linking/de-cross-linking, which is valuable for developing polymers for specific applications. Some tips for improving these materials include: • The attention to the structure of the materials used in the design of the molecular structure, chain length, functional groups, soft and hard segments structure, and ratio, etc. • The addition of reinforcing agents including fibers, particles, and sheets to improve mechanical properties and surface modification of reinforcing agents to participate in the self-healing process. • The utilization of inexpensive, available, manageable and remote controllable external stimuli such as electromagnetic field, electric current, light, etc. • Combination of several different self-healing approaches together to prepare materials with new properties like: (1) multi-responsive feature which results in healing under various conditions, (2) tunable and higher mechanical properties due to more interaction between structures (e.g., supramolecular network 1 dynamic network)

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• The utilization of shape-memory polymers and vitrimers as materials with many capabilities in the self-healing field. • Finally as furan derivatives can be obtained from hemicellulose (an abundant agricultural waste product), furan-based DA systems are partially renewable. So, in addition to healability, they are considered as partially bioresourced sustainable systems capable to be recycled and reused. • From a long-term point of view, while noteworthy progress has been made in recent years, the topic is still in its early stages. Lots of ambiguities such as degradation behavior or lifetime and mechanical properties especially at elevated temperatures require more understandings and investigations. On the other hand, the challenges which researchers have to deal with in the future, are consisting of improving the selfhealing efficiency, increasing the number of healing cycles and decreasing the costs of self-healing thermosets production.

References [1] S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev. 39 (2010) 1973 1985. Available from: https://doi.org/10.1039/b904502n. [2] Q. Zhang, L. Liu, C. Pan, D. Li, Review of recent achievements in self-healing conductive materials and their applications, J. Mater. Sci. 53 (2018) 27 46. Available from: https://doi.org/10.1007/s10853-017-1388-8. [3] Y. Yang, X. Ding, M.W. Urban, Chemical and physical aspects of self-healing materials, Prog. Polym. Sci. 49 (2015) 34 59. Available from: https://doi.org/10.1016/j.progpolymsci.2015.06.001. [4] U. Lafont, C. Moreno-Belle, H. Van Zeijl, S. Van Der Zwaag, Self-healing thermally conductive adhesives, J. Intell. Mater. Syst. Struct. 25 (2014) 67 74. Available from: https://doi.org/10.1177/1045389X13498314. [5] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science. (80- ) 295 (2002) 1698 1702. Available from: https://doi.org/10.1126/ science.1065879. [6] B.C.K. Tee, C. Wang, R. Allen, Z. Bao, An electrically and mechanically self-healing composite with pressureand flexion-sensitive properties for electronic skin applications, Nat. Nanotechnology 7 (2012) 825 832. Available from: https://doi.org/10.1038/nnano.2012.192. [7] M. Abdolah Zadeh, S. Van Der Zwaag, S.J. Garcia, Adhesion and long-term barrier restoration of intrinsic self-healing hybrid sol-gel coatings, ACS Appl. Mater. Interfaces 8 (2016) 4126 4136. Available from: https://doi.org/10.1021/acsami.5b11867. [8] C. Wang, H. Wu, Z. Chen, M.T. McDowell, Y. Cui, Z. Bao, Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries, Nat. Chem. 5 (2013) 1042 1048. Available from: https://doi.org/10.1038/nchem.1802. [9] K. Jud, H.H. Kausch, J.G. Williams, Fracture mechanics studies of crack healing and welding of polymers, J. Mater. Sci. 16 (1981) 204 210. Available from: https://doi.org/10.1007/BF00552073. [10] K. Urdl, A. Kandelbauer, W. Kern, U. Mu¨ller, M. Thebault, E. Zikulnig-Rusch, Self-healing of densely crosslinked thermoset polymers—a critical review, Prog. Org. Coat. 104 (2017) 232 249. Available from: https:// doi.org/10.1016/j.porgcoat.2016.11.010. [11] B.J. Blaiszik, M. Baginska, S.R. White, N.R. Sottos, Autonomic recovery of fiber/matrix interfacial bond strength in a model composite, Adv. Funct. Mater. 20 (2010) 3547 3554. Available from: https://doi.org/ 10.1002/adfm.201000798. [12] A.R. Jones, B.J. Blaiszik, S.R. White, N.R. Sottos, Full recovery of fiber/matrix interfacial bond strength using a microencapsulated solvent-based healing system, Compos. Sci. Technol. 79 (2013) 1 7. Available from: https://doi.org/10.1016/j.compscitech.2013.02.007. [13] A.R. Jones, A. Cintora, S.R. White, N.R. Sottos, Autonomic healing of carbon fiber/epoxy interfaces, ACS Appl. Mater. Interfaces 6 (2014) 6033 6039. Available from: https://doi.org/10.1021/am500536t.

Self-Healing Polymer-Based Systems

228

8. Self-healing Diels Alder engineered thermosets

[14] M.R. Kessler, N.R. Sottos, S.R. White, Self-healing structural composite materials, Compos. Part A Appl. Sci. Manuf. 34 (2003) 743 753. Available from: https://doi.org/10.1016/S1359-835X(03)00138-6. [15] M.R. Kessler, S.R. White, Self-activated healing of delamination damage in woven composites, Compos. Part A Appl. Sci. Manuf. 32 (2001) 683 699. Available from: https://doi.org/10.1016/S1359-835X(00) 00149-4. [16] E.B. Murphy, F. Wudl, The world of smart healable materials, Prog. Polym. Sci. 35 (2010) 223 251. Available from: https://doi.org/10.1016/j.progpolymsci.2009.10.006. [17] C.M. Dry, N.R. Sottos, Passive smart self-repair in polymer matrix composite materials, North. Am. Conf. Smart Struct. Mater. 1916 (1993) 438 444. Available from: https://doi.org/10.1117/12.148501. [18] S.M. Bleay, C.B. Loader, V.J. Hawyes, L. Humberstone, P.T. Curtis, A smart repair system for polymer matrix composites, Compos. Part A Appl. Sci. Manuf. 32 (2001) 1767 1776. Available from: https://doi.org/ 10.1016/S1359-835X(01)00020-3. [19] R.S. Trask, I.P. Bond, Biomimetic self-healing of advanced composite structures using hollow glass fibres, Smart Mater. Struct. 15 (2006) 704 710. Available from: https://doi.org/10.1088/0964-1726/15/3/005. [20] H.R. Williams, R.S. Trask, I.P. Bond, Self-healing composite sandwich structures, Smart Mater. Struct. 16 (2007) 1198 1207. Available from: https://doi.org/10.1088/0964-1726/16/4/031. [21] C. Yuan, M.Z. Rong, M.Q. Zhang, Z.P. Zhang, Y.C. Yuan, Self-healing of polymers via synchronous covalent bond fission/radical recombination, Chem. Mater. 23 (2011) 5076 5081. Available from: https://doi.org/ 10.1021/cm202635w. [22] S. Telitel, Y. Amamoto, J. Poly, F. Morlet-Savary, O. Soppera, J. Laleve´e, et al., Introduction of self-healing properties into covalent polymer networks via the photodissociation of alkoxyamine junctions, Polym. Chem. 5 (2014) 921 930. Available from: https://doi.org/10.1039/c3py01162c. [23] M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, et al., Self-healing supramolecular gels formed by crown ether based host-guest interactions, Angew. Chem. 51 (2012) 7011 7015. Available from: https://doi. org/10.1002/anie.201203063. [24] D. Xia, M. Xue, A supramolecular polymer gel with dual-responsiveness constructed by crown ether based molecular recognition, Polym. Chem. 5 (2014) 5591 5597. Available from: https://doi.org/10.1039/ c4py00590b. [25] Y.L. Rao, A. Chortos, R. Pfattner, F. Lissel, Y.C. Chiu, V. Feig, et al., Stretchable self-healing polymeric dielectrics cross-linked through metal-ligand coordination, J. Am. Chem. Soc. 138 (2016) 6020 6027. Available from: https://doi.org/10.1021/jacs.6b02428. [26] M. Enke, R.K. Bose, S. Bode, J. Vitz, F.H. Schacher, S.J. Garcia, et al., A metal salt dependent self-healing response in supramolecular block copolymers, Macromolecules 49 (2016) 8418 8429. Available from: https://doi.org/10.1021/acs.macromol.6b02108. [27] L. Guadagno, L. Vertuccio, C. Naddeo, E. Calabrese, G. Barra, M. Raimondo, et al., Self-healing epoxy nanocomposites via reversible hydrogen bonding, Compos. Part B Eng. 157 (2019) 1 13. Available from: https:// doi.org/10.1016/j.compositesb.2018.08.082. [28] Y.G. Luan, X.A. Zhang, S.L. Jiang, J.H. Chen, Y.F. Lyu, Self-healing supramolecular polymer composites by hydrogen bonding interactions between hyperbranched polymer and graphene oxide, Chin. J. Polym. Sci. 36 (2018) 584 591. Available from: https://doi.org/10.1007/s10118-018-2025-y. [29] A. Zhang, L. Yang, Y. Lin, L. Yan, H. Lu, L. Wang, Self-healing supramolecular elastomers based on the multi-hydrogen bonding of low-molecular polydimethylsiloxanes: synthesis and characterization, J. Appl. Polym. Sci. 129 (2013) 2435 2442. Available from: https://doi.org/10.1002/app.38832. [30] R.J. Varley, S. Van der Zwaag, Development of a quasi-static test method to investigate the origin of selfhealing in ionomers under ballistic conditions, Polym. Test. 27 (2008) 11 19. Available from: https://doi. org/10.1016/j.polymertesting.2007.07.013. [31] R.J. Varley, S. van der Zwaag, Autonomous damage initiated healing in a thermo-responsive ionomer, Polym. Int. 59 (2010) 1031 1038. Available from: https://doi.org/10.1002/pi.2841. [32] N. Garcı´a-Huete, W. Post, J.M. Laza, J.L. Vilas, L.M. Leo´n, S.J. Garcı´a, Effect of the Blend Ratio on the Shape Memory and Self-Healing Behaviour of Ionomer-Polycyclooctene Crosslinked Polymer Blends, Eur. Polym. J. 98 (2018) 154 161. Available from: https://doi.org/10.1016/j.eurpolymj.2017.11.006. [33] J. Kamada, K. Koynov, C. Corten, A. Juhari, J.A. Yoon, M.W. Urban, et al., Redox responsive behavior of thiol/disulfide-functionalized star polymers synthesized via atom transfer radical polymerization, Macromolecules 43 (2010) 4133 4139. Available from: https://doi.org/10.1021/ma100365n.

Self-Healing Polymer-Based Systems

References

229

[34] J.A. Yoon, J. Kamada, K. Koynov, J. Mohin, R. Nicolay¨, Y. Zhang, et al., Self-healing polymer films based on thiol disulfide exchange reactions and self-healing kinetics measured using atomic force microscopy, Macromolecules (2011) 142 149. Available from: https://doi.org/10.1021/ma2015134. [35] A. Ruiz De Luzuriaga, R. Martin, N. Markaide, A. Rekondo, G. Caban˜ero, J. Rodrı´guez, et al., Epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites, Mater. Horiz. 3 (2016) 241 247. Available from: https://doi.org/10.1039/ c6mh00029k. [36] R. Araya-Hermosilla, A.A. Broekhuis, F. Picchioni, Reversible polymer networks containing covalent and hydrogen bonding interactions, Eur. Polym. J. 50 (2014) 127 134. [37] A. Laquie`vre, S. Barrau, D. Fournier, G. Stoclet, P. Woisel, J.M. Lefebvre, Thermally reversible crosslinked copolymers: solution and bulk behavior, Polymer 117 (2017) 342 353. Available from: https://doi.org/ 10.1016/j.polymer.2017.04.042. [38] C. Lakatos, K. Czifra´k, J. Karger-Kocsis, L. Daro´czi, M. Zsuga, S. Ke´ki, Shape memory crosslinked polyurethanes containing thermoreversible Diels Alder couplings, J. Appl. Polym. Sci. 133 (2016) 1 9. Available from: https://doi.org/10.1002/app.44145. [39] K. Roos, E. Dolci, S. Carlotti, S. Caillol, Activated anionic ring-opening polymerization for the synthesis of reversibly cross-linkable poly(propylene oxide) based on furan/maleimide chemistry, Polym. Chem. 7 (2016) 1612 1622. Available from: https://doi.org/10.1039/c5py01778e. [40] M.A.C. Stuart, W.T.S. Huck, J. Genzer, M. Mu¨ller, C. Ober, M. Stamm, et al., Emerging applications of stimuli-responsive polymer materials, Nat. Mater. 9 (2010) 101 113. Available from: https://doi.org/ 10.1038/nmat2614. [41] P. Theato, B.S. Sumerlin, R.K. O’reilly, T.H. Epp, Stimuli responsive materials, Chem. Soc. Rev. 42 (2013) 7055 7056. Available from: https://doi.org/10.1039/c3cs90057f. [42] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Silica-like malleable materials from permanent organic networks, Sci. (80- ) 334 (2011) 965 969. [43] W. Denissen, J.M. Winne, F.E.D. Prez, Vitrimers: permanent organic networks with glasslike fluidity, Chem. Sci. 7 (2016) 30 38. Available from: https://doi.org/10.1039/c5sc02223a. [44] D.J. Fortman, J.P. Brutman, G.X.D. Hoe, R.L. Snyder, W.R. Dichtel, M.A. Hillmyer, Approaches to sustainable and continually recyclable cross-linked polymers, ACS Sustain. Chem. Eng. 6 (2018) 11145 11159. Available from: https://doi.org/10.1021/acssuschemeng.8b02355. [45] O. Diels, K. Alder, Syntheses in the hydroaromatic series. I. Addition of “diene” hydrocarbons, Justus Liebigs Ann. Chem. 460 (1928) 98 122. Available from: https://doi.org/10.1002/jlac.19284600106. [46] J. Ax, G. Wenz, Thermoreversible networks by diels-alder reaction of cellulose furoates with bismaleimides, Macromol. Chem. Phys. 213 (2012) 182 186. Available from: https://doi.org/10.1002/macp.201100410. [47] K.N. Houk, Frontier molecular orbital theory of cycloaddition reactions, Acc. Chem. Res. 8 (1975) 361 369. Available from: https://doi.org/10.1021/ar50095a001. [48] N. Roy, B. Bruchmann, J.M. Lehn, Dynamers: dynamic polymers as self-healing materials, Chem. Soc. Rev. 44 (2015) 3786 3807. Available from: https://doi.org/10.1039/c5cs00194c. [49] E. Dolci, V. Froidevaux, G. Michaud, F. Simon, R. Auvergne, S. Fouquay, et al., Thermoresponsive crosslinked isocyanate-free polyurethanes by Diels Alder polymerization, J. Appl. Polym. Sci. 134 (2017) 1 11. Available from: https://doi.org/10.1002/app.44408. [50] F. Carey, R. Giuliano, Organic Chemistry, McGraw-Hill Educ, 2013. [51] M. Zolghadr, A. Shakeri, M.J. Zohuriaan-mehr, A. Salimi, Self-healing semi-IPN materials from epoxy resin by solvent-free furan maleimide Diels Alder polymerization, J. Appl. Polym. Sci. 136 (2019) 48015. Available from: https://doi.org/10.1002/app.48015. [52] R.J. Sheridan, C.N. Bowman, A simple relationship relating linear viscoelastic properties and chemical structure in a model Diels Alder polymer network, Macromolecules 45 (2012) 7634 7641. Available from: https://doi.org/10.1021/ma301329u. [53] Y.L. Liu, C.Y. Hsieh, Y.W. Chen, Thermally reversible cross-linked polyamides and thermo-responsive gels by means of Diels Alder reaction, Polymer 47 (2006) 2581 2586. Available from: https://doi.org/10.1016/j. polymer.2006.02.057. [54] N.K. Guimard, J. Ho, J. Brandt, C. Lin, M. Namazian, J.O. Mueller, et al., Harnessing entropy to direct the bonding/debonding of polymer systems based on reversible chemistry, Chem. Sci. 4 (2013) 2752. Available from: https://doi.org/10.1039/c3sc50642h.

Self-Healing Polymer-Based Systems

230

8. Self-healing Diels Alder engineered thermosets

[55] A. Gandini, Progress in polymer science the furan/maleimide diels Alder reaction: a versatile click unclick tool in macromolecular synthesis, Prog. Polym. Sci. 38 (2013) 1 29. Available from: https://doi.org/ 10.1016/j.progpolymsci.2012.04.002. [56] A. Duval, H. Lange, M. Lawoko, C. Crestini, Reversible crosslinking of lignin via the furan-maleimide Diels Alder reaction, Green. Chem. 17 (2015) 4991 5000. Available from: https://doi.org/10.1039/ c5gc01319d. [57] R. Araya-Hermosilla, G. Fortunato, A. Pucci, Thermally reversible rubber-toughened thermoset networks via Diels Alder chemistry, Eur. Polym. J. 74 (2016) 229 240. Available from: https://doi.org/10.1016/j. eurpolymj.2015.11.020. [58] V. Gaina, O. Ursache, C. Gaina, E. Buruiana, Novel thermally-reversible epoxy-urethane networks, Des. Monomers Polym. 15 (2012) 63 73. Available from: https://doi.org/10.1163/156855511X606155. [59] J.M. Craven Cross-Linked Thermally Reversible Polymers Produced From Condensation Polymers With Pendant Furan Groups Cross-Linked With Maleimides, U.S. Patent 3,435,003, 1969. [60] K.K. Oehlenschlaeger, J.O. Mueller, J. Brandt, Adaptable hetero diels alder networks for fast self-healing under mild conditions, Adv. Mater. 26 (2014) 3561 3566. Available from: https://doi.org/10.1002/ adma.201306258. [61] X. Chen, F. Wudl, A.K. Mal, H. Shen, S.R. Nutt, New thermally remendable highly cross-linked polymeric materials, Macromolecules 36 (2003) 1802 1807. [62] J. Li, G. Zhang, L. Deng, K. Jiang, S. Zhao, Y. Gao, et al., Thermally reversible and self-healing novolac epoxy resins based on Diels Alder chemistry, J. Appl. Polym. Sci. 132 (2015) 1 7. Available from: https://doi.org/ 10.1002/app.42167. [63] D.H. Turkenburg, H.R. Fischer, Diels Alder based, thermo-reversible cross-linked epoxies for use in selfhealing composites, Polymer 79 (2015) 187 194. Available from: https://doi.org/10.1016/j. polymer.2015.10.031. [64] Z. Karami, M.J. Zohuriaan-mehr, A. Rostami, Biobased Diels Alder engineered network from furfuryl alcohol and epoxy resin: preparation and mechano-physical characteristics, ChemistrySelect 3 (2018) 12099 12105. Available from: https://doi.org/10.1002/slct.201702387. [65] M. Iqbal, R.A. Knigge, H.J. Heeres, Diels Alder-crosslinked polymers derived from Jatropha Oil, Polym. (Basel) 10 (2018) 1177. Available from: https://doi.org/10.3390/polym10101177. [66] N. Bai, G.P. Simon, K. Saito, Characterisation of the thermal self-healing of a high crosslink density epoxy thermoset, N. J. Chem. 39 (2015) 3497 3506. Available from: https://doi.org/10.1039/C5NJ00066A. [67] Z. Karami, M.J. Zohuriaan-Mehr, A. Rostami, Bio-based thermo-healable non-isocyanate polyurethane DA network in comparison with its epoxy counterpart, J. CO2 Util. 18 (2017) 294 302. Available from: https:// doi.org/10.1016/j.jcou.2017.02.009. [68] C. Garcia-Astrain, I. Algar, A. Gandini, Hydrogel synthesis by aqueous Diels Alder reaction between furan modified methacrylate and polyetheramine-based bismaleimides, Polym. Chem. (2014) 699 708. Available from: https://doi.org/10.1002/pola.27495. [69] G. Li, P. Xiao, S. Hou, Y. Huang, Rapid and efficient polymer/graphene based multichannel self-healing material via Diels Alder Reaction, Carbon 147 (2019) 398 407. Available from: https://doi.org/10.1016/j. carbon.2019.03.021. [70] E. Dolci, G. Michaud, F. Simon, S. Fouquay, R. Auvergne, Diels Alder thermoreversible isocyanate-free polyurethanes, Green. Mater. 4 (2017) 160 170. Available from: https://doi.org/10.1680/jgrma.16.00018. [71] N. Okhaya, N. Mignarda, C. Jegata, Diels Alder thermoresponsive networks based on high maleimidefunctionalized urethane prepolymers, Des. Monomers Polym. 16 (2013) 475 487. Available from: https:// doi.org/10.1080/15685551.2012.747166. [72] O. Guaresti, R.H. Aguirresarobe, A. Eceiza, N. Gabilondo, Synthesis of stimuli responsive chitosan based hydrogels by Diels Alder cross linking ‘click’ reaction as potential carriers for drug administration, Carbohydr. Polym. 183 (2018) 278 286. Available from: https://doi.org/10.1016/j.carbpol.2017.12.034. [73] S. Li, L. Wang, X. Yu, C. Wang, Z. Wang, Synthesis and characterization of a novel double cross-linked hydrogel based on Diels Alder click reaction and coordination bonding, Mater. Sci. Eng. C. 82 (2018) 299 309. Available from: https://doi.org/10.1016/j.msec.2017.08.031.

Self-Healing Polymer-Based Systems

References

231

[74] M. Rezakazemi, A. Dashti, M. Asghari, S. Shirazian, H2-selective mixed matrix membranes modeling using ANFIS, PSO-ANFIS, GA-ANFIS, Int. J. Hydrogen Energy 42 (2017) 15211 15225. Available from: https:// doi.org/10.1016/j.ijhydene.2017.04.044. [75] T.P. Sathishkumar, S. Satheeshkumar, J. Naveen, Glass fiber-reinforced polymer composites 2 a review, J. Reinf. Plast. Compos. 33 (2014) 1258 1275. Available from: https://doi.org/10.1177/0731684414530790. [76] J. Li, Q. Liu, D. Ho, S. Zhao, S. Wu, L. Ling, et al., Three-dimensional graphene structure for healable flexible electronics based on Diels Alder chemistry, ACS Appl. Mater. Interfaces 10 (2018) 9727 9735. Available from: https://doi.org/10.1021/acsami.7b19649. [77] R. Araya-hermosilla, A. Pucci, P. Raffa, Electrically-responsive reversible polyketone/MWCNT network through Diels Alder chemistry, Polymers (2018) 10. Available from: https://doi.org/10.3390/ polym10101076. [78] S. Sung, S. Young, T. Hee, Thermally reversible polymer networks for scratch resistance and scratch healing in automotive clear coats, Prog. Org. Coat. 127 (2019) 37 44. Available from: https://doi.org/10.1016/j. porgcoat.2018.10.023. [79] S. Terryn, J. Brancart, D. Lefeber, G. Assche, B. Vanderborght, A pneumatic artificial muscle manufactured out of self - healing polymers that can repair macroscopic damages, IEEE Robot. Autom. Lett. 3 (2018) 16 21. Available from: https://doi.org/10.1109/LRA.2017.2724140. [80] S. Terryn, G. Mathijssen, J. Brancart, D. Lefeber, G. Assche, B. Vanderborght, Development of a self-healing soft pneumatic actuator: a first concept, Bioinspir Biomim. 10 (2015) 46007. Available from: https://doi.org/ 10.1088/1748-3190/10/4/046007. [81] S. Terryn, J. Brancart, D. Lefeber, G. Assche, B. Vanderborght, Self-healing soft pneumatic robots, Sci. Robot. 2 (2017) 1 13. Available from: https://doi.org/10.1126/scirobotics.aan4268. [82] L.J. Smith, S.M. Taimoory, R.Y. Tam, Tam, Y. Roger, E.G. Alexander, et al., Diels Alder click-crosslinked hydrogels with increased reactivity enable 3d cell encapsulation, Biomacromolecules 12 (2018) 926 935. Available from: https://doi.org/10.1021/acs.biomac.7b01715. [83] B. Bi, M. Ma, S. Lv, R. Zhuo, X. Jiang, In-situ forming thermosensitive hydroxypropyl chitin-based hydrogel crosslinked by Diels Alder reaction for three dimensional cell culture, Carbohydr. Polym. 212 (2019) 368 377. Available from: https://doi.org/10.1016/j.carbpol.2019.02.058. [84] O. Guaresti, C.G. Astrain, L. Urbina, A. Eceiza, N. Gabilondo, Reversible swelling behaviour of Diels Alder clicked chitosan hydrogels in response to pH changes, Express Polym. Lett. Polym. Lett. 13 (2019) 27 36. [85] M. Zhang, J. Wang, Z. Jin, Supramolecular hydrogel formation between chitosan and hydroxypropyl β-cyclodextrin via Diels Alder reaction and its drug delivery, Biol. Macromol. 114 (2018) 381 391. Available from: https://doi.org/10.1016/j.ijbiomac.2018.03.106. [86] C. Shao, M. Wang, H. Chang, F. Xu, J. Yang, A self-healing cellulose nanocrystals-poly(ethylene glycol) nanocomposite hydrogel via Diels Alder click reaction, ACS Sustain. Chem. Eng. 5 (2017) 6167 6174. Available from: https://doi.org/10.1021/acssuschemeng.7b01060. [87] J.Y. Oh, S. Kim, H. Baik, U. Jeong, Conducting polymer dough for deformable electronics, Adv. Mater. 28 (2016) 4455 4461. Available from: https://doi.org/10.1002/adma.201502947. [88] L.M. Polgar, F. Criscitiello, E.M. Van, R. Araya-hermosilla, N. Migliore, M. Lenti, et al., Thermoreversibly cross-linked epm rubber nanocomposites with carbon nanotubes, Nanomaterials (2018) 58. Available from: https://doi.org/10.3390/nano8020058. [89] L.M. Polgar, G. Fortunato, R. Araya-hermosilla, M.V. Duin, A. Pucci, F. Picchioni, Cross-linking of rubber in the presence of multi-functional cross-linking aids via thermoreversible Diels Alder chemistry, Eur. Polym. J. 82 (2016) 208 219. Available from: https://doi.org/10.1016/j.eurpolymj.2016.07.018. [90] L. Yuan, Z. Wang, M.S. Ganewatta, A. Rahman Md, M.E. Lamma, C. Tang, A biomass approach to mendable bio-elastomers, Soft Matter 13 (2017) 1306 1313. Available from: https://doi.org/10.1039/C6SM02003H. [91] M. Deng, F. Guo, D. Liao, Z. Hou, Y. Li, Aluminium-catalyzed terpolymerization of furfuryl glycidyl ether with epichlorohydrin and ethylene oxide: synthesis of thermoreversible polyepichlorohydrin elastomers with furan/maleimide covalent crosslinks, Polym. Chem. 9 (2018) 98 107. Available from: https://doi.org/ 10.1039/C7PY01516J. [92] V.K. Thakur, M.R. Kessler, Self-healing polymer nanocomposite materials: a review, Polymer 69 (2015) 369 383. Available from: https://doi.org/10.1016/j.polymer.2015.04.086.

Self-Healing Polymer-Based Systems

232

8. Self-healing Diels Alder engineered thermosets

[93] N. Kato, Y. Matsui, D.K. Jesthi, S. Swarup, A review on Diels-Alder based self-healing polymer composites a review on Diels Alder based self-healing polymer composites, Mater. Sci. Eng. (2018) 377. Available from: https://doi.org/10.1088/1757-899X/377/1/012007. [94] S. Wu, J. Li, G. Zhang, Y. Yao, G. Li, R. Sun, et al., Ultrafast self-healing nanocomposites via infrared laser and their application in flexible electronics, ACS Appl. Mater. Interfaces 9 (2017) 3040 3049. Available from: https://doi.org/10.1021/acsami.6b15476. [95] G.M. Fierro, F. Pinto, S. Iacono, Dello, Monitoring of self-healing composites: a nonlinear ultrasound approach, Smart Mater. Struct. 26 (2017) 115015. Available from: https://doi.org/10.1088/1361-665X/ aa89a8. [96] A. Martone, S.D. Iacono, S. Xiao, F. Xu, E. Amendola, An integrated strategy to promote self-healing at interface in composites based on Diels Alder epoxy, AIP Conf. Proc. (2018) 20046. Available from: https:// doi.org/10.1063/1.5045908. [97] W. Zhang, J. Duchet, J.F. Gerard, Effect of epoxy matrix architecture on the selfhealing ability of thermoreversible interfaces based on Diels Alder reactions: demonstration on a carbon fiber/epoxy microcomposite, RSC Adv. 6 (2016) 114235. Available from: https://doi.org/10.1039/C6RA23246A. [98] K. Park, C. Shin, Y. Song, et al., Recyclable and mendable cellulose-reinforced composites crosslinked with diels alder adducts, Polymers (2019) 11. Available from: https://doi.org/10.3390/polym11010117. [99] C. In, R.B. Holland, J. Kim, K.E. Kurtis, L.F. Kahn, L.J. Jacobs, Monitoring and evaluation of self-healing in concrete using diffuse ultrasound, NDT&E Int. 57 (2013) 36 44. Available from: https://doi.org/10.1016/j. ndteint.2013.03.005. [100] D. Voiry, J. Yang, J. Kupferberg, R. Fullon, C. Lee, H.Y. Jeong, et al., High-quality graphene via microwave reduction of solution-exfoliated graphene oxide, Science 80 (2016) 1413 1416. [101] J. Li, G. Zhang, C. Wong, A covalently cross-linked reduced functionalized graphene oxide/polyurethane composite based on Diels Alder chemistry and its potential application in healable flexible electronics, J. Mater. Chem. C. 5 (2017) 220 228. Available from: https://doi.org/10.1039/c6tc04715g. [102] G. Fu, L. Yuan, G. Liang, A. Gu, Heat-resistant polyurethane film with great electrostatic dissipation capacity and very high thermally reversible selfhealing efficiency based on multi-furan and liquid multimaleimide polymers, J. Mater. Chem. A 4 (2016) 4232 4241. Available from: https://doi.org/10.1039/ C6TA00953K. [103] K. Pyo, D.H. Lee, Y. Kim, J. Kim, Extremely rapid and simple healing of a transparent conductor based on Ag nanowires and polyurethane with a Diels Alder network, J. Mater. Chem. C. 4 (2016) 972 977. Available from: https://doi.org/10.1039/C5TC04030B. [104] C.R. Oh, D.I. Lee, J.H. Park, D.S. Lee, Thermally healable and recyclable graphene-nanoplate/epoxy composites via an thermally healable and recyclable in-situ Diels Alder reaction on the graphene-nanoplate/ epoxy composites via an graphene-nanoplate surface in-situ Diels Alder reaction on the gr, Polymers (2019) 11. Available from: https://doi.org/10.3390/polym11061057. Available from: www.mdpi.com/journal/polymers. [105] J. Handique, S.K. Dolui, A thermally remendable multiwalled carbon nanotube/epoxy composites via Diels Alder bonding, J. Polym. Res. (2019) 26. Available from: https://doi.org/10.1007/s10965-019-1804-7. [106] S. Scha¨fer, G. Kickelbick, Self-healing polymer nanocomposites based on Diels Alder-reactions with silica nanoparticles: the role of the polymer matrix, Polymer 69 (2015) 357 368. Available from: https://doi.org/ 10.1016/j.polymer.2015.03.017. [107] Y.H. Lee, Y.N. Zhuang, H.T. Wang, Fabrication of self-healable magnetic nanocomposites via Diels 2 Alder click chemistry, Appl. Sci. (2019) 9. Available from: https://doi.org/10.3390/app9030506. [108] J. Li, J. Liang, L. Li, F. Ren, W. Hu, J. Li, et al., Healable capacitive touch screen sensors based on transparent composite electrodes comprising silver nanowires and a furan / maleimide Diels Alder cycloaddition polymer, ACS Nano 8 (2014) 12874 12882. [109] C. Gong, J. Liang, W. Hu, X. Niu, S. Ma, H.T. Hahn, et al., A healable, semitransparent silver nanowirepolymer composite conductor, Adv. Mater. 25 (2013) 4186 4191. Available from: https://doi.org/10.1002/ adma.201301069. [110] X. Luo, P.T. Mather, Shape memory assisted self-healing coating, ACS Macro Lett. 2 (2013) 152 156. [111] L. Fang, J. Chen, Y. Zou, Z. Xu, Thermally-induced self-healing behaviors and properties of four epoxy l coatings with different network architectures, Polymers (2017) 9. Available from: https://doi.org/10.3390/ polym9080333.

Self-Healing Polymer-Based Systems

References

233

[112] D.H. Turkenburg, Y. Durant, H.R. Fischer, Progress in organic coatings bio-based self-healing coatings based on thermo-reversible Diels Alder reaction, Prog. Org. Coat. 111 (2017) 38 46. Available from: https://doi.org/10.1016/j.porgcoat.2017.05.006. [113] G. Postiglione, S. Turri, M. Levi, Effect of the plasticizer on the self-healing properties of a polymer coating based on the thermoreversible Diels Alder reaction, Prog. Org. Coat. 78 (2015) 526 531. Available from: https://doi.org/10.1016/j.porgcoat.2014.05.022. [114] P.K. Behera, P. Mondal, N.K. Singha, Self-healable and ultrahydrophobic polyurethane-poss hybrids by diels 2 alder “click” reaction: a new class of coating material, Macromolecules 51 (2018) 4770 4781. Available from: https://doi.org/10.1021/acs.macromol.8b00583. [115] J. Brancart, G. Scheltjens, T. Muselle, B.V. Mele, H. Terryn, G.V. Assche, Atomic force microscopy based study of self-healing coatings based on reversible polymer network systems, J. Intell. Mater. Syst. Struct. 25 (2014) 40 46. Available from: https://doi.org/10.1177/1045389X12457100. [116] H. Turkenburg, H.V. Bracht, M. Schmider, Polyurethane adhesives containing Diels Alder based thermoreversible bonds, J. Appl. Polym. Sci. 134 (2017) 44972. Available from: https://doi.org/10.1002/app.44972. [117] Z. Gou, Y. Zuo, S. Feng, Thermally self-healing silicone-based networks with potential application in recycling adhesives, RSC Adv. 6 (2016) 73140 73147. Available from: https://doi.org/10.1039/C6RA14659G. [118] A.M. Asadirad, S. Boutault, Z. Erno, N.R. Branda, Controlling a polymer adhesive using light and a molecular switch, J. Am. Chem. Soc. 136 (2014) 3024 3027. [119] E. Dolci, G. Michaud, F. Simon, Remendable thermosetting polymers for isocyanate-free adhesives: a preliminary study, Polym. Chem. 6 (2015) 7851 7861. Available from: https://doi.org/10.1039/C5PY01213A. [120] Y.K. Guo, H. Li, P. Zhao, X.F. Wang, D.A. Shuai, Thermo-reversible MWCNTs/epoxy polymer for use in self-healing and recyclable epoxy adhesive, Chin. J. Polym. Sci. 6 (2017) 728 738. Available from: https:// doi.org/10.1016/j.polymer.2004.10.060. [121] B. Willocq, F. Khelifa, J. Brancart, G.V. Assche, P. Dubois, J. Raquez, One-component Diels Alder based polyurethanes: a unique way to self-heal, RSC Adv. 7 (2017) 48047 48053. Available from: https://doi.org/ 10.1039/C7RA09898G.

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C H A P T E R

9 Self-healing polymeric coatings containing microcapsules filled with active materials S. Mojtaba Mirabedini and Farhad Alizadegan Iran Polymer and Petrochemical Institute, Tehran, Iran

9.1 Introduction The application of self-repairing materials is one of the cost-effective and promising approaches to prolong the service durability of polymeric coatings throughout their lifetime. Self-healing material can restore the structural integrity of coatings after cracks and damages appear and extend the service life of the product [1]. Self-healing mechanisms for spontaneously restoring damaged areas can be classified into intrinsic or extrinsic (Fig. 9.1). In intrinsic form, repairing of the polymer matrix can be triggered without the interference of any extrinsic agent and begins with ionomeric arrangements, molecular diffusion, or reversible hydrogen bonding, while it is a task to bring in or embed a former healing agent in a polymer matrix in the extrinsic healing mechanism. This scope will be realizable by one of three different carriers: hollow glass fibers [2,3], capillaries [4 6], or microcapsules [7 10]. The liquid-form healing material is contained in a protective carrier and can flow if released. The healing liquid flows into the damaged area due to the rupturing of the carrier and repairs the affected area by various mechanisms. Encapsulation enables shell property variations, hence controlling the release of the active core materials. This is a process that protects the active substance from the external medium by forming microcapsules. Microcapsulating the healing materials prevents them from being released into the coating composition, resulting in no change in the coating properties. The encapsulation process also increases the shelf life of the healing material and consequently prolongs the coating’s service life.

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FIGURE 9.1 The extrinsic-based self-healing systems provided by: (A) microcapsules, (B) vascular, and (C) intrinsic self-healing by functional terminal groups of the medium with an external trigger. Source: Reprinted with permission from K. Urdl, A. Kandelbauer, W. Kern, U. Mu¨ller, M. Thebault, E. Zikulnig-Rusch, Self-healing of densely crosslinked thermoset polymers—a critical review. Prog. Org. Coat. 104 (2017) 232 249 [11].

The healing material in the vascular-based systems is generally stored in a network of unfilled tubes that are distributed throughout the polymer matrix. To attain microcapsulebased self-healing systems, the healing material is stored in separate microcapsules [12]. The great advantage of microcapsule-containing systems over other methods is the easy dispersion of microcapsules in the polymer matrix. Among the different methods available for the preparation of self-coatings, the use of microcapsules filled with active liquid has been shown to be a promising approach [1]. The performance of self-healing coatings can be prominent in the absence of unwanted physical stresses or even through the shear applied during the manufacturing process [2]. Since this chapter focuses on microcapsule filled self-healing systems, only microcapsules and systems containing them will be discussed. The essential key points for designing a proper self-healing coating are discussed in the next section.

9.2 Requirements for designing a self-healing coating Conventional polymers without self-healing ability cannot convert physical energy, such as pressure, into a chemical/physical reaction to repair the scratched or defected area. The most important necessity to achieve self-healing property is the presence of a structure that can respond dynamically to external motivation and, in doing so, restore the material properties [13]. The polymer requires sensing the exterior energy, converting it into a convenient response (healing) at the defected area. Self-healing materials can restore the structural integrity of the damaged coating and extend its service life after repairing [14]. Self-healing polymeric materials follow a threestep process. The first stage is stimulation in the affected area, which occurs immediately after the damage is created. The second stage involves moving the healing material to the affected area, which is a quick step. The third stage, which varies depending on the repair mechanism, is the chemical restoration process, such as polymerization, entanglement, and reversible crosslinking, in the affected area [15]. Since the design of self-healing systems is a systematic process, it is necessary to properly select the core and shell materials

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related to the polymeric coating structure, as well as have appropriate knowledge about the chemistry of the process. The basic requirements for self-healing systems were initially stated, but these requirements may differ by the final application. For example, chemical stability and proper thermal resistance of self-healing components, in addition to high healing efficiency at low temperatures, are essential requirements of aeronautic self-healing material [16].

9.3 Microcapsule-based self-healing systems In self-healing systems containing microcapsules, the healing material is encapsulated in a separate capsule and dispersed along with a catalyst or curing agent within the matrix [1,16,17]. In the microencapsulation process, a dispersed phase is isolated from the external environment by a protective shell material [18]. Microcapsules comprising repairing material can spontaneously inhibit propagation of the defective area [2,19]. After rupturing the microcapsule’s shell, the core content flows inside the cracked areas, in contact with active substances, and, after curing or drying, repairs the affected areas [20]. Typically microcapsules can be size-classified as macro, micro, or nano sizes. As a simple definition, microcapsules are small spherical containers with a diameter of 1 μm to 1 mm and with a shell thickness of 0.5 150 μm [21]. Microcapsules may be classified into three different morphologies: mononuclear (core shell), multinuclear (poly-core), and matrix types, as displayed in Fig. 9.2 [22]. Microcapsule morphology mostly depends on the core material and the microencapsulation procedure. In monocore microcapsules, the core content is enclosed in a shell. However, poly-core microcapsules comprise several small core droplets surrounded by a single shell. Finally, in a matrix-type microcapsule, the core material is repeatedly dispersed within the shell material. Because of higher core content than other types of microcapsules, it is anticipated that a coating film containing mononuclear microcapsules exhibits better self-healing properties [23]. In the 1950s Green and Low [25,26] first introduced microcapsules in the form of microencapsulated dyes used for the preparation of carbonless copying paper. In 2001 White et al. and Wu et al. [1,27] first systematically introduced self-healing polymeric composites comprising polyurea-formaldehyde (PUF)-based microcapsules containing dicyclopentadiene (DCPD) in the presence of dispersed Grubb’s catalyst. When the polymer composite is subjected to scratch or extensive mechanical stresses/elongation, the microcapsule shell ruptures and DCPD material is released into the damaged area. The self-healing property of DCPD-based microcapsules is activated in contact with Grubb’s catalyst by ringopening polymerization (ROP) of DCPD [12,28]. Fig. 9.3 illustrates the formation of a polyDCPD network through ring-opening metathesis polymerization (ROMP) of DCPD in the presence of Grubb’s catalyst. After 2 years, the same group of researchers reported the preparation of self-healing coatings containing PUF-microcapsules filled with DCPD via in situ polymerization route [29]. Microencapsulation of di-n-butyl tin dilaurate (DBTL) catalyst in a polyurethane (PU) shell and dispersion of poly dimethyl siloxane with hydroxyl functional groups (HOPDMS) as a healing agent in a vinyl ester matrix resulted in a self-healing coating [30].

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FIGURE 9.2 (A) Different types of microcapsule morphology and (B) optical images of latex film containing microcapsules with different morphologies. Source: Reprinted with permission from (A) R. Al Shannaq, M.M. Farid, Microencapsulation of phase change materials (PCMs) for thermal energy storage systems, in: Advances in Thermal Energy Storage Systems, Elsevier, 2015, pp. 247 284; (B) S.M. Mirabedini, I. Dutil, L. Gauquelin, N. Yan, R.R. Farnood, Preparation of self-healing acrylic latex coatings using novel oil-filled ethyl cellulose microcapsules, Prog. Org. Coat. 85 (2015) 168 177 [24].

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FIGURE 9.3 Poly-DCPD network formation by ring-opening metathesis polymerization of DCPD and Grubbs’ catalyst. DCPD, Dicyclopentadiene. Source: Reprinted with permission from D.A. Rusakov, A.A. Lyapkov, E.I. Korotkova, N.V. Thanh, T.Q. Cuong, M.K. Zamanova, Correlation between temperature setting and DCS complex peak energy and in ROMP of dicyclopentadiene, Procedia Chem. 10 (2014) 490 493.

Despite high reactivity and a proper healing rate, the DCPD-based/Grubb’s catalyst system exhibits some serious limitations. Grubb’s catalyst is very expensive and loses its primary activity with increasing temperature and humid conditions. Also, appropriate distribution and dispersion of the material in the polymer matrix are rather tricky, and it tends to aggregate in a matrix. Therefore these shortcomings have led to the examination of other systems. The use of a dual-layer capsule-based system is one of the suggested approaches to overcome these limitations. Jin et al. [31] introduced microencapsulation of EPON 815C epoxy resin in a PU PUF shell and polyoxypropylenetriamine (POPTA), EPIKURE 3274, as a curing agent in a PUF microcapsule shell with almost 90% healing efficiency [31 33]. Fig. 9.4 shows some characteristic test results of the amine-containing microcapsules [33]. In recent years, several materials have been introduced for preparation of PUF-microcapsules; [4,34 36] including melamine urea formaldehyde (MUF) [36,37], melamine phenol formaldehyde (MPF) [38], methylene diphenyl diisocyanate (MDI) [39], melamine formaldehyde [40], poly(methyl methacrylate) (PMMA) [41,42], and many other compounds. Li and coworkers [43] reported the preparation of a self-repairing epoxy system comprising PMMA microcapsules containing diglycidyl ether of bisphenol A epoxy (DGEBA) and polyether amine hardener using a solvent evaporation technique. With the addition of 5 and 15 wt.% PMMA microcapsules, the healing efficiency of about 43.5% and 84.5% was reported at room temperature after 24 h, respectively. In this system, by controlling the Self-Healing Polymer-Based Systems

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FIGURE 9.4 Amine microcapsules characterization: (A) SEM micrograph of amine microcapsules, (B) OM of capsules immersed in EPIKURE 3274, and (C) FTIR spectra of EPIKURE 3274 and capsule inside material content. Source: Reprinted with permission from H. Jin, C.L. Mangun, D.S. Stradley, J.S. Moore, N.R. Sottos, S.R. White, Selfhealing thermoset using encapsulated epoxy-amine healing chemistry, Polym. (Guildf.) 53 (2) (2012) 581 587.

curing temperature and time, epoxy can be cured in contact with its hardener [44]. Many studies have been conducted to obtain similar two-part self-healing systems [33,45 51]. To deal with these limitations, and for replacing DCPD, many methods and materials have been proposed, such as monomeric [39,52] and bulky isocyanates [53,54] and plant air-drying oils [9,23,55,56]. These systems are usually triggered by absorbing moisture or oxygen from the atmosphere. Air-drying oils, such as linseed, rapeseed, and Tung oils, have received magnificent attention because of their low cost, ecofriendly nature, easy accessibility, and simplistic processing [9,37,57]. Suryanarayana and coworkers [9] reported microencapsulation of linseed oil (LO) in PUF shell, based on the method suggested by Brown et al. [29] and using polyvinyl alcohol (PVA) as a surfactant, instead of ethylene-maleic anhydride copolymer. In another study, Nesterova et al. [37] reduced the LO ratio to half of the amount used by

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Suryanarayana et al. [9] to prevent microcapsule aggregation due to the free and/or leaked LO from the thin-shell microcapsules. In 2010 Samadzadeh and coworkers [57] introduced self-healing epoxy coatings containing Tung oil-filled PUF-microcapsules. Tung and linseed oils have similar properties; conversely, a higher proportion of the unsaturated Tung oil may speed up its polymerization and reduce drying time. However, Behzadnasab et al. [56] claimed that Tung oil has a very long drying time and may not be a suitable selection for this purpose. In 2014 Behzadnasab and coworkers [56] studied the effect of synthesizing PUF-based microcapsules filled with LO along with cobalt (II)-based drier on the mechanical properties of an epoxy coating. The properties of the LO-filled PUF microcapsules were optimized by adjusting the effective parameters, such as reaction time, temperature, pH, and mixing rate. They claimed the preparation of microcapsules with the maximum quantity of LO maintained free-flowing and non-sticky properties during the storage time. Fig. 9.5 shows an optical microscopic image and SEM micrograph of LO-filled PUF-microcapsules prepared at a mixing speed of 700 rpm [56]. In their research, the effect of core:shell ratio and mixing speed on the microcapsule size and morphology was studied. The results revealed that applying a higher mixing speed led to a decrease in the microcapsule size, as well as an optimum core:shell ratio. It has been shown [56,58] that if the microencapsulating system is correctly formulated, the released oil absorbs and reacts with oxygen to form a new protective film, thereby requiring no addition of an extra hardener or catalyst within the coating. It has been reported that the loading quantity of the healing materials strongly depends on the size and amount of microcapsules in the coating formulation; [59,60] however, in self-healing coatings, additional shortcomings, such as film thickness, appearance, optical, mechanical, and rheological properties of the coating, have to be considered. Therefore optimizing the size distribution and loading of microcapsules are keys to the successful preparation of a self-healing coating. It is recommended that the microcapsule size be less

FIGURE 9.5 (A) Optical microscopic image and (B) SEM micrograph of linseed oil-filled PUF-microcapsules prepared at an agitation rate of 700 rpm. PUF, Polyurea-formaldehyde. Source: Reprinted with permission from M. Behzadnasab, S.M. Mirabedini, M. Esfandeh, R.R. Farnood, Evaluation of corrosion performance of a self-healing epoxybased coating containing linseed oil-filled microcapsules via electrochemical impedance spectroscopy, Prog. Org. Coat. 105 (2017) 212 224.

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than one-third the thickness of the coating; therefore the mechanical/optical properties of the coating are not significantly reduced. Self-healing systems comprising microcapsules are widely used in solvent-based and thermosetting systems, such as an epoxy-based matrix. However, in water-borne coatings, despite their widespread performance in decorative paints, packaging applications, and so on [18,61], little success has been achieved in the use of microcapsules in such coatings [9,61,62]. Mirabedini and coworkers [23] developed a versatile technique for the microencapsulating of plant oils (linseed and rapeseed) in ethyl cellulose (EC) shell using a low toxic solvent. The results revealed that spherical microcapsules were obtained in the range size of 10 50 μm with a porous shell at a thickness of 0.65 1.55 μm. Optical microscopy and SEM study revealed that the oil was successfully encapsulated in the ethyl-cellulose shell. Fig. 9.6 shows optical images of either rapeseed oil or LO-filled microcapsules with 60:40 core:shell wt.% using chloroform and ethyl acetate solvents [23]. In further work, Mirabedini et al. [24] studied the self-healing properties of a waterborne carboxylated styrene/butadiene coating containing EC-based microcapsules filled with rapeseed oil. The results revealed the successful preparation of spherical microcapsules with a rough porous shell and with a particle size of 10 45 μm. The addition of rapeseed oil-filled microcapsules improved the mechanical properties of the host latex film. The self-healing properties, as well as the plasticizing mechanism of the released oils, FIGURE 9.6 Optical images of (I) rapeseed oilfilled and (II) linseed oilfilled microcapsules, with 60:40 core:shell weight ratio, using (A) chloroform and (B) ethyl acetate solvent. Source: Reprinted with permission from S.M. Mirabedini, I. Dutil, R.R. Farnood, Preparation and characterization of ethyl cellulose-based core shell microcapsules containing plant oils, Colloids Surf. A Physicochem. Eng. Asp. 394 (2012) 74 84.

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were studied. The results showed that elongation of the specimens resulted in the release of rapeseed oil throughout the coating film. Therefore the coating films were plasticized, and, as a result, the mechanical properties of the pre-elongated films were restored. Due to the special chemical structure and the ability to interact with the microcapsule shell and the host polymeric matrix, special attention has been paid to the silane adhesion promoters. A few research works have introduced silane bonding of microcapsules shell to improve the interaction between the polymer matrix and microcapsule shell [63,64]. Es-haghi and coworkers [65] studied the effect of different trimethoxysilane silane derivative treatments on LO-filled EC microcapsules and the interactions between microcapsules and water-borne acrylic paint. The results revealed that the mechanical properties of the coating containing silane-treated microcapsules were improved compared to their counterparts containing neat oil-filled EC microcapsules, due to the chemical interaction between the trialkoxy groups of the silane compounds and the hydroxyl groups on the microcapsule’s shell. However, in self-healing systems containing oil-filled microcapsules, insufficient mechanical properties and very low drying rates of the released oil in the damaged area are two main deficiencies. Therefore the use of a one-component self-healing microcapsule-based system comprising isocyanate compounds that can be cured with air humidity is of great interest. Consideration has been given to the use of reactive isocyanate-based repairing materials that can easily fill cracks by reacting with atmospheric moisture [53]. Among the many isocyanate-based compounds [6,53,54,66,67] that have been microencapsulated, the most attention has been focused on isophorone diisocyanate (IPDI). In 2003 Sottos et al. [20] first reported the encapsulation of hexamethylene diisocyanate (HDI). In another work, the encapsulation of the IPDI monomer was introduced by Sottos et al. [52]. In their work, IPDI was encapsulated into a PU shell comprising a toluene diisocyanate-based prepolymer and 1,4-butanediol using the interfacial polymerization technique. Yang and coworkers [39] reported encapsulation of HDI, which is a reactive aliphatic diisocyanate, inside the PU shell via in situ reaction of an MDI-based prepolymer with 1,4-butanediol. Using SEM they studied the self-healing property of an epoxy-based coating containing 10 wt.% microcapsules by evaluating a healed area. The results revealed the healing of the scratched area after the coated sample was immersed in NaCl solution for 48 h. In a further study, the encapsulation of IPDI monomer into a double-layer shell composed of either PU and PUF as a single layer shell or PU/PUF(PUF) as a bilayer shell was reported by Di Credico et al. [68]. The scratched area in the epoxy coating comprising 15 wt. % of the aforementioned microcapsules was repaired after 48 h immersion in NaCl solution. However, despite various benefits, the low rate of curing reaction with atmospheric moisture and under catalyst-free conditions, as well as the low physical/chemical performance of the healed area, are two main drawbacks of these types of systems [53]. One approach to deal with these issues is to use higher Mw multifunctional isocyanate derivatives as a healing material. Haghayegh et al. [54] optimized the synthesis of novel single-layer PU-based microcapsules comprising bulky isocyanate molecules. The microcapsules were added to an epoxybased coating and the scratch-healing properties of the material were evaluated and

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compared with their counterpart system containing monomeric IPDI-filled microcapsules. This study aimed to find more in-depth information on the ability of the released healing agent to protect a substrate exposed to corrosive media and to restore the original tensile strength of the matrix coating. Recently, Alizadegan et al. [69,70] reported the microencapsulation of monomeric and bulky forms of IPDI via interfacial polymerization into a PU shell containing 2,4-toluene diisocyanate (TDI)-based prepolymer and 1,4-butanediol as a chain extender in an oil-inwater emulsion. Fig. 9.7 shows a schematic of IPDI-loaded PU microcapsule synthesis. Instead of commercial toxic solvents, n-butyl acetate was used as a low-toxic solvent in the synthesis of both prepolymer and microcapsules. The healing effectiveness of PU-based coating samples comprising 1 10 wt.% microcapsules was evaluated. Natural salt spray test and tensile strength measurements were used to evaluate corrosion performance and mechanical properties of the PU coating samples containing 1 wt.% nanoclay and various wt.% microcapsules. Improved corrosion performance and increased mechanical properties were reported for coating samples containing clay nanoparticles compared to neat PU and microcapsule-embedded coating samples. The researchers attributed this improvement is due to the mineral nature of the nanoparticles and the improvement of the barrier property of the coating films. Cong and coworkers [71] introduced a simple method for preparation of dual pH and UV-responsive microcapsules by Pickering emulsions that were stabilized by SiO2 and TiO2 nanoparticles. The synthesized microcapsules were added to a water-borne

FIGURE 9.7

Preparation route for IPDI-loaded PU microcapsules using n-butyl acetate solvent. IPDI, Isophorone diisocyanate; PU, polyurethane. Source: Reprinted with permission from F. Alizadegan, S.M. Mirabedini, S. Pazokifard, S.G. Moghadam, R. Farnood, Improving self-healing performance of polyurethane coatings using PU microcapsules containing bulky-IPDI-BA and nano-clay, Prog. Org. Coat. 123 (2018) 350 361.

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polysiloxane latex. In addition to self-healing properties, this coating also exhibited a rapid response to pH and UV stimuli. As described so far, different materials are usually used as the core and shell components, depending on the application and desired expectations of the coating. Therefore based on the type of materials and their applications, different methods are used for the microencapsulation process. In many cases, the type of core and shell materials and their sensitivity to the environment play a key role in choosing the appropriate preparation method. In the following section, some of the major microencapsulation methods will be discussed.

9.4 Microcapsule preparation methods Microencapsulation of materials can be performed using numerous chemical and physical methods, such as solvent evaporation [34], phase change materials [29,30], phase separation [28], spray drying [35,72,73], miniemulsion polymerization [32,33], interfacial polymerization [54,70], in situ polymerization [25,31], coacervation [2,74], complex coacervation [75,76], and/or, nonsolvent encapsulation [35], and incompatible polymer phase separation [36]. Although many procedures have been introduced for microencapsulation of different healing materials, they can generally be divided into two main categories: chemical and physical methods. The selection of the encapsulation method depends on the type and nature of the material to be encapsulated, as well as on the application and expected properties of the microcapsules. It should also be noted that the encapsulation method affects the microcapsule morphology. For example, the use of a solvent evaporation method in the preparation of microcapsules leads to microcapsules with polycore and/or matrix morphologies. Meng et al. [77] used acetone-in-polydimethylsiloxane oil-in-oil (O/O) emulsion solvent evaporation system (Fig. 9.8) to encapsulate oxalic acid (OA) into acrylonitrile butadiene styrene (ABS) shell compound. If the material used to prepare the microcapsule shell contains small molecules, monomers, or prepolymers (i.e., in liquid form under normal conditions), the microencapsulation process involves chemical reactions. In contrast, when using macromolecules or polymeric materials as the shell component, usually no chemical reactions are implemented, and only shape establishment occurs during the encapsulation process. Consequently, the appropriate choice of preliminary materials and methods results in the preparation of microcapsules. Phase separation, spray drying, fluidized bed coating, melt solidification, coextrusion process, vapor deposition, and fluidized bed spray coating are among the physical methods of encapsulation [22]. The principal methods for the preparation of microcapsules can be recognized as emulsification, layer-by-layer (LBL) assembly, coacervation, and internal phase separation [16,78]. The majority of the capsules used in self-healing materials are synthesized via in situ and/or interfacial polymerizations, which are the most common chemical methods for encapsulating liquid materials and take place in an oil-in-water or water-in-oil emulsion systems [52]. In 2008 Yang et al. [52] introduced the encapsulation of IPDI into a PU shell using the interfacial polymerization technique (Fig. 9.9). In the case of interfacial polymerization, as can be deduced from the term “interfacial,” the polymerization of two different monomers takes place at the interface of two phases Self-Healing Polymer-Based Systems

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[79]. Microcapsules are formed when two phases are carefully agitated into small droplets dispersed in the continuous phase. A suitable stabilizer is needed to prevent the droplets from coalescing or particles from coagulating during the polycondensation process and capsule formation. A large number of polymers, consisting of polyuria [80,81], PU [69,70,82], polyamide [83], even polyester-urethane [84], are reported to have been used as the microcapsule’s shell and produced by interfacial polycondensation.

FIGURE 9.8 Microencapsulation of OA core with ABS shell by Ac/PDMS emulsion solvent evaporation. ABS, Acrylonitrile butadiene styrene; OA, oxalic acid. Source: Reprinted with permission from F. Meng, S. Wang, Y. Wang, H. Liu, X. Huo, H. Ma, et al., Microencapsulation of oxalic acid via oil-in-oil (O/O) emulsion solvent evaporation, Powder Technol. 320 (2017) 405 411. FIGURE 9.9 Interfacial polymerization portrait for PU microcapsules preparation. PU, Polyurethane. Source: Reprinted with permission from J. Yang, M.W. Keller, J.S. Moore, S.R. White, N.R. Sottos, microencapsulation of isocyanates for self-healing polymer, Macromolecules 41 (24) (2008) 9650 9655.

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The other approach used for the preparation of microcapsules is in situ polymerization. In situ polymerization is a promising method with the ability to control the size and shell thickness of microcapsules, ease of the process, low cost, and prospective use in practical applications. Nevertheless, in situ polymerization requires a longer time despite other encapsulation methods. During in situ polymerization, oil-in-water or water-in-oil emulsions are formed at high agitation speeds, and the core compounds act as a dispersed phase. Monomers and initiators, which form the microcapsule shell, are dissolved in the continuous phase. Polymerization occurs at the core surface as the polymer synthesized from the monomers is insoluble in the emulsion [85]. In situ polymerization and interfacial polymerization are quite similar. However, in situ polymerization does not have any reactive monomers in the organic phase, and all polymerization is carried out in the continuous phase. One of the well-known examples of in situ polymerization is the synthesis of PUF microcapsules. Yuan et al. [10,86] reported the preparation of PUF microcapsules containing epoxy resin through a two-step reaction. In the first step, the urea formaldehyde prepolymer was synthesized. The core material was then added to the prepolymer diluted in a solvent to incorporate oil-in-water emulsion, as shown in Fig. 9.10.

FIGURE 9.10 Schematic encapsulation process of n-octadecane core into resorcinol-modified MF shell by in situ polymerization. Source: Reprinted with permission from H. Zhang, X. Wang, Fabrication and performances of microencapsulated phase change materials based on N-octadecane core and resorcinol-modified melamine formaldehyde shell, Colloids Surf. A Physicochem. Eng. Asp. 332 (2 3) (2009) 129 138 [87].

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FIGURE 9.11 Synthesis procedure of Gel-GA/PUF double-walled microcapsules containing BPO paste via complex coacervation and in situ polymerization. BPO, Benzoyl peroxide; Gel-GA, gelatin-gum Arabic; PUF, polyurea-formaldehyde. Source: Reprinted with permission from M. Raeesi, S.M. Mirabedini, R.R. Farnood, Preparation of microcapsules containing benzoyl peroxide initiator with gelatin-gum arabic/polyurea-formaldehyde shell and evaluating their storage stability, ACS Appl. Mater. Interfaces 9 (24) (2017) 20818 20825.

Raeesi and coworkers [76] reported a versatile method for preparation of gelatin-gum Arabic (Gel-GA)/PUF dual-layer microcapsules containing dispersed benzoyl peroxide (BPO) in dibutyl phthalate (DBP) via a two-step method using complex coacervation and in situ polymerization techniques, respectively. In Fig. 9.11, a schematic of the synthesis procedure of Gel-GA/PUF double-layer microcapsules comprising BPO dispersed in DBP via complex coacervation and in situ polymerization is shown [76]. Not all materials are suitable for use as core and shell components in the microcapsule preparation, as discussed in the following section.

9.5 Materials selection for core and shell components of microcapsules As stated earlier, the proper selection of core and shell materials is a fundamental issue that should be considered before the beginning of the encapsulation process. For a successful encapsulation, these materials must have specific parameters, such as reactivity, solubility, volatility, viscosity, pH, flow-ability, and fast integration [12]. Some important parameters in the preparation of microcapsules should be considered as effective factors in the encapsulation, such as mixing device geometry, type, and concentration of used surfactant, agitation speed, the viscosity of the media, and temperature. A surfactant is a surface-active substance that is capable of being adsorbed at the interface of the system to hold two different thermodynamically heterogeneous phases

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together. In other words, an emulsifier is a surfactant that is usually used to hold the mixture of immiscible well-dispersed fluids that result from reducing the surface tension or interfacial tension between two liquids [88]. The equilibrium between hydrophilicity and lipophilicity of emulsifiers is called HLB value and varies from 0 to 20. Surfactants with low HLB are oil-soluble or dispersed. The surfactant molecules have a hydrophilic tail and a hydrophobic head. The polar and nonpolar segments are oriented toward aqueous and oil phases, respectively, and therefore micelles are formed after the agitation of two phases into each other. The optimum conditions of emulsion formation and thus encapsulation can be obtained by measuring the interfacial tension and dilational rheology [79]. Surfactants are selected based on the type of emulsion (hydrophobic or hydrophilic), compatibility, and creation of stable emulsions [89]. As the surfactant concentration increases, the size and thickness of the microcapsules decrease and then increase, respectively. The precise process of the effect of surfactant concentration on the formation of microcapsules has not yet been elucidated [52]. But it can be noted that as the surfactant concentration increases, the viscosity of the solution increases, and the shear force of the mixture is broken down into smaller droplets. For the synthesis of small-size microcapsules with a narrow particle-size distribution, the surfactant concentration appears to increase the emulsion viscosity, thereby increasing shear forces due to the higher speed of the mixer [68]. Increasing surfactant concentration reduces the diameter of microcapsules by affecting the interfacial tension of the solution before the critical micelle concentration (CMC) point. Passing the CMC point, with increasing surfactant concentration, there is no change in tension, and the diameter of the microcapsules remains almost constant, but as evident in Fig. 9.12, there is no significant change in the core fraction of the microcapsules [39]. The high agitation rate and mixing blades with special geometry overcome the solution interfacial tension by creating high shear forces. As a result, the shell diameter and thickness of the microcapsules will decrease (Fig. 9.13). The viscosity of the core material influences its releasing behavior. Higher viscosity results in a slower release of core content, so the lower it takes for the healing of the

FIGURE 9.12

Effect of different concentrations of surfactant on diameter and core fraction of PU microcapsules. PU, Polyurethane. Source: Reprinted with permission from M. Huang, J. Yang, Facile microencapsulation of HDI for self-healing anticorrosion coatings, J. Mater. Chem. 21 (30) (2011) 11123 11130.

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FIGURE 9.13 The effect of different mixing speeds on diameter and shell thickness PU microcapsules. PU, Polyurethane. Source: Reprinted with permission from M. Huang, J. Yang, Facile microencapsulation of HDI for self-healing anticorrosion coatings, J. Mater. Chem. 21 (30) (2011) 11123 11130.

affected area. Therefore the viscosity must be an optimum value so that the core material flows to the damaged area after the microcapsules rupture [90]. The rising temperature and increasing solvent content have adverse effects on the microcapsules’ morphology and permeability. For example, the shell thickness is increased due to a significant shrinkage, and a decrease in the core content occurs during 30 min of heat treatment at 70 C [91]. Mueller et al. [92] claimed that an increase in temperature would result in a significant reduction in microcapsule permeability. The structural rearrangement of the microcapsule shell, due to its continuous shrinkage with increasing temperature, was responsible for the fortification of chain correlations in the shell, which lead to sealing microcapsules’ leaks [93]. In another study, Tong et al. [94] reported increasing permeation of microcapsules by conducting polar solvents, such as ethanol 20. Encapsulation efficiency and microcapsule stability are two important factors in selecting the appropriate material for the microcapsule shell. Not all materials are suitable for use in shell formation, but one common point is that all shell materials must protect core content without reacting with it [21]. Core material physical properties and target applications are important elements that influence microcapsule shell selection and the encapsulation process [95]. The core material can be small solid particles (active constituents, stabilizers, diluents, or accelerators), liquid droplets (dispersed and/or dissolved material), or gas composition [96]. But the conventional healing materials inside the shell must be in liquid form to flow through the cracked area [57]. The variety of core and shell materials results in the preparation of microcapsules with a wide range of desired properties. Suitable shell materials (interconnected, chemically compatible, and nonreactive with the core material) must have certain properties, including strength and stability, flexibility, impermeability, optical properties, cohesiveness, permeance, solubility, and transparency [96]. The microcapsule shell in the self-healing polymer should have two additional characteristics: be mechanically vigorous enough to stay pristine during preparation and transportation and, if necessary, rupture and release the core material [37]. Not only core and shell materials but also solvents used in the encapsulation process should have specific properties. Not all solvents are suitable to be used in the

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TABLE 9.1 Characteristic properties of some suitable solvents used in the microencapsulation process [97,98]. Solvent

Dielectric constant (ε)

Boiling point ( C)

Flash point ( C)

Phenylacetate

5.4

193

77

Butylacetate

5.07

126.1

23

Chlorobenzene

5.7

132

27

Cyclohexanone

18.3

155.6

44

encapsulation process. Ideal solvents have dielectric constant (ε) between 5 and 38 [44,51]. High boiling and flashpoints are also recommended for the solvent used in the encapsulation process. Some of these solvent classes are listed in Table 9.1. The most important factors in selecting a liquid healing material can be summarized as follows: (1) Long-term stability and lifetime: the healing agent must remain stable during the microencapsulation process and inside the microcapsule shell and be released during the repair process and at the appropriate time. (2) Physical and chemical properties: in addition to low viscosity, the healing material should have low volatility to fill the damage site at the highest possible speed and to react at the appropriate time. (3) Chemical activity: the rate of chemical reactions of a healing agent in the presence of a catalyst or exposed to environmental factors in a free catalyst system should be fast enough to complete the healing process with other involuntary processes, such as evaporation and absorption. (4) Physical mechanical properties: the healing material must be selected in such a way as not to lose the physical and mechanical properties of the matrix after its release. (5) Thermal stability: the healing material should have a low freezing point and a high boiling point to minimize the possibility of phase change at different temperature conditions, after application. Some of the important factors for selecting suitable microcapsule shell materials can be concluded as follows: (1) Matrix compatibility: to prevent any defects or cracks, the microcapsule shell must have the highest compatibility with the surrounding polymeric matrix. (2) Chemical compatibility: the shell compound must not chemically react with the core material and the surrounding environment. (3) Mechanical properties: the strength of the microcapsule shell must be lower than the failure resistance of the matrix while being high enough to remain intact during the process. (4) Prohibition of penetration: the microcapsule shell must prevent the penetration and dispersion of the healing material in the matrix. (5) Thermal stability: the microcapsule must be stable in terms of thermal properties over a wide range of temperatures for different application conditions. Given the mentioned

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properties, urea-formaldehyde, melamine-formaldehyde, PU, or acrylate is often used to prepare the microcapsule shell [2].

9.6 Limitations and shortcomings of microcapsule-embedded coatings Microencapsulation has plenty of remarkable benefits, including controlled and steadystate release, core protection against evaporation and degradation, releasing core content to the defected area, and protection of the materials from the surrounding environment [99]. The main advantage of the extrinsic healing system is the local response to stimulants and reaction with existing components containing catalysts or cross-linkers in the surrounding environment [100]. However, disregarding the disadvantages and limitations of the microcapsule can destroy all the benefits and expectations. The most important shortcoming in the use of microcapsules is the limitation in the repeatability of healing in the damaged conditions and the lack of reproducibility of the previously repaired area [100]. If the liquid core material remains in the affected area, the instinctual healing is likely to be repeated, but there is no perception that the healing material is present in the damaged part and is not fully consumed and converted [68]. Another memorable problem is the maximum amount of microcapsule that can be used in the coating composition. The amount of microcapsules in the composition should be such that no appreciable decrease in the initial properties of the coating occurs. In other words, the designed system should be highly compatible with the polymer matrix and add the minimum level of microcapsules required to the system. The effect of incremental addition of embedded microcapsules on Young’s modulus and stress of epoxy coating was evaluated by Zeszutko et al. [101]. Tensile strength measurement is an important technique to evaluate microcapsules’ effect on the mechanical properties of coating [53,70]. Increasing the concentration of microcapsules results in a weak interface of the microcapsule/polymer matrix, as well as the lack of mechanical properties of the polymer matrix. Inappropriate dispersion of microcapsules (aggregation) and the presence of air bubbles in the polymer matrix can act as a stress concentration (defects points) [70]. Poor compatibility between the coating and microcapsules that cause agglomeration and defects at the microcapsule/coating interface is undesirable in terms of barrier and adhesion properties [102]. An 8.6% reduction of the elastic modulus of epoxy films containing 3 wt.% LO-filled urea-formaldehyde microcapsules have been reported [56]. Haghayegh et al. [53] also reported a reduction in the elastic modulus of epoxy film from 1385 MPa for the neat sample to 991 MPa for coating containing 3 wt.% IPDIfilled PU-based microcapsules. Another limitation of using microcapsules in the coating composition is the thickness of the coating. The size of the microcapsule should be almost one-third of the dried coating film thickness [59]. Usually, the thickness of the coating is less than 200 μm, which limits the size of the microcapsules that can be incorporated in the coating formulation. According to a study performed by Roll et al. [59], the larger microcapsule size leads to more self-healing healing material and better healing properties. However, the probability of a large microcapsule rupturing is much greater than that of a group of smaller

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microcapsules with an equivalent volume. If the microcapsules have a very small size, the released healing material is very low and not sufficient to fill the affected area. Otherwise, the addition of capsules larger than the thickness of the coating detracts from the mechanical properties [103]. Another limitation is the method of adding microcapsules to the coating formulation since conventional mixing techniques with very high shear stress can rupture the microcapsule shell. The tendency of the microcapsules to aggregate, the type of solvent used, the interaction of the microcapsules and resins on each other, and the viscosity of the resin have a significant effect on the choice of the method, rate, and quality of mixing. As stated, the most important limitation in the microcapsule-embedded system is the one-time use of microcapsules. Therefore when the coating is under strain or stress and the microcapsules rupture, there is no capability to reheal the damaged area [57]. Microcapsules are used to prevent defects in the effectiveness of the coating due to the appearance of damage, but in most cases, the coatings must be repaired or replaced at an appropriate time.

9.7 Summary Application of smart self-repairing materials is among the cost-effective, long-lasting, and efficient approaches that extend the service life of polymer coatings. The simplicity of adding microcapsules to the polymer matrix, compared to the other self-healing systems, is an advantage of these types of self-healing systems. Microencapsulation has significant benefits, such as the controlled and steady-state release of the healing material, core content protection against evaporation and destruction, optimal release in a particular location, safety for the user, and environmental shield. On the other hand, a coating containing microcapsules has the following limitations: one-time self-repairing, restrictions on the use of large quantities of microcapsules, low quantity of healing material due to the presence of small-size microcapsules in a thin-layer coating, and lack of coating strength in a high concentration of microcapsule loading. The diversity of prepared microcapsules is broad and is based on what they can do: types of core materials (all three morphologies), shell material, applications, number of healing agents (one or two-component systems), such as air-drying and isocyanate (e.g., system with catalyst and healing agents together, respectively). Not all materials are suitable for the synthesis of microcapsules as the core or shell material. The materials must have specific properties, for instance, their compatibility and thermal and mechanical stability. The characteristic properties of the other materials present in the process, such as the solvent and surfactant, should be considered. Despite the numerous studies and reports suggesting suitable materials for encapsulation and the use of various preparation methods, as well as gaining some outstanding properties, the use of microcapsules in polymer coatings has not yet been widely incorporated. Limitations, such as the effect of adding microcapsules to the apparent, optical and mechanical properties of the coatings are among the issues on which further research should focus.

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References [1] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794. [2] E.B. Murphy, F. Wudl, The world of smart healable materials, Prog. Polym. Sci. 35 (1 2) (2010) 223 251. [3] R.S. Trask, G.J. Williams, I.P. Bond, Bioinspired self-healing of advanced composite structures using hollow glass fibres, J. R. Soc. Interface 4 (13) (2006) 363 371. [4] T.C. Mauldin, M.R. Kessler, Self-healing polymers and composites, Int. Mater. Rev. 55 (6) (2010) 317 346. [5] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (8) (2007) 581. [6] B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, Self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (2010) 179 211. [7] X. Liu, X. Sheng, J.K. Lee, M.R. Kessler, Synthesis and characterization of melamine-urea-formaldehyde microcapsules containing ENB-based self-healing agents, Macromol. Mater. Eng. 294 (6-7) (2009) 389 395. [8] J.K. Lee, S.J. Hong, X. Liu, S.H. Yoon, Characterization of dicyclopentadiene and 5-ethylidene-2-norbornene as self-healing agents for polymer composite and its microcapsules, Macromol. Res. 12 (5) (2004) 478 483. [9] C. Suryanarayana, K.C. Rao, D. Kumar, Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings, Prog. Org. Coat. 63 (1) (2008) 72 78. [10] L. Yuan, G. Liang, J. Xie, L. Li, J. Guo, Preparation and characterization of poly (urea-formaldehyde) microcapsules filled with epoxy resins, Polym. (Guildf.) 47 (15) (2006) 5338 5349. [11] K. Urdl, A. Kandelbauer, W. Kern, U. Mu¨ller, M. Thebault, E. Zikulnig-Rusch, Self-healing of densely crosslinked thermoset polymers—a critical review, Prog. Org. Coat. 104 (2017) 232 249. [12] A.C.M. Silva, A.D. Moghadam, P. Singh, P.K. Rohatgi, Self-healing composite coatings based on in situ micro nanoencapsulation process for corrosion protection, J. Coat. Technol. Res. (2017) 1 29. [13] W.H. Binder, Self-Healing Polymers: From Principles to Applications, John Wiley & Sons, 2013. [14] S.J. Garcı´a, H.R. Fischer, S. Van Der Zwaag, A critical appraisal of the potential of self healing polymeric coatings, Prog. Org. Coat. 72 (3) (2011) 211 221. [15] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (17) (2013) 7446 7467. [16] A.P. Esser-Kahn, S.A. Odom, N.R. Sottos, S.R. White, J.S. Moore, Triggered release from polymer capsules, Macromolecules 44 (14) (2011) 5539 5553. [17] S.K. Ghosh, Self-Healing Materials: Fundamentals, Design Strategies, and Applications, Wiley Online Library, 2009. [18] Y. Hennequin, N. Pannacci, C.P. De Torres, G. Tetradis-Meris, S. Chapuliot, E. Bouchaud, et al., Synthesizing microcapsules with controlled geometrical and mechanical properties with microfluidic double emulsion technology, Langmuir 25 (14) (2009) 7857 7861. [19] E.N. Brown, S.R. White, N.R. Sottos, Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite—part II: in situ self-healing, Compos. Sci. Technol. 65 (15 16) (2005) 2474 2480. [20] M.R. Kessler, N.R. Sottos, S.R. White, Self-Healing structural composite materials, Compos. Part. A Appl. Sci. Manuf. 34 (8) (2003) 743 753. [21] P.T.D. Silva, L.L.M. Fries, C.R.D. Menezes, A.T. Holkem, C.L. Schwan, E´.F. Wigmann, et al., Microencapsulation: concepts, mechanisms, methods and some applications in food technology, Cieˆnc. Rural. 44 (7) (2014) 1304 1311. [22] R. Al Shannaq, M.M. Farid, Microencapsulation of phase change materials (PCMs) for thermal energy storage systems, Advances in Thermal Energy Storage Systems, Elsevier, 2015, pp. 247 284. [23] S.M. Mirabedini, I. Dutil, R.R. Farnood, Preparation and characterization of ethyl cellulose-based core shell microcapsules containing plant oils, Colloids Surf. A Physicochem. Eng. Asp. 394 (2012) 74 84. [24] S.M. Mirabedini, I. Dutil, L. Gauquelin, N. Yan, R.R. Farnood, Preparation of self-healing acrylic latex coatings using novel oil-filled ethyl cellulose microcapsules, Prog. Org. Coat. 85 (2015) 168 177. [25] B.K. Green, S. Lowell, Oil Containing Microscopic Capsules and Method of Making Them, The National Cash Register Company, Dayton, OH, US Patent 2,800,457, July 23, 1957, p. 11. [26] B.K. Green, Oil Containing Microscopic Capsules and Method of Making Them, The National Cash Register Company, Dayton, OH, US Patent 2,800,458, July 23, 1957, p. 7. [27] D.Y. Wu, S. Meure, D. Solomon, Self-healing polymeric materials: a review of recent developments, Prog. Polym. Sci. 33 (5) (2008) 479 522.

Self-Healing Polymer-Based Systems

References

255

[28] D.A. Rusakov, A.A. Lyapkov, E.I. Korotkova, N.V. Thanh, T.Q. Cuong, M.K. Zamanova, Correlation between temperature setting and DCS complex peak energy and in ROMP of dicyclopentadiene, Procedia Chem. 10 (2014) 490 493. [29] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, In situ poly (urea-formaldehyde) microencapsulation of dicyclopentadiene, J. Microencapsul. 20 (6) (2003) 719 730. [30] S.H. Cho, H.M. Andersson, S.R. White, N.R. Sottos, P.V. Braun, Polydimethylsiloxane-based self-healing materials, Adv. Mater. 18 (8) (2006) 997 1000. [31] H. Jin, C.L. Mangun, A.S. Griffin, J.S. Moore, N.R. Sottos, S.R. White, Thermally stable autonomic healing in epoxy using a dual-microcapsule system, Adv. Mater. 26 (2) (2014) 282 287. [32] K.S. Toohey, N.R. Sottos, S.R. White, Characterization of microvascular-based self-healing coatings, Exp. Mech. 49 (5) (2009) 707 717. [33] H. Jin, C.L. Mangun, D.S. Stradley, J.S. Moore, N.R. Sottos, S.R. White, Self-healing thermoset using encapsulated epoxy-amine healing chemistry, Polym. (Guildf.) 53 (2) (2012) 581 587. [34] T. Nesterova, K. Dam-Johansen, L.T. Pedersen, S. Kiil, Microcapsule-based self-healing anticorrosive coatings: capsule size, coating formulation, and exposure testing, Prog. Org. Coat. 75 (4) (2012) 309 318. [35] N. Selvakumar, K. Jeyasubramanian, R. Sharmila, Smart coating for corrosion protection by adopting nano particles, Prog. Org. Coat. 74 (3) (2012) 461 469. [36] X. Liu, H. Zhang, J. Wang, Z. Wang, S. Wang, Preparation of epoxy microcapsule based self-healing coatings and their behavior, Surf. Coat. Technol. 206 (23) (2012) 4976 4980. [37] T. Nesterova, K. Dam-Johansen, S. Kiil, Synthesis of durable microcapsules for self-healing anticorrosive coatings: a comparison of selected methods, Prog. Org. Coat. 70 (4) (2011) 342 352. [38] Z.-X. Hu, X.-M. Hu, W.-M. Cheng, W. Lu, Influence of synthetic conditions on the performance of melamine phenol formaldehyde resin microcapsules, High. Perform. Polym. 31 (2) (2019) 197 210. [39] M. Huang, J. Yang, Facile microencapsulation of HDI for self-healing anticorrosion coatings, J. Mater. Chem. 21 (30) (2011) 11123 11130. [40] M. de la Paz Miguel, C.I. Vallo, Influence of the emulsifying system to obtain linseed oil-filled microcapsules with a robust poly (melamine-formaldehyde)-based shell, Prog. Org. Coat. 129 (2019) 236 246. [41] F. Ahangaran, M. Hayaty, A.H. Navarchian, Y. Pei, F. Picchioni, Development of self-healing epoxy composites via incorporation of microencapsulated epoxy and mercaptan in poly (methyl methacrylate) shell, Polym. Test. 73 (2019) 395 403. ¨ zcan, Thermal energy storage characteristics of myristic acid-palmitic [42] A. Sarı, A. Bicer, C. Alkan, A.N. O eutectic mixtures encapsulated in PMMA shell, Sol. Energy Mater. Sol. Cell 193 (2019) 1 6. [43] Q. Li, N.H. Kim, D. Hui, J.H. Lee, Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy, Compos. Part B Eng. 55 (2013) 79 85. [44] M.M. Caruso, D.A. Delafuente, V. Ho, N.R. Sottos, J.S. Moore, S.R. White, Solvent-promoted self-healing epoxy materials, Macromolecules 40 (25) (2007) 8830 8832. [45] H. Zhang, J. Yang, Etched glass bubbles as robust micro-containers for self-healing materials, J. Mater. Chem. A 1 (41) (2013) 12715 12720. [46] H. Zhang, J. Yang, Development of self-healing polymers via amine epoxy chemistry: i. properties of healing agent carriers and the modelling of a two-part self-healing system, Smart Mater. Struct. 23 (6) (2014) 65003. [47] D.A. McIlroy, B.J. Blaiszik, M.M. Caruso, S.R. White, J.S. Moore, N.R. Sottos, Microencapsulation of a reactive liquid-phase amine for self-healing epoxy composites, Macromolecules 43 (4) (2010) 1855 1859. [48] H. Choi, K.Y. Kim, J.M. Park, Encapsulation of aliphatic amines into nanoparticles for self-healing corrosion protection of steel sheets, Prog. Org. Coat. 76 (10) (2013) 1316 1324. [49] J. Li, A.D. Hughes, T.H. Kalantar, I.J. Drake, C.J. Tucker, J.S. Moore, Pickering-emulsion-templated encapsulation of a hydrophilic amine and its enhanced stability using poly (allyl amine), ACS Macro Lett. 3 (10) (2014) 976 980. [50] H. Yi, Y. Deng, C. Wang, Pickering emulsion-based fabrication of epoxy and amine microcapsules for dual core self-healing coating, Compos. Sci. Technol. 133 (2016) 51 59. [51] B.J. Blaiszik, M.M. Caruso, D.A. McIlroy, J.S. Moore, S.R. White, N.R. Sottos, Microcapsules filled with reactive solutions for self-healing materials, Polym. (Guildf.) 50 (4) (2009) 990 997. [52] J. Yang, M.W. Keller, J.S. Moore, S.R. White, N.R. Sottos, Microencapsulation of isocyanates for self-healing polymers, Macromolecules 41 (24) (2008) 9650 9655.

Self-Healing Polymer-Based Systems

256

9. Self-healing polymeric coatings containing microcapsules filled with active materials

[53] M. Haghayegh, S.M. Mirabedini, H. Yeganeh, Preparation of microcapsules containing multi-functional reactive isocyanate-terminated-polyurethane-prepolymer as healing agent, Part II: corrosion performance and mechanical properties of a self healing coating, RSC Adv. 6 (56) (2016) 50874 50886. [54] M. Haghayegh, S.M. Mirabedini, H. Yeganeh, Microcapsules containing multi-functional reactive isocyanateterminated polyurethane prepolymer as a healing agent. Part 1: synthesis and optimization of reaction conditions, J. Mater. Sci. 51 (6) (2016) 3056 3068. [55] S. Ataei, S.N. Khorasani, R.E. Neisiany, Biofriendly vegetable oil healing agents used for developing selfhealing coatings: a review, Prog. Org. Coat. 129 (2019) 77 95. [56] M. Behzadnasab, M. Esfandeh, S.M. Mirabedini, M.J. Zohuriaan-Mehr, R.R. Farnood, Preparation and characterization of linseed oil-filled urea formaldehyde microcapsules and their effect on mechanical properties of an epoxy-based coating, Colloids Surf. A Physicochem. Eng. Asp. 457 (2014) 16 26. [57] M. Samadzadeh, S.H. Boura, M. Peikari, S.M. Kasiriha, A. Ashrafi, A review on self-healing coatings based on micro/nanocapsules, Prog. Org. Coat. 68 (3) (2010) 159 164. [58] M. Behzadnasab, S.M. Mirabedini, M. Esfandeh, R.R. Farnood, Evaluation of corrosion performance of a selfhealing epoxy-based coating containing linseed oil-filled microcapsules via electrochemical impedance spectroscopy, Prog. Org. Coat. 105 (2017) 212 224. [59] J.D. Rule, N.R. Sottos, S.R. White, Effect of microcapsule size on the performance of self-healing polymers, Polym. (Guildf.) 48 (12) (2007) 3520 3529. [60] S.M. Mirabedini, I. Dutil, L. Gauquelin, N. Yan, R.R. Farnood, Enhancing mechanical properties of acrylic latex self-healing composite coatings containing oil filled ethyl cellulose microcapsules. in: Composites/ Nano Engineering (ICCE 21), Tenerife, Spain, 2013. [61] C. Andersson, L. Ja¨rnstro¨m, A. Fogden, I. Mira, W. Voit, S. Zywicki, et al., Preparation and incorporation of microcapsules in functional coatings for self-healing of packaging board, Packag. Technol. Sci. An. Int. J. 22 (5) (2009) 275 291. [62] T. Szabo´, L. Molna´r-Nagy, J. Bogna´r, L. Nyikos, J. Telegdi, Self-healing microcapsules and slow release microspheres in paints, Prog. Org. Coat. 72 (1 2) (2011) 52 57. [63] H. Es-Haghi, S.M. Mirabedini, M. Imani, R.R. Farnood, Preparation and characterization of pre-silane modified ethyl cellulose-based microcapsules containing linseed oil, Colloids Surf. A Physicochem. Eng. Asp. 447 (2014) 71 80. [64] S.M. Mirabedini, M. Esfandeh, R.R. Farnood, P. Rajabi, Amino-silane surface modification of ureaformaldehyde microcapsules containing linseed oil for improved epoxy matrix compatibility. Part I: optimizing silane treatment conditions, Prog. Org. Coat. (2019) 105242. [65] H. Es-Haghi, S.M. Mirabedini, M. Imani, R.R. Farnood, Mechanical and self-healing properties of a waterbased acrylic latex containing linseed oil filled microcapsules: effect of pre-silanization of microcapsules’ shell compound, Compos. Part B Eng. 85 (2016) 305 314. [66] Y. Wang, D.T. Pham, C. Ji, Self-healing composites: a review, Cogent Eng. 2 (1) (2015) 1075686. [67] L.-T.T. Nguyen, X.K.D. Hillewaere, R.F.A. Teixeira, O. van den Berg, F.E. Du Prez, Efficient microencapsulation of a liquid isocyanate with in situ shell functionalization, Polym. Chem. 6 (7) (2015) 1159 1170. [68] B. Di Credico, M. Levi, S. Turri, An efficient method for the output of new self-repairing materials through a reactive isocyanate encapsulation, Eur. Polym. J. 49 (9) (2013) 2467 2476. [69] F. Alizadegan, S. Pazokifard, S.M. Mirabedini, M. Danaei, R. Farnood, Polyurethane-based microcapsules containing reactive isocyanate compounds: study on preparation procedure and solvent replacement, Colloids Surf. A Physicochem. Eng. Asp. 529 (2017) 750 759. [70] F. Alizadegan, S.M. Mirabedini, S. Pazokifard, S.G. Moghadam, R. Farnood, Improving self-healing performance of polyurethane coatings using PU microcapsules containing bulky-IPDI-BA and nano-clay, Prog. Org. Coat. 123 (2018) 350 361. [71] Y. Cong, K. Chen, S. Zhou, L. Wu, Synthesis of PH and UV dual-responsive microcapsules with high loading capacity and their application in self-healing hydrophobic coatings, J. Mater. Chem. A 3 (37) (2015) 19093 19099. [72] A. Gharsallaoui, G. Roudaut, O. Chambin, A. Voilley, R. Saurel, Applications of spray-drying in microencapsulation of food ingredients: an overview, Food Res. Int. 40 (9) (2007) 1107 1121. [73] B. Shu, W. Yu, Y. Zhao, X. Liu, Study on microencapsulation of lycopene by spray-drying, J. Food Eng. 76 (4) (2006) 664 669.

Self-Healing Polymer-Based Systems

References

257

[74] M.N.A. Hawlader, M.S. Uddin, M.M. Khin, Microencapsulated PCM thermal-energy storage system, Appl. Energy 74 (1 2) (2003) 195 202. [75] K.S. Mayya, A. Bhattacharyya, J. Argillier, Micro-encapsulation by complex coacervation: influence of surfactant, Polym. Int. 52 (4) (2003) 644 647. [76] M. Raeesi, S.M. Mirabedini, R.R. Farnood, Preparation of microcapsules containing benzoyl peroxide initiator with gelatin-gum arabic/polyurea-formaldehyde shell and evaluating their storage stability, ACS Appl. Mater. Interfaces 9 (24) (2017) 20818 20825. [77] F. Meng, S. Wang, Y. Wang, H. Liu, X. Huo, H. Ma, et al., Microencapsulation of oxalic acid via oil-in-oil (O/O) emulsion solvent evaporation, Powder Technol. 320 (2017) 405 411. [78] D.G. Shchukin, M. Zheludkevich, H. Mo¨hwald, Feedback active coatings based on incorporated nanocontainers, J. Mater. Chem. 16 (47) (2006) 4561 4566. [79] L. Janssen, A. Boersma, K. Te Nijenhuis, Encapsulation by interfacial polycondensation. III. Microencapsulation; the influence of process conditions on wall permeability, J. Memb. Sci. 79 (1) (1993) 11 26. [80] S.V. Ley, C. Ramarao, A.-L. Lee, N. Østergaard, S.C. Smith, I.M. Shirley, Microencapsulation of osmium tetroxide in polyurea, Org. Lett. 5 (2) (2003) 185 187. [81] D. Berthier, M. Jacquemond, N. Paret, L. Ouali, Process for Preparing Polyurea Microcapsules, United States Patent Application US 15/564,817, April 26, 2018. [82] Y. Ma, Z. Li, H. Wang, H. Li, Synthesis and optimization of polyurethane microcapsules containing [BMIm] PF6 ionic liquid lubricant, J. Colloid Interface Sci. 534 (2019) 469 479. [83] A. Trojanowska, V. Marturano, N.A.G. Bandeira, M. Giamberini, B. Tylkowski, Smart microcapsules for precise delivery systems, Funct. Mater. Lett. 11 (05) (2018) 1850041. [84] Y. Fuchs, H. Witteler, M. Bratz, Microcapsule Comprising a Polyester-Urethane Shell and a Hydrophilic Core Material, Unit States Patent Application US 15/777,334, December 13, 2018. [85] C.E. Diesendruck, N.R. Sottos, J.S. Moore, S.R. White, Biomimetic self-healing, Angew. Chem. Int. Ed. 54 (36) (2015) 10428 10447. [86] L. Yuan, A. Gu, G. Liang, Preparation and properties of poly (urea formaldehyde) microcapsules filled with epoxy resins, Mater. Chem. Phys. 110 (2 3) (2008) 417 425. [87] H. Zhang, X. Wang, Fabrication and performances of microencapsulated phase change materials based on N-octadecane core and resorcinol-modified melamine formaldehyde shell, Colloids Surf. A Physicochem. Eng. Asp. 332 (2 3) (2009) 129 138. [88] F. Leal-Calderon, V. Schmitt, J. Bibette, Emulsion Science: Basic Principles, Springer Science & Business Media, 2007. [89] A. Sharipova, S. Aidarova, B. Mutaliyeva, A. Babayev, M. Issakhov, A. Issayeva, et al., The use of polymer and surfactants for the microencapsulation and emulsion stabilization, Colloids Interfaces 1 (1) (2017) 3. [90] G. Murtaza, M. Ahamd, N. Akhtar, F. Rasool, A comparative study of various microencapsulation techniques: effect of polymer viscosity on microcapsule characteristics, Pak. J. Pharm. Sci. 22 (3) (2009). [91] J.-W. Cui, R.-J. Zhang, Z.-G. Lin, L. Li, W.-R. Jin, The effect of temperature and solvent on the morphology of microcapsules doped with a europium β-diketonate complex, Dalt. Trans. (2008) 895 899. No. 7. [92] R. Mueller, K. Ko¨hler, R. Weinkamer, G. Sukhorukov, A. Fery, Melting of PDADMAC/PSS capsules investigated with AFM force spectroscopy, Macromolecules 38 (23) (2005) 9766 9771. [93] K. Glinel, G.B. Sukhorukov, H. Mo¨hwald, V. Khrenov, K. Tauer, Thermosensitive hollow capsules based on thermoresponsive polyelectrolytes, Macromol. Chem. Phys. 204 (14) (2003) 1784 1790. [94] W. Tong, H. Song, C. Gao, H. Mo¨hwald, Equilibrium distribution of permeants in polyelectrolyte microcapsules filled with negatively charged polyelectrolyte: the influence of ionic strength and solvent polarity, J. Phys. Chem. B 110 (26) (2006) 12905 12909. [95] S.N. Gan, N. Shahabudin, Applications of microcapsules in self-healing polymeric materials, MicroencapsulationProcesses, Technologies and Industrial Applications, IntechOpen, 2019. [96] H. Umer, H. Nigam, A.M. Tamboli, M.S.M. Nainar, Microencapsulation: process, techniques and applications, Int. J. Res. Pharm. Biomed. Sci. 2 (2) (2011) 474 481. [97] D.R. Lide, CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data, CRC Press, 1995. [98] A.A. Maryott, E.R. Smith, Table of Dielectric Constants of Pure Liquids, National Bureau of Standards, Gaithersburg, MD, 1951.

Self-Healing Polymer-Based Systems

258

9. Self-healing polymeric coatings containing microcapsules filled with active materials

[99] R. Meirowitz, Microencapsulation technology for coating and laminating, Smart Textile Coatings and Laminates, Elsevier, 2019, pp. 117 154. [100] S.J. Garcia, H.R. Fischer, Self-healing polymer systems: properties, synthesis and applications, Smart Polymers and their Applications, Elsevier, 2014, pp. 271 298. [101] A.A. Rzesutko, E.N. Brown, N.R. Sottos, Tensile properties of self-healing epoxy, TAM. Tech. Rep. (2003) 1041 1055. [102] F. Zhang, P. Ju, M. Pan, D. Zhang, Y. Huang, G. Li, et al., Self-healing mechanisms in smart protective coatings: a review, Corros. Sci. 144 (2018) 74 88. [103] T. Nesterova, K. Dam-Johansen, L.T. Pedersen, S. Kiil, Self-Healing Anticorrosive Coatings (Doctoral dissertation), Department of Applied Chemistry, Danmarks Tekniske Universitet, Institut for Anvendt Kemi, Denmark, 2012.

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C H A P T E R

10 Capsule-based self-healing polymers and composites Maria Kosarli1, Dimitrios Bekas2, Kyriaki Tsirka1 and Alkiviadis S. Paipetis1 1

Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece 2 Structural Integrity and Health Monitoring Group, Department of Aeronautics, Imperial College London, South Kensington Campus, London, United Kingdom

10.1 Introduction An alternative approach to achieve self-repairing polymeric materials is the incorporation of capsules within the polymer [1]. Inside these microcapsules lies the healing agent, which is delivered to the damaged area upon rupture of the capsule. In self-healing approaches using microencapsulated healing agents, at least one component of the healing agent must be of low viscosity and capable of being stored in the microcapsules. In addition, the capsules shall be strong enough to permit handling and dispersion processes for their integration in polymers, but also should be easily breakable upon cracking of the matrix, thereby releasing the healing agent to the target areas via capillary effects. Subsequently the damaged areas of the matrix are healed through chemical and/or physical interactions with the healing agent. The design cycle for capsule-based self-healing materials includes: (1) the encapsulation process; (2) the integration of the capsules into the matrix; (3) the mechanical characterization; (4) the triggering and release of the healing agent to the damaged area; and finally (5) the evaluation of the healing efficiency [2]. Five types of encapsulated healing agent systems have already been proven efficient and are further discussed in (Fig. 10.1) [3]. The single-capsule system has at least one healing agent encapsulated, which can be a reactive chemical, a solvent, or a low-melting point metal. The capsule/dispersed catalyst healing system is related to the encapsulation of a self-healing agent within brittle capsules and the dispersion of the catalyst/polymerizer within the matrix. Damage in the form of propagating cracks causes the breakage of the capsules releasing the

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00010-6

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FIGURE 10.1 Capsule-based self-healing systems, (A) single capsules, (B) capsule (green)/dispersed catalyst (orange), (C) phase-separated droplet/capsules (green), (D) double-capsule (blue capsules with hardener, red capsules with healing agent) and (E) all-in-one microcapsules (multiple shell walls depicted with different colors). Source: Reprinted with permission from D.Y. Zhu, M.Z. Rong, M.Q. Zhang, Self-healing polymeric materials based on microencapsulated healing agents: From design to preparation, Prog. Polym. Sci. 49 50 (2015) 175 220, DOI:10.1016/j. progpolymsci.2015.07.002, Copyright (2019), Progress in Polymer Science.

monomer which comes in contact with the catalyst and polymerization occurs. In the third approach of the phase separated droplet/capsule systems, at least one of the healing components undergoes phase separation, whereas the other component is encapsulated. These two fluids react with each other upon release. The double-capsule system includes the encapsulation of one or more reactive liquid healing agents or polymerizers while the all-in-one microcapsules system is entirely self-contained. In this approach, the monomer and the catalyst are either integrated into the core and shell wall of the same multilayer capsule, isolated from each other by layers or are encapsulated in separate smaller spheres that are stored within a larger sphere, generating a capsule-in-capsule system. Upon breakage, the released healing agent first fills the crack area and when it is exposed to the catalyst heals the damaged area.

10.2 Capsule synthesis and characterization There are many encapsulation techniques such as centrifugal extrusion, pan coating, spray drying, and emulsion-based methods [4]. Each method exhibits different characteristics of the produced capsules, as their size, the shell wall composition, thickness, and strength. One of the most significant criteria for choosing the encapsulation method is the compatibility of the shell wall with the core materials. Other factors are the encapsulation efficiency, the permeability of the wall, and the microstructural characteristics of the

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capsules exterior surface which affects the interfacial properties between the capsules and the host matrix. The most common techniques for self-healing applications are emulsionbased methods where the shell wall forms at the aqueous/organic interface of the droplets of the healing agent. This category includes the emulsification polymerization, which is the mostly used method, the layer-by-layer assembly, the coacervation, and the internal phase separation methods (Fig. 10.2). Brown and his team performed one of the first studies on microencapsulation [5]. By changing the agitation rate, urea formaldehyde (UF) microcapsules containing dicyclopentadiene (DCPD) were prepared by an oil-in-water emulsion polymerization. Microcapsules diameters ranged from 10 to 1000 μm within a range of stirring rates from 200 to 2000 rpm and a linear relationship between them was observed. Fill content was determined to 83 92 wt.% by elemental analysis using carbon hydrogen nitrogen (CHN) analyzer. In another study, Blaiszik et al. [6] prepared UF microcapsules with diameters from 10 to 300 μm via in situ emulsification polymerization by changing the stirring rate. For the preparation of small capsules with a diameter at 300 nm, the use of sonication and stabilization processes were necessary. Capsules were filled with different epoxy resins (diclycidyl ether or bisphenol A or F) and solvents (ethyl phenylacetate, phenylacetate, and chlorobenzene) had a rough exterior shell wall for better interlocking between capsules and epoxy matrix. Further studies evaluated the mechanism of microencapsulation with urea formaldehyde polymer [7]. Capsules, in this case, were prepared in two steps. The first step involves the preparation of UF prepolymer and the preparation of the emulsion, while the second step consists of the shell wall formation and the final development of the microcapsule. Results proved that the UF polymerization occurred concurrently in the emulsion and at the microcapsule surface. In addition, at higher temperatures, the efficiency of microencapsulation decreased while the quantity of the UF microparticles at the shell increased. In addition, the size distribution and mean diameter were reduced when the reaction time and homogenization increased. The most sufficient capsule characteristics were achieved for capsules prepared at 50 C, with 30 min of homogenization and 3 h of reaction time. A similar study by McIlroy et al. focused on the microencapsulation of a reactive amine [8]. These capsules were prepared by interfacial polymerization of an isocyanate and amine stabilization by an inverse pickering emulsion. This method was based on the reverse-phase emulsion which created the amine droplets while the interfacial polymerization took place at the interface of the droplet and the oil. Capsules after the incorporation of an epoxy adhesive film and after crashing, reacted with the epoxy, thus making them competent for application in self-healing epoxy composites. Lus Sa´nchez-Silva et al. [9] considered the effect of methyl methacrylate monomer (MMA) on the phase change materials (PCM) microencapsulation by means of suspension-like polymerization. They also reported the influence of the MMA/styrene (St) and monomers/paraffin mass ratio on microcapsule properties. The results showed that the MMA/St mass ratio affected the conversion and the time at which the identity point was reached, while the MMA/St mass ratio had a remarkable influence on the polymerization rate. As the amount of MMA was increased the reaction time and the mean particle size decreased. The optimum mass ratios of MMA/St and monomers/paraffin for the

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FIGURE 10.2 Emulsion-based encapsulation methods (A) in situ emulsification polymerization (shell wall depicted with dark pink and core with light pink), (B) layer-by-layer assembly (LbL) (shell wall depicted with red/green), (C) coacervation (shell wall depicted with red/blue and core with pink), and (D) internal phase separation (shell wall depicted with blue and core with pink). Source: Reprinted with permission from A.P. Esser-Kahn, S. A. Odom, N.R. Sottos, S.R. White, J.S. Moore, Triggered release from polymer capsules, Macromolecules 44 (2011) 5539 5553, DOI:10.1021/ma201014n, Copyright (2019), American Chemical Society.

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production of this kind of microcapsules were 4.0 and 3.0, respectively. Monomers/paraffin mass ratio lower than 3.0 did not allow the microencapsulation of the paraffin wax. The higher reactivity and solubility of the methyl methacrylate in water, when compared to styrene, favored paraffin microencapsulation—as shown by the average storage energy capacity of the microcapsules obtained using this copolymer. Thermogravimetric analysis (TGA) measurements showed similar thermal stability for both the virgin copolymer and the microcapsules containing PCMs when P (St-co-MMA) was used as the shell material. A very interesting concept developed by Dong Yu Zhu and coworkers [10], constitutes the construction and development of multilayered microcapsules used for self-healing thermoplastics. By optimizing the synthesis conditions, robust poly(melamine-formaldehyde) (PMF)-walled microcapsules containing fluidic glycidyl methacrylate (GMA) monomer with proper size and core content were produced. The other two layers, second and third (outer/protective), comprised of living poly(methyl methacrylate) (PMMA-Br) and wax, respectively. The performance and stability (thermal and chemical) of the multilayered microcapsules indicated that they might be applicable for manufacturing not only self-healing thermoplastics but also self-healing thermosets. Ming You and coworkers [11] fabricated microencapsulated n-octadecane with a shell of styrene divinybenzene (DVB) copolymer by suspension-like polymerization. The results indicated that the majority of the microencapsulated n-octadecane (MicroC18) showed large concaves and the inner structure of microencapsulated n-octadecane compared to in situ polymerization showed to be more complex. The thickness of the microcapsule shell was 3 4 μm. They also noticed that the average diameters of MicroC18 decreased with an increase of stirring rate. When the mass ratio of monomers/octadecane was 1:1 the enthalpy of microencapsulated n-octadecane reached the highest value of 26 J g21. Furthermore they also reported that with the increase of the content of DVB, the enthalpy of the microencapsulated n-octadecane decreased. The thermal decomposition temperature of MicroC18 was about 230 C which is higher than that of MicroC18 using melamine formaldehyde shell. Microcapsules could also be used for the restoration of the electrical properties. The first attempt for the encapsulation of suspensions of carbon nanotubes was in 2009 from Caruso et al. [12]. Single-walled carbon nanotubes dispersed in chlorobenzene (PhCl) and ethyl phenylacetate (EPA) were encapsulated into UF microcapsules with a mean diameter of 280 350 μm. DC conductivity measurements proved the encapsulation of carbon nanotubes since after the crush, the core material was released and the conductivity increased. In another study, two different types of nanomaterials such as suspensions of singlewalled carbon nanotubes and graphene were encapsulated in polymeric capsules to provide restoration of conductivity in gold lines as suggested by Odom et al. [13]. Capsules were incorporated in gold lines while after fracture the core content released, bridged the gap of damaged circuits and partially restored the conductivity of the system. These capsules were proposed as promising materials for use as lithium-ion battery anode materials. Another approach was the encapsulation of liquid metals which was used for the restoration of the electrical properties in a mechanically damaged circuit [14]. The encapsulation of nanomodified epoxy resin could also slightly influence the exterior wall. Poly(methyl methacrylate) (PMMA) capsules were produced by Icduygu and his coworkers [15] containing epoxy resin modified with up to 2 wt.% multiwalled carbon

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nanotubes. Microcapsules filled with modified core exhibited a rougher surface morphology since SEM images showed the presence of nanotubes at the surface and also a more brittle shell wall. To better understanding the morphology of microcapsules, the same team in a recent study, performed a three-dimensional nanomorphology investigation of PMMA capsules containing carbon nanotubes and epoxy resin [16]. The novel laser scanning confocal microscopy (LSCM) was used as a nondestructive technique for the threedimensional microcapsule imaging which revealed information for the interior morphology (Fig. 10.3). TGA results indicated that the amount of encapsulated agent was not affected by the polymerization process parameters while the stirring rate and the content of the CNTs affected the capsules surface which was smooth with a variety of core shell morphologies. A vital parameter for the successful healing process is the mechanical performance of the shell of the capsules. The wall must be thin and brittle for easy breakage but strong enough to avoid the rupture before damage or during the dispersion of the capsules into the matrix material. Sun and Zhang [17] investigated the mechanical properties of capsules made of three different shell wall materials, namely melamine formaldehyde (MF), urea formaldehyde (UF) and gelatin gum arabic coacervate. Measurements were performed at single microcapsules by a micromanipulation technique in which capsules were compressed between two surfaces. MF and UF microcapsules indicated a viscoelastic behavior with a plastic deformation of 19% and 17% and a deformation at breakage at 68% and 35%, respectively, while gelatin capsules exhibited an elastic behavior and they did not rupture under compression. In another study, Keller and Sottos [18] evaluated the mechanical properties of UF microcapsules regarding their size. An average wall elastic FIGURE 10.3 Three dimensional LSCM imaging of microcapsules containing carbon nanotubes (A) Capsules produced at 300 rpm, (B) Capsules produced at 1000 rpm. LSCM, Laser scanning confocal microscopy. Source: Reprinted from M.G. Icduygu, M. Asilturk, M.A. Yalcinkaya, Y.K. Hamidi, M.C. Altan, Three-dimensional nano-morphology of carbon nanotube/epoxy filled poly(methyl methacrylate) microcapsules, Materials (Basel). 12 (2019) 26, DOI:10.3390/ ma12091387, under a Creative Commons license, Copyright (2019), Materials.

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modulus was estimated at 3.7 GPa, while the capsule size was found to be depended on the failure strength with smaller capsules sustaining higher loads. The incorporation of nanoparticles into the capsule shell could also influence the micromechanical and surface properties. Ghorbanzadeh Ahangari et al. [19] studied the addition of nanoparticles (carbon nanotubes and nanoalumina) in the exterior wall of UF microcapsules. The results indicated an increase of 14% and 46% of the elastic modulus in the case of nanoalumina and carbon nanotubes, respectively. In addition, the hardness of the shell increased with the nanoalumina capsules providing stiffer and harder wall, while the surface roughness decreased in all the cases. In a similar study, Freidoon et al. [20] examined the thermal properties and the morphology of the same capsules. The addition of nanoparticles did not affect the core content but reduced the mean diameter in both cases. Thermal properties were also improved, while contact angle measurements exhibited an increase from 44 to 50degrees for microcapsules modified with carbon nanotubes. Additionally Hu and coworkers [21] examined the mechanical properties of melamine formaldehyde (MF) microcapsules containing DCPD. Environmental scanning electron microscopy (ESEM) and optical microscopy were used to characterize the surface as shown in Fig. 10.4. The microcapsule size was directly dependent on the agitation rate and the strength of the capsules, with larger capsules to be weaker. The mechanical strength could also be affected by the reaction parameters while changing the reaction time led to variable shell thickness and different mechanical properties. Su et al. [22] investigated the effect of the size of MF capsules and the shell thickness on the micromechanical properties using the nanoindentation method. In this case, the elastic modulus and the stiffness increased with increasing capsule size. Lee et al. [23] produced microcapsules filled with fragrant oil via in situ polymerization of melamine formaldehyde (MF) as the shell wall material. Increasing the formaldehyde/melamine ratio and the pH of the mixture, the encapsulation efficiency increased and the morphology of the surface became smoother. Cosco et al. [24] utilized a new method to estimate the amount of encapsulated epoxy resin. They produced UF microcapsules with a variety of experimental conditions such as

FIGURE 10.4 Optical microscopy (A) and ESEM-Environmental scanning electron microscopy (B) images from the melamine formaldehyde microcapsules. Source: Reprinted with permission from J. Hu, H.Q. Chen, Z. Zhang, Mechanical properties of melamine formaldehyde microcapsules for self-healing materials, Mater. Chem. Phys. 118 (2009) 63 70, DOI:10.1016/j.matchemphys.2009.07.004, Copyright (2019), Materials Chemistry and Physics.

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reaction temperature, stirring rate, and reaction time. Morphological and thermal analysis showed rougher outer surface and lower thermal properties at higher reaction temperatures, while increased stirring rates led to increased encapsulation percentage. Therefore the encapsulation efficiency was directly correlated with the stirring rate, that is, reducing the stirring rate, encapsulation percentage decreased. In a similar study [25], the authors evaluated the relation of the polymerization conditions such as reaction time, temperature, and stirring rate on the encapsulation yield. Lower temperatures and stirring rate had a significant influence and decreased the encapsulation yield while the reaction time did not affect the polymerization. Another method for the microencapsulation yield assessment was the TGA as proposed by Carmona et al. [26]. In this study, poly(urea-formaldehyde) (PUF) fragrance microcapsules with a diameter at 2 8 μm were produced in a variety of core:shell ratios. FTIR and SEM images confirmed the successful formation of the microcapsules. TGA and derivative thermogravimetric (dTG) curves were used for the quantification of the encapsulated core which increased by about 15% as the core:shell ratio increased. In a more analytical study, Katoueizadeh and coworkers [27] investigated the effect of urea/formaldehyde molar ratio, reaction time, temperature, and pH on the formation of UF microcapsules. The most sufficient properties (formation and morphology of the UFMs, thermal stability) were observed at microcapsules produced at pH value 3, temperature at 85 C, a U/F ratio at 2.81, and reaction time at 4 h. Additionally Yuan Li and his team [28] produced a series of UF microcapsules by applying different parameters during encapsulation including surfactant type and concentration, heating rate, and pH value. An increment of the surface roughness was observed when they increased surfactant concentration or reducing pH, adjusting time and increasing the heating rate. Microcapsule size was reduced in the first case while there was no significant influence in the second case. Among different surfactants the optimum type was found to be the sodium dodecylbenzene sulfonate (SDBS) which also provided excellent solvent resistance, good storage stability, and sufficient mechanical strength. Apart from the well-known poly(urea-formaldehyde)-shell microcapsules, a generalized silica coating on the shell was developed by Jackson et al. [29]. In order to functionalize and protect submicron and micron size DCPD monomer-filled capsules and Grubbs’ catalyst particles, fluoride-catalyzed silica condensation chemistry was selected for the construction of the protective and functional silica coatings. This coating improved the dispersion efficiency of the capsules and catalyst particles inside the epoxy matrix. Unlike many other studies, a successful incorporation of both capsules into the epoxy was achieved without significant loss of healing agent. Matsuda et al. [30] produced microcapsules that are sensitive to pH changes. Cerium nitrate-containing pH-MC (Ce-MC) for corrosion inhibition and aluminum nitratecontaining pH-MC (Al-MC) microcapsules for the investigation of the release property were produced by water-in-oil emulsion polymerization. These capsules were coated on metal substrates and placed in corrosive (acidic or alkaline) environments for the shell wall decomposition and the release of the healing agent to occur. Al-MCs were found sensitive to acidic and basic environments while they were inactive in neutral environments. Additionally Ce-MC microcapsule coated metals were proved to inhibit corrosion through open circuit potential measurements.

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FIGURE 10.5 Binary microcapsule architecture with UF-DBP capsules in the shell wall. UF-DBP, Urea formaldehyde-dibutylphthalate. Source: Reprinted with permission from S.D. Mookhoek, B.J. Blaiszik, H.R. Fischer, N.R. Sottos, S.R. White, S. Van Der Zwaag, Peripherally decorated binary microcapsules containing two liquids, J. Mater. Chem. 18 (2008) 5390 5394, DOI:10.1039/b810542a, Copyright (2019), Royal Society of Chemistry.

Mookhoek et al. [31] demonstrated binary microcapsules containing two distinct liquids. In detail, UF microcapsules filled with dibutylphthalate (DBP) with an average size of 1.4 μm and a shell wall thickness of 75 nm were used as stabilizers in oil/water emulsions. New, larger microcapsules were synthesized with a mean diameter of 140 μm and shell wall thickness of 3 9 μm filled with dicyclopentadiene (DCPD) via an isocyanate alcohol interfacial polymerization. The small capsules (DBP) formed a layer around the core (DCPD) and were polymerized into the PU shell wall (Fig. 10.5). The confirmation of the presence of both capsules was performed via DSC scans with a volume fraction of small capsules of 8.8% which was in agreement with the theoretical calculations of the volume fraction that were based on the observed architecture and dimensions.

10.3 Self-healing polymers and composites The application of capsules into polymer matrices is the first step for the evaluation of the self-healing efficiency of capsule-based materials. Thermoset matrices like epoxy, polyurethane, cyanate ester, or even thermoplastic matrices were chosen for the integration of the capsule systems depending on the final application. The incorporation of the capsules into the polymer matrices is mainly achieved through dispersion processes where the capsule shell shall be strong enough to sustain the stirring forces. Then these capsule modified polymers can be further used as coatings, or as a matrix in composites. The following paragraphs describe some representable examples of capsule-based polymers. The first capsule-based self-healing system was proposed by White et al. [32] in 2001. In this study microcapsules containing a healing agent and catalyst particles were embedded into a polymer matrix exhibiting a satisfying self-healing efficiency. Dicyclopentadiene (DCPD) filled urea formaldehyde (UF) microcapsules with an average size of 220 μm and Grubbs’ catalyst particles were incorporated into a polymer matrix using the tapered double cantilever beam (TDCB) geometry. For the first time, both the influence of the selfhealing system incorporation in polymers and the healing process were investigated and

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self-healing efficiency was 60%. Since then capsule systems were studied in a variety of cases by many researchers due to their ease of applicability in polymers or composites and their potential for mass production. In order to determine the optimal combination of the maximum healing efficiency along with the minimum reduction in the mechanical properties of polymer epoxy matrix after the incorporation of the self-healing system, Kosarli et al. [33] investigated the effect of capsule size using the TDCB geometry. Results indicated that as the capsule size increased the healing efficiency and the reduction of mechanical properties also increased. Despite the fact that smaller capsules were filled with a higher percentage than the bigger ones, their healing efficiency was lower. These could be attributed to: (1) the higher load for the smaller capsules breakage; (2) the insufficient dispersion in the polymer matrix; (3) capsule pull out phenomena; and (4) the smaller quantity of the healing agent that was released locally. Li Yuan et al. [34] demonstrated the self-healing ability of a cyanate ester (CE) resin by the addition of microcapsules within the volume of the material. The capsules consisted of a poly(urea-formaldehyde) shell filled with a bisphenol A epoxy (EP). Diaminodiphenyl sulfone (DDS) catalyst was also employed in the CE formulation in order decrease the temperature of the polymerization reaction. The healing efficiency of the system was 85% demonstrating the effectiveness of the microcapsule approach for the development of selfhealing polymeric and polymer-based composite materials. In a very interesting research, Zhang and his coworkers [35] reported a skin-inspired, fully autonomous, self-warning, and self-healing polymeric material under damaging events. In detail, a dual microcapsule system containing an epoxy monomer, dyed with a pH indicator and a polyamine, was embedded into an epoxy matrix. After the impact damage, the material was able to warn the user by changing its color to red. The recovery of structural properties was increased from 70% to 95% by increasing the concentration of microcapsules from 5 to 10 wt.%, respectively. Ma et al. [36] demonstrated UF microcapsules filled with ethyl phenylacetate (EPA). Self-healing performance was accomplished by manual and in situ measurements. Manual experiments included the manual injection of EPA at the crack plane whereas during in situ experiments the UF microcapsules were incorporated in TDCB specimens. The healing efficiency was correlated with the swelling process and healing occurred due to solvent diffusion phenomena. Jin et al. [37] studied the encapsulation of epoxy and amine reactants in distinct polymeric microcapsules. The epoxy resin was encapsulated in a polyurethane (PU)-poly(ureaformaldehyde) (PUF) double shell wall. The healing agent was a bisphenol-A epoxy resin diluted with a low viscosity solvent (o-cresyl glycidyl ether). The amine capsules were produced with a method of vacuum infiltration of polyoxypropylenetriamine (POPTA) into polymeric hollow (PUF walled) microcapsules, providing thus a simple method for the encapsulation of a highly reactive core material. Microcapsules were then embedded into a polymer matrix (Araldite/Aradur 8615) while taking into account the required stoichiometry. Maintaining the total capsule concentration at 10 wt.% while varying the ratio of epoxy to amine capsules, which was at an equal mass ratio of amine to epoxy capsules (5:5), they managed to obtain the highest average healing efficiency. It was demonstrated that higher exposure temperature caused increased loss of core contents for both types of capsules leading to poor mixing of the reactants in the damaged area.

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Yan Chao Yuan et al. [38] investigated the influence of a series of factors on the healing chemistry of epoxy-based self-healing polymer composites embedded with dual microcapsules, which contained a two-part healing agent, unreacted epoxy as the polymerizable component and mercaptan and tertiary amine catalyst as the hardener. They found out that the strong alkaline character of the catalyst, the high activity of mercaptan, and the low viscosity of the encapsulated epoxy prepolymer ensured high healing efficiency and a fast rate of healing. The healing reaction was facilitated when the catalyst possessed strong alkalinity and suitable concentration, the encapsulated epoxy prepolymer was highly flowable, and mercaptan had a sufficiently large amount of hydrosulfide groups. The size and the fraction of the epoxy and hardener loaded microcapsules should be matched with each other in order to offer high healing efficiency. They concluded that larger microcapsules and/or higher microcapsules concentration were favored for filling up the larger cracks while on the other hand for smaller cracks, smaller microcapsules were preferred since mixing and interdiffusion of the healant’s components plays the leading role instead. In an effort to improve the self-healing efficiency of epoxy resin, Qi Li and his coworkers [39] prepared a dual-component microcapsule of DGEBA and polyether amine (hardener) using a water-in-oil-in-water emulsion solvent evaporation technique with polymethyl methacrylate (PMMA) as the shell material. It was shown that the content and ratio of the dual-component microcapsules affected the healing efficiency of epoxy system. Self-healing was carried out successfully at room temperature, but, as was indicated, an increase in temperature led to higher levels of the self-healing efficiency. Zhang and Yang [40] created two types of healing agent carriers, that is, microcapsules containing epoxy solution (Epolam 5015 and hardener 5015) and etched hollow glass bubbles (HGBs) loaded with amine solution (diethylenetriamine and ethyl phenylacetate) which were then incorporated in a self-healing epoxy system (Epolam 5015 and hardener 5015). The characterization of both capsules and bubbles were performed using TGA, SEM, and optical microscopy. The amine in the etched HGBs showed high thermal stability during the curing stage. A mathematical model has also been formulated in order to calculate the available healants and the diffusion distance on the crack plane of a two-part epoxy-amine. The diffusion distance of the released healing agent was calculated according to the simple cubic array model and it was found to be inversely proportional to the cubic root of the concentration of the healing agent carrier. A novel approach that concerned the implementation of a Lewis acid-catalyzed selfhealing system into epoxy-based fiber reinforced polymer (FRP) composite materials was investigated by Coope and his coworkers [41]. In their work, self-healing performance was quantified using a TDCB test specimen and the effects of several parameters such as the capsule content, healing temperature, and time were also investigated. The Lewis acidcatalyzed self-healing epoxy resin exhibited good compatibility and adhesion between the healing agent and the host matrix, while a recovery value of greater than 80% fracture strength was reported. In a recent study conducted by Hia et al. [42], alginate multicored microcapsules were developed and embedded into an epoxy matrix to produce a capsule-based self-healing composite system. The microcapsules were produced via the electrospraying method while two different epoxy resins (EPIKOTE 828 and ARALDITE 506) were employed as core material. The produced self-healing systems were tested via fracture (TDCB geometry)

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and impact tests. Results obtained from impact tests showed multiple healing events up to three cycles due to the controlled release of multicore capsules system, while in the case of the TDCB specimens showed only a single healing cycle which was attributed to the slow and steady crack propagation of the test. In a very promising study, Sun et al. [43] demonstrated phenol formaldehyde (PF) capsules with encapsulated dicyclopentadiene (DCPD). This process included the following steps: (1) preparation of polystyrene (PS) spheres; (2) application of PF coating on spheres; (3) removal of PS spheres; and (4) amine modification and induction of DCPD. The mean diameter of this batch was estimated of 500 nm with a shell wall thickness of 50 nm and core content at 45 wt.%. With the incorporation of 15 wt.% capsules into an epoxy matrix, fracture toughness and tensile strength were increased about 81.4% and 26.6%, respectively, with a healing efficiency of about 91.8%. A dual-capsule system was demonstrated by Ahangaran et al. [44] in a recent study. PMMA capsules containing an epoxy prepolymer (EC157) as healing agent and PMMA capsules filled with mercaptan as the curing agent were produced via internal phase separation and integrated into the epoxy polymer matrix. The mean diameter was found at 21 and 25 μm, respectively. With the increase of the capsule concentration up to 10%, the initial tensile strength and Young’s modulus decreased. However, results indicated a recovery of the mechanical properties by about 80% with 10 wt.% capsules in the matrix while the healing proceeded at room temperature for 24 h. A new healing agent, aminefunctionalized polydimethylsiloxane (PDMS-a), was recently suggested by Weihermann et al. [45] which was encapsulated into UF capsules. Both PDMS-microcapsules and triethylenetetramine (TETA) capsules, as the catalyst, were embedded into the DGEBA epoxy polymer matrix. The highest recovery of the mechanical properties (up to 100%) was accomplished with a 2.5 wt.% microcapsules for the healing process at 80 C for 48 h. On the other hand, this system was found to be inappropriate for healing at room temperature. In addition, a greater energy release rate was performed due to the PDMS which reduced the stiffness of the matrix. He et al. [46] demonstrated isophorone diisocyanate filled PVE polyurea microcapsules for application at polyurethanes and epoxy resins. Capsules were produced at five different sizes and exhibited a sufficient interlocking with the two matrices. The self-healing efficiency of both epoxies and polyurethanes increased, as the microcapsule size increased with the capsules with a mean diameter of B96 μm restoring the mechanical properties at 105% and 101%, respectively. In a numerical study, Mookhoek et al. [47] studied the effect of elongated capsules on the healing efficiency and the comparison with the spherical capsules. The model predicted the amount of healing agent that released regarding the capsule size, volume fraction aspect ratio, and capsule orientation for the elongated capsules. Results indicated that healing efficiency improved in the case of the elongated capsules with the most significant factor to be the capsule orientation. In another modeling effort, a hybrid computational model was used to simulate the fluid mechanism of the delivery of the healing agent into cracks from amphiphilic microcapsules. The addition of hydrophobic nanoparticles into microcapsules led to a delivery system in which the microcapsule released nanoparticles into the damaged hydrophobic areas and then moved into other cracked areas. This system was called “repair-and-go” [48].

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The ultimate aim of the aforementioned efforts is to apply the capsule-based systems as self-healing matrices in fiber reinforced polymer composites for structural applications. As early as 2003, Kessler et al. [49] demonstrated a 66% recovery of the initial mechanical properties of a microcapsule-based self-healing graphite fiber reinforced epoxy matrix composite using width-TDCB fracture tests. These values were achieved for a composite filled with 20 wt.% microcapsules containing DCPD and 5 wt.% of a Grubbs’ catalyst when the healing process was performed at 80 C, while an average recovery of 38% was achieved for healing at room temperature. Some years later, Brown et al. [50] studied the crack-growth behavior of a similar microcapsule system based on ruthenium (Ru) catalyst via tests relying on fatigue life extension. The authors demonstrated a prolonged fatigue life by up to 213% for moderate cyclic stress intensities. The authors identified several crack-tip shielding mechanisms that contributed to crack arrest and increased fatigue life when the rate of in situ healing exceeded the rate of crack-growth. The healing agent exhibited a viscous flow in the crack plane which delayed the growing crack and then polymerized there providing adhesion and finally total crack closure or a decrease in the crack length (partial closure of the crack). The degree of life extension was dependent on the stress frequency, amplitude, rest periods, and the in situ healing rate. In another study, Kim et al. [51] evaluated the self-healing capability of a fatigue damage in a cross-ply glass fiber reinforced epoxy composite structure. Polyurethane/poly (urea-formaldehyde) (PU/UF) microcapsules with a 2.5 μm diameter, filled with an Epon 862/EPA solvent mixture were incorporated into prepreg composites. The laminates were mechanically tested in tension tension fatigue loading for 15,000 cycles at 30% of the maximum strength. The healing process took place for 24 h at room temperature and exhibited a 52.4% recovery of Young’s modulus. Manfredi and coworkers [52] produced glass fiber reinforced polymer (CFRP) containing a solvent [ethyl phenylacetate (EPA)] capsule-based healing system using the vacuum assisted resin infusion molding technique. Capsules were manually dispersed into the composite at the maximum pressure threshold, in order to avoid premature capsule rupture was calculated at 0.3 bar. It must be pointed out that the healing process is based on the swelling mechanism of the polymeric matrix (Epon 828/DETA) in the presence of EPA solvent which results in filling the defects that have been created due to static loading in Mode I and II. Self-healing glass fabric/epoxy composites included epoxy and mercaptan filled microcapsules were tested under a low-velocity impact damage by Yuan and his team [53]. A dual microcapsule system was used while the healing process started without external intervention and took place at room temperature. The most significant parameter for the successful healing was found to be the impact energy since at small energies, the damage was healed faster. Another important factor was the capsule size with bigger capsules leading to an increased rate of damage area reduction. The full recovery of interfacial shear strength (IFSS) in a glass/epoxy was reported by Jones and coworkers [54] using a single capsule chemistry. The employed self-healing methodology involved the use of single capsules containing a solvent-based healing system. Microcapsules contained EPON 862 (diglycidyl ether of bisphenol-F) dissolved in EPA while the shell material consisted of poly(urea-formaldehyde) (pUF). Moreover

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several parameters that can affect the healing efficiency of the system like the resin solvent ratio, the capsule coverage, and the capsule size were also examined. Results indicated that the critical resin solvent ratio in order to obtain submicron capsules (0.6 μm diameter) was 30:70 in which a total of 83% recovery of IFSS was reported. Ahmed and Sanada investigated the effective elastic properties of 5, 10, and 20 vol.% UF/DCPD microcapsule-based epoxy composites both theoretically and experimentally [55]. During this study, DMA was used for the evaluation of the elastic modulus on neat epoxy, while the elastic modulus of the microcapsules with 32 μm size and 0.29 μm shell thickness was assessed both by single-microcapsule compression tests and finite element modeling. An analytical model based on Eshelby and Mori-Tanaka models was reformulated through an iterative homogenization of two-constituent composite systems until obtaining the effective elastic properties of the multiphase composite by considering every phase of constituents. These results were compared with predictions of the rule of mixtures and the differential effective medium theory. Finally the theoretical predictions were compared with the experimental results. The analytical models and the reformulated model showed good agreement with the experimental results with the least accuracy shown by the rule of mixtures. The authors concluded that an increase in the volume fraction of the microcapsules caused a decrease of the tensile storage modulus or in other words a decrease in the Poisson’s ratio of the composites up to a deflection point, where further increase in the volume fraction only slightly affects the elastic modulus of the microcapsule-based composite. The effects of (1) the test uncertainty, (2) the microcapsule geometric parameters (diameter variation and shell thickness) for the same batch, and (3) the shell elastic modulus and thickness on the effective elastic properties of the composite were all insignificant. Sanada et al. also examined the interfacial debonding phenomena of carbon fiber reinforced self-healing polymer composites as well as their repair efficiency [56]. The authors utilized the critical fracture loads recorded during single edge-notched tensile tests of fiber strands coated with an epoxy mixture containing 30 wt.% DCPD microcapsules and 2.5 wt.% Grubbs catalyst. A fluorescent dye was also contained in the coating mixture in order to aid the optical confirmation of the healing procedure under ultraviolet light. Finite element analyses were also performed based on a 3D model for the prediction of the damage progression in the selected specimen geometry which showed similar trends with the experimental results. The primary mode of failure after the healing process was interfacial debonding, while complete healing of the damage was observed for several specimens. The thickness of the specimens was found to slightly affect the healing probability with thinner specimens showing increased values in comparison to thicker ones. The critical fracture load was only weakly affected by the coating properties. In another study, Moll et al. evaluated the self-sealing behavior of plain weave E-glass epoxy composite where the matrix was modified with microencapsulated DCPD and Grubbs’ catalyst coated with paraffin wax [57]. The healing behavior was assessed by detecting the flow of nitrogen under pressure through the thickness direction in damaged composites using a pressure cell apparatus. The cyclic indentation was utilized in order to introduce a controlled amount of microcracking for the investigation of the effect of capsule concentration and size on the self-sealing performance of the composites. The authors monitored the sealing efficiency, the percentage of samples that fully sealed, as well as the

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initial leakage rate. The study concluded that complete healing can be achieved only when enough healing agent is delivered through an optimal combination of minimum capsule size and concentration. This combination is dependent on the size of the damage volume in comparison to the volume of the delivered healing agent. A composite with 6.5 wt.% capsules with diameters of 51 μm had the best performance between the studied systems. In comparison to the microcapsule-based systems, Blaiszik and his team [58] produced via in situ polymerization UF nanocapsules filled with dicyclopentadiene (DCPD). The small size of capsules was achieved with the simultaneous use of sonication and mixing while the mean diameter of capsules was estimated at 220 nm and the shell wall thickness at 77 nm. The incorporation of nanocapsules in epoxy/capsule composites also increased significantly (by 59%) the mode-I fracture toughness with a capsule volume fraction of Φf 5 0.015.

10.4 Self-healing coatings Corrosion is a natural process that highly affects the performance of materials and structures and may result in severe safety and environmental hazards. The impact of corrosion on society is often described in economic terms. Several financial studies indicated that industrialized nations lose approximately 3% of their gross domestic product (GDP) due to premature materials degradation [59]. Over the last decades, efforts have been focused on the development of both organic and inorganic coatings for the protection of materials and especially metals against corrosion. Among the anticorrosion methodologies, polymeric protective coatings are the most widely employed, and their costs add up to two-thirds of all anticorrosion expenditures. Polymeric coatings act as excellent barriers against corrosive ions and moisture and can easily be applied to a variety of materials such as metals or ceramics. However, the damage created within these coatings, during service or transportation, may significantly reduce their performance and thus compromise the integrity of the entire material or structure. Damage mechanisms in anticorrosive coatings involve but are not limited to, stressrelated micro-cracks, localized cracks, or delamination between the substrate and the coating. If the created damage is not detected and repaired on time, it may lead to conduits for the rapid intake of corrosive media and eventually to premature coating failure. In most of the cases, damaged coatings are either artificially repaired or replaced by another part. Both approaches raise great concerns since they are time-consuming and involve increased associated costs. A possible solution may be provided by self-healing polymers which possess the nature-inspired ability to repair cracks and thus may contribute to significantly extended service lives of anticorrosive coatings. The healing effects in protective coatings can be manifested by restoring the physical coating barriers via defect closing or sealing, or by inhibiting the corrosion reactions that occur at the coating defects. The following paragraphs present some characteristic examples of the application of capsule-based systems in self-healing coatings. In one of the first studies on self-healing coatings, Cho et al. [60] reported the development of a self-healing coating for the corrosion protection in cold-rolled steel. In this study,

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polydimethylsiloxane (PDMS) prepolymers and a dimethyldineodecanoate (DMDNT) catalyst were successfully encapsulated in PUF and polyurethane microcapsules, respectively. The developed capsules were embedded in a polymeric matrix which was used as an anticorrosive coating. Upon damage, the capsules were ruptured and the polymerization of PDMS took place within the crack plane at 50 C. At the end of the healing process, the polymeric coating regained its initial anticorrosive properties showing no evidence of rust formation during salt water exposure tests. In a more recent study, Yi et al. [61] synthesized microcapsules containing an epoxy or tetraethylenepentamine (TEPA) curing agent in polyurea (PU) microcapsules using Pickering emulsion polymerization. To evaluate the healing performance of the system, the PU capsules were incorporated within an epoxy coating, and the system was tested via brine-submersion corrosion-accelerating experiment. The obtained results indicated that the dual-core self-healing coating was able to fully restore the anticorrosive functionality of the coating by insulating the corrosion hazard from the metallic substrate. Liu et al. [62] prepared a smart self-healing coating consisting of an epoxy resin diglycidyl ether of bisphenol-A (DGEBA) as matrix and microcapsules filled with the same polymer as the curing agent. Capsules were synthesized by interfacial polymerization of epoxy droplets with ethylenediamine (EDA). These microcapsules exhibited high shell strength but were able to rupture under external force, releasing the healing agent to the damaged area. It should be noted that the complete absence of catalyst along with the high level of healing efficiency makes epoxy-capsule loaded polymers excellent candidates for the development of self-healing films. Yao Jialan et al. [63] prepared self-healing microcapsules by using in situ polymerization with urea formaldehyde as the wall material and E-51 epoxy resin as the core material. This research team studied the amount of epoxy resin, sodium dodecylbenzene sulfonate (SDBS), ammonium chloride, resorcinol and also the stirring rate on the particle size of the microcapsules. The results indicated that when the ratio was 24%, 8%, 5.6%, 10.4%, of the core material, SDBS, ammonium chloride, content of resorcinol, respectively and the stirring speed was 450 rpm, the average size of the microcapsules was 55.7 μm. The temperature of the microcapsules was stable at 250 C. Compared to the blank coating, the coating mixed with 2% microcapsules, the effective service life was elongated by about four times.

10.5 Conclusions and future trends Self-healing polymers and polymeric composites have attracted increasing attention over the last 30 years and self-healing technology is striving for a shift from prototyping of materials to the production of high-value components in order to prove its full maturation. The question regarding the durability of self-healing materials to perform under long-term environmental exposure remains open. However, research on novel healing methods, encapsulation methodologies, and engineering models for design and optimization accelerate the transition from laboratory demonstrations into useful and practical real-scale applications in several industrial sectors. To enable such a transition and maximize efficiency while minimizing cost, the design of capsule-based self-healing systems should

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focus on localized and targeted embedment of the self-healing functionality in large-scale components. In addition, the detrimental effect of capsules embedment in the host matrix should be taken into great consideration when selecting the most suitable capsule-based system. Due to their widespread use and the requirements for moderate mechanical properties, the most promising candidates that can adopt the capsule-based self-healing technology are polymeric coatings. Polymer coatings exhibited the great potential to be transformed into a self-healing system that will be able to regain its initial properties such as the anticorrosion, after a damage event. Other applications involve materials or structures employed in the electronics and optics where there are no requirements for increased mechanical properties. Addressing the challenges associated with the design and integration of capsule-based functionality in polymers will pave the way for the development of smart, multifunctional self-healing components that will be able to find applications in several industrial sectors.

References [1] D.G. Bekas, K. Tsirka, D. Baltzis, A.S. Paipetis, Self-healing materials: A review of advances in materials, evaluation, characterization and monitoring techniques, Comp. Part B. 87 (2016) 92 119. Available from: https://doi.org/10.1016/j.compositesb.2015.09.057898728408. [2] S.R. White, B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, Self-healing polymers and composites, Am. Sci. 99 (2011) 392 399. Available from: https://doi.org/10.1179/095066010X12646 898728408. [3] D.Y. Zhu, M.Z. Rong, M.Q. Zhang, Self-healing polymeric materials based on microencapsulated healing agents: from design to preparation, Prog. Polym. Sci. 49 50 (2015) 175 220. Available from: https://doi. org/10.1016/j.progpolymsci.2015.07.002. [4] A.P. Esser-Kahn, S.A. Odom, N.R. Sottos, S.R. White, J.S. Moore, Triggered release from polymer capsules, Macromolecules 44 (2011) 5539 5553. Available from: https://doi.org/10.1021/ma201014n. [5] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene, J. Microencapsul. 20 (2003) 719 730. Available from: https://doi.org/10.1080/ 0265204031000154160. [6] B.J. Blaiszik, M.M. Caruso, D.A. McIlroy, J.S. Moore, S.R. White, N.R. Sottos, Microcapsules filled with reactive solutions for self-healing materials, Polymer (Guildf.) 50 (2009) 990 997. Available from: https://doi. org/10.1016/j.polymer.2008.12.040. [7] B. Yogyakarta, Mechanism of microencapsulation with urea formaldehyde polymer Rochmadi, Agus Prasetya and Wahyu Hasokowati Department of Chemical Engineering, Faculty of Engineering, Am. J. Appl. Sci. 7 (2010) 739 745. Available from: https://doi.org/10.3844/ajassp.2010.739.745. [8] D.A. McIlroy, B.J. Blaiszik, M.M. Caruso, S.R. White, J.S. Moore, N.R. Sottos, Microencapsulation of a reactive liquid-phase amine for self-healing Epoxy composites, Macromolecules 43 (2010) 1855 1859. Available from: https://doi.org/10.1021/ma902251n. [9] L. Sa´nchez-Silva, J.F. Rodrı´guez, A. Romero, A.M. Borreguero, M. Carmona, P. Sa´nchez, Microencapsulation of PCMs with a styrene methyl methacrylate copolymer shell by suspension-like polymerisation, Chem. Eng. J. 157 (2010) 216 222. Available from: https://doi.org/10.1016/j.cej.2009.12.013. [10] D.Y. Zhu, M.Z. Rong, M.Q. Zhang, Preparation and characterization of multilayered microcapsule-like microreactor for self-healing polymers, Polymer (UK) 54 (2013) 4227 4236. Available from: https://doi.org/ 10.1016/j.polymer.2013.06.014. [11] M. You, X. Wang, X. Zhang, L. Zhang, J. Wang, Microencapsulated n-octadecane with styrene divinybenzene co-polymer shells, J. Polym. Res. 18 (2011) 49 58. Available from: https://doi.org/10.1007/ s10965-010-9390-8.

Self-Healing Polymer-Based Systems

276

10. Capsule-based self-healing polymers and composites

[12] M.M. Caruso, S.R. Schelkopf, A.C. Jackson, A.M. Landry, P.V. Braun, J.S. Moore, Microcapsules containing suspensions of carbon nanotubes, J. Mater. Chem. 19 (2009) 6093 6096. Available from: https://doi.org/ 10.1039/b910673a. [13] S.A. Odom, T.P. Tyler, M.M. Caruso, J.A. Ritchey, M.V. Schulmerich, S.J. Robinson, et al., Autonomic restoration of electrical conductivity using polymer-stabilized carbon nanotube and graphene microcapsules, Appl. Phys. Lett. 101 (2012). Available from: https://doi.org/10.1063/1.4737935. [14] B.J. Blaiszik, S.L.B. Kramer, M.E. Grady, D.A. McIlroy, J.S. Moore, N.R. Sottos, et al., Autonomic restoration of electrical conductivity, Adv. Mater. 24 (2012) 398 401. Available from: https://doi.org/10.1002/adma.201102888. [15] M.G. Icduygu, M. Asilturk, M.C. Altan, Microcapsules of poly(methyl metharcylate) containing epoxy resin and multi-walled carbon nanotubes, AIP Conf. Proc. 1914 (2017). Available from: https://doi.org/10.1063/ 1.5016704. [16] M.G. Icduygu, M. Asilturk, M.A. Yalcinkaya, Y.K. Hamidi, M.C. Altan, Three-dimensional nano-morphology of carbon nanotube/epoxy filled poly(methyl methacrylate) microcapsules, Materials (Basel) 12 (2019) 26. Available from: https://doi.org/10.3390/ma12091387. [17] G. Sun, Z. Zhang, Mechanical strength of microcapsules made of different wall materials, Int. J. Pharm. 242 (2002) 307 311. Available from: https://doi.org/10.1016/S0378-5173(02)00193-X. [18] M.W. Keller, N.R. Sottos, Mechanical properties of microcapsules used in a self-healing polymer, Exp. Mech. 46 (2006) 725 733. Available from: https://doi.org/10.1007/s11340-006-9659-3. [19] M. Ghorbanzadeh Ahangari, A. Fereidoon, M. Jahanshahi, N. Sharifi, Effect of nanoparticles on the micromechanical and surface properties of poly(urea-formaldehyde) composite microcapsules, Compos. Part. B Eng. 56 (2014) 450 455. Available from: https://doi.org/10.1016/j.compositesb.2013.08.071. [20] A. Fereidoon, M. Ghorbanzadeh Ahangari, M. Jahanshahi, Effect of nanoparticles on the morphology and thermal properties of self-healing poly(urea-formaldehyde) microcapsules, J. Polym. Res. 20 (2013). Available from: https://doi.org/10.1007/s10965-013-0151-3. [21] J. Hu, H.Q. Chen, Z. Zhang, Mechanical properties of melamine formaldehyde microcapsules for self-healing materials, Mater. Chem. Phys. 118 (2009) 63 70. Available from: https://doi.org/10.1016/j.matchemphys. 2009.07.004. [22] J.F. Su, X.Y. Wang, H. Dong, Micromechanical properties of melamine formaldehyde microcapsules by nanoindentation: effect of size and shell thickness, Mater. Lett. 89 (2012) 1 4. Available from: https://doi. org/10.1016/j.matlet.2012.08.072. [23] H.Y. Lee, S.J. Lee, I.W. Cheong, J.H. Kim, Microencapsulation of fragrant oil via in situ polymerization: effects of pH and melamine formaldehyde molar ratio, J. Microencapsul. 19 (2002) 559 569. Available from: https://doi.org/10.1080/02652040210140472. [24] S. Cosco, V. Ambrogi, P. Musto, C. Carfagna, Properties of poly(urea-formaldheyde) microcapsules containing an epoxy resin, J. Appl. Polym. Sci. 105 (2007) 1400 1411. Available from: https://doi.org/10.1002/ app.26263. [25] S. Cosco, V. Ambrogi, P. Musto, C. Carfagna, Urea formaldehyde microcapsules containing an epoxy resin: influence of reaction parameters on the encapsulation yield, Macromol. Symp. 234 (2006) 184 192. Available from: https://doi.org/10.1002/masy.200650224. [26] C.G. Carmona, M.J.L. Arias, O.G. Carmona, F. Maesta´, M.P. Andreu, N. De, et al., Microencapsulation yield assessment using TGA, J. Miner. Met. Mater. Eng. 2019 (2019) 1 11. [27] E. Katoueizadeh, S.M. Zebarjad, K. Janghorban, Investigating the effect of synthesis conditions on the formation of urea formaldehyde microcapsules, J. Mater. Res. Technol. (2018) 1 12. Available from: https://doi. org/10.1016/j.jmrt.2018.04.013. [28] L. Yuan, A. Gu, G. Liang, Preparation and properties of poly(urea-formaldehyde) microcapsules filled with epoxy resins, Mater. Chem. Phys. 110 (2008) 417 425. Available from: https://doi.org/10.1016/j. matchemphys.2008.02.035. [29] A.C. Jackson, J.A. Bartelt, K. Marczewski, N.R. Sottos, P.V. Braun, Silica-protected micron and sub-micron capsules and particles for self-healing at the microscale, Macromol. Rapid Commun. 32 (2011) 82 87. Available from: https://doi.org/10.1002/marc.201000468. [30] T. Matsuda, N. Jadhav, K.B. Kashi, M. Jensen, V.J. Gelling, Release behavior of pH sensitive microcapsules containing corrosion inhibitor, Prog. Org. Coat. 132 (2019) 9 14. Available from: https://doi.org/10.1016/j. porgcoat.2019.03.032.

Self-Healing Polymer-Based Systems

References

277

[31] S.D. Mookhoek, B.J. Blaiszik, H.R. Fischer, N.R. Sottos, S.R. White, S. Van Der Zwaag, Peripherally decorated binary microcapsules containing two liquids, J. Mater. Chem. 18 (2008) 5390 5394. Available from: https:// doi.org/10.1039/b810542a. [32] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794 797. Available from: https://doi.org/10.1038/35057232. [33] M. Kosarli, D.G. Bekas, K. Tsirka, D. Baltzis, D. Vaimakis-Tsogkas, S. Orfanidis, et al., Microcapsule-based self-healing materials: healing efficiency and toughness reduction vs. capsule size, Compos. Part B Eng 171 (2019) 78 86. Available from: https://doi.org/10.1016/j.compositesb.2019.04.030. [34] L. Yuan, S. Huang, A. Gu, G. Liang, F. Chen, Y. Hu, et al., A cyanate ester/microcapsule system with low cure temperature and self-healing capacity, Compos. Sci. Technol. 87 (2013) 111 117. Available from: https://doi.org/10.1016/j.compscitech.2013.08.005. [35] H. Zhang, X. Zhang, C. Bao, X. Li, F. Duan, K. Friedrich, et al., Skin-inspired, fully autonomous self-warning and self-repairing polymeric material under damaging events, Chem. Mater. (2019). Available from: https:// doi.org/10.1021/acs.chemmater.9b00398. [36] W. Ma, W. Zhang, Y. Zhao, H. Yu, Preparation and assessment of a self-healing material based on microcapsules filled with ethyl phenylacetate, J. Appl. Polym. Sci. 133 (2016). Available from: https://doi.org/ 10.1002/app.43430. [37] H. Jin, C.L. Mangun, A.S. Griffin, J.S. Moore, N.R. Sottos, S.R. White, Thermally stable autonomic healing in epoxy using a dual-microcapsule system, Adv. Mater. 26 (2014) 282 287. Available from: https://doi.org/ 10.1002/adma.201303179. [38] Y.C. Yuan, M.Z. Rong, M.Q. Zhang, G.C. Yang, Study of factors related to performance improvement of selfhealing epoxy based on dual encapsulated healant, Polymer (Guildf.) 50 (2009) 5771 5781. Available from: https://doi.org/10.1016/j.polymer.2009.10.019. [39] Q. Li, H. Siddaramaiah, N.H. Kim, D. Hui, J.H. Lee, Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy, Compos. Part B Eng. 55 (2013) 79 85. Available from: https://doi.org/10.1016/j.compositesb.2013.06.006. [40] H. Zhang, J. Yang, Development of self-healing polymers via amine-epoxy chemistry: I. Properties of healing agent carriers and the modelling of a two-part self-healing system, Smart Mater. Struct. 23 (2014). Available from: https://doi.org/10.1088/0964-1726/23/6/065003. [41] T.S. Coope, U.F.J. Mayer, D.F. Wass, R.S. Trask, I.P. Bond, Self-healing of an epoxy resin using scandium(III) triflate as a catalytic curing agent, Adv. Funct. Mater. 21 (2011) 4624 4631. Available from: https://doi.org/ 10.1002/adfm.201101660. [42] I.L. Hia, P. Pasbakhsh, E.S. Chan, S.P. Chai, Electrosprayed multi-core alginate microcapsules as novel selfhealing containers, Sci. Rep. 6 (2016) 1 8. Available from: https://doi.org/10.1038/srep34674. [43] T. Sun, X. Shen, C. Peng, H. Fan, M. Liu, Z. Wu, A novel strategy for the synthesis of self-healing capsule and its application, Compos. Sci. Technol. 171 (2019) 13 20. Available from: https://doi.org/10.1016/j. compscitech.2018.12.006. [44] F. Ahangaran, M. Hayaty, A.H. Navarchian, Y. Pei, Development of self-healing epoxy composites via incorporation of microencapsulated epoxy and mercaptan in poly (methyl methacrylate) shell, Polym. Test. 73 (2019) 395 403. Available from: https://doi.org/10.1016/j.polymertesting.2018.11.041. [45] W.R.K. Weihermann, M.M. Meier, S.H. Pezzin, Microencapsulated amino-functional polydimethylsiloxane as autonomous external self-healing agent for epoxy systems, J. Appl. Polym. Sci. 47627 (2019) 1 9. Available from: https://doi.org/10.1002/app.47627. [46] Z. He, S. Jiang, N. An, X. Li, Q. Li, J. Wang, et al., Self-healing isocyanate microcapsules for efficient restoration of fracture damage of polyurethane and epoxy resins, J. Mater. Sci. 54 (2019) 8262 8275. Available from: https://doi.org/10.1007/s10853-018-03236-3. [47] S.D. Mookhoek, H.R. Fischer, S. van der Zwaag, A numerical study into the effects of elongated capsules on the healing efficiency of liquid-based systems, Comput. Mater. Sci. 47 (2009) 506 511. Available from: https://doi.org/10.1016/j.commatsci.2009.09.017. [48] J.Y. Lee, G.A. Buxton, A.C. Balazs, Using nanoparticles to create self-healing composites, J. Chem. Phys. 121 (2004) 5531 5540. Available from: https://doi.org/10.1063/1.1784432. [49] M.R. Kessler, N.R. Sottos, S.R. White, Self-healing structural composite materials, Compos. Part A Appl. Sci. Manuf. 34 (2003) 743 753. Available from: https://doi.org/10.1016/S1359-835X(03)00138-6.

Self-Healing Polymer-Based Systems

278

10. Capsule-based self-healing polymers and composites

[50] E.N. Brown, S.R. White, N.R. Sottos, Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite—part II: manual infiltration, Compos. Sci. Technol. 65 (2005) 2466 2473. Available from: https://doi.org/10.1016/j.compscitech.2005.04.020. [51] S.Y. Kim, N.R. Sottos, S.R. White, Self-healing of fatigue damage in cross-ply glass/epoxy laminates, Compos. Sci. Technol. 175 (2019) 122 127. Available from: https://doi.org/10.1016/j.compscitech.2019.03.016. [52] E. Manfredi, A. Cohades, I. Richard, V. Michaud, Assessment of solvent capsule-based healing for woven E-glass fibre-reinforced polymers, Smart Mater. Struct. 24 (2015) 015019. Available from: https://doi.org/ 10.1088/0964-1726/24/1/015019. [53] Y.C. Yuan, Y. Ye, M.Z. Rong, H. Chen, J. Wu, M.Q. Zhang, et al., Self-healing of low-velocity impact damage in glass fabric/epoxy composites using an epoxy-mercaptan healing agent, Smart Mater. Struct. 20 (2011). Available from: https://doi.org/10.1088/0964-1726/20/1/015024. [54] A.R. Jones, B.J. Blaiszik, S.R. White, N.R. Sottos, Full recovery of fiber/matrix interfacial bond strength using a microencapsulated solvent-based healing system, Compos. Sci. Technol. 79 (2013) 1 7. Available from: https://doi.org/10.1016/j.compscitech.2013.02.007. [55] A. Ahmed, K. Sanada, Micromechanical modeling and experimental verification of self-healing microcapsules-based composites, Mech. Mater. (2019). Available from: https://doi.org/10.1016/j.mechmat. 2019.01.020. [56] K. Sanada, Y. Mizuno, Y. Shindo, Damage progression and notched strength recovery of fiber-reinforced polymers encompassing self-healing of interfacial debonding, J. Compos. Mater. 0 (2014) 1 12. Available from: https://doi.org/10.1177/0021998314540193. [57] J.L. Moll, S.R. White, N.R. Sottos, A self-sealing fiber-reinforced composite, J. Compos. Mater. 44 (2010) 2573 2585. Available from: https://doi.org/10.1177/0021998309356605. [58] B.J. Blaiszik, N.R. Sottos, S.R. White, Nanocapsules for self-healing materials, Compos. Sci. Technol. 68 (2008) 978 986. Available from: https://doi.org/10.1016/j.compscitech.2007.07.021. [59] M.P.H. Gerhardus, H. Koch, N.G.T.Y.P.V.J.H.P. Brongers, Corrosion costs and preventive strategies in the United States, NACE Rep. (2002) 1 12. doi:FHWA-RD-01-156. [60] S.H. Cho, S.R. White, P.V. Braun, Self-healing polymer coatings, Adv. Mater. 21 (2009) 645 649. Available from: https://doi.org/10.1002/adma.200802008. [61] H. Yi, Y. Deng, C. Wang, Pickering emulsion-based fabrication of epoxy and amine microcapsules for dual core self-healing coating, Compos. Sci. Technol. 133 (2016) 51 59. Available from: https://doi.org/10.1016/j. compscitech.2016.07.022. [62] X. Liu, H. Zhang, J. Wang, Z. Wang, S. Wang, Preparation of epoxy microcapsule based self-healing coatings and their behavior, Surf. Coat. Technol. 206 (2012) 4976 4980. Available from: https://doi.org/10.1016/j. surfcoat.2012.05.133. [63] Y. Jialan, Y. Chenpeng, Z. Chengfei, H. Baoqing, Preparation process of epoxy resin microcapsules for selfhealing coatings, Prog. Org. Coat. 132 (2019) 440 444. Available from: https://doi.org/10.1016/j. porgcoat.2019.04.015.

Self-Healing Polymer-Based Systems

C H A P T E R

11 Ionomers as self-healing materials S. Mojtaba Mirabedini and Farhad Alizadegan Iran Polymer and Petrochemical Institute, Tehran, Iran

11.1 Introduction Damage occurrence is inevitable in natural or artificial materials. A variety of materials [1,2] and methods [3 6] have been introduced to deal with this concern. Meanwhile, the self-healing phenomenon has been revealed to be one of the most promising and costeffective methods [5,6]. Many researchers have worked on this topic and introduced different approaches to providing polymeric materials with self-healing capability. Self-healing is a smart property of materials that can be typically accomplished through intrinsic or extrinsic mechanisms. When a polymer matrix contains a latent functionality that actuates repairing damages via hydrogen bonding, thermally reversible reactions, ionomeric materials, or molecular diffusion and entanglement, it is called intrinsic self-healing [7,8]. In systems comprising an extrinsic mechanism, healing materials are added or induced into the polymer matrix through different approaches, such as fibers [9,10], capillaries [11 13], or microcapsules [14 17]. The task of self-healing materials is to extend the efficient service life of materials by implementing the concept of spontaneous repair rather than increasing their primary performance. Ionic systems play a key role in self-healing. An ion is an atom or molecule with positive [ 1 ] or negative [ 2 ] charges. However, an ionomer is not only a polymer with ionic functional groups but also it is a polymer with repeating units of ionic groups and their counterions. The first ionomer polymer was introduced in 1964 by DuPont Co. [18,19] under the trade name Surlyn, which was comprised of ethylene and methacrylic acid copolymers (M-PEMA, where the first M denotes the metal cation) [20,21] that were partially neutralized with sodium or zinc ion. In Fig. 11.1, the chemical structure of Surlyn ionomers is illustrated. Ionomeric polymers usually comprise up to 20 mol.% of ionic species. For self-healing objectives, ionomers mainly contain an organic backbone [21 23]. The ionomer polymers are prepared in the form of chains with ionic groups and add a sufficient counterion to the polymer matrix. By changing the ionic content, the properties of a self-healing

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FIGURE 11.1 Chemical structure of Surlyn ionomers.

ionomeric polymer can be varied. Numerous types of ionomers are available, which vary in the percentage and type of cations used to neutralize acidic groups (e.g., sodium, zinc, magnesium, and lithium) [24 27]. The majority of studied ionomers comprise a polyvinyl or polydiene backbone, which carries anionic groups (anionomers) with Na1 and Zn21 as counterions. Three major ionized groups are: carboxylate ( COO2), sulfonate ( SO32), and phosphonate ( PO322), which differ in the strength of the ionic interaction [28 30].

11.2 Materials, chemistry, and fundamentals Microphase separation into a polar phase, a cluster-forming ionic phase, and a nonpolar polymer phase lead to improvements in intermolecular interactions and thermally reversible cross-links. The formation of these cross-links is accomplished by the interaction of the metal ion with the carboxylate group, which increases the proliferation and clustering of these reversible bonds [31,32]. Ionomers are nanostructured materials, similar to block copolymers and nanocomposites with very short ionic blocks that attract each other [33,34]. Small angle X-ray scattering (SAXS) technique [35] and scanning electron microscopy [35,36] were used to show the microstructure of the ionomers. Zinc-neutralized poly(styrene-co-styrene sulfonate) ionomers form Zn-rich aggregates and can have two solid sphere shapes ranging in diameter from 4 to 10 nm, and vesicles aggregate with a diameter ranging from 9 to 55 nm [37]. Kirkmeyer et al. [37] used scanning transmission electron microscopy to show the spherical structure with 75% neutralization of sulfonated polystyrene ionomers. As shown in Fig. 11.2, ionomers have an individual feature, which is their supramolecular structure. SAXS analyses [35] indicated that the nanodomains have an order of 1 nm in diameter with an average separation of 2 5 nm. The mechanical properties and dynamics of these materials are affected by covalently attached ion-pairs to the polymer chains [38]. Ionomers are prepared in two different modes, either by direct copolymerization of an ionic with a nonionic monomer or via postpolymerization treatment, such as sulfonation and quaternization. Due to the difficulty of dissolving ionic and nonionic monomers, in

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11.2 Materials, chemistry, and fundamentals

FIGURE

11.2

of ionomer attached to chain.

Schematic microstructure the polymer

addition to the compatibility of ionic monomers with the polymerization technique, the second method is typically used [39]. Experimental studies have made significant advances in the direct copolymerization of ionomers in two categories: controlled free-radical polymerization (CFRP) [40] and ionic liquids [41]. The CFRP route fabricates polymers with controlled molecular weight and monomer distribution in comparison to anionic and cationic polymerization products. For instance, for the preparation of sulfonated polystyrene ionomers, Okamura et al. [42] utilized nitroxide-treated, free-radical polymerization via copolymerization of styrene sulfonate ester. Recently for direct polymerization of ionomers, a variety of ionic liquids have been introduced as monomers. First, low melting point ionic liquids, such as acyclic diene, are useful for the preparation of ionomers, such as polyolefins, with functional groups of imidazolium through direct polymerization [43]. Second, some ionic liquids monomers, such as N-methyl-2-pyrrolidone (NMP), which are oleophilic, admit their direct copolymerization for synthesizing of neutral-cationic-neutral triblock block copolymers through hydrophobic monomers, such as poly(n-butyl acrylate) and poly(vinyl tri-alkyl benzyl phosphonium chloride) [44]. In another study, cationic ionomers were prepared using methacrylate-based imidazolium and n-butyl methacrylate monomers by free-radical copolymerization [45]. Innovative ionomeric materials have been developed, and the earliest and foremost of them were thermoplastic elastomers (TPEs), such as acrylonitrile [46] and acrylic acid copolymer [47]. Another introduced ionic elastomer was obtained from the sulfonation of polyethylene chlorine. Due to the curing of metal oxides, ionomers form a blend of physical and ionic cross-links [48]. Neutralization has major effects, such as improving the

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toughness and flexibility of these materials. The optical clarity is obtained from a lowdensity polyethylene or a key ionic elastomer achieved by sulfonation of polyethylene/ propylene diene monomer (EPDM). The elastomeric properties of these products are directly controlled by the amount of sulfonic agent that is used [49 52]. The other key group of ionomers is styrene-based thermoplastic polymers that can be prepared by postreaction modification [49,53,54] or copolymerization [55 57]. Polytetra fluoroethylene-based ionomers comprised of ionic side groups in ether bonds are used as the materials for the preparation of semipermeable membranes because of their ability to withstand severe chemical environments [48]. Semipermeable ionomeric membranes or “ion-selective membranes” are thin and smart pieces of polymeric materials that selectively allow materials to pass through them. For example, these ion-selective membranes can separate ions by permitting water to pass through and prevent metal ions [58]. Another group of ionomers is Halato-telechelic polymers, which are salts derived from carboxyl-terminated materials. Some of the other reported ionomers are based on linear telechelic polyisobutylene sulfonate and star-shaped trialkenes [59]. Also for sodium sulfonate, polysulfone-based ionomers, a content of 3% 30% mole sulfur, and no phase separation confirmation have been reported [60,61]. Fig. 11.3 shows clearly the orientation of polar ionic groups in nonpolar chains to form a small cluster. Ionic groups tend to depart from each other, but they are attached to polyethylene chains. These clusters allow thermoplastic ionomers to act like chemically cross-linked or blocked copolymers [58]. Ionomers are reversibly cross-linked plastics rather than chemically cross-linked polymers that lose their attractions due to heating, and the chains can move freely around (Fig. 11.4). When the temperature rises sufficiently, the groups can no longer remain in the clusters, resulting in the chains spinning around rapidly. These ionomers are sometimes known as TPEs. Structurally ionic groups can be randomly dispersed in different parts of the polymer chain (Fig. 11.5A), arranged in blocked copolymers (Fig. 11.5B), most of which can be directly attached to the polymer backbone such as polyethylene, in the side chains (Fig. 11.5C), or at the end of the polymer chains (Fig. 11.5D) [62].

FIGURE 11.3 The orientation of polar ionic groups in nonpolar chains.

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11.3 The self-healing mechanisms

FIGURE 11.4 Reversibility of ionomers in hot condition. FIGURE 11.5 Different structures of ionomer: (A) random, (B) block, (C) side chain, and (D) end of polymer.

11.3 The self-healing mechanisms The ionomers’ self-repairing mechanism is not an autonomous process because these components need to be “activated,” and this activation is controlled by polymer structure and morphology. The benefit of intrinsic ionomer healing systems is that the process is potentially repeatable many times. During the collision, the frictional forces heat up the affected polymer and transfer energy to it. After increasing the temperature of the region, the affected polymer is locally melted. A high scale of elongation occurs because of the elastic nature of the molten polymer, and the healing of the cavity, hole, or damaged zone takes place in polymer rebounding. For classifying, the healing process can be divided into two stages. The first is in the elastic/viscoelastic recovery, and the second is in the full recovery and repair due to the local melting/solidification process [24,63,64]. When sufficient entanglements appear after the slow diffusion of the chemical species into the discontinuous part, complete healing occurs [18]. A considerable fact in the case of selfhealing is that ionomers can act as thermoplastics and thermosets systems. Ionic domains or “aggregates” are manufactured in the presence ionic species, which means several ion pairs with sufficient flexibility in the polymer backbone, steric factors, the size of ion couples, and the dielectric constant, determine the size and number of ion couples [65 67]. Stephen and Ward [24] and Kalista [68] hypothesized a three-stage self-healing mechanism for ionomeric systems. The first stage is the elastic reconstitution of the molten material after heating to Tg or semicrystalline area. The second stage involves healing the damage by interdiffusion through melting ionomeric clusters. Finally, the third stage is

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continuously restoring the mechanical properties of the affected area with the formation of new ionic aggregates [69].

11.4 Activation methods As stated, activation of the healing process in the ionomers is an integral part of the process that starts with the adequate transfer of high thermal energy forces [70]. In another study, local heating was reported as an efficient activation mode in alternating magnetic fields in the NBR matrix. Magnetic nanoparticles of cobalt magnetite, and cobalt ferrite are the product of the activating heating [71]. Smart and controlled self-healing activation of elastomeric ionomers was achieved by integrating magnetic nanoparticles with a variety of navigates into the polymer matrix [72]. Polyethylene-co-methacrylic acid (EMAA) is also known as a heat-activated thermoplastic healing agent [73 75]. The ionic groups under a certain temperature endure the transitions and relaxations due to the mobility of the polymer network [76]. Kalista et al. [75] showed that the direct effect of neutralization amount on temperature range for activation of the healing process. They reported that 30% of acid groups neutralized with sodium ions, exhibited the largest temperature range in which self-healing behavior appeared. Atom transfer radical polymerization (ATRP) method also was utilized to evaluate the ionomer self-healing system based on n-butyl acrylate copolymer. Sodium, zinc, or cobalt salts were used as neutralization agents for neutralizing the carboxylic acid group and to prepare related ionomer [62].

11.5 Applications The thermally reversible ionic compounds have some outstanding advantages. Indeed, the thermal reversibility makes them suitable for processing through compression molds. The diversity of applications of ionomeric polymers is due to flexibility in their composition, matrix versatility, ionic content, the nature of counterions, and the neutralization amount. Ionomers have the potential to be used in mechanical, petrochemical, electrochemical, biotechnological, and medical fields. The most considerable applications of ionomeric polymers are in the packaging of films, coatings, and membranes in fuel cells, as well as in additives, adhesives, and polymer modifiers or catalysts [62]. The stiffness and clearness of ionomers are the main reasons for exploitation in nondurable products like food packaging and the cosmetic industry [62,77]. Ionomers can be used as additives, such as a modifier for improving coating properties. Therefore ionmade blending generally has superior properties compared to individual components. Ionomer-based bullet-proof composites made by sandwiching two layers of glass were prepared [78]. In the past decade, smart ionomer-based self-healing materials have been developed [62]. Rhaman et al. [79] investigated the healing performance of mixtures based on an ethylene/methacrylic acid ionomers copolymer with ethylene/vinyl alcohol (EVA) or epoxidized natural rubber (ENR). The healing behavior of these materials for ballistic puncture tests revealed that for neat EMNa, complete cavity closure occurred compared to pure EVA, where no healing improvement was observed. By the addition of 15 and 30 wt. % of EVA, self-healing was achieved. However, when the amount of EVA increased to

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50 wt.%, the hole was not completely healed, and only a limited repairing occurred because the crystallinity of EVA leads to higher stiffness and brittleness. In another study conducted by Baba et al. [80], the self-healing behavior of ionomeric surlyn 8940 comprising carbon fibers due to medium-velocity impact and resistive heating was investigated. Surlyn is a copolymer of ethylene/methacrylic acid (E/MAA), in which MAA groups are partly Na1-neutralized. Carbon fiber with properties of thermal resistance, electrical resistance, high strength, and low density was utilized as a heating element. SEM micrographs were obtained from the sample after a medium-velocity impact test, to determine if any damaged area remained in the specimen. In Fig. 11.6, a trace of the impact, as well as the following reconstruction of damaged Surlyn after heating at 95 C, is shown. Self-repairing systems based on ethylene methacrylic acid (EMA) ionomers having acid groups were incompletely neutralized with metal ions (Na1 and Zn21), and ENR blends have been studied [81]. The self-repairing of ENR and ionomers based on their thermal and mechanical properties was evaluated after the ballistic puncture test. ENR molecules have three different functionalities (i.e., double bonds, epoxy, and acid groups in the main chain); therefore three potential cross-links can be formed. Both ionomers containing Na and Zn ions were also blended with ENR in the presence of a free-radical generator of dicumyl peroxide (DCP) to generate different amounts of rubbery phase cross-linking, therefore to better figure out the character of the functionalized rubbery phase in the self-relief behavior of the blend. The samples, including ENR, which was cross-linked with 0.2, 0.5, and 0.8 wt.% of DCP (Fig. 11.7A and B), as well as pure EMNa and EMZn, illustrated complete healing after ballistic puncture tests (Fig. 11.7C and D). FIGURE 11.6 SEM micrographs of impacted and healed Surlyn: (A) entry path damage, (B) exit path damage, (C) entry path damage, then subsequently healed, and (D) exit path damage, then subsequently healed.

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Remarkably the self-repairing behavior of blends with the Na ion-containing ionomer is better than those containing zinc, which demonstrates fragmental healing. The OM images of the fully healed area for a variety of amounts from 15 up to 50 wt.% of ENR in EMNa/ ENR blends after bullet impact are depicted in Fig. 11.8. In contrast, the defective mending of EMZn/ENR blends containing 15, 20, 30, and 50 wt.% of ENR is illustrated in Fig. 11.9. In wounded zones of different compositions, fragile surfaces are seen, and airflow tests established the incomplete healing for blends with EMZn. Also, ionomers material can be used in sports items, such as golf balls [82]. In recent years researchers have paid much attention to this class of polymers because of their mechanical properties and their ability to provide immediate and reproducible healing of damages without any external interference [25,66]. One of the first classes of self-healing polymer ionomers is ethylene-co-methacrylic acid copolymer [63]. The elastic response of Surlyn ionomer, LLDPE, and PP polymers after striking a 9-mm disk was evaluated by optical micrographs, as shown in Fig. 11.10. Ionomers materials are used in fuel cells as proton exchange layers, for the chloralkali process, separators for redox flow batteries, ion-exchange membranes, and recently for reverse osmosis and other water purification processes [18]. Ionomers have recently been utilized to obtain shape memory polymers [83,84]. In conclusion, the major employment of ionomers is as sporting goods, membranes, polymer blends (for instance as compatibilizers), the film and packaging, coatings, and adhesives [85]. It has been claimed that PEMA ionomers can be widely used in film packaging applications because of their transparency, toughness, flexibility, oil resistance, high-level gas permeability, and low sealing temperature [86]. Pure polyethylene compounds have less

FIGURE 11.7 OM images of bullet impact exit zone of ENR cross-linked with (A) 0.2 wt.% DCP and (B) 0.5 wt.% DCP (C) neat EMNa (D) neat EMZn. DCP, Dicumylperoxide; ENR, epoxidized natural rubber.

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FIGURE 11.8 The healed bullet exit zone OMs of samples holding (A) 15 wt.%, (B) 20 wt.%, (C) 30 wt.%, and (D) 50 wt.% of ENR in EMNa/ENR blends. ENR, Epoxidized natural rubber.

FIGURE 11.9 The partially healed bullet exit zone OMs of samples holding (A) 15 wt.%, (B) 20 wt.%, (C) 30 wt.%, and (D) 50 wt.% of ENR in EMZn/ENR blends. ENR, Epoxidized natural rubber.

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FIGURE 11.10 The comparison of elastic response or shape memory of Surlyn ionomer, LLDPE, and PP in the bump region [25].

deflate resistance, less sealing range, more sensitivity to contamination, more ambiguity and less flexibility compared to ionomers [18].

11.6 Summary To summarize, ionomers are polymers consisting of up to 20 mol.% ionic groups and their counterions, which form new ionomers with different monomers and countercations. Ionomers can be prepared by direct copolymerization or postpolymerization methods. The dispersed distribution of these compounds in the main polymer chain leads to various structures, such as random, block, side chain, and attached to the end of the polymer. These smart materials are thermally reversible, and their self-repairing properties are controlled via the morphology and chemical structure of the ionomer polymers. Therefore it is expected that the healing property of ionomer polymers is repeatable several times when the polymer is exposed to thermal energy. Ionic groups undergo transitions and relaxations at specific temperatures, which reveal the mobility of the polymer network. In the molten conditions, the ionic groups remain stable; therefore the polymer can highly elongate and return to primary size elastically when the accumulated energy is discharged at the breakdown.

References [1] M. Behzadnasab, S.M. Mirabedini, K. Kabiri, S. Jamali, Corrosion performance of epoxy coatings containing silane treated ZrO2 nanoparticles on mild steel in 3.5% NaCl solution, Corros. Sci. 53 (1) (2011) 89 98. [2] M. Sabzi, S.M. Mirabedini, J. Zohuriaan-Mehr, M. Atai, Surface modification of TiO2 nano-particles with silane coupling agent and investigation of its effect on the properties of polyurethane composite coating, Prog. Org. Coat. 65 (2) (2009) 222 228. [3] N. Sottos, S. White, I. Bond, Introduction: self-healing polymers and composites, J. R. Soc. Interface 4 (13) (2007) 347 348. [4] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794. [5] E.N. Brown, R.W. Scott, R.S. Nancy, Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite—part II: in situ self-healing, Compos. Sci. Technol. 65 (15-16) (2005) 2474 2480. [6] E.N. Brown, R.S. Nancy, R.W. Scott, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (4) (2002) 372 379. [7] C.D. Ding, J.H. Xu, L. Tong, G.C. Gong, W. Jiang, J. Fu, Design and fabrication of a novel stimulus-feedback anticorrosion coating featured by rapid self-healing functionality for the protection of magnesium alloy, ACS Appl. Mater. Interfaces 9 (24) (2017) 21034 21047. [8] M.K. Wiliams, T.L. Gibson, S.T. Jolley, A.J. Caraccio-Meier, Self-healing technologies healing technologies for wiring and surfaces in aerospace and deep space exploration applications, in: Smart Coatings Conference, Orlanda, FL, 2017.

Self-Healing Polymer-Based Systems

References

289

[9] E.B. Murphy, F. Wudl, The world of smart healable materials, Prog. Polym. Sci. 35 (1-2) (2010) 223 251. [10] R.S. Trask, G.J. Williams, I.P. Bond, Bioinspired self-healing of advanced composite structures using hollow glass fibres, J. R. Soc. Interface 4 (2007) 363 371. [11] T.C. Mauldin, M.R. Kessler, Self-healing polymers and composites, Int. Mater. Rev. 55 (6) (2010) 317 346. [12] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (8) (2007) 581. [13] B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, Self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (2010) 179 211. [14] X. Liu, X. Sheng, J.K. Lee, M.R. Kessler, Synthesis and characterization of melamine-urea-formaldehyde microcapsules containing ENB-based self-healing agents, Macromol. Mater. Eng. 294 (6-7) (2009) 389 395. [15] J.K. Lee, S.J. Hong, X. Liu, S.H. Yoon, Characterization of dicyclopentadiene and 5-ethylidene-2-norbornene as self-healing agents for polymer composite and its microcapsules, Macromol. Res. 12 (5) (2004) 478 483. [16] C. Suryanarayana, K.C. Rao, D. Kumar, Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings, Prog. Org. Coat. 63 (1) (2008) 72 78. [17] L. Yuan, G. Liang, J.Q. Xie, L. Li, J. Guo, Preparation and characterization of poly (urea-formaldehyde) microcapsules filled with epoxy resins, Polymer 47 (15) (2006) 5338 5349. [18] L. Zhang, R.B. Nicole, K.A. Cavicchi, R.A. Weiss, Perspective: ionomer research and applications, Macromol. React. Eng. 8 (2) (2014) 81 99. [19] ,http://www.dupont.com/products-and-services/plastics-polymersresins/ethylenecopolymers/brands/ surlyn-ionomer-resin.html. (accessed 04.07.17). [20] R.W. Rees, Ionomers: a new thermoplastic, Mod. Plast. 42 (1964) 98 102. [21] R.W. Rees, D.J. Vaughan, Physical Structure of Ionomers, Polymer Preprints, American Chemical Society, Division of Polymer Chemistry, vol. 6, 1965, p. 287. [22] C.A. Finch, Ionic polymers, Br. Polym. J. 7 (1975) 275-276. https://doi.org/10.1002/pi.4980070408. [23] F.C. Wilson, R. Longworth, D.J. Vaughan. Polymer Preprints, American Chemical Society, Division of Polymer Chemistry, vol. 9, 1968, p. 505. [24] K. Jr Stephen, T.C. Ward, Thermal characteristics of the self-healing response in poly (ethylene-comethacrylic acid) copolymers, J. R. Soc. Interface 4 (13) (2006) 405 411. [25] R.J. Varley, Svd Zwaag, Towards an understanding of thermally activated self-healing of an ionomer system during ballistic penetration, Acta Mater. 56 (19) (2008) 5737 5750. [26] A.M. Grande, L. Castelnovo, L.D. Landro, C. Giacomuzzo, A. Francesconi, M.A. Rahman, Rate-dependent self-healing behavior of an ethylene-co-methacrylic acid ionomer under high-energy impact conditions, J. Appl. Polym. Sci. 130 (3) (2013) 1949 1958. [27] B. Hird, A. Eisenberg, Phase dominance, multiple glass transitions, and apparent activation energies in some poly (styrene-co-alkali methacrylate) ionomers, J. Polym. Sci. Part B: Polym. Phys. 28 (10) (1990) 1665 1675. [28] B. Wang, G. Wan, J. Zhao, Y. Feng, J. Yin, Synthesis and mechanical properties of chlorinated polyvinyl chloride (CPVC) carboxylate ionomer, Polym. Technol. Eng. 47 (11) (2008) 1101 1104. [29] Y. Shu, X.-W. Chen, J.-H. Wang, Ionic liquid polyvinyl chloride ionomer for highly selective isolation of basic proteins, Talanta 81 (1-2) (2010) 637 642. [30] H.-Q. Xie, Y. Chen, W. Yang, D. Xie, Synthesis of sulfonated rubber ionomers by the ring-opening reaction of epoxidized styrene butadiene rubber, their characterization, and their properties, J. Appl. Polym. Sci. 101 (5) (2006) 3090 3096. [31] R.K. Bose, N. Hohlbein, J.G. Santiago, A.M. Schmidt, Svd Zwaag, Connecting supramolecular bond lifetime and network mobility for scratch healing in poly (butyl acrylate) ionomers containing sodium, zinc and cobalt, Phys. Chem. Chem. Phys. 17 (3) (2015) 1697 1704. [32] R.K. Bose, N. Hohlbein, J.G. Santiago, A.M. Schmidt, Svd Zwaag, Relationship between the network dynamics, supramolecular relaxation time and healing kinetics of cobalt poly (butyl acrylate) ionomers, Polymer 69 (2015) 228 232. [33] I.A. Nyrkova, R.K. Alexei, M. Doi, Microdomains in block copolymers and multiplets in ionomers: Parallels in behavior, Macromolecules 26 (14) (1993) 3601 3610. [34] A.N. Semenov, I.A. Nyrkova, A.R. Khokhlov, Polymers with strongly interacting groups: Theory for nonspherical multiplets, Macromolecules 28 (22) (1995) 7491 7500. [35] R.A. Register, R.K. Prud’Homme, Melt rheology, in: M.R. Tant, K.A. Mauritz, G.L. Wilkes (Eds.), Ionomers: Synthesis, Structure, Properties and Applications, Chapman and Hall, New York, 1997, pp. 208 260.

Self-Healing Polymer-Based Systems

290

11. Ionomers as self-healing materials

[36] C.E. Williams, C. Colliex, J. Horrion, R. Jerome, Multiphase polymers: blends and ionomers, in: L.A. Utracki, R.A. Weiss (Eds.), ACS Symposium Series, vol. 395, American Chemical Society, 1989, pp. 439 445. [37] B.P. Kirkmeyer, A.W. Robert, I.W. Karen, Spherical and vesicular ionic aggregates in Zn-neutralized sulfonated polystyrene ionomers, J. Polym. Sci. Part B: Polym. Phys. 39 (5) (2001) 477 483. [38] D.J. Yarusso, L.C. Stuart, Microstructure of ionomers: interpretation of small-angle x-ray scattering data, Macromolecules 16 (12) (1983) 1871 1880. [39] M. Bouix, J. Gouzi, B. Charleux, J.-P. Vairon, P. Guinot, Synthesis of amphiphilic polyelectrolyte block copolymers using living radical polymerization. Application as stabilizers in emulsion polymerization, Macromol. Rapid Commun. 19 (4) (1998) 209 213. [40] T.P. Davis, Handbook of Radical Polymerization, Wiley-Interscience, 2002. [41] B. Kirchner, Ionic liquids from theoretical investigations, In Ionic liquids, Springer, Berlin, Heidelberg, 2008, pp. 213 262. [42] H. Okamura, Y. Takatori, M. Tsunooka, M. Shirai, Synthesis of random and block copolymers of styrene and styrenesulfonic acid with low polydispersity using nitroxide-mediated living radical polymerization technique, Polymer 43 (11) (2002) 3155 3162. [43] B.S. Aitken, M. Lee, M.T. Hunley, H.W. Gibson, K.B. Wagener, Synthesis of precision ionic polyolefins derived from ionic liquids, Macromolecules 43 (4) (2010) 1699 1701. [44] S. Cheng, L.B. Frederick, B.D. Mather, R.B. Moore, T.E. Long, Phosphonium-containing ABA triblock copolymers: controlled free radical polymerization of phosphonium ionic liquids, Macromolecules 44 (16) (2011) 6509 6517. [45] H. Chen, J.-H. Choi, D.S.-dl Cruz, K.I. Winey, Y.A. Elabd, Polymerized ionic liquids: the effect of random copolymer composition on ion conduction, Macromolecules 42 (13) (2009) 4809 4816. [46] S. Tungchaiwattana, M.S. Musa, J. Yan, P.A. Lovell, P. Shaw, B.R. Saunders, The role of acrylonitrile in controlling the structure and properties of nanostructured ionomer films, Soft matter 10 (26) (2014) 4725 4734. [47] J.E. Jenkins, M.E. Seitz, C.F. Buitrago, K.I. Winey, K.L. Opper, T.W. Baughman, et al., The impact of zinc neutralization on the structure and dynamics of precise polyethylene acrylic acid ionomers: a solid-state 13C NMR study, Polymer 53 (18) (2012) 3917 3927. [48] C.W. Lantman, W.J. MacKnight, R.D. Lundberg, Structural properties of ionomers, Annu. Rev. Mater. Sci. 19 (1) (1989) 295 317. [49] R.D. Lundberg, H.S. Makowski, A comparison of sulfonate and carboxylate ionomers, Ions Polym. 187 (1980) 21 36. [50] R. Neumann, W.J. MacKnight, R.D. Lundberg, Dielectric and mechanical studies of sulfonate ionomers, Am. Chem. Soc. 19 (1978) 298 303. [51] H.S. Makowski, R.D. Lundberg, L. Westerman, J. Bock, Plasticization of metal sulfonate containing Epdm with fatty-acid derivatives, Polym. Prep, 176 (1978) 304-309. [52] B. Siadat, R.D. Lundberg, R.W. Lenz, Preparation of an ionomer elastomer by continuous sulfonation in an extruder and neutralization in static mixers, Polym. Eng. Sci. 20 (8) (1980) 530 534. [53] W.J. MacKnight, R.D. Lundberg, Elastomeric ionomers, Rubber Chem. Technol. 57 (3) (1984) 652 663. [54] R.D. Lundberg, The solution behavior of selected ionic polymers, Polym. Prep. ACS Div. Polym. Chem. 19 (1978) 455. [55] A. Eisenberg, M. Navratil, Ion clustering and viscoelastic relaxation in styrene-based ionomers. II. Effect of ion concentration, Macromolecules 6 (4) (1973) 604 612. [56] R.A. Weiss, S.R. Turner, R.D. Lundberg, Sulfonated polystyrene ionomers prepared by emulsion copolymerization of styrene and sodium styrene sulfonate, J. Polym. Sci.: Polym. Chem. Ed. 23 (2) (1985) 525 533. [57] S.R. Turner, R.A. Weiss, R.D. Lundberg, The emulsion copolymerization of styrene and sodium styrene sulfonate, J. Polym. Sci.: Polym. Chem. Ed. 23 (2) (1985) 535-548. [58] ,https://pslc.ws/macrog/ionomer.htm.. [59] Y. Mohajer, S. Bagrodia, G.L. Wilkes, R.F. Storey, J.P. Kennedy, New polyisobutylene-based model elastomeric ionomers. VI. The effect of excess neutralizing agents on solid-state mechanical properties, J. Appl. Polym. Sci. 29 (6) (1984) 1943 1950. [60] B.C. Johnson, I. Yilgo¨r, C. Tran, M. Iqbal, J.P. Wightman, D.R. Lloyd, et al., Synthesis and characterization of sulfonated poly (acrylene ether sulfones), J. Polym. Sci.: Polym. Chem. Ed. 22 (3) (1984) 721 737. [61] J.F. O’Gara, D.J. Williams, W.J. Macknight, F.E. Karasz, Random homogeneous sodium sulfonate polysulfone ionomers: preparation, characterization, and blend studies, J. Polym. Sci. Part B: Polym. Phys. 25 (7) (1987) 1519 1536.

Self-Healing Polymer-Based Systems

References

291

[62] W.H. Binder (Ed.), Self-Healing Polymers: From Principles to Applications, John Wiley & Sons, 2013. [63] A. Grande, Self-healing Ionomer Based Systems for Aerospace Applications, Ph.D. Dissertation, Italy, 2014. [64] K. Jr, Stephen, T.C. Ward, Z. Oyetunji, Self-healing of poly (ethylene-co-methacrylic acid) copolymers following projectile puncture, Mech. Adv. Mater. Struct. 14 (5) (2007) 391 397. [65] D. Jan, R.K. Bose, S. Zechel, S.J. Garcia, Svder Zwaag, et al., A new approach toward metal-free self-healing ionomers based on phosphate and methacrylate containing copolymers, Macromol. Chem. Phys. 218 (23) (2017) 1700340. [66] R. Varley, Ionomers as self healing polymers, in: Self Healing Materials, Springer, Dordrecht, 2007, pp. 95 114. [67] W. Deng, Y. You, A. Zhang, Supramolecular network-based self-healing polymer materials, in: Recent Advances in Smart Self-healing Polymers and Composites, Woodhead Publishing, 2015, pp. 181 210. [68] S.J. Kalista, Self-healing of thermoplastic poly (ethylene-comethacrylic acid) copolymers following projectile puncture, Master of Science, Virginia Polytechnic Institute and State University, Virginia, USA, 2003. [69] R.J. Varley, S. Shen, S. van der Zwaag, The effect of cluster plasticisation on the self healing behaviour of ionomers, Polymer 51 (3) (2010) 679 686. [70] R.J. Varley, S. van der Zwaag, An investigation of the self-healing mechanism of ionomer based system. in: Proceedings of the First International Conference on Self-Healing Materials, Noordwijk aan Zee, The Netherlands, 2007. [71] N. Hohlbein, T. Pelzer, J. Nothacker, M. Von Tapavicza, A. Nellesen, H. Datta et al. Schmidt, Self-healing processes in ionomeric elastomers, In ICSHM 2013: Proceedings of the 4th international conference on selfhealing materials, Ghent, Belgium, 2013, pp. 16-20. [72] A. Shaaban, N. Hohlbein, A.M. Schmidt, Heal-on-demand by Local Heating in Ionomeric Nanocomposites, in: ICSHM 2015: the 5th international conference on self-healing materials, Duke University, Durham NC, USA, 22-25 June, 2015. [73] R.K. Shah, D.L. Hunter, D.R. Paul, Nanocomposites from poly (ethylene-co-methacrylic acid) ionomers: effect of surfactant structure on morphology and properties, Polymer 46 (8) (2005) 2646 2662. [74] T. Tomkovic, G.H. Savvas, Nonlinear rheology of poly (ethylene-co-methacrylic acid) ionomers, J. Rheology 62 (6) (2018) 1319 1329. [75] S.J. Kalista, R.P. John, J.V. Russell, Effect of ionic content on ballistic self-healing in EMAA copolymers and ionomers, Polym. Chem. 4 (18) (2013) 4910 4926. [76] S.J. Garcia, H.R. Fischer, Self-healing polymer systems: properties, synthesis and applications, in: Smart Polymers and their Applications, Woodhead Publishing, 2014, pp. 271 298. [77] L.A. Utracki, History of commercial polymer alloys and blends (from a perspective of the patent literature), Polym. Eng. Sci. 35 (1) (1995) 2 17. [78] W.N. Smith, N. Bolton, Ionomer Resin Films and Laminates Thereof, U.S. Patent 5,763,062, issued June 9, 1998. [79] M.A. Rhaman, M. Penco, G. Spagnoli, A.M. Grande, L.D. Landro, Self-healing behavior of blends based on ionomers with ethylene/vinyl alcohol copolymer or epoxidized natural rubber, Macromol. Mater. Eng. 296 (12) (2011) 1119 1127. [80] S.V. Baba, A. Morgan, M. Castellucci, Self-healing of ionomeric polymers with carbon fibers from mediumvelocity impact and resistive heating, Smart Mater. Res. 2013 (() (2013). [81] R.M. Arifur, M. Penco, I. Peroni, G. Ramorino, A.M. Grande, L.D. Landro, Self-repairing systems based on ionomers and epoxidized natural rubber blends, ACS Appl. Mater. Interface 3 (12) (2011) 4865 4874. [82] J. Chen, Stearic-Modified Ionomers for Golf Balls, U.S. Patent Application 10/315,526, filed August 21, 2003. [83] D. Hager Martin, S. Bode, C. Weber, U.S. Schubert, Shape memory polymers: past, present and future developments, Prog. Polym. Sci. 49 (2015) 3 33. [84] J. Zhang, M. Huo, M. Li, T. Li, N. Li, J. Zhou, et al., Shape memory and self-healing materials from supramolecular block polymers, Polymer 134 (2018) 35 43. [85] R.D. Lundberg, Elastomers and fluid applications, in: M.R. Tant, K.A. Mauritz, G.L. Wilkes (Eds.), Ionomers: Synthesis, Structure, Properties and Application, Blackie Academic Press, London, 1997 (Chapter 12). [86] B.A. Morris, New developments in ionomer technology for film applications, J. Plast. Film Sheet. 23 (2) (2007) 97 108.

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C H A P T E R

12 Self-healing materials utilizing supramolecular interactions James F. Reuther, Randall A. Scanga, Ali Shahrokhina and Priyanka Biswas Department of Chemistry, University of Massachusetts Lowell, MA, United States

12.1 Intrinsic self-healing systems Today, synthetic polymers have become fully integrated into the everyday lives of human beings owing to their attractive, chemically tunable mechanical properties and relatively low cost of development and utilization. Due to the excellent chemical and thermal stability of polymeric materials, the global production of plastics has shown steady, exponential growth over the past 30 years to an estimated 322 million metric tons per year in 2015 owing to their wide-applicability [1]. Their hallmark stability, however, has led to the truly unfortunate realization that disposal of these plastic materials is not trivial leading to floods of plastic consumables occupying landfills around the world by the billions of tons [2]. A constant, resonating question now exists of what to do once a plastic material has served its purpose, experiences mechanical failure, and/or loses specific properties (i.e., degradation) that give the material its utility? Recycling plastics has become common place among communities around the world, but, due to poor properties of remanufactured materials, their applications are limited resulting in a meager 9% of waste plastics actually being incorporated in recycled material-based products [1]. Some recent research has begun to turn the tides [3,4], but much still must be done from a scientific standpoint to better produce polymeric materials that are readily reusable. Mechanical failure of synthetic materials through damage, degradation, or other means is an inevitability that cannot be circumvented. Many polymeric materials are developed and engineered to be placed under great deals of mechanical stress such as high shear, flow, impact, pressure, temperature, etc. These types of stresses will ultimately result in material failure at some point over extended periods of time. Thus there is an obvious need for polymers that can quantitatively recover or heal mechanical strength and thermal

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properties after these types of mechanical stress yield overall failure within the material. This type of theoretical material is rapidly becoming a reality with a large research thrust growing in the area of self-healing materials. Biological materials such as tissue and bone are naturally occurring examples of selfhealing materials whose repair mechanisms have not been truly reproduced synthetically. For a wound to heal, an organism typically goes through a three-step process including the initial inflammatory response (e.g., blood clotting), followed by cell proliferation, and biological matrix remodeling [5]. The healing mechanism for synthetic, self-healing materials varies but, generally, fit into three different categories. These include capsule-based, vascular-based, and intrinsic self-healing materials. For the purposes of this chapter, we will only focus on intrinsic self-healing materials. Intrinsic self-healing refers to repair mechanisms that are initiated through reversible bond breakage and reformation. In many cases, these polymeric materials are cross-linked using reversible interactions such as metal-coordination [6
8], electrostatic interactions [9], and/or dynamic supramolecular bonds [10
12] (Fig. 12.1). The break point during a mechanical failure event typically occurs at the weakest bond within a network which, in the case of intrinsic self-healing materials, is the dynamic cross-links. When these bonds are broken, due to their thermodynamically and kinetically dynamic nature, they can be reformed under specific conditions to repair cracks and tears in bulk materials inducing reentanglement of polymer chains along the break point to reinitiate mechanical strength. The true limitation of intrinsic self-healing materials are the relatively low damage volume that can be healed in after mechanical failure. Intrinsic self-healing materials do,

FIGURE 12.1 Depiction of intrinsic self-healing through various types of reversible bond formation including metal-coordination, electrostatic interactions, and supramolecular/dynamic covalent (DC) bonds.

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however, have many advantages including theoretically unlimited repair cycles possible unlike capsule and vascular-based healing mechanisms which can only be regenerated once. Additionally some specific intrinsic self-healing materials have been shown to fully recover mechanical properties after multiple break
heal cycles [13,14]. Additionally healing can be controlled through specific triggers such as thermal or photo-responsive curing of dynamic bonding moieties to initiate healing only at desired times. Before diving into the different intrinsic self-healing materials, it is first important to review the different types of supramolecular interactions that make them possible.

12.1.1 Supramolecular bonds Interest in research areas of supramolecular chemistry skyrocketed following the awarding of the 1987 Nobel Prize in Chemistry to Jean-Marie Lehn, Donald Cram, and Charles Pederson for their contributions developing “molecules with structure-specific interactions of high selectivity.” [15] Today, supramolecular chemistry has become its own subdivision of chemical research and encompasses a suite of different, noncovalent interactions such as metal ion coordination, hydrogen bonding, host
guest complexation, electrostatic interactions, halogen bonding, pi
pi interactions, and hydrophobic interactions (i.e., van der Walls forces). Additionally dynamic covalent chemistry is often included when discussing supramolecular chemistry due to the kinetic dynamicity of the covalent bonds under specific conditions allowing for triggered on
off states of covalent interactions. Before describing the utilization of supramolecular bonding in self-healing materials, it is important to first briefly discuss some of the different classes of these intermolecular interactions and how they have evolved over the years.

12.1.2 Hydrogen bonding Controlling the assembly of molecules via supramolecular chemistry was first imagined through bio-inspiration where natural biomolecules and biomacromolecules in nature utilize various intermolecular interactions to exquisitely fold and assemble imparting unique, selective functions such as catalysis [16], ion gating [17], and molecular motion [18], to name a few. The benchmark assembly of nucleotide-based biopolymers are the orthogonal hydrogen bonding complex formed from complementary Watson-Crick base-pairs essential for deoxyribonucleic acid and ribonucleic acid double helical assembly. Synthetically chemists have long studied hydrogen bond association and strengths of such interactions can range greatly from 0.2 to 39 kcal mol21 in gas-phase calculations [19]. These intermolecular interactions are most typically between partially positive, polar covalent hydrogen atoms attached to heteroatoms such as nitrogen or oxygen (i.e., hydrogen bond donors) and lone pair containing heteroatoms capable of donating electron density (i.e., hydrogen bond acceptors). Through molecular design, the typically weak association of hydrogen bonds can be made much stronger through installation of multiple H-bonds working in unison [20]. One of the most studied is the quadruple H-bonding ureido-pyrimidone (UPy) functionalities popularized by Meijer and coworkers [21]. Utilizing such designed intermolecular

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interactions, complex supramolecular assemblies can be constructed synthetically including folded polymer structures, engineered cocrystals, linear supramolecular polymers, and dynamic network materials [20]. In Section 12.7, we discuss the utilization of hydrogen bonding for the development of supramolecular, self-healing materials.

12.1.3 Metal coordination Organic ligand
metal coordination chemistry (i.e., organometallic chemistry) has reshaped catalytic synthetic chemistry over the past century leading to unique, high-yielding, atom-efficient chemical transformations that hold massive utility in organic, inorganic, and polymer chemistries. Though organo
metal complexes had been observed before, the field of organometallic chemistry as a whole was truly born in the early 1900s when Victor Grignard discovered organo
magnesium metal-carbonanion coordination complexes; reagents that still bear his name and continue to find utility in modern synthetic chemistry [22,23]. The impact of organometallic chemistry is exemplified by the number of Nobel Prizes awarded for research in this field starting the 1912 award for chemistry to Victor Grignard. Over the past 50 1 years, 6 additional Nobel Prizes have been presented for research in organometallic chemistry and catalysis, with the most recent award going to Heck, Negishi, and Suzuki for palladium-catalyzed cross-coupling reactions in 2010. Organometallic chemistry has greatly impacted polymer and material science as well with influence ranging from controlled supramolecular assembly of macromolecules using directional coordination bonds (e.g., metal organic frameworks) [24,25] to polymer synthesis with unprecedented level of control over molecular weight (MW; e.g., atom transfer radical polymerization) [26] and tacticity (e.g., Ziegler
Natta polymerizations) [27]. Herein, we describe the use of organometallic, coordination species in the development of self-healing materials. The dynamic nature of metal
ligand bonds often makes them ideal fits for self-repair on material failure. This topic will be reviewed in detail in section 12.3.

12.1.4 Host
guest interactions One of the most unique supramolecular interaction is that of noncovalent binding of “guest” molecules within “host” cavities forming what is known as “host
guest” complexes. Typically host
guest interactions encompass a range of different supramolecular interactions including hydrogen bonding, van der Walls forces, and hydrophobic and electrostatic interactions [28,29]. Such interactions have attracted much attention from researchers from a molecular recognition stand-point opening up many applications in biological, chemical, and environmental sensing [30
32]. One of the most popular sensing mechanisms of host
guest systems, popularized by Anslyn and coworkers, is known as an indicator displacement assay which takes advantage of optical responses on competitive binding of a specific analyte (i.e., biologically relevant molecule, environmental contaminant, or dangerous chemical) that displaces an optical reporter or “indicator” from its bound state within the host [33]. On removal, the optical reporter will typically experience a “turn-on” in absorption/fluorescence leading to quantitative sensing of the analyte in question.

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Host
guest chemistry has also proved useful in polymer science allowing for dynamic, stimuli
responsive behavior in a range of materials. One of the most notable examples involves host
guest interactions between poly(ethylene glycol) (PEG) and β-cyclodextrin (β-CD) in the preparation of slide-ring hydrogels [34]. These types of materials contain covalent cross-linked junctions at host
guest sites allowing for significantly enhanced deformation from mechanical stress due to free movement of β-CD along the high MW PEG chain. Furthermore, the dynamic, tunable nature of host
guest interactions offers a potential route to self-repair leading to the development of a variety of self-healing materials utilizing various host
guest complexes. These materials will be discussed in detail in section 12.5.

12.1.5 Dynamic covalent chemistry Supramolecular chemistry encompasses a suite of noncovalent interactions with one exception being specific covalent bonds that are kinetically dynamic at ambient conditions or on response to specific stimuli. Such dynamic covalent bonds (DCBs) are unique because they combine the reversibility of supramolecular bonds with the robust mechanical properties of covalent bonds making them ideal for dynamic network materials [35,36]. Many DCBs show true orthogonality [37] allowing for the introduction of multiple dissimilar DCBs into a single system without cross-reactivity between noncomplementary DCB pairs providing multi-responsiveness to a range of external stimuli such as pH, heat, light, and redox responses. In section 12.6, we will discuss in detail how the use of DCBs have impacted the field of self-healing materials.

12.2 Main-chain supramolecular polymers Conventionally polymers are comprised of covalent bonds linking many repeating units through chemical reactions with small molecule monomers. Traditionally these covalent bonds are static and irreversible leading to difficulties in reconfiguring materials once set. Noncovalent interactions, conversely are weaker and reversible allowing for potential reconfigurability and repair. Incorporating noncovalent interactions into polymer backbones in turn, offers a route self-healing polymeric material. The inherent reversibility of bonding in the polymer matrix gives supramolecular polymers the ability to repair themselves after rupture [38]. We can distinguish self-healable polymeric materials into two broad categories based on whether an external stimulus [39,40] is needed to assist the healing process. Autonomous self-healable materials can recover their physical and/or mechanical properties after damage without any external involvement. Whereas, stimuli
responsive selfhealable materials need the specific external stimulus (e.g., light, heat, and pH) to recover their physical and/or mechanical properties. In both cases, the healable polymers must be able to reform multiple bonds in and around the ruptured area by utilizing the constituents from within its original polymer structure. Several distinguishable approaches have been used to address this challenge including the encapsulated monomer approach,

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reversible covalent bond formation, and supramolecular bond formation. In this section, we provide an overview of the utilization of main-chain supramolecular polymers for different material applications [41]. When the covalent interactions in the monomeric units of a polymer are substituted with directional noncovalent interactions, main-chain supramolecular polymers are formed [42
46]. As the repeat units in main chain supramolecular polymers are reversibly associated, they can show specific stimuli
responsiveness allowing for selective polymerization
depolymerization equilibrium triggered on demand [47].

12.2.1 Supramolecular polymerizations Supramolecular polymers can be prepared by the polymerization of monomers having two or more functional groups that can interact in a supramolecular fashion. Supramolecular polymerizations are generally reversible thus implying that depolymerization cannot be neglected. This section is mainly dedicated to linear main-chain supramolecular systems which can be divided into three important polymerization mechanisms. Herein, we will detail these different types of supramolecular polymerizations including isodesmic supramolecular polymerization (ISP), ring-chain mediated supramolecular polymerization and cooperative supramolecular polymerization (CSP) [48].

12.2.2 Isodesmic supramolecular polymerization Also known as multistage open association [42], ISP can be identified by formation of one identical, noncovalent bond between monomers first forming oligomers and later polymers, analogous to step-growth polymerizations with covalent bonds. The word isodesmic comes from two Greek words iso meaning equal and desmos meaning bond. The reactive moieties of the monomers, oligomers, and polymer have equal reactivity and hence the bonds formed are identical throughout the whole polymer backbone. The equilibrium constant for each step (K) is also independent of the chain length. Both bifunctional, rod-like, or flexible monomers can be used to form a linear supramolecular polymer as shown in the Fig. 12.2 [49]. Different types of monomers, such as single monomers with self-complementary interactions or bifunctional monomers, with two binding groups connected with a spacer, have been utilized. Since supramolecular polymers formed contain relatively weak bonds linking monomer units, the accurate determination of their MW is difficult. Commonly used analytical methods for conventional polymers such as matrix-assisted laser desorptionionization time of flight mass spectroscopy (MALDI-TOF MS) and size exclusion chromatography (SEC) prove largely ineffective for MW characterization. For MALDI-TOF MS,

FIGURE 12.2 Schematic representation of isodesmic supramolecular polymerization of a monomer into a linear supramolecular polymer [49].

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fragmentation of the weak monomer linkages during ionization skew results. For SEC, significant dilution is required for characterization which will affect the equilibrium constant between high MW polymers and bifunctional monomers. Alternatively spectroscopic techniques [such as nuclear magnetic resonance (NMR)], isothermal calorimetry and analytical ultracentrifugation can be used to calculate the MW of supramolecular polymers [48,49]. Moore and coworkers have significantly contributed to the field of ISP by studying the supramolecular organizations of arylene
ethynylene macrocycles (AEMs). Having large aromatic surfaces and a rigid backbone, AEMs have a propensity to stack in solution by van der Waals and pi
pi interactions. Supramolecular polymers can be formed by the macrocycles as shown in the Fig. 12.3 below [50,51].

12.2.3 Ring-chain supramolecular polymerization Ring-chain supramolecular polymerization can be loosely defined as the reversible polymerization of a bi-functional, flexible monomers in which every linear unit (including the monomers and oligomers) in the polymerization process is in equilibrium with its cyclic analog [49]. Two types of reactions are prevalent in this type of polymerization: intramolecular reactions of the end groups forming a macrocycle and intermolecular reaction forming linear oligomers as depicted in Fig. 12.4. It is crucial to know the concept of effective molarity (EM) in ring-chain supramolecular polymerization. EM is the ratio of the intramolecular and intermolecular equilibrium constants. Linear chains are favored when the value of EM is less than 1 and cyclization is favored when EM is greater than 1 [48]. Stoddart and coworkers have studied the formation of pseudorotaxane supramolecules in the ring-chain supramolecular polymerization of crown ether derivatives with positively charged amines. These crown ether derivatives can be regarded as a single, self-

FIGURE 12.3 Chemical structure of arylene
ethylene macrocycle (A) and packing of three layers of H-bonded aggregates of macrocycles in a layered sequence of close cubic packing (B) [51].

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FIGURE 12.4 Schematic representation of a ring-chain polymerization where Kinter represents the intermolecular constant for bimolecular association and Kintra represents the intramolecular ring constant for ring closure.

FIGURE 12.5 Secondary amine functionalized 24-crown-8 macrocycle (A) is utilized for pH responsive supramolecular polymerization forming mixtures of linear oligomers [a1]
[a3] and intermolecular cages [c1]
[c3] (B) [52]. Source: Reprinted from S.J. Cantrill, G.J. Youn, J.F. Stoddart, D.J. Williams, Supramolecular daisy chains, J. Org. Chem. 66 (21) (2001) 6857
6872 with permission from American Chemical Society; Copyright 2001.

complimentary monomer which can self-assemble both cyclic and acyclic superstructures (Fig. 12.5) [52].

12.2.4 Cooperative supramolecular polymerization Cooperative supramolecular polymerization (CSP) and ISP initially appear similar with monomers linking together to form oligomers [48]. However, contrary to ISP, CSPs have at least two equilibrium constants associated with polymerization and depolymerization rather than one for ISP [49]. The most common form of CSP is known as nucleation
elongation polymerization (NEP) [48,50]. NEP is characterized using two equilibrium constants; one for nucleation (KN) and one for chain elongation (KE). Those equilibrium

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constants are cooperative in that one reinforces the other (i.e., as KN increases, so does KE) as shown in Fig. 12.6. The majority of supramolecular polymers known today self-assemble in a fixed supramolecular structure whose chains are dynamic, with the local structure constrained by the relative arrangements of the interacting functional groups. An example of CSP utilizes bisurea-based monomers that form H-bonded supramolecular polymers with two unique structures. The bis-urea monomer shown in Fig. 12.7 is proven, by small angle neutron scattering, to self-assemble in a cooperative manner (i.e., one molecular assembly reinforces another) into two long, rod-like supramolecular structures recognized as filament or tube structures depending on the solvent and temperature [53]. The origin of the cooperativity comes from the polarization of the urea functional groups after the formation of Hbonded dimers. The filament structure is formed when the chains of monomers are linked by hydrogen bonds and arranged in such a manner that the cross-section of the filament contains a single monomer. Whereas, the tube structure has 2.2 times larger measured packing density which implies that they are two or three times thicker than the filament structures and the cross section contains two or three monomers [53].

FIGURE 12.6 Schematic representation of cooperative supramolecular polymerization (CSP) with two equilibrium constants associated with nucleation (KN) and chain elongation (KE) [49].

FIGURE 12.7

Formation of filament (left) and tube (right) structures through the CSP of the bis-urea monomer shown [53]. CSP, Cooperative supramolecular polymerization.

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12.2.5 Directional noncovalent interactions Noncovalent interactions include, but are not limited to, hydrogen bonding [54], metal
ligand coordination [55,56], and host
guest interactions [57]. The way in which a supramolecular unit coheres is significant as there are various means in which binding can take place between two units. These different types of interactions are illustrated in Fig. 12.8. These binding mechanisms are important to understand as it will help to develop a polymer structure fully utilizing the discrete noncovalent interactions used [47]. Self-complementary one-component and complementary two-component binding are the most common types of noncovalent interactions used to form supramolecular polymers. To understand these two mechanisms better, de Greef and coworkers have demonstrated the hydrogen bond-mediated A
A homocoupling and A
B heterocoupling are of two types of monomers. Methyl substituted 2-UPy and amino substituted UPy are used as self-complimentary A type of monomers which can reversibly associate with itself or 2,7diamido-1,8 naphthyridine (NaPy) monomers (B). These associations display concentration dependent selectivity alternating between A
B and A
A complexation behavior of UPy units. Above a concentration of 0.1 M, UPy
NaPy heterocomplexation is favored over UPy
UPy homodimerization by a factor of greater than 20:1. This A
B type of supramolecular polymerization is based on the strong interaction between the self-dimerizing UPy and NaPy units and is demonstrated in Fig. 12.9 [58]. Hydrogen bonds hold a notable place supramolecular polymer chemistry because of their directionality and versatility, although they are not the strongest among all noncovalent interactions [59]. Very stable complexes can be formed when combining multiple Hbonds cooperatively magnifying the strength and specificity of the interactions [59]. The first main-chain supramolecular polymer network (SPN) based on hydrogen bonding was reported by Lehn et al. [47]. Later, Meijer et al. reported the application of UPy hydrogen bonding units to form supramolecular polymers [54]. UPy molecules contain selfcomplementary quadrupole hydrogen bonding units that dimerize in a stable but reversible manner. These characteristics of UPy make it suitable for development of commercial supramolecular materials. The Dutch company SupraPolix BV has developed a commercial UPy polymer (trade name SupraB). SupraB, like UPy-based plastics, can be more durable than conventional plastics due to their self-healing properties [60]. Hydrogen-bonded supramolecular networks have been explored in single phase thermoset materials including work by Leibler et al. [12], that detailed the use of fatty acid FIGURE 12.8 Different classifications of noncovalent interactions used to develop supramolecular polymers: (A) self-complementary one-component binding, (B) complementary two-component binding, and (C) complementary three-component binding [47].

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FIGURE 12.9

Equilibrium between two common supramolecular moieties used in supramolecular polymerizations: equilibrium between methyl-substituted UPy dimer and 2,7-diamido-1,8-naphthyridine (top); equilibrium between dibutylamino-substituted UPy dimer and 2,7-diamido-1,8-naphthyridine (bottom) [58]. UPy, Ureidopyrimidinone.

UPy derivatives to form supramolecular networks. These molecules are associated together via hydrogen bonding interactions to form supramolecular cross-links. The resulting thermo-responsive rubbers formed using supramolecular assembly displayed selfhealing properties due to the reversible association of the UPy groups at break points. Unlike the conventional cross-linked rubbers, these systems when cut could be repaired by bringing together the ruptured surfaces at room temperature [12]. Meijer and coworkers have reported a synthetic methodology for functionalizing hydroxyl-terminated, telechelic poly(ethylene/butylene) with UPy functionalities linked with a reactive isocyanate group (Fig. 12.10). Poly(ethylene/butylene) is nonpolar and amorphous which helps to increase the strength of hydrogen bonds when the electrophilic isocyanate reacts with the hydroxyl group of the telechelic poly(ethylene/butylene). This concept can be generally applied to telechelic polycarbonates, polyethers, and polyesters, thus leading to a new set of highly processable functional supramolecular polymers [61]. More recent examples include two-phase materials in which properties are obtained due to polymer
polymer microphase separation. Guan et al. has designed H-bonded selfhealing brush copolymer systems with hard, mechanically reinforcing segments, and soft-healing segments. This supramolecular brush copolymer contains a polystyrene backbone (hard) and polyacrylate (soft) brushes with pendant amide moieties self-assembled into a microphase-separated domains, resulting in tough materials with the self-healing

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FIGURE 12.10 Supramolecular polymerization between poly(ethylene/butylene) end functionalized with UPy quadruple H-bonding moieties [61]. UPy, Ureido-pyrimidinone.

capability of dynamic supramolecular assemblies. The reversible hydrogen bonding interactions in the soft matrix (polyacrylate amide) exhibits self-healing behavior at room temperature [62]. A concept based on coupling conductive ionic liquid and quadrupole hydrogen bonding UPy units was explored by Wang et al. to form an inherently recyclable and selfhealing conductive supramolecular polymer. It was prepared by the free radical polymerization of UPy functionalized 2-hydroxyethyl methacrylate (HEMA) (functionalized vinyl monomer) and 1-vinyl-3-ethyl imidazolium acetate (ionic liquid). This polymer system when broken could totally recover it original mechanical properties in 30 min at the room temperature [63]. Promising research has been conducted in the field of self-healing wearable devices utilizing supramolecular polymers. Li and coworkers reported a supramolecular polymer composite matrix that have potential applications as motion-monitoring devices in underwater environments. They fabricated telechelic UPy-based supramolecular polymer film by synthesizing two UPy end-functional polymers: poly(ethylene-co-butylene) and poly(propylene glycol-co-ethylene glycol). These polymers were then blended with the multi-walled carbon nanotubes (MWCNTs) suspensions and cast into films to form self-healable thermo-responsive composites. The MWCNTs conferred superhydrophobicity to the blended films and, coupled with the UPy terminal groups, allowed for noncontact self-healing characteristics using near-infrared(NIR) light irradiation at 808 nm underwater at relevant body temperatures (33 C
34 C) [64]. Recent advancements by Johnson et al. detail the formation of main-chain zwitterionic supramolecular polymers derived from N-heterocyclic carbene
carbodiimide (NHC-CDI) adducts. The reversible interaction here is the DCB between the NHC and the center carbon of the carbodiimide. Although DCBs are technically covalent bonds, they are typically included with supramolecular interactions due to their stimuli
responsive reversibility. Thermodynamically these DCBs are stable but are kinetically reversible allowing for facile on
off switching between polymer and monomer in equilibrium. These supramolecular zwitterionic main chain polymers are predicted to show self-healing properties [65,66].

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12.3 Self-healing materials driven by metal coordination 12.3.1 Early development In 1992, Constable presented what would later prove to be a useful design paradigm for the assembly of coordination polymers [67]. Although, the successful synthesis of such a material was not reported at the time, it was proposed that interaction of suitable metals and complementary multidentate ligands would permit the generation of metallosupramolecular polymers (MSPs) by coordination driven self-assembly. This initial concept of metallosupramolecular polymerization proposed the use of back-to-back bis-terpyridine ligands, ditopic linkers that would later prove to be viable structural elements for the construction of supramolecular systems. While bis-terpyridines are now known to be competent ligands for the production of MSPs, the first report of a main-chain supramolecular polymer formed by dynamic metal
ligand complexation did not come until 2001, when Rehahn and coworkers utilized di-nucleating linkers featuring phenanthroline ligands to yield high MW MSPs [68]. On addition of Cu(I) and/or Ag(I), MSPs were obtained via coordination polymerization as confirmed experimentally through a combination of 1H and 13C NMR. The resultant polymers were dynamic in nature, exhibiting solvent dependent ligand exchange rates, with rapid exchange occurring in coordinating solvents and slow exchange in noncoordinating media. In 2002, Schubert et al. further discussed the use of bipyridine and terpyridine ligands for the creation of coordination polymers (Fig. 12.11) [69]. Nonetheless, with few convincing reports, the field was still largely in its infancy. While at the time, the presence of two spacer-linked, polydentate functions formed the most basic structural requirement, pyridine ligands were specifically chosen due to their ability to spontaneously self-assemble with a variety of metals, affording stable complexes. Although, bipyridine ligands such as 2,2’bipyridine do indeed yield stable complexes, they also result in the generation of stereocenters, and therefore lead to added geometric and spatial complexity. Consequently poorly defined photophysical, electrochemical, and electronic properties are often obtained, whereas the tridentate ligand 2,2’: 6’,2”-terpyridine leads to the generation of single isomer. Early attempts to characterize the MW of terpyridine derived MSPs were made by Schubert and coworkers. However, the main fragments observed were simply free ligand and a mononuclear complex, with no evidence of higher oligomers; a result attributed to the metal
ligand bond dissociation induced by the measurement itself [70]. Now, despite these challenges it has been shown that ligands of this class provide access to molecular

FIGURE 12.11

Interaction of bis-functional tridentate terpyridine ligands with a six-coordinate metal center forming MSPs with n repeat units [80]. MSPs, Metallosupramolecular polymers.

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architectures ranging from linear [71], block [72,73], star [74,75], dendritic [76], and network polymers [77
79]. Coordination polymers have been reported with variety of metals including Fe(II), Zn(II), Ni(II), Cu(II), Co(II), Ru(II), Ag(I) and other transition, main group, and lanthanide metals.

12.3.2 Network formation and self-healing To create SPNs, ligands must be employed that possess at least two linkers joined within a central core for directional metal
ligand binding. The corresponding choice of metal species influences the dynamics of supramolecular interaction and ultimately the stability of the generated complex. Through careful consideration of thermodynamic and kinetic parameters (e.g., equilibrium constant and association/dissociation rate), a material may be rendered responsive to a given environmental stimulus, and this responsiveness forms an important prerequisite for the emergence of efficient self-repair behaviors. Autonomous healing responses mediated by external agents such as the microcapsulebased approaches [81] are limited in the regard that they cannot achieve repeated selfrepair to the same areas, effectively limiting the useable lifetimes of such materials. In contrast, internally mediated, intrinsic self-healing has the potential to self-repair sites that may be routinely affected in their chosen operating environment. Under mechanical stress supramolecular binding motifs serve as sacrificial sites of cleavage, as it is frequently the weaker bonds of system that fail under stress. Owing to the dynamic behavior of supramolecular interactions, recovery occurs due to the subsequent regeneration of these broken linkages. The degree to which a particular system undergoes stress-recovery or “healing” is defined in Eq. 12.1 [82]. Healing efficiency 5

Mechanical value Mechanical valuepristine

 100

(12.1)

As a reversible process, metal
ligand complexation may be controlled in a variety of ways, including thermally, electrochemically, or through alteration of pH. These mechanisms provide a tractable means to impart a variety of stimuli
responsive properties to macromolecular systems. Furthermore, the kinetic and thermodynamic properties of ligand
metal interactions are highly tunable and, thus, allow one to exert finite control over the mechanical properties of a given material. Therefore coordination strength may be varied to control reversibility, with weakly coordinating systems yielding interesting dynamic behaviors. Using a combination of analytical modeling and Monte-Carlo simulation, Chen et al. examined the formation of linear and ring-like structures for a class of terpyridine ligands (2,2’: 6’,2”) [83]. The results of these studies suggested that high MWs are obtained over a relatively narrow compositional range of 2:1 ligand-to-metal ratios and that MW is found to decrease at 1:1 ligand
metal ratios. Their findings further indicate that metal
ligand complexation cooperativity can serve to limit the proportion of 1:1 ligand
metal complexes. Meanwhile, dilution results in a lowering of MW due to an increased tendency toward macrocycle formation which is generally disfavored and only results at high dilution.

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In addition, to their innate capability for self-repair, yet another attractive feature of these materials, is their ease of processability. This added benefit means that metalcomplexation driven self-assembly does not require specialized processing strategies and conventional polymer processing techniques are still applicable (i.e., blowing, compounding, extrusion, spin coating, and even ink-jet printing) [84]. However, it should be noted that while this reversibility frequently proves advantageous, it also complicates synthesis, purification, and molecular characterization (e.g., MW determination). Therefore MSP systems present unique technical challenges as well. Additionally as a thermodynamically driven process, the self-assembly of metallopolymers is subject to the influence of parameters such as temperature and composition, further complicating characterization. Whereas synthetically the presence of metal centers capable of exerting competeing catalytic functions can serve to limit the use of established polymerization techniques. Historically this has often meant that defect ladden, low MW metallopolymers, that were intractable both in terms of solubility and characterization, were obtained. Since these early efforts toward their synthesis, various approaches to polymerization (e.g., crosscoupling, ROP, anionic, and living) have been used in conjunction with metalcoordination driven self-assembly to yield new MSPs and this newly obtained degree of synthetic diversity has permitted study of structure-property relationships.

12.3.3 Ligand systems The judicious selection of appropriate multi-valent, metal
ligand coordination complexes provides materials with a balance of stability and reversibility. Synthetic strategies ranging from orthogonal supramolecular self-assembly, coordination driven self-assembly, and postassembly polymerization can be successfully implemented resulting in the production of well-defined, MSP architectures (Fig. 12.12). For instance, Craig and coworkers exploited metal
ligand coordination of ditopic organometallic Pt(II) and Pd(II) complexes with poly(4-vinylpyridine) to afford a 3-dimensional network polymers exhibiting frequency dependent mechanical properties [79,85]. These properties are largely governed by relaxations arising due to the dissociation of cross-links through ligand exchange, with stress
relaxation behavior being primarily influenced by the dynamic nature of chain entanglements and cross-links. Whereas, Lehn and coworkers reported the selective selfassembly of MSPs from rationally designed precursors and demonstrated the dynamic nature of their “metallodynamers” as synthesized [86]. Ligand and metal ions were found to be interchangeable in neat polymers requiring neither solvent nor catalyst for this reported system. Indeed, a great many chelating ligands have been successfully applied to the production of MSP systems, resulting in a structurally diverse assortment of potential design elements. Conveniently simplified self-healing systems that can be readily generated from inexpensive, commercially available materials have also been reported. One such example reported by Urban and coworkers, is a system comprised of polyethylenimine
copper complexes [87]. The Diaz-Diaz group has likewise reported a simplified design-based on complexation of oxalic acid with Cu(II) acetate which results in the formation of selfrepairing supramolecular metallogels [88]. Importantly healed samples of this material

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FIGURE 12.12 Representative chemical structures of ligands suitable for the generation of self-healing MSPs. MSPs, Metallosupramolecular polymers.

were found to have much the same conductive and rheological properties as pristine, undamaged samples; an observation that later permitted the production of self-healing conductive composites. Similarly Bielawski and coworkers successful applied NHC ligand
metal coordination to the creation of self-healing conductive materials [89]. Exploiting changes in the electrical resistance of the conductive polymer network,

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localized heating was realized; a phenomenon that led to self-repair when exposed to solvent. It should be noted that appropriate metal
ligand combinations may be chosen by matching redox potentials thereby improving electronic communication and, by extension, conductivity [90]. On the other hand, in the pursuit of desirable mechanical properties, Yan et al. developed tetrazolyl derivatives which when combined with Pd(II) displayed significant load-bearing capacity, rapid recovery from mechanical deformation, as well as stress and solvent induced self-healing [91]. Li et al. reported binding of 2,6-pyridinedicarboxamide (PDCA) functions via 6-coordinate interaction to Fe(III). An interaction in which pyridyl nitrogen and carboxamide oxygen atoms work in concert with two strong bonds resulting from pyridyl groups, two bonds of intermediate strength from amide linkages, and weak bonds resembling hydrogen bonds formed by interaction of Fe
O [92]. The synergistic effects of these individual contributors give rise to a framework which is ideally suited for the creation of an elastomer. Accordingly the incorporation of PDCA ligands onto a PDMS backbone yields a crosslinkable elastomer stretchable to 45 times its original length and is capable of autonomous self-healing; further mechanistic insights were later provided by Mo et al. [93]. Craig and coworkers have prepared model pincer complexes in an effort to inform the rational design of tunable stimuli
responsive materials [79]. While thermodynamic considerations are often central to the design of SPNs, the importance of molecular dynamics cannot be overstated. The nonequilibrium conditions brought about by mechanical stress alter the dynamics of polymer chain entanglements and cross-links. Through the resulting dissociation and reassociation of intermolecular interactions, the network is provided a means of stress relaxation. Investigation into the kinetic behavior of two isostructural model complexes revealed molecular contributions to rate determining processes. Simple steric effects were employed (i.e., substitution where R 5 Me or Et) to achieve independent control of Kd and Keq at the metal center of square planar Pd(II) and Pt(II) complexes (Fig. 12.13). Generally ligand exchange occurs in these complexes through associative FIGURE 12.13 Structure of dynamic pincer-based crosslinks (top) and the model system used for the determination of kinetic and thermodynamic parameters of pyridine coordination.

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FIGURE 12.14

Steady shear viscosity as a function of density of cross-linker 3a (red triangle) or 3b (blue circle), expressed as functional group equivalents of metal relative to pyridine side group. 100 mg mL21 3’PVP in DMSO. DMSO, Dimethylsulfoxide; PVP, Polyvinylpyrrolidone. Source: Reproduced from W.C. Yount, D.M. Loveless, S.L. Craig, Small-molecule dynamics and mechanisms underlying the macroscopic mechanical properties of coordinatively cross-linked polymer networks, J. Am. Chem. Soc. 127 (41) (2005) 14488
14496 with permission from the American Chemical Society; Copyright 2005 [94].

processes, whereby an attacking nucleophile joins the metal center prior to loss of the original ligand. This pentacoordinate transition state is subject to considerable crowding relative to both the parent and product complexes. Therefore steric effects imposed by spectator ligands exert significant influence over the rate of ligand exchange. Here, the dynamics of network cross-linking were found to serve as the primary determinant of bulk viscosity. Moreover, across a broad range (several orders of magnitude) the frequency dependent dynamic properties of the bulk material resulted due to specific solvent mediated ligand displacement reactions. The dynamic mechanical properties of the system are specifically driven by these dissociation events as opposed to nondissociative cross-link migration. By simply substituting one metal for another (i.e., Pt for Pd) kinetic control could be realized; kinetics could be made up to a 100 times faster by virtue of modest R group substitutions. Importantly well resolved 1 H NMR signals correlated to free (solvent-coordinated) and bound (pyridine-coordinated) metal centers permitted direct quantitation of exchange kinetics by exchange-spectroscopy. Given the tremendous differences in equilibrium and exchange rates, vastly different mechanical properties are obtained, with measured viscosities ranging several orders of magnitude [94]. This indicates that it is the dynamics of the molecular cross-links themselves rather than thermodynamics which are primarily responsible for the viscoelastic properties of such SPNs (Fig. 12.14).

12.3.4 Naturally occuring and bioinspired self-healing systems A mounting body of evidence indicates the involvement of metal
ligand coordination chemistry in both the self-assembly and adhesive properties of certain biological systems. Specifically the environmental challenges faced by marine mollusks necessitated the evolution of chemical systems capable of self-assembling soluble precursors into complex physical structures. One such structure, the byssus, is a collection of secreted filaments comprised of muscle foot proteins (MFPs); importantly MFP 1 typically contains between

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10% and 15% 3,4-dihydroxyphenylalanine (DOPA) amino acids. Early Raman spectroscopy studies demonstrated that these catechol containing proteins combine with inorganic iron ions resulting in the formation of a naturally occurring MSP [95]. The submicrometer resolution of Raman spectroscopy permitted the localized detection of metal
catecholate interactions where the majority of peaks can be attributed to the resonance enhanced interaction of iron with ligand functions. Complexation of this type occurs via donation of nonbonding catechol election pairs to the vacant orbital of the transition metal. Cross-links resulting from the formation of these coordination complexes exhibit favorable formation kinetics and considerably high strength. The differential cross-linking density of these structures are responsible for their unique mechanical behavior, combining both strength and stretchability (i.e., greater than 100% strain). In some species (Mytilus), a component of the byssus known as the cuticle is found to consist of a granular matrix capable of sustaining strains well beyond that which would cause ordinary, manmade materials to fail. Due to its granular structure, this matrix closely resembles microparticle composite systems where this granularity serves to reinforce the material. Raman peak intensities of sample cross-sections indicate that this apparent phase heterogeneity results due to local variations in metal
DOPA cross-linking density. The pH dependent coordination chemistry of catechol and Fe(III) was first utilized by Holten-Andersen et al. to generate polymer networks exhibiting moduli that rival that of covalently cross-linked networks [96]. Similarly Krogsgaard et al. presented a biomimetic system which successfully implemented Fe(III) cross-linking to produce pH responsive hydrogel materials [97]. Cross-linking density was effectively modulated through the presence of mono, di, and tri catechol-Fe adducts giving rise to a polymer network with highly dynamic behavior displaying both pH controllable mechanical properties and self-healing (Fig. 12.15). Although, the bonding interactions of DOPA ligands and metal ions are substantially weaker than covalent ones, these interactions are shown to be highly reversible (i.e., tolerant of hundreds of cycles). In contrast to prior synthetic systems based on neutral polymers, the design incorporates a polyallylamine backbone. The pH responsive behavior of the backbone more closely mimics the cationic nature of mussel plaque proteins (mfp-3, mfp-5) which arises due to their substantial lysine content (21 mol% in mfp-5). Consequently a bistable system is obtained wherein the gel collapses at the pI value (the isoelectric point) of the polymer; the point at which it possesses no electrical charge. The resultant multi-pH responsive system likely achieves maximum mechanical strength around the pI value, with increasing pH, cross-linking density is similarly increased with the formation of the bis- and tris(DOPA)Fe complexes. As the pH is raised further, cross-linking density continues to increase with a concomitant reduction in polymer charge. This reduction in charge leads to decreased polymer hydrophilicity and interchain repulsion, ultimately leading to collapse of the gel. Fig. 12.16 depicts the storage modulus G’ of unmodified polyallylamine: Fe(III) (blue) were purely viscous behavior is observed. Conversely DOPA functionalized polyallylamine:Fe(III) shows an increase in storage modulus G’ from nealy zero to c. 7000 Pa at a pH of 9.3 The imidazole
Zn interaction is another mode of metallosupramolecular binding commonly encountered in biological systems with rapid exchange dynamics. Guan et al.

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FIGURE 12.15

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Increase in cross-link density with increasing pH due to the formation of bis-and tris-Fe(III)

complexes.

FIGURE 12.16 Plot of storage modulus G’ as a function of the molar amount of added NaOH to DOPA 2 polyallylamine: Fe(III) hydrogels (black). For reference polyallylamine:Fe(III) samples are shown in blue. DOPA, Dihydroxyphenylalanine. Source: Reproduced from M. Krogsgaard, M.A. Behrens, J.S. Pedersen, H. Birkedal, Self-healing mussel-inspired multi-pH-responsive hydrogels, Biomacromolecules, 14 (2) (2013) 297
301 with permission from the American Chemical Society; Copyright 2013 [97].

presented a two-phase system resembling the granular DOPA matrix of the MFP-based byssus [98]. The reported microphase separated system is comprised of a hard/soft, two-phase brush polymer system found to simultaneously yield favorable mechanical properties and spontaneous self-healing. The polymer matrix consisted of a polystyrene

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macro-CTA backbone densely functionalized with imidazole brushes by reversible addition fragmentation chain-transfer (RAFT) polymerization. Significantly the mechanical and dynamic properties of this system were shown to be tunable when degree of polymerization, brush density, and ligand density were varied. Relative to previously introduced systems, such as terpyridine M
L complexes, imidazole ligands have a comparatively small binding constant to metal ions and so complexes of this type exhibit fast exchange kinetics. The incorporation of imidazole pendant functions into the soft phase (low Tg matrix) of the two-phase hard/soft polymer system results in the creation of a mechanically robust material with thermoplastic elastomer (TPE) behavior; with the PS phase contributing to both the high Young’s modulus and TPE behavior. Significantly the mechanical behavior of the imidazole containing polymer (ICP) system may be tuned by altering three molecular level parameters; percentage incorporation of imidazole containing acrylate, ligand
metal ratio and finally hard phase volume fraction/brush density (Fig. 12.17). Specifically the volume fraction of the hard PS phase may be controlled by altering the brush density. When the brush density is decreased from 10% to 5% an increase in extensibility, yield strength, tensile strength and Young’s modulus is observed. Critically despite the dynamic nature of the polymer network, the ICPs exhibit adequate creep resistance as FIGURE 12.17 Static tensile tests of ICP samples (A). TPE-like stress 2 strain behavior is observed and mechanical properties are found to be tunable over a wide range through variation of the incorporation percentage of imidazole, the ligand:metal ratio (L/Zn), and brush density. Creep-recovery experiments of the ICP after application of constant stress followed by relaxation of stress (B). ICP, Imidazole containing polymer; TPE, Thermoplastic elastomer. Source: Reproduced from D. Mozhdehi, S. Ayala, O.R. Cromwell, Z. Guan, Selfhealing multiphase polymers via dynamic metal
ligand interactions. J. Am. Chem. Soc. 136 (46) (2014) 16128
16131 with permission from the American Chemical Society; Copyright 2014.

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evidenced by their stress
strain behavior. Application of a 5 3 104 Pa stress for 300 min results in a final strain of 39%, on release, the sample is found to completely recover its original dimension, exhibiting little residual strain (,1%). When a stress of 1.0 3 105 Pa is applied for the same duration, a final stress of 70% is obtained, with release resulting in a residual strain of ,5%. Additionally the material displays exemplary capacity for selfrepair, once reunited, severed interfaces are found to quickly and quantitatively recover toughness, yield stress and Young’s modulus under ambient conditions in 3 h (Fig. 12.18). This is in contrast to conventional terpyridine-metal complexes which require exposure to solvent or heat to achieve similar self-healing behavior. Furthermore, the system is largely insensitive to moisture and thereby applicable to a broader range of operating environments than self-healing materials created through the use of hydrogen bonding interactions.

12.3.5 Photoresponsive systems In 2003, Rowan and coworkers successfully applied a reversible metal
ligand strategy for the construction of stimuli
responsive MSPs [99]. By combining 2:1 transition metal [i.e., Co(II) or Zn(II)] and 3:1 lanthanide ligand
metal binding, MSP gels were obtained through the formation of chain extensions and cross-links, respectively (Fig. 12.19). The approach allowed for associated properties of the chosen metal to be effectively incorporated into the gel networks; in particular, luminescence. The presence of a UV active bis (2,6-bis(10 -methylbenzimidazolyl)-4-hydroxypyridine tridentate) ligand serves to impart light-responsive behavior, serving as a molecular “antenna” and, thereby, permitting ligand
metal energy transfer. This energy-transfer pathway results in metal ion-based luminescence originating from the spectroscopically active La(III) species. The resultant gels also exhibited thermally responsive behavior, with the Co/La system undergoing a reversible sol
gel transition on heating to 100 C; a finding attributed to

FIGURE 12.18 Self-healing tests for ICP samples at room temperature and ambient humidity, stress 2 strain curve for ICP. Error bars indicate standard deviation of triplicate measurements. ICP, Imidazole containing polymer. Source: Reproduced from D. Mozhdehi, S. Ayala, O.R. Cromwell, Z. Guan, Self-healing multiphase polymers via dynamic metal
ligand interactions. J. Am. Chem. Soc. 136 (46) (2014) 16128
16131 with permission from American Chemical Society; Copyright 2014.

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dissociation of La/Ligand interactions and predicated on color changes observed in the material. Attenuation of lanthanide-based emission is observed on heating as well while ligand luminescence was found to be unperturbed providing evidence for the persistence of transition metal/ligand interactions at elevated temperature. Gels were shown to be mechanically responsive as well, exhibiting thixotropic behavior. Notably the invloved fblock lanthanide interactions are predominately ionic in nature and do not allow direct HOMO
LOMO band-gap tuning in the same way that transition
metal complexes can. However, the weaker interactions of the lanthanide species promote favorable reversibility, whereas, transition metal
ligand complexes yield strong, static, nearly covalent bonds. Thermochromic materials have also been prepared by Tew and coworkers which exploit the unique emission of lanthanide
polymer alloy materials [100]. In a subsequent report, the Rowan group described optically mediated self-healing of 2,6-bis-(methylbenzimidazolyl)pyridine ligands linked by a central poly(ethylene-co-butylene) core [11]. Photoexcitation of metal
ligand complexes within the MSPs resulted in temporary bond dissociation, an event which was simultaneously accompanied by the liberation of heat. These dissociation events and their corresponding reductions in MW serve to lower viscosity, promoting diffusion, and reentanglement of polymer chains at the damage site leading to self-repair (Fig. 12.20). Viscosity lowering accomplished through

FIGURE 12.19 Synthesis of Rowan’s stimuli
responsive MSP by reversible metal
ligand coordination. MSP, Metallosupramolecular polymer. FIGURE 12.20 Proposed mechanism of optically healable MSPs. MSPs, Metallosupramolecular polymers.

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transient metal
ligand bond disruption permits rearrangements to occur on rapid timescales thereby accelerating the self-repair process. As noted previously phase separated materials generally exhibit favorable mechanical stability. Here, the poly(ethylene-co-butylene) served as the hydrophobic core-forming block and the metal
ligand component acted as the polar constituent with small angle xray scattering (SAXS) and transmission electron microscopy supporting the formation of microphase-separated lamellar phases. These morphological features result from the disparate nature of the rubbery poly(ethylene-co-butylene) block and the stiff metal
ligand motifs with the latter acting as physical cross-links, increasing toughness. Chain-end disengagements at these hard
soft interfaces are accompanied by a decrease in MW and a consequent lowering of viscosity. Healing efficiencies were shown to be quantitative, with self-repair time-scales on the order of 1 min on exposure to 320
390 nm irradiation. In similar fashion to other material behaviors, the extent of self-repair was found to be influenced by metal
ligand stoichiometry with samples utilizing higher metal to ligand ratios exhibiting less efficient healing. Importantly healing was not observed on exposure to wavelengths of light not coinciding with the ligand
metal absorption eliminating the possibility of infrared heating and providing confirmation that the intended mode of healing was indeed the operative mechanism. Quantitative evaluations of healing behavior were performed by examining stress
strain responses of pristine and damaged samples. Photo-thermal induced self-healing behaviors have emerged as a general feature of supramolecular systems requiring only that a binding function possess adequately dynamic behavior. Substitution of one ligand
chromophore species for another can permit selective tuning of the wavelength required for healing. Interestingly Gunnlaugsson and coworkers reported the nonequilibrium coordination driven self-assembly of luminescent supramolecular Eu(III) and Tb(III) lanthanide gels wherein the interplay of 1:2 and 1:3 recognition processes gave rise to a 3D coordinative network [101]. The high coordination number of lanthanides and excess number of available carboxylic acid ligands are credited with the material’s self-healing features.

12.3.6 Self-assembly: directional bonding approaches The directional bonding approach has proven to be a particularly useful strategy for the creation of large self-assembled structures of controlled size, shape, and geometry (Fig. 12.21). In contrast to directionally insensitive noncovalent interactions such as hydrogen bonding and van der Waals interaction, metal
ligand bonding is highly directional [102]. The groups of Mirkin, Lehn [103], and Fujita [104] have championed the development of directional bonding approaches to coordination driven self-assembly. When appropriately selected, election-rich organic linkers can be combined with rigid, electron poor metals to form arrays of 2D and 3D coordination geometries (i.e., polygonal and polyhedral). Electron-deficient square planar complexes of Pt(II) and Pd(II) are frequently combined with complementary, electron-rich, nitrogenous ligands to achieve coordinative self-assembly. The requirement of rigidity is generally met through the use of aromatic or acetylenic functions which serve to maintain bond directionality. While these dative

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317 FIGURE 12.21 Representation of the coordination-driven selfassembly approach to the construction of 2D metallacycles from rigid, predesigned di-Pt(II) acceptors (blue) and ditopic organic donors (red).

modes of bonding are significantly stronger than other noncovalent interactions, thermodynamic control is still required to yield kinetically stable structures. Two highly influential factors to be considered for the controlled generation of supramolecular structures are the size and shape of the assembly’s constituent parts. The shape of both the donor and acceptor functions in a given assembly rely heavily on the turning angle which is formed between them (Fig. 12.21). Moving from the relatively simple case of ditopic electron donors to interactions of increased valency involving nonrigid blocks an assortment of polygonal metallocages may be obtained, and sphere-in-sphere shells by orthogonal self-assembly [105]. Stang and coworkers utilized an orthogonal strategy in which tandem coordination driven self-assembly and host
guest complexation gave rise to dual-responsive MSP networks (Fig. 12.22) with thermal and cation induced sol
gel transitions [106]. The structure was first templated by coordination driven self-assembly of a highly directional ditopic pyridyl donor to obtain hexagonal metallacycles featuring benzo-21-crown-7 moieties. As obtained crown-7 functionalized hexagons were then polymerized to yield a SPN by host
guest complexation of bis-ammonium salts. Similarly Yin and coworkers used orthogonal self-assembly to obtain a multi-responsive self-healing system. SPNs were generated by inclusion complexation of dibenzylammonium salts (DBASs) within 24-crown-8 ether derivatives and metal
ligand coordination of Zn-terpyridines or Pd-1,2,3-triazoles [107]. Molecular characterization was accomplished through a combination 1H and DOSY NMR, UV
Vis spectroscopy, rheology, and viscosity measurements. Network topology was shown to be tunable, transitioning from monomer to main-chain supramolecular polymer to SPN. At sufficiently high concentration, SPNs exhibit both stimuli
responsive and self-healing behavior.

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FIGURE 12.22 Selfassembly of crown etherfunctionalized metallacycle 1 (A) and generation of a cross-linked 3D supramolecular polymer network and (B) by self-assembly of metallacycle 1 and cationic ammonium salt guest 4 [106]. Source: Reproduced from X. Yan, T.R. Cook, J.B. Pollock, P. Wei, Y. Zhang, Y. Yu, et al., Responsive supramolecular polymer metallogel constructed by orthogonal coordination-driven self-assembly and host/guest interactions. J. Am. Chem. Soc. 136 (12) (2014) 4460
4463 with permission from the American Chemical Society; Copyright 2014.

Zheng et al. coupled coordination driven self-assembly with postassembly living polymerization to obtain stimuli
responsive supramolecular star polymers [108]. Macrocycles bearing trithiocarbonate (TTC) chain transfer agents (CTA) were first self-assembled and subsequently polymerized by RAFT, (Fig. 12.23). Polymers with dispersity indices (Ð) of 1.25 and 1.28 were obtained demonstrating that the polymerization was living in nature. Postpolymerization NMR analysis and atomic force microscopy (AFM) provided confirmation of the desired star-shaped topology. Likewise, ESI-TOF-MS was minimally perturbing to the molecular architecture, and thus served as a useful tool for molecular characterization and provided further confirmation for the formation of hexagonal metallacycles. The resultant polymers displayed lower critical solution temperature behavior due to the presence of poly(N-isopropylacrylamide) (PNIPAM) which further promoted self-repair.

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FIGURE 12.23 Synthesis of Pt(II) metallacycles and star supramolecular polymers by tandem coordination driven self-assembly and RAFT polymerization. CTA containing 120 degrees dipyridyl donor (1), 120 degrees diPt(II) acceptors (2, 3), CTA functionalized metallacycles (4, 5), star supramolecular polymers (6, 7). CTA, chain transfer agent; RAFT, Reversible addition fragmentation chain-transfer.

Notably addition of bromide anions inhibited self-healing, an observation further supporting the involvement of metal
ligand coordination in the repair process. Gel-phase transitions are routinely determined by rheology where the temperature at which G’ (the elastic modulus) curve intersects the G” (viscous modulus) generally indicates the gel-phase transition. Accordingly oscillatory temperature ramps were recorded from 20 C to 35 C with a heating rate of 1.0 K min21 (with fixed stress and frequency) for the SPN hydrogels. At temperatures below 25 C G’ and G” were low, with G’ remaining larger than G”, a condition indicative of the gel state. However, when the temperature was raised beyond 25 C, both G’ and G” were found to increase substantially with G” increasing faster than G’ leading to a crossover point at 29 C. This crossover point is correlated with the phase transition of the PNIPAAM hydrogel (Fig. 12.24). Since the hydrogel exhibited a volume phase transition rather than sol
gel transition on heating beyond 25 C, irregular changes in the G’ and G” were observed, an observation indicating the collapse of the hydrogel. Utilizing metallosupramolecular assembly Johnson and coworkers created polymer networks bearing photoresponsive metal-organic cages and demonstrated that the resulting MSPs could undergo reversible changes in topology (Fig. 12.25) [109]. The photoinduced dimerization of bis-pyridyl dithienylethene (DTE) ligands at opposing ends of the ditopic linker introduced stimuli
responsive behavior to the networks endowing them with uniquely tunable properties. Photoswitching permits alteration of several material properties simultaneously, such as shear modulus, self-healing, branch functionality, and stressrelaxation behavior. The use of photocyclizable ligands in conjunction with Pd21 species

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FIGURE 12.24 Oscillatory shear rheology frequency sweep, clearly showing hydrogel-like behavior of star polymer (6, Fig. 12.13) with G’ and G” curves are all being linear and parallel with G’ being dominant across all examined frequency ranges (A). Plot of the temperature dependence of dynamic shear moduli for a 3.0 wt.% hydrogel of star SPN (B). SPN, Supramolecular polymer network. Source: Reproduced from W. Zheng, L.-J. Chen, G. Yang, B. Sun, X. Wang, B. Jiang, et al., Construction of smart supramolecular polymeric hydrogels cross-linked by discrete organoplatinum(II) metallacycles via post-assembly polymerization, J. Am. Chem. Soc. 138 (14) (2016) 4927
4937 with permission from the American Chemical Society; Copyright 2016 [108].

FIGURE 12.25 Schematic representation of photomediated interconversion of network topology by UV induced ring-closure corresponding to the Pd24L48 form and green light induced ring-opening to the Pd3L6 form [110].

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allow for alterations in network geometry, wherein small rings (Pd3L6) are interconverted to large rhombicuboctahedra (Pd24L48) on exposure to UV light and subsequently returned to their original state after irradiation with green light. This morphological transition results in alterations to cross-link density of the material leading to the observed mechanical property changes. SAXS analysis of networks featuring the open form of DTE ligand/ linker revealed the presence of small spherical forms of radius 0.79 nm, an observation consistent with the presence of Pd3L6. Remarkably gels produced from open forms of DTE exhibited self-healing while gels derived from photocyclized DTE did not. This observation is attributed to the slow network dynamics of the closed form DTE gels leading to photoswitchability between nonhealing and self-healing states. The slow kinetics of the closed/cyclized DTE form limits molecular diffusion necessary for requisite network rearrangements utilized in self-repair. In contrast, the open DTE linker form possesses sufficient chain mobility to permit selfhealing. 12.3.6.1 Metal-organic framework To date, practical applications of solar-to-fuel conversion technology have been limited due to degradation pathways associated with the requisite molecular catalysts. Development of a suitable self-healing material enabling continuous repair and regeneration of the active catalyst would serve to circumvent this issue. To this end, Kim et al. described a spontaneously self-repairing molecular catalyst for the hydrogen evolving reaction (HER) embedded within a metal
organic framework (MOF) (Fig. 12.26) [110]. The MOF’s capacity for self-repair is exemplified by its ability to maintain catalytic activity for extended periods when compared to its homogeneous counterpart (days vs hours). Spontaneous self-repair was realized for the MOF supported two-component system of catalyst and photosensitizer, effectively extending the useful lifetime of the catalytic system. Mechanistically this self-healing behavior is attributed to the presence of diimine sites located about the H2-evolving catalyst and photosensitizer. Specifically the reticular chemistry of the MOF provides a high density of neatly arranged sites adjacent to embedded Pt-catalyst and Ir-sensitizer. This proximity promotes self-repair of broken diimine-metal bonds occuring during photocatalysis. FIGURE 12.26 Schematic representation of the synthesis of Ptn and Ir containing MOF. MOF, Metal
organic framework.

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12.4 Self-healing mediated by electrostatic interactions 12.4.1 Electrostatic self-assembly In a seminal report, Iler described the development of a new technique in which oppositely charged colloidal particles could be assembled in a step-wise manner on glass substrates to achieve multi-layer adsorption (Fig. 12.27) [111]. The newly established method enabled the production of lamellar films with uniform layer thicknesses and controlled chemical characteristics. This step-wise, alternating physisorption of polyelectrolytes with complementary ionic charges is referred to electrostatic self-assembly (ESA) or, sometimes simply, the layer-by-layer (LbL) method. The generality of ESA makes it an appealing alternative to other approaches such as the chemisorption of silanes and thiols or the Langmuir-Blodgett technique; another attractive quality of ESA is that the resultant films display unique self-healing properties [112,113]. In addition, to colloidal particles, charged nanostructures [114] and aggregates [115] are applicable to the ESA process although polyelectrolyte systems are most common [113]. The versatility of this strategy has enabled the production of a diverse array of architectures such as the LbL assembly of polyelectrolytes and ferromagnetic [116], semiconductor [117], and silicate [118] nanoparticles. Furthermore, a variety of nanosheets [119
121] have been exploited to yield composite films; surface modifications of nanopores [122], and nanochannels [123] have also been reported. Polyions composed of functional groups such as amines, ammonium cations, quaternized amines, sulfonate anions, and carboxylic acids are frequently utilized (Fig. 12.28). The ESA process is presumed to be entropically driven as evidenced by the diffusion of small counterions on binding to polyionic species leading to increased disorder [124]. Notably interactions in ESA are not adequately explained merely by Coulomb law and specific ions are found to preferentially associate [125]. The role of ionomer molecular structure has been systematically assessed in addition to interactions of ionomers with the underlying support from which films are grown. Generally the support presents a complimentary charge to which the first layer of ionomer is bound, then the alternating, FIGURE 12.27 General depiction of electrostatic selfassembly (ESA) showing the physisorption of charged macromolecules in a layer-bylayer fashion forming complex, uniform films with self-healing characteristics via electrostatic charge pairing of different polyelectrolytes and substrates.

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FIGURE 12.28 Ionomer structures useful for the creation of self-healing thin films. poly(diallyldimethylammonium chloride) (PDADMAC, 1); poly (acrylic acid) (PAA, 2) and branched poly(ethylene imine) (bPEI, 3); poly(2-[(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl)-ammonium hydroxide) (MEDSAH, 4).

step-wise deposition of polyanions and polycations leads to the production of ordered multilayer films. The process is simple, fast, and amenable to automation. Under appropriate off-stoichiometry conditions, surface charge inversion occurs with the addition of each subsequent layer, a process which can be experimentally verified by measuring ζ-potential [126]. The self-healing behaviors observed in ESA films are attributed to the charge overcompensation which results due to this off-stoichiometric adsorption, the ability of a particular polyelectrolyte system to self-repair has also been shown to be MW dependent [118]. South et al. reported the autonomous self-healing behavior of poly(diallyldimethylammonium chloride) (PDADMAC, 1) hydrogel thin films [127]. The films exhibit rapid selfhealing, with reorganization and repair time-scales on the order of seconds on immersion in water (Fig. 12.28). Micron-scale defects can be repaired without discernible residual damage and without external influence, unlike other materials that may require external modulation of temperature (e.g., heating above Tg) to generate sufficient molecular mobility among chains. So as to permit interdiffusion and thereby promote healing, the thin-films can rapidly reorganize when solvated with water to realize truly autonomous self-healing. In the presented design, microgels particles were obtained by aqueous free radical polymerization of N-isopropylacrylamide (NIPAm) and acrylic acid (AAc) with a PEG diacrylate cross-linker. Silanization of oxidized PDMS substrates with (3-aminopropyl) trimethoxysilane (APTMS) then provided amine terminated self-assembled monolayers to which the pendant carboxylic acid functions of the microgel particles were bound via EDC coupling. Finally diffusion of PDADMAC into the negatively charged surface yielded a Coulombicaly cross-linked 3D network. Wang et al. reported polyelectrolyte multilayer (PEM) films derived from codeposition of poly(acrylic acid) (PAA, 2) and branched poly(ethylene imine) (bPEI, 3) where the presence of reversible ion
ion interactions gave rise to self-healing behavior [128]. On mechanical disruption, visually apparent healing is observed in the bPEI/PAA film in seconds, with complete healing occurring within minutes. In contrast to water immersion promoted self-healing, a humid operating environment is sufficient (100% relative humidity at room

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temperature) to permit complete healing of the bPEI/PAA film within 1 h. The interactions of the polyelectrolyte components with the underlying support were studied by confocal laser scanning microscopy using bPEI-fluorescein isothiocyanate and PAA-lucifer yellow cadaverine conjugates. The strong binding of the substrate to the thin-films limits diffusion of molecular components responsible for self-healing behavior. Rapid exponential growth of the films is observed and rationalized by invoking a previously proposed and verified “in-and-out” diffusion mechanism. Both layer thickness and cross-linking density are thought to be pH dependent. AFM indentation was utilized to evaluate the effect of solvation on LbL-derived coatings. The indentation loading curve slope at the point of contact in is representative of the film stiffness with substantial softening of the coating being observed when immersed in water. Based on the experimentally derived loading curves, Young’s modulus may be calculated using the Hertz model. For a representative case of a (bPEI6.5/PAA3) 3 300 coating, where 300 denotes the number of deposition cycles, Young’s modulus was found to decrease by a factor of 87 in water compared to the identical film in the dry state. PEM films of this type readily absorb water owing to their ionically cross-linked network structure and the hydrophilic character of their constituent polyelectrolytes. Importantly this softening permits increased chain interdiffusion and thereby promotes self-healing. Zhang et al. examined the self-healing of bPEI/PAA in the bulk and compared its behavior to that of PEM films [129]. While small fractures in thin-films can be rapidly and completely repaired with deionized (DI) water treatment at modest pH, the mechanical properties of the bulk material were not recovered as efficiently and requires exposure to salt solution or altered pH (Fig. 12.29). The rheological behavior of bPEI/PAA is strongly affected by pH and salt concentration. Since these factors largely determine the cross-link density and molecularly mobility, the self-healing behavior is similarly affected. Distinct

FIGURE 12.29 Stress
strain curves of healed bulk PEI/PAA complex (A) without DI water incubation (B) with DI water incubation. (C) Failure of “healed” polyelectrolyte complex (PEC) after treatment with only water (no salt) and (D) yield during tensile test. DI, Deionized; PAA, polyacrylic acid; PEI, polyethylene imine. Source: Reproduced from H. Zhang, C. Wang, G. Zhu, N.S. Zacharia, Self-healing of bulk polyelectrolyte complex material as a function of pH and salt, ACS Appl. Mater. Interfaces 8 (39) (2016) 26258
26265 with permission from the American Chemical Society; Copyright 2016 [129].

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material properties are observed depending on the identity of the metal salt, with alkali metals giving rise to increased chain mobility though effective charge screening, and transition metals increasing network rigidity through the formation of dynamic coordination cross-links and electrostatic interactions between carboxylic acid and amine groups. By altering the extent to which ionic functions engage in such interactions, one is provided a ready means of tuning material properties and therefore the material’s capability for selfrepair as well. Wang et al. reported the preparation of zwitterionic polymer networks for antibiofouling applications [130]. Zwitterionic surfaces are frequently prepared to this end with the Cu(I) catalyzed surface-initiated atom transfer radical polymerization being a commonly employed route for their production. Brush polymer coatings produced using such living polymerization methods are highly stable and have controllable layer thicknesses, although, the approach is generally less scalable due to the oxygen sensitivity of the copper catalyst. Furthermore, the densely grafted polymer chains obtained possess limited molecular mobility potentially impairing mechanisms for self-repair. Key structural features are the electrostatically associated ammonium and sulfonate ion pairs of 2[(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)-ammonium hydroxide] that dissociate on mechanical disruption and reform in the presence of water, these charged moieties are also responsible for the protein repellency displayed by this system (Fig. 12.30). When the underlying surface is exposed due to mechanical damage, protein absorption is found to occur at the affected areas, and after self-healing, the protein repellency of the surface recovers. A reporter strategy based on an Alexa 488-BSA fluorophore-protein conjugate permitted quantitative monitoring of this process. Gaddes et al. described PEM films derived by the LbL deposition of squid ring teeth (SRT) proteins on textile substrates [131]. Researchers found that polyelectrolyte diffusion leads to exponential growth resulting in the formation of thick films (μm scale) with intermingled components rather than lamellar type structure. Furthermore, exposure to water speeds up molecular diffusion thereby facilitating faster self-repair within damaged materials. One shortcoming of existing systems is their tendency to crack or fracture when

FIGURE 12.30

Schematic depiction of protein adsorption and desorption on damaged and healed zwitterionic

polymer networks.

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exposed to dry environments for extended periods, conveniently SRT proteins can be isolated on gram-scale with relative ease and display persistent material properties in both hydrated and dried states. Importantly SRT protein PEM films were shown to repair macroscopic defects and maintain enzymatic activity of encapsulated Urease. Amorphous regions of the protein are credited for its supramolecular self-assembly and adhesive properties, where the ability to deform above the Tg while simultaneously retaining hydrogen bonds likely provides the operative mode of stabilization. The coated textiles could potentially find application in the second skin program, protecting wearers from chemical and biological war-agents.

12.4.2 Bulk ionomer self-healing materials Miwa et al. used dynamic ionic cross-linking to prepare an ultrastrechable elastomeric material, where the dynamic nature of these linkages permitted the production of materials with a balance of strength and stretchability [132]. The incorporation of carboxylate functionalities into the commodity polymer polyisoprene results in the generation of a random copolymer with B1.7% of repeats bearing carboxyl groups. The degree of neutralization was experimentally varied and ranged from 24% to 90% of carboxyl groups existing as sodium carboxylate salts. Copolymer ionic functions are said to be in a continual state of flux, hopping between ionic aggregates at room temperature, leading to dynamic network behavior. Further kinetic control of hopping rate can be achieved by tuning the neutralization level. Autonomous self-healing behavior is also reported for this material and the rheological behavior is shown to be dependent on the deformation time scale with fast regimes yielding strong elastic responses and slow regimes resulting in highly stretchable, viscoelastic behavior (Fig. 12.31). This phenomenon was previously suggested by Leibler et al. wherein the “effective” cross-linking density in dynamic cross-linked elastomers will necessarily depends on the deformation speed [133]. In contrast to fast regimes, slow deformation timescales permit molecular diffusion, and it is this diffusion that is

FIGURE 12.31 Schematic illustration of ion-hopping induced viscoelastic response under low-frequency deformation and elastic deformation in the fast regime, with ionic aggregates serving as physical cross-links.

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ultimately responsible for viscoelastic behavior. Ionic aggregates and metal
ligand interactions are directionally insensitive allowing network rearrangements to occur on a timescale that is faster than hydrogen bonds. Local variation in the Tg was noted for ionic aggregates which were studied by ESR, the radius of aggregates as measured by SAXS was consistently 0.96 nm, with no dependence on degree of neutralization. The propensity of ionic groups contained within a polymer matrix to form aggregates capable of undergoing rapid relaxation and reformation after disruption is well documented [133
135]. Commercial materials have already been developed to capitalize on this capability, namely poly(ethylene-co-methacrylic acid) EMAA a random copolymer partially neutralized with either sodium or zinc, sold under the trade name of Surlyn by DuPont. During self-repair, ionic aggregates reassemble effectively acting as cross-links, and this reorganization leads to repair to the damage site. The ballistic self-healing of EMAA has been studied using acoustic wave and ultrasonic resonant spectroscopy by Pestka et al. [136]. A balance of properties are required to achieve the desired healing response; these properties can easily be modulated by altering the degree of neutralization of the carboxylic acid side chains [137]. Inclusion of nanoparticles into a polymer matrix can improve the mechanical stability of an otherwise brittle material. Huang et al. implemented this strategy and targeted the development of personalized, wearable electronic devices, in particular those capable of energy storage [138]. They soug ht to replace conventional polyvinyl alcohol (PVA) polyelectrolytes with polyacrylic acid (PAA), the design employs dual cross linking of PAA by vinyl functionalized silica nanoparticles and hydrogen bonding. Stretchability up to 600% strain was achieved using this design showing remarkable improvement compared to the majority of reports for other systems where this value does not exceed 100%. Samples could be stretched to 3700% without cracking suggesting that the cross-linking of the polymer-particle network is dynamic in nature and endows the material with mechanisms for stress and energy dissipation transfer. Comparable electrode performance to conventional PVA systems was achieved, as were nonautonomic healing efficiencies approaching unity over 20 cycles, this represents a factor of four improvement relative to other systems. The PAA electrolyte displayed excellent ionic conductivity when compared to conventional PVA, wherein the mobile protons of the polyelectrolyte yield capacitance values similar to those in the commonly encountered PVA/H3PO4 system. Self-healing is found to occur at room temperature, with repaired samples exhibiting ionic properties that closely resemble those of virgin samples.

12.4.3 Self-healing poly(ionic liquids) Another interesting class of polyelectrolyte materials with applications in energy storage are known as poly(ionic liquids) or polymerized ionic liquids (PILs). A central thrust for this particular line of research has also been the development of polymer electrolytes for fuel cells and batteries. The first report of such materials came in 1973 year when Salamone and co-workers reported the production of a polyimidazolium salt by free radical polymerization of a vinylimidazolium monomer [139]. The field then lay mostly dormant for more than two decades until Ohno began the development of conductive PIL

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materials [140]. Various strategies have been utilized to derive PILs, Cui presented a selfhealing PIL platform produced by ring-opening metathesis polymerization of an imidazolium-functionalized norbornene monomer with Grubb’s third-generation catalyst (Scheme 12.1) [141]. Interaction intensities for a variety of counter-ions were determined using 1H NMR analysis and Tg was found to be primarily influenced by counter-ion size. Sufficiently large counter ions were found to associate into loosely formed aggregates effectively functioning as weak cross-links, with noted plasticizing effects as well. Conversely counter-ions with smaller ionic radii led to the production of tightly packed clusters as evidenced by x-ray scattering. These ionic interactions act as physical crosslinks thereby restricting polymer chain mobility and increasing mechanical strength. Simultaneously the as-described material possesses sufficient molecular mobility to permit dynamic rearrangements which promote self-healing. These favorable properties were found to be tunable when PIL species with varying counter-ions, and hence, varying Tg values were coformulated. Others have examined counter-ion effects in PILs, including a report by Guo et al. detailing the pairing of materials with bulky anions that led to dynamic ion-paring and increased chain mobility. Here, they found that those with FSI-counter-ions could completely self-heal mechanical damage when heated to 55 C for 7.5 h. Further the ionic conductivity can be tuned through counterion substitution [142]. Another notable example of self-healing behavior enabled by ionic interactions was discovered in functional bromobutyl rubber (BIIR). After functionalization with imidazolium, the material was capable of ionic association
dissociation despite low concentrations of the requisite bromine group (B1.13 wt.%) [143].

SCHEME 12.1 Synthesis of PILs by ROMP of imidazolium functionalized norbornene monomer and structure of counter-ions permitting tuning of material properties. PIL, Polymerized ionic liquids; ROMP, Ring-opening metathesis polymerization.

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Shinde et al. synthesized polythiophenes with pendant carboxyl groups which on reaction with difunctional amine cross-linkers yield electrostatically associated ammonium carboxylate ion pairs, resulting in the formation of π-conjugated polymer ionic networks (PINs) [144]. They propose that the obtained PINs could serve as potential candidates for the production of a new class of self-healing conductive polymers (Scheme 12.2). Depending on the identity and steric bulk of the amine cross-linker either soluble or insoluble materials were obtained at room temperature. Dynamic strain amplitude testing of the resulting gels indicated that the ionic network is dynamic in nature and capable of reforming after mechanical disruption. This reversibility, coupled with the thixotropic properties also revealed by rheological study, demonstrate that the approach is indeed a viable strategy for the production of self-healing materials. A variety of 2 and 3 amines were tested, with primarily 2 amines yielding strong gels. Strain sweep experiments conducted on the π-PIN gels revealed a linear viscoelastic regime below c. 1% strain, within this regime, storage modulus (G’) was greater than loss modulus (G”), indicating that the gels are elastic (Fig. 12.32A). When the applied strain was increased, G’ steadily decreased, while G” was found to increase to a maximum before finally decreasing. This rheological behavior is attributed to a strain-induced imbalance between the rate of bond formation and disruption for cross-linking functions within the network. The gel to sol transition (G’ and G” crossover) was observed at c. 5% strain, a finding suggestive of network disruption and consequently that chains are free to flow. G’ derived from the plateau modulus for 2 amines (c. 65
100 Pa) is B2
3 3 higher than those measured for 3 amines (c. 30 Pa) despite lower cross-linker concentrations (20 mol %) (Fig. 12.32B). Additionally the G’ of the 2 nBu network is found to increase with increasing amine ratios, indicating an increased cross-linking density (Fig. 12.32C). Importantly G’ and G” are found to be reversible at both high and low strain for greater than five cycles, providing further confirmation for the proposed thixotropic behavior of the π-PIN gels; further, viscosity reduces with increased shear rate (Fig. 12.32E). Tamesue et al. prepared nanocomposite hydrogel materials by cross-linking clay nanosheets (CNS) with linear and dendritic binders [145]. The CNS surface is densely

SCHEME 12.2 General schematic of the synthesis of π-conjugated polymer ionic networks (PINs). PINs, Polymer ionic networks.

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FIGURE 12.32 Strain sweep of the 2 nBu gels (A); storage moduli of the complexes (B); amine composition dependent storage moduli of the 2 nBu gels (C); dynamic strain amplitude cyclic test of the 2 nBu gel (D); and shear rate dependent viscosity of 2 nBu gel (E). Source: Reproduced from S. Shinde, J.L. Sartucci, D.K. Jones, N. Gavvalapalli, Dynamic π-conjugated polymer ionic networks, Macromolecules 50 (19) (2017) 7577
7583 with permission from the American Chemical Society; Copyright 2017 [144].

populated with oxyanions which interact electrostatically with positively charged guanidinium pendants of the binder. Linear binders are introduced as effective and synthetically tractable alternatives to previously prepared dendritic analogues. The materials are capable of producing organogels with suitable solvents and ionic liquids. Notably the supramolecular networks also exhibit shape memory, thixotropic behavior, and self-healing. Cuthbert et al. presented self-healing materials that are capable of performing favorably at biologically relevant salt concentrations. Polystyrene homopolymers with pendant alkyl phosphonium groups were formed by free radical polymerization of either poly(triethyl(4vinylbenzyl)phosphonium chloride) (P-Et-P) or poly(tributyl(4-vinylbenzyl)phosphonium chloride) (P-Bu-P). Polymer networks were then generated at slightly alkali pH (pH 8) with PAA (Scheme 12.3). Moduli were found to be of similar magnitude to those encountered in soft tissues which leads to potential biomedical applications but, when the phosphonium R group is butyl rather than ethyl, poor mechanical properties result [146]. Self-healing is shown to be dependent on salt concentration with 0.5 mm defects completely healing in 24 h in 0.1 M NaCl, while appreciable healing was not observed in DI water (Scheme 12.4). Xiong reported the coformulation of a imidazolium PIL with a PAA-azo brush in DMF to produce organogels with autonomic self healing and a modulus which could be tuned photoswitchably. In the as-presented design, cation-π and hydrogen bonding interactions

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SCHEME 12.3 Synthesis of phosphonium PILs and their combination with PAA to form associated ion pairs. PAA, Polyacrylic acid; PILs, Polymerized ionic liquids.

SCHEME 12.4 Schematic depiction of supramolecular organogel formed by interaction of (Azo)-grafted PAA and an imidazolium-based PIL. PAA, Polyacrylic acid; PILs, Polymerized ionic liquids.

work in concert to confer these properities [147]. Both storage (G’) and loss (G”) moduli were found to be significantly higher for the PIL@PAA-cis-Azo than the PIL@PAA-transAzo, suggesting an interaction of higher strength between the PIL and the PAA-Azo brush. Further confirmation of photo-modulated binding ability was provided by isothermal titration calorimetry. These features permit photo-controlled alterations to the material’s bulk modulus. The resultant organogels are found to heal when servered interfaces are reunited and in the absence of external force (i.e., pressure). The presence of cation
π interactions are ubitious in natural systems, potentially even including those which exhibit self-healing behavior [148]. Surface force measurements indicate that the adhesive forces between cationic and aromatic functions by short range interaction may actually be stronger than the electrostatic interactions between oppositely charged polyelectrolytes [149]. Kim et al. showed

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that complex coacervation originating from the interaction of two like-charged polyelectrolyte systems could provide a new route to bioinspired self-healing materials [150]. Given the diverse array of functionalities capable of engaging in the favorable electrostatic interactions that impart self-healing behavior, and the breadth of materials properties accessible by this unique class of materials, ionomers are poised to find application in the next generation of devices and functional materials.

12.5 Host
guest interactions in self-healing materials 12.5.1 Introduction to host
guest interactions Host
guest interactions are important types of noncovalent interactions that allow for introduction of various functional properties such as self-healing, shape memory, and selective molecular adhesion in a facile and effective manner [151,152]. Donald J. Cram coined the expression host
guest chemistry and defined the host
guest relationship [153]. According to Cram, the host
guest relationship entails a complementary stereoelectronic organization of binding sites for hosts and guests. In a highly structured molecular complex comprising of at least one host and one guest, the host is typically an organic molecule or ion whose binding sites converge in the complex and the guest is a molecule whose binding sites diverge in the complex. Hosts are generally characterized by their dimensionality, connectivity, and cyclic topology. Guests can be metal ions, metal
ligand assemblies, organic ions or neutral organic compounds. The binding site is defined as the molecular space where hosts and guests interact and is characterized by the appropriate size, shape, chemical nature, and electronic properties (i.e., charge, polarity, polarizability, van der Waals attraction, and repulsion) to form noncovalent, supramolecular interactions [153]. Molecular recognition is important for understanding and fabricating self-healing supramolecular polymers using host
guest interactions. As defined by Lehn, molecular recognition is the information, energy, and function involved in the selection and binding of other molecules by a given receptor molecule. The lock and key principle developed by Emil Fisher in 1894 was one of the first principles used to understand molecular recognition and selective binding [154]. It states that the key (guest) has a complementary size, shape, and electronics to the lock’s (host) binding site. Werner further stated that selective binding must involve attraction or mutual affinity between the host and guest [155]. The binding of a guest by a host species is an equilibrium process and the corresponding equilibrium constant is known as the binding constant (KA). The binding constant, also known as the association constant, provides a quantitative description of the degree of supramolecular association. In a general classification, “and” denotes a free host and “ate” denotes the combination of host and guest [156].

12.5.2 Mechanism of self-healing based on host
guest interactions The self-healing ability of supramolecular polymers may depend on at least two factors: the strength of supramolecular interactions across the interface and the mobility of

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polymer chains. The formation of host
guest inclusion complexes is an equilibrium process as shown in Eqs. (12.2 and 12.3). The binding capability of the host and guest to form an inclusion complex is expressed as the complex stability/association constant KA. nH 1 mG 5 nH:mG KA 5

½nH:mG ½Hn ½Gm

(12.2) (12.3)

In the above equations, H and G are the host and guest molecules, respectively, and n and m are the number of host and guest molecules, respectively. Restoring the mechanical strength at the damaged interface is a complex process. The presence of free host and guest moieties is essential for the self-healing process to be successful. When supramolecular polymers are cut, the host
guest interactions near the fractured interface are easily dissociated since these associations are the weakest bond in the material. On fracture, this creates new, unbound host and guest moieties, but since these interactions are dynamic, these molecules can re-associate to “heal” the material’s properties. There may also be existing, free host and guest moieties due to their constant equilibrium between free and bound states. On the fractured interface, the number of healing junctions between the host and guest moieties are expected to be proportional to their concentration. As the host
guest interactions are the major healing forces, the restoring strength between the twodamaged interfaces depends mainly on two factors: the binding capacity of the host
guest inclusion complex and the number of links/joints at the damaged interfaces. Molecular mobility also influences self-healing performance. For stiff supramolecular polymers, employing stimulus to increase molecular mobility has been implemented to improve the self-healing performance. Host
guest interactions can also work in concert with other reversible interactions such as hydrogen bonding, metal
ligand interactions, and electrostatic interactions [156,157].

12.5.3 Design of self-healing, host
guest materials Hosts (molecular receptors), as discussed above are organic molecules that can selectively bind to ionic or molecular substrates by various intermolecular interactions. Based on their shape and appearance, hosts can be majorly classified into acyclic and cyclic (macrocycles, macrobicycles, or macrotricycles). For fabricating supramolecular polymers utilizing host
guest interactions, macrocyclic structures are of significant interest. Macrocyclic hosts contain intramolecular cavities into which the guest substrates can fit often with high selectivity based on size. They are utilized as hosts as they are relatively large molecules with cavities of suitable size and shape for guest molecules to occupy. They provide a way for the organization of structural groups, binding sites, and reactive functions. Cyclodextrins (CDs) [158,159], crown ethers [160,161], cucurbiturils (CBs) [162,163], and calixarenes [164] are several examples of hosts used in fabricating selfhealing supramolecular polymers based on host
guest interactions. The last few decades, have revealed numerous polymeric architectures that utilize host
guest chemistry to construct self-healing supramolecular polymers. The formation of different examples of polymeric host
guest architectures [165] (Fig. 12.33) includes

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FIGURE 12.33

12. Self-healing materials utilizing supramolecular interactions

Different categories of supramolecular polymers formed using host
guest interactions.

supramolecular polymers formed with linear structure and three-dimensional network using multifunctional cross-linkers (Fig. 12.33A and B, respectively). The linear supramolecular polymer in (1) can be formed by A
B or A
A/B
B monomers and supramolecular networks like in (2) can be formed using AxBy type monomers where x $ 2, y $ 2, and x 1 y $ 5) supramolecular network formed by polymer chains containing host and(or) guest pendant groups. The supramolecular network in Fig. 12.33C is formed by a mixture of polymers with host or guests as pendant groups and bifunctional cross-linkers.

12.5.4 Self-healing materials utilizing cyclodextrin CDs are macrocyclic oligosaccharides with different ring sizes in which α-CD, β-CD, and γ-CD which consist of 6, 7, and 8 glucopyranose units, respectively, linked by 1,4- glycosidic bonds. CDs are often employed as host moieties as they are nontoxic, environmentally benign, and cheap. For fabricating self-healing polymers using host
guest interactions, the hydrophobic cavities of CDs have been utilized [156]. Self-healing supramolecular gels can be prepared by host
guest interactions in two useful ways: (1) from a combination of host and guest polymers (Fig. 12.34A and B) from polymerization of host and guest inclusion complexes as dynamic cross-linkers (Fig. 12.34B) [151]. Harada and coworkers have reported an innovative method for the formation of transparent supramolecular hydrogels with self-healing properties. PAA functionalized with β-CD was used as the host polymer for dynamic cross-linking with ferrocene functionalized PAA as the guest polymer (Fig. 12.35). The host polymer was synthesized by reacting either 6-amino-α-CD or 6-amino-β-CD with PAA forming the amide linked PAA-CD. 1H NMR confirmed that 4%
5% of the carboxylic acid groups of PAA were substituted by 6amino-β-CD. The guest polymer was synthesized using similar PAA amidation

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FIGURE 12.34 Different methods for forming supramolecular hydrogels utilizing host
guest interactions including mixing of guest and host-containing polymers (A) and polymerization of inclusion complexes as supramolecular cross-linkers (B).

FIGURE 12.35 Structures of host and guest PAA functionalized with α or β-CD (host) and ferrocene pendants via amidation [167]. PAA, Polyacrylic acid.

procedures where 2.7% of the carboxylic acid groups were substituted with ferrocene guest pendants as shown in Fig. 12.35. Due to the host
guest complementarity and multipoint cross-links, hydrogelation between the host and guest polymers resulted forming dynamic, self-healing supramolecular hydrogels. To test the self-healing properties of these hydrogels, a cube-shaped supramolecular hydrogel was cut, and rejoined showing complete disappearance of the crack after 24 h and an adhesion strength recovery of 84% at the healed interface. This self-healing property was observed due to the formation of new host
guest inclusion complexes between the ferrocene and β-CD groups of the host and guest polymers [151,156,166]. In another example, Harada and coworkers have utilized β-CD-based inclusion complexes to form self-healing supramolecular hydrogels with dissimilar hydrophobic guests (Fig. 12.36). According to their hypothesis, preorganization of host and guest monomers into inclusion complexes prior to polymerization can improve the self-healing efficacy of the resulting networks. To form inclusion complexes, the hydrophobic guest monomers n-butyl acrylate (nBuAc) and N-adamantane-1-yl-acrylamide (AAm-Ad), were copolymerized with α- and β-CD functional host monomers in water to form supramolecular networks as shown in Fig. 12.36. The supramolecular hydrogels were formed using homogeneous radical polymerization of the prepared inclusion complex with acrylamide (AAm). When the surfaces are cut and rejoined, the gel mends mechanical properties substantially within 5 s and recovers completely after 24 h. In general, CD-guest-based hydrogels show effective self-healing properties as the supramolecular hydrogels due to these reversible host
guest interactions between polymer chains [156,167]. Self-Healing Polymer-Based Systems

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FIGURE 12.36

Preparation of host
guest supramolecular hydrogels via copolymerization of CD-guest inclusion complex with acrylamide [168]. CD, Cyclodextrin.

12.5.5 Self-healing materials utilizing crown ethers Since the pioneering work by Pedersen, crown ethers have been extensively utilized in the preparation of self-healing polymers based on host
guest interactions [159,168]. Crown ethers are preferred due their facile, well known synthetic methodologies and the versatility that can be achieved in their molecular architecture. Two techniques can be used to construct self-healing supramolecular gels based on crown ether host
guest interactions: (1) form supramolecular polymers using low MW monomers and (2) by utilizing polymers with crown ether hosts and complementary guests as side groups [169,170]. Huang and coworkers reported cross-linked crown ether-based polymer gels [171]. The host polymers were formed by intermixing poly(methyl methacrylate) (PMMA) copolymers with incorporated comonomers containing pendant dibenzo[24]crown-8 (DB24C8) units. Two different bis-ammonium cross-linkers with distinct end group sizes were utilized for supramolecular cross-linking via host
guest complexation as shown in Fig. 12.37. The crown ether containing polymers and the bis-ammonium cross-linker form host
guest pairs via cation stabilization within the polyether ring. The resulting gel showed remarkable self-healing properties after mechanical failure with 95% recovery in the storage modulus values in less than 30 s on contact after fracture. It can be predicted that the reversible host
guest interactions were involved in self-healing and the regeneration of the electrostatic/hydrogen bonding interactions between the DB24C8 and the cross-linker units led to the fast recovery of the gel. This gel also displayed pH tunable

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FIGURE 12.37 Formation of dynamic self-healing hydrogels via mixing crown ether containing PMMADB24C8 copolymer and bis-ammonium cross-linkers with distinct end-groups [173]. PMMA, Poly(methyl methacrylate).

stimuli
responsive behaviors. On addition of trifluoroacetic acid (TFA) or triethylamine (TEA), the hydrogel exhibited reversible sol
gel or gel
sol transitions, respectively, due to pH-controllable host
guest interaction between bis-ammonium cross-linkers and DB24C8 [156,172]. Ikeda and coworkers reported the formation of self-healing polymers utilizing a yet another polymer scaffold functionalized with crown ether pendants [172]. This self-healing supramolecular organogel was synthesized via the mixing of glycidyl triazole polymers functionalized with crown ether (host) and secondary ammonium (guest) attached as side groups. The dibenzo[24]crown-8 (DB24C8) and DBAS form host
guest pairs causing supramolecular organogelation as depicted in the Fig. 12.38. The macroscopic self-healing behavior of the organogel was demonstrated by cut
heal tests both using visual assessment and via qualification of recovered mechanical properties. The recovered gel could be pulled, sheared, and bent at the fracture point with no additional damage. The selfhealing behavior was also characterized by dynamic mechanical analysis (DMA) via measuring stress
strain curves to determine the storage moduli of pristine and repaired material. The experiment involved application of a high magnitude strain (750%), at which point the modulus value decreases significantly and the structure breaks. After the reduction of the strain to 1%, the modulus value fully recovered in less than 10 s owing to the material’s fast recovery initiated by inclusion complexation between host (DB24C8) and guest (DBAS) units [156,173].

12.5.6 Self-healing materials utilizing cucurbiturils CB are macrocyclic molecules generally containing 5
8 or 10 repeating glycouril units. These molecules have large cavity volumes (e.g., CB[8] has an internal diameter of 8.8 angstroms) enabling the encapsulation/binding of two-guest moieties forming a threecomponent supramolecular complex and leading to enhanced selectivity of molecular recognition [166,174]. Common guest pairs used for these complexes include electrondeficient aromatic guests such as viologen derivatives paired with electron-rich aromatic guests such as derivatives of pyrene or naphthol. These can form charge
transfer interactions and moreover can be stabilized by the CB host. There has been a significant Self-Healing Polymer-Based Systems

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FIGURE 12.38

Preparation of crown ether and secondary ammonium functionalized GTPs utilized for host
guest complexation forming self-healing supramolecular networks [173]. GTP, Glycidyl triazole polymer. Source: Reproduced from ref. 173 with permission from the American Chemical Society; Copyright 2013.

development in the synthesis of CB-based systems mainly because of the high binding equilibrium constants and good solubility of CB-based complexes [156]. Overall, combining two seemingly contrasting properties, high storage modulus and rapid self-healing, in the field of self-healing materials has been challenging. Scherman and coworkers have designed a robust method to prepare hydrogels by cooperatively combining mechanically strong colloidal reinforcements and highly dynamic supramolecular interactions [175,176]. The three-component host
guest system of CB[8] with two guests, namely viologen and naphthyl substituted polymethacrylate and vinyl alcohol, were selected. Cellulose nanocrystals (CNCs) were used as reinforcing nanoscale materials into this system. The naphthyl guest molecules were introduced into polymethacrylate (NpMA) while the electron-rich naphthyl guests were introduced into polymer brushes covalently attached to CNC nanomaterials. When mixed with CB[8], these materials form dynamic, supramolecular networks cross-linked via host
guest interactions. Performing cut
heal tests demonstrated the rapid self-healing ability of the hydrogel which recovered mechanical properties in as little as 30 s. It also showed temporal stability with fast healing of fractured hydrogel even after aging for 4 months [177].

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12.5.7 Enhancing mechanical properties in host
guest self-healing materials While various parameters such as (1) healing rate, (2) healing times at the same fracture point, and (3) storage time after which the fractured surface still shows healing property are taken into account to evaluate self-healing behavior, it is also important to consider healing efficiency which is essentially the ratio of mechanical property after and before healing [156]. Although the fabrication of self-healing materials with quantitative property recovery and robust mechanical performance remains a challenge, researchers have made efforts to synthesize self-healing polymers with superior integrated performance by (1) incorporating strong reinforcing nanoscale additives into host
guest systems and (2) combining host
guest interactions with other reversible interactions. As such, these materials are typically characterized using traditional rheometrical methods used for determination of mechanical strength such as tensile testing and DMA. Yu and coworkers have utilized a rapid, noncontact procedure of magnetically induced frontal polymerization which is achieved via magnetocaloric effect to synthesize novel host
guest supramolecular gels with enhanced mechanical properties. β-CD and N-vinylimidazole (VI) have been used as the hosts and guests, respectively to synthesize Fe3O4 doped self-healing gels. Three types of gels were synthesized (gels 1
3) with increasing concentration of CD showing greatly improves the mechanical properties. When CD/VI weight ratios increase from 1:10 to 4:10, stress intensity increases from 7.1 3 104 to 1.25 3 105 Pa for 1, from 5.3 3 104 to 7.0 3 104 Pa for 2, and from 2.7 3 104 to 6.7 3 104 Pa for 3. Also gels 1 2 3 with CD/VI 5 4:10 w/w show the elongation at break over 826%, 683%, and 967%. The increase in mechanical strength is speculated to result from the host
guest interaction that increase as the amount of host facilitates the effective supramolecular interaction [178]. In another example, Li and coworkers synthesized a supramolecular polymer gel utilizing the host
guest interactions between a DB24C8-based bis-(crown ether) and a copolymer containing the dibenzyl ammonium moiety. Their self-healing behavior was studied by evaluating the viscoelastic properties via examining the storage modulus (G0 ) and loss modulus (G00 ) as a function of angular frequency. At the lower frequency, the value of G00 is larger than G0 , implying that the viscous property of the gel dominates, whereas at high frequency, the value of G0 is larger than G00 , implying that the elasticity dominates. Also at higher frequency, the value of G0 reaches its maximum of 14.8 kPa suggestive of the supramolecular polymer gel having high mechanical strength. Strain sweeping experiments revealed that both the storage modulus and loss modulus decreases rapidly with strain implying that the gel state collapses under mechanical strain. The supramolecular gel exhibits very rapid recovery of its mechanical strength after a large-amplitude oscillatory breakdown. Notably the gel could immediately recover to nearly 100% of its initial G0 and G00 values and return to a quasi-solid state when the amplitude is decreased at the same frequency. This recovery is achieved for at least 3 cycles in less than 13 s, which shows the excellent self-healing property of the supramolecular polymer gel and stems from the reversible host
guest interactions and formation of the cross-linked supramolecular networks [179].

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12.6 Dynamic covalent self-healing materials 12.6.1 Introduction to dynamic covalent bonds A covalent bond can be categorized as dynamic if it can display reversible breaking and reformation under equilibrium conditions when confronting different environmental stimuli [180]. Incorporation of these types of dynamic functionalities into polymeric materials can elicit unique properties such as macroscopic reconfigurability and stimuli
responsiveness. One of the more useful properties of dynamic covalent (DC) materials are related to the repair of mechanical damage to restore mechanical and physical properties (i.e., self-healing). Polymeric materials are always prone to different types of mechanical stress, leading to reduction of the materials’ efficiency and usefulness over time. The utilization of DCBs allow for one potential remedy to this limitation because these bonds possess both the reversibility of supramolecular bonds and the robust nature of covalent bonds. To achieve these self-healable characteristics, bond formation and reversal must be on the time-scale of milliseconds to minutes (i.e., under equilibrium conditions) [181]. The studied DC bonding, self-healing systems can be divided into four classes according to the reaction mechanism, modification, and/or the stimuli
responsiveness. These classes include condensation reactions, cycloaddition reactions, catalytic exchange reactions, and photo-induced DCBs. The first two classes consist of pH responsive functional groups whose equilibrium is heavily influenced by changes in global pH under aqueous conditions. Some other stimuli
responsive materials include thermal and photoresponsive materials that experience healing events (i.e., DC equilibrium) as a function of changes in temperature and light.

12.6.2 Self-healing from dynamic condensation reactions The most ubiquitous and widely studied DCB mechanisms fall under the broad category of dynamic condensation reactions and include acylhyrazones [182], boronate esters [183], and imine bonds [184], to name a few. Changes in pH alter the equilibrium constant in these dynamic functional groups resulting in unique pH responsive behaviors. Because of this unique pH-sensitive degradability, hydrogels comprised of these DCB cross-links have wide applicability in the biomedical applications, most notably drug delivery. 12.6.2.1 Acylhydrazone bonds The acylhydrazone DCB is formed via the condensation reaction of acylhydrazide functional groups onto aldehydes or ketones, as illustrated in Scheme 12.5. The dynamic equilibrium between products (hydrazone) and starting materials (hydrazide and aldehyde/ ketone) is sensitive to solution pH. The equilibrium heavily favors hydrazone formation at neutral to mildly alkaline pH, but begins to favor reaction reversal under acidic pH conditions [185]. Thus sol
gel transitions can be observed in dynamic polymer networks containing these DCBs as reversible cross-links. Further self-healing characteristics are also possible as long as an appreciable amount of product reforms at the fracture point of the polymer material.

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SCHEME 12.5 Reactions between acylhydrazides and aldehydes or ketones forming dynamic, pH-responsive acylhydrazone bonds.

FIGURE 12.39 pH-responsive, self-healable hydrogel formed on dynamic acylhydrazone cross-linking of the polymers shown [184].

Chang et al. reported the formation of self-healable polymer networks via cross-linking pendant hydrazide functionalities with aldehyde end-functional poly(ethylene oxide) (PEO) [186]. In this report, the researchers prepared PNIPAM copolymerized with acylhydrazide containing monomers (Fig. 12.39). This copolymer was cross-linked with dialdehyde terminated PEO to form reversible cross-links mediated by DCBs. The hydrogel displayed self-healing properties via the exchange of dynamic acylhydrazone bonds, which can reform after fracture at specific pH. The hydrogel also showed pHresponsiveness based on the dynamic property of acylhydrazone bond where addition of HCl or TFA caused the hydrogel to dissolve and, after neutralization of the acid with TEA, the hydrogel again reformed (i.e., sol
gel transitions). One can expand the self-healing characteristics in dynamically cross-linked polymeric materials via incorporation of multiple, orthogonal DCBs, such as acylhydrazones and disulfide bonds. The utilization of both DCBs as cross-links can extend the self-healing properties to acidic pH ranges in addition to basic conditions. Further disulfide linkages show redox responsiveness via interconversion between thiol (reduced) and disulfide (oxidized). Deng et al. synthesized hydrogels containing both acylhydrazone and disulfide bonds within cross-links providing the capability to repair damage under both acidic (pH 3
6) and basic (pH 9) conditions through acylhydrazone and disulfide exchange, respectively (Scheme 12.6) [187]. Additionally using a redox trigger such as 1,4-dithiothreitol (DTT), reduction of the disulfide cross-links dissociated the hydrogel allowing for redoxresponsive sol
gel transitions.

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SCHEME 12.6 Dual-responsive cross-linkers utilizing dynamic, orthogonal acylhydrazone and disulfide DCBs to form multi-responsive, self-healing hydrogels. DCBs, Dynamic covalent bonds.

12.6.2.2 Boronate ester bonds Self-healable materials have also been synthesized using reversible boronate ester DCBs, formed from condensation reactions between diols and boronic acids [188]. The equilibrium, governing the extent of the forward reaction, is heavily dependent on the solution pH and the pKa of the boronic acid component. Boronate ester bonds are formed at solution pH higher than the boronic acid utilized. When at lower pH, the dissociation of boronate ester back to diol and boronic acid occurs [189
191]. Deng et al. synthesized and characterized self-healable hydrogels containing reversible boronate ester cross-links [192]. These gels were created on condensation of poly(2-acrylamidophenylboronic acid-co-N,Ndimethylacrylamide) P(2APBA-co-DMA) with poly(vinyl alcohol) (PVOH) or acrylamide copolymer containing dopamine pendant groups, P(DOPAAm-co-DMA) (Fig. 12.40). To confirm the self-healing ability, samples were examined by cut/heal tests. The hydrogel comprised of P(2APBA-co-DMA) and PVOH fully healed within 60 min, as verified by the gradual vanishing of the scar at the damaged site (Fig. 12.40). The recovery of mechanical properties during the self-healing process was studied during this investigation. The PVOH hydrogels possessed an overall higher equilibrium modulus (B1 kPa) than the DOPAAm hydrogels (B350 kPa) which can be explained by the higher cross-link density for PVOH hydrogels since the concentration of diols participating in cross-linking was considerably higher. Furthermore, the self-healing capabilities of the resulting gels was also found to be highly dependent on the overall gel stiffness and, in turn, the cross-link density of boronate ester bonds. Rheological analysis using frequency-sweeping experiments, at all frequencies above 0.1 rad s21 (constant strain of 5%), of the gels with 10 and 15 mol% 2APBA revealed G’ . G” suggesting that these networks behave similar to permanently crosslinked, elastic gels with independent frequency moduli. Thus the time scale utilized in these experiments was confirmed to be shorter than that of the boronate ester cross-link reversal yielding materials perturbing self-repair. In contrast, on exposure to increasing strain (c. 10 rad s21) in strain-sweeping experiments (Fig. 12.41), the materials experienced a crossover of moduli yielding transitions from elastic solid to viscous fluid allowing for rapid recovery of original mechanical properties on self-healing. Researchers have long studied methods for lowering the pKa of boronic acids to facilitate boronic ester formation with diols at neutral pH. The most popularized method utilizes synthetic means to incorporate aminomethyl substituents ortho to phenylboronic acids to facilitate binding. Cromwell et al. utilized different small molecule boronic acid cross-linkers to tailor the malleability and self-healing efficiencies in the final product [193]. They examined two phenylboronic acid self-transesterification reactions

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FIGURE 12.40 Healing process for hydrogels formed containing P(2APBA-co-DMA) and PVOH. (A) Undamaged gel; (B) cut gel; (C) gel halves placed in contact; (D) gel after healing in 1 h; (E) healed gel prone to its own weight; and (F) hydrogel synthesis between P(2APBA-co-DMA) with PVOH and P(DOPAAm-co-DMA) [192]. APBA, Acrylamidophenylboronic acid; DMA, Dimethylacrylamide; PVOH, polyvinyl alcohol. Source: Reproduced from C.C. Deng, W.L.A. Brooks, K.A. Abboud, B.S. Sumerlin, Boronic acid-based hydrogels undergo self-healing at neutral and acidic pH. ACS Macro. Lett. 4 (2) (2015) 220
224 with permission from American Chemical Society; Copyright 2015.

(compounds 1 and 2) with different substituents in the presence of excess of neopentyl glycol (Scheme 12.7). Since nitrogen atom of the o-aminomethyl acts as a proximal base to facilitate the proton transfer between the leaving group diol on the boronate ester, compound 2 showed faster self-transesterification. This concept was utilized to dynamically cross-link 1,2 diol-containing polymer backbones, exhibiting specific characteristics including lower resistance to flow, quantitative self-healing ability, and faster reprocessing for materials cross-linked with compound 4. He et al. synthesized self-healable hydrogels through thiol-Michael addition between poly(ethylene glycol diacrylate) and DTT and boric acid (Borax) as a catalyst/cross-linker. As illustrated in Scheme 12.8, the boric acid has two roles: (1) the catalyst for Michael polyaddition reaction between the thiol and acrylate and (2) generation of dynamic boronate ester cross-links between the diols from the DTT repeat unit. The obtained hydrogel displayed pH and thermal responsivity along with the self-healing properties based on dynamic boronate ester exchange (near complete recovery of mechanical properties following cut-heal tests). It was also demonstrated that increasing boric acid concentration effectively increased the cross-link density resulting in higher relaxation time for the Self-Healing Polymer-Based Systems

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FIGURE 12.41 Strain sweeping experiments of hydrogels comprised of P(2APBA-co-DMA) (10 mol % 2APBA) with P(DOPAAm-co-DMA) (10 mol % DOPAAm) at pH 4.0 (A) and hydrogels generated by P(2APBAco-DMA) (10 mol% 2APBA) with PVOH at pH 4.0 (B) (constant angular frequency at 10 rad s21 and 10 wt.% original polymer solution). APBA, Acrylamidophenylboronic acid; DMA, Dimethylacrylamide; PVOH, polyvinyl alcohol. Source: Reproduced from C.C. Deng, W.L.A. Brooks, K.A. Abboud, B.S. Sumerlin, Boronic acid-based hydrogels undergo self-healing at neutral and acidic pH. ACS Macro. Lett. 4 (2) (2015) 220
224 with permission from the American Chemical Society; Copyright 2015 [192].

chains, increased hydrogel strength, and less transient behavior in the network (the crossover of storage and loss modulus) [194]. 12.6.2.3 Imine and enamine bonds The reversible condensation reaction between a primary amine and aldehydes or ketones form dynamic imine bonds which are substantially more reversible compared to acylhydrazones [184,195]. Imine formation, in general, can be divided into three distinct

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SCHEME 12.7 Controlling the exchange kinetics in boronic ester cross-linked materials by tailoring the neighboring groups on cross-linkers.

SCHEME 12.8 One-pot thiol-acrylate Michael addition polymerization of DTT and PEGDA and Borax-diol DCB cross-linking forming self-healing hydrogels in situ. DCB, Dynamic covalent bonds; DTT, Dithiothreitol; PEGDA, Poly(ethylene glycol diacrylate).

mechanisms, as depicted in Scheme 12.9. The first example depicts the reversible condensation reaction between unreacted aldehydes and primary amines. The second example displays an exchange reaction, where a second amine exchanges out the condensed amine through competition. Finally example three shows imine metathesis where two imines

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SCHEME 12.9

Mechanisms for imine and enamine generation dynamic covalent bonds.

dynamically exchange their R groups. Dynamic imine cross-links have demonstrated utility in reversible thermoset materials with ambient temperature self-healing in the presence of water at neutral or acidic pH [196]. Chao et al. synthesized amine-terminated PEO with different molecular weights (Mn 5 1000, 1500, and 3000 Da) and formed dynamic imine cross-links using 1,3,5-triformylbenzene in solvents with a broad range of polarity [e.g., acetonitrile (AN), N,N-dimethylformamide (DMF), benzyl alcohol, ethanol (EtOH), tetrahydrofuran (THF), and toluene] at room temperature and in neat conditions at 50 C [197]. The time required to reach the gel point (crossover for G’ and Gv in the rheological test) increased in polar solvents when compared to nonpolar solvents related to faster dynamic imine bond exchange. Using higher MW-PEO caused a decrease in elastic modulus and malleability for the bulk materials. Further the self-healing ability of the material was studied by preparing two different gels containing either fluorescein or rhodamine B flurophores. The two gels were cut and the two halves were reattached to separate, fluorophore-free gels at the cut point. After about 5 min, the cut point had been fully repaired allowing for bending without fracture. Additionally the diffusion of fluorophores into each half was observed, suggesting that molecular recovery of imine bonds is occurring at the interface. Furthermore, the stress
strain measurements showed that, after 30 min, the strength of damaged gel was comparable to that in the pristine gel; however, no tangible improvement was observed after 48 h. Similar to imine DCBs, enamine bond formation (Scheme 12.9) displays dynamic bond formation/breaking characteristics. Conversely enamines are prepared from condensation of secondary amines onto ketones or acetoacetates. Liu and coworkers, synthesized the polysaccharide-based hydrogels generated by the reaction of cellulose acetoacetate and chitosan [198]. After the cutting the hydrogel and reattaching at cut interfaces, the material recovered its primary shape and showed near quantitative mechanical recovery when compared to the pristine material.

12.6.3 Reversible cycloaddition reactions 12.6.3.1 Reversible Diels
Alder One of the most studied and common thermally induced self-healing systems utilizes dynamic Diels
Alder (DA) reactions [199]. These reactions goes through a [4 1 2] cycloaddition mechanisms between a diene and dienophile. Depending on the nature of diene Self-Healing Polymer-Based Systems

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(typically furan groups) and dienophile (typically maleimide groups), the DA reaction can be reversible (retroDA) across different temperature ranges, allowing for development of self-healing materials [200]. One common dynamic polymeric network utilizes the direct reaction between multifunctional monomers with furan and/or maleimide functional groups [201,202]. Chen et al. utilized two monomers, one with four furan groups and one with three maleimide functionalities, to synthesize thermally repairable polymeric networks (Scheme 12.10) [203]. Using the temperature-modulated DSC (TM
DSC) and 13C NMR analysis for samples with different thermal history, the reversibility of the furan
maleimide (FMI) DCB was confirmed. Furthermore, the mechanical properties recovery in the fracture tests revealed that samples heated to 150 C and 120 C showed healing efficiencies of 50% and 41% respectively, indicating the influence of temperature to drive the reaction toward the retroDA. This DC self-healing mechanism has been expanded to many different polymeric materials such as polyesters [204], polyamides [205], and polyurethanes (PUs) [206] leading to improvements in chemical and solvent resistance. Du et al. synthesized and compared two linear PUs, one with DA adducts within the linear backbone (PU
DA) and the other, a traditional linear PU (Scheme 12.11) [206]. The thermal, microscopic, and spectroscopic analysis of these materials, including TGA, DSC, 13C NMR, AFM and mechanical analysis, revealed that the dynamic PU
DA polymers had higher thermal stability and better microphase separation. Further because of the reversibility of the DA bonds, these

SCHEME 12.10

The creation of the polymeric network, comprised of monomers with furan and maleimide

functionalities.

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SCHEME 12.11 Thermally reversible polyurethanes (PU
DA) synthesized via dynamic Diels
Alder reactions between bis-furan end- functional polyurethanes and bis-maleimides [204].

polymers showed good recovery of mechanical properties after thermal self-healing. For this process after mechanical failure, the PU
DA materials were heated to 120 C (for retroDA), followed by cooling to 60 C and holding at constant temperature for 24 h. Still, these materials showed modest recovery of mechanical properties with 80% and 66% observed breaking tensile strengths after the first and second self-healing cycle, respectively, compared to the pristine sample. Self-healing polymer nanocomposites have attracted much attention from researchers because of their superior mechanical properties and unique applications in the drug delivery [207,208]. Researchers have coupled thermally reversible DA self-healing with nanofillers such as silica [209], carbon fibers [210], graphene oxide [211], and silver nanowires [212] utilized as inorganic phases in nanocomposites. It has been verified that the number of DA groups on the surface of the building blocks and their relative degrees of freedom are the crucial parameters correlated to self-healing capacity of such materials [213]. Scha¨fer et al. studied the DA mechanism on the modified surface of magnetite/maghemite (spherical super-paramagnetic iron oxide nanoparticles). Different functionalized organophosphonic acid coordinating ligands were examined, containing either maleimide or furan groups. FTIR spectroscopy, DSC, and CHN elemental analysis revealed that increased steric hindrance of coordinating DA functionalities (associated to the length of the spacer from propyl to dodecyl) could hamper the DA mechanism, resulting a deterioration in self-healing efficiency for the prepared film. Further these materials displayed magnetic responsive selfhealing behaviors due to the paramagnetic properties of the iron oxide nanoparticles. Yu and coworkers synthesized multifunctional hydrogels utilizing two, orthogonal DCB cross-links (i.e., acyl hydrazones and maleimide
furan DA reactions) with differing mechanisms and stimuli
responsiveness [214]. They compared two samples, one with only acylhydrazone cross-links and the other with both DA and acylhydrazone DCBs (Fig. 12.42). It was demonstrated that the DA bond preserved the integrity of the compound and acylhydrazone bond provided the self-healing properties via the pH stimulus. Even though both samples showed an acceptable recovery after the cut
heal test (nearly the same as the neat sample in the stress
strain test), the compound with two orthogonal DCBs had higher compressive modulus, enhanced stability, and better tissue adhesive property independent of solution pH.

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349

FIGURE 12.42 Schematic representation of polymeric networks containing both acylhydrazone and Diels
Alder bonds [210]. Source: Reproduced from J.S. Park, T. Darlington, A.F. Starr, K. Takahashi, J. Riendeau, H. Thomas Hahn, Multiple healing effect of thermally activated self-healing composites based on Diels
Alder reaction, Compos. Sci. Technol. 70 (15) (2010) 2154
2159 with permission from the American Chemical Society; Copyright 2015.

12.6.4 DCB exchange through chemical or catalytic stimuli Different catalytic systems can be utilized to impart self-healing in polymer systems, relying on the rearrangement of cross-links through catalyzed DC reactions. The amount and activity of the catalyst are important factors used to control the kinetics of self-healing systems governed by catalytic response. Researchers have demonstrated that transesterification can be used to generate self-healing epoxy-acid and epoxy-anhydride networks, dependent on the temperature, time, and concentration of zinc
acetate catalysts [215]. By increasing the temperature, the rearrangement of ester groups in the system caused the network to behave like a viscoelastic liquid and on cooling, the transesterification slows down leading to fixing of the cross-linked networks [215,216]. The carbon
carbon bond, due to its robust stability, is difficult to utilize for DC selfhealing applications. One example, however, includes olefin metathesis which, on addition of catalytic ruthenium complexes, will dynamically exchange functional olefins opening up the possibility for self-healing applications [217]. Lu et al. studied a self-healing system based on the cross-linked polybutadiene network and Grubbs’ second-generation ruthenium catalysts [218]. It was demonstrated that only minuscule amounts of catalyst were required to progress the self-healing process. However, increasing catalyst loading and temperature accelerated the healing process greatly by increasing the dynamicity of the olefin metathesis reaction (Fig. 12.43).

12.6.5 Photo-induced dynamic covalent self-healing Light is often a preferred stimuli for controlling DCBs in self-healing systems due to orthogonal reactivity of different wavelengths, high degree of spatial and temporal control, and facile modulation of intensity [219,220]. Photo-responsive DCBs mainly consist of radical transfer/crossover reactions or photo-dynamic dimerization reactions and include some systems with TTC, diselenide, and thiuram disulfide (TDS) bonds (vide infra) built into cross-linked network. Amamoto and coworkers focused on systems with TTC-based cross-links and demonstrated self-healing via photo-stimulated, DC reshuffling of TTC units [221]. Here, polymer networks were formed by RAFT copolymerization of n-butyl acrylate (BA) and a TTC cross-linker and demonstrated the self-healing ability of this material in solution and neat. However, due to limited chain mobility in the bulk, the

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FIGURE 12.43 Dynamic olefin exchange mechanism for self-healing by transition-metal-catalyzed olefin metathesis [218]. Source: Reproduced from Y.-X. Lu, Z. Guan, Olefin metathesis for effective polymer healing via dynamic exchange of strong carbon
carbon double bonds, J. Am. Chem. Soc. 134 (34) (2012) 14226
14231 with permission from the American Chemical Society; Copyright 2012.

self-healing ability was slower and less efficient compared to in the presence of solvent. Furthermore, even though this system showed repeatable self-healing processes, the reactivity of the radical (after the cleavage of TTC) toward oxygen diminished the real-world applicability of this system. Ji et al. utilized diselenide bonds in PU materials and demonstrated visible-light triggered self-healing capabilities [222]. They synthesized a series of PUs containing various amounts of deselenide DCBs incorporated into the backbone of the polymer. Under visible light irradiation, the reversibility of the diselenide bond is increased dramatically leading to mechanical recovery of the materials properties. Furthermore, utilizing laser irradiation as the trigger accelerated the healing process from 24 h to as low as 30 min showing good healing efficiencies of B80% using 457 nm blue laser irradiation. Amamoto et al. synthesized cross-linked PU networks with incorporated TDS units via polyaddition reactions between hexamethylene diisocyanate and TDS diol (photo-reactive monomer unit), tetra(ethylene glycol) (TEG) as a chain extending monomer, TEA crosslinker, and in the presence of dibutyl tin dilaurate as a catalyst [223]. The resulting copolymer, in the presence of visible light, showed self-healing characteristics due to the dynamic scrambling of TDS linkages through radical cleavage. The polymer, when exposed to visible light, showed self-healing efficiencies of near 100% after the cut
heal tests. However, by increasing the cross-link density and restricting chain mobility, the self-healing efficiencies subsided dramatically. Furthermore, increasing the time for the reaction under visible light irradiation decreased the self-healing efficiencies via the expulsion of CS2 or migration of TDS groups from the damaged surface to the bulk. In comparison, photo-reversible cyclization reactions have showed responsiveness to visible or ultraviolet irradiation and have been demonstrated for cinnamate [224], chalcone [225], anthracene [226], and coumarin [227] functional groups. The coumarin group can undergo a photo-reversible dimerization reaction on photoexcitation with 365 nm UV light. Abdallh et al. utilized a coumarin cross-linking agent copolymerized with a variety of acrylate monomers to create networks with a photo-responsive self-healing ability [227]. On exposing the material to 254 nm UV light, the coumarin dimer cross-links will undergo the reverse [2 1 2] cycloaddition reaction to break the four-member ring. If this is

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12.7 Hydrogen bonding in self-healing systems

conducted at the break or scratch point of the material, the cross-links will effectively dissociate allowing for mending of cross-links on recyclization using 365 nm UV light (Fig. 12.44). Optical microscope images demonstrated that scratches in some areas of samples completely vanished, with overall, bulk healing efficiencies around 80%.

12.7 Hydrogen bonding in self-healing systems 12.7.1 Hydrogen bonding Supramolecular interactions are powerful tools utilized to tailor the materials’ properties, including viscosity, storage modulus, loss modulus, etc. The dynamic behavior in these interactions can induce stimuli
responsiveness in materials, making them a reliable candidate for the self-healing systems [38,228
233]. After damage, the unbound/broken supramolecular agents at a fracture point can recombine noncovalently to reinitiate chain entanglements repairing mechanical properties of the material. The time necessary for the bonds to recombine is the first factor to analyze for self-healing materials. The second factor is correlated with the chain dynamic characteristics, that is, the mobility of supramolecular functional groups. Hydrogen bonds have been counted as one of the most promising supramolecular bonds for self-healing since their mobility and association strength can be tuned over a broad range [12,59,233
235]. The number of hydrogen bonds in the material network is a tunable factor to tailor and control the self-healing mechanisms and final properties. FIGURE 12.44 The selfhealing mechanism for the polymer, including coumarin cross-linking agent.

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12. Self-healing materials utilizing supramolecular interactions

Herein, we will discuss many different examples of self-healing polymer materials that utilize hydrogen bonding as the dynamic repair source.

12.7.2 Self-complementary hydrogen bonding in self-healing materials The self-associating, quadruple hydrogen bonding of ureidopyrimidinone (UPy) functionalities are one of the most ubiquitous compounds utilized in self-healing materials. These groups are particularly useful due to their strong but reversible dimerization and facile incorporation allowing their integration into a wide variety of polymer systems, generating materials with increased mechanical properties. This quadruple hydrogen bonding unit is influential due to its self-complementary nature, synthetic accessibility (i.e., reaction of methyl isocytosine with any isocyanate), reversibility at room temperature, and very strong dimerization (Kdim 5 6 3 107 M21 in chloroform) [21,236,237]. Oya et al. investigated the healing capabilities of supramolecular polymers consisting of UPy-tolylene functionalized poly(ethylene adipate) (PEA-Toly-UPy) and UPyhexamethylene-telechelic poly(ethylene adipate), (PEA-Hex-UPy) (Fig. 12.45). Here, they controlled the healing process according to the crystallization capabilities of the material demonstrating that slow crystallization in the material fracture provides the required chain mobility for self-healing. The slow crystallization of PEA-Toly-UPy, for example, allowed for molecular mobility near the cut surfaces for longer periods of time, even at room temperature. By preserving the contact between cut surfaces, hydrogen bonds could be reformed between the free UPy units, thus, reattaching and repairing the cut surfaces. In the case of PEA-Hex-UPy, the fast crystallization inhibited the healing process although several free UPy units might be generated in the cut surfaces [238]. Tensile testing revealed 85% maximum stress recovery following the first healing cycle for PEA-Toly-UPy materials with slow crystallization processes. Further time between break and healing played a large role in overall healing efficiencies. Zhu and coworkers synthesized network materials with low cross-link densities based on hydroxyethyl acrylate and poly(ethylene glycol) methacrylate (PEGMA) accompanying UPy functionalized monomer, 2-(UPy)ethyl methacrylate (Scheme 12.12) [239]. The

FIGURE 12.45

Chemical structure of telechelic, UPy functional polyesters, PEA-Toly-UPy and PEA-Hex-UPy.

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353

SCHEME 12.12 Chemical structure of MAUPy, cross-linker, RAFT agent utilized for synthesis of UPy containing copolymers for self-healing materials based on quadruple hydrogen bonds. RAFT, Reversible addition fragmentation chain-transfer.

polymers displayed different thermal and mechanical properties with PHEA-UPy showing a higher glass transition temperature (Tg) and, thus, no self-healing at room temperature due to the lack of chain mobility. In contrast, PEGMA360-UPy and PEGMA500-UPy had longer flexible side-chains, lower Tg, and dominantly viscous behavior enabling the UPy functional groups to rapidly reestablish hydrogen bonding networks for self-healing applications. Furthermore, lower molecular polarity, lower surface energy, and higher cohesive energy in the PHEA materials made it a perfect candidate as a potential self-healing adhesive at room temperature while the PPEG360-UPy and PPEG500-UPy were suitable adhesives at lower temperatures. Faghihnejad et al. fabricated self-healing poly(butyl acrylate) (PBA) copolymers containing comonomers with UPy quadruple hydrogen bonding groups as polymer pendants (PBA-UPy4.0 and PBA-UPy7.2 containing 4.0 and 7.2 mol% of UPy comonomers, respectively) and studied their surface interactions and adhesion mechanisms (Fig. 12.46). Fractured PBA
UPy films could fully reassociate and regenerate their self-adhesion strength to 40%, 81%, and 100% in 10 s, 3 h, and 50 h, respectively, with almost zero external load. Furthermore, it was shown that the higher adhesion force of PBA-UPy7.2 was related to the increased amount of UPy groups at the interface which, at higher temperature and humidity, displayed elastic behavior and enhanced interpenetration of polymer chains across the contact interface [240]. Cheng et al. synthesized a supramolecular polyurea (PU) containing urea
cytosine (UrCy) quadruple hydrogen bonding units installed within the polymer backbone. Cytosine terminated poly(propylene glycol) (Cy-PPG) was synthesized via Michael addition of cytosine on telechelic diacrylate functional PPG [237]. The final polyurea (PUUrCy), possessing quadruple hydrogen bonding units in the backbone, was generated via reaction of Cy-PPG with hexane-1,6-diisocyanate. The material displayed well-defined

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FIGURE 12.46 The chemical structure of P(BA-r-UPy) (A) which can self-associate via the quadruple hydrogen bonds of the UPy group (B) and to form dynamically cross-linked, self-healing hydrogels (C).

SCHEME 12.13 Chemical structure of PU-UrCy and switching between folded and unolded conformation causing dynamic cross-linking and network dissociation [237]. PU, Polyurethanes.

fibrous morphologies and self-assembled into cross-linked network structures via dynamic interactions between dimerized UrCy moieties. Furthermore, a transition between the dynamically favorable quadruple hydrogen bonded array (unfolded at room temperature) to a kinetically and thermodynamically favorable dual hydrogen bonded array (folded at higher temperature) demonstrating the stimuli
responsiveness of the material and the dynamicity of the cross-linked network (Scheme 12.13). Cut
heal tests for the sample revealed time-dependent healing behavior, with about 82% and .98% recovery of the original mechanical strength after 3 and 24 h, respectively, considered as a relatively high room temperature healing efficiency [241].

12.7.3 Self-healing polymers utilizing weak hydrogen bonding In contrast with the former section, many systems only rely on a lesser amounts of hydrogen bonds to fine tune materials properties and reprocessability. These systems are more ubiquitous in their application to hydrogels [242,243]. Teng et al. synthesized a polymeric network by free radical copolymerization of oligo(ethylene glycol)methacrylate (OEGMA) and methacrylic acid (MAA) without any cross-linking agents introduced [244]. It was demonstrated that the properties of the material could be tailored via alterations to OEGMA/MAA monomer incorporation and pH variation. The mass ratio of 2:1 OEGMA/ MAA with the total monomer concentration of 25 wt.% was chosen for hydrogel formation

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355

with the highest possible amount of hydrogen bonding. Two disc-shaped hydrogels (one dyed with rhodamine B for visualization) were cut into two parts and the two fractions chosen from each hydrogel were put in contact to investigate the self-healing process. On contact, the integrated hydrogel disks could recombine showing transfer of the rhodamine B across the interface and the hydrogel remained intact prone to its own weight after half an hour. Increasing the healing time caused better healing efficiency with 90% recovery in the stretchability compared to a pristine sample due to increase probability of reassociation via hydrogen bonding. In contrast, variations to temperature showed little effect on healing efficiencies, but pH was found to play a fundamental role. It was observed that, at pH 5 12, the electrostatic repulsion correlated with the deprotonation of pendant carboxylic acid groups caused self-healing efficiencies to drop dramatically. At pH 5 4, however, synergistic effects of complementary quadruple hydrogen bonding units and protonated carboxylic acid hydrogen bonding caused enhanced healing efficiencies. In many similar reports, the efficiency for healing process has not been reported such as the fabrication of designed hydrogels based on acryloyl derivatized 6-aminocaproic acid [245] and glycinamide [243].

12.7.4 Combination of hydrogen bonding and dynamic covalent bonds Combining DC and supramolecular chemistry for the fabrication of self-healing materials has recently attracted the attention of researchers. The fast healing process can cause significant creep leading to undesired properties as the healing process is repeated through many cycles. To overcome this limitation, the combination of bonds with different dynamic characteristics provide more efficient self-healing that occurs through multiple mechanisms expanding the utility of the studied materials [246
248]. For example, Zhan et al. synthesized poly(2-hydroxyethyl acrylate) (PHEA), containing UPy (as a highly dynamic and weak cross-link) and FMI DA (slow and strong) adducts both as reconfigurable, repairable cross-links [249]. In 7 h, the material demonstrated partial healing at room temperature recovering approximately 50% of the strain at break and 50% of the peak stress while at 90 C resulting in B90% of the pointed properties being restored. The FMI DA cross-link exchanges slowly which results in limited creep and complete stress relaxation. Furthermore, due to the dynamic characteristic of these two bonds, the material possessed an acceptable malleability by heating from reprocessing. In another instance, Qiao and coworkers synthesized a polymeric network via the dialdehyde-terminated PEG (PEG
CHO) and adipic dihydrazide-modified alginate (ALGADH) [250]. The obtained dynamic acylhydrazone and multiple hydrogen bonds endowed material self-healing characteristics with mechanical properties repair up to 85% recovery of tensile modulus after cut-heal tests. It is noteworthy to mention that the cross-linking agent concentration had a prominent influence on the gelation kinetics and pore diameter in the hydrogel micromorphology. It was demonstrated that 5 mol% of cross-linking agent was the optimal amount for improved mechanical properties (higher storage modulus) and provided the minimum pore diameter in the hydrogel. Compared to other hydrogels, containing imine [251], imine and acylhydrazone [252], acylhydrazone and DA [214], and disulfide and acylhydrazone [253] cross-links with compressive modulus in the range of 7
174 kPa, the novel material showed enhanced toughness with G5 288 kPa.

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Furthermore, the material displayed an acceptable malleability for molding into different shapes, owing to its superior reprocessability. Finally the material was considered as a potential candidate for drug delivery due to the dynamic nature of both acylhydrazone and hydrogen cross-links at neutral pH.

12.8 Conclusions and future outlook With the ever growing quantity of plastic waste products reaching critical levels, the developmental need for polymeric materials that are readily reprocessable with desired mechanical properties is becoming a necessity. To this end, self-healing materials offer one platform for improvement due to their dynamic reconfigurability. The development of intrinsic self-healing materials that utilize reversible, supramolecular bonds is one of the most active areas of polymer science research today with thousands of journal articles published yearly. This field has grown to include all types of supramolecular bonds including electrostatic interactions, metal
ligand coordiniation, host
guest complexation, hydrogen bonding, and DC bonding. Due to the reversible nature of these interactions, they are ideally suited for the dynamic repair of materials properties on mechanical failure through supramolecular bond reformation at damaged interfaces. Many of these interactions show orthogonality and compatibility allowing for the development of responsive materials with multiple stimuli affecting mechanical, thermal, and/ or morpholgical properties in unison. This has led to complex designs of materials utilizing multiple supramolecular bonds in unison. Smart materials that can reconfigure specific properties on introduction of specific stimuli are rapidly becoming a reality. As these materials become more sophisticated and the cost begins to decrease, we will begin to see the phasing out of more commodity plastics to include materials that can be readily depolymerized or networks that can be readily dissociated on demand for reprocessing to serve additional uses. For these applications, intrinsic self-healing materials that utilize supramolecular bonds are ideally suited. Challenges still remain in the development of robust, thermally, and mechanically tough self-healing polymers that implement these dynamic bonds. Further quantitative recovery of mechanical properties on repeated healing cycles is necessary for infinitely repairable materials. Current research focused on optimizing reprocessing procedures, integrating dynamic bonds with triggerable alterations to bond strength, and combinations of inorganic nanofillers for mechanical reinforcement are currently underway. This research and more should provide the next-generation of polymeric materials that can be infinitely reprocessed and recycled providing truly green materials.

References [1] R. Geyer, J.R. Jambeck, K.L. Law, Production, use, and fate of all plastics ever made, Sci. Adv. 3 (7) (2017) e1700782. [2] J.R. Jambeck, R. Geyer, C. Wilcox, T.R. Siegler, M. Perryman, A. Andrady, et al., Plastic waste inputs from land into the ocean, Science 347 (6223) (2015) 768
771. [3] C. Creton, Molecular stitches for enhanced recycling of packaging, Science 355 (6327) (2017) 797
798.

Self-Healing Polymer-Based Systems

References

357

[4] J.M. Eagan, J. Xu, R. Di Girolamo, C.M. Thurber, C.W. Macosko, A.M. LaPointe, et al., Combining polyethylene and polypropylene: enhanced performance with PE/iPP multiblock polymers, Science 355 (6327) (2017) 814
816. [5] B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, Self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (1) (2010) 179
211. [6] D.E. Fullenkamp, L. He, D.G. Barrett, W.R. Burghardt, P.B. Messersmith, Mussel-inspired histidine-based transient network metal coordination hydrogels, Macromolecules 46 (3) (2013) 1167
1174. [7] A. Winter, U.S. Schubert, Synthesis and characterization of metallo-supramolecular polymers, Chem. Soc. Rev. 45 (19) (2016) 5311
5357. [8] S.Y. Zheng, H. Ding, J. Qian, J. Yin, Z.L. Wu, Y. Song, et al., Metal-coordination complexes mediated physical hydrogels with high toughness, stick-slip tearing behavior, and good processability, Macromolecules 49 (24) (2016) 9637
9646. [9] R. Xing, K. Liu, T. Jiao, N. Zhang, K. Ma, R. Zhang, et al., An injectable self-assembling collagen-gold hybrid hydrogel for combinatorial antitumor photothermal/photodynamic therapy, Adv. Mater. 28 (19) (2016) 3669
3676. [10] T. Aida, E.W. Meijer, S.I. Stupp, Functional supramolecular polymers, Science 335 (6070) (2012) 813
817. [11] M. Burnworth, L. Tang, J.R. Kumpfer, A.J. Duncan, F.L. Beyer, G.L. Fiore, et al., Optically healable supramolecular polymers, Nature 472 (7343) (2011) 334
337. [12] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (7181) (2008) 977
980. [13] X. Luo, R. Ou, D.E. Eberly, A. Singhal, W. Viratyaporn, P.T. Mather, A Thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion, ACS Appl. Mater. Interfaces 1 (3) (2009) 612
620. [14] T.A. Plaisted, S. Nemat-Nasser, Quantitative evaluation of fracture, healing and re-healing of a reversibly cross-linked polymer, Acta Materialia 55 (17) (2007) 5684
5696. [15] The Nobel Prize in Chemistry. https://www.nobelprize.org/prizes/chemistry/1987/summary/, 1987. [16] A.G. Doyle, E.N. Jacobsen, Small-molecule H-bond donors in asymmetric catalysis, Chem. Rev. 107 (12) (2007) 5713
5743. [17] S.A. Pless, J.D. Galpin, A.P. Niciforovic, H.T. Kurata, C.A. Ahern, Hydrogen bonds as molecular timers for slow inactivation in voltage-gated potassium channels, eLife 2 (2013) e01289. [18] L. Zhang, V. Marcos, D.A. Leigh, Molecular machines with bio-inspired mechanisms, Proc. Natl. Acad. Sci. 115 (38) (2018) 9397
9404. [19] T. Steiner, The hydrogen bond in the solid state, Angew. Chem. Int. Ed. 41 (1) (2002) 48
76. [20] D.C. Sherrington, K.A. Taskinen, Self-assembly in synthetic macromolecular systems multiple hydrogen bonding interactions, Chem. Soc. Rev. 30 (2) (2001) 83
93. [21] F.H. Beijer, R.P. Sijbesma, H. Kooijman, A.L. Spek, E.W. Meijer, Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding, J. Am. Chem. Soc. 120 (27) (1998) 6761
6769. [22] V. Grignard, Mixed organomagnesium combinations and their application in acid, alcohol, and hydrocarbon synthesis, Ann. Chim. Phys. 24 (1901) 433
490. [23] V. Grignard, Some new organometallic combinations of magnesium and their application to the synthesis of alcohols and hydrocarbons, C. R. Hebd. Seances Acad. Sci. 130 (1900) 1322
1324. [24] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Commercial metal-organic frameworks as heterogeneous catalysts, Chem. Commun. 48 (92) (2012) 11275
11288. [25] H.-C. Zhou, J.R. Long, O.M. Yaghi, Introduction to metal-organic frameworks, Chem. Rev. 112 (2) (2012) 673
674. [26] K. Matyjaszewski, Atom transfer radical polymerization (ATRP): current status and future perspectives, Macromolecules 45 (10) (2012) 4015
4039. [27] Q. Zhou, A. Wang, H. Li, Z. Luo, T. Zheng, L. Zhang, et al., Microstructure of polypropylene and active center in Ziegler-Natta catalyst: effect of novel salicylate internal donor, RSC Adv. 6 (79) (2016) 75023
75031. [28] J.W. Lee, S. Samal, N. Selvapalam, H.-J. Kim, K. Kim, Cucurbituril homologues and derivatives: new opportunities in supramolecular chemistry, Acc. Chem. Res. 36 (8) (2003) 621
630. [29] O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Synthetic strategies, structure patterns, and emerging properties in the chemistry of modular porous solids, Acc. Chem. Res. 31 (8) (1998) 474
484.

Self-Healing Polymer-Based Systems

358

12. Self-healing materials utilizing supramolecular interactions

[30] K.S. Mali, J. Adisoejoso, E. Ghijsens, I. De Cat, S. De Feyter, Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface, Acc. Chem. Res. 45 (8) (2012) 1309
1320. [31] M.A. Kobaisi, S.V. Bhosale, K. Latham, A.M. Raynor, S.V. Bhosale, Functional naphthalene diimides: synthesis, properties, and applications, Chem. Rev. 116 (19) (2016) 11685
11796. [32] E. Arunkumar, P. Chithra, A. Ajayaghosh, A controlled supramolecular approach toward cation-specific chemosensors: alkaline earth metal ion-driven exciton signaling in squaraine tethered podands, J. Am. Chem. Soc. 126 (21) (2004) 6590
6598. [33] B.T. Nguyen, E.V. Anslyn, Indicator-displacement assays, Coord. Chem. Rev. 250 (23 1 24) (2006) 3118
3127. [34] T. Murakami, B.V.K.J. Schmidt, H.R. Brown, C.J. Hawker, One-pot “Click” fabrication of slide-ring gels, Macromolecules 48 (21) (2015) 7774
7781. [35] E.A. Appel, J. del Barrio, X.J. Loh, O.A. Scherman, Supramolecular polymeric hydrogels, Chem. Soc. Rev. 41 (18) (2012) 6195
6214. [36] C.J. Kloxin, T.F. Scott, B.J. Adzima, C.N. Bowman, Covalent adaptable networks (CANs): a unique paradigm in cross-linked polymers, Macromolecules 43 (6) (2010) 2643
2653. [37] J.F. Reuther, S.D. Dahlhauser, E.V. Anslyn, Tunable orthogonal reversible covalent (TORC) bonds: dynamic chemical control over molecular assembly, Angew. Chem. Int. Ed. 58 (1) (2019) 74
85. [38] W. Deng, Y. You, A. Zhang, 7 - Supramolecular network-based self-healing polymer materials, in: G. Li, H. Meng (Eds.), In Recent Advances in Smart Self-healing Polymers and Composites, Woodhead Publishing, 2015, pp. 181
210. [39] F. Liu, M.W. Urban, Recent advances and challenges in designing stimuli-responsive polymers, Prog. Poly Sci. 35 (1) (2010) 3
23. [40] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (17) (2013) 7446
7467. [41] S. Burattini, B.W. Greenland, D. Chappell, H.M. Colquhoun, W. Hayes, Healable polymeric materials: a tutorial review, Chem. Soc. Rev. 39 (6) (2010) 1973
1985. [42] A. Ciferri, Supramolecular polymerizations, Macromol. Rapid Commun. 23 (9) (2002) 511
529. [43] L. Brunsveld, B.J.B. Folmer, E.W. Meijer, R.P. Sijbesma, Supramolecular polymers, Chem. Rev. 101 (12) (2001) 4071
4097. [44] T.F.A. de Greef, E.W. Meijer, Supramolecular polymers, Nature 453 (2008) 171. [45] D.M. Rudkevich, Supramolecular polymers, 2nd Edition, edited by Alberto Ciferri. J. Am. Chem. Soc. 2006, 128 (21), 7110. [46] R. Hofsaeβ, P. Ensslen, H.-A. Wagenknecht, Control of helical chirality in supramolecular chromophoreDNA architectures, Chem. Commun. 55 (9) (2019) 1330
1333. [47] E.A. Appel, U. Rauwald, O.A. Scherman, 6.16 - Dynamic Supramolecular Polymers, in: K. Matyjaszewski, M. Mo¨ller (Eds.), In Polymer Science: A Comprehensive Reference, Elsevier, Amsterdam, 2012, pp. 587
62x8. [48] A. Winter M.D. Hager, U.S. Schubert, Supramolecular polymers, in: Reference Module in Materials Science and Materials Engineering, Elsevier, (2016). [49] T.F.A. De Greef, M.M.J. Smulders, M. Wolffs, A.P.H.J. Schenning, R.P. Sijbesma, E.W. Meijer, Supramolecular polymerization, Chem. Rev. 109 (11) (2009) 5687
5754. [50] D. Zhao, J.S. Moore, Nucleation-elongation: a mechanism for cooperative supramolecular polymerization, Org. Biomol. Chem. 1 (20) (2003) 3471
3491. [51] D. Zhao, J.S. Moore, Shape-persistent arylene ethynylene macrocycles: syntheses and supramolecular chemistry, Chem. Commun. 7 (2003) 807
818. [52] S.J. Cantrill, G.J. Youn, J.F. Stoddart, D.J. Williams, Supramolecular daisy chains, J. Org. Chem. 66 (21) (2001) 6857
6872. [53] B. Isare, S. Pensec, M. Raynal, L. Bouteiller, Bisurea-based supramolecular polymers: from structure to properties, C. R. Chim. 19 (1-2) (2016) 148
156. [54] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J.B. Folmer, J.H.K.K. Hirschberg, R.F.M. Lange, et al., Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding, Science 278 (5343) (1997) 1601
1604. [55] S.H. Chen, A.C. Su, H.L. Chou, K.Y. Peng, S.A. Chen, Phase behavior and molecular aggregation in bulk poly(2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene, Macromolecules 37 (1) (2004) 167
173. [56] W. Gao, M. Bie, F. Liu, P. Chang, Y. Quan, Self-healable and reprocessable polysulfide sealants prepared from liquid polysulfide oligomer and epoxy resin, ACS Appl. Mater. Interfaces 9 (18) (2017) 15798
15808.

Self-Healing Polymer-Based Systems

359

References

[57] A. Harada, Preparation and structures of supramolecules between cyclodextrins and polymers, Coord. Chem. Rev. 148 (1996) 115
133. [58] T.F.A. de Greef, G. Ercolani, G.B.W.L. Ligthart, E.W. Meijer, R.P. Sijbesma, Influence of selectivity on the supramolecular polymerization of AB-type polymers capable of both A A and A B interactions, J. Am. Chem. Soc. 130 (41) (2008) 13755
13764. [59] A.W. Bosman, R.P. Sijbesma, E.W. Meijer, Supramolecular polymers at work, Mater. Today 7 (4) (2004) 34
39. [60] G.M.L. van Gemert, J.W. Peeters, S.H.M. Soentjens, H.M. Janssen, A.W. Bosman, Self-healing supramolecular polymers in action, Macromol. Chem. Phys. 213 (2) (2012) 234
242. [61] B.J.B. Folmer, R.P. Sijbesma, R.M. Versteegen, J.A.J. van der Rijt, E.W. Meijer, Supramolecular polymer materials: chain extension of telechelic polymers using a reactive hydrogen-bonding synthon, Adv. Mater. 12 (12) (2000) 874
878. [62] Y. Chen, A.M. Kushner, G.A. Williams, Z. Guan, Multiphase design of autonomic self-healing thermoplastic elastomers, Nat. Chem. 4 (2012) 467. [63] H. Liao, S. Liao, X. Tao, C. Liu, Y. Wang, Intrinsically recyclable and self-healable conductive supramolecular polymers for customizable electronic sensors, J. Mater. Chem. C. 6 (47) (2018) 12992
12999. [64] B. Li, L. Kan, S. Zhang, Z. Liu, C. Li, W. Li, et al., Planting carbon nanotubes onto supramolecular polymer matrices for waterproof non-contact self-healing, Nanoscale 11 (2) (2019) 467
473. [65] N.M. Gallagher, A.V. Zhukhovitskiy, H.V.T. Nguyen, J.A. Johnson, Main-chain zwitterionic supramolecular polymers derived from N-heterocyclic carbene
carbodiimide (NHC
CDI) adducts, Macromolecules 51 (8) (2018) 3006
3016. [66] W.C. Yount, H. Juwarker, S.L. Craig, Orthogonal control of dissociation dynamics relative to thermodynamics in a main-chain reversible polymer, J. Am. Chem. Soc. 125 (50) (2003) 15302
15303. [67] E.C. Constable, A.M.W.C. Thompson, Multinucleating 2,20 : 60 ,2v-terpyridine ligands as building blocks for the assembly of co-ordination polymers and oligomers, J. Am. Chem. Soc. 24 (1992) 3467
3475. [68] B. Lahn, M. Rehahn, Coordination polymers from kinetically labile copper(I) and silver(I) complexes: true macromolecules or solution aggregates? Macromol. Symposia 163 (1) (2001) 157
176. [69] U.S. Schubert, C. Eschbaumer, Macromolecules containing bipyridine and terpyridine metal complexes: towards metallosupramolecular, Polym. Angew. Chem. Int. Ed. 41 (16) (2002) 2892
2926. [70] S. Schmatloch, M.F. Gonza´lez, U.S. Schubert, Metallo-supramolecular diethylene glycol: water-soluble reversible polymers, Macromol. Rapid Commun. 23 (16) (2002) 957
961. [71] B.J. Holliday, T.B. Stanford, T.M. Swager, Chemoresistive gas-phase nitric oxide sensing with cobaltcontaining conducting metallopolymers, Chem. Mater. 18 (24) (2006) 5649
5651. [72] D.A. Rider, K.A. Cavicchi, K.N. Power-Billard, T.P. Russell, I. Manners, Diblock copolymers with amorphous atactic polyferrocenylsilane blocks: synthesis, characterization, and self-assembly of polystyrene-block-poly (ferrocenylethylmethylsilane) in the bulk state, Macromolecules 38 (16) (2005) 6931
6938. [73] R.G.H. Lammertink, M.A. Hempenius, J.E. van den Enk, V.Z.H. Chan, E.L. Thomas, G.J. Vancso, Nanostructured thin films of organic
organometallic block copolymers: one-step lithography with poly(ferrocenylsilanes) by reactive ion etching, Adv. Mater. 12 (2) (2000) 98
103. [74] G.L. Fiore, C.L. Fraser, Iron-centered star polymers with pentablock bipyridine-centered PEG-PCL-PLA macroligands, Macromolecules 41 (21) (2008) 7892
7897. [75] S.J. Payne, G.L. Fiore, C.L. Fraser, J.N. Demas, Luminescence oxygen sensor based on a ruthenium(II) star polymer complex, Anal. Chem. 82 (3) (2010) 917
921. [76] A. Kishimura, T. Yamashita, K. Yamaguchi, T. Aida, Rewritable phosphorescent paper by the control of competing kinetic and thermodynamic self-assembling events, Nat. Mater. 4 (7) (2005) 546
549. [77] M. Serrano Ruiz, A. Romerosa, B. Sierra-Martin, A. Fernandez-Barbero, A. Water, Soluble diruthenium
gold organometallic microgel, Angew. Chem. Int. Ed. 47 (45) (2008) 8665
8669. [78] J.M.J. Paulusse, D.J.M. van Beek, R.P. Sijbesma, Reversible switching of the sol 2 gel transition with ultrasound in rhodium(I) and iridium(I) coordination networks, J. Am. Chem. Soc. 129 (8) (2007) 2392
2397. [79] W.C. Yount, D.M. Loveless, S.L. Craig, Small-molecule dynamics and mechanisms underlying the macroscopic mechanical properties of coordinatively cross-linked polymer networks, J. Am. Chem. Soc. 127 (41) (2005) 14488
14496.



Self-Healing Polymer-Based Systems



360

12. Self-healing materials utilizing supramolecular interactions

[80] P.R. Andres, U.S. Schubert, New functional polymers and materials based on 2,20 :60 ,2v-terpyridine metal complexes, Adv. Mater. 16 (13) (2004) 1043
1068. [81] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794
797. [82] F. Herbst, D. Do¨hler, P. Michael, W.H. Binder, Self-healing polymers via supramolecular forces, Macromol. Rapid Commun. 34 (3) (2013) 203
220. [83] C.-C. Chen, E.E. Dormidontova, Supramolecular polymer formation by metal 2 ligand complexation: monte carlo simulations and analytical modeling, J. Am. Chem. Soc. 126 (45) (2004) 14972
14978. [84] G.R. Whittell, M.D. Hager, U.S. Schubert, I. Manners, Functional soft materials from metallopolymers and metallosupramolecular polymers, Nat. Mater. 10 (2011) 176. [85] M.J. Serpe, S.L. Craig, Physical organic chemistry of supramolecular polymers, Langmuir 23 (4) (2007) 1626
1634. [86] C.-F. Chow, S. Fujii, J.-M. Lehn, Metallodynamers: neutral dynamic metallosupramolecular polymers displaying transformation of mechanical and optical properties on constitutional exchange, Angew. Chem. Int. Ed. 46 (26) (2007) 5007
5010. [87] Z. Wang, M.W. Urban, Facile UV-healable polyethylenimine
copper (C2H5N
Cu) supramolecular polymer networks, Poly Chem. 4 (18) (2013) 4897
4901. [88] T. Feldner, M. Ha¨ring, S. Saha, J. Esquena, R. Banerjee, D.D. Dı´az, Supramolecular metallogel that imparts self-healing properties to other gel networks, Chem. Mater. 28 (9) (2016) 3210
3217. [89] K.A. Williams, A.J. Boydston, C.W. Bielawski, Towards electrically conductive, self-healing materials, J. R. Soc. Interface 4 (13) (2007) 359
362. [90] B.J. Holliday, T.M. Swager, Conducting metallopolymers: the roles of molecular architecture and redox matching, Chem. Commun. (1)(2005) 23
36. [91] L. Yan, S. Gou, Z. Ye, S. Zhang, L. Ma, Self-healing and moldable material with the deformation recovery ability from self-assembled supramolecular metallogels, Chem. Commun. 50 (85) (2014) 12847
12850. [92] C.-H. Li, C. Wang, C. Keplinger, J.-L. Zuo, L. Jin, Y. Sun, et al., A highly stretchable autonomous self-healing elastomer, Nat. Chem. 8 (2016) 618. [93] S.-R. Mo, J.-C. Lai, K.-Y. Zeng, D.-P. Wang, C.-H. Li, J.-L. Zuo, New insights into the mechanical and selfhealing properties of polymers cross-linked by Fe(iii)-2,6-pyridinedicarboxamide coordination complexes, Poly Chem. 10 (3) (2019) 362
371. [94] W.C. Yount, D.M. Loveless, S.L. Craig, Strong means slow: dynamic contributions to the bulk mechanical properties of supramolecular, Netw. Angew. Chem. Int. Ed. 44 (18) (2005) 2746
2748. [95] M.J. Harrington, A. Masic, N. Holten-Andersen, J.H. Waite, P. Fratzl, Iron-clad fibers: a metal-based biological strategy for hard flexible coatings, Science 328 (5975) (2010) 216. [96] N. Holten-Andersen, M.J. Harrington, H. Birkedal, B.P. Lee, P.B. Messersmith, K.Y.C. Lee, et al., pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with nearcovalent elastic moduli, Proc. Natl Acad. Sci. 108 (7) (2011) 2651. [97] M. Krogsgaard, M.A. Behrens, J.S. Pedersen, H. Birkedal, Self-healing mussel-inspired multi-pH-responsive hydrogels, Biomacromolecules 14 (2) (2013) 297
301. [98] D. Mozhdehi, S. Ayala, O.R. Cromwell, Z. Guan, Self-healing multiphase polymers via dynamic metal
ligand interactions, J. Am. Chem. Soc. 136 (46) (2014) 16128
16131. [99] J.B. Beck, S.J. Rowan, Multistimuli, multiresponsive metallo-supramolecular polymers, J. Am. Chem. Soc. 125 (46) (2003) 13922
13923. [100] R. Shunmugam, G.N. Tew, Unique emission from polymer based lanthanide alloys, J. Am. Chem. Soc. 127 (39) (2005) 13567
13572. [101] M. Martı´nez-Calvo, O. Kotova, M.E. Mo¨bius, A.P. Bell, T. McCabe, J.J. Boland, et al., Healable luminescent self-assembly supramolecular metallogels possessing lanthanide (Eu/Tb) dependent rheological and morphological properties, J. Am. Chem. Soc. 137 (5) (2015) 1983
1992. [102] B.H. Northrop, H.-B. Yang, P.J. Stang, Coordination-driven self-assembly of functionalized supramolecular metallacycles, Chem. Commun. (45)(2008) 5896
5908. [103] J.-M. Lehn, Toward self-organization and complex matter, Science 295 (5564) (2002) 2400. [104] M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa, K. Biradha, Molecular paneling coordination, Chem. Commun. 6 (2001) 509
518.

Self-Healing Polymer-Based Systems

References

361

[105] Q.-F. Sun, T. Murase, S. Sato, M. Fujita, A. Sphere-in-Sphere, Complex by orthogonal self-assembly, Angew. Chem. Int. Ed. 50 (44) (2011) 10318
10321. [106] X. Yan, T.R. Cook, J.B. Pollock, P. Wei, Y. Zhang, Y. Yu, et al., Responsive supramolecular polymer metallogel constructed by orthogonal coordination-driven self-assembly and host/guest interactions, J. Am. Chem. Soc. 136 (12) (2014) 4460
4463. [107] J. Zhan, M. Zhang, M. Zhou, B. Liu, D. Chen, Y. Liu, et al., Supramolecular polymer gel network based on multiple orthogonal interactions, Macromol. Rapid Commun. 35 (16) (2014) 1424
1429. [108] W. Zheng, L.-J. Chen, G. Yang, B. Sun, X. Wang, B. Jiang, et al., Construction of smart supramolecular polymeric hydrogels cross-linked by discrete organoplatinum(II) metallacycles via post-assembly polymerization, J. Am. Chem. Soc. 138 (14) (2016) 4927
4937. [109] Y. Gu, E.A. Alt, H. Wang, X. Li, A.P. Willard, J.A. Johnson, Photoswitching topology in polymer networks with metal
organic cages as crosslinks, Nature 560 (7716) (2018) 65
69. [110] D. Kim, D.R. Whang, S.Y. Park, Self-healing of molecular catalyst and photosensitizer on metal
organic framework: robust molecular system for photocatalytic H2 evolution from water, J. Am. Chem. Soc. 138 (28) (2016) 8698
8701. [111] R.K. Iler, Multilayers of colloidal particles, J. Colloid Interface Sci. 21 (6) (1966) 569
594. [112] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (5330) (1997) 1232. [113] P. Bertrand, A. Jonas, A. Laschewsky, R. Legras, Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: suitable materials, structure and properties, Macromol. Rapid Commun. 21 (7) (2000) 319
348. [114] J.H. Fendler, Self-assembled nanostructured materials, Chem. Mater. 8 (8) (1996) 1616
1624. [115] M. Coustet, J. Irigoyen, T.A. Garcia, R.A. Murray, G. Romero, M. Susana Cortizo, et al., Layer-by-layer assembly of polymersomes and polyelectrolytes on planar surfaces and microsized colloidal particles, J. Colloid Interface Sci. 421 (2014) 132
140. [116] F.G. Aliev, M.A. Correa-Duarte, A. Mamedov, J.W. Ostrander, M. Giersig, L.M. Liz-Marza´n, et al., Layerby-layer assembly of core-shell magnetite nanoparticles: effect of silica coating on interparticle interactions and magnetic properties, Adv. Mater. 11 (12) (1999) 1006
1010. [117] N.A. Kotov, I. Dekany, J.H. Fendler, Layer-by-layer self-assembly of polyelectrolyte-semiconductor nanoparticle composite films, J. Phys. Chem. 99 (35) (1995) 13065
13069. [118] N.A. Kotov, S. Magonov, E. Tropsha, Layer-by-layer self-assembly of alumosilicate 2 polyelectrolyte composites: mechanism of deposition, crack resistance, and perspectives for novel membrane materials, Chem. Mater. 10 (3) (1998) 886
895. [119] N.A. Kotov, I. De´ka´ny, J.H. Fendler, Ultrathin graphite oxide
polyelectrolyte composites prepared by self-assembly: transition between conductive and non-conductive states, Adv. Mater. 8 (8) (1996) 637
641. [120] J. Shen, Y. Pei, P. Dong, J. Ji, Z. Cui, J. Yuan, et al., Layer-by-layer self-assembly of polyelectrolyte functionalized MoS2 nanosheets, Nanoscale 8 (18) (2016) 9641
9647. [121] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, et al., Layer-bylayer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations, Chem. Mater. 11 (3) (1999) 771
778. [122] H. Alem, F. Blondeau, K. Glinel, S. Demoustier-Champagne, A.M. Jonas, Layer-by-layer assembly of polyelectrolytes in nanopores, Macromolecules 40 (9) (2007) 3366
3372. [123] J.P. DeRocher, P. Mao, J. Han, M.F. Rubner, R.E. Cohen, Layer-by-layer assembly of polyelectrolytes in nanofluidic devices, Macromolecules 43 (5) (2010) 2430
2437. [124] B. Philipp, H. Dautzenberg, K.-J. Linow, J. Ko¨tz, W. Dawydoff, Polyelectrolyte complexes — recent developments and open problems, Prog. Poly Sci. 14 (1) (1989) 91
172. [125] P. Ko¨berle, A. Laschewsky, Hybrid materials from organic polymers and inorganic salts, Macromol. Symposia 88 (1) (1994) 165
175. [126] N.G. Hoogeveen, M.A. Cohen Stuart, G.J. Fleer, M.R. Bo¨hmer, Formation and stability of multilayers of polyelectrolytes, Langmuir 12 (15) (1996) 3675
3681. [127] A.B. South, L.A. Lyon, Autonomic self-healing of hydrogel thin films, Angew. Chem. Int. Ed. 49 (4) (2010) 767
771. [128] X. Wang, F. Liu, X. Zheng, J. Sun, Water-enabled self-healing of polyelectrolyte multilayer, Coat. Angew. Chem. Int. Ed. 50 (48) (2011) 11378
11381.

Self-Healing Polymer-Based Systems

362

12. Self-healing materials utilizing supramolecular interactions

[129] H. Zhang, C. Wang, G. Zhu, N.S. Zacharia, Self-healing of bulk polyelectrolyte complex material as a function of pH and salt, ACS Appl. Mater. Interfaces 8 (39) (2016) 26258
26265. [130] Z. Wang, E. van Andel, S.P. Pujari, H. Feng, J.A. Dijksman, M.M.J. Smulders, et al., Water-repairable zwitterionic polymer coatings for anti-biofouling surfaces, J. Mater. Chem. B 5 (33) (2017) 6728
6733. [131] D. Gaddes, H. Jung, A. Pena-Francesch, G. Dion, S. Tadigadapa, W.J. Dressick, et al., Self-Healing textile: enzyme encapsulated layer-by-layer structural proteins, ACS Appl. Mater. Interfaces 8 (31) (2016) 20371
20378. [132] Y. Miwa, J. Kurachi, Y. Kohbara, S. Kutsumizu, Dynamic ionic crosslinks enable high strength and ultrastretchability in a single elastomer, Commun. Chem. 1 (1) (2018) 5. [133] L. Leibler, M. Rubinstein, R.H. Colby, Dynamics of reversible networks, Macromolecules 24 (16) (1991) 4701
4707. [134] A.V. Tobolsky, P.F. Lyons, N. Hata, Ionic clusters in high-strength carboxylic rubbers, Macromolecules 1 (6) (1968) 515
519. [135] H. Tachino, H. Hara, E. Hirasawa, S. Kutsumizu, K. Tadano, S. Yano, Dynamic mechanical relaxations of ethylene ionomers, Macromolecules 26 (4) (1993) 752
757. [136] K.A. Pestka, J.D. Buckley, S.J. Kalista, N.R. Bowers, Elastic evolution of a self-healing ionomer observed via acoustic and ultrasonic resonant spectroscopy, Sci. Rep. 7 (1) (2017) 14417. [137] S.J. Kalista, J.R. Pflug, R.J. Varley, Effect of ionic content on ballistic self-healing in EMAA copolymers and ionomers, Poly Chem. 4 (18) (2013) 4910
4926. [138] Y. Huang, M. Zhong, Y. Huang, M. Zhu, Z. Pei, Z. Wang, et al., A self-healable and highly stretchable supercapacitor based on a dual crosslinked polyelectrolyte, Nat. Commun. 6 (2015) 10310. [139] J.C. Salamone, S.C. Israel, P. Taylor, B. Snider, Synthesis and homopolymerization studies of vinylimidazolium salts, Polymer 14 (12) (1973) 639
644. [140] S. Grubjesic, S. Lee, M.A. Firestone, The design of polymeric ionic liquids for the preparation of functional materials AU - green, omar, Polym. Rev. 49 (4) (2009) 339
360. [141] J. Cui, F.-M. Nie, J.-X. Yang, L. Pan, Z. Ma, Y.-S. Li, Novel imidazolium-based poly(ionic liquid)s with different counterions for self-healing, J. Mater. Chem. A 5 (48) (2017) 25220
25229. [142] P. Guo, H. Zhang, X. Liu, J. Sun, Counteranion-mediated intrinsic healing of poly(ionic liquid) copolymers, ACS Appl. Mater. Interfaces 10 (2) (2018) 2105
2113. [143] M. Suckow, A. Mordvinkin, M. Roy, N.K. Singha, G. Heinrich, B. Voit, et al., Tuning the properties and self-healing behavior of ionically modified poly(isobutylene-co-isoprene) rubber, Macromolecules 51 (2) (2018) 468
479. [144] S. Shinde, J.L. Sartucci, D.K. Jones, N. Gavvalapalli, Dynamic π-conjugated polymer ionic networks, Macromolecules 50 (19) (2017) 7577
7583. [145] S. Tamesue, M. Ohtani, K. Yamada, Y. Ishida, J.M. Spruell, N.A. Lynd, et al., Linear versus dendritic molecular binders for hydrogel network formation with clay nanosheets: studies with ABA triblock copolyethers carrying guanidinium ion pendants, J. Am. Chem. Soc. 135 (41) (2013) 15650
15655. [146] T.J. Cuthbert, J.J. Jadischke, J.R. de Bruyn, P.J. Ragogna, E.R. Gillies, Self-healing polyphosphonium ionic networks, Macromolecules 50 (14) (2017) 5253
5260. [147] Y. Xiong, Z. Chen, H. Wang, L.-M. Ackermann, M. Klapper, H.-J. Butt, et al., An autonomic self-healing organogel with a photo-mediated modulus, Chem. Commun. 52 (98) (2016) 14157
14160. [148] D.A. Dougherty, Cation-π interactions in chemistry and biology: a new view of benzene, phe, tyr, and trp, Science 271 (5246) (1996) 163. [149] Q. Lu, D.X. Oh, Y. Lee, Y. Jho, D.S. Hwang, H. Zeng, Nanomechanics of cation
π interactions in aqueous solution, Angew. Chem. Int. Ed. 52 (14) (2013) 3944
3948. [150] S. Kim, J. Huang, Y. Lee, S. Dutta, H.Y. Yoo, Y.M. Jung, et al., Complexation and coacervation of likecharged polyelectrolytes inspired by mussels, Proc. Natl Acad. Sci. 113 (7) (2016) E847. [151] Y. Takashima, A. Harada, Stimuli-responsive polymeric materials functioning via host-guest interactions, J. Incl. Phenom. Macrocycl. Chem. 88 (3-4) (2017) 85
104. [152] S. Nomimura, M. Osaki, J. Park, R. Ikura, Y. Takashima, H. Yamaguchi, et al., Self-healing alkyl acrylate-based supramolecular elastomers cross-linked via host-guest interactions, Macromolecules 52 (7) (2019) 2659
2668. [153] E.P. Kyba, R.C. Helgeson, K. Madan, G.W. Gokel, T.L. Tarnowski, S.S. Moore, et al., Host-guest complexation. 1. Concept and illustration, J. Am. Chem. Soc. 99 (8) (1977) 2564
2571.

Self-Healing Polymer-Based Systems

References

363

[154] D.E. Koshland, Jr., The lock-and-key principle and the induced-fit theory. Angew. Chem. 1994, 106 (23/24), 2468
272 (See also Angew. Chem., Int. Ed. Engl., 1994, 33(23/24), 2475
2478). [155] A. Werner, Beitrag zur konstitution anorganischer verbindungen, Z. fu¨r anorganische Chem. 3 (1) (1893) 267
330. [156] X. Yang, H. Yu, L. Wang, R. Tong, M. Akram, Y. Chen, et al., Self-healing polymer materials constructed by macrocycle-based host-guest interactions, Soft Matter 11 (7) (2015) 1242
1252. [157] K. Guo, M.-S. Lin, J.-F. Feng, M. Pan, L.-S. Ding, B.-J. Li, et al., The deeply understanding of the self-healing mechanism for self-healing behavior of supramolecular materials based on cyclodextrin-guest interactions, Macromol. Chem. Phys. 218 (10) (2017). [158] M.L. Bender, M. Komiyama, Cyclodextrin chemistry, Springer, 1978, p. 96. [159] A. Harada, Y. Takashima, M. Nakahata, Supramolecular polymeric materials via cyclodextrin
guest interactions, Acc. Chem. Res. 47 (7) (2014) 2128
2140. [160] C.J. Pedersen, Cyclic polyethers and their complexes with metal salts, J. Am. Chem. Soc. 89 (26) (1967) 7017
7036. [161] C.J. Pedersen, The discovery of crown ethers (noble lecture), Angew. Chem. Int. Ed. 27 (8) (1988) 1021
1027. [162] R. Behrend, E. Meyer, F. Rusche, I. ueber condensationsproducte aus glycoluril und formaldehyd, Justus Liebigs Annalen der Chem. 339 (1) (1905) 1
37. [163] J. Lagona, P. Mukhopadhyay, S. Chakrabarti, L. Isaacs, The cucurbit[n]uril family, Angew. Chem. Int. Ed. 44 (31) (2005) 4844
4870. [164] A. Ikeda, S. Shinkai, Novel cavity design using calix[n]arene skeletons: toward molecular recognition and metal binding, Chem. Rev. 97 (5) (1997) 1713
1734. [165] T. Kakuta, Y. Takashima, A. Harada, Highly elastic supramolecular hydrogels using host
guest inclusion complexes with cyclodextrins, Macromolecules 46 (11) (2013) 4575
4579. [166] M. Nakahata, Y. Takashima, H. Yamaguchi, A. Harada, Redox-responsive self-healing materials formed from host-guest polymers, Nat. Commun. 2 (Oct) (2011) 1521/1
1521/6. [167] T. Kakuta, Y. Takashima, M. Nakahata, M. Otsubo, H. Yamaguchi, A. Harada, Preorganized hydrogel: selfhealing properties of supramolecular hydrogels formed by polymerization of host-guest-monomers that contain cyclodextrins and hydrophobic guest groups, Adv. Mater. 25 (20) (2013) 2849
2853. [168] S. Dong, B. Zheng, F. Wang, F. Huang, Supramolecular polymers constructed from macrocycle-based host
guest molecular recognition motifs, Acc. Chem. Res. 47 (7) (2014) 1982
1994. [169] K. Morino, M. Oobo, E. Yashima, Helicity induction in a poly(phenylacetylene) bearing aza-18-crown-6 ether pendants with optically active bis(amino acid)s and its chiral stimuli-responsive gelation, Macromolecules 38 (8) (2005) 3461
3468. [170] Z. Ge, J. Hu, F. Huang, S. Liu, Responsive supramolecular gels constructed by crown ether based molecular recognition, Angew. Chem. Int. Ed. 48 (10) (2009) 1798
1802. [171] Y. Suzaki, T. Taira, K. Osakada, Physical gels based on supramolecular gelators, including host
guest complexes and pseudorotaxanes, J. Mater. Chem. 21 (4) (2011) 930
938. [172] M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, et al., Self-healing supramolecular gels formed by crown ether based host-guest interactions, Angew. Chem, Int. Ed. 51 (28) (2012). 7011-7015, S7011/1-S7011/ 19. [173] D. Liu, D. Wang, M. Wang, Y. Zheng, K. Koynov, G.K. Auernhammer, et al., Supramolecular organogel based on crown ether and secondary ammoniumion functionalized glycidyl triazole polymers, Macromolecules 46 (11) (2013) 4617
4625. [174] J. Rebek, Introduction to the molecular recognition and self-assembly special feature, Proc. Natl Acad. Sci. 106 (26) (2009) 10423. [175] C. Hilger, M. Draeger, R. Stadler, Molecular origin of supramolecular self-assembling in statistical copolymers, Macromolecules 25 (9) (1992) 2498
2501. [176] J.-M. Lehn, Supramolecular chemistry—scope and perspectives molecules, supermolecules, and molecular devices (nobel lecture), Angew. Chem. Int. Ed. 27 (1) (1988) 89
112. [177] J.R. McKee, E.A. Appel, J. Seitsonen, E. Kontturi, O.A. Scherman, O. Ikkala, Healable, stable and stiff hydrogels: combining conflicting properties using dynamic and selective three-component recognition with reinforcing cellulose nanorods, Adv. Funct. Mater. 24 (18) (2014) 2706
2713.

Self-Healing Polymer-Based Systems

364

12. Self-healing materials utilizing supramolecular interactions

[178] C. Yu, C.-F. Wang, S. Chen, Robust self-healing host
guest gels from magnetocaloric radical polymerization, Adv. Funct. Mater. 24 (9) (2014) 1235
1242. [179] S. Li, H.-Y. Lu, Y. Shen, C.-F. Chen, A stimulus-response and self-healing supramolecular polymer gel based on host
guest interactions, Macromol. Chem. Phys. 214 (14) (2013) 1596
1601. [180] S.J. Rowan, S.J. Cantrill, G.R.L. Cousins, J.K.M. Sanders, J.F. Stoddart, Dynamic covalent chemistry. 2002, 41 (6), 898
952. [181] Aida, T.; Meijer, E.W.; Stupp, S.I., Functional supramolecular polymers. 2012, 335 (6070), 813
817. [182] R. Narayan, U.Y. Nayak, A.M. Raichur, S. Garg, Mesoporous silica nanoparticles: a comprehensive review on synthesis and recent advances, Pharmaceutics 10 (3) (2018) 118. [183] L. He, D.E. Fullenkamp, J.G. Rivera, P.B. Messersmith, pH responsive self-healing hydrogels formed by boronate
catechol complexation, Chem. Commun. 47 (26) (2011) 7497
7499. [184] M.E. Belowich, J.F. Stoddart, Dynamic imine chemistry, Chem. Soc. Rev. 41 (6) (2012) 2003
2024. [185] D.E. Apostolides, C.S. Patrickios, Dynamic covalent polymer hydrogels and organogels crosslinked through acylhydrazone bonds: synthesis, characterization and applications, 67 (6) (2018) 627
649. [186] R. Chang, X. Wang, X. Li, H. An, J. Qin, Self-activated healable hydrogels with reversible temperature responsiveness, ACS Appl. Mater. Interfaces 8 (38) (2016) 25544
25551. [187] G. Deng, F. Li, H. Yu, F. Liu, C. Liu, W. Sun, et al., Dynamic hydrogels with an environmental adaptive self-healing ability and dual responsive sol
gel transitions, ACS Macro Lett. 1 (2) (2012) 275
279. [188] J. Xu, D. Yang, W. Li, Y. Gao, H. Chen, H. Li, Phenylboronate-diol crosslinked polymer gels with reversible sol-gel transition, Polymer 52 (19) (2011) 4268
4276. [189] D. Roy, J.N. Cambre, B.S. Sumerlin, Triply-responsive boronic acid block copolymers: solution selfassembly induced by changes in temperature, pH, or sugar concentration, Chem. Commun. 16 (2009) 2106
2108. [190] D. Roy, J.N. Cambre, B.S. Sumerlin, Sugar-responsive block copolymers by direct RAFT polymerization of unprotected boronic acid monomers, Chem. Commun. 21 (2008) 2477
2479. [191] J.N. Cambre, D. Roy, B.S. Sumerlin, Tuning the sugar-response of boronic acid block copolymers. 50 (16) (2012) 3373
3382. [192] C.C. Deng, W.L.A. Brooks, K.A. Abboud, B.S. Sumerlin, Boronic acid-based hydrogels undergo self-healing at neutral and acidic pH, ACS Macro Lett. 4 (2) (2015) 220
224. [193] O.R. Cromwell, J. Chung, Z. Guan, Malleable and self-healing covalent polymer networks through tunable dynamic boronic ester bonds, J. Am. Chem. Soc. 137 (20) (2015) 6492
6495. [194] L. He, D. Szopinski, Y. Wu, G.A. Luinstra, P. Theato, Toward self-healing hydrogels using one-pot thiol
ene click and borax-diol chemistry, ACS Macro Lett. 4 (7) (2015) 673
678. [195] Z.Q. Lei, P. Xie, M.Z. Rong, M.Q. Zhang, Catalyst-free dynamic exchange of aromatic Schiff base bonds and its application to self-healing and remolding of crosslinked polymers, J. Mater. Chem. A 3 (39) (2015) 19662
19668. [196] P. Taynton, K. Yu, R.K. Shoemaker, Y. Jin, H.J. Qi, W. Zhang, Heat- or water-driven malleability in a highly recyclable covalent network polymer, Adv. Mater. (Deerfield Beach, Fla.) 26 (23) (2014) 3938
3942. [197] A. Chao, I. Negulescu, D. Zhang, Dynamic covalent polymer networks based on degenerative imine bond exchange: tuning the malleability and self-healing properties by solvent, Macromolecules 49 (17) (2016) 6277
6284. [198] H. Liu, X. Sui, H. Xu, L. Zhang, Y. Zhong, Z. Mao, Self-healing polysaccharide hydrogel based on dynamic covalent enamine bonds, Macromol. Mater. Eng. 301 (6) (2016) 725
732. [199] H.K.K. King, The reverse Diels
Alder or retrodiene reaction, Chem. Rev. 68 (1968) 415
447. [200] T. Paulo¨hrl, A.J. Inglis, C. Barner-Kowollik, Reversible Diels
Alder chemistry as a modular polymeric color switch. 22 (25) (2010) 2788
2791. [201] F. Ghezzo, D.R. Smith, T.N. Starr, T. Perram, A.F. Starr, T.K. Darlington, et al., Development and characterization of healable carbon fiber composites with a reversibly cross linked polymer, J. Composite Mater. 44 (13) (2010) 1587
1603. [202] K. Inoue, M. Yamashiro, M. Iji, Recyclable shape-memory polymer: poly(lactic acid) crosslinked by a thermoreversible Diels
Alder reaction. 112 (2) (2009) 876
885. [203] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (5560) (2002) 1698
1702.

Self-Healing Polymer-Based Systems

References

365

[204] N. Yoshie, M. Watanabe, H. Araki, K. Ishida, Thermo-responsive mending of polymers crosslinked by thermally reversible covalent bond: polymers from bisfuranic terminated poly(ethylene adipate) and tris-maleimide, Polym. Degrad. Stab. 95 (5) (2010) 826
829. [205] Y.-L. Liu, Y.-W. Chen, Thermally reversible cross-linked polyamides with high toughness and self-repairing ability from maleimide- and furan-functionalized aromatic polyamides. 208 (2) (2007) 224
232. [206] P. Du, X. Liu, Z. Zheng, X. Wang, T. Joncheray, Y. Zhang, Synthesis and characterization of linear selfhealing polyurethane based on thermally reversible Diels
Alder reaction, RSC Adv. 3 (35) (2013) 15475
15482. [207] K. Adachi, A.K. Achimuthu, Y. Chujo, Synthesis of organic 2 inorganic polymer hybrids controlled by Diels 2 Alder reaction, Macromolecules 37 (26) (2004) 9793
9797. [208] T.T. N’Guyen, H.T. Duong, J. Basuki, V. Montembault, S. Pascual, C. Guibert, et al., Functional iron oxide magnetic nanoparticles with hyperthermia-induced drug release ability by using a combination of orthogonal click reactions, Angew. Chem. Int. Ed. 52 (52) (2013) 14152
14156. [209] S. Scha¨fer, G. Kickelbick, Self-healing polymer nanocomposites based on Diels
Alder-reactions with silica nanoparticles: the role of the polymer matrix, Polymer 69 (2015) 357
368. [210] J.S. Park, T. Darlington, A.F. Starr, K. Takahashi, J. Riendeau, H. Thomas Hahn, Multiple healing effect of thermally activated self-healing composites based on Diels
Alder reaction, Compos. Sci. Technol. 70 (15) (2010) 2154
2159. [211] T. Engel, G. Kickelbick, Thermoreversible reactions on inorganic nanoparticle surfaces: Diels
Alder reactions on sterically crowded surfaces, Chem. Mater. 25 (2) (2013) 149
157. [212] J. Li, J. Liang, L. Li, F. Ren, W. Hu, J. Li, et al., Healable capacitive touch screen sensors based on transparent composite electrodes comprising silver nanowires and a furan/maleimide Diels-Alder cycloaddition polymer, ACS nano 8 (12) (2014) 12874
12882. [213] T. Engel, G. Kickelbick, Furan-modified spherosilicates as building blocks for self-healing materials, Eur. J. Inorg. Chem. 2015 (7) (2015) 1226
1232. [214] F. Yu, X. Cao, J. Du, G. Wang, X. Chen, Multifunctional hydrogel with good structure integrity, self-healing, and tissue-adhesive property formed by combining Diels
Alder click reaction and acylhydrazone bond, ACS Appl. Mater. Interfaces 7 (43) (2015) 24023
24031. [215] M. Capelot, D. Montarnal, F. Tournilhac, L. Leibler, Metal-catalyzed transesterification for healing and assembling of thermosets, J. Am. Chem. Soc. 134 (18) (2012) 7664
7667. [216] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Silica-like malleable materials from permanent organic networks. 334 (6058) (2011) 965
968. [217] Y.-X. Lu, F. Tournilhac, L. Leibler, Z. Guan, Making insoluble polymer networks malleable via olefin metathesis, J. Am. Chem. Soc. 134 (20) (2012) 8424
8427. [218] Y.-X. Lu, Z. Guan, Olefin metathesis for effective polymer healing via dynamic exchange of strong carbon
carbon double bonds, J. Am. Chem. Soc. 134 (34) (2012) 14226
14231. [219] Y.-Z. You, C.-Y. Hong, R.-K. Bai, C.-Y. Pan, J. Wang, Photo-initiated living free radical polymerization in the presence of dibenzyl trithiocarbonate. 203 (3) (2002) 477
483. [220] R. Ran, Y. Yu, T. Wan, Photoinitiated RAFT polymerization in the presence of trithiocarbonate, J. Appl. Polym. Sci. 105 (2) (2007) 398
404. [221] Y. Amamoto, J. Kamada, H. Otsuka, A. Takahara, K. Matyjaszewski, Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units, 50 (7) (2011) 1660
1663. [222] S. Ji; W. Cao, Y. Yu, H. Xu, Visible-light-induced self-healing diselenide-containing polyurethane elastomer, 27 (47) (2015) 7740
7745. [223] Y. Amamoto, H. Otsuka, A. Takahara, K. Matyjaszewski, Self-healing of covalently cross-linked polymers by reshuffling thiuram disulfide moieties in air under visible light, 24 (29) (2012) 3975
3980. [224] N. Oya, P. Sukarsaatmadja, K. Ishida, N. Yoshie, Photoinduced mendable network polymer from poly (butylene adipate) end-functionalized with cinnamoyl groups, Polym. J. 44 (2012) 724. [225] R. Gaur, A.S. Pathania, F.A. Malik, R.S. Bhakuni, R.K. Verma, Synthesis of a series of novel dihydroartemisinin monomers and dimers containing chalcone as a linker and their anticancer activity, Euro J. Med. Chem. 122 (2016) 232
246. [226] P. Froimowicz, H. Frey, K. Landfester, Towards the generation of self-healing materials by means of a reversible photo-induced approach, 32 (5) (2011) 468
473.

Self-Healing Polymer-Based Systems

366

12. Self-healing materials utilizing supramolecular interactions

[227] M. Abdallh, M.T.W. Hearn, G.P. Simon, K. Saito, Light triggered self-healing of polyacrylate polymers crosslinked with 7-methacryloyoxycoumarin crosslinker, Poly Chem. 8 (38) (2017) 5875
5883. [228] L. de Lucca Freitas, R. Stadler, Thermoplastic elastomers by hydrogen bonding 4. Influence of hydrogen bonding on the temperature dependence of the viscoelastic properties, Colloid Polym. Sci. 266 (12) (1988) 1095
1101. [229] R. Stadler, L. de Lucca Freitas, Thermoplastic elastomers by hydrogen bonding 1. Rheological properties of modified polybutadiene, Colloid Polym. Sci. 264 (9) (1986) 773
778. [230] W.H. Binder, R. Zirbs, Supramolecular polymers and networkswith hydrogen bonds in the main- and sidechain, in: W. Binder, (Ed.) Hydrogen Bonded Polymers, Springer Berlin Heidelberg: Berlin, Heidelberg, 2007; pp. 1
78. [231] Y. Yang, M.W. Urban, Self-healing of polymers via supramolecular chemistry, Adv. Mater. Interfaces 5 (17) (2018) 1800384. [232] X. Yan, D. Xu, J. Chen, M. Zhang, B. Hu, Y. Yu, et al., A self-healing supramolecular polymer gel with stimuli-responsiveness constructed by crown ether based molecular recognition, Poly Chem. 4 (11) (2013) 3312
3322. [233] Z.R. Hinton, A. Shabbir, N.J. Alvarez, Dynamics of supramolecular self-healing recovery in extension, Macromolecules 52 (6) (2019) 2231
2242. [234] O.A. Scherman, G.B.W.L. Ligthart, H. Ohkawa, R.P. Sijbesma, E.W. Meijer, Olefin metathesis and quadruple hydrogen bonding: a powerful combination in multistep supramolecular synthesis, Proc. Natl Acad. Sci. 103 (32) (2006) 11850
11855. [235] K.P. Nair, V. Breedveld, M. Weck, Complementary hydrogen-bonded thermoreversible polymer networks with tunable properties, Macromolecules 41 (10) (2008) 3429
3438. [236] B. Zhu, N. Jasinski, A. Benitez, M. Noack, D. Park, A.S. Goldmann, et al., Hierarchical nacre mimetics with synergistic mechanical properties by control of molecular interactions in self-healing polymers, Angew. Chem. Int. Ed. 54 (30) (2015) 8653
8657. [237] G. Zhang, Y. Chen, Y. Deng, T. Ngai, C. Wang, Dynamic supramolecular hydrogels: regulating hydrogel properties through self-complementary quadruple hydrogen bonds and thermo-switch, ACS Macro Lett. 6 (7) (2017) 641
646. [238] N. Oya, T. Ikezaki, N. Yoshie, A crystalline supramolecular polymer with self-healing capability at room temperature, Polym. J. 45 (2013) 955. [239] D. Zhu, Q. Ye, X. Lu, Q. Lu, Self-healing polymers with PEG oligomer side chains based on multiple Hbonding and adhesion properties, Poly Chem. 6 (28) (2015) 5086
5092. [240] A. Faghihnejad, K.E. Feldman, J. Yu, M.V. Tirrell, J.N. Israelachvili, C.J. Hawker, et al., Adhesion and surface interactions of a self-healing polymer with multiple hydrogen-bonding groups, Adv. Funct. Mater. 24 (16) (2014) 2322
2333. [241] C.-C. Cheng, F.-C. Chang, J.-K. Chen, T.-Y. Wang, D.-J. Lee, High-efficiency self-healing materials based on supramolecular polymer networks, RSC Adv. 5 (122) (2015) 101148
101154. [242] X. Hu, M. Vatankhah-Varnoosfaderani, J. Zhou, Q. Li, S.S. Sheiko, Weak hydrogen bonding enables hard, strong, tough, and elastic hydrogels, Adv. Mater. 27 (43) (2015) 6899
6905. [243] X. Dai, Y. Zhang, L. Gao, T. Bai, W. Wang, Y. Cui, et al., A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel, Adv. Mater. 27 (23) (2015) 3566
3571. [244] L. Teng, Y. Chen, M. Jin, Y. Jia, Y. Wang, L. Ren, Weak hydrogen bonds lead to self-healable and bioadhesive hybrid polymeric hydrogels with mineralization-active functions, Biomacromolecules 19 (6) (2018) 1939
1949. [245] A. Phadke, C. Zhang, B. Arman, C.C. Hsu, R.A. Mashelkar, A.K. Lele, et al., Rapid self-healing hydrogels, Proc. Natl Acad. Sci. 109 (12) (2012) 4383
4388. [246] B.V.S. Iyer, I.G. Salib, V.V. Yashin, T. Kowalewski, K. Matyjaszewski, A.C. Balazs, Modeling the response of dual cross-linked nanoparticle networks to mechanical deformation, Soft Matter 9 (1) (2013) 109
121. [247] F. Sordo, S.-J. Mougnier, N. Loureiro, F. Tournilhac, V. Michaud, Design of self-healing supramolecular rubbers with a tunable number of chemical cross-links, Macromolecules 48 (13) (2015) 4394
4402. [248] N. Roy, E. Buhler, J.-M. Lehn, Double dynamic self-healing polymers: supramolecular and covalent dynamic polymers based on the bis-iminocarbohydrazide motif, Poly Int. 63 (8) (2014) 1400
1405.

Self-Healing Polymer-Based Systems

References

367

[249] B. Zhang, Z.A. Digby, J.A. Flum, E.M. Foster, J.L. Sparks, D. Konkolewicz, Self-healing, malleable and creep limiting materials using both supramolecular and reversible covalent linkages, Poly Chem. 6 (42) (2015) 7368
7372. [250] L. Qiao, C. Liu, C. Liu, L. Yang, M. Zhang, W. Liu, et al., Self-healing alginate hydrogel based on dynamic acylhydrazone and multiple hydrogen bonds, J. Mater. Sci. 54 (11) (2019) 8814
8828. [251] Y. Zhang, L. Tao, S. Li, Y. Wei, Synthesis of multiresponsive and dynamic chitosan-based hydrogels for controlled release of bioactive molecules, Biomacromolecules 12 (8) (2011) 2894
2901. [252] Z. Wei, J.H. Yang, Z.Q. Liu, F. Xu, J.X. Zhou, M. Zrı´nyi, et al., Novel biocompatible polysaccharide-based self-healing hydrogel, Adv. Funct. Mater. 25 (9) (2015) 1352
1359. [253] X. Yang, G. Liu, L. Peng, J. Guo, L. Tao, J. Yuan, et al., Highly efficient self-healable and dual responsive cellulose-based hydrogels for controlled release and 3D cell culture, Adv. Funct. Mater. 27 (40) (2017) 1703174.

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C H A P T E R

13 Self-healing hydrogels Imtiaz Hussain1,2 and Guodong Fu1 1

School of Chemistry and Chemical Engineering Southeast University, Nanjing, P.R. China 2 College of Science, Nanjing Forestry University, Nanjing, P.R. China

13.1 Introduction The key feature of biomaterials are the spontaneously healing of the injury which increases the lifetime and survivability of most animals and plants. Inspired by nature, the engineers and scientists have developed self-healing materials to improve their lifetime, safety, environmental impact, and energy efficiency of the man-made materials. Several strategies have been reported for the development of self-healing hydrogels, for example, incorporation of healing agents, the formation of irreversible and reversible covalent bonds, and dynamic noncovalent bonding interaction as the healing motifs. To design intrinsic self-healing hydrogel various strategies of supramolecular interactions inclusive hydrogen bonding, ionic interactions, π
π stacking, hydrophobic interactions, metal
ligand interaction, and host
guest interactions have been reported in the literature. The three-dimensional (3D) smart networks, that swell greatly in an aqueous medium but do not dissolve in water are called hydrogels. They exhibiting an outstanding capability to absorb water into the intertwined structure. Such a fundamental feature is an inferior of substantial interest in scientific research, covering their potential in the hi-tech applications. Significant improvement has been made in the field of hydrogel materials over the past decades, and further, explorations in all directions are continuously being made for their extensive usage at an accelerated pace [1]. Hydrogels are the polymeric cross-linked materials which play a significant role in various fields of life. Hydrogel came into being when the first synthetic hydrogel was synthesized by Wichterle and Lim [2]. Hydrogels are the insoluble, 3D cross-linked, and tissues such as polymeric network system which retain biological fluids and a huge amount of water in their swollen state. The interaction between water or biological fluids and polymeric chain networks occur through osmotic, capillary, and hydration forces, and their counter-balancing result an expansion of chain networks [3]. The magnitude of these opposing effects have an adverse effect on the equilibrium state of the hydrogel, that

Self-Healing Polymer-Based Systems DOI: https://doi.org/10.1016/B978-0-12-818450-9.00013-1

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determine some intrinsic properties of the hydrogel, including diffusion characteristics, internal transport, and mechanical strength [4]. Hydrogels have been defined in a number of ways over the years by researchers. The most commonly used definition is that “the hydrogel is cross-linked and a water-swollen polymeric network, which is produced by the simple reaction of one or more monomer/ polymer/cross-linker units.” It can also be defined as, “the polymeric material which has the ability to swell and retain a considerable amount of water in its 3D network, but will not dissolve in water is called hydrogel” [5]. They are defined another way as, “the polymeric network systems that have the ability to swell in water, and retaining a significant fraction of water ( . 20%) inside their 3D structure, without dissolving in water is called hydrogel” [6]. In a broader term, “the hydrophilic three-dimensionally cross-linked polymeric network structure, which is formed from a class of synthetic/natural polymeric materials, that absorbs a substantial amount of water is called hydrogels.” The insolubility of hydrogel materials in water is due to the cross-linking which generate the ionic interaction and hydrogen bonding in the hydrogel networks [7]. The cross-linking also affords the vital mechanical strength and the physical integrity of the polymeric hydrogels [8]. Hydrogels can be used in the various fields of life such as food additives, biomedical implants, pharmaceuticals, regenerative medicines and tissue engineering, cellular immobility, diagnostics, cell encapsulation, separation of cells or biomolecules and as some barrier materials for the regulation of biosensor, biological adhesions, and biomedical micro-electromechanical devices [9]. Furthermore, they have been used in miscellaneous applications, such as controlled drug delivery, in the production of artificial muscles, contact lenses, biosensors, super-absorbents, and wound dressing [10,11]. In recent years, hydrogels appear as a probable candidate that competes, with numerous of the prevailing smart functional materials effectively that are used for immeasurable applications. Supplementary, the evergrowing variety of functional monomers and macromeres extend its applicability [5,10]. Generally hydrogels can be considered as polymeric materials which can absorb a large amount of water (free/freezable or bound/nonfreezable water) and offer good integral biocompatibility. These comprise of two phases, a liquid, and a solid phase. The solid phase derived from extremely hydrophilic monomers comprises of a 3D polymeric network of the hydrogels, which are eventually turned into insoluble materials because of cross-linking. When a liquid phase, for example, water is merged into their network, the hydrogels can attain a good and soft elastomeric mechanical properties [12]. Additionally the expulsion or absorption of free water can easily change the shape and size of the hydrogels in response to external stimuli. Further the water facilitates the monomeric, reactive, and potentially polymerizable species transportation into the aperture of the hydrogel matrix and take up the empty spaces. These are also combine with the chain segments or pendant moieties of the hydrogel matrix [13]. The fibrous network of the hydrogels may not possess the same properties, in the absence of water.

13.1.1 Gel and hydrogel Substantially diluted cross-linked system, which is basically categorized as strong or weak gels based on their nature of flow in steady-state is called a gel. Usually biomaterials and food scientists used the term gels and hydrogels interchangeably in literature, to Self-Healing Polymer-Based Systems

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describe the polymeric cross-linked network structures [14]. Gelling polysaccharides (hydrocolloids) are used widely in the food industry as edible gels [15]. While Hydrogel is a 3D network structure of natural or synthetic polymers, which can absorb and hold a significant quantity of water [16]. Its structure is usually constructed by the hydrophilic domains or groups in the polymeric network as a result of hydration in an aqueous environment.

13.1.2 Hydrosol and hydrogel A sol is formed if the interactions between a liquid and a polymer are more favored than the interactions between both liquid
liquid and polymer
polymer in a dual system of a polymer and a liquid. If the polymer is hydrophilic and the liquid is water, the product of the liquid-polymer interaction is called a hydrosol [17]. This type of reaction is mostly dependent on the type of functional groups, a number of ions and the structure of polymer used, and the reaction conditions, for example, pH and temperature. The dissolution ability of the hydrophilic polymer can be prevented in water by the addition of a cross-linker either via a chemical or a physical process, and the resulting cross-linked hydrosol is then called hydrogel. The swelling of the hydrogels in the surrounding liquid depends on the cross-linking density.

13.2 Self-healing In nature living organism acquire some functionalities such as self-cleaning, superhydrophobicity antireflection, and self-healing [18]. Among these functions the self-healing is an aptitude of a material or a system, that can repair itself and regenerate their function on the castigation of damage, and this property of materials expanded several academic curiosities. This charming property would encompass the working lifespan and extended their applications of creatures. Inspired by this, scientists have been trying to their best for designing and impart enthralling self-healing properties to well-suited materials with low manufacture costs and improved safety. As an ideal of soft matters with extensive applications, hydrogels with a self-healing feature have been grown rapidly as “smart” materials [19]. Self-healing, self-recovery, and self-repair are used synonymously and refers to a material’s ability to self-mend damage and regain its associated mechanical properties. Self-recovery can be used to quantify a material’s capability to restore the mechanical viscoelastic properties from the internal damage. The type of cross-linking in a hydrogel system is responsible for their healing efficiency, for example, physically cross-linked hydrogel networks exhibit about 100% healing efficiency while the hydrogel network having both physical and chemical cross-linking points does not show 100% recovery [20].

13.3 Self-healing and its characterization Based on self-healing chemistry, there are two types of self-healing mechanism namely extrinsic and intrinsic. Self-Healing Polymer-Based Systems

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13.3.1 Extrinsic self-healing The self-healing which do not show naturally occurring healing capability after the damage is called extrinsic self-healing. White and coworker designed the most common strategy toward the extrinsic self-healing biomaterials [21,22]. The self-healing occurs as a result of some additional healing agents and a catalyst is incorporated into the matrix by catalyzing a polymerization process which causes the release of the healing agent from the capsule. Micro capsulation and microvascular network designs are frequently used to prepare such materials, and, in both cases, the self-healing process is caused by the external healing agent. The extrinsic biomaterials usually consist of a matrix phase which contains a specific catalyst, healing liquid encapsulated, and the embedded microcapsule [22]. The main drawback of extrinsic self-healing is that the microcapsules heal the resin once and the vascular systems are more complex which can reduce its industrial production.

13.3.2 Intrinsic self-healing The self-healing process which takes place naturally without any external healing agent or external intervention is called intrinsic self-healing. The typical dynamic bonds involved in intrinsic self-healing are reversible covalent bonds including Diels
Alder’s (DA) bond, disulfide exchange, and ester exchange. The chemistry involved in the intrinsic selfhealing is a noncovalent or dynamic interaction including hydrogen bonding, metal
ligands interactions, π
π stacking, host
guest interaction. These noncovalent integrations enable a biomaterial to reform the broken bonds in the material due to their reversible nature [23
25].

13.4 Chemistry involved in intrinsic self-healing 13.4.1 Reversible covalent bonds In designing self-healing materials, the dissociation and association rates and the mobility of chain are the significant factors. For developing self-repairing polymers, reversible covalent bonds are the best candidate because of their high bond strength, which in turn enhance their mechanical strength of the materials. However, the main drawback of this type of bonds repairing is their high activation energy. Four types of reversible covalent bonds are used in the self-healing polymeric system of hydrogel materials (Fig. 13.1). 13.4.1.1 Reversible cycloaddition reactions In cycloaddition reactions, the formation of a cyclic product takes place, in which the two π bond containing components are joined together by two σ bonds and are usually the reversible reaction. The backward reaction is assigned as retro-cycloaddition or a cyclo-reversion. The most prominent cycloaddition reversible reaction is the DA reaction which is introduced into the self-healing polymeric materials. It is a [4 1 2] cycloaddition reaction among an electron-poor dienophile and an electron-rich diene, which results in a stable cyclohexene adduct and it is one of the most important reactions in organic

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chemistry. Owing to the thermally reversible nature of DA reaction, the retro-DA reactions have a momentous role in the designing of new self-healing polymers with well-defined architectures and properties. On heating (about 150 C), the DA adduct undergoes the reversible reaction by breaking the old covalent bonds. While on cooling to a lower temperature (80 C), it results in the cycloaddition and reforms the new bonds which repair the crack. To construct such a reversible network system various compounds can be used but the most useful and prominent compounds are furan and maleimide. The with two electrons withdrawing CQO groups in maleimide make its unsaturated CQC bond more electron-insufficient and more active toward the diene group in DA reaction. DA pairs have been merged in epoxies, polyesters, and polyamides, which enable the formation of a self-healing network with different properties. 13.4.1.2 Exchange reactions On the basis of reversible covalent bonds, the incorporation of dynamic exchange reactions provides another opportunity to design self-healing materials. The versatile tool for constructing self-healing hydrogels are the disulfide groups, which can undergo various reversible reactions including, thiol-disulfide exchange, radical-mediated disulfide

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fragmentation, disulfide exchange, and reduction
oxidation [26]. The disulfide can be used as a cross-linker in the side chain or can be incorporated into the main chain [27]. Based on the literature survey, the disulfide exchange occurs in the presence of a catalyst, at room temperature under ultraviolet (UV) radiation or at a moderate temperature. Along with base catalyst, phosphines, typically tertiary amines have also been used as a catalyst to accelerate the reaction in a polar solvent at mild conditions [28]. Canadell et al. added disulfide moieties into a covalently cross-linked rubber to introduce the self-healing ability in the network [29]. During the healing experiment, the freshly cut material was healed completely in one hour at 60 C and the expected healing mechanism was due to the exchange of disulfide bonds, which facilitate cross-links across the damaged area. Later Lei et al. also reported disulfide cross-linked polymers that were self-healed at room temperature due to the exchange of dynamic disulfide bonds [30]. It showed about 91% self-healing efficiency after 24 h healing period at room temperature and can be repeated multiple times. Similar healing efficiency can be obtained in 2 h time when the healing takes place at room temperature. Pepels et al. also studied various thermoset systems based on the disulfide
thiol exchange reactions which showed almost complete healing ability after 24 h healing time [31]. Disulfide cross-linked redox-responsive stars were prepared by Yoon et al., which possess a potential self-healing ability [32]. With the addition of reducing agent, thiol-functionalized soluble star was produced due to the cleavage of disulfide bonds. On the reoxidation, the reformation of S
S bonds took place to form an insoluble network. Recently Fengtao et al. synthesized a new type of trimers with the double sulfide bonds which shows self-healing and recyclable properties on the basis of double-disulfide bond exchange [33]. 13.4.1.3 Stable free radical-mediated reshuffle reactions Dynamic reshuffling reactions that involve a stable free-radical formation endeavor an interesting alternative for the self-healing substances. Mechanical damage of polymer network results in the formation of free radical by the cleavages of covalent bonds within the polymer network. The recoupling of the free radicals from the opposite cut surfaces will recombine the cut pieces as a result of the formation of a new covalent linkage. In alkoxyamine units, the reversible nature of C
ON bonds exhibits a promising role in the designing of new and novel self-healing materials among the self-healing system that is based on dynamic covalent bonds. Amamoto et al. reported another important system, by reshuffling thiuram disulfide moieties, which were able to self-heal at ambient temperature [34]. Yuan et al. reported simultaneously covalent bond breaking/radical recombination by introducing alkoxyamine moieties which act as intermolecular linker points in polystyrene. On heating, the fission of covalent bonds and synchronously recombination of radical intervene among alkoxyamine moieties. The damaged parts were rejoined repeatedly, without losing load bearing ability and integrity of the materials even above Tg [35]. 13.4.1.4 Heterocyclic compounds and carbohydrates in self-healing polyurethanes In the case of evenly distributed the mechanical forces within the network, the rupturing of the bond will take place at lowest bond strength. However, for polymer network due to the presence of cross-linking points and chain entanglements, the external force is

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unevenly distributed. Therefore strong covalent and noncovalent bonds may also be cleaved along with weakest bonds, which depends on the magnitude of the force [36]. The self-healing process study usually focused on one type of bond reconstruction that reform multiple cracked bonds. Different free-radical intermediates such as
C,
N, and oxygen
O will be generated by cleavage of cross-linked polyurethanes on mechanical damages. The self-repairing by coupling of these free radicals in their stable condition. Heterocyclic compounds with low ring opening activation energy such as oxetanes (OXEs) and oxolanes, usually produce stable free radical is used in designing self-repairing polyurethane network. Biswajit et al. reported oxetane substituted chitosan polyurethane polymeric material which can self-heal when put under UV light. When subjected to mechanical stress, the four-member oxetane rings will open and producing two reactive ends. On UV light the cleavage of the chitosan chains occurs, and it forms cross-links with the oxetane reactive ends and results repairing of the network in less than an hour [37].

13.4.2 Supramolecular chemistry Supramolecular chemistry was defined by a Nobel Prize winner Lehn as “the chemistry of molecular assemblies and intermolecular bonds” or simply as “the chemistry beyond the molecule” [38]. Supramolecular chemistry offers fast reversibility by dynamic dissociation and association of bonds, chain relaxation, bonding directionality, the decay of the chain relaxation after damage at ambient conditions which results exceptionally fast reaching equilibrium state. The multiple noncovalent interactions supramolecular interactions, including hydrogen bonding, π
π stacking, metal
ligand complexation, ionic interactions, host
guest, and hydrophobic interactions (Fig. 13.1) are responsible to achieve mechanical integrity using supramolecular chemistry which is discussed here in detail 13.4.2.1 Hydrogen bonds A hydrogen bond is a convenient and extensively applied in the supramolecular chemistry as a noncovalent integration, which was first of all suggested by Moore and Windmill [39]. It is a type of intermolecular forces of attraction which is weaker the typical covalent and ionic bonds. Although hydrogen bond is  10 times weaker (bonding energy 5
30 kJ mol21) than the typical hydrogen bonds (bonding energy  345 kJ mol21), it has a prominent effect on the viscoelastic properties, phase separation, and the degree of crystallinity of the polymers. Single hydrogen bonds do not have the ability to induce the supramolecular assembles due to its weak nature (5
6 kJ mol21). Multiple hydrogen bonds are required for the formation of the strong intermolecular network in a directional manner resulting in a strong binding affinity. The strength of the hydrogen bonds also dependent on the order of the acceptor and donor in the binding array. Instead of hydrogen binding array, secondary interaction also has a significant effect on the binding strength. Zimmeman et al. proved, that the AA
DDD array has a greater binding constant (Ka . 105) than DAA
AAD and ADA
DAD array which exhibit about 104 versus 102 M21, respectively [40].

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13.4.2.2 π
π Stacking interactions π
π stacking is a type of noncovalent interaction, in which the π orbitals of aromatic rings are involved which play a considerable role in various biological processes such as RNA and DNA interactions, molecular recognition, and protein folding. They are usually introduced in the designing of reversible self-healing polymeric network on the thermally triggered process, by the interaction of π-electron-rich aromatic rings with the end-capped π-electron deficient group. To achieve better self-healing at a broad range of temperature (  50 C
100 C), the glass transition temperature (Tg) can be tuned by adapted the composition of the blend and changing the spacers. Heating will cause the interruption of the π
π stacking, which results disengaged of the end-capped chain polymer, and flow of spacer due to its soft and flexible nature. In this way, the dynamic nature of the component of the polymeric network will reconstruct newly π
π stacking bond interaction resulting self-healing of the system which regains the original mechanical strength. To gain efficient thermal healing ability, we can combine both the π
π stacking and hydrogen bonding interaction within one supramolecular network by using polybutadiene containing spacers as a urea and urethane groups [41]. Nitrobenzoxadiazole (NBD) having a cholesterol derivative also possess a unique hydrogen bonding and π
π stacking interactions, in which the spacer length connecting the Chol and NBD units together playing a critical role in controlling gel and healing ability. The π
π stacking interaction can also be increased by combining it with the metallophilic interaction such as Pt
Pt interaction having a reversible property within the range of equilibrium constant of 50
500 M21 [42]. 13.4.2.3 Metal
ligand interaction Metal
ligand interaction is a type of noncovalent interaction, which is particularly attractive due to its strong nature (bond energy c. 40
120 kJ mol21), highly directional, easy access to an organic ligand and a large number of metal ions in nature [43,44]. Metal
ligand supramolecular gel has got greater interest, which combines the viscoelastic properties of the organic polymer and the physical properties for instance, electronic, magnetic, optical, mechanical, and catalytic properties of the inorganic metal component [45]. There are three main categories of such polymeric system, that is, metal
organic framework, long-chain-1D macromolecules and 3D network which is usually called metallosupramolecular polymers (MSPs) [46]. In these, the metal
ligand coordination is usually reversible and dynamic. The nature of ligand and metal ions, both influence the binding strength, solubility, and reversibility and we can tune their properties by changing one of them or both. By using multivalent metal ions and multidentate ligands, the strength in the binding can be increased which result in a high binding constant K [47]. The dynamic nature of M
L bond is the critical property of MSPs. The binding kinetics and thermodynamics of the MSPs depend on the nature of metal ions used in the complexation, which make these systems more versatile. By choosing the suitable metal
ligand pairs, the thermodynamics, and binding kinetics can be tuned finely and the hydrogel with the desired self-healing can be achieved easily. For example, the ligands such as ketones, ethers, imines, phosphines, pyridines, amines, and nitriles can form complexes more efficiently with different metal ions. The structure of ligands, for example, linear, highly branched, starshaped, or dendritic moieties has also an adverse effect on the strength of coordination

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bonds which can tune the strength of MSPs. Generally the increase in metal salt concentration results in an increase in viscosity of the solution, but a higher concentration of metal ions reduces the degree of polymerization, which causes a decrease in the mechanical strength of the MSPs [48]. Recently Hussain et al. explored the effect of metal ion concentration on the mechanical properties and self-healing ability of the hydrogels. They find out the beyond a specific Fe31 ions concentration, there was a decrease both in mechanical stress and self-healing efficiency. Similarly the monomer and natural polymer concentration also have a considerable effect on the strength of hydrogels [49]. 13.4.2.4 Ionic interaction The attraction between the opposite charges present at the end of polymer chains results in the formation of gelation of the ionic liquids and generating supramolecular hydrogels. Their electrostatic interactions give mechanical strength and stimuli
responsive capability to the hydrogel materials and have a considerable application in the field of drug delivery, microfluidics, and artificial organ fabrication [50]. The interaction of ions with a polar molecule results in ion-dipolar interaction or induce a dipole character in the nonpolar molecule. The dielectric constant, electrostatic charge density, and temperature has a great effect on the strength of ionic interactions. The representative ionic groups including sulfonate (
SO32), carboxylate (
COO2), and phosphonate (2PO22 3 ) present on ionomers or polyelectrolyte are responsible to form ionic clusters, which acts as physical cross-linking points and form a tough supramolecular hydrogel network [51]. The strength of ionic interaction also plays a critical role in the selfhealing, diffusion, and elastic behavior of hydrogel at various temperatures. High ionic content results in a dense cross-linking polymeric network, which causes a decrease in mobile polymer structures, self-healing ability, and elasticity of the hydrogels at low temperature due to immobilizing polymer chains. In polyacrylates, the self-healing efficiency can be improved by adding a small amount of ionic salts, such as NaCl, which also indicates that the dynamics of the hydrogels can be initiated by interrupting the ionic interactions by adding the salts, which cause an increase in the chain mobility. On the other hand, water, polar solvent, and salt also have a significant impact on the self-healing. By adding water or any polar solvent, the dissociation occurs due to strong electrostatic interaction among the polyelectrolytes and polar solvent or salt [52]. Ionic interactions between the multianionic and multicationic small molecules usually form a transparent material which also shows rheological and self-healing properties at room temperature. Similarly zwitterionic polymers also form ionic interaction as they have an equal number of cationic and anionic groups on each chain. By using zwitterionic polymers as a coating material, exhibit excellent healing within 1 min when immersed in water [53]. 13.4.2.5 Host
guest interaction It is a type of supramolecular interaction between two or more molecules as a result of different noncovalent dynamic interaction such as electrostatic interaction, hydrogen bonds, charge transfer, hydrophobic interaction, and van der Waals forces of attraction. In host
guest supramolecular chemistry, the inside of the guest moiety is decorated with a

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macrocyclic host moiety to form inclusion complexation with a unique structure. Various macrocyclic molecules such as calixarene, pillararenes, crown ethers, cyclodextrins (CDs), and cucurbiturils are incorporated into functional guests’ molecules to fabricate supramolecular materials because of the host
guest interaction. Host
guest interaction has been broadly used in the designing of self-healing hydrogels. In hydrogels, host
guest interaction usually relies on external stimuli, including, temperature, pH, redox potential, and light, which trigger the healing process [54]. The combination of multiple noncovalent interactions between two complementary compounds results in a good binding agreement and form complexes directionality with fixed host
guest geometry [55]. Supramolecular polymeric materials can be synthesized with three different host
guest interaction approaches. They may be prepared from linear supramolecular polymers or from the mixtures of host and guest polymers. They can also be formed by the polymerization of host and guest monomer. CDs-modified host polymers attracting great attention to the self-healing hydrogel because of their soluble nature in water, multistimuli
responsiveness, low cost, biocompatibility, and favorable association/dissociation dynamics. Self-healing performance has been observed in hydrogels by using guest polymers contain n-butyl acrylate, ferrocene, N-adamantane-1-yl-acrylamide, adamantine, and N-vinyl imidazole.

13.5 Self-healing process On the basis of the external trigger or energy required for the self-healing of hydrogel during the healing process, they are divided into different, that is, nonautonomic and autonomic self-healing hydrogels.

13.5.1 Autonomic self-healing hydrogels The property of materials that automatically and intrinsically heal the damages and restore itself to normality is called autonomous self-healing. Hydrogels which do not need the addition of external trigger or no manual intervention and the self-healing event of the damaged area take place is called autonomic self-healing hydrogels. For example, the multiple ionic noncovalent interactions between Fe31 ions, and the carboxyl groups of polyacrylic acid (PAA) enable the hydrogel network to heal at room temperature autonomously [56].

13.5.2 Nonautonomic self-healing hydrogels Hydrogels that require a moderate external trigger, inclusive of heat or light for the process of self-healing are called nonautonomic self-healing hydrogels. Trigger
-responsive hydrogels acclimate to the encompassing environment, and recompensing to external stimuli are emerging as “smart hydrogels.” For example, Alginate-Ca21 cross-linked smart hydrogels were designed and were ultra-sonicated to speed up the drug release, but in physiological fluids, the Ca21 would allow cross-linking to self-healing when the stimulus

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was removed [57]. Similarly graphene
poly(N,N0 -dimethyl acrylamide) hydrogel displays a thermally triggered (heated at 37 C in the air for 12 h) self-healing ability, which is analogous to the healing of wounds in human tissues (37 C). The near-infrared (NIR) laser-triggered (NIR laser irradiation treatment for 2 h), is similar to a minimally invasive surgical procedure, self-healing behavior [57].

13.6 Classification of self-healing hydrogels According to the materials used in the synthesis of hydrogel network, self-healing hydrogels are divided into three groups, that is, inorganic-based, polymer-based, and nanocomposite hydrogels. The critical aspects and their trends in the self-healing process of hydrogels are given below.

13.6.1 Inorganic-based self-healing hydrogels These hydrogels contain inorganic portion as a building block in their network structure with or without metal ions or organic macromolecules [58]. They are different from typical nanocomposite hydrogels due to the presences of a polymer as main building blocks, and the physically cross-linking points are usually provided by the nanofillers [59]. Inorganic hydrogels with one- or two-dimensional (2D) basic building blocks, draw excessive consideration because the appealing properties of inorganic may import supplementary functions to macroscopic hydrogels [60]. Plentiful fascinating properties of innovative inorganic graphene have convinced scientists to interpret its 2D sheets into complex 3D macrostructures. The first hydrogel based on graphene were synthesized by Shi’s and his coworker, by one spot hydrothermal reduction of concerted graphene oxide (GO) aqueous dispersion [61]. GO sheets can immediately self-assemble into hydrogels in water with the compensation of different promoters, containing multivalent metal ions such as Ca21, Mg21, Fe31, and polymer [62]. Clay is another conventional inorganic substance broadly spread on earth, which circulates inhomogeneous charge on its disk-like structure and generates hydrogel with a special structure called “house-of-cards” structures [63].

13.6.2 Polymer-based self-healing hydrogels As compare to the inorganic-based hydrogels, polymeric hydrogels are extra biocompatible because of their resemblances with the biological system in various ways, which make them the most suitable candidates for self-healing polymer hydrogels. Li et al. designed a biocompatible and degradable poly(L-glutamic acid) (PLGA)-based self-healing hydrogels by introducing host
guest interaction. The self-healing aptitude was confirmed macroscopically and qualitatively [64]. Poly(vinyl alcohol) is a biocompatible and nontoxic synthetic polymer having a crystalline nature has been used for the synthesis of hydrogel via freezing/thawing method and exhibit highly self-healing ability without any peripheral stimulus at room temperature which is described in Fig. 13.2A [65]. Itamar and his

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FIGURE 13.2 Photographs showing the self-healing behavior of; (A) the two cut pieces of PVA hydrogel at room temperature in air for 12 h without any external stimuli that can be bend and stretch without any visible fracture; (B) the dopamine/DTEc and self-complementary nucleic acid cross-linked using redox triggers or photochemical triggers; and (C) the DN-SHPE hydrogel healed at 60  C that could be stretched without any crack.

coworkers recently reported DNA/bipyridinium dithienylethene based multitriggered supramolecular self-healing hydrogel with stimuli
responsive and shape memory features via donor
acceptor interactions. The fabricated hydrogels behave between high and low stiffness state triggered by light, redox agents, or K1/crown ether and their self-healing capacity is displayed in Fig. 13.2B [66]. Binghua et al. reported a new strategy for the synthesis of selfhealing polymer electrolyte through the double network for the application of flexible lithium-ion batteries. The double network structure was fabricated by cross-linked with a quadruple hydrogen bonding and chemical bonding, which impart the self-healing and mechanically robust properties to the hydrogels which are demonstrated in Fig. 13.2C [67].

13.6.3 Nanocomposite-based self-healing hydrogels Nanocomposite hydrogels generally assigned to those cross-linked polymer networks, which swells in water containing nanostructures or nanoparticles [59]. Over the past few years, clay nanoplatelets are most frequently used in nanocomposite hydrogels which are fabricated by in situ free-radical polymerizations [68]. GO nanosheets have also been used in the synthesis of nanocomposite self-healing hydrogels, in which the GO is introduced either by self-assembling or in situ polymerization. Shi et al. reported nanocomposite hydrogel by self-assembling the GO and DNA into the hydrogels, which enhance the mechanical strength along with the absorption capacity. After heating the hydrogels at

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FIGURE 13.3 Optical images showing the self-healing properties of; (A) the cut pieces of the GO/DNA selfassembled hydrogels by heating at 90  C in air for 30 min; (B) the TA@CNC ionic gels and their typical stressstrain curves with different healing time; and (C) the multifunctional PVA hydrogel and their response to the magnet that can be used as the conductive bulk in a simple circuit through the cutting/healing operations.

90 C for 3 min, the hydrogels show excellent self-healing as Fig. 13.3A [69]. Zhong et al. reported tough and extremely stretchable physical hydrogels by a facile one-pot method, consuming vinyl-hybrid silica nanoparticle as multivalent covalent cross-linking points and ferric ions as an ionic cross-linkers. The resulting hydrogels exhibit highly stretchability, toughness, and self-healing ability without the addition of any organic cross-linkers [70]. Most recently Changyou et al. reported cellulose-based nanocomposite self-healing hydrogels through the construction of synergistic reversible and multiple coordination bonds PAA chain and the tannic acid coated cellulose nanocrystal. This fabricated hydrogels accumulatively possess the adhesive, self-healing, and strain-sensitive features which enable it to be used as a flexible strain sensor to monitor and distinguish both large motion and sublet motions and their self-healing ability is shown in Fig. 13.3B [71]. Kai et al. also reported nanoparticle-hydrogels composites with self-healing, magnetic, and conductive properties. The took nano-fibrillated cellulose as a substrate and the conductive properties were inducted by the in situ polymerization of aniline and the magnetic properties were induced by the loading of MnFe2O4 on the NFC via chemical coprecipitation method. The facile approach, magnetism, conductivity, and self-healing ability (Fig. 13.3C) impart the novelty to the hydrogels and can be used in various applications such as rechargeable batteries, electrochemical display devices, and electromagnetic interface shielding [72].

13.7 Mechanism of self-healing of hydrogels The self-healing mechanism of hydrogels can be illustrated in two broad categories, that is, physically and chemically. Self-Healing Polymer-Based Systems

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13.7.1 Physically (diffusion) self-healing mechanism From the physical point of view, the self-healing of hydrogels occurs due to the movement of molecular segments across the cut interfaces, which rejoin the cut hydrogels pieces together and this mechanism is so-called molecular diffusion. This type of mechanism can be visualized by ultraviolet excitation method and digital monitoring, scanning electron microscopy techniques [56,73,74]. Various types of supramolecular interactions involved in the physical self-healing process of hydrogels are listed in Fig. 13.4.

FIGURE 13.4

Various interactions involve in the physical interaction of self-healing hydrogels.

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13.7.2 Chemically self-healing mechanism Chemically hydrogels can be self-healed by the rebonding of broken dynamic covalent and noncovalent interactions subsequently mechanical damage, which is notably critical [19]. Various types of noncovalent interactions including hydrogen bonding, hydrophobic, host
guest, crystallization, a polymer nanocomposite, and manifold intermolecular interactions between molecules or polymer chains, are depicted in Fig. 13.5. Similarly the dynamic covalent bonds including reversible DA cycloaddition (cyclohexene), disulfide (S
S bonds), phenylboronate ester (B
O bonds), imine and acylhydrazone (C
N bonds), and reversible radical reactions (C
C/C
S) also involves in the process of self-healing of hydrogels (Fig. 13.6). Noncovalent interactions demonstrate a higher dynamic behavior as compared to the dynamic covalent bonds. Thus physical diffusion of molecules and chemically recombination of the cleaved bonds assist the process of self-healing in hydrogels.

FIGURE 13.5 Outstanding properties of metal-containing polymer hydrogels reproduced.

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FIGURE 13.6 Various types of bonds in chemical selfhealing.

13.8 Factors impact on self-healing mechanism Various parameters that impact/or accelerate the molecular diffusion which in turn accelerate the self-healing mechanism [75] are given below.

13.8.1 Separation time With the increase in separation time of the cut pieces, the self-healing ability of the hydrogels decreases. This is because of a decrease in the number of free and active groups on the cut surfaces, which in turn decreases the number of new bonds formation [65].

13.8.2 Self-healing time Healing time has a direct relationship to the self-healing efficiency. Hydrogels healed for a long time show excellent healing efficiency which is due to the formation of stronger interactions across the interfaces [76].

13.8.3 Temperature Temperature usually affects the rate of diffusion of polymer chains, which in turn facilitate the self-healing process of hydrogels. At higher temperature the rate of molecular diffusion increases which result in good healing efficiency [77].

13.8.4 Chain length Chain length has an inverse relation with the molecular diffusion but direct relation with the strong recovery. Short chains cannot diffuse deeper however they facilitate fast diffusion, while long chain deliberate high strength recovery at the cut interface of the hydrogels [78].

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13.8.5 The content of nanomaterials Nanomaterials, such as clay and GO, usually retard the diffusion of molecular chains which shorten the self-recovery time of the hydrogel. With the increase of nanomaterials content, result in the chain length between the nanosheet become smaller [79].

13.9 Sacrificial bonds Sacrificial bonds have been accepted as one of the structural origins for the unique combination of high strength and high toughness for biological materials. For example, nacre in abalone shells, bones in mammals, and byssus in mollusks are known for their extraordinary stiffness, strength, and toughness with the capability of self-repair [80,81]. The excellent performance of biological materials originate from a growing hierarchical structure with an ordered arrangement of hard inorganic and soft organic building blocks [82]. Recently it is reported that the energy dissipating mechanisms via sacrificial bonds are the vital influences which avail strong and tough aspects of natural materials. Inordinate development has been achieved in the synthesis of polymeric materials having sacrificial bonds in their polymeric structure. Inspired by the sacrificial bonding structures in biological materials, scientists have been engineered various noncovalent bonds and covalent ones as sacrificial bonds into artificial polymeric materials such as hydrogels and elastomers, resulting in significant improvements in strength, and fracture toughness [83].

13.10 Nature and mechanisms of sacrificial bonds 13.10.1 Sacrificial bonds in biological materials The high toughness of the biological materials originates from the dynamics of the organic components. The rupture of the sacrificial bonds and the release of hidden lengths dissipate a huge amount of energy. Sacrificial bonds in nacre dissipate a large amount of energy in a successive pulling of collagen molecule of bone in the presences of divalent Ca21 ions in a buffer [84]. It is due to the formation of sacrificial bonds between the negatively charged group on collagen molecule and the positive cation Ca21 and this Ca21mediated sacrificial bonds in collagen molecule of bone play a vital role in increasing the stiffness and energy dissipation [85]. There are three types of hypothetical sacrificial bonds in the bone tissue, that is, (1) within the molecule; (2) between two molecules; and (3) a molecule and the surface of a mineral plate [86]. Thus the sacrificial bonding system can dissipate a large amount of energy against an entropic force to toughen a biological material on stretching out the hidden lengths that are exposed when sacrificial bonds rupture. Sacrificial bonding system is also found in other biological materials, such as tendon [87], silk [88], gastroliths [89], and byssus [90]. Hydrogen bonds and metal
ligand coordination bonds were found to be sacrificial bonds in spider silk proteins [91] and mussel byssal threads [92], respectively.

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Simple, when biological material is stretched or loaded, a large amount of energy is dissipated in the biopolymer through the rupture of sacrificial bonds, and the release of hidden lengths, thereby ensuring high toughness of the biological material. Moreover, these sacrificial bonds, such as ionic bonds, hydrogen bonds, and metal
ligand coordination bonds, are usually weaker than covalent bonds, and reversible. They can rupture prior to covalent bonds and self-repair to maintain the integrity of the biological materials.

13.10.2 Constitutive theories of sacrificial bonding systems The functioning of a sacrificial bonding system relies on the dynamic state of its sacrificial bonds. With the formation of hidden lengths (folded or looped chains) sacrificial bonds restrict some part of the hidden lengths from contributing to the endwise distance. This restriction correlates the reduction in the biopolymer chain entropy and an increase in the initial stiffness of the polymer chain. After rupturing the sacrificial bonds, the hidden lengths are released, and a huge amount of energy is dissipated and the chain entropy reduces as the hidden lengths are straight out. Recently several constitutive theories have been developed to illustrate the mechanical behavior of sacrificial bonding systems. The worm-like chain model has been used to interpret the mechanical behavior of a bone collagen
polymer containing a sacrificial bonding system [93,94].

13.11 Inspired sacrificial bonds in artificial polymeric materials 13.11.1 Sacrificial covalent bonds Gong et al. synthesize double network hydrogels via two-step polymerization by the formation of sacrificial covalent bond interactions [95]. They further explored that the sacrificial covalent bonds in the first network dissipate energy under deformation and protect the integrity of DN hydrogels. Extensive studies on the DN gels toughening mechanism have been suggested, that the introduction of any potent motifs of sacrificial bonds which dissipate energy under deformation, will toughen the materials. This approach allows preparing highly toughened soft materials with multiple networks, such as DN gels and elastomers, in which covalent bonds in the first brittle network acts as “sacrificial bonds” and the entangled polymer chains in the second/third ductile network act as “hidden lengths” [96]. The first brittle network breaks into small clusters by sacrificing partial covalent bonds to dissipate a huge amount of energy at relatively low stress. These small clusters serve as sliding cross-linking points in the ductile networks, in which the “hidden lengths” of the second/third network chains are straightened by successive stretching. The concept of sacrificial bonding is reflected by a typical yielding and necking phenomenon in the loading
unloading tests of DN gels. Sacrificial covalent bonds do not recover themselves from fracture, leading to irreversible, and permanent damage to the materials. As a result, these materials exhibit softening and decreased hysteresis loops when subjected to repeated large deformation [97,98].

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13.12 Sacrificial bonds in hydrogels

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13.11.2 Sacrificial noncovalent bonds Sacrificial noncovalent bonds play a role of physical cross-links in polymeric networks. Physical cross-links based on noncovalent bonds are much weaker than chemical ones based on covalent bonds in a polymeric network. Sacrificial noncovalent bonds can preferably dissociate under a mechanical load or an external stimulus (e.g., pH, temperature, and ionic strength) to dissipate energy for protecting the integrity of polymer chains. After dissociation of sacrificial noncovalent bonds, the detached polymer chains can relax and straighten under stress to further dissipate mechanical energy in polymer networks. Sacrificial noncovalent bonds can endow artificial materials with both high toughness due to the dissipation of mechanical energy and recoverable and self-healing abilities due to the reversibility of the sacrificed noncovalent bonds [99,100]. The reversibility of sacrificial noncovalent bonds can sustain the same hysteresis over cyclic loading, leading to antifatigue materials [101]. However, since recovered sacrificial noncovalent bonds are usually out of their original positions, artificial materials that only use noncovalent bonds as crosslinks show plastic deformation under mechanical loads [102]. Therefore the elastic mechanism should be introduced into reversible cross-linked systems to maintain the shape of the artificial materials.

13.12 Sacrificial bonds in hydrogels Various types of sacrificial bonds have been engineered into the hydrogel network for the development of strong and tough hydrogels to acquire better mechanical properties. Some of them are discussed below.

13.12.1 Sacrificial ionic bonds Sacrificial ionic bonds can be constructed between oppositely charged organic and inorganic components in organic/inorganic composites. The force between the oppositely charged ions is called the electrostatic forces, therefore, ionic bonds are also called electrostatic bonds. Ionic bonds are versatile in terms of the valence and size of the ion, and there is a large range of dynamic bond situations. Different from the covalent bonds and hydrogen bonds, ionic bonds can be tuned by photo-induced valence changes or via electrochemistry [103,104]. Thus tuning nanoscale ionic interactions can tailor macroscopic mechanical performance. Tang et al. designed nacre-mimetic composite films from positively charged PDDA and negatively charged montmorillonite (MTM) platelets by layerby-layer (LBL) technique [105,106]. Na1 cations are the major counter-ions in charged montmorillonite clay (CMC)/MTM nacre-mimetic composite due to the use of Na1 CMC and Na1 MTM. To introduce the sacrificial dynamic super-molecular cross-links, the mono-valent Na1 cations can be exchanged with multivalent ones by infiltration of the latter in solution. Sacrificial ionic bonds in hydrogels can be formed between oppositely charges polyelectrolyte in polymeric networks. Pendant cationic and anionic groups are randomly distributed along polyelectrolyte chains by random polymerization of ionic monomers. The

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randomly distributed charges form multiple ionic bonds of a vast dispersion of strengths through inter- and intra-chain interactions, in which strong ionic bonds act as eternal cross-links to impart elasticity whereas weak ones act as reversible sacrificial bonds to strengthen the materials. Luo et al. developed polyion complexes (PIC) hydrogels which consisted of oppositely charged polyelectrolytes [99,107]. Strong and tough PIC hydrogels with self-healing properties were synthesized by two steps sequential homopolymerization of cationic and anionic monomers. All the ionic bonds were formed through inter-chain complexation where the weak ionic bonds served as sacrificial bonds.

13.12.2 Sacrificial hydrogen bonds Hydrogen bonds have been found to serve as sacrificial bonds in spider silk protein [108]. However, an isolate hydrogen bond is relatively weak (5
6 kcal mol21) [109] and does not resist much to force unless there are a large number of hydrogen bonds. Thus sacrificial hydrogen bonds generally refer to multiple hydrogen bonds or clusters of hydrogen bonds. Moieties having several hydrogen bonding accepting and donating sites, such as 20 -deoxyguanosine 50 -monophosphate and 2-ureido-4-pyrimidone (UPy) [110,111], are capable of forming architectures with multiple hydrogen bonds. They have been successfully incorporated into artificial materials to form multiple hydrogen bonds severing as sacrificial bonds, that dissipate a huge amount of energy under deformation of the materials. Similarly the UPy moieties reported by Meijer’s group are often used to build sacrificial hydrogen bonds because of their ability to form quadruple self-complementary hydrogen bonds [112,113]. Recently weak hydrogen bonds were developed to serve as “sacrificial bonds” in hydrogels [114]. Hu et al. designed a new hybrid network system, based on a tunable composition of strong bonds and clusters of sacrificial hydrogen bonds [115].

13.12.3 Sacrificial metal
ligand coordination bonds Metal
ligand coordination bonds in biological materials are thoroughly documented in the literature [116]. They are formed when the ligand donates a lone pair of electrons to the metal ion empty orbitals. From the perspective of sacrificial bods, mono-valent metal
ligand coordination bonds are excluded because they will not break but slid when polymer chains involved in the ligand are stretched. Typical sacrificial metal
ligand coordination bonds are formed between divalent/multivalent metal ions (e.g., Zn21, Cu21, Ca21, and Fe31) and ligands capable of donating lone pairs of electrons (e.g., carboxylate groups, amino groups, and 3,4-dihdroxyphenylanine) [116]. Typically, trivalent cations Fe31 may be an ideal choice to form efficient sacrificial metal
ligand coordination bonds because of high complexing strength. In polysaccharides such as alginate, gellan gum, and carrageenan are ionic polymers with abundant carboxyl gropes in the main chains, and cations could form sacrificial coordination bonds with carboxyl groups of polysaccharides. Under a stress, these sacrificial coordination bonds are easily dissociated to dissipate energy, and thus toughen the hydrogels. When the stress is removed, the coordination bonds can reform and restore the integrity of the hydrogels. For example, Panhuis’s group

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developed a series of IPNs of coordinative cross-linked polysaccharides and a covalent cross-linked neutral polymer [117]. Similarly, Sun et al. designed extremely stretchable and tough hybrid hydrogels by combining an ionically cross-linked alginate network and covalently cross-linked PAAm network. In the presence of enough Ca21 cations, zip-like metal
ligand coordinating cross-links were formed between the G segments in different alginate chains [118]. Lu et al. also reported highly stretchable and tough hydrogels, consisting of a carrageenan network cross-linked with cations-mediated (K1/Ca21) coordination bonds and a PAAm network cross-linked with covalent bonds. The metal
ligand coordination bonds that linked the carrageenan double helices served as reversible sacrificial bonds to stiffen and toughen the hydrogel through energy dissipation [119]. Jun et al. recently reported a self-healing electrolyte based on P(AA-co-AAm) cross-linked with a divalent cobalt Co12 ion which exhibit excellent mechanical properties, flexibility, and self-recovering properties along with high capacitances (90%) after 5000 cycles [120].

13.12.4 Sacrificial hydrophobic interactions Hydrophobic interaction is one of the significant driving forces for protein folding, cell membranes and many other biological assemblies in aqueous solutions [121]. The hydrophobic effect is important in the case of protein folding. As proteins have a hydrophobic clustered of the amino group within the protein structure. Water-soluble proteins bear a hydrophobic center in which the side chains are covered from water, which maintains the folded state. Polar sides and charged chains are located on the solvent-exposed surface, which causes their interactions with surrounding water molecules. Recently hydrophobic interaction has been widely applied to molecular self-assembly of hydrous artificial polymers. Gong’s group demonstrated that hydrophobic association could serve as reversible sacrificial bonds in hydrophobic association hydrogels [122,123]. The latter is composed of amphiphilic polymers and water. Amphiphilic polymers are macromolecules that have both hydrophilic and hydrophobic segments. The molecular structure of amphiphilic polymers is critical to constructing a sacrificial hydrophobic association that results from selfassembly of pendant hydrophobic groups on a hydrophilic backbone. One strategy is to prepare amphiphilic polymers through radical copolymerization of water-soluble monomers and a few comonomers with extended hydrophobic groups [124,125]. Water-soluble monomers such as acrylamide and acrylic acid form hydrophilic polymer backbones, while comonomers such as stearyl acrylate and octyl phenol polyethoxy ether acrylate act as hydrophobic monomers to form hydrophobic associations (micelles) due to the selfassembly of their long alkyl chains. The resulting hydrophobic association serves as reversible physical cross-links of copolymers.

13.12.5 Sacrificial host
guest complexes The sacrificial host
guest complexes can usually be realized by the “bottom
up” synthesis methodology and hierarchical construction in multiple length scales. Host
guest complexes are generally composed of two molecules that are held together in unique structures through molecular recognition and noncovalent bonding. In principle, a

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host
guest complex consists of a receptor moiety (host) and a ligand moiety (guest) through noncovalent bonding. Common hosts include crown ethers, CDs, cucurbiturils, calixarenes, and pillararenes, while guests can be organic molecules, ions, and even nanoparticles. Host
guest complexes are formed by embedding guests into host cavities. Incorporating host
guest complexes into supramolecular systems can provide distinctive properties. Recently host
guest complexes have been incorporated into supramolecular polymer networks to act in a sacrificing manner to endow the networks with high toughness as well as self-recovery [126,127]. Typical hosts for constructing sacrificial bonds include CDs and cucurbiturils [127] Guests should be selected based on the association constants of host
guest pairs. The higher the association constants, the more stable the host
guest complexes.

13.13 Metal
ligand polymer hydrogels Hydrogels containing metal
ligand hydrogels gain curiosity in recent years because of their excellent properties, for instance, self-healing, biocompatibility, redox activity, and/ or recoverability (Fig. 13.5) [128]. A synthetic or natural hydrogel formed by physical and/ or chemical cross-linking is a hydrophilic polymer network containing a high ratio of water to dry gel. Recently appreciable curiosity has proceeded in the designing and application of metal-containing polymers because of the presence of both inorganic and organic components in a single network [129]. The hard metal centers impart unique functions and soft polymers processing advantages. Fundamentally the organic polymers impart solubility, flexibility, and processability, while the inclusion of metal elements enable multiple functionalities such as catalytic, redox activity, magnetic, electronic, and responsive properties [130]. Therefore metal-containing polymers with particular and adaptable properties should be the most suitable candidate for construction functional hydrogels. On the basis of interactions present metal-containing polymer hydrogels, they can be divided into covalently cross-linked hydrogels [131], cross-linked hydrogels via metal coordination [132], and hybrid cross-linked hydrogels [133]. Currently the most widely designed metal-containing hydrogels are cross-linked hydrogels via metal coordination, that is grasped together by metal ions. However, the covalently cross-linked metalcontaining hydrogels are rare and negligible explored due to the cytotoxicity of metals and the incorporation of metal ions into the macromolecular network become difficult.

13.13.1 Cross-linked hydrogels via metal coordination Metal ions in hydrogel network act cross-linking points and coordination interactions established between the functional groups in polymer chains and metal ions. These interactions are generally used as cross-linking junctions for the manufacturing of metalcontaining hydrogels. Coordination is a distinct type of chemical bond wherein one atom donates a pair of electrons and is different from a typical covalent bond where mutual sharing of electron take place. Metal coordination is an example of Lewis-acid
base interactions and is enough stronger than most noncovalent interactions, while it is weaker than

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the typical covalent bond (e.g., C
H, C
C, C
O, etc.). Thus Ca-alginate hydrogels [134], which can be fabricated by metal coordination interaction possess the properties of either chemical or physical gel. Coordination interactions in hydrogel formed by metal
polymers interactions exhibited many outstanding properties such as recoverability, biocompatibility, redox activity, and self-healing [130,135]. Polysaccharide-based polyvalent metal ions associated metal
ligand hydrogels have been reported in the literature [118,136]. Anionic alginate solution can form hydrogels with metal cations (Fe31, Ca21, Sr21, Ba21), which can promote coordination with carboxylate groups of alginates. The guluronic acid blocks in various alginate chains inaugurate cross-links through metal coordination and finally leading to the formation of a metal
alginate hydrogel. Coordination associating other anionic groups and metal ions would be utilized to introduce the gelation progression. Andzelm and his coworkers explored that hydrogels would be fabricated by the insertion of divalent or trivalent cations (Ca21, Cu21, Zn21, Al31, and Fe31) into a dispersion medium of cellulose nanofibrils [137]. They also subjugated the ionic interactions between metal ions and carboxylate groups to launch the gelation process. The storage moduli of hydrogels depend on the choice of cations, the valency of the metal ions and their binding strength with carboxylate groups.

13.13.2 Covalently cross-linked hydrogels Metal
ligand polymer hydrogel network can be synthesized by the incorporation of the metal ion as a cross-linking point into the polymer chain which can be held as a pendant group on the side chains. The bonding linkages that connect the metal centers with the functional groups of the polymer may be reversibly or strongly covalently bonded, and dynamically coordinated [138]. These polymers can be used directly to fabricate metal-containing hydrogels by communicable superiority of prototypical covalent bonds as cross-linking points. Ferrocene, as a principal member of the metallocene family, which is incorporated into the main chain to design metal-containing polymer hydrogels [131]. Cationic cobaltoceniumcontaining polymers have been recently acquiring a great consideration [139]. Tang and his coworker reported the cationic cobaltocenium chunk into polymeric networks by means of free-radical polymerization of cobaltocenium methacrylate with a PEG functionalized crosslinker [140]. The metal-containing hydrogel can also be created by the electrostatic attraction between metal complex cations and anionic functional groups of polymer chains [141].

13.13.3 Hybrid cross-linked hydrogels Metal-containing polymer hydrogels with high mechanical properties are useful in many situations in various practical uses. Therefore the ionic interactions and other interactions, for example, coordination and covalent bonds can be combined as cross-linking junctions and is favored to design metal-containing hybrid cross-linked hydrogels exhibiting enhanced mechanical performance. Xie and his coworker reported a one-pot method for the preparation of hydrogels by free-radical polymerization of acrylic acid and N,N0 methylenebisacrylamide (MBA) is used as a covalent cross-linker, and Fe31 from Fe(NO3)3

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aqueous solution was inserted as the ionic cross-linker [142]. A number of reversible crosslinks are generated from the coordination interactions between Fe31 ions and carboxyl functional groups of PAA within the sparse, which are covalently cross-linked and impart high mechanical toughness and excellent stretchability to the hydrogel’s materials.

13.14 Self-healing gels mechanism based on constitutional dynamic chemistry On the basis of the self-healing mechanism, gels are classified into two categories, that is, physical and chemical self-healing gels. In physical self-healing, the gel can reform the networks by the formation of dynamic noncovalent attraction between the molecules and polymer chains inclusive of hydrogen bonding, host
guest interactions, crystallization, polymer nanocomposite interactions, multiple intermolecular interactions, and hydrophobic interactions. While in chemical self-healing the network can be reestablished by the formation of dynamic covalent bonds such as sulfur
sulfur bonds, boron
oxygen bonds, carbon
nitrogen bonds, cyclohexenes, and carbon
carbon/carbon
sulfur bonds. But in both the cases the dynamic equilibrium can be achieved by the dissociation and recombination of chemical bonds or physical interaction. To establish this type of interaction, there must be some functional sites in the hydrogel network which should interact physically or chemically with each other to reform new bonds after undergoing mechanical damage and recover the network and desired mechanical strength of the hydrogels [19]. Two important features of the gel system to self-heal like other self-healing polymer materials, that they produce a “mobile phase” in or around the crack areas after damage, and in this way, they will fill and bridge the damaged zone to facilitate their healing [143]. To generate the mobile phase, they need the outstanding flowability of the gels. Gels have a particular advantage over polymers are in accomplish self-healing performance. This is because of the solvent present in the physical networks, which facilitates the formation of a “mobile phase” around cracks. Secondly self-healing gels may be automatically or nonautomatically self-healable which depends on the additional external energy, trigger, or intervention (e.g., light, heat, pH, or catalyst) whether they need or not to regain their original structures and properties.

13.15 Natural polymer-based hydrogels Natural polymers have some unique properties such as they are safe, easy to handle, biocompatible, hydrophilic, and biodegradable nature which make them suitable candidate in the field of self-healing hydrogels, which received enormous consideration in the past decades [144]. Several natural polymers, for example, polysaccharides [alginate, cellulose, chitosan, and hyaluronic acid (HA)], proteins (gelatin, collagen), and DNA, etc. have some functionalities to form a 3D network, that holds a huge quantity of water, therefore they can form a hydrogel. Natural polymer-based functionalized hydrogels have been explored for the pharmaceutical, medical, and biological applications due to their biocompatibility, biodegradability, renewability, and accessibility [145]. However,

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they have distinct drawbacks, for example; trouble in controlling their degradation and structure owing to their fabrication method, unsatisfactory mechanical properties, and potential immunogenicity, which limits their applications for more considerable uses [146]. To eliminate these drawbacks, scientists, and researcher have made a significant improvement in these regards by the modification of natural polymers, the method of synthesis and fabrications, which could possible their use in various fields in near future. Naturally derived hydrogels act as a native extracellular matrix (ECM), with high water content, and fine 3D network, versatile fabrication methods, and good biocompatibility. Thus naturally consequential hydrogels that are such as instinctive ECMs, having a fine 3D network, high water content, good biocompatibility, and multipurpose fabrication methods, have arisen as hopeful matrices for the assembly and designing of hydrogels. To date, plentiful natural polymer-based hydrogels have been manufactured, such as polysaccharide-based (alginate, HA, cellulose, and chitosan), protein-based hydrogels (collagen and gelatin), and DNA-based hydrogels a summarized in Fig. 13.2. Different types of natural polymers and their hydrogels have been discussed here.

13.15.1 Alginate Alginate can be isolated from seaweed, is an anionic polysaccharide, which has mannuronic acid and guluronic acid units in its backbone structure. The linear and flexible conformation of alginate is due to the mannuronic acid segment in which the mannuronic acid form β-(1-4) linkages. While the guluronic acids are linked with α-(1-4) linkages, which inaugurate a steric hindrance in the vicinity of carboxyl groups. Alginate usually forms hydrogels via the use of an ionic bonding interaction between the carboxylate group on its backbone and a cationic (metal ions) as a cross-linking agent [8]. Usually divalent cations, such as Ca21, Ba21, Zn21, or trivalent cations, for example, Al31 may exist in alginate hydrogels. Alginate hydrogels are providing to have wide applications in various such as biomaterials, for example, their potential applications have been broadly explored as drug delivery, as scaffolds for tissue engineering, and as model extracellular matrices for basic biological studies [147]. Zhao et al. reported polysaccharide-based novel and biocompatible self-healing hydrogels mixing the oxidized sodium alginate solution into the mixture of ADH and N-carboxyethyl chitosan. The designed hydrogels exhibited 95% self-healing efficiency without any external intervention, good cytocompatibility and cell release, which make this hydrogel a suitable candidate in biomedical fields. The two dynamic bonds, imine bond and acylhydrazone bond in the hydrogel network are responsible for the self-healing ability of the hydrogel and the macroscopic self-healing test is shown in Fig. 13.7A [148]. Shunli et al. also reported oxidized alginate-based self-healing conductive hydrogels, in which the oxidized sodium alginate was conjugated with acrylamide through Schiff base reaction. The self-healing and high mechanical properties were attributed to the dynamic hydrogen bonding and Schiff base reaction, which also control the conductivity and stretchability which is also displayed in Fig. 13.7C [149]. Yin and his group recently reported dual functionalized alginate-based injectable and self-healing hydrogels through Schiff base reaction. They grafted dopamine on the aldehyde backbones of the oxidized alginate were

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FIGURE 13.7 Photographs showing the self-healing process of; (A) the cut pieces of CEC-I-OSA-I-ADH hydrogels were healed at 25  C without any external interventions and they retained their shapes after immersed in PBS (pH 7.0) for 3 h; (B) the cut halves of PLGA/ALG-CHO-Catechol hydrogels and the injectability of hydrogels via single syringe which healed after injection; and (C) the four cut pieces of the OSA-PAM hydrogels at room temperature for 6 h that can be bent and exhibit large tensile deformation.

cross-linked with the hydrazide modified poly(L-glutamic acid) to fabricate self-healing (Fig. 13.7B) and adhesive hydrogels with hemostatic ability/cytocompatibility [150].

13.15.2 Agarose-based self-healing hydrogels Agarose is a biocompatible polysaccharide which can be extracted from marine red algae has been frequently used in cosmetics, food, and pharmaceutical industries. Agarose-based systems have been designed for the various applications in medical and biological research in the last few decades. It contains repetitions of agarobiose (a disaccharide of D-galactose and 3,4-anhydro-L-galactopyranose) and can be used in thermalreversible gel formations. It is the principal component of agar, which can be obtained by the extraction of agaropectin from agar [151]. Depending on its concentration, molecular weight, and a number of its side groups, its gelation and meting pints alters from 30 C
40 C to 80 C
90 C, respectively. It dissolves in hot water, dimethylformamide, dimethyl sulfoxide, N-methylformamide, formamide, and 1-Butyl-3-methylimidazolium chloride (BmimCl) [152]. The self-gelling feature of agarose is due to the presence of hydrogen and oxygen on the side chain of this natural carbohydrate polymer. The gelation process of agarose takes place in three steps that are induction, gelation, and pseudo-equilibrium. In these processes, the electrostatic interaction and hydrogen bonding resulted in a helical structure of agarose molecule which causes gelation. The presence of hydrogen bonding enables the agarose hydrogel to be formed without any need for toxic Self-Healing Polymer-Based Systems

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cross-linking agents such as genipin [153]. Agarose is a promising candidate of electrophoresis on the basis of the negative charge on pyruvate and sulfate groups that confirm the mobility of DNA, protein extraction, microfluid, gold separation, cell encapsulation, biosensor, nano/ micro drug carriers, scaffold, and in vitro models for neuroscience and radiotherapy [154]. Agarose is a natural polysaccharide exhibiting some fruitful properties such as its excellent biocompatibility, physiochemical features, and thermo-reversible gelation behaviors, making it a suitable candidate to use it as a biomaterial for localized drug delivery and cell growth. Due to the resemblance of agarose with the ECM, excellent biocompatibility, agarose hydrogels have been extensively studied toward their application in biochemical, pharmaceutical and other biomedical-related fields. Mohammad et al. recently reported agar-based physical cross-linked double network selfhealing hydrogels in which the strong agar biopolymer gels acts as the first network and the tough polyvinyl alcohol biopolymer gel as the second network. These hydrogels possess multiple energy dissipating mechanisms to possess a high modulus up to 2200 kPa with a toughness of 2111 kJ m23 and autonomous self-healing efficiency of 67% within 10 min [155]. Navid et al. also reported agar-based self-healing hydrogen physically cross-linked triple network hydrogels in which they incorporated graphene nanoplates as the third network. It was very interesting that the network of the nanocomposite hydrogel was crosslinked physically through dynamic hydrogen bonding association. The presence of hydrogen bonding gives the high mechanical strength and autonomous self-healing ability in a short time of 10 min to the hydrogel materials which is demonstrated in Fig. 13.8A [156].

FIGURE 13.8 The images representing (A) the proposes structure of Agar/PVA TN nanocomposite hydrogels and the self-healing process of the cut pieces of the agar hydrogel that could withstand high stretching after 10 minutes healing time; and (B) the fabrication and interaction of the PEDOT:PSS/agarose nanocomposite hydrogels that exhibit thermoplastic reversibility (sol-gel transition) and excellent bending and twisting properties. The hydrogels also showing the NIR laser-triggered local self-healing properties.

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Youngsang et al. reported very simple and aqueous solution base strategy to construct an effective photothermal agarose nanocomposite self-healing hydrogels with thermoprocessability, excellent antibacterial activity, and light-triggered self-healing ability. They reported that these hydrogels can kill 100% of the pathogenic bacteria within 2 min of NIR irradiation due to the increase on temperature which is also shown in Fig. 13.8B [157].

13.15.3 Chitosan Chitosan is a linear polysaccharide which is produced by the exhaustive deacetylation of chitin in which the β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine units are randomly distributed. Chitosan is the second most abundant natural biopolymer on earth after cellulose. It can be electrodeposited on the electrode as a hydrogel due to its filmforming and pH-responsiveness properties [158,159]. These properties and the porous network of the deposited chitosan hydrogel enable the entrapping of various biocomponents, which enhance their advanced applications. The entrapment of drugs, such as ibuprofen [160], by the application of electrical signals can be used for controlled release. Similarly the entrapment of redox-active molecules, such as catechol [161] or enzymes [162], for biosensors also broaden the applications of chitosan hydrogels. Further chitosan hydrogels can be fabricated via physical or chemical crosslinking [163,164]. Chitosan hydrogels are admirable biomaterials subsequently they are nontoxic, biodegradable stable, and serializable. Additionally these features render chitosan a very multipurpose material, with widespread knowledge, and potential applications in several fields such as biotechnological and biomedical, etc. [144]. Wei and his group synthesized chitosan-based self-healing hydrogels by a facile, inexpensive and environmentally friendly method through Schiff base linkage with multiresponsive and dynamic nature for controlled vitamin B6. The self-healing ability was attributed to the Schiff base interaction between the amino group on chitosan chain and the aldehyde functional group at the end of PEG chain which is displayed in Fig. 13.9A [165]. Xin et al. reported chitosan/GO based self-healing electroactive hydrogels by the incorporation of polydopamine, which reduce the GO to form an electric pathway into the hydrogel network. The simple approach and excellent properties such as conductivity, adhesion, and self-healing fit it a potential candidate in the application of electroactive tissue engineering and their general schematic diagram is given in Fig. 13.9B [166]. Guo and his coworker recently reported quaternized chitosan-based self-healing hydrogels via Schiff base reaction, which shows injectable, extensible, compressible, and antibacterial properties, and are used for wound dressing applications, and their various macroscopic self-healing tests are shown in Fig. 13.9D [167]. Yong et al. synthesized chitosan-based self-healing hydrogel by intermolecular hydrogenation and ionic cross-linking copolymer, with ductile and corrosive environments tolerant, for oil/water separation and Fig. 13.9D shows its self-healing ability [168].

13.15.4 Cellulose Cellulose is a biodegradable, renewable, and the most abundant natural polymer on the earth. Cellulose was first of all isolated and discovered by Anselme Payen in 1838. It is

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FIGURE 13.9

The photographs demonstrating (A) the multi-responsive and self-healing behavior of Chitosan-based hydrogels at various time intervals; (B) the formation and interaction between CS and GO in CSDA-GO composite hydrogel; the self-healing, self-adhesion and electrical conductivity mechanism of the Chitosan based hydrogels; (C) the self-healing capability of cut pieces of quaternized chitosan (QCS) hydrogel in a cylindrical mold at 25  C for 2 h that exhibit rapid self-healing performance ( , 3 s); and (D) the self-healing and adhesion behavior between the two freshly cut surfaces of the Chitosan composite hydrogel.

one of the most promising raw materials due to its easy availability, abundance, and low cost. The polysaccharide chain of cellulose is named as cellobiose, mainly composed of Dglucose unit and it is the fundamental constituent of cellulose. Cellulose contains two glucose residues which are linked through β-1,4-linked anhydro-D-glucose units (glycosidic bonds). In its structure, every unit is further coiled with 180 degrees with respect to its neighbors. The replication of segments occurs regularly in such a way that it forms a dimer of glucose, known as cellobiose [169]. Cellulose is extremely crystalline and generally insoluble in water. The intramolecular hydrogen bond ranging from O(30 )-H hydroxyl to the O(5) ring oxygen of the subsequent unit diagonally the glycosidic linkage and from the O(2)-H hydroxyl to the O(60 )-H hydroxyl of the following residue. This makes it one outstanding material for tissue engineering applications [170,171]. Cellulose-based materials have been extensively explored in the last few years for tissue engineering applications due to their admirable biodegradability, biocompatibility, and low cytotoxicity [171]. The absence of side chain on branching enable cellulose chain in a regularly ordered with a semi-crystalline polymer structure, having 60% crystallinity [172]. Recently cellulose has been widely explored owing to its unique properties, for example, low cost, hydrophilicity, biocompatibility, nontoxicity and biodegradability, makes it

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FIGURE 13.10 Applications of cellulose-based hydrogels in various fields.

an excellent candidate for the fabrication of biocompatible hydrogels. The presence of plentiful hydrophilic carboxyl, hydroxyl, and aldehyde functional groups on the cellulose and its derivatives backbone enable them to form hydrogels with enthralling properties and structures, which make it an auspicious candidate in biomedical applications such as tissue engineering, bioimaging, wound dressing, a wearable sensor, and drug delivery, which are depicted in Fig. 13.10 [173]. Yang and his coworkers have reported a facile strategy for the preparation of TEMPOoxidized cellulose nanofibrils self-healing hydrogels via hydrogen bonding and dual coordination bond interactions with the ferric ions. The multiple noncovalent interactions act as a dynamic and stable association which results in an effective self-healing efficiency (90%). The biocompatible and biodegradable nature of the carbon nanofibers enable to form dynamic and tunable hydrogels which expand their application in the biomedical field [174]. Various macroscopic experimental tests and mechanical stress-strain curves are given in Fig. 13.11A, which demonstrate the excellent self-healing ability of the hydrogel. Xuefeng et al. reported cellulose-based self-healing hydrogels via dynamic covalent Acylhydrazone linkage with the dual response for 3D cell culture and controlled release. These hydrogels were shown excellent self-healing capability with a healing efficiency of 96% along with efficient mechanical properties. They display redox/pH dual responsive behavior and successfully applied for the controlled release of doxorubicin, and their biocompatible and reversible cross-linked nature makes them suitable for 3D culture scaffolds

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Images Showing the self-healing performance of (A) the cut halves of PAA-CNF-Fe31 physical hydrogels that sustain stretching and can lift a weight without breaking, the stress-strain curves of original and healed of cellulose-based hydrogels, and their microscopic self-healing performance; and (B) the macroscopic and microscopic self-healing behavior of cellulose based hydrogel, and the stress-strain curves of original and healed samples at different time intervals.

FIGURE 13.11

for tissue engineering [175]. Some of the macroscopic and microscopic self-healing tests along with the mechanical tensile curves are depicted in Fig. 13.11B.

13.15.5 Hydroxyethyl cellulose Cellulose ethers are used in biomedical applications due to their water-soluble and swellable nature. Different cellulose ethers inclusive hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), methylcellulose (MC), and hydroxypropyl methylcellulose (HPMC) have been investigated for the physicochemical factors affecting the drug release kinetics. Among these cellulose ethers, the HEC is a water-soluble, tasteless, odorless, nonionic, nontoxic. These properties and the presence of the β-linkage makes it a promising candidate for tissue engineering applications and in pharmaceutical areas [176]. It is present in a wide range of viscosity grades, low solubility in an organic solvent as linked to other cellulose derivatives, such as HPC and HPMC. Furthermore, HEC exhibits a pHindependent release due to its nonionic nature [177,178]. Hydroxyethyl cellulose (HEC) can be used as a thickening agent, binding, suspending, emulsifying, stabilizing agent, form films, retain water, dispersion, and provide protective colloid action. It is readily soluble in hot or cold water and form solutions with a

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FIGURE 13.12 The digital images showing the self-healing behavior of; (A) the cut pieces of disk shapes with different colours of HEC/PAA-Fe31 hydrogel; and (B) the small cut pieces of rod shaped of HEC/P(AA-coAAm)-Fe31 hydrogels that can form a thin film after pressing between two glass slides, which show high mechanical strength.

wide range of viscosities. HEC has many industrial applications like contributing the favorite body in paints [179], textile finishing [180], thickening cement mortar [181], and papermaking [182]. Hussain et al. recently synthesis hydroxyethyl cellulose-based self-healing hydrogel via metal
ligand interactions with enhanced mechanical properties. The fabricated hydrogels show about 1660 % extensive fracture strain and 1.35 MPa tensile stress, about 8.8 MJ23 toughness and an excellent self-healing efficiency of 87% in 24 h healing time at room temperature as shown in Fig. 13.12A [49]. Hussain et al. further enhance the mechanical properties of hydroxyl ethyl cellulosebased self-healing hydrogel by introducing more functional groups to the hydrogel network by adding acrylamide and systematically studied the concentration effect of HEC on the mechanical strength of the hydrogels. These hydrogels exhibited about 3.50 MPa mechanical tensile stress and 32 MPa compression stress and can achieve about 98% healing efficiency within 24 h healing time at room temperature with the addition of any external healing agent which is demonstrated in Fig. 13.12B [183].

13.15.6 Dextrin-based self-healing hydrogels Dextrin is an oligosaccharide with a low molecular weight which is produced by acid or enzymatic hydrolysis of glycogen or starch. Dextrin exhibiting the α-(1-4)- and α-(1-4),60-Glc branched structure of amylopectin and α-(1-4)-Glc structure of amylose. The total reducing power is measured in terms of “dextrose equivalent” (DE), which indicate their extent of hydrolysis [184,185]. Dextrin having same DE value but they show different physical and chemical behavior such as viscosity, hygroscopicity, solubility, Self-Healing Polymer-Based Systems

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bioavailability, fermentability, gelation, stability, sweetness due to their different structural features. The properties of dextrin depend on their native source, hydrolysis conditions for their desired and valuable applications. Dextrin is regarded as a safe (GRAS), biocompatible, degradable, and can be widely used in a variety of applications such as foods, adhesives, textile, and cosmetics [186]. Dextrin a suitable candidate for hydrogel production and several types of dextrin-based hydrogels have been reported via radical polymerization [187]. The set of cyclic oligosaccharides which consist of α-1,4-linkage of repeating D-glucose is called cyclodextrins (CDs), which is used in various fields such as medicine, daily chemical and agriculture, and food. The number of D-glucose units comprises different types of CDs such as α-, β-, and γ-cyclodextrins consist of 6, 7, and 8 D-glucose units, respectively. The molecular 3D structure of CDs has both the outer hydrophilic surface and hydrophobic inner cavity like a truncated cavity. The cavity is about 0.79 nm in depth while the diameter of the top and bottom cavity depends on the number of D-glucose units [188,189]. The most significant role of CDs is its reactivity with different guests of appropriate geometry and physicochemical properties to form inclusion complexes. The guest molecule fit themselves partially or completely in the internal hydrophobic cavity while exposing the outer hydrophilic surface to the aqueous environment. This spontaneous process involves various types of interaction such as hydrogen bonding, hydrophobic, electrostatic, and van der Waals interactions and the. geometric compatibility [190,191]. Feng Liang and his group recently reported a facile strategy for the preparation of flexible conductive hydrogel via host
guest interaction based on the amphiphilic configuration of α-cyclodextrin. These hybrid hydrogels exhibit excellent self-healing as shown in Fig. 13.13A, thermoresponsiveness, and high conductive (0.64 S m21) properties [192].

FIGURE 13.13 The digital images showing the self-healing behavior of; (A) the cut pieces of disk shapes with different colours of HEC/PAA-Fe31 hydrogel; and (B) the small cut pieces of rod shaped of HEC/P(AA-coAAm)-Fe31 hydrogels that can form a thin film after pressing between two glass slides, which show high mechanical strength.

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Zhifang et al. recently reported rapidly self-healing hydrogels with high mechanical strength via host
guest supramolecular chemistry between a host of modified β-cyclodextrin and an organic guest to form three-arm host supramolecular first interaction. While the second strong covalent bonds were generated by UV-initiated polymerization to form a cross-link in the hydrogels. On cutting the fresh surfaces were rapidly reorganized to each other to heal the hydrogel by recombination of the cut surfaces [193]. The self-healing ability was determined by both macroscopically and microscopically and was also recorded their tensile stress-strain curve to find their self-healing efficiency, which is collectively shown in Fig. 13.13B. Nigus et al. reported thermal frontal polymerization of β-cyclodextrin-based smart and intrinsic self-healing hydrogels via host
guest supramolecular interaction which exhibits about 99% recovery efficiency [194]. The mechanical strength and self-healing efficiency of these hydrogel have been displayed in Fig. 13.13C. He Tian and his coworkers also reported β-cyclodextrin host polymer-based self-healing hydrogel via supramolecular host
guest recognition without using any gelator, which can self-heal within 1 min time without external gelator at ambient atmosphere [195].

13.15.7 Guar gum-based self-healing hydrogels Guar gum (GG) is a natural hydrophilic and nonionic polysaccharide, which can be extracted from the seed of guar plant, a species of Leguminosae family known as Cyamopsis tetragonolobus which is usually grown Pakistan, India, Sudan, and in the United States. In GG the galactose side chains are linked on the mannose backbone to form a polymeric structure. The D-mannose units linked together in a straight chain by β-(1-4) glycoside linkage while the D-galactose units are held together in an alternate pattern via (1-6) linkage and the ratio of mannose to galactose sugar units ranges from 1.8:1 to 2:1. The presence of hydroxyl groups on the polymeric chain facilitates the modification of GG for various industrial applications. Because of its nontoxicity, low cost, biocompatibility, biodegradability, high water solubility, and high viscosity, it is widely used in many industrial applications such as in the textile and paper industries it is used as a finishing and sizing agents, in the cosmetics and food industries as stabilizer, binder and thickener agent, and in mining and hydraulic fracturing process it uses a fracturing additive. In the pharmaceutical field, it is used for the controlled release of drugs in the gastrointestinal tract, for example, carrier for colon targeted drugs, in the treatment of colorectal as an anticancer drug, and in the treatment of cholera in adults for oral rehydration solutions. Ni et al. synthesized GG-based silver nanocomposite hydrogels with dual functionality with stimuli
responsive and rapid self-healing ability [196]. Guo et al. recently reported a facile strategy for the synthesis of hydroxypropyl GG-based self-healable hydrogels which exhibited higher stress, elongations and self-healing efficiency due to the presence of Hbonding and dynamic covalent bonding interactions and the microscopic self-healing test to show the self-healing ability of these hydrogels is given in Fig. 13.14A [197]. Ni Yonghao and his groups also reported a one flask method for fabricating GG-based hydrogels by taking the advantages of regioselective oxidation of GG in the presences of NaIO4. The resulted hydrogels obtained without any addition cross-linker under ambient

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FIGURE 13.14 Photographs showing the self-healing properties of (A) the two freshly cut pieces of hydroxypropyl guar gum hydrogels at room temperature in 30 minutes; (B) the formation, self-healing and thermo responsive behavior of guar gum hydrogels; and (C) the self-healing and stretching ability of the healed boraxguar gum hydrogels.

conditions demonstrate significant self-healing ability and thermally responsive properties. This hydrogel can also have the ability to remove copper ions from the aqueous environment and they achieved efficiency of absorbance of 944 mg g21 GG in 5 h time [198]. The schematic synthesis and self-healing tests are shown in Fig. 13.14B. Nan et al. has recently synthesized multifunctional GG-based self-healing hydrogels through dynamic and reversible noncovalent interactions. Along with self-healing ability, they also show outstanding reform-ability and injectable capability and the macroscopic and microscopic self-healing tests are depicted in Fig. 13.14C [199]. Xiaofeng et al. synthesized GG
glycerol ionic hydrogel in glycerol/water solution as a dispersion medium, while borax was added as a cross-linking agent. The hydrogels exhibited self-adhesive, strain sensitivity, antifreeze, and ultrafast self-healing properties [200].

13.15.8 Gelatin-based self-healing hydrogels Gelatin which can be obtained by the partial hydrolysis of collagen is natural and biodegradable functional protein present mainly in the connective tissue in the body. Collagen is a fibrous protein which is responsible to maintain the integrity of connective tissues such as the tendons, bones, skin, cartilages, ligaments, corneas dentin, and blood vessels, and it can also form gels under ambient conditions. Gelatin consist of 18 amino

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acids in which three of them predominating in the structure and all the three amino acids are linked in repeating unit sequence -P-G-R- ordered fashion. Where p represent the proline/hydroxyl proline, G is glycine, and the R represent the side chain which was first of all proposed by Astbury and finally the accepted structure of gelatin is a linear structure with small branches [201]. Gelatin-based hydrogels such as gelatin/chitosan, gelatin/alginate, gelatin/fibrinogen, gelatin/hyaluronan, etc., have unique characteristics including rapid biodegradability, excellent biocompatibilities, and nonimmunogenicities. The natural polymer gelatin has many functional groups on its backbone including the carboxyl and amino groups, enable it a promising candidate for the preparation of hydrogels and is widely used in the field of drug delivery. Recently Gerecht and his coworker designed gelatin-based self-healing hydrogels crosslinked by oxidized dextrin through reversible imine linkage by Schiff base reaction. This dynamic imine bond between gelatin and oxidized dextrin confer the self-healing properties of the hydrogels. They also reported that the designed hydrogels exhibit injectable and the presence of bio-functional cell adhesive peptides sites on gelatin chains promoting the vascular morphogenesis endothelial colony-forming cells (ECFCs). The imine bond is a class of dynamic covalent bonds, which can offer the intrinsic self-healing by the bond association and dissociation at physiological conditions (37 C and pH 7.4). The only byproduct in this reaction is water which minimizing the cytotoxicity of the hydrogels, which enable these self-healing hydrogels to be used in the delivery of ECFCs and also reconstruct the injured and diseased tissue [202]. The self-healing and injectable tests are shown in Fig. 13.15A. Wang et al. reported gelatin-based self-healing hydrogels by combining the ionic and multiple hydrogen bonding interactions with injectable properties. They also studied the effect of the ionic interaction and quadruple hydrogen bonds on the mechanical strength of the hydrogel and Fig. 13.15B shows the self-healing ability of the prepared hydrogels [203]. Yueyuan et al. recently reported injectable hydrogel for wound dressing through electrostatic complexation between gelatin and gallen which demonstrate remarkable self-recovery and shear-thinning properties [204]. Mohammad et al. also synthesized gelatin-based self-healing hydrogels via dynamic Schiff base linkage with polyethylene glycol dibenzaldehyde by just mixing their aqueous solution at adjusted pH carefully. The loading and delivery performance was carried out with clindamycin hydrochloride as a model antibacterial drug and the antibacterial activity was evaluated against Staphylococcus aureus [205]. The visual self-healing experiment showing the self-healing behavior of this hydrogel is demonstrated in Fig. 13.15C.

13.15.9 Glycogen-based self-healing hydrogels Glycogen, a polysaccharide which contain five glucosyl residues was discovered by a French physiologist Claude Bernard in 1887 [206]. Glycogen is present in the cytosol of the cell as hydrated granules with a diameter between 1 and 4 μm which forms complexes with proteins and enzymes and most abundantly it is found in liver and muscle. The chemical structure of glycogen is similar to the amylopectin; therefore, it is also referred to as animal starch. As compare to amylopectin, glycogen is highly branched, more compact,

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FIGURE 13.15 Photographs showing the (A) the self-healing and injectable capability of Gtn-I-Odex hydrogel in which three cylindrical hydrogels with different dyes were self-healed for 20 min at 37  C and are stretchable by tweezers, the disk like Gtn-I-Odex hydrogel were injected into tiny PDMS disc mold by a syringe including Gtn hydrogel. CEC-I-Odex hydrogel and Gtn-I-Odex hydrogels, strain sweep and rheology recovery; (B) optical self-healing of gelatin-UPy-Fe hydrogels in which the cut pieces were healed instantaneously that was strong enough to support its own weight and illustrating the process of self-healing; and (C) the intermixing of GA/ P4K/0.5 hydrogels with different colors, resulting a purple hydrogel that can be stretched, indicating the selfhealing property of hydrogels.

and enough large that its molecular weight reaches up to 108 Da which is approximately equal to 60,000 glucose units. The compact structure of glycogen is due to the coiling of the polymer chain and this allows a large amount of carbon energy which is to be stored in a small volume and has little effect on cellular osmolarity. The highly branched structure of glycogen increases the solubility and the rate at which glucose can be stored and released. The linear chain of glycogen is linked by α-(1-4) glycosidic bonds while the branched chain glycosidic linkage occurs by α-(1-6) linkage at every 8
10 glucose residue [207]. This is the reason that glycogen has a higher molecular weight and behave differently than amylopectin. Every glycogen molecule has a dimeric protein, glycogen which is covalently attached via the hydroxyl groups of a specific tyrosine to the C1 of the first glucose unit at the reducing end of the chain [208,209]. Recently Hussain et al. reported glycogen-based self-healing hydrogels via sacrificial metal
ligand bonds interaction with flexible, ultra-stretchable, and enhanced mechanical strength. They constructed the hydrogel network through multiple hydrogen bonding interactions and metal coordination bonds and these supramolecular bonding interactions were responsible for their best self-healing efficiency which was about 94% within 24 h healing time without an external healing agent. The self-healing efficiency of the healed sample was determined by the tensile test and the macroscopic self-healing experiment was performed with a dyed color hydrogel samples for their better visualization as shown

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FIGURE 13.16 Photographs showing the (A) the self-healing and injectable capability of Gtn-I-Odex hydrogel in which three cylindrical hydrogels with different dyes were self-healed for 20 min at 37  C and are stretchable by tweezers, the disk like Gtn-I-Odex hydrogel were injected into tiny PDMS disc mold by a syringe including Gtn hydrogel. CEC-I-Odex hydrogel and Gtn-I-Odex hydrogels, strain sweep and rheology recovery; (B) optical self-healing of gelatin-UPy-Fe hydrogels in which the cut pieces were healed instantaneously that was strong enough to support its own weight and illustrating the process of self-healing; and (C) the intermixing of GA/ P4K/0.5 hydrogels with different colors, resulting a purple hydrogel that can be stretched, indicating the selfhealing property of hydrogels.

in Fig. 13.16A [210]. Fu and his coworker also reported a facile and one-pot method for the synthesis of glycogen-based self-healing hydrogels with tunable mechanical properties and extremely flexible nature. These hydrogels exhibited about 98% healing efficiency in strain and 96% healing efficiency in stress along with 91% healing efficiency in their toughness [211]. The self-healing ability was determined by both tensile testing machine and macroscopic self-healing experiment, which shows about 98% self-healing efficiency as described in Fig. 13.16B.

13.15.10 Hyaluronic acid-based hydrogels Hyaluronic acid (HA) is a nonsulfated glycosaminoglycan polysaccharide. It consists of alternating units of D-glucuronic acid and D-N-acetylglucosamine. They are linked together via alternating β-1,4 and β-1.3-glycosidic bonds [212]. Naturally HA is an anionic polysaccharide, occurs in the form of a straight chain and is broadly spread all over the human body. Under ambient conditions, it can be cross-linked physically by freeze-thaw techniques, without any organic solvent or toxic cross-linking agents [213]. In addition, the functional hydroxyl, carboxylic acid and N-acetyl groups of HA facilitate its modification. The hydrogel can be formed from the modified HA via enzymatic cross-linking, chemical cross-linking, or photo-cross-linking. HA also accounts for a major portion of the cartilage,

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ECMs of skin, and vitreous humor. Therefore it has been extensively used for clinical applications, especially in wound healing, cellular signaling, cell motility, matrix organization, and angiogenesis for over thirty years [144].

13.15.11 Xanthan-gum-based self-healing hydrogels Xanthan-gum is a water-soluble, anionic, and high molecular weight (2 million g mole21) polysaccharide which can be obtained by the aerobic microbial fermentation of corn, sugar, or their derivatives and in the presence of the bacterium Xanthomonas campestris in is transferred into a soluble gum [214,215]. This gum can be converted into xanthangum by precipitating it in a nonsoluble solvent. It is an acidic biopolymer pentasaccharide repeated units comprising D-glucosyl, D-mannosyl, and D-glucuronyl acid residues in the molar ratio of 2:2:1 and variable proportion of pyruvyl and O-acetyl residues. Xanthangum has a unique structure with trisaccharide’s main side chain form a cellulosic structure is composed of mannose (β-1,4) glucuronic acid (β-1,2) mannose which is attached alternatively by α-1,3 linkages in the backbone. At pH higher than 4.5, the deportation of pyruvyl and O-acetyl residue take place which results in an increase in the negative charge density around the xanthan chain, which enabled their physical cross-linking with cations such as Ca21 [216]. Xanthan-gum has many tunable hydroxyl groups and acts as an efficient polyelectrolyte which can be used in medicine, pharmaceuticals, suspending agents, agricultural, additive in food as a thickening agent, and wastewater treatment [217]. Yashveer Sing and his group recently reported xanthan-based self-healing hydrogels for controlled drug and 3D cell culture. They obtained the hydrogel network by cross-linking the oxidized xanthan and 8-arm PEG hydrazine via biodegradable and pH-responsive hydrazine linkage. The excellent self-healing was determined by the recovery in strain when the hydrogel was subjected to an alternating cycle of varying strains of 1% and 800%. The self-healing and injectability characteristics of these hydrogels are depicted in Fig. 13.17A [218]. Xiuwen Wu and his coworker also reported the synthesis of self-healing hydrogel via self-crossing of carboxymethyl-modified chitosan and aldehyde-modified xanthan which shows multifunctionalities such as antienzymatic hydrolysis, regelifying, biodegradability, and biocompatibility [219]. The self-healing process and local injection ability of these hydrogels are depicted in Fig. 13.17B. Ningxiao et al. also reported the synthesis of double network hydrogels by cross-linking xanthan-gum with polyacrylamide, which exhibited high fracture stress (3.64 MPa), and compressive stress of 50 MPa at 99% fracture strain. The hydrogel demonstrated excellent self-healing, fatigue resistance, high stability in various environments, notch-insensitivity properties [220]. The self-healing ability was determined by the macroscopic test with dyed samples for better visualization and was also confirmed via microscopic self-healing test. The self-healing efficiency of these hydrogels were calculated to form their tensile stress-strain curves of the healed sample and this is summarized in Fig. 13.17C. The microwave synthesis of xanthan-gum hydrogels have been reported in the literature in which the acrylic acid is grafted onto the xanthan-gum biopolymer and N,N0 -methylenebisacrylamide (MBA) was used as a cross-linker and ammonium persulfate as an initiator. They further state that these hydrogels are environmentally benign, recyclable, and readily recoverable materials and have excellent absorption capacity in the removal of dyes [221]. Self-Healing Polymer-Based Systems

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FIGURE 13.17 The Photographs demonstrating the (A) self-healing, injectability, extrudability, and selfhealing mechanism of Xanthan-based hydrogels; (B) the self-healing process and local injection of Xan-CHO/ NOCC hydrogels and fibrin gel in PBS, and the cumulative releasing ration of BSA-FITC after local injection in PBS; and (C) the self-healing behavior of PAM/XG-10 hydrogels in which 4 cut pieces were healed in sealed polyethylene bag in a water bath at 70  C; microscopic images showing the changes of the incision over time, stressstrain curves of the original and healed hydrogels for 48 h, and self-healing efficiency of the PAM/XG-2.5 hydrogels for different time at different temperature.

13.16 Recent development in miscellaneous application fields Recent development address critical needs, which include, however not limited to the following spectrum of hydrogels. Some of their important applications are summarized in Fig. 13.18.

13.16.1 Superabsorbent hybrid hydrogels Superabsorbent polymers as hydrogels fascinate and hold extraordinary large amounts of water or aqueous solutions [222]. These hydrogels can absorb a high amount of water [between 1000% and 100,000% (10
1000 g g21)], while the absorption capacities of common hydrogels are ,100% (1 g g21) [223]. Mostly acrylics monomers are used to make superabsorbent polymers. In the manufacture of superabsorbent polymer hydrogels, acrylic acid and its potassium or sodium salts, and acrylamide are used. Superabsorbent hybrid hydrogels are very useful due to their outstanding water absorption capacity, which improves fertilizer retention in soil, reduces water consumption in irrigation, increases the plant’s growth and lowers the death rate of plants. For the renewal of desert and dry environments, the use of superabsorbent hydrogels as

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FIGURE 13.18

Applications of hydrogels in various fields.

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water-saving materials have been reported very well in literature [224,225]. The pollutants present in the environment contaminates wastewater to a larger extent and this originates adverse environmental issues in various parts of the world. In this case, the dyes and toxic heavy metal ions can be absorbed by using superabsorbent hydrogel. The functional groups present in these hydrogels inclusive
OH,
CONH2,
NH2,
COOH, and
SO3 groups, are responsible for the modification, activation, and the properties of hydrogels. For example, ghatti/polyaniline, polyacrylamide/gum, interpenetrating hydrogel network exhibited admirable properties for the elimination of malachite green dye from the polluted water [226]. Special superabsorbent hydrogel materials are commonly employed as hygienic materials, mainly as lady napkins and disposable diapers, to absorb the secreted liquids, for example, blood and urine [6]. Superabsorbent hybrid hydrogels also facilitate targeted drug delivery, and nano/controlled drug delivery.

13.16.2 Conductive polymer hydrogels Conductive polymer hydrogels (CPHs) are a special class of soft and smart materials, that relates the beneficial and facial characteristics of both the organic conductors and hydrogel materials. Conducting polymer hydrogels serves as an outstanding interface between the ionic-transporting phases (electrolyte), and electronic-transporting (electrode). They also acts as a bridge between synthetic and natural biological systems and between hard and soft materials. Consequently conducting polymeric hydrogels determined upand-coming results for wide-ranging current applications, extending from energy storage devices, such as supercapacitors and biofuel cells, to molecular level inclusive, bioelectronics and medical electrodes [227]. CPHs can also be used in screen printed into micropatterns or ink-jet printed. Hydrogels on the basis of conducting polymers, associate various properties of the polymeric hydrogels with the optical and electrical characteristic of metal or semiconductors. Therefore contributing a collection of features, such as 3D microstructured conducting frameworks which encourage the transport of ions, charges, and molecules [228]. Polypyrrole (PPy), polyaniline (PANii), and polythiophene (PTh) structures are some appropriate raw materials used for the designing of CPHs. CPHs have been used as a potential candidate in chemical mimicry of neural networks, electro-stimulated drug release, and implantable electrochemical biosensors, etc. [229]. CPHs have been recommended as probable conductive and flexible electrodes for supercapacitor applications [230]. They can also show suitable and useful applications in various fields such as, for bioelectronics, energy storage devices, sensitivity, and as glucose enzyme biosensors with high sensing speed [227]. CPHs have auspicious applications in lithium-ion battery technology due to their admirable electronic and electrochemical properties. CPHs can be used to report the tasks faced by the coming generation of high capacity alloy-based anodes, for example, silicon and germanium [231]. CPHs also have special, potential, and advanced applications in actuators, bioactive electrode coating, and tissue engineering field [232,233]. In the field of biosensors another important application of CPHs is the integration of biological sensing elements, such as

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antibodies, enzymes, cells, nucleic acid, etc. with an electronic amplifier by an electronic transducer equipped [231]. CPHs possess many advantages, such as providing improved electrode interface between the ionic transport phases, and the electronic, which make the possibility of molding into different flexible, and complex shapes and the preparation of micro-patterns by ink-jet printing or spray coating [230].

13.16.3 Polysaccharide-based natural hydrogels Polysaccharides are a class of natural polymeric materials that were mostly used to stabilize the designed nanoparticles and hydrogels growth. Polysaccharides have some exceptional properties which make the polymer group, widest ranging knowledge and the longest standing in terms of advanced biomedical applications, such as water solubility, nontoxicity, high capacity for swelling, a wide variety of chemical structures, and induced by simple chemical modifications [234]. Additionally polysaccharides are biodegradable, biocompatible, abundant biostable, and inclined to enzymatic digestion in the human body. Functionality (biocompatibility and biodegradability) is specifically beneficial for the release of drugs at a convinced time and at a convinced site in the body [235]. Currently the polysaccharides are being improved to get innovative biomaterials, as a support material for gene delivery, and controlled drug delivery, cell culture, and tissue engineering [236]. Moreover, the compliance of their chemical network structures permit the improvement of advanced functionalized materials which can meet an array of necessities. In the advanced biomedical area, the degradation of natural polymers into functional metabolites produces them a marvelous candidate for an extensive variety of advanced applications, together with regenerative medicine. Established from natural, nontoxic, renewable, and biodegradable sources, polysaccharide-based hydrogels are admissibly observed as “ecologicallyfriendly” products [234]. Polysaccharide includes alginate, agarose cellulose, chitosan, carrageenan, gelatin, GG, gellan gum, pectin, and xanthan-gum, etc. are accomplished of making hydrogels. Though, in the designing of a significant network of hydrogels, polysaccharide polymers are more gorgeous than synthetic polymers, because of their functional properties such as biodegradation, good hydrophilicity, and biocompatibility. Owing to these properties, they are used in environmental, biomedical, and industrial applications. In various industries applications, polysaccharides are commonly used such as paper, agro-food, textile, cosmetic, pharmaceutical, and biomedical, on account of their rheological characteristics such as vilifying, gelling, and stabilization of dispersions. In contrast, attributable to its good tissue compatibility, they have been broadly used in the field of tissue engineering together with regeneration of cartilage, skin, liver, and bone, and in the handling of radiating wounds [237,238]. Bioactive coating (e.g., catheter and stent), replacement of cellular scaffold and nucleus pulpous (artificial organs) are fairly and manifestly probable with polysaccharide-based hydrogels. Microporous hydrogels have been widely used for blood purification, controlled drug release, removing the metal ions/anionic dyes from polluted water [239], regenerative medicine wound dressing materials and for cell delivery [240,241]. Acidic cellulose
chitin hybrid gel (as novel electrolyte) has been established for an electric double layer capacitor.

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13.16.4 Protein-based hydrogels Among the natural polymers, proteins are the most under-utilized and under-rated feeds stocks with a variance to their cutting-edge industrial applications. Proteins such as gelatin, actin, collagen, myosin, silk, elastin keratin, and fibrinogen are extensively used for the manufacture of hydrogels [242]. This extensive assortment of natural materials can form noncytotoxic polymeric hydrogels. Protein-based polymeric hydrogels have similar features such as the extracellular system, therefore they take the probability to encourage the movement, growth for tissue regeneration, for cell encapsulation and wound healing [243]. Recently fibrous protein-based hydrogels getting attention due to their mechanical and structural similarity with the native ECM and their comparatively humble processability under mild, cell-compatible conditions [243]. Hydrogels synthesized from silk fibroin take momentous interstitial fluid support, which achieved high compressive moduli between 10 and 50 kPa, and this is most suitable to covers the region of articular cartilage repairs [244]. Elastin-like protein hydrogels have been utilized to encourage the outgrowth and to improve the property of common polymeric hydrogels strategy limitations on neuronal cultures. Elastin-like proteins are formed via resilient engineered protein polymers, recombinant protein synthesis, that is made wholly of amino acids. Elastin-based polymeric hydrogels have revealed the enormous potential for the advanced engineering of elastic tissues, such as lung, skin, and vasculature. The presence of elastic polymeric fibers in blood vessels allow the vessels to stretch and relax more than a billion times during their lifetime [245].

13.16.5 Hydrogels for energy applications Recently hydrogels have been used for a lot of energy applications. They can be used as electrodes in electrocatalytic applications such as full cells, metal
air batteries, supercapacitors, water electrolyzers, and lithium-ion batteries [246]. Arumugam Manthiram and his coworker designed a sectionalized MnO2-Co3O4 electrodes by electrodeposition of metal ions in hydrogels which enhanced the catalytic activity in metal
air batteries. This novel structure enhances the catalytic activities (ORR/OER) as compared to the bared metal catalyst. This sectionalized electrode also shows excellent cycling performance and long-term stability in rechargeable Zn-air batteries and thus has potential for wide applications in the fabrication of other electrode structures employing various metal oxides [247]. Nitrogen and oxygen dual-doped carbon hydrogel film for highly efficient oxygen evaluation reaction was fabricated by LBL assembly of chemically converted graphene and CNTs through a simple filtration procedure followed by N-doping with ammonia. This self-supported hydrogel film surprisingly proved high OER activity which were higher that the noble (IrO2) and some transition metal catalyst. Further this material also exhibit stability in both alkaline and strong acidic solutions [248]. Hongxia et al. reported a facile inorganic
organic carbon nanostructured hydrogel framework via ultrafine Sn
Fe alloy (average size B 2.7 nm) in 3D double network for enhanced lithium storage. This fine structure enables the Sn-Fe@C framework electrode to exhibit long cycle life (516 mA h g21 after 500 cycle at 0.1 A g21) and high rate capability which provides new insight for improving energy storage properties [249]. An efficient two steps hydrothermal synthetic approach was demonstrated for a novel

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References

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heterostructure molybdenum disulfide nanosheets and nitrogen-doped reduced GO hydrogels which exhibit a high-performance for lithium-ion batteries. Due to an open pore structure, high nitrogen content and large surface area, the anode manifests a high specific capacities of 1140 mA h g21 at a current density of 100 mA g21. This MoS2/n-RGO hybrid delivers a maximum energy density of 890 W h kg21 with the power density of 130 W kg21 which highlights the importance of the nitrogen-doped hydrogel as a novel and advanced electrode for lithium-ion batteries [250]. Tao Chen and his group recently reported healable and highly stretchable DN hydrogel electrolyte supercapacitors by cross-linking a copolymer with double point linker of Laponite and GO. The designed hydrogels simultaneously exhibit excellent ionic conductivity, high mechanical stretchability (1000%), and superior healable performance (900%) which extends their use in portable and wearable energy related devices with multifunction [251].

References [1] K. Varaprasad, G.M. Raghavendra, T. Jayaramudu, M.M. Yallapu, R. Sadiku, A mini review on hydrogels classification and recent developments in miscellaneous applications, Mater. Sci. Eng.: C 79 (2017) 958
971. [2] O. Wichterle, D. Lim, Hydrophilic gels for biological use, Nature 185 (4706) (1960) 117. [3] W. Roorda, H. Bodde, A. De Boer, H. Junginger, Synthetic hydrogels as drug delivery systems, Pharmaceutisch Weekbl. 8 (3) (1986) 165
189. [4] S.J. Buwalda, K.W. Boere, P.J. Dijkstra, J. Feijen, T. Vermonden, W.E. Hennink, Hydrogels in a historical perspective: from simple networks to smart materials, J. Controlled Rel. 190 (2014) 254
273. [5] E.M. Ahmed, Hydrogel: preparation, characterization, and applications: a review, J. Adv. Res. 6 (2) (2015) 105
121. [6] M.M. Zohourisan, K. Kabiri, Superabsorbent polymer materials- a review. Iranian Poly. J. 17 (6) (2008) 55246346. [7] N. Peppas, P. Bures, W. Leobandung, H. Ichikawa, Hydrogels in pharmaceutical formulations, Eur. J. Pharm. Biopharm. 50 (1) (2000) 27
46. [8] J.A. Rowley, G. Madlambayan, D.J. Mooney, Alginate hydrogels as synthetic extracellular matrix materials, Biomaterials 20 (1) (1999) 45
53. [9] K. Varaprasad, K. Vimala, G.M. Raghavendra, T. Jayaramudu, E. Sadiku, K. Ramam, Cell encapsulation in polymeric self-assembled hydrogels, Nanotechnology Applications for Tissue Engineering, Elsevier, 2015, pp. 149
171. [10] W.A. Laftah, S. Hashim, A.N. Ibrahim, Polymer hydrogels: a review, Polym. Technol. Eng. 50 (14) (2011) 1475
1486. [11] K. Sharma, B. Kaith, V. Kumar, S. Kalia, V. Kumar, H. Swart, Water retention and dye adsorption behavior of Gg-cl-poly (acrylic acid-aniline) based conductive hydrogels, Geoderma 232 (2014) 45
55. [12] C. Chang, L. Zhang, Cellulose-based hydrogels: present status and application prospects, Carbohydr. Polym. 84 (1) (2011) 40
53. [13] N. Peppas, Y. Huang, M. Torres-Lugo, J. Ward, J. Zhang, Physicochemical foundations and structural design of hydrogels in medicine and biology, Annu. Rev. Biomed. Eng. 2 (1) (2000) 9
29. [14] J.D. Ferry, Viscoelastic Properties of Polymers, John Wiley & Sons, 1980. [15] K. Draget, G. Philips, P. Williams, Handbook of Hydrocolloids, Norwegian University of Science and Technology, 2000, pp. 379
395. [16] J.M. Rosiak, F. Yoshii, Hydrogels and their medical applications, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 151 (1-4) (1999) 56
64. [17] H. Omidian, K. Park, in: J. Siepmann, R. Siegel, M. Rathbone (Eds.), Fundamentals and Applications of Controlled Release Drug Delivery, Spinger, New York, 2012, pp. 75
106. [18] K. Liu, L. Jiang, Multifunctional integration: from biological to bio-inspired materials, Acs Nano 5 (9) (2011) 6786
6790.

Self-Healing Polymer-Based Systems

414

13. Self-healing hydrogels

[19] Z. Wei, J.H. Yang, J. Zhou, F. Xu, M. Zrı´nyi, P.H. Dussault, et al., Self-healing gels based on constitutional dynamic chemistry and their potential applications, Chem. Soc. Rev. 43 (23) (2014) 8114
8131. [20] D.L. Taylor, M.I.H. Panhuis, Self-healing hydrogels, Adv. Mater. 28 (41) (2016) 9060
9093. [21] S.R. White, N. Sottos, P. Geubelle, J. Moore, M.R. Kessler, S. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794. [22] A.S. Gladman, A.D.N. Celestine, N.R. Sottos, S.R. White, Autonomic healing of acrylic bone cement, Adv. Healthc. Mater. 4 (2) (2015) 202
207. [23] M. Burnworth, L. Tang, J.R. Kumpfer, A.J. Duncan, F.L. Beyer, G.L. Fiore, et al., Optically healable supramolecular polymers, Nature 472 (7343) (2011) 334. [24] M.V. Biyani, E.J. Foster, C. Weder, Light-healable supramolecular nanocomposites based on modified cellulose nanocrystals, ACS Macro Lett. 2 (3) (2013) 236
240. [25] M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, et al., Self-healing supramolecular gels formed by crown ether based host
guest interactions, Angew. Chem. 124 (28) (2012) 7117
7121. [26] A.P. Bapat, J.G. Ray, D.A. Savin, B.S. Sumerlin, Redox-responsive dynamiC
Covalent assemblies: stars and miktoarm stars, Macromolecules 46 (6) (2013) 2188
2198. [27] H. Otsuka, S. Nagano, Y. Kobashi, T. Maeda, A. Takahara, A dynamic covalent polymer driven by disulfide metathesis under photoirradiation, Chem. Commun. 46 (7) (2010) 1150
1152. [28] R.J. Sarma, S. Otto, J.R. Nitschke, Disulfides, imines, and metal coordination within a single system: interplay between three dynamic equilibria, Chemistry A Eur. J. 13 (34) (2007) 9542
9546. [29] J. Canadell, H. Goossens, B. Klumperman, Self-healing materials based on disulfide links, Macromolecules 44 (8) (2011) 2536
2541. [30] Z.Q. Lei, H.P. Xiang, Y.J. Yuan, M.Z. Rong, M.Q. Zhang, Room-temperature self-healable and remoldable cross-linked polymer based on the dynamic exchange of disulfide bonds, Chem. Mater. 26 (6) (2014) 2038
2046. [31] M. Pepels, I. Filot, B. Klumperman, H. Goossens, Self-healing systems based on disulfide
thiol exchange reactions, Polym. Chem. 4 (18) (2013) 4955
4965. [32] J.A. Yoon, J. Kamada, K. Koynov, J. Mohin, R. Nicolay¨, Y. Zhang, et al., Self-healing polymer films based on thiol
disulfide exchange reactions and self-healing kinetics measured using atomic force microscopy, Macromolecules 45 (1) (2011) 142
149. [33] F. Zhou, Z. Guo, W. Wang, X. Lei, B. Zhang, H. Zhang, et al., Preparation of self-healing, recyclable epoxy resins and low-electrical resistance composites based on double-disulfide bond exchange, Compos. Sci. Technol. 167 (2018) 79
85. [34] Y. Amamoto, H. Otsuka, A. Takahara, K. Matyjaszewski, Self-healing of covalently cross-linked polymers by reshuffling thiuram disulfide moieties in air under visible light, Adv. Mater. 24 (29) (2012) 3975
3980. [35] C.E. Yuan, M.Z. Rong, M.Q. Zhang, Z.P. Zhang, Y.C. Yuan, Self-healing of polymers via synchronous covalent bond fission/radical recombination, Chem. Mater. 23 (22) (2011) 5076
5081. [36] M.K. Beyer, The mechanical strength of a covalent bond calculated by density functional theory, J. Chem. Phys. 112 (17) (2000) 7307
7312. [37] B. Ghosh, M.W. Urban, Self-repairing oxetane-substituted chitosan polyurethane networks, Science 323 (5920) (2009) 1458
1460. [38] J. Lehn, Cryptates: inclusion complexes of macropolycyclic receptor molecules, Pure Appl. Chem. 50 (9-10) (1978) 871
892. [39] T.S. Moore, T.F. Winmill, CLXXVII.—The state of amines in aqueous solution, J. Chem. Society, Trans. 101 (1912) 1635
1676. [40] T.J. Murray, S.C. Zimmerman, New triply hydrogen bonded complexes with highly variable stabilities, J. Am. Chem. Soc. 114 (10) (1992) 4010
4011. [41] S. Burattini, B.W. Greenland, D.H. Merino, W. Weng, J. Seppala, H.M. Colquhoun, et al., A healable supramolecular polymer blend based on aromatic π2π stacking and hydrogen-bonding interactions, J. Am. Chem. Soc. 132 (34) (2010) 12051
12058. [42] M.G. Hill, J.A. Bailey, V.M. Miskowski, H.B. Gray, Spectroelectrochemistry and dimerization equilibria of chloro (terpyridine) platinum (II). Nature of the reduced complexes, Inorg. Chem. 35 (16) (1996) 4585
4590. [43] A.J. Goshe, I.M. Steele, C. Ceccarelli, A.L. Rheingold, B. Bosnich, Supramolecular recognition: on the kinetic lability of thermodynamically stable host
guest association complexes, Proc. Natl Acad. Sci. 99 (8) (2002) 4823
4829.

Self-Healing Polymer-Based Systems

References

415

[44] G.A. Lawrance, Introduction to Coordination Chemistry, John Wiley & Sons, 2013. [45] J. Brassinne, C.-A. Fustin, J.-F. Gohy, Polymer gels constructed through metal
ligand coordination, J. Inorg. Organomet. Polym. Mater. 23 (1) (2013) 24
40. [46] X. Wang, R. McHale, Metal-containing polymers: building blocks for functional (Nano) materials, Macromol. Rapid Commun. 31 (4) (2010) 331
350. [47] R. Dobrawa, M. Lysetska, P. Ballester, M. Gru¨ne, F. Wu¨rthner, Fluorescent supramolecular polymers: metal directed self-assembly of perylene bisimide building blocks, Macromolecules 38 (4) (2005) 1315
1325. [48] S. Schmatloch, A.M. Van den Berg, A.S. Alexeev, H. Hofmeier, U.S. Schubert, Soluble high-molecular-mass poly (ethylene oxide) s via self-organization, Macromolecules 36 (26) (2003) 9943
9949. [49] I. Hussain, S.M. Sayed, S. Liu, F. Yao, O. Oderinde, G. Fu, Hydroxyethyl cellulose-based self-healing hydrogels with enhanced mechanical properties via metal-ligand bond interactions, Eur. Polym. J. 100 (2018) 219
227. [50] P. Calvert, Hydrogels for soft machines, Adv. Mater. 21 (7) (2009) 743
756. [51] Y. Yang, M.W. Urban, Self-healing of polymers via supramolecular chemistry, Adv. Mater. Interfaces (2018) 1800384. [52] X. Wang, F. Liu, X. Zheng, J. Sun, Water-enabled self-healing of polyelectrolyte multilayer coatings, Angew. Chem. 123 (48) (2011) 11580
11583. [53] Z. Wang, E. Van Andel, S.P. Pujari, H. Feng, J.A. Dijksman, M.M. Smulders, et al., Water-repairable zwitterionic polymer coatings for anti-biofouling surfaces, J. Mater. Chem. B 5 (33) (2017) 6728
6733. [54] Y. Liu, S.-h Hsu, Synthesis and biomedical applications of self-healing hydrogels, Front. Chem. 6 (2018). [55] A. Harada, R. Kobayashi, Y. Takashima, A. Hashidzume, H. Yamaguchi, Macroscopic self-assembly through molecular recognition, Nat. Chem. 3 (1) (2011) 34. [56] Z. Wei, J. He, T. Liang, H. Oh, J. Athas, Z. Tong, et al., Autonomous self-healing of poly (acrylic acid) hydrogels induced by the migration of ferric ions, Polym. Chem. 4 (17) (2013) 4601
4605. [57] N. Huebsch, C.J. Kearney, X. Zhao, J. Kim, C.A. Cezar, Z. Suo, et al., Ultrasound-triggered disruption and self-healing of reversibly cross-linked hydrogels for drug delivery and enhanced chemotherapy, Proc. Natl. Acad. Sci. 111 (27) (2014) 9762
9767. [58] H. Bai, C. Li, X. Wang, G. Shi, A pH-sensitive graphene oxide composite hydrogel, Chem. Commun. 46 (14) (2010) 2376
2378. [59] P. Schexnailder, G. Schmidt, Nanocomposite polymer hydrogels, Colloid Polym. Sci. 287 (1) (2009) 1
11. [60] T. Ogoshi, Y. Takashima, H. Yamaguchi, A. Harada, Chemically-responsive sol 2 gel transition of supramolecular single-walled carbon nanotubes (SWNTs) hydrogel made by hybrids of SWNTs and cyclodextrins, J. Am. Chem. Soc. 129 (16) (2007) 4878
4879. [61] Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS nano 4 (7) (2010) 4324
4330. [62] C. Li, G. Shi, Functional gels based on chemically modified graphenes, Adv. Mater. 26 (24) (2014) 3992
4012. [63] D. Yang, S. Peng, M.R. Hartman, T. Gupton-Campolongo, E.J. Rice, A.K. Chang, et al., Enhanced transcription and translation in clay hydrogel and implications for early life evolution, Sci. Rep. 3 (2013) 3165. [64] G. Li, J. Wu, B. Wang, S. Yan, K. Zhang, J. Ding, et al., Self-healing supramolecular self-assembled hydrogels based on poly (L-glutamic acid), Biomacromolecules 16 (11) (2015) 3508
3518. [65] H. Zhang, H. Xia, Y. Zhao, Poly (vinyl alcohol) hydrogel can autonomously self-heal, ACS Macro Lett. 1 (11) (2012) 1233
1236. [66] Z. Li, G. Davidson-Rozenfeld, M. Va´zquez-Gonza´lez, M. Fadeev, J. Zhang, H. Tian, et al., Multi-triggered supramolecular DNA/bipyridinium dithienylethene hydrogels driven by light, redox, and chemical stimuli for shape-memory and self-healing applications, J. Am. Chem. Soc. 140 (50) (2018) 17691
17701. [67] B. Zhou, C. Zuo, Z. Xiao, X. Zhou, D. He, X. Xie, et al., Self-healing polymer electrolytes formed via dual-networks:a new strategy for flexible lithium metal batteries, Chemistry 24 (72) (2018) 19200
19207. [68] K. Haraguchi, M. Ebato, T. Takehisa, Polymer
clay nanocomposites exhibiting abnormal necking phenomena accompanied by extremely large reversible elongations and excellent transparency, Adv. Mater. 18 (17) (2006) 2250
2254. [69] Y. Xu, Q. Wu, Y. Sun, H. Bai, G. Shi, Three-dimensional self-assembly of graphene oxide and DNA into multifunctional hydrogels, ACS Nano 4 (12) (2010) 7358
7362. [70] M. Zhong, X.-Y. Liu, F.-K. Shi, L.-Q. Zhang, X.-P. Wang, A.G. Cheetham, et al., Self-healable, tough and highly stretchable ionic nanocomposite physical hydrogels, Soft Matter 11 (21) (2015) 4235
4241.

Self-Healing Polymer-Based Systems

416

13. Self-healing hydrogels

[71] C. Shao, M. Wang, L. Meng, H. Chang, B. Wang, F. Xu, et al., Mussel-inspired cellulose nanocomposite tough hydrogels with synergistic self-healing, adhesive, and strain-sensitive properties, Chem. Mater. 30 (9) (2018) 3110
3121. [72] K. Liu, X. Pan, L. Chen, L. Huang, Y. Ni, J. Liu, et al., Ultrasoft self-healing nanoparticle-hydrogel composites with conductive and magnetic properties, JACS Sustain. Chem. 6 (5) (2018) 6395
6403. [73] Y. Fang, C.-F. Wang, Z.-H. Zhang, H. Shao, S. Chen, Robust self-healing hydrogels assisted by cross-linked nanofiber networks, Sci. Rep. 3 (2013) 2811. [74] Z. Xu, J. Peng, N. Yan, H. Yu, S. Zhang, K. Liu, et al., Simple design but marvelous performances: molecular gels of superior strength and self-healing properties, Soft Matter 9 (4) (2013) 1091
1099. [75] K. Haraguchi, K. Uyama, H. Tanimoto, Self-healing in nanocomposite hydrogels, Macromol. Rapid Commun. 32 (16) (2011) 1253
1258. [76] X. Xing, L. Li, T. Wang, Y. Ding, G. Liu, G. Zhang, A self-healing polymeric material: from gel to plastic, J. Mater. Chem. A 2 (29) (2014) 11049
11053. [77] A. Phadke, C. Zhang, B. Arman, C.-C. Hsu, R.A. Mashelkar, A.K. Lele, et al., Rapid self-healing hydrogels, Proc. Natl Acad. Sci. 109 (12) (2012) 4383
4388. [78] S. Basak, J. Nanda, A. Banerjee, Multi-stimuli responsive self-healing metallo-hydrogels: tuning of the gel recovery property, Chem. Commun. 50 (18) (2014) 2356
2359. [79] J. Liu, G. Song, C. He, H. Wang, Self-healing in tough graphene oxide composite hydrogels, Macromol. Rapid Commun. 34 (12) (2013) 1002
1007. [80] M.H. Suhre, M. Gertz, C. Steegborn, T. Scheibel, Structural and functional features of a collagen-binding matrix protein from the mussel byssus, Nat. Commun. 5 (2014) 3392. [81] M.A. Meyers, J. McKittrick, P.-Y. Chen, Structural biological materials: critical mechanics-materials connections, Science 339 (6121) (2013) 773
779. [82] G. Mayer, Rigid biological systems as models for synthetic composites, Science 310 (5751) (2005) 1144
1147. [83] X. Zhou, B. Guo, L. Zhang, G.-H. Hu, Progress in bio-inspired sacrificial bonds in artificial polymeric materials, Chem. Soc. Rev. 46 (20) (2017) 6301
6329. [84] J.B. Thompson, J.H. Kindt, B. Drake, H.G. Hansma, D.E. Morse, P.K. Hansma, Bone indentation recovery time correlates with bond reforming time, Nature 414 (6865) (2001) 773. [85] J. Currey, Biomaterials: sacrificial bonds heal bone, Nature 414 (6865) (2001) 699. [86] G.E. Fantner, T. Hassenkam, J.H. Kindt, J.C. Weaver, H. Birkedal, L. Pechenik, et al., Sacrificial bonds and hidden length dissipate energy as mineralized fibrils separate during bone fracture, Nat. Mater. 4 (8) (2005) 612. [87] T. Gutsmann, T. Hassenkam, J.A. Cutroni, P.K. Hansma, Sacrificial bonds in polymer brushes from rat tail tendon functioning as nanoscale velcro, Biophys. J. 89 (1) (2005) 536
542. [88] N. Becker, E. Oroudjev, S. Mutz, J.P. Cleveland, P.K. Hansma, C.Y. Hayashi, et al., Molecular nanosprings in spider capture-silk threads, Nat. Mater. 2 (4) (2003) 278. [89] E. Thormann, H. Mizuno, K. Jansson, N. Hedin, M.S. Ferna´ndez, J.L. Arias, et al., Embedded proteins and sacrificial bonds provide the strong adhesive properties of gastroliths, Nanoscale 4 (13) (2012) 3910
3916. [90] M.J. Harrington, H.S. Gupta, P. Fratzl, J.H. Waite, Collagen insulated from tensile damage by domains that unfold reversibly: in situ X-ray investigation of mechanical yield and damage repair in the mussel byssus, J. Struct. Biol. 167 (1) (2009) 47
54. [91] E. Oroudjev, J. Soares, S. Arcidiacono, J. Thompson, S. Fossey, H. Hansma, Segmented nanofibers of spider dragline silk: atomic force microscopy and single-molecule force spectroscopy, Proc. Natl. Acad. Sci. 99 (suppl 2) (2002) 6460
6465. [92] A. Reinecke, L. Bertinetti, P. Fratzl, M.J. Harrington, Cooperative behavior of a sacrificial bond network and elastic framework in providing self-healing capacity in mussel byssal threads, J. Struct. Biol. 196 (3) (2016) 329
339. [93] C.K. Lieou, A.E. Elbanna, J.M. Carlson, Sacrificial bonds and hidden length in biomaterials: a kinetic constitutive description of strength and toughness in bone, Phys. Rev. E 88 (1) (2013) 012703. [94] A.E. Elbanna, J.M. Carlson, Dynamics of polymer molecules with sacrificial bond and hidden length systems: towards a physically-based mesoscopic constitutive law, PLoS One 8 (4) (2013) e56118. [95] J.P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada, Double-network hydrogels with extremely high mechanical strength, Adv. Mater. 15 (14) (2003) 1155
1158. [96] J.P. Gong, Materials both tough and soft, Science 344 (6180) (2014) 161
162.

Self-Healing Polymer-Based Systems

References

417

[97] Q. Chen, L. Zhu, C. Zhao, Q. Wang, J. Zheng, A robust, one-pot synthesis of highly mechanical and recoverable double network hydrogels using thermoreversible sol-gel polysaccharide, Adv. Mater. 25 (30) (2013) 4171
4176. [98] H.R. Brown, A model of the fracture of double network gels, Macromolecules 40 (10) (2007) 3815
3818. [99] F. Luo, T.L. Sun, T. Nakajima, T. Kurokawa, Y. Zhao, K. Sato, et al., Oppositely charged polyelectrolytes form tough, self-healing, and rebuildable hydrogels, Adv. Mater. 27 (17) (2015) 2722
2727. [100] F. Luo, T.L. Sun, T. Nakajima, D.R. King, T. Kurokawa, Y. Zhao, et al., Strong and tough polyion-complex hydrogels from oppositely charged polyelectrolytes: a comparative study with polyampholyte hydrogels, Macromolecules 49 (7) (2016) 2750
2760. [101] T.L. Sun, T. Kurokawa, S. Kuroda, A.B. Ihsan, T. Akasaki, K. Sato, et al., Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity, Nat. Mater. 12 (10) (2013) 932. [102] X. Zhao, N. Huebsch, D.J. Mooney, Z. Suo, Stress-relaxation behavior in gels with ionic and covalent crosslinks, J. Appl. Phys. 107 (6) (2010) 063509. [103] F. Peng, G. Li, X. Liu, S. Wu, Z. Tong, Redox-responsive gel 2 sol/sol 2 gel transition in poly (acrylic acid) aqueous solution containing fe (iii) ions switched by light, J. Am. Chem. Soc. 130 (48) (2008) 16166
16167. [104] P. Calvo-Marzal, M.P. Delaney, J.T. Auletta, T. Pan, N.M. Perri, L.M. Weiland, et al., Manipulating mechanical properties with electricity: electroplastic elastomer hydrogels, ACS Macro Lett. 1 (1) (2011) 204
208. [105] Z. Tang, N.A. Kotov, S. Magonov, B. Ozturk, Nanostructured artificial nacre, Nat. Mater. 2 (6) (2003) 413. [106] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (5330) (1997) 1232
1237. [107] F. Luo, T.L. Sun, T. Nakajima, T. Kurokawa, A.B. Ihsan, X. Li, et al., Free reprocessability of tough and selfhealing hydrogels based on polyion complex, ACS Macro Lett. 4 (9) (2015) 961
964. [108] E. Oroudjev, J. Soares, S. Arcdiacono, J. Thompson, S. Fossey, H. Hansma, Segmented nanofibers of spider dragline silk: atomic force microscopy and single-molecule force spectroscopy, Proc. Natl. Acad. Sci. U. S. A. 99 (14) (2002). [109] S.-Y. Sheu, D.-Y. Yang, H. Selzle, E. Schlag, Energetics of hydrogen bonds in peptides, Proc. Natl. Acad. Sci. 100 (22) (2003) 12683
12687. [110] N. Houbenov, J.S. Haataja, H. Iatrou, N. Hadjichristidis, J. Ruokolainen, C.F. Faul, et al., Self-assembled polymeric supramolecular frameworks, Angew. Chem. Int. Ed. 50 (11) (2011) 2516
2520. [111] R.P. Sijbesma, F.H. Beijer, L. Brunsveld, B.J. Folmer, J.K. Hirschberg, R.F. Lange, et al., Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding, Science 278 (5343) (1997) 1601
1604. [112] F.H. Beijer, R.P. Sijbesma, H. Kooijman, A.L. Spek, E. Meijer, Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding, J. Am. Chem. Soc. 120 (27) (1998) 6761
6769. [113] B.J. Folmer, R.P. Sijbesma, E. Meijer, Unexpected entropy-driven ring-opening polymerization in a reversible supramolecular system, J. Am. Chem. Soc. 123 (9) (2001) 2093
2094. [114] Y. Zhang, Y. Li, W. Liu, Dipole
dipole and H-bonding interactions significantly enhance the multifaceted mechanical properties of thermoresponsive shape memory hydrogels, Adv. Funct. Mater. 25 (3) (2015) 471
480. [115] X. Hu, M. Vatankhah-Varnoosfaderani, J. Zhou, Q. Li, S.S. Sheiko, Weak hydrogen bonding enables hard, strong, tough, and elastic hydrogels, Adv. Mater. 27 (43) (2015) 6899
6905. [116] E. Degtyar, M.J. Harrington, Y. Politi, P. Fratzl, The mechanical role of metal ions in biogenic protein-based materials, Angew. Chem. Int. Ed. 53 (45) (2014) 12026
12044. [117] S.E. Bakarich, G.C. Pidcock, P. Balding, L. Stevens, P. Calvert, Recovery from applied strain in interpenetrating polymer network hydrogels with ionic and covalent cross-links, Soft Matter 8 (39) (2012) 9985
9988. [118] J.-Y. Sun, X. Zhao, W.R. Illeperuma, O. Chaudhuri, K.H. Oh, D.J. Mooney, et al., Highly stretchable and tough hydrogels, Nature 489 (7414) (2012) 133. [119] X. Lu, C.Y. Chan, K.I. Lee, P.F. Ng, B. Fei, J.H. Xin, et al., Super-tough and thermo-healable hydrogel
promising for shape-memory absorbent fiber, J. Mater. Chem. B 2 (43) (2014) 7631
7638. [120] L.X. Dai, W. Zhang, L. Sun, X.H. Wang, W. Jiang, Z.W. Zhu, et al., Highly stretchable and compressible self-healing P(AA-co-AAm)/CoCl2 hydrogel electrolyte for flexible supercapacitors, ChemElectroChem 6 (2) (2019) 467
472. [121] G. Bao, S. Suresh, Cell and molecular mechanics of biological materials, Nat. Mater. 2 (11) (2003) 715.

Self-Healing Polymer-Based Systems

418

13. Self-healing hydrogels

[122] M. Haque, G. Kamita, T. Kurokawa, K. Tsujii, J.P. Gong, Unidirectional alignment of lamellar bilayer in hydrogel: one-dimensional swelling, anisotropic modulus, and stress/strain tunable structural color, Adv. Mater. 22 (45) (2010) 5110
5114. [123] M.A. Haque, T. Kurokawa, G. Kamita, J.P. Gong, Lamellar bilayers as reversible sacrificial bonds to toughen hydrogel: hysteresis, self-recovery, fatigue resistance, and crack blunting, Macromolecules 44 (22) (2011) 8916
8924. [124] Y. Geng, X.Y. Lin, P. Pan, G. Shan, Y. Bao, Y. Song, et al., Hydrophobic association mediated physical hydrogels with high strength and healing ability, Polymer 100 (2016) 60
68. [125] G. Jiang, C. Liu, X. Liu, G. Zhang, M. Yang, F. Liu, Construction and properties of hydrophobic association hydrogels with high mechanical strength and reforming capability, Macromol. Mater. Eng. 294 (12) (2009) 815
820. [126] J.R. McKee, E.A. Appel, J. Seitsonen, E. Kontturi, O.A. Scherman, O. Ikkala, Healable, stable and stiff hydrogels: combining conflicting properties using dynamic and selective three-component recognition with reinforcing cellulose nanorods, Adv. Funct. Mater. 24 (18) (2014) 2706
2713. [127] J. Liu, C.S.Y. Tan, Z. Yu, Y. Lan, C. Abell, O.A. Scherman, Biomimetic supramolecular polymer networks exhibiting both toughness and self-recovery, Adv. Mater. 29 (10) (2017). [128] H. Li, P. Yang, P. Pageni, C. Tang, Recent advances in metal-containing polymer hydrogels, Macromol. Rapid Commun. (2017). [129] A.S. Abd-El-Aziz, Organometallic polymers of the transition metals, Macromol. Rapid Commun. 23 (17) (2002) 995
1031. [130] Y. Yan, J. Zhang, L. Ren, C. Tang, Metal-containing and related polymers for biomedical applications, Chem. Soc. Rev. 45 (19) (2016) 5232
5263. [131] M.A. Hempenius, C. Cirmi, F.L. Savio, J. Song, G.J. Vancso, Poly (ferrocenylsilane) gels and hydrogels with redox-controlled actuation, Macromol. Rapid Commun. 31 (9-10) (2010) 772
783. [132] M.-O.M. Piepenbrock, G.O. Lloyd, N. Clarke, J.W. Steed, Metal-and anion-binding supramolecular gels, Chem. Rev. 110 (4) (2009) 1960
2004. [133] Q. Chen, H. Chen, L. Zhu, J. Zheng, Fundamentals of double network hydrogels, J. Mater. Chem. B 3 (18) (2015) 3654
3676. [134] H. Yuk, T. Zhang, S. Lin, G.A. Parada, X. Zhao, Tough bonding of hydrogels to diverse non-porous surfaces, Nat. Mater. 15 (2) (2016) 190. [135] Y. Yang, X. Ding, M.W. Urban, Chemical and physical aspects of self-healing materials, Prog. Polym. Sci. 49 (2015) 34
59. [136] G.E. Giammanco, A.D. Ostrowski, Photopatterning the mechanical properties of polysaccharide-containing gels using Fe31 coordination, Chem. Mater. 27 (14) (2015) 4922
4925. [137] H. Dong, J.F. Snyder, K.S. Williams, J.W. Andzelm, Cation-induced hydrogels of cellulose nanofibrils with tunable moduli, Biomacromolecules 14 (9) (2013) 3338
3345. [138] G.R. Whittell, M.D. Hager, U.S. Schubert, I. Manners, Functional soft materials from metallopolymers and metallosupramolecular polymers, Nat. Mater. 10 (3) (2011) 176. [139] L. Ren, C.G. Hardy, C. Tang, Synthesis and solution self-assembly of side-chain cobaltocenium-containing block copolymers, J. Am. Chem. Soc. 132 (26) (2010) 8874
8875. [140] J. Zhang, J. Yan, P. Pageni, Y. Yan, A. Wirth, Y.-P. Chen, et al., Anion-responsive metallopolymer hydrogels for healthcare applications, Sci. Rep. 5 (2015) 11914. [141] N. Sahiner, Soft and flexible hydrogel templates of different sizes and various functionalities for metal nanoparticle preparation and their use in catalysis, Prog. Polym. Sci. 38 (9) (2013) 1329
1356. [142] M. Zhong, Y.-T. Liu, X.-Y. Liu, F.-K. Shi, L.-Q. Zhang, M.-F. Zhu, et al., Dually cross-linked single network poly (acrylic acid) hydrogels with superior mechanical properties and water absorbency, Soft Matter 12 (24) (2016) 5420
5428. [143] R.P. Wool, Self-healing materials: a review, Soft Matter 4 (3) (2008) 400
418. [144] Z. Shi, X. Gao, M.W. Ullah, S. Li, Q. Wang, G. Yang, Electroconductive natural polymer-based hydrogels, Biomaterials 111 (2016) 40
54. [145] F. Song, L.-M. Zhang, J.-F. Shi, N.-N. Li, Novel casein hydrogels: formation, structure and controlled drug release, Colloids Surf. B Biointerfaces 79 (1) (2010) 142
148. [146] N. Annabi, A. Tamayol, J.A. Uquillas, M. Akbari, L.E. Bertassoni, C. Cha, et al., 25th anniversary article: rational design and applications of hydrogels in regenerative medicine, Adv. Mater. 26 (1) (2014) 85
124.

Self-Healing Polymer-Based Systems

References

419

[147] A.D. Augst, H.J. Kong, D.J. Mooney, Alginate hydrogels as biomaterials, Macromol. Biosci. 6 (8) (2006) 623
633. [148] Z. Wei, J.H. Yang, Z.Q. Liu, F. Xu, J.X. Zhou, M. Zrı´nyi, et al., Novel biocompatible polysaccharide-based self-healing hydrogel, Adv. Funct. Mater. 25 (9) (2015) 1352
1359. [149] S. Liu, M. Kang, K. Li, F. Yao, O. Oderinde, G. Fu, et al., Polysaccharide-templated preparation of mechanically-tough, conductive and self-healing hydrogels, Chem. Eng. J. 334 (2018) 2222
2230. [150] S. Yan, W. Wang, X. Li, J. Ren, W. Yun, K. Zhang, et al., Preparation of mussel-inspired injectable hydrogels based on dual-functionalized alginate with improved adhesive, self-healing, and mechanical properties, J. Mater. Chem. B 6 (40) (2018) 6377
6390. [151] M.H. Shin, D.Y. Lee, G. Wohlgemuth, I.-G. Choi, O. Fiehn, K.H. Kim, Global metabolite profiling of agarose degradation by Saccharophagus degradans 2-40, Nat. Biotechnol. 27 (2) (2010) 156
168. [152] S. Wang, R. Zhang, Y. Yang, S. Wu, Y. Cao, A. Lu, et al., Strength enhanced hydrogels constructed from agarose in alkali/urea aqueous solution and their application, Chem. Eng. J. 331 (2018) 177
184. [153] F. Campos, A.B. Bonhome-Espinosa, G. Vizcaino, I.A. Rodriguez, D. Duran-Herrera, M.T. Lo´pez-Lo´pez, et al., Generation of genipin cross-linked fibrin-agarose hydrogel tissue-like models for tissue engineering applications, Biomed. Mater. 13 (2) (2018) 025021. [154] P. Zarrintaj, S. Manouchehri, Z. Ahmadi, M.R. Saeb, A.M. Urbanska, D.L. Kaplan, et al., Agarose-based biomaterials for tissue engineering, Carbohydr. Polym. 187 (2018) 66
84. [155] M. Sabzi, N. Samadi, F. Abbasi, G.R. Mahdavinia, M. Babaahmadi, Bioinspired fully physically cross-linked double network hydrogels with a robust, tough and self-healing structure, Mater. Sci. Eng. C. 74 (2017) 374
381. [156] N. Samadi, M. Sabzi, M. Babaahmadi, Self-healing and tough hydrogels with physically cross-linked triple networks based on Agar/PVA/Graphene, Int. J. Biol. Macromolecules 107 (2018) 2291
2297. [157] Y. Ko, J. Kim, H.Y. Jeong, G. Kwon, D. Kim, M. Ku, et al., Antibacterial poly (3, 4-ethylenedioxythiophene): poly (styrene-sulfonate)/agarose nanocomposite hydrogels with thermo-processability and self-healing, Carbohydr. Polym. 203 (2019) 26
34. [158] X.-Q. Wei, G.F. Payne, X.-W. Shi, Y. Du, Electrodeposition of a biopolymeric hydrogel in track-etched micropores, Soft Matter 9 (7) (2013) 2131
2135. [159] X.-W. Shi, L. Qiu, Z. Nie, L. Xiao, G.F. Payne, Y. Du, Protein addressing on patterned microchip by coupling chitosan electrodeposition and ‘electro-click’ chemistry, Biofabrication 5 (4) (2013) 041001. [160] Y. Liu, K. Yan, G. Jiang, Y. Xiong, Y. Du, X. Shi, Electrical signal guided ibuprofen release from electrodeposited chitosan hydrogel, Int. J. Polym. Sci. 2014 (2014). [161] E. Kim, T. Gordonov, W.E. Bentley, G.F. Payne, Amplified and in situ detection of redox-active metabolite using a biobased redox capacitor, Anal. Chem. 85 (4) (2013) 2102
2108. [162] X.-L. Luo, J.-J. Xu, Q. Zhang, G.-J. Yang, H.-Y. Chen, Electrochemically deposited chitosan hydrogel for horseradish peroxidase immobilization through gold nanoparticles self-assembly, Biosens. Bioelectron. 21 (1) (2005) 190
196. [163] M.J. Moura, H. Faneca, M.P. Lima, M.H. Gil, M.M. Figueiredo, In situ forming chitosan hydrogels prepared via ionic/covalent co-cross-linking, Biomacromolecules 12 (9) (2011) 3275
3284. [164] N. Bhattarai, J. Gunn, M. Zhang, Chitosan-based hydrogels for controlled, localized drug delivery, Adv. Drug Deliv. Rev. 62 (1) (2010) 83
99. [165] Y. Zhang, L. Tao, S. Li, Y. Wei, Synthesis of multiresponsive and dynamic chitosan-based hydrogels for controlled release of bioactive molecules, Biomacromolecules 12 (8) (2011) 2894
2901. [166] X. Jing, H.-Y. Mi, B.N. Napiwocki, X.-F. Peng, L.-S. Turng, Mussel-inspired electroactive chitosan/graphene oxide composite hydrogel with rapid self-healing and recovery behavior for tissue engineering, Carbon 125 (2017) 557
570. [167] J. Qu, X. Zhao, Y. Liang, T. Zhang, P.X. Ma, B. Guo, Antibacterial adhesive injectable hydrogels with rapid self-healing, extensibility and compressibility as wound dressing for joints skin wound healing, Biomaterials 183 (2018) 185
199. [168] Y. Liu, M. Su, Y. Fu, P. Zhao, M. Xia, Y. Zhang, et al., Corrosive environments tolerant, ductile and selfhealing hydrogel for highly efficient oil/water separation, Chem. Eng. J. 354 (2018) 1185
1196. [169] Y. Habibi, L.A. Lucia, O.J. Rojas, Cellulose nanocrystals: chemistry, self-assembly, and applications, Chem. Rev. 110 (6) (2010) 3479
3500.

Self-Healing Polymer-Based Systems

420

13. Self-healing hydrogels

[170] F. Ding, H. Deng, Y. Du, X. Shi, Q. Wang, Emerging chitin and chitosan nanofibrous materials for biomedical applications, Nanoscale 6 (16) (2014) 9477
9493. [171] K. Ku¨mmerer, J. Menz, T. Schubert, W. Thielemans, Biodegradability of organic nanoparticles in the aqueous environment, Chemosphere 82 (10) (2011) 1387
1392. ˘ S. Sarmad, Cellulose graft copolymers: synthesis, properties, and applications, Polysaccharide [172] G. Gu¨rdag, Based Graft Copolymers, Springer, 2013, pp. 15
57. [173] L.-H. Fu, C. Qi, M.-G. Ma, P. Wan, Multifunctional cellulose-based hydrogels for biomedical applications, J. Mater. Chem. B (2019). [174] C. Shao, H. Chang, M. Wang, F. Xu, J. Yang, High-strength, tough, and self-healing nanocomposite physical hydrogels based on the synergistic effects of dynamic hydrogen bond and dual coordination bonds, ACS Appl. Mater. Interfaces 9 (34) (2017) 28305
28318. [175] X. Yang, G. Liu, L. Peng, J. Guo, L. Tao, J. Yuan, et al., Highly efficient self-healable and dual responsive cellulose-based hydrogels for controlled release and 3D cell culture, Adv. Funct. Mater. 27 (40) (2017) 1703174. [176] F. Fayazpour, B. Lucas, C. Alvarez-Lorenzo, N.N. Sanders, J. Demeester, S.C. De Smedt, Physicochemical and transfection properties of cationic hydroxyethylcellulose/DNA nanoparticles, Biomacromolecules 7 (10) (2006) 2856
2862. [177] M. Matsuo, K. Arimori, C. Nakamura, M. Nakano, Delayed-release tablets using hydroxyethylcellulose as a gel-forming matrix, Int. J. Pharmaceutics 138 (2) (1996) 225
235. [178] R.M. Domingues, M.E. Gomes, R.L. Reis, The potential of cellulose nanocrystals in tissue engineering strategies, Biomacromolecules 15 (7) (2014) 2327
2346. [179] A.G. Dal-Bo´, R. Laus, A.C. Felippe, D. Zanette, E. Minatti, Association of anionic surfactant mixed micelles with hydrophobically modified ethyl (hydroxyethyl) cellulose, Colloids Surf. A Physicochem. Eng. Asp. 380 (1-3) (2011) 100
106. [180] S. Gorgieva, V. Kokol, Synthesis and application of new temperature-responsive hydrogels based on carboxymethyl and hydroxyethyl cellulose derivatives for the functional finishing of cotton knitwear, Carbohydr. Polym. 85 (3) (2011) 664
673. [181] L. Patural, P. Marchal, A. Govin, P. Grosseau, B. Ruot, O. Deves, Cellulose ethers influence on water retention and consistency in cement-based mortars, Cem. Concr. Res. 41 (1) (2011) 46
55. [182] C. Kugge, V.S. Craig, J. Daicic, A scanning electron microscope study of the surface structure of mineral pigments, latices and thickeners used for paper coating on non-absorbent substrates, Colloids Surf. A Physicochem. Eng. Asp. 238 (1-3) (2004) 1
11. [183] I. Hussain, S.M. Sayed, S. Liu, O. Oderinde, M. Kang, F. Yao, et al., Enhancing the mechanical properties and self-healing efficiency of hydroxyethyl cellulose-based conductive hydrogels via supramolecular interactions, Eur. Polym. J. 105 (2018) 85
94. [184] I.S. Chronakis, On the molecular characteristics, compositional properties, and structural-functional mechanisms of maltodextrins: a review, Crit. Rev. Food Sci. Nutr. 38 (7) (1998) 599
637. [185] D.R. White Jr, P. Hudson, J.T. Adamson, Dextrin characterization by high-performance anion-exchange chromatography
pulsed amperometric detection and size-exclusion chromatography
multi-angle light scattering
refractive index detection, J. Chromatogr. A 997 (1-2) (2003) 79
85. [186] K. Alvani, X. Qi, R.F. Tester, Use of carbohydrates, including dextrins, for oral delivery, Starch-Sta¨rke 63 (7) (2011) 424
431. [187] D. Das, R. Das, J. Mandal, A. Ghosh, S. Pal, Dextrin crosslinked with poly (lactic acid): a novel hydrogel for controlled drug release application, J. Appl. Polym. Sci. 131 (7) (2014). [188] S. Li, W.C. Purdy, Cyclodextrins and their applications in analytical chemistry, Chem. Rev. 92 (6) (1992) 1457
1470. [189] Y. Kang, Y. Ma, S. Zhang, L.-S. Ding, B.-J. Li, Dual-stimuli-responsive nanoassemblies as tunable releasing carriers, ACS Macro Lett. 4 (5) (2015) 543
547. [190] K. Miyake, S. Yasuda, A. Harada, J. Sumaoka, M. Komiyama, H. Shigekawa, Formation process of cyclodextrin necklace 2 analysis of hydrogen bonding on a molecular level, J. Am. Chem. Soc. 125 (17) (2003) 5080
5085. [191] Y. Kang, K. Guo, B.-J. Li, S. Zhang, Nanoassemblies driven by cyclodextrin-based inclusion complexation, Chem. Commun. 50 (76) (2014) 11083
11092.

Self-Healing Polymer-Based Systems

References

421

[192] Y. Zhu, S. Liu, X. Shi, D. Han, F. Liang, A thermally responsive host
guest conductive hydrogel with selfhealing properties, Mater. Chem. Front. 2 (12) (2018) 2212
2219. [193] Z. Wang, Y. Ren, Y. Zhu, L. Hao, Y. Chen, G. An, et al., Rapidly self-healing host-guest supramolecular hydrogel with high mechanical strength and excellent biocompatibility, Angew. Chem. Int. Ed. (2018). [194] N.M. Tsegay, X.Y. Du, S.S. Liu, C.F. Wang, S. Chen, Frontal polymerization for smart intrinsic self-healing hydrogels and its integration with microfluidics, J. Polym. Sci. Part. A Polym. Chem. 56 (13) (2018) 1412
1423. [195] H. Chen, X. Ma, S. Wu, H. Tian, A rapidly self-healing supramolecular polymer hydrogel with photostimulated room-temperature phosphorescence responsiveness, Angew. Chem. 126 (51) (2014) 14373
14376. [196] L. Dai, B. Nadeau, X. An, D. Cheng, Z. Long, Y. Ni, Silver nanoparticles-containing dual-function hydrogels based on a guar gum-sodium borohydride system, Sci. Rep. 6 (2016) 36497. [197] X. Lu, Y. Li, W. Feng, S. Guan, P. Guo, Self-healing hydroxypropyl guar gum/poly (acrylamide-co-3-acrylamidophenyl boronic acid) composite hydrogels with yield phenomenon based on dynamic PBA ester bonds and H-bond, Colloids Surf. A Physicochem. Eng. Asp. 561 (2019) 325
331. [198] L. Dai, L. Zhang, B. Wang, B. Yang, I. Khan, A. Khan, et al., Multifunctional self-assembling hydrogel from guar gum, Chem. Eng. J. 330 (2017) 1044
1051. [199] N. Li, C. Liu, W. Chen, Facile access to guar gum based supramolecular hydrogels with rapid self-healing ability and multi-stimuli responsive gel-sol transitions, J. Agric. Food Chem. 67 (2019). [200] X. Pan, Q. Wang, D. Ning, L. Dai, K. Liu, Y. Ni, et al., Ultraflexible self-healing guar gum-glycerol hydrogel with injectable, antifreeze, and strain-sensitive properties, ACS Biomater. Sci. Eng. 4 (9) (2018) 3397
3404. [201] K.B. Djagny, Z. Wang, S. Xu, Gelatin: a valuable protein for food and pharmaceutical industries, Crit. Rev. Food Sci. Nutr. 41 (6) (2001) 481
492. [202] Z. Wei, S. Gerecht, A self-healing hydrogel as an injectable instructive carrier for cellular morphogenesis, Biomaterials 185 (2018) 86
96. [203] G. Zhang, L. Lv, Y. Deng, C. Wang, Self-healing gelatin hydrogels cross-linked by combining multiple hydrogen bonding and ionic coordination, Macromol. Rapid Commun. 38 (12) (2017) 1700018. [204] Y. Zheng, Y. Liang, D. Zhang, X. Sun, L. Liang, J. Li, et al., Gelatin-based hydrogels blended with gellan as an injectable wound dressing, ACS Omega 3 (5) (2018) 4766
4775. [205] M. Vahedi, J. Barzin, F. Shokrolahi, P. Shokrollahi, Self-healing, injectable gelatin hydrogels cross-linked by dynamic schiff base linkages support cell adhesion and sustained release of antibacterial drugs, Macromol. Mater. Eng. 303 (9) (2018) 1800200. [206] F.G. Young, Claude Bernard and the discovery of glycogen, Br. Med. J. 1 (5033) (1957) 1431. [207] P.C. Calder, Glycogen structure and biogenesis, Int. J. Biochem. 23 (12) (1991) 1335
1352. [208] D.J. Manners, Recent developments in our understanding of glycogen structure, Carbohydr. Polym. 16 (1) (1991) 37
82. [209] M. Vetrik, M. Pradny, L. Kobera, M. Slouf, M. Rabyk, A. Pospisilova, et al., Biopolymer-based degradable nanofibres from renewable resources produced by freeze-drying, RSC Adv. 3 (35) (2013) 15282
15289. [210] I. Hussain, S.M. Sayed, S. Liu, O. Oderinde, F. Yao, G. Fu, Glycogen-based self-healing hydrogels with ultra-stretchable, flexible, and enhanced mechanical properties via sacrificial bond interactions, Int. J. Biol. Macromol. 117 (2018) 648
658. [211] I. Hussain, S.M. Sayed, G. Fu, Facile and cost-effective synthesis of glycogen-based conductive hydrogels with extremely flexible, excellent self-healing and tunable mechanical properties, Int. J. Biol. Macromol. 118 (2018) 1463
1469. [212] J.A. Burdick, G.D. Prestwich, Hyaluronic acid hydrogels for biomedical applications, Adv. Mater. 23 (12) (2011). [213] H. Zhang, F. Zhang, J. Wu, Physically crosslinked hydrogels from polysaccharides prepared by freeze
thaw technique, Reactive Funct. Polym. 73 (7) (2013) 923
928. [214] R. Geremia, M. Rinaudo, Biosynthesis, structure, and physical properties of some bacterial polysaccharides, Polysaccharides: Structural Diversity and Functional Versatility, 15, 2005, pp. 411
430. [215] S. Dumitriu, Polysaccharides: Structural Diversity and Functional Versatility, CRC Press, 2004. [216] A.F. Da´rio, L.M. Horteˆncio, M.R. Sierakowski, J.C.Q. Neto, D.F. Petri, The effect of calcium salts on the viscosity and adsorption behavior of xanthan, Carbohydr. Polym. 84 (1) (2011) 669
676.

Self-Healing Polymer-Based Systems

422

13. Self-healing hydrogels

[217] F. Garcıa-Ochoa, V. Santos, J. Casas, E. Gomez, Xanthan gum: production, recovery, and properties, Biotechnol. Adv. 18 (7) (2000) 549
579. [218] P.K. Sharma, S. Taneja, Y. Singh, Hydrazone-linkage-based self-healing and injectable xanthan
poly (ethylene glycol) hydrogels for controlled drug release and 3D cell culture, ACS Appl. Mater. Interfaces 10 (37) (2018) 30936
30945. [219] J. Huang, Y. Deng, J. Ren, G. Chen, G. Wang, F. Wang, et al., Novel in situ forming hydrogel based on xanthan and chitosan re-gelifying in liquids for local drug delivery, Carbohydr. Polym. 186 (2018) 54
63. [220] N. Yuan, L. Xu, H. Wang, Y. Fu, Z. Zhang, L. Liu, et al., Dual physically cross-linked double network hydrogels with high mechanical strength, fatigue resistance, notch-insensitivity, and self-healing properties, ACS Appl. Mater. Interfaces 8 (49) (2016) 34034
34044. [221] E. Makhado, S. Pandey, J. Ramontja, Microwave assisted synthesis of xanthan gum-cl-poly (acrylic acid) based-reduced graphene oxide hydrogel composite for adsorption of methylene blue and methyl violet from aqueous solution, Int. J. Biol. Macromolecules 119 (2018) 255
269. [222] H. Warson, Modern Superabsorbent Polymer Technologyin: F.L. Buchholz, A.T. Graham (Eds.), WileyVCH, New York, 1998, pp. xvii 1 279, ISBN priced 85.00, Polymer Int. 49(11) (2000) 1548
1548. [223] H. Omidian, M. Zohuriaan-Mehr, K. Kabiri, K. Shah, Polymer chemistry attractiveness: synthesis and swelling studies of gluttonous hydrogels in the advanced academic laboratory, J. Polym. Mater. 21 (3) (2004) 281
291. [224] B. Narjary, P. Aggarwal, A. Singh, D. Chakraborty, R. Singh, Water availability in different soils in relation to hydrogel application, Geoderma 187 (2012) 94
101. [225] L. Wu, M. Liu, Slow-release potassium silicate fertilizer with the function of superabsorbent and water retention, Ind. Eng. Chem. Res. 46 (20) (2007) 6494
6500. [226] K. Sharma, B. Kaith, V. Kumar, V. Kumar, S. Som, S. Kalia, et al., Synthesis and properties of poly (acrylamide-aniline)-grafted gum ghatti based nanospikes, RSC Adv. 3 (48) (2013) 25830
25839. [227] L. Pan, G. Yu, D. Zhai, H.R. Lee, W. Zhao, N. Liu, et al., Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity, Proc. Natl. Acad. Sci. 109 (24) (2012) 9287
9292. [228] A.S. Hoffman, Hydrogels for biomedical applications, Adv. Drug Deliv. Rev. 64 (2012) 18
23. [229] Y. Lu, W. He, T. Cao, H. Guo, Y. Zhang, Q. Li, et al., Elastic, conductive, polymeric hydrogels and sponges, Scientific reports 4 (2014) 5792. [230] N. Abu-Thabit, Y. Umar, Electrically conductive polyacrylamide-polyaniline superabsorbing polymer hydrogels, in: 1st International Electronic Conference on Materials (ECM), Montreal, Canada, 2014. [231] Y. Zhao, B. Liu, L. Pan, G. Yu, 3D nanostructured conductive polymer hydrogels for high-performance electrochemical devices, Energy Environ. Sci. 6 (10) (2013) 2856
2870. [232] T. Jayaramudu, H.-U. Ko, L. Zhai, Y. Li, J. Kim, Preparation and characterization of hydrogels from polyvinyl alcohol and cellulose and their electroactive behavior, Soft Mater. 15 (1) (2017) 64
72. [233] K.Y. Lee, D.J. Mooney, Hydrogels for tissue engineering, Chem. Rev. 101 (7) (2001) 1869
1880. [234] D. Pasqui, M. De Cagna, R. Barbucci, Polysaccharide-based hydrogels: the key role of water in affecting mechanical properties, Polymers 4 (3) (2012) 1517
1534. [235] A. Uliniuc, M. Popa, T. Hamaide, M. Dobromir, New approaches in hydrogel synthesis—click chemistry: a review, Cellulose Chem. Technol. 46 (1) (2012) 1. [236] S. Jana, A. Gandhi, K. Sen, S. Basu, Natural polymers and their application in drug delivery and biomedical field, J. PharmaSciTech 1 (2011) 16
27. [237] K. Shalumon, K. Anulekha, S.V. Nair, S. Nair, K. Chennazhi, R. Jayakumar, Sodium alginate/poly (vinyl alcohol)/nano ZnO composite nanofibers for antibacterial wound dressings, Int. J. Biol. Macromol. 49 (3) (2011) 247
254. [238] F. Camponeschi, A. Atrei, G. Rocchigiani, L. Mencuccini, M. Uva, R. Barbucci, New formulations of polysaccharide-based hydrogels for drug release and tissue engineering, Gels 1 (1) (2015) 3
23. [239] A.T. Paulino, L.A. Belfiore, L.T. Kubota, E.C. Muniz, E.B. Tambourgi, Efficiency of hydrogels based on natural polysaccharides in the removal of Cd2 1 ions from aqueous solutions, Chem. Eng. J. 168 (1) (2011) 68
76. [240] Y.M. Mohan, K. Vimala, V. Thomas, K. Varaprasad, B. Sreedhar, S. Bajpai, et al., Controlling of silver nanoparticles structure by hydrogel networks, J. Colloid Interface Sci. 342 (1) (2010) 73
82.

Self-Healing Polymer-Based Systems

References

423

[241] T. Jayaramudu, G.M. Raghavendra, K. Varaprasad, R. Sadiku, K. Ramam, K.M. Raju, Iota-Carrageenanbased biodegradable Ag0 nanocomposite hydrogels for the inactivation of bacteria, Carbohydrate Polymers 95 (1) (2013) 188
194. [242] A. Vasconcelos, A.C. Gomes, A. Cavaco-Paulo, Novel silk fibroin/elastin wound dressings, Acta Biomate. 8 (8) (2012) 3049
3060. [243] R. Silva, B. Fabry, A.R. Boccaccini, Fibrous protein-based hydrogels for cell encapsulation, Biomaterials 35 (25) (2014) 6727
6738. [244] M. Parkes, C. Myant, D. Dini, P. Cann, Tribology-optimised silk protein hydrogels for articular cartilage repair, Tribol. Int. 89 (2015) 9
18. [245] N. Annabi, S.M. Mithieux, G. Camci-Unal, M.R. Dokmeci, A.S. Weiss, A. Khademhosseini, Elastomeric recombinant protein-based biomaterials, Biochem. Eng. J. 77 (2013) 110
118. [246] J. Anjali, V.K. Jose, J.M. Lee, Carbon-based hydrogels: synthesis and their recent energy applications, J. Mater. Chem. A 7 (26) (2019) 15491
15518. [247] G.P. Kim, H.H. Sun, A. Manthiram, Design of a sectionalized MnO2-Co3O4 electrode via selective electrodeposition of metal ions in hydrogel for enhanced electrocatalytic activity in metal-air batteries, Nano Energy 30 (2016) 130
137. [248] S. Chen, J.J. Duan, M. Jaroniec, S.Z. Qiao, Nitrogen and oxygen dual-doped carbon hydrogel film as a substrate-free electrode for highly efficient oxygen evolution reaction, Adv. Mater. 26 (18) (2014) 2925
2930. [249] H.X. Shi, Z.W. Fang, X. Zhang, F. Li, Y.W. Tang, Y.M. Zhou, et al., Double-network nanostructured hydrogel-derived ultrafine Sn-Fe alloy in three-dimensional carbon framework for enhanced lithium storage, Nano Lett. 18 (5) (2018) 3193
3198. [250] N. Lingappan, D.J. Kang, Molybdenum disulfide nanosheets interconnected nitrogen-doped reduced graphene oxide hydrogel: a high-performance heterostructure for lithium-ion batteries, Electrochim. Acta 193 (2016) 128
136. [251] H.L. Li, T. Lv, H.H. Sun, G.J. Qian, N. Li, Y. Yao, et al., Ultrastretchable and superior healable supercapacitors based on a double cross-linked hydrogel electrolyte, Nat. Commun. 10 (2019) 8.

Self-Healing Polymer-Based Systems

C H A P T E R

14 A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers Amir Shojaei1,2 and Guoqiang Li2 1

Varian Medical Systems, Palo Alto, CA, United States 2Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA, United States

14.1 Introduction Modeling of the elastic and inelastic responses of metallic, ceramic, and polymeric materials has been under constant developments during past decades. In general the models have been developed within three length scales: molecular-scale, mesoscale, and continuum. While the molecular mechanics and dynamics are becoming a standard tool for hierarchical characterizations, their high computational costs limit the simulations’ time, temperature, and length scales, and they still cannot address most of the real engineering problems [1,2]. The mesoscale modeling approach provides more computational efficiency by bridging the micro- and macro-scales. This task is accomplished by the averaging techniques which utilize the microscale properties to provide internal state variables in the macroscale continuum level models [3
8]. Despite extensive studies on the molecular scale simulations, continuum mechanics is still considered the most practical tool to study most of the engineering problems. Due to the fact that many of the up-to-date self-healing systems are polymeric based, we limit our discussions to elastic
plastic as well as damage and healing modeling schemes related to the polymeric systems. Nevertheless, the discussed continuum damage healing mechanics (CDHM) is applicable to any self-healing material system. Elastic
plastic continuum models are classified into thermodynamic consistent and phenomenological approaches. In terms of phenomenological modeling approaches which are based on nonlinear and finite deformation spring and dashpot elements, for polymers one may mention works by [9
14]. Thermodynamic consistent models for polymers have

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been investigated by [15
19] and also the performance of the micromechanics-based multiscale approaches in predicting the viscoplastic behavior of semicrystalline polymers have been examined by [20
23]. The main deficiency associated with these continuum level modeling approaches is the high number of material parameters which need to be found through calibration processes between simulations and experimental data. Most recently Shojaei and Li [13] proposed a computationally effective viscoplastic model in which the number of the material parameters are minimized and also most of the material parameters are obtained directly from the experiments without cumbersome numerical curve fitting techniques [13]. Despite the nature of the material system, all of the design products will experience specific type of damage mechanisms during their service life, such as low/high cycle fatigue, ductile, or impact damages. The service life damage category is called ME damages hereinafter [13]. Another active damage mechanism, which is specific to selfhealing materials with the shape memory polymer (SMP) or shape memory alloy (SMA) elements, is thermomechanical (TM) damage; and that is associated with the processes of programming and recovery of the smart material system. To formulate the ME and TM damages, the continuum damage mechanics (CDM) framework is utilized by Shojaei and Li [13]. In their approach the ME damage variable represents the various types of service condition damages at the microscale level, such as microcracks, voids, and microcavities [13]. While it has been argued that two independent damage variables are required to describe accurately the case of isotropic ME damage [24], it was shown by Lemaitre that the assumption of isotropic damage gives sufficient accuracy to predict the load carrying capacity, the number of cycles, or the time to local failure in structural components [25]. Shojaei and Li developed a separate set of damage parameters to take into account the TM damage effects [13]. Their approach provides a comprehensive description for the active damage mechanisms in a self-healing system made of SMP or SMA components. Thus the material designers can predict the whole spectrum of the active damage mechanisms in self-healing materials by superposing ME and TM damages. Damage healing in composite structures has become a popular topic recently and several healing schemes have been reported in the literature for polymeric-, metallic-, and ceramic-based material systems. The healing schemes can be classified into two broad categories, which are extrinsic healing, which needs incorporation of external healing agent, and intrinsic healing, which can be healed by the polymer itself [26]. The extrinsic healing can be further divided into two subcategories: (1) embedding liquid healing agent within the material system: the pioneering works by White et al. demonstrated that the microcracks with narrow opening in polymeric systems can be healed by injecting liquid healing agent into the opened surfaces that will be solidified in presence of catalysts. The liquid healing agent is embedded via microencapsulating processes [27], hollow fibers [28], or microvascular networks [29], in which wall fracture of these containers release the healing agent and the liquid is solidified on contact with embedded catalysts, see Fig. 14.1. (2) Incorporating solid healing agents: in these systems a chemically compatible solid healing agent is blended into the material system and during the healing process it is meletd, difussed into the crack space, solidified by cooling, and bond the fracture surfaces together [30]. For wider opened cracks, the cracks can be narrowed or closed first through constrained expansion of SMP matrix [30
37], or through the constrained shrinkage of

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427

FIGURE 14.1 Self-healing system with embedded microcapsules.

embedded SMP fibers [38
41] or polymeric artificial muscle [42
44], followed by healing. This has been named as close-then-heal (CTH) scheme [26]. For intrinsic healing, many systems have been imvestigated. One may mention ionomers [45,46], thermally reversible covalent bonds (TRCBs) [47], and reversible covalent bond via transesterification reaction [48
51]. Thermoset polymers integrated with dynamic covalent bonds and shape memory (SM) effect were also synthesized, making healing of wide opened cracks possible without the need for external healing agent [52]. The proposed CDHM framework in this chapter is capable to address damage-healing for all of the abovementioned self-healing systems, because the CDHM scheme is developed based on the physics of the damage and healing rather than specific mechanisms. Because some damages, such as impact damages, are usually in the structural-length scale, the main challenge to heal these macrocracks remains the closure of such a wide opening. One of the disadvantages of some of the existing healing systems is their inability to effectively heal macroscopic cracks. For instance, in the case of the microencapsulated liquid healing agent, a large amount of healing agent is needed to heal macrocracks. However, incorporation of a large amount of healing agent will significantly alter the physical/ME properties of the host structure. Also large capsules/thick hollow fibers themselves may become potential defect sites when the encased healing agent is released. For ionomers and TRCB polymers, they need external help to bring the fractured surfaces in contact before chemical bonds can be established. While Kirkby et al. [53] proposed a very smart idea by incorporating SMA wires to close the cracks before healing, one limitation is that the SMA recovery force cannot be effectively transferred because the polymer matrix becomes soft at the SMA recovery temperature while the SMA is very stiff. Another limitation is that the SMA wires have small recovery strain, limiting the width of the cracks that can be closed. Also the “run-off” of the liquid monomer in wide-opened crack is another challenge before polymerization occurs, although efforts have been made to fill in the crack space by fast gelling healing agent [54]. Therefore the grand challenge facing the scientific community is how to heal structural-length scale damage such as impact damage repeatedly, efficiently, and molecularly. Most recently Li and Shojaei [23,55] have proposed a bioinspired self-healing system in which SMP fibers are

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incorporated as crack sealing agent and thermoplastic particles (TPs) are utilized as the solid healing agent. Fig. 14.2 represents a schematic of this healing system in which shape recovery of the SMP fibers closes the crack and melting of the TPs provides molecular level healing. The difussion process is accompained with recovery pressure by the external SMP fibers which closes the crack opening and improves the difussion process. The proposed system belongs to the broader category of CTH strategy [33,34] and is validated to repeatedly and molecularly heal macroscopic crack [23,30,32,34,38,40,55]. Most of the up-to-date works on the self-healing systems are focused on trial-and-error manufacturing processes to demonstrate the capability of these systems to heal damages, although physically consistent models are required to promote these systems into the real applications. Due to the computational and experimental difficulties facing molecular level modeling techniques, the alternative continuum level modeling approach is considered herein, such as those developed by Barbero et al. [16
18,56,57]. Although the discussions on the elastic and inelastic analysis are dedicated to the polymeric material systems, the proposed CDHM scheme is applicable to any self-healing system. Some suggestions for future development are also presented at the end of this chapter. Both indicial and bold notations are utilized to indicate tensors and light letters represent scalar variables.

FIGURE 14.2

Close-then-heal system with embedded SMP fibers, after Li and Shojaei [23,55].

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429

14.2 Finite deformation kinematics: elastic, plastic, damage, and healing in polymers The deformation mechanisms in polymeric material systems are commonly associated with large deformations and finite deformation kinematics and are an essential essence for formulating the elastic, plastic, damage, and healing processes. These processes follow different physics and require different constitutive relations. In general the elastic deformation of polymeric networks is associated with reversible chain stretches and conformational rotations. The plastic deformation is commenced when the stretched chains or deformed configurations will not attain their original shapes on unloading, caused by cooperated segmental rotation. The damage mechanisms in polymers can be described on chain failure in which the molecular chains will fail once the applied stretch is greater than their locking stretch limit. These processes are discussed in detail by [16,18] and Fig. 14.3 represents schematically the elastic
plastic damage deformation in polymers. The multiplicative decomposition of the deformation gradient is a conventional assumption which provides enough flexibility to introduce separate constitutive laws for each of the deformation processes and it reads: ðpÞ dÞ ðhÞ Fij 5 FðikeÞ Fkl Fðlm Fmj ;

(14.1)

where F(e), F(p), F(d), and F(h) tensors are, respectively, the elastic, plastic, damage, and healing part of the total deformation gradient tensor F. The polar decomposition of the deformation gradient tensors to the rotational and stretching tensors is an alternative description for the finite deformation kinematic [58,59]: Fij 5 Rik Ukj

(14.2)

where U 5 (FTF)1/2 is the right Cauchy stretch tensor and R 5 FU21 is the rotation tensor. The pull-back/push-forward approach [60] is applicable to both of the decomposition

FIGURE 14.3 (A) Undeformed body, (B) microcrack formation due to breakage of polymer chain, (C) stretched chains after saturation of conformational changes, and (D) conformational changes due to external loading, after [23].

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approaches (presented in Eqs. 14.1 and 14.2) to interchange between the associated values in the material, intermediate relaxed, and spatial configurations, in the case of Eq. (14.1), and material, stretched, and spatial configurations in the case of Eq. (14.2). Generally to facilitate the formulation process, the polar decomposition is applied to each of the deformation gradients in Eq. (14.1). In other words, the intermediate relaxed configuration can be represented by the polar decomposition of the inelastic right stretch tensor, that is, U(p) 5 (F(p)T F(p))1/2, and proper orthogonal rotation tensor, Rp, as follows: ðpÞ

ðpÞ

ðpÞ

Fij 5 Ril Ulj

(14.3)

This polar decomposition is applied for all the transformation between the configurations and provides the material designers tools to investigate the finite kinematics process in details. The Eulerian strain tensor, E, is then given by: Eij 5

 1
21 δij 2 F2T ik Fkj 2

(14.4)

_ 21 , and where δ is the Kronecker delta. The spatial velocity gradient is defined as: L 5 FF the strain rate tensor E_ , and spin tensor ω are, respectively, given by the following relationships: E_ ij 5

  1 1 Lij 1 Lji and ωij 5 Lij 2 Lji : 2 2

(14.5)

where ð:Þ  @=@t denotes the time derivative. The additive decomposition of the elastic, plastic, damage, and healing strain rates is an alternative constitutive assumption to multiplicative decomposition and it also provides the flexibility of defining separate constitutive relations for each stage of the elastic, plastic, damage, and healing deformations. In the case of finite deformation elastoplastic kinematics it is shown that if the elastic strains are sufficiently small, the additive decomposition of the elastic and plastic strain rates gives acceptable computational accuracy during each time increment [61]. Accordingly the total strain rate tensor, E_ , is decomposed into the ðe Þ undamaged elastic, E_ , plastic, E_ ðpÞ , damage, E_ ðdÞ , and healing E_ ðhÞ strain rates: ðe Þ

ðpÞ

_ ðhÞ E_ ij 5 E_ ij 1 E_ ij 1 E_ ðdÞ ij 1 E ij :

(14.6)

Regardless of the additive or multiplicative assumption, the constitutive relations for each of the strain components can be investigated separately. The additive or multiplicative decompositions are also useful in developing the return mapping solution algorithms, as discussed by [17,60]. The additive decomposition in finite deformations is consistent with the solid mechanics thermodynamic principles [62,63]. As discussed by [59], if the developed constitutive relations are to be applicable to a wide range of dynamic energy densities, for example, associated with different impact velocities, it is convenient to further decompose the undamaged elastic, E_ ðeÞ , plastic, E_ ðpÞ , damage, E_ ðdÞ , and healing, E_ ðhÞ , strain _ # Þ and shear ðγ_ # Þ components as follows: rates into their dilatational ðM E_ #ij 5 γ_ #ij 1

1 _# M δij : 3

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(14.7)

14.3 Plastic deformation in polymers

431

where “#” is substituted by “e,” “p,” “d,” or “h” to indicate elastic, plastic, damage, and healing, respectively. Using the formulation approach in Eq. (14.7), the underlying deformation mechanisms for a wide range of dynamic problems can be readily explained. For example, in the case of the low-energy density dynamic problems, the effect of the hydrostatic pressure changes, that is, Σ, is generally negligible; this is unlike the high-energy density problems where hydrostatic pressure has a significant effect on the deformation and damage mechanisms. Eq. (14.7) allows the elastic, inelastic, damage, and healing mechanics, induced by the hydrostatic and deviatoric part of the applied stresses, to be formulated separately. In a finite deformation problem there are a number of available elastic constitutive relations. When the undamaged elastic strains are small, the Hooke’s Law reads: 1 r 1 _ ðe Þ 1 r ðeÞ r _ ðe Þ sij and M E_ ij 5 L21 5 Σ ijkl σkl ; γ ij 5 2μ 3 3K

(14.8)

where L is the fourth order elastic stiffness tensor, “r” indicates the Jaumann rate, σ is the Cauchy stress tensor, s 5 σ 2 Σδ is the deviatoric stress tensor, Σ 5 1=3σkk denotes the applied hydrostatic pressure (mean stress), and μ and K are, respectively, undamaged shear and bulk elastic moduli. Eq. (14.8) represents undamaged elastic strains that are distinguished from their damaged counterparts by “
” notation.

14.3 Plastic deformation in polymers Inelastic deformation mechanisms in polymers include several distinctive events and a physically consistent model should be able to address them. Polymers show irregular elastic
plastic behavior which cannot be modeled by many classical plasticity constitutive equations. Many solid polymers exhibit necking in both tensile and compression tests [64]. The behavior of glassy polymers at temperatures below their transition temperature is usually evaluated through compression or shear deformation because most glassy polymers break at this temperature before any plastic deformation under tensile stresses [65]. A typical compression stress
strain test with polymers subjected to constant compression strain rate includes four regions which are (1) elastic region, (2) softening region, (3) stationary region, and (4) strain hardening region prior to failure. The overall ME resistance to the straining of a polymer mainly comes from two distinct sources: (1) the temperatureand rate-dependent intermolecular resistance and (2) the entropy driven molecular network orientation resistance. Several mechanisms based phenomenological models have been developed to capture this nonlinear behavior by decomposing the stress response into an equilibrium time-dependent component representing the viscoplastic behavior and an equilibrium time-independent component representing the rubber-like behavior [9,11,12,66]. The main drawback in these formulations is the need for a large number of material parameters without clear calibration procedures. Most recently Shojaei and Li developed a mathematically simple and computationally efficient viscoplastic model for the polymers. The number of material parameters is

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minimized and the model is able to capture temperature, rate and hardening effects [13,59]. The plastic strain rate constitutive relation is introduced as follow:    n1  p jτ j γ_  5 γ_ 3 1 1 C1 ln γ_ (14.9) 3 0 μ γ_ 0 where γ_ 0 is a reference strain rate, which indicates the minimum strain rate in a set of experiments, γ_ is the applied strain rate, C1 is the rate sensitivity parameter, n1 is a hardening exponent, μ is the shear modulus, and jτ j is the effective shear stress, to be defined in the following. Basically a backstress tensor is required to capture the subsequent strain hardening which is associated with the polymer chain segment/network stretching after the initial post yield softening response in glassy polymers. Accordingly the softening at yield point in polymers is attributed to overcoming the conformational rotation resistance of the segments in which the applied external energy is dissipated in rearrangement of the polymer networks. The conformational dissipative mechanisms are saturated once most of the polymer chains are aligned in the direction of the external force field. In this stage the molecular network start to stretch and the polymer show the subsequent hardening effect. Boyce and Arruda have utilized the Langevin function to simulate the hardening effect at high strain level [66
69]. Shojaei and Li have developed a physically consistent kinematic hardening model which incorporates the statistical mechanics basis for describing the chain stretching proP3 p cess. The mean inelastic stretch scalar variable I p 5 1=3 λ is utilized to characterize j51 j p the magnitude of the polymer chain stretches where λj denotes the principal inelastic stretch vector. On saturation of the conformational rotations, the chain stretching process dominates the inelastic deformation in the molecular network where the variable I p is utilized to capture this effect. In the developed statistical framework, the process of the chain extension is distributed over a range of inelastic strains which is prescribed by the variance parameter σα . The subscript “α” herein indicates the inelastic stretching process. Also the stretching process is centered at a specific inelastic stretch range which is constitutively p prescribed by the mean value Iα . In other words the chain stretching process is initiated at p p inelastic strain level of I p 5 Iα 2 σα and it will end at I p 5 Iα 1 σα , beyond which the chains may start to fail. Based on these justifications the Gaussian (normal) distribution for the chain stretching process is formulated as follows [13]:  p p 2 ! I 2Iα 1 p p dnormα ðI ; Iα ; σα Þ 5 pffiffiffiffiffiffi exp 2 (14.10) 2σ2α σα 2π pnormα ðI

p

; Iαp ; σα Þ 5

ð Ip 0

  dnormα I p ; Iαp ; σα :dI p

(14.11)

Once the inelastic stretches in most of the SMP chains reaches the limiting value of p I p DI L , an appropriate damage formulation is required to model the chain failures, such as continuum damage statistic mechanics models. Now the Gaussian distribution is utilized to prescribe the back stress tensor, αij, for the kinematic hardening effects in SMPs:
 αij 5 @3 I p 1 2 pnormg pnormα δij (14.12)

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433

where @3 (MPa) is a material parameter and it is determined from the numerical curve fit techniques, such as least square of deviation between the simulation and experimental data points. One may realize that when the value of @3 is higher, higher strain hardening effects will be observed. Eqs. (14.9)
(14.12) provide a mathematically simple and effective tool to simulate the kinematic strain hardening effect in polymers. The driving stress for producing the inelastic deformation rate is computed based on the following relation [22,23]: sij 5 sij 2 Xij   where Xij 5 αij 2 1=3αkk δij is the deviatoric part of the back stress tensor, and sij 5 σij 2 1=3σkk δij is the deviatoric Cauchy stress. The effective shear stress, jτ j, is computed by [22]: rffiffiffiffiffiffiffiffiffiffiffiffi 1   s s jτ j 5 (14.13) 2 ij ij Due to the fact that the deviatoric part of the applied stress governs the inelastic flow in polymers, the 3D flow rule is proposed as follows:   sij p Dij 5 γ_ p  pffiffiffi 2jτ j

(14.14)

A nonunified viscoplastic model is developed in this work. Thus the von Mises yield criterion reads: rffiffiffiffiffiffiffiffiffiffiffiffi 1   s s 2 τy # 0 (14.15) 2 ij ij where τ y ðT; E_ Þ is the temperature- and rate-dependent yield stress and it is defined by:  p 

1 2 pnormg γ_ τ y 5 τ 0 3 1 1 Cln (14.16) 3 γ_ 0 1 1 pnormg where τ 0 is the yield stress at loading rate γ_ 0 , C is a material parameter which is available for polymers in the literature [70,71], and the last term in Eq. (14.16) takes into account the temperature effect on the yield stress. The proposed statistics based “kinematic hardening flow rule” and the viscoplastic constitutive relations are well correlated with the observed experimental data. The material p parameters Iα and σα are easily obtainable from a simple tensile (or compression) test in which the former indicates the central inelastic strain level and the latter is the inelastic strain bandwidth of the stretching process. The performance of these equations is parametrically studied in Fig. 14.4. In Fig. 14.4A the effect of various central values for the strain p p hardening effect, that is, Iα , is studied in which two different Iα s result in two distinguishable responses. In Fig. 14.4B the role of different bandwidths values, that is, σα , are depicted.

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p

FIGURE 14.4

Parametric study for the developed viscoplastic model: (A) effect of different mean values Iα and (B) effect of different bandwidth values, σα , after [13].

14.4 Continuum damage and healing mechanics Healing causes a reduction in the volume of damage and recovers the loss stiffness due to the damage mechanisms in the material. Thus for any type of healing mechanism the healing process can be described within CDHM framework. To extend the applicability of the CDHM scheme, general physical meaning of the damage and healing processes are considered in this section without targeting any specific healing or damage type. The concept of CDHM has been proposed by Barbero et al. [56], and later Voyiadjis et al. formulated the CDHM within the CDM framework [16
18,57]. Since then this formulation has been widely recognized as basis of the continuum approach for modeling the damage-healing processes [72
74]. The concept of representative volume element (RVE) is used in which the presence of microcracks and microvoids are

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14.4 Continuum damage and healing mechanics

considered through reduction of the load carrying capacity of RVE; while healing increases the load carrying capacity of the medium. In Section 14.4.1 a scalar damagehealing parameter is developed for the case of isotropic damage and healing and in Section 14.4.2 anisotropy problem is studied where tensorial correlations are developed. The proposed healing variables capture the healing effect by two distinctive measurement methods that are direct and indirect methods. In the direct method the healing is observed by the measurement of the effective cross section changes after the healing, while the indirect method captures elastic stiffness changes to calibrate the healing. The relationships between classical damage variables with these new healing variables are investigated and it is shown that the developed damage-healing variables reduce to the classical damage parameters when healing is eliminated.

14.4.1 Scalar damage-healing variables for isotropic problems The effective and real configurations in CDHM have been introduced in [17,57] in the case of isotropic damage-healing mechanisms in which two new scalar healing variables are formulated on the basis of CDHM. To accomplish this task the CDM framework, which was formerly proposed by [75], was modified to measure the changes in density of defects [17,57]. Let the initial undeformed and undamaged configuration of the body is represented by C0 , and the damaged and deformed configuration is represented by C. The effective configuration is a fictitious state where all damages including microcracks and voids have been removed from the deformed body and it is shown by C. In Fig. 14.5 these configurations are depicted schematically. For the case of isotropic damage the change in the load carrying area of the RVE is represented by: φ5

A2A A

(14.17)

The scalar healing variable is introduced through defining three new configurations within the CDHM. The fictitious state of fully damaged and deformed configuration, that is,Cd , represents the removed area from the RVE due to the damage mechanisms and it is

FIGURE 14.5 (A) Initial undeformed and undamaged configuration: C0 , (B) damaged and deformed configuration: C, and (C) effective fictitious undamaged and deformed configuration: C after [17,57].

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obtained by subtracting the fictitious undamaged and deformed cross section A in the configuration C from the total cross section A in the configuration C as follows [17,57]: Ad 5 A 2 A 5 A 2 Að1 2 φÞ 5 Aφ

(14.18)

where Ad is the total damaged cross section (removed area due to damage). As discussed by[17,57] the healing process can be assumed to act only on the pure damaged cross section, that is, Ad . The fictitious healed and deformed configuration Ch is defined by removing the healed portion of damages from Ad as shown in Fig. 14.6 and the scalar healing variable reads [17,57]: h5

Ad 2 Ah ; 0,h,1 Ad

(14.19)

where Ah is the healed portion of the cross section Ad . The case of h 5 1 corresponds to zero percent healing of the damaged area (Ah 5 0), and h 5 0 relates to 100% healing of the damaged area (Ah 5 Ad ). Finally the effective fictitious fully healed and deformed configuh ration C is obtained by removing all the remaining damages from Ch , as shown in Fig. 14.7. According to the basic concept of effective configuration, the damaged area does

FIGURE 14.6 (A) Fictitious total removed area due to damage: Cd and (B) fictitious healed and damaged configuration: Ch, after [17,57].

FIGURE 14.7 (A) Fictitious healed and deformed configuration: Ch and (B) effective fictih tious fully healed and deformed configuration: C , after [17,57].

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437

not sustain load. However, after healing the healed cross section Ah can carry load Tv as shown in Figs. 14.6 and 14.7. The cross section of the hypothetical healed configuration healed C is assumed to be the summation of the cross section of the effective fictitious healed h and deformed configuration C and the cross section of the effective fictitious undamaged and deformed configuration C as shown in Fig. 14.8 [17,57]. healed The transformation equation between stresses in C and C configurations is derived by applying the equilibrium between the healed and damaged states as follows: σ (14.20) σ5 ðð1 2 φÞ 1 φð1 2 hÞÞ The transformation relation between damaged-healed elastic modulus, that is, Eðφ; hÞ, and the fictitious undamaged elastic modulus, that is, E, is obtained based on one of the two following hypothesizes [17,57]: 1. Hypothesis of elastic strain equivalence: in this case, the strains in both configurations is assumed to be the same and E 5 E. 2. Hypothesis of elastic strain energy equivalence: in this case elastic strain energy in both cases is assumed to be equivalent. While both of these hypotheses are being used by researchers in the field of damage mechanics, it is believed that the hypothesis of elastic energy equivalence is more general since it is based on an energy formation [17,57,76]. In the following for sake of brevity only the hypothesis of elastic strain energy equivalence is utilized to derive the evolutions laws for damage and healing processes. The hypothesis of elastic strain energy, U, equivalence between the effective configurahealed tion, that is, C , and the real damaged-healed configuration, that is, C, is incorporated by [17,57] to derive the correlation between the damaged-healed and undamaged elastic moduli between these two states. This hypothesis reads: U5

1 1 2 σ2 5 σ 2Eh ðφ; hÞ 2E

(14.21)

FIGURE 14.8 (A) Virgin material, (B) damaged and deformed configuration: C, and (C) hypothetical healed configuration: C

healed

, after [17,57].

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Substituting σ from Eq. (14.20) into (14.21) results in the relation between the elastic modulus in the two configurations as follows [17,57]: Eh ðφ; hÞ 5 Eðð12φÞ1φð12hÞÞ2

(14.22)

The state of 100% healing yields: Eh ðφ; h 5 0Þ 5 E, which shows the corresponding elastic modulus is fully recovered after the healing process. In the case of zero healing, Eq. (14.22) results in Eh ðφ; h 5 1Þ 5 Eð12φÞ2 which is the well-known correlation in the classical CDM concept [76,77]. Accordingly the damage parameters based on the area reduction are hard to calibrate in practical applications and requires sophisticated measurement techniques to calibrate the cross section area reduction due to the damage or cross section area increase due to the healing. To facilitate the damage and healing measurement, an indirect measurement method for the damage, proposed by Lemaitre and Dufailly [78], was generalized by Voyiadjis and Kattan to facilitate the damage and healing measurements [77]. In their approach the elastic modulus changes are correlated to the healing process. Let the scalar damage variable l measures the elastic modulus changes [78]: l5

E 2 Ed ðlÞ Ed ðlÞ

(14.23)

Another scalar healing variable, that is, h0 , is defined to measure the change in the elastic modulus during the healing process [17,57]: h0 5

Eh ðl; h0 Þ 2 Ed ðlÞ Ed ðlÞ

(14.24)

where E is the elastic modulus for a virgin material and Ed ðlÞ is the damaged elastic modulus in the damaged and deformed configuration, that is, C, and Eh ðl; h0 Þ is the elastic modulus in the healed and deformed configuration, that is, Ch . The relation between the three moduli is defined as follows [17,57]: Ed ðlÞ # Eh ðl; h0 Þ # E

(14.25)

To obtain the transformation equation between the effective and healed elastic modulus one may substitute Ed ðlÞ from Eq. (14.23) into (14.24): Eh ðl; h0 Þ 5

ð1 1 h0 ÞE ð1 1 lÞ

(14.26)

where in Eq. (14.26) h0 5 0 indicates zero percent healing and results in 0 Eh ðl; h 5 0Þ 5 Ed ðlÞ 5 E=ð1 1 lÞ which is consistent with the result of pure damage without healing [79]. By setting Eh ðl; h0 5 lÞ 5 E, the upper bound for h0 is obtained as h0 5 l. Then this healing variable can change in the range of 0 # h0 # l which shows consistency with the general concept of healing. The elastic energy equivalence is again applied to derive the correlation between damage variables, φ and l, and healing variables h and h0 and also the stress transformation between the damaged state C and the hypothetical healed states

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439

healed

C . Again the hypothesis of elastic strain energy equivalence, see Eq. (14.21), is utilized to obtain [17,57]: rffiffiffiffiffiffiffiffiffiffiffiffi l11 σ5σ h0 1 1

(14.27)

where σ is the healed stress in the healed configuration and stress σ is in the damaged configuration. To derive the relation between the damage variables l and φ and the healing variables h and h0 in the case of elastic energy equivalence, one may substitute E from Eq. (14.26) into (14.22) [17,57]: 1 12 φ5 h

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! ð 1 1 h0 Þ ð1 1 lÞ

(14.28)

If the healing is eliminated from the system it means that h0 5 0, and h 5 1 and Eq. (14.28) reduces to: pffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 1 lÞ 2 1 φ 5 pffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 1 lÞ

(14.29)

which is consistent with the published results in the literature [80].

14.4.2 Anisotropic damage-healing problems In many cases the scalar damage-healing parameters performs quite well in capturing the response of the smart material systems due to the damaging or healing processes. While in highly anisotropic processes the scalar variables fail in reproducing exact changes in material properties and tensorial parameters that are crucial. For example in the case of fiber reinforced composites the damage or healing process highly depends on the material orientation and a single damage or healing parameter cannot represent the actual state of the smart material system. Voyiadjis et al. aimed to extend the classical CDM concept, developed by [75,81], to a generalized anisotropic CDHM framework [57]. This approach is schematically depicted in Fig. 14.9. Fig. 14.9A depicts the real state of damaged material, which is assumed to be decomposed to a fictitious effective configuration in Fig. 14.9B and a fictitious fully damaged configuration in Fig. 14.9C. The effective configuration carries the load, while the fully damaged state cannot sustain load [57]. Let the damage tensor φij represents the transformation between the real damaged area vector dAni , Fig. 14.9A, and effective fictitious area vector dAni , Fig. 14.9B. This transformation is mathematically prescribed by [81]:   dAnj 2 dAnj ; φij ni 5 dA


1=2 0 # φij φij #1

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(14.30)

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14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

FIGURE 14.9 Schematic representation of (A) real damaged configuration, (B) fictitious effective configuration, and (C) fictitious damaged state [57].

The pure damaged area vector, that is, dAd ndi , Fig. 14.9C, is then obtained as a function of the damage variable tensor, φij , and the area vector, dAnj , in the real damaged configuration. Eq. (14.30) is rearranged to obtain [57]: dAd ndi 5 dAni 2 dAni 5 φij nj dA

(14.31)

According to the physics of the healing process, some of microscale damages are removed during the healing process. This is simulated by increasing the effective area, which carries the load. To represent this phenomenon, it is assumed that the fictitious fully damaged configuration, Fig. 14.9C, undergoes the healing process. This process for an anisotropic healing case is shown in Fig. 14.10. Fig. 14.10A shows the pure damaged sate without load carrying capacity, Fig. 14.10B represents the healed configuration and Fig. 14.10C and D shows the fictitious effective healed configuration and remaining damages after accomplishing the healing process, respectively. A second rank anisotropic healing variable tensor hij is proposed by [57] as follows: hij ndi 5

φjk dAnk 2 dAh nhj dAd

;

 1=2 0 # hij hij #1

(14.32)

where hij captures the transformation between real damaged area vector dAni in Fig. 14.11A and fictitious healed area vector dAvnhi in Fig. 14.11B. Fig. 14.11 shows the overall mapping procedure between real damaged and fictitious healed effective configurations. Due to the complex mapping techniques, required in direct measurement of damage and healing variables based on defect density, one may use an indirect measurement Self-Healing Polymer-Based Systems

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FIGURE 14.10 (A) Fictitious damaged state, (B) fictitious healed state, (C) fictitious effective healed configuration, and (D) fictitious remained damaged state, after [57].

FIGURE 14.11

(A) Damaged configuration and (B) fictitious effective configuration after healing process, after [57].

method such as calibrating damage and healing based on elastic modulus changes [78]. 0 Fourth order anisotropic damage tensor, that is, κijkl , and healing tensor, hijkl , are proposed by [57] to capture the elastic modulus change and is defined as follows:
 21 d κð1Þ ijkl 5 Eijmn 2 Eijmn Emnkl
 (14.33) 21 d κð2Þ 5E E 2 E mnkl ijmn mnkl ijkl

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14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

where superscript (1) and (2) indicate the two different mathematical tensorial expressions for the damage tensor when normalizing it with reference to the inverse of the undamaged elasticity tensor Eijkl , and Edijkl is the damaged elastic modulus. To measure the healing 0 indirectly, the fourth rank healing variable tensor hijkl is introduced as follows [57]:  21  0 ð1Þ hijkl 5 Ehijmn 2 Edijmn Edmnkl (14.34)  0 21  ð2Þ hijkl 5Edijmn Ehmnkl 2 Edmnkl 0

where Ehijkl is the elastic modulus of the healed material in which hijkl 5 0ijkl represents no healing. One may assume that the maximum healing is obtained when all produced 0 max damages are cured, and hijkl 5 κmax shows the elastic modulus is fully recovered. ijkl d Substituting Eijkl from Eq. (14.33) into Eq. (14.34) results in the following expression for the healed elastic modulus Ehijkl :
0  ð1 Þ 1Þ 1Þ 0 ð1Þ 2 κðijkl 2 κðpqkl hijpq Ehijmn 5 Eijmn 1 Eklmn hijkl
0  (14.35) ð2 Þ 2Þ 2Þ 0 ð2Þ Ehijmn 5 Eijmn 1 Eijpq hpqmn 2 κðpqmn 2 κðpqkl hklmn The transformation rules between the damaged, Fig. 14.11A, and effective healed configurations, Fig. 14.11B, are obtained by introducing a new fourth rank damage-healing transformation tensor, Qijkl , as follows [57]: σij 5 Qijkl σkl

(14.36)



 21 21 21 Qijkl 5 M21 ijkl 1 Iijmn 2Mijmn Hmnkl

(14.37)

with

where  h
i21=2 Mijkl 5 Iij 2φij Ikl 2φkl

21=2 Hijkl 5 hij hkl

(14.38)

where Iij denotes the unity tensor. The correlation between damage tensors φij and κijkl 0 and healing tensors hij and hijkl are given in [57].

14.5 Physically consistent evolution laws for the damage and healing processes The mathematical correlations between damage and healing processes are represented in Section 14.4. The next step is to define evolution laws for the damage and healing processes in which the physics of the damage and healing mechanisms are utilized to establish their flow rules. In the case of continuum damage, the loading condition, for example, fatigue, monotonic loading, impact, etc., together with the material constitutive behavior and environmental conditions are dominant factors to be considered. Many empirical, for example, Johnson and Cook [59,70] for polycrystalline materials, phenomenological, for

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example, Shojaei et al. [82] for porous rocks, and thermodynamic consistent, for example, Voyiadjis et al. [16,18] for polymers, damage evolution laws exist and we cannot cite all related works herein. Despite extensive theoretical works on damage process in materials, little works have been done in the case of modeling healing. This might be due to the limited application of smart self-healing materials in the industry, but it is expected that with advancement in new high throughput self-healing materials the need for modeling schemes surges. To formulate the healing process the nature of the healing system should be defined that is a function of many parameters including the healing method which is used. Types of damages and constitutive behavior of the constituent material are other important factors during this analysis. Due to the inefficiency of the molecular dynamics or mechanics in simulating realtime healing at an acceptable length and time scale, the continuum mechanics is adopted here to represents the healing process. There are two general methods to formulate the healing within the continuum mechanics. One approach utilizes thermodynamic rules to formulate the healing process. This concept, which incorporates macroscopic internal variables to define damage, healing, and plasticity, has been practiced by [16,18]. Another approach is to investigate the healing through mechanisms based phenomenological models. Both of these methods are represented in the following two subsections.

14.5.1 Thermodynamic consistent damage and healing model Thermodynamic restrictions including energy consideration is used to derive the damage and healing constitutive relations. In this subsection the tensorial parameters are printed in bold and scalar parameters are printed in light letters. Let u denotes the specific internal energy, which is a function of entropy s, elastic strain tensor ee , damage variable tensor ζ d , plastic deformation tensor ζ p , and healing tensor ζ h . The thermodynamic variables might be observable or internal variables. The specific internal energy u is then defined as follows [16,18]:
 u 5 u s; ee ; ζ d ; ζ p ; ζ h (14.39) where the dimension of ζ d ; ζ p , and ζ h is the number of internal variables which is used to describe each phenomenon in damage, plasticity and healing, respectively. Time derivative of Eq. (14.39) yields: u_ 5

@u @u @u d @u p @u h s_ 1 e :_ee 1 d :ζ_ 1 p :ζ_ 1 h :ζ_ @s @e @ζ @ζ @ζ

(14.40)

where “:” indicates contraction over two tensorial indices. The second law of thermodynamic states that the change in entropy is always positive and it can be expressed in the ClausiusDuhem inequality as follows:   q σ:_ee 2 ρ u_ 1 sT_ 2 :rT $ 0 (14.41) T where ρ is the density which is assumed to be constant and σ is the Cauchy stress and T is the absolute temperature. The following restrictions are used in this formulation [83]: (1)

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purely ME theory is used (no heat source and no heat flux in the body) and (2) infinitesimal deformation state is considered. An additive elastic and plastic state is the result of the second statement. By substituting Eq. (14.40) into (14.41) and eliminating the heat flux term one obtains:    @u @u @u d @u p @u h s_ 1 e :_ee 1 d :ζ_ 1 p :ζ_ 1 h :ζ_ 1 Ts_ $ 0 σ:_ee 2 ρ (14.42) @s @e @ζ @ζ @ζ Rearranging Eq. (14.42) yields:       @u @u @u _ d @u _ p @u _ h e s_ 2 ρ :ζ 1 p :ζ 1 h :ζ $ 0 σ 2 ρ e :_e 1 ρ T 2 @e @s @ζ @ζ d @ζ

(14.43)

The conjugate thermodynamic forces related to the flux of the entropy s and the elastic strain ee are obtained as follows: T5

@u @u ; and σ 5 ρ e ; @s @e

The power of dissipation Γ is expressed as follows:   @u _ d @u _ p @u _ h : ζ 1 : ζ 1 : ζ Γ52ρ @ζ p @ζ d @ζ h

(14.44)

(14.45)

The dissipative power Γ is used to define the following conjugate thermodynamic forces: yd 5

@u @ζ

d

; yp 5

@u @u ; yh 5 h @ζ p @ζ

(14.46)

where yd , yp , and yh are damage, plasticity, and healing thermodynamic conjugate forces, respectively. Finally the second law of thermodynamics reduces to: Γ$0

(14.47)

Helmholtz free energy function Ψ is obtained through Legendre transformation of the internal energy as follows: Ψ 5 u 2 Ts

(14.48)

Using the Helmholtz free energy yields the same result for the dissipative power Γ as shown in Eq. (14.47). The only difference between internal energy definition and Helmholtz free energy definition is that the internal energy is a function of entropy and ME variables but Helmholtz free energy is a function of temperature and ME variables. For the case of isentropic processes internal energy formulation is used and for the case of isothermal processes the Helmholtz potential is used. The first law of thermodynamic of an infinitesimal quasistatic process states that the change of energy for a system is equal to the sum of ME input and output works. The first law based on the internal variable formulation reduces to the following expression: u_ 5

1 σ:_ee ρ

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(14.49)

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14.5 Physically consistent evolution laws for the damage and healing processes

Substituting Eq. (14.49) into (14.40) while incorporating Eq. (14.44) results in:   @u _ d @u _ p @u _ h : ζ 1 : ζ 1 : ζ Ts_ 5 2 @ζ p @ζ d @ζ h

(14.50)

As discussed by [16,18], plasticity internal variables ζ p may include: (1) second-order plastic strain tensor eP, (2) second-order tensor α to express kinematic hardening due to inelastic deformation, which represents the shift in the center of the yield surface f p in the stress space, and (3) tensor p to express isotropic hardening, which shows change in the size of the yield surface f p in different directions during a plastic deformation (distortion in plasticity yield surface) [16,18,84
88]. In the case of the damage variable tensor ζ d , in the most general form, they can represent three variables: (1) generalized damage tensor d which measure overall degradation of materials, (2) damage tensor dK which describes the kinematic hardening due to the damage process and it indicates the shift in the center of the damage surface f d , and (3) damage tensor dI to represent isotropic hardening which shows the change in the size of the damage surface f d [77,84]. The most general form of the healing variable tensor ζ h may be introduced as follows: (1) generalized healing tensor K h which measures the overall healing in the material, (2) healing variable tensor h which describes the kinematic hardening during the healing process, which is the change in the center of the healing surface f h , and (3) healing tensor hI to describe the isotropic hardening/softening during a healing process, which is the change in the size of the healing surface f h in different directions. The physical meaning of the kinematic and isotropic hardening for the plasticity, damage, and healing are discussed by [16,18]. Substituting the introduced internal variables into the Helmholtz free energy Ψ and decomposing the Helmholtz free energy into quadratic forms of potential functions for each corresponding internal variable yields:   1 Ψ e; ep ; α; p; d; dK ; dI ; h; hK ; hI 5 ðe 2 ep Þ:Eðd; hÞ:ðe 2 ep Þ 2 1 1 1 1 k1 α:α 1 k2 p2 1 k3 dK :dK 2 2 2 1 1 1 1 k4 dI2 2 k5 hK :hK 2 k6 hI2 2 2 2

(14.51)

where E is the elastic modulus, e is total strain tensor, ep is the plastic strain tensor, and ki ði 5 1 to 6Þ are material-dependent constants and they may be linked to the dislocation density for plasticity related parameters k1 and k2 , defect density or surface energy changes for the damage and healing related constants k3 to k6 . The thermodynamic associated variables are given for each internal variable as follows: σ5ρ

@Ψ @Ψ @Ψ @Ψ 5 k1 α; ypI 5 ρ 5 k2 p; ydK 5 ρ K 5 k3 dK ; 5 Eðd; hÞ:ðe 2 ep Þ; ypK 5 ρ e @e @α @p @d

ydI 5 ρ

@Ψ @Ψ @Ψ 5 k4 dI ; yhK 5 ρ K 5 2 k5 hK and yhI 5 ρ I 5 2 k6 hI : @dI @h @h (14.52)

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14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

To derive the evolution equations for the internal variables associated with plasticity, damage and healing, the power of dissipation, Eq. (14.45), is rewritten in terms of the Helmholtz free energy Ψ, Eq. (14.52). Thus the power of dissipation reads: K I K I _ 1 ypI :p_ 1 yd :d_ 1 ydK :d_ 1 ydI :d_ 1 yh :h_ 1 yhK :h_ 1 yhI :h_ Γ 5 σ:_ep 1 ypK :α |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Γp

Γd

(14.53)

Γh

All three plasticity Γp , damage Γd , and healing Γh dissipative power influence each other, through interrelated plasticity, damage and healing mechanisms via their corresponding conjugate forces. To derive explicit expressions between internal variables and associated conjugate forces (flow rules) the complementary formalism of the dissipation processes is required. The Legendre transformation is used to obtain the conjugate dissipative potentials as a function of the conjugate forces [16,18]. Using the Lagrangian multipliers, the final form of the functional γ  to be extermized is given by: γ  5 Γ 2 λ_ Fp 2 λ_ Fd 2 λ_ Fh p

d

h

(14.54)

where three plastic Fp , damage Fd , and healing Fh potentials are assumed to describe each process separately. While these potentials are decoupled for each process, and there is no any explicit coupling between these potentials, implicit couplings exist between conjugate forces and all potential functions are coupled with each other through plasticity, damage and healing internal variables. Necessary conditions @γ  =@σ 5 0, @γ  =@yd 5 0, and @γ  =@yh 5 0 are applied to minimize the functional Eq. (14.54) and they result in the following coupled evolution equations for the plastic strain, damage and healing variables [16,18]: p @F d @F h @F e_ p 5 λ_ 1 λ_ 1 λ_ ; @σ @σ @σ p

d

h

p @F d @F h @F d_ 5 λ_ 1 λ_ 1 λ_ ; @yd @yd @yd p

d

h

(14.55)

p @F d @F h @F h_ 5 λ_ 1 λ_ 1 λ_ : @yh @yh @yh p

d

h

These evolution equations hold when consistency conditions for each respective process p d are satisfied. They are Fp 5 0 and F_ 5 0 for plasticity, Fd 5 0 and F_ 5 0 for damage, and h Fh 5 0 and F_ 5 0 for healing process. In the case that one of them does not hold the corresponding evolution law and variable are eliminated. To simplify the problem it is assumed that there is no coupling between plasticity, damage, and healing processes for deriving the hardening variables. The remaining evolution laws for the internal variables are obtained by applying the flow rule as follows: p p d d p @F _ p @F ; d_ K 5 2 λ_ d @F ; d_I 5 2 λ_ d @F ; _ _ 5 2 λ_ α ; p 5 2 λ @ypK @ypI @ydK @ydI h h K h @F _I 5 2 λ_ h @F h_ 5 2 λ_ ; h @yhK @yhI

Self-Healing Polymer-Based Systems

(14.56)

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Thus the problem is simplified to defining proper potential functions for the plasticity Fp , damage Fd , and healing Fh events. There are two common approaches in thermodynamics of solids for defining these potentials and establishing the flow rules. Those are (1) associative flow rules: the potential functions are the same as initiation criterion for each process and (2) nonassociative flow rules: in which the potential functions need to be defined separately. Some examples of associative and nonassociative flow rules are practiced by [16,18].

14.5.2 Mechanisms-based phenomenological healing models Among many of the self-healing systems, we address here a bioinspired healing mechanism which is called CTH [23,33,34,55]. In the CTH mechanism, the cracks are closed through the constrained shape recovery of the SMP matrix/fiber, or SMA fibers, or polymeric artificial muscles, where external triggers, for example, heat, activate the shape recovery process. Once the crack is closed different healing mechanisms such as liquid healing agent [27,29,53], or solid healing agent such as molten TPs [30,89,90] may be incorporated to obtain a molecular level healed configuration. Most recently [23,40,55] proposed a new biomimetic self-healing system, which includes cold-drawn SMP fibers as grid skeleton (or unidirectional fibers or chopped fibers) and conventional thermosetting polymer as matrix. In the self-healing systems with embedded solid healing agent, the molten healing phase, for example, TP will diffuse into the polymer material and provides molecular level healing by regenerating of covalent bonds or physical entanglement. The main concerns to be addressed in this work are the role of the internal stresses, due to the SM process; and, the effect of the TP molecular level properties, for example, molecular weight and viscosity, volume content, and interaction with SMP molecules in the diffusion process. In general the induced SM internal stresses will help to seal the fractured surfaces and acts as an external driving force to improve the diffusion process of TP particles into the matrix. The content of the TP will affect the concentration gradient, which is basically another driving force in the self-healing diffusion process. As already discussed, the healing process can be calibrated with respect to an appropriate healing parameter which relates some ME properties in the healed state to the damaged state. For example, fracture toughness, failure stress/strain, fatigue life, impact resistance in a damaged material state is less than its virgin values. During a healing process the deteriorated material properties, ζ di , will be gradually recovered toward their virgin values, ζ i : ζ di -ζ i with i 5 1; 2; . . .; n

(14.57)

where the relation between damaged ζ di and undamaged ζ i configurations is given based on damage parameter d: ζ di 5 ð12dÞ2 ζ i

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(14.58)

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14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

Now let us assume the healing process recovers the damaged properties of the material system, that is, ζ di , to a healed state, that is, ζ hi . Then one may define the relation between damaged and healed material parameters as: ζ hi 5 R 3 ζ di

(14.59)

where R is a recovery function which depends on the time, t, temperature, T, pressure, P, of the healing process, and also on the microstructure of the diffused healing agent including molecular weight, and viscosity. Fig. 14.12 depicts a composite which is constituted from a thermosetting SMP matrix and a dispersed phase of TPs as solid healing agents. Fig. 14.12A shows the generated cross-links and physical entanglements between molecular chains of these two phases. On applying deformation gradient, F, the molecular configuration stretches and rotates to the elastoplastically deformed state, Fig. 14.12B. The total applied deformation then can be multiplicatively decomposed into elastic, F e , and plastic, F p , deformation gradient tensors. Fig. 14.12C represents the damaged state of molecular configuration in which the molecular chains are stretched beyond their failure limits. The red lines in Fig. 14.12C indicate failed molecular chains. The damage deformation gradient, F d , captures the deformation associated with the damaging process. The CTH molecular level healing is then classified to: (1) bringing the failed molecular chains in contact with each other and (2) forming Covalent bonds or physical entanglement between those two molecular chains. Fig. 14.13B represents the approaching step (i), where the SM deformation gradient, F s , is applied; and Fig. 14.13C shows the healing step (ii) with the associated deformations healing gradient, F h . As shown in Fig. 14.13B, the diffusion depth of the TP molecules into the SMP network is represented by χ and it saturates to an asymptotic value of χN ultimately in Fig. 14.13C. Although, Figs. 14.12 and 14.13 provide a molecular length scale view of the healing steps; a microscale point of view is required for describing more complex phenomena involved in the healing process of self-healing polymers. Fig. 14.14 represents a microscopic view of a cracked SMP sample. The microscale SMP constituent phase is shown by

FIGURE 14.12 Molecular level damaging process in a particulate self-healing polymer: (A) intact molecular configuration, (B) elastoplastically stretched and rotated state, and (C) elastoplastic deformed and damaged state of the molecular configuration.

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FIGURE 14.13 Molecular level healing process: (A) elastoplastic deformed and damaged configuration, (B) state of the molecule after application of the shape recovery induced deformation gradient, Fs, and (C) healed configuration. Fh is the healing associated deformation gradient, the parameter χ denotes the diffusion penetrated distance, and χN stands for equilibrium diffusion distance.

FIGURE 14.14

Microstructural length scale view of the close-then-heal process, (A) cracked surfaces, (B) closed crack surfaces in which the SMP is activated and applies shape memory pressure, Ps, (C) diffused TP phase into opposite fractured areas, and (D) healed surface after lowering the temperature.

solid regions and TP healing agents are marked with hatched regions. Color changes indicate the temperature variations. Speaking in general, the healing in microscale is classified into: (1) SM induced surface approaching, (2) wetting of fractured surfaces at T . Th by molten TPs, (3) diffusion of the healing agent into the matrix, and (4) formation of physical

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14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

entanglements between matrix and healing agent. Fig. 14.14A shows the state of an open cracked area at room temperature. In Fig. 14.14B the activation temperature of the SMP matrix is met, that is, T . Ts , and the shape recovery of the SMP is activated. The shape recovery pressure, Ps, closes the open area and improves the wetting and diffusion processes. On further heating the TPs are molten, that is, T . Th , and they can diffuse into the SMP matrix. Fig. 14.14D shows the healed configuration in which the diffused TPs have formed ME entanglements and provided a molecular level healing. Due to the fact that the original molecular entanglements between SMP matrix and healing agent will not be regenerated on healing, and only newly formed TP-TP and/or TP-SMP bonds will contribute to the healed fracture toughness. It is expected that healed fracture toughness, Σh , be less than virgin toughness, Σ. However, as discussed by [35] only a small TP content, for example, 3%, can recover up to 65% of the Σ. The wetting and diffusion stages will dominantly determine the healing efficiency in which the ME properties are recovered via the intrinsic healing function, R. The SM induced pressure, Ps , has a major role in this process. Thus our task is to correlate the magnitude of the recoverable properties to Ph (which is assumed to be equal to Ps here), Th and th in such a way that when χ approaches χN and wetting process saturates to its maximum value, the healing function R reaches its limiting value: R-Rmax

(14.60)

Consequently there are two major events, which need to be considered in derivation of the intrinsic healing function, R: (1) wetting of the fractured surface by molten TPs which forms adhesive bonds on cooling, and (2) diffusion of TPs into the SMP matrix which provides molecular interlocks between the two phases. From a micromechanics point of view, the wetting process can be contributed to the volume fractions of the available TPs on the cracked surfaces and the diffusion process can be linked to the SMP matrix volume fraction. Thus the healing efficiency can be described based on molecular level mechanisms that are involved in the healing process. Most recently the multiscale healing model, developed by Shojaei et al. has been implemented into and standard FEA package via user-defined materials. They demonstrated the performance of the model under a TM cycle, where activated SMP sample undergoes wetting and diffusion processes on contact of fractured surfaces Shojaei et al.

14.6 Concluding remarks Bioinspired healing schemes are studied in this chapter and modeling techniques for simulation of elastic, viscoplastic, damage, and healing mechanisms in self-healing material are revisited. The large deformation kinematics is utilized to develop the elastoplastic constitutive relations. A mechanism-based visoplasticity model is also presented for capturing the rate- and temperature-dependent TM responses of polymers including SMPs. The concept of continuum damage and healing mechanics is elaborated in which scalar damage/healing parameters are developed for the case of isotropic damage and healing; and tensorial damage/healing parameters are developed for the case of anisotropic damage and healing processes. The thermodynamic consistent and mechanisms based healing

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References

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modeling approaches are also presented. The proposed CDHM and thermodynamics frameworks are applicable to all healing systems including reversible cross-linking polymer-based self-healing materials, ionomeric polymer-based self-healing materials, supramolecular network-based self-healing polymer materials, microcapsule-based self-healing materials, microvascular-based self-healing materials, and novel self-healing material systems. However, providing physically consistent molecular level descriptions for the damage and healing events remain a grand challenge in the research community. It is envisioned that by increasing the computational power in the near future molecular mechanics/dynamics simulations can provide more insight into the healing mechanisms. Also linking the discrete and continuum levels is another challenge that should be overcome for using molecular level simulations. Before overcoming these barriers, phenomenological- and thermodynamic-based continuum models remain the most reliable tools for the material designers. The proposed modeling frameworks in this chapter may assist designers in predicting the strength, healing efficiency, and life of the self-healing structures.

References [1] D.N. Theodorou, U.W. Suter, Detailed molecular structure of a vinyl polymer glass, Macromolecules 18 (1985) 1467
1478. [2] D.N. Theodorou, U.W. Suter, Atomistic modeling of mechanical properties of polymeric glasses, Macromolecules 19 (1986) 139
154. [3] J. Fish, Practical Multiscaling, John Wiley & Sons, UK, 2013. [4] V. Kafka, Shape memory polymers: a mesoscale model of the internal mechanism leading to the SM phenomena, Int. J. Plast. 24 (2008) 1533
1548. [5] V. Kafka, D. Vokoun, On backstresses, overstresses, and internal stresses represented on the mesoscale, Int. J. Plast. 21 (2005) 1461
1480. [6] C. Regrain, L. Laiarinandrasana, S. Toillon, K. Saı¨, Multi-mechanism models for semi-crystalline polymer: constitutive relations and finite element implementation, Int. J. Plast. 25 (2009) 1253
1279. [7] M. Uchida, N. Tada, Micro-, meso- to macroscopic modeling of deformation behavior of semi-crystalline polymer, Int. J. Plast. 49 (2013) 164
184. [8] M. Uchida, N. Tada, Sequential evaluation of continuous deformation field of semi-crystalline polymers during tensile deformation accompanied by neck propagation, Int. J. Plast. 27 (2011) 2085
2102. [9] R.B. Dupaix, M.C. Boyce, Constitutive modeling of the finite strain behavior of amorphous polymers in and above the glass transition, Mech. Mater. 39 (2007) 39
52. [10] E. Krempl, F. Khan, Rate (time)-dependent deformation behavior: an overview of some properties of metals and solid polymers, Int. J. Plast. 19 (2003) 1069
1095. [11] G. Li, W. Xu, Thermomechanical behavior of thermoset shape memory polymer programmed by cold-compression: testing and constitutive modeling, J. Mech. Phys. Solids 59 (2011) 1231
1250. [12] H.J. Qi, M.C. Boyce, Stress
strain behavior of thermoplastic polyurethanes, Mech. Mater. 37 (2005) 817
839. [13] A. Shojaei, G. Li, Thermomechanical constitutive modeling of shape memory polymer including continuum functional and mechanical damage effects, Proc. R. Soc. A Math. Phys. Eng. Sci. 470 (2170) (2014). paper number 20140199. [14] Q. Yang, G. Li, Temperature and rate dependent thermomechanical modeling of shape memory polymers with physics based phase evolution law, Int. J. Plast. 80 (2016) 168
186. [15] S. Govindjee, J. Simo, A micro-mechanically based continuum damage model for carbon black-filled rubbers incorporating Mullins’ effect, J. Mech. Phys. Solids 39 (1991). 87-1. [16] G.Z. Voyiadjis, A. Shojaei, G. Li, A thermodynamic consistent damage and healing model for self healing materials, Int. J. Plast. 27 (2011) 1025
1044. [17] G.Z. Voyiadjis, A. Shojaei, G. Li, A generalized coupled viscoplastic-viscodamage-viscohealing theory for glassy polymers, Int. J. Plast. 28 (2012) 21
45.

Self-Healing Polymer-Based Systems

452

14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

[18] G.Z. Voyiadjis, A. Shojaei, G. Li, P. Kattan, Continuum damage-healing mechanics with introduction to new healing variables, Int. J. Damage Mech. 21 (2011) 391
414. [19] W. Xu, G. Li, Constitutive modeling of shape memory polymer based self-healing syntactic foam, Int. J. Solids Struct. 47 (2010) 1306
1316. [20] S. Ahzi, D.M. Park, A.S. Argon, Estimats of the overall elastic properties in semi-crystalline polymers, in: AMD—Vol. 203, Current Research in the Thermo-Mechanics of Polymers in the Rubbery-Glassy Range; 1995, pp. 31
44, American Society of Mechanical, ISBN-10: 0791813185, ISBN-13: 978-0791813188. [21] B.J. Lee, D.M. Parks, S. Ahzi, Micromechanical modeling of large plastic deformation and texture evolution in semi-crystalline polymers, J. Mech. Phys. Solids 41 (1993) 1651
1687. [22] A. Shojaei, G. Li, Viscoplasticity analysis of semicrystalline polymers: a multiscale approach within micromechanics framework, Int. J. Plast. 42 (2013) 31
49. [23] A. Shojaei, G. Li, G.Z. Voyiadjis, Cyclic viscoplastic-viscodamage analysis of shape memory polymers fibers with application to self-healing smart materials, J. Appl. Mech. 80 (2013) 011014. [24] A. Cauvin, R.B. Testa, Damage mechanics: basic variables in continuum theories, Int. J. Solids Struct. 36 (1999) 747
761. [25] J. Lemaitre, How to use damage mechanics, Nucl. Eng. Des. 80 (1984) 233
245. [26] G. Li, Self-Healing Composites: Shape Memory Polymer Based Structures, John Wiley & Sons, Inc., West Sussex, UK, 2014. [27] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794
797. [28] J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791
1799. [29] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (2007) 581
585. [30] J. Nji, G. Li, A biomimic shape memory polymer based self-healing particulate composite, Polymer 51 (2010) 6021
6029. [31] M. John, G. Li, Self-healing of sandwich structures with grid stiffened shape memory polymer syntactic foam core, Smart Mater. Struct. 19 (2010). paper number 075013. [32] G. Li, M. John, A self-healing smart syntactic foam under multiple impacts, Compos. Sci. Technol. 68 (2008) 3337
3343. [33] G. Li, D. Nettles, Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam, Polymer 51 (2010) 755
762. [34] G. Li, N. Uppu, Shape memory polymer based self-healing syntactic foam: 3-D confined thermomechanical characterization, Compos. Sci. Technol. 70 (2010) 1419
1427. [35] J. Nji, G. Li, Damage healing ability of a shape-memory-polymer-based particulate composite with small thermoplastic contents, Smart Mater. Struct. 21 (2012) 025011. [36] P. Zhang, D.J. Arceneaux, Z. Liu, P. Nikaeen, A. Khattab, G. Li, A crack healable syntactic foam reinforced by 3D printed healing-agent based honeycomb, Compos. Part B: Eng. 151 (2018) 25
34. [37] J. Champagne, S.S. Pang, G. Li, Effects of partial confinement and local heating on healing efficiencies of selfhealing particulate composites, Composites Part B: Eng. 97 (2016) 344
352. [38] G. Li, O. Ajisafe, H. Meng, Effect of strain hardening of shape memory polymer fibers on healing efficiency of thermosetting polymer composites, Polymer 54 (2013) 920
928. [39] G. Li, H. Meng, J. Hu, Healable thermoset polymer composite embedded with stimuli-responsive fibers, J. R. Soc. Interface 9 (2012) 3279
3287. [40] G. Li, P. Zhang, A self-healing particulate composite reinforced with strain hardened short shape memory polymer fibers, Polymer 54 (2013) 5075
5086. [41] P. Zhang, B. Ogunmekan, S. Ibekwe, D. Jerro, S.S. Pang, G. Li, Healing of shape memory polyurethane fiber reinforced syntactic foam subjected to tensile stress, J. Intell. Mater. Syst. Struct. 27 (2016) 1792
1801. [42] P. Zhang, U. Ayaugbokor, S. Ibekwe, D. Jerro, S.S. Pang, P. Mensah, et al., Healing of polymeric artificial muscle reinforced ionomer composite by resistive heating, J. Appl. Polym. Sci. 133 (2016). Paper number 43660. [43] P. Zhang, G. Li, Healing-on-demand composites based on polymer artificial muscle, Polymer 64 (2015) 29
38. [44] P. Zhang, G. Li, Fishing line artificial muscle reinforced composite for impact mitigation and on-demand damage healing, J. Composite Mater. 50 (2016) 4235
4249.

Self-Healing Polymer-Based Systems

References

453

[45] T.A. Plaisted, S. Nemat-Nasser, Quantitative evaluation of fracture, healing and re-healing of a reversibly cross-linked polymer, Acta Mater. 55 (2007) 5684
5696. [46] R.J. Varley, S. van der Zwaag, Towards an understanding of thermally activated self-healing of an ionomer system during ballistic penetration, Acta Mater. 56 (2008) 5737
5750. [47] Y.L. Liu, Y.W. Chen, Thermally reversible cross-linked polyamides with high toughness and self-repairing ability from maleimide- and furan-functionalized aromatic polyamides, Macromol. Chem. Phys. 208 (2007) 224
232. [48] A. Li, J. Fan, G. Li, Recyclable thermoset shape memory polymer with high stress and energy output via facile UV-curing, J. Mater. Chem. A 6 (2018) 11479
11487. [49] L. Lu, J. Pan, G. Li, Recyclable high performance epoxy based on transesterification reaction, J. Mater. Chem. A 5 (2017) 21505
21513. [50] Q. Shi, K. Yu, M.L. Dunn, T. Wang, H.J. Qi, Solvent assisted pressure-free surface welding and reprocessing of malleable epoxy polymers, Macromolecules 49 (2016) 5527
5537. [51] K. Yu, Q. Shi, H. Li, J. Jabour, H. Yang, M.L. Dunn, et al., Interfacial welding of dynamic covalent network polymers, J. Mech. Phys. Solids 94 (2016) 1
17. [52] L. Lu, J. Fan, G. Li, Intrinsic healable and recyclable thermoset epoxy based on shape memory effect and transesterification reaction, Polymer 105 (2016) 10
18. [53] E.L. Kirkby, V.J. Michaud, J.A.E. Ma˚nson, N.R. Sottos, S.R. White, Performance of self-healing epoxy with microencapsulated healing agent and shape memory alloy wires, Polymer 50 (2009) 5533
5538. [54] S.R. White, J.S. Moore, N.R. Sottos, B.P. Krull, W.A. Santa Cruz, R.C.R. Gergely, Restoration of large damage volumes in polymers, Science 344 (2014) 620
623. [55] G. Li, A. Shojaei, A viscoplastic theory of shape memory polymer fibres with application to self-healing materials, Proc. R. Soc. A: Math. Phys. Eng. Sci. 468 (2012) 2319
2346. [56] E.J. Barbero, F. Greco, P. Lonetti, Continuum damage-healing mechanics with application to self-healing composites, Int. J. Damage Mech. 14 (2005) 51
81. [57] G.Z. Voyiadjis, A. Shojaei, G. Li, P.I. Kattan, A theory of anisotropic healing and damage mechanics of materials, Proc. R. Soc. A: Math. Phys. Eng. Sci. 468 (2012) 163
183. [58] S. Nemat-Nasser, Plasticity: A Treatise on Finite Deformation of Heterogeneous Inelastic Materials, Cambridge University Press, Cambridge, 2006. [59] A. Shojaei, G.Z. Voyiadjis, P.J. Tan, Viscoplastic constitutive theory for brittle to ductile damage in polycrystalline materials under dynamic loading, Int. J. Plast. 48 (2013) 125
151. [60] J.C. Simo, T.J.R. Hughes, Computational Inelasticity, Springer, New York, 1997. [61] J.C. Simo, K.S. Pister, Remarks on rate constitutive equations for finite deformation problems: computational implications, Comput. Methods Appl. Mech. Eng. 46 (1984) 201
215. [62] A.E. Green, P.M. Naghdi, Some remarks on elastic-plastic deformation at finite strain, Int. J. Eng. Sci. 9 (1971) 1219
1229. [63] P.M. Naghdi, J.A. Trapp, Restrictions on constitutive equations of finitely deformed elastic-plastic materials, Q. J. Mech. Appl. Math. 28 (1975) 25
46. [64] C. G’Sell, J.M. Hiver, A. Dahoun, Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking, Int. J. Solids Struct. 39 (2002) 3857
3872. [65] M.B.M. Mangion, J.Y. Cavaille´, J. Perez, A molecular theory for the sub-Tg plastic mechanical response of amorphous polymers, Philos. Mag. A 66 (1992) 773
796. [66] E.M. Arruda, M.C. Boyce, Evolution of plastic anisotropy in amorphous polymers during finite straining, Int. J. Plast. 9 (1993) 697
720. [67] E.M. Arruda, M.C. Boyce, A three-dimensional constitutive model for the large stretch behavior of rubber elastic materials, J. Mech. Phys. Solids 41 (1993). 389-4. [68] E.M. Arruda, M.C. Boyce, H. Quintus-Bosz, Effects of initial anisotropy on the finite strain deformation behavior of glassy polymers, Int. J. Plast. 9 (1993) 783
811. [69] M.C. Boyce, E.M. Arruda, An experimental and anaiytical investigation of the large strain compressive and tensile response of glassy polymers, Polym. Eng. Sci. 30 (1990) 1288
1298. [70] G.R. Johnson, W.H. Cook, Fracture characteristics of three metals subjected to various strains, strain rates, temperatures and pressures, Eng. Fract. Mech. 21 (1985) 31
48. [71] D.J. Steinberg, Equation of State and Strength Properties of selected Materials, Tech. Rep. UCRL-MA-106439, Lawrance Livermore National Labratory, Livermore, CA; 1996.

Self-Healing Polymer-Based Systems

454

14. A continuum mechanics approach to the healing efficiency of extrinsic self-healing polymers

[72] M.K. Darabi, R.K. Abu Al-Rub, D.N. Little, A continuum damage mechanics framework for modeling microdamage healing, Int. J. Solids Struct. 49 (2012) 492
513. [73] J. Mergheim, P. Steinmann, Phenomenological modelling of self-healing polymers based on integrated healing agents, Comput. Mech. 52 (2013) 681
692. [74] G. Voyiadjis, P.I. Kattan, Healing and super healing in continuum damage mechanics, Int. J. Damage Mech. 21 (3) (2013) 391
414. [75] L.M. Kachanov, Rupture time under creep conditions, Izvestija Academii Nauk. SSSR 8 (1958) 26
31 (Reprinted in Int. J. Fract., 97, 11-18). [76] J. Lemaitre, J.L. Chaboche, Mechanics of Solid Materials, Cambridge University Press, Cambridge, 1990. [77] Z. Voyiadjis, P.I. Kattan, Advances in Damage Mechanics, Elsevier, London, 2006. [78] J. Lemaitre, J. Dufailly, Damage measurements, Eng. Fract. Mech. 28 (1987) 643
661. [79] G.Z. Voyiadjis, P.I. Kattan, Mechanics of small damage in fiber-reinforced composite materials, Composite Struct. 92 (2010) 2187
2193. [80] G.Z. Voyiadjis, P.I. Kattan, A comparative study of damage variables in continuum damage mechanics, Int. J. Damage Mech. 18 (2009) 315
340. [81] S. Murakami, Mechanical modeling of material damage, J. Appl. Mech. 55 (1988) 280
286. [82] A. Shojaei, A. Taleghani, G. Li, A continuum damage failure model for hydraulic fracturing of porous rocks, Int. J. Plast. 59 (2014) 199
212. [83] J. Lubliner, On the thermodynamic foundations of non-linear solid mechanics, Int. J. Non-Linear Mech. 7 (1972) 237
254. [84] N.R. Hansen, H.L. Schreyer, A thermodynamically consistent framework for theories of elastoplasticity coupled with damage, Int. J. Solids Struct. 31 (1994) 359
389. [85] G. Thiagarajan, G.Z. Voyiadjis, Directionally constrained viscoplasticity for metal matrix composites, J. Aerosp. Eng. 13 (2000) 92
99. [86] G.Z. Voyiadjis, M. Foroozesh, Anisotropic distortional yield model, J. Appl. Mech. 57 (1990) 537
547. [87] G.Z. Voyiadjis, G. Thiagarajan, An anisotropic yield surface model for directionally reinforced metal-matrix composites, Int. J. Plast. 11 (1995) 867
894. [88] J.L. Chaboche, Thermodynamic formulation of constitutive equations and application to the viscoplasticity and viscoelasticity of metals and polymers, Int. J. Solids Struct. 34 (1997) 2239
2254. [89] J. Nji, G. Li, A self-healing 3D woven fabric reinforced shape memory polymer composite for impact mitigation, Smart Mater. Struct. 19 (2010) 035007. [90] M. Zako, N. Takano, Intelligent material systems using epoxy particles to repair microcracks and delamination damage in GFRP, J. Intell. Mater. Syst. Struct. 10 (1999) 836
841.

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C H A P T E R

15 Self-healing fiber-reinforced polymer composites for their potential structural applications Nazrul Islam Khan1,2 and Sudipta Halder1,3 1

Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar, India 2Department of Mechanical Engineering, GMRIT, Srikakulam, India 3Department of Civil, Construction and Environmental Engineering, The University of Alabama Engineering, Tuscaloosa, AL, United States

15.1 Introduction Self-healing materials are polymers, metals, ceramics, and their composites that when damaged by an operational use has the ability to fully or partially recover its original set of properties. Self-healing is a bioinspired technology which can heal micro- or nanolevel cracks generated in polymeric composites without any external interventions. In the present scenario, fiber-reinforced polymer (FRP) composites are being used in wide range of applications such as automobile, aerospace, energy, construction, and sporting equipment applications because of their high strength and stiffness-to-weight ratio, excellent corrosion resistance, and high thermal stability [1 5]. Delamination occurring between the interply region and poor fracture toughness of matrix is the main failure mode occurring in FRP composites [5 7]. Delamination of FRP composites occurs due to micro- or nanolevel cracks generated during in service period which propagates and finally led to catastrophic failure of the material. The cracks are hardly detectable and can be repaired. Therefore a huge demand is originated for self-healing of the micro- or nanolevel cracks generated in FRP-laminated composites at their nascent stage where repairing is too much challengeable. The concept of self-healing polymeric materials was proposed in 1980s with the advent concept of healing invisible micro/nanocracks to increase the service life of polymer composites [8]. Mainly two approaches have been adopted to repair internal cracks: (1)

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incorporation of healing agent externally in the polymeric matrix also called extrinsic healing approach and (2) development of special intrinsic chemical bonds that can be rebonded or reversible after being broken in certain conditions also called intrinsic healing approach [9 13]. The concept of extrinsic self-healing is based on incorporation of microcapsules [14 18] and microvascular or hollow channel [19 22] containing healing agent in the polymer matrix which can release them on the crack surface and heals the matrix by self-polymerization. Disadvantage of this concept is single-time healing and synthesis complexities associated with it [23]. The second concept of intrinsic healing is based on introduction of reversible bonds such as supramolecular bonds [24,25], [2 1 2] cycloaddition reaction [26], host guest chemistry-based self-healing [27], and thermoreversible Diels Alder (DA) bonds [28,29]. Among these concepts, self-healing based on thermoreversible DA chemistry in polymer composites is found as the best choice of healing because of their multiple times healing capability, better mechanical properties, and compatibility with the commercially available different epoxy resin systems [28,30]. Moreover, DA reaction between diene and dienophile form an efficient thermoreversible bond which can be effectively used for designing self-healing polymer composites [31,32]. Self-healing DA adduct can be incorporated either in the polymer matrix or on the surface of nanofillers and finally reinforces them in the matrix [33 36]. Different self-healing concepts such as microencapsulation technique [37], microvascular technique [38], and mendable polymer technique [39] are being used in FRP composites to repair the microcracks generated in the FRPs, thereby inhabit its further propagation called delamination. Most of these techniques reduce the mechanical performance of the composites and very difficult for their industrialization. Graphene having 2D honeycomblike nanostructure has been found as an effective filler to improve mechanical performance of hybrid FRP composites by improving matrix fracture toughness and interfacial adhesion between fiber and matrix [40 43]. If self-healing DA adduct is attached on the surface of graphene which can form covalent thermoreversible bond with the DA-based matrix system when incorporated in them, we can expect better mechanical and interfacial healing from the resulting polymeric composites. In this chapter, brief introduction on the scope of self-healing in FRP composites and different self-healing approaches has been covered. The use of extrinsic and intrinsic selfhealing approaches, their advantages and limitations have also been covered in this chapter. The different test methods to assess the healing behavior have also been discussed in this chapter.

15.2 Scope of self-healing in fiber-reinforced polymer composites FRP is a composite material prepared with a polymer matrix reinforced with fibers as shown in Fig. 15.1. The fibers generally used for the preparation FRP composites are glass fiber, carbon fiber, aramid or basalt, and matrix materials used are polyesters, vinylesters, epoxies, bismaleimides, and polyamides. The most commonly used fabricating methods of FRP laminates are resin transfer molding, vacuum-assisted resin transfer molding (VARTM), injection double VARTM, flow flooding chamber method, hand lay-up, and hot pressing method.

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15.2 Scope of self-healing in fiber-reinforced polymer composites

Among the different fibers, glass fiber, carbon fiber, and aramid fibers are mostly used for fabrication of FRP composites. Each type of fibers possess different properties and hence used for different applications. The typical tensile properties of carbon, glass, and aramid fibers are shown in Table 15.1. However, when we compare the fibers in their different nine aspects such as mechanical, thermal, and economic analyses and set them in the scale of best, moderate, and good as shown in Table 15.2, we observe that of the nine, six properties are found best in case of carbon fiber-reinforced polymer (CFRP) composites. As already mentioned, CFRP composites are gaining tremendous interests in automobile industry because of their high strength and stiffness-to-weight ratio. In automobile industry, the major factor that governs the design parameter is the light weight of the component which can increase the fuel efficiency. Moreover, to reduce carbon emission, demand of electric vehicle is increasing in the globe. To maximize the life time of the battery, the automobile industry such as BMW is replacing different heavy components with CFRP composites as shown in Fig. 15.2. It has been reported in composite reports that in FIGURE 15.1 Components of FRP showing polymer matrix and carbon fiber. FRP, Fiber-reinforced polymer.

TABLE 15.1 Comparison of the physical properties of commonly used grades of carbon fiber, glass fiber, and aramid fiber [44]. Property

Carbon fiber (T300)

Glass fiber (S-2)

Aramid fiber (49)

Density (g cm23)

1.76

2.46

1.45

230

86.9

112

Specific tensile modulus (GPa cm g )

131

35.3

77.2

Tensile strength (GPa)

3.53

4.89

3.00

Specific tensile strength (GPa cm g )

2.010

1.990

2.070

Tensile strain (ductility) (%)

1.5

5.7

2.4

Compressive strength (GPa)

0.87

1.60

Tensile modulus (GPa) 3

3

26

CTE (axial, 10

21

K )

21

21

2 0.41

2.9

CTE, Coefficient of thermal expansion.

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15. Self-healing fiber-reinforced polymer composites for their potential structural applications

TABLE 15.2 Depicting comparison of mechanical, thermal, and economical analysis of aramid, carbon, and glass fiber. S. no.

Property

Aramid

Carbon

Glass

1

Tensile strength

B

A

B

2

Compressive strength

C

A

C

3

Flexural strength

C

A

B

4

Impact strength

A

C

B

5

ILSS

B

A

A

6

Fatigue resistance

B

A

C

7

Thermal insulation

A

C

B

8

Low thermal expansion

A

A

A

9

Low cost

C

C

A

A, best; B, moderate; C, good.

FIGURE 15.2 Different components of BMW i3 electric car made of CFRP composites. CFRP, Carbon fiberreinforced polymer. Source: Different Google images assembled together.

2010, the demand of CFRP across globe was 51,000 ton which is increasing exponentially and by 2022, the demand will be around 194,000 ton as shown in Fig. 15.3. The overall performance of the FRP composites is governed by the load-bearing capacity of fiber and matrix and load transferring capability between them. But, fracture

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toughness of single component fiber-reinforced composites is very poor. Moreover, the interfacial interaction between fiber and matrix is very poor because of the chemical inert surface of the fibers. The crack generally propagates through the interfacial zone between fiber and matrix (as shown in Fig. 15.4) as this is the weakest zone in FRP composites. The poor interfacial interaction between fiber and matrix accelerates fiber matrix debonding and led to defused intralaminar damage [45 47]. In addition to this, delamination or ply to ply separation is the most common crack growth mode which results in acute depression in in-plane strength and stiffness [48]. All these drawbacks led to a catastrophic failure of the whole FRP laminates. To improve the delamination resistance behavior of the FRP laminates, several techniques have been introduced such as stitching of fiber, Z pinning, designing 3D fabric design [49], fiber surface modification [50,51], and

FIGURE 15.3

Global demand of CFRP in 1000 ton reported by carbon composites. CFRP, Carbon fiberreinforced polymer.

FIGURE 15.4

Schematic showing interfacial zone between fiber and matrix.

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matrix modifications. The crack generation through the interfacial zone between fiber and matrix of FRP composites is very hard to detect, but it can be healed at their nascent stage to restrict their further propagation. The restriction of the crack propagation will indirectly restrict the delamination occurring in the FRP-laminated composites, thereby increase the durability of the FRP composites. Thus development of healing technique has tremendous scope for their application in FRP composites. Traditionally when a damage has been detected or visible in an FRP composites, this is repaired by simplistic external patches. However, this technique of repair is very difficult and costly to apply. Moreover, the internal defects are hardly detectable and repairable. Therefore engineers mostly design the FRPs so that it has the ability to regain strength in presence of defects by adding some extra weight and can sustain for some stipulated time period. Thus the demand of self-repairing in FRP composites is increasing day by day. The internal and undetectable defects can be easily repaired by incorporating self-healing agents (SHAs) within intra- or interply region of the FRP composites. The self-healing approach is mainly categorized as extrinsic self-healing using microcapsules, hollow fibers, and microvascular network and intrinsic self-healing approach using some ionic bonding or thermoreversible bonding as explained below.

15.3 Extrinsic self-healing approaches for fiber-reinforced polymer composites The extrinsic self-healing as already explained is based on some healing agent container which may be solid particles, microcapsules, hollow fibers, or microvascular agents which are embedded in FRP composites externally. Hayes et al. [52,53] incorporated thermoplastic solid-state healing agent in the matrix of an E-glass fiber-based laminated composites which were capable to recover fracture by 50% 70% when tested by Charpy impact test. But, this glass fiber-reinforced polymeric (GFRP) composites showed decrease in healing efficiency to around 30% when tested to investigate repeated healing behavior. The extrinsic self-healing approaches adopted for healing of FRP composites are categorized as microcapsule-based self-healing, hollow fiber-based self-healing, and microvascular-based self-healing as discussed subsequently.

15.3.1 Microcapsule-based self-healing Self-healing of FRP composites through microencapsulation involves the encapsulation of the healing agent which is mostly monomer with a brittle and thin-walled shell and then dispersion within the polymer matrix. When the crack propagates through the intraply or interply region of the FRP composites, the capsule gets broken and then release the monomer into the damage site. A series of publications on microcapsule-based self-healing approaches have been demonstrated by the pioneer White group [54]. The microcapsulebased healing of GFRP composites is first demonstrated by Kessler et al. [55,56], with lot of difficulties such as dispersion and reduction in volume fraction of fiber which has great impact on the mechanical and healing properties of the laminated composites. Despite the challenges, about 45% healing efficiencies at ambient temperature to 80% at 80 C were

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successfully demonstrated when tested with tapered double-cantilever beam (TDCB). After that, further research on microcapsule-based self-healing of FRP composites was demonstrated by decreasing the size of microcapsule and improving dispersion in the polymer matrix already containing the catalyst of the monomer. Later, Yin et al. [57,58] showed a new approach of microcapsule-based self-healing of GFRP composites using epoxy monomer within urea formaldehyde capsules of typically of 30 70 μm size. The FRP composites showed B70% healing efficiency when tested with fracture toughness and healed at 130 C for 1 h. Jones et al. [59] dispersed reactive epoxy resin on the surface of carbon fiber as shown in Fig. 15.5 and investigated the healing behavior from interfacial shear strength (IFSS) of the resulting CFRP composites. They found B91% recovery of IFSS from the resulting composites. However, the microcapsulebased healing in FRP composites showed degradation in tensile properties. Moreover, the need of high temperature to heal the crack showed same challenges as can be found in previous approach.

15.3.2 Hollow fiber-based self-healing After the failure of the concept of microcapsule-based healing of FRP composites with various challenges, self-healing with hollow fibers containing healing agent was demonstrated by various workers [60 62]. The principle of healing in this approach remains same with the advantage of large amount of healing agent present in the fiber which can improve healing efficiency of the composites. Another advantage of this approach is the ease of integration of healing agent containing fiber into the FRP composites having fibrous architecture. Moreover, the chemistry of the healing agent chemistry to activate much easier when compared with the matrix modification or microencapsulation-based healing methods. FIGURE 15.5 Microcapsules deposited on the surface of carbon fiber for self-healing GFRP composites. GFRP, Glass fiber-reinforced polymer. Source: Adapted from A.R. Jones, A. Cintora, S.R. White, N.R. Sottos, Autonomic healing of carbon fiber/epoxy interfaces, ACS Appl. Mater. Interfaces 6 (2014) 6033 6039.

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15. Self-healing fiber-reinforced polymer composites for their potential structural applications

Bleay et al. [63] demonstrated healing of GFRP composites using commercially available hollow glass fiber containing two-part epoxy system in them and then dispersed in such a way that the two part of epoxy containing fibers remains adjacent to each other as schematically shown in Fig. 15.6. The diameter of the fiber was B15 μm. The healing efficiency was calculated from impact test and the results showed that application of compression on fracture surface was capable to improve the healing efficiency by B10%. However, the limited flow ability of the highly viscous epoxy resin was the major challenge in this approach which can only be overcome by using large diameter hollow fibers. Pang and Bond [64] investigated self-healing of similar fibers using an ultraviolet fluorescent dye to detect the healing of the cracks as shown in Fig. 15.7. There results showed FIGURE 15.6 Schematic diagram of smart repair concepts considered for polymer matrix composites. Source: Adapted from S.M. Bleay, C.B. Loader, V.J. Hawyes, L. Humberstone, P.T. Curtis, A smart repair system for polymer matrix composites, Compos. Part A Appl. Sci. Manuf. 32 (2001) 1767 1776.

FIGURE 15.7 Optical micrographs of fibers and composites manufactured at Bristol (A) hollow glass fibers of 60 m external diameter with a hollowness of 50% and (B) the same fibers within a Hexcel 913 epoxy matrix. Source: Adapted from J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791 1799.

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463

FIGURE 15.8 Photograph of a vascular sandwich beam subjected to the impact of a 2.8-kg cylindrical impactor dropped from 130 mm. The damage void has been infiltrated with premixed epoxy resin healing agent through the vascular network in the core. Source: Adapted from J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791 1799.

significant recovery of flexural strength by 97% as large extent of healing can be visualized from the use of the dye.

15.3.3 Microvascular-based self-healing The limitations of dispersion of the healing agent container in the matrix system to get better self-healing systems within an FRP was later addressed by Williams et al. [64]. They designed and developed a pervasive vascular network within the FRP sandwich structure as shown in Fig. 15.8. The vascular network had negligible influence on the mechanical performance of the resulting FRP composites. The resulting FRP composites were able to recover B100% when tested in flexural strength and the healing was demonstrated using a two-part epoxy healing agent. However, the study had the challenges of choice of appropriate healing system and the design complexities associated with them to prepare the vascular network.

15.4 Intrinsic self-healing approach for fiber-reinforced polymer composites Because of the various challenges in case of extrinsic self-healing approach, people started to adopt intrinsic self-healing approach for healing of FRP composites because of their various advantages such as auto repair without any external intervention. The intrinsic healing approach is based on some intrinsic property of the polymeric material and repairs the newly generated surfaces via reversible bonding. In case of FRP composites, the first successful intrinsic healing approach is the use of mendable polymers which can heal cracks under thermal stimuli. The development of intrinsic healing approaches for FRP applications can be divided into three categories: (1) use of reversible covalent bonds, (2) use of supramolecular interactions, and (3) use of polymer blends. In a recent study, self-healing of GFRP composite was demonstrated with disulfide containing organic inorganic thermoset matrices having a fiber volume fractions of B50% [65]. The resulting GFRP composites showed a full recovery of interlaminar fracture toughness when heated for 16 h at 85 C. The advantage of this healing approach is the polymers system which sustain the mechanical properties very close to the commercial

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15. Self-healing fiber-reinforced polymer composites for their potential structural applications

epoxies or unsaturated polyesters. An interlaminar fracture among different intrinsic healing, the most widely used healing approach is the DA reaction. In DA-based healing, electron rich dienes such as furans and electron poor dienophiles such as maleimides are reacted to get the thermoreversible bond. When heat is applied (above 120 C), about 30% of the monomer linkage disconnect and when cooled, reconnect the bonds, thereby help in healing. Chen et al. [32] first demonstrated the healing of polymer composites using this technique and showed that B57% recovery of fracture load was possible. However, the healing efficiency was found reduced after few number of healing cycles. The self-healing chemistry using different existing DA reaction systems was reviewed by Liu and Chuo [66] and Bergman and Wudl [67]. Later Park et al. [68] showed that the approach can be used for healing FRP composites successfully. They investigated healing of CFRP composites and assessed damage healing through flexural test. It was found that B90% healing can be found for three successive damaging cycles when heated 150 C using resistive heating treatment. After Chen et al. [32], Heo and Sodano successfully demonstrated healing of same composites using short-beam-shear specimens prepared with a VARTM process [69]. The healing efficiencies of the respective specimens showed B85% and B73% for the first and second healing cycles without degrading the mechanical properties of the CFRP composites prepared using conventional carbon fiber/epoxy systems. However, to avoid the use of high temperature in healing, noncovalent interactionbased healing approaches have been developed [70]. But, these systems have relatively poor mechanical performance when compared with DA-based self-healing approaches when integrated in FRPs. Thus to avoid reduction of mechanical properties and to heal at room temperature, hybrid systems of covalent, and noncovalent bonds were established [71,72]. Accordingly bifunctional and tetrafunctional epoxy resins were hybridized by Sordo [71] and were able to get full recovery of fracture when healed for 24 h at ambient temperature without degradation of the tensile properties. Sordo and Michaud [73] later showed that these systems can be used for healing of FRP composites and get recoveries of 65% and 72% of flexural strength and bending stiffness, respectively. Even though the FRP composites showed high healing and damping properties, till now, the low stiffness of the resulting FRP limits their use to semi structural applications. At the same time, lot of efforts have also been made to demonstrate self-healing in FRP composites consisting of a particulate thermoplastics in the matrix phase [74,75]. The selfhealing approaches demonstrate the use of the ionomeric copolymer poly(ethylene-comethacrylic) acid (EMAA) in the epoxy system because of the advantages of better adhesion between them and better thermal expansion of EMAA than that of epoxy. The EMAA-dispersed epoxy system was integrated into FRPs and the healing behavior was assessed from mode-I double-cantilever beam (DCB) test. The resulting composites showed 63% improvement in fracture toughness and B156% healing efficiency. Thus the potential of EMAA in healing of FRPs has been further demonstrated by integrated in the form of layers [76] or stitches [77]. However, EMAA is highly sensitive to humidity which restricts their use in various applications of FRP composites. In another healing approach, particulate thermosets are embedded in a thermoplastic matrix system for thermal healing of FRPs [78]. In these systems, the thermoset resin forms interconnected network with the thermoplastic particles at moderate temperatures. Luo et al. [79] first developed self-healing polymer using 15.5 wt.% epoxy-polycaprolactone

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465

(PCL) in the matrix. In a recent study, Cohades et al. [80] blended 25 vol.% PCL in the matrix system which exhibited a healing efficiency of B70%, without degradation of mechanical properties at ambient temperature and were proposed as promising candidates to be integrated as a matrix for healable FRPs.

15.5 Thermoreversible healing of FRP Thermoreversible linkages through DA or retro DA reaction, use of mendable thermoplastic polymer, and shape memory polymer are found successful approach to demonstrate self-healing in FRP composites [53,81]. Hayes et al. [53] dispersed 7.5 wt.% solidstate thermoplastic particles of polybisphenol-A-co-epichlorohydrin in bisphenol-A-based thermosetting resin and demonstrated 65% healing efficiency of the glass-FRP composites when tested with interlaminar shear test. Inspired from this work, Meure et al. [39] demonstrated self-healing of carbon fiber reinforced using poly[ethylene-co-(methacrylic acid)] thermoplastic healing agent as discrete particles and fiber mesh in bisphenol-A-based thermosetting resin. Their result showed repeatable self-healing with 100% healing efficiency investigated from interlaminar fracture toughness test. Moreover, Pingkarawat et al. [82] used three different thermoplastic healing agents such as EMAA, poly(ethylene-co-glycidyl methacrylate), and ethylene vinyl acetate at 5 and 15 wt.% for demonstration of thermoreversible healing of CFRP composites. Although DA-based self-healing chemistry has been developed to some extent for polymeric composites, but their demonstration in FRP composites are found very limited mainly because of the reduction of mechanical properties due to the incorporation of healing agents. Peterson et al. [35] demonstrated DA-based self-healing GFRP composites by incorporating maleimide on the surface of glass fiber and furan in the thermosetting resin system as shown in Fig. 15.9. The healing efficiency was evaluated from the microdroplet debonding test. A maximum healing efficiency of 100% was observed for some specimens and up to 5 times healing was successfully demonstrated. Zhang et al. [83] demonstrated for the first time healing of CFRP composites by DA chemistry. The carbon fiber surface was modified with bismaleimide and the epoxy matrix system was modified with furan group. The healing efficiency was evaluated from the debonding test. It has been reported that maximum healing efficiency of 82% can be achieved by this method. However, this technique reduced the mechanical properties of the composites due to which the authors might not have reported the mechanical performance of the resulting composites. In another recently published work, Kotrotsos et al. [84] demonstrated healing of CFRP composites by DA chemistry. The incorporation of SHA in the DA-based polymer matrix improved the mode-I fracture toughness, but reduced the flexural properties of the composites. Most of the reported self-healing techniques are found detrimental effect on mechanical properties of the nanocomposites or their FRP composites. Moreover, after few cycle of healing, the efficiency are found drastically reduced. The incorporation of thermoplastic healing agent in the thermosetting matrix reduces the glass transition temperature of the composites. Therefore incorporation of secondary nanofiller in the thermoreversible matrix in their pristine form or functionalized

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15. Self-healing fiber-reinforced polymer composites for their potential structural applications

FIGURE 15.9 Scheme of DA-based thermoreversible healing of GFRP composites. DA, Diels Alder. Source: Adapted from A.M. Peterson, R.E. Jensen, G.R. Palmese, Thermoreversible and remendable glass-polymer interface for fiberreinforced composites, Compos. Sci. Technol. 71 (2011) 586 592.

form can improve the mechanical as well healing performance of the nanocomposites and hybrid FRP composites.

15.6 Assessment of self-healing efficiency for fiber-reinforced polymer composites Healing of polymer composites are referred to recovery of different mechanical properties such as tensile, fracture toughness, flexural, and fatigue. [9,85,86]. Wool and O’Connor [87] demonstrated basic method for determining extent of healing in the polymer composites. However, till now, both quantitative as well as qualitative assessment of healing efficiency in polymer composite and FRP composites are reported. The mostly used method for quantification of self-healing efficiency is the recovery of fracture toughness (KIC) as shown in the following equation: Healing Efficiency; η 5

healed KIC virgin

KIC

3 100 virgin

(15.1)

healed is the fracture toughness of healed sample and KIC is the fracture toughness where KIC of virgin sample. The static fracture toughness of the polymer composites are calculated

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15.6 Assessment of self-healing efficiency for fiber-reinforced polymer composites

467

from single-edge notched bending (SENB) test. In case of polymeric composites, mode-I fracture properties are also measured from compact tension (CT) [88], DCB [89] and TDCB [14], SENB [16], three-point bending test or flexural test [90], double cleavage drilled compression [91], and short-beam-shear (SBS) test [92] as shown in Fig. 15.10. Brown et al. [93] demonstrated the quantification of healing efficiency of microcapsulebased self-healing polymer composites from fracture toughness measured by TDCB test and showed 100% healing efficiency of the resulting composites. For quantification of healing efficiency of intrinsic self-healing composites, small damage is incorporated in the control sample so that the fracture surface remains attached very closely to promote healing. Therefore intrinsic self-healing efficiency is mostly measured from fracture properties obtained from SENB and CT test. Luo et al. [79] investigated self-healing behavior of thermally mendable polymer composites by SENB test and found more than 100% healing efficiency by this test method. Moreover, Hayes et al. [52] quantified the healing efficiency of solid-state repaired thermosetting composite by CT method and found 50% 70% healing efficiency by this method. The quantitative assessment of self-healing efficiency is also performed from dynamic fatigue test using the following equation [94]: ηd 5

Nhealed 2 Nvirgin 3 100 N

(15.2)

where Nhealed is the total number of cycles before failure for the healed specimen and Nvirgin is the total number of cycle before failure for virgin sample. In case of FRP composites, the healing efficiency is mostly quantified from different interlaminar delamination test such as SBS test, and DCB test [84,95]. Kotrotsos et al. [84] investigated the healing performance of CFRP composites by DCB test and found 32% healing after first cycle. The qualitative analysis to determine extent of healing has been

FIGURE 15.10

Assessment of healing efficiency from different test methods.

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15. Self-healing fiber-reinforced polymer composites for their potential structural applications

performed by scratch healing test made by sharp razor blade in different reports [92]. The scratch made on the sample surface was investigated under optical microscope before and after healing to assess the extent of healing.

15.7 Conclusions In summary, we have observed that self-healing has a bright scope in engineered structures to get auto repaired the internal defects present in FRP composites. In general, the majority of self-healing studies although reported on healing behavior, but do not report on in-plane and out-plane mechanical behavior of the composites which are structurally useful. Most of the literature showed that the mechanical properties are significantly affected due to incorporation of SHA. Moreover, the healing efficiency reduces after number of healing cycles. Most of the successful self-healing approaches used in FRP composites require high temperature to heal the cracks. Although the self-healing technology has been developed in recent years, still much research has to be carried to bring them for commercial application in the structures of aerospace, aircraft, cars, and wind turbines.

Acknowledgments We thank the Department of Science and Technology, India under DST-FIST Program 2014 with Grant No. SR/ FST/ETI-373/2014. The authors also want to thank NMHS project with Grant No. NMHS/2016 2017/SG 18/07 for financial support. This work was initiated under the project head ‘Synthesis and Fracture Property Evaluation of Polymer Nanocomposites’ supported by National Institute of Technology Silchar, Assam, India [Project number (RC)/457/122].

References [1] K. Dransfield, C. Baillie, Y. Mai, K. Dransfield, C. Baillie, Y. Mai, Improving the delamination resistance of CFRP by stitching: a review, Compos. Sci. Technol. 50 (1994) 305 317. [2] M.J. Palmeri, K.W. Putz, T. Ramanathan, L.C. Brinson, Multi-scale reinforcement of CFRPs using carbon nanofibers, Compos. Sci. Technol. 71 (2011) 79 86. [3] S. Ozcan, J. Tezcan, P. Filip, Microstructure and elastic properties of individual components of C/C composites, Carbon N. Y 47 (2009) 3403 3414. [4] E. Fitzer, The future of carbon-carbon composites, Carbon N. Y 25 (1987) 163 190. [5] A.C. Garg, Failure mechanisms in toughened epoxy resins: a review, Compos. Sci. Technol. 31 (2006) 179 223. [6] A.S. Argon, R.E. Cohen, Toughenability of polymers, Polymer 44 (2003) 6013 6032. [7] N.S. Choi, A.J. Kinloch, J.G. Williams, Delamination fracture of multidirectional carbon-fiber/epoxy composites under mode I, mode II and mixed-mode I/II loading, J. Compos. Mater. 33 (1999) 73 100. [8] R.P. Wool, K.M. O’Connor, A theory of crack healing in polymers, J. Appl. Phys. 52 (1981) 5953 5963. [9] D.Y. Wu, S. Meure, D. Solomon, Self-healing polymeric materials: a review of recent developments, Prog. Polym. Sci. 33 (2008) 479 522. [10] R.P. Wool, Self-healing materials: a review, Soft Matter 4 (2008) 400. [11] Y. Yang, X. Ding, M.W. Urban, Chemical and physical aspects of self-healing materials, Prog. Polym. Sci. 49 50 (2015) 34 59. [12] B.J. Blaiszik, S.L.B. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, MR40CH08-white self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (2010) 179 211. [13] Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (2013) 7446.

Self-Healing Polymer-Based Systems

References

469

[14] J.D. Rule, N.R. Sottos, S.R. White, Effect of microcapsule size on the performance of self-healing polymers, Polymer. 48 (2007) 3520 3529. [15] Q. Li, A.K. Mishra, N.H. Kim, T. Kuila, K.T. Lau, J.H. Lee, Effects of processing conditions of poly(methylmethacrylate) encapsulated liquid curing agent on the properties of self-healing composites, Compos. Part B Eng. 49 (2013) 6 15. [16] Q. Li, Siddaramaiah, N.H. Kim, D. Hui, J.H. Lee, Effects of dual component microcapsules of resin and curing agent on the self-healing efficiency of epoxy, Compos. Part B Eng. 55 (2013) 79 85. [17] L. Yuan, A. Gu, G. Liang, Preparation and properties of poly(urea-formaldehyde) microcapsules filled with epoxy resins, Mater. Chem. Phys. 110 (2008) 417 425. [18] D.Y. Zhu, M.Z. Rong, M.Q. Zhang, Preparation and characterization of multilayered microcapsule-like microreactor for self-healing polymers, Polymer 54 (2013) 4227 4236. [19] C.M. Dry, N.R. Sottos, Passive smart self-repair in polymer matrix composite materials, North. Am. Conf. Smart Struct. Mater. 1916 (1993) 438 444. [20] J.H. Huang, J. Kim, N. Agrawal, A.P. Sudarsan, J.E. Maxim, A. Jayaraman, et al., Rapid fabrication of bioinspired 3D microfluidic vascular networks, Adv. Mater. 21 (2009) 3567 3571. [21] H. Williams, R. Trask, P. Weaver, I. Bond, Minimum mass vascular networks in multifunctional materials, J. R. Soc. Interface 5 (2008) 55 65. [22] H. Williams, R. Trask, A. Knights, E. Williams, I. Bond, Biomimetic reliability strategies for self-healing vascular networks in engineering materials, J. R. Soc. Interface 5 (2008) 735 747. [23] M. Scheiner, T.J. Dickens, O. Okoli, Progress towards self-healing polymers for composite structural applications, Polymer 83 (2016) 260 282. [24] W. Deng, Y. You, A. Zhang, Supramolecular Network-Based Self-Healing Polymer Materials, Elsevier Ltd., 2015. [25] G. Li, J. Wu, B. Wang, S. Yan, K. Zhang, J. Ding, et al., Self-healing supramolecular self-assembled hydrogels based on poly(L-glutamic acid), Biomacromolecules 16 (2015) 3508 3518. [26] C.M. Chung, Y.S. Roh, S.Y. Cho, J.G. Kim, Crack healing in polymeric materials via photochemical [2 1 2] cycloaddition, Chem. Mater. 16 (2004) 3982 3984. [27] M. Nakahata, Y. Takashima, A. Harada, Highly flexible, tough, and self-healing supramolecular polymeric materials using host-guest interaction, Macromol. Rapid Commun. 37 (2016) 86 92. [28] X. Chen, F. Wudl, A.K. Mal, H. Shen, S.R. Nutt, X. Chen, et al., New thermally remendable highly crosslinked polymeric materials new thermally remendable highly cross-linked polymeric materials, Macromolecules 36 (2003) 1802 1807. [29] S. Scha¨fer, G. Kickelbick, Self-healing polymer nanocomposites based on Diels-Alder-reactions with silica nanoparticles: the role of the polymer matrix, Polymer 69 (2015) 357 368. [30] T.S. Coope, D.H. Turkenburg, H.R. Fischer, R. Luterbacher, H. Van Bracht, I.P. Bond, Novel Diels-Alder based self-healing epoxies for aerospace composites, Smart Mater. Struct. 25 (2016) 1 9. [31] C. Shao, M. Wang, H. Chang, F. Xu, J. Yang, A self-healing cellulose nanocrystals-poly (ethylene glycol) nanocomposite hydrogel via Diels Alder click reaction, ACS Sustainable Chem. Eng. 5 (2017) 6167 6174. [32] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698 1702. [33] P. Du, X. Liu, Z. Zheng, X. Wang, T. Joncheray, Y. Zhang, Synthesis and characterization of linear selfhealing polyurethane based on thermally reversible Diels Alder reaction, RSC Adv. 3 (2013) 15475. [34] P. Mineo, V. Barbera, G. Romeo, F. Ghezzo, E. Scamporrino, F. Spitaleri, et al., Thermally reversible highly cross-linked polymeric materials based on furan/maleimide Diels-Alder adducts, J. Appl. Polym. Sci. 132 (2015) 1 9. [35] A.M. Peterson, R.E. Jensen, G.R. Palmese, Thermoreversible and remendable glass-polymer interface for fiber-reinforced composites, Compos. Sci. Technol. 71 (2011) 586 592. [36] Q. Tian, M.Z. Rong, M.Q. Zhang, Y.C. Yuan, Synthesis and characterization of epoxy with improved thermal remendability based on Diels-Alder reaction, Polym. Int. 59 (2010) 1339 1345. [37] J. Lee, D. Bhattacharyya, M.Q. Zhang, Y.C. Yuan, Mechanical properties of a self-healing fibre reinforced epoxy composites, Compos. Part B Eng. 78 (2015) 515 519. [38] T.S. Coope, D.F. Wass, R.S. Trask, I.P. Bond, Repeated self-healing of microvascular carbon fibre reinforced polymer composites, Smart Mater. Struct. 23 (2014).

Self-Healing Polymer-Based Systems

470

15. Self-healing fiber-reinforced polymer composites for their potential structural applications

[39] S. Meure, S. Furman, S. Khor, Poly[ethylene-co-(methacrylic acid)] healing agents for mendable carbon fiber laminates, Macromol. Mater. Eng. 295 (2010) 420 424. [40] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Graphene based materials: past, present and future, Prog. Mater. Sci. 56 (2011) 1178 1271. [41] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymer nanocomposites, Polymer 52 (2011). [42] W. Choi, I. Lahiri, R. Seelaboyina, Y.S. Kang, Synthesis of graphene and its applications: a review, Crit. Rev. Solid State Mater. Sci. 35 (2010) 52 71. [43] R.J. Young, I.A. Kinloch, L. Gong, K.S. Novoselov, The mechanics of graphene nanocomposites: a review, Compos. Sci. Technol. 72 (2012) 1459 1476. [44] D.D.L. Chung, Processing-structure-property relationships of continuous carbon fiber polymer-matrix composites, Mater. Sci. Eng. R 113 (2017) 1 29. [45] N. De Greef, L. Gorbatikh, A. Godara, L. Mezzo, S.V. Lomov, I. Verpoest, The effect of carbon nanotubes on the damage development in carbon fiber/epoxy composites, Carbon N. Y 49 (2011) 4650 4664. [46] N. De Greef, L. Gorbatikh, S.V. Lomov, I. Verpoest, Damage development in woven carbon fiber/epoxy composites modified with carbon nanotubes under tension in the bias direction, Compos. Part A Appl. Sci. Manuf. 42 (2011) 1635 1644. [47] G. Lubineau, A. Rahaman, A review of strategies for improving the degradation properties of laminated continuous-fiber/epoxy composites with carbon-based nanoreinforcements, Carbon N. Y. 50 (2012) 2377 2395. [48] F. Gao, L. Boniface, S. Ogin, P. Smith, R. Greaves, Damage accumulation in wovenfabric CFRP laminates under tensile loading: part 1. Observations of damage accumulation, Compos. Sci. Technol 59 (1999) 123 136. [49] A.K. Pathak, M. Borah, A. Gupta, T. Yokozeki, S.R. Dhakate, Improved mechanical properties of carbon fiber/graphene oxide-epoxy hybrid composites, Compos. Sci. Technol. 135 (2016) 28 38. [50] J.D. Schaefer, A.J. Rodriguez, M.E. Guzman, C.S. Lim, B. Minaie, Effects of electrophoretically deposited carbon nanofibers on the interface of single carbon fibers embedded in epoxy matrix, Carbon N. Y 49 (2011) 2750 2759. [51] X. Zhang, X. Fan, C. Yan, H. Li, Y. Zhu, X. Li, et al., Interfacial microstructure and properties of carbon fiber composites modified with graphene oxide, ACS Appl. Mater. Interfaces 4 (2012) 1543 1552. [52] S.A. Hayes, F.R. Jones, K. Marshiya, W. Zhang, A self-healing thermosetting composite material, Compos. Part A Appl. Sci. Manuf. 38 (2007) 1116 1120. [53] S.A. Hayes, W. Zhang, M. Branthwaite, F.R. Jones, Self-healing of damage in fibre-reinforced polymer-matrix composites, J. R. Soc. Interface 4 (2007) 381 387. [54] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794 797. [55] M.R. Kessler, S.R. White, Self-activated healing of delamination damage in woven composites, Compos. Part A Appl. Sci. Manuf. 32 (2001) 683 699. [56] M.R. Kessler, N.R. Sottos, S.R. White, Self-healing structural composite materials, Compos. Part A Appl. Sci. Manuf. 34 (2003) 743 753. [57] T. Yin, M.Z. Rong, M.Q. Zhang, G.C. Yang, Self-healing epoxy composites - preparation and effect of the healant consisting of microencapsulated epoxy and latent curing agent, Compos. Sci. Technol. 67 (2007) 201 212. [58] T. Yin, L. Zhou, M.Z. Rong, M.Q. Zhang, Self-healing woven glass fabric/epoxy composites with the healant consisting of micro-encapsulated epoxy and latent curing agent, Smart Mater. Struct. 17 (2008). [59] A.R. Jones, A. Cintora, S.R. White, N.R. Sottos, Autonomic healing of carbon fiber/epoxy interfaces, ACS Appl. Mater. Interfaces 6 (2014) 6033 6039. [60] G. Williams, R. Trask, I. Bond, A self-healing carbon fibre reinforced polymer for aerospace applications, Compos. Part A Appl. Sci. Manuf. 38 (2007) 1525 1532. [61] R.S. Trask, I.P. Bond, Biomimetic self-healing of advanced composite structures using hollow glass fibres, Smart Mater. Struct. 15 (2006) 704 710. [62] R.S. Trask, G.J. Williams, I.P. Bond, Bioinspired self-healing of advanced composite structures using hollow glass fibres, J. R. Soc. Interface 4 (2007) 363 371. [63] S.M. Bleay, C.B. Loader, V.J. Hawyes, L. Humberstone, P.T. Curtis, A smart repair system for polymer matrix composites, Compos. Part A Appl. Sci. Manuf 32 (2001) 1767 1776.

Self-Healing Polymer-Based Systems

References

471

[64] J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791 1799. [65] W. Post, A. Cohades, V. Michaud, S. van der Zwaag, S.J. Garcia, Healing of a glass fibre reinforced composite with a disulphide containing organic-inorganic epoxy matrix, Compos. Sci. Technol. 152 (2017) 85 93. [66] Y.L. Liu, T.W. Chuo, Self-healing polymers based on thermally reversible Diels-Alder chemistry, Polym. Chem. 4 (2013) 2194 2205. [67] S.D. Bergman, F. Wudl, Mendable polymers, J. Mater. Chem. 18 (2008) 41 62. [68] J.S. Park, H.S. Kim, H. Thomas Hahn, Healing behavior of a matrix crack on a carbon fiber/mendomer composite, Compos. Sci. Technol. 69 (2009) 1082 1087. [69] Y. Heo, H.A. Sodano, Thermally responsive self-healing composites with continuous carbon fiber reinforcement, Compos. Sci. Technol. 118 (2015) 244 250. [70] F. Herbst, D. Do¨hler, P. Michael, W.H. Binder, Self-healing polymers via supramolecular forces, Macromol. Rapid Commun. 34 (2013) 203 220. [71] F. Sordo, Design of Self-Healing Supramolecular Rubbers with a Tunable Number of Chemical Cross-Links, 2008. [72] Jinhui Li, Guoping Zhang, Rong Sun and Ching-Ping Wong, A covalently cross-linked reduced functionalized graphene oxide/polyurethane composite based on Diels Alder chemistry and its potential application in healable flexible electronics, J. Mater. Chem. C, 5 (2017) 220-228. [73] F. Sordo, V. Michaud, Processing and damage recovery of intrinsic self-healing glass fiber reinforced composites, Smart Mater. Struct. 25 (2016) 1 9. [74] K. Pingkarawat, C. Dell’Olio, R.J. Varley, A.P. Mouritz, Poly(ethylene-co-methacrylic acid) (EMAA) as an efficient healing agent for high performance epoxy networks using diglycidyl ether of bisphenol A (DGEBA), Polymer 92 (2016) 153 163. [75] K. Pingkarawat, T. Bhat, D.A. Craze, C.H. Wang, R.J. Varley, A.P. Mouritz, Healing of carbon fibre-epoxy composites using thermoplastic additives, Polym. Chem. 4 (2013) 5007 5015. [76] C.H. Wang, K. Sidhu, T. Yang, J. Zhang, R. Shanks, Interlayer self-healing and toughening of carbon fibre/ epoxy composites using copolymer films, Compos. Part A Appl. Sci. Manuf. 43 (2012) 512 518. [77] K. Pingkarawat, C.H. Wang, R.J. Varley, A.P. Mouritz, Effect of mendable polymer stitch density on the toughening and healing of delamination cracks in carbon-epoxy laminates, Compos. Part A Appl. Sci. Manuf. 50 (2013) 22 30. [78] A. Cohades, V. Michaud, Thermal mending in E-glass reinforced poly(ε-caprolactone)/epoxy blends, Compos. Part A Appl. Sci. Manuf. 99 (2017) 129 138. [79] X. Luo, R. Ou, D.E. Eberly, A. Singhal, W. Viratyaporn, P.T. Mather, A thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion, ACS Appl. Mater. Interfaces 1 (2009) 612 620. [80] A. Cohades, E. Manfredi, C.J.G. Plummer, V. Michaud, Thermal mending in immiscible poly(E-caprolactone)/epoxy blends, Eur. Polym. J. 81 (2016) 114 128. [81] G. Rivero, L.T.T. Nguyen, X.K.D. Hillewaere, F.E. Du Prez, One-pot thermo-remendable shape memory polyurethanes, Macromolecules 47 (2014) 2010 2018. [82] K. Pingkarawat, C.H. Wang, R.J. Varley, A.P. Mouritz, Mechanical properties of mendable composites containing self-healing thermoplastic agents, Compos. Part A Appl. Sci. Manuf. 65 (2014) 10 18. [83] W. Zhang, J. Duchet, J.F. Ge´rard, Self-healable interfaces based on thermo-reversible Diels-Alder reactions in carbon fiber reinforced composites, J. Colloid Interface Sci. 430 (2014) 61 68. [84] A. Kotrotsos, P. Tsokanas, S. Tsantzalis, V. Kostopoulos, Healing of carbon fiber reinforced plastics by Diels Alder based polymers: effects of healing agent concentration and curing cycle, J. Appl. Polym. Sci. 136 (2019) 1 12. [85] Ming Qiu Zhang, Min Zhi Rong, Self-healing polymers and polymer composites. doi: 10.1002/9781118082720 [86] B. Aı¨ssa, D. Therriault, E. Haddad, W. Jamroz, Self-healing materials systems: overview of major approaches and recent developed technologies, Adv. Mater. Sci. Eng. 2012 (2012) 1 17. [87] R.P. Wool, W.R. O’Connor, A theory crack healing in polymers, J. Appl. Phys. 5953 (1981). [88] M.S. Md Jamil, F.R. Jones, N.N. Muhamad, S.M. Makenan, Solid-state self-healing systems: the diffusion of healing agent for healing recovery, Sains Malaysiana. 44 (2015) 843 852. [89] H. Jin, G.M. Miller, N.R. Sottos, S.R. White, Fracture and fatigue response of a self-healing epoxy adhesive, Polymer 52 (2011) 1628 1634.

Self-Healing Polymer-Based Systems

472

15. Self-healing fiber-reinforced polymer composites for their potential structural applications

[90] J.W. Nazrul Islam Khan, S. Halder, Diles-Alder based epoxy matrix and interfacial healing of bismaleimide grafted GNp infused hubrid nanocomposites, Polym. Test. 74 (2019) 138 151. [91] A.R. Hamilton, N.R. Sottos, S.R. White, Pressurized vascular systems for self-healing materials, J. R. Soc. Interface 9 (2012) 1020 1028. [92] Y. Heo, M.H. Malakooti, H.A. Sodano, Self-healing polymers and composites for extreme environments, J. Mater. Chem. A 4 (2016) 17403 17411. [93] E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (2002) 372 379. [94] E.N. Brown, S.R. White, N.R. Sottos, Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite - part II: in situ self-healing, Compos. Sci. Technol. 65 (2005) 2474 2480. [95] Jong Se, Healing behavior of a matrix crack on a carbon fiber/mendomer composite, Compos. Sci. Technol. 69 (2009) 1082 1087.

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16 Self-healing polymeric coating for corrosion inhibition and fatigue repair Vikas S. Hakke1, Uday D. bagale1, Shirish H. Sonawane1, Dipak Pinjari2, S. Manigandan3 and Shriram Sonawane4 1

Department of Chemical Engineering, National Institute of Technology, Warangal, India National Center for Nanoscience and Nanotechnology, University of Mumbai, Mumbai, India 3 Department of Chemical Engineering, Indian Institute of Technology, Ropar, India 4 Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India 2

16.1 Background of self-healing and corrosion inhibition Engineering materials are used to provide the mechanical strength and stiffness. There are different classes of engineering materials which include composites, ceramics, polymers, and metals. These materials have an intrinsic property to oppose the damage due to the presence of the microstructures inside the materials. Recently the advent of nanotechnology made lots of changes in the materials science area and development of the advanced engineering materials. However, there are number of the functional applications which are based on the release mechanism on the requirement of damage. The best example of the delaying the damage is liberation of the blood after the cut happens on the skin. The corrosion is one of the prevailing problems in the petrochemical and petroleum refining industries. It causes loss of materials and damage in the pipelines or equipment. Usually polymer-based coatings are being used as protective barricade to the substrate to protect substrate from the external environment. Polymeric coatings are class of engineering material which gives the protection to the metal surfaces. Coating contains the important ingredients consisting of main polymer vehicle called resin base along with drying agents, curing agents, active pigments, etc. Earlier the chromium-based pigments are being used in barrier corrosion inhibition coating. However, the leaching of chromium pigments causes damage to marine life and environment. Hence, now new class of engineering coatings is always preferred to have green corrosion inhibition pigments. A number of

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strategies are being developed to prevent the corrosion. Traditional approach for inhibition of the corrosion involves the selection of material, cathodic protection, and addition of corrosion inhibition. As per the National Association of Corrosion Engineers report, polymer coating has a major role in the protection of metals (about 66%). Self-healing coatings are recently being used for the protection of the metal from the environmental damage. Selfhealing materials have the ability to heal itself automatically. Self-healing materials provide control over corrosion and formation of healable polymerized film on damage site. Healing of invisible microcracks or fatigue in coating by self-healing was introduced in late 1980. There are number of limitations observed for the self-healing coatings. As the cost of the healing agent is very high in which diphenyl phalate is used, upon the formation of the crack, the release of the healing agent happen from core and cross-link with the substrate. This may in turn contribute to weaker mechanical properties such as flexibility and hardness of coating. Water-based coating is having high demand for the industry in which there is no volatile organic compounds. There are number of advantages with the selfhealing coating such as automatic repairing process, barrier protection of underlying substrate, autopreservation of surface appearance, and restoration of mechanical properties. In this chapter, the overview of the self-healing materials and corrosion-inhibiting agents is reported, using two case studies. The green synthesis of self-healing corrosion-inhibiting coating using neem oil as a self-healing agent and corrosion inhibitor and polymer capsules for releasing the agent and the action of inorganic halloysite clay is reported.

16.2 Self-healing and corrosion inhibitor materials In ancient era, pyramid was best long-lasting material made construction site, which is base of smart material [1]. Later in 1970s, Malinskii et al. [2] reported the first smart material base on aluminum foil-poly(vinyl acetate). Later in years of 1980, Wool studied the use of hard elastomeric thermoplastic polymer such block copolymer of styrene butadiene styrene with carbon black-filled natural rubber and polypropylene [3]. Furthermore Wang et al. reported the polymer-based healing using solvent process [4]. In solvent process, polymer is swelled or softened after dissolving in solvent and repairs the damage themselves [4]. However, repair or replacement of this material with conventional approaches (manual interventions) is mostly limited. In such situation, required polymeric material has the self-healing ability. The polymer can extend life of material as well as save the maintenance cost, which is great advantage in many maintenance applications [5,6]. In early 21st century, White et al. was the first who reported the use autonomous self-healing by microcapsular approach [7]. They demonstrated that dicyclopentadiene (DCPD) as healing agent encapsulates in microcapsule of shell made from polymer. These microcapsules have the ability to release the DCPD (healing agent), once it cracks to the coating specimen. Following that work, many researchers worked on self-healing system with different approaches and a number of research articles were published in this field.

16.2.1 Self-healing materials Based on self-healing chemistries, two parts of self-healing system occur, namely, intrinsic and extrinsic. In intrinsic self-healing, healing occurs within the material itself through Self-Healing Polymer-Based Systems

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its structure. The self-healing was achieved through the reversible reaction between the healing materials (polymer matrix). The process of healing can be done using several techniques/approaches such as hydrogen bonding, thermally reversible reaction, molecular diffusion and entanglement, and ionomeric arrangements [8 11]. Wool and O’Connor [12] proposed one of the most accepted theories leading to interfacial physical healing which is the one based on molecular interdiffusion, leading to chain entanglements. Based on their main molecular principle, the existing chemistries can be grouped into two broad categories: (1) reversible covalent bonds [9 11] and (2) supramolecular interactions [10,11,13]. On the other hand, extrinsic self-healing system, healing material stored in container or reservoir to form microcapsules or vascular. Examples of vascular-based healing system are one-dimensional two-pack epoxy [14 16], 3D epoxy system with Grubbs catalyst [17], and two-pack epoxy with more number of healing cycles [18]. However, this has a limitation due to the swelling for preserving the integrity resin matrix, lack of good compatibility to ensure adhesion, and time-consuming step of fiber dissolution and infusion of healing agents. Thus moving toward capsule-based system has more attraction. The main benefit of this capsule-based system is that the material regains its original esthetic look with better self-healing ability [19]. 16.2.1.1 Single catalyst with microcapsules White et al. [7] was the first who reported about the Grubb’s catalyst-based self-healing. Subsequently Brown et al. [20] reported its limitations regarding the concentration of catalyst and microcapsule used in self-healing material. They reported that these high concentrations affect fracture toughness and healing efficiency. Maximum healing efficiencies was achieved by the addition of 2 wt.% catalyst and 15 wt.% microcapsule. Jin et al. [21] fabricated microcapsule-based self-healing coatings by incorporation of the endo-DCPD in double wall polyurethane (PU)/urea-formaldehyde (UF) shell. They obtained coatings of thickness 750 mm cured at higher temperature (110 C for 3 h) with recovery in range of 20% 58%. Rule et al. [22] reported a novel self-healing system based on wax-coated DCPD microcapsule. These capsules are being protected from external environment because of nonwax DCPD-based capsule. Owing to its high cost and incompatibility of Grubbs with host matrix, second-generation Grubbs were required to be replaced with other material. Kamphaus et al. [23] reported that the tungsten hexachloride (WCl6) was economical for the polymerization of DCPD. However, they also reported that tungsten hexachloride alone cannot be effective for original fracture recovery. So use of phenylacetylene as alkylating agent and nonylphenol as dissolution agent provides activation for the WCl6 precursor. Cooper et al. [24] reported the excellent chemistry between Lewis acids such as scandium (III) triflate catalyst and diglycidyl ether of bisphenol A-based epoxy capsule for self-healing. 16.2.1.2 Dual capsule-based system Freitas et al. [25] reported the preparation of dual microencapsulation of epoxy and amine, respectively, using solvent encapsulation approach. Later Pang and Bond [26] reported that epoxy and amine microcapsules were incorporated in hollow fibers for selfrepairing effect in plastic composite with healing efficiency around 40% 60%. Yuan et al. [27] reported the dual microcapsule-based epoxy self-healing system comprising of epoxy (base) and mercaptan (hardner) microcapsules prepared by in situ polymerization

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approach encapsulate. Hart et al. [28] reported the novel 2-ethyl-4-methylimidazole (24EMI) as latent catalyst for new self-healing system which can be healed for multiple times at moderate temperature. 24-EMI along with liquid low viscous epoxy monomer and cycloaliphatic amine was encapsulated in host matrix. With help of TDCB fracture test method, 90% recovery fracture with addition of 10% 24-EMI in coating was obtained. Li et al. [29] reported a dual microcapsule prepared by solvent evaporation approach. Polyetheramine (PEA) and poly(methyl methacrylate) were used as core and shell material, respectively. Another type of self-healing system reported by Caruso et al. [30] was solvent-based microcapsules for self-healing system. A series of different solvents such as chlorobenzene, xylene, toluene, hexane, methylene chloride, nitrobenzene, and dimethylacetamide were incorporate in epoxy and amine microcapsule using an in situ polymerization approach. Cho et al. [31] reported the novel siloxane-based self-healing system for mild steel substrate. The main concept of this work was based on phase separation in which siloxane material was not encapsulated (phase separation form) but the catalyst is encapsulated. The used materials hydroxyl end-functionalized polydimethylsiloxane (HOPDMS) and polydiethoxysiloxane were cost effective, easily available, and chemically stable under any environmental condition during self-healing process. Keller et al. [32] reported the new elastomer self-healing system based on PDMS. Two different core materials, namely blend of vinyl-based PDMS resin, platinum catalyst, and activator for vinylPDMS were encapsulated in UF shell material for the formation of microcapsules. Apart from above system, epoxy-oil-based self-healing system has another important application for corrosion inhibition coating based on linseed oil [33 35], tung oil [36], linseed oil and mercaptobenzotriazole [37], neem oil [38], etc.

16.2.2 Corrosion inhibitors In paint industries, corrosion inhibitors were used as corrosion resistance material which contains chromates, molybdates, nitrates, and phosphate. The basic principle of corrosion inhibitor is that it causes the formation of a passive layer on the metal surface, which prevents the attack of corrosive species onto the substrate. Generally inhibitor is chemically adsorbed on the metal surface, which forms a barrier layer between inhibitor and substrate. This forms an oxide layer, which eliminates chance of corrosion species on substrate. The inhibitor reacts with a potential corrosive component present in aqueous media and forms complex with corrosion components [39 42]. The efficiency of inhibitor can be calculated as follows: η5

Ci 2 Co Co

(16.1)

where η is the inhibitor efficiency (%), Ci is corrosion rate of metal with inhibitor, and Co is corrosion rate of metal without inhibitor [42]. 16.2.2.1 Types of corrosion inhibitors Depending on their mechanism and nature of substance, corrosion inhibitors can be classified as (1) anodic, (2) cathodic, (3) organic, (4) precipitation, (5) vapor phase, and (6)

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ohmic inhibitors. An emerging new addition to the class of corrosion inhibitors is the green corrosion inhibitors. 16.2.2.2 Anodic corrosion inhibitors The inhibitors which reduce the anodic reaction, that is, they support the natural reaction of passivation to metal surface while blocking the anodic reaction. This inhibitors form an insoluble film by adsorption on the metal which is initially reactive with the corrosive species [43,44]. The corrosion inhibitors affect in a way that, potential value is shifted from negative to side that is more positive with decrease in current density value if more concentration of inhibitors is used. If less concentration of inhibitor is used, then there will be chance of breakdown of passivation film, which causes the pitting type of corrosion. Examples are nitrites, molybdates, phosphates, hydroxides, and chromates. 16.2.2.3 Nitrites It is an anodic corrosion inhibitor having tendency of reduce the anode reaction, thereby increasing potential value and anodic polarization with decrease in current density. A number of articles were reported on corrosion inhibition capacity of nitrite which can be effectively utilized in the case of the formation of rust on steel surface, as adhesion protective oxide, the concentration effect with time, and their correlation with oxygen [45]. Since nitrite contains a strong oxidizing power, it oxidizes the surface and forms Fe2O3. The formation rate of protective film due to nitrite is very fast and thus among the several corrosion inhibitors, nitrite shows good performance [46]. Sodium nitrite and calcium nitrite are commonly used nitrites. 16.2.2.4 Molybdates Molybdates were recognized in late 1990s as because of its simple, less toxicity, and less water solubility. It has general formula as M2MoO4, molybdate ions were generally available in solid and liquid phases. Calcium, zinc, and sodium molybdates have slight water solubility that makes them to use in corrosion-inhibiting coating system. In comparison to chromates, they are very weak oxidant agent but are more compatible. At present there is no evidence about the mechanism molybdates ion action. But, generally it forms an insoluble film of iron molybdate with active passive transition on the surface of steel. Due to this electrode polarization, Mo (VI) stabilizes native oxide on unetched steel which increases the impendence [47]. 16.2.2.5 Cathodic corrosion inhibitor This inhibitor reduces the corrosion rate by increasing the depolarization overvoltage, that is, cathodic reaction. Cathodic inhibitor reduces the diffusion of corrosion species at metal surface, which causes inhibiting cathodic reaction. Depending on their inhibition mechanism, cathodic inhibitors are classified as cathodic poisons, cathodic precipitates, and oxygen scavengers. Examples of cathodic inhibitor are the phosphate groups, used in cooling system by the formation of barrier layer on cathode. Sulfite acts as oxygen scavengers with decrease in corrosion rate. Cobalt or manganese is used to catalyze the reaction between oxygen and sulfites at low temperature [39 41].

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16.2.2.6 Organic inhibitors With limitation of chromate and other corrosion inhibitor due to its toxicity, inorganic inhibitors were replaced by organic inhibitors. These inhibitors forming strong hydrophobic adsorption layer reduce the diffusion of species and provide strong corrosion resistance. Inhibitor can work as anode or cathode or both with less concentration of inhibitor. These inhibitors have its own functional group which make them different from other inhibitors such as sulfur, oxygen, organic heterocyclic polar group, and nitrogen. Organic inhibitors cover the entire surface area of the corroding metal with a thick film consisting of several monolayers and change the structure of the double layer at the metal interface, decreasing depolarization rate. They may also act as a barrier film by blocking anodic and cathodic active sites by decreasing electroactive species transport rate to and from the metal surface. Examples of such inhibitor reported are benzotriazole (BTA), mercaptobenzotriazole, amine, heterocyclic nitrogen group, urea, sulfur composing compounds, and other acid such ascorbic acid and succinic acid [39 44].

16.3 Case study for self-healing material and corrosion inhibitors 16.3.1 Green synthesis of self-healing corrosion-inhibiting coating using neem oil as self-healing and corrosion inhibitor Interest in developing smart polymer coating with adequate corrosion inhibition or quick healing based on the use of natural materials has been significantly increased in recent years and is considered as a green processing approach. In this case study, use of sonication for the improved synthesis of nanocapsules using green corrosion inhibitor species (Azadirachta indica) derived from neem seed oil have been reported. As reported in literature the neem oil (A. indica) are water soluble, electrochemically active, and have high concentrations of alkaloids and fatty acids. The nitrogen and oxygen groups present in the structure of neem oil promote the corrosion inhibition properties in the corrosive environment [38]. As reported by Swaroop et al. [48] neem leaf in 3% in biotic medium gives better corrosion inhibition properties to copper surface for more than 3 weeks. Parthipan et al. [49] reported the use of neem leaf in a high concentration render microbial resistance and corrosion inhibition properties on carbon steel surface.

16.3.2 Synthesis of nanocapsules using ultrasound and conventional method As reported in the literature [38] nanocapsules were prepared by in situ oil in water emulsion polymerization. The modified procedure for emulsion polymerization using conventional and ultrasound is as follows: ultrasound-assisted synthesis of nanocapsules was performed using a probe sonicator. Initially a surfactant solution was prepared using 35 mL of distilled water, 0.3 g span 80, and 30 mL 5% PVA solution. Subsequently 2.5 g urea, 0.21 g resorcinol, and 0.21 g ammonium chloride were added to this prepared solution. The pH value of the solution was then adjusted to 3.5 by the addition of hydrochloric acid for further reaction. Dropwise addition of corrosion inhibitor, that is, A. indica to above solution resulted in the formation of an emulsion. The solution was allowed to

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stabilize under the action of ultrasound. After stabilization, 5 g of a 37% aqueous solution of formaldehyde was added to maintain the molar ratio of urea and formaldehyde as 1:2. The reaction mixture was sonicated at 20 kHz with power of 120 W for 40 min and then the reaction mixture was heated to 50 C for 2 h. Finally the emulsion was cooled to room temperature and then vacuum filtration was used to yield the nuances capsules. Subsequently the obtained capsules were washed with water and xylene to remove the suspended oil and dried at room temperature (30 C), whereas the capsules were prepared using conventional method at 200 rpm for 4 h reaction time at 60 C. Fig. 16.1 shows the procedure for nanocapsules formation using ultrasonication. Resorcinol-based urea-formaldehyde nanocapsules have been prepared using neem oil as core material under constant agitation at 400 rpm in the presence of ultrasonic probe of diameter 20 mm, operating at frequency 20 kHz with power 120 W and also using the conventional approach based on agitation at 400 rpm. The FESEM analysis of the capsules is shown in Fig. 16.2 for both the approaches. From the FESEM image (Fig. 16.2A), it reveals that core (A. indica) is completely encapsulated in the UF shell material. The capsules were spherical in shape with inner smooth surface and rough outer surface. In the case of ultrasound-assisted approach, the inner surface showed thickness of

FIGURE 16.1 Procedure for nanocapsules formation using ultrasonication.

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FIGURE 16.2 FESEM image of capsules for (A) ultrasound method and (B) conventional method [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

50 100 nm, whereas the conventional approach showed thickness of 5 μm. Agglomerates were formed for a case of convention method, which was due to sticky nature of the capsules and also due to the insufficient surfactant concentration in the capsules. In the case of ultrasound-assisted approach, the degree of agglomeration was reduced and the mean particle size was lower. This was due to high intense shear rate generated during ultrasonication process, which created more pressure on liquid emulsion for creation of bubble. As sonication time for reaction exceeded than that needed for the growth of the bubble, the collapsing of bubble took place which produced smaller particles which was in dispersed form. Also the obtained nanocapsules show better dispersion stability in aqueous phase in the case of ultrasound-assisted approach as reported in Fig. 16.3. Images show the stable nanocapsules-based emulsion for ultrasound-assisted approach. Capsules synthesized using conventional approaches were found to be unstable (settling observed which is shown in Fig. 16.3), after 2 days of preparation, whereas for the ultrasoundassisted approach, no such problem was observed. The observed results can be attributed to the insufficient amount and improper distribution of surfactant molecules in system which resulted in the formation of clusters as shown in FESEM image 16.2 (b). Using the particle size analyzer, as shown in Fig. 16.4, it was confirmed that ultrasound-based capsules had narrow PSD with mean size of 65 nm, whereas for conventional method, broad PSD was observed with the mean particle size of around 50 μm. As seen in Fig. 16.3 in conventional method-based capsules, capsules were having different intensity of particle size (means particle size distribution of capsules varies from 5 to 100 μm). Better performance for the ultrasound-assisted approach could be attributed to the intense turbulence and microscale mixing introduced by the cavitational effects induced by the passage of ultrasound [38].

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FIGURE 16.3

Pictorial image of stable nanocapsules emulsion for (A) ultrasound and (B) conventional method [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

FIGURE 16.4 Particle size distribution of capsule for (A) ultrasound and (B) conventional method [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

In the mentioned work, ultrasound radiation has been used to produce nanocapsules. Droplet size and polydispersity are key attributes that govern the functionality and stability of emulsions of nanocapsules. The intense shear forces generated during ultrasonic cavitation can be used to create emulsions with very small and relatively uniformly sized droplets reported by Thomas et al [50]. FESEM and particle size data also confirmed that the use of sonication resulted in smaller size of nanocapsules [38].

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Nanocapsules were added into a solution of epoxy resin, which was prepared by diluting epoxy resin o-xylene/n-butyl alcohol mixture. With specified curing ratio, polyamide (hardener) and reactive diluent were used. Fig. 16.5 depicts the effect of loading of nanocapsules on the corrosion rate and it was observed that the corrosion rate decreased with the incorporation of the nanocapsules in the clear epoxy-polyamide coating. Loading of 2 wt.% nanocapsules resulted in significant improvement in the corrosion inhibition property and further increase in the loading to 4 wt.% which resulted in marginal improvement. The actual corrosion rate for different coatings of clear epoxy-polyamide, coating with 2 wt.% nanocapsules, and coating with 4 wt.% nanocapsules were observed to be 0.1239, 0.077, and 0.0612 cm year21, respectively. The presented results clearly showed that corrosion rate substantially decreased (around 50%) when nanocapsules (at 2 wt.% loading) were incorporated in epoxy-polyamide clear coating. Fig. 16.6 shows the Tafel plot for bare metal and clear epoxy polyamide coating containing nanocapsules (0 4 wt.%). It was observed that the corrosion current density decreased with an increase in the concentration of encapsulated A. indica-based capsules in the epoxy-polyamide clear coating. It was found that Icorr value is 0.015 A cm22 for the bare metal which decreased to 0.0011 A cm22 (for clear epoxy-polyamide coating) and to 3.15 3 1025 A cm22 when coating was applied with 2 wt.% of encapsulated A. indica as core. The Icorr value further decreased to 5.22 3 1027 A cm22 for the 4 wt.% encapsulated A. indica. The Ecorr values was also shifted from negative to positive side confirming that corrosion inhibition increased with an increase in the loading of green corrosion inhibition agent. The observed trend could be attributed to the fact that A. indica formed a passivating layer on the surface of substrate which resisted the coating from corrosion. This result was also supported by earlier result of weight study for the corrosion. The healing property of coated panel was evaluated by immersing in salt solution. The coated panels with dry thickness of 120 μm after curing of 7 days were scrubbed manually and immediately placed in 3.5 wt.% NaCl solutions at room temperature. The panel was kept immersed in FIGURE 16.5 Effect of nanocapsules loading on corrosion rate after loading into epoxy-polyamide coating [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

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FIGURE 16.6 Tafel plots for the MS stripes coated with different coatings of nanocapsules in 0.5 M HCl solution (green: bare metal, gray: epoxy neat, black: 2% neem oil, and red 4% neem oil) [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

FIGURE 16.7 Immersion study of nanocapsules in 5% NaCl solution for 20 days [38]. Source: Reprinted with permission from Green Process Synthesis 2018 (7) 147 159, De Gruyter.

the solution for a period of 20 days. The images were captured for monitoring the selfhealing behavior. The obtained photographic images shown in Fig. 16.7 confirmed that the uncoated panel shows corrosion on exposure to the salt solution. The nanocapsules of epoxy amine-coated panel showed corrosion resistance attributed mainly due to the presence of nanocapsules as

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it also showed better healing properties. Here, the core material could automatically heal the crack observed during the damage without any external trigger [38].

16.4 Polymer capsules-based self-healing coating for corrosion inhibition Self-healing materials can exhibit a core shell structure, that is, polymeric capsules filled with the healing agent. When the capsules containing liquid is triggered open, the self-healing materials recover the component from the damage without any human intervention. It is designed such that the healing takes place automatically just like the self-healing process takes place in many living organisms. There are several self-healing materials reported by various researchers. They include thermoplastic polymers, thermoset polymers, elastomers, etc. [1 7]. In recent years, research articles published in the area of self-healing were focused on the studies pertaining to the synthesis and chemistry of the self-healing materials. Different types of materials such as polymers, concretes, and metals follow a specific type of mechanism. Depending on the end use applications, suitable multifunctional materials are incorporated to recover their properties such as corrosion resistance, toughness, mechanical strength, and conductivity for the relevant operational use [7]. In general, the self-healing polymer capsule consists of the following ingredients: • Core—self-healing agent, • Shell—polymer capsule, and • Catalysts. In the event of catastrophic failure or crack, the polymer capsules are ruptured and liquid active agents from these reservoirs are released into the cracks where it get healed in the presence of preloaded catalysts. As this process starts automatically, it does not require human intervention or external aid. Fig. 16.8 displays the classification of materials based on their autonomic and nonautonomic self-healing mechanisms. In this section, we will FIGURE 16.8 Classification of materials based on their selfhealing mechanism.

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16.4 Polymer capsules-based self-healing coating for corrosion inhibition

485

place our emphasis on the concepts of extrinsic self-healing chemistries and a general overview of various self-healing systems dealing with the polymer.

16.4.1 Self-healing capsules based on polymeric materials Nanocontainers with polymer shell can be prepared by using emulsion polymerization. The shell, in this case, is a rigid structure and has a healing agent which is part of a liquid-like core structure. In this context, White et al. introduced the self-healing materials based on capsule approach [7]. In that study, it was reported that an in situ polymerization with urea formaldehyde can be induced to form microcapsule around the emulsion droplets containing DCPD. The underlying mechanism of the autonomous capsule-based self-healing process is displayed in Fig. 16.9. As shown by the figure, when the microcapsule embedded into structural composite matrix containing Grubbs’ catalyst gets ruptured, the autonomous healing process takes place into following three steps: (1) crack forms at the expense of damage (Fig. 16.9A) and (2) crack propagates and ruptures the microcapsule which eventually releases healing agent (Figs. 16.9B and 16.3) the catalyst which is in contact with the healing agent triggers the polymerization to heal the crack regions (Fig. 16.9C). This strategy had laid a benchmark and opened up a new avenue in the area of corrosion inhibition and fatigue repair. Cho et al. demonstrated the self-healing of materials based on the polycondensation of HO-PDMS [31]. In that, two self-healing approaches were explored by these authors. In the first approach, the catalyst is microencapsulated and healing agent (siloxane) is dispersed in the form of droplets into the matrix, along with the catalysts. In the second method, both the healing agents and catalyst are encapsulated in different capsules and then embedded into the matrix. However, when we use the first approach, the PDMS healing agent comes into contact with the FIGURE 16.9 Schematic description showing the underlying mechanism of self-healing process.

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matrix and becomes susceptible to a matrix-initiated reaction. For instance, the presence of amine curing agents in the matrix catalyzes the polycondensation reaction with PDMS. To overcome this issue, the strategy formulated by the second approach was used by dispersing PDMS-filled UF capsules and tetrakis(acetoxydibutyl tinoxy)silane (TKAS) catalystfilled PU capsules into the matrix. By incorporating the dual capsule, it was demonstrated that the fast and room temperature sensitive and autonomous self-healing can be achieved by initiating polycondensation reaction. Alternatively Garcia et al. introduced the selfhealing studies based on a new encapsulated water-reactive organic agent, octyldimethylsilyloleate (silyl ester) without the use of any catalyst material [51]. The method proposed by these authors does not necessitate any catalyst to accelerate the reaction and is thus devoid of the presence of a catalyst into the system. In this method, the self-healing capsule containing silyl ester was prepared by in situ poly (urea formaldehyde) microencapsulation. The proposed idea works based on the reaction between moisture (H2O) and silyl ester. The chemistry between these two triggers the formation of a hydrophobic layer (silanol) between the metal and polymer surface which eventually protect the metallic substrate from the contact with moisture. However, tunability of microcapsules for its size and mass production has posed challenges and was addressed by several researchers in the past two decades. Therefore the search for an alternative method to tackle corrosion by eliminating contact with moisture and air has been increased simultaneously over the past 10 years. Several researchers have explored different types of self-healing materials to be encapsulated and demonstrated the corrosion inhibition with some of them as shown in Fig. 16.9. They include DCPD, diisocyanates, epoxy, and drying oil. Although several researchers have demonstrated a method based on the use of oil as a healing agent to protect metal from moisture and air, Suryanarayana et al. was the first to introduce a protocol to produce linseed oil in PUF capsules and demonstrated the improved performance by tuning the size of micro/nanocapsule [33]. They reported a suitable method to control the size of microcapsule by devising an agitation rate. In situ polymerization of urea-formaldehyde resin was used to form a shell over the droplets containing linseed oil. The shell thickness of size 0.2 μm was achieved by setting the speed of the stirrer at 250 rpm. Boura et al. investigated the performance of micro- and nanosized capsules filled with linseed oil [34]. Ultrasonic energy was used to break down bigger emulsion droplets into a smaller size and produce nanocapsules. The study conducted by these authors proved that the composite matrix containing nanocapsules provided improved adhesion and corrosion resistance and eventually self-healing performance. Although several studies reported the procedure to encapsulate the linseed oil as a healing agent, very few have provided the details of the choice of stabilizers and their effects on the mechanical properties when added into the matrix. Lang et al. have clearly investigated the effect of stabilizer on the final properties of linseed oil-filled PUF capsules [52]. The intriguing part of the study is the ease of tunability of the amount of encapsulated material present in the form of capsules. The results indicated that as the molecular weight of stabilizer (PVA) increases, the mean diameter of microcapsules prepared using PUF (shell material) and linseed oil (core material) decreases. Table 16.1 presents the summary of a various fabrication method as discussed above to curb corrosion via self-healing based on polymer capsule containing self-healing agent.

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16.5 Nanocontainer-based self-healing approach for corrosion inhibition

TABLE 16.1

Methodology

Summary of different fabrication methods based on the polymer capsule. Core part (healing agent)

References

I: 220 μmII: 2.5% Grubbs’ catalyst: composition in matrix 10% microcapsules

DCPD

White et al. [7]

PUF/PU

I: 60 μm (PDMS-filled UF capsule), 90 μm (TKAS catalyst-filled PU capsule)II: 3% adhesion promoter, 3% TKAS catalyst-filled PU capsules, and 14% PDMS-filled UF capsules

Siloxane based

Cho et al. [31]

PUF

I: 100 μmII: 17.7% PUF microcapsules

Silyl ester based

Garcia et al. [51]

PUF

I: 5 100 μm

Linseed oil

Suryanarayana et al. [33]

PUF

I: 0.75 6 μmII: 5% 20% PUF microcapsules

Linseed oil

Boura et al. [34]

PUF

I: 48 138 μmII: 10% PUF microcapsules

Linseed oil

Lang et al. [52]

Shell part Capsule size (I) and matrix formulation (II)

UF polymerization PUF

DCPD, Dicyclopentadiene; PDMS, polydimethylsiloxane; PU, polyurethane; TKAS, tetrakis(acetoxydibutyl tinoxy)silane; UF, urea formaldehyde.

In summary, different kind of polymer-based self-healing capsules is discussed in this section. This section provides required insight into various self-healing processes that exploit the use of capsules containing healing agents or corrosion inhibitors. As outlined in the previous section, there are a plethora of opportunities from the perspectives of polymer-based self-healing capsule systems owing to its attractive features such as durability, light weight, and low cost. However, the central issues such as poor adhesion and mechanical strength need further investigation. Polymer nanocapsules could be one of the better candidates to curb the problem related to adhesion. On the other hand, it is indeed very difficult to deal with the problem coming from the deterioration of mechanical or physical properties typically caused by embedding of polymeric materials into the matrix. One way to tackle this issue could be by utilizing polymer nanocomposites instead of the polymer alone. However, very little attention has been paid in these nanocomposite systems. Repeated attempts must be made to shed some light on this area to understand about its suitability for their multifunctional applications.

16.5 Nanocontainer-based self-healing approach for corrosion inhibition Recent studies have shown that incorporation of nanocontainer into the paint film enhances the corrosion inhibition performance [53 55]. The important function of nanocontainer is that, when it is dispersed properly into paint film/coating matrix, forms a passive layer in which inhibitor is trapped. Nanocontainer provides the protection layer and when it is exposed to corrosive environment, it releases the inhibitor and protects the metal surface. Nanocontainer must be chosen in such a way that they provide all the benefits. Bigger

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16. Self-healing polymeric coating for corrosion inhibition and fatigue repair

microcapsules size-based system provides excellent self-healing with thermal stability. However, the use of these microcapsules limits its use in thin coating where we cannot develop the nanocontainer based on large size microcapsule. The purpose of addition of nanocontainer in coating is to make responsive to both internal and external corrosion trigger whenever, it is required. Nanocontainer is well distributed in coating matrix, which keep away corrosion inhibitor from direct with coating matrix. The corrosion inhibitor released from nanocontainer attacks onto the damage sites which provide superior corrosion resistance. Shchukin et al. [56] reported the first layer-by-layer (LBL) approach for the preparation of nanocontainer. They have taken silica nanoparticle (SiO2) with BTA as corrosion inhibitor, poly(ethylene imine) (PEI)/poly(styrene sulfonate) (PSS) as polyelectrolyte layer for formation of SiO2/PEI/PSS/BTA/PSS/BTA nanocontainer with size 70 nm. Zheludkevich et al. [57] reported the formation SiO2/PEI/PSS/BTA/PSS/BTA nanocontainer of size 70 nm and their incorporation in hybrid silica-zirconia sol gel coating for self-healing application. These coatings were applied on metallic substrate and provided long-term corrosion inhibition and control release of inhibitor. Grigoriev et al. [58] reported the nanocontainer based on polyelectrolyte inhibitor complex along with polymeric shell [i.e., SiO2/PEI/PSS (BTA/PSS)n nanocontainer] using LBL approach. These containers of size 100 nm were encapsulated in sol gel coating for self-healing. Based on the stimuli responsive trigger mechanism, controlled release of BTA occur and provide excellent self-healing ability to alloy surface. Skorb et al. [59] reported novel active smart material-based coating in the form mesoporous silica nanocontainer. 2-(Benzothiazol-2-ylsulfanyl)-succinic acid corrosion inhibitor was encapsulated in silica nanoparticle a using LBL approach. This container-based coating was system mainly focussed for industrial application due to its corrosion issue on metallic surfaces. Selfhealing evaluation of such coating was carried out using SVET. Borisova et al. [60] reported the design of coating based on 2-mercaptobenzothizole (MBT)-loaded silica nanocontainer with respect to its position and concentration in coating. They reported that nanocontainer first acted as top coat and another one act as primer for aluminum alloys surface. Kartsonakis et al. [61] reported a novel approach for cerium molybdate nanocontainer using pickering emulsion. They synthesized nanocontainer of size 145 nm with more surface area. 8-HQ and 1-H-benzotriazole-4-sulfonic acid as corrosion inhibitors were encapsulated in nanocontainer and later incorporated in a coating for protection of AA2024-T3 or DC01 steel surface. Sonawane et al. [62] reported the synthesis of LBL assembled zinc oxide nanocontainer and its kinetic mechanism for controlled release of corrosion inhibitor from container. BTA as corrosion inhibitor was loaded in the nanocontainer and later added in alkyd-based coating. Bhanvase et al. [63] reported that LBL assembled calcium zinc phosphate nanocontainer using sonochemical approach and its release followed by kinetic studies. They prepared container by sonochemical approach, and small and uniform particle size distribution was observed. Nanocontainer showed pH responsive release mechanism. Nanocontainer with increase in concentration from 0% to 4%, decreases the corrosion rate from 2.2 to 0.15 mm year21. Another approach for nanocontainer preparation using pickering emulsion was studied by many researchers. Haase et al. [64] reported the multifunctional container in the form silica-coated polystyrene with different corrosion inhibitor of different concentration of 8-HQ and MBT. They also reported that with this type of container shows better technique for the fabrication of ecofriendly waterborne coating by the control of the size of nanocontainer and increase of encapsulation percentage of inhibitor in container as compared to earlier

Self-Healing Polymer-Based Systems

16.6 Clay-based self-healing materials for corrosion inhibition

489

reported works [59 61]. Uday et al [70] reported the use of halloysite nanotube as potential nanocontainer for self-healing and corrosion inhibition coating. Halloysite [Al2Si2O5(OH) 4 3 nH2O] is a two-layered (1:1) aluminosilicate. The dimension of halloysite tubes may range of 500-1,000 nm in length and 15 100 nm in inner diameter (lumen) depending on the dispersion. This container is suitable for corrosion inhibitor incorporation and their controlled release. These inhibitors-clay embedded with epoxy matrix was suitable for self-healing corrosion inhibition application [70].

16.6 Clay-based self-healing materials for corrosion inhibition Navarchian et al. [65] reported the use of modified montmorillonite (MMT) clay with polyaniline encapsulated in epoxy coating for corrosion inhibition. Effect of different cation and clay incorporation in epoxy coating was analyzed. EIS studies were reported and result showed that it exhibited excellent corrosion inhibition compared to epoxy-based coating without clay. Truc et al. [66] reported the modified clay-based corrosion inhibition coating. Indole-3-butyric acid was encapsulated in between layer of MMT using cation exchange approach. The 2% of these modified clays were then incorporated in epoxy coating for corrosion resistance. The evaluation of coating tested by using EIS analysis and the result showed that modified clay containing coating provided higher corrosion inhibition compared with neat epoxy coating. Motte et al. [67] reported the alternative to chromate conversion coating in the form of silane sol gel coating. However, it showed a drawback of corrosion when coating film was cracked. To overcome this issue they added modified clay nanoparticle (which is prepared through cation ion exchange approach) in coating film for self-healing performance as well as corrosion inhibition. Cerium modified MMT clay along with rare earth corrosion inhibitor were encapsulated in TEOS silane-based sol gel coating and evaluated it self-healing properties using EIS analysis. The release of Ce ions from clay modified nanoreserviors was triggered by the presence of sodium or zinc ions. Zheludkevich et al. [68] reported the novel layer double hydroxide (LDH)-based nanocontainer system for self-healing application. Container composed of nanosize double layer of Mg/Al and Zn/Al hydroxides. Divanadate anions were present at the inner layer of container. These nanocontainers released the vanadate ions for corrosion protection. This type of coating is ecofriendly, and exhibited good self-healing effect as compared to chromate-based coating. Montemor et al. [69] reported the novel material by the blending of cerium molybdate nanocontainer and LDH nanocontainer in automotive sector. MBT as organic corrosion inhibitor was encapsulated in both these nanocontainers and reported their self-healing performance in terms of EIS and SVET analysis. The result showed that the anodic activity was less which favorable for the self-healing effect for these two nanocontainer-based coatings compared to neat coating.

16.6.1 Commercial applications and future prospectus There are many major developments in the field of self-healing coatings. There is a high demand from industries where the multilayer coating with self-healing functionality along with specific requirements for engineered coating applications. There are also

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16. Self-healing polymeric coating for corrosion inhibition and fatigue repair

requirements to develop the antiwear and antiscratch self-healing coatings. There is a large demand for the microcapsules which have multifunctional ability as a chemical agent, antimicrobial agent, etc. Coating which can able to heal the crack based on the influence of heat specifically using sunlight is in high demand. It is also required to design the coating which consists of dense polymers containing flexible linkages. In the self-healing area for the newer applications, cheaper products with unique customized self-healing coating are in large demand. The major key players are Bayer Material Science, Michelin, Dow Chemicals, Nissan, Akzo Nobel, and Autonomic Materials. Nissan Motor Co. Ltd. has commercialized world’s first self-healing clear coating for car surfaces. “Scratch Guard Coat.” This hydrophobic paint repair scratches (arising from car washings, off-road driving, etc.) on coated car surfaces are effective for 3 years. The Bayer Material Science developed two component PU clear coats (2 K clear coat) with trade name as Desmodur and Desmophen. Automotive industries, petroleum industries, marine industries, etc. are the major industries which will have large demand for self-healing corrosion inhibition coatings in future.

Conclusions This chapter discussed the innovative methods in self-healing materials which are being used for the corrosion inhibition at damage site. There are two case studies which are highlighted in this chapter; one is based on the addition of green inhibitor such as neem oil and second one is use of halloysite clay materials. Ultrasound-assisted preparation of the shell and core is innovative method which gives smaller particle size and improves the self-healing corrosion inhibition performance without sacrifice of the mechanical properties. Poor adhesion is crucial problem of the self-healing-based coatings; this problem can be solved with the use of polymer-based nanocapsules without sacrificing the mechanical properties of the coating matrix.

References [1] W.H. Binder, Self-Healing Polymers: From Principles to Applications, Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA, 2013. [2] Y.M. Malinskii, V.V. Prokopenko, N.A. Ivanova, V.A. Kargin, Investigation of self-healing of cracks in polymers I. Effect of temperature and crosslinks on self-healing of cracks in polyvinyl acetate, Mekhanika Polimerov 2 (1969) 271 275. [3] S.M. Wiederhorn, P.R. Townsend, Crack healing in glass, J. Am. Ceram. Soc. 53 (9) (1970) 486 489. [4] P.P. Wang, S. Lee, J.P. Harmon, Ethanol-induced crack healing in poly (methyl methacrylate), J. Polym. Science: Part B Polym. Phys. 32 (1994) 1217 1227. [5] R. Talrega, Damage Mechanics of Composite Materials, NY: Elsevier, 1994, pp. 139 247. [6] J. Raghavan, R.P. Wool, Interfaces in repair, recycling, joining and manufacturing of polymers and polymer composites, J. Appl. Polym. Sci. 71 (1999) 775 785. [7] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, et al., Autonomic healing of polymer composites, Nature 409 (2001) 794 797. [8] G.H. Deng, C.M. Tang, F.Y. Li, H.F. Jiang, Y.M. Chen, Covalent cross-linked polymer gels with reversible sol gel transition and self-healing properties, Macromolecules 43 (2010) 1191 1194.

Self-Healing Polymer-Based Systems

References

491

[9] M. Pepels, I. Filot, B. Klumperman, H. Goossens, Self-healing systems based on disulfide thiol exchange reactions, Polym. Chem. 4 (2013) 4955 4965. [10] Y.X. Lu, Z.B. Guan, Olefin metathesis for effective polymer healing via dynamic exchange of strong carbon carbon double bonds, J. Am. Chem. Soc. 134 (2012) 14226 14231. [11] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic, L. Leibler, Self-healing and thermoreversible rubber from supramolecular assembly, Nature 451 (2008) 977 980. [12] R.P. Wool, K.M. O’Connor, A theory crack healing in polymers, J. Appl. Phys. 52 (1981) 5953 5963. [13] R.J. Varley, S. van der Zwaag, Autonomous damage initiated healing in a thermo-responsive ionomer, Polym. Interface 59 (2010) 1031 1038. [14] B.J. Blaiszik, S.L. Kramer, S.C. Olugebefola, J.S. Moore, N.R. Sottos, S.R. White, Self-healing polymers and composites, Annu. Rev. Mater. Res. 40 (2010) 179 211. [15] M. Lee, An Seongpil, S.S. Yoon, A. Yarin, Advances in self-healing materials based on vascular networks with mechanical self-repair characteristics, Adv. Colloid Interface Sci. (2017). Available from: https://doi.org/10.1016/j.cis.2017.12.010. [16] V. Vahedi, P. Pasbakhsh, C.S. Piao, C.E. Seng, A facile method for preparation of self-healing epoxy composites: using electrospun nanofibers as microchannels, J. Mater. Chem. A 3 (2015) 16005 16012. [17] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (8) (2007) 581 585. [18] K.S. Toohey, C.S. Hansen, J.A. Lewis, S.R. White, N.R. Sottos, Delivery of two-part self-healing chemistry via microvascular networks, Adv. Funct. Mater. 19 (9) (2009) 1399 1405. [19] H. Ullah, K.A. Azizli, Z.B. Man, M.C. Ismai, M.I. Khan, The potential of microencapsulated self-healing materials for microcracks recovery in self-healing composite systems: a Review, Polym. Rev. (2016) 1 57. [20] E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (4) (2002) 372 379. [21] H. Jin, G.M. Miller, S.J. Pety, A.S. Griffin, D.S. Stradley, D. Roach, et al., Fracture behavior of a self-healing, toughened epoxy adhesive, Int. J. Adhes. Adhes. 44 (2013) 157 165. [22] J.D. Rule, E.N. Brown, N.R. Sottos, S.R. White, J.S. Moore, Wax-protected catalyst microspheres for efficient self-healing materials, Adv. Mater. 17 (2) (2005) 205 208. [23] J. Kamphaus, J.D. Rule, N.R. Sottos, S.R. White, J.S. Moore, A new self-healing epoxy with tungsten (VI) chloride catalyst, J. R. Soc. Interface 5 (18) (2008) 95 103. [24] T.S. Coope, U.F. Mayer, D.F. Wass, R.S. Trask, I.P. Bond, Self-healing of an epoxy resin using scandium (III) triflate as a catalytic curing agent, Adv. Funct. Mater. 21 (24) (2011) 4624 4631. [25] S. Freitas, H.P. Merkle, B. Gander, Microencapsulation by solvent extraction/evaporation: reviewing the state of the art of microsphere preparation process technology, J. Control. Rel. 102 (2005) 313 332. [26] J.W. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (11) (2005) 1791 1799. [27] Y.C. Yuan, M. Rong, M. Zhang, G.C. Yang, Study of factors related to performance improvement of selfhealing epoxy based on dual encapsulated healant, Polymer 50 (2009) 5771 5781. [28] K.R. Hart, N.R. Sottos, S.R. White, Repeatable self-healing of an epoxy matrix using imidazole initiated polymerization, Polymer 67 (2015) 174 184. [29] Q. Li, A.K. Mishra, N.H. Kim, T. Kuila, K.T. Lau, J.H. Lee, Effects of processing conditions of poly (methyl methacrylate) encapsulated liquid curing agent on the properties of self-healing composites, Compos. Part B Eng. 49 (2013) 6 15. [30] M.M. Caruso, D.A. Delafuente, V. Ho, N.R. Sottos, S.R. White, J.S. Moore, Solvent-promoted self-healing epoxy materials, Macromolecules 40 (25) (2007) 8830 8832. [31] S.H. Cho, H.M. Andersson, N.R. Sottos, S.R. White, P.V. Braun, Polydimethylsiloxane-based self-healing materials, Adv. Mater. 18 (8) (2006) 997 1000. [32] M.W. Keller, S.R. White, N.R. Sottos, A self-healing poly(dimethyl siloxane) elastomer, Adv. Funct. Mater. 17 (2007) 2399 2404. [33] C. Suryanarayana, K.C. Rao, D. Kumar, Preparation and characterization of microcapsules containing linseed oil and its use in self-healing coatings, Prog. Org. Coat. 63 (1) (2008) 72 78. [34] S.H. Boura, M. Peikari, A. Ashrafi, M. Samadzadeh, Self-healing ability and adhesion strength of capsule embedded coatings: micro and nano sized capsules containing linseed oil, Prog. Org. Coat. 75 (2012) 292 300.

Self-Healing Polymer-Based Systems

492

16. Self-healing polymeric coating for corrosion inhibition and fatigue repair

[35] T. Nesterova, K. Dam-Johansen, L.T. Pedersen, S. Kiil, Microcapsule-based self-healing anticorrosive coatings: capsule size, coating formulation, and exposure testing, Prog. Org. Coat. 75 (4) (2012) 309 318. [36] M. Samadzadeh, S.H. Boura, M. Peikari, A. Ashrafi, M. Kasiriha, Tung oil: an autonomous repairing agent for self-healing epoxy coatings, Prog. Org. Coat. 70 (4) (2011) 383 387. [37] T. Siva, S. Sathiyanarayanan, Self-healing coatings containing dual active agent loaded urea formaldehyde (UF) microcapsules, Prog. Org. Coat. 82 (2015) 57 67. [38] U.D. Bagale, S.H. Sonawane, B.A. Bhanvase, R.D. Kulkarni, P.R. Gogate, Green synthesis of nanocapsules for self-healing anticorrosion coating using ultrasound-assisted approach, Green Process. Synth. 7 (2018) 147 159. [39] K. Sayin, D. Karkas, Inhibitors, Corros. Sci. 77 (2013) 37 45. [40] U. Straetmans, M. Soltau, F. Straetmans, Polymeric Corrosion Inhibitor for Metal Surfaces and the Production Thereof, US20130330564 A1, 2013. [41] V.S. Sastri, Green Corrosion Inhibitors: Theory and Practice, John Wiley & Sons, Inc., NJ, 2011, pp. 107 115. [42] C.G. Dariva, A.F. Galio, Corrosion inhibitors principles, mechanisms and applications, Dev. Corros. Prot. (2014) 365 379. [43] V. Gentil, Corrosa˜o, fourth ed., Rio de Janeiro: LTC, 2003. [44] P.R. Roberge, Handbook of Corrosion Engineering, New York: McGraw-Hill Handbook, 1999. [45] K.T. Kim, H.W. Kim, H.Y. Chang, B.T. Lim, H.B. Park, Y.S. Kim, Corrosion inhibiting mechanism of nitrite ion on the passivation of carbon steel and ductile cast iron for nuclear power plants, Adv. Mater. Sci. Eng. (2015) 1 16. [46] R. Mehra, A. Soni, Inhibition of corrosion of mild steel by nitrite, hydrogen phosphate and molybdate ions in aqueous solution of sodium chloride, Indian. J. Eng. Mater. Sci. 9 (2) (2002) 141 146. [47] S. Mohammadi, F.B. Ravari, A. Dadgarinezhad, Improvement in corrosion inhibition efficiency of molybdatebased inhibitors via addition of nitroethane and zinc in stimulated cooling water, ISRN Corrosion (2012) 9. [48] B.S. Swaroop, S.N. Victoria, R. Manivannan, Azadirachta indica leaves extract as inhibitor for microbial corrosion of copper by Arthrobacter sulfureus in neutral pH conditions a remedy to blue green water problem, J. Taiwan. Inst. Chem. Eng. 64 (2016) 269 278. [49] P. Parthipan, J. Narenkumar, P. Elumalai, P.S. Preethi, A.U.R. Nanthini, A. Agrawal, et al., Neem extract as a green inhibitor for microbiologically influenced corrosion of carbon steel API 5LX in a hypersaline environments, J. Mol. Liq. 240 (2017) 121 127. [50] S.H. Leong Thomas, J.O. Martin Gregory, M. Ashokkumar, Ultrasonic encapsulation a review, Ultrason. Sonochem. 35 (2017) 605 614. [51] S.J. Garcı´a, H.R. Fischer, P.A. White, J. Mardel, Y. Gonza´lez-Garcı´a, J.M.C. Mol, et al., Self-healing anticorrosive organic coating based on an encapsulated water reactive silyl ester: synthesis and proof of concept, Prog. Org. Coat. 70 (2-3) (2011) 142 149. [52] S. Lang, Q. Zhou, Synthesis and characterization of poly (urea-formaldehyde) microcapsules containing linseed oil for self-healing coating development, Prog. Org. Coat. 105 (2017) 99 110. [53] H. Fischer, Self-repairing material systems-a dream or a reality, Nat. Sci. 2 (2010) 873 901. [54] N. Oya, T. Ikezaki, N. Yoshie, A crystalline supramolecular polymer with self-healing capability at room temperature, Polym. J. 45 (2013) 955 961. [55] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698 1702. [56] D.G. Shchukin, M. Zheludkevich, K. Yasakau, S. Lamaka, M.G.S. Ferreira, H. Mohwald, Layer-by-layer assembled nanocontainers for self-healing corrosion protection, Adv. Materials 18 (2006) 1672 1678. [57] M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Mohwald, M.G.S. Ferreira, Anticorrosion coatings with self- healing effect based on nanocontainers impregnated with corrosion inhibitor, Chem. Mater. 19 (2007) 402 411. [58] D.O. Grigoriev, K. Kohler, E. Skorb, D.G. Shchukin, H. Mohwald, Polyelectrolyte complexes as a smart depot for self-healing anticorrosion coatings, Soft Matter 5 (2009) 1426 1432. [59] E.V. Skorb, D. Fix, D.V. Andreeva, H. Mohwald, D.G. Shchukin, Surface-modified mesoporous SiO2 containers for corrosion protection, Adv. Funct. Mater. 19 (2009) 2373 2379. [60] D. Borisova, H. Mo¨hwald, D.G. Shchukin, Influence of embedded nanocontainers on the efficiency of active anticorrosive coatings for aluminum alloys part I: influence of nanocontainer concentration, ACS Appl. Mater. Interfaces 4 (2012) 2931 2939.

Self-Healing Polymer-Based Systems

References

493

[61] A. Kartsonakis, G. Kordas, Synthesis and characterization of cerium molybdate nanocontainers and their inhibitor complexes, J. Am. Ceram. Soc. 93 (2010) 65 73. [62] S.H. Sonawane, B.A. Bhanvase, A.A. Jamali, S.K. Dubey, S.S. Kale, D.V. Pinjari, et al., Improved active anticorrosion coatings using layer-by-layer assembled ZnO nanocontainers with benzotriazole, Chem. Eng. J. (2012) 464 472. 189-190. [63] B.A. Bhanvase, Y. Kutbuddin, R.N. Borse, N.R. Selokar, D.V. Pinjari, P.R. Gogate, et al., Ultrasound assisted synthesis of calcium zinc phosphate pigment and its application in nanocontainer for active anticorrosion coatings, Chem. Eng. J. 231 (2013) 345 354. [64] M.F. Haase, D.O. Grigoriev, H. Mohwald, D.G. Shchukin, Development of nanoparticle stabilized polymer nanocontainers with high content of the encapsulated active agent and their application in water-borne anticorrosive coatings, Adv. Mater. 24 (2012) 2429 2435. [65] A.H. Navarchian, M. Joulazadeh, F. Karimi, Investigation of corrosion protection performance of epoxy coatings modified by polyaniline/clay nanocomposites on steel surfaces, Prog. Org. Coat. 77 (2014) 347 353. [66] A. Truc, T.T.X. Hang, V.K. Oanh, E. Dantras, C. Lacabanne, D. Oquab, et al., Incorporation of an indole-3 butyric acid modified clay in epoxy resin for corrosion protection of carbon steel, Surf. Coat. Technol. 202 (2008) 4945 4951. [67] C. Motte, M. Poelman, A. Roobroeck, M. Fedel, F. Deflorian, M.G. Olivier, Improvement of corrosion protection offered to galvanized steel by incorporation of lanthanide modified nanoclays in silane layer, Prog. Org. Coat. 74 (2012) 326 333. [68] M.L. Zheludkevich, S.K. Poznyak, L.M. Rodrigues, D. Raps, T. Hack, L.F. Dick, et al., Active protection coatings with layered double hydroxide nanocontainers of corrosion inhibitor, Corros. Sci. 52 (2010) 602 611. [69] M.F. Montemor, D.V. Snihirova, M.G. Taryba, S.V. Lamaka, I.A. Kartsonakis, A.C. Balaskas, et al., Evaluation of self-healing ability in protective coatings modified with combinations of layered double hydroxides and cerium molybdate nanocontainers filled with corrosion inhibitors, Electrochim. Acta 60 (2012) 31. [70] U.D. Bagale, R. Desale, S.H. Sonawane, R.D. Kulkarni, An active corrosion inhibition coating of two pack epoxy polyamide system using halloysite nanocontainer, Prot. Met. Phys. Chem. Surf. 54(2) (2018) 230 239, ISSN 2070-2051.

Self-Healing Polymer-Based Systems

C H A P T E R

17 Applications of self-healing polymeric systems Jomon Joy1, Elssa George1, S. Anas1,2 and Sabu Thomas1,3 1

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India 2Advanced Molecular Materials Research Centre, Mahatma Gandhi University, Kottayam, India 3 International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

17.1 Introduction The term “self-healing materials” commonly implies substances or systems of materials that are able to partially or fully restore their initial properties after being mechanically damaged and rapidly developed a trend in materials science [1]. Restoring may refer to any initial characteristics such as shape, appearance, and various properties [1]. An ideal self-healing material is supposed to automatically, without any external action, recover its characteristics and integrity in a minimum amount of time necessary after the suffering damage and repeat this countless times. Artificial self-healing materials that would be similar to biological objects in terms of restoring characteristics would offer huge opportunities, especially in the cases where it is necessary to provide the reliability of materials in not easily accessible zones as long as possible during their service life [2]. This circumstance makes the development of such systems and materials increasingly promising and subsequently results in the increase in number of patents and patent applications [3]. The current applications of self-healing systems are mostly concentrated in construction, electronics, medicine, aerospace materials, special-purpose materials [4,5], etc. Self-healing materials has grown significantly in the area of aviation, and these type of materials have attracted in recent years for the development of composite materials. Composites strengthened by hollow fibers have a potential candidate to recover cracking or damages. Self-healing polymers acquired a vital role in the space and construction applications. Self-healing corrosion resistant coatings may well be useful for structural gilded parts such as steel for achieving long service life with reduced maintenance value. Self-healing materials are also

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used in biomedical applications Biocompatible self-healing composite may extend the service life of artificial bone, artificial teeth, etc. Self-healing hydrogels are widely use as drug delivery, tissue engineering, wound dressing, 3D printing applications. Self-healing rubber could find applications in toy industry [6]. Since it is a growing area, these materials find applications in various fields. This chapter discusses the main applications of selfhealing materials such as wound healing, tissue engineering, 3D printing, drug delivery, anticorrosion coatings, aerospace, and electronics applications. It is to be noted that this overview is not focused on so-called shape-memory materials, which form another field of research in science and technology.

17.2 Application in wound healing Skin, as the largest organ in the body, is vulnerable to external attacks. Various commercially available dressings, such as polyurethane (PU) films, fibers, and hydrogels, are already used for the treatment of wounds [7]. Hydrogels as the wound dressing material have a lot of inherent advantages, such as providing moist environments, absorbing wound exudates, allowing oxygen to permeate, and cooling the wound surfaces. In addition to the above properties, hydrogels possess unique properties, such as adequate flexibility, elasticity, biocompatibility, high water content, and great sensitivity to physiological environments. In recent years, significant efforts have been devoted to developing hydrogels that facilitate wound healing [8]. In particular, hydrogels with intrinsic self-healing ability have attracted much attention because they can self-heal after damage similar like a biological tissue. Shi et al. [9] developed a self-healing hydrogel based on bisphosphonatemodified hyaluronic acid (HA) through dynamic metal
ligand coordination bonds. Resulted hydrogel exhibited an in vitro antibacterial property and could promote wound healing and tissue regeneration in a rat skin defect model. Supramolecular self-healing hydrogel through cross-linkable supramonomers were prepared by Xu et al. [10]. Due to the dynamic nature of the supramonomers, the hydrogel was easy to remove, which would alleviate pain and shorten wound-healing time for patients. Sodium alginate possess good properties, such as nontoxicity, biodegradability, biocompatibility, stability and gelling ability, and has been widely employed for preparing tough and self-healing hydrogels. These hydrogels often possess an excellent adhesive property and biological affinity, but their adhesive strength decreases dramatically in physiological conditions, which greatly limits their clinical applications. Balakrishnan et al. endowed sodium alginate with aldehyde groups through an oxidation reaction and then made a hydrogel by cross-linking this with gelatin [11]. Mussel materials have given researchers great inspiration for the development of hydrogels with exceptional tissue adhesiveness. Dopamine is a kind of derivative from tyrosine in mussel proteins. It has been used to improve the adhesive property of hydrogel, especially in wetting conditions. Most research found that catechols in dopamine are easy to oxidize and transform into semiquinones or quinones which would further react with amine or thiol containing substrates via aryl
aryl coupling or possibly via Michaeltype addition reactions. Han et al. developed a polydopamine
polyacrylamide single network hydrogel with remarkable tissue adhesiveness [12].

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Additionally wound dressings with good antibacterial properties can prevent wounds from infection. The presence of ammonium group in the acid environment results in inherent antibacterial activity of chitosan. However, this antibiotic property is limited in the nonacidic environment. The quaternized chitosan has a superb antibacterial property in all condition. Zhao et al. developed a series of quaternized chitosan-based injectable and self-healing hydrogels, referred as quaternized chitosan-g-polyaniline (QCSP)/poly(ethylene glycol)-co-poly (glycerol sebacate) (PEGS-FA) hydrogel, (the number after the copolymer name meant the mass concentration of component) for wound healing [13]. The QCSP3/PEGS-FA1.5 hydrogel showed the significantly better effect of wound healing than the commercial film dressing due to the synergistic effect between quaternized chitosan and polyaniline. In addition the chitosan-based hydrogels encapsulated antibacterial drugs or polyaminoacid are excellent candidates as wound dressings. Zhao et al. prepared a series of injectable conductive selfhealed hydrogels based on QCSP and benzaldehyde group functionalized PEGS-FA as antibacterial, antioxidant, and electroactive dressing agent for cutaneous wound healing. Schematic representation of the preparation of hydrogel is shown in Fig. 17.1

FIGURE 17.1 Preparation of quarternized chitosan-g-polyaniline (A), benzaldehyde functionalized PEGS (B), and hydrogel (C). Reprinted with permission from X. Zhao, H. Wu, B. Guo, R. Dong, Y. Qiu, P.X. Ma, Antibacterial anti-oxidant electroactive injectable hydrogel as self-healing wound dressing with hemostasis and adhesiveness for cutaneous wound healing, Biomaterials 122 (2017) 34
47. Available from https://doi.org/10.1016/j.biomaterials.2017.01.011 [13].

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The characteristics of the hydrogel such as good self-healing, electroactivity, antibacterial activity, adhesiveness, and biocompatibility can be used for wound healing applications. Hydrogel with an optimal cross linker concentration of 1.5 wt.% PEGS-FA presented excellent in vivo blood clotting capacity with significantly enhanced in vivo wound healing activity in a full-thickness skin defect model than quaternized chitosan/PEGS-FA hydrogel and commercial dressing (Tegaderm film) by upregulating the gene expression of growth factors including vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), and transforming growth factor-beta (TGF) band and then promoting granulation tissue thickness and collagen deposition. The antibacterial electroactive injectable hydrogel dressing prolonged the lifespan of dressing relying on self-healing ability and significantly promoted in vivo wound healing process. This is attributed to its multifunctional properties, suggesting that they could be excellent candidates for full-thickness skin wound healing. Generally an ideal hydrogel for wound dressing should have several typical properties, such as efficient self-healing ability, high toughness, outstanding cell affinity, and tissue adhesiveness.

17.3 Application in tissue engineering The aim of tissue engineering is to develop tissue and organ substitutes for maintaining, restoring, or augmenting functions of their injured or diseased counterparts in vivo. In recent decades, increasing demand for biomaterials capable of aiding in regeneration or replacement of damaged tissue motivated the development of new tissue engineering constructs [14]. The demand for engineered tissues has risen rapidly owing to the limited availability of donor tissues and organs for transplantation [15]. Smart hydrogels can serve as promising cell delivery vehicles for therapeutic healing and tissue regeneration because of their high water content, as well as the responsiveness to various environmental stimuli such as temperature, pH, and enzymes. The properties of hydrogels follow from their structure: namely their highly swollen, hydrophilic 3D cross-linked polymer network that may be either chemically or physically cross-linked to form a material that mimics advantageous properties of the highly hydrated extracellular matrix (ECM) and facilitates nutrient and oxygen transport due to its porous structure. Hydrogels have a long history as tools for tissue regeneration and 3D cell culture as they may be engineered to mimic the desired aspects of the native local ECM depending on their intended usage [16]. Supramolecular hydrogels have emerged as a promising tool for tissue regeneration as they can be biocompatible and recapitulate the viscoelastic nature of the ECM better than their elastic, covalently cross-linked counterparts due to the presence of dynamic linkages. The resulting viscoelastic and dynamic behavior of these linkages are responsible for other advantages such as self-healing and injectability [17]. Supramolecular hydrogels can selfheal after damage either spontaneously or in the presence of a physiological stimulus. This characteristic extends the lifetime of materials and makes them ideal candidates for applications involving repeated mechanical stress or injection. Ideally hydrogels for tissue engineering should enable cell infiltration as well as encapsulate and deliver cells and biologics and be able to autonomously, rapidly, and repeatedly heal in situ at physiological conditions [18].

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17.4 Application in three-dimensional printing Additive manufacturing technologies, such as 3D printing, enable the fabrication of such complex, multiscale structures through computer-aided design and hardware capable of depositing materials, including for bioprinting cells [19]. Limitations of 3D printing are often related to the materials being printed. Hydrogel inks are particularly useful in biomedical applications to mimic or reproduce features of cellular environments and to form constructs that can be used to replace, augment, or model tissues. Cell-laden hydrogels are commonly used in 3D bioprinting and are termed as “bioinks” as they can mimic or reproduce features of cellular environments [20,21]. However, 3D bioprinting that allows the printing of materials into 3D space at any specified point with high resolution and multiple length scales are currently unrealized. A dual-cross-linking HA system [adamantine (Ad) and β-cyclodextrin (β-CD) modified HA guest
host complex] was studied by Burdick and coworkers as a printable hydrogel ink, which consists of both shear-thinning and self-healing behaviors via guest
host bonding, as well as covalent cross-linking for stabilization using photopolymerization [22]. The hydrogels can quickly self-heal due to their noncovalent and reversible bonds, which allows both “ink” and “support” gels being disrupted of a physical stimulus such as shear stress while maintaining material localization into any position within a 3D space of the “support” gel after the printing. Patterning of mesenchymal stem cells (MSCs) into 3T3 fibroblasts containing support gel was accomplished by this 3D printing process. Such structure could be maintained after several days in culture with minimal loss of cells in viability ( . 90% viable). To maintain the mechanical properties that is necessary for longterm stability or perfusion of the 3D printed hydrogel, the authors also introduced methacrylates into the HA macromers and applied photocross-linking to introduce a secondary, covalent crosslinking mechanism in either ink or support materials to selectively stabilized against physical or chemical perturbations. The secondary crosslinking did not affect the printing process. The ink gel without the secondary crosslinking can be removed by flow and left behind an open channel with the printed patterns and dimensions in photocrosslinked support gels. Self-healing hydrogels, based on the host
guest interaction of β-CD and Ad-modified HA, were used in the 3D printing of high-resolution structures through printing of shearing-thinning ink hydrogel into self-healing support hydrogel [21]. The multi cellular structures could be expediently patterned, such as printing of MSCs within an ink hydrogel into a support hydrogel containing 3T3 fibroblasts. The channellike structure was achieved by writing the ink hydrogel into the methacrylate-modified support hydrogel, followed by UV irradiation for secondary covalent crosslinks of support hydrogel and removal of the physical (i.e., host
guest) ink hydrogel. Meanwhile, the selfsupporting structure was obtained by covalently crosslinking the ink hydrogel and removing the noncovalent support hydrogel. This system supported the patterning of multiple inks, cells, and channels in 3D space. Mechanically robust 3D nanostructure chitosanbased hydrogels with autonomic self-healing properties have been reported [23]. The hydrogel nanocomposites proved to have a very good film forming properties, high modulus and strength, acceptable electrical conductivity, and excellent self-healing properties at neutral pH. The high mechanical strength coupled with self-healing capabilities, electrical conductivity, and the good film-forming properties may guarantee the use of

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MWCNT
COOH-reinforced self-healing CS/ZnPcTa hydrogels in a variety of fields, and the natural origin of the constituents may render the hydrogels the suitable candidates for biomedical applications.

17.5 Application in drug delivery Injectable hydrogels are promising for controlled release of encapsulated therapeutics (cell therapy and drug delivery) by being injected as a liquid and gelated in situ to accommodate irregular defects of desired position [24
26]. However, traditional injectable hydrogels with slow gelation may lead to cargo loss (cell/drug) and diffusion from the target site, while the extremely rapid gelation would result in undesired premature solidification. These practical problems can be solved by using self-healing hydrogels. The major difference between traditional drug delivery by intravenous cycling and in situ drug delivery by self-healing hydrogel represented schematically (Fig. 17.2). Xing et al. reported an injectable and self-healing collagen gold hybrid hydrogel with adjustable mechanical properties [28]. This colloidal gel was ready through electrostatic interaction between charged sclera protein chains and charged tetrachloroaurate [AuCl4]2 ions and more noncovalent interactions between ensuant biomineralized gold nanoparticles and scleroprotein. The hydrogel was developed for localized delivery and sustained release of the photosensitive drug. By combinatorial photothermal and photodynamic therapies, the significantly enhanced antitumor efficacy was demonstrated through an in vivo antitumor test in the subcutaneous mouse model. Self-healing hydrogels based on glycol chitosan and DFPEG (GC-DP) have been developed for intratumor therapy in vivo. GC-DP hydrogel containing antitumor drug was FIGURE 17.2 Schematic representation of the difference between traditional drug delivery (A) by intravenous cycling and in situ drug delivery (B) by self-healing hydrogel. Reprinted with permission from L. Yang, Y. Li, Y. Gou, X. Wang, X. Zhao, L. Tao, Improving tumor chemotherapy effect using an injectable self-healing hydrogel as drug carrier, Polym. Chem. 8 (2017) 5071
5076. Available from: https://doi.org/10.1039/c7py00112f [27].

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injected into the disease position with a steady release in situ [27]. Moreover, the ionic GC-DP hydrogel exhibited microwave susceptibility to produce high-temperature hyperthermia for tumor ablation [29]. A multiantitumor system was developed based on GC-DP hydrogel containing doxorubicin/docetaxel-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles and iron oxide for chemotherapy and magnetic hyperthermia. The system showed the greater in vivo antitumor efficacy under the alternative magnetic field compared to the hydrogel containing doxorubicin/docetaxel-loaded PLGA nanoparticles. Moreover, to improve remote controllability of the drug delivery system, magnetic self-healing hydrogels have attracted particular attention. The fragments of self-healing hydrogels with magnetism can be easily guided and automatically merge together under a magnetic field, rendering self-healing and magnetic remote control. Zhang’s group [30] embedded carboxy modified Fe3O4 nanoparticles into chitosan
PEG self-healing hydrogels, affording dual functionalities, i.e., self-healing and magnetism, to the hydrogel In another development, researchers developed an ultrasound-triggered drug delivery strategy to improve chemotherapy via ionically cross-linked self-healing hydrogel. The hydrogels cross-linked from alginate and Ca21 could be disrupted by ultrasonic and reversibly reformed in the presence of physiological level of Ca21. Introduction of mitoxantrone into the hydrogels demonstrated ultrasound-triggered “burst” release. In vivo studies of self-healing hydrogel in treatment of xenograft human breast tumors in nude mice confirmed ultrasound-triggered drug release inhibited tumor growth, since the hydrogel induced apoptosis compared with systemic drug delivery in the short term [31]. This study validates the importance of cross-linked self-healing hydrogel in drug delivery as well as cancer therapy.

17.6 Application in anticorrosion coating It is 21st century; the world and its people are running at its peak pace, as such is the industrialization and its boom catching up. When we mention about “catching up,” anything that delays a process in an industry is a bottleneck that restricts the usual flow of work. One of the top notch destructive processes that result in high economic causalities in the industries are due to the “corrosion” of metals. In recent studies it has been estimated that the direct cost of corrosion accounts upto about 2%
5% of the nation’s GDP. It requires expensive efforts to effectively reduce its impact and usually has to be planned and executed from the design stage itself. Corrosion protection is one of the essential methods to avoid process disruptions caused by metal destruction, and it could be achieved through organic, inorganic, or metallic coatings. For example, paint coatings are used widely to prevent corrosion [32
36]. But most of these methods are not fool proof and may fail due to misapplication, defects, wrong choice, etc. In certain cases, mechanical forces might be the culprit; minute cracks could break inside and expose the inner parts of the metals to oxygen leading to corrosion. In such cases, the damage might go unnoticed until the device/equipment affected gets fully damaged. In such instances, the recently developed class of smart materials known as the “self-healing polymers” could do the autonomic repair without any manual detection or intervention [37
42]. These polymers are designed to withstand the

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mechanical and chemical impacts from the external environment, and it is able to mimic the original properties of the substrate that it is coated. The most effective self-healing anticorrosion coatings have been employed by chromium (VI) compounds [43]. However, this species is highly dangerous to human life, and it can even alter DNA and causes several diseases, including cancer. [44] Therefore current researches are aiming to develop substitutes for such coatings and come with self-healing polymers. Polymers and composites with properties such as oxidizing, cross linking ability, swelling, and low temperature production are mostly chosen to be used as self-repair coatings due to the ease of its modification [45
59]. These list includes polyaniline [45,46], polypyrrole [52], vinyl ester polymers [56], epoxy [45,60], cellulose [47], polystyrene [59], etc. Encapsulation, reversible chemistry, micro vascular networks, nanoparticle phase separation, polyionomers, hollow fibers, monomer phase separation, etc., are different ways of achieving the healing functionality. However, all these could not be used as a coating material due to its mechanical and chemical limitations. Industrially important coatings are currently based on siloxane materials, and it has two major approaches for implementation [61,62]. In the first approach, siloxanes are present as phase-phase-separated droplets as the catalyst gets micro encapsulated. In the latter approach, the siloxanes were also encapsulated and dispersed in the coating matrix. Encapsulation of both phases (the catalyst and the healing agent) is advantageous in cases where the matrix can react with the healing agent. This healing chemistry of siloxane is attractive because it is air stable and water stable and remains active even after exposure to elevated temperatures, enabling its use in systems requiring a thermal cure. While the mechanical properties of the resultant cross-linked siloxane are not exceptional, in a coating system the mechanical strength of the healed matrix is of secondary importance, compared to chemical stability and passivating ability, two areas where siloxanes show exceptional performance. There are many recent studies going on in the case of coatings based on epoxy resins, one of which involves an alternative chemistry on Michael addition between bismaleimides and amines or thiols [50]. The maleimide conjugation reaction with amines ensures chemical bonding of the applied substrate to that of the coating during crack healing. Another study shows higher coating resistance on mild steel when coated with epoxy containing polyo-phenylenediamine (PoPD) nanotubes [63]. In the self-healing process of epoxy vinyl ester matrix, dimethyldineodecanoatetin (DMDNT) catalyst solution is encapsulated in PU capsules, and the phase separated polydimethylsiloxane (PDMS) based healing agents were dispersed in the polymer matrix [64]. Damaged and healed coated steel samples were corrosion tested, and the experiment resulted in an autonomic corrosion protection, which can be achieved by self-healing under ambient environmental conditions. A novel polyaniline/organic
inorganic hybrid sol
gel coating was successfully prepared and found that the impedance of the coating remained stable for up to 24 months in neutral 3.5% NaCl solution (Fig. 17.3). In addition SVET tests conducted in neutral pH solution showed that the PANI/sol
gel system also provides corrosion protection via a “self-repair” mechanism when the underlying substrate is exposed to the chloride solution. This behavior is assigned to the ability of the polyaniline to undergo oxidation
reduction reactions [46].

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FIGURE 17.3 SEM image of sol
gel-only coated sample after 16 days immersion in neutral 3.5% NaCl solution. Reprinted with permission from R. Akid, M. Gobara, H. Wang, Corrosion protection performance of novel hybrid polyaniline/sol-gel coatings on an aluminium 2024 alloy in neutral, alkaline and acidic solutions, Electrochim. Acta. 56 (2011) 2483
2492. Available from: https://doi.org/10.1016/j.electacta.2010.12.032 [46].

The self-healing property of TiO2 particle
vinyl ester polymers composite was evaluated on the surface of an aluminum alloy [57]. An artificial defect was imposed on the alloy and was monitored under seawater at a temperature of 308 C. The polarization resistance spectrum of the alloy increased over time, proving the self-healing properties of the coating. The list of self-healing polymers also extends to coatings based on polystyrene sulfonate exchange resin [59] biopolymers such as chitosan with Ce(NO3)3 addictive [48,65], polypyrrole dopes with heteropolyanions of PMo12O4o3, HPO42 [52], etc. The world of these polymers is yet to explore, and this could be one of the key attributes to the future of all our industries and mankind itself when fully utilized.

17.7 Application in electronic application Recently organic materials-based electronics have great development in performance, stability, and lowering of costs of materials and processing, thereby providing enhanced possibility for commercialization. The term electronic skin (or E-skin) demonstrates stretchable, flexible, and dexterous electronics, which in some cases can mimic the properties of human skin. A biomimetic E-skin should process self-repairing ability to function as a protective barrier for external damages similar to that of human skin. Both mechanical sensing and repeatable self-healing capabilities owned by human skin are not completely satisfied by the electronic skin devices mainly the repeatable self-healing property at the same damage location. The ability of human skin to recognize both medium pressure (10
100 kPa, suitable for object manipulation) and low pressure (,10 kPa, comparable to gentle touch) perturbations should be mimic by the E-skin to make it replica of human skin [66,67]. There has been intense research into self-healing materials in the field of electronics, prosthetics, artificial intelligence, systems for robotics, personal health monitoring, and

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communication devices [68
74]. Self-healing materials can be broadly classified into two based on the nature of healing process via nonautonomic and the autonomic. The external factors such as light, heat, or chemicals should trigger the self-healing of nonautonomic self-healing materials whereas damage resulted itself initiates the healing of autonomic self-healing materials. A lack of electrical conductivity makes the E-skin materials to restrict their use in electronic applications. The microcapsules containing various types of liquid precursor healing agents for structural healing and various conductive agents or solvents are used for autonomous electrical healing of thin metal films. But in these cases healing agents are drained after a single healing process. The electric properties have to be incorporated into the materials for the polymers to be E-skin. Self-healing electrical polymers can be divided into two main categories, reversible type with dynamic reversible bonds and irreversible type with healant capsules. The former one is the most direct method to develop a selfhealing conductor. The first report on self-healing conductor using electric polymer (Fig. 17.4) where the conjugated cores are cross-linked by the reversible bond between N-heterocyclic carbenes (NHCs) and transition metals, which are both electrically conductive and structurally dynamic [75]. Due to the dynamic feature of the polymer, the micron sized cracks are healed by treating at 150 C in DMSO vapor. The presence of solvent vapor enhances the dynamic ability of the complex. In addition to this organometallic polymer, several other conductive polymers are also studied for the E-skin application. Supramolecular rubber with Ga-In microchannels [20], silver nanowires coated by polymer electrolyte multilayers [76], poly(urea-formaldehyde) microcapsules with tetrathiafulvalene (TTF)
tetracyanoquinodimethane (TCNQ) [77], hydrogen bonding polymers with Ni particles [78], PU embedded with capsules of silver paste [79], etc. are some of the examples. Incorporating conductive inorganic material into nonconductive polymers is another approach of developing self-healing conductors. The inorganic part provides conductivity, and polymer part provides self-healing ability to the material. For the proper functioning of self-healing material, the inorganic material should be in good compatible with the polymer matrix for the easy movement of inorganic part along with polymer during the crack healing and to avoid the phase separation thereby providing electrical healing. The polymer host with a hydrogen bond network with flower such as Ni microparticles was reported recently as a novel conductive self-healing material that can heal at room temperature [78]. The thin oxide layer that covered the conductive filler form hydrogen bonding with polymer host makes the μNi compatible to polymer material. A high conductivity of 40 S cm21 was observed above percolation threshold (15 vol.% of Ni).

FIGURE 17.4

Dynamic covalent chemistry in self-healing organomettalic polymer.

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The composite shows electric and mechanical self-healing even at 30 vol.% of μNi at room temperature, making it suitable for self-healing conductors. Sensing is another important ability of human skin. The above mentioned supramolecular composite also serve as self-healing. The composites are sensor to external forces both flexion and tactile pressure, when the volume percentage is below the threshold value, and the μNi particles are spaced apart. The flexion sensor become active when the composite resistance get reduced as the μNi particles came closer due to build up stresses arise from the bending of material. Similar effect serves the tactile sensor from the external pressure. A self-healable pressure sensor nanocomposite based on polysiloxane was fabricated by incorporating graphene nanosheets into the elastomer [80]. The prepared intrinsic selfhealable pressure sensor exhibited a high sensitivity of 0.765 kPa21 and a gauge factor of 4.87 which promises potential application in wearable sensors. The self-healing property of the elastomer was impacted by dynamic Diels
Alder bonds that cross-linked the polysiloxane elastomer network. Excellent tensile stress and electromechanical property were achieved by the addition of graphene sheets into the thermal reversible polysiloxane elastomer. The self-healing polymers could also incorporate CNT to form a conductive composite for humidity sensor [81]. Bottom-up approach was implemented for the construction of the composite. Through π
π stacking, pyrene-modified β-CD is attached to the surface of CNT, and then self-healing conductive composite was built using host
guest chemistry of 2-hydroxyethyl methacrylate (HEMA) and β-CD. At the end, the constructed composite was cross-linked through polymerization in the presence of ethylene glycol dimethacrylate. A detailed discussion on different self-healing polymers for analyte sensing was reported by Huynh and Haick [82]. A complete self-healing E-skin can be developed by integrating sensor materials with the conductors where both the conductive electrodes and the pressure sensor can self-heal. The material can be molded into a fat piece or mounted onto PET substrate into a flexion sensor [78]. A tactile sensor has a parallel plate structure with the piezo-resistive composites and sandwiched between the conductive composite with its piezo-resistive response. Both sensors were then mounted on the palm and elbow joint of a humanoid mannequin. The light intensity of LEDs was used as indicators of mechanical forces, and at different pressure and different flexion angles, the LED was shown to have different intensities, as it responds to the different forces and movements. Supercapacitors, field effect transistors, and perovskite solar cells are the other applications of self-healing electronics. The deformation or breakage caused due to mechanical damage shortens the life time of these energy storage devices. Fabricating supercapacitors using self-healing materials is a good alternative to restore the electrical properties after mechanical damage. In 2014 first self-healing supercapacitor was reported [83]. A planarlike electrode containing SWCNT that spreaded on self-healing polymer material (LSHP) restores electrical conductivity upto 85.7% after damage. Later much efficient wire shaped supercapacitor using same self-healing polymer with three different conducting materials [84] and a yarn shaped capacitor wrapped by magnetic electrodes in self-healing PU polymer were produced [85]. The introduction of perovskite solar cells has intensively changed the photovoltaics (PV) technology due to the versatile properties of the organic
inorganic metal halide

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perovskites such as suitable bandgap, high-absorption coefficient, and long-charge carrier diffusion length, but Their poor stability towards oxygen, moisture, and UV lights was the main concern of scientist. [86
90] Zhao et al. reported a self-healing moisture absorber, polyethylene glycol (PEG) that can help to improve the stability and boost the commercial viability of the perovskite PV devices [91].

17.8 Application in aerospace applications Any industrial application was once just an idea generated in a laboratory and then implemented in the same place before being introduced into the industries. There are many developing areas of research which are still in the laboratory levels and waiting for entering industrial requirements. Self-healing polymers are such a developing area of research with plenty applications in aeronautic or aerospace field. Aerospace industry projects a lot of challenges for the selection of materials while building of each components of a space craft due to their severe working conditions. The space crafts are subjected to impact damages caused by unfavorable weather conditions, stones, or rocks on the keel while landing, accidents, collisions during maintenance, hard strike, etc. These damages can affect the strength and safety of the aircraft and even end up in a catastrophe [92]. Most of the damages can be only a weakening of the integrity of the structure, and this will affect the total strength of the structure [93]. Modern research has developed many solutions for the aerospace industry challenges. Carbon fiber reinforced plastics (CFRPs) are one among them. The commercially available carbon fiber has a compressive strain to failure of around 1%; whereas CFRPs are assigned an allowable compressor strain level of ,0.4% [94,95]. As discussed above, the reverse working conditions of aircrafts and aerospace crafts may damage the integrity of the structure and permit ingress of contaminants into the structure. Damages can be result in the formation of micro voids which later expand to generate the micro cracks within the surface. These internal damages are difficult to diagnose and repair once developed. The maintenance and detection of such damages are very difficult and expensive, and CFRPs are really sensitive to impact damages. As CFRPs are prone to the damages and require always inspection and repairs; the concept of self-healing is a big solution to the limitations of CFRP. Self-healing polymer is a better replacement for the conservative design and takes away the requirements for performing temporary repair to damaged structures. It has a great potential for sandwich structures or as a single structural materials. An aircraft made from self-healing polymers can reduce the thicker structures to thinner structures and thus bringing more efficiency in long time by reducing airframe weight and cost-effective. There are two categories of self-healing polymers—intrinsic and extrinsic. The classification of self-healing polymers is based on the ways of healing. Intrinsic self-healing polymers can heal cracks by itself while extrinsic self-healing polymers need preembedded healing agent inside them [96,97]. In extrinsic self-healing polymers due to some chemical interactions of the functionality present, the polymer matrix undergoes healing process when certain stimulations occur. They do not possess autorepair ability, and a healing agent has to be stored in the vessel. They are to be embedded into the polymeric matrix in

Self-Healing Polymer-Based Systems

17.8 Application in aerospace applications

507

advance during the manufacturing of the material. When a crack occurs the fragile vessel which stores healing agents are ruptured releasing the healing agent into the crack planes due to capillary forces and heal the cracks. The extrinsic self-healing system is of great preference in aeronautical structural applications as they make easier damage diagnosis and reduce further crack progression. The University of Illinois developed the first capsule based self-healing system for thermo setting polymer [98,99]. It was based on the ring opening metathesis polymerization (ROMP) for dicyclopentadiene (DCPD) healing agent with a transition metal catalyst (Grubb’s) [100]. DCPD was made into microcapsules and then dispersed in epoxy resin along with Grubb’s catalyst. In the presence of a crack, the DCPD microcapsules rupture and release DPCD monomer into the crack. The polymerization of DCPD is initiated when they get in contact with the embedded Grubb’s catalyst and thus the crack plain is rebonded. Above 90% fracture recovery was yielded with self-healing epoxy, and they also provided extended fatigue life [101]. As the operating range of aircraft varies roughly from 250 C to 160 C, the self-healing functionality should be effective at this range of temperatures also. Hence the self-healing materials used for aeronautic and aerospace must satisfy high thermal stability of selfhealing components of catalysts and vessels filled with healing agent, chemical stability, and high healing efficiency at lower temperatures. To be a healing agent, the liquid monomer that microencapsulated should possess a long shelf life, high reactivity, easy deliverability, and low volume shrinkage upon polymerization [102]. DCPD is the common monomer used as the healing agent for the system [97,102]. In all the thermosetting autorepair polymers systems, Grubbs’ first-generation catalyst (G1) [38,103
105] and recently the possibility of applying other ruthenium catalysts, such as second-generation Grubbs’ catalyst (G2) and Hoveyda
Grubbs’ secondgeneration catalyst (HG2), are under investigation [106,107]. Recently Merle and coworkers reported the use of 5-ethylidene-2-norbornene (ENB) as a healing agent active at low temperature [108]. Later Guadagno et al. analyzed the ROMP of this healing agent using G1, G2, and HG1 catalysts, and obtained results were compared with those obtained using the most common healing agent (DCPD monomer) [109,110]. The result showed (Table 17.1) that the metathesis polymerization of ENB activated by HG1 catalyst offers best selfhealing function because of its thermal stability and strong activity at lower temperature, making them suitable for the self-healing materials in aeronautic. Recently an electrically conductive self-healing resin based on reversible hydrogen bonds (RHB) interaction has been developed [111]. The presence of hydrogen bonding moieties on functionalized MWCNT walls was able to establish RHB interactions with
OH of epoxy resin. A CuAAC “click” reaction was performed for the functionalization of the MWCNTs with hydrogen bonding moieties. The chosen functional groups, which are covalently attached on the walls of carbon nanotubes, are able to activate hydrogen bonds via electron donation and electron acceptance. The stress resulted in the breaking of hydrogen bonds, which have the ability to regenerate the electron donor/acceptor interaction without any external stimulus. This approach have combined advantages for the design and manufacturing of self-healing nanocomposites with potential for fulfilling many requirements of structural applications in many technological fields, such as aerospace, automotive, etc.

Self-Healing Polymer-Based Systems

508

17. Applications of self-healing polymeric systems

TABLE 17.1 Comparison of ROMP reaction of DCPD and ENB with G1, G2, and HG1 catalysts. Catalyst/monomer ratio

Temperature ( C)

Time (min)

Yield (%)

1:1000 G1/DCPD

25

15

34

1:1000 G1/DCPD

0

5

0

1:1000 G1/DCPD

10

30

0

1:1000 G1/ENB

25

0.5

100

1:1000 G1/ENB

0

13

100

1:1000 G1/ENB

2 10

60

17

1:1000 G1/ENB

2 20

1440

79

1:1000 G1/ENB

2 30

1440

74

1:1000 G1/ENB

2 40

1440

52

1:1000 G2/ENB

25

1

99

DCPD, Dicyclopentadiene; ENB, 5-ethylidene-2-norbornene; ROMP, ring opening metathesis polymerization.

17.9 Conclusions Self-healing materials have great potential for advanced engineering systems, and these types of systems respond without external intervention to environmental stimuli in a nonlinear and productive fashion. Over the last decade, self-healing technology and its science have been developed at a faster rate and led to the development of new polymers, polymer blends, polymer composites, and smart materials with self-healing capabilities. Implementation of self-healing properties in polymeric materials can now effectively solve numerous damage mechanisms at molecular and structural levels. Self-healing materials find applications in various fields such as biomedical, electronic, aerospace, and coatings due to its exceptional properties. Precision and design of self-healing materials are very significant for the commercial production of these materials in different fields. In addition to above mentioned areas, self-healing rubber, self-healing concrete, etc., have gained much attention for the scientists. There are several areas that are well unexplored and have great opportunities in developing new self-healing polymers remain as a fertile area of future research.

References [1] V.K. Thakur, M.R. Kessler, Self-healing polymer nanocomposite materials: a review, Polymer (Guildf.) 69 (2015) 369
383. Available from: https://doi.org/10.1016/j.polymer.2015.04.086. [2] Y. Yang, X. Ding, M.W. Urban, Chemical and physical aspects of self-healing materials, Prog. Polym. Sci. 49
50 (2015) 34
59. Available from: https://doi.org/10.1016/j.progpolymsci.2015.06.001. [3] S.J. Garcia, Effect of polymer architecture on the intrinsic self-healing character of polymers, Eur. Polym. J. 53 (2014) 118
125. Available from: https://doi.org/10.1016/j.eurpolymj.2014.01.026.

Self-Healing Polymer-Based Systems

References

509

[4] H.M. Jonkers, A. Thijssen, G. Muyzer, O. Copuroglu, E. Schlangen, Application of bacteria as self-healing agent for the development of sustainable concrete, Ecol. Eng. 36 (2010) 230
235. Available from: https://doi. org/10.1016/j.ecoleng.2008.12.036. [5] S.J. Benight, C. Wang, J.B.H. Tok, Z. Bao, Stretchable and self-healing polymers and devices for electronic skin, Prog. Polym. Sci. 38 (2013) 1961
1977. Available from: https://doi.org/10.1016/j.progpolymsci.2013.08.001. [6] R. Zhang, T. Yan, B.D. Lechner, K. Schro¨ter, Y. Liang, B. Li, et al., Heterogeneity, segmental and hydrogen bond dynamics, and aging of supramolecular self-healing rubber, Macromolecules 46 (2013) 1841
1850. Available from: https://doi.org/10.1021/ma400019m. [7] N. Annabi, D. Rana, E. Shirzaei Sani, R. Portillo-Lara, J.L. Gifford, M.M. Fares, et al., Engineering a sprayable and elastic hydrogel adhesive with antimicrobial properties for wound healing, Biomaterials 139 (2017) 229
243. Available from: https://doi.org/10.1016/j.biomaterials.2017.05.011. [8] A. Phadke, C. Zhang, B. Arman, C.C. Hsu, R.A. Mashelkar, A.K. Lele, et al., Rapid self-healing hydrogels, Proc. Natl. Acad. Sci. U. S. A. 109 (2012) 4383
4388. Available from: https://doi.org/10.1073/pnas.1201122109. [9] L. Shi, Y. Zhao, Q. Xie, C. Fan, J. Hilborn, J. Dai, et al., Moldable hyaluronan hydrogel enabled by dynamic metal
bisphosphonate coordination chemistry for wound healing, Adv. Healthc. Mater. 1700973 (2017) 1
9. Available from: https://doi.org/10.1002/adhm.201700973. [10] W. Xu, Q. Song, J.F. Xu, M.J. Serpe, X. Zhang, Supramolecular hydrogels fabricated from supramonomers: a novel wound dressing material, ACS Appl. Mater. Interfaces 9 (2017) 11368
11372. Available from: https:// doi.org/10.1021/acsami.7b02850. [11] B. Balakrishnan, N. Joshi, A. Jayakrishnan, R. Banerjee, Self-crosslinked oxidized alginate/gelatin hydrogel as injectable, adhesive biomimetic scaffolds for cartilage regeneration, Acta Mater. Inc. (2014). Available from: https://doi.org/10.1016/j.actbio.2014.04.031. [12] L. Han, L. Yan, K. Wang, L. Fang, H. Zhang, Y. Tang, et al., Tough, self-healable and tissue-adhesive hydrogel with tunable multifunctionality, NPG Asia Mater. 9 (2017). Available from: https://doi.org/10.1038/am.2017.33. [13] X. Zhao, H. Wu, B. Guo, R. Dong, Y. Qiu, P.X. Ma, Antibacterial anti-oxidant electroactive injectable hydrogel as self-healing wound dressing with hemostasis and adhesiveness for cutaneous wound healing, Biomaterials 122 (2017) 34
47. Available from: https://doi.org/10.1016/j.biomaterials.2017.01.011. [14] F. Gelain, D. Silva, A. Caprini, F. Taraballi, A. Natalello, O. Villa, et al., BMHP1-derived self-assembling peptides: hierarchically assembled structures with self-healing propensity and potential for tissue engineering applications, ACS Nano 5 (2011) 1845
1859. Available from: https://doi.org/10.1021/nn102663a. [15] A. Hasan, A. Khattab, M.A. Islam, K.A. Hweij, J. Zeitouny, R. Waters, et al., Injectable hydrogels for cardiac tissue repair after myocardial infarction, Adv. Sci. 2 (2015) 1
18. Available from: https://doi.org/10.1002/ advs.201500122. [16] M.E. Furth, A. Atala, M.E. Van Dyke, Smart biomaterials design for tissue engineering and regenerative medicine, Biomaterials 28 (2007) 5068
5073. Available from: https://doi.org/10.1016/j.biomaterials.2007.07.042. [17] L. Saunders, P.X. Ma, Self-healing supramolecular hydrogels for tissue engineering applications, Macromol. Biosci. 19 (2019) 1
11. Available from: https://doi.org/10.1002/mabi.201800313. [18] G.D. Nicodemus, S.J. Bryant, Cell encapsulation in biodegradable hydrogels for tissue engineering applications, Tissue Eng. - Part. B Rev. 14 (2008) 149
165. Available from: https://doi.org/10.1089/ten.teb.2007.0332. [19] X. Kuang, K. Chen, C.K. Dunn, J. Wu, V.C.F. Li, H.J. Qi, 3D printing of highly stretchable, shape-memory, and self-healing elastomer toward novel 4D Printing, ACS Appl. Mater. Interfaces 10 (2018) 7381
7388. Available from: https://doi.org/10.1021/acsami.7b18265. [20] E. Palleau, S. Reece, S.C. Desai, M.E. Smith, M.D. Dickey, Self-healing stretchable wires for reconfigurable circuit wiring and 3D microfluidics, Adv. Mater. 25 (2013) 1589
1592. Available from: https://doi.org/ 10.1002/adma.201203921. [21] C.B. Highley, C.B. Rodell, J.A. Burdick, Direct 3D printing of shear-thinning hydrogels into self-healing hydrogels, Adv. Mater. 27 (2015) 5075
5079. Available from: https://doi.org/10.1002/adma.201501234. [22] L. Ouyang, C.B. Highley, C.B. Rodell, W. Sun, J.A. Burdick, 3D printing of shear-thinning hyaluronic acid hydrogels with secondary cross-linking, ACS Biomater. Sci. Eng. 2 (2016) 1743
1751. Available from: https://doi.org/10.1021/acsbiomaterials.6b00158. [23] A.R. Karimi, A. Khodadadi, Mechanically robust 3D nanostructure chitosan-based hydrogels with autonomic self-healing properties, ACS Appl. Mater. Interfaces. 8 (2016) 27254
27263. Available from: https://doi.org/ 10.1021/acsami.6b10375.

Self-Healing Polymer-Based Systems

510

17. Applications of self-healing polymeric systems

[24] A. Pal, B.L. Vernon, M. Nikkhah, Therapeutic neovascularization promoted by injectable hydrogels, Bioact. Mater. 3 (2018) 389
400. Available from: https://doi.org/10.1016/j.bioactmat.2018.05.002. [25] G. Chang, Y. Chen, Y. Li, S. Li, F. Huang, Y. Shen, et al., Self-healable hydrogel on tumor cell as drug delivery system for localized and effective therapy, Carbohydr. Polym. 122 (2015) 336
342. Available from: https://doi.org/10.1016/j.carbpol.2014.12.077. [26] J.H. Lee, Injectable hydrogels delivering therapeutic agents for disease treatment and tissue engineering, Biomater. Res. 22 (2018) 1
14. Available from: https://doi.org/10.1186/s40824-018-0138-6. [27] L. Yang, Y. Li, Y. Gou, X. Wang, X. Zhao, L. Tao, Improving tumor chemotherapy effect using an injectable self-healing hydrogel as drug carrier, Polym. Chem. 8 (2017) 5071
5076. Available from: https:// doi.org/10.1039/c7py00112f. [28] R. Xing, K. Liu, T. Jiao, N. Zhang, K. Ma, R. Zhang, et al., An injectable self-assembling collagen-gold hybrid hydrogel for combinatorial antitumor photothermal/photodynamic therapy, Adv. Mater. 28 (2016) 3669
3676. Available from: https://doi.org/10.1002/adma.201600284. [29] J. Wang, D. Wang, H. Yan, L. Tao, Y. Wei, Y. Li, et al., An injectable ionic hydrogel inducing high temperature hyperthermia for microwave tumor ablation, J. Mater. Chem. B 5 (2017) 4110
4120. Available from: https://doi.org/10.1039/c7tb00556c. [30] Y. Zhang, B. Yang, X. Zhang, L. Xu, L. Tao, S. Li, et al., A magnetic self-healing hydrogel, Chem. Commun. 48 (2012) 9305
9307. Available from: https://doi.org/10.1039/c2cc34745h. [31] W. Hu, Z. Wang, Y. Xiao, S. Zhang, J. Wang, Advances in crosslinking strategies of biomedical hydrogels, Biomater. Sci. 7 (2019) 843
855. Available from: https://doi.org/10.1039/c8bm01246f. [32] Z.W. Wicks, F.N. Jones, S.P. Pappas, D.A. Wicks, Organic Coatings: Science and Technology, 2006. Available from: https://doi.org/10.1002/9780470079072. [33] J. Karger-Kocsis, Paints, coatings and solvents: edited by D. Stoye. VCH, Weinheim, 1993 (409 pp.). ISBN 3527-28623-3. Price: d80.00, Compos. Sci. Technol. 51 (1994) 613
614. Available from: https://doi.org/ 10.1016/0266-3538(94)90094-9. [34] P.A. Schweitzer, Corrosion of Linings and Coatings: Cathodic and Inhibitor Protection and Corrosion Monitoring, CRC press, 2006. [35] D.G. Weldon, Failure Analysis of Paints and Coatings: Revised Edition, 2009. Available from: https://doi. org/10.1002/9780470744673. [36] T. Siva, K. Kamaraj, V. Karpakam, S. Sathiyanarayanan et al., Soft template synthesis of poly(o-phenylenediamine) nanotubes and its application in self healing coatings, Progress in Organic Coatings 76 (2013) 581
588. [37] A. Yabuki , T. Nishisaka, Self-healing capability of porous polymer film with corrosion inhibitor inserted for corrosion protection, Corrosion Science 53 (2011) 4118
4123. [38] J.S. Moore, S. Viswanathan, S.R. Sriram, N.R. Sottos, S.R. White, M.R. Kessler, et al., Autonomic healing of polymer composites, Nature 409 (2002) 794
797. Available from: https://doi.org/10.1038/35057232. [39] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, et al., A thermally re-mendable cross-linked polymeric material, Science 295 (2002) 1698
1702. Available from: https://doi.org/10.1126/science.1065879. [40] X. Chen, F. Wudl, A.K. Mal, H. Shen, S.R. Nutt, New thermally remendable highly cross-linked polymeric materials, Macromolecules 36 (2003) 1802
1807. Available from: https://doi.org/10.1021/ma0210675. [41] J.Y. Lee, G.A. Buxton, A.C. Balazs, Using nanoparticles to create self-healing composites, J. Chem. Phys. 121 (2004) 5531
5540. Available from: https://doi.org/10.1063/1.1784432. [42] J.W.C. Pang, I.P. Bond, A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility, Compos. Sci. Technol. 65 (2005) 1791
1799. Available from: https://doi.org/ 10.1016/j.compscitech.2005.03.008. [43] J.H. Osborne, Observations on chromate conversion coatings from a sol-gel perspective, Prog. Org. Coatings 41 (2001) 280
286. Available from: https://doi.org/10.1016/S0300-9440(01)00143-6. [44] R.L. Twite, G.P. Bierwagen, Review of alternatives to chromate for corrosion protection of aluminum aerospace alloys, Prog. Org. Coatings 33 (1998) 91
100. Available from: https://doi.org/10.1016/S0300-9440(98)00015-0. [45] R. Arefinia, A. Shojaei, H. Shariatpanahi, J. Neshati, Anticorrosion properties of smart coating based on polyaniline nanoparticles/epoxy-ester system, Prog. Org. Coatings 75 (2012) 502
508. Available from: https:// doi.org/10.1016/j.porgcoat.2012.06.003. [46] R. Akid, M. Gobara, H. Wang, Corrosion protection performance of novel hybrid polyaniline/sol-gel coatings on an aluminium 2024 alloy in neutral, alkaline and acidic solutions, Electrochim. Acta. 56 (2011) 2483
2492. Available from: https://doi.org/10.1016/j.electacta.2010.12.032.

Self-Healing Polymer-Based Systems

References

511

[47] A. Yabuki, T. Nishisaka, Self-healing capability of porous polymer film with corrosion inhibitor inserted for corrosion protection, Corros. Sci. 53 (2011) 4118
4123. Available from: https://doi.org/10.1016/j. corsci.2011.08.022. [48] M.L. Zheludkevich, S.C.M. Fernandes, A.J.D. Silvestre, A. Gandini, J. Tedim, C.S.R. Freire, et al., Chitosanbased self-healing protective coatings doped with cerium nitrate for corrosion protection of aluminum alloy 2024, Prog. Org. Coatings 75 (2012) 8
13. Available from: https://doi.org/10.1016/j.porgcoat.2012.02.012. [49] J. Yuan, X. Fang, L. Zhang, G. Hong, Y. Lin, Q. Zheng, et al., Multi-responsive self-healing metallosupramolecular gels based on ‘‘click’’ ligand, J. Mater. Chem 22 (2012) 11515
11522. [50] S. Billiet, W. Van Camp, X.K.D. Hillewaere, H. Rahier, F.E. Du Prez, Development of optimized autonomous self-healing systems for epoxy materials based on maleimide chemistry, Polymer (Guildf.) 53 (2012) 2320
2326. Available from: https://doi.org/10.1016/j.polymer.2012.03.061. [51] V. Karpakam, K. Kamaraj, S. Sathiyanarayanan, G. Venkatachari, S. Ramu, Electrosynthesis of polyanilinemolybdate coating on steel and its corrosion protection performance, Electrochim. Acta. 56 (2011) 2165
2173. Available from: https://doi.org/10.1016/j.electacta.2010.11.099. [52] D. Kowalski, M. Ueda, T. Ohtsuka, Self-healing ion-permselective conducting polymer coating, J. Mater. Chem. 20 (2010) 7630
7633. Available from: https://doi.org/10.1039/c0jm00866d. [53] Z. Zhang, Y. Hu, Z. Liu, T. Guo, Synthesis and evaluation of a moisture-promoted healing copolymer, Polymer (Guildf.) 53 (2012) 2979
2990. Available from: https://doi.org/10.1016/j.polymer.2012.04.048. [54] T.F. da Conceicao, N. Scharnagl, W. Dietzel, D. Hoeche, K.U. Kainer, Study on the interface of PVDF coatings and HF-treated AZ31 magnesium alloy: determination of interfacial interactions and reactions with self-healing properties, Corros. Sci. 53 (2011) 712
719. Available from: https://doi.org/10.1016/j.corsci.2010.11.001. [55] B. Aı¨ssa, R. Nechache, E. Haddad, W. Jamroz, P.G. Merle, F. Rosei, Ruthenium Grubbs’ catalyst nanostructures grown by UV-excimer-laser ablation for self-healing applications, Appl. Surf. Sci. 258 (2012) 9800
9804. Available from: https://doi.org/10.1016/j.apsusc.2012.06.032. [56] A. Yabuki, K. Okumura, Self-healing coatings using superabsorbent polymers for corrosion inhibition in carbon steel, Corros. Sci. 59 (2012) 258
262. Available from: https://doi.org/10.1016/j.corsci.2012.03.007. [57] A. Yabuki, W. Urushihara, J. Kinugasa, K. Sugano, Self-healing properties of TiO2 particle-polymer composite coatings for protection of aluminum alloys against corrosion in seawater, Mater. Corros. 62 (2011) 907
912. Available from: https://doi.org/10.1002/maco.201005756. [58] D.O. Grigoriev, K. Ko¨hler, E. Skorb, D.G. Shchukin, H. Mo¨hwald, Polyelectrolyte complexes as a “smart” depot for self-healing anticorrosion coatings, Soft Matter. 5 (2009) 1426
1432. Available from: https://doi. org/10.1039/b815147d. [59] G. Williams, S. Geary, H.N. McMurray, Smart release corrosion inhibitor pigments based on organic ionexchange resins, Corros. Sci. 57 (2012) 139
147. Available from: https://doi.org/10.1016/j.corsci.2011.12.024. [60] S.H. Cho, S.R. White, P.V. Braun, Self-healing polymer coatings, Adv. Mater. 21 (2009) 645
649. Available from: https://doi.org/10.1002/adma.200802008. [61] M.W. Keller, S.R. White, N.R. Sottos, A self-healing poly(dimethyl siloxane) elastomer, Adv. Funct. Mater. 17 (2007) 2399
2404. Available from: https://doi.org/10.1002/adfm.200700086. [62] B. Zhang, P. Zhang, H. Zhang, C. Yan, Z. Zheng, B. Wu, et al., A transparent, highly stretchable, autonomous self-healing poly(dimethyl siloxane) elastomer, Macromol. Rapid Commun. 38 (2017) 1
9. Available from: https://doi.org/10.1002/marc.201700110. [63] T. Siva, K. Kamaraj, V. Karpakam, S. Sathiyanarayanan, Soft template synthesis of poly(o-phenylenediamine) nanotubes and its application in self healing coatings, Prog. Org. Coatings 76 (2013) 581
588. Available from: https://doi.org/10.1016/j.porgcoat.2012.11.009. [64] S.H. Cho, H.M. Andersson, S.R. White, N.R. Sottos, P.V. Braun, Polydiniethylsiloxane-based self-healing materials, Adv. Mater. 18 (2006) 997
1000. Available from: https://doi.org/10.1002/adma.200501814. [65] A. Gandini, S. Kallip, A. Lisenkov, J. Tedim, S.C.M. Fernandes, M.L. Zheludkevich, et al., Self-healing protective coatings with “green” chitosan based pre-layer reservoir of corrosion inhibitor, J. Mater. Chem. 21 (2011) 4805. Available from: https://doi.org/10.1039/c1jm10304k. [66] Y.-S. Kim, R.-H. Kim, M. Li, A. Islam, R. Ma, L. Xu, et al., Epidermal electronics, Science 333 (2011) 838
843. Available from: https://doi.org/10.1126/science.1206157. [67] S.W. Hong, S.-H. Lee, I.Y. Kim, G.S. Jeong, H.C. Jung, D.-H. Baek, et al., Solderable and electroplatable flexible electronic circuit on a porous stretchable elastomer, Nat. Commun. 3 (2012) 977
978. Available from: https://doi.org/10.1038/ncomms1980.

Self-Healing Polymer-Based Systems

512

17. Applications of self-healing polymeric systems

[68] S. Cheng, Z. Wu, A microfluidic, reversibly stretchable, large-area wireless strain sensor, Adv. Funct. Mater. 21 (2011) 2282
2290. Available from: https://doi.org/10.1002/adfm.201002508. [69] A.N. Sokolov, C.J. Bettinger, Z. Bao, Chemical and engineering approaches to enable organic field-effect transistors for electronic skin applications, Acc. Chem. Res. 45 (2012) 361
371. [70] V. Maheshwari, R. Saraf, Tactile devices to sense touch on a par with a human finger, Angew. Chem. - Int. Ed. 47 (2008) 7808
7826. Available from: https://doi.org/10.1002/anie.200703693. [71] K. Takei, T. Takahashi, J.C. Ho, H. Ko, A.G. Gillies, P.W. Leu, et al., Nanowire active-matrix circuitry for low-voltage macroscale artificial skin, Nat. Mater. 9 (2010) 821
826. Available from: https://doi.org/ 10.1038/nmat2835. [72] M.S. Shur, S. Wagner, Sensitive skin, IEEE Sens. J. 1 (2001) 41
51. [73] J.H. Ahn, J.H. Je, Stretchable electronics: materials, architectures and integrations, J. Phys. D. Appl. Phys. 45 (2012). Available from: https://doi.org/10.1088/0022-3727/45/10/103001. [74] D.S. Gray, J. Tien, C.S. Chen, High-conductivity elastomeric electronics, Adv. Mater. 16 (2004) 393
397. Available from: https://doi.org/10.1002/adma.200306107. [75] K.A. Williams, A.J. Boydston, C.W. Bielawski, Towards electrically conductive, self-healing materials, J. R. Soc. Interface. 4 (2007) 359
362. Available from: https://doi.org/10.1098/rsif.2006.0202. [76] Y. Li, S. Chen, M. Wu, J. Sun, Polyelectrolyte multilayers impart healability to highly electrically conductive films, Adv. Mater. 24 (2012) 4578
4582. Available from: https://doi.org/10.1002/adma.201201306. [77] A.M. Prokup, J.S. Moore, S.R. White, S.A. Odom, J.H. Leonard, N.R. Sottos, et al., Restoration of conductivity with TTF-TCNQ charge-transfer salts, Adv. Funct. Mater. 20 (2010) 1721
1727. Available from: https://doi. org/10.1002/adfm.201000159. [78] B.C.K. Tee, C. Wang, R. Allen, Z. Bao, An electrically and mechanically self-healing composite with pressureand flexion-sensitive properties for electronic skin applications, Nat. Nanotechnol. 7 (2012) 825
832. Available from: https://doi.org/10.1038/nnano.2012.192. [79] S.A. Odom, S. Chayanupatkul, B.J. Blaiszik, O. Zhao, A.C. Jackson, P.V. Braun, et al., A self-healing conductive ink, Adv. Mater. 24 (2012) 2578
2581. Available from: https://doi.org/10.1002/adma.201200196. [80] L. Zhao, B. Jiang, Y. Huang, Self-healable polysiloxane/graphene nanocomposite and its application in pressure sensor, J. Mater. Sci. 54 (2019) 5472
5483. Available from: https://doi.org/10.1007/s10853-018-03233-6. [81] K. Guo, D.L. Zhang, X.M. Zhang, J. Zhang, L.S. Ding, B.J. Li, et al., Conductive elastomers with autonomic self-healing properties, Angew. Chem. - Int. Ed. 54 (2015) 12127
12133. Available from: https://doi.org/ 10.1002/anie.201505790. [82] T.-P. Huynh, H. Haick, Self-healing materials for analyte sensing, in: Nanomaterials Design for Sensing Applications, Elsevier Inc., 2019. Available from: https://doi.org/10.1016/b978-0-12-814505-0.00010-2. [83] X. Chen, W. Jiang, W.R. Leow, B. Zhu, H. Wang, H. Wang, et al., A mechanically and electrically self-healing supercapacitor, Adv. Mater. 26 (2014) 3638
3643. Available from: https://doi.org/10.1002/adma.201305682. [84] H. Sun, X. You, Y. Jiang, G. Guan, X. Fang, J. Deng, et al., Self-healable electrically conducting wires for wearable microelectronics, Angew. Chem. - Int. Ed. 53 (2014) 9526
9531. Available from: https://doi.org/ 10.1002/anie.201405145. [85] Y. Huang, Y. Huang, M. Zhu, W. Meng, Z. Pei, C. Liu, et al., Magnetic-assisted, self-healable, yarn-based supercapacitor, ACS Nano. 9 (2015) 6242
6251. Available from: https://doi.org/10.1021/acsnano.5b01602. [86] P. Docampo, G.E. Eperon, H.J. Snaith, A. Goriely, V.M. Burlakov, Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells, Adv. Funct. Mater. 24 (2013) 151
157. Available from: https://doi.org/10.1002/adfm.201302090. [87] S. Pathak, G. Grancini, J.J. van Franeker, D.W. deQuilettes, H.J. Snaith, B.J. Bruijnaers, et al., The importance of moisture in hybrid lead halide perovskite thin film fabrication, ACS Nano. 9 (2015) 9380
9393. Available from: https://doi.org/10.1021/acsnano.5b03626. [88] H.J. Snaith, S. Pathak, M.M. Lee, T. Leijtens, A. Abate, G.E. Eperon, Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells, Nat. Commun. 4 (2013) 1
8. Available from: https://doi.org/10.1038/ncomms3885. [89] N. Klein-Kedem, D. Cahen, G. Hodes, Effects of light and electron beam irradiation on halide perovskites and their solar cells, Acc. Chem. Res. 49 (2016) 347
354. Available from: https://doi.org/10.1021/acs. accounts.5b00469. [90] D. Wang, M. Wright, N.K. Elumalai, A. Uddin, Stability of perovskite solar cells, Sol. Energy Mater. Sol. Cells. 147 (2016) 255
275. Available from: https://doi.org/10.1016/j.solmat.2015.12.025.

Self-Healing Polymer-Based Systems

References

513

[91] H. Li, Q. Zhao, W. Zhou, D. Yu, J. Wei, Y. Zhao, et al., A polymer scaffold for self-healing perovskite solar cells, Nat. Commun. 7 (2016) 1
9. Available from: https://doi.org/10.1038/ncomms10228. [92] J. Chen, Thermal and mechanical cracking in TIPS-pentacene thin films, J. Polym. Sci. Part. B Polym. Phys. 45 (2007) 1390
1398. Available from: https://doi.org/10.1002/polb. [93] C. Dry, W. McMillan, A novel method to detect crack location and volume in opaque and semi-opaque brittle materials, Smart Mater. Struct. 6 (1997) 35
39. Available from: https://doi.org/10.1088/0964-1726/6/1/004. [94] G. Zhou, The use of experimentally-determined impact force as a damage measure in impact damage resistance and tolerance of composite structures, Compos. Struct. 42 (1998) 375
382. Available from: https:// doi.org/10.1016/0306-3747(90)90340-8. [95] M.O.W. Richardson, M.J. Wisheart, Review of low-velocity of composite materials, Compos. Part. A. 27A (1996) 1123
1131. Available from: https://doi.org/10.1016/1359-835X(96)00074-7. [96] Y.C. Yuan, T. Yin, M.Z. Rong, M.Q. Zhang, Self healing in polymers and polymer composites. Concepts, realization and outlook: a review, Express Polym. Lett. 2 (2008) 238
250. Available from: https://doi.org/ 10.3144/expresspolymlett.2008.29. [97] M.Q. Zhang, M.Z. Rong, Self-healing polymers and polymer composites, Self-Healing Polym. Polym. Compos. (2011). Available from: https://doi.org/10.1002/9781118082720. [98] M.R. Kessler, N.R. Sottos, S.R. White, Self-healing structural composite materials, Compos. Part. A Appl. Sci. Manuf. 34 (2003) 743
753. Available from: https://doi.org/10.1016/S1359-835X(03)00138-6. [99] M.R. Kessler, S.R. White, Self-activated healing of delamination damage in woven composites, Compos. Part. A Appl. Sci. Manuf. 32 (2001) 683
699. Available from: https://doi.org/10.1016/S1359-835X(00)00149-4. [100] P. Schwab, R.H. Grubbs, J.W. Ziller, R.V. August, Synthesis and applications of RuCl2(dCHR0 )(PR3)2.pdf, J. Am. Chem. Soc. 2 (1996) 100
110. [101] W. Dong, C. Zhang, H. Jiang, Y. Su, Z. Dai, RSC Advances correlation on the mechanical and thermodynamic, RSC Adv. 5 (2015) 103082
103090. Available from: https://doi.org/10.1039/C5RA23189B. [102] K.S. Toohey, N.R. Sottos, J.A. Lewis, J.S. Moore, S.R. White, Self-healing materials with microvascular networks, Nat. Mater. 6 (2007) 581
585. Available from: https://doi.org/10.1038/nmat1934. [103] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene, J. Microencapsul. 20 (2003) 719
730. Available from: https://doi.org/10.1080/ 0265204031000154160. [104] M. Motuku, G.M. Janowski, U.K. Vaidya, Parametric studies on self-repairing approaches for resin infused composites subjected to low velocity impact, Smart Mater.Struct. 8 (1999) 623. [105] T.C. Mauldin, M.R. Kessler, T.C. Mauldin, M.R. Kessler, Self-healing polymers and composites self-healing polymers and composites, Int. Mater. Rev. 6608 (2013) 317
346. Available from: https://doi.org/10.1179/ 095066010X12646898728408. [106] C. Naddeo, G. Iannuzzo, V. Vittoria, A. Mariconda, S. Russo, L. Guadagno, et al., Use of Hoveyda
Grubbs’ second generation catalyst in self-healing epoxy mixtures, Compos. Part. B Eng. 42 (2010) 296
301. Available from: https://doi.org/10.1016/j.compositesb.2010.10.011. [107] N.R. Sottos, G.O. Wilson, J.S. Moore, S.R. White, M.M. Caruso, N.T. Reimer, Evaluation of ruthenium catalysts for ring-opening metathesis polymerization-based self-healing applications, Chem. Mater. 20 (2008) 3288
3297. Available from: https://doi.org/10.1021/cm702933h. [108] M. Sells, B, a, t, 28 (2008) 2
3. Available from: https://doi.org/10.1037//0033-2909.127.5.629. [109] L. Guadagno, M. Raimondo, C. Naddeo, A. Mariconda, R. Corvino, Longo, Pasquale (Capaccio, IT)Process for preparing self-healing composite materals of high efficiency for structural applications, Patent no:US 8.481,615 B2. [110] J. Ehrenkranz, (12) Patent Application Publication (10) Pub. No.: US 2006/0222585 A1 Fig. 1, 002 (2017) 7. Available from: https://doi.org/10.1037/t24245-000. [111] L. Guadagno, L. Vertuccio, C. Naddeo, E. Calabrese, G. Barra, M. Raimondo, et al., Reversible self-healing carbon-based nanocomposites for structural applications, Polymer (Basel) 11 (2019). Available from: https://doi.org/10.3390/polym11050903.

Self-Healing Polymer-Based Systems

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A A6ACA. See Acryloyl-6-aminocaproic acid (A6ACA) AAc. See Acrylic acid (AAc) AAm. See Acrylamide (AAm) Abrasion, 76 ABS. See Acrylonitrile
butadiene
styrene (ABS) AC. See Alternating current (AC) AC-β-CD. See Acryloyl-β-cyclodextrin (AC-β-CD) Acetone, 20
22 Acetone-in-polydimethylsiloxane oil-in-oil, 245 Acoustical microscopy, 124
125 Acrylamide (AAm), 41, 56 Acrylates, 20
22 Acrylic acid (AAc), 323 Acrylonitrile
butadiene
styrene (ABS), 245 copolymer, 105
106 Acryloyl-6-aminocaproic acid (A6ACA), 172 Acryloyl-β-cyclodextrin (AC-β-CD), 106
107 Activation methods in ionomers, 284 Active damage mechanism, 426 Acylhydrazones, 20 acylhydrazone-based polymeric materials, 35
36 bonds, 178
179, 340
341 ADA. See Alginate dialdehyde (ADA) Adamantine (Ad), 196
197, 499 Adhesion, 40 strategy, 181
182 AEMs. See Arylene
ethynylene macrocycles (AEMs) Aerospace, 1 applications, 117, 506
507 AFM. See Atomic force microscopy (AFM) agar-PAAm. See Agar-polyacrylamide (agar-PAAm) Agar-polyacrylamide (agar-PAAm), 57 Agarose-based self-healing hydrogels, 394
396 Agglomerates, 479
480 Aggregates, 283 AgNWs. See Silver nanowires (AgNWs) Air-drying oils, 240 Al-MC. See Aluminum nitrate-containing pH-MC (Al-MC) Alginate, 168, 393
394 Alginate dialdehyde (ADA), 33

Aliphatic disulfide, 27
28 Alkenes, 3
4 Alkoxyamine junction, 5 moiety homolytically cleaves, 32 Alkynes, 3
4 Allyl sulfides, 28
29 α-CD-nBu. See n-butyl acrylate and host to α-cyclodextrin (α-CD-nBu) Alternating current (AC), 135 Aluminum nitrate-containing pH-MC (Al-MC), 266 Amine-functionalized polydimethylsiloxane (PDMS-a), 270 Animal starch, 404
405 Anisotropic damage-healing problems, 439
442 Anodic corrosion inhibitors, 477 Anthracene
maleimide DA system, 4
5 Antibacterial surfaces, 192
194 Anticorrosion coating, 501
503 Antifouling surface, 192
194 Aramid fiber, 457, 457t, 458t Arylene
ethynylene macrocycles (AEMs), 298
299, 299f Asphalt, 157, 158f, 160f Atom transfer radical polymerization (ATRP), 22, 62
63, 284 Atomic force microscopy (AFM), 134, 134f, 222, 224f, 318
319 ATRP. See Atom transfer radical polymerization (ATRP) AuNPs. See Gold nanoparticles (AuNPs) Autohesive tack, 19 Autonomic response, 1 Autonomic self-healing hydrogels, 378 Autonomous healing, 306 Autonomous process, 283 Axolotl, 75
76 Aza-Michael addition, 36
37 Azadirachta indica. See Neem oil (Azadirachta indica) Azide-alkyne click chemistry, 17
18, 61
62, 61f, 62f Azobenzene-containing hydrogen bonding-based hydrogel, 189

515

516 B BA. See Butyl acrylates (BA) BACA. See N,N-bis(acryloyl)cystamine (BACA) BDM. See 4,40 -Diphenylmethane bismaleimide (BDM) Beam on elastic foundation (BOEF), 152
153, 152f method, 152
153, 153f Bending testing, 127 Bentonite, 146
147 Benzaldehyde-terminated Pluronic F127, 197
198 Benzoimidazole MMT (BIAMMT), 146
147 Benzotriazole (BTA), 147
148, 478 Benzoxaborole
catechol complexation, 31 Benzoyl peroxide (BPO), 248 β-cyclodextrin (β-CD), 297, 499 BIAMMT. See Benzoimidazole MMT (BIAMMT) BIIR. See Brominated poly(isobutylene-co-isoprene) rubber (BIIR) Binding site, 332 Biobased epoxy resin, 214 Biocompatibility, 168 Biocompatible copolymer, 190
191 Biocompatible injectable hydrogels, 196 Biodegradability, 168 Bioelectronics, 17
18, 410 Bioinks, 499 Bioinspired self-healing systems, 310
314 Biological hydrogels, 216
217 Biological materials, 294 Biomass-derived compounds, 22 Biomass-derived furan functional units, 20
22 Biomaterials, 167 surfaces, 191 Biosensor, 186
188 Bis (2,6-bis(10 -methylbenzimidazolyl)-4hydroxypyridine tridentate), 314 2,6-Bis(10 -methylbenzimidazolyl)pyridine (Mebip), 49 Bis(heptamethylcyclotetrasiloxanyl)ethane, 37 2,5-Bis(hydroxymethyl), 22 Bismaleimide (BMI), 211
212 Bismalemide (BM), 23
25 Bisphenol A, 261 Bisphenol-A diglycidyl ether, 33 Bisphosphonate (BP), 47
48, 174 Bisurea units (IU unit), 199
200 Bitumen, 152, 155
158, 156f, 161f sample-mixing with microcapsules, 155 self-healing process, 153
155 Bituminous material, 156
157 BM. See Bismalemide (BM) BMA. See Butyl methacrylate (BMA) BMI. See Bismaleimide (BMI) BmimHSO4. See 1-Butyl-3-methylimidazolium hydrogen sulfate (BmimHSO4)

Index

BOEF. See Beam on elastic foundation (BOEF) Boric acid, 29
30 Boronate ester bonds, 342
344 dynamic covalent bond, 29
30 Boronic acid, 29
30 Boronic esters, 29
30 hydrogels, 31 BP. See Bisphosphonate (BP) BP-modified hyaluronic acid (HA-BP), 189, 190f BPEI/PAA. See Bulk-branched poly(ethylene imine) and poly(acrylic acid) (BPEI/PAA) bPEI
PAA/HA. See Nanofibrous poly(ethylenimine)
poly(acrylic acid)/hyaluronic acid (bPEI
PAA/ HA) BPO. See Benzoyl peroxide (BPO) Brominated poly(isobutylene-co-isoprene) rubber (BIIR), 45 Bromobutyl rubber, 45, 328 Brønsted acid-promoted dynamic exchange reaction, 37, 38f BTA. See Benzotriazole (BTA) Bulk ionomer self-healing materials, 326
327 Bulk-branched poly(ethylene imine) and poly(acrylic acid) (BPEI/PAA), 46 Burdick group, 196
197, 196f Butyl acrylates (BA), 31, 48
49 Butyl methacrylate (BMA), 22 1-Butyl-3-methylimidazolium hydrogen sulfate (BmimHSO4), 104

C CA. See Citric acid (CA) Calcium ion (Ca21), 47
48 Calcium ions-based self-healing mechanism, 47
48 Calixarenes (CAs), 52
53 Capillary forces, 155
156 Capsule synthesis and characterization, 260
267 binary microcapsule architecture, 267f emulsion-based encapsulation methods, 262f Capsule-based polymers, 267 Capsule-based self-healing polymers and composites capsule synthesis and characterization, 260
267 capsule-based self-healing systems, 260f self-healing coatings, 273
274 self-healing polymers and composites, 267
273 Capsules, 190
191 Carbon, 6
7 carbon-centered radicals, 32 carbon-free carbon paper, 149
150 Carbon fiber, 457, 457t, 458t Carbon fiber reinforced plastics (CFRPs), 506 composites, 457
458, 458f

Index

Carbon nanotubes (CNTs), 174 CNT-based DA nanocomposites, 221
222 Carbon
hydrogen
nitrogen (CHN), 261 Carbonyl hydrazide donor, 36
37 Carboxyl-functionalized nitrile rubber, 9
10 CAs. See Calixarenes (CAs) Catalyst, 2, 18 Cathodic corrosion inhibitor, 477 CBs. See Cucurbiturils (CBs) C
C bond formation, 20 CDHM. See Continuum damage healing mechanics (CDHM) CDM. See Continuum damage mechanics (CDM) CDs. See Cyclodextrins (CDs) CE. See Cyanate ester (CE) Ce-MC. See Cerium nitrate-containing pH-MC (Ce-MC) Cell affinity, 198 coculture, 185
186, 187f scaffolds, 182
183 therapy, 182 Cellobiose, 396
397 Cellulose, 396
399 fibers, 220 Cellulose nanocrystals (CNCs), 217
218, 338 Cellulose nanofibers
polypyrrole (CNFs
PPy), 6
7 Centrifugal extrusion, 260
261 Cerium nitrate-containing pH-MC (Ce-MC), 266 CFRP. See Controlled free-radical polymerization (CFRP) CFRPs. See Carbon fiber reinforced plastics (CFRPs) Chain transfer agents (CTA), 318
319 Chemical attack, 209 Chemical restoration process, 236
237 Chemical transformations involved in self-healing dynamic chemistry of selenium, 29 dynamic exchange of disulfide bonds, 27
29 thiol-ene click chemistry, 26
27 through involved reaction in self-healing dynamic covalent exchange network in polyesters, 37 dynamic siloxane bond exchange, 37 exchangeable hydrazide Michael adduct linkages, 36
37 self-healing based on exchangeable reactions, 37
38 Chemically self-healing mechanism, 383, 383f Chemistries involving in microcapsule-based self-healing polymeric system, 60
64 Chitosan, 168, 396 Chlorobenzene (PhCl), 263 CHN. See Carbon
hydrogen
nitrogen (CHN)

517

Chondrocytes, 182
183 Cinnamoyl group, 25
26 Citric acid (CA), 143
144 Classical engineering approach, 75
76 Clay nanosheets (CNS), 329
330 Clay-based self-healing materials, 489
490 Close-then-heal scheme (CTH scheme), 426
427, 428f CMC. See Critical micelle concentration (CMC) CNCs. See Cellulose nanocrystals (CNCs) CNFs
PPy. See Cellulose nanofibers
polypyrrole (CNFs
PPy) CNS. See Clay nanosheets (CNS) CNTs. See Carbon nanotubes (CNTs) Coaxial electrospinning techniques, 2
3 Collagen, 403
404 Commercial rubber, 45 Compact tension (CT), 466
467 Composites, 108
110 Conductive polymer hydrogels (CPHs), 410
411 Confocal laser scanning microscopy (CLSM). See Confocal microscopy Confocal microscopy, 129, 131f, 179
180, 263
264, 264f Constitutional dynamic chemistry, 392 Constitutive theories of sacrificial bonding systems, 386 Construction industries, 1 Continuum damage healing mechanics (CDHM), 425, 434
442 anisotropic damage-healing problems, 439
442 scalar damage-healing variables, 435
439 Continuum damage mechanics (CDM), 426 physically consistent evolution laws, 442
450 mechanisms-based phenomenological healing models, 447
450 thermodynamic consistent damage and healing model, 443
447 Controlled free-radical polymerization (CFRP), 281 Controlled radical polymerization (CRP), 62
63 in microcapsule system, 62
63 Conventional method, 478
484 Conventional polymers, 236 Conventional thermosets, 22
23 Cooperative supramolecular polymerization (CSPs), 300
301 Coordination polymers, 305 Copolymerization, 55, 56f Copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC), 61 Core and shell components of microcapsules, 248
252 Corrosion, 129, 130f, 222, 273 assessment tests, 134
135 EIS, 135 potentiostatic and potentiodynamic techniques, 135

518

Index

Corrosion (Continued) inhibitors, 80 Corrosion inhibition clay-based self-healing materials, 489
490 and corrosion inhibitor, 474
478 corrosion inhibitors, 476
478 nanocapsules formation using ultrasonication, 479f self-healing materials, 474
476, 478
484 nanocontainer-based self-healing approach, 487
489 polymer capsules-based self-healing coating, 484
487 self-healing and, 473
474 Cosmetics, 149
150 Covalent bonds, 79 Covalent self-healing, 99
103 Covalently cross-linked hydrogels, 391 CP-DCPD. See Cyclopentadiene-dicyclopentadiene (CP-DCPD) CPHs. See Conductive polymer hydrogels (CPHs) Cracks, 76, 455, 458
459 growth method, 151 Critical micelle concentration (CMC), 248
249 Cross-linked (PHFBA-co-PFMA)/BM polymer, 23
25, 24f Cross-linked hydrogels, 390
391 Cross-linked PCL (n-PCL), 108, 109f Cross-linkers, 18 Crosslinked polymer network, 78
79, 214 Crown ethers, 336
337 CRP. See Controlled radical polymerization (CRP) Crystalline temperature (Tc), 108 Crystallization, 176 CSPs. See Cooperative supramolecular polymerization (CSPs) CT. See Compact tension (CT) CTA. See Chain transfer agents (CTA) CTH scheme. See Close-then-heal scheme (CTH scheme) CuAAC. See Copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) Cucurbiturils (CBs), 52
53, 333, 337
338 Curcumin, 189 Cyanate ester (CE), 268 [4 1 4] Cycloaddition reactions, 26 [2 1 2] Cycloaddition, 25
26 Cyclodextrins (CDs), 52
53, 173, 333
335, 401 Cyclopentadiene-dicyclopentadiene (CP-DCPD), 20
22

D DA. See Dopamine (DA) DA reactions. See Diels
Alder reactions (DA reactions) DA-based healable polymer adhesives, 223
225

composites, 218
222 healing of DA-based polymer composites, 220 self-healing DA-based nanocomposites, 220
222 DA-based healable rubbers, 218 DA-based healable thermosets, 214
225 applications of DA-based self-healing networks, 216
225 intrinsic thermohealability of scratched film surface, 215f DA-based healing, 463
464 DA-based reversible polyurethane network (DA-PU network), 222 DA-based self-healing actuators and robots, 225 network applications, 216
225 DA-based healable polymer adhesives, 223
225 DA-based healable polymer composites, 218
222 DA-based healable rubbers, 218 healable DA-based hydrogels, 216
218 healable DA-based polymer coatings, 222
223 DA-PU network. See DA-based reversible polyurethane network (DA-PU network) DAA. See Diacetone acrylamide (DAA) DABBF. See Diarylbibenzofuranones (DABBF) Damage management, 75
76 mechanisms in polymers, 429
431 DBASs. See Dibenzylammonium salts (DBASs) DBP. See Dibutyl phthalate (DBP) DBTL. See Di-n-butyl tin dilaurate (DBTL) DC. See Dynamic covalent (DC) DCB test. See Double-cantilever beam test (DCB test) DCBs. See Dynamic covalent bonds (DCBs) DCHs. See Diblock copolypeptide hydrogels (DCHs) DCP. See Dicumyl peroxide (DCP) DCPD. See Dicyclopentadiene (DCPD) DDS. See Diaminodiphenyl sulfone (DDS) DE. See Dextrose equivalent (DE) Defense, 1 Delamination, 455
456 Depolymerization, 84
85 Derivative thermogravimetric (dTG), 266 Design of experiment (DOE), 145
146 Dextran (DEX), 173 Dextrin-based self-healing hydrogels, 400
402 Dextrose equivalent (DE), 400
401 DGEBA. See Diglycidyl ether of bisphenol A (DGEBA) Di-n-butyl tin dilaurate (DBTL), 237 Diacetone acrylamide (DAA), 179
180 2,7-Diamido-1,8 naphthyridine (NaPy), 302 Diaminodiphenyl sulfone (DDS), 268 Diarylbibenzofuranones (DABBF), 85
86 Dibenzo-24-crown-8 (DB24C8), 53
54

Index

Dibenzylammonium salts (DBASs), 317 Diblock copolypeptide hydrogels (DCHs), 176 Dibutyl phthalate (DBP), 248, 267 Diclycidyl ether, 261 Dicumyl peroxide (DCP), 285 Dicyclopentadiene (DCPD), 17
18, 60, 123, 237, 261, 267
268, 270, 273, 474, 507 DCPD-based/Grubb’s catalyst system, 239 Diels
Alder reactions (DA reactions), 3
4, 17
18, 20
22, 21f, 22f, 89
90, 99
100, 101f, 115f, 177, 210
214, 212f, 219f, 346
347 bonds, 455
456 of furan/maleimide, 213
214 kinetics and thermodynamics, 211
213 Diene, 210
211, 211f moieties, 214 Dienophile, 210
211 moieties, 214 Diethylenetriamine, 33 Differential scanning calorimetry (DSC), 125
126, 126f, 211
212, 212f Diffusion, 95, 144
145, 382 pathways, 6
7 Diglycidyl ether of bisphenol A (DGEBA), 214, 239
240, 274 3,4-Dihydroxy benzaldehyde, 216
217 3,4-Dihydroxyphenylalanine (DOPA), 310
311 1,6-Diisocyanatohexane (HDI), 31 Dimethyldineodecanoate (DMDNT), 273
274, 502 4,40 -Diphenylmethane bismaleimide (BDM), 99
100 Direct tensile tests, 153
157 Directional bonding approaches, 316
321 Diselenide, 29 Disulfide bonds, dynamic exchange of, 27
29, 29f Disulfides, 28
29 exchangeable reaction, 27
28 Divinybenzene (DVB), 263 DMA. See Dynamic mechanical analysis (DMA) DMAAm. See N,N-dimethyl acrylamide (DMAAm) DMDNT. See Dimethyldineodecanoate (DMDNT) DMTA. See Dynamic mechanical thermal analyses (DMTA) DN. See Double network (DN) DOE. See Design of experiment (DOE) DOPA. See 3,4-Dihydroxyphenylalanine (DOPA) Dopamine (DA), 178, 496 Double network (DN), 30
31 Double-cantilever beam test (DCB test), 464 Double-capsule system, 259 Drug delivery, 500
501 Drug/cell delivery systems, 167 Drug/gene delivery systems, 188
191 injectable hydrogels, 188
189

519

particles and capsules, 190
191 DSC. See Differential scanning calorimetry (DSC) dTG. See Derivative thermogravimetric (dTG) Dual capsule-based system, 475
476 Duromers, 87
89 DVB. See Divinybenzene (DVB) Dynamic chemistry of selenium, 29 Dynamic covalent (DC), 340. See also Reversible covalent bonding, 176
180 acylhydrazone bonds, 178
179 Diels
Alder reaction, 177 imine bonds, 178 oxime bonds, 179
180 thiol
disulfide exchange, 177
178 chemistry, 99, 297 exchange network in polyesters, 37 network in self-healing, 20 Dynamic covalent bonds (DCBs), 297. See also Sacrificial bonds exchange through chemical or catalytic stimuli, 349 and hydrogen bonding, 355
356 photo-induced dynamic covalent self-healing, 349
351 reversible cycloaddition reactions, 346
348 self-healing from, 340
346 Dynamic exchange of disulfide bonds, 27
29, 29f Dynamic mechanical analysis (DMA), 128
129 Dynamic mechanical thermal analyses (DMTA), 128
129 Dynamic noncovalent supramolecular interactions, 17
18 Dynamic PVA-borate network, 30
31 Dynamic reversibility of hindered urea bond, 31 Dynamic reversible boronate ester bond, 29
31, 30f Dynamic siloxane bond exchange, 37 Dynamic thiol-Michael chemistry, 26
27 Dynamically reversible alkoxyamines fission/radical recombination, 32
33, 33f alkoxyamine-based self-healing chemistry containing epoxy system, 34f

E E-skin. See Electronic skin (E-skin) EC. See Ethyl cellulose (EC) ECFCs. See Endothelial colony-forming cells (ECFCs) ECM. See Extracellular matrix (ECM) EDA. See Ethylenediamine (EDA) EGF. See Epidermal growth factor (EGF) EIS. See Electrochemical impedance spectroscopy (EIS) Elastic foundation method, 152
153 Elastic process in polymers, 429
431 Elastic
plastic continuum models, 425
426

520 Elastomers, 28, 77
79, 87
89, 112
113 direct ink write-based 3D printer, 114f Electrochemical impedance spectroscopy (EIS), 135 Electron donor groups, 211 Electronic application, 503
506 Electronic skin (E-skin), 197
198, 199f, 503
504 Electrostatic bonds, 387 forces, 387 self-assembly, 322
326 Electrostatic interactions, 42, 170
171, 322
339. See also Host
guest interactions bulk ionomer self-healing materials, 326
327 electrostatic self-assembly, 322
326 self-healing poly(ionic liquids), 327
332 EMA. See Ethylene methacrylic acid (EMA) EMAA. See Ethylene-methacrylic acid (EMAA) 24-EMI. See 2-Ethyl-4-methylimidazole (24-EMI) Emulsification polymerization, 260
261 Emulsifier, 248
249 Emulsion-based methods, 260
261 Enamine bonds, 344
346 ENB. See 5-Ethylidene-2-norbornene (ENB) Encapsulated monomer, 80 Encapsulation process, 235, 248, 260
261 Endo isomeric DA linkers, 4
5 Endo stereoisomer, 211
212 Endo-transition state, 211 Endothelial colony-forming cells (ECFCs), 404 ENR. See Epoxidized natural rubber (ENR) Enthalpy, 213 Entropic response, 96
97 Environmental scanning electron microscopy (ESEM), 159
161, 159f, 265, 265f EP. See Epoxy (EP) EPA. See Ethyl phenylacetate (EPA) EPDM. See Polyethylene/propylene diene monomer (EPDM) Epidermal growth factor (EGF), 498 Epoxidized natural rubber (ENR), 284
285 Epoxidized soybean oil (ESO), 143
144 Epoxy (EP), 22
23, 268 epoxy-amine cross-linking, 17
18 epoxy-based coatings, 115
116, 129 matrix, 242 self-healing polymer composites, 269 systems, 7 resin, 20
22, 33 Equilibrium, 85
86 ESEM. See Environmental scanning electron microscopy (ESEM) ESO. See Epoxidized soybean oil (ESO)

Index

Ethyl cellulose (EC), 242 Ethyl phenylacetate (EPA), 263, 268, 271 2-Ethyl-4-methylimidazole (24-EMI), 475
476 Ethylene methacrylic acid (EMA), 285 Ethylene vinyl acetate copolymer (EVA copolymer), 113 Ethylene-methacrylic acid (EMAA), 42, 148, 284
285, 464 Ethylene/vinyl alcohol (EVA), 284
285 Ethylenediamine (EDA), 274 5-Ethylidene-2-norbornene (ENB), 507 Eutectic gallium indium particles, 9
10 EVA. See Ethylene/vinyl alcohol (EVA) EVA copolymer. See Ethylene vinyl acetate copolymer (EVA copolymer) Exchange reactions, 373
374 Exchangeable hydrazide Michael adduct linkages, 36
37, 36f Exchangeable reactions involving hypervalent iodine, 37
38 Exo isomeric DA linkers, 4
5 Exo stereoisomer, 211
212 Extracellular matrix (ECM), 392
393, 498 environment, 188
189 Extrinsic healing system, 252, 455
456 Extrinsic self-healing, 18, 372 approach for FRP composites, 460
463 hollow fiber-based self-healing, 461
463 microcapsule-based self-healing, 460
461 microvascular-based self-healing, 463 biomaterials, 167
168 materials, 1
2, 79 in polymeric systems, 2
3 self-healing by hollow fibers, 3f polymers, 79
81, 210 mechanistic aspects of extrinsic self-healing materials, 82t microcapsule approach and microvascular network approach, 81f Extrinsic shape memory-assisted self-healing, 114
116, 116f

F Fabricating self-healing materials on polymeric systems, 11
13 FE-SEM. See Field-emission scanning electron microscopy (FE-SEM) Fe3O4 nanoparticle-doped hydrogels, 53 FER. See Furfurylated epoxy resin (FER) Ferrocene, 391 FGE. See Furfuryl glycidyl ether (FGE) Fiber-reinforced composites, 33 Fiber-reinforced polymer composites (FRP composites), 117, 269, 455

Index

assessment of self-healing efficiency for, 466
468 components, 457f extrinsic self-healing approaches, 460
463 intrinsic self-healing approach, 463
465 self-healing in, 456
460 thermoreversible healing of, 465
466 Fiber-reinforced polymers, 77 Fibrin, 168 Field-emission scanning electron microscopy (FE-SEM), 124, 125f Films, self-healing, 183
188 applications in tissue engineering, 185
188 mechanism, 183
185 Finite deformation kinematics, 429
431 First law of thermodynamic, 444
445 Fixing deformation, 97 FLP. See Frustrated Lewis pair (FLP) Fluorescence microscope, 180 Fluorinated cross-linked copolymer, 23
25 Fluorinsated-decyl polyhedral oligomeric silsesquioxane, 23
25 Fluoropolymers, 20
25 FMA. See Furfuryl methacrylate (FMA) FMI. See Furan
maleimide (FMI) Food, 149
150 4-arm furan (4F), 20
22 FRP composites. See Fiber-reinforced polymer composites (FRP composites) Frustrated Lewis pair (FLP), 48 polymers as responsive self-healing gels, 48 Functional surfaces, 167
168, 194 self-healing, 191
194 Furan-functionalized ethylene polyethylene rubber, 218 Furan-functionalized resin, 211
212 Furan-modified polymer precursors, 20
22 Furan/maleimide, 213
214 Furan
maleimide (FMI), 346
347 chemistry, 3
4 Furfuryl glycidyl ether (FGE), 99
100, 218 Furfuryl methacrylate (FMA), 22
25 Furfuryl-terminated MWCNT, 6
7 Furfurylated epoxy resin (FER), 214

G GDP. See Gross domestic product (GDP) Gel, 370
371 Gel-GA. See Gelatin-gum arabic (Gel-GA) Gel-phase transitions, 319 Gelatin-based self-healing hydrogels, 403
404 Gelatin-gum arabic (Gel-GA), 248 coacervate, 264
265 Gel-GA/PUF dual-layer microcapsules, 248 Gene therapy, 188

521

GFRP composites. See Glass fiber-reinforced polymeric composites (GFRP composites) GG. See Guar gum (GG) Glass fiber, 457, 457t, 458t Glass fiber reinforced polymer (CFRP), 271 Glass fiber-reinforced polymeric composites (GFRP composites), 460 Glass transition temperature (Tg), 18
19, 84
85, 96
97, 99
100 Glassy polymers, 77 Glutathione peroxidase, 29 (3-Glycidoxypropyl)trimethoxysilane-modified nanosilica, 6
7 Glycidyl methacrylate (GMA), 62
63 Glycogen-based self-healing hydrogels, 404
406 GO. See Graphene oxide (GO) Gold nanoparticles (AuNPs), 41 composite polyelectrolyte gel, 143
144 Graphene, 6
7, 456 graphene-based DA nanocomposites, 220
221 Graphene oxide (GO), 2
3, 379 Green synthesis of self-healing corrosion-inhibiting coating, 478 Gross domestic product (GDP), 273 Grubb’s catalyst, 79
80, 507 Guar gum (GG), 402 GG-based self-healing hydrogels, 402
403

H H-boning interactions in self-healing polymeric systems, 41, 42f HA. See Hyaluronic acid (HA) HA-BP. See BP-modified hyaluronic acid (HA-BP) HABI. See Hexaarylbiimidazole (HABI) Halato-telechelic polymers, 282 Halloysite, 146
147, 487
489 nanotubes, 147
148, 147f HBFP. See Hyperbranched fluorinated polymer (HBFP) HBPU. See Hyper-branched polyurethane (HBPU) HBPUMSA-ABS. See Hyper-branched polyurethanemercaptosuccinic acid-acrylonitrile butadiene styrene (HBPUMSA-ABS) HDI. See 1,6-Diisocyanatohexane (HDI) HEA. See 2-Hydroxyethyl acrylate (HEA) HEAA. See N-hydroxyethyl acrylamide (HEAA) Healable DA-based hydrogels, 216
218 Healing action, 2 of DA-based polymer composites by nonthermal methods, 220 efficiency, 126
127 of hydrophobicity, 132
133 mechanism, 141
142

522 Healing (Continued) process, 18
19 in polymers, 429
431 time, 180 Healing index (HI), 145
146 Heat-activated thermoplastic healing agent, 284 Heat-shrinkable shape memory products, 116
117 HEC. See Hydroxyethyl cellulose (HEC) HEMA. See 2-Hydroxyethyl methacrylate (HEMA) HER. See Hydrogen evolving reaction (HER) Hexaarylbiimidazole (HABI), 26 HFBA. See Perfluorobutyl acrylate (HFBA) HG2. See Hoveyda
Grubbs’ second-generation catalyst (HG2) HGBs. See Hollow glass bubbles (HGBs) HI. See Healing index (HI) High-performance polymers, 117 self-healing polymers, 20
22 HO-PDMS. See Hydroxyl end-functionalized polydimethylsiloxane (HO-PDMS) Hollow fiber-based self-healing, 461
463 Hollow glass bubbles (HGBs), 269 Hollow glass fiber approach, 2 Host
guest chemistry, 52
53, 297, 332
334, 505 Host
guest interactions, 52
54, 53f, 103
104, 106
107, 108f, 173
174, 196
197, 296
297, 332, 377
378. See also Electrostatic interactions design of self-healing, 333
334 mechanical properties in, 339 mechanism of self-healing based on, 332
333 self-healing materials utilizing CDs, 334
335 utilizing crown ethers, 336
337 utilizing cucurbiturils, 337
338 “House-of-cards” structures, 379 Hoveyda
Grubbs’ second-generation catalyst (HG2), 507 HPC. See Hydroxypropyl cellulose (HPC) HPMC. See Hydroxypropyl methylcellulose (HPMC) Human skin, 198 Human
machine interactions, 9
10 interfaces, 220 Hyaluronic acid (HA), 174, 496 HA-based hydrogels, 406
407 Hybrid cross-linked hydrogels, 391
392 Hydrogels, 45, 49, 56, 97
98, 216, 369
371, 496. See also Self-healing hydrogels for energy applications, 412
413 gel and, 370
371 hydrosol and, 371 mechanism

Index

dynamic covalent bonding, 176
180 noncovalent bonding, 170
176 metal
ligand polymer, 390
392 natural polymer-based, 392
407 sacrificial bonds in, 387
390 self-healing, 169
183 applications in tissue engineering, 180
183 evaluation of self-healing efficiency, 180 Hydrogen bonding, 17
18, 103
104, 172
173, 295
296, 351
352 dynamic covalent bonds and, 355
356 self-complementary hydrogen bonding in, 352
354 self-healing polymers, 354
355 in self-healing systems, 351
356 bonds, 141
142, 375 Hydrogen evolving reaction (HER), 321 Hydrogen-bonded supramolecular networks, 302
303 Hydrolyzed fluorinated alkyl silane, 23
25 Hydrophilic mercaptosuccinic acid (MSA), 105
106 Hydrophilicity/hydrophobicity property, 132 Hydrophobic interactions, 40, 55
57, 103
104, 174
176 Hydrophobicity healing, 132
133 Hydrosilylation reactions, 17
18 Hydrosol, 371 2-Hydroxyethyl acrylate (HEA), 29
30 Hydroxyethyl cellulose (HEC), 399
400 2-Hydroxyethyl methacrylate (HEMA), 304, 505 Hydroxyl end-functionalized polydimethylsiloxane (HO-PDMS), 475
476 Hydroxypropyl cellulose (HPC), 399 Hydroxypropyl methylcellulose (HPMC), 399 Hygroscopic poly(ester amide) elastomer, 115 Hyper-branched polyurethane (HBPU), 105
106, 106f Hyper-branched polyurethane-mercaptosuccinic acid-acrylonitrile butadiene styrene (HBPUMSA-ABS), 56
57 Hyperbranched fluorinated polymer (HBFP), 193 Hypervalent iodine compounds, 37
38

I ICP system. See Imidazole containing polymer system (ICP system) IFSS. See Interfacial shear strength (IFSS) Imidazole containing polymer system (ICP system), 313 Imidazole
Zn interaction, 311
313 Imine bonds, 20, 178, 344
346 In situ polymerization, 247 Indicator displacement assay, 296 Inelastic deformation mechanisms, 431

Index

Infrared laser (IR laser), 221 Injectable hydrogels, 188
189, 500 Injectable self-healing hydrogels, 169
170, 197
198 Inorganic NPs, self-healing polymer using, 144
148 Inorganic-based self-healing hydrogels, 379 Interatomic covalent bonds, 141
142 Interfacial polymerizations, 245, 246f Interfacial shear strength (IFSS), 271
272, 461 Interpenetrating network (IPN), 57, 58f, 59f Interpenetrating polymer network for self-healing, 57
60 Interpolated network model, 98
99 Intrinsic healability, 214 Intrinsic healing approach, 455
456 Intrinsic self-healing, 18, 279, 372
378 approach, 128 for FRP composites, 463
465 materials, 1
2 in polymeric systems, 3
5 micro/nanocapsule-embedded self-healing systems, 4f SEM images of Au film electrode, 6f polymers, 82
89, 99
107, 210 covalent self-healing, 99
103 healing efficiency as function of E-modulus, 88f healing times for two reversible polymer systems, 87f mechanisms for covalent-based intrinsic self-healing materials, 85f mechanistic aspects of intrinsic self-healing materials, 88t noncovalent self-healing, 103
107 parameters for modeling of healing times, 86t stages of crack healing represented by two polymer coils, 83f systems, 99 reversible covalent bonds, 372
375 supramolecular chemistry, 375
378 systems, 293
297 dynamic covalent chemistry, 297 host
guest interactions, 296
297 hydrogen bonding, 295
296 metal coordination, 296 supramolecular bonds, 295 types, 294f Ion, 279 Ion-selective membranes, 282 Ionic domains, 283 Ionic interaction, 17
18, 59
60, 103
104, 377 Ionic mechanism, 42
46, 44f Ionomer(s), 279
280 activation methods, 284 applications, 284
288

523

materials, chemistry, and fundamentals, 280
282 microstructure to polymer chain, 281f orientation of polar ionic groups, 282f self-healing mechanisms, 283
284 Surlyn ionomers, 280f Ionomeric arrangements, 235 Ionomeric mechanism, 42
46 Eisenberg
Hird
Moore ionic model of ionomer, 43f IONPs-MWCNTs. See Iron oxide nanoparticlesdecorated MWCNT (IONPs-MWCNTs) IPDI. See Isophorone diisocyanate (IPDI) IPN. See Interpenetrating network (IPN) IR laser. See Infrared laser (IR laser) Iron oxide nanoparticles-decorated MWCNT (IONPs-MWCNTs), 222 Isodesmic supramolecular polymerization (ISP), 298 Isophorone diisocyanate (IPDI), 31, 243, 244f ISP. See Isodesmic supramolecular polymerization (ISP)

K Kinetics of DA reaction, 211
213

L l-PCL. See Linear PCL (l-PCL) Laser confocal scanning microscopy (LCSM). See Confocal microscopy Layer double hydroxide-based nanocontainer system (LDH-based nanocontainer system), 489 Layer-by-layer technique (LBL technique), 44, 245, 387, 487
489 Layered montmorillonite nanoclay structure, 146
147, 146f Ligament-cartilage-bone, 185
186 Ligand systems, 307
310 Linear PCL (l-PCL), 108, 109f Linseed oil (LO), 240
241 Liquid crystalline elastomers, 97
98 liquid-form healing material, 235 oligomers, 80 LO. See Linseed oil (LO) Long-term stability and lifetime, 251 Low-boiling solvent solution, 20
22 Lower unoccupied molecular orbital (LUMO), 213 Lysine/arginine
alanine, 170

M Macrocyclic host
guest interaction, 173 Macrovascular system, 64 Magnesium ions (Mg21), 46
47

524 Magnesium ions (Mg2 1 ) (Continued) magnesium ions-based self-healing mechanism, 46
47 Main-chain supramolecular polymers, 297
304 CSPs, 300
301 Directional noncovalent interactions, 302
304 ISP, 298
299 Ring-chain supramolecular polymerization, 299
300 supramolecular polymerizations, 298, 303f MALDI-TOF MS. See Matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF MS) Maleimide, 23
25 derivatives, 20
22 maleimide-treated glass fibers, 219 Matrix microcracks, 141 Matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF MS), 298
299 Matrix-reinforcing interphase, 218
219 MBA. See N,N0 -methylenebisacrylamide (MBA) MBT. See 2-Mercaptobenzothizole (MBT) MC. See Methylcellulose (MC) MDI. See Methylene diphenyl diisocyanate (MDI) Mebip. See 2,6-Bis(10 -methylbenzimidazolyl)pyridine (Mebip) Mechanical abrasion, 209 Mechanical failure, 168
169 of synthetic materials, 293
294 Mechanisms-based phenomenological healing models, 447
450 Medicine, 149
150 Melamine
formaldehyde (MF), 264
265 Melamine
phenol
formaldehyde (MPF), 239 Melamine
urea
formaldehyde (MUF), 239 2MEP4F. See Self-mendable bis-maleimide tetrafuran (2MEP4F) Mercaptobenzimidazole, 147
148 Mercaptobenzothiazole, 147
148 2-Mercaptobenzothizole (MBT), 487
489 Mesenchymal stem cells (MSCs), 499 Meso-scale modeling approaches, 98 Metal coordination, 296 self-healing materials by early development, 305
306 ligand systems, 307
310 naturally occurring and bioinspired self-healing systems, 310
314 network formation and self-healing, 306
307 photoresponsive systems, 314
316 self-assembly, 316
321, 318f Metal NPs, self-healing polymer using, 143
144, 143f Metal-organic framework (MOF), 321

Index

Metal
ligand coordination, 174, 175f self-healing materials, 199
200 interaction, 376
377 polymer hydrogels, 390
392 covalently cross-linked hydrogels, 391 cross-linked hydrogels, 390
391 hybrid cross-linked hydrogels, 391
392 Metallogels, 49 Metallosupramolecular polymers (MSPs), 305, 376
377 3-(Methacryloylamin)propyl trimethylammonium chloride (MPTC), 45 Methyl methacrylate (MMA), 22, 31, 48
49, 261
263 Methylcellulose (MC), 399 Methylene diphenyl diisocyanate (MDI), 239 4,40 -Methylenebis(phenyl urea) (MPU unit), 199
200 MF. See Melamine
formaldehyde (MF) MFPs. See Muscle foot proteins (MFPs) Micellar copolymerization, 175 Micro-computed tomography (μCT), 125 Micro-level cracks, 455
456 Micro/nanocapsules embedment, 2 MicroC18. See Microencapsulated n-octadecane (MicroC18) Microcapsule-based self-healing polymeric materials, 17
18 Microcapsule-based self-healing polymeric system. See also Self-healing polymeric systems azide-alkyne click chemistry, 61
62 CRP in microcapsule system, 62
63 microcapsule mediated ring-opening metathesis polymerization, 60 vascular-based self-healing system, 64 Microcapsule-based self-healing systems, 237
245, 238f amine microcapsules characterization, 240f optical images, 242f optical microscopic image, 241f poly-DCPD network formation, 239f Microcapsules, 20, 81, 150, 150f, 158, 237 core and shell components, 248
252, 250f properties of suitable solvents in microencapsulation process, 251t CRP in microcapsule system, 62
63 mediated ring-opening metathesis polymerization, 60, 60f microcapsule-based self-healing, 460
461 microcapsule-embedded coatings, 252
253 microcapsule-embedded systems, 63 preparation methods, 245
248, 247f single catalyst with, 475 Microcracks, 141
142 self-healing mechanism, 144
145 Microencapsulated healing agents, 259

Index

Microencapsulated n-octadecane (MicroC18), 263 Microencapsulation, 149
150, 237, 245, 252 Microvascular interpenetrating networks, 2
3 Microvascular system, 2 Microvascular-based self-healing, 64, 463 MMA. See Methyl methacrylate (MMA) Modified montmorillonite (MMT), 489 MOF. See Metal-organic framework (MOF) Molar mass, 83
84 Molecular diffusion, 235, 382 Molybdates, 477 Monochromatic radiation, 26 Monomers, 18, 247 Monoselenide, 29 MPF. See Melamine
phenol
formaldehyde (MPF) MPTC. See 3-(Methacryloylamin)propyl trimethylammonium chloride (MPTC) MPU unit. See 4,40 -Methylenebis(phenyl urea) (MPU unit) MSA. See Hydrophilic mercaptosuccinic acid (MSA) MSCs. See Mesenchymal stem cells (MSCs) MSPs. See Metallosupramolecular polymers (MSPs) MUF. See Melamine
urea
formaldehyde (MUF) Multi-walled carbon nanotube (MWCNT), 221, 304 Multiantitumor system, 500
501 Multifunctional furan (TF), 20
22 Multilayers, 44 Multistage open association. See Isodesmic supramolecular polymerization (ISP) Muscle foot proteins (MFPs), 310
311 Mussel materials, 496 MWCNT. See Multi-walled carbon nanotube (MWCNT) Mytilus, 311

N N,N-bis(acryloyl)cystamine (BACA), 11 N,N-dimethyl acrylamide (DMAAm), 29
30, 179
180 N,N0 -methylenebisacrylamide (MBA), 391
392 N-acryloyl glycinamide (NAGA), 41 n-butyl acrylate and host to α-cyclodextrin (α-CD-nBu), 52
53 N-heterocyclic carbenes (NHCs), 504 N-heterocyclic carbine
carbodiimide (NHC-CDI), 304 N-hydroxyethyl acrylamide (HEAA), 193 N-isopropylacrylamide (NIPAM), 106
107, 174, 323 N-methyl-2-pyrrolidone (NMP), 281 n-PCL. See Cross-linked PCL (n-PCL) Na-montmorillonite (Na-MMT), 146
147 NAGA. See N-acryloyl glycinamide (NAGA) Nanocapsules, 142
143 formation, 479f, 482 synthesis of, 478
484

525

Nanocellulose, 6
7 Nanocomposite hydrogel, 217
218 Nanocomposite-based self-healing hydrogels, 380
381 Nanocontainer-based self-healing approach, 487
489 Nanofibrous poly(ethylenimine)
poly(acrylic acid)/ hyaluronic acid (bPEI
PAA/HA), 10
11 Nanofillers role in self-healing polymeric systems, 6
7 Nanolevel cracks, 455
456 Nanoparticles (NPs), 123, 143 in self-healing of polymeric systems inorganic NPs, self-healing polymer using, 144
148 metal NPs, self-healing polymer using, 143
144 organic NPs, self-healing polymer using, 148
161 Nanosilica, 6
7, 145
146 Nanotechnology, 142
143 Nanotubes, 6
7 Naphthalene-diimide chains, 54
55 NaPy. See 2,7-Diamido-1,8 naphthyridine (NaPy) NaSS. See Sodium p-styrenesulfonate (NaSS) Natural polymer-based hydrogels, 392
407 agarose-based self-healing hydrogels, 394
396 alginate, 393
394 cellulose, 396
399 chitosan, 396 dextrin-based self-healing hydrogels, 400
402 gelatin-based self-healing hydrogels, 403
404 glycogen-based self-healing hydrogels, 404
406 guar gum-based self-healing hydrogels, 402
403 HEC, 399
400 hyaluronic acid-based hydrogels, 406
407 xanthan-gum-based self-healing hydrogels, 407 Natural polymers, 168 Natural rubber (NR), 19 Naturally occurring self-healing systems, 310
314 NBD. See Nitrobenzoxadiazole (NBD) Near-infrared (NIR), 304 light, 174 Neem oil (Azadirachta indica), 478 NEP. See Nucleation
elongation polymerization (NEP) NHC-CDI. See N-heterocyclic carbine
carbodiimide (NHC-CDI) NHCs. See N-heterocyclic carbenes (NHCs) NIPAM. See N-isopropylacrylamide (NIPAM) NIPU. See Nonisocyanate polyurethane (NIPU) NIR. See Near-infrared (NIR) Nitrites, 477 Nitrobenzoxadiazole (NBD), 54
55, 376 NMP. See N-methyl-2-pyrrolidone (NMP) NMR spectroscopy. See Nuclear magnetic resonance spectroscopy (NMR spectroscopy) Nonautonomic self-healing hydrogels, 378
379 Noncovalent bonding, 170
176

526

Index

Noncovalent bonding (Continued) crystallization, 176 electrostatic interaction, 170
171 host
guest interaction, 173
174 hydrogen bonding, 172
173 hydrophobic interactions, 174
176 metal
ligand coordination, 174 Noncovalent interactions, 20, 39
40 Noncovalent self-healing, 103
107 Noncovalent supramolecular interactions, 18 Nondestructive testing, 142 Nonisocyanate polyurethane (NIPU), 214, 215f Nonreversible deformation, 77 Nonselective electrostatic and H-bonding interactions, 53
54 Nonthermal methods, 220 NPs. See Nanoparticles (NPs) NR. See Natural rubber (NR) Nuclear magnetic resonance spectroscopy (NMR spectroscopy), 216, 298
299 Nucleation
elongation polymerization (NEP), 300
301 Nucrels, 42 Nylon-6 promote polymer chain alignment, 40

O O/O emulsion. See Oil-in-oil emulsion (O/O emulsion) OA. See Oxalic acid (OA) OCP. See Open circuit potential (OCP) Octamethylcyclotetrasiloxane, 37 Oil-in-oil emulsion (O/O emulsion), 245 Oil-in-water emulsion, 247 Olefin moiety, 25 One-dimensional rheological model (1D rheological model), 98 Open circuit potential (OCP), 135 Organic inhibitors, 478 Organic micro-or nanocapsules, 149
151 Organic NPs, self-healing polymer using evaluation of self-healing capability, 151
157 real application of self-healing nanocapsules, 157
161 self-healing by organic micro-or nanocapsules, 149
151 by shape-memory organic NPs, 148
149 Organometallic chemistry, 296 OSA. See Oxidized sodium alginate (OSA) Ossipiv group, 46
47 Oxalic acid (OA), 245, 246f Oxidized sodium alginate (OSA), 178 Oxime bonds, 179
180

P P-Et-P. See Poly(triethyl(4-vinylbenzyl)phosphonium chloride) (P-Et-P)

P(VBIm-Cl). See Polyionic liquid 1-methyl-3(4-vinylbenzyl) imidazolium chloride (P(VBIm-Cl)) P(NaSS). See Poly(sodium p-styrenesulfonate) (P(NaSS)) PAA. See Poly(acrylic acid) (PAA) pAAM. See Polyacrylamide (pAAM) PAM. See Polyazomethine (PAM) PAN. See Polyacrylonitrile (PAN) Pan coating, 260
261 Particles, 190
191 PBA. See Polybutyl acrylate (PBA) PBMA. See Poly(butyl methacrylate) (PBMA) PCL. See Polycaprolactone (PCL) PCLF. See Polycaprolactone
poly(furfuryl glycidylether) copolymer (PCLF) PCM. See Phase change materials (PCM) PDA. See Polydopamine (PDA) PDADMAC. See Poly(diallyldimethylammonium chloride) (PDADMAC) PDCA. See 2,6-Pyridinedicarboxamide (PDCA) pDMAAm. See Poly(N,N-dimethyl acrylamide) (pDMAAm) PDMS. See Polydimethylsiloxane (PDMS) PDMS-a. See Amine-functionalized polydimethylsiloxane (PDMS-a) PEA. See Polyetheramine (PEA) PEG. See Polyethylene glycol (PEG) PEG-PVA. See Poly(ethylene glycol)-poly(vinyl alcohol) (PEG-PVA) PEGS-FA. See Poly(ethylene glycol)-co-poly (glycerol sebacate) (PEGS-FA) PEI. See Poly(ethylene imine) (PEI) PEM. See Polyelectrolyte multilayer film (PEM) PEO. See Poly(ethylene oxide) (PEO) Peptide-conjugated PEG, 170 Peptide
polymer conjugates, 26
27 Perfluoroalkyl chains, 23
25 Perfluorobutyl acrylate (HFBA), 23
25 Perfluorooctyl triethoxysilane (POTS), 6
7 Pesticides, 149
150 PF. See Phenol
formaldehyde (PF) PF127. See Poly-(ethylene glycol)-b-poly (propyleneglycol)-b-poly(ethylene glycol) (PF127) PFMA-co-PBMA. See Poly(2-ethylhexyl acrylate) and butyl methacrylate (PFMA-co-PBMA) PGA. See Poly(glycolic acid) (PGA) PGS. See Poly(glycerol sebacate) (PGS) PGS-graft-UPy (PGS-U), 183
184 Phase change materials (PCM), 261
263 Phase transition-based viscoelastic behavior, 98 PhCl. See Chlorobenzene (PhCl) pHEA. See Poly(2-hydroxyethyl acrylate) (pHEA)

Index

Phenol
formaldehyde (PF), 270 resin, 80 Phenomenological approach, 425
426 Phenylborate
catechol complexation-based PEG hydrogel, 189 pHMAAm. See Poly(2-hydroxymethyl acrylamide) (pHMAAm) Phosphates, 47
48 Photo-induced dynamic covalent self-healing, 349
351 Photo-induced self-healing, 25
26 [4 1 4] cycloaddition reactions, 26 self-healing via covalent bond reformation, 26f Photocyclization reactions, 28
29 Photodimerization/photoscission, 25
26 Photomediated reversible [2 1 2] cycloaddition chemistry, 25 Photoresponsive systems, 314
316 Photovoltaics technology (PV technology), 505
506 Physical/chemical compositions, 17
18 Physically self-healing mechanism, 382, 382f π-conjugated imine-containing polymers, 89 π-electron-rich pyrenyl units, 5 π-interactions based self-healing polymer, 54
55, 55f PIB. See Polyisobutylene (PIB) PIC. See Polyion complexes (PIC) PINs. See Polymer ionic networks (PINs) π
π stacking interactions, 103
104, 376 PLA. See Poly(L-lactic acid) (PLA) Plastic deformation in polymers, 431
433 Plastic process in polymers, 429
431 Plasticity, 431 PLGA. See Poly(lactic-co-glycolic) acid (PLGA) PMF. See Poly(melamine-formaldehyde) (PMF) PMMA. See Polymethyl methacrylate (PMMA) pNIPAAm. See Poly(N-isopropylacrylamide) (pNIPAAm) PNIPAM. See Poly(N-isopropylacrylamide) (PNIPAM) Poly-(ethylene glycol)-b-poly(propyleneglycol)-b-poly (ethylene glycol) (PF127), 106
107 Poly(2-ethylhexyl acrylate) and butyl methacrylate (PFMA-co-PBMA), 22 Poly(2-hydroxyethyl acrylate) (pHEA), 53 Poly(2-hydroxymethyl acrylamide) (pHMAAm), 53 Poly(2,5-furan dimethylene succinate-co-butylene succinate), 22 Poly(acrylic acid) (PAA), 8, 46, 187
188, 323
324 Poly(alkylene-thiourea), 41 Poly(allylamine hydrochloride), 8 Poly(butyl methacrylate) (PBMA), 57 Poly(diallyldimethylammonium chloride) (PDADMAC), 323 Poly(dicyclopentadiene), 79
80, 123 Poly(ether-thiourea), 41

527

Poly(ethylene glycol)-based polymer gel, 26 Poly(ethylene glycol)-co-poly (glycerol sebacate) (PEGS-FA), 497 Poly(ethylene glycol)-poly(vinyl alcohol) (PEG-PVA), 57 Poly(ethylene imine) (PEI), 487
489 Poly(ethylene oxide) (PEO), 35
36 Poly(glycerol sebacate) (PGS), 183
184 Poly(glycolic acid) (PGA), 168 Poly(ionic liquids), 327
332 Poly(L-lactic acid) (PLA), 168 Poly(lactic-co-glycolic) acid (PLGA), 168, 190
191, 191f, 500
501 Poly(melamine-formaldehyde) (PMF), 263 Poly(N,N-dimethyl acrylamide) (pDMAAm), 53 Poly(N-isopropylacrylamide) (pNIPAAm), 53 Poly(N-isopropylacrylamide) (PNIPAM), 318
319 Poly(sodium p-styrenesulfonate) (P(NaSS)), 44, 57
59 Poly(styrene sulfonate) (PSS), 487
489 Poly(styrene-acrylic acid) core
shell nanoparticles, 5 Poly(triethyl(4-vinylbenzyl)phosphonium chloride) (P-Et-P), 46 Poly(α-fluoro acrylate) materials, 23
25 Polyacrylamide (pAAM), 53 Polyacrylonitrile (PAN), 2
3 Polyacryloyl hydrazide, 36
37 Polyacylhyrazone, 35
36 Polyampholyte, 170
171, 171f Polyazomethine (PAM), 7 Polybutyl acrylate (PBA), 41 Polycaprolactone (PCL), 98
99, 168, 464
465 Polycaprolactone
poly(furfuryl glycidylether) copolymer (PCLF), 222 Polycondensation process, 245
246 Polydiethoxysiloxane, 475
476 Polydimethylsiloxane (PDMS), 5, 51, 54
55, 104
105, 185
186, 273
274 Polydopamine (PDA), 178, 181f Polyelectrolyte multilayer film (PEM), 193, 194f, 323
324 Polyelectrolyte multilayers, 44 Polyesters, 39 dynamic covalent exchange network in, 37 Polyetheramine (PEA), 475
476 Polyethers, 39 Polyethyleimine, 9
10 Polyethylene glycol (PEG), 8, 33, 39, 47
48, 172
174, 172f, 178, 192
193, 217
218, 297 Polyethylene/propylene diene monomer (EPDM), 281
282 Polyion complexes (PIC), 176 Polyionic liquid 1-methyl-3-(4-vinylbenzyl) imidazolium chloride (P(VBIm-Cl)), 44, 57
59

528

Index

Polyisobutylene (PIB), 25
26 Polymer ionic networks (PINs), 329 Polymer(s), 95, 124
125, 209 capsules-based self-healing coating, 484
487 fabrication methods, 487t self-healing capsules, 485
487 self-healing mechanism, 484f chain segments, 20 materials, 17
18, 151 matrix using NPs, 144
145, 145f network, 26
27 polymer-based self-healing composite, 142 polymer-based self-healing hydrogels, 379
380 polymer-containing hydroxyl group, 29
30 Polymeric coatings, self-healing behavior of, 129
135 qualitative methods, 129
132 quantitative methods, 132
135 Polymeric composites, self-healing behavior of qualitative methods, 124
126 quantitative methods, 126
129 Polymeric electrolytes, 8 Polymeric membranes, 11 Polymeric networks, 18
19 Polymeric systems, 1 extrinsic self-healing in, 2
3 intrinsic self-healing in, 3
5 Polymerization, 173, 236
237 Polymerized ionic liquids (PILs). See Poly(ionic liquids) Polymer
nanofiller interfacial movement, 6
7 Polymethyl methacrylate (PMMA), 53
54, 57, 62
63, 210, 239, 263
264, 269, 475
476 Polyo-phenylenediamine (PoPD), 502 Polyolefins, 110
112 Polyoxypropylenetriamine (POPTA), 239, 268 Polypropylene (PP), 110
112 Polypyrrole (PPY), 174 Polysaccharide-based natural hydrogels, 411 Polysiloxane, 5, 20
22, 39 Polystyrene (PS), 270 Polytetra fluoroethylene-based ionomers, 282 Polyurea (PU), 31, 32f, 274 Polyurea-formaldehyde (PUF), 237, 266, 268, 271
272 Polyurethane (PU), 20
22, 99
100, 110, 111f, 237, 268, 475, 496 Polyurethane/poly(urea-formaldehyde) (PU/UF), 271 Polyvinyl alcohol (PVA), 6
7, 30
31, 44, 52, 57
59, 104, 143
144, 240
241 PoPD. See Polyo-phenylenediamine (PoPD) POPTA. See Polyoxypropylenetriamine (POPTA) Potentiodynamic technique, 135 Potentiostatic technique, 135 POTS. See Perfluorooctyl triethoxysilane (POTS) PP. See Polypropylene (PP)

PPR. See Pseudopolyrotaxane (PPR) PPY. See Polypyrrole (PPY) Protein-based hydrogels, 412 PS. See Polystyrene (PS) Pseudopolyrotaxane (PPR), 174 PSS. See Poly(styrene sulfonate) (PSS) PU. See Polyurea (PU); Polyurethane (PU) PU/UF. See Polyurethane/poly(urea-formaldehyde) (PU/UF) PUF. See Polyurea-formaldehyde (PUF) PV technology. See Photovoltaics technology (PV technology) PVA. See Polyvinyl alcohol (PVA) Pyrenyl-tweezer-ended polyamide, 5 2,6-Pyridinedicarboxamide (PDCA), 309

Q QCSP. See Quaternized chitosan-g-polyaniline (QCSP) Qualitative methods acoustical microscopy, 124
125 confocal microscopy, 129 evaluation of self-healing reaction heat, 125
126 SECM, 130
131 SEM, 129 SVET, 131
132 visual inspection and optical microscopy, 129 visualization techniques, 124 X-ray microtomography, 125 Quantitative methods, 132
135 AFM, 134 ballistic impact, 128 bending testing, 127 corrosion assessment tests, 134
135 DMTA, 128
129 healing of hydrophobicity, 132
133 TDCB, 128 tensile testing, 126
127 tribological properties, 134 Quaternized chitosan, 197
198 Quaternized chitosan-g-polyaniline (QCSP), 497

R Radical thiol-ene reactions, 26
27 Radiography, 142 RAFT. See Reversible addition-fragmentation chain-transfer (RAFT) Randomization, 95, 144
145 rDA reaction. See Retro-Diels
Alder reaction (rDA reaction) Real application of self-healing nanocapsules, 157
161 Resin, 473
474 Resorcinol-based urea-formaldehyde nanocapsules, 479 Responsiveness, 306

Index

Retro-Diels
Alder reaction (rDA reaction), 20
22, 213, 217f, 221f rDA chemistry, 20
25 retroDA reactions. See Reversible Diels
Alder reactions (retroDA reactions) Reversible addition-fragmentation chain-transfer (RAFT), 22, 311
313 Reversible alkoxyamine, 20 Reversible bonding and debonding, 31 Reversible covalent. See also Dynamic covalent (DC) acylhydrazone bond in self-healing chemistry, 35
36 bonds, 99, 372
375 exchange reactions, 373
374 reversible cycloaddition reactions, 372
373 self-healing polyurethanes, 374
375 stable free radical-mediated reshuffle reactions, 374 reaction involved in self-healing dynamic reversibility of hindered urea bond, 31 dynamic reversible boronate ester bond, 29
31 dynamically reversible alkoxyamines fission/ radical recombination, 32
33 reversible covalent acylhydrazone bond in self-healing chemistry, 35
36 reversible dynamic covalent Schiff-base (imine) linkage-based self-healing chemistry, 33
34 Reversible cycloaddition reactions, 346
348, 372
373 Reversible Diels
Alder reactions (retroDA reactions), 184, 346
348 adducts, 222 Reversible dynamic covalent Schiff-base (imine) linkage-based self-healing chemistry, 33
34, 34f Reversible hydrogen bonds (RHB), 5, 235, 507 Reversible metallosupramolecular polymer, 49
52 metal
ligand coordination motifs, 51f self-healing reversible metal binding, 50f Reversible polymer networks, 82, 84 Reversible sol
gel process, 32
33 RHB. See Reversible hydrogen bonds (RHB) Ring-chain supramolecular polymerization, 299
300, 300f Ring-opening metathesis polymerization (ROMP), 17
18, 237, 507 Ring-opening polymerization (ROP), 237 Robotic applications, 225 Robots, 225 ROMP. See Ring-opening metathesis polymerization (ROMP) ROP. See Ring-opening polymerization (ROP) Rouse friction coefficient, 85
86 Rubber elastic foundation, 152 Rubber-modified epoxy network, 5 Ruthenium (Ru), 271

529

S Sacrificial bonds, 385
386. See also Dynamic covalent bonds (DCBs) in hydrogels, 387
390 sacrificial covalent bonds, 386 sacrificial noncovalent bonds, 387 Sacrificial covalent bonds, 386 Sacrificial host
guest complexes, 389
390 Sacrificial hydrogen bonds, 388 Sacrificial hydrophobic interactions, 389 Sacrificial ionic bonds, 387
388 Sacrificial metal
ligand coordination bonds, 388
389 Sacrificial noncovalent bonds, 387 SAXS. See Small angle x-ray scattering (SAXS) SBR. See Styrene-butadiene rubber (SBR) SBS test. See Short-beam-shear test (SBS test) Scalar damage-healing variables, 435
439 Scanning electrochemical microscopy (SECM), 130
131, 132f Scanning electron microscopy (SEM), 124, 129, 131f, 179
180, 221 Scanning vibrating electrode technique (SVET), 131
132, 133f Scratch-induced damage, 77 Scratches, 76 SDBS. See Sodium dodecylbenzene sulfonate (SDBS) SDS. See Sodium dodecyl sulfate (SDS) SEC. See Size exclusion chromatography (SEC) SECM. See Scanning electrochemical microscopy (SECM) Secondary amide-containing polymers, 40 Selective host
guest exchange, 53
54 Selenium, dynamic chemistry of, 29 Self-assembly, 40, 316
321, 318f Self-contained dynamic reaction, 211 Self-healable hydrogels, 29
30 Self-healable sulfur vulcanized NR, 28 Self-healing, 17
18, 75
76, 108, 218, 279, 371
372, 455 anticorrosion coatings, 89 based on exchangeable reactions involving hypervalent iodine, 37
38, 38f behavior of polymeric coatings, 129
135 of polymeric composites, 124
129 biomaterials on polymeric systems characterization of self-healing, 194
195 in drug/gene delivery systems, 188
191 self-healing films, 183
188 self-healing functional surfaces, 191
194 self-healing hydrogels, 169
183 in tissue engineering, 168
188 capability evaluation BOEF method, 152
153

530 Self-healing (Continued) crack growth method, 151 direct tensile tests, 153
157 characterization, 194
195 chemical aspects in, 17
18 extrinsic and intrinsic self-healing, 18 coatings, 273
274 requirements for designing, 236
237 and corrosion inhibition, 473
474 cycles, 156
157 DA engineered thermosets DA reaction, 210
214 DA-based healable thermosets, 214
225 DA-based nanocomposites, 220
222 carbon nanotube-based DA nanocomposites, 221
222 graphene-based DA nanocomposites, 220
221 silver nanowires-based DA nanocomposites, 222 efficiency, 131
132, 180, 466
468 films, 183
188, 184f in FRP composites, 456
460 functional surfaces, 191
194 antibacterial and antifouling surfaces, 192
193 challenges, 194 surface-mediated drug delivery, 193 fundamentals, 209 glass fabric/epoxy composites, 271 glass-fiber-reinforced composites, 22
23 injectable hydrogels, 167 intrinsic, 372
378 involved through electrostatic interactions calcium ions-based self-healing mechanism, 47
48 frustrated Lewis pair polymers as responsive self-healing gels, 48 ionomeric or ionic mechanism, 42
46 magnesium ions-based self-healing mechanism, 46
47 through ionic salts, 46 key requirements of, 18
20 damage
repair cycle in polymeric materials, 19f materials, 2, 141
142, 495 mechanism, 96
107, 235 intrinsic self-healing of polymers, 99
107 ionomers, 283
284 opportunities and challenges, 195
200 3D printing, 195
197 electronic skin, 197
198 wound dressing, 195
197 by organic micro-or nanocapsules, 149
151 performance of SNPP hydrogels, 11, 12f polymer, 250 films, 186

Index

using inorganic NPs, 144
148 using metal NPs, 143
144 using organic NPs, 148
161 polyurethanes, 374
375 coatings, 82 process, 378
379 property, 5 reaction heat evaluation, 125
126 real application of self-healing nanocapsules, 157
161, 162f reversible covalent acylhydrazone bond in, 35
36 self-healing system with embedded microcapsules, 427f types, 210 by shape-memory organic NPs, 148
149 superhydrophobic fluoropolymer, 23
25 technology, 141 on van der Waals force of attraction, 48
49 Self-healing agents (SHAs), 460 Self-healing hydrogels, 169
183, 369 classification, 379
381 CPHs, 410
411 development in, 408
413 hydrogels for energy applications, 412
413 mechanism, 381
383 chemically self-healing mechanism, 383, 383f factors impact on, 384
385 physically self-healing mechanism, 382, 382f metal
ligand polymer hydrogels, 390
392 natural polymer-based hydrogels, 392
407 polysaccharide-based natural hydrogels, 411 protein-based hydrogels, 412 self-healing gels mechanism, 392 superabsorbent hybrid hydrogels, 408
410 Self-healing polymeric coatings extrinsic-based self-healing systems, 236f limitations and shortcomings of microcapsuleembedded coatings, 252
253 materials selection for core and shell components of microcapsules, 248
252 microcapsule preparation methods, 245
248 microcapsule-based self-healing systems, 237
245 requirements for designing self-healing coating, 236
237 Self-healing polymeric materials, 123, 236
237 self-healing behavior of polymeric coatings, 129
135 of polymeric composites, 124
129 Self-healing polymeric systems, 6
7 extrinsic self-healing in polymeric systems, 2
3 fabricating self-healing materials on polymeric systems, 11
13

Index

intrinsic self-healing in polymeric systems, 3
5 key developments in field, 8
11 role of nanofillers in, 6
7 self-healing action of energy storage device, 9f Self-healing polymeric systems. See also Microcapsulebased self-healing polymeric system applications aerospace applications, 506
507 anticorrosion coating, 501
503 drug delivery, 500
501 dynamic covalent chemistry, 504f electronic application, 503
506 three-dimensional printing, 499
500 tissue engineering, 498 wound healing, 496
498 chemical aspects, 17
18 chemical transformations, 26
29 through involved reaction, 36
38 chemistries involving in microcapsule-based, 60
64 dynamic covalent network, 20 H-boning interactions in, 41, 42f key requirements, 18
20 photoinduced self-healing, 25
26 reversible covalent reaction, 29
36 supramolecular noncovalent interaction, 39
60 thermoreversible Diels
Alder and retro Diels
Alder chemistry, 20
25 Self-healing polymers, 75
76, 501
502 and composites, 267
273 concepts for design of self-healing polymers, 79 extrinsic self-healing polymers, 79
81 intrinsic self-healing polymers, 82
89 macroscopic level, 76f mechanism of, 76
79 mechanistic aspects, 89
90 molecular level, 77f scratch map of polymer, 78f Self-mendable bis-maleimide tetrafuran (2MEP4F), 3
4 Self-repairing, 502 materials, 235 polymeric materials, 259 systems, 285 SEM. See Scanning electron microscopy (SEM) Semi-interpenetrated polymer networks (semi-IPNs), 113, 216 Semipermeable ionomeric membranes, 282 SENB test. See Single-edge notched bending test (SENB test) Sensing, 505 Shape memory (SM), 426
427 materials, 495
496 mechanism, 96
107, 98f

531

organic NPs, 148
149 Shape memory alloy (SMA), 426 Shape memory effect (SME), 95, 123 Shape memory polymers (SMPs), 89
90, 95, 96f, 426 Shape memory-assisted self-healing (SMASH), 89
90, 89f, 96, 112f elastomers, 112
113 extrinsic shape memory-assisted self-healing, 114
116 polymer systems, 107
116 applications, 116
117 shape memory and self-healing mechanisms, 96
107 urethane
thiourethane networks, 114 thermoplastic polymers, 107
112 thermoset polymers, 113
114 Shape recovery, 97 SHAs. See Self-healing agents (SHAs) Short-beam-shear test (SBS test), 466
467 Silicone-based materials, 37 Silk fibroin, 168 Siloxanes, 37 Silver (Ag), 11 Silver nanowires (AgNWs), 222 silver nanowires-based DA nanocomposites, 222 Single catalyst with microcapsules, 475 Single-component self-healing polyurethanes, 224
225 Single-edge notched bending test (SENB test), 466
467 Size exclusion chromatography (SEC), 298
299 SM. See Shape memory (SM) SMA. See Shape memory alloy (SMA) Small angle x-ray scattering (SAXS), 280, 316 Smart coatings of self-healing polymer, 25
26 Smart hydrogels, 498 Smart material-based coating, 487
489 Smart prosthetics, 9
10 SMASH. See Shape memory-assisted self-healing (SMASH) SME. See Shape memory effect (SME) SMPs. See Shape memory polymers (SMPs) Sodium dodecyl sulfate (SDS), 55
56, 173 Sodium dodecylbenzene sulfonate (SDBS), 266, 274 Sodium p-styrenesulfonate (NaSS), 45, 170
171 Sol
gel method, 131
132 Solid
liquid
solid transformation, 9
10 SPEEK. See Sulfonated poly(ether ketone) (SPEEK) SPN. See Supramolecular polymer network (SPN) Spray drying, 260
261 Stable free radical-mediated reshuffle reactions, 374 Stimuli-responsiveness, 162, 170, 174, 176, 178, 183, 189, 298 Stress
strain behavior, 76
77

532 Stress
strain curves, 127, 127f Styrene (St), 261
263 acrylonitrile, 210 styrene-based thermoplastic polymers, 282 Styrene-butadiene rubber (SBR), 6
7 Sulfonated poly(ether ketone) (SPEEK), 8, 23
25 Superabsorbent hybrid hydrogels, 408
410 Supercapacitors, 505 Superhydrophobic coatings, 23
25 Supramolecular bonds, 295 Supramolecular chemistry, 99, 295, 375
378 host
guest interaction, 377
378 hydrogen bonds, 375 ionic interaction, 377 metal
ligand interaction, 376
377 π
π stacking interactions, 376 Supramolecular forces, 40 Supramolecular hydrogels, 170, 498 Supramolecular interactions, 104
105, 210 host
guest interactions, 332
339 hydrogen bonding in self-healing systems, 351
356 intrinsic self-healing systems, 293
297 main-chain supramolecular polymers, 297
304 self-healing materials dynamic covalent, 340
351 by metal coordination, 305
321 self-healing mediated by electrostatic interactions, 322
332 Supramolecular noncovalent interaction, 39
60 host
guest interactions, 52
54 hydrogen-bonding-based self-healing, 39
41 hydrophobic interactions, 55
57 interpenetrating polymer network for self-healing, 57
60 reversible metallosupramolecular polymer, 49
52 self-healing involved through electrostatic interactions, 42
48 self-healing on van der Waals force of attraction, 48
49 π-interactions based self-healing polymer, 54
55 Supramolecular polymer network (SPN), 302 Supramolecular polymeric systems, 5 linear, 53
54 Supramolecular polymerizations, 298 Supramolecular rubber, 85
86 Supramolecular self-healing hydrogel, 496 Supramolecular systems, 84
85 Surface approach, 95, 144
145 coatings, 117 rearrangement, 95, 144
145 segregation, 82
83

Index

surface-mediated drug delivery, 193 Surfactant, 175, 248
249, 249f Surlyn, 42, 285 ionomers, 279, 280f surlyn-type EMAA materials, 43 Sustainability, 17
18 SVET. See Scanning vibrating electrode technique (SVET) Synthetic polymeric materials, 75 Synthetic polymers, 180
181, 293

T Tapered double cantilever beam (TDCB), 128, 128f, 267
268, 460
461 tBAEMA. See 2-(Tert-butylamino)ethyl methacrylate (tBAEMA) TCE. See 1,1,2-Trichloroethane (TCE) TCNQ. See Tetracyanoquinodimethane (TCNQ) TDCB. See Tapered double cantilever beam (TDCB) TDI. See 2,4-Toluene diisocyanate (TDI) TDS. See Thiuram disulfide (TDS) TEM. See Transmission electron microscopy (TEM) Temperature-dependent Raman spectroscopy, 26
27 Tensile testing, 126
127 Tension test sample, 154, 154f TEP. See Triepoxy (TEP) TEPA. See Tetraethylenepentamine (TEPA) Terpolymerization, 218 2-(Tert-butylamino)ethyl methacrylate (tBAEMA), 31 TETA. See Triethylenetetramine (TETA) Tetracyanoquinodimethane (TCNQ), 504 Tetraethylenepentamine (TEPA), 274 Tetrafunctional epoxy compound-containing furan
bismaleimide adduct linkages, 22
23 Tetrakis(acetoxydibutyl tinoxy)silane (TKAS), 485
486 Tetrathiafulvalene (TTF), 504 TGA. See Thermogravimetric analysis (TGA) TGF. See Transforming growth factor-beta (TGF) Thermal decomposition, 209 Thermally reversible covalent bonds (TRCBs), 426
427 Thermally reversible DA reactions, 3
4 Thermally reversible ionic compounds, 284 Thermally reversible nematic/isotropic transition, 97
98 Thermally reversible order/disorder transition, 97
98 Thermodynamic consistent models, 425
426 damage and healing model, 443
447 Thermodynamics of DA reaction, 211
213 Thermogravimetric analysis (TGA), 261
263 Thermohealable polymer network, 223 Thermomechanical damage (TM damage), 426 Thermoplast, 78
79

Index

Thermoplastic elastomers (TPEs), 281
282, 313 Thermoplastic particles (TPs), 427
428 Thermoplastic polymers, 76
77, 107
112, 148
149 self-healing for synthesized waterborne polyurethane films, 112f Thermoreversible Diels
Alder, 20
25, 24f monomeric systems utilized in self-healings polymeric systems, 25f Thermoreversible healing of FRP composites, 465
466 Thermoreversible physical interactions, 99 Thermoreversible reactions, 5 Thermoset polymers, 113
114 Thermosetting polymer, 33, 60, 225 Thiol-ene “click” reaction, 100
102 Thiol-ene click chemistry, 26
27, 27f Thiol-Michael reaction, 26
27 Thiol
disulfide exchange, 177
178, 179f Thiuram disulfide (TDS), 28
29, 102
103, 104f 3-arm maleimide (3M), 20
22 Three-dimensional printing, 195
197, 499
500 Three-dimensional rheological model (3D rheological model), 98 Tissue adhesiveness, 198 Tissue adhesives, 180
182 Tissue engineering, 498 applications of self-healing hydrogels in cell scaffolds, 182
183 tissue adhesives, 180
182 self-healing biomaterials in, 168
188 TKAS. See Tetrakis(acetoxydibutyl tinoxy)silane (TKAS) TM damage. See Thermomechanical damage (TM damage) 2,4-Toluene diisocyanate (TDI), 244 TPEs. See Thermoplastic elastomers (TPEs) TPs. See Thermoplastic particles (TPs) Transforming growth factor-beta (TGF), 498 Transmission electron microscopy (TEM), 124 TRCBs. See Thermally reversible covalent bonds (TRCBs) Trial-and-error, 428 Trialkoxysilane, 219 Tribometers, 134 1,1,2-Trichloroethane (TCE), 26 Triepoxy (TEP), 100
102, 103f Triethylenetetramine (TETA), 270 Trimethoxysilane silane derivative treatments, 243 1,1,1-Tris-(cinnamoyloxymethyl)ethan, 25
26 Trithiocarbonate (TTC), 28
29, 102
103, 318
319 TTC. See Trithiocarbonate (TTC) TTF. See Tetrathiafulvalene (TTF) Tungsten hexachloride (WCl6), 475 Tweezer-type bis-pyrenyl end groups, 54
55

533

U UF. See Urea
formaldehyde (UF) Ultrasonication, 478
480, 479f Ultrasound, 124
125, 142, 478
484 Ultraviolet (UV), 9
10 radiation, 102
103, 373
374 Unsaturated polyester resins, 22
23 Unvulcanized elastomeric materials, 19 Upy. See 2-Ureido-4-pyrimidinone (Upy) UPy. See Ureidopyrimidinone (UPy) Urea
formaldehyde (UF), 261, 264
265, 267
268, 475 prepolymer, 247 2-Ureido-4-pyrimidinone (Upy), 39, 40f, 41, 172
173, 172f Ureidopyrimidinone (UPy), 103
104, 183
184, 295
296, 352 UV. See Ultraviolet (UV) UV
vis spectroscopy, 25
26

V Vacuum-assisted resin transfer molding (VARTM), 456 van der Waals force of attraction, 48
49, 49f van der Waals interactions, 19
20 VARTM. See Vacuum-assisted resin transfer molding (VARTM) Vascular endothelial growth factor (VEGF), 498 Vascular-based self-healing system, 64 macrovascular system, 64 microvascular-based self-healing, 64 Vascular-based systems, 236 Vault particle, 191 VEGF. See Vascular endothelial growth factor (VEGF) Vinyl ester matrix, 237 2-((4-Vinylbenzyloxy) methyl) furan, 11 4,40 -Vinylphenyl-N,N-bis(4-tert-butylphenyl) benzenamine, 22 Viscosity, 210 modulation, 40 Visual inspection and optical microscopy, 129 Visualization techniques, 124 Vitrimers, 210

W Waste water, 9
10 Water-swollen network, 188
189 Wearable health monitoring devices, 9
10 Wetting, 95 Wound dressing, 195
197 Wound healing, 496
498

X X-ray microtomography, 125

534 X-ray photoelectron spectroscopy, 221 Xanthan-gum-based self-healing hydrogels, 407

Y Young’s modulus, 100
102, 126
127, 170
171

Index

Z Zinc dimethacrylate (ZDMA), 45 Zinc-MMT (Zn-MMT), 146
147 Zinc-neutralized poly(styrene-co-styrene sulfonate) ionomers, 280 Zuilhof group, 192
193