Interfaces in Particle and Fibre Reinforced Composites: Current Perspectives on Polymer, Ceramic, Metal and Extracellular Matrices (Woodhead Publishing Series in Composites Science and Engineering) 008102665X, 9780081026656

Table of contents :
Cover
Interfaces in Particle and Fibre Reinforced CompositesCurrent Perspectives on Polymer, Ceramic, Metal and Extracellular Mat ...
Copyright
Contributors
Foreword
References
Acknowledgements
1. Introduction
References
Part One: General perspectives
2. Structured interfaces and their effect on composite performance: computational studies1
1. Introduction
2. Biological materials and interface effects
2.1 Nanostructures in nacre
2.2 Multiscale structure of timber
3. Composites and nanocomposites: computational modeling of the effect of interfaces on the mechanical properties
3.1 Interfaces and fiber sizing in fiber reinforced composites
3.2 Nanoclay/polymer interface and region of perturbed structure around the clay: effective interfaces model
3.3 Hierarchical fiber reinforced composites with nanoeingineered interfaces
4. Nanocrystalline metals: grain boundaries and their effect on the mechanical properties
4.1 Ultrafine grained titanium: effect of dislocation density and non-equilibrium state of grain boundaries
4.2 Precipitates in grain boundaries of UFG metals
5. Conclusions
Acknowledgements
References
3. Characterization studies of biopolymeric matrix and cellulose fibres based composites related to functionalized fibre-matri ...
1. Introduction
2. Natural fibres
3. Chemical composition of natural fibre
4. Microstructure of fibre
5. Nanostructure of fibre
6. Mechanical properties of nanocellulose
7. Processing of polymer composites
8. Theories of adhesion and type of bonding
9. Physio-chemical characterisation of interphase
10. Effect of fibre loading, size, and biopolymer composite composition (variable parameters) on mechanical properties
10.1 Tensile strength
10.2 Flexural strength
10.3 Impact strength
11. Effect of fibre loading, size, and biopolymer composite composition on physical properties
11.1 Water absorption
11.2 Thickness swelling
11.3 Density
12. Morphological analysis of fibre and matrix interfacial bonding by using scanning electron microscope (SEM)
12.1 Analysis on the effect of treated fibre adhesion on matrix composite
12.2 Analysis on the effect of fibre loading on matrix composites
12.3 Analysis on the effect of matrix adhesion on untreated fibre
13. Thermal gravimetric analysis (TGA) on types of fibre and matrix composites
13.1 Effect of various fibre loadings on TGA
13.2 Effect of various matrix composites on TGA
13.3 Effect of thermal degradation on various fibres and matrices
13.4 Residue effect on various fibre and matrix composites
14. Effect of nanoparticle size, nanoparticle/matrix interface adhesion and nanoparticle loading on the mechanical properties o ...
14.1 Young's modulus and tensile strength
14.2 Effect of particle size
14.3 Effect of particle loading
14.4 Effect of nanocellulose/matrix interfacial adhesion
14.5 Effect of nanocellulose in a starch matrix
14.6 Effect of various sources of nanocellulose reinforced biopolymer
15. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM)
16. Fourier transform infrared (FTIR) spectroscopy
17. X-ray diffraction (XRD)
18. Nuclear magnetic resonance (NMR) spectroscopy
19. Measurement of contact angle
20. Raman spectroscopy (RS)
21. Concluding remarks
References
4. Filler matrix interfaces of inorganic/biopolymer composites and their applications
1. Introduction
2. Filler-matrix interface of polymer composite
3. Common types of biopolymer composites
3.1 Metal based biopolymer composites
3.2 Metal oxide based biopolymer composites
3.3 Carbon based biopolymer composites
3.4 Other biopolymer composites
4. Practical applications of biopolymer composites
5. Conclusions
References
Part Two: Polymer matrix
5. A critical role of interphase properties and features on mechanical properties of poly(vinyl alcohol) (PVA) bionanocomposit ...
1. Introduction
2. Materials
3. PVA/NBC bionanocomposite fabrication
4. Characterisation techniques
5. Results and discussion
5.1 Particle size and elastic modulus of NBCs
5.2 Nanomechanical properties of PVA/NBC bionanocomposites
5.3 Interphase characterisation of PVA/NBC bionanocomposites
5.3.1 Modelling approach
5.3.2 Interphase elastic properties
5.4 Interphase dimensions
6. Conclusions
Acknowledgements
References
6. Effect of nanoclay filler on mechanical and morphological properties of Napier/ epoxy composites
1. Introduction
2. Natural fibre
3. Napier grass fibre
4. Natural fibre reinforced epoxy composites
5. Nanoclay reinforced composites
6. Nanoclay filled Napier/epoxy composites
7. Fabrication and flexure test of nanoclay filled Napier/epoxy composites
8. Flexural strength and modulus of nanoclay filled Napier/epoxy composites
9. Morphology of nanoclay filled Napier reinforced epoxy composites
10. Conclusion
References
7. A review on the interfacial characteristics of natural fibre reinforced polymer composites
1. Introduction
2. Types of fibre and matrix
3. Interfacial characterisation methods
4. Effects of the types of fibres and matrix
5. Effects of fibre extraction method
6. Effects of fibre embedded length
7. Effects of fibre surface treatment
8. Effects of strain rate
9. Effects of environmental attack
10. Conclusions and future recommendations
Acknowledgements
References
8. Interfaces in sugar palm fibres reinforced composites: A review
1. Introduction
2. Classification and structures of sugar palm fibres
2.1 Chemical compositions and properties of sugar palm fibres
2.2 Sugar palm particles reinforced composites
2.2.1 Physical properties
2.2.2 Mechanical properties
2.2.2.1 Sugar palm particle reinforced thermoset matrices
2.2.2.2 Sugar palm particle reinforced thermoplastic matrices
2.2.3 Thermal properties
3. Research methodology
3.1 Fibre modification
3.2 Chemical modification
3.2.1 Alkaline treatments
3.2.2 Silane treatment
3.2.3 Seawater treatment
3.3 Characterization methods
3.4 Scanning electron microscope
3.5 Atomic force microscope
3.6 X-Ray photo electron spectroscope
3.7 Applications and challenges
4. Conclusion
Acknowledgements
References
9 - Characterization studies of polymer-based composites related to functionalized filler-matrix interface
1. Introduction
1.1 Classification of polymers
2. Classifications based on application
2.1 Polymer composite
2.2 Types of polymer composites
2.2.1 Particulate reinforced composites
2.2.2 Fiber reinforced composites
2.2.3 Hybrid composites
2.2.4 Laminates
2.3 Polymer nanocomposites
3. Effects of additives on composite
3.1 Reinforcements
3.2 Fillers
3.3 Different types of fillers
3.3.1 Natural and renewable fillers
3.3.2 Zeolites
3.3.3 Dense fillers
3.3.4 Expandable microspheres
3.3.5 Nano-fillers
3.3.5.1 Cellulose-based nanofillers
3.3.5.2 Clay
3.3.6 Molecular fillers
3.3.7 Functional fillers
4. Characterization of polymer composites for filler matrix interface
4.1 SEM and TEM analysis
4.2 Atomic force microscopy (AFM)
4.3 Thermal analysis
4.3.1 Differential scanning calorimetry (DSC)
4.3.2 Thermogravimetric analysis (TGA)
4.4 X-ray diffraction (XRD)
4.5 FTIR analysis
4.6 XPS analysis
5. Conclusions
Acknowledgements
References
10. Performance of 3D printed poly(lactic acid)/halloysite nanocomposites
1. Introduction
2. Materials and methods
2.1 Overview
2.2 Materials
2.3 Fabrication of PLA-based nanocomposite filaments by melt compounding
2.4 Fabrication of PLA/epoxy/HNT nanocomposites for fill compositing
2.5 Impact and tensile testing of 3D printed specimens
2.6 Morphological analysis
3. Results and discussion
3.1 Filament diameter and quality
3.2 Impact tests
3.3 Tensile testing of PLA/HNT filament specimens
3.4 Fracture morphology
3.5 Interfacial interactions
4. Conclusion
References
11. Role of ionic liquids in eliminating interfacial defects in mixed matrix membranes
1. Introduction
2. Transport of gases through membranes
2.1 Permeation through dense membranes
2.2 Sorption in polymer membranes
2.3 Diffusion in polymer membranes
3. Membrane synthesis materials
3.1 Polymer
3.1.1 Glassy polymers
3.1.2 Rubbery polymers
3.2 Inorganic materials
4. Mixed matrix membranes
4.1 The concept of MMM
4.2 Selected reports on MMM
4.3 Issues and challenges in MMM
5. Ternary MMM
6. Ionic liquid embedded ternary mixed matrix membranes
7. Conclusion and outlook
Acknowledgement
References
12. Advancement in flame retardancy of natural fibre reinforced composites with macro to nanoscale particulates additives
1. Introduction
2. Standard and guidelines in fire safety of composite materials
2.1 Background
2.2 Fire test techniques
2.2.1 Bunsen burner test
2.2.2 Limiting oxygen index test
2.2.3 Heat release and mass loss rate tests
2.2.4 Smoke generation and toxicity tests
3. Marco to nanoscale particulate additives in flame retardancy of NFRC
3.1 Macroscale flame retardant particulate additives
3.1.1 Mineral hydroxide flame retardant
3.1.2 Expandable graphite
3.1.3 Hydroxycarbonates flame retardant
3.1.4 Borates based flame retardant
3.1.5 Phosphorous based flame retardant
3.1.6 Halogenated flame retardant
3.1.7 Hybrid flame retardants with synergistic effect
3.2 Nanoscale particulates flame retardant additives
3.2.1 Layered silicates
3.2.2 Carbon-family nanomaterials
4. Prospects of macro- to nano-particulate flame retardants in NFRC
References
Part Three: Ceramic matrix
13. Current review on the utilization of nanoparticles for ceramic matrix reinforcement
1. Introduction
2. Properties and applications of nanoparticles in reinforcement of ceramics
3. Common nanomaterials used in reinforcement of ceramics
3.1 Alumina based materials
3.2 Titanium dioxide
3.3 Carbon based materials
3.3.1 Carbon nanotubes
3.3.2 Carbon fibres
3.3.3 Graphene
3.4 Zirconia and rare earth materials
3.5 Hydroxyapatite and phosphates
3.6 Metal particles
4. Surface modification of nanoparticles
5. Conclusions
References
14. Characterization studies of ceramic-based composites related to functionalized filler-matrix interface
1. Background
1.1 Make-up and characteristics of a composite
2. Classification of composites
2.1 Matrix material
2.2 Reinforcement/filler phase
2.2.1 Particulate reinforcements
2.2.2 Ceramic material as particulate reinforcement
3. Types of ceramic used in composites
3.1 Ceramic-ceramic composites
3.2 Ceramic-metal composites
3.3 Graphite powder composites
3.4 Fiber-reinforced ceramics
4. Filler matrix interfaces
4.1 Particle-matrix interface
4.2 Carbon fiber matrix interface
4.3 Glass fiber-matrix interface
4.4 Ceramic fiber-matrix interface
4.5 Ceramic whisker-matrix interface
4.6 Oxide fiber-matrix interface
4.7 Non-oxide fiber-matrix interface
5. Characterization of interfaces in reinforced ceramic composites
6. Conclusions and future trends
Acknowledgements
References
15. Band-gap engineering using metal-semiconductor interfaces for photocatalysis and supercapacitor application: a nanoparticle ...
1. Introduction
2. Methodologies for synthesis of composite nanomaterials with improved physical and chemical properties
2.1 Chemical method
2.2 Thermal method
2.3 Deposition-precipitation method
2.4 Photodeposition method
2.5 Sputtering method
2.6 Chemical vapor deposition (CVD)
2.7 Anodization technique
3. The concept of band-gap engineering for the development of visible light active photocatalysts for energy harvesting applic ...
3.1 Schottky barrier
3.2 Ohmic contacts
3.3 The basic principle of photocatalytic water-splitting and purification of toxic water/air systems
3.4 Essentialities to be a photocatalyst
3.5 Bang-gap engineering
3.5.1 Composite semiconductors (formation of semiconductor heterojunctions)
3.5.2 Cation/anion doping
3.5.3 Surface co-catalysts
3.5.4 Semiconductor alloys
3.5.5 Nanodesign
4. Probable mechanistic pathways at the interfaces of particle reinforced nanocomposites in energy storage devices
4.1 Electrode materials
4.1.1 Carbon materials
4.1.1.1 Activated carbon
4.1.1.2 Carbide-derived carbons (CDC)
4.1.1.3 Carbon nanotubes (CNT)
4.1.1.4 Graphene
4.1.2 Metal-organic frameworks (MOFs)
4.1.3 Metal oxide
4.1.4 Metal chalcogenides (MX; X=S, Se)
4.2 Electrolyte
4.2.1 Aqueous and organic electrolytes
4.2.2 Ionic liquids
5. Conclusions
References
Part Four: Metal matrix
16. Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading
1. Introduction
2. Simulation facts
3. Thermal loading
3.1 Stress contour
3.2 Scattering of principal stress
3.3 Distribution of Von-Mises stress
4. Tensile loading
4.1 Influence of particle content on stress distribution
4.2 Influence of particle size on stress distribution
4.3 Influence of particles' shape on stress distribution
4.4 Effect of reinforcement content on strain field
4.5 Effect of reinforcement size on strain
4.6 Effect of particles' shape on strain field
5. Conclusions
References
17. Interface tailoring and thermal conductivity enhancement in diamond particles reinforced metal matrix composites
1. Diamond particles reinforced Al matrix composites
1.1 Fabrication of Al/diamond composites
1.2 Characterization of composite microstructure
1.3 Characterization of Al/diamond interface
1.4 Thermal conductivity of Al/diamond composites
2. Diamond particles reinforced Cu matrix composites
2.1 Modification of Cu/diamond interface by metal matrix alloying
2.1.1 Fabrication of Cu/diamond composites
2.1.2 Characterization of composite microstructure
2.1.3 Characterization of Cu/diamond interface
2.1.4 Thermal conductivity of Cu/diamond composites
2.2 Modification of Cu/diamond interface by diamond surface coating
2.2.1 Fabrication of Cu/diamond composites
2.2.2 Characterization of composite microstructure
2.2.3 Characterization of Cu/diamond interface
2.2.4 Thermal conductivity of Cu/diamond composites
3. Summary
Acknowledgements
References
18. Particle reinforced nanocomposites, interfaces, strength and tribological properties depending on the reinforcement type
1. Introduction
2. Materials and methods
2.1 Materials and fabrication
2.2 Characterization and testing
3. Microstructure and hardness analysis
4. Effect of load on wear loss and friction coefficient
5. Effect of sliding speed on wear loss and friction coefficient
6. Wear debris analysis
7. Conclusions
References
Part Five: Extracellular matrix
19. Stress induced at the bone-particle-reinforced nanocomposite interface: a finite element approach
1. Introduction
2. Finite element approach
2.1 Overview
2.2 Some key issues
3. Structures of human bone
3.1 Overview
3.2 Bone remodeling
4. Application of FEA in biomedical engineering and dentistry
4.1 Overview
4.2 Bone tissue engineering and scaffolds
4.3 Dentistry
5. Conclusion
References
20. Current understanding of interfacial stress transfer mechanisms in connective tissue
1. Introduction
2. Early studies on interfacial stress transfer mechanisms
2.1 Basic concepts, from an engineering perspective
2.2 Early analysis of interfibrillar shear stress
2.3 Insights derived from materials engineering and biological-based experiments
3. Later studies on interfacial stress transfer mechanisms
3.1 Engineering perspective: nanofibre-like particles reinforcing composites
3.2 Evidence against proteoglycans regulating collagen fibril stress uptake
3.3 Interfibrillar shear stress is responsible for collagen fibril stress uptake
4. Conclusion
References
Index
A
B
C
D
E
F
G
H
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K
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M
N
O
P
R
S
T
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Y
Z
Back Cover

Citation preview

Woodhead Publishing Series in Composites Science and Engineering

Interfaces in Particle and Fibre Reinforced Composites Current Perspectives on Polymer, Ceramic, Metal and Extracellular Matrices

Edited by

Kheng Lim Goh Aswathi M.K. Rangika Thilan De Silva Sabu Thomas

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2020 Elsevier Ltd. 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. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102665-6 For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gewn Jones Editorial Project Manager: Mariana L Kuhl Production Project Manager: Debasish Ghosh Cover Designer: Alan Studholme Typeset by TNQ Technologies

Contributors

M.N.M. Ansari Department of Mechanical Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia; Institute of Power Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia Mochamad Asrofi Department of Mechanical Engineering, Andalas University, Padang, West Sumatera, Indonesia M.K. Aswathi Mahatma Gandhi University, Kottayam, Kerala, India M.S.N. Atikah Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia A. Atiqah Department of Mechanical Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia A.M. Noor Azammi Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; Automotive Engineering Technology Section, UniKL-MFI, B.B. Bangi, Selangor, Malaysia A.K. Basak

Adelaide Microscopy, University of Adelaide, SA, Adelaide, Australia

Rishika Chakraborty Department of Chemistry, National Institute of Technology Meghalaya, Shillong, Meghalaya, India S. Chattopadhyaya Department of Mechanical Engineering, Indian School of Mines, Dhanbad, Jharkhand, India Andy H. Choi

Faculty of Science, University of Technology Sydney, Australia

Kishore Debnath Department of Mechanical Engineering, National Institute of Technology Meghalaya, Shillong, Meghalaya, India Rangika Thilan De Silva Sri Lanka Institute of Nanotechnology Pvt Ltd. (SLINTEC), Nanotechnology and science park, Homagama, SriLanka D.M.S.N. Dissanayake Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka

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Contributors

A.R. Dixit Department of Mechanical Engineering, Indian School of Mines, Dhanbad, Jharkhand, India Yu Dong School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia Kheng Lim Goh Advanced Composites Research Group, Newcastle Research & Innovation Institute (NewRIIS), Singapore; Newcastle University, Faculty of Science, Agriculture and Engineering, Newcastle Upon Tyne, United Kingdom Sreerag Gopi Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India Rushdan Ibrahim Pulp and Paper Branch, Forest Research Institute Malaysia, Kepong, Selangor, Malaysia R.A. Ilyas Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Serdang, Selangor, Malaysia H.A. Israr School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia E. Jackcina Stobel Christy Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India M. Jawaid Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia UPM Serdang, Selangor, Malaysia K. Jayaraj Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India M. Johar School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia Moon J. Kim

University of Texas at Dallas, Richardson, TX, United States

Ing Kong School of Engineering and Mathematical Sciences, La Trobe University, Bendigo, VIC, Australia K.H. Lim School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia G. Littlefair

The University of Auckland, Auckland, Auckland, New Zealand

M.S. Abdul Majid School of Manufacturing Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia; School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia

Contributors

xv

Hafiz Abdul Mannan Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia M.M.M.G.P.G. Mantilaka Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka; Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka M.T. Mastura Faculty of Mechanical and Manufacturing Engineering Technology, Universiti Teknikal Malaysia, Durian Tunggal, Melaka, Malaysia U.G. Mihiri Ekanayake Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka Leon Mishnaevsky, Jr. Department of Wind Energy, Technical University of Denmark, Roskilde, Denmark Dzeti Farhah Mohshim Petroleum Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia Mohanad Mousa School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia; Shatrah Technical Institute, Southern Technical University, Baghdad, Iraq Hilmi Mukhtar Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia Rizwan Nasir Department of Chemical Engineering, Faculty of Engineering, University of Jeddah, Jeddah, Saudi Arabia W.K. Ng School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia Mohumed Bilal Panjwani School of Engineering, Mechanical Engineering Discipline, Monash University Malaysia, Bandar Sunway, Selangor, Malaysia Pooria Pasbakhsh School of Engineering, Mechanical Engineering Discipline, Monash University Malaysia, Bandar Sunway, Selangor, Malaysia Maduranga Pillai Sri Lanka Institute of Nanotechnology Pvt Ltd. (SLINTEC), Nanotechnology and science park, Homagama, SriLanka Anitha Pius Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India Mukul Pradhan Department of Chemistry, National Institute of Technology Meghalaya, Shillong, Meghalaya, India A. Pramanik School of Civil and Mechanical Engineering, Curtin University, Bentley, WA, Australia

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Contributors

H.G. Prashantha Kumar Metal Matrix Composite (MMC) Laboratory, School of Mechanical Engineering, Vellore Institute Technology, Vellore, Tamil Nadu, India R.B.S.D. Rajapakshe Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka A. Rajeswari Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India Samantha Prabath Ratnayake Sri Lanka Institute of Nanotechnology, Homagama, Pitipana, Sri Lanka; School of Science, RMIT University, Victoria, Australia M.J.M. Ridzuan School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia K.A.A. Ruparathna Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka S.A.L. Sameera Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka S.M. Sapuan Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia Anoja Senthilnathan Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka Maizatul Shima Shaharun Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia M.G.G.S.N. Thilakarathna Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka S. Thomas Mahatma Gandhi University, Kottayam, Kerala, India Kim Yeow Tshai Faculty of Engineering, University of Nottingham Malaysia, Semenyih, Selangor, Malaysia Xitao Wang University of Science and Technology Beijing, Beijing, China Jinguo Wang University of Texas at Dallas, Richardson, TX, United States W.P.S.L. Wijesinghe Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka

Contributors

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K.J. Wong School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia M. Anthony Xavior Metal Matrix Composite (MMC) Laboratory, School of Mechanical Engineering, Vellore Institute Technology, Vellore, Tamil Nadu, India Hailong Zhang

University of Science and Technology Beijing, Beijing, China

Foreword

Some time ago, the publisher, Elsevier, had asked me whether I would like to prepare a foreword to a book entitled “Interfaces in Particle and Fibre Reinforced Composites”. When I recognized the four editors, I could not turn down the invitation. This is because one of the editors is a close research collaborator of mine for many years, and I also know many of the scientific contributions of the other co-editors. It is therefore a pleasure for me to write this foreword. Allow me to say a few words about the editors of this book. I first met Professor Sabu Thomas in the 1980’s, and in 1997 I spent a short visit with him at the Mahatma Gandhi University in Kerala to deliver a set of lectures to his students. He is an outstanding researcher in the fields of polymer blends, particulate and fibre reinforced polymers, natural composites, and nanocomposites. Sabu has provided fundamental understanding of polymer degradation, recycling and electronic properties. Two of his former doctoral students spent a one-year research stay at our institute funded by the prestigious Alexander von Humboldt Foundation. I note that Dr. Kheng Lim Goh is presently an Associate Professor in mechanical engineering and Director of Research at Newcastle University in Singapore. His research interests are focussed on the fundamentals of fiber and particle reinforced composites, design, manufacturing and repair of composite material, and structurefunction relationship of composite material based on concepts derived from biological materials, namely extracellular matrix of connective tissues. He has delivered several keynote speeches at many international conferences about his work on stress transfer mechanisms in composite material. He is an editorial board member of several journals. I also note that Dr. Rangika Thilan De Silva is presently Head of Technology Transfer at the Sri Lanka Institute of Nanotechnology. He is an experienced researcher in the field of nanotechnology and has strong skills in materials science in general and nanomaterials in particular. I am aware that Dr. M. K. Aswathi did a lot of research at the Mahatma Gandhi University and at other places and has published a variety of scientific contributions, e.g. on recycling of PET bottles, advanced carbon based foam materials, and EMI shielding of polymer nanocomposites using multi-walled carbon nanotubes. There are several journal publications related to the main subject of this book but only a handful of books have attempted to summarize the state of the art in this field. The last one dates back to the year 2011 [1e5]. After more than 8 years, clearly it is the

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Foreword

right time to renew the knowledge on interfacial effects in composite materials by a new book. Knowing the authors’ working knowledge in this field, this book will hold what it promises. Although this book comprises of several single contributions by various authors, it has been prepared based on a well-thought-out approach. The contents in the book cover a wide range of topics. To facilitate the reader, introductory interfacial problems in fibre and particle reinforced composite materials are presented in Chapters 1 to 3. This is followed by specialised sections categorised according to the type of material found in the matrix phase of the composite material. Thus, Chapters 4 to 12 focus on polymer matrices. The first chapter on polymer matrix seeks to address the critical role of interface problems on the mechanical properties of poly(vinyl) alcohol bionanocomposites (Chapter 5), Chapters 6, 7 and 8 deal with natural fibres and particles in polymer matrices, reflecting the current urgency and importance of producing green composite material for sustainability. Chapter 9 specialises in functionalized fillermatrix interfaces in polymer-based composites. Chapter 10 deals with a new and exciting emerging technique using additive manufacturing approach to 3D-print polylactide acid/halloysite nanocomposites. Chapter 11 deals with the use of ionic liquids in eliminating interfacial defects in mixed matrix membranes. The last chapter of the polymer matrix section covers an interesting and important topic on the advancement in the study of flame retardancy of natural fibre reinforced composites with macro-to nano-scale particulate additives (Chapter 12). Chapters 13, 14 and 15 focus on ceramic matrices. The utilization of nanoparticles for ceramic matrix reinforcement is emphasized in Chapter 13. Chapter 14 reports on the characterization studies of ceramic based composites related to the functionalized filler-matrix interface. An interesting topic on band-gap engineering using metal-semiconductor interfaces for photocatalysis and particle reinforced composites for supercapacitor applications is described in Chapter 15. Chapters 16, 17 and 18 focus on metal matrices. In particular, Chapter 16 outlines the stress formation in the interfaces of metal matrix composites (MMCs) under thermal and tensile loadings. Chapter 17 reports on a very recent study about interface tailoring and thermal conductivity enhancement of diamond particle reinforced metal matrix composites. Finally, Chapter 19 and 20 focus on the fourth type of matrix, namely extracellular matrix in hard and soft connective tissues, respectively. In particular, finite element analysis of stresses induced at the boneparticle reinforced nanocomposite interface is discusseddfrom a general perspectivedin Chapter 19. Chapter 20 outlines the basic concepts of stress transfer used in understanding interfaces in collagen reinforced ECM and the latest measurements derived from new experiments that attempt to revisit the study of interfacial stress transfer mechanisms in soft connective tissues (Chapter 20). Overall, the book is well-balanced with regard to experimental findings, mathematical modelling, as well as numerical simulation. The readers of the book can benefit from the latest knowledge about interfacial effects on the properties of particle and fibre reinforced composites with regard to

Foreword

xxi

extending their horizon on future developments, as well as finding new starting points for their own research activities in this field. The book has an excellent author and subject index. It is well-written and copiously illustrated. Anyone who needs to study interfaces in composites must read this work. The book is expected to be beneficial to practicing engineers, designers and graduate students, and should serve as a valuable reference book. I would like to thank the editors for the very interesting book content. Last, but not least, I also hope that the publisher will have a good success with this new book. August 27, 2019 Prof. Dr.-Ing. Dr. h.c. mult. Klaus Friedrich Retired Professor and Research Consultant Institute for Composite Materials (IVW GmbH) Technical University of Kaiserslautern Kaiserslautern, Germany

References [1] E.P. Pluedemann, Interfaces in Polymer Matrix Composites, Academic Press, 1974. [2] Controlled interfaces in composite materials, in: H. Ishida (Ed.), Proc. 3rd Int. Conf. on Composite Interfaces (ICCI-III), Cleveland, USA, 1990. [3] J.-K. Kim, Y.-W. Mai, Engineered Interfaces in Fiber Reinforced Composites, Elsevier, 1998. [4] Y. Ivanov, V. Cheskov, M. Natora, Polymer Composite Materials e Interface Phenomena & Processes, Springer, 2001. [5] S.-J. Park, M.-K. Seo, Interface Science and Composites, Academic Press, 2011.

Acknowledgements

This book was constructed with a lot of help from expert reviewers. We sincerely thank: Dr Vahdat Vahedi, Dr Asanka Roshan Bandara, Prof. Dr Azman bin Hassan, Dr Amit Chauhan, Prof. Ilaria Cacciotti, Dr Marcos Mariano, Dr Jinling Liu, Dr Reza Taherzadeh Mousavian, Dr Syed Sohail Akhtar, Dr Norhayani Othman and Dr Vee San Cheong for their helpful comments on the chapters.

Kheng Lim Goh

Rangika Thilan De Silva

Aswathi M.K.

Sabu Thomas

Introduction a, b

c

d

c

Kheng Lim Goh , M.K. Aswathi , Rangika Thilan De Silva , S. Thomas a Advanced Composites Research Group, Newcastle Research & Innovation Institute (NewRIIS), Singapore; bNewcastle University, Faculty of Science, Agriculture and Engineering, Newcastle Upon Tyne, United Kingdom; cMahatma Gandhi University, Kottayam, Kerala, India; dSri Lanka Institute of Nanotechnology Pvt Ltd. (SLINTEC), Nanotechnology and science park, Homagama, SriLanka

1

This book is concerned with the nature of the interface in composite materials reinforced by fibres or particles. The fibres can be long (continuous) or short (discontinuous) [1]. The particles may be made from a variety of materials, and can assume a variety of sizes and shapes [2e6]. In any case, the desired outcome is that the fibres and particles must reduce the load in the matrix by taking up a share of the load [2e4,7]. Interactions between the fibre (or particle) and the matrix are responsible for the mechanical properties of the composite material. The interface between the fibre (or particle) and the matrix is responsible for transferring the load from the matrix to the fibre (or particle) [5,6,8e12]. Where interfacial adhesion exists, the adhesion controls the mode of propagation of the microcracks at the fibre ends or around the vicinity of a particle. When a strong bond exists between the fibre (or particle) and the matrix, the cracks do not propagate easily along the surfaces of the fibre and particle [1,3]. With regard to fibres, the reinforcement may remain effective even after the fibre fragments at several points along its length [1,3]. A strong bond is important for other reasons, namely endowing the composite with high transverse strength and protecting the composite from deterioration sustained from continuous use. As an additional safeguard, good adhesion can enhance water resistance of, e.g. polymer matrix composites. Other bulk properties, such as thermal conductivity, are also important but how the strength of the interfacial adhesion relates precisely to these properties is not well understood. These relationships are important with regard to designing new composite materials for sustainability. On this note, there is a growing interest in using fibres derived from natural resources, such as oil palm, pineapple leaf, coir, jute, flax, sisal and hemp [13e18]. While these natural fibres present several advantages, namely strength and environmental friendliness [18], the key challenge is overcoming the hydrophilicity so that the good interfacial adhesion can be achieved when the fibres are blended in polymer matrices [18]. How effective is the bonding of the fibre (or particle) to the matrix depends on the materials for making the fibre, particle and matrix and these are important considerations when designing the composite material with enhanced interfacial adhesion [19,20]. More and more advanced materials and fabrication strategies have been developed for the fibre, particle and matrix [21,22], to ensure they are fit for the purpose, as composite materials continue to attract a wide range of applications from aerospace [23] to biomedical engineering [24]. Alternatively, one may pursue chemical and Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00001-7 Copyright © 2020 Elsevier Ltd. All rights reserved.

2

Interfaces in Particle and Fibre Reinforced Composites

physical modifications to the fibre and particle surfaces to improve the compatibility between the fibre (or particle) and the matrix [15e17,25e28]. The study of the types of fibre, particle and matrix are important for understanding how fibres and particles, when blended into the matrix material, provide reinforcement to the composite material. Typically, the matrix material dominates the bulk of the composite material. The different types of matrix materials may be categorised into four, namely polymers, metals, ceramics and biological materials. The first three categories deserve a short technical comment here. It is interesting to note that an order-of-magnitude comparison of these materials reveals that metals exhibit a wide range of values of stiffness (10e103 GPa) similar to those of ceramics but higher than those of polymers (10 3 to 10 GPa) [29]. With regard to yield strength, ceramics exhibit a range of values (102e105 MPa) which are much higher than those of metals (1e103 MPa), but polymers exhibit the lowest values (0.1e102 MPa), albeit some overlaps, e.g. ceramics-metals and metals-polymers [29]. With regard to toughness, metals exhibit the widest range of values (10 1e103 kJ/m2), with values at the lower end overlapping those of polymers (10 1e10 kJ/m2); ceramics exhibit the lowest values (10 3e10 1 kJ/m2) [29]. These observations suggest that the physical properties of metals are not always superior to those of ceramics and polymers and the physical properties of polymers are not always the most inferior of the three. The fourth category of matrix material, i.e. biological matrix, deserves a brief introduction here. The biological matrix broadly refers to the naturally occurring interfibrillar materials in extracellular matrix of soft and hard connective tissues. With regard to soft tissues, such as tendons and ligaments [30] the fibrillar components are collagen fibrils and elastic fibres, which provide reinforcement to the biological matrix. The biological matrix comprises a hydrated ground substance made up of biological macromolecules [31]. Although no direct measurements have yet been made, the physical properties, such as stiffness, strength and toughness, of the interfibrillar matrix material of soft tissues are expected to be far inferior to those of metals, ceramics and polymers. However, from the tissue engineering perspective, as the decellularized tissue yields good biocompatibility with the human body it lends to useful applications, such as scaffolds for medical implants [32]. Bone, which belongs to the hard tissue category, contains an organic component, called osteoid, which is mainly made up of collagen type 1 fibrils, and an inorganic component, which is predominated by hydroxyapatite crystals [Ref: Zioupos, P., 2001, Accumulation of in-vivo fatigue microdamage and its relation to biomechanical properties in ageing human cortical bone, Journal of Microscopy, Vol. 201, p270e278]. In recent years, advanced characterisation methods have been applied to test materials from macroscopic to nanoscopic length scale, complemented by computational approaches [30,33]. These methods are increasingly directed at composite materials reinforced by fibres and particles. Significant progress has also been made to apply these approaches to explore the interface between the matrix and the fibre (or particle), leading to a variety of insights into how the fibre, particle and the matrix material interact and function. More importantly, the results lead to important insights about the nature of the interface at the fundamental level, to inform the manufacturing

Introduction

3

engineer on the choice of the fibre, particle and matrix materials that can produce the composite material with the required mechanical and other physical characteristics. This book arises from our motivation to provide an up-to-date book of fibre and particle reinforced composites, in part because of the rapid advancement in composite materials. This book describes current research on selected aspects of the matrix, with a focus on the four types of matrix materials and the interfacial effects. It is not intended to be comprehensive. Each chapter in this book describes findings on the fibre- and particle-matrix interface with regard to either metal-based, ceramic-based, polymer-based and biological matrices. Some chapters are concerned with reviewing the findings of others while other chapters are concerned with reports from experimental studies carried out by the contributors. Although each chapter is concerned with a highly specialised topic, it is hope that this book will both inspire and help the reader to gain new insights that may be useful for addressing fundamental problems across all length scales and across all traditional materials at the cutting edge of composite technology. The book recognizes that many challenging and exciting problems facing fibre and particle reinforcing composites lie at the boundaries between the traditional materials and our understanding of the varying length scales.

References [1] K.L. Goh, Discontinuous-fibre Reinforced Composites. Fundamentals of Stress Transfer and Fracture Mechanics, first ed., Springer-Verlag, London, 2017. [2] K.L. Goh, R.M. Aspden, K.J. Mathias, D.W.L. Hukins, Effect of fibre shape on the stresses within fibres in fibre-reinforced composite materials, Proc. R. Soc. Lond. 455 (1999) 3351e3361. [3] K.L. Goh, R.M. Aspden, D.W.L. Hukins, Review: finite element analysis of stress transfer in short-fibre composite materials, Compos. Sci. Technol. 64 (2004a) 1091e1100. [4] K.L. Goh, R.M. Aspden, K.J. Mathias, D.W.L. Hukins, Finite-element analysis of the effect of material properties and fibre shape on stresses in an elastic fibre embedded in an elastic matrix in a fibre-composite material, Proceedings of the Royal Society of London A, 2004b, pp. 2339e2352. [5] R.T. De Silva, P. Pasbakhsh, K.L. Goh, S.-P. Chai, J. Chen, Synthesis and characterisation of poly (lactic acid)/halloysite bionanocomposite films, J. Compos. Mater. 48 (2014a) 3705e3717. [6] R.T. De Silva, M. Soheilmoghaddam, K.L. Goh, M.U. Wahit, H. Abd bee, C.S.P. Sharifah, P. Pasbakhsh, Influence of the processing methods on the properties of poly (lactic acid)/halloysite nanocomposites, Polym. Compos. 37 (2016) 861e869. [7] S.Y. Fu, X.Q. Feng, B. Lauke, Y.W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate-polymer composites, Compos. B Eng. 39 (2008) 933e961. [8] C. Galiotis, A. Paipetis, C. Marston, Unification of fibre/matrix interfacial measurements with Raman microscopy, J. Raman Spectrosc. 30 (1999) 899e912. [9] C. Galiotis, R.J. Young, P.H.J. Yeung, D.N. Batchelder, The study of model polydiacetylene/epoxy composites: Part 1 the axial strain in the fibre, J. Mater. Sci. 19 (1984) 3640e3648.

4

Interfaces in Particle and Fibre Reinforced Composites

[10] R.B. Nath, D.N. Fenner, C. Galiotis, The progressional approach to interfacial failure in carbon reinforced composites: elasto-plastic finite element modelling of interface cracks, Compos. Appl. Sci. Manuf. 31 (2000) 929e943. [11] A. Paipetis, C. Galiotis, Y.C. Liu, J.A. Nairn, Stress transfer from the matrix to the fibre in a fragmentation test: Raman experiments and analytical modeling, J. Compos. Mater. 33 (1999) 377e399. [12] R.T. De Silva, P. Pasbakhsh, K.L. Goh, L. Mishnaevsky, 3-D Computational Model of Poly (Lactic Acid)/halloysite Nanocomposites: Predicting Elastic Properties and Stress Analysis. Polymer (United Kingdom), vol. 55, 2014. [13] S.A.S.M. Buana, P. Pasbaskhsh, K.L. Goh, F. Bateni, M.R.H.M. Haris, Elasticity, microstructure and thermal stability of foliage and fruit fibres from four tropical crops, Fibers Polym. 14 (2013) 623e629. [14] S. Thomas, Y.-K. Woh, R. Wang, K.L. Goh, Probing the Hydrophilicity of Coir Fibres: Analysis of the Mechanical Properties of Single Coir Fibres, Procedia Engineering, 2017. [15] A.K.F. Dilfi, A. Balan, H. Bin, G. Xian, S. Thomas, Effect of surface modification of jute fiber on the mechanical properties and durability of jute fiber- reinforced epoxy composites, Polym. Compos. 39 (2018) E2519eE2528. [16] A. Ajith, G. Xian, H. LI, Z. Sherief, S. Thomas, Surface grafting of flax fibres with hydrous zirconia nanoparticles and the effects on the tensile and bonding properties, J. Compos. Mater. 50 (2016) 627e635. [17] K.M. Praveen, S. Thomas, Y. Grohens, M. Mozetic, I. Junkar, G. Primc, M. Gorjanc, Investigations of plasma induced effects on the surface properties of lignocellulosic natural coir fibres, Appl. Surf. Sci. 368 (2016) 146e156. [18] S. Manna, P. Saha, S. Chowdhury, S. Thomas, Alkali treatment to improve physical, mechanical and chemical properties of lignocellulosic natural fibers for use in various applications, in: A. Kuila, V. Sharma (Eds.), Lignocellulosic Biomass Production and Industrial Applications, Scrivener Publishing LLC, 2017. [19] C. Zabihi, M. Ahmadi, S. Nikafshar, K.C. Preyeswary, M. Naebe, A technCical review on epoxy-clay nanocomposites: structure, properties, and their applications in fiber reinforced composites, Composites Part B 135 (2018) 1e24. [20] A. Kausar, Review on polymer/halloysite nanotube nanocomposite, Polym. Plast. Technol. Eng. 57 (2018) 548e564. [21] L. Lavagna, D. Massella, M.F. Pantano, F. Bosia, N.M. Pugno, M. Pavese, Grafting Carbon Nanotubes onto Carbon Fibres Doubles Their Effective Strength and the Toughness of the Composite, Composites Science and Technology, 2018. [22] G. Mittal, K.Y. Rhee, V. Miskovic-Stankovic, D. Hui, Reinforcements in multi-scale polymer composites: processing, properties, and applications, Composites Part B 138 (2018) 122e139. [23] M.Z. Naser, A.I. Chehab, Materials and design concepts for space-resilient structures, Prog. Aerosp. Sci. 98 (2018) 74e90. [24] X. Liu, D. Liu, J.H. Lee, Q. Zheng, X. Du, X. Zhang, H. Xu, Z. Wang, Y. Wu, X. Shen, J. Cui, Y.W. Mai, J.K. Kim, Spider web-inspired stretchable graphene woven fabric for highly sensitive, transparent, wearable strain sensors, ACS Appl. Mater. Interfaces 11 (2018) 2282e2294. [25] P. Saha, S. Chowdhury, D. Roy, B. Adhikari, J.K. Kim, S. Thomas, A brief review on the chemical modifications of lignocellulosic fibers for durable engineering composites, Polym. Bull. 73 (2016) 587e620.

Introduction

5

[26] L.P. Tan, C.Y. Yue, K.C. Tam, Y.C. Lam, X. Hu, Effect of compatibilization in injectionmolded polycarbonate and liquid crystalline polymer blend, J. Appl. Polym. Sci. 84 (2002) 568e575. [27] L.P. Tan, C.Y. Yue, K.C. Tam, Y.C. Lam, X. hu, K. Nakayama, Relaxation of liquidcrystalline polymer fibers in polycarbonateeliquid-crystalline polymer blend system, J. Polym. Sci. B Polym. Phys. 41 (2003) 2307e2312. [28] K.L. Goh, L.P. Tan, Micromechanical fibre-recruitment model of liquid crystalline polymer reinforcing polycarbonate composites, in: M. Tamin (Ed.), Damage and Fracture of Composite Materials and Structures, Springer-Verlag, Berlin, 2011. [29] M.F. Ashby, D.R.H. Jones, Engineering Materials 1: An Introduction to Properties, Applications, and Design, 2009. [30] S.W. Cranford, J. de Boer, C. Van Blitterswijk, M.J. Buehler, Materiomics: an -omics Approach to biomaterials research, Adv. Mater. 25 (2013) 802e824. [31] J.M. Mattson, R. Turcotte, Y. Zhang, Glycosaminoglycans contribute to extracellular matrix fiber recruitment and arterial wall mechanics, Biomechanics Model. Mechanobiol. 16 (2017) 213e225. [32] J.M. Aamodt, D.W. Grainger, Extracellular matrix-based biomaterial scaffolds and the host response, Biomaterials 86 (2016) 68e82. [33] Z. Zhao, H. Dang, C. Zhang, G.J. Yun, Y. Li, A multi-scale modeling framework for impact damage simulation of triaxially braided composites, Composites Part A 110 (2018) 113e125.

Structured interfaces and their effect on composite performance: computational studies1

2

Leon Mishnaevsky, Jr. Department of Wind Energy, Technical University of Denmark, Roskilde, Denmark

1. Introduction Various strategies and techniques of the microstructure modification have been developed to ensure the better service properties of materials [1e11]. Interfaces, phase and grain boundaries represent often relatively unstable and deformable regions of materials. The typical deformation and degradation scenario of materials includes the formation and development of highly deformed regions (e.g., shear bands), defects, cracks in the deformable regions. One of the ways to control the deformation scenario is to modify the structure of the less stable, deformable elements of the material at the lower scale level, thus, influencing the deformation development and damage initiation processes. The introduction of geometrical structural inhomogeneities into unstable phases, like defects, nanoscale reinforcements, structural gradients, can make it possible to control the local stress concentration, localization of deformation in weaker phases and thus microstructure evolution and microstructural adaptation of the material. Toughening the weak regions, interfaces or other defects (which otherwise serve as sites of damage initiation) channels the deformation energy into the lower scale level. Thus, nanomodification of weak regions and structural defects can be used to influence the damage evolution and improve the damage resistance of the material. In this paper, we provide a short overview of the computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials. We consider several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) and explore (using numerical experiments) how the interface structures influence the properties of the materials. Considering wood (multilayered nanoreinforced cellular material), fiber reinforced polymer composites, nanocomposites and hybrid composites, as well as ultrafine grained metals, we demonstrate that the availability of special structures in grain boundaries/phase boundaries/interfaces represents an important and promising source of the enhancement of the materials strength. 1

Some part of this paper are reprinted from L. Mishnaevsky Jr., Nanostructured interfaces for enhancing mechanical properties of materials: Computational micromechanical studies, Composites Part B, Vol. 68, 2015, pp. 75e84.

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00002-9 Copyright © 2020 Elsevier Ltd. All rights reserved.

10

2.

Interfaces in Particle and Fibre Reinforced Composites

Biological materials and interface effects

Natural biological materials demonstrate often extraordinary strength, damage resistance and toughness. They are often stronger and tougher, than could be assumed from averaging their component properties [12]. Typically, strong biological materials (bones, sea shells, insect cuticles) represent composites, consisting of hard (mineral) and soft (biopolymer) phases, organized in complex multilevel structures [13,14].

2.1

Nanostructures in nacre

Mayer and Sarikaya [15] reviewed the structures and properties of rigid biological composites, and noted that biointerfaces are seldom sharp, and typically have very complex structure. Interfaces can be interpenetrating and form the wide transitional zone, or change from columnar calcitic structure to aragonite nacreous structure (in the shell of red abalone). The broad, structured interfaces with interpenetrating, gradient nanostructures increase the toughness of the biomaterials. For instance, nacre of mollusks contains 95% of aragonite platelets and only 5% of biopolymer. Thin layers of biopolymer between the platelets can be considered as interfaces in this material [16]. The biopolymer layers have in fact a very complex structure: they represent organic macromolecules, containing polysacharides and protein fibers. Furthermore, the biopolymer thin layers of nacre contain nanopores and also inorganic mineral bridges, linking the aragonite platelets [16e19], see Fig. 2.1. The adhesive fibers elongate in a stepwise manner, when nacre is loaded, as folded domains or loops are pulled open. Sawtooth pattern of the forceeextension curve in the

Aragonite platelets Biopolymer layer

Mineral bridges in biopolymer layer

Fig. 2.1 Schema of microarchitecture of nacre: aragonite platelets, biopolymer layer and mineral bridges. After F. Song, Y.L. Bai, Effects of nanostructures on the fracture strength of the interfaces in nacre, J. Mater. Res., 18 (8) (2003). F.Song, A.K. Soh, Y.L. Bai, Structural and mechanical properties of the organic matrix layers of nacre Biomaterials 24 (2003) 3623e3631.

Structured interfaces and their effect on composite performance: computational studies

11

protein is a result of the successive domain unfolding. During the crack propagation, the energy is absorbed by the interface debonding and by the shearing of the protein layer. Katti et al. [20] simulated the effect of nanoasperities in the platelets/biopolymer interfaces of nacre on the mechanical properties. While only marginal effect of nanoasperites on the mechanical properties was proved in the simulations, the authors noted that the positive role of nanoasperities can include the effects of larger surface area of minerals and confinement of polymer. Qi et al. [21] modeled the mechanical behavior of nacre numerically, taking into account the unfolding of protein molecules in the organic matrix. The nonlinear stress-strain behavior was observed, with an apparent “yield” stress (related with the unfolding events in the organic layers and to the mitigation of load transfer to the aragonite tablets) and hardening (related with the shear in the organic layers). Song and Bai [16] evaluated the fracture toughness in the “brick bridge mortar” structure of nacre and showed that the availability of nanostructures in the nacre interfaces (i.e., mineral bridges between aragonite platelets located in the bioolymer layers) is one of the reasons for the high toughness of nacre. The mineral bridges reinforce the weak interface, and control the crack propagation in the interfaces (biopolymer layers). Thus, nanostructuring of interfaces and thin biopolymer layers between mineral platelets play an important role for the strength and toughness of biocomposites, and represent one of the sources of high strength and toughness of these materials.

2.2

Multiscale structure of timber

Timber can be considered as a natural composite with very high strength/weight ratio [22]. The structure of softwood is usually described at four different structural levels (see Fig. 2.2). At the macroscale, it is the annual rings (alternated light and dark rings, called earlywood and latewood respectively; the earlywood is characterized by cells with larger diameters and thinner cell walls than in latewood). At the mesoscale, it is a cellular material, built up by hexagon-shaped-tube cells oriented fairly parallel to the stem direction. At the microscale, cell walls of wood consists of 4 layers with different microstructures and properties, which are called usually P, S1, S2 and S3, and middle layer M acts as bonding material. At the nanoscale, the layers in the secondary wall of a tracheid cell are built of several hundred individual lamellae with varied volume fractions and characteristic microfibril angles (MFAs). The layers, building the hexagon cells, can be considered as one voluminous element (the thick layer S2) and several almost two-dimensional layers, M, P, S1, S3. The thickness of the S2 layer is 10 . 100 times higher than that of other layers. The mechanical behavior and strength of wood are determined by the complex interaction between all the elements at different scales. In order to analyze the effect of nanostructures (angle and distribution of microfibrils in each layer) on the mechanical properties of the wood, Qing and Mishnaevsky Jr [23,24]. developed and employed 3D multiscale computational model of wood as layered fibril reinforced

12

Interfaces in Particle and Fibre Reinforced Composites

Fig. 2.2 Multiscale model of wood as layered, cellular (A) and fibril reinforced (B) material [23,24] 2. The thin layers S1, S3 with fibrils perpendicular to the fibrils in thick S2 layer control the buckling and fracture resistance.

cellular material. In the numerical experiments, the authors demonstrated that the variation of microfibril angles represents a rather efficient mechanism of the control of stiffness of wood. By increasing the MFAs (microfibril angle, i.e., the angle between microfibrils and horizontal line), the drastic increase of shear stiffness in 1e2 direction is achieved, without any sizable losses of the transverse Young modulus and shear modulus in the 23 plane. The thick layer S2 is responsible mainly for the stiffness and deformation behavior of wood. The nanostructures of thin layers (microfibril angles in S1, S3) have still a rather strong effect on the peak stress in earlywood under tensile loading (22% higher peak stress when the fibril angles changes from 70 to 50
in the thin S1 layer). Still, the thin layers play different roles. The microfibrils in S1 and S3 layers are perpendicular to those in S2 layer. So, the S1 and S3 layers are responsible mainly for the resistance against buckling, collapse and fracture, and shear moduli. The layer S1 (which is much thicker in compression wood) ensuring the stability of wood under compression (S1) [25]. According to Ref. [26], the fibril distribution in thin layers (S1, S3) controls the trans- and intra-wall crack propagation, resisting the development of transwall cracks in transverse direction and preventing the intrawall cracks from becoming transwall cracks. Thus, while the properties of wood are generally controlled by complex interplay of layered, cellular, fibrous structures at many scale levels, the variation of nanoscale structures in thin layers binding voluminous structural elements play very important roles in ensuring the optimal output properties, deformation and strength of wood.

2

Reprinted with kind permission from Elsevier.

Structured interfaces and their effect on composite performance: computational studies

13

3. Composites and nanocomposites: computational modeling of the effect of interfaces on the mechanical properties The strength and lifetime of fiber reinforced composites are controlled by the strength of fibers, matrix and interfaces. While the matrix is responsible for the material integrity, fibers control the stiffness of the composite in the case of glass or carbon fiber reinforced polymers. The strength of interface plays a mixed role: while the crack propagation into the fiber/matrix interface delays the matrix failure (and, thus, increases the strength of the composite), too weak interfaces lead to quick fiber pull-out and also to quick failure. So, the idea of interface based design of fiber reinforced composites was formulated (see, e.g. Ref. [3]).

3.1

Interfaces and fiber sizing in fiber reinforced composites

In order to analyze the role of interfaces and mechanisms of composite degradation, a series of computational studies has been carried out [27e31]. A number of multifiber unit cell models were implemented in the framework of finite element method. The concept of “third material layer” was applied to simulate the interface properties and interface degradation. Namely, since surfaces of fibers can be rather rough [32], and the interface regions in many composites contain interphases [33,34], the interface debonding was considered not as a two-dimensional opening of two contacting plane surfaces, but rather as a three-dimensional process in a thin layer (“third material layer” between the homogeneous fiber and matrix materials [35]). In the numerical studies, it was observed that the formation of interface cracks under tensile loading takes place often after and as a result the fiber cracking, and in the vicinity of the fiber cracks. If however the interface is weak, the interface damage begins much earlier than the fiber cracking and can be accelerated by the matrix defects. On the other side, if the interface is pre-damaged, that can lead to a slightly lower stress level in the corresponding fiber: while the stresses in the vicinity of the interface crack are rather high, the stresses in the fiber are lower than those in fibers with undamaged interfaces. The fiber cracks cause interface damage, but not vice versa. In further numerical experiments, the competition between the matrix cracking and the interface debonding was observed. In the area, where the interface is damaged, no matrix crack forms; vice versa, in the area, where the long matrix cracks is formed, the fiber cracking does not lead to the interface damage. Fig. 2.3 shows the crack evolution in fiber reinforced polymer composites, obtained in the simulations. The weak interfaces of composites, as such, have a negative effect of the composite properties: ultimately, the homogeneously weak interfaces will debond, and the composite will behave as a dry fiber bundle. However, the results of these studies demonstrated that local weak places in composite interfaces can be rather beneficial for the composite strength and toughness: they can prevent the matrix failure (by channeling

14

Interfaces in Particle and Fibre Reinforced Composites

Fig. 2.3 Simulated damage mechanism in fiber reinforced polymer composites: Competition of damage mechanisms. (A and B). Interface is damaged (red region) in a region far from the first cracked fiber; matrix crack is formed far from the region with damaged interface. (C) Stress in a fiber is lower than in other fibers if its sizing is pre-damaged. Reprinted from L. Mishnaevsky Jr and P. Brøndsted, Micromechanisms of damage in unidirectional fiber reinforced composites: 3D computational analysis, Compos. Sci. Technol. 69 (7e8) (2009) 1036e10441.

the fracture energy into interface defects), and even delay the fiber failure. Practically, it means that a heterogeneous interface (interface with both weak and strong regions) can prevent the matrix failure, and therefore, ensure the integrity of the material. This suggests that microporous, heterogeneously pre-damaged interfaces in composites can be beneficial for the strength of materials [28]. The concept of porous interface which encourage the crack deflection into the interface has been discussed also by Evans, Zok and colleagues [9e11]. The porous interface was also realized in US Patent 6121169 A by Northrop Grumman Corporation.

3.2

Nanoclay/polymer interface and region of perturbed structure around the clay: effective interfaces model

Nanoinclusions in polymer matrix have much stronger reinforcing effect, than microscale particles. While the stiffening and strengthening of polymers by microscale particles can be roughly described by the rule-of-mixture, the addition of even very small amount of nanoparticles (of the order of few percents) can lead to the drastic improvements in modulus, strength and other properties, much above the rule-of-mixture estimations [36]. For instance, 34% higher Young’s modulus and 25% higher tensile strength were achieved in nanoclay/epoxy composite by adding only 5 wt% of nanoclay [37]. 38% higher Young’s modulus, 10.5% higher flexural strength and 25% higher microhardness of epoxy/glass fibers/nanoclay hybrid composites (HC) were achieved by adding only 5% of nanoclay [38]. The strong, non-proportional strengthening of nanoreinforcement is related with the large interfacial area of nanoparticles, interacting with the matrix and perturbing the molecular structure of the polymer matrix (another reason is the high aspect ratio of

Structured interfaces and their effect on composite performance: computational studies

15

most nanoparticles). Thus, the polymer is reinforced not only by the nanoscaled particles, but also by the layers of modified, constrained polymers surrounding each nanoparticle. In order to simulate this effect, Odegard and his colleagues [39] proposed the effective interface model (EIM). This model allows to generalize the micromechanical models of composites onto nanocomposites. In this model, the interfacial region of nanocomposites (consisting, e.g. of perturbed polymer and interfacial molecules) is presented as a layer with properties different from those of the rest of matrix [39]. The properties of the effective interface can be determined from molecular dynamics, or inverse modeling. Since the effective interface model is not applicable for the case of high volume fraction of nanoparticles, intercalated and clustered microstructures, when the particles might touch one another, Wang, Peng and colleagues [40e42] developed a generalized effective interface model in which the effective interface layer consist of several sublayers, with different properties, and some of the outer layers can be allowed to overlap. As demonstrated in Ref. [40], elastic properties of nanoclay reinforced polymers increase proportionally to the stiffness and the fraction of the interfacial layer. Fig. 2.4 shows two finite element models of nanocomposites (for exfoliated and intercalated structures of nanocomposites), and (c) Young’s modulus of the polyimide/silica nanocomposite plotted versus the silica particle volume fraction for different interface properties (phenoxybenzene silica nanoparticle/polyimide system with the Young’s modulus 0.3 GPa, functionalized silica nanoparticle/polyimide system with the Young’s modulus 3.5 GPa and a model with higher stiffness interface whose Young’s modulus is 8.4 GPa; the matrix module was 4.2 GPa) [40,41]. It can be seen that the interface properties strongly influence both the nanocomposite stiffness and reinforcing effect of nanoparticles. In order to determine the elastic properties of the interphase, the inverse modeling approach was employed in Ref. [36]. According to Ref. [43], 50% increase in the initial modulus of the polymer is observed for the 5% weight content of nanoclay. Considering the nanoclay/polymer composite with data from Ref. [43] (clay length 1000 nm, thickness 1 nm, Em ¼ 2.05 GPa, Enc ¼ 176 GPa) and varying the fraction of the intercalated nanoclay particles from zero (fully exfoliated material) to 100% (only clusters), as well as the amount of nanoparticles per cluster, one could demonstrate that for the case of fully exfoliated structure, Young’s modulus of the interphase can be 2.9 times of that of polymer matrix. For the more realistic case of partially intercalated microstructure (with the fraction of 25%e50% of nanoparticles in clusters), the Young’s modulus of the interphase becomes around 5 . 8 times that of the polymer matrix. These results are similar to the estimations by Yang and Cho [44] (from 2.44 Em and higher), Tsai, Tzeng [45] and Mesbah et al. [46] (5 . 8 Em). Thus, the layer of the polymer material with perturbed molecular structure, surrounding nanoreinforcing particles is stiffer than the rest of polymer, and represents the main reason for the un-proportionally strong reinforcing effect of nanoinclusions. This layer is formed as a result of interaction between large surface area of nanoinclusions and surrounding polymer chains. By modifying the nanoparticle surfaces (for instance, by oxidation of nanoreinforcement like graphene, or functionalizing the

16

Interfaces in Particle and Fibre Reinforced Composites

Fig. 2.4 Generalized equivalent interface (GIF) model for the analysis of nanoparticles in polymers (A, B) Finite element models with GEIF for exfoliated and intercalated (clustered) structures of nanocomposites, and (C) Effect of GIF properties on the properties of nanocomposite. Reprinted from H.W. Wang et al., Nanoreinforced polymer composites: 3D FEM modeling with effective interface concept, Compos. Sci. Technol., 71 (7) (2011) 980e988. R.D. Peng et al. Modeling of nano-reinforced polymer composites: microstructure effect on the Young’s modulus, Comput. Mater. Sci., 60 (2012) 19e311.

surface), one can influence the polymer-nanosurface interaction effect, and control the reinforcement degree of the nanoinclusions and mechanical properties of nanocomposites.

3.3

Hierarchical fiber reinforced composites with nanoeingineered interfaces

Nanostructuring of the matrix and/or fiber/matrix interfaces of fiber reinforced composites enhances the lifetime, fatigue resistance and strength of the materials in many cases. One can list a number of examples when the hierarchical design of fiber reinforced composites, with nanomodified sizings or matrix, lead to the enhancement of the material properties. For instance, 85% increase in fracture toughness was

Structured interfaces and their effect on composite performance: computational studies

17

achieved introducing 4 phr nanoclay in the matrix of carbon fiber reinforced epoxy/ clay nanocomposites [47]. 0.5 wt% CNT addition of carbon nanotubes (CNTs) lead to the 80% improvement of fracture toughness of carbon fiber reinforced epoxy composites [48]. 45% increase in shear strength is achieved by adding 0.015 wt% nanotubes into glass fiber reinforced vinyl ester composite with [49]. Strong positive effect is achieved if the nanoreinforcing elements are placed in the fiber sizing or fiber matrix interface. So, 30% enhancement of the interlaminar shear strength was achieved by deposition of multi and single walled CNT on woven carbon fabric fibers in epoxy matrix [50,51]. Interlaminar toughness and strength of alumina fiber reinforced plastic laminates were improved by 76% and 9% due to the radially aligned CNTs in both interlaminar and intralaminar regions [52]. Chatzigeorgiou and colleagues [53] analyzed the effect of coating from radially aligned carbon nanotubes on carbon fibers (“fuzzy fibers”) on the mechanical properties, and demonstrated that fuzzy fibers show improved transverse properties as compared with uncoated one. Even small additions of CNTs have very strong effect of these properties. Comparing the CNT reinforcement in polymer resine and CNTs grown/deposited on the surface of different fibers [54e57], one can see that the shear strength of the composites with CNTs in resin increases typically in the range 7 . 45%. At the same time, the increase of the interfacial shear strength due to the CNTs grown/deposited on fibers is between 30% and 150% (and for carbon fibers even 475%). For the computational analysis of the effect of nanostructuring in matrix and in the fiber/matrix interface on the mechanical properties of the composites, a 3D multiscale finite element model based on the macro-micro multiple-step modeling strategy was developed [58]. Here, the glass fiber/epoxy matrix/nanoclay reinforced composites under compression cyclic loading were considered. The microscale (lower level) unit cell includes the nanoplatelets reinforcement (exfoliated nanoplatelets and intercalated nanoplatelets/cluster) in matrix and/or interfaces. The model is shown schematically in Fig. 2.5. Using the model and the XFEM (eXtended Finite Element Method), the authors simulated the damage evolution in hierarchical composites subject to cyclic compressive loading, considering different structures and distributions of nanoreinforcements. In particular, nanoclay platelets randomly distributed in the matrix and localized in the glass fiber sizing have been considered (as well as different orientations and different degrees of clustering of these platelets). Fig. 2.5 shows the crack paths in the sizing of fibers, reinforced with aligned nanoclay platelets. Analyzing the effects of the secondary reinforcement on the fatigue resistance of composites, the authors [58] demonstrated that the crack path in the composite with nanomodified interfaces (fiber sizings) is much more rough than in the composite with nanomodified matrix. In the case of the matrix without nanoreinforcement, the crack grows straightforward, without deviations. The parameter of the crack deviation (Y-coordinate/height of crack peak divided by the X-coordinate of the crack peak) is 50 . 85% higher for the cases when the nanoplatelets are localized in the fiber sizing and not throughout the matrix. This parameter is related with the fracture toughness, and it suggests that the stress of crack initiation becomes much higher for the case of nanostructured fiber/matrix interface.

18

Interfaces in Particle and Fibre Reinforced Composites Crack growth direction

(a) II

Matrix

Crack

I

Interface Crack

Fiber

Crack growth direction

(b) III

II

I Crack

Matrix Interface Fiber

Fig. 2.5 Crack development in global and submodel a. in a model with nanoparticles in fiber sizing, b. with nanoparticles in a matrix Reprinted from L.Mishnaevsky Jr, Micromechanical analysis of nanocomposites using 3D voxel based material model, Compos. Sci. Technol., 72 (2012) 1167e1177. G.M. Dai, L. Mishnaevsky Jr, Fatigue of multiscale composites with secondary nanoplatelet reinforcement: 3d computational analysis , Composites Science and Technology Composites Science and Technology, 91 (2014) 71e811.

Further, the fatigue behavior of hierarchical composites with secondary nanoplatelet reinforcement in the polymer matrix, in the fiber/matrix interface and without the secondary reinforcement was compared. Composites with nanoreinforcement achieve the same fatigue life (taken exemplarily at 5.68*107 cycles) as neat composites, but subject to 2 . 3.5 times higher loadings. Further, composites with the nanoplatelets localized in the fiber/matrix interface layer (fiber sizing) ensure much higher fatigue lifetime than those with the nanoplatelets in the matrix. For instance, for the selected lifetime of 5.68*107 cycles, the applied stress can be 43 . 49% higher for the composites with the nanoplatelets localized in the fiber/matrix interfaces. Thus, the nanomodification of weaker phases in the fiber reinforced composites (polymer matrix and fiber/matrix interface layers) ensures the drastic increase in the fatigue lifetime. In particularly, the nanoreinforcements in fiber sizing (fiber/

Structured interfaces and their effect on composite performance: computational studies

19

matrix interface) lead to the drastic increase in the fatigue lifetime of the composites. From the short overview in this section, it can be seen that the interface/interphase regions of polymer composites and nanocomposites influence the strength and mechanical properties of these materials to a large degree. The layers of modified constrained polymer chains formed around nanoparticles due to the nanoparticle/polymer interfacial interaction determine the unusually high strength and mechanical properties of nanocomposites. The nanostructuring of fiber/matrix interfaces in fiber reinforced composites (porosity of fiber sizing/coatings, nanoreinforcement in fiber coatings) allow to control the mechanisms of the composite degradation, increase the lifetime and toughness of the composites.

4. Nanocrystalline metals: grain boundaries and their effect on the mechanical properties A very promising group of advanced materials for various applications are nanocrystalline metallic materials, e.g. materials with nanosized grains. As demonstrated in a number of works, these materials have better mechanical properties, higher ductility and strength, as compared with usual, coarse grained materials (CGM) [61e63]. One of technologies on nanostructuring of materials is the severe plastic deformation (SPD), which allows to fabricate bulk samples of the materials with the grain sizes 100e500 nm. These materials are called ultrafine grained (UFG) materials. An example of application of such materials is medical and dental implants made from ultrafine grained titanium [59,60]. Peculiarities of structures of nanocrystalline and ultrafine grained materials as compared with CGMs include the higher fraction of grain boundary (GB) phases, different atomic structure and availability of long-range stresses, enhanced atomic mobility and sometimes segregations in grain boundaries [64], as well as different deformation mechanisms (like grain boundary sliding and diffusion controlled flow). The concept “grain boundary engineering” for the materials improvement was developed by Watanabe [65]. Observing that an increase in the fraction of the special grain boundaries (i.e., boundaries with low reciprocal number densities of lattice sites) leads to better corrosion, creep and fracture resistance properties of materials, Watanabe suggested to use it to improve the properties of crystalline materials. This approach is especially important for ultrafine grained materials, due to the high fraction of grain boundaries [66]. Characterizing the grain boundary with the use of concident site lattice (CSL) model, considering misorientation of adjoining crystals, one can calculate a relative fraction of grain boundaries with the concident site lattice [67]. It was shown [68,69] that the materials with low relative fraction of grain boundaries which concident site lattice show, among other, high resistance to sliding, fracture and corrosion.

20

Interfaces in Particle and Fibre Reinforced Composites

Another approach to the enhancement of mechanical properties of nanocrystalline materials is based on the concept of non-equilibrium grain boundaries [66,72]. The grain boundaries which are characterized by higher energies, large amount of dislocations, higher diffusion coefficient, larger free volume in grain boundaries as well as the concentration of alloying elements and formation of their segregations are considered as non-equilibrium grain boundaries in nanocrystalline and ultrafine grained metals. Frolov et al. [74] also demonstrated in numerical simulations that multiple grain boundary phases with different atomic structures and densities are available in metallic grain boundaries. Reversible first order phase transitions between these phases can take place as a result of injecting point defects or varying temperatures. These interfacial phase transitions, observed in FCC metals, can have a strong effect on the materials properties. Below, we show several examples on how the structures and defects in grain boundaries of UFG titanium influence the mechanical properties of the material.

4.1

Ultrafine grained titanium: effect of dislocation density and non-equilibrium state of grain boundaries

The high density of dislocations in grain boundaries of ultrafine grained SPD produced metals is a result of SPD processing, and one of characteristics of non-equilibrium grain boundaries in nanomaterials. As noted in Ref. [70], the dislocation density in GBs grows with deformation passes, and is higher in GBs than in grain interior. Generally, the dislocation density in grain boundaries is estimated about 30 times higher after IV deformation stage [71]. In order to analyze effect of non-equilibrium grain boundaries of UFG titanium on its mechanical properties, a series of computational experiments were carried out in Refs. [72,73]. Computational models of UFG titanium were developed in Refs. [72,73] on the basis of “composite” representation of nanotitanium, as a hexagon or using Voronoi tessellation with grains surrounded by grain boundary layers (Fig. 2.6). For the description of deformation of grain boundary phase and the grain interior phase, the dislocation density based model was used, which took into account the dislocations immobilization at stored dislocations, storage of a geometrically necessary dislocation density in the interface between boundaries and interiors, mutual annihilation of dislocations of opposite sign, with a proportionality coefficient characterizing the probability of dislocations leaving their slip plane, e.g. by cross slip. For the grain boundaries, an additional term of the second annihilation mechanism is included where two stored dislocations of opposite sign may climb towards each other and annihilate eventually. More details about the materials properties and simulations conditions are given elsewhere [72,73]. In the simulations, it was observed that increasing the dislocation density in grain boundaries of ultrafine grained metals leads to the increased flow stress. This effect is

Structured interfaces and their effect on composite performance: computational studies

(a)

21

(b)

0.40 0.35

Maximum damage value

0.30 0.25 0.20 0.15

No precipitate Precipitate in GI Precipitate in GB

0.10 0.05 0.00 0.0

0.2

0.4

0.6

0.8

1.0

True strain

Fig. 2.6 Computational model of UFG titanium with precipitates in grain boundary phase and in grain interior (A) and the effect of the precipitate distribution on the damage evolution in UFG titanium (B). Reprinted from H.S Liu, W. Pantleon, L.Mishnaevsky Jr, Non-equilibrium grain boundaries in UFG titanium: computational study of sources of the material strengthening, Comput. Mater. Sci. 83 (2014) 318e330. H.S Liu, L.Mishnaevsky Jr, Gradient ultrafine-grained titanium: computational study of mechanical and damage behavior, Acta Materialia, 71, 2014, pp. 220e2331

especially strong for the nanoscale grain sizes: the yield stress increases by 18% in a material with grain size 250 nm, and by 51% in a material with grain size 50 nm, when initial dislocation density in GBs changes from 1.0  1015/m2 to 1.0  1018/m2. Apparently, the material with smaller grains is much more sensitive to the dislocation density in grain boundary, and, thus, to the non-equilibrium state of GB than a materials with larger grains. But also the damage value in the materials increases drastically if the initial dislocation density in GB increases [73]: for instance, the simulated highest damage value (under applied strain 0.22) increases by 95% (from 0.22 to 0.43) if the initial dislocation density in GB increases from 1015 to 1018. Thus, the high dislocation density in GB improves the flow stress, but also creates higher stress gradient and stress triaxiality in triple junction due to big difference in dislocation density and properties in GB and grain interior, and, ultimately, to the higher damage parameter. From the technology viewpoint, the decrease of grain size and the increase of the initial dislocation density are achieved by increasing the number of passes of the equal-channel angular pressing via the conform scheme (ECAP-C) fabrication technology [75].

4.2

Precipitates in grain boundaries of UFG metals

Another physical feature of non-equilibrium state of grain boundaries of ultrafine grained metals is related with the precipitates, segregations and foreign atoms formed

22

Interfaces in Particle and Fibre Reinforced Composites

in the grain boundaries. Impurity atoms, oxygen and carbon precipitates located in the GBs [76] interact with surrounding atoms of titanium, preventing the dislocation movement in their neighborhood. These precipitates, with content of the order of 0.5 at.% and of atomistic size, are rather spread, and their influence on the mechanical properties is still not well known. Another group of nanoscale secondary phases are dispersoids, e.g., titanium silicides or carbides [77]. Fig. 2.6(A) shows the computational unit cell models of the ultrafine grained titanium with precipitates in grain boundaries and in grains. In order to simulate the effect of low content, atomistic size precipitates on the macroscale mechanical properties of Ti, computational models of ultrafine grained titanium with precipitates in grain boundaries and in grains [72,73] were developed, in which small round foreign inclusions (which might represent physically dispersoids or precipitates or foreign impurity atoms and the regions of their immediate neighborhood with changed properties) are distributed randomly in GBs, grain interior or GB/grain borders. These precipitates are considered as round inclusions, elastic and impenetrable for dislocations. Fig. 2.6(B) shows the damage (i.e., maximum damage parameter in the model calculated by formulas from Refs. [78,79]) plotted versus the strain curves for the unit cell models for 3 cases: precipitates in GBs, in grain interior and no precipitates. (As shown in Ref. [73], the damage formula derived in Refs. [78,79] give the most correct damage distribution for nanomaterials). One can see from the curves that the availability of precipitates strongly delays the damage growth: while the damage level 0.2 is achieved in pure UFG titanium at the applied strain 0.1 . 0.13, the same level of damage is achieved for the materials with precipitates at 0.22 (precipitates in grain interior) . 0.35 strains (precipitates in grain boundaries). This means 83% increase in the critical strains due to the precipitates, and around 300% increase due to the precipitates located in grain boundaries. Flow stress was the highest for the material with the dispersoids in the grain boundary [73]. For instance, the flow stress at the applied strain 1.0 was 8% higher (for precipitates in GB) and 5.8% higher (for precipitates in grain interior), than in UFG Ti without precipitates. Thus, both the defects (dislocations) and nanoscale structural elements (precipitates, dispersoids) have a strong potential to increase the damage resistance and mechanical properties od advanced nanoscaled materials.

5.

Conclusions

In this work, we considered several groups of materials, characterized by high strength, and damage resistance. The interface structure versus strength and mechanical properties relationships for these groups of materials have been studied with the use of computational micromechanical models, reflecting the structures and architectures of these materials at several scale levels.

Structured interfaces and their effect on composite performance: computational studies

23

From these studies, one can conclude that the purposeful nanostructuring of interfaces and grain boundaries represents an important reserve of the improvement of the materials properties. Since the material deformation is often localized in and around defects (interfaces and grain boundaries), the structuring of these regions (adding specially arranged and oriented nanoreinforcements, or adding nanoscale defects, changing the local properties) allows to control the deformation and fracture behavior of these weak areas, thus, determining the degradation process in the whole material. The effect of nanostructured interfaces, phase and grain boundaries (PGB) on the strength behavior of the material can be realized by several ways: •

•

•

Defects in interfaces and PGB: increasing deformability of interfaces, one can channel the deformation energy from the main, load bearing (fibers in composites) or integrity ensuring (matrix) elements into non-critical areas. The examples of such effects are the microporous interfaces (fiber sizing) in fiber reinforced composites, and, to a some degree, high initial dislocation density in the grain boundaries of ultrafine grained metals. Modifying the constitutive behavior of grain boundaries and interfaces: By varying the mechanical behavior of the interfaces and GBs, the load transfer conditions as well deformation behavior can be controlled. The examples of such modifications are the non-equilibrium grain boundaries of ultrafine grained metals and also varied microfibril angles in this layers in wood cells. Nanoreinforcing the interfaces and PGBs: while the nanoreinforcement changes the elastic properties of these areas only weakly, it does change the damage mechanisms. It can lead to nanoscale crack bridging, crack deviation and blocking, what drastically changes the crack initiation and crack propagation toughness in the interface region. Thus, while the deformation is still localized in these regions, their damage resistance can be increased. The examples of such effects are the nanoreinforcing platelets in the sizing of fiber reinforced composites, as well as dispersoids and precipitates in the grain boundaries of ultrafine grained metals and mineral bridges in biopolymer layers in nacre.

On the interface structures-properties relationships, one can see from the listed examples that the heterogeneous interfaces have the highest potential as ways to improve the materials properties. Quite often, the interfaces with low stiffness lead to the localization of deformation, while the internal structures of the interfaces (like mineral bridges in nacre, or nanoplatelets in sizing of fiber reinforced composites) allow to control the deformation, damage initiation and fracture processes locally. Such a mechanism can allow to control and increase the material toughness and strength. Another mechanism is related with pre-damaged, porous interfaces, which cause the damage initiation in interfaces, but prevent the crack propagation. Apparently, the interface reinforcement oriented normally to the main reinforcing elements (like mineral bridges and aragonite platelets, fibrils in S1 and S2 layers of wood, or nanoplatelets aligned normally to fiber axes) ensure the highest toughness and optimal properties. Further investigations should be directed toward qualitative analysis of the service properties-interface structures relationships, and toward the optimal design of interface structures to enhance the strength, toughness and fatigue resistance of materials.

24

Interfaces in Particle and Fibre Reinforced Composites

Acknowledgements The author gratefully acknowledges the financial support of the Innovation Foundation of Denmark in the framework of the Grand Solutions project DURALEDGE, Durable leading edges for high tip speed wind turbine blades, File nr.: 8055-00012A.

References [1] L. Mishnaevsky Jr., Computational Mesomechanics of Composites, John Wiley, 2007, p. 280. [2] J.K. Kim, Y.W. Mai, High strength, high fracture toughness fibre composites with interface controlda review, Compos. Sci. Technol. 41 (4) (1991) 333e378. [3] Interface Design of Polymer Matrix Composites, in: B.F. Sørensen, et al. (Eds.), Proc. 28th Risø International Symposium on Materials Science, Risø National Laboratory, Roskilde, Denmark, 2007. [4] Controlled interphases in composite materials, in: H. Ishida (Ed.), Proc. 3rd Int Conference on Composite Interfaces (ICCI-III), 1990 Cleveland, Ohio, USA, 1990. [5] S.M. Mukhopadhyay, A. K Karumuri, Nanotube attachment for prevention of interfacial delamination, J. Phys. D Appl. Phys. 43 (7pp) (2010) 365301, https://doi.org/10.1088/ 0022-3727/43/36/365301. [6] S.M. Mukhopadhyay, A. Karumuri, I.T. Barney, Hierarchical nanostructures by nanotube grafting on porous cellular surfaces, J. Phys. D Appl. Phys. 42 (2009) 195503. [7] W. Wiedemann, L. Sims, A. Abdellah, A. Exner, R. Meier, K.P. Musselman, J.L. MacManus-Driscoll, P. Mueller-Buschbaum, G. Scarpa, P. Lugli, L. Schmidt-Mende, Nanostructured interfaces in polymer solar cells, Appl. Phys. Lett. 96 (26) (2010). [8] Y. Gao, A. Marconnet, M. Panzer, S. LeBlanc, S. Dogbe, Y. Ezzahri, A. Shakouri, K.E. Goodson, Nanostructured interfaces for thermoelectrics, J. Electron. Mater. 39 (2010) 1456e1462. [9] F.W. Zok, C.G. Levi, Mechanical properties of porous-matrix ceramic composites, Adv. Eng. Mater. 3 (1e2) (2001) 15e23. [10] A.G. Evans, F.W. Zok, J.B. Davis, The role of interfaces in fiber-reinforced brittle matrix composites, Compos. Sci. Technol. 42 (1991) 3e24. [11] J.B. Davis, J.P.A. Lofvander, A.G. Evans, Fiber coating concepts for brittle matrix composites, J. Am. Ceram. Soc. 76 (5) (1993) 1249. [12] P. Ramachamndra Rao, Biomimetics, Sadhan,a 28 (3e4) (2003) 657e676. [13] M.A. Meyers, P.-Y. Chen, A.Y.M. Lin, Y. Seki, Biological materials: structure and mechanical properties, Prog. Mater. Sci. 53 (2008) 1e206. [14] H. Ehrlich, H. Worch, Sponges as Natural Composites: From Biomimetic Potential to Development of New Biomaterials, Porifera Research: Biodiversity, Innovation and Sustainability, 2007, pp. 303e311. [15] G. Mayer, M. Sarikaya, Rigid biological composite materials, Structural examples for biomimetic design Experimental Mechanics 42 (4) (2002) 395e403. [16] F. Song, Y.L. Bai, Effects of nanostructures on the fracture strength of the interfaces in nacre, J. Mater. Res. 18 (8) (2003). [17] F. Song, A.K. Soh, Y.L. Bai, Structural and mechanical properties of the organic matrix layers of nacre, Biomaterials 24 (2003) 3623e3631.

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25

[18] T.E. Schaeffer, C. Ionescu-Zanetti, R. Proksch, M. Fritz, D.A. Walters, N. Almquist, C.M. Zaremba, A.M. Belcher, B.L. Smith, G.D. Stucky, D.E. Morse, P.K. Hansma, Does abalone nacre form by heteroepitaxial nucleation or by growth through mineral bridges? Chem. Mater. 9 (1997) 1731e1740. [19] B. Smith, T. Schaffer, M. Viani, J. Thompson, N. Frederick, J. Kindt, A. Belcher, G. Stucky, D. Morse, P. Hansma, Molecular mechanistic origin of the toughness of natural adhesives, fibres and composites, Nature 399 (1999) 761e763. [20] D. R Katti, S.M. Pradhan, K.S Katti Modeling the organic-inorganic interfacial nanoasperities in a model bio-nanocomposite, Nacre. Rev.Adv.Mater.Sci 6 (2004) 162e168. [21] H.J. Qi, B.J.F. Bruet, J.S. Palmer, C. Ortiz, M.C. Boyce, in: G.A. Holzapfel, R.W. Ogden (Eds.), Micromechanics and Macromechanics of the Tensile Deformation of Nacre, Mechanics of Biological Tissues, Springer-Verlag, Graz, 2005, pp. 175e189. [22] L. Mishnaevsky Jr., H. Qing, Micromechanical modelling of mechanical behaviour and strength of wood: state-of-the-art review, Comput. Mater. Sci. 44 (2) (2008) 363e370. [23] H. Qing, L. Mishnaevsky Jr., 3D multiscale micromechanical model of wood: from annual rings to microfibrils, Int. J. Solids Struct. 47 (9) (2010) 1253e1267. [24] H. Qing, L. Mishnaevsky Jr., 3D hierarchical computational model of wood as a cellular material with fibril reinforced, heterogeneous multiple layers, Mech. Mater. 41 (9) (2009) 1034e1049. [25] R.E. Booker, J. Sell, The nanostructure of the cell wall of softwood and its functions in a living tree, Holz als Roh- Werkst. 56 (1998) 1e8. [26] R.E. Booker, The reason for the microfibril orientations in the cell wall of trees, in: L.A. Donaldson, A.P. Singh, B.G. Butterfield, J. Whitehouse (Eds.), Recent Advances in Wood Anatomy, NZ Forest Research Institute Ltd, 1995, pp. 273e282. [27] L. Mishnaevsky Jr., P. Brøndsted, R. Nijssen, D.J. Lekou, T.P. Philippidis, Materials of large wind turbine blades: recent results in testing and modelling, Wind Energy 15 (1) (2012) 83e97. [28] L. Mishnaevsky Jr., P. Brøndsted, Micromechanisms of damage in unidirectional fiber reinforced composites: 3D computational analysis, Compos. Sci. Technol. 69 (7e8) (2009) 1036e1044. [29] H. Qing, L. Mishnaevsky Jr., Unidirectional high fiber content composites: automatic 3D FE model generation and damage simulation, Comput. Mater. Sci. 47 (2) (2009) 548e555. [30] L. Mishnaevsky Jr., P. Brøndsted, Three-dimensional numerical modelling of damage initiation in UD fiber-reinforced composites with ductile matrix, Mater. Sci. Eng. 498 (1e2) (2008) 81e86. [31] H.W. Wang, et al., Single fibre and multifibre unit cell analysis of strength and cracking of unidirectional composites, Comput. Mater. Sci. 46 (4) (2009) 810e820. [32] O.K. Buryan, V.U. Novikov, Modeling of the interphase of polymer-matrix composites: determination of its structure and mechanical properties, Mech. Compos. Mater. 38 (3) (2002) 187e190. [33] T.D. Downing, R. Kumar, W.M. Cross, L. Kjerengtroen, J.J. Kellar, Determining the interphase thickness and properties in polymer matrix composites using phase imaging atomic force microscopy and nanoindentation, J. Adhes. Sci. Technol. 14 (14) (2000) 1801e1812. [34] Y. Huang, J. Petermann, Interface layers of fiber-reinforced composites with transcrystalline morphology, Polym. Bull. 36 (4) (1996) 517e524.

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[35] L.E. Asp, L.A. Berglund, R. Talreja, Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites Composites, Sci. Technol. 56 (6) (1996) 657e665. [36] L. Mishnaevsky Jr., Micromechanical analysis of nanocomposites using 3D voxel based material model, Compos. Sci. Technol. 72 (2012) 1167e1177. [37] M.L. Chan, et al., Mechanism of reinforcement in a nanoclay/polymer composite, Compos. B Eng. 42 (6) (2011) 1708e1712. [38] J.J. Karippal, H.N. Narasimha Murthy, K.S. Rai, M. Sreejith, M. Krishna, Study of mechanical properties of epoxy/glass/nanoclay hybrid composites, J. Compos. Mater. 22 (2011), https://doi.org/10.1177/002199831038908. [39] G.M. Odegard, T.C. Clancy, T.S. Gates, Modeling of the mechanical properties of nanoparticle/polymer composites, Polymer 46 (2) (2005) 553e562. [40] H.W. Wang, et al., Nanoreinforced polymer composites: 3D FEM modeling with effective interface concept, Compos. Sci. Technol. 71 (7) (2011) 980e988. [41] R.D. Peng, et al., Modeling of nano-reinforced polymer composites: microstructure effect on the Young’s modulus, Comput. Mater. Sci. 60 (2012) 19e31. [42] G.M. Dai, L. Mishnaevsky Jr., Damage evolution in nanoclay-reinforced polymers: a three-dimensional computational study, Compos. Sci. Technol. 74 (2013) 67e77. [43] J.J. Luo, I.M. Daniel, Characterization and modeling of mechanical behavior of polymer/ clay nanocomposites, Compos. Sci. Technol. 63 (2003) 1607e1616. [44] S. Yang, M. Cho, Scale bridging method to characterize mechanical properties of nanoparticle/polymer nanocomposites, Appl. Phys. Lett. 93 (2008) 043111, https://doi.org/ 10.1063/1.2965486. [45] J.L. Tsai, S.H. Tzeng, Characterizing mechanical properties of particulate nanocomposites using micromechanical approach, J. Compos. Mater. 42 (22) (2008) 2345e2361. [46] A. Mesbah, et al., Experimental characterization and modeling stiffness of polymer/clay nanocomposites within a hierarchical multiscale framework, J. Appl. Polym. Sci. 114 (2009) 3274e3291. [47] Y. Xu, S.V. Hoa, Mechanical properties of carbon fiber reinforced epoxy/clay nanocomposites, Compos. Sci. Technol. 68 (3e4) (2008) 854e861. [48] A. Godara, et al., Influence of carbon nanotube reinforcement on the processing and the mechanical behaviour of carbon fiber/epoxy composites, Carbon 47 (12) (2009) 2914e2923. [49] J. Zhu, et al., Processing a glass fiber reinforced vinyl ester composite with nanotube enhancement of interlaminar shear strength, Compos. Sci. Technol. 67 (7e8) (2007) 1509e1517. [50] E. Bekyarova, et al., Functionalized single-walled carbon nanotubes for carbon fibere epoxy composites, J. Phys. Chem. C 111 (2007) 17865e17871. [51] E. Bekyarova, E.T. Thostenson, A. Yu, H. Kim, J. Gao, J. Tang, et al., Multiscale carbon nanotube-carbon fiber reinforcement for advanced epoxy composites, Langmuir 23 (7) (2007) 3970e3974. [52] S.S. Wicks, R.G. de Villoria, B.L. Wardle, Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes, Compos. Sci. Technol. 70 (No 1) (2010) 20e28. [53] G. Chatzigeorgiou, G. Don Seidel, D. C. LagoudasEffective mechanical properties of “fuzzy fiber” composites, Compos. B Eng. 43 (6) (2012) 2577e2593. [54] P.C. Ma, Y. Zhang, Perspectives of carbon nanotubes/polymer nanocomposites for wind blade materials, Renew. Sustain. Energy Rev. 30 (2014) 651e660.

Structured interfaces and their effect on composite performance: computational studies

27

[55] P.C. Ma, J.K. Kim, Carbon Nanotubes for Polymer Reinforcement, BocaCRC Press, Raton, 2011. [56] H. Qian, E.S. Greenhalgh, M.S.P. Shaffer, A. Bismarck, Carbon nanotube-based hierarchical composites: a review, J. Mater. Chem. 20 (2010) 4751e4762. [57] S.U. Khan, J.K. Kim, Impact and delamination failure of multiscale carbon nanotubeefiber reinforced polymer composites: a review, Int J Aeronaut Space Sci 12 (2011) 115e133. [58] G.M. Dai, L. Mishnaevsky Jr., Fatigue of multiscale composites with secondary nanoplatelet reinforcement: 3d computational analysis, Comp. Sci. Technol. 91 (2014) 71e81. [59] L. Mishnaevsky Jr., E. Levashov, Editorial, Computational Materials Science, 76, 2013, pp. 1e2. [60] L. Mishnaevsky Jr., E. Levashov, R. Valiev, E. Rabkin, E. Gutmanas, et al., Nanostructured titanium based materials for medical implants: modeling and development, Mater. Sci. Eng. R (2014), https://doi.org/10.1016/j.mser.2014.04.002. [61] R.Z. Valiev, Nanostructuring of metals by severe plastic deformation for advanced properties, Nat. Mater. 3 (2004) 511e516. [62] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Bulk nanostructured materials from severe plastic deformation, Prog. Mater. Sci. 45 (2000) 103e189. [63] L. Mishnaevsky Jr., E. Levashov, Editorial Notes, Computational Materials Science, 76, 2013, pp. 1e2. [64] X. Sauvage, G. Wilde, S.V. Divinski, et al., Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena, Mater. Sci. Eng. A 540 (1) (2012) 1e12. [65] T. Watanabe, An Approach to grain-boundary design for strong and ductile polycrystals, Res. Mech. 11 (1984) 47e84. [66] R.Z. Valiev, I.V. Alexandrov, N.A. Enikeev, et al., Towards enhancement of properties of UFG metals and alloys by grain boundary engineering using SPD processing, Rev. Adv. Mater. Sci. 25 (2010) 1e10. [67] V. Randle, Refined approaches to the use of the coincidence site lattice, JOM 50 (2) (1998) 56e59. [68] G. Palumbo, et al., Applications for grain-boundary engineered materials, J. Occup. Med. 50 (2) (1998) 40e43. [69] G. Palumbo, E.M. Lehockey, P. Lin, U. Erb, K.T. Aust, A grain boundary engineering approach to materials reliability, MRS Proc. Symp. Interf. Eng. Opt. Prop. 458 (1997) 273e383. [70] M. Besterci, et al., Formation of ultrafine-grained (UFG) structure and mechanical properties by severe plastic deformation (SPD), Metalurgija 47 (4) (2008) 295e299. [71] Y. Estrin, L. Toth, A. Molinari, et al., A dislocation-based model for all hardening stages in large strain deformation, Acta Mater. 46 (1998) 5509e5522. [72] H. S Liu, W. Pantleon, L. Mishnaevsky Jr., Non-equilibrium grain boundaries in UFG titanium: computational study of sources of the material strengthening, Comput. Mater. Sci. 83 (2014) 318e330. [73] H. S Liu, L. Mishnaevsky Jr., Gradient ultrafine-grained titanium: computational study of mechanical and damage behavior, Acta Materialia 71 (2014) 220e233. [74] T. Frolov, D.L. Olmsted, M. Asta, Y. Mishin, Structural phase transformations in metallic grain boundaries, Nat. Commun. 4 (2013) 1899. [75] D.V. Gunderov, et al., Evolution of microstructure, macrotexture and mechanical properties of commercially pure Ti during ECAP-conform processing and drawing, Mater. Sci. Eng. 562 (2013) 128e136.

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Interfaces in Particle and Fibre Reinforced Composites

[76] I. Semenova, et al., Enhanced strength and ductility of ultrafine-grained Ti processed by severe plastic deformation, Adv. Eng. Mat. 12 (8) (2010) 803e807. [77] D. Handtrack, C. Sauer, B. Kieback, Microstructure and properties of ultrafine-grained and dispersion-strengthened titanium materials for implants, J. Mater. Sci. 43 (2008) 671e679. [78] J. Lin, Y. Liu, T.A. Dean, A Review on damage mechanisms, models and calibration methods under various deformation conditions, Int. J. Damage Mech. 14 (2005) 4 299e319. [79] J. Lin, D.R. Hayhurst, B.F. Dyson, The standard ridges uniaxial testpiece: computed accuracy of creep strain, J. Strain Anal. 28 (2) (1993) 101e115.

Characterization studies of biopolymeric matrix and cellulose fibres based composites related to functionalized fibre-matrix interface

3

A.M. Noor Azammi a,b , R.A. Ilyas a, c , S.M. Sapuan a , Rushdan Ibrahim d , M.S.N. Atikah e , Mochamad Asrofi f , A. Atiqah g a Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; bAutomotive Engineering Technology Section, UniKL-MFI, B.B. Bangi, Selangor, Malaysia; cLaboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; dPulp and Paper Branch, Forest Research Institute Malaysia, Kepong, Selangor, Malaysia; eDepartment of Chemical and Environmental Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia; fDepartment of Mechanical Engineering, Andalas University, Padang, West Sumatera, Indonesia; gDepartment of Mechanical Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia

1. Introduction To overcome the obvious drawbacks of biopolymers, such as low strength, low stiffness and low water resistance, and to expand their applications in various sectors, natural fibre, for instance micro/nano-kenaf fibre (Hibiscus cannabinus), sugar palm fibre (Arenga pinnata), cassava bagasse (Manihot esculenta), and oil palm fibre (Elaeis guineensis) are often incorporated in the process of biopolymer composites, which normally combines the advantages of their constituent phases [1e5]. Natural fibres can modify the mechanical and physical properties of biopolymers in many ways. Fibre reinforced biopolymer composite technology is based on gaining advantages of high stiffness and strength of the fibre by reinforcing with the biopolymer matrix material and developing inevitable interfaces. In fibre composite, both the matrix and fibre preserve their original chemical and physical identities. Therefore, reinforcing both materials would create high physical and mechanical properties of the composite, in which these properties would not be achieved when either of the materials acts alone. This might be attributed to the presencestarch of an interface between these two materials. Fig. 3.1 demonstrates the schematic illustration of the interphase

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00003-0 Copyright © 2020 Elsevier Ltd. All rights reserved.

30

Interfaces in Particle and Fibre Reinforced Composites

Bulk matrix

Thermal, chemical, mechanical environment

Matrix of different properties

Adsorbed material Surface layer Bulk fiber

Fig. 3.1 Schematic illustration of the components of the three-dimensional (3D) interphase between fibre and matrix [7].

concept according to Drzal et al. [6], where numerous processing conditions were carried out on the interphase to allow volumetric changes and chemical reactions to take place, as well as stresses to be generated. The matter of understanding the properties and composition of interfaces in fibre reinforced matrix composite materials is still growing although many works have been done and published to investigate this field. The reason behind the growing research is due to the interdisciplinary nature of the subject. There are several important topics that will be critically reviewed in this chapter, which are classification and composition of natural fibres, micro- and nanostructure of natural fibres, processing of biopolymer composites, theories of adhesion and type of bonding, and the analytical techniques to characterise the interface of macro- to nano-sized fibre reinforced polymer matrices. The type of natural fibres, as well as their mechanical and chemical compositions are reviewed because the different stiffness of fibre would affect the overall mechanical properties of the composite. Besides, the size of fibre also plays an important role in the fibre-matrix interphase. Examples of specific nanofibre reinforced biopolymer matrices are provided along with their corresponding mechanism of adhesion. This review is concerned with the characterisation studies of biopolymer composites related to functionalised fibre-matrix interface.

2.

Natural fibres

World fibre production in 2016 was expected to surpass 100 million tonnes, where 30% of the total fibre productions were classified as natural fibres [8] as shown in Fig. 3.2. Moreover, in 2016, the natural fibre production increased to 30 million tonnes as compared to 28 million tonnes in 2015. The increase was due to increasing buyer’s willingness and human population. Natural fibres can be classified according to their origins, such as mineral, animal and plant sources, whereas plant fibres can be

Characterization studies of biopolymeric matrix and cellulose fibres

31

World fiber production (FAO) 120,000,000 Manmade fibres

Metric tons

100,000,000

Natural fibres

80,000,000 60,000,000 40,000,000 20,000,000 0

2008

2009

2010

2011

2012

2013

2014

2015

2016

Year

Fig. 3.2 World fibre production developed from data provided by FAO statistics (http://faostat3. fao.org/).

Plant fibres Nonwood of biofibres Trunk/stem/ frond

Straw

Bast

Examples

Examples Banana stems Date palm Coconut tree Oil palm Sugar palm

Wheat Corn straws Rice Barley

Wood

Leaf

Seed/fruit

Examples Examples

Sisal

Kenaf

Leaf fibre

Flax Jute

Mengkuang Pineapple leaf fibre (PALF) Henequen Abaca Bowstring hemp Phormium

Hemp Esparto Hoopvine Beans Linden bast Nettles Ramie Papyrus

Yucca Banana leaf

Examples Milkweed Luffa Coir Cocoa Pressed fibre Empty fruit bunch (EFB) Cotton Jatropha

Grass

Recycled wood fibres

Examples

Examples:

Elephant grass Bamboo fibre Trefoil

Newspaper / magazine fibres

Lucerne Ryegrass Wheat grass

Examples Soft and hard woods (pine, teak, rubber wood, acacia)

Fig. 3.3 Schematic representation of plant fibre classification.

classified into two main groups, which are wood and non-wood bio-fibres (Fig. 3.3). Mineral fibres can be classified into four main groups, which are asbestos, graphite, ceramic and metal (Fig. 3.4), while animal fibres can be classified into eight main groups, which are animal hair, silk fibres, avian fibres, collagen, keratin, fibroin, chitin, and chitosan (Fig. 3.5). Generally, the term ‘natural fibres’ is utilised to define various types of fibre that are naturally produced by minerals, plants and animals. Therefore, it is important to justify that in this manuscript, ‘natural fibres’ refer to ‘plant fibres’, also called as ‘lignocellulosic fibres’, ‘cellulosic fibres’, or ‘vegetable fibres’. Natural fibres are basically comprised of three main constituents, namely cellulose, hemicellulose and lignin, however, other minor constituents, such as pectin, pigments and extractive can also

32

Interfaces in Particle and Fibre Reinforced Composites

Mineral fibres Asbestos

Graphite

Ceramic

Metal Boron carbide

Silicon carbide

Aluminum oxide

Anthophyllite

Gold

Amphiboles

Silver

Quartz

Serpentine

Aluminum fibres

Glass wood

Glass fibres

Fig. 3.4 Schematic representation of mineral fibre classification. Animal fibres Animal hair Examples sheep’s wool

Silk fibres

Example cocoons

Avian fibres

Collagen

Keratin

Fibroin

Chitin

Chitosan

Example birds feather

goat hair alpaca hair horse hair

Fig. 3.5 Schematic representation of animal fibre classification.

be traced in lower quantities [9,10]. The natural fibre chemical composition and cell structures are quite complicated as they differ in different plant parts and origins. Each fibre is a composite by nature, in which rigid cellulose microfibrils are reinforced in the amorphous matrix that composed of hemicellulose and lignin. Therefore, natural fibres can also be referred to as cellulosic or lignocellulosic fibres [5]. The mechanical, thermal and physical properties of the natural fibres are different from one another as they depend on their cellulose crystallinity [11]. Natural fibres have been used since early civilisations to form fabrics, twines, and ropes, and played important roles in human society. They are sustainable resources, as they are biodegradable, renewable and carbon neutral, and they can be used without depleting or damaging the environment [12,13]. Presently, in this modern world, natural fibres play a vital role in supporting the world’s population needs by allowing communication to be done more effectively with the use of papers. These fibres can also be utilised as the reinforcement components of composite materials, in which the orientation of fibre would enhance the mechanical, thermal and water barrier properties of the composite [14,15]. Moreover, natural fibres can be used for high-tech applications, such as composite parts for aerospace, medical and automobiles [12,16,17].

Characterization studies of biopolymeric matrix and cellulose fibres

33

3. Chemical composition of natural fibre The chemical composition of plant fibre is different depending on the type of fibre. The main components of natural fibres are cellulose, hemicellulose, and lignin [5,18,19]. Generally, natural fibres are composed of 30%e80% cellulose, 7%e40% hemicellulose and 3%e33% lignin (Table 3.1). The properties of each component contribute to the overall properties of natural fibres. Cellulose is an important structural component of the primary cell wall of natural fibre walls that surround the natural fibres, making plant cells, leaves, branches and stems strong [20]. Cellulose serves many functions, including connecting cells to form tissue, providing structural support, providing strong resistance to stress, and preventing the cell from bursting in a hypotonic solution. It is also thermally stable (decomposition occurs between 315 and w400
C). Cellulose fibres are assembled together by pectin fibres, which bind the cellulose together to produce tighter cell walls in natural fibres, accounting for their strength that provides resistance to lysis in the presence of water. On the other hand, hemicelluloses are responsible for moisture absorption, biodegradation and thermal degradation (occurs between 220 and w315
C) of natural fibres as they display the least resistance as compared to celluloses. Lignin is thermally stable (decomposition occurs between 165 and w900
C), however, it is responsible for ultraviolet (UV) degradation [9]. The chemical composition percentage of each of these constituents varies with the different parts of fibre, as well as the types of natural fibre. The mechanical, thermal and physical properties of natural fibres are different from one another as they depend on their cellulose crystallinity [21,22]. A highly compact system of cellulose crystalline packing is formed as a result of the intra- and inter-molecular hydrogen bonding among the cellulose chains [23]. Table 3.1 shows the chemical composition of natural fibres and their crystallinity.

4. Microstructure of fibre The natural fibres consist of cellulose microfibres reinforced amorphous matrix of hemicellulose and lignin while cellulose microfibres consist of thousands of nanofibrils connected along the length of the natural fibres. The hydrogen bonds and others linkages provide the necessary stiffness and strength to the fibres.

5. Nanostructure of fibre Cellulose is the product of biosysthesis from plant fibres. The term ‘nanocellulose’ refers to the processed materials or cellulosic isolates with nano-sized structural dimensions. Nanocellulose can be classified into three types of nanomaterials: (1) bacterial cellulose, also referred to as bacterial nanocellulose (BNC), biocellulose and microbial cellulose, (2) nanofibrillated cellulose (NFC), also referred to as cellulose nanofibrils (CNFs), microfibrillated cellulose (MFC), nanofibrils, microfibrils, nanofibrils and (3) nanocrystalline cellulose (NCC), also referred to as cellulose nanocrystals (CNCs),

34

Table 3.1 Chemical composition of natural fibres from different plant and different part. Holocellulose (wt%) Cellulose (wt%)

Hemicellulose (wt%)

Lignin (wt%)

Ash (wt%)

Extractives (wt%)

Crystallinity (%)

References

Sugar palm fibre

43.88

7.24

33.24

1.01

2.73

55.8

[9]

Wheat straw fibre

43.2  0.15

34.1  1.2

22.0  3.1

e

e

57.5

[24]

Soy hull fibre

56.4  0.92

12.5  0.72

18.0  2.5

e

e

59.8

[24]

Arecanut husk fibre

34.18

20.83

31.60

2.34

e

37

[25]

Helicteres isora plant

71  2.6

3.1  0.5

21  0.9

e

e

38

[26]

Pineapple leaf fibre

81.27  2.45

12.31  1.35

3.46  0.58

e

e

35.97

[27]

Ramie fibre

69.83

9.63

3.98

e

e

55.48

[28]

Oil palm mesocarp fibre (OPMF)

28.2  0.8

32.7  4.8

32.4  4.0

e

6.5  0.1

34.3

[29]

Oil palm empty fruit bunch (OPEFB)

37.1  4.4

39.9  0.75

18.6  1.3

e

3.1  3.4

45.0

[29]

Oil palm frond (OPF)

45.0  0.6

32.0  1.4

16.9  0.4

e

2.3  1.0

54.5

[29]

Oil palm empty fruit bunch (OPEFB) fibre

40  2

23  2

21  1

e

2.0  0.2

40

[30]

Rubber wood

45  3

20  2

29  2

e

2.5  0.5

46

[30]

Curauna fibre

70.2  0.7

18.3  0.8

9.3  0.9

e

e

64

[31]

Banana fibre

7.5

74.9

7.9

0.01

9.6

15.0

[32]

Sugarcane bagasse

43.6

27.7

27.7

e

e

76

[33]

Interfaces in Particle and Fibre Reinforced Composites

Fibres

63.5  0.5

17.6  1.4

12.7  1.5

2.2  0.8

4.0  1.0

48.2

[34]

Phoenix dactylifera palm leaflet

33.5

26.0

27.0

6.5

e

50

[35]

Phoenix dactylifera palm rachis

44.0

28.0

14.0

2.5

e

55

[35]

Kenaf core powder

80.26

23.58

e

e

48.1

[36]

Water hyacinth fibre

42.8

20.6

4.1

e

e

59.56

[37]

Wheat straw

43.2  0.15

34.1  1.2

22.0  3.1

e

e

57.5

[38]

Sugar beet fibre

44.95  0.09

25.40  2.06

11.23  1.66

17.67  1.54

e

35.67

[39]

Mengkuang leaves

37.3  0.6

34.4  0.2

24  0.8

2.5  0.02

55.1

[40]

Characterization studies of biopolymeric matrix and cellulose fibres

Kenaf bast

35

36

Interfaces in Particle and Fibre Reinforced Composites

Amorphous region

Fiber deconstruction

Crystalline region

Pretreatment and shear

Nanofibrillated cellulose (NFC)

Acid hydrolysis

Nanocrystalline cellulose (NCC)

Fig. 3.6 Schematic illustration of NFC and NCC production from fibre cell walls via mechanical and chemical treatments, respectively [9,21].

whiskers, nanowhiskers, and cellulose microcrystals [41,42]. Different methods are utilised to extract nanofibrils from cellulose sources, which result in nanofibrils with varied mechanical properties, crystallinity, and surface chemistries [5,43,44]. Fig. 3.6 shows the schematic illustration of NFC and NCC production from fibre cell walls via mechanical and chemical treatments, respectively. The detailed properties, processing and reinforcement of nanocellulose can be found elsewhere [2,5,43,45]. Nanocellulose can be isolated by using top-down chemical or mechanical processes. There is an increasing demand for nanocellulose production due to its incredible properties, such as biodegradable, renewable, non-toxic material, abundant surface hydroxyl group, large specific surface area, high aspect ratio, high crystallinity, high thermal resistance, and good mechanical properties [5,46]. These properties make nanocellulose more compatible to be reinforced within the polymer matrix.

6.

Mechanical properties of nanocellulose

Materials with good mechanical properties are suitable to be reinforced with polymer matrices as they can help improve the mechanical properties of the composite structure. However, the good mechanical properties of nanofibre alone are not enough because without having a good interfacial bond between the nanofibre and matrix as the resultant composite would not have good mechanical properties. Nanocellulose consists of highly ordered crystal formed and the abundant inter-chain hydrogen offers high stiffness and good mechanical strength to be reinforced with polymer matrices.

Characterization studies of biopolymeric matrix and cellulose fibres

37

Nanocellulose also obtains high elastic modulus and Young’s modulus of up to 145 and 150 GPa, respectively, which are five times higher than mild steel or magnesium alloy [5,47,48]. The mechanical properties of nanocellulose depend on many factors, such as plant maturity, separating processes, and chemical or mechanical process used to isolate nanocellulose. The nanocellulose modulus is estimated to result from the crystalline and amorphous regions. Therefore, based on the theory and structures of the NCC and NFC, NCC should be higher than NFC due to the elimination of the amorphous regions during the hydrolysis process, which results in the formation of higher crystallinity region. These outstanding mechanical properties make nanocellulose the ideal candidate to be reinforced with polymer matrix nanocomposites [5]. Besides, the Young’s modulus of the nanocellulose is the highest as compared to that of glass fibres and Kevlar, at 50e150 GPa, 60e125 GPa and 70 GPa, respectively. Nanocellulose also has the lowest density of around 1.05 g cm3 as compared to that of glass fibre and Kevlar, at 2.6 and 1.45 g cm3, thus making it the potential candidate for nanocomposite reinforcement [41,44]. Moreover, the specific Young’s modulus (ratio of Young’s modulus and density of material) of the nanocellulose is the highest at approximately 85 J g1 for nanocrystalline cellulose, followed by 65 J g1 for microfibrils and around 25 J g1 for steel.

7. Processing of polymer composites The reinforcement of the biopolymer matrix and fibres has improved the mechanical and physical properties of composites [15,49e51]. Biopolymers are polymers produced by living organisms, which break down after their planned use. There are many biopolymers that have been produced commercially on a huge scale for various applications. Information on these biopolymers is summarised in Table 3.2 and the detailed information can be found elsewhere [52]. These natural raw materials are renewable, abundant and biodegradable, thus making them the attractive sources for the production of a new generation of environmentally friendly bioplastics. Fig. 3.7 shows the current classification of bioplastics based on their production routes. Biocomposite processing is a process of reinforcing the fibre and matrix together, whether the fibre or matrix, or both are biodegradable in nature [53,54]. In biocomposites, the fibres act as the reinforcement agents by improving the stiffness and strength of the resulting composite structure. Details regarding the processing of biocomposites were reported by Mohanty et al. [55], Yu et al. [56], and Huber et al. [57]. Nanocellulose has a strong tendency for self-agglomeration due to the highly interacting surface hydroxyl groups. This property of inter-particle interactions can cause agglomeration during the nanocomposite preparation, thus hindering the potential of mechanical reinforcement within the host polymer matrix. Agglomeration can occur easily as the size of the particle decreases, resulting from the increase in specific surface area. Therefore, different approaches have been taken to overcome this problem as reported in various literature in order to disperse and homogeneously mix the nanocellulose within a polymeric matrix. These different approaches are illustrated in Fig. 3.8 and critical review regarding this information can be found elsewhere [5,58,59].

38

Interfaces in Particle and Fibre Reinforced Composites

Table 3.2 Description of bioplastics. Bioplastics

Descriptions

Petroleumebased bioplastics

Including in this category are poly(butylene adipate-coterephthalate) (PBAT), polycaprolactone (PCL) and Poly(vinyl alcohol) (PVOH). These petroleum-based polymers are produced from petroleum resources, however they are biodegradable in nature.

Poly(lactic acid) (PLA)

PLA is a transparent plastic obtained by either direct polycondensation of lactic acid or by ring opening polymerization (ROP) of lactide.

Polyhydroxyalkanoates (PHAs)

PHAs are the family of biopolyesters produced in nature by bacterial/microorganism fermentation of lipids or sugar. They are obtained when bacteria are exposed to carbon source while other necessary nutrient becomes limited.

Starch

Starch is made up of both branched and linear polysaccharides known as amylopectin and amylose, respectively. Plasticizer such as water, glycerol and sorbitol are added to increase the free volume, thus decreasing the softening temperatures and glass transition. Both polysaccharides varies with their botanical origin. They can be obtained via corn, wheat, potatoes, sugar palm, etc.

Cellulose

Cellulose is an abundant and ubiquitous natural polymer obtained from fibres, cotton and wood as well as non-plant resources such as bacteria and tunicates. Cellulose pulp is extracted from agricultural byproduct such as stalks, sugar palm fibres, crops straws and bagasse. Cellulose bioplastics are mainly composed of cellulose esters, which including nitrocellulose and cellulose acetate, and their derivatives, including celluloid.

Proteinous plastic

This type of plastics is produced from proteins which is random copolymer of different amino acids. Protein can be classified based own their origin such as animal proteins (e.g., gelatin, casein, whey and keratin) and plant proteins (e.g., wheat, canola, pea, soy protein). Generally, fatty acids, water, oils and glycerols are used as plasticizer for protein.

Bioplastics from mixed sources

These bioplastics are made from the incorporation of petroleum and bio-based monomers (e.g., poly (trimethylene terephthalate) (PTT) which is synthesized using petroleum derived terephthalic acid and biologically derived 1,3propanediol.

Characterization studies of biopolymeric matrix and cellulose fibres Petroleum based biodegradable polymers

Renewable resource based polymers

39

Polymers from mixed sources (bio-/petro-)

Poly (lactic acid) (PLA)

Aliphatic polyesters

Polyesters

Ex: Poly(D-lactide) (PDLA), poly(DL-lactide) (PDLLA)

Ex: Polycaprolactone (PCL), poly(butylene succinate) (PBS)*

Ex: Poly (trimethylene terephthalate) (PTT)

Polyhydroxyalkanotes (PHAs) Aliphatic-aromatic polyesters

Thermosets

Ex: Polyhydroxybutyrate (PHB), poly(hydroxybutyrate-cohydroxyvalerate) (PHBV)

Ex: Poly(butylene adipate–coterephthalate) (PBAT)

Ex: Biobased epoxy, biobased polyur ethane

Starch plastics Poly (vinyl alcohol) (PVOH)

Ex: Wheat/potato/corn-based plastics

Cellulosics Ex: Cellulose esters

Proteineous plastics Ex: Plant and animal proteins based plastics

Fig. 3.7 Classification of bioplastics based on their production routes [60].

Cellulosic nanoparticles Liquid medium

Non-aqueous suspension

Aqueous suspension Non-aqueous polar medium

Hydrosoluble polymer Latex

Casting/ evaporation

Surfactant

Electrospinning

Dried nanoparticles Meltprocessing

Solvent mixture/ exchange Chemical modification

Impregnation

LBL assembly

Fig. 3.8 Processing of nanocellulose reinforced polymer nanocomposite matrices [59].

8. Theories of adhesion and type of bonding The nature of bonding depends on several factors, which are: (I) diffusivity of element materials, (II) morphological properties of natural fibres, (III) chemical composition and molecular conformation, and (IV) atomic arrangement of fibres and matrices. Generally, adhesion can be defined as the process or action of adhering to a surface

40

(a)

Interfaces in Particle and Fibre Reinforced Composites

(d)

A

A B

(b)

(e)

(c)

(f)

A B

A B

A

A B

B

B

Fig. 3.9 Interface bonds formed by (a) molecular entanglement; (b) electrostatic attraction; (c) inter-diffusion of elements; (d) chemical reaction between Group A on one surface and Group B on the other surface; (e) chemical reaction following the formation of a new compound(s), particularly in metal matrix composites (MMCs); (f) mechanical interlocking [7].

or object. Specifically, it can be well-defined as the tendency of different particles or surfaces to adhere to one another. The mechanism that causes adhesion can be divided into several types, which are adsorption, inter-diffusion, electrostatic attraction, reaction bonding, chemical bonding, and mechanical bonding. Fig. 3.9 shows several schematics of interface bonds. Details regarding this interface bond can be found elsewhere [7].

9.

Physio-chemical characterisation of interphase

Material characterisation can be classified into microscopy characterisation, spectroscopy characterisation and macroscopic testing. However, for the interphase characterisation of composites, only several of the listed characterisations in Fig. 3.10 are used. Generally, the interphase of composites occurs in various forms of different

Small-angle X-ray scattering (SAXS)

Fourier transform infrared spectroscopy (FTIR)

Thermoluminescence (TL)

Photoluminescence (PL)

X-ray diffraction topography (XRT)

Atomic force microscope (AFM)

Scanning probe microscopy (SPM)

Auger electron spectroscopy (AES)

Transmission electron microscope (TEM)

Fig. 3.10 Material characterizations.

X-ray photoelectron spectroscopy (XPS)

Field ion microscope (FIM)

X-ray photon correlation spectroscopy (XPCS)

Electron energy loss spectroscopy (EELS)

Scanning tunneling microscope (STM)

Wavelength dispersive xray spectroscopy (WDX, WDS)

Energy-dispersive x-ray spectroscopy (EDX, EDS)

X-ray diffraction (XRD)

X-ray

Ultraviolet-visible spectroscopy (UV-vis)

Optical radiation

Spectroscopy

Scanning electron microscope (SEM)

Optical microscope

Microscopy

Secondry ion mass spectrometry (SIMS)

MALDI-TOF

Thermal ionization mass spectrometry (TI-MS)

Electron ionization (EI)

Mass spectrometry

Material characterization

Mössbauer spectroscopy (MBS)

Rutherford backscattering spectrometry (RBS)

Small-angle neutron scattering (SANS)

Electron paramagnetic/spin resonance (EPR,ESR)

Terahertz spectroscopy (THz)

Nuclear magnetic resonance spectroscopy (NMR)

Photon correlation spectroscopy/Dynamic light scattering (DLS)

Other

Impulse excitation technique (IET)

Tensile

Compressive

Torsional

Creep

Fatigue

Dielectric thermal analysis (DEA, DETA)

Differential thermal analysis (DTA)

Tounghnes

Hardness

Mechanical

Thermogravimetric analysis (TGA)

Differentiaal scanning calorimetry (DSC)

Thermal

Macroscopic testing

Resonant ultrasound spectroscopy

Ultrasonic testing

Ultrasound

Characterization studies of biopolymeric matrix and cellulose fibres 41

42

Interfaces in Particle and Fibre Reinforced Composites

materials [61]. The interphase characterisation of composites can be done by analysing the composite constituent surface before they are reinforced together. Although the surface layers of composites show only a slight portion of the total volume of the bulk material, they can provide important information in estimating the overall performance and properties of the composites. The parameters of the physico-chemical surface area analysis are depth, atomic/microscopic structure, purity, chemical composition, as well as the distribution of fibre within the composite matrices [61]. The characterisation techniques that are mostly used to determine the interface of the composites are the measurement of the contact angle, laser Raman spectroscopy, Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), wide-angle X-ray scattering (WAXS), X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy and infrared (IR) spectroscopy. Besides that, to determine the surface interaction between both surface of fibre and polymer, the microscopic visualisation techniques can be used, such as scanning electron microscopy (SEM), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), scanning tunneling microscopy (STM) and atomic force microscopy (AFM). These techniques can provide information about the physico-chemical interaction, surface morphology, surface depth profile and fibre-matrix concentration. Table 3.3 shows the techniques to determine the composition and surface structure of composites.

10.

Effect of fibre loading, size, and biopolymer composite composition (variable parameters) on mechanical properties

This section discusses the variable parameters used and their effects towards the mechanical properties of composite, i.e., tensile strength, flexural strength, and impact strength.

10.1

Tensile strength

Several studies that investigated fibre loading effect on polymer composites found that it had a good relationship with tensile strength. Studies on fibre loading effect that led to the tensile strength were observed [62]. It was demonstrated that the optimum fibre loading for kenaf/thermoplastic polyurethane composites was 30% [63]. Other studies regarding kenaf fibre and phenol-formaldehyde (KF/PF) composites reported that kenaf fibre loading up to 43% showed the best tensile strength for the composites [64]. A study on Twaron fibre with blended natural rubber (NR) and linear low-density polyethylene (LLDPE) investigated the fibre loading effect for the tensile properties. It was found that at 20% of the Twaron fibre loading, the optimum tensile properties were observed [62]. Studies on bagasse fibre with the combination of two chemical treatments, namely sodium hydroxide (NaOH) with acrylic acid (AA) showed higher tensile properties with

Technique

Atomic process and description

Microscopy Scanning probe microscope (SPM)

Atomic force microscope (AFM)AFM is a very high resolution type of SPM. It can demonstrate resolution on the order of fractions of nanometer, which is 1000 times better than optical diffraction limit. The information is collected by touching the surface with a mechanical probe. Based on the block diagram of AFM using beam deflection detection, as the cantilever is displaced via its interaction with the surface, so too will the reflection of the laser beam be displaced on the surface of the photodiode. The interaction forces formed between the tip and specimen, when they are close, can be van der Waals, electrostatic or magnetic force. AFM can be used for both non-conductive and conductive specimens, without having to apply a high vacuum, presenting a major advantage over STM. Recurrent tracking microscope (RTM) RTM is based on the quantum recurrence phenomena of an atomic wave packet, which is used to determine the nano-structure on the surface of the specimen. It includes the visualisation and measurement of the specimen surface features in the size of one nanometer. It is also comprised of a magneto-optic trap (MOT) where (a) super cold atoms are trapped inside; (b) a dielectric surface above which the evanescent wave mirror is obtained by the total internal reflection of a monochromatic laser from the dielectric film; and (c) a cantilever is attached to the dielectric film with its other end above the surface under investigation.

Characterization studies of biopolymeric matrix and cellulose fibres

Table 3.3 Techniques to determine the composition and surface structure of composites.

Scanning tunnelling microscopy (STM) STM is a device for imaging surface at the atomic level. The resolution of STM is considered to be 0.1 nm lateral resolution and 0.01 nm (10 pm) depth resolution. It is based on the quantum tunnelling concept. A bias (voltage difference) applied between the tip and the specimen surface when they are brought very near can allow electrons to tunnel through the vacuum between them. The resulting tunnelling current is a function of tip position, applied voltage, and the local density of states (LDOSs) of the sample. Information is obtained by monitoring the current as the tip’s position scans across the surface, and is usually displayed in image form. Continued 43

44

Table 3.3 Continued Technique

Atomic process and description

Electron microscope

Transmission electron microscope (TEM) In TEM, a two-dimensional (2D) image of an ultrathin section is produced by capturing electrons that have passed through a specimen. The specimen is most often an ultrathin specimen less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The interaction degree between stained specimen and electron affects the kinetic energy of electron, which is collected by the fluorescent plate. The light of varying intensity produced is directly proportional to the electron’s kinetic energy and used to produce the image. Scanning electron microscope (SEM) SEM is used to make a three-dimensional (3D) image of the composite specimen’s surface. A beam of electrons is passed over the stained surface of the composite specimen. Some electrons reflected (backscatter electrons) and some (secondary electrons) are emitted from the metallic stain. The 3D image is generated from the captured electrons.

Spectroscopy

Interfaces in Particle and Fibre Reinforced Composites

Field emission scanning electron microscopy (FESEM) FESEM is used to provide topographical and elemental information at magnifications of 1,000,000 with sub-1 nm resolution, and virtually unlimited depth of field. The principle of operation for FESEM is the same as SEM. FESEM produces clearer, less electrostatically distorted images with spatial resolution down to 1 1/2 nm, which are three to six times better as compared to SEM. Advantages of FESEM include (a) high quality, low voltage images with negligible electrical charging of samples (accelerating voltages ranging from 0.5 to 30 kV), (b) reduced penetration of low-kinetic-energy electrons probes closer to the immediate material surface, and (c) the ability to examine smaller-area contamination spots at electron accelerating voltages compatible with energy dispersive spectroscopy (EDS). Usually, before conducting FESEM, all specimens were sputter coated with goldepalladium to avoid charging.

NMR is an important spectroscopic technique used to observe local magnetic fields around atomic nuclei, especially for organic specimens. NMR can be performed by using nuclei atoms instead of electrons. The specimen is placed in a magnetic field and the NMR signal is produced by excitation of the nuclei sample with radio waves into nuclear magnetic resonance, which is detected with sensitive radio receivers. The intramolecular magnetic field around an atom in a molecule changes the resonance frequency, and thus giving access to details of the electronic structure of a molecule and its individual functional groups. In NMR, the specimen is exposed to a strong magnetic field. Upon exposure, certain nuclei transition or resonate between discreet energy levels. The energy gap between these levels can be measured and visualised as spectra. The data can be used to elucidate the chemical structure of the sample.

Infrared (IR) and Fourier transform infrared (FTIR) spectroscopy

FTIR spectroscopy is an experimental technique used initially for qualitative and quantitative analysis of organic compounds, providing specific information on molecular structure, chemical bonding and molecular environment. FTIR uses an incandescent source of light to emit a bright ray in the IR wavelength range. FTIR spectrometers collect all wavelengths simultaneously. The absorption of infrared radiation generates characteristic vibrational movements in molecules, defined as bending and stretching because of the changing of electric dipole; the molecule changes its vibrational state as it passes from fundamental vibrational state to excited vibrational state.

Secondary ion mass spectroscopy (SIMS)

SIMS is an experimental technique used for the analysis of atomic monolayer composition on solid material surfaces (e.g., organic compound) and thin films by sputtering the specimen surface with a focused primary ion beam. As a result of the energetic sputtering process, some of the ejected material (either atoms or molecules) is ionised, and can thus be directed towards the mass spectrometer where detection is made.

Auger electron spectroscopy (AES)

AES is a common analytical technique used specifically in the study of surfaces and in the area of materials science to a depth of about 2 nm for elements above helium (He). It is also used to study the surface compositional changes during physical property measurement. Underlying the spectroscopic technique is the Auger effect, which is based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events.

Characterization studies of biopolymeric matrix and cellulose fibres

Nuclear magnetic resonance (NMR) spectroscopy

Continued

45

46

Table 3.3 Continued Atomic process and description

Raman spectroscopy (RS)

RS is an optical spectroscopic technique based on the inelastic scattering of light by the matter (the molecule of interest for instance) that is used to observe vibrational, rotational, and other low-frequency modes in a system. RS event occurs because a molecular vibration can change the polarisability of molecule that interacts with incident light. It is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.

Ion scattering spectroscopy (ISS)

ISS is an analytic technique in which a beam of ions (typically Heþ or Arþ) is directed at the specimen surface and scattered from the atoms at the surface. As the ions lose kinetic energy, their resultant kinetic energy is measured and a spectrum is produced where peaks are observed at different kinetic energies related to the mass difference between the ion and the atom. Clearly two atoms in a surface that have very different masses (e.g., oxygen and iron) will scatter ions differently. ISS is also used in the detection of top most surface elements and measurements of adsorbed species on single crystal surfaces.

X-ray scattering

Small angle scattering (SAS) and small-angle X-ray scattering (SAXS) SAXS is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified with molecular weight from about 5 kDa up to 100 MDa. This means that it can determine nanoparticle size distribution, particle shape, particle structure (e.g., core-shell), specific surface area, agglomeration behaviour of nanoparticles, pore size distribution, liquid crystalline phases, pore sizes, characteristic distances of partially ordered materials, and many more. Samples that can be run by SAXS are liquid nanoparticle dispersions/colloids, nanopowders, nanocomposites, polymers, surfactants, microemulsions, biomacromolecules, liquid crystals and mesoporous materials. X-ray diffraction (XRD) XRD is a non-destructive analytical technique to determine the crystallinity of a compound. It is based on observing the scattered intensity of an X-ray beam hitting a specimen as a function of incident and scattered angle, polarisation, and wavelength or energy. Usually, it is used for the identification of crystalline material (for regulatory purposes or during development), identification of different polymorphic forms (“fingerprints”), structural information, orientation, layer thickness, distinguishing between amorphous and crystalline material and quantification of the percent crystallinity of a specimen.

Interfaces in Particle and Fibre Reinforced Composites

Technique

Characterization studies of biopolymeric matrix and cellulose fibres

47

the use of 20% fibre loading [65]. Previous studies on fibre length correlation to tensile strength were conducted by several researchers. Studies on wheat husk length reinforced rubber composites showed that the highest tensile strength achieved was the medium length of fibre (125e250mm), where the fibre was arranged longitudinally [66].

10.2

Flexural strength

Flexural strength is the ability of composites or materials to resist bending deflection when energy is applied to the structure. Studies on kenaf and bagasse fibre reinforced with bio-degradable corn starch resin showed that the length of fibre influences the flexural strength obtained. Both fibres at 10 mm length resulted in high flexural strength [67]. Research on bamboo fibre showed a potential result for flexural strength. The combination of 1e6 mm bamboo fibre length and 3 mm glass fibre length in polypropylene (PP) and maleic anhydride polypropylene (MAPP) matrix with the ratio of 20% bamboo fibre loading and 10% glass fibre loading showed the highest flexural strength result [68]. Treated kenaf short fibre mixed with polypropylene (PP) with different fibre loadings also showed a good result in flexural strength. In another study, it was reported that 40% of treated kenaf fibre loading achieved the highest flexural strength result [69].

10.3

Impact strength

Impact strength is one of the basic mechanical properties that needs to be investigated when a new material is created. Impact strength is related to energy absorbent or vibration resistance. Influence of fibre length towards impact strength was clearly shown in a study on kenaf fibre reinforced with thermoplastic polyurethane, where kenaf fibre length between 125 and 300 mm was the optimum length to achieve the highest impact strength [70]. Moreover, the combination of different fibres and orientations could contribute to impact strength [71,72]. A study on the combination of sisal fibre and coconut sheath with different orientations showed that the combination did contribute to impact strength properties [73]. Another sample of fibre loading that did contribute to impact strength is the treated torch ginger fibre coated with liquid epoxidised natural rubber. In this study, 5% of the torch ginger fibre loading showed the highest result compared to the rest [74]. A study on the combination of un-woven kenaf/banana fibre with unsaturated polyester composites was done. Both had 40% fibre loading and 10 mm fibre length, and were compared with a woven technique for both fibres with two types of fibre treatment, namely sodium oxide (NaOH) and sodium lauryl sulphate (SLS). The finding showed that the woven kenaf/banana fibre with both chemical treatments had much higher impact strength than the treated un-woven kenaf/banana fibre [75].

48

11.

Interfaces in Particle and Fibre Reinforced Composites

Effect of fibre loading, size, and biopolymer composite composition on physical properties

This section discusses how the variable parameters affect the physical properties of composite.

11.1

Water absorption

There are quite a number of research on natural fibre composites conducted to create water-resistant or water barrier materials [15,76]. Studies on kenaf/carbon hybrid using epoxy resin as the matrix were done to observe the water absorption effect. Treated and non-treated fibres were compared and liquid epoxidised natural rubber (LENR) was used to increase their toughness. The results showed that the epoxy reinforced kenaf/carbon hybrid with the addition of LENR had the lowest water absorption. Moreover, the increase in fibre loading of kenaf/carbon hybrid had decreased the water absorption percentage [77]. Many studies have reported on kenaf/fibreglass hybrid with polyester composites for water absorption test. In a previous study, a comparison was done between pure 20% kenaf fibre loading and the kenaf/fibreglass hybrid with a ratio of 20% and 16% fibre loadings, respectively. The water absorption test was conducted by using three different water sources, which were rainwater, salt water, and tap water. The results showed that the kenaf/fibreglass hybrid can keep water from entering the polyester composites [78]. Low water absorption is able to reduce fungus inhabitation and degrade the structure strength of composite due to humidity, such as a table made from compressed wood [76].

11.2

Thickness swelling

An investigation on sugar palm fibre (SPF) with seaweed/thermoplastic sugar palm starch agar (TPSA) composites was conducted to identify the swelling effect due to water absorption. The hybridised seaweed/SPFs with weight ratios of 25:75, 50:50 and 75:25 were prepared by using TPSA as the matrix. The experiment was conducted within a duration from 0.5 to 2 h for the water uptake. The results showed that the SPF had more rigid structure and less hydrophilic character, thus providing better resistance to swelling when exposed to water [79]. A study on rice husk (RH) was done, where polyethylene glycol was used as the polyol. A comparison was done by determining the percentage of RH load, the effect of RH hydroxyl (OH) and the size of the RH. The immersion test that was done using dimethylformamide (DMF) showed that absorption and swelling decreased as the percentage of RH was increased [80].

11.3

Density

A study was conducted on kenaf-derived cellulose (KDC)-filled poly(lactic acid) (PLA) composites. The lignin was removed from the kenaf fibre (delignification),

Characterization studies of biopolymeric matrix and cellulose fibres

49

causing the kenaf fibre colour to change from light brown to white. Then, the kenaf fibre was repeatedly soaked with NaOH and finally neutralised by adding 10% of acetic acid for 5 min. The cellulose was washed, filtered and finally oven dried overnight at 100e105
C by using a vacuum oven. The composition of the KDC/PLA composites was between 0% and 60% loading. The result showed that the KDC with 60% loading had the highest density and flexural modulus values [81]. A comparison study was conducted between kenaf and hemp fibres on the effect of alkaline fibre treatment towards density. The density test was done by comparing the untreated fibres for both types of fibre. The result showed that the treated fibre for both types of fibre had density increment between 1.2% and 2.4% [82].

12.

Morphological analysis of fibre and matrix interfacial bonding by using scanning electron microscope (SEM)

This section discusses the effects of fibre adhesion on matrices in composites. The fibre adhesion on matrices will be shown using morphological pictures taken using SEM.

12.1

Analysis on the effect of treated fibre adhesion on matrix composite

There are many examples of fibre studied; either the natural or artificial fibre that is used in various matrix composites [83]. This section discusses the effect of treated fibre adhesion on matrices through SEM images as studied by several researchers. Research on treated kenaf fibre using sodium oxide (NaOH) was conducted by many researchers. Treated kenaf filled thermoplastic polyurethane (TPU)-natural rubber (NR) blend was studied and the fidings are shown in Fig. 3.11. From Fig. 3.11(a) (a)

(b)

Fibres debonding to the matrix

Fibres bonding to the matrix

Fibres surface 30 mm

30 mm

Fig. 3.11 (a) Untreated kenaf fibre and (b) treated kenaf fibre. Courtesy of Polym. J.

50

(a)

Interfaces in Particle and Fibre Reinforced Composites

(b)

Fig. 3.12 SEM micrograph of (a) TGF treated with NaOH and (b) TGF treated with 5% LENR composition.

below, the fibre adhesion on the untreated kenaf fibre shows better bonding than the treated fibre, which shows the debonding condition between kenaf fibre and the matrices as shown in Fig. 3.11(b) [84]. This shows that the natural condition of the fibre, which is hydrophilic is able to mix with NR and TPU as compared to the treated fibre, where the hydrogen from the NaOH has good hydrogen bonding between fibre and NR. Studies were done on treated torch ginger fibre (TGF) using NaOH with thermoplastic natural fibre composites using liquid epoxidised natural fibre as the compatibiliser. The results showed that the treated TGF with an addition of 5% LENR in the thermoplastic natural rubber composites achieved the best mechanical result compared to others. Fig. 3.12 clearly shows that the fibre adhesion between the matrices is bonding with each other. The 5% LENR, which acts as a compatibiliser, does help to improve bonding adhesion between the fibre and the matrices [74]. Another study examined treated kenaf fibre (KF) hybrid with carbon fibre (CF) in thermoplastic natural rubber (TPNR) as composites. Both fibres were treated differently; the KF was treated using maleic anhydride polypropylene (MAPP) and the CF was treated using sulphuric acid. The fibre was mixed with different ratios into the composites. The results, as shown in Fig. 3.13, showed that the fibre adhesion with the matrix was not truly bonding. Therefore, the tensile result was much lower than the untreated fibre. There was no significant improvement in tensile for the hybrid system [85].

12.2

Analysis on the effect of fibre loading on matrix composites

This section discusses the effect of structure of fibre loading and composites from the perspective of SEM micrograph results of related studies. Studies on two types of aramid fibre were done using thermoplastic polyurethane (TPU) as the matrices. The two types of aramid fibre used were m-aramid (Teijin-Connex) and copolymer (p-aramid) (Technora). Both fibres were mixed with TPU according to the percentage of fibre loading, i.e., 3%, 7% and 10%. The SEM micrograph result, as shown in Fig. 3.14, showed the difference between 3% Technora fibre and 7% Connex fibre

Characterization studies of biopolymeric matrix and cellulose fibres

51

Fig. 3.13 SEM micrograph of 30CF/70KF compositions.

Fig. 3.14 3% Technora fibre composites and 7% Connex fibre composites.

loading. Moreover, the Technora fibre showed the highest result in storage modulus compared to the Connex fibre [86].

12.3

Analysis on the effect of matrix adhesion on untreated fibre

Adhesion between treated fibre and matrices can easily be shown via SEM micrograph results. Moreover, the condition of matrices can easily be observed, i.e., rough or smooth surface of the matrices. The changes in matrix surface of composites are due to the chemical reaction from the treated fibre.

52

Interfaces in Particle and Fibre Reinforced Composites

Fig. 3.15 Fibre-matrix interfacial adhesion between treated and untreated kenaf fibres.

Fibres break surface

Surface of polymer composite matrix

80 mm

80 mm

Fig. 3.16 Comparison of treated and untreated NaOH kenaf fibres in TPU-NR composites.

Fig. 3.15 show the difference between treated and untreated kenaf fibres using MAPP with blended thermoplastic elastomers (TPEs), polypropylene (PP) and ethylene-propylene-diene monomer (EPDM). The hybrid matrix surface of MAPP treated kenaf fibre was very rough throughout the composites, while the matrix surface of untreated kenaf fibre was smooth and flat. From the SEM micrograph examination, there was no fibre-matrix adhesion for the treated fibre [87]. In another research on fibre treatment using NaOH, the kenaf fibre (KF) was treated using 6% of NaOH in blended TPU-NR composites. The result of SEM micrograph, as shown in Fig. 3.16, showed that the matrix surface of the treated kenaf fibre was rough throughout the composites, while the matrix surface of the untreated fibre was smooth and flat. This shows that the fibre-matrix roughness surface can indicate better wetting and impregnation of the fibre, thus increasing the bonding with matrices and improving the impact properties [84].

13.

Thermal gravimetric analysis (TGA) on types of fibre and matrix composites

The thermal stability of composites towards heat is important to ensure the physical and chemical properties do not change. The ability of composites to withstand heat

Characterization studies of biopolymeric matrix and cellulose fibres

53

can be analysed via TGA. With this analysis, researchers will be able to identify the maximum heat where composite starts to degrade, and the amount of residue left behind. Moreover, they will be able to investigate the weight loss of composites with respect to temperature increase.

13.1

Effect of various fibre loadings on TGA

A study on kenaf fibre in polypropylene (PP) composites was conducted to investigate the thermal stability by using TGA. Maleic anhydride-polypropylene (MAPP) was used as the compatibiliser, while ammonium polyphosphate was used as the fire retardant element. It was found that kenaf fibre itself had reduced the thermal stability of PP and ammonium polyphosphate (APP) filled PP composites. The APP content in the PP composites had better flammability resistant and good mechanical properties than PP [88]. An article regarding natural fibre research was published to determine the effect of fibre loading and length on the thermal properties of hybridised kenaf/pineapple leaf fibre (PALF) reinforced with high-density polyethylene (HDPE) [89]. According to the article, the hybridised kenaf PALF fibre composites showed a decrease in thermal stability with the increase in fibre loading as compared to the neat HDPE.

13.2

Effect of various matrix composites on TGA

A research was conducted on the thermal behaviour of natural fibre kenaf with epoxy composites. The experiment compared the alkaline treated kenaf fibre with the untreated ones. In this research, the kenaf fibre with epoxy composites showed thermal and charring stability before reaching 100
C. At temperatures above 100
C, the voids started to occur for the untreated fibre as the moisture content in the fibre decreased with the increase in temperature [90,91]. A study on the percentage of alkaline treatment effect on natural fibre composite matrix investigated the thermal stability. The FTIR experiment showed that the untreated kenaf fibre had more H-bonding than all treated composites. The TGA result also showed that the untreated kenaf fibre was more stable in thermal stability [92].

13.3

Effect of thermal degradation on various fibres and matrices

Studies that focused on the thermal degradation of composites were conducted to determine various temperature differences and matrix conditions. Each material has its own decomposition temperature, but when it is combined or blended with other materials, the physical characteristic of the material is changed and the decomposition temperature of the new material will also change [93e95]. Thermal degradation analysis on the various types of matrix was conducted on alkaline treated/untreated kenaf fibre with epoxy. Glass fibre with epoxy and the neat epoxy were used for comparison purposes. The result showed that the decomposition temperature occurred as early as 372
C, which was from the neat epoxy [96]. There was an article regarding an experiment on low-density polyethylene (LDPE) blended

54

Interfaces in Particle and Fibre Reinforced Composites

with thermoplastic sago starch (TPSS) containing kenaf core fibre with fibre loading ranged between 10% and 40%. The TGA experiment showed that the first decomposition temperature occurred at 176.2
C for the 0% and 40% kenaf core fibre loading. The second decomposition temperature occurred at 332
C for all types of composition [97].

13.4

Residue effect on various fibre and matrix composites

Every natural fibre composite undergoes thermal degradation process. The early stages can determine the first temperature where the composite starts to degrade and the amount of residue or char that is left at the end of the degradation temperature. A study related to the use of hemp fibre in polypropylene was conducted for the thermal degradation experiment. 30% of fibre loading was used in the experiment. Three types of hemp fibre condition were prepared, which were bleach hemp fibre, conttonised hemp fibre, and long hemp fibre. These types of fibre condition were then mixed with the polypropylene. As a result, the bleach hemp fibre had the lowest residue, which was 0.20% at a temperature higher than the other two types of hemp fibre condition at 800
C [98]. A study was conducted on the natural fibre of kenaf with the blends of thermoplastic polyurethane (TPU) and natural rubber (NR) to study the thermal effect of kenaf blended composites. The experiment was designed with various matrix compositions. From the thermal degradation experiment, the result showed that the highest amount of TPU in the composites had the lowest char residue at 600
C [99]. A few characteristics need to be determined each time a new material is combined or blended together with the presence of a synthetic or natural fibre. Firstly, the purpose of the new material; either it is for a rigid or flexible material. At this point, the fibre bonding condition of the matrix is determined. Secondly, the environmental condition that the new material is exposed to. At this stage, determining the decomposition temperature of the matrix is important. Last but not least is the final design of the new material that is suitable with the strength of the end product.

14.

Effect of nanoparticle size, nanoparticle/matrix interface adhesion and nanoparticle loading on the mechanical properties of particulate-polymer biocomposites

This section discusses how the type of fibre, nano-sized fibre loading, fibre length, and its biopolymer composite affect the physical and mechanical properties of particulatepolymer biocomposites.

14.1

Young’s modulus and tensile strength

Young’s modulus is the stiffness (the ratio between stress and strain) of a material at the elastic stage of the tensile test. It can be enhanced by adding or reinforcing micro/

Characterization studies of biopolymeric matrix and cellulose fibres

55

nanofibre to a polymer matrix as the fibre has higher stiffness values than the matrix polymer [1]. The mechanical properties of the reinforcing polymer composites are considerably higher than those of un-reinforced polymer composites. Generally, the mechanical properties of the fibre/polymer composites are dominated by the addition of fibre to the polymer composites. There are four key factors that govern the contribution of fibre: (1) the surface interaction of fibre and resin; (2) the orientation of the fibres in the composite; (3) the basic mechanical properties of the fibre itself, and (4) the amount of fibre in the composite (fibre volume fraction). The basic mechanical property of common biocomposites is fibre selection. The surface interaction or surface interphase of matrix and fibre is affected by the degree of bonding formed between each surface. This phenomenon is highly influenced by the mechanical or chemical treatment or combination of both treatments given to the surface of fibre. The manufacturing process used to fabricate the composites would be affected by the amount of fibre. The reinforcement of fibre with closely packed fibres will provide a higher fibre volume fraction (FVF) within the matrix than the composite that is fabricated with coarse fibre or fibres with poor arrangement that form large gaps between the fibre bundles. According to Eq. (3.1), the FVF ðVÞ is determined from the fractions in weight ðwwÞ by using the density of each component ðdÞ. Eqs (3.2) and (3.3) show the formulas for FVF from fibre weight fraction (FWF) and FWF from FVF, respectively [100]. wwi di Vi ¼ P ww i di

(3.1)

FVF from FWF: FVF ¼ 

1   rf 1 1 1þ rm FWF

(3.2)

FWF from FVF: FWF ¼

rf  FVF rm þ ððrf  rm Þ  xFVFÞ

(3.3)

where, FVF is the fibre volume fraction, FWF is the fibre weight fraction, rm is the density of cured resin/hardener matrix (g/cm3) and rf is the density of fibre (g/cm3). Besides, the diameter of fibre also plays an important role in the surface interaction between the fibre and matrix. Smaller diameters of fibre would be attributed to higher fibre surface area, spreading the fibre/matrix interfacial loads. The strength and stiffness of composite would increase in proportion to the amount of fibre present. However, at certain loadings, i.e., around 60%e70% FVF (depending on the way in

56

Interfaces in Particle and Fibre Reinforced Composites

which the fibres are packed together) the mechanical properties of the composite will reach a peak and then begin to decrease due to the lack of sufficient resin to hold the fibres together properly. The excess fibre would cause a slight reduction in the composite with higher fibre loading, which is presumed to be due to the insufficient ability of the matrix-polymer to cover the fibre completely and the agglomeration of fibre that limits the interfacial bonding of fibre with the matrix [1]. The increasing mechanical properties of the composite might be attributed to (a) the presence of small-sized fibre particle, (b) elevated aspect ratio and surface area of fibre, (c) strong interaction between the fibre and matrix, and (d) homogeneous distribution of fibre in the matrix [1,15,101,102]. Moreover, the orientation of the fibres creates highly specific direction properties in the composite as the reinforcing fibres are designed to be loaded along their length. This anisotropic feature of the composites can be used in designs, with the majority of fibres being placed along the orientation of the main load paths. This would minimise the amount of parasitic material that is put in orientations where there is little or no load.

14.2

Effect of particle size

Extensive studies can be found in the literature regarding the reinforcement of micro and nano fibres in biopolymeric matrices and the final mechanical properties of these materials [103]. The development of high-performance nanocellulose/polymer composites that requires homogeneous dispersion of nanocellulose in the polymeric matrix is important for the composites’ performance [104]. The performance of the stress transfer and Young’s modulus between the nanocellulose and the biopolymer matrix material is also reported to play an important role in the biocomposite’s interface performance to achieve efficient load transfer from the matrix to the nanocellulose [105]. Fig. 3.17 shows the tensile strength and Young’s modulus of the micro- and nanocellulose reinforced sugar palm starch. A study conducted by Sanyang et al. [106] reported that the micro-sized cellulose sugar palm fibre (10.24  3 mm) reinforced sugar palm starch had increased the Young’s modulus. It was observed that by incorporating 1 wt% micro-sized cellulose sugar palm fibre into sugar palm starch, the tensile strength and Young’s modulus were increased by 10.5 and 31.38 MPa, respectively (Fig. 3.17). The incorporation of the micro-sized cellulose sugar palm fibre also decreased the molecular mobility of starch biopolymer, thus making the biocomposite stiffer, less stretchable and resistant to break as compared to the neat sugar palm starch biopolymer film. Moreover, a study conducted by Ilyas et al. [1] reported that the incorporation of 0.5 wt% sugar palm nanocellulose had increased the tensile strength and Young’ modulus by 11.47 and 178.83 MPa, respectively, in which the Young’s modulus of nanocomposite film was six times higher than the micro-sized cellulose sugar palm fibre reinforced sugar palm starch. This was because the sugar palm nanocellulose was nanoscale in size. Besides, it had a large aspect ratio and abundant hydroxyl group in a large surface area (14.47 m2/g). Furthermore, it was also due to the ability of the starch biopolymer to mechanically interlock with the nanofibres [9]. Although the nanocellulose has a strong interaction with each other, the process of ultra-sonication was used to destroy its bonding interaction. Therefore, when

10.00

8.60 e

8.15 d,e 8.00 4.80 a

4.00 2.00

98.10 b

120.00

107.98 b,c

122.93 d

117.19 c

100.00 80.00 60.00

53.97 a

40.00

–1

.5

Cs

–0 SP

S/

SP

NC

Cs SP

S/

SP

NC

NC SP S/ SP

SP

S/

SP

NC

Cs

Cs

–0

–0

.4

.3

.2 Cs NC SP S/

SP

SP S/

(d)

25

–0

–0 Cs NC

S/

SP

SP

NC

S

Cs

.1

–1

0.00

SP

SP S/ SP

SP S/ SP

NC

NC

Cs

Cs

–0

–0

.5

.4

.3 –0 Cs NC SP S/

SP

SP

S/

SP

NC

NC

Cs

Cs

–0

–0

.2

.1

S SP SP S/ SP

100 90

20

Tensile modulus (MPa)

Tensile strength (MPa)

133.94 e

140.00

20.00

0.00

(c)

160.00

SP

6.00

7.78 c,d

7.19 b,c

6.60 b

178.83 e

180.00

11.47 f

12.00

Tensile modulus (MPa)

Tensile strength (MPa)

(b) 200.00

15 10 5

80 70

Characterization studies of biopolymeric matrix and cellulose fibres

(a) 14.00

60 50 40 30 20 10

0 SPS

SPS-C1

SPS-C3

SPS-C5

SPS-C10

0 SPS

SPS-C1

SPS-C3

SPS-C5

SPS-C10

Fig. 3.17 Tensile strength and Young’s modulus of the micro- and nanocellulose reinforced sugar palm starch. (a and b) Nanocomposite film samples. (c and d) Composite film samples. 57

58

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b)

Fig. 3.18 FESEM micrograph of (a) sugar palm starch/sugar palm nanocellulose, and (b) sugar palm starch/sugar palm cellulose [106].

incorporating with a starch biopolymer, a new strong interfacial adhesion between sugar palm nanocellulose and starch biopolymer is formed upon processing the film. Fig. 3.18 shows the FESEM micrograph of (a) sugar palm starch/sugar palm nanocellulose, and (b) sugar palm starch/sugar palm cellulose. Fig. 3.18 also shows the strong interaction with highly homogeneous distribution and dispersion of nanocellulose in sugar palm starch biopolymer [1]. Besides, the high surface area in the nanocellulose not only reduces polymer chain mobility, but also serves as a nucleation agent that is increasing crystallinity. Film crystallinity increases with increasing nanocellulose content, indicating the nanocellulose’s role as a nucleating agent that promotes crystallisation. The high functionality of nanocellulose coupled with its high specific strength and surface area make it desirable as a reinforcing and nucleating agent in polymers. Ashley et al. [107] reported that nanocellulose is a very efficient nucleating agent for the biodegradable aliphatic poly(ethylene succinate) (PESu).

14.3

Effect of particle loading

The effect of particle loading on Young’s modulus, tensile strength, and elongation at break of biopolymer composites can be analysed in Fig. 3.17. Ilyas et al. [1] proved that the tensile strength and Young’s modulus of the sugar palm nanocellulose nanocomposites films increased with the increase of cellulose concentration by 0.1e0.5 wt%. The tensile strength and Young’s modulus of the neat starch biopolymer were 4.80 and 53.97 MPa, respectively. The addition of the nanocellulose from 0.1 to 0.5 wt% significantly increased the Young’s modulus values of the biopolymer nanocomposites from 6.60 to 11.47 MPa and from 90.10 to 178.83 MPa respectively. Therefore, at the maximum nanocellulose loading (0.5 wt%), the tensile strength of nanocomposites film improved by 140%, while the tensile modulus was 98.48% higher than that of the neat sugar palm starch film. This tensile behaviour can be attributed to the favourable interaction between the nanocellulose and starch biopolymer

Characterization studies of biopolymeric matrix and cellulose fibres

59

matrices, which facilitated adequate interfacial nano-adhesion because of their chemical similarities [3]. The findings are similar to the work done by Sanyang et al. [106], which incorporated micro-sized cellulose sugar palm with sugar palm starch biopolymer. However, the tensile strength and Young’s modulus values gained by Sanyang et al. [106] with similar loading values were lower than that of Ilyas et al. [1]. This was due to the nanoscale of the fibre with abundant hydroxyl groups that interlocked the biopolymer starch molecules. Besides, the tensile strength and Young’s modulus of the nanocomposites were observed to be optimised at 0.5 wt% followed by the reduction in both values at 1.0 wt% due to the agglomeration of nanocellulose. Moreover, these phenomena occurred due to the agglomeration and uneven distribution of the nanocellulose within the starch biopolymer, in which the nanocellulose failed to act as the reinforcement agent in the starch biopolymer. Furthermore, it was also caused by the excess of nanocellulose content that was likely attributed to the poor particle distribution, phase separation, and large agglomerates formation, which led to the poor mechanical properties of nanocomposites [1,108e110]. The reinforcement of nanocellulose and starch biopolymer showed similar effects with the previously reported results of flax [111], and potato peel [112] on the mechanical properties of starch-based nanocomposites. The presence of nanocellulose within a polymer matrix might affect the mechanical properties of the nanocomposite film. According to Ramires and Dufresne [113], there are three main factors that can affect the mechanical performances of the nanocomposite material: (1) the dimension and morphology of the nanofibre, (2) the method of processing and (3) the micro/nanostructure of the matrix and matrix/fibre interface. Nanofibres with high aspect ratio are particularly interesting because of their high specific surface area that give better reinforcing effects. Besides that, according to Tonoli et al. [114], the nanofibres are responsible for the net adhesion formation within the matrix composite during the dehydrating stage of the manufacturing process. By increasing the nanofibres’ aspect ratio and specific surface area with a rough surface, and decreasing the diameter of fibres, the nanofibre/matrix adhesion could be improved, and thus providing better mechanical performance [1]. A study on polymer nanocomposite films based on the poly(oxyethylene) PEO polymer as the matrix and high aspect cellulose whiskers isolated from ramie plant as the reinforcing phase was done via casting/evaporation and extrusion processes by Alloin et al. [109]. Microscopic observations by using scanning electron microscopy (SEM) displayed aggregations of nanocellulose and a small decrease of the whiskers aspect ratio for extruded sample, but for both processes employed, the films displayed homogeneous surfaces [1,15,102,109]. Besides, a thermal stabilisation of the modulus of the cast/evaporated nanocomposite films for temperatures higher than the PEO melting temperature was reported. High mechanical properties of the casting/evaporation process were ascribed to the formation of a rigid cellulosic network within the matrix. However, for the extruded composites polymer, the rheological behaviour through the viscoelastic and creep measurements showed a liquidlike behaviour. However, for the extruded nanocomposite films, the reinforcing effect of whiskers was reduced due to the absence of a strong mechanical network or at least the presence of a poor whisker percolating network. This weak mechanical

60

Interfaces in Particle and Fibre Reinforced Composites

reinforcement after the PEO melting temperature might be caused by the extrusion process that prevented the formation of a strong whisker network for the whisker content used as compared to the evaporated films. Therefore, to overcome this drawback of aggregation and the decrease of the aspect ratio of whiskers during the extrusion process, one has to increase the nanofibre content [109].

14.4

Effect of nanocellulose/matrix interfacial adhesion

The interface area is a contact surface area formed between the matrix and fibre. This area becomes a transfer load from the matrix to reinforcement. The composite interface influences the characteristics of the composite as it plays an important role in transferring the load between the matrix and reinforcement [115]. A strong interface provides a high strength. The interface can be a simple atomic bond (between starch and starch) and it can also be an inter-matrix reaction (starch and nanocellulose fibre). The interface greatly affects the strength, stiffness, and toughness. In general, there are several theories about the mechanism of adhesion, namely, adsorption, mechanical bonding, electrostatic bonding, and chemical bonding [7]. However, the debonding phenomenon also often occurs in composites. Debonding is a releasing mechanism of interface bonds between the matrix and fibre during load transfer. This is indicated by a bad interfacial bonding between the matrix and fibre. The increase in the debonding surface between matrix and fibre is due to the increase in deformation. After debonding, the fibre loses its ability to retain the load in the direction of debonding. Even so, the fibre still distributes the load to the matrix through its other bound part. The interfacial bond between matrix and fibre is an important factor in the debonding phenomena [116]. Several studies have reported the increase of tensile strength of nanocellulose fibre reinforced starch matrix. This is due to the small interface bonding between nanocellulose fibre and starch matrix [117]. The presence of nanocellulose in starch matrix makes the hydrogen bonds to be well-formed and results in a smaller distance between the molecules. The strong interaction between nanocellulose and starch matrix can be retained when the external loads are given [117]. The increase in the tensile strength of bionanocomposite is also caused by a good dispersion of nanocellulose in starch matrix [37]. Higher nanocellulose fraction in starch matrix gives better interaction bonding and good compatibility between the two. Actually, starch has hydrophilic properties, while cellulose fibres are hygroscopic. This causes a trigger factor in increasing tensile strength [37]. The interfacial adhesion between matrix and fibre can be increased by providing ultrasonic vibration while undergoing gelatinisation [118]. Previous report showed that the addition of ultrasonic vibrations to biocomposite gel of oil palm empty fruit bunch fibre filled tapioca starch had successfully increased its tensile strength. This was due to the kinetic energy of the ultrasonic bath that spread the agglomerated fibre evenly in the matrix. The kinetic energy improved the interfacial bonding between the matrix and fibre, which indicated a reduction in the number of free OH bonding. This phenomenon produced compact structures and good mechanical properties [118,119].

Characterization studies of biopolymeric matrix and cellulose fibres

61

Nanostructured fibres play an important role in biocomposite fabrication since they bring various desired functionalities to the composites. NFC gels can be converted to films by dilution and dispersion in water and then either cast [15,102,120,121] or vacuum filtered [122e124]. When water is removed from the NFC gel, a cellulose nanofibre network is formed with interfibrillar hydrogen bonding, resulting in the formation of stiff and strong films. Henriksson et al. [123] reported that the mechanical properties of NFC films were reduced when immersed in water but much of the structure was retained. The nanofibres in the film were not redispersible in water due to the strong interaction between adjacent nanofibres after drying, most likely dominated by hydrogen bonding. Despite random in-plane NFC orientation, NFC films have interesting mechanical properties. As discussed by Berglund [125], the Young’s modulus may approach 20 GPa and strength can reach 240 MPa. However, most literature indicates lower modulus and strength values. Besides improving the mechanical properties of starch, the addition of NFC to the matrix can result in a decrease of water uptake at equilibrium and water diffusion coefficient. Mondrag
on et al. [126] applied glyceryl monostearate (GMS) as a surfactant in TPS-NFC nanocomposites prepared via solution casting. As expected, cellulose nanofibres derived from husks and corncobs increased the Young’s modulus and tensile strength of TPS films due to the strong interactions between the starch matrix and the high aspect ratio of nanofibres [127]. The properties of fibre-reinforced composites depend on many factors, including fibre size, fibre/matrix adhesion, volume fraction of fibre, fibre aspect ratio, fibre orientation, and stress/transfer efficiency through the interface [128]. Fibre content in composites is a critical factor since fibre agglomeration through hydrogen bonding tends to occur at higher fibre loadings. Beyond the optimum amount, further addition of fibre into these composite systems resulted in a gradual decrement of mechanical properties of the materials. The diminished performance of the composites at higher amounts of fibre loading is possibly due to the lack of resin infusion and/or the presence of so many fibre ends in the materials, which could cause crack initiation and hence, potential composite failure beforehand. The cellulosic network is surrounded by a soft phase and the interactions between the fibre and the matrix are strongly reduced [129].

14.5

Effect of nanocellulose in a starch matrix

In the last decade, the potential of using nano-sized materials has become a big concern among researchers. Nanomaterial has a diameter between 1 and 100 nm. Usually, this material has unique properties, such as higher contact surface areas than bulk material [41,130]. This advantage is able to increase the chemical reactivity and improve mechanical properties. The characteristics are based on the number of surface atoms. The smaller size of the material affected on high exposed of a surface atom of materials [41]. One of the trending materials in the last two decades is cellulose. Cellulose is a natural polymer formed by linear glucose chains and connected by glyosidic b-1.4 bonds. Because of its linear structure, cellulose is categorised in the crystalline structure and it is not easy to dissolve [131,132]. Various sources of cellulose fibre have been produced, such as sugar palm [9], water hyacinth [133], oil palm empty fruit bunch

62

Interfaces in Particle and Fibre Reinforced Composites

(OPEFB) [134], ramie [28], pineapple leaf [135], kenaf bast [136], cotton [137], arecanut husk (Areca catechu) [25] and sugarcane bagasse [138]. The modification of nano-sized cellulose is one way of improving its performance. Several methods can be used to obtain nanocellulose, one of which provides a combination between chemical (pulping, bleaching, acid hydrolysis) and mechanical treatments (ultrasonication, high shear homogenisation, high-pressure homogenisation). Nanocellulose is a cellulose in diameter range of below 100 nm and length of hundreds of nanometers. Nanocellulose has several advantages, such as good mechanical properties, abundant availability, low density and environmentally friendly [41]. This material is generally used as reinforcement in various polymer matrices. The effects of nanocellulose in a starch-based matrix were widely reported by previous researcher [1,139,140]. They reported that the addition of nanocellulose in the starch matrix improved the mechanical properties. This can be due to several factors, namely, good adhesion bonding between nanocellulose and matrix, good fibre dispersion, minimum porosity and absence of agglomeration [37]. Starch-nanocellulose composites have been used in several applications, such as in food packing application (especially permeability). Nanocellulose fibres have many potential applications from flexible optoelectronic to scaffolds for tissue regeneration. Cellulose is a biosynthetic product that originates from animal, bacteria or plant. The term ‘nanocellulose’ refers to cellulose that has been extracted and has a nano-scale dimension [141]. Table 3.4 shows several polymer components, their manufacturing technique and applications. Nanocellulose has a large contact surface area, and because of its surface, strong interactions will be formed when combined with water, organic compounds, nanoparticles and starch [41]. A possible interaction mechanism is the hydrogen bond, where the hydroxyl group of cellulose will bind to the hydroxyl group of starch [142]. However, if the cellulose size is in nanometer, the contact surface will become large. The number of hydrogen bonds formed will also increase [132]. As a result, the number of free OH in cellulose and starch matrix bonding will decrease. This will result in the improvement of bionanocomposite performance [118].

14.6

Effect of various sources of nanocellulose reinforced biopolymer

Recently, due to the high aspect ratio and mechanical strength of nanocellulose, it has been used as the load-bearing component in developing new and inexpensive biodegradable materials. This section discusses the use of starch as a polymer in which the solution casting method is mostly used in the data collected. Table 3.5 summarises the mechanical properties of some nanocrystalline cellulose reinforced starch-based nanocomposites. It can be summarised that different types of starch and nanocellulose sources result in different mechanical properties. The difference between the current work and other works listed in Tables 3.5 and 3.6 is the source of fibres used in the production of nanocrystalline cellulose (NCC) and nanofibrillated cellulose (NFC). Natural fibres from various plants are used to isolate NCC, through the process of delignification, mercerisation, and hydrolysis.

Characterization studies of biopolymeric matrix and cellulose fibres

63

Table 3.4 Polymer component reinforced nanocelluloses, their manufacturing technique and applications [141,143]. Polymer component

Manufacturing technique

Applications

References

Maize amylopectin

Solution casting

Continuous papermaking

[144]

PVA

Solution casting

Flexible displays, optical devices, food packaging and automobile windows

[145]

Carboxymethyl cellulose

Solution casting

Edible coatings and packaging materials

[146]

Polyethylene

Extrusion

High performance cellulosics

[147]

Environmentally friendly HDPE

[137]

Evaluation of cotton filler in LDPE

[148]

Polyethylene glycol

PEG-g-CNF ribbons via stretching hydrogel

Ultra-high tensile strength and modulus for optoelectronic and medical devices

[149]

Amorphous dialcohol cellulose

Oxidation þ reduction of CNF surface

Barrier film

[150]

Polyvinyl amine

Layer by layer

Self-healing polymer film, films with good mechanical, optical thermal and oxygen barrier properties

[151]

Poly(butylene adipate-coterephthalate)

Injection molding

Light-weight and high performance materials for defense, infrastructure and energy

[141]

Cellulose esterified with lauroyl chloride

Solution casting and thermorpressing

Interface melting

[152]

Ethyl acrylate; methylmethacrylate

Solution mixing

Drug carrier

[153]

Ethylene-covinyl acetate rubber

Solution mixing and vulcanization

Transparent, rubbery materials

[154]

Continued

64

Interfaces in Particle and Fibre Reinforced Composites

Table 3.4 Continued Polymer component

Manufacturing technique

Applications

References

Maleic-anhydride grafted PLA

Electrospinning

Bone tissue engineering

[155]

Methylcellulose

Hydrogel by aqueous dispersion

Thermoreversible and tunable nanocellulosebased hydrogels

[156]

PC

Matster batch melt extrusion process

Optical devices

[157]

PC based polyurethane blend

Solution casting

Smart actuators and sensors

[158]

Plasticized PLA

Twin-screw extruder

Film blowing, packaging

[159]

Plasticized starch

Solution casting

Transparent materials

[160]

PU

Solution casting

High temperature biomedical devices

[161]

PVA

Solution casting

Stretchable photonic devices

[162]

PVA

Solution casting

Wound diagnosis/biosensor scaffolds

[163]

PVA

Solution casting

conductive materials

[164]

Starch

Blending, solution casting

Air permeable, resistant, surface-sized paper, food packaging

[165,166]

Starch

Solution casting

Food packaging

[167]

Starch

Solution casting

Food packaging

[168]

Wheat starch

Solution casting

Food packaging

[169]

Tuber native potato

Solution casting

Packaging

[170]

Cereal corn

Solution casting

Packaging

[170]

Legume pea

Solution casting

Packaging

[170]

Waterborne acrylate

Solution mixing

Corrosion protection

[171]

Wheat straw hemicelluloses

Solution casting

Packaging

[172]

PVA

Solution casting

Food packaging

[173]

Chitosan

Solution casting

Food coating/packaging

[174]

Table 3.5 Examples of starch based polymer, NCC nanocomposites and their mechanical properties. Tensile strength (MPa)

Young’s modulus (MPa)

References

Year

Tunicin

55 wt% H2SO4/ 20 min

Solution-casting

0.24e20

51e315

[177,178]

2000, 2001

Maize starch

Waxy maize starch

H2SO4/5 days

Solution-casting

1e15

11e320

[179,180]

2004, 2006

Wheat starch

Cottonseed linter

64 wt% H2SO4/4 h

Solution-casting

2.5e7.8

36e301

[181]

2005

Wheat starch

Ramie

64 wt% H2SO4/4 h

Solution-casting

2.8e6.9

56e480

[169]

2006

Potato starch

MCC

64 wt% H2SO4/2 h

Solution-casting

13.7

460

[182]

2007

Pea starch

Hemp

64 wt% H2SO4/4 h

Solution-casting

3.9e11.5

31.9e823.9

[183]

2008

Pea starch

Flax

64 wt% H2SO4/4 h

Solution-casting

3.9e11.9

31.9e498.2

[111]

2008

Maize starch

Tunicate

e

Solution-casting

42

208e838

[184]

2008

Pea starch

Bamboo

50 wt% H2SO4/ 48 h

Solution-casting

2.5e12

20.4e210.3

[167]

2010

Wheat starch

Microcrystalline cellulose (MCC)

36.5 wt% HCl

Solution-casting

3.15e10.98

e

[185]

2010

Potato starch

Potato peel waste

64 wt% H2SO4/ 90 min

Solution-casting

e

460

[112]

2012

Plasticized starch

Cotton cellulose powders

H2SO4

Solution-casting

e

e

[165]

2014

Maize starch

Sugarcane bagasse

64 wt% H2SO4/3 h

Solution-casting

17.4

520

[166]

2014

Potato starch

Cotton linter

64 wt% H2SO4/1 h

Solution-casting

4.93

e

[186]

2016

Sugar Palm Starch

Sugar palm fibre

60 wt% H2SO4/ 45 min

Solution-casting

11.5

178

[1]

2018

NCC sources

Maize starch

65

Manufacturing technique

Characterization studies of biopolymeric matrix and cellulose fibres

Isolation chemical/ time of NCC

Starch-based polymers

66

Table 3.6 Examples of starch based polymer, NFC nanocomposites and their mechanical properties. Tensile strength (MPa)

Young’s modulus (MPa)

References

Year

Hydrolyzed in 6.5 M H2SO4/40 min

Solution-casting

4.8

84.3

[168]

2009

Wheat

e

Solution-casting

8.76

322.05

[187]

2009

Maize starch

Wheat straw

High Pressurize Homogenizer/15 min

Solution-casting

6.75

220

[188]

2010

Maize starch

Cotton cellulose

Hydrolyzed in 6.5 M sulfuric acid/75 min

Solution-casting

0.35

3.12

[189]

2011

Potato starch

Softwood wood flour

Super masscolloider

Solution-casting

17.5

1317.0

[190]

2013

Potato starch

Rice straw

Ultrasonication

Solution-casting

5.01

160

[175]

2014

Maize starch

Kenaf

Super masscolloider

Solution-casting

2.35

53.6

[176]

2014

Corn starch

Kenaf

Super masscolloider

Solution-casting

38.0

141.0

[191]

2015

Corn starch

Bamboo fibre

e

Solution-casting

11.2

12.4

[192]

2017

Sugar Palm Starch

Sugar palm fibre

High Pressurize Homogenizer, 500 bar

Solution-casting

e

e

[21]

2018

NFC sources

NFC preparation

Cassava starch

Cassava bagasse

Mango puree

Interfaces in Particle and Fibre Reinforced Composites

Manufacturing technique

Starch-based polymers

Characterization studies of biopolymeric matrix and cellulose fibres

67

In an experiment conducted by Ilyas et al. [9], delignification (by using sodium chlorite, NaClO2) and mercerisation (by using sodium hydroxide, NaOH) treatments were used to remove the amorphous structure in fibres, such as lignin and hemicellulose, respectively, which also split the fibres into smaller fibrils known as cellulose microfibrils. After that, the cellulose macrofibrils underwent the hydrolysis process (1200 rpm, 45 min, 45
C, 60 wt%, H2SO4) to remove the amorphous region within the cellulose [9]. However, for the NFC, high pressure homogeniser (HPH) mechanical, super mass colloider [175] and ultrasonication [176] processes were used to extract NFC from the cellulose fibre [21]. Tables 11.5 and 11.6 summarise the mechanical properties, source and preparation of NFC and NCC, respectively, as well as the method of manufacturing some nanofibrillated cellulose reinforced starch-based nanocomposites. It can be concluded that different types of starch and NFC reinforcement result in different mechanical properties.

15.

Atomic force microscopy (AFM), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM)

Usually, AFM is used to determine the surface roughness of composites or fibres by using high resolution, non-destructive analysis. Recently, a study by Sexena et al. [193] examined the reinforcement of xylan/sorbitol films with nanocrystalline cellulose, bleached softwood kraft fibres and acacia fibres and their impact on water transmission. They used SEM and AFM image characterisation to analyse the structural morphology of the resulting nanocomposite films as shown in Figs 3.19 and 3.20. From the analysis, it was observed that the control film containing sorbitol and xylan

(a)

(b)

Fig. 3.19 AFM images of fractured surface (a) control xylan and (b) 10% sulphuric nanocrystalline cellulose film [193].

68

(a)

Interfaces in Particle and Fibre Reinforced Composites

(b)

Fig. 3.20 ESEM images of (a) surface image of control xylan and (b) surface image of xylan sulphuric nanocrystalline composite film [194].

had a more open structure than the xylan-sorbitol film reinforced nanocrystalline cellulose. This interfacial hydrogen bond between the surface of nanocellulose and matrix was supported by the FTIR analysis, which showed a strong interaction at the OeH band (3200e3450cm1). According to Sexena et al. [193], this intensity band was observed to increase with the increment of nanocellulose concentration. Another study conducted by Grande et al. [194] on the development of self-assembled bacterial cellulose-starch nanocomposite showed a strong interaction between starch biopolymer and bacterial nanocellulose. In their study, the self-assembled BC (bacterial cellulose) reinforced starch nanocomposites showed a coherent morphology that was analysed by using the environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM) as shown in Fig. 3.21. Besides, they also observed that most of the BC nanofibrils were fully covered by a starch layer (white arrows), which indicated the layer seemed to be quasi homogeneous at meso- and microlevels as shown in Fig. 3.21(e) and (f). The homogeneous dispersion of the nanocellulose within the starch biopolymer can affect the mechanical properties, which can also be related to the strong interaction between those materials [2]. However, due to the deficiency of regularity of the starch layer (Fig. 3.21(b) and (c)), it can be observed that some of the BC nanofibrils are not covered or partially covered by starch biopolymer. The reinforcement between nanocellulose and chitosan nanocomposite film was investigated by Azeredo et al. [195] by using the AFM analysis. They observed a good dispersion of nanocellulose and good nanocellulose-matrix interactions, which resulted in good performance of the nanocomposite films (Fig. 3.22). In 2018, Ilyas et al. [1] published a paper entitled “Development and characterization of sugar palm nanocrystalline cellulose reinforced sugar palm starch bionanocomposites” in which they indicated that the optimum concentration of nanocellulose formed a strong interaction with starch biopolymer by using FESEM

Characterization studies of biopolymeric matrix and cellulose fibres

(a)

295.40 nm

69

(d)

132.33 nm

(b)

144.73 nm

(e)

–85.95 nm

(c)

165.84 nm

(f)

–93.10 nm

Fig. 3.21 AFM micrographs of (a) Pure BC sheet; (b) BCepotato starch nanocomposite sheet and (c) BCecorn starch nanocomposite sheet, and ESEM micrograph of (d) Pure BC sheet, (e) BCepotato starch nanocomposite showing the starch covering layer, and some uncovered nanofibrils (white arrows) and (f) BCecorn starch nanocomposite showing the starch covering layer and some uncovered nanofibrils (white arrows) [194].

micrograph (Fig. 3.23). This was due to the nanocellulose that dispersed more homogeneously within the biopolymer and was fully covered by the biopolymer. Therefore, this phenomenon would create a strong solid interfacial adhesion between nanocellulose and sugar palm starch (SPS) biopolymer matric, thus improving the mechanical properties.

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 3.22 AFM images (3 mm  3 mm) of films: on the left, chitosan film with CNF (15 g/100 g) and glycerol (18 g/100 g); on the right, chitosan films with glycerol (18 g/100 g). The grey scale in the right side of each image indicates the respective height values [195].

(a)

(b)

(c)

Fig. 3.23 FESEM micrographs of the (a) surface, (b) 2 mm cross-section, and (c) 100 mm crosssection of SPS/SPNCCs-0.5 [1].

16.

Fourier transform infrared (FTIR) spectroscopy

FTIR spectroscopy is used in the characterisation of material for the interpretation of the functionalities present in the chemical structure of composites. It is an experimental technique used initially for the qualitative and quantitative analysis of organic compounds, providing specific information on molecular structure, chemical bonding and molecular environment [1]. This can be verified by Huq et al. [196] in their research on NCC reinforced alginate nanocomposite films. According to them, the incorporation of nanocellulose within the biopolymer had increased the interfacial bond between both polymers. It can be observed from Fig. 3.24 that after increasing the concentration of 5% and 8% NCC into the alginate biopolymer, a slight increase of typical sharpen peak was seen at 3335 cm1 related to the OeH vibration of crystalline NCC. Besides that, the width and intensity of the overall band OeH band (3200e3600 cm1) increased with the increasing concentration of NCC, suggesting an increase of interfacial hydrogen bonding between NCC and alginate biopolymer.

Characterization studies of biopolymeric matrix and cellulose fibres O-H stretching vibration 3600-3200 cm–1 3335 cm–1

71

COO• stretching vibration 1595 cm–1

C-H stretching vibration 2930 cm–1

(d)

(c) A

(b)

(a) 4000,0

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800 650,0

cm–1

Fig. 3.24 FTIR spectra of (a) Pure NCC film, (b) native alginate, (c) Alginate þ 5% (w/w) NCC and (d) Alginate þ 8% (w/w) NCC [196].

These outcomes were also supported by Ilyas et al. [1], Asrofi et al. [197], Chang et al. [185], Chen et al. [112], Yang et al. [165], Slavutsky and Bertuzzi [166], and Noshirvani et al. [186], where they summarised that the interactions by hydrogen bonding between hydroxyl groups of nanocellulose and biopolymer were formed with the addition of nanocellulose nanofibre. Besides, in another experiment conducted by Flauzino et al. [198] on the mechanical properties of natural rubber nanocomposites reinforced with high aspect ratio cellulose nanocrystals isolated from soy hulls, they reported that the FTIR spectra for natural rubber matrix and the nanocomposite film reinforced with 5 wt% nanocellulose soy hull (CNCSH) (NR5%), two spectra were recorded. One corresponded to the lower face (in contact with the Petri dish during water evaporation) and the other corresponded to the upper face. In addition to these typical peaks associated to natural rubber, a new prominent peak appeared at 1059 cm1 for the lower face of the nanocomposite film. It corresponded to the CeO stretching and CeH rock vibrations of cellulose [38]. This indicated that the lower face of the film was richer in nanocellulose as compared to the upper face. Moreover, a probable gradient of nanocellulose concentration existed within the thickness of the film due to a possible sedimentation during the film processing by casting/evaporation. According to Flauzino et al. [198] the possibility of sedimentation of the nanocellulose depends mainly on the viscosity of the medium. Therefore, sedimentation could be avoided, or at least limited, by using more concentrated suspensions. Nevertheless, it would be more difficult to mix two too concentrated suspensions. This sedimentation phenomenon can also result from the inherent incompatibility and insufficient molecular scale interaction between

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Interfaces in Particle and Fibre Reinforced Composites

nanocellulose and natural rubber, which may restrict the overall performance of the nanocomposite material.

17.

X-ray diffraction (XRD)

XRD is used to determine the crystalline material (used for regulatory purposes or during development), different polymorphic forms (“fingerprints”), structural information, orientation, and layer thickness. It can also distinguish between amorphous and crystalline materials and quantification of the percent crystallinity of a specimen. It is a non-destructive analytical technique in the determination of crystallinity of a compound. In this section, XRD of the composites was studied to determine the effect of fibre concentration on the crystallinity of biopolymer composites. The XRD results were related to the mechanical, barrier and thermal properties as well as the chemical interactions between fibre and biopolymer determined by the FTIR analysis. A number of authors have considered the effects of fibre incorporated biopolymer composites on the crystallinity index [79,127,135,139,159,186,188,191,196,199e216]. Fig. 3.25 illustrates the XRD pattern of nanocellulose reinforced biopolymer [1]. According to Ilyas et al. [1], the crystallinity index of the biopolymer nanocomposites increased as the concentration of nanocellulose increased. The relative crystallinity of the neat SPS was 22.8%. The reinforcement of sugar palm nanocrystalline cellulose (SPNCC) nanofibre led to an increase in the relative crystallinity from 22.81% to 41.84%. Besides that, they reported that only one well-defined peak at 2q ¼ 22.6 degrees instead of two of peaks (2q ¼ 15 degrees) was observed when the concentration of nanocellulose increased by 1.0 wt%. This phenomenon might be attributed to the concentration of cellulose that was added into the SPS matrix. These diffraction peaks (2q ¼ 22.5 degrees) can be seen in the 100 wt% of nanocellulose pattern at the same angle (Fig. 3.26) [9], supporting the fact that these diffraction peaks in the nanocomposite films were attributable to the nanofibre [9]. The incorporation of fibre into the 1400 g 1200

f

Intensity

1000

e

800 600

d c

400

b a

200 0

5

10

15

20 25 2θ (degree)

30

35

40

Fig. 3.25 XRD patterns of (a) SPS, (b) SPS/SPNCCs-0.1, (c) SPS/SPNCCs-0.2, (d) SPS/ SPNCCs-0.3, (e) SPS/SPNCCs-0.4, (f) SPS/SPNCCs-0.5, and (g) SPS/SPNCCs-1.0 [1].

Characterization studies of biopolymeric matrix and cellulose fibres

73

1000

d

800

Intensity

a 600 b 400 c 200

0 5

10

15

25 20 2θ (degree)

30

35

40

Fig. 3.26 XRD patterns of (a) sugar palm fibres, (b) bleaching fibres, (c) alkali-treated fibres, and (d) 100 wt% of nanocellulose SPNCCs [1].

biopolymer had attributed to the boost in the crystallinity and interfacial bond of the medium, which led to the improvement in the mechanical properties of the biopolymer composites accordingly [217].

18.

Nuclear magnetic resonance (NMR) spectroscopy

The content for the crystalline phase of materials can be determined by using highresolution solid-state 13C NMR spectroscopy. In a study conducted by Avella et al. [218], structural characterisation was done to determine the dispersion of clay into the starch biopolymer matrix by using NMR spectroscopy as shown in Fig. 3.27. They concluded that a good intercalation of the biopolymer matrix into clay montmorillonite interlayer galleries in the case of starch/clay sample by NMR structural analysis was observed. Besides, a reinforcing effect of the clay montmorillonite particles on the modulus and tensile strength of the starch was observed to increase as the concentration of fibre increased, which showed that there was a good correlation interphase between the starch biopolymer and clay montmorillonite. However, the addition of the polyester phase seemed to negatively affect the reinforcing effect of the clay particles [218]. A study conducted by Akhlagi et al. [219] on the modification of cellulose nanocrystals with chitosan oligosaccharide for drug delivery applications using NMR spectroscopy showed that the NMR spectrum of oxidized CNC (CNC-OX) displayed a peak at 175 ppm, which was due to the carbonyl group of the carboxylic acids (Fig. 3.28). The significant reduction of the carbonyl peak in the spectra of cellulose nanocrystal grafted with chitosan oligosaccharide (CNCeCSOS) proved the grafting of CSOS on CNC-OX and the formation of amide bonds. Sain and Kokta [220] studied the performance of bismaleimide modification to enhance the properties of wood fibre

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Interfaces in Particle and Fibre Reinforced Composites

(a)

27AI

MAS-NMR

Clay

100

50

0

–50

ppm

100

50

0

–50

ppm

0

–50

ppm

(b)

PS/C

(c)

PE/PS/C

100

Fig. 3.27

50

27

Al MAS NMR spectroscopy of clay, PS/C and PS/PE/C [218].

composites by using PP as the bonding material. Write and Mathias [221] demonstrated the synergistic incorporation of balsa wood composites by using styrene and ethyl-a-(hydroxymethyl)acrylate (EHMA). The reinforcement of this composite improved its properties due to the strong interaction between the fibre and polymer. The strong interaction and improvement of dimensional stability between both materials were confirmed by solid-state SEM and NMR.

19.

Measurement of contact angle

Academic literature on the measurement of contact angle of composites has revealed the emergence of several contrasting themes. Recently, many composites are being generated, which benefit from the fundamental understanding of composite surface. Kim and Mai [7] mentioned that the measurement of contact angle can be classified into three, which are (1) contact angle on a flat surface, (2) contact angle on a rough

Characterization studies of biopolymeric matrix and cellulose fibres

(a)

75

CNC

180 175

170 ppm

(b) CNC-OX

180 175

(c)

170 ppm

CNC-CSOS

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170 ppm

200 190 180 170 160 150 140 130 120 110 100

Fig. 3.28

90

80

70

60

50

40

30

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ppm

13

C solid-state NMR spectra of (a) CNC, (b) CNC-OX, and (c) CNCe CSOS [219].

surface and (3) contact angle on a cylindrical surface. The measurement of contact angle is illustrated in Fig. 3.29. The contact angle measurement is useful to determine the wettability of the solid surface by water. A common method used to measure the contact angle is the ‘sessile drop method’. This method can measure the contact angle by directly dropping water on the flat surface of a solid material. The water is left to rest Sessile Drops r

h θ

θ θ

Sessile bubble

Fig. 3.29 The use of sessile drops or bubbles for the determination of contact angles [7].

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Interfaces in Particle and Fibre Reinforced Composites

X2 θ

Fiber

X1

l

Fig. 3.30 Liquid droplet attached to a monofilament [7].

before the measurement is taken. However, to determine the contact angle on a rough surface or when the surface of composite consists of small patches of various kinds, the equation must be modified or some derivations must be done as described by Kim and Mai [7] and Whalen [222]. Besides, in order to determine the contact angle on a cylindrical surface, Kim and Mai [7] proposed a direct and simple method called the ‘droplet aspect ratio method’ as illustrated in Fig. 3.30. This method can be done by spotting the shape of the liquid droplet attached to a single fibre. According to Pandey et al. [223], a polymer surface that displays water contact angles greater than 90 degrees is being considered as a hydrophobic material. To better understand the mechanism of the interaction of composites with water and its effects, Ferrer et al. [224] analysed the water contact angle of nanofibrillated cellulose and empty palm fruit bunch fibres (EPFBFs) nanopaper. They reported that the water contact angle of the composite increased with the lignin content because lignin is less hydrophilic than cellulose. Besides, the water contact angle values were almost similar and ranged from 50 to 55 degrees for all nanopaper composites because surface roughness (as examined via AFM analysis) is strongly associated with the solid-liquid-gas interfaces of macroporous surfaces. In another study conducted by Hou et al. [225], they concluded that as the amount the of polysiloxane nanoparticles is increased, the surface of the nanocomposite hydrogels becomes more hydrophobic at all temperatures, ranging from 10 to 40
C. This phenomenon is attributed to the strong interaction bond between the polysiloxane nanoparticles and hydrogel matrix, which hinders the water interaction with the nanocomposites. Mathew et al. [184] reported on the significant effect of plasticisation by water on the performance of tunicate nanocellulose reinforced starch composite. When the RH level is above 75%, a thin layer of water molecules may form and accumulate at the nanocellulose/amylopectin interphase. This phenomenon would restrict efficient stress between the fibre and matrix. In others words, the interfacial bond between the fibre and matrix becomes weak. This can also be proved through the contact angle measurements, where water has more attraction to cellulose as compared to the starch biopolymer. This agrees with the hypothesis that water can accumulate at the interface between cellulose and starch biopolymer [184].

20.

Raman spectroscopy (RS)

RS is commonly used in chemistry to provide a structural fingerprint by which the molecules can be identified. The crystallinity index (CI) of materials (cellulose,

Characterization studies of biopolymeric matrix and cellulose fibres

77

5500 Epoxy + MC cellulose Intensity (arbitrary units)

Epoxy 5000

4500

4000

3500 1100

1120

1140

1160

Raman wavenumber (cm–1)

Fig. 3.31 Raman spectrum for microcrystalline cellulose reinforced epoxy composite.

microcrystalline cellulose, and nanocrystalline cellulose) can be determined via different techniques including RS, X-ray diffraction (XRD), infrared (IR) spectroscopy, and solid-state 13C NMR. In RS, CI is determined as the relative intensity ratio 1 1 of the Raman line I1481 cm and I1462 cm , reflecting the amorphous and crystalline regions of materials. RS can also be used in the deformation of composite specimens. Eichhorn and Young [226] studied the Young’ modulus and micromechanical properties of a particulate form of cellulose, namely microcrystalline cellulose reinforced epoxy composite by using RS (Fig. 3.31). They concluded that RS is a powerful tool to estimate the Young’s modulus of microcrystalline cellulose reinforced epoxy composite.

21.

Concluding remarks

The novel characteristic of composite material needs to be determined when a material is combined or blended together with synthetic or natural fibre. There are a few methods that can be applied to examine the reaction in the composite at the macro or nano level, such as scanning probe microscopy (SPM), electron microscope, spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) and fourier transform infrared (FTIR) spectroscopy, secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), Raman spectroscopy (RS), ion scattering spectroscopy (ISS) and X-ray scattering. The determination of the final properties of the composite material needs to be done in order to know the mechanical properties, physical properties and interfacial bond between the fibre and polymer. Fibre interface adhesion must be determined either in fibre bonding or fibre debonding condition. Besides, fibre treatment techniques (chemical and mechanical treatments) are important because they would affect the interfacial bond between the fibre and matrix of the composites. For the detail investigation and deformation in determining the surface structures and composite compositions between the fibre and matrix, a full examination must be implemented by using AFM, RTM, STM or FESEM. There are huge

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Interfaces in Particle and Fibre Reinforced Composites

effects of fibre size on the fibre/matrix interface adhesion, and fibre loading on the mechanical properties of polymer biocomposites. The interfacial adhesion between natural fibre and polymer can be improved significantly within the composite through functionalisation treatments (mechanical or chemical treatment). The smaller the fibre, the higher the surface area of the fibre, and the better the interfacial bond between the fibre and matrix. Besides, improving the mechanical properties of biocomposites, especially their strength, involve the development of functionalisation and compatibilisation strategies. The formation of the interphase with better interfacial adhesion between the fibre and polymer can be achieved through optimum loading and stress transfer.

References [1] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Development and characterization of sugar palm nanocrystalline cellulose reinforced sugar palm starch bionanocomposites, Carbohydr. Polym. 202 (2018) 186e202, https://doi.org/10.1016/j.carbpol.2018.09.002. [2] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, M.S.N. Atikah, Characterization of Sugar Palm Nanocellulose and its Potential for Reinforcement with a Starch-Based Composite. Sugar Palm Biofibers, Biopolymers, and Biocomposites, First edition, CRC Press, Taylor & Francis Group, Boca Raton, FL , 2018, pp. 189e220, https:// doi.org/10.1201/9780429443923-10. [3] S.M. Sapuan, R.A. Ilyas, M.R. Ishak, Z. Leman, M.R.M. Huzaifah, I.M. Ammar, et al., Development of Sugar PalmeBased Products: A Community Project. Sugar Palm Biofibers, Biopolymers, and Biocomposites, first ed., CRC Press, Taylor & Francis Group, Boca Raton, FL , 2018, pp. 245e266, https://doi.org/10.1201/9780429443923-12. [4] M.L. Sanyang, R.A. Ilyas, S.M. Sapuan, R. Jumaidin, Sugar Palm Starch-Based Composites for Packaging Applications. Bionanocomposites for Packaging Applications, Springer International Publishing, Cham, 2018, pp. 125e147, https://doi.org/10.1007/ 978-3-319-67319-6_7. [5] R.A. Ilyas, S.M. Sapuan, M.L. Sanyang, M.R. Ishak, E.S. Zainudin, Nanocrystalline cellulose as reinforcement for polymeric matrix nanocomposites and its potential applications: a Review, Curr. Anal. Chem. 14 (2018) 203e225, https://doi.org/10.2174/ 1573411013666171003155624. [6] L.T. Drzal, M.J. Rich, P.F. Lloyd, Adhesion of graphite fibers to epoxy matrices: I. The role of fiber surface treatment, J. Adhes. 16 (1983) 1e30, https://doi.org/10.1080/ 00218468308074901. [7] J.-K. Kim, Y.-W. Mai, Engineered Interfaces in Fiber Reinforced Composites [Internet], first ed., Elsevier Science Ltd., 1998 https://doi.org/10.1016/B978-008042695-2/500014. [8] FAO Statistics [Internet], 2017. Available: http://faostat3.fao.org/. [9] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, Isolation and characterization of nanocrystalline cellulose from sugar palm fibres (Arenga Pinnata), Carbohydr. Polym. 181 (2018) 1038e1051, https://doi.org/10.1016/j.carbpol.2017.11.045. [10] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Effect of delignification on the physical, thermal, chemical, and structural properties of sugar palm fibre, BioResources 12 (2017) 8734e8754, https://doi.org/10.15376/biores.12.4.8734-8754.

Characterization studies of biopolymeric matrix and cellulose fibres

79

[11] D.J. Cosgrove, Growth of the plant cell wall, Nat. Rev. Mol. Cell Biol. 6 (2005) 850e861, https://doi.org/10.1038/nrm1746. [12] S.M. Sapuan, R.A. Ilyas, Sugar Palm: Fibers, Biopolymers and Biocomposites, INTROPica, Serdang, Selangor, 2017, pp. 5e7. [13] S.M. Sapuan, M.R. Ishak, Z. Leman, R.A. Ilyas, M.R.M. Huzaifah, Development of Products from Sugar Palm Trees (Arenga Pinnata Wurb. Merr): A Community Project, INTROPica, Serdang, Selangor, 2017, pp. 12e13. Available: http://www.introp.upm. edu.my/upload/dokumen/2018091416164313.5_Prof._Sapuan.pdf. [14] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, M.S.N. Atikah, M.R.M. Huzaifah, Water barrier properties of biodegradable films reinforced with nanocellulose for food packaging application : a review, in: 6th Postgraduate Seminar on Natural Fiber Reinforced Polymer Composites 2018. Serdang, Selangor, 2018, pp. 55e59. [15] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Water transport properties of bionanocomposites reinforced by sugar palm (Arenga Pinnata) nanofibrillated cellulose, J. Adv. Res. Fluid Mech. Therm. Sci. J. 51 (2018) 234e246. [16] M.L. Sanyang, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Recent developments in sugar palm (Arenga pinnata) based biocomposites and their potential industrial applications: a review, Renew. Sustain. Energy Rev. 54 (2016) 533e549, https://doi.org/ 10.1016/j.rser.2015.10.037. Elsevier. [17] S.M. Sapuan, M.R. Ishak, Z. Leman, M.R.M. Huzaifah, R.A. Ilyas, I.M. Ammar, et al., in: S.M. Sapuan (Ed.), Pokok Enau: Potensi dan pembangunan produk, first ed., Penerbit Universiti Putra Malaysia, Serdang, Selangor, 2017. [18] M.A. Martins, P.K. Kiyohara, I. Joekes, Scanning electron microscopy study of raw and chemically modified sisal fibers, J. Appl. Polym. Sci. 94 (2004) 2333e2340, https:// doi.org/10.1002/app.21203. [19] F.V. Ferreira, M. Mariano, S.C. Rabelo, R.F. Gouveia, L.M.F. Lona, Isolation and surface modification of cellulose nanocrystals from sugarcane bagasse waste: from a micro- to a nano-scale view, Appl. Surf. Sci. 436 (2018) 1113e1122, https://doi.org/10.1016/ j.apsusc.2017.12.137. [20] M.C. Jarvis, Structure of native cellulose microfibrils, the starting point for nanocellulose manufacture, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 376 (2018), https://doi.org/ 10.1098/rsta.2017.0045, 20170045. [21] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Sugar palm nanofibrillated cellulose (Arenga pinnata (Wurmb.) Merr): effect of cycles on their yield, physic-chemical, morphological and thermal behavior, Int. J. Biol. Macromol. 123 (2019) 379e388, https://doi.org/10.1016/j.ijbiomac.2018.11.124. Elsevier B.V. [22] V.I. Kovalenko, Crystalline cellulose: structure and hydrogen bonds, Russ. Chem. Rev. 79 (2010) 231e241, https://doi.org/10.1070/RC2010v079n03ABEH004065. [23] S.P. Rowland, P.S. Howley, Hydrogen bonding on accessible surfaces of cellulose from various sources and relationship to order within crystalline regions, J. Polym. Sci. A Polym. Chem. 26 (1988) 1769e1778, https://doi.org/10.1002/pola.1988.080260708. [24] A. Alemdar, M. Sain, Isolation and characterization of nanofibers from agricultural residues - wheat straw and soy hulls, Bioresour. Technol. 99 (2008) 1664e1671, https:// doi.org/10.1016/j.biortech.2007.04.029. [25] C.S. Julie Chandra, N. George, S.K. Narayanankutty, Isolation and characterization of cellulose nanofibrils from arecanut husk fibre, Carbohydr. Polym. 142 (2016) 158e166, https://doi.org/10.1016/j.carbpol.2016.01.015. Elsevier Ltd.

80

Interfaces in Particle and Fibre Reinforced Composites

[26] C.J. Chirayil, J. Joy, L. Mathew, M. Mozetic, J. Koetz, S. Thomas, Isolation and characterization of cellulose nanofibrils from Helicteres isora plant, Ind. Crop. Prod. 59 (2014) 27e34, https://doi.org/10.1016/j.indcrop.2014.04.020. Elsevier B.V. [27] B.M. Cherian, A.L. Le~ao, S.F. de Souza, S. Thomas, L.A. Pothan, M. Kottaisamy, Isolation of nanocellulose from pineapple leaf fibres by steam explosion, Carbohydr. Polym. 81 (2010) 720e725, https://doi.org/10.1016/j.carbpol.2010.03.046. [28] E. Syafri, A. Kasim, H. Abral, A. Asben, Cellulose nanofibers isolation and characterization from ramie using a chemical-ultrasonic treatment, J. Nat. Fibers 00 (2018) 1e11, https://doi.org/10.1080/15440478.2018.1455073. Taylor & Francis. [29] L.N. Megashah, H. Ariffin, M.R. Zakaria, M.A. Hassan, Properties of cellulose extract from different types of oil palm biomass, IOP Conf. Ser. Mater. Sci. Eng. 368 (2018), https://doi.org/10.1088/1757-899X/368/1/012049. [30] M. Jonoobi, A. Khazaeian, P.M. Tahir, S.S. Azry, K. Oksman, Characteristics of cellulose nanofibers isolated from rubberwood and empty fruit bunches of oil palm using chemomechanical process, Cellulose 18 (2011) 1085e1095, https://doi.org/10.1007/s10570011-9546-7. [31] A.C. Corrêa, E. de Morais Teixeira, L.A. Pessan, L.H.C. Mattoso, Cellulose nanofibers from curaua fibers, Cellulose 17 (2010) 1183e1192, https://doi.org/10.1007/s10570-0109453-3. [32] H. Tibolla, F.M. Pelissari, F.C. Menegalli, Cellulose nanofibers produced from banana peel by chemical and enzymatic treatment, LWT Food Sci. Technol. 59 (2014) 1311e1318, https://doi.org/10.1016/j.lwt.2014.04.011. Elsevier Ltd. [33] E. De Morais, T. Jessika, K. Bruna, R. Teodoro, A. Carolina, J. Manoel, et al., Sugarcane bagasse whiskers: extraction and characterizations, Ind. Crop. Prod. 33 (2011) 66, https:// doi.org/10.1016/j.indcrop.2010.08.009. Elsevier B.V. [34] M. Jonoobi, J. Harun, A. Shakeri, M. Misra, K. Oksmand, Chemical composition, crystallinity, and thermal degradation of bleached and unbleached kenaf bast (Hibiscus cannabinus) pulp and nanofibers, BioResources 4 (2009) 626e639, https://doi.org/ 10.15376/biores.4.2.626-639. [35] A. Bendahou, Y. Habibi, H. Kaddami, A. Dufresne, Physico-chemical characterization of palm from Phoenix Dactylifera-L, preparation of cellulose whiskers and natural rubberbased nanocomposites, J. Biobased Mater. Bioenergy 3 (2009) 81e90, https://doi.org/ 10.1166/jbmb.2009.1011. [36] C.H. Chan, C.H. Chia, S. Zakaria, I. Ahmad, A. Dufresne, Production and characterisation of cellulose and nano- crystalline cellulose from kenaf core wood, BioResources 8 (2013) 785e794, https://doi.org/10.15376/biores.8.1.785-794. [37] H. Abral, M.H. Dalimunthe, J. Hartono, R.P. Efendi, M. Asrofi, E. Sugiarti, et al., Characterization of tapioca starch biopolymer composites reinforced with micro scale water hyacinth fibers, Starch Staerke 70 (2018) 1e8, https://doi.org/10.1002/ star.201700287. [38] A. Alemdar, M. Sain, Biocomposites from wheat straw nanofibers: morphology, thermal and mechanical properties, Compos. Sci. Technol. 68 (2008) 557e565, https://doi.org/ 10.1016/j.compscitech.2007.05.044. [39] M. Li, L.J. Wang, D. Li, Y.L. Cheng, B. Adhikari, Preparation and characterization of cellulose nanofibers from de-pectinated sugar beet pulp, Carbohydr. Polym. 102 (2014) 136e143, https://doi.org/10.1016/j.carbpol.2013.11.021. Elsevier Ltd. [40] R.M. Sheltami, I. Abdullah, I. Ahmad, A. Dufresne, H. Kargarzadeh, Extraction of cellulose nanocrystals from mengkuang leaves (Pandanus tectorius), Carbohydr. Polym. 88 (2012) 772e779, https://doi.org/10.1016/j.carbpol.2012.01.062. Elsevier Ltd.

Characterization studies of biopolymeric matrix and cellulose fibres

81

[41] A. Dufresne, Nanocellulose: a new ageless bionanomaterial, Mater. Today 16 (2013) 220e227, https://doi.org/10.1016/j.mattod.2013.06.004. Elsevier Ltd. [42] I. Capron, O.J. Rojas, R. Bordes, Behavior of nanocelluloses at interfaces, Curr. Opin. Colloid Interface Sci. 29 (2017) 83e95, https://doi.org/10.1016/j.cocis.2017.04.001. Elsevier Ltd. [43] R.A. Ilyas, S.M. Sapuan, M.L. Sanyang, M.R. Ishak, Nanocrystalline Cellulose Reinforced Starch-Based Nanocomposite: A Review. 5th Postgraduate Seminar on Natural Fiber Composites, Universiti Putra Malaysia, Serdang, Selangor, 2016, pp. 82e87. Available: https://www.researchgate.net/publication/315675302_Nanocrystalline_ cellulose_reinforced_starch-based_nanocomposites_A_Review. [44] R.A. Ilyas, S.M. Sapuan, R. Ibrahim, H. Abral, M.R. Ishak, E.S. Zainudin, et al., Sugar palm (Arenga pinnata (Wurmb.) Merr) cellulosic fibre hierarchy: a comprehensive approach from macro to nano scale, J. Mater. Res. Technol. (2019), https://doi.org/ 10.1016/j.jmrt.2019.04.011. Korea Institute of Oriental Medicine. [45] I. Usov, G. Nystr€om, J. Adamcik, S. Handschin, C. Sch€ utz, A. Fall, et al., Understanding nanocellulose chirality and structureeproperties relationship at the single fibril level, Nat. Commun. 6 (2015) 7564, https://doi.org/10.1038/ncomms8564. [46] C. Driemeier, M.T.B. Pimenta, G.J.M. Rocha, M.M. Oliveira, D.B. Mello, P. Maziero, et al., Evolution of cellulose crystals during prehydrolysis and soda delignification of sugarcane lignocellulose, Cellulose 18 (2011) 1509e1519, https://doi.org/10.1007/ s10570-011-9592-1. [47] M. Jacob, S. Thomas, Biofibers and biocomposites, Carbohydr. Polym. (2008) 343e364. [48] S. Mondal, Preparation, properties and applications of nanocellulosic materials, Carbohydr. Polym. (2017), https://doi.org/10.1016/j.carbpol.2016.12.050. [49] A. Nazrin, S.M. Sapuan, R.A. Ilyas, Thermoplastic starch blended poly(lactic) acid for Food packaging application : mechanical properties, in: 6th Postgraduate Seminar on Natural Fiber Reinforced Polymer Composites 2018, Serdang, 2018, pp. 79e84. [50] M.D. Hazrol, S.M. Sapuan, R.A. Ilyas, Electrical and surface resistivity of polymer composites : a review, in: 6th Postgraduate Seminar on Natural Fiber Reinforced Polymer Composites 2018, Serdang, Selangor, 2018, pp. 44e47. [51] R. Syafiq, S.M. Sapuan, R.A. Ilyas, Starch-based polymer film incorporated with antimicrobial plant essential oil : a review, in: 6th Postgraduate Seminar on Natural Fiber Reinforced Polymer Composites 2018, Serdang, Selangor, 2018, pp. 74e78. [52] M.M. Reddy, S. Vivekanandhan, M. Misra, S.K. Bhatia, A.K. Mohanty, Biobased plastics and bionanocomposites: current status and future opportunities, Prog. Polym. Sci. 38 (2013) 1653e1689, https://doi.org/10.1016/j.progpolymsci.2013.05.006. Elsevier Ltd. [53] M.R.M. Huzaifah, S.M. Sapuan, Z. Leman, M.R. Ishak, R.A. Ilyas, Effect of soil burial on water absorption of sugar palm fibre reinforced vinyl ester composites, in: 6th Postgraduate Seminar on Natural Fiber Reinforced Polymer Composites 2018, Serdang, Selangor, 2018, pp. 52e54. [54] N. Mazani, S.M. Sapuan, M.L. Sanyang, A. Atiqah, R.A. Ilyas, Design and fabrication of a shoe shelf from kenaf fiber reinforced unsaturated polyester composites, Lignocellul. Fut. Bioecon. (2019) 315e332, https://doi.org/10.1016/B978-0-12-816354-2.00017-7. Elsevier. [55] A.K. Mohanty, M. Misra, G. Hinrichsen, Biofibres, biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng. 276e277 (2000) 1e24, https://doi.org/ 10.1002/(SICI)1439-2054(20000301)276:13.0.CO;2-W.

82

Interfaces in Particle and Fibre Reinforced Composites

[56] L. Yu, K. Dean, L. Li, Polymer blends and composites from renewable resources, Prog. Polym. Sci. 31 (2006) 576e602, https://doi.org/10.1016/j.progpolymsci.2006.03.002. [57] T. Huber, J. M€ussig, O. Curnow, S. Pang, S. Bickerton, M.P. Staiger, A critical review of all-cellulose composites, J. Mater. Sci. 47 (2012) 1171e1186, https://doi.org/10.1007/ s10853-011-5774-3. [58] G. Siqueira, J. Bras, A. Dufresne, Cellulosic bionanocomposites: a review of preparation, properties and applications, Polymers 2 (2010) 728e765, https://doi.org/10.3390/ polym2040728. [59] A. Dufresne, Processing of polymer nanocomposites reinforced with cellulose nanocrystals: a challenge, Int. Polym. Process. 27 (2012) 557e564, https://doi.org/10.3139/ 217.2603. [60] M.M. Reddy, M.M.A. Misra, Bio-based materials in the new bio-economy, Chem. Eng. Prog. 108 (2012) 37e42. [61] E.J. Foster, R.J. Moon, U.P. Agarwal, M.J. Bortner, J. Bras, S. Camarero-Espinosa, et al., Current characterization methods for cellulose nanomaterials, Chem. Soc. Rev. 47 (2018) 2609e2679, https://doi.org/10.1039/C6CS00895J. [62] I. Ahmad, A. Baharum, I. Abdullah, Effect of extrusion rate and fiber loading on mechanical properties of twaron Fiber-Thermoplastic Natural Rubber (TPNR) composites, J. Reinf. Plast. Compos. 25 (2006) 957e965, https://doi.org/10.1177/ 0731684406065082. [63] Y.A. El-Shekeil, S.M. Sapuan, K. Abdan, E.S. Zainudin, Influence of fiber content on the mechanical and thermal properties of kenaf fiber reinforced thermoplastic polyurethane composites, Mater. Des. 40 (2012) 299e303, https://doi.org/10.1016/ j.matdes.2012.04.003. Elsevier Ltd. € [64] S. Ozturk, Effect of fiber loading on the mechanical properties of kenaf and fiberfrax fiber-reinforced phenol-formaldehyde composites, J. Compos. Mater. 44 (2010) 2265e2288, https://doi.org/10.1177/0021998310364265. [65] V. Vilay, M. Mariatti, R. Mat Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber-reinforced unsaturated polyester composites, Compos. Sci. Technol. 68 (2008) 631e638, https://doi.org/10.1016/ j.compscitech.2007.10.005. [66] M.S. Sobhy, M.T. Tammam, The influence of fiber length and concentration on the physical properties of wheat husk fibers rubber composites, Int. J. Polym. Sci. 2010 (2010), https://doi.org/10.1155/2010/528173. [67] S. Shibata, Y. Cao, I. Fukumoto, Press forming of short natural fiber-reinforced biodegradable resin: effects of fiber volume and length on flexural properties, Polym. Test. 24 (2005) 1005e1011, https://doi.org/10.1016/j.polymertesting.2005.07.012. [68] M.M. Thwe, K. Liao, Environmental effects on bamboo-glass/polypropylene hybrid composites, J. Mater. Sci. 38 (2003) 363e376, https://doi.org/10.1023/A:1021130019435. [69] M.N. Akhtar, A.B. Sulong, M.K.F. Radzi, N.F. Ismail, M.R. Raza, N. Muhamad, et al., Influence of alkaline treatment and fiber loading on the physical and mechanical properties of kenaf/polypropylene composites for variety of applications, Prog. Nat. Sci. Mater. Int. 26 (2016) 657e664, https://doi.org/10.1016/j.pnsc.2016.12.004. Elsevier. [70] Y.A. El-Shekeil, M.S. Salit, K. Abdan, E.S. Zainudin, Development of a new kenaf bast fiber-reinforced thermoplastic polyurethane composite, BioResources 6 (2011) 4662e4672. [71] D.S. LeCorre, J. Bras, A. Dufresne, Influence of the botanic origin of starch nanocrystals on the morphological and mechanical properties of natural rubber nanocomposites, Macromol. Mater. Eng. 297 (2012) 969e978, https://doi.org/10.1002/mame.201100317.

Characterization studies of biopolymeric matrix and cellulose fibres

83

[72] A. Kutvonen, G. Rossi, S.R. Puisto, N.K.J. Rostedt, T. Ala-Nissila, Influence of nanoparticle size, loading, and shape on the mechanical properties of polymer nanocomposites, J. Chem. Phys. 137 (2012) 214901, https://doi.org/10.1063/1.4767517. [73] K.S. Kumar, I. Siva, N. Rajini, P. Jeyaraj, J.W. Jappes, Tensile, impact, and vibration properties of coconut sheath/sisal hybrid composites: effect of stacking sequence, J. Reinf. Plast. Compos. 33 (2014) 1802e1812, https://doi.org/10.1177/ 0731684414546782. [74] N.U.R. Kahirul, N. Kamaruddin, I. Abdullah, I. Ahmad, Reinforcement of thermoplastics natural rubber composite with liquid epoxidized natural rubber treated, Torch Ginger Fiber 1 (2015) 25e36. [75] M. Thiruchitrambalam, A. Alavudeen, A. Athijayamani, N. Venkateshwaran, A.E. Perumal, Improving mechanical properties of banana/kenaf polyester hybrid composites using sodium laulryl sulfate treatment, Mater. Phys. Mech. 8 (2009) 165e173. [76] A. Bentur, S. Mindess, Fibre Reinforced Cementitious Composites, second ed., Taylor & Francis, 2007. [77] M.A. Abu Bakar, S. Ahmad, S. Kasolang, M.A. Ahmad, N.R. Nik Roseley, S. Norazlini, et al., Characterization of rubber toughened epoxy reinforced hybrid kenaf/carbon fiber via water absorption and thermal degradation, J. Teknol. 76 (2015) 81e85, https:// doi.org/10.11113/jt.v76.5518. [78] Z. Salleh, K.M. Hyie, M.N. Berhan, Y.M. Taib, N.R. Nik Roseley, A.R.M. Razib, Effect of water absorption on mechanical properties of kenaf and kenaf hybrid polyester, Appl. Mech. Mater. 465e466 (2013) 967e972, https://doi.org/10.4028/www.scientific.net/ AMM.465-466.967. [79] R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Thermal, mechanical, and physical properties of seaweed/sugar palm fibre reinforced thermoplastic sugar palm Starch/Agar hybrid composites, Int. J. Biol. Macromol. 97 (2017) 606e615, https:// doi.org/10.1016/j.ijbiomac.2017.01.079. Elsevier B.V. [80] H.D. Rozman, Y.S. Yeo, G.S. Tay, A. Abubakar, The mechanical and physical properties of polyurethane composites based on rice husk and polyethylene glycol, Polym. Test. 22 (2003) 617e623, https://doi.org/10.1016/S0142-9418(02)00165-4. [81] I.S.M.A. Tawakkal, R.A. Talib, K. Abdan, C.N. Ling, Mechanical and physical properties of kenaf-derived cellulose (KDC)-filled polylactic acid (PLA) composites, BioResources 7 (2012) 1643e1655, https://doi.org/10.15376/biores.7.2.1643-1655. [82] S.H. Aziz, M.P. Ansell, The effect of alkalization and fibre alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites: part 1 - polyester resin matrix, Compos. Sci. Technol. 64 (2004) 1219e1230, https://doi.org/10.1016/ j.compscitech.2003.10.001. [83] F.V. Ferreira, I.F. Pinheiro, M. Mariano, L.S. Cividanes, J.C.M. Costa, N.R. Nascimento, et al., Environmentally friendly polymer composites based on PBAT reinforced with natural fibers from the amazon forest, Polym. Compos. (2019), https://doi.org/10.1002/ pc.25196. [84] A.M.N. Azammi, S.M. Sapuan, M.R. Ishak, M.T.H. Sultan, Mechanical properties of kenaf fibre thermoplastic polyurethane-natural rubber composites, Polimery 63 (2018) 45, https://doi.org/10.14314/polimery.2018.7.6. [85] H. Anuar, S.H. Ahmad, R. Rasid, S. Zakaria, Tensile behavior and morphological studies of TPNR-KF-CF hybrid composite, Am. J. Appl. Sci. (2005). Special Is: 34e39. [86] C. Vajrasthira, T. Amornsakchai, S. Bualek-Limcharoen, Fiber-matrix interactions in aramid-short-fiber-reinforced thermoplastic polyurethane composites, J. Appl. Polym. Sci. 87 (2003) 1059e1067, https://doi.org/10.1002/app.11484.

84

Interfaces in Particle and Fibre Reinforced Composites

[87] H. Anuar, A. Zuraida, Improvement in mechanical properties of reinforced thermoplastic elastomer composite with kenaf bast fibre, Compos. Part B Eng. 42 (2011) 462e465, https://doi.org/10.1016/j.compositesb.2010.12.013. Elsevier Ltd. [88] A. Ismail, A. Hassan, A. Abu Bakar, M. Jawaid, Flame retardancy and mechanical properties of kenaf filled polypropylene (PP) containing ammonium polyphosphate (APP), Sains Malays. 42 (2013) 429e434. [89] I.S. Aji, E.S. Zainudin, A. Khalina, S.M. Sapuan, M.D. Khairul, Thermal property determination of hybridized kenaf/PALF reinforced HDPE composite by thermogravimetric analysis, J. Therm. Anal. Calorim. 109 (2012) 893e900, https://doi.org/10.1007/ s10973-011-1807-z. [90] Z.N. Azwa, B.F. Yousif, Characterisation of kenaf fibre/epoxy composites subjected to thermal degradation, in: Malaysian Postgraduate Conference (MPC), 2013, pp. 256e264. [91] Y. Peng, D.J. Gardner, Y. Han, A. Kiziltas, Z. Cai, M.A. Tshabalala, Influence of drying method on the material properties of nanocellulose I: thermostability and crystallinity, Cellulose 20 (2013) 2379e2392, https://doi.org/10.1007/s10570-013-0019-z. [92] Y.A. El-Shekeil, S.M. Sapuan, A. Khalina, E.S. Zainudin, O.M. Al-Shuja’A, Effect of alkali treatment on mechanical and thermal properties of kenaf fiber-reinforced thermoplastic polyurethane composite, J. Therm. Anal. Calorim. 109 (2012) 1435e1443, https:// doi.org/10.1007/s10973-012-2258-x. [93] D. L
opez-Gonz
alez, M. Fernandez-Lopez, J.L. Valverde, L. Sanchez-Silva, Thermogravimetric-mass spectrometric analysis on combustion of lignocellulosic biomass, Bioresour. Technol. 143 (2013) 562e574, https://doi.org/10.1016/ j.biortech.2013.06.052. [94] S.Y. Yorulmaz, A.T. Atimtay, Investigation of combustion kinetics of treated and untreated waste wood samples with thermogravimetric analysis, Fuel Process. Technol. 90 (2009) 939e946, https://doi.org/10.1016/j.fuproc.2009.02.010. [95] M. Mariano, R. Cercen
a, V. Soldi, Thermal characterization of cellulose nanocrystals isolated from sisal fibers using acid hydrolysis, Ind. Crop. Prod. 94 (2016) 454e462, https://doi.org/10.1016/j.indcrop.2016.09.011. [96] Z.N. Azwa, B.F. Yousif, Thermal degradation study of kenaf fibre/epoxy composites using thermo gravimetric analysis, in: 3rd Malaysian Postgraduate Conference (MPC2013), 2013, pp. 256e264. [97] N. Sarifuddin, H. Ismail, Z. Ahmad, The effect of kenaf core fibre loading on properties of low density polyethylene/thermoplastic sago starch/kenaf core fiber composites, J. Phys. Sci. 24 (2013) 97e115, https://doi.org/10.4028/www.scientific.net/AMR.626.1048. [98] M.C. Khoathane, O.C. Vorster, E.R. Sadiku, Hemp fiber-reinforced 1-pentene/polypropylene copolymer: the effect of fiber loading on the mechanical and thermal characteristics of the composites, J. Reinf. Plast. Compos. 27 (2008) 1533e1544, https:// doi.org/10.1177/0731684407086325. [99] A.M. Noor Azammi, S.M. Sapuan, M.R. Ishak, M.T.H. Sultan, Mechanical and thermal properties of kenaf reinforced thermoplastic polyurethane (TPU)-natural rubber (NR) composites, Fibers Polym. 19 (2018) 446e451, https://doi.org/10.1007/s12221-018-7737-7. [100] L. Avérous, N. Boquillon, Biocomposites based on plasticized starch: thermal and mechanical behaviour, Carbohydr. Polym. 56 (2004) 111e122, https://doi.org/10.1016/ j.carbpol.2003.11.015. [101] M.J. Halimatul, S.M. Sapuan, M. Jawaid, M.R. Ishak, R.A. Ilyas, Effect of sago starch and plasticizer content on the properties of thermoplastic films: mechanical testing and cyclic soaking-drying, Polimery 64 (2019) 32e41, https://doi.org/10.14314/ polimery.2019.6.5.

Characterization studies of biopolymeric matrix and cellulose fibres

85

[102] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Sugar palm nanocrystalline cellulose reinforced sugar palm starch composite: degradation and water-barrier properties, IOP Conf. Ser. Mater. Sci. Eng. (2018), https://doi.org/10.1088/1757-899X/368/1/012006. [103] H.M. Hassanabadi, D. Rodrigue, Effect of particle size and shape on the reinforcing efficiency of nanoparticles in polymer nanocomposites, Macromol. Mater. Eng. 299 (2014) 1220e1231, https://doi.org/10.1002/mame.201300442. [104] G. Siqueira, J. Bras, A. Dufresne, Cellulose whiskers versus microfibrils: influence of the nature of the nanoparticle and its surface functionalization on the thermal and mechanical properties of nanocomposites, Biomacromolecules 10 (2009) 425e432, https://doi.org/ 10.1021/bm801193. [105] S.-Y. Fu, X.-Q. Feng, B. Lauke, Y.-W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulateepolymer composites, Compos. Part B Eng. 39 (2008) 933e961, https://doi.org/10.1016/ j.compositesb.2008.01.002. [106] M.L. Sanyang, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Effect of sugar palmderived cellulose reinforcement on the mechanical and water barrier properties of sugar palm starch biocomposite films, BioResources 11 (2016) 4134e4145, https://doi.org/ 10.15376/biores.11.2.4134-4145. [107] A. Clarke, A.A. Vasileiou, M. Kontopoulou, Crystalline nanocellulose in biodegradable polyester nanocomposites prepared by in situ polymerization, in: AIP Conference Proceedings, 2017, p. 1914, https://doi.org/10.1063/1.5016697. [108] M.T. Islam, M.M. Alam, M. Zoccola, Review on modification of nanocellulose for application in composites, Int. J. Innov. Res. Sci. Eng. Technol. 2 (2013) 5444e5451. [109] F. Alloin, A. D’Aprea, A. Dufresne, N. El Kissi, F. Bossard, Poly(oxyethylene) and ramie whiskers based nanocomposites: influence of processing: extrusion and casting/evaporation, Cellulose 18 (2011) 957e973, https://doi.org/10.1007/s10570-011-9543-x. [110] J. Sapkota, S. Kumar, C. Weder, E.J. Foster, Influence of processing conditions on properties of poly (vinyl acetate)/cellulose nanocrystal nanocomposites, Macromol. Mater. Eng. 300 (2015) 562e571, https://doi.org/10.1002/mame.201400313. [111] X. Cao, Y. Chen, P.R. Chang, A.D. Muir, G. Falk, Starch-based nanocomposites reinforced with flax cellulose nanocrystals, Express Polym. Lett. 2 (2008) 502e510, https:// doi.org/10.3144/expresspolymlett.2008.60. [112] D. Chen, D. Lawton, M.R. Thompson, Q. Liu, Biocomposites reinforced with cellulose nanocrystals derived from potato peel waste, Carbohydr. Polym. 90 (2012) 709e716, https://doi.org/10.1016/j.carbpol.2012.06.002. [113] E.C. Ramires, A. Dufresne, A review of cellulose nanocrystals and nanocomposites, Tappi J. 10 (2011) 9e16. [114] G.H.D. Tonoli, A.P. Joaquim, M.-A. Arsene, K. Bilba, H. Savastano, Performance and durability of cement based composites reinforced with refined sisal pulp, Mater. Manuf. Process. 22 (2007) 149e156, https://doi.org/10.1080/10426910601062065. [115] T.J. Gutiérrez, V.A. Alvarez, Cellulosic materials as natural fillers in starch-containing matrix-based films: a review, Polym. Bull. 74 (2017) 2401e2430, https://doi.org/ 10.1007/s00289-016-1814-0. Springer Berlin Heidelberg. [116] L. Zhuang, A. Pupurs, J. Varna, R. Talreja, Z. Ayadi, Effects of inter-fiber spacing on fiber-matrix debond crack growth in unidirectional composites under transverse loading, Compos. Appl. Sci. Manuf. 109 (2018) 463e471, https://doi.org/10.1016/ j.compositesa.2018.03.031.

86

Interfaces in Particle and Fibre Reinforced Composites

[117] A. Awal, M. Rana, M. Sain, Thermorheological and mechanical properties of cellulose reinforced PLA bio-composites, Mech. Mater. 80 (2015) 87e95, https://doi.org/10.1016/ j.mechmat.2014.09.009. [118] H. Abral, G.J. Putra, M. Asrofi, J.W. Park, H.J. Kim, Effect of vibration duration of high ultrasound applied to bio-composite while gelatinized on its properties, Ultrason. Sonochem. 40 (2018) 697e702, https://doi.org/10.1016/j.ultsonch.2017.08.019. [119] M. Asrofi, H. Abral, Y.K. Putra, S.M. Sapuan, H.J. Kim, Effect of duration of sonication during gelatinization on properties of tapioca starch water hyacinth fiber biocomposite, Int. J. Biol. Macromol. 108 (2018) 167e176, https://doi.org/10.1016/j.ijbiomac.2017.11.165. Elsevier B.V. [120] A. Dufresne, J.-Y. Cavaill, M.R. Vignon, Mechanical behavior of sheets prepared from sugar beet cellulose microfibrils, J. Appl. Polym. Sci. 64 (1997) 1185e1194, https:// doi.org/10.1002/(SICI)1097-4628(19970509)64:63.0.CO;2-V. [121] T. Saito, S. Kimura, Y. Nishiyama, A. Isogai, Cellulose nanofibers prepared by TEMPOmediated oxidation of native cellulose, Biomacromolecules 8 (2007) 2485e2491. [122] S. Iwamoto, K. Abe, H. Yano, The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics, Biomacromolecules 9 (2008) 1022e1026, https:// doi.org/10.1021/bm701157n. [123] M. Henriksson, L.A. Berglund, Structure and properties of cellulose nanocomposite films containing melamine formaldehyde, J. Appl. Polym. Sci. 106 (2007) 2817e2824, https:// doi.org/10.1002/app.26946. [124] J.K. Pandey, H. Takagi, A.N. Nakagaito, H.-J. Kim (Eds.), Handbook of Polymer Nanocomposites. Processing, Performance and Application [Internet], Springer Berlin Heidelberg, Berlin, Heidelberg, 2015, https://doi.org/10.1007/978-3-642-45232-1. [125] L. Berglund, New concepts in natural fiber composites, in: Proceedings of the 27th Ris¢ International Symposium on Materials Science: Polymer Composite Materials for Wind Power Turbines, 2006, pp. 1e9. [126] M. Mondrag
on, K. Arroyo, J. Romero-García, Biocomposites of thermoplastic starch with surfactant, Carbohydr. Polym. 74 (2008) 201e208, https://doi.org/10.1016/ j.carbpol.2008.02.004. [127] I. Sir
o, D. Plackett, Microfibrillated cellulose and new nanocomposite materials: a review, Cellulose 17 (2010) 459e494, https://doi.org/10.1007/s10570-010-9405-y. [128] M.A.S.A. Samir, F. Alloin, J.Y. Sanchez, A. Dufresne, Cross-linked nanocomposite polymer electrolytes reinforced with cellulose whiskers, Macromolecules 37 (2004) 4839e4844, https://doi.org/10.1021/ma049504y. [129] N. Soykeabkaew, N. Laosat, A. Ngaokla, N. Yodsuwan, T. Tunkasiri, Reinforcing potential of micro- and nano-sized fibers in the starch-based biocomposites, Compos. Sci. Technol. 72 (2012) 845e852, https://doi.org/10.1016/j.compscitech.2012.02.015. Elsevier Ltd. [130] N. Lavoine, I. Desloges, A. Dufresne, J. Bras, Microfibrillated cellulose - its barrier properties and applications in cellulosic materials: a review, Carbohydr. Polym. 90 (2012) 735e764, https://doi.org/10.1016/j.carbpol.2012.05.026. Elsevier Ltd. [131] J. Biener, A. Wittstock, T.F. Baumann, J. Weissm€uller, M. B€aumer, A.V. Hamza, Surface chemistry in nanoscale materials, Materials 2 (2009) 2404e2428, https://doi.org/ 10.3390/ma2042404. [132] W. Chen, H. Yu, S.Y. Lee, T. Wei, J. Li, Z. Fan, Nanocellulose: a promising nanomaterial for advanced electrochemical energy storage, Chem. Soc. Rev. 47 (2018) 2837e2872, https://doi.org/10.1039/c7cs00790f. Royal Society of Chemistry.

Characterization studies of biopolymeric matrix and cellulose fibres

87

[133] M. Asrofi, H. Abral, A. Kasim, A. Pratoto, M. Mahardika, J.W. Park, et al., Isolation of nanocellulose from water hyacinth fiber (WHF) produced via digester-sonication and its characterization, Fibers Polym. 19 (2018) 1618e1625, https://doi.org/10.1007/s12221018-7953-1. [134] N.S. Lani, N. Ngadi, A. Johari, M. Jusoh, N.S. Lani, N. Ngadi, A. Johari MJ, Isolation, characterization and application of nanocellulose from oil palm empty fruit bunch, J. Nanomater. (2014), https://doi.org/10.1155/2014/702538. [135] M. Mahardika, H. Abral, A. Kasim, S. Arief, M. Asrofi, Production of nanocellulose from pineapple leaf fibers via high-shear homogenization and ultrasonication, Fibers 6 (2018) 28, https://doi.org/10.3390/fib6020028. [136] F.A. Sabaruddin, M.T. Paridah, Effect of lignin on the thermal properties of nanocrystalline prepared from kenaf core, IOP Conf. Ser. Mater. Sci. Eng. 368 (2018) 012039, https://doi.org/10.1088/1757-899X/368/1/012039. [137] J. Li, Z. Song, D. Li, S. Shang, Y. Guo, Cotton cellulose nanofiber-reinforced high density polyethylene composites prepared with two different pretreatment methods, Ind. Crop. Prod. 59 (2014) 318e328, https://doi.org/10.1016/j.indcrop.2014.05.033. [138] D. Bhattacharya, L.T. Germinario, W.T. Winter, Isolation, preparation and characterization of cellulose microfibers obtained from bagasse, Carbohydr. Polym. 73 (2008) 371e377, https://doi.org/10.1016/j.carbpol.2007.12.005. [139] N.R. Savadekar, S.T. Mhaske, Synthesis of nano cellulose fibers and effect on thermoplastics starch based films, Carbohydr. Polym. 89 (2012) 146e151, https://doi.org/ 10.1016/j.carbpol.2012.02.063. [140] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, M.S.N. Atikah, Nanocellulose reinforced starch polymer composites : a review of preparation, properties and application, in: Proceeding: 5th International Conference on Applied Sciences and Engineering (ICASEA, 2018), Global Academic Excellence (M) Sdn Bhd, Capthorne Hotel, Cameron Highlands, Malaysia, 2018, pp. 325e341. [141] T. Abitbol, A. Rivkin, Y. Cao, Y. Nevo, E. Abraham, T. Ben-Shalom, et al., Nanocellulose, a tiny fiber with huge applications, Curr. Opin. Biotechnol. 39 (2016) 76e88, https://doi.org/10.1016/j.copbio.2016.01.002. Elsevier Ltd. [142] H. Abral, A. Basri, F. Muhammad, Y. Fernando, F. Hafizulhaq, M. Mahardika, et al., A simple method for improving the properties of the sago starch films prepared by using ultrasonication treatment, Food Hydrocolloids 93 (2019) 276e283, https://doi.org/ 10.1016/j.foodhyd.2019.02.012. [143] R.A. Ilyas, S.M. Sapuan, M.N. Norizan, M.S.N. Atikah, M.R.M. Huzaifah, A.M. Radzi, et al., Potential of natural fibre composites for transport industry : a review, in: S.M. Sapuan, Z. Leman, E.S. Zainudin, M.Y.M. Zuhri, R.A. Ilyas (Eds.), Prosiding Seminar Enau Kebangsaan 2019, Institute of Tropical Forest and Forest Products (INTROP), Universiti Putra Malaysia, Bahau, Negeri Sembilan, Malaysia, 2019, pp. 2e11. [144] K. Prakobna, S. Galland, L.A. Berglund, High-performance and moisture-stable celluloseestarch nanocomposites based on bioinspired coreeshell nanofibers, Biomacromolecules 16 (2015) 904e912, https://doi.org/10.1021/bm5018194. [145] S. Xiao, R. Gao, L. Gao, J. Li, Poly(vinyl alcohol) films reinforced with nanofibrillated cellulose (NFC) isolated from corn husk by high intensity ultrasonication, Carbohydr. Polym. 136 (2016) 1027e1034, https://doi.org/10.1016/j.carbpol.2015.09.115. [146] A.A. Oun, J.-W. Rhim, Preparation and characterization of sodium carboxymethyl cellulose/cotton linter cellulose nanofibril composite films, Carbohydr. Polym. 127 (2015) 101e109, https://doi.org/10.1016/j.carbpol.2015.03.073.

88

Interfaces in Particle and Fibre Reinforced Composites

[147] N. Volk, R. He, K. Magniez, Enhanced homogeneity and interfacial compatibility in melt-extruded cellulose nano-fibers reinforced polyethylene via surface adsorption of poly(ethylene glycol)- block -poly(ethylene) amphiphiles, Eur. Polym. J. 72 (2015) 270e281, https://doi.org/10.1016/j.eurpolymj.2015.09.025. [148] N. Farahbakhsh, R.A. Venditti, J.S. Jur, Mechanical and thermal investigation of thermoplastic nanocomposite films fabricated using micro- and nano-sized fillers from recycled cotton T-shirts, Cellulose 21 (2014) 2743e2755, https://doi.org/10.1007/ s10570-014-0285-4. [149] H. Tang, N. Butchosa, Q. Zhou, A transparent, hazy, and strong macroscopic ribbon of oriented cellulose nanofibrils bearing poly(ethylene glycol), Adv. Mater. 27 (2015) 2070e2076, https://doi.org/10.1002/adma.201404565. [150] P.A. Larsson, L.A. Berglund, L. Wågberg, Ductile all-cellulose nanocomposite films fabricated from coreeshell structured cellulose nanofibrils, Biomacromolecules 15 (2014) 2218e2223, https://doi.org/10.1021/bm500360c. [151] T. Kurihara, A. Isogai, Mechanism of TEMPO-oxidized cellulose nanofibril film reinforcement with poly(acrylamide), Cellulose 22 (2015) 2607e2617, https://doi.org/ 10.1007/s10570-015-0680-5. [152] L. Timhadjelt, A. Serier, M.N. Belgacem, J. Bras, Elaboration of cellulose based nanobiocomposite: effect of cellulose nanocrystals surface treatment and interface “melting”, Ind. Crop. Prod. 72 (2015) 7e15, https://doi.org/10.1016/j.indcrop.2015.02.040. Elsevier B.V. [153] J. Letchford, B. Wasserman, H.W. Ye, H. Burt, The use of nanocrystalline cellulose for the binding and controlled release of drugs, Int. J. Nanomed. (2011) 321, https://doi.org/ 10.2147/IJN.S16749. [154] P. Ma, L. Jiang, M. Hoch, W. Dong, M. Chen, Reinforcement of transparent ethylene-covinyl acetate rubber by nanocrystalline cellulose, Eur. Polym. J. 66 (2015) 47e56, https:// doi.org/10.1016/j.eurpolymj.2015.01.037. Elsevier Ltd. [155] C. Zhou, Q. Shi, W. Guo, L. Terrell, A.T. Qureshi, D.J. Hayes, et al., Electrospun bionanocomposite scaffolds for bone tissue engineering by cellulose nanocrystals reinforcing maleic anhydride grafted PLA, ACS Appl. Mater. Interfaces 5 (2013) 3847e3854, https://doi.org/10.1021/am4005072. [156] J.R. McKee, S. Hietala, J. Seitsonen, J. Laine, E. Kontturi, O. Ikkala, Thermoresponsive nanocellulose hydrogels with tunable mechanical properties, ACS Macro Lett. 3 (2014) 266e270, https://doi.org/10.1021/mz400596g. [157] M. Mariano, N. El Kissi, A. Dufresne, Melt processing of cellulose nanocrystal reinforced polycarbonate from a masterbatch process, Eur. Polym. J. 69 (2015) 208e223, https:// doi.org/10.1016/j.eurpolymj.2015.06.007. Elsevier Ltd. [158] Y. Li, H. Chen, D. Liu, W. Wang, Y. Liu, S. Zhou, pH-responsive shape memory poly(ethylene glycol)ePoly(ε-caprolactone)-based polyurethane/cellulose nanocrystals nanocomposite, ACS Appl. Mater. Interfaces 7 (2015) 12988e12999, https://doi.org/ 10.1021/acsami.5b02940. [159] N. Herrera, A.M. Salaberria, A.P. Mathew, K. Oksman, Plasticized polylactic acid nanocomposite films with cellulose and chitin nanocrystals prepared using extrusion and compression molding with two cooling rates: effects on mechanical, thermal and optical properties, Compos. Appl. Sci. Manuf. 83 (2016) 89e97, https://doi.org/10.1016/ j.compositesa.2015.05.024. Elsevier Ltd. [160] R. Nasseri, N. Mohammadi, Starch-based nanocomposites: a comparative performance study of cellulose whiskers and starch nanoparticles, Carbohydr. Polym. 106 (2014) 432e439, https://doi.org/10.1016/j.carbpol.2014.01.029. Elsevier Ltd.

Characterization studies of biopolymeric matrix and cellulose fibres

89

[161] J.C. Liu, D.J. Martin, R.J. Moon, J.P. Youngblood, Enhanced thermal stability of biomedical thermoplastic polyurethane with the addition of cellulose nanocrystals, J. Appl. Polym. Sci. 132 (2015) 1e8, https://doi.org/10.1002/app.41970. [162] B. Wang, A. Walther, Self-assembled, iridescent, crustacean-mimetic nanocomposites with tailored periodicity and layered cuticular structure, ACS Nano 9 (2015) 10637e10646, https://doi.org/10.1021/acsnano.5b05074. [163] B. Schyrr, S. Pasche, G. Voirin, C. Weder, Y.C. Simon, E.J. Foster, Biosensors based on porous cellulose nanocrystalepoly(vinyl alcohol) scaffolds, ACS Appl. Mater. Interfaces 6 (2014) 12674e12683, https://doi.org/10.1021/am502670u. [164] S. Montes, P.M. Carrasco, V. Ruiz, G. Caba~nero, H.J. Grande, J. Labidi, et al., Synergistic reinforcement of poly(vinyl alcohol) nanocomposites with cellulose nanocrystalstabilized graphene, Compos. Sci. Technol. 117 (2015) 26e31, https://doi.org/10.1016/ j.compscitech.2015.05.018. [165] S. Yang, Y. Tang, J. Wang, F. Kong, J. Zhang, Surface treatment of cellulosic paper with starch-based composites reinforced with nanocrystalline cellulose, Ind. Eng. Chem. Res. 53 (2014) 13980e13988, https://doi.org/10.1021/ie502125s. [166] A.M. Slavutsky, M.A. Bertuzzi, Water barrier properties of starch films reinforced with cellulose nanocrystals obtained from sugarcane bagasse, Carbohydr. Polym. 110 (2014) 53e61, https://doi.org/10.1016/j.carbpol.2014.03.049. Elsevier Ltd. [167] D. Liu, T. Zhong, P.R. Chang, K. Li, Q. Wu, Starch composites reinforced by bamboo cellulosic crystals, Bioresour. Technol. 101 (2010) 2529e2536, https://doi.org/10.1016/ j.biortech.2009.11.058. Elsevier Ltd. [168] E.M. de Teixeira, D. Pasquini, A.A.S.S. Curvelo, E. Corradini, M.N. Belgacem, A. Dufresne, Cassava bagasse cellulose nanofibrils reinforced thermoplastic cassava starch, Carbohydr. Polym. 78 (2009) 422e431, https://doi.org/10.1016/j.carbpol.2009.04.034. Elsevier Ltd. [169] Y. Lu, L. Weng, X. Cao, Morphological, thermal and mechanical properties of ramie crystallitesdreinforced plasticized starch biocomposites, Carbohydr. Polym. 63 (2006) 198e204, https://doi.org/10.1016/j.carbpol.2005.08.027. [170] B. Montero, M. Rico, S. Rodríguez-Llamazares, L. Barral, R. Bouza, Effect of nanocellulose as a filler on biodegradable thermoplastic starch films from tuber, cereal and legume, Carbohydr. Polym. 157 (2017) 1094e1104, https://doi.org/10.1016/ j.carbpol.2016.10.073. [171] Y. He, Y. Boluk, J. Pan, A. Ahniyaz, T. Deltin, P.M. Claesson, Corrosion protective properties of cellulose nanocrystals reinforced waterborne acrylate-based composite coating, Corros. Sci. (2019), https://doi.org/10.1016/j.corsci.2019.04.038. Elsevier Ltd. [172] P.H.F. Pereira, K.W. Waldron, D.R. Wilson, A.P. Cunha, E.S. de Brito, T.H.S. Rodrigues, et al., Wheat straw hemicelluloses added with cellulose nanocrystals and citric acid. Effect on film physical properties, Carbohydr. Polym. 164 (2017) 317e324, https://doi.org/10.1016/j.carbpol.2017.02.019. [173] E. Fortunati, D. Puglia, F. Luzi, C. Santulli, J.M. Kenny, L. Torre, Binary PVA bionanocomposites containing cellulose nanocrystals extracted from different natural sources: part I, Carbohydr. Polym. 97 (2013) 825e836, https://doi.org/10.1016/j.carbpol.2013.03.075. Elsevier Ltd. [174] M. Pereda, A. Dufresne, M.I. Aranguren, N.E. Marcovich, Polyelectrolyte films based on chitosan/olive oil and reinforced with cellulose nanocrystals, Carbohydr. Polym. 101 (2014) 1018e1026, https://doi.org/10.1016/j.carbpol.2013.10.046.

90

Interfaces in Particle and Fibre Reinforced Composites

[175] B. Nasri-Nasrabadi, T. Behzad, R. Bagheri, Preparation and characterization of cellulose nanofiber reinforced thermoplastic starch composites, Fibers Polym. 15 (2014) 347e354, https://doi.org/10.1007/s12221-014-0347-0. [176] S. Karimi, P. Tahir, A. Dufresne, A. Karimi, A. Abdulkhani, A comparative study on characteristics of nanocellulose reinforced thermoplastic starch biofilms prepared with different techniques, Nord. Pulp Pap. Res. J. 29 (2014) 41e45. [177] M.N. Angles, A. Dufresne, Plasticized starch/tunicin whiskers nanocomposite materials. 2. Mechanical behavior, Macromolecules 34 (2001) 2921e2931, https://doi.org/10.1021/ ma001555h. [178] M.N. Angles, A. Dufresne, Plasticized starch/tunicin whiskers nanocomposites. 1. Structural analysis, Macromolecules 33 (2000) 8344e8353, https://doi.org/10.1021/ ma0008701. [179] H. Angellier, S. Molina-Boisseau, P. Dole, A. Dufresne, Thermoplastic starchwaxy maize starch nanocrystals nanocomposites, Biomacromolecules 7 (2006) 531e539, https://doi.org/10.1021/bm050797s. [180] H. Angellier, L. Choisnard, S. Molina-Boisseau, P. Ozil, A. Dufresne, Optimization of the preparation of aqueous suspensions of waxy maize starch nanocrystals using a response surface methodology, Biomacromolecules 5 (2004) 1545e1551, https://doi.org/10.1021/ bm049914u. [181] Y. Lu, L. Weng, X. Cao, Biocomposites of plasticized starch reinforced with cellulose crystallites from cottonseed linter, Macromol. Biosci. 5 (2005) 1101e1107, https:// doi.org/10.1002/mabi.200500094. [182] I. Kvien, J. Sugiyama, M. Votrubec, K. Oksman, Characterization of starch based nanocomposites, J. Mater. Sci. 42 (2007) 8163e8171, https://doi.org/10.1007/s10853007-1699-2. [183] X. Cao, Y. Chen, P.R. Chang, M. Stumborg, M.A. Huneault, Green composites reinforced with hemp nanocrystals in plasticized starch, J. Appl. Polym. Sci. 109 (2008) 3804e3810, https://doi.org/10.1002/app.28418. [184] A.P. Mathew, W. Thielemans, A. Dufresne, Mechanical properties of nanocomposites from sorbitol plasticized starch and tunicin whiskers, J. Appl. Polym. Sci. 109 (2008) 4065e4074, https://doi.org/10.1002/app.28623. [185] P.R. Chang, R. Jian, P. Zheng, J. Yu, X. Ma, Preparation and properties of glycerol plasticized-starch (GPS)/cellulose nanoparticle (CN) composites, Carbohydr. Polym. 79 (2010) 301e305, https://doi.org/10.1016/j.carbpol.2009.08.007. Elsevier Ltd. [186] N. Noshirvani, B. Ghanbarzadeh, H. Fasihi, H. Almasi, Starch-PVA nanocomposite film incorporated with cellulose nanocrystals and MMT: a comparative study, Int. J. Food Eng. 12 (2016) 37e48, https://doi.org/10.1515/ijfe-2015-0145. [187] H.M.C. Azeredo, L.H.C. Mattoso, D. Wood, T.G. Williams, R.J. Avena-Bustillos, T.H. McHugh, Nanocomposite edible films from mango puree reinforced with cellulose nanofibers, J. Food Sci. 74 (2009), https://doi.org/10.1111/j.1750-3841.2009.01186.x. [188] A. Kaushik, M. Singh, G. Verma, Green nanocomposites based on thermoplastic starch and steam exploded cellulose nanofibrils from wheat straw, Carbohydr. Polym. 82 (2010) 337e345, https://doi.org/10.1016/j.carbpol.2010.04.063. Elsevier Ltd. [189] E.D.M. Teixeira, C. Lotti, A.C. Corrêa, K.B.R. Teodoro, J.M. Marconcini, L.H.C. Mattoso, Thermoplastic corn starch reinforced with cotton cellulose nanofibers, J. Appl. Polym. Sci. 120 (2011) 2428e2433, https://doi.org/10.1002/app.33447. [190] M. Hietala, A.P. Mathew, K. Oksman, Bionanocomposites of thermoplastic starch and cellulose nanofibers manufactured using twin-screw extrusion, Eur. Polym. J. 49 (2013) 950e956, https://doi.org/10.1016/j.eurpolymj.2012.10.016.

Characterization studies of biopolymeric matrix and cellulose fibres

91

[191] M. Babaee, M. Jonoobi, Y. Hamzeh, A. Ashori, Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers, Carbohydr. Polym. 132 (2015) 1e8, https://doi.org/10.1016/j.carbpol.2015.06.043. Elsevier Ltd. [192] J.H.R. Llanos, C.C. Tadini, Preparation and characterization of bio-nanocomposite films based on cassava starch or chitosan, reinforced with montmorillonite or bamboo nanofibers, Int. J. Biol. Macromol. (2017), https://doi.org/10.1016/j.ijbiomac.2017.09.001. [193] A. Saxena, T.J. Elder, A.J. Ragauskas, Moisture barrier properties of xylan composite films, Carbohydr. Polym. 84 (2011) 1371e1377, https://doi.org/10.1016/j.carbpol.2011.01.039. Elsevier Ltd. [194] C.J. Grande, F.G. Torres, C.M. Gomez, O.P. Troncoso, J. Canet-Ferrer, J. MartínezPastor, Development of self-assembled bacterial cellulose-starch nanocomposites, Mater. Sci. Eng. C 29 (2009) 1098e1104, https://doi.org/10.1016/j.msec.2008.09.024. [195] H.M.C. Azeredo, L.H.C. Mattoso, R.J. Avena-Bustillos, G.C. Filho, M.L. Munford, D. Wood, et al., Nanocellulose reinforced chitosan composite films as affected by nanofiller loading and plasticizer content, J. Food Sci. 75 (2010) 1e7, https://doi.org/ 10.1111/j.1750-3841.2009.01386.x. [196] T. Huq, S. Salmieri, A. Khan, R.A. Khan, C. Le Tien, B. Riedl, et al., Nanocrystalline cellulose (NCC) reinforced alginate based biodegradable nanocomposite film, Carbohydr. Polym. 90 (2012) 1757e1763, https://doi.org/10.1016/j.carbpol.2012.07.065. Elsevier Ltd. [197] M. Asrofi, H. Abral, A. Kasim, A. Pratoto, M. Mahardika, F. Hafizulhaq, Characterization of the sonicated yam bean starch bionanocomposites reinforced by nanocellulose water hyacinth fiber (Whf): the effect of various fiber loading, J. Eng. Sci. Technol. 13 (2018) 2700e2715. [198] W.P. Flauzino Neto, M. Mariano, I.S.V. da Silva, H.A. Silvério, J.L. Putaux, H. Otaguro, et al., Mechanical properties of natural rubber nanocomposites reinforced with high aspect ratio cellulose nanocrystals isolated from soy hulls, Carbohydr. Polym. 153 (2016) 143e152, https://doi.org/10.1016/j.carbpol.2016.07.073. Elsevier Ltd. [199] G.H.D.H.D. Tonoli, E.M.M. Teixeira, A.C.C. Corrêa, J.M.M. Marconcini, L.A.A. Caixeta, M.A.A. Pereira-Da-Silva, et al., Cellulose micro/nanofibres from Eucalyptus kraft pulp: preparation and properties, Carbohydr. Polym. 89 (2012) 80e88, https://doi.org/10.1016/j.carbpol.2012.02.052. Elsevier Ltd. [200] H.-M.M. Ng, L.T. Sin, T.T. Tee, S.T. Bee, D. Hui, C.Y. Low, et al., Extraction of cellulose nanocrystals from plant sources for application as reinforcing agent in polymers, Compos. Part B Eng. 75 (2015) 176e200, https://doi.org/10.1016/j.compositesb.2015.01.008. Elsevier Ltd. [201] Y. Chen, C. Liu, P.R. Chang, X. Cao, D.P. Anderson, Bionanocomposites based on pea starch and cellulose nanowhiskers hydrolyzed from pea hull fibre: effect of hydrolysis time, Carbohydr. Polym. 76 (2009) 607e615, https://doi.org/10.1016/ j.carbpol.2008.11.030. [202] A.C.W. Leung, E. Lam, J. Chong, S. Hrapovic, J.H.T. Luong, Reinforced plastics and aerogels by nanocrystalline cellulose, J. Nanoparticle Res. 15 (2013) 1636, https:// doi.org/10.1007/s11051-013-1636-z. [203] P. Balakrishnan, M.S. Sreekala, M. Kunaver, M. Huski
c, S. Thomas, Morphology, transport characteristics and viscoelastic polymer chain confinement in nanocomposites based on thermoplastic potato starch and cellulose nanofibers from pineapple leaf, Carbohydr. Polym. 169 (2017) 176e188, https://doi.org/10.1016/j.carbpol.2017.04.017. [204] H.A. Silvério, W.P. Flauzino Neto, N.O. Dantas, D. Pasquini, Extraction and characterization of cellulose nanocrystals from corncob for application as reinforcing agent in

92

[205]

[206]

[207]

[208]

[209] [210]

[211]

[212]

[213]

[214]

[215]

[216]

[217]

[218]

Interfaces in Particle and Fibre Reinforced Composites

nanocomposites, Ind. Crop. Prod. 44 (2013) 427e436, https://doi.org/10.1016/ j.indcrop.2012.10.014. Elsevier B.V. J.S. Gonzalez, L.N. Ludue~na, A. Ponce, V.A. Alvarez, Poly(vinyl alcohol)/cellulose nanowhiskers nanocomposite hydrogels for potential wound dressings, Mater. Sci. Eng. C 34 (2014) 54e61, https://doi.org/10.1016/j.msec.2013.10.006. Elsevier B.V. V.P. Cyras, L.B. Manfredi, M.-T. Ton-That, A. V
azquez, Physical and mechanical properties of thermoplastic starch/montmorillonite nanocomposite films, Carbohydr. Polym. 73 (2008) 55e63, https://doi.org/10.1016/j.carbpol.2007.11.014. F.M. Pelissari, M.M. Andrade-Mahecha, A. Sobral PJ do, F.C. Menegalli, Nanocomposites based on banana starch reinforced with cellulose nanofibers isolated from banana peels, J. Colloid Interface Sci. 505 (2017) 154e167, https://doi.org/10.1016/ j.jcis.2017.05.106. M. Abdollahi, M. Alboofetileh, M. Rezaei, R. Behrooz, Comparing physico-mechanical and thermal properties of alginate nanocomposite films reinforced with organic and/or inorganic nanofillers, Food Hydrocolloids 32 (2013) 416e424, https://doi.org/10.1016/ j.foodhyd.2013.02.006. Elsevier Ltd. M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites : preparation , properties and uses of a new class of materials, Mater. Sci. Eng. A 28 (2000) 1e63. B. Chen, J.R.G. Evans, Thermoplastic starcheclay nanocomposites and their characteristics, Carbohydr. Polym. 61 (2005) 455e463, https://doi.org/10.1016/ j.carbpol.2005.06.020. R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Characteristics of thermoplastic sugar palm starch/agar blend: thermal, tensile, and physical properties, Int. J. Biol. Macromol. 89 (2016) 575e581, https://doi.org/10.1016/j.ijbiomac.2016.05.028. Elsevier B.V. R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Effect of seaweed on physical properties of thermoplastic sugar palm starch/agar composites, J. Mech. Eng. Sci. 10 (2016) 2214e2225, https://doi.org/10.15282/jmes.10.3.2016.1.0207. R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Effect of seaweed on mechanical, thermal, and biodegradation properties of thermoplastic sugar palm starch/agar composites, Int. J. Biol. Macromol. 99 (2017) 265e273, https://doi.org/10.1016/j.ijbiomac.2017.02.092. Elsevier B.V. R. Jumaidin, S. Sapuan, M. Jawaid, M. Ishak, J. Sahari, Effect of agar on flexural, impact, and thermogravimetric properties of thermoplastic sugar palm starch, Curr. Org. Synth. 14 (2017) 200e205, https://doi.org/10.2174/1570179413666160921110732. R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Effect of agar on physical properties of thermoplastic starch derived from sugar palm tree, Pertanika J. of Sci. Technol. 25 (2017) 1235e1248. J. Marett, A. Aning, E.J. Foster, The isolation of cellulose nanocrystals from pistachio shells via acid hydrolysis, Ind. Crop. Prod. 109 (2017) 869e874, https://doi.org/10.1016/ j.indcrop.2017.09.039. Elsevier. S. Karimi, A. Dufresne, P. Tahir, A. Karimi, A. Abdulkhani, Biodegradable starch-based composites: effect of micro and nanoreinforcements on composite properties, J. Mater. Sci. 49 (2014) 4513e4521, https://doi.org/10.1007/s10853-014-8151-1. M. Avella, J.J. De Vlieger, M.E. Errico, S. Fischer, P. Vacca, M.G. Volpe, Biodegradable starch/clay nanocomposite films for food packaging applications, Food Chem. 93 (2005) 467e474, https://doi.org/10.1016/j.foodchem.2004.10.024.

Characterization studies of biopolymeric matrix and cellulose fibres

93

[219] S.P. Akhlaghi, R.C. Berry, K.C. Tam, Surface modification of cellulose nanocrystal with chitosan oligosaccharide for drug delivery applications, Cellulose 20 (2013) 1747e1764, https://doi.org/10.1007/s10570-013-9954-y. [220] M.M. Sain, B.V. Kokta, Polyolefinewood filler composite. I. Performance of m-phenylene bismaleimide-modified wood fiber in polypropylene composite, J. Appl. Polym. Sci. 54 (1994) 1545e1559, https://doi.org/10.1002/app.1994.070541019. [221] J.R. Wright, L.J. Mathias, New lightweight materials: balsa wood-polymer composites based on ethyl a-(hydroxymethyl)acrylate, J. Appl. Polym. Sci. 48 (1993) 2241e2247, https://doi.org/10.1002/app.1993.070481217. [222] J.W. Whalen, Physical chemistry of surfaces, J. Chem. Educ. 60 (1983) A322, https:// doi.org/10.1021/ed060pA322.2. [223] J.K. Pandey, W.S. Chu, C.S. Kim, C.S. Lee, S.H. Ahn, Bionanocomposites of grass, Adv. Mater. Res. 47 (50) (2008) 435e438, https://doi.org/10.4028/www.scientific.net/ AMR.47-50.435. [224] A. Ferrer, I. Filpponen, A. Rodríguez, J. Laine, O.J. Rojas, Valorization of residual empty palm fruit bunch fibers (EPFBF) by microfluidization: production of nanofibrillated cellulose and EPFBF nanopaper, Bioresour. Technol. 125 (2012) 249e255, https:// doi.org/10.1016/j.biortech.2012.08.108. Elsevier Ltd. [225] Y. Hou, A.R. Matthews, A.M. Smitherman, A.S. Bulick, M.S. Hahn, H. Hou, et al., Thermoresponsive nanocomposite hydrogels with cell-releasing behavior, Biomaterials 29 (2008) 3175e3184, https://doi.org/10.1016/j.biomaterials.2008.04.024. [226] Eichhorn, R.J. Young, The young’s modulus of a microcrystalline cellulose, Cellulose 8 (2001) 197e207, https://doi.org/10.1023/A:1013181804540.

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W.P.S.L. Wijesinghe a , M.M.M.G.P.G. Mantilaka a,b , K.A.A. Ruparathna b , R.B.S.D. Rajapakshe b , S.A.L. Sameera a , M.G.G.S.N. Thilakarathna b a Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka; bPostgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka

1. Introduction Polymers are no end in sight, molecular chains consisting repeating units called monomers which are covalently bonded together [1,2]. A large deviation of synthetic polymeric materials has been used on a large scale as biomaterials. They are the most widely used materials in biomedical applications. The dominant advantage of a polymer as biomaterial compared to metal or ceramic materials has manufacturability to produce various shapes. Also, advantage of secondary process ability, reasonable cost, flexibility, resistance to biochemical challenge, lightweight and accessible [2] in a wide abnormality of compositions mutually adequate physical and technical properties [3,4]. Also, the stiffness of polymeric materials is essentially closer to the stiffness of bone in contrast to metals or ceramics for certain applications [5]. When choosing polymers as biomaterials, the most important considerations are biocompatibility, satisfying mechanical and physical properties, sterilization techniques and manufacturability. Compatibility issues with specific systems, biopolymers have earned special attention due to their biocompatibility and biodegradability properties, as well as low-cost and renewable sources [2]. Biopolymers often refer to the polymers that are capable of biodegrading. They are generally produced with the materials extracted by living organisms or directly by the activities of microorganisms. These polymeric biomolecules contain covalently bonded monomeric units which create larger polymer structures. Using the method of production, type of the monomeric unit used and the structure of the biopolymer, all the biopolymers can be classified into two main classes including natural biopolymers and synthetic biopolymers [6,7]. Some of the natural biopolymers are extracted from bio mass, such as polynucleotides (DNA and RNA), polypeptides, polysaccharides and lipids. Synthetic biopolymers are produced using the available monomeric components. There are many synthetic biopolymers such as polyvinyl alcohol (PVA), polyvinyl acetate as well as other aliphatic polyesters (polyglycolic acid (PGA), polycaprolactone (PCL) and polylactic acid (PLA)). Biopolymers are

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00004-2 Copyright © 2020 Elsevier Ltd. All rights reserved.

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renewable and they are sustainable and carbon neutral. These biopolymers can be modified and assemble with other components to develop different types of biocomposites [8,9]. Composites can be described as substances that are made up of two or more materials which have separate phases and constituents. Homogenous polymer-based material with specific properties often refers as a polymer composite. A composite material mostly consisted with micro- or nano-structures. With the decreasing particle size, the effectiveness for the formation of a composites increases. There are a few other biopolymers as well, such as melanin, lignin, rubber and suberin [9e13]. Biopolymers composites are synthesised using numerous methods the following three preparation methods of biopolymer composites are important [14]. These are in situ reaction, solution casting method and melt mixing technique. In situ polymer techniques are mostly delivered in the presence of suitable initiators like heat, radiation, etc [15,16]. Here, the particles get intermixed in monomer solution or with liquid monomer. In the solution casting technique, the principle of Stokes’ law is followed. In this process, the polymer and pre-polymer are similarly soluble in the suitable solution and the particles used to prepare the composite are dispersed in same solution or different solutions before mixing the both. In preparing clay-based biopolymer composites, the solution swells into clay. Due to weak forces such that the layer filled one by one, the clay gets dispersed. The surface is then mixed in clay dispersion, once solution swelling is over. Melt Mixing Technique is carried out in the molten state and the components get mixed in that condition. In this process, molten polymer composite continued long period which is called peeling of platelets. In melting process, temperature and pressure depend on degradation of biopolymer [11]. Apart from these techniques some other simple synthesizing methods are also used to prepare biopolymer composites, such as sol gel techniques, inside out methods and Acid Catalyzed Condensation Polymerization [17].

2.

Filler-matrix interface of polymer composite

Due to the controlled composition of the polymer composites, polymer matrix composite materials characteristically consist of two or more components that contain significantly diverse with chemical and physical properties. Polymer composite materials are generally composed into two materials as the one typically being the continuous phase (matrix) and other being discontinues phase (reinforcement/dispersed) [18,19]. Fig. 4.1 clearly describes the arrangement of these two phases via interface. Herein, reinforcing particles are dispersed in matrix and properties of the composites are mainly depend on the interaction between matrix and dispersed materials. The area between matrix and dispersed particle is called interface which is formed through the chemical and physical bonds between surface atoms of discontinues phase materials and surrounding matrix atoms of discontinues phase. When compare to the bulk materials, majority of atoms of nanomaterials are in the surface of nanomaterials and interact with the matrix to form composites. Therefore, nanomaterial surfaces provide the direct impact on the composite properties [20]. Several models have been used to

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Continues phase (polymer matrix)

Interface

Discontinues phase (reinforcement)

Fig. 4.1 Arrangement of both continuous phase (matrix) and discontinues phase (reinforcement) via interface of the composites.

explain the interfaces of polymer composites. Huang et al. [114] have introduced simple model to explain the interface of polymer composite. They have incorporated silica to the polymer matrix and morphological model was introduced based on the characterization results. This model shows that interface of nanoparticles and polymers are developed through the covalent and hydrogen bonds. According to their explanation, discontinues particles distribute in the outer polymer region through the covalent bonds also matrix and particles are mutually interconnected through the hydrogen bonds to form network structure [21]. Tanaka et al. have schematically illustrated this model (Fig. 4.2) [22,23].

Si

Si

O

Polymer

Continues region

Interface

Inter distance (center to center)

Fig. 4.2 Huangs’ model for polymer composite interface [23].

lym

er

O O

Discontinues region

Po

O

Si

O

er

lym

O Po

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d2 d1

d3 d4

d1> d2> d3 >D4

Filler particles

Tightly bound polymer

Loosely bound polymer

Fig. 4.3 Tsagaropoulos model for polymer composite interface [24].

Tsagaropoulos et al. [115] have proposed a model for polymer and filler interface which describes how the reduction of average inter filler particle distance with the incorporated filler amount due to the interactions of polymer chains and filler particles [24,25]. Those created interactions change the vicinity of the fillers generating regions mutually control the mobility of the filler materials [26]. Also the interactions of polymer chains with very fine particles with high surface area restrict the mobility of the chains and lead to the formation of tightly bound and loosely bound polymer. Fig. 4.3 describes the reduction of inter particle distance of filler materials with filler materials loading [24]. The morphological and structural changes of the composites with an increase of filler concentration have been described by O’Brien et al. [27,28]. Lewis et al. have proposed a model which describes duty of the interface. They describe regions with altered electrochemical and electro-mechanical process [29]. Two phases have been defined as the phase A and phase B which are illustrated in Fig. 4.4 [30]. According to the model, polymer and nanoparticles have diverse electrochemical and electromechanically physical or chemical properties It means some polymers and their attached fillers have various properties such as the electric field, the local dielectric permittivity or an optical parameter and the electrochemical force

Intensity Interphase A

Interphase B

{ Interface of two phases }

Distance

Fig. 4.4 Intensity model of interface between two phases 1 and 2 [30].

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B

99

A

Diffuse double layer [a b]

Fig. 4.5 Distribution of the electrical charge layers in the interface ab in response [30].

[30]. This described that an electrical charge layer is created around the nano-structures in the interface region d as well (Fig. 4.4) [30]. There are three charged layers containing defuse interfaces and double layer (Fig. 4.5). Double layer with whole of a higher charge dens layer (Stern (Helmholtz) layer), exists attached to the A side. Adsorbed ions and dipoles form the A side of the Stern (Helmholtz) layer and the B tag end is energetic by ions attracted separately rest of charge on phase A. The Gouy-Chapman layer is beyond the Stern (Helmholtz) layer, formed by the separation of mobile positive and negative charges from phase B. The charge is significant by the time mentioned, if crystal ball gazer B has a polar component. The fact of containing mobile ions in medium B can cause them to migrate to uphold a diffuse electric double layer around particle A [31,32]. Tanaka et al. [23] have introduced a model to describes the chemical, physical and electrical structural properties of the interface formed between spherical nanoparticles and biopolymer matrix which is shown in Fig. 4.3. This model [32,33], suggest that the term interface is replaced by the term interface whereas their characteristics depend on the filler concentration, particles size and type of polymers and nanofillers. According to the hypothesis, the maximum of interface volume is reached for a distinct filling concentration and a certain interface thickness is assumed to be obtained an ideal dispersion of nanoparticles in a polymer matrix (see Fig. 4.5) [32,33]. Normally, the three layers of interfaces are shown on the nanoparticle as a bonded layer (the first layer), a bound layer (the second layer) and a serene layer (the hot box layer) [23]. At the alternative layer, chemical bonding occurs between the biological polymer matrix and inorganic particles. A layer of polymer chains which are strongly bound to the first layer and the surface of inorganic nanoparticles process as an interfacial region forms the second layer. The third layer is a region which is loosely coupling and interacting mutually with the second layer, having different morphology compared to the others [23]. In polymer nanocomposites, particles may interact electrically with the nearest neighbouring particles due to the far-field effect resulting in a collaborative effect [23]. This model can explain different electrical properties such as, partial discharge resistance of PA/LS nanocomposites [23] (Fig. 4.6).

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The first layer Matrix inter particle distance (surface to surface) 40 – 100 nm

The second layer The third layer Nano particle

Gouy-chapman diffuse layer charge distribution

Diameter of nano particle: 20 – 50 nm Thickness of layer: 10 – 30 nm

+ + + +

– – – –

Gouy-chapman diffuse layer will overlap the three layers of multi-core model.

Electric double layer when a nano particle is positively charged

Fig. 4.6 Tanakas’ multi-core model [23].

There are several models that were proposed besides the four models of the interface region between nanoparticles and polymer matrix. These models are used in practical situations where there should be some modifications. The alignment of polymer chains on nanoparticles via the interface depends on the surface of the nanoparticle and the effect of nanoparticle surface shown in fig. 4.7. Without the involvement of modified nanoparticles (Fig. 4.7(a)), the interactions between the polymer matrix and the nanoparticles becomes low. A restructuring of the polymer matrix can take place by modifying the surfaces of nanoparticles (Fig. 4.7(b)). This is due to the reactions between the polymer and modified layer groups where an alignment layer of the polymer chains perpendicular to the nanoparticle surface appears and the adjacent polymer region is affected as well [34].

(a)

Non-aligned layer

(b)

Affected layer Aligned layer Surface modification

Nanoparticle Surface

Fig. 4.7 Polymer Chain Alignment of Andritschs’ model (a) without and (b) with surface modifications [34].

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Water shell QDC

Filler particles

Fig. 4.8 Schematic representation of the water shell Zous’ model [35].

Zou et al. in 2008 [35] proposed a water shell model based on Lewis’ and Tanaka’s models and that explains the effect of water absorption in epoxy nanocomposites, when they are exposed to humidity [24]. This model is schematically represented in fig. 4.8. In this model, nanoparticles are concentrated with water molecules while the polymer matrix is less concentrated with water. Through overlapping water shells, percolative paths which affect the dielectric properties of epoxy nanocomposites are formed when water concentration around the nanoparticles gets high. The physical, chemical and electrical properties of the interface between nanoparticles and polymer matrix composite can be explained by the above models of polymer nanocomposites but as the interface regions in polymer nano-composites have not been made visible until now, limitations still exist (Fig. 4.8).

3. Common types of biopolymer composites 3.1

Metal based biopolymer composites

Metal nanoparticles are widely used to synthesise metal based biopolymer composites because of properties of metal nanoparticles including optical polarizability, antibacterial activity, electrical conductivity, chemical properties and biocompatibility. The disadvantages of polymer matrix have been overcome by using metal based polymer composites. This has resulted in upgrading packaging applications, biomedical applications, electronic applications and environmental decontamination. Metal nanoparticles are synthesised under different methods such as spray pyrolysis, sol-gel process, chemical vapour deposition (CVD), electrodeposition and chemical methods, rapid solidification and so on [36e38]. Nanostructured metal has been incorporated as a filler material [39] to synthesise polymer matrices. Ag and Au are mainly used metal nanoparticles to synthesise the biopolymer composite with all kind of biopolymers such as PVA, PCL chitosan and many more synthetic and natural biopolymers [40e46]. Metal nanoparticles and polymers interact each other through their interface. Biopolymers are part of a very promising family of natural and

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synthetic molecules that have also been described to interact with metal nanoparticles. Variety of functional groups present in their structure support the good interactions between the metal surfaces, which is responsible for the metal nanoparticle dispersion in the polymer matrix. In general, pure metal particles are mixed under low acidic conditions which provide the positive charge on the metal surfaces therefore negatively charged functional groups in biopolymers are combine with metal particles through electrostatic attractions. Also, surfaces of metal nanoparticles are modified by providing functional groups to develop covalent bonds between polymer and metal nanoparticles [47,48]. For instance, Barros et al. have studied the surface electrostatic interactions of gold nanoparticles and biopolymers (chitosan and gum arabic) which was developed through the surfactant [47].

3.2

Metal oxide based biopolymer composites

Metal oxides are also used to synthesise biopolymer composite to incorporate the properties of them to polymer matrix. Preparation and usage of metal oxide incorporated polymer composites is similar to the metal based biopolymer composites [37,38,49]. Large number of metal oxides such as ZnO, TiO2, SnO2, SiO2, ZrO2, and etc. have been used to combine with all types of biopolymers in order to synthesise metal oxide based biopolymer composites [49e60]. Because of the amazing properties of metal oxide nanoparticles in antibacterial activity, optical polarizability, thermal conductivity, good chemical properties, and biocompatibility, metal oxide nanoparticles are used to prepare biopolymer composites for various applications such as environmental decontamination, biomedical applications, packaging applications and electronic applications [61e67]. Nanoparticles of metal oxides were included during suspension polymerization to produce hybrid metal oxides-alginate-containing and producing biopolymer matrix with several polymers [68,69]. Unlike metal nanoparticles, metal oxide nanoparticles are containing oxygen atoms and therefore, these metal oxide nanoparticles can easily be dispersed in polar biopolymer matrices without any surface modifications. Chemical bonds and electrostatic interactions are created between oxygen atoms in metal oxide surfaces and functional groups in polymer matrix while making the interface which is responsible for nanoparticle dispersion in the polymer matrix [70,71].

3.3

Carbon based biopolymer composites

The applications of carbon fillers incorporated biopolymers can be seen in environmental based sensors, photocatalytic materials, electronic devices and so on [72]. Carbon based materials are excellent candidates for thermal and electrical conductivity applications. Therefore, carbon based materials are used as a filer materials in biopolymer composites to increase the thermal and electrical conductivities in the composites [73]. Major carbon based filler materials are carbon nanotubes (CNT), graphene, graphene oxide, graphene derivatives and carbon black [73,74]. Carbon nanomaterials are used as an outstanding candidate having light weight, high surface area and high mechanical strength [75]. For instance, Graphene, a single-layer carbon sheet

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with a hexagonal packed lattice structure, has shown many unique properties such as the quantum hall effect (QHE), high carrier mobility at room temperature (10,000 cm2 V1 S1) [76], large theoretical specific surface area [77], good optical transparency (97.7%) [78], high Young’s modulus (1 TPa) [75] and excellent conductivity [79]. Graphene-based polymer composites depict the superior properties of graphene compared to polymers in terms of thermal, mechanical, gas barrier, flame retardant and electrical properties. It was also reported that the improvement in mechanical and electrical properties of graphene-based polymer composites are much better in comparison to that of clay or other carbon fibre based polymer composites [79e84]. The physiochemical properties of the nanocomposite depend on the distribution of carbon materials in the polymer matrix as well as interfacial bonding between the graphene layers and polymer matrix. There are some limitations in interface formation of polymer composites with functional group-free carbon materials such as pristine graphene and carbon nanotubes due to their tendency to form polymer matrix agglomerates and also due to their incompatibility with organic polymers and inability to form homogeneous composites [72,73,75,85,86]. Therefore, surface modifications should be done to form interfaces. Hydrogen bonds, ester bonds, van der Waals bonds and electrostatic attractions are involving to form the interface of biopolymer composites. In contrast, because of the various functional groups, organic polymers are more compatible with Graphene oxide (GO) [84,87e90]. Hence GO can be considered as a promising nanofiller for polymer nanocomposites. Graphene oxide is unsuitable to produce conducting nanocomposites as it is an electrical insulator. For instances, Laredo et al. have shown the improvement of mechanical and electrical properties in CNT incorporated PCl/PLA matrix. The poly(vinylalcohol) (PVA)-grafted GO and CNTs were successfully loaded with Camptothecin (CPT). A comparison and evaluation of the ability to kill cancer cells by GO-PVA-CPT and CNT-PVACPT complexes has been conducted [91].

3.4

Other biopolymer composites

Other types of filler materials such as layered silicates, phosphates and carbonates etc [92,93]. They can successfully interact with biopolymer matrix are used to prepare polymer composites because all of them are having active groups to interact with the functional groups in polymer matrix. Silicates are one dimensional nanomaterials. In the past few years due to the effective thermal and mechanical properties, polymer clay or polymer layered silicate nanocomposites are used in the field of automobile industry, fuel tanks, electronic, coating and packaging industry [94,95]. Layered silicates (mostly clays) are basically hydrated magnesium and alumina silicates. Hydrophilic nature and oxygen groups of clay layers are involved to form the interface via hydrogen bonds, coordination bonds and electrostatic interactions. Also composites are formed with intercalation of polymer into layered structure of clay [95]. When clay is added as fillers into composites due to the nanolayers in clay structures, it results in large aspect ratio, high surface area and microscopic dispersion of filler into the matrix. For instance, montmorillonite has an aspect ratio of 10e1000 and a large

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surface area range of 750 m2/g [96]. Synthetic clays like silicate bioactive glass (SiO2eNa2OeCaOeP2O5) and phosphate glasses (P2O5eCaOe Na2O), Silicon nitride-bioglass etc are used as filler materials to synthesised biopolymer composites and interface of these composites similar to the clay polymer composites. In addition, hydroxyapatite and calcium phosphate like other biocompatible inorganic fillers are used to synthesise biopolymer composites while targeting biomedical applications [91]. Interface between theses fillers and biopolymers is formed similar to the other composites via different types of interactions.

4.

Practical applications of biopolymer composites

According to the properties of the biopolymer composites, they are used in different applications such as electronic applications biomedical applications, environmental decontamination and packaging applications which depend on the properties of incorporated filler (reinforced) materials. In electronic applications, mainly high thermal and electrical conductive biopolymer composites are widely used. A few biopolymers have very much recorded properties as natural semiconductors [97]. Biopolymer material based on DNA has unique electromagnetic and optical properties, such as ultralow optical and microwave low misfortune, low and tunable electrical resistivity, natural light producing diodes (LED) and natural field impact transistors [98]. Contrasted to those produced using different materials, nonlinear optical polymer electro-optic modulators produced from biopolymers have shown better execution [99]. At room temperature, naturally occurring biopolymer of Guar gum, changed artificially with polyaniline, exhibits electrical conductivity in the scope of 1.6  10 2 S/cm [100]. When Mallick and Sakar [116] studied about the electrical conductivity of gum arabica which is found in different types of Acacia babul (Acacia Arabica) it was found that its electrical properties reflect engineered polymers doped with inorganic salts and are proton leading in nature. Two procedures are involved during the charge transport in biomolecular materials; Super trade transport and bouncing transport. Super trade is a chain intervened burrowing transport. In here, a thoroughly secluded extension is used to exchange electrons or gaps from a giver to acceptor and the scaffold orbitals are just used as coupling media. The electron briefly resides on the extension for a brief span in the jumping component while going from one redox focus to the next. Yet in the supertrade, the electron is passed between the benefactor and acceptor though a medium of conjugated scaffold [101]. The process of decaying exponentially with the length of the particle is referred as burrowing. Limited potential hindrance at the metalseparator interface is accepted as a typical burrowing model. It indicates free electron stream for a short division into the example from the metal contact. Simmons relations depict the charge transfer at low Voltages [102], but it is the Fowler-Nordheim process that explains burrowing at higher voltages. Super exchange procedure can be either rational or non-sound. In rational burrowing procedure, A charge bearer travels from a contributor to acceptor quick enough with the end goal that there is no dephasing by atomic movements of the scaffold [103]. Thus, charges

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do not trade vitality with the particles. Anyway this procedure does not happen at altogether long separations. Muddled super exchange is a multi-step process in which warm movement of the atoms produces a restricted accuse transporter cooperates of phonons [104]. Furthermore, biomedical filed is the one of major application areas of biopolymer composites [105,106]. Tissue building is a rising zone in prescription including cells control to advance the recovery what’s more, mending of damaged normal tissues or the improvement of organic substitutes that re-establish, keep up or enhance tissue and organ capacities. The manufactured frameworks with cells can be utilised as an alternation to both organ transplantation as well as implantation of mechanical therapeutic gadgets that are unfit to play out all components of a tissue or an organ. A number of composite materials can be identified to be used in applications as framework for cell development in tissue designing [105e109]. In order to enhance sensory tissue and bone recovery, experiments are being conducted to improve the framework that joins electrical qualities and natural properties. A few investigations are dedicated to the improvement of a framework which joins explicit natural properties and electrical qualities, enhancing bone and sensory tissue recovery. Hyaluronic corrosive and polypirrone composite materials are described. Great outcomes are demonstrated when the composite was tried in vitro and in vivo. A composite comprising of a threedimensional work made of 80 wt% polycaprolactone (PCL) filaments with 20 wt% HA particles are used for tissue building of bone and ligament [110]. Further examinations are required as the perfect platform material has not yet been grown. HA with different morphologies and the blend of them have been investigated. The in situ polymerization and invasion of ε-caprolactone monomer into permeable apatite squares produce the composite [111]. As the material is biodegradable, it also has applications in ligament recovery, as cancellous or trabecular bone swap materials. The molecular weight of the polymer increases with the polymerization time, prompting a moved forward compressive quality. Severe functional diseases requiring surgical treatments can be caused Sundry congenital or acquired lesions cognate to the heart and blood vessels. Therapeutic fortifies for heart and blood vessels are the cardiovascular contrivances. The biggest issue for materials placed in contact with blood is Thrombus formation and it strongly dependents on implantation site, surface characteristics, and local hemodynamics. The development of a perfect hemocompatible material is still an unsolved dilemma. Currently, even though there are some studies on the possibility to utilise composites as vascular grafted composites, they are not used in clinical practice. Porous polyethylene terephthalate (PET), polyurethane (PU), or polytetrafluoroethylene (PTFE) impregnated with collagen or gelatin have been studied [112]. Graft impregnation facilitates the percolating process to be evaded and that is necessary to minimise blood leakage; by that the dimensional stability of the graft amends lead to the sealing of the pores. The porous wall of the graft sanctions the ingrowth and firm anchorage of the host tissue to the material after in vivo degradation of the collagen and gelatin. Filament winding technology was used to obtain an arterial prosthesis that is made up of PU fibres and a coalescence of PU and PELA a copolymer of lactic acid and polyethylene glycol (PEG) [113].

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The graft is dense at the time of implantation avoiding any blood loss. However, formation of a porous wall during the rejuvenating process that sanctions the tissue ingrowth into the pores was resulted due to the resorbable matrix in vivo. To match the elastic and mechanical properties of the natural vessel, the Fibre orientation, amount and properties are culled in. Photocatalytic and antimicrobial materials incorporated biopolymer composites have been used as environmental decontamination applications like waste water treatment and packaging applications. Photocatalytic materials like TiO2 and ZnO have been incorporated in different types of bio polymermatrices such as PVA, PCL and PLA etc. for use in water treatment applications and packaging applications [61e67].

5.

Conclusions

In this chapter, interfaces of biopolymer composites, fillers and applications of biopolymers have been reviewed. Also, formation of interfaces via different interactions between polymer matrix and fillers have been discussed. There were five models including Huangs’, Tsagaropoulos’, Tanakas’, Lewis and Andritschs’ models used to explain the interfaces of the polymer composites. Different types of filler nanoparticles such as metals, metal oxides, carbon based nanomaterials and layered silicates etc have been widely used in dispersion of polymer matriceswhile introducing important properties to polymer composites such as mechanical strengths, high stiffness, antimicrobial, photocatalytic, electrical conductivity and thermal conductivity. According to the properties of the biopolymer composites, they have used in different areas of applications such as biotechnological, electronics environmental decontaminants and energy conservation applications. Further enhancement of properties of this composite materials will be used to fulfil future development of technology.

References [1] G. Binyamin, B.M. Shafi, C.M. Mery, Biomaterials: a primer for surgeons, in: Seminars in Pediatric Surgery, vol. 15, Elsevier, 2006, pp. 276e283. [2] S. Kara, A Roadmap of Biomedical Engineers and Milestones, InTech, 2012. [3] N.R. Patel, P.P. Gohil, A review on biomaterials: scope, applications & human anatomy significance, Int. J. Emerg. Technol. Adv. Eng. 2 (4) (2012) 91e101. [4] P. Parida, A. Behera, S.C. Mishra, Classification of Biomaterials Used in Medicine, 2012. [5] K. Pielichowska, S. Blazewicz, Bioactive Polymer/Hydroxyapatite (Nano) Composites for Bone Tissue Regeneration, 2010 (April). doi: 10.1007/12. [6] M. Tavangar, Y. Sadeghi, R. Davalloo, F. Darabi, S. Mahmudi, S. Seyednejad, Research Paper: A Comparative Study of Shear Bond Strength of Bulk-Fill Composites, Packable and Flowable, as Well as Conventional Light-Curing Composite, vol. 6, 2018, pp. 115e122 (4). [7] B. Sharma, P. Malik, P. Jain, Biopolymer reinforced nanocomposites: a comprehensive review, Mater. Today Commun. 16 (2018) 353e363. [8] L.T. Drzal, M. Misra, A.K. Mohanty, Natural Fibers, Biopolymers, and Biocomposites, Taylor & Francis, 2005.

Filler matrix interfaces of inorganic/biopolymer composites and their applications

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[9] M.J. Sandoval, G.T. Rocca, I. Krejci, M. Mandikos, In Vitro Evaluation of Marginal and Internal Adaptation of Class II CAD/CAM Ceramic Restorations with Different Resinous Bases and Interface Treatments, 2015, https://doi.org/10.1007/s00784-015-1449-9. [10] C. José, Polymerization shrinkage stress of composite resins and resin cements e what do we need to know ? 31 (2017) 49e63. [11] G. Eda, Y.-Y. Lin, S. Miller, C.-W. Chen, W.-F. Su, M. Chhowalla, Transparent and conducting electrodes for organic electronics from reduced graphene oxide, Appl. Phys. Lett. 92 (23) (2008) 209. [12] A.D. McNaught, A. Wilkinson, Compendium of Chemical Terminology. IUPAC Recommendations, 1997. [13] M. Vert, Y. Doi, K.-H. Hellwich, et al., Terminology for biorelated polymers and applications (IUPAC Recommendations 2012), Pure Appl. Chem. 84 (2) (2012) 377e410. [14] D. Schmidt, D. Shah, E.P. Giannelis, New advances in polymer/layered silicate nanocomposites, Curr. Opin. Solid State Mater. Sci. 6 (3) (2002) 205e212. [15] D.D.L. Chung, Advanced industrial and engineering polymer research thermoelectric polymer-matrix structural and nonstructural composite materials, Adv. Ind. Eng. Polym. Res. (2018) 1e5, https://doi.org/10.1016/j.aiepr.2018.04.001. [16] O.M. Folarin, E.R. Sadiku, A. Maity, Polymer-Noble Metal Nanocomposites, 2011. [17] L. Valentini, M. Cardinali, S.B. Bon, et al., Use of butylamine modified graphene sheets in polymer solar cells, J. Mater. Chem. 20 (5) (2010) 995e1000. [18] P. Amdjadi, A. Ghasemi, F. Najafi, H. Nojehdehian, Pivotal Role of Filler/Matrix Interface in Dental Composites : Review, vol. 28, 2017, pp. 1054e1065 (3). [19] F.L. Matthews, R.D. Rawlings, Composite Materials: Engineering and Science, CRC Press, 1999. [20] G. Kickelbick, Introduction to Hybrid Materials, 2007, https://doi.org/10.1002/ 9783527610495.ch1. [21] T. Tanaka, T. Imai, Advances in nanodielectric materials over the past 50 years, IEEE Electr. Insul. Mag. 29 (1) (2013) 10e23, https://doi.org/10.1109/MEI.2013.6410535. [22] O. Yakovenko, L. Matzui, G. Danylova, V. Zadorozhnii, L. Vovchenko, Y. Perets, Electrical Properties of Composite Materials with Electric Field-Assisted Alignment of Nanocarbon Fillers, 2017, https://doi.org/10.1186/s11671-017-2244-0. [23] T. Tanaka, M. Kozako, N. Fuse, Y. Ohki, Proposal of a multi-core model for polymer nanocomposite dielectrics, IEEE Trans. Dielectr. Electr. Insul. 12 (2005) 669e681. [24] G. Tsagaropoulos, A. Eisenberg, Dynamic mechanical study of the factors affecting the two glass transition behavior of filled polymers. Similarities and differences with random ionomers, Macromolecules 28 (18) (1995) 6067e6077, https://doi.org/10.1021/ ma00122a011. [25] B. Drubi-filho, F.D.C.P. Pires-de-souza, Color stability of modern composites subjected to different periods of accelerated, Artif. Aging 23 (2012) 575e580. [26] D. Pitsa, M.G. Danikas, Interfaces features in polymer nanocomposites: a review of proposed models, Nano 6 (6) (2011) 497e508, https://doi.org/10.1142/ S1793292011002949. [27] L. Calabrese, F. Fabiano, M. Curr
o, et al., Hydroxyapatite Whiskers Based Resin Composite versus Commercial Dental Composites : Mechanical and Biocompatibility Characterization, vol. 2016, 2016. [28] J. O’Brien, E. Cashell, G.E. Wardell, V.J. McBrierty, An NMR investigation of the interaction between carbon black and cis-polybutadiene, Macromolecules 9 (4) (1976) 653e660, https://doi.org/10.1021/ma60052a025.

108

Interfaces in Particle and Fibre Reinforced Composites

[29] T.J. Lewis, Nanometric dielectrics, IEEE Trans. Dielectr. Electr. Insul. 1 (5) (1994) 812e825, https://doi.org/10.1109/94.326653. [30] T.J. Lewis, Interfaces: nanometric dielectrics, J. Phys. D Appl. Phys. 38 (2) (2005) 202e212, https://doi.org/10.1088/0022-3727/38/2/004. [31] A. Tsujimoto, W.W. Barkmeier, N.G. Fischer, et al., Wear of resin composites : current insights into underlying mechanisms, evaluation methods and influential factors ଝ, Jpn. Dent. Sci. Rev. (2017) https://doi.org/10.1016/j.jdsr.2017.11.002. [32] J. Seiler, J. Kindersberger, Insight into the Interface in Polymer Nanocomposites, vol. 21, 2014, https://doi.org/10.1109/TDEI.2013.004388. [33] S. Raetzke, J. Kindersberger, Role of interface on the resistance to high-voltage arcing, on tracking and erosion of silicone/SiO2 nanocomposites, IEEE Trans. Dielectr. Electr. Insul. 17 (2010). [34] T. Andritsch, R. Kochetov, P.H.F. Morshuis, J.J. Smit, Proposal of the polymer chain alignment model, in: 2011 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2011, pp. 624e627, https://doi.org/10.1109/CEIDP.2011. 6232734. [35] C. Zou, J.C. Fothergill, S.W. Rowe, The effect of water absorption on the dielectric properties of epoxy nanocomposites, IEEE Trans. Dielectr. Electr. Insul. 15 (1) (2008) 106e117, https://doi.org/10.1109/T-DEI.2008.4446741. [36] C.C. Review, Current Concepts in Scaffolding for Bone Tissue Engineering, vol. 90, 2018, pp. 90e99. [37] Y. Li, X. Qu, Z. Zheng, S. Yu, Manufacture of high-dispersed W-Cu composite powder by mechano-thermal process, J. Cent. South Univ. Technol. 10 (3) (2003) 168e172, https://doi.org/10.1007/s11771-003-0002-1. [38] V. Provenzano, N.P. Louat, M.A. Imam, K. Sadananda, Ultrafine superstrength materials, Nanostructured Mater. 1 (1) (1992) 89e94. https://doi.org/10.1016/0965-9773(92) 90058-6. [39] M. Muccini, A bright future for organic field-effect transistors, Nat. Mater. 5 (8) (2006) 605. [40] O.G. Abdullah, Y.A.K. Salman, S.A. Saleem, In-situ synthesis of PVA/HgS nanocomposite films and tuning optical properties, Phys. Mater. Chem. 3 (2) (2015) 18e24, https://doi.org/10.12691/pmc-3-2-1. [41] B. Ankamwar, M. Chaudhary, M. Sastry, Gold nanotriangles biologically synthesized using tamarind leaf extract and potential application in vapor sensing, J. Synth. React. Inorg. Metal-Org. Nano-Metal Chem. 35 (1) (2005) 19e26, https://doi.org/10.1081/SIM200047527. [42] A.J. Varma, S.V. Deshpande, J.F. Kennedy, Metal complexation by chitosan and its derivatives: a review, Carbohydr. Polym. 55 (1) (2004) 77e93. https://doi.org/10.1016/j. carbpol.2003.08.005. [43] S. Kobayashi, T. Kiyosada, S. Shoda, Synthesis of artificial Chitin: irreversible catalytic behavior of a glycosyl hydrolase through a transition state analogue substrate, J. Am. Chem. Soc. 118 (51) (1996) 13113e13114, https://doi.org/10.1021/ja963011u. [44] R.A. Jain, The manufacturing techniques of various drug loaded biodegradable poly (lactide-co-glycolide)(PLGA) devices, Biomaterials 21 (23) (2000) 2475e2490. [45] J.-W. Rhim, Effect of PLA lamination on performance characteristics of agar/k-carrageenan/clay bio-nanocomposite film, Food Res. Int. 51 (2) (2013) 714e722. [46] O.G. Abdullah, Y.A.K. Salman, S.A. Saleem, In-situ synthesis of PVA/HgS nanocomposite films and tuning optical properties, Phys. Mater. Chem. 3 (2) (2015) 18e24.

Filler matrix interfaces of inorganic/biopolymer composites and their applications

109

[47] H. Ribeiro, D. Barros, L. Piovan, et al., Surface interactions of gold nanorods and polysaccharides : from clusters to individual nanoparticles, Carbohydr. Polym. 152 (2016) 479e486, https://doi.org/10.1016/j.carbpol.2016.07.018. [48] M.K. Corbierre, N.S. Cameron, M. Sutton, S.G.J. Mochrie, L.B. Lurio, A. Ru, PolymerStabilized Gold Nanoparticles and Their Incorporation into Polymer Matrices, 2001, pp. 10411e10412. [49] Y.-H. Choa, J.-K. Yang, B.-H. Kim, et al., Preparation and characterization of metal/ ceramic nanoporous nanocomposite powders, J. Magn. Magn. Mater. 266 (1) (2003) 12e19. https://doi.org/10.1016/S0304-8853(03)00450-5. [50] A. Nazari, S. Riahi, The effects of SnO2 nanoparticles on physical and mechanical properties of high-strength self-compacting concrete, J. Exp. Nanosci. 7 (5) (2012) 559e577. [51] A.G. El-Shamy, W. Attia, K.M. Abd El-Kader, The optical and mechanical properties of PVA-Ag nanocomposite films, J. Alloy. Compd. 590 (2014) 309e312. https://doi.org/10. 1016/j.jallcom.2013.11.203. [52] K.L. Choy, Chemical vapour deposition of coatings, Prog. Mater. Sci. 48 (2) (2003) 57e170. https://doi.org/10.1016/S0079-6425(01)00009-3. [53] P. Kanmani, J.-W. Rhim, Properties and characterization of bionanocomposite films prepared with various biopolymers and ZnO nanoparticles, Carbohydr. Polym. 106 (2014) 190e199. https://doi.org/10.1016/j.carbpol.2014.02.007. [54] S. Saravanan, S. Nethala, S. Pattnaik, A. Tripathi, A. Moorthi, N. Selvamurugan, Preparation, characterization and antimicrobial activity of a bio-composite scaffold containing chitosan/nano-hydroxyapatite/nano-silver for bone tissue engineering, Int. J. Biol. Macromol. 49 (2) (2011) 188e193. [55] J.-W. Rhim, S.-I. Hong, H.-M. Park, P.K.W. Ng, Preparation and characterization of chitosan-based nanocomposite films with antimicrobial activity, J. Agric. Food Chem. 54 (16) (2006) 5814e5822. [56] M. Cho, H. Chung, W. Choi, J. Yoon, Linear correlation between inactivation of E. coli and OH radical concentration in TiO2 photocatalytic disinfection, Water Res. 38 (4) (2004) 1069e1077. https://doi.org/10.1016/j.watres.2003.10.029. [57] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (5) (2003) 53e229. https://doi.org/10.1016/S0167-5729(02)00100-0. [58] M. Gericke, A. Pinches, Biological synthesis of metal nanoparticles, Hydrometallurgy 83 (1) (2006) 132e140. https://doi.org/10.1016/j.hydromet.2006.03.019. [59] C. Zhan, F. Chen, J. Yang, D. Dai, X. Cao, M. Zhong, Visible light responsive sulfated rare earth doped TiO2@fumed SiO2 composites with mesoporosity: enhanced photocatalytic activity for methyl orange degradation, J. Hazard. Mater. 267 (2014) 88e97. https://doi.org/10.1016/j.jhazmat.2013.12.038. [60] C. Chen, W. Ma, J. Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (11) (2010) 4206e4219, https://doi.org/ 10.1039/B921692H. [61] A. Sirelkhatim, S. Mahmud, A. Seeni, et al., Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism, Nano-Micro Lett. 7 (3) (2015) 219e242. [62] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2) (2014) 95. [63] V. Subramanian, E. Wolf, P.V. Kamat, Semiconductor metal composite nanostructures. To what extent do metal nanoparticles improve the photocatalytic activity of TiO2 films? J. Phys. Chem. B 105 (46) (2001) 11439e11446.

110

Interfaces in Particle and Fibre Reinforced Composites

[64] V. Subramanian, E.E. Wolf, P.V. Kamat, Influence of metal/metal ion concentration on the photocatalytic activity of TiO2 Au composite nanoparticles, Langmuir 19 (2) (2003) 469e474. [65] D.P. Macwan, P.N. Dave, S. Chaturvedi, A review on nano-TiO2 solegel type syntheses and its applications, J. Mater. Sci. 46 (11) (2011) 3669e3686. [66] V.A. Sinani, D.S. Koktysh, B.-G. Yun, et al., Collagen coating promotes biocompatibility of semiconductor nanoparticles in stratified LBL films, Nano Lett. 3 (9) (2003) 1177e1182. [67] L.S. Acosta-Torres, L.M. Lopez-Marín, R.E. Nunez-Anita, G. Hernandez-Padr on, V.M. Casta~no, Biocompatible metal-oxide nanoparticles: nanotechnology improvement of conventional prosthetic acrylic resins, J. Nanomater. 2011 (2011) 12. [68] T. Uemura, N. Yanai, S. Kitagawa, Polymerization reactions in porous coordination polymers, Chem. Soc. Rev. 38 (5) (2009) 1228e1236. [69] G. Kickelbick, Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale, Prog. Polym. Sci. 28 (1) (2003) 83e114. [70] S. Sarkar, E. Guibal, F. Quignard, A.K. SenGupta, Polymer-supported metals and metal oxide nanoparticles: synthesis, characterization, and applications, J. Nanoparticle Res. 14 (2) (2012) 715. [71] R.B. Grubbs, Hybrid metalepolymer composites from functional block copolymers, J. Polym. Sci. Part A Polym. Chem. 43 (19) (2005) 4323e4336. [72] G. Zhao, T. Wen, C. Chen, X. Wang, Synthesis of graphene-based nanomaterials and their application in energy-related and environmental-related areas, RSC Adv. 2 (25) (2012) 9286e9303. [73] R.F. Gibson, A review of recent research on mechanics of multifunctional composite materials and structures, Compos. Struct. 92 (12) (2010) 2793e2810. [74] T.-W. Chou, L. Gao, E.T. Thostenson, Z. Zhang, J.-H. Byun, An assessment of the science and technology of carbon nanotube-based fibers and composites, Compos. Sci. Technol. 70 (1) (2010) 1e19. [75] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science (80- ) 321 (5887) (2008) 385e388. [76] K.S. Novoselov, A.K. Geim, S.V. Morozov, et al., Electric field effect in atomically thin carbon films, Science (80- ) 306 (5696) (2004) 666 LPe669, https://doi.org/10.1126/ science.1102896. [77] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498e3502, https://doi.org/10.1021/nl802558y. [78] R.R. Nair, P. Blake, A.N. Grigorenko, et al., Fine structure constant defines visual transparency of graphene, Science (80- ) 320 (5881) (2008) 1308 LPe1308, https:// doi.org/10.1126/science.1156965. [79] A.A. Balandin, S. Ghosh, W. Bao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902e907, https://doi.org/10.1021/nl0731872. [80] S. Stankovich, D.A. Dikin, G.H.B. Dommett, et al., Graphene-based composite materials, Nature 442 (2006) 282. [81] T. Ramanathan, A.A. Abdala, S. Stankovich, et al., Functionalized graphene sheets for polymer nanocomposites, Nat. Nanotechnol. 3 (2008) 327. [82] Y.R. Lee, A.V. Raghu, H.M. Jeong, B.K. Kim, Properties of waterborne polyurethane/ functionalized graphene sheet nanocomposites prepared by an in situ method, Macromol. Chem. Phys. 210 (15) (2009) 1247e1254, https://doi.org/10.1002/macp.200900157.

Filler matrix interfaces of inorganic/biopolymer composites and their applications

111

[83] Y. Xu, Y. Wang, J. Liang, et al., A hybrid material of graphene and poly (3,4ethyldioxythiophene) with high conductivity, flexibility, and transparency, Nano Res. 2 (4) (2009) 343e348, https://doi.org/10.1007/s12274-009-9032-9. [84] H. Quan, B. Zhang, Q. Zhao, R.K.K. Yuen, R.K.Y. Li, Facile preparation and thermal degradation studies of graphite nanoplatelets (GNPs) filled thermoplastic polyurethane (TPU) nanocomposites, Compos. Part A Appl. Sci. Manuf. 40 (9) (2009) 1506e1513, https://doi.org/10.1016/j.compositesa.2009.06.012. [85] T.S. Sreeprasad, V. Berry, How do the electrical properties of graphene change with its functionalization? Small 9 (3) (2013) 341e350. [86] Y. Cao, J. Feng, P. Wu, Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly (lactic acid) composites, Carbon N.Y. 48 (13) (2010) 3834e3839. [87] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solutionprocessed reduced graphene oxide films as transparent conductors, ACS Nano 2 (3) (2008) 463e470, https://doi.org/10.1021/nn700375n. [88] D.A. Dikin, S. Stankovich, E.J. Zimney, et al., Preparation and characterization of graphene oxide paper, Nature 448 (2007) 457. [89] J.L. Vickery, A.J. Patil, S. Mann, Fabrication of grapheneepolymer nanocomposites with higher-order three-dimensional architectures, Adv. Mater. 21 (21) (2009) 2180e2184, https://doi.org/10.1002/adma.200803606. [90] M.J. McAllister, J.-L. Li, D.H. Adamson, et al., Single sheet functionalized graphene by oxidation and thermal expansion of graphite, Chem. Mater. 19 (18) (2007) 4396e4404, https://doi.org/10.1021/cm0630800. [91] M. Dziadek, E. Stodolak-Zych, K. Cholewa-Kowalska, Biodegradable ceramic-polymer composites for biomedical applications: a review, Mater. Sci. Eng. C 71 (2017) 1175e1191. https://doi.org/10.1016/j.msec.2016.10.014. [92] J.D. Schiffman, C.L. Schauer, A review: electrospinning of biopolymer nanofibers and their applications, Polym. Rev. 48 (2) (2008) 317e352. [93] D.N. Saheb, J.P. Jog, Natural fiber polymer composites: a review, Adv. Polym. Technol. J. Polym. Process. Inst. 18 (4) (1999) 351e363. [94] R.A. Vaia, H. Ishii, E.P. Giannelis, Synthesis and properties of two-dimensional nanostructures by direct intercalation of polymer melts in layered silicates, Chem. Mater. 5 (12) (1993) 1694e1696. [95] L. Liu, Z. Qi, X. Zhu, Studies on nylon 6/clay nanocomposites by melt-intercalation process, J. Appl. Polym. Sci. 71 (7) (1999) 1133e1138. [96] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi, O. Kamigaito, Synthesis of nylon 6eclay hybrid by montmorillonite intercalated with ε-caprolactam, J. Polym. Sci. Part A Polym. Chem. 31 (4) (1993) 983e986. [97] D.D. Eley, N.C. Lockhart, C.N. Richardson, Electrical properties and structural transitions in the mitochondrion, J. Bioenerg. Biomembr. 9 (5) (1977) 289e301, https:// doi.org/10.1007/BF00743216. [98] J.A. Hagen, W. Li, A.J. Steckl, J.G. Grote, Enhanced emission efficiency in organic lightemitting diodes using deoxyribonucleic acid complex as an electron blocking layer, Appl. Phys. Lett. 88 (17) (2006) 171109, https://doi.org/10.1063/1.2197973. [99] A. Tiwari, S.P. Singh, Synthesis and characterization of biopolymer-based electrical conducting graft copolymers, J. Appl. Polym. Sci. 108 (2) (2008) 1169e1177, https:// doi.org/10.1002/app.27789.

112

Interfaces in Particle and Fibre Reinforced Composites

[100] A. Priel, A.J. Ramos, J.A. Tuszynski, H.F. Cantiello, A biopolymer transistor: electrical amplification by microtubules, Biophys. J. 90 (12) (2006) 4639e4643. https://doi.org/10. 1529/biophysj.105.078915. [101] A. Shah, B. Adhikari, S. Martic, et al., Electron transfer in peptides, Chem. Soc. Rev. 44 (4) (2015) 1015e1027, https://doi.org/10.1039/C4CS00297K. [102] S.A. DiBenedetto, A. Facchetti, M.A. Ratner, T.J. Marks, Molecular self-assembled monolayers and multilayers for organic and unconventional inorganic thin-film transistor applications, Adv. Mater. 21 (14e15) (2009) 1407e1433, https://doi.org/10.1002/ adma.200803267. [103] E.A. Weiss, J.K. Kriebel, M.A. Rampi, G.M. Whitesides, The study of charge transport through organic thin films: mechanism, tools and applications, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 365 (1855) (2007) 1509e1537, https://doi.org/10.1098/ rsta.2007.2029. [104] M.R. Singh, G. Bart, M. Zinke-Allmang, Polaron hopping in nano-scale poly (dA)ePoly (dT) DNA, Nanoscale Res. Lett. 5 (3) (2010) 501. [105] S. Kalia, L. Avérous, Biopolymers: Biomedical and Environmental Applications, vol. 70, John Wiley & Sons, 2011. [106] S. Ebnesajjad, Handbook of Biopolymers and Biodegradable Plastics: Properties, Processing and Applications, William Andrew, 2012. [107] S.-B. Park, E. Lih, K.-S. Park, Y.K. Joung, D.K. Han, Biopolymer-based functional composites for medical applications, Prog. Polym. Sci. 68 (2017) 77e105. [108] C.A. Haynes, W. Harvey, P.W. Watt, Biopolymer Composites, August 1997. [109] W. Harvey, N.D. Light, C.A. Haynes, Composite Surgical Material, December 1999. [110] D. Prisco, R. De Santis, F. Mollica, L. Ambrosio, S. Rengo, L. Nicolais, Fiber post adhesion to resin luting cements in the restoration of endodontically-treated teeth, Oper. Dent. 28 (5) (2003) 515e521. [111] H.H.K. Xu, J.B. Quinn, D.T. Smith, J.M. Antonucci, G.E. Schumacher, F.C. Eichmiller, Dental resin composites containing silica-fused whiskers - effects of whisker-to-silica ratio on fracture toughness and indentation properties, Biomaterials 23 (3) (2002) 735e742, https://doi.org/10.1016/S0142-9612(01)00178-8. [112] E. Pinho, M. Grootveld, G. Soares, M. Henriques, Cyclodextrins as encapsulation agents for plant bioactive compounds, Carbohydr. Polym. 101 (2014) 121e135. https://doi.org/ 10.1016/j.carbpol.2013.08.078. [113] V. Kontogiorgos, A.M. Smith, G.A. Morris, The parallel lives of polysaccharides in food and pharmaceutical formulations, Curr. Opin. Food Sci. 4 (2015) 13e18. https://doi.org/ 10.1016/j.cofs.2015.04.001. [114] H. Huang, G.L. Wilkes, “Structure-property behavior of new hybrid materials incorporating oligomeric poly (tetramethylene oxide) with inorganic silicates by a sol-gel process”, Polymer Bulletin 18.5 (1987) 455e462. [115] G. Tsagaropoulos, A. Eisenberg, Dynamic mechanical study of the factors affecting the two glass transition behavior of filled polymers. Similarities and differences with random ionomers, Macromolecules 28 (18) (1995) 6067e6077, https://doi.org/10.1021/ ma00122a011. [116] H. Mallick, A. Sarkar, “An experimental investigation of electrical conductivities in biopolymers.”, Bulletin of Materials Science 23.4 (2000) 319e324.

A critical role of interphase properties and features on mechanical properties of poly(vinyl alcohol) (PVA) bionanocomposites: nanoscaled characterisation and modelling

5

Mohanad Mousa a, b , Yu Dong a a School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia; b Shatrah Technical Institute, Southern Technical University, Baghdad, Iraq

1. Introduction The rapid growth in the field of nanocomposite materials [1,2] has generated a market share of nearly 3.4 and 6.1 billion US dollars in 2014 and 2016 [3], respectively. While the uptake in the applications of nanocomposites is increasing, little is known about how nanomaterial characteristics and material performance are related to nanoscaled properties such as interface/interphase properties. This may further hinder the drive towards precisely tailored morphological structures and desirable material characteristics of nanocomposites, as well as implementation of upscaling methodologies for industrial applications. However, notwithstanding abundant emerging applications in nanocomposites, holistic nanomaterial characterisation and detailed evaluation of material performance for nanocomposites on a nanoscaled level according to their interface/interphase properties are in an urgent demand, which may further broaden their wide scope to clearly understand a critical processing-structure-property relationship. It is well-known that the material performance of nanocomposites can be significantly enhanced when low nanofiller contents typically less than 5 wt% are used when compared with corresponding virgin polymers or microcomposites with the same compositions [4,5]. In a nanocomposite system, “nano effect” for reinforcements stems from a dramatic increase in interphase volume fraction with decreasing the nanofiller size [6,7]. In general, interphases are referred to as a material volume particularly affected by interfacial interactions between fillers and polymer matrices, which, however, possess distinct chemical/physical properties and morphological structures from those of polymer matrices and nanofillers alone. Additionally, interphase dimensions and properties are critical to determine mechanical properties

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00005-4 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Interfaces in Particle and Fibre Reinforced Composites

of bulk nanocomposites (from the perspective of volumetric phase) in that nanointerphases tend to become more dominant volumetric phase for bulk nanocomposites when nanoparticle size is reduced accordingly [8,9]. When the particle size is reduced from micro-, nano-to subnanoscaled level (i.e. molecular size), interphases become increasingly prevalent [10,11], which arises from large surface areas and uniform particle dispersion [2]. The existence of interphase and their corresponding properties can be detected by a variety of characterisation techniques. For instance, nuclear magnetic resonance (NMR) spectroscopy was employed to characterise interphases between ethylene propylene dine monomer (EPDMs) and carbon black by means of polymeric chain dynamics [12]. It has been observed that the chain mobility of EPDMs could be constrained near carbon particles in contrast with that when surrounding polymer matrices owing to the existence of interphases [12]. Besides, broadband dielectric spectroscopy and small angle X-ray scattering were also utilised to directly measure the interphase thickness in the range of 4e6 nm in poly(2-vinylpyridine)/silica nanocomposites [13]. Other techniques worth mentioning are related to the determination of different glass transition temperatures via dynamic mechanical analysis (DMA) for interphases of elastomer nanocomposites [14], as well as interphase identification with the aid of differential scanning calorimetry (DSC) in polypropylene/glass fibre composites [15]. The major drawback of aforementioned techniques lies in the underpinning qualitative or semi-quantitative analysis of nanocomposite interphases, which, to a great extent, reduces the accuracy of the measurements. Recent work [16e19] has confirmed that atomic force microscopy (AFM) can be considered as a more effective and straightforward technique to investigate surface morphology and nanomechanical properties of nanocomposites. The technique involves the determination of the interactions between AFM probe and sample surfaces in relation to their interphases. Consequently, it is possible to identify interphase dimensions and properties that are distinct from those of bulk materials. Wang et al. [17] employed the AFM in a conventional tapping mode to assess the topographical features and nanomechanical properties of natural rubber (NR)/carbon nanotube (CNT) composites as well as mechanical interfacial regions formed around CNTs. Typical cellular structures were clearly observed resulting in greatly enhanced mechanical properties of composites because mechanical interfacial regions and CNTs divided elastomer matrices into small “cells”. Qu et al. [20] implemented an alternative torsional harmonic AFM-based technique for the characterisation of interphase nanomechanical properties in elastomer/carbon black nanocomposites, which determined that an average interphase thickness appeared to be approximately 19
8 nm. However, the quantitative evaluation of interphase dimensions and properties still encounters major challenges such as limited lateral resolutions and position capability of indenter probe generally used in nanoindentation. Interphase thickness in thermosetting polymer/glass fibre composites is much smaller when compared with those of individual fibre and matrix components [21,22]. Plastic deformation occurs when materials undergo a high portion of applied load, thus leading to a minimum allowable distance between two indentation spots, which generally reflects upon the reduction of testing lateral resolutions [22,23]. In recent years, quantitative measurements of

A critical role of interphase properties and features on mechanical properties

117

nanomechanical properties of nanocomposites comprising their stiffness and adhesion have gained the popularity along with acquired corresponding dimensions via peak force quantitative nanomechanical mapping (PFQNM) as a new AFM tapping mode [19,24,25]. PFQNM technique can achieve high-resolution topographical imaging analysis and quantitative mapping of mechanical properties with typical measurements of adhesion force between AFM tip and sample surfaces, as well as surface deformation by avoiding the difficult operation in relation to lateral forces. In this chapter, we described a study to determine nanomechanical properties of poly(vinyl alcohol) (PVA)/nanodiameter bamboo charcoal (NBC) bionanocomposite films. This covers a novel modelling approach to determine the nanoelastic behavior of the films, as well as 3D interphase dimensions and moduli with respect to interphase surface area and interface volume. In this way, a clear guidance can be offered towards the accurate prediction of bulk mechanical properties of nanocomposites.

2. Materials A water-soluble biopolymer PVA (material type: MFCD00081922; molecular weight of 89000e98000 g/mol; degree of hydrolysis at 99.0%e99.8%) was purchased from Sigma Aldrich Pty. Ltd, NSW, Australia. NBC nanoparticles were purchased from US Research Nanomaterials, Inc. TX, USA with the material compositions of 84.14% carbon, 4.6% oxygen, 2.22% hydrogen, 0.66% nitrogen and 8.34% ash [26]. NBCs yield much higher absorption capacity of PVA molecular chains into their porous structur es, thus allowing for effective interfacial bonding between PVA matrices and NBCs, as evidenced by larger BrunauereEmmetteTeller (BET) surface area (SBET) and micropore volume (Vmic) of NBCs at 624.81 m2/g and 0.17 cm3/g, respectively [26].

3. PVA/NBC bionanocomposite fabrication PVA/NBC bionanocomposite films at different NBC contents of 0, 3, 5 to 10 wt% were manufactured using a solution casting method. Initially, 5 wt%/v PVA aqueous solution was prepared by stirring 10 g PVA and 190 mL deionised water at 400 rpm and 90  C for 3 h in order to achieve the complete dissolution. Meanwhile, NBCs were mechanically mixed in deionised water at 405 rpm and 40  C for 2 h prior to the ultrasonication process using an ELMA TieHe5 ultrasonicating bath at 25 kHz and 40  C with the power intensity of 70% for 1 h. Subsequently, NBC suspension was added into PVA solutions in a dropwise manner and resulting mixtures were stirred for 1 h and sonicated for additional 30 min to obtain uniform NBC dispersion. About 20 mL prepared solutions were poured onto a glass Petri dish and dried in an air-circulating oven at 40  C for 48 h. Finally, PVA/NBC bionanocomposite films were stored in a silica gel-containing desiccator for further material characterisation.

118

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Interfaces in Particle and Fibre Reinforced Composites

Characterisation techniques

According to the previous studies [27,28] on the fundamental principle of AFM technique using PFQNM, the probe is made to come into contact with the sample surface for a short period of time to enable the recording of force-distance curves at each pixel of AFM images, as illustrated in Fig. 5.1. Such a technique used in the feedback loop can control the peak force applied on AFM tip in order to significantly reduce the deformation between material samples and the AFM tip. In particular, forcedistance curves are obtained after the conversion from cantilever deflection and piezo position involving tip-material sample interaction through five key steps of “approach”, “attraction”, “repulsion”, “adhesion” and “pull-off”. It is worth noting that accurate force-distance data primarily rely on the high deflection sensitivity for the deflected distance of the cantilever [27].

(a)

(b) Ca

ntil

eve

r

Force DMT fit modulus

Nano

Adhesion force

Force

Peak force

(3)

comp

osite

samp

le

(1) (2) Dissipation

(5)

Approach

(4) Withdraw Deformation Distance

(1)

(2) Approach

(4)

(3) Attraction

Repulsion

(5) Adhesion

Pull-off

Fig. 5.1 (a) Schematic diagram of force vs. tip-sample separation obtained from AFM tapping on the sample surface where the separation is calculated from z piezo position and cantilever deflection (Blue and red curves denote loading and unloading portions, respectively and green dash line is obtained by DMT fitting). The minimum force in the withdraw curve is used for mapping adhesion force. The steps from (1) to (5) represent tip-sample interactions throughout 0.5 ms, and (b) schematic diagram of a probe tapping process for nanocomposite samples [19].

A critical role of interphase properties and features on mechanical properties

119

Nanomechanical properties such as adhesion force between the AFM tip and material sample, elastic modulus, deformation and dissipated energy during the interaction can be explicitly measured. In between, elastic modulus is generally determined by fitting Derjaguin-Muller-Toropov (DMT) model [29] with a portion of force-distance curve where AFM tip and material sample are in contact, as evidenced by the green dash line depicted in Fig. 5.1. Such a model is developed to take into account low adhesion force generated between AFM tip and sample surfaces. The load force on the cantilever FLc can be given in the following formula [29]. 4 FLc ¼ E  3

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rðz  zo Þ3 þ Fadh

(5.1)

where FLC and Fadh are cantilever force and adhesion force, respectively. E  is the reduced elastic modulus, R is the tip-end radius, z  zo is the deformation on the sample, which can be referred to as the difference between current piezo position z and original position z0. The sample modulus Es can be calculated based on E and elastic modulus of AFM tip Etip as well as vs and vtip as poisson’s ratios of the sample and AFM tip, respectively using the following equation [29]: " # 2 1 2 1  v 1  v tip s E ¼ þ Es Etip

(5.2)

In this study, a Bruker Dimension Fastscan AFM system was used to determine nanomechanical properties and topographical features of PVA/NBC bionanocomposite films at 25  C with relative humidity about 30%. Nanomechanical mapping was carried out with the aid of a Bruker RTESPA 525A probe (Nominal spring constant: 200 N/m, nominal tip radius: 8 nm and nominal resonant frequency: 525 kHz). Before each measurement, the calibration of deflection sensitivity was performed on stiff sapphire-12 surface with the spring contant being detected using a thermal tuning method [11,27,30]. AFM imaging analysis was made via RTESPA probes (nominal spring constant: 40 N/m and tip radius: 8 nm) with the image scan rate of 2 Hz and 256256 digital pixel resolution. Resulting topographical images underwent the first-order flattened operation via Flatten command in Burker Nanoscope 1.5 software in order to correct surface tilt and bow effect.

5. Results and discussion 5.1

Particle size and elastic modulus of NBCs

With the aid of PFQNM, average particle diameter and thickness of NBCs were determined to be 69.43
3.29 and 6
0.73 nm, respectively based on holistic evaluation of 1356 NBCs, as illustrated in Fig. 5.2. Uniformly dispersed disk-like

120

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b)

(c)

(d)

Frequency distribution (%)

60

40

20

0 2

2–3 3–4 4–5 5–6 6–7 7–8 8–9 NBC thickness (nm)

Fig. 5.2 AFM image of individual NBCs deposited on steel substrates from aqueous suspensions at (a) low magnification (the inset shows an individual NBC) and (c) high magnification, as well as frequency distribution diagrams for (b) NBC diameter and (d) NBC thickness.

anisotropic NBCs appeared to be evident, which is most likely ascribed to our used dropwise material-processing approach for NBCs [26]. On the other hand, five different zones of interest consisting dispersed NBCs in PVA/NBC bionanocomposite films were selected for nanomechanical mapping, Fig. 5.3. A set of Young’s moduli were estimated using DMT model [31,32], thus also known as DMT moduli, for each targeted zone by obtaining corresponding DMT modulus distribution curves. According to curve mapping using Gaussian distribution, average Young’s modulus (or DMT modulus) of NBCs was estimated to be 84.5
3.6 GPa, which is higher than those of carbon-based fillers such as T300 carbon fibres (generally in the range from 20 to 40 GPa [33]) and carbon nanoparticles (i.e. 43.89 GPa) [34]. On the contrary, the elastic modulus of NBCs determined in this study is still much lower than the moduli of graphene oxides (GOs) in range of 200e250 GPa [35] and multi-walled CNTs (MWCNTs) at 0.9 TPa [36]. Nonetheless, GO sheets tend to yield

A critical role of interphase properties and features on mechanical properties

121

3.0 2.5

195.0 GPa Frequency (%)

Zone 1 18 .0 GPa

89.3 GPa

2.0 1.5 1.0 0.5 0.0

Zone 1 0

50

100

150 E (GPa)

200

250

300

1.6 1.4

Zone 2 183.0 GPa Frequency (%)

1.2

26.3 GPa

83.2 GPa

1.0 0.8 0.6 0.4

Zone 2

0.2 0.0

Zone 3

40

60

80

100 120 140 E (GPa)

160

180

200

3.0

179.4 GPa

2.5 Frequency (%)

31.5 GPa

87.2 GPa

2.0 1.5 1.0

Zone 3

0.5 0.0

Zone 4

50

0

100 E (GPa)

150

200

3.0

185.6 GPa

Frequency (%)

2.5

13.0 GPa

80.5 GPa

2.0 1.5 1.0 0.5

Zone 5

0.0

200.5 GPa

Zone 4 100 E (GPa)

0

200

4.0 3.5

23.0 GPa

82.4 GPa

1 5.0 um

2 1

3 2

3

4 4

5.0 um

Frequency (%)

3.0 2.5 2.0 1.5 1.0

Zone 5

0.5 0.0

0

100

200

300

E (GPa)

Fig. 5.3 3D DMT modulus mapping images and DMT modulus histogram of NBCs at five different zones of interest. The determination of elastic moduli is based on fitting Gaussian distribution curves for data histograms.

particle agglomeration [37] with detrimental effect on human cells by decreasing A549 cell viability at higher GO content levels [38]. Furthermore, the material demerits of CNTs lie in their high material cost and nanotoxicity owing to the CNT accummulation in cytoplasm for the damage of human cells under certain inhalation conditions [1]. As such, NBCs can be considered as potential carbon-based nanofillers to possibly replace currently used GOs and CNTs from the viewpoint of cost-effectiveness and safe material handling of nanofillers.

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Interfaces in Particle and Fibre Reinforced Composites

5.2

Nanomechanical properties of PVA/NBC bionanocomposites

Both the height and DMT modulus mapping images of neat PVA films are shown in Fig. 5.4(a and b), respectively. There are two distinct phases represented by crystalline phase (in bright white colour) and amorphous phase (in dark colour), which is a very common phenomenon in semicrystalline polymers like PVA [39,40]. A typical section A1-B1 was chosen in Fig. 5.4(b) to analyse DMT modulus. Its profile yielded a high modulus level of 24
4.2 GPa and phase width of 20e76 nm, mainly resulting from the bundles of semicrystalline stacks in crystalline phase. Such a DMT modulus value was far higher than the elastic modulus of local PVA-poly(acrylic acid) (PAA) nanophase at 9.9 GPa [41], but was very similar to corresponding modulus of 23.69 GPa obtained from PVA/chitosan composite film coating [42]. In contrast, PVA amorphous phase possessed a relatively low modulus of 11.4
3.1 GPa with the phase width of 18e65 nm accordingly. Overall, the DMT moduli for both PVA phases appreared to be significantly high when compared with the elastic modulus of PVA films at 2.08 GPa at a macroscopic level, based on our previously determined

(a)

(b)

6.5 nm

39 GPa

0.3 nm

9.5 GPa

(c)

(d)

40 Crystalline phase

37.46 nN

E (GPa)

30

20

10

16.19 nN

Amorphous phase 0 0.0

0.2

0.4

0.6

0.8

1.0

Scan distance (µm)

Fig. 5.4 2D AFM mapping images of PVA films for (a) height and (b) DMT modulus, (c) DMT modulusescan distance curve for corresponding typical section A1-B1 and (d) 2D adhesion force mapping image of PVA films [19].

A critical role of interphase properties and features on mechanical properties

123

tensile test results [26]. Such a large discrepancy generally occurs between nanomechanical properties and bulk properties of composite materials [19,33,42]. Gu et al. [43] reported the elastic modulus of epoxy/carbon fibre composites at the nanoscaled level at 17 GPa as opposed to approximately 3e4 GPa for that of bulk composites. In a similar manner, an elastic modulus in magnitude of 11.9 GPa was found for the local PVA regions, which is higher than the elastic modulus (¼2.08 GPa) of bulk PVA films in this study [26]. Such great variations can be attributed to three major reasons. First, although important PFQNM parameters were calibrated prior to each measurement, the shape function of the probe topmost tip might not be accurate under the condition of very low penetration depth [22,44]. The use of low indentation force led to low indentation depth to induce residual stress and plastic deformation from next indents with higher lateral resolution capability particularly on the nanoscale. Second, when the penetration depth for measuring elastic modulus was considered at a nanoscaled level, the difference in morphological structures between the outer skin on material surfaces and bulk materials [22,43] was believed to contribute to the significant variations between nanomechanical properties of local material phases and bulk properties. Third, distinct measurement mechanisms between PFQNM and conventional tensile testing also played an important role in determining local DMT modulus and tensile modulus for bulk properties [45]. The application of adhesion force mapping is also an effective approach to characterise PVA crystalline and amorphous phases (Fig. 5.4(d)). Adhesion forces for crystalline and amorphous phases were determined to be 19.7
0.6 and 33
2.8 nN, respectively. Relatively high adhesion force obtained for amorphous PVA phase could be associated with typical viscoelastic behaviour taking place in amorphous polymers in a non-glassy state. In general, amorphous phase tended to have lower density and surface tension as opposed to crystalline phase [40]. On the other hand, Fig. 5.5(a) illustrated a 3D height mapping image on PVA/NBC bionanocomposites reinforced with 3 wt% NBCs where a typical section A2-B2 was selected to plot corresponding DMT modulus profile for more detailed analysis in Fig. 5.5(b). Apparently, circled peaks with relatively high modulus values were indicative of the existence of NBCs within PVA matrices. A clear low-high modulus cyclic behaviour depicted in Fig. 5.5(b) could be explained by an alternating sequence of amorphous and crystalline phases of PVA [19,39,40]. PVA/NBC bionanocomposites reinforced with 3 wt% NBCs shown in Fig. 5.5(b) possessed totally different DMT modulus profile when compared with that of neat PVA, as depicted in Fig. 5.4(c) in terms of the degree of structural orderliness. This finding could be related to significant reinforcing effect of nanoparticles to restrict the chain mobility of polymer matrices and significant influence on the structures and properties of both crystalline and amorphous phases for PVA matrices [46]. To further investigate morphological structures of PVA/NBC bionanocomposites, both height and adhesion force mapping images are presented at high magnifications in Fig. 5.5(c and d). The data analysis of DMT modulus profile based on a typical section A3-B3 in Fig. 5.5(c) is demonstrated in Fig. 5.5(e). The crystalline and amorphous phase widths of such bionanocomposites were reported to be in range of 5e53 and 4e35 nm, respectively, which appeared to be much lower than those of corresponding PVA

124

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b) A2

B2

80

1.0 um

5.3 nm 0.3 nm

0.8 1.0 um

17 nm

0.4 0.6

E(GPa)

0.6 0.8

40

0.2 0.4

0.2

16 nm

1.0 um 0.2

0.6

0.4

0.8

0 1

0 Scan distance (µm)

(d)

(c)

400 nm

5.4 nm 0.3 nm

41.3 nN 0.25 nN

400 nm

300

300

200

400 nm

400 nm

100

200 300

300 100

200

200 300

100

100 100

300

200

400 nm

400 nm

200 100

(e)

(f) Crystalline phase

40

E(GPa)

PVA

Lamellar stacks

Stack bundles 20 Amorphous phase PVA/3 wt% NBC bionanocomposites 0 0

250

500

Scan distance (nm)

Fig. 5.5 (a) 3D AFM height mapping image of PVA/NBC bionanocomposites and (b) DMT modulus-scan distance curve for corresponding typical section A2-B2 in (a), 3D AFM mapping images of PVA/NBC bionanocomposites for (c) height and (d) adhesion force at high magnifications, (e) DMT modulus-scan distance curve for corresponding typical section A3-B3 in (c), and (f) schematic diagram for bundles of lamellar stacks in both PVA and PVA/NBC bionanocomposite films [19].

phases exhibited in Fig. 5.4(c). The decrease in phase width yields the reduction of stack sizes while the number of lamellae stacks per unit volume from neat polymer to nanocomposites increases shown in Fig. 5.5(f), thus resulting in the enhancement of tensile strengths of nanocomposites. This is because tensile stress highly depends

A critical role of interphase properties and features on mechanical properties

125

on the grain or particle size according to the Hall-Petch relation [47,48], as evidenced by much higher tensile strength of 147.94 MPa for PVA/NBC bionanocomposites when compared to 70.32 MPa for neat PVA in our previous work [26]. The reinforcing effect of NBCs within PVA matrices not only lies in the reduction of PVA phase widths, but also the interphase properties and dimensions [11,26] in terms of efficient interfacial bonding between nanofillers and matrices for achieving excellent material performance of nanocomposites.

5.3

Interphase characterisation of PVA/NBC bionanocomposites

5.3.1

Modelling approach

The elastic properties of interphases like elastic modulus in PVA/NBC bionanocomposites were determined on the basis of a set of data collection from PVA/NBC interphases surrounding 75 different NBCs along with 25 line scan regions (LSRs). 3D interphase dimensions could be characterised by interphase width WiInterphase, interphase length Lj Interphase and interphase height Hk Interphase on individual PVA/NBC phases via PFQNM. Since interphases possess completely different nanomechanical properties from those of nanofillers and polymer matrices in nanocomposites, it is thus much easier to distinguish interphase dimensions from those of NBCs and PVA. For example, WiInterphase was determined by scanning along the ith transverse plane (i ¼ 1, 2, 3 .) for PVA/NBC phases. In a similar manner, LjInterphase and HkInterphase along the jth longitudinal plane (j ¼ 1, 2, 3 .) and kth height plane (k ¼ 1, 2, 3 .) could be obtained, respectively. More procedure details can be found elsewhere [11]. On the other hand, uniform nanoparticle dispersion is assumed to occur in possession of two typical scenarios of fully and partially embedded NBCs within PVA matrices in bionanocomposites, as depicted in Fig. 5.6. Apparently it is evident that PVA/NBC interphases are generally surrounded between inner interface area and outer interface area bound by nanofillers and PVA matrices, respectively. Based upon analytical equations obtained from Behmer and Hawkins [49] to calculate surface areas of anisotropic shapes, one can establish the following Eqs. (5.3)e(5.6) in order to determine surface areas of outer interface (SAouter Interface) and inner interface (SAinner Interface) in a wide range of fully and partially embedded NBCs. Subscripts of ‘f’ and ‘p’ denote fully and partially embedded NBCs, respectively, as demonstrated in Fig. 5.6. 2 2 ðSAouter Interface Þf ¼ a1 þ b1 L2Interphase þ c1 WInterphase þ d1 HInterphase

(5.3)

ðSAouter Interface Þp ¼ a2 þ b2 L0 Interphaseeffective þ c2 W 0 Interphaseeffective 2

2

þ d2 H 0 Interphaseeffective 2

2 2 ðSAinner Interface Þf ¼ a3 þ b3 L2NBC þ c3 WNBC þ d3 HNBC

(5.4) (5.5)

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Interfaces in Particle and Fibre Reinforced Composites

(a)

(b) PVA matrices Inner interface

PVA matrices Effective interphase

Interphase NBC

NBC

Inner interface Outer interface Outer interface

Fig. 5.6 Schematic diagrams of (a) single fully embedded NBC and (b) single partially embedded NBC with particle-matrix interactions in PVA/NBC bionanocomposites [11].

ðSAinner Interface Þp ¼ a4 þ b4 L0 NBCeffective þ c4 W 0 NBCeffective 2

þ d4 H 0 NBCeffective 2

2

(5.6)

where L0 Interphase-effective, W0 Interphase-effective and H0 Interphase-effective refer respectively to maximum length, width and thickness for effective interphases. Whereas, L0 NBC0 0 effective, W NBC-effective and H NBC-effective represent maximum length, width and thickness for effective NBCs. a1, b1, c1, d1, a2, b2, c2, d2, a3, b3, c3, d3 and a4, b4, c4, d4 are constants determined by curve fitting. For example, the constants of a1 , b1 , c1 and d1 in PVA/NBC bionanocomposites were determined to be 0.7439, 0.3627, 0.7006, and 0.9979, respectively, by fitting Eq. (5.3) to experimental data of surface area (SAouter Interface)f obtained from AFM measurements (see more details in [11]). After surface area data were obtained, the relevant results were implemented to detect interphase/NBC volume (VNBC/Interphase) and NBC volume (VNBC) in terms of both fully and partially embedded NBCs, according to the following modified equations derived from Behmer and Hawkins [49]: f1 ðSAouter Interface Þf ¼ e1 VNBC=Interphase

(5.7)

f2 ðSAouter Interface Þp ¼ e2 VNBC=Interphaseeffective

(5.8)

f3 ðSAinner Interface Þf ¼ e3 VNBC

(5.9)

f4 ðSAinner Interface Þp ¼ e4 VNBCeffective

(5.10)

A critical role of interphase properties and features on mechanical properties

127

e1, f1, e2, f2, e3, f3 and e4, f4 are constants determined by curve fitting. For example, the constants of e1 , f1 were estimated to be 0.3824 and 0.3825, respectively, by fitting Eq. (5.7) to the value of VNBC/Interphase based on the AFM measurement (see more details in [11]). The final step involves the determination of interphase volume VInterphase for fully or partially embedded NBCs in PVA bionanocomposites (Fig. 5.6) using the equations given below: ðVInterphase Þf ¼ VNBC=Interphase  VNBC

(5.11)

ðVInterphase Þp ¼ VNBC=Interphaseeffective  VNBCeffective

(5.12)

5.3.2

Interphase elastic properties

As shown in Fig. 5.7(a), strong interactions are generated between NBCs and PVA matrices, which are found not only on NBC surfaces, but also in porous NBC particles with the penetration of PVA molecules. A typical topographical image of PVA/NBC bionanocomposite sample and corresponding variations of elastic moduli from PVA matrices, interphases to NBCs are displayed in Fig. 5.7(b and c), respectively. A typical section A4-B4 in Fig. 5.7(c) indicates that elastic modulus is significantly increased from the lowest modulus level of 14.8 GPa with respect to PVA matrices to the highest level of 72.86 GPa assigned to NBC zones. Moreover, interphase elastic modulus clearly demonstrates an alomost linearly increasing trend as a function of scan distance apart from interphase boundary regions surrounding PVA matrices and NBCs alone, ranging from 17.1 GPa (near PVA) to 64.9 GPa (around NBCs) with interlayer thickness of 31.8 nm [11]. Final interphase modulus was acquired from the solid black curve by best fitting discrete modulus data set according to 25 representative LSRs in Fig. 5.7(d). The same applies to elastic moduli of NBCs and PVA matrices detected to be 78.4
4.9 and 24.25
4.2 GPa, respectively. Significantly high interphase modulus is associated with highly porous structures of NBCs that are distinct from other carbon based nanofillers. Such porous structures can facilitate the generation of highly positive capilary pressure in order to drive PVA molecular chains into NBC pores, thus resulting in the formation of typical chemical bonding in nanocomposites [55,56]. On the other hand, it is worth mentioning that “mechanical anchoring” can also happen by creating a mechanical interlocking mechanism when PVA molecules penetrate into NBC pores in their bionanocomposites. The modulus gradient effect from NBCs to PVA matrices at the interphase regions lies in the difference in the number of hydrogen bonding from nanofiller surfaces to polymer matrices, which is considered as an important factor to well tailor mechanical properties of polymer nanocomposites [50,51].

5.4

Interphase dimensions

The variations in height and adhesion force mapping images enable the determination of the interphase dimensions of PVA/NBC bionanocomposites, as shown in

128

Interfaces in Particle and Fibre Reinforced Composites

(a)

0.4 nm Partially embedded NBCs

Fully embedded NBCs

160 nm

0.4 nm 120

0.0 nm 80 160 nm

40 120

80

40

0.0 nm

(b)

(c) Elastic modulus (E) (GPa)

225 150

300 nm 225

75

150 75

Interphase

PVA

75

300 nm

B4

A4 NBC

50

25

0 0

50 Scan distance (nm)

100

(d) 100 PVA zone 90 80

Elastic modulus (E) (GPa)

Interphase zone

NBC zone

E=15.38/(1+exp(Sd-104.2)/13.7) 2

R =0.83 70 60 50 40 30 20 10 0 0

50

100

150

200

Scan distance (Sd) (nm)

Fig. 5.7 (a) Proposed scheme for typical PVA/NBC interactions with fully and partially embedded NBCs, (b) 3D AFM modulus mapping image of PVA/3 wt% NBC bionanocomposites, (c) modulus profile for PVA/3 wt% NBC bionanocomposites taken along typical section A4-B4, and (d) typical data sets of modulus profiles along 25 line scan regions (LSRs) of corresponding bionanocomposites with the best-fit curve in which Sd represents scan distance [11].

Fig. 5.8(a and b), as well as Fig. 5.8(c and d), respectively. The height of PVA is less than those of NBCs with a nearly linearly increasing tendency from interphase regions near PVA matrices to those close to NBCs, Fig. 5.8(a and b). It appears to be much easier to differentiate NBCs and PVA matrices on account of their differet adhesion

A critical role of interphase properties and features on mechanical properties

(a)

(b)

129

A5

8 PVA

B5 Interphase NBC Interphase 16 nm 14 nm 13 nm

PVA

7 5.0 nm

6 Height (nm)

1.0 nm

400 nm 300

5 4 3

200

400 nm

2

300

100

200

1

100

0

(c)

20

40

60 80 100 Scan distance (nm)

120

140

(d) A5 16

B5 PVA

PVA Interphase NBC Interphase 14 nm 13 nm 16 nm

14 12 Adhesion (nN)

15.0 nN 3.0 nN

400 nm 300

10 8 6 4

200

400 nm 300 100

2

200 100

0 0

20

40

60

80

100

120

140

Scan distance (nm)

Fig. 5.8 (a) 3D AFM height mapping image of PVA/3 wt% NBC bionanocomposites, (b) height profile of corresponding bionanocomposites taken along typical section A5-B5, (c) 3D AFM adhesion mapping image of PVA/3 wt% NBC bionanocomposites, and (d) adhesion profile of corresponding bionanocomposites taken along typical section A5-B5 [11].

properties. The AFM measurements revealed that adhesion force to PVA matrices was approximately 10.76
3.42 nN, which was much higher in magnitude than that of NBCs at 2.1
0.87 nN. Consequently, scan distances of 16 and 13 nm in Fig. 5.8(d) represented the values of interphase thickness with a sharply increasing adhesion gradient taking place from NBCs to PVA matrices on both sides of selected material regions. Such results are in good accordance with the interphase thickness of 13 and 12.5 nm, previously reported for epoxy/graphene nanoplatelet (GNP) composites and epoxy/graphene oxide (GO) nanocomposites, respectively [52]. As far as 3D interphase dimensions are concerned, interphase thickness tInterphase should be more appropriately deemed as a non-uniform and non-constant geometric parameter [11] as opposed to constant interphase thickness with one-dimensional interphase layers for simplicity, which, however, is generally assumed in current modelling framework of nanocomposites. In fact, the uniformity of tInterphase is ascribed to the number of chemical hydrogen bonds as well as physical roughness of NBC surfaces [53e55]. To overcome the above-mentioned disadvantages, our study focused on the successful identification of actual 3D interphases in term of interphase width (WiInterphase), length (Lj Interphase) and height (Hk Interphase) among individual NBCs.

130

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(a)

(b) 20

15

10 15 10 5 5 0

0

20

40

60

80

100

0 0

Scan distance (Sd) (nm)

(c)

5

10

15

20

25

30

(d) 160

15.0

140 2

R =0.71

12.5

2

R =0.86

120 10.0

100 80

7.5

60 5.0 40 2.5

20 0

0.0 0

5

10

15

20

25

30

0

35

10

20

30

40

50

60

70

80

90

100 110 120 130

(e) 120

2

100

R =0.77

80

60

40

20

0 0

10

20

30

40

50

60

70

80

90

100

Fig. 5.9 (a) Maximum interphase dimensions along typical PVA/NBC interphases and relationships between NBC and interphase dimensions in PVA/NBC bionanocomposites: (b) interphase thickness (tInterphase) and NBC thickness (tNBC), (c) maximum interphase height (HInterphase-max) and NBC height (HNBC), (d) maximum interphase length (LInterphase-max) and NBC length (LNBC) as well as (e) maximum interphase width (WInterphase-max) and NBC width (WNBC) [11].

As demonstrated in Fig. 5.9(a), LInterphase, W Interphase and H Interphase were determined to be 97, 40 and 8 nm, respectively. However, there is no clear relationship between tInterphase and NBC thickness (tNBC), which suggests that interphase thickness is indepnedent of nanoparticle thickness. On the other hand, when observed from Fig. 5.9(bee), it is manifested that H Interphase tends to decrease in a linear manner

A critical role of interphase properties and features on mechanical properties

131

with an increase in H NBC. On the contrary, LInterphase and W Interphase linearly increase with an increase in LNBC and W NBC, respectively. This phenomenon is associated with large contact surface areas between polymer matrices and nanoparticles in a nanocomposite system [1]. Fig. 5.10 shows a proposed mechanism model in relation to the effects of NBC dispersion and particle-matrix interaction. Two major NBC dispersion states within PVA matrices in bionanocomposites including uniform NBC dispersion and highly aggregated NBC dispersion take place according to our previous study in material morphology [26]. Additionally, two further categories of PVA/NBC interactions, namely fully embedded and partially embedded NBCs within PVA matrices, are

(a)

(b) 5.1 nm

8.1 nm

1.0 nm

1.6 nm

(d)

80

40 Inner interface-fully embedded NBC Inner interface-partially embedded NBC Outer interface -fully embedded NBC Outer interface-partially embedded NBC

Elastic modulus (E) (GPa)

Interphase modulus (EInterphase) (GPa)

(c)

PVA matrices

Interphase (2)

NBC

(1)

0 0

2500 (SA)Interphase (nm2)

5000

Distance (nm)

(e) Interphase modulus (EInterphase) (GPa)

100 80 60 40 20

Fully embedded NBC Partially embedded NBC

0 0

4000

8000 V Interphase

12000

(nm3)

Fig. 5.10 2D AFM height mapping images of PVA/NBC bionanocomposites: (a) fully embedded NBCs and (b) partially embedded NBCs, (c) relationship between SAInterphase and EInterphase, (d) schematic diagram for two proposed interphase regions 1 and 2 and (e) relationship between VInterphase and EInterphase [11].

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illustrated in Fig. 5.10(a and b). The former results in more effective interphases surrounding each side of NBCs with full coverage for effective load transfer from matrices to nanoparticles. In comparison, the latter yields less effective interphases because some NBCs tend to be free of contact with PVA matrices, which gives rise to the noticeable reduction of effective interfacial regions. VInterphase is a critical parameter to assess the influence of fully or partially embedded NBCs in PVA/NBC bionanocomposites in expression of (V Interphase)f and (V Interphase)p, respectively. The relationship between interphase modulus (EInterphase) and surface areas (i.e. SAouter Interface and SAinner Interface) for both fully and partially embedded NBCs within PVA matrices in bionanocomposites is illustrated in Fig. 5.10(c). Evidently, EInterphase appears to be enhanced remarkably in a non-linear manner with increasing SAouter Interface and SAinner Interface. As such, it can be suggested that interphase modulus highly depends on interphase dimensions. Overall, fully embedded NBCs yield consistently higher EInterphase irrespective of the use of either inner or outer interface as opposed to partially embedded NBCs in bionanocomposites. It is proven that interphases plays a more important role in the enhancement of mechanical properties of PVA/NBC bionanocomposites, which is most likely to be ascribed to active matrixfiller interactions and strong interfacial bonding. Furthermore, relatively high overall interphase modulus has been detected when SAinner Interface is taken into consideration in comparison with SAouter Interface. As demonstrated in Fig. 5.10(d), interphase regions with modulus gradient effect are classified into two distinct regions, namely region 1 and region 2 where region 1 represents the interphase region close to NBC zones with a relatively high interphase density as opposed to region 2 in good accordance with Liu et al. [52]. Moreover, our high interphase modulus results, based on inner interface area are in good agreement with Fan et al. [56], which indicates the enhancement of interphase modulus with increasing interphase density. This is because inner interface is adjacent to NBC zones with higher material density and elastic modulus, as depicted in Fig. 5.10(d). On the other hand, the correlation between EInterphase and VInterphase is plotted in Fig. 5.10(e) for both fully and partially embedded NBCs in bionanocomposites. Apparently, EInterphase is significantly enhanced as VInterphase increases, which is similar to the aforementioned case by increasing SAInterphase. It is again confirmed that fully embedded NBCs are more favorable morphological structures towards greater EInterphase in terms of VInterphase when compared with partially embedded counterparts in PVA/NBC bionanocomposites.

6.

Conclusions

In this chapter, we have described an investigation of nanomechanical properties of PVA and PVA/NBC bionanocomposites. The PFQNM measurement indicates PVA crystalline phase has a higher elastic modulus as compared to the corresponding crystalline phase. Both phase widths were found to decrease with the incorporation of NBCs, resulting in the increase in the number of lamellae stacks per unit volume in order to significantly improve tensile properties of PVA/NBC bionanocomposites.

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A pioneering modelling approach that we have developed has been used for determining the nanoelastic behaviour of PVA/NBC bionanocomposites, as well as 3D interphase dimensions and elastic moduli, in terms of Interphase surface area and interphase volume. Our results confirm that interphase thickness is non-uniform among individual nanoparticles, which is independent of nanoparticle thickness. Furthermore, interphase modulus increases with increasing interphase volume, and nanomechanical properties and dimensions of interphases depend primarily on nanofiller dispersion states in PVA/NBC bionanocomposites. This study concentrates on the explicit interphase characterisation in nanocomposite systems so that the implementation of actual interphase properties and dimensions can offer an essential guidance towards accurate theoretical and numerical modelling work in the future in place of simple assumption of uniform interphase layer structures.

Acknowledgements Mr M. Mohanad would like to express his gratitude to the Higher Committee for Education Development (HCED) in Iraq for awarding a PhD scholarship at Curtin University. Dr Thomas Becker at Nanochemistry Research Institute, Curtin University is also acknowledged for technical support in PFQNM measurements.

References [1] M.H. Mousa, Y. Dong, I.J. Davies, Recent advances in bionanocomposites: Preparation, properties, and applications, Int. J. Polym. Mater. Polym. Biomater. 65 (5) (2016) 225e254. [2] H. Huang, I. Dobryden, N. Ihrner, M. Johansson, H. Ma, J. Pan, P.M. Claesson, Temperature-dependent surface nanomechanical properties of a thermoplastic nanocomposite, J. Colloid Interface Sci. 494 (2017) 204e214. [3] Y. Huang, L. Wang, Experimental studies on nanomaterials for soil improvement: a review, Environ. Earth. Sci. 75 (2016) 497. [4] S.K. Kumar, N. Jouault, B. Benicewicz, T. Neely, Nanocomposites with polymer grafted nanoparticles, Macromolecules 46 (9) (2013) 3199e3214. [5] D. Paul, L.M. Robeson, Polymer nanotechnology: Nanocomposites, Polymer 49 (15) (2008) 3187e3204. [6] H. Hu, L. Onyebueke, A. Abatan, Characterizing and modeling mechanical properties of nanocomposites-review and evaluation, J. Miner. Mater. Charact. Eng. 9 (4) (2010) 275e319. [7] D.S. Simmons, An emerging unified view of dynamic interphases in polymers, Macromol. Chem. Phys. 217 (2) (2016) 137e148. [8] C.L. Wu, M.Q. Zhang, M.Z. Rong, K. Friedrich, Tensile performance improvement of low nanoparticles filled-polypropylene composites, Compos. Sci. Technol. 62 (10e11) (2002) 1327e1340.

134

Interfaces in Particle and Fibre Reinforced Composites

[9] M. Mousa, Y. Dong, I.J. Davies, Eco-friendly polyvinyl alcohol (PVA)/bamboo charcoal (BC) nanocomposites with superior mechanical and thermal properties, Adv. Compos. Mater. 27 (5) (2018) 499e509. [10] T. Lewis, Interfaces: nanometric dielectrics, J. Phys. D Appl. Phys. 38 (2) (2005) 202e212. [11] M. Mousa, Y. Dong, Novel three-dimensional interphase characterisation of polymer nanocomposites using nanoscaled topography, Nanotechnology 29 (2018) 385701. [12] V. Litvinov, P. Steeman, EPDMCarbon black interactions and the reinforcement mechanisms, as studied by low-resolution 1H NMR, Macromolecules 32 (25) (1999) 8476e8490. [13] A.P. Holt, P.J. Griffin, V. Bocharova, A.L. Agaopov, A.E. Imel, M.D. Dadmun, J.R. Sangoro, A.P. Sokolov, Dynamics at the polymer/nanoparticle interface in poly (2-vinylpyridine)/silica nanocomposites, Macromolecules 47 (5) (2014) 1837e1843. [14] W.K. Goertzen, M. Kessler, Dynamic mechanical analysis of fumed silica/cyanate ester nanocomposites, Compos. Appl. Sci. Manuf. 39 (5) (2008) 761e768. [15] G. Pompe, E. M€ader, Experimental detection of a transcrystalline interphase in glass-fibre/ polypropylene composites, Compos. Sci. Technol. 60 (11) (2000) 2159e2167. [16] L. Tadiello, M. D’Arienzo, B.D. Credico, T. Hanel, L. Matejka, M. Mauri, F. Morazzoni, R. Simonutti, M. Spirkova, R. Scotti, The fillererubber interface in styrene butadiene nanocomposites with anisotropic silica particles: morphology and dynamic properties, Soft Matter 11 (20) (2015) 4022e4033. [17] D. Wang, S. Fujinami, K. Nakajima, K. Niihara, S. Inukai, H. Ueki, A. Magario, T. Noguchi, M. Endo, T. Nishi, Production of a cellular structure in carbon nanotube/ natural rubber composites revealed by nanomechanical mapping, Carbon 48 (13) (2010) 3708e3714. [18] N. Ning, T. Mi, G. Chu, L.Q. Zhang, L. Liu, M. Tian, H.T. Yu, Y.L. Lu, A quantitative approach to study the interface of carbon nanotubes/elastomer nanocomposites, Eur. Polym. J. 102 (2018) 10e18. [19] M. Mousa, Y. Dong, Elastic behavior of nanophases in polyvinyl alcohol (PVA)/bamboo charcoal (BC) nanocomposite films, Front. Mater. 5 (2018) 44. [20] M. Qu, F. Deng, S.M. Kalkhoran, A. Gouldstone, A. Robisson, K.J.V. Vliet, Nanoscale visualization and multiscale mechanical implications of bound rubber interphases in rubberecarbon black nanocomposites, Soft Matter 7 (3) (2011) 1066e1077. [21] S.L. Gao, E. M€ader, S.F. Zhandarov, Carbon fibers and composites with epoxy resins: topography, fractography and interphases, Carbon 42 (3) (2004) 515e529. [22] C.A. Clifford, M.P. Seah, Quantification issues in the identification of nanoscale regions of homopolymers using modulus measurement via AFM nanoindentation, Appl. Surf. Sci. 252 (5) (2005) 1915e1933. [23] S.L. Gao, E. M€ader, Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites, Compos. Part A: Appl. Sci. Manuf. 33 (4) (2002) 559e576. [24] S. Jafarzadeh, P.M. Claesson, P.E. Sundell, J. Pan, E. Thormann, Nanoscale electrical and mechanical characteristics of conductive polyaniline network in polymer composite films, ACS Appl. Mater. Interfaces 6 (21) (2014) 19168e19175. [25] M. Sababi, J. Kettle, H. Rautkoski, P.M. Claesson, E. Thormann, Structural and nanomechanical properties of paperboard coatings studied by peak force tapping atomic force microscopy, ACS Appl. Mater. Interfaces 4 (10) (2012) 5534e5541. [26] M. Mousa, Y. Dong, Strong poly (vinyl alcohol)(PVA)/bamboo charcoal (BC) nanocomposite films with particle size effect, ACS Sustain. Chem. Eng. 6 (1) (2017) 467e479.

A critical role of interphase properties and features on mechanical properties

135

[27] B. Pittenger, N. Erina, C. Su, Quantitative mechanical property mapping at the nanoscale with PeakForce QNM, Bruker Application Note # 128 (2010) 1e12. [28] K.O. Van Der Werf, C.A. Putman, B.G. de Grooth, J. Greve, Adhesion force imaging in air and liquid by adhesion mode atomic force microscopy, Appl. Phys. Lett. 65 (9) (1994) 1195e1197. [29] B.V. Derjaguin, V.M. Muller, Y.P. Toporov, Effect of contact deformations on the adhesion of particles, J. Colloid Interface Sci. 53 (2) (1975) 314e326. [30] B. Ohler, Practical advice on the determination of cantilever spring constants, 2007, pp. 1e12. Bruker Application Note# AN94. [31] T. Pirzada, S.A. Arvidson, C.D. Saquing, S.S. Shah, S.A. Khan, Hybrid silicaePVA nanofibers via solegel electrospinning, Langmuir 28 (13) (2012) 5834e5844. [32] X. Yang, L. Li, S. Shang, X. Tao, Synthesis and characterization of layer-aligned poly (vinyl alcohol)/graphene nanocomposites, Polymer 51 (15) (2010) 3431e3435. [33] Y.F. Niu, Y. Yang, S. Gao, J.W. Yao, Mechanical mapping of the interphase in carbon fiber reinforced poly (ether-ether-ketone) composites using peak force atomic force microscopy: interphase shrinkage under coupled ultraviolet and hydro-thermal exposure, Polym. Test. 55 (2016) 257e260. [34] M.A. Hossain, S. Islam, F.A. Chowdhury, K. Uchida, T. Tamura, K. Sugawa, T. Mochida, J. Otsuki, T.G. Mohiuddin, M.S. Alam, Structural, mechanical, and electrical properties of carbon nanoparticles synthesized from diesel, Fullerenes, Nanotub. Carbon Nanostruct. 24 (1) (2016) 43e51. [35] J.W. Suk, R.D. Piner, J. An, R.S. Ruoff, Mechanical properties of monolayer graphene oxide, ACS Nano 4 (11) (2010) 6557e6564. [36] B. Demczyk, Y.M. Wang, J. Cumings, M. Hetman, W. Han, A. Zettl, R.O. Ritchie, Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes, Mater. Sci. Eng. A 334 (1e2) (2002) 173e178. [37] K.W. Putz, O.C. Compton, M.J. Palmeri, S.T. Nguyen, L.C. Brinson, High-nanofillercontent graphene oxideepolymer nanocomposites via vacuum-assisted self-assembly, Adv. Funct. Mater. 20 (19) (2010) 3322e3329. [38] Y. Chang, S.T. Yang, J.H. Liu, E. Dong, Y.W. Wang, A. Cao, Y. Liu, H. Wang, In vitro toxicity evaluation of graphene oxide on A549 cells, Toxicol. Lett. 200 (3) (2011) 201e210. [39] D.M. Panaitescu, A.N. Frone, M. Ghiurea, I. Chiulan, Influence of storage conditions on starch/PVA films containing cellulose nanofibers, Ind. Crops Prod. 70 (2015) 170e177. [40] A. Voss, R.W. Stark, C. Dietz, Surface versus volume properties on the nanoscale: elastomeric polypropylene, Macromolecules 47 (15) (2014) 5236e5245. [41] A. Pakzad, J. Simonsen, R.S. Yassar, Gradient of nanomechanical properties in the interphase of cellulose nanocrystal composites, Compos. Sci. Technol. 72 (2) (2012) 314e319. [42] S.K. Mishra, S. Kannan, Development, mechanical evaluation and surface characteristics of chitosan/polyvinyl alcohol based polymer composite coatings on titanium metal, J. Mech. Behav. Biomed. Mater. 40 (2014) 314e324. [43] Y. Gu, M. Li, J. Wang, Z. Zhang, Characterization of the interphase in carbon fiber/ polymer composites using a nanoscale dynamic mechanical imaging technique, Carbon 48 (11) (2010) 3229e3235. [44] S.B. Yedla, M. Kalukanimuttam, R.M. Winter, S.K. Khanna, Effect of shape of the tip in determining interphase properties in fiber reinforced plastic composites using nanoindentation, J. Eng. Mater. Technol. 130 (4) (2008) 041010.

136

Interfaces in Particle and Fibre Reinforced Composites

[45] S.A. Syed Asif, K.J. Wahl, R.J. Colton, O.L. Warren, Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation, J. Appl. Phys. 90 (3) (2001) 1192e1200. [46] K. Manna, S.K. Srivastava, V. Mittal, Role of enhanced hydrogen bonding of selectively reduced graphite oxide in fabrication of poly (vinyl alcohol) nanocomposites in water as EMI shielding material, J. Phys. Chem. C 120 (30) (2016) 17011e17023. [47] E. Hall, The deformation and ageing of mild steel: III discussion of results, Proc. Phys. Soc. B 64 (9) (1951) 747e753. [48] K. Song, Y. Zhang, M.L. Minus, Polymer interphase self-reinforcement and strengthening mechanisms in low-loaded nanocomposite fibers, Macromol. Chem. Phys. 216 (12) (2015) 1313e1320. [49] D.J. Behmer, C.P. Hawkins, Effects of overhead canopy on macroinvertebrate production in a Utah stream, Freshw. Biol. 16 (3) (1986) 287e300. [50] K. Tashiro, H. Tadokoro, Calculation of three-dimensional elastic constants of polymer crystals. 3. a and g forms of nylon 6, Macromolecules 14 (3) (1981) 781e785. [51] K. Tashiro, M. Kobayashi, Theoretical evaluation of three-dimensional elastic constants of native and regenerated celluloses: role of hydrogen bonds, Polymer 32 (8) (1991) 1516e1526. [52] Y. Liu, A.L. Hamon, P. Haghi-Ashtiani, T. Reiss, B. Fan, D. He, J. Bao, Quantitative study of interface/interphase in epoxy/graphene-based nanocomposites by combining STEM and EELS, ACS Appl. Mater. Interfaces 8 (49) (2016) 34151e34158. [53] Y. Li, Y. Huang, T. Krentz, B. Natarajan, Neely, T, Schadler, LS, Polymer nanocomposite interfaces: the hidden lever for optimizing performance in spherical nanofilled polymers, in: A.N. Netravali, K.L. Mittal (Eds.), Interface/interphase in Polymer Nanocomposites, Scrivener Publishing, 2016, pp. 3e69. [54] N. Widjojo, T.S. Chung, M. Weber, C. Maletzko, V. Warzelhan, The role of sulphonated polymer and macrovoid-free structure in the support layer for thin-film composite (TFC) forward osmosis (FO) membranes, J. Membr. Sci. 383 (1e2) (2011) 214e223. [55] N. D’Souza, S. Pendse, L. Sahu, A. Ranade, S. Vidhate, Impact of crystallization on the interface in polymer nanocomposites, in: A.N. Netravali, K.L. Mittal (Eds.), Interface/ interphase in Polymer Nanocomposites, Scrivener Publishing, 2016, pp. 139e170. [56] H. Fan, C. Hartshorn, T. Buchheit, D. Tallant, R. Assink, R. Simpson, D.J. Kissel, D.J. Lacks, S. Torquato, C.J. Brinker, Modulusedensity scaling behaviour and framework architecture of nanoporous self-assembled silicas, Nat. Mater. 6 (2007) 418e423.

Effect of nanoclay filler on mechanical and morphological properties of Napier/ epoxy composites

6

M.S. Abdul Majid a,b , M.J.M. Ridzuan b , K.H. Lim b a School of Manufacturing Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia; bSchool of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, Arau, Perlis, Malaysia

1. Introduction Over the past decade, the adoption of natural fibres is gradually increasing as reinforcing resources in particular applications for composites structural industries all over the world. This is due to the establishment of environmental awareness, the tendency of renewable product, pollution issues and financial problem. Thus, many engineering composite applications are now adopting natural fibre resources instead of using synthetic fibres [1]. Although synthetic fibres have yield mechanical properties and better reinforcement in polymer composites, they are non-renewable, non-biodegradable and quite expensive resources. Therefore, a lot of industrial and structural applications such as aerospace, automotive, marine, construction and packaging are improving the utilisation of natural fibre reinforced composites due to their cost-effectiveness, high specific strength, toxic free, renewability, low density and biodegradable behaviour [2]. Moreover, some researchers have investigated on several kinds of natural fibre resources such as hemp, jute, bamboo, kenaf, oil palm, coconut seed, kapok and Raphia and its have contributed significantly on composite materials [3]. However, poor adhesion bonding between fibres and matrix, low durability, low fibres partition and poor thermal properties are still concerned issues to be addressed in reinforced composite applications. The mechanical properties, such as tensile, impact and flexural properties as well as the morphological behaviour of the natural fibres are still of research interest to be addressed [4]. Natural fibres reinforced composites are widely utilised in the structural applications due to their relatively high mechanical performance, low cost and environmentally friendly applications. Epoxy resins are commonly selected matrix used in fabrication natural fibres reinforced composites. They are one of the outstanding resins amongst the class of thermosetting resins. Epoxy resins are contributing to better mechanical, thermal, adhesive and electrical properties [5]. Their primary function is to enhance the adhesive structure when reinforcing with natural fibres. However,

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00006-6 Copyright © 2020 Elsevier Ltd. All rights reserved.

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epoxy resins do have limitations when it comes to high-performance applications due to their inherent brittleness behaviour. Therefore, various studies of epoxy resin have been carried out to overcome the shortcomings, hence enhance their performance such as additional of nanoparticles, optimum fibre orientation, fibre weight fraction with matrix and surface modification [6]. In recent year, a new approach is employed to enhance the performance of the composite by adding nanoclay filler into the matrix system. Addition of nanoclay filler into matrix found to improve the mechanical properties such as flexural strength, interlaminar shear strength and tensile toughness behaviour [7]. Montmorillonite (MMT) is the common nanoclay which can be used to increase the mechanical properties of polymers. It contains 25e30 wt% trimethyl stearyl ammoniums that able to also improve the modulus and enhance chemical resistance. Also, montmorillonite clay can also reduce crack propagation and improve the flexural strength; consequently due to it consists of platelets with an inner octahedral layer inserted between two silicates tetrahedral [8]. The low density of nanoclay particles plays a significant role in minimising the overall weight of the resin matrix [9]. The surface modification also can be achieved efficiently for better dispersion and distribution in the resin phase by adding nanoclay filler. This proper dispersion in nanoscale makes altering in flexural strength properties and prevents crack propagation of composites.

2.

Natural fibre

Natural fibre is a fibrous fibre; extracted from three subdivided classes such as plant fibres, animal fibres and mineral source fibres. The widely used of natural fibre are usually derived from the plant. Plant fibres consist of five basic types of natural fibre which include seed fibres, core fibres, bast fibres, leaf fibres, grass and reed fibres [10]. Table 6.1 shows that the types of natural fibres with particular examples. Recently, there are many investigations on several kinds of natural fibres, such as coconut, jute, flax, hemp kenaf, ramie, bamboo, Napier grass, reeds, banana, sisal, pineapple and kapok for uses in many applications especially as reinforcement in composite materials. Fig. 6.1 shows the more prevalently natural fibres which are widely utilised in particular industries. The natural fibres provide many significant Table 6.1 Types of natural fibres with examples [10]. No.

Types of natural fibres

Examples

1.

Bast fibres

Jute, flax, hemp, ramie, kenaf

2.

Leaf fibres

Banana, sisal, agave, pineapple

3.

Seed fibres

Coir, cotton, kapok, coconut

4.

Core fibres

Kenaf, hemp, jute, bamboo, Napier

5.

Grass and stem fibres

Wheat, corn, rice

Effect of nanoclay filler on mechanical and morphological properties

139

Fig. 6.1 Type of natural fibres: (A) Bamboo; (B) Coconut; (C) Hemp; (D) Jute; (E) Kenaf; (F) Sisal [12e17].

advantages and potentials such as low cost when compared to synthetic fibres, low density, lightweight, excellent mechanical properties, biodegradability, renewable source, high specific stiffness, acoustic insulation and non-toxicity [11]. Their excellent characteristics are utilised for use in aerospace, automotive, plastics and packaging industries. From the 90s, natural fibres are in demand for many engineering applications as a response to the laws of environmental. This is because the growing uses of synthetic fibre which consequently produce non-biodegradable waste disposal that leads to serious pollution concerns [17]. Thus the needs for the substitution of synthetic fibres; e.g. glass or carbon to natural fibres for reinforcing of polymer composites is now more essential [18]. Liu et al. reported that high performance of flax fibre was able to replace carbon or glass fibres as reinforcement’s materials for plastics. The author also mentioned about the superior properties of thermoplastic and thermosetting composites reinforced with flax fibre was obtained from a mixture of epoxy resin and soybean oil and generated “Green” composites from renewable materials or resources [19]. Natural fibres are seen as potential fibres for many engineering applications. However, there are still critical issues causing limitations; performance during service, long-term performance and fracture behaviour. Thus, it is essential to perform the necessary studies to improve their performance. Other factors, i.e., high moisture

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sensitivity, poor wettability, low thermal degradation temperature and low chemical resistance are the critical disadvantages of natural fibres that need overcome. Thus, the natural fibres must be significantly examined methodically to investigate their physical, morphological, chemical, flexural and tensile performance [20]. Rao K et al. studied the chemical and physical compositions of natural fibres to determine their physical properties. They investigated their fibre structure, density cellulose content and cross-sectional shape [21]. Valadez et al. also reported that the application of chemical treatment on the surface of the fibre to improve the adhesion characteristics and bonding intensity of natural fibre reinforced composites. The author strongly stressed that there was an enhancement of the adhesion characteristics of natural fibre surfaces after going through chemical treatment [22]. Besides, there was a cost-effective way to enhance the fibre surface such as chemical treatment. This kind of treatment could remove all the existing impurities on the fibre surface like hemicelluloses, lignin or waxy substances to avoid the moisture absorption from the surrounding. Overall, it is necessary to continue the low-cost modification of natural fibres to fulfil the need of the today composites market.

3.

Napier grass fibre

As a result of environmental awareness, much research in engineering applications are focused on natural fibres as substitution of synthetic fibres for fibre reinforced composites because of their unique characteristics and eco-friendly advantages. Advantages like lightweight, environmental sustainability, low material cost, low density, renewability and potential mechanical properties are playing a vital role in making composites nowadays [23]. In the recent review, a natural fibre extracted from Napier grass which is also known as ‘Pennisetum purpureum’ or elephant grass was investigated. Napier grass is a native of Africa, and it was promoted to South America, Australia and Asia as food or provisions for animals. Napier grass is the core fibres type of which the fibres presence inside the bast fibres of the centre of the plant’s stems. It is similar to bamboo, yellowish colour stem and highly sustainable throughout Malaysia. It could be harvested 4 to 6 times a year. Napier fibres are described by long narrow fibres with thick-cell wall on stems. Napier grass is natural to grow by underground stems, deeply rooted and densely clump plant; can grow up to 7.5 m tall. Moreover, it can be perennial grow and fast growing in the drought weather [24]. Napier grass especially when young is mainly providing good taste and highly nutritious of fodder crop for animals. It has been used widely before for biomass production due to its high sustainability. In addition, the mechanical properties of the grass, like tensile, impact and flexural strength have been reviewed by the earlier researchers [25] (Fig. 6.2). After the conventional water retting process, a manual extraction is applied to extract the fibre from Napier stems in order to remove all the hemicelluloses, wax or impurities on the surface. Rao et al. studied the effect of extracting elephant grass

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141

Fig. 6.2 Napier Grass Fibre: (A) Napier Grass; (B) Napier stem; (C) Napier fibre strand.

fibres via chemical and retting process on the tensile strength. They found that the retting process yielded superior tensile strength and elastic modulus of the fibres by almost 1.5 times greater than those chemically extracted fibres [26]. Ridzuan et al. reviewed the properties of P. purpureum fibres and found that it has high moisture substances due to their multi-cellular structures. The surface morphological was observed and emphatically proved that there is the existence of contamination on the surface of untreated P. purpureum fibres [27]. Moreover, research reported that the surface behaviour of the Napier fibres becomes uneven and rougher after alkali treatment and it has improved the structural, thermal, and tensile properties. The author also mentioned that Napier grass fibres are excellent raw material to strengthen in the fabrication of bio-composites [28]. Kommula et al. had also investigated the mechanical performances, water absorption, and chemical resistance of Napier grass fibre reinforced epoxy resin composites. They found that 20 wt% Napier fibre strands reinforcing composites was able to obtain optimum mechanical properties. Besides, short fibres strand-epoxy composites displayed poor mechanical performances compared to long linear fibre reinforced epoxy composites. There is also an improvement in interfacial bonding between matrix and fibre strand after undergoing alkali treatment on fibre strands [29]. M. Haameem et al. have studied and characterised the mechanical behaviours of Napier fibres reinforced polyester composites. The flexural and tensile strength of the untreated Napier fibre reinforced polyester composites are dependent on the volume fraction of fibres. The result showed that a 25% volume fraction of Napier fibre in the composite is enough to produce optimum tensile and flexural properties. Furthermore, the author also found that more than 80% improvement on the tensile strength of the alkaline-treated Napier fibres compared to the untreated fibres [30]. Furthermore, the tensile and flexural strength of Napier grass also has been investigated through the previous researches on fabricating of Napier fibre-reinforced polyester laminates. Subsequently, the results obtained proved that the Napier fibrereinforced polyester laminates performing good properties compared to other natural fibre-based composites.

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Interfaces in Particle and Fibre Reinforced Composites

To conclude, the use of Napier grass has the potential to be promoted as a possible source for reinforcing materials in composites industries. This could positively contribute to many engineering applications today especially for reinforcement in composites.

4.

Natural fibre reinforced epoxy composites

In the past decades, the creation of synthetic fibre reinforced polymer composites resulting in widespread use in various engineering industries. The wastage from the manufacturing and removal of composites structures are all non-biodegradable material. It is major concerns to the environment because there is still have not any perfect methods for the disposal of non-degradable synthetic fibre materials. In recent years, scientists and engineers have been actively exploring to search the elements as substitution of using synthetic fibres to overcome the pollution problems. The natural fibres successfully validated to be an alternative source for reinforcing composites. Natural fibre composites have been gradually used for many engineering applications over recent years. For example, natural fibre composites have become attractive for structural applications such as construction, marine, aerospace and automobile industries. This is due to their low specific mass, high mechanical performance, biodegradable and low in cost [31]. There are still however poor characteristics of natural fibres in reinforced composites such as low thermal properties, poor fibre partition, low durability and poor interfacial strength between resin and fibres due to incompatibility issues to be addressed. Researchers have focussed on enhancing the mechanical properties and interfacial bonding intensity of natural fibre composites. Mohanty et al. studied the effect of different chemical surface treatments on mechanical properties of jute fibres composites. Alkali treatment and cyanoethylation surface modification result in improved tensile and bending strength [32]. Furthermore, Sreekumar et al. indicated that the damping factor, loss modulus and storage modulus of fibrereinforced polyester composites could immensely change by fibres surface treatments at a wide range of temperatures [33]. Fahmi Idris found that the higher fibre volume fraction, the higher the peak load and energy absorption. Addition of Napier fibre improves the impact response of the epoxy composites. Besides, the higher the fibre volume fraction of the reinforced composite, the longer the time contact and energy absorption when the force applied on it which means that it has a low brittle fracture and more ductile [34]. Also, J.H. Song reported the effects of strain rate on sisal fibre reinforced polymerematrix composites. The epoxy resin composite was reinforced by sisal fibre through the resin transfer moulding (RTM) method. The mechanical properties of composites were tested with different surface modification such as permanganate, silane and non-treated treatments. The results showed that untreated fibres composites yielded a stable fracture and deformation energy while permanganate-treated fibre reinforced composites had the highest tensile strength compared to other surface treatments [35].

Effect of nanoclay filler on mechanical and morphological properties

143

Santosh et al. examined the difference in mechanical properties of banana fibres which reinforced with both epoxy and vinyl ester resin. The study found that banana fibres reinforced epoxy composites had a higher impact, flexural and tensile strength compared to banana fibres reinforced vinyl ester composites. Besides, the mechanical properties like impact, flexural and tensile strength of both banana fibres epoxy and vinyl ester composite can be enhanced by alkali treatment with 5% of NaOH [36]. Physical, mechanical and thermal properties of jute and bamboo fibre reinforced unidirectional epoxy composites had been investigated by Subhankar Biswas et al. The unidirectional composites were produced by using vacuum infusion technique and found that jute and Bamboo reinforced epoxy composites performed high flexure strength with arrangement transverse and longitudinal respectively. This study reviewed jute fibre reinforced epoxy composites had higher Young’s modulus while Bamboo fibre reinforced epoxy performed stronger tensile strength [37]. Madhukiran et al. reviewed the hardness and tensile characteristics of reinforcing banana and pineapple fibres with epoxy composites. Both banana and pineapple fibres were extracted by water retting and manual cleaning process. The results showed that the tensile strength had increased gradually with the increase in fibre weight fraction. This revealed that hybridisation of reinforcement composites could obtain a better result than a single type of natural fibres [38]. On a similar note, the mechanical properties of jute fibre reinforced composites with polyester, and epoxy resin matrices were examined by Ajith Gopinath [39]. The research reported that jute epoxy composites obtained superior flexural and tensile properties compared to jute polyester composite. However, the processing time required for jute epoxy composite is much longer than jute polyester composite. The author also mentioned that jute epoxy composites are more suitable for fabricating automotive applications rather than jute polyester composites. Overall, natural fibres reinforced composites can be great significance than metals and ceramics due to its high stiffness and stronger. Besides, natural fibres reinforced composites are very light in weight, thus the ratios of strength to weight and stiffness to weight is much stronger than aluminium and steel. It is also possible to achieve the mechanical properties of metals, ceramics polymer.

5. Nanoclay reinforced composites In the past decade, fibre reinforced polymer composites have been used in large quantity for structural applications, for example, an automobile, marine and construction industries as a result of their superior mechanical properties. Besides, the high mechanical, thermal and electrical performance of the epoxy resins were also widely used as a matrix material in fibre reinforced polymer composites. There were some concerns about their limitation for many high-performance applications due to their low impact resistance and inherent brittleness properties. However, research then revealed that incorporation of nanoparticle into matrix composites would overcome some of the limitations of their composites properties. Over the recent years, carbon nanofillers have

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Interfaces in Particle and Fibre Reinforced Composites

been investigated for reinforcing fibres epoxy composites and found that addition of carbon nanofillers into epoxy resins can enhance the mechanical, functional, structural and adhesive properties [40]. Past research found that the nanofiller loadings range from 1 to 3 vol% was the maximum volume to improvise the mechanical properties of the epoxy composites. If the nanofiller loading volume is more than the mentioned range, a reduction could occur on both tensile strength and tensile modulus of the filled epoxy composites. Sandler et al. reported that adding 1 wt % of CNT with oriented randomly into the matrix was enough to obtain a better state of dispersion and mechanical properties improvement [41]. Martone et al. had examined the influences on the bending modulus of the composites by adding multi-walled carbon nanotube (MWCNT) into an epoxy system. The outcomes indicated that the MWCNT could maximise the reinforcement efficiency like aspect ratio and bending modulus with very low volume loading 0.05% w/w. This is because low filler loading could obtain better dispersion and optimise the interfacial bonding between fibre and matrix [42]. Furthermore, Mittal et al. investigated that graphene and carbon nanotube (CNT) has a great deal potential to improvise the properties of composites for several engineering applications because of their satisfied structural, functional properties and its suitability. The authors mentioned various factors that can optimize the functions of multi-walled carbon nanotube (MWCNT) or graphene nanoplatelet (GNP) reinforced epoxy composites such as quality of fillers, aspect ratio, amount of fillers loading, surface modification, state of dispersion of filler in matrix, the selection of matrix and as well as adhesion bonding between fillers and matrix [43]. In addition, Guo et al. reviewed the effects of on the mechanical performances and fracture morphologies of composites reinforced with multi-walled carbon nanotube filler (MWCNT). This study found that increasing the volume loading of MWCNT lead increasing in the tensile strength and fracture strain of composites, yet Young’s modulus dropped at the same time. This is because of the addition of MWCNT could harden the composites and the outside layer of MWCNT damaged by oxidation after mixedacid treatment [44]. To summary, nanofiller could be used to improve and modify the mechanical performance in composites. Nanofiller able to enhance the surface finishing on the composites while maintaining minimum health hazards.

6.

Nanoclay filled Napier/epoxy composites

Nanoclay or nano-layered silicate are optimised-clay minerals with several enhancements properties and have become much popular as reinforcing fillers for composites amongst the various nanoparticles. This is due to their high aspect ratio, potentially exfoliation characteristics and better mechanical performance. It is like montmorillonite which the most commonly used in materials applications. Nanoclay consisted of approximately 1 nm thick alumina silicate layer surface and stacked with around 10 nm sized of multilayer stacks. Thus it has an excellent aspect ratio and specific surface area with approximately 657m2 /g. Many researchers found promising that

Effect of nanoclay filler on mechanical and morphological properties

145

nanoclay filler enables to strengthen and altering the mechanical properties dramatically of fibre reinforced polymer when nanoclay was well dispersed in epoxy resin composites [45]. Moreover, few research studies have been carried out to test the effect of nanoclay filler on the mechanical properties of fibre reinforced polymer. The presence of nanoclay of less than 10% by weight of epoxy found to improve the mechanical properties; tensile strength and tensile modulus. The brittle behaviour was reported when a large amount of nanoclay loading into epoxy resin system. This was suspected of causing by agglomeration of nanoclay fillers in the epoxy resin. Aidah et al. reported the effect of nanoclay filler on the mechanical properties of glass fibre reinforced polymer (GFRP). The morphological properties on specimen surface have been carried out through FESEM technique. Crack propagation was observed on the epoxy matrix due to the addition of a certain amount of nanoclays filler. Besides, there was a fracture surface occurred on the interface bonding between fibre and epoxy of the GFRP without nanoclay filler loading. This clearly showed that it has a weak adhesion bonding between fibre and matrix. This study highlighted that 5 wt% of nanoclay filler enable to optimised in the flexural properties of the GFRP composites. This was possibly caused by better interface bonding between the nanoclay filler, fibre and matrix, improvement in stiffness properties of the composites and well adhesion between fibre and matrix with the help of nanoclay filler [46]. Furthermore, a study also indicated that nanoclay particles could dramatically drop the whole weight of the resin matrix compared to other nanoparticles due to its lower density. This was a significant advantage and essential on fabricating a composite with a low weight ratio of nanoclay filler to reinforcement properties [47]. Montmorillonite (MMT) clay is one of the available forms of nanoclay known as layered silicate materials. It has been widely used as inorganic fillers in polymer applications due to the ability to increase and modify the mechanical properties of polymers significantly such as reduction in crack propagation and enhancement in flexural strength. It is low in cost consequently on using this kind of method for the improvement of the properties of polymers [48]. Yuanxin Zhou et al. investigated the effect of montmorillonite clay on mechanical and thermal properties of epoxy composites. The flexural tests have been carried out to assess the mechanical properties behaviour. The result displayed that 2.0 wt% of nanoclay loading in epoxy resin performance most significant improvement in flexural strength as compared to without nanoclay loading in epoxy resin. Addition of 2 wt% nanoclay has increased by 31.6% and 27% in flexural modulus and strength respectively [49]. The purpose of nanoclays fillers can be divided into two main reasons; to enhance the properties of the materials and bring down the cost of the component. Recently, nanoclay particles have been proved and widely used in many engineering applications for enhancement of the tensile toughness and flexural strength properties of epoxy composites as well as surface finishing. To conclude, adding nanoclay filler in the Napier fibres reinforced epoxy composites can increase the roughness of the surface and maximise in the contact area between the epoxy resins on the fibres. Thus, it may dramatically enhance in their interfacial bonding leading to improvise the mechanical and morphological properties of the composites when modifying by adding of nanoclay filler.

146

7.

Interfaces in Particle and Fibre Reinforced Composites

Fabrication and flexure test of nanoclay filled Napier/epoxy composites

First and foremost, the extracted raw Napier fibres were oven dried at 55
C for 30 min to remove excess moistures. The composite samples of Napier fibre reinforced epoxy resin was fabricated with 75% of epoxy resin and 25% of Napier fibre weight content (with ratio 3:l). Specimens from each subgroup were fabricated by using the vacuum infusion technique. The vacuum infusion technique is one of the effective methods for fabricating composite components, and it brought many advantages such as operates with low-cost tooling, unlimited setup time, high aspect ratio, consistent resin usage and wasted no resin. At the beginning of the vacuum infusion technique, the glass surface was polished by wax, and then the resin feed spiral was installed at one side of the glass plate. Few layers of Napier fibres were then positioned and oriented randomly on the glass surface. Next, the vacuum connector and resin feed spiral were connected. The infusion mesh was placed after the peel ply layer of the infusion was put. Moreover, the resin feed spiral and vacuum hose were sealed and positioned while the vacuum bag was then being taped. After that, the vacuum pump was connected and switched on to withdraw the air inside. The resin was then infused and flow slowly onto the surface of Napier fibres. While the resin was distributed until the end of the edge, the excess resin flowed into a prepared resin trap. The process was continued run under room temperature for around 6 h to ensure the epoxy resin with nanoclay filler full covered onto the Napier fibres (Fig. 6.3). Lastly, the Napier fibre epoxy composite was done fabricating and stored for thoroughly dried and hardened. The same procedures were repeated with different amount of nanoclay filler volume fraction specimens [50]. Five different nanoclay filler1 loading of Napier/epoxy composites such as neat, 2 wt%, 3 wt%, 4 wt% and 5 wt% were fabricated by vacuum infusion technique. There are some procedures, and precaution steps need to comply with to fabricate a satisfactory composite. The epoxy was first stirred by hand with nanoclay filler for 20 min at room temperature to get sound dispersion. Next, the epoxy hardener was added at a ratio of 3:1 by weight (epoxy: hardener) and then slowly stirred for another 20 min to remove small tiny air bubbles inside. After that, the mixtures were ready to be infused at room temperature until the Napier fibres fully covered by the epoxy matrix. The thickness of the composite was controlled by a Perspex moulding, and the thickness of composite fabricated was within ranges of 3e4 mm. It was hard to fabricate Napier/epoxy composite with nanoclay filler beyond 5 wt% because of the nanoclay filler would make the epoxy mixture itself became very viscous and stagnant. As a result, the more the nanoclay filler loaded, the more viscous of the epoxy mixture. Consequently, the surface of the composites was full of voids and holes because of the epoxy matrix unable to adequately cover all the fibres (Fig. 6.3). The three-point bending test was carried out by using INSTRON micro-tester. The cross-head speed should not set higher than 2 mm/min in order to get the compliance 1

Nanoclay filler refers to Montmorillonite.

Effect of nanoclay filler on mechanical and morphological properties

147

Fig. 6.3 Experimental setup for vacuum infusion process.

curve determination. Besides, the load applied to the centre of the specimen to get an accurate result as shown in Fig. 6.4(A, B). Eight specimens were tested for each type of composite, and the results were recorded. The results obtained; the load and extension data are being used to find the flexural strength and modulus, as well as the strain to failure. The flexure stress shows the ability of the composites to withstand deformation under bending load. This stress can be obtained and calculated on the load-deflection curve by using the equation given; sf ¼

3pl 2bd2

where sf ¼ flexural strength (MPa); p ¼ maximum load (N); l ¼ support span (mm); b ¼ width of specimen (mm) and d ¼ thickness of specimen (mm).

Flexural strain illustrates the minimal changes in the length of the test specimen when undergoing deformation, where the maximum strain happens at midspan. It can also be calculated for any deflection using; εf ¼

6Dd l2

where εf ¼ flexural strain(mm/mm). D ¼ maximum deflection of the centre of the beam (mm); d ¼ thickness of specimen (mm); l ¼ support span (mm).

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 6.4 (A) Before three-point bending testing, (B) During the three-point bending test, (C) Fractured specimen after the test.

Flexural modulus often called modulus elasticity which is used to demonstrate the bending stiffness of the composites. It can be calculated through mathematical calculation or by tangential of the initial straight line portion of the stress-strain curve. E¼

ml3 4bd3

where E ¼ flexural modulus (N/m2 ) and. m ¼ initial slope of the load deflection (N/mm).

Finally, the morphology of the nanoclay filled Napier reinforced epoxy composites were observed by using field emission scanning electron microscope (FESEM). The images obtained used to evaluate the surface of the specimen and characterise their morphological properties. The microscope was acquired with 3e5 kV accelerating voltages, and the magnification was focused in a range of 50e200 times. Thus, the specimens were prepared with diameter 10 mm  10 mm. Before scanning, the

Effect of nanoclay filler on mechanical and morphological properties

149

specimens have to be consistently coated with a thin layer of platinum in order to prevent charging and caused a white spot on the scanned pictures.

8. Flexural strength and modulus of nanoclay filled Napier/epoxy composites The flexural stress-strain trend of the neat, 2 wt%, 3 wt%, 4 wt% and 5 wt% nanoclay filled Napier/epoxy composite is illustrated in Fig. 6.5. It clearly shows that the composites undergo only an elastic region and no plastic deformation. In the beginning, the stress increases proportionally with the strain until reaching the elastic limit. According to Hooke’s law, the flexural modulus of the composite is calculated at the initial flexural strain of 1% (0.01 mm/mm). Furthermore, the strain gradually increased beyond the elastic limit until it reached to the yield stress as the load was increased, where there is no more increment in load applied for further deformation of composite. After the yield strength point, the composite did not further undergo any plastic strain, and it suddenly decreased sharply, where the composite was broken at that time. Only neat Napier/epoxy composite that yields and then breaks before the 3.5% strain limit. On the other hand, the 2 wt%, 3 wt%, 4 wt% and 5 wt% nanoclay filled Napier/epoxy composite that breaks within strain limit of 2%e3% before yielding because no obvious yield point was found in the stress-strain curve. The trend of this stressstrain curve shows similar characteristics to those of brittle materials. Therefore, it reveals that the composites become more brittleness and stiffness as the nanoclay filler loading increases. In addition, the presences of nanoclay filler enable to improve the flexural stressstrain response of the Napier/epoxy composite especially 3 wt% of nanoclay filler enhances the most. This is indicated by the area under the stress-strain curve of the nanoclay filled composites increases compared to neat Napier/epoxy composite. Stress strain curve

Stress (Mpa)

70 60

Without nanocaly filler 2% of nanoclay filler

50

3% of nanoclay filler

40

4% of nanoclay filler

30

5% of nanoclay filler

20 10

0 -10

0

0.01

0.02

0.03

0.04

Strain (mm/mm)

Fig. 6.5 Flexural stress-strain for various nanoclay-filled loading Napier/epoxy composites. Nanoclay filler refers to Montmorillonite.

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Interfaces in Particle and Fibre Reinforced Composites

It is due to the proper dispersion of nanoclay filler in the epoxy matrix where improving the fracture toughness of the material. There is no constant increment to the stress-strain curve may be due to the uncontrollable failure during the vacuum infusion process. During processing, the epoxy matrix can form small bubbles under high vacuum conditions. Consequently, the bubbles then trapped in the fabric and sometimes voids occurred after the epoxy resin solidifying. However, this is one of the reasons to affect the results on the mechanical properties. Fig. 6.6 presents the strain to failure with the different nanoclay filler contents for Napier/epoxy composites. It is clearly shown from the above figure that the strain to failure values decreased with increases the amount of nanoclay filler content of the composites. This indicated that the nanoclay fillers cause the epoxy matrix to behave more brittle-like. The results show that the maximum strain to failure values was observed with neat Napier/epoxy composites at 3.08% while the least strain to failure seen with 5 wt% of nanoclay filled composite at 2.21%. Besides, the stain to failure value for 2 wt%, 3 wt% and 4 wt% of nanoclay filler slightly reduced with 3.0%, 2.98% and 2.73% respectively. The drop in strain to failure values of the nanoclay filled Napier/epoxy composite could be explained by the presence of the nanoclay filler in the epoxy matrix changes their properties from elastic behaviour to more brittle-like behaviour. This is because the nanoclay fillers limit the mobility and flexibility of the matrix, and thus the higher the amount for nanoclay filler content, the higher the brittleness of the composite. The flexural strength of nanoclay filled Napier/epoxy composites from 2 wt% to 5 wt% is much better than neat Napier/epoxy composites as shown in Fig. 6.7. It was illustrated that the nanoclay fillers are creating strong bonding between Napier fibres and epoxy resin. Thus, it resulted in increases the capability of the epoxy composite to sustain the stress during bending test. The flexural strength of filled epoxy composite increases with a rise in nanoclay filler loading from neat to 3 wt%, and there were slight decreases with further increase in filler loading. However, Table 4.3 shows that the addition of 3 wt% of nanoclay filler exhibits the highest flexural strength with 0.035 3.08%

3.00%

2.98%

0.030

2.73%

Strain to failure

0.025

2.21%

0.020 Without nanoclay filler 0.015

2% nanoclay filler 3% nanoclay filler

0.010

4% nanoclay filler 5% nanoclay filler

0.005 0.000 Filler loading (%)

Fig. 6.6 Strain to failure for various nanoclay-filled loading Napier/epoxy composites. Nanoclay filler refers to Montmorillonite.

Effect of nanoclay filler on mechanical and morphological properties

151

70

Flexural strength (MPa)

60 50 Without nanoclay filler

40

2% of nanoclay filler 3% of nanoclay filler

30

4% of nanoclay filler 20

5% of nanoclay filler

10 0

Filler loading (%)

Fig. 6.7 Flexural strength for various nanoclay-filled loading Napier/epoxy composites. Nanoclay filler refers to Montmorillonite.

the value 57.72 MPa, followed by 5 wt%, 4 wt%, 2 wt% and last; without nanoclay filler respectively. By comparing to the neat Napier/epoxy composites, the addition of 3 wt% of nanoclay filler had significantly increased the composite’s flexural strength with an improvement of approximately 163%. The improvement is achieved due to the nanoclay filler plays an essential role in the curing of epoxy resin which could significantly improve the interface interaction and created exfoliated structure in the composite. Also, with a further increase in the filler loading of nanoclay to 4 wt% and 5 wt%, the flexural strength Napier/epoxy composite dropped to 43.77 MPa and 48.60 MPa respectively. In general, the flexural strength of 4 wt% of nanoclay filled Napier/epoxy composites are expected to be higher than 5 wt% of nanoclay filled yet experimental testing revealed unexpected results. The flexural strength of 4 wt% of nanoclay filled composites is found somewhat lower than the 5 wt% of nanoclay filled composites by almost 10%. This might be due to the poor dispersion of nanoclay filler in the epoxy composites. Furthermore, the gradual decreased in the flexural strength with addition more than 3 wt% of nanoclay fillers are suspected due to the inappropriate dispersion of the nanoclay filler and formation of agglomeration in the epoxy resin. This causes the non-rich nanoclay of the epoxy resin to crack easily from the stress applied in the epoxy resin. This might be the critical reason that leads to crack propagation and reduction in flexural strength of the Napier/epoxy composite [52]. Therefore, excellent dispersion of nanoclay filler in the epoxy is vital in resulting flexural strength and physical properties of the composite materials. In Solhi et al.’s [53] and Zukas et al.’ [54] investigation, results show that; increasing the % wt of nanoparticles exceeding a threshold point; the flexural strength experienced significant reduction. Furthermore, past research investigated the flexural strength of the nanoclay/E-glass epoxy composites, revealed that the highest flexural strength was observed at 3 wt% addition of nanoclay, beyond which it subsequently dropped. Nevertheless, the presence of higher amounts of nanoclay up to 5 wt% gradually decrease the flexural strength due to the agglomeration of the nanoclay fillers at higher ratios and consequently embrittlement of the matrix [51].

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Interfaces in Particle and Fibre Reinforced Composites

Similarly, Li et al. [55] and Adabo et al. [56] reported that increasing the amount of nanofiller could have a significant improvement on flexural strength, but then the strength dropped eventually by further increasing the amount nanofiller in the resin metric. As a result, the flexural strength will undergo significant reduction when the amount of nanoscale filler adding beyond the particular percentage. Moreover, the high viscosity of the epoxy resin phase will weaken the bonding between fibres and epoxy resin. Thus, the interfacial adhesion between the epoxy matrix and the Napier fibres deteriorated, resulting in the strength reduction. Napier/epoxy composites ranked among the highest flexural modulus with approximately mean value of 3.2 GPa. There was over 180% of improvement compared to neat Napier/epoxy composites. This could be due to the formation of the exfoliated structure resulting in the strong interfacial interaction between the epoxy matrix and nanoclay filler [57]. Besides, the flexural modulus for 2 wt%, 3 wt% and 4 wt% of nanoclay filler also displayed significant enhancement with 42%, 132% and 130% respectively. The flexural modulus of 3 wt% and 4 wt% nanoclay filler loading showed no significant differences which, unexpected. This may be due to the agglomeration of clay and intercalated structure in the epoxy resin that contribute no increment in flexural modulus value. The presence of nanoclay filler in the epoxy matrix contributed to stronger the bonding via rearranging the chain structure of the epoxy, and it enables to tighten the chain for preventing the chain free to move. Thus, it significantly improves the flexural modulus as a consequence of the addition of nanoclay filler in the epoxy metric. Based on Chisholm et al.‘s study, a significant enhancement was obtained in flexural modulus of epoxy by adding nanoparticles due to higher surface energy [58]. Besides, the highly dispersed of nanoclay filler could provide great bonding with epoxy matrix and thus the efficiency of stress transferring boosted up subsequently, leading to ameliorate the flexural modulus. Consequently, the neat Napier/epoxy composite was rated as the lowest in flexural modulus due to its weak interfacial bonding between the epoxy resin and Napier fibres and thus impaired their adhesion chains which can potentially hinder the flexural modulus. Past researchers investigated that nanoclay fillers facilitate to build up strong interface bonding and adhesion to fibres and epoxy matrix and then enhance the mechanical properties. The rigid nanoclay fillers bonding expected to improve the flexural modulus, stiffness and toughness, however also resulting in an increase of its brittleness and reduction in strain to failure. Krushnamurty et al. mentioned that nanoclay filler could improve matrix toughness because of their rough fracture surface and as well as strong inter-filament bonding of the nanoclay filled glass/epoxy composites [51]. According to Siddiqui et al., the flexural modulus of epoxy composites naturally improve with the presence of the organo-clay and the increment of the flexural modulus was at the expense of a reduction in flexural strength [59]. Also, the high flexural modulus of the composites also attributes to the brilliant performance of the thermoset epoxy. This is because it has low viscosity and thus low processing temperature is needed for fabricating composites. It provides an advantage to facilitate the creation of a better fibre/matrix composite compared to the use of thermoplastic resins.

Effect of nanoclay filler on mechanical and morphological properties

153

9. Morphology of nanoclay filled Napier reinforced epoxy composites The fracture surface of Napier/epoxy composites with and without nanoclay filler through SEM are shown in Fig. 6.8. Fig. 6.8(A) shows the fractured surface of neat nanoclay filled composites; it is clear with a smooth surface. It can be observed that a large number of tiny white particles are inspected on the fracture surface of the composites. Composite fractured surface with 2 wt% of nanoclay filler indicates smaller areas and fewer nanoclay fillers as shown in Fig. 6.8(B), whereas for 5 wt% nanoclay filled composites, large areas with presence of nanoclay filler is observed as shown in Fig. 6.8(E). The increased loading of nanoclay filled Napier/epoxy composites, the tinier white particles found on the epoxy interface. In order to achieve the better mechanical performance of the composites, the nanoclay filler should be well distributed in the epoxy matrix. Excellent and homogeneous dispersion of nanoclay filler indicates that the clay fillers are thoroughly dispersed and bounded by the epoxy. The insufficient nanoclay fillers in the composites can cause clay agglomeration due to a non-homogeneous mixture [60]. Thus, it could contribute to crack propagation and affect the flexural strength as well as its modulus. From Fig. 6.8(B) of 2 wt% of nanoclay filler, it shows that the nanoclay filler was poorly distributed in the epoxy matrix and aggregations were observed. Therefore, the mechanical performances were not as good as expected and the composites cracked under loading. Nonetheless, for the 3 wt% and 5 wt% of nanoclay loading, the SEM images displayed that the nanoclay fillers are well distributed throughout the composites without apparent agglomeration observed. Thus, it brings significant advantages to the flexural strength and modulus properties. Fig. 6.9 shows the various loading of nanoclay filler displayed varying degrees of surface roughness. It can be seen that a relatively smooth fracture surface is displayed by the neat Napier/epoxy composites as shown in Fig. 6.9(A). Compared to the neat 3500

Flexural modulus (MPa)

3000 2500 Without nanoclay filler 2% of nanoclay filler 3% of nanoclay filler 4% of nanoclay filler 5% of nanoclay filler

2000 1500 1000 500 0

Filler loading (%)

Fig. 6.8 Flexural modulus for various nanoclay-filled loading Napier/epoxy composites. Nanoclay filler refers to Montmorillonite.

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Interfaces in Particle and Fibre Reinforced Composites

Napier/epoxy composite, the fracture surface of the nanoclay filler composites exhibited a much rougher fracture surface as shown in Fig. 6.9(BeD). It is because of the presence of nanoclay fillers in the epoxy matrix influences the fracture path to be more complicated and very rough surface occurred. These rough surfaces could promote better interfacial bonding between the Napier fibres and the epoxy resin. It is

(a) N

D5.8 x50

2 mm

(c)

(b) N

D5.8 x50

2 mm

N

D6.6 x50

2 mm

N

D6.1 x50

N

D7.5 x50

2 mm

(e)

(d)

2 mm

Fig. 6.9 SEM images of the fracture surface of the Napier/epoxy composites at various loadings of nanoclay. (A) Neat, (B) 2 wt%, (C) 3 wt%, (D) 4 wt%, (E) 5 wt% of nanoclay filler. Nanoclay filler refers to Montmorillonite.

Effect of nanoclay filler on mechanical and morphological properties

155

illustrated that the addition of nanoclay filler into the Napier/epoxy composites results in improvement in toughness. This is supported by the increment in the flexural modulus as the nanoclay filler loading increases. Moreover, it can also be observed that the fractured surface of the nanoclay filled Napier/epoxy composites shows a textured surface compared with that of the neat Napier/epoxy composite. As a result, the composites become tougher in behaviour. By comparison, the surface of 5 wt% of nanoclay filler composites is rougher thus yield the highest flexural modulus. S. Parija et al. mentioned that the higher amount of nanoclay filler contents in the polymer matrix could provide high efficiency of stress transfer in the composites and created excellent flexural modulus [61]. Researchers also found that the nanoclay particles could create stress disturbance to the polymer composites and enable to enhance the toughening mechanism of the composites. In addition, the nanoclay fillers can act as additional reinforcement to the epoxy matrix and enhances the interface structures due to the smaller in size of nanoclay filler able to bundle between Napier fibres and epoxy matrix (Fig. 6.10). Consequently, this can improve the flexural strength and modulus where the mobility was reduced (Fig. 6.10). Fig. 6.11 exhibits apparent agglomerates which were observed in SEM micrograph with 200  magnifications. The agglomeration of the nanoclay filler in the composites

(a) N

D5.6 x200

500 µm

N

D6.6 x100

1 mm

(b) N

D5.6 x100

N

D7.5 x100

(c)

1 mm

(d) 1 mm

Fig. 6.10 SEM images of the roughness surface of the Napier/epoxy composites at various loading of nanoclay. (A) Neat, (B) 3 wt%, (C) 4 wt%, (D) 5 wt% of nanoclay filler. Nanoclay filler refers to Montmorillonite.

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Interfaces in Particle and Fibre Reinforced Composites

(a) N

D5.7 x200

(b)

500 µm

N

D6.6 x200

500 µm

(c) N

D7.5 x200

500 µm

Fig. 6.11 SEM images of the agglomeration of nanoclay on the fracture surface of the Napier/ epoxy composites at various loading of nanoclay. (A) 3 wt%, (B) 4 wt%, (C) 5 wt% of nanoclay filler.

causes the stress concentration and consequently crack propagation initiated. This reduces the flexural strength of the composites and yield brittle fracture behaviour. This appearance assures the stress concentrated on the particular points and causes the lesser rich area of nanoclay to cracks easily [62,63]. As shown in the SEM image of Fig. 6.12(A), the neat Napier/epoxy composite demonstrated that the fractures took place along the bonding interface between the epoxy resin and Napier fibres. The Napier fibres are completely separate from the fractured surface that proves poor interphase bonding between the Napier fibre and epoxy matrix and consequently, more Napier fibres pull out. In contrast, with the presence of nanoclay fillers in the composites, the fibres pull-out has been greatly reduced, of which the epoxy matrix is completely secured the Napier fibres at the surface of failure. As can be observed from the 5 wt% nanoclay fillers, lesser fibre pull-out observed under 100  magnifications. As a result, it indicates an enhancement of adhesion capability between the Napier fibres and epoxy matrix with the presence of the nanoclay filler. The previous study reported similar findings; nanoclay filler improved the cohesion properties between the adjacent and the interlayer of composite layers [51].

Effect of nanoclay filler on mechanical and morphological properties

157

(a) N

D5.6 x100

1 mm

(c)

(b) N

D5.9 x100

1 mm

N

D7.5 x100

1 mm

Fig. 6.12 SEM images of the fibre pull-out observed from the Napier/epoxy composites at various loading of nanoclay. (A) Neat, (B) 2 wt%, (C) 5 wt% of nanoclay filler.

However, this advantage can lead to increase of flexural strength and modulus of natural fibres reinforced composites. It can be concluded that better adhesion of nanoclay filled Napier/epoxy able to produce better stress transfer in the matrix, thus subsequently enhances their strength, toughness and modulus.

10.

Conclusion

The effect of nanoclay filler loading on the mechanical and morphological properties of Napier/epoxy composites was investigated. The Napier/epoxy composites reinforced with nanoclay filler were successfully fabricated using vacuum infusion technique. Five different nanoclay filler loading of Napier/epoxy composites such as neat, 2 wt%, 3 wt%, 4 wt% and 5 wt% were fabricated. Three-point bending test was carried out according to ASTM 790 standard. Based on the experimental results, it can be concluded that the reinforcement of nanoclay fillers in Napier/epoxy composites has a significant impact on the flexural properties compared to neat Napier/epoxy composite. The fracture mode is no longer ductile with plastic deformation, but rather

158

Interfaces in Particle and Fibre Reinforced Composites

becoming brittle-like behaviour. This was confirmed by the decreased in the strain to failure with the loading increase of nanoclay filler. In addition, the stress-strain curve shows similar characteristics to brittle materials. The findings can also be supported by the rougher surface on the fractured surface of the composites. The incorporation of nanoclay filler had enhanced the flexural strength of Napier/ epoxy composites. However, it was investigated that there was a maximum improvement limit. The highest tensile strength was achieved by 3 wt% of nanoclay filled composites at 57.72 MPa. However, there is a reduction in flexural strength with a further increase in nanoclay fillers. The high nanoclay filler contents affect the filler-filler interaction which consequently agglomeration occurred and thus caused a reduction in the flexural strength of the composites. From the observation under SEM, the nanoclay filler distribution was quite uniformly in the epoxy matrix although there is the problem of the agglomeration of the nanoclay filler at higher nanoclay filler ratios within the epoxy matrix. Therefore, the proper dispersion and distribution of nanoclay filler particles in the epoxy matrix act as an essential role in the mechanical properties of the composites. From this morphology, it was concluded that the fibres become roughened with the addition of nanoclay. By comparison, the fracture surface of 5 wt% of nanoclay filler composite is rougher, and it ranked as the highest in flexural modulus. Also, fibre pull-out failure was observed on the flexural fracture surfaces of the neat Napier/epoxy composite. With the addition of nanoclay filler, it could help to reduce the fibre pull-out failure problem due to the enhancement of adhesion capability between the Napier fibres and epoxy matrix. Overall, the SEM shows an improvement in interfacial bonding between the Napier fibres and epoxy matrix upon the nanoclay filler was added. As a conclusion, the presence of nanoclay filler loading with a range of 3 wt% to 5 wt% in the Napier/epoxy composites shows the significant improvement in mechanical and morphological properties.

References [1] D. Nabi Saheb, J.P. Jog, Natural fiber polymer composites: a review, Adv. Polym. Technol. 18 (4) (1999) 351e363. [2] D.H.H. Cheung, M.P. Ho, K.T. Lau, F. Cardona, Natural fibre-reinforced composites for bioengineering and environmental engineering applications, Compos. B Eng. 40 (7) (2009) 655e663. [3] M. Hughes, Defects in natural fibres: their origin, characteristics and implications for natural fibre-reinforced composites, J. Mater. Sci. 47 (2) (2012) 599e609. [4] K.O. Reddy, C.U. Maheswari, D.J.P. Reddy, A.V. Rajulu, Thermal properties of Napier grass fibers, Mater. Lett. 63 (27) (2009) 2390e2392. [5] W.W. Amir, A. Jumahat, A. Kalam, Characterization of Nanoclay-Modified Epoxy Polymers Using X-Ray Diffraction Analysis, Applied Mechanics and Materials, 2015, pp. 175e180. [6] S. Jagtap, V.S. Rao, D. Ratna, Preparation of flexible epoxy/clay nanocomposites: effect of preparation method, clay modifier and matrix ductility, J. Reinf. Plast. Compos. 32 (2012) 183e196.

Effect of nanoclay filler on mechanical and morphological properties

159

[7] S.M. Mousavinasab, M. Atai, B. Alavi, To compare the microleakage among experimental adhesives containing nanoclay fillers after the storages of 24 hours and 6 month, Open Dent. J. 5 (2011) 52e57. [8] D.R. Paul, L.M. Robeson, Polymer nanotechnology: nanocomposites, Polymer (2008) 3187e3391. [9] M. Atai, L. Solhi, A. Nodehi, S.M. Mirabedini, S. Kasraei, K. Akbari, PMMA-grafted nanoclay as novel filler for dental adhesives, Dent. Mater. 25 (2009) 339e386. [10] D.B. Dittenber, H.V.S. Gangarao, Critical review of recent publications on use of natural composites in infrastructure, Compos. Part A Appl. Sci. Manuf. 43 (8) (2012) 1419e1429. [11] C.R. Mothé, Ara
ujo “Thermal and mechanical characterisation of polyuretane composites with Curaua fibers”, Polímeros 14 (2004) 274e278. [12] A. Jurries, The Bamboo Backlash, The GearCaster, December 30, 2015 [Online]. Available: http://www.thegearcaster.com/2015/12/the-bamboo-backlash.html. [13] M. Legarda, 11 Reasons Why Virgin Coconut Oil Should be on Your Travel Packing Essentials List, March 16, 2016 [Online]. Available: http://www.huffingtonpost.com/ melissa-legarda/10-reasons-why-virgin-coc_b_9429310.html. [14] K. Maloney, 4 Best Reason to Legalize Hemp, August 21, 2015 [Online]. Available: http:// truththeory.com/2015/08/21/4-best-reasons-to-legalize-hemp/. [15] A. Anwar, The Golden Fibres, January 16, 2006 [Online]. Available: http://exporter-ofjute-products.blogspot.my/2006/01/golden-copper-and-silver-fibers-of.html. [16] K. Oluwatayo, Farmers Sensitize on Kenaf Plant, September 02, 2016 [Online]. Available: http://thehopenewspapers.com/2016/09/farmers-sensitize-kenaf-plant/. [17] J. Jayaramudu, B.R. Guduri, A. Varada Rajulu, Characterization of new natural cellulosic fabric Grewia tilifolia, Carbohydr. Polym. 79 (4) (2010) 847e851. [18] D. Nabi Saheb, J.P. Jog, Natural fiber polymer composites, Adv. Polym. Technol. 18 (4) (1999) 351e363. [19] Z. Liu, S.Z. Erhan, D.E. Akin, F.E. Barton, “ ‘‘Green” composites from renewable resources: preparation of epoxidized soybean oil and flax fiber composites, J. Agric. Food Chem. 54 (2006) 2134e2141. [20] K.M.M. Rao, K.M. Rao, Extraction and tensile properties of natural fibers: vakka, date and bamboo, Compos. Struct. 77 (3) (Feb. 2007) 288e295. [21] A. Valadez-Gonzalez, J.M. Cervantes-Uc, R. Olayo, P.J. Herrera-Franco, “Effect of fiber surface treatment on the fiberematrix bond strength of natural fiber reinforced composites, Compos. B Eng. 30 (3) (1999) 309e329. [22] K.O. Reddy, C.U. Maheswari, D.J.P. Reddy, A.V. Rajulu, Thermal properties of Napier grass fibers, Mater. Lett. 63 (27) (2009) 2390e2392. [23] S.V. Joshi, L.T. Drzal, A.K. Mohanty, S. Arora, Compos. Part A Appl. Sci. Manuf. 35 (2004) 371. [24] K.R. Woodard, G.M. Prine, Dry matter accumulation of elephant grass, energy cane and elephantmillet in a subtropical climate, Crop Sci. 33 (1993) 818e824. [25] K. Kovier, Fiber Reinforced Concrete, The Cement and Concrete institute, Midrand, 2001. [26] K. Murali Mohan Rao, A.V. Ratna Prasad, M.N.V. Ranga Babu, K. Mohan Rao, A.V.S.S.K.S. Gupta, “, Tensile properties of elephant grass fiber reinforced polyester composites”, J. Mater. Sci. 42 (2007) 3226e3272. [27] M.J.M. Ridzuan, M.S. Abdul Majid, M. Afendi, S.N. Aqmariah Kanafiah, J.M. Zahri, A.G. Gibson, Characterisation of natural cellulosic fibre from Pennisetum purpureum stem as potential reinforcement of polymer composites, Mater. Des. 89 (November 2016) 839e847.

160

Interfaces in Particle and Fibre Reinforced Composites

[28] V.P. Kommula, K. Obi Reddy, M. Shukla, T. Marwala, A. Varada Rajulu, Physicochemical, tensile and thermal characterization of Napier grass (native african) fiber strands, Int. J. Polym. Anal. Charact. 18 (2013) 303e314. [29] V.P. Kommula, K.O. Reddy, M. Shukla, A.V. Rajulu, S. Africa, S. Africa, A. Park, S. Africa, M.N. National, “Mechanical properties , water absorption , and chemical resistance of Napier grass fiber strand e reinforced epoxy resin composites, Int. J. Polym. Anal. Charact. 18 (2014) 37e41. [30] M. Haameem J.A, M.S. Abdul Majid, M. Afendi, H.F.A. Marzuki, I. Fahmi, A.G. Gibson, Mechanical properties of Napier grass fibre/polyester composites, Compos. Struct. 136 (February 2016) 1e10. [31] C. Mothe, C.R. Araujo, Thermal and mechanical characterization of polyuretane composites with Curaua fibers, Polimeros 14 (2004) 274e278. [32] A. Mohanty, M. Khan, G. Hinrichsen, Influence of chemical surface modification on the properties of biodegradable jute fabrics- polyester amide composites, Compos. Part A Appl. Sci. Manuf. 31 (2) (2000) 143e150. [33] P.A. Sreekumar, R. Saiah, J.M. Saiter, N. Leblanc, K. Joseph, G. Unnikrishnan, S. Thomas, Dynamic mechanical Properties of sisal fiber reinforced polyester composites fabricated by resin transfer molding, Polym. Compos. 1 (2009) 768e775. [34] I. Fahmi, M.S. Abdul Majid, M. Afendi, E.A. Helmi, M. Haameem J.A, Low-velocity impact responses of Napier fibre/polyester composites, Int. J. Automot. Mech. Eng. 13 (1) (June 2016) 3226e3237. [35] J.H. Song, S.D. Mun, C.S. Kim, Mechanical properties of sisal natural fiber composites according to strain rate and absorption ratio, Polym. Compos. 32 (2011) 1174e1180. [36] J. Santhosh, N. Balanarasimman, R. Chandrasekar, S. Raja, Study of properties of banana fibre reinforced composites, Intl. J. Res. Eng. Technol. 3 (11) (November 2014) 144e150. [37] S. Biswas, S. Shahinur, M. Hasan, Q. Ahsan, Physical , mechanical and thermal properties of jute and bamboo fiber reinforced unidirectional epoxy composites, Procedia Eng. 105 (2015) 933e939. [38] J. Madhukiran, Dr S. Srinivasa Rao, Madhusudan, Tensile and Hardness Properties of Banana/Pineapple Natural Fibre Reinforced Hybrid Composites, Intl. J. Res. Eng. Technol 2 (7) (July 2013) 1260e1264. [39] A. Gopinath, M. Senthil Kumar, A. Elayaperumal, Experimental Investigations on Mechanical Properties of Jute Fiber Reinforced Composites with Polyester and Epoxy Resin Matrices.12th Global Congress on Manufacturing and Management, GCMM 2014, Procedia Engineering, 2014. [40] J. Ervina, M. Mariatti, S. Hamdan, Effect of filler loading on the tensile properties of multiwalled carbon nanotube and graphene nanopowder filled epoxy composites, Procedia Chem. 19 (2016) 897e905. [41] J.K.W. Sandler, J.E. Kirk, I.A. Kinloch, M.S.P. Shaffer, A.H. Windle, Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites, Polymer 44 (19) (September 2003) 5893e5899. [42] A. Martone, C. Formicola, M. Giordano, M. Zarrelli, Reinforcement efficiency of multiwalled carbon nanotube/epoxy nano composites, Compos. Sci. Technol. 70 (7) (2010) 1154e1160. [43] G. Mittal, V. Dhand, K.Y. Rhee, S.-J. Park, W.R. Lee, A review on carbon nanotubes and graphene as fillers in reinforced polymer nanocomposites, J. Ind. Eng. Chem. 21 (2015) 11e25.

Effect of nanoclay filler on mechanical and morphological properties

161

[44] P. Guo, X. Chen, X. Gao, H. Song, H. Shen, Fabrication and mechanical properties of welldispersed multiwalled carbon nanotubes/epoxy composites, Compos. Sci. Technol. 67 (2007) 3331e3337. [45] B.S.H. John, F. Timmerman, J.C. Seferis, Nanoclay reinforcement effects on the cryogenic microcracking of carbon fiber/epoxy composites, Compos. Sci. Technol. 62 (2002) 1249e1258. [46] W. Wahyuni Amir, A. Jumahat, J. Mahmud, Effect of nanoclay content on flexural properties of glass fiber reinforced polymer (GFRP) composites, J. Technol. 76 (3) (2015) 31e35. [47] M. Atai, L. Solhi, A. Nodehi, S.M. Mirabedini, S. Kasraei, K. Akbari, PMMA-grafted nanoclay as novel filler for dental adhesives, Dent. Mater. 25 (2009) 339e386. [48] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C. Chung, Polypropylene/montmorillonite nanocomposites. Review of the synthetic routes and materials properties, Chem. Mater. 13 (2001) 3516e3523. [49] Y. Zhou, F. Pervin, A. Mohammad, Biswas , Shaik Jeelani, “ Fabrication and characterization of montmorillonite clay-filled SC-15 epoxy, Mater. Lett. 60 (7) (April 2006) 869e873. [50] S.D. Salman, M.J. Sharba, Z. Leman, M.T.H. Sultan, M.R. Ishak, F. Cardona, Physical, mechanical, and morphological properties of woven kenaf/polymer composites produced using a vacuum infusion technique, Int. J. Polym. Sci. 10 (January 2015) 315e323. [51] K. Krushnamurty, I. Srikanth, B. Rangababu, S.K. Majee, R. Bauri, C. Subrahmanyam, Effect of nanoclay on the toughness of epoxy and mechanical, impact properties of E-glassepoxy composites, Adv. Mater. 6 (March 2015) 684e689. [52] I. Ikejima, R. Nomoto, J.F. McCabe, Shear punch strength and flexural strength of model composites with varying filler volume fraction, particle size and silanation, Dent. Mater. 19 (2003) 206e217. [53] L. Solhi, M. Atai, A. Nodehi, M. Imani, A novel dentin bonding system containing poly (methacrylic acid) grafted nanoclay: synthesis, characterization and properties, Dent. Mater. 28 (2012) 1041e1050.  [54] T. Zukas, V. Jankauskaite, K. Zukien_ e, The influence of nanofillers on the mechanical properties of carbon fibre reinforced methyl methacrylate composite, Mater. Sci. 18 (2012) 250e255. [55] Y. Li, M. Swartz, R. Phillips, B. Moore, T. Roberts, Materials science effect of filler content and size on properties of composites, J. Dent. Res. 64 (1985) 1396e1799. [56] G.L. Adabo, C.A. dos Santos Cruz, R.G. Fonseca, L.G. Vaz, The volumetric fraction of inorganic particles and the flexural strength of composites for posterior teeth, J. Dent. 31 (2003) 353e562. [57] S.S. Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (11) (2003) 1539e1641. [58] N. Chisholm, H. Mahfuz, V.K. Rangari, A. Ashfaq, S. Jeelani, Fabrication and mechanical characterization of carbon/SiC-epoxy nanocomposites, Compos. Struct. 67 (2005) 115e139. [59] N.A. Siddiqui, R.S.C. Woo, J.K. Kim, C.C.K. Leung, Munir, Mode I interlaminar fracture behavior and mechanical properties of CFRPs with nanoclay-filled epoxy matrix, Composites Part A 38 (2007) 449e460. [60] M.S. Ibrahim, S.M. Sapuan, A.A. Faieza, Mechanical and thermal properties of composites from unsaturated polyester filled with oil palm ash, J. Mech. Eng. Sci. 2 (2012) 133e147.

162

Interfaces in Particle and Fibre Reinforced Composites

[61] S. Parija, S.K. Nayak, S.K. Verma, S.S. Tripathy, Studies on physico-mechanical properties and thermal characteristics of polypropylene/layered silicate nanocomposites, Polym. Compos. 25 (2004) 646e652. [62] I. Ikejima, R. Nomoto, J.F. McCabe, Shear punch strength and flexural strength of model composites with varying filler volume fraction, particle size and silanation, Dent. Mater. 19 (2003) 206e211. [63] M.F. Nunes, E.J. Swift, J. Perdig~ao, Effects of adhesive composition on microtensile bond strength to human dentin, Am. J. Dent. 14 (2001) 340e343.

A review on the interfacial characteristics of natural fibre reinforced polymer composites

7

W.K. Ng, M. Johar, H.A. Israr, K.J. Wong School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia

1. Introduction In recent decades, natural fibres are getting more attention as reinforcements for polymer composites due to their advantages of low weight, low cost and environmentally friendly (renewable, recyclable and biodegradable) [1]. The applications of natural fibres include automotive [2] and building [3] industries. However, the fibre/ matrix interface is generally weak due to the incompatibility between hydrophilic fibre and hydrophobic matrix, leading to early debonding of the fibre upon loading [4]. Consequently, the debonded fibres are not able to carry the load and this would lead to the premature failure of the composites. Hence, it is important to understand the parameters that influence the interfacial characteristics of natural fibre/polymer composites in order to explore the opportunities to enhance the overall performance of the composites. The study of interfacial adhesion of natural fibres in various different types of polymers has been carried out by many researchers. This includes bamboo [5e11], basalt [12], cotton [13,14], flax [13,15e35], hemp [12,13,17,20,36e39], henequen [1,4,40,41], jute [7,39,42e45], kenaf [15,44,46,47], lyocell [15,48,49], pineapple leaf [50], ramie [18,48,49], sisal [20,51e55], and wheat straw pulp [56]. The important parameters and results of interfacial shear strength (IFSS) studies of various natural fibre/polymer matrix systems from the literature are summarised in Appendix 1. All values of findings from the literature quoted in this chapter are average values. The comparison among different parameters will be carried out in the following sections.

2. Types of fibre and matrix Fig. 7.1 classifies some of the common cellulosic natural fibres [57e59]. In general, natural fibres are having a significant lower density compared to E-glass, and also comparable to Kevlar and carbon fibres [58]. Therefore, they are having high specific strength and modulus. This is a great advantage as a lightweight material.

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00007-8 Copyright © 2020 Elsevier Ltd. All rights reserved.

164

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems. Lc (mm)

IFSS (MPa)

S.D (MPa)

References

PO

e

e

3.2

e

[5]

Untreated

PO

e

e

8.4

e

[5]

VE

steam-exploded extraction

PO

0.5

e

10.8

e

[6]

bamboo

VE

NaOH extraction (1.5 N)

PO

0.5

e

6.4

e

[6]

bamboo

VE

chemical extraction

PO

0.5

e

11.5

e

[6]

bamboo

PP

Untreated

FM

0.25

3.50

4.9

0.3

[7]

bamboo

UP

untreated, Le ¼ 1 mm

PO

1

e

3.3

1.0

[8]

bamboo

UP

untreated, Le ¼ 3 mm

PO

1

e

7.9

1.4

[8]

bamboo

UP

untreated, Le ¼ 5 mm

PO

1

e

14.2

1.2

[8]

bamboo

UP

NaOH (1 wt%), Le ¼ 1 mm

PO

1

e

6.2

0.9

[8]

bamboo

UP

NaOH (1 wt%), Le ¼ 3 mm

PO

1

e

15.1

1.3

[8]

bamboo

UP

NaOH (1 wt%), Le ¼ 5 mm

PO

1

e

26.2

1.7

[8]

bamboo

UP

NaOH (3 wt%), Le ¼ 1 mm

PO

1

e

9.5

1.0

[8]

bamboo

UP

NaOH (3 wt%), Le ¼ 3 mm

PO

1

e

19.1

1.0

[8]

bamboo

UP

NaOH (3 wt%), Le ¼ 5 mm

PO

1

32.6

2.4

[8]

bamboo

UP

NaOH (5 wt%), Le ¼ 1 mm

PO

1

e

11.1

0.7

[8]

bamboo

UP

NaOH (5 wt%), Le ¼ 3 mm

PO

1

e

22.2

1.6

[8]

bamboo

UP

NaOH (5 wt%), Le ¼ 5 mm

PO

1

e

36.5

2.0

[8]

bamboo

UP

untreated, Le ¼ 3 mm

PO

1

e

1.8

0.8

[11]

Matrix

Fibre/Matrix modificationb

Test

bamboo

PP

Untreated

bamboo

PVDF

bamboo

c

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

untreated, Le ¼ 5 mm

PO

1

e

2.0

1.1

[11]

bamboo

UP

untreated, Le ¼ 7 mm

PO

1

e

1.4

0.7

[11]

bamboo

UP

untreated, Le ¼ 10 mm

PO

1

e

1.7

0.8

[11]

bamboo

UP

NaOH (1 wt%), Le ¼ 3 mm

PO

1

e

4.3

1.0

[11]

bamboo

UP

NaOH (1 wt%), Le ¼ 5 mm

PO

1

e

4.6

1.4

[11]

bamboo

UP

NaOH (3 wt%), Le ¼ 3 mm

PO

1

e

4.8

1.5

[11]

bamboo

UP

NaOH (3 wt%), Le ¼ 5 mm

PO

1

e

5.3

0.9

[11]

bamboo

UP

NaOH (5 wt%), Le ¼ 3 mm

PO

1

e

3.6

1.0

[11]

bamboo

UP

NaOH (5 wt%), Le ¼ 5 mm

PO

1

e

3.7

0.9

[11]

bamboo

UP

NaOH (7 wt%), Le ¼ 5 mm

PO

1

e

3.8

1.2

[11]

bamboo

UP

NaOH (7 wt%), Le ¼ 7 mm

PO

1

e

2.9

2.0

[11]

bamboo

UP

NaOH (10 wt%), Le ¼ 7 mm

PO

1

e

5.5

2.5

[66]

bamboo

VE

Untreated

PO

2

e

10.7

e

[9]

bamboo

PLA

Untreated

MB

e

e

4.3

e

[10]

bamboo

PLA

NaOH extraction (1.5 N)

MB

e

e

9.8

e

[10]

bamboo

PLA

steam-exploded extraction

MB

e

e

12.6

e

[10]

basalt

PP

Untreated

MB

e

0.64

2.5

0.4

[12]

basalt

PP

(SFO þ MA)PP

MB

e

1.04

4.1

0.5

[12]

cotton

PP

MAPP (2%)

FM

0.2

5.03

0.7

e

[13]

cotton

PP

Untreated

FM

1

0.34

6.1

e

[14]

cotton

PP

crystallisation (t ¼ 1 min)

FM

1

0.26

8.4

e

[14]

cotton

PP

crystallisation (t ¼ 5 min)

FM

1

0.19

12.0

e

[14]

flax

PLA

Untreated

PO

1

1.95

28.3

10.9

[15]

165

UP

A review on the interfacial characteristics of natural fibre

bamboo

Continued

166

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

PO

1

3.42

17.9

10.5

[15]

MAPP (2%)

PO

1

1.87

24.3

11.1

[15]

PP

Untreated

FM

0.2

3.99

9.8

6.8

[15]

flax

PP

MAPP (2%)

FM

0.2

2.80

15.8

14.5

[15]

flax

PP

MAPP

FM

0.1

2.60

12.3

1.3

[18]

flax

PP

MAPP

FM

0.2

2.42

13.9

1.3

[18]

flax

PP

MAPP

FM

0.3

2.45

12.8

1.2

[18]

flax

PP

MAPP

FM

0.5

2.51

12.7

0.3

[18]

flax

PP

MAPP

FM

2

2.60

11.2

1.9

[18]

flax

PP

MAPP

FM

0.1

3.00

9.1

1.3

[18]

flax

PP

Untreated

FM

0.2

4.86

4.5

0.7

[18]

flax

PLA

Untreated

FM

e

e

e

e

[13]

flax

PP

MAPP (2%)

FM

0.2

3.2

12.0

e

[13]

flax

PP

Untreated

MB

1

5.01

3.4

0.6

[20]

flax

MaterBi

Untreated

MB

1

4.26

4.2

0.6

[20]

flax

PuraSorb

Untreated

MB

1

2.35

9.0

3.0

[20]

flax

PP

Untreated

PO

e

e

7.8

e

[23]

flax

PP

VTMO (2.5 wt%)

PO

e

e

8.0

e

[23]

flax

PP

maleic anhydride (10%)

PO

e

e

6.1

e

[23]

flax

PP

MAPP (10%)

PO

e

e

8.9

e

[23]

Matrix

Fibre/Matrix modificationb

Test

flax

PP

Untreated

flax

PP

flax

c

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

NaOH (20 wt%)

PO

e

e

8.4

e

[23]

flax

VE

Untreated

FM

0.1

0.54

28.0

11.0

[24]

flax

VE

acrylic acid (low)

FM

0.1

0.45

25.0

4.0

[24]

flax

VE

acrylic acid (high)

FM

0.1

0.33

31.0

6.0

[24]

flax

VE

VTMO (low)

FM

0.1

0.66

20.0

2.0

[24]

flax

VE

VTMO (high)

FM

0.1

0.60

21.0

3.0

[24]

flax

UP

Untreated

FM

0.1

0.94

13.0

2.0

[24]

flax

UP

VTMO (low)

FM

0.1

0.81

15.0

3.0

[24]

flax

UP

VTMO (high)

FM

0.1

1.09

13.0

5.0

[24]

flax

UP

VTMO (high)

FM

0.1

0.88

14.0

2.0

[24]

flax

EP

Untreated

FM

0.1

0.40

33.0

7.0

[24]

flax

EP

maleic anhydride

FM

0.1

0.49

24.0

3.0

[24]

flax

PP

Untreated

MB

0.05

e

8.9

e

[25]

flax

PP

MAPP (5%)

MB

0.05

e

9.9

e

[25]

flax

PP

MAPP (10%)

MB

0.05

e

9.4

e

[25]

flax

PP

maleic anhydride (5%)

MB

0.05

e

7.2

e

[25]

flax

PP

maleic anhydride (10%)

MB

0.05

e

6.8

e

[25]

flax

PP

VTMO (2.5 wt%)

MB

0.05

e

9.8

e

[25]

flax

PP

Untreated

FM

e

0.98

13.0

e

[26]

flax

PP

MAPP (1 wt%)

FM

e

0.52

28.0

e

[26]

flax

PP

Untreated

FM

e

3.80

8.0

e

[26]

flax

PP

MAPP (1 wt%)

FM

e

2.80

12.0

e

[26]

flax

VE

Untreated

FM

0.1

0.58

24.0

4.0

[27]

167

PP

A review on the interfacial characteristics of natural fibre

flax

Continued

168

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

FM

0.1

0.34

38.0

6.0

[27]

acrylic acid (high)

FM

0.1

0.33

42.0

8.0

[27]

VE

VTMO (low)

FM

0.1

0.49

30.0

4.0

[27]

flax

VE

VTMO (high)

FM

0.1

0.45

26.0

4.0

[27]

flax

UP

Untreated

FM

0.1

0.70

18.0

3.0

[27]

flax

UP

acrylic acid (low)

FM

0.1

0.61

22.0

2.0

[27]

flax

UP

acrylic acid (high)

FM

0.1

0.82

22.0

5.0

[27]

flax

UP

VTMO (high)

FM

0.1

0.66

18.0

3.0

[27]

flax

PHB

Untreated

MB

0.5

e

8.8

e

[28]

flax

PHB

Acetylation

MB

0.5

e

10.3

e

[28]

Matrix

Fibre/Matrix modificationb

Test

flax

VE

acrylic acid (low)

flax

VE

flax

c

flax

PHB

plasma (0.5 cm /s, 50 W)

MB

0.5

e

12.4

e

[28]

flax

PP

MAPP

PO

e

e

11.4

e

[29]

flax

PP

Untreated

MB

1

e

7.2

1.8

[30]

flax

PP

MAPP (5 wt%)

MB

1

e

7.2

2.1

[30]

flax

PP

Untreated

FM

1

1.78

6.3

e

[33]

flax

PP

acetylation (1 h)

FM

1

1.33

8.4

e

[33]

flax

PP

acetylation (2 h)

FM

1

1.19

9.5

e

[33]

flax

PP

acetylation (4 h)

FM

1

0.97

11.6

e

[33]

flax

PP

stearic acid (12 h)

FM

1

1.52

7.4

e

[33]

flax

PP

stearic acid (36 h)

FM

1

1.19

9.5

e

[33]

3

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

PP

stearic acid (90 h)

FM

1

2.13

4.6

e

[33]

flax

UP

Untreated

PO

e

e

17.1

e

[35]

flax

LDPE

Untreated

PO

e

e

5.4

e

[35]

flax (dr)

EP

no immersion

MB

0.1

e

22.5

2.1

[16]

flax (dr)

EP

deionised water 15 min

MB

0.1

e

15.0

4.0

[16]

flax (dr)

EP

deionised water 840 min

MB

0.1

e

11.1

3.8

[16]

flax (dr)

EP

deionised water 5160 min

MB

0.1

e

8.2

1.0

[16]

flax (dr)

EP

deionised water 8100 min

MB

0.1

e

8.9

1.1

[16]

flax (dr)

PLA

untreated

MB

0.1

e

>20

e

[17]

flax (dr)

PLA

cooling in air

MB

0.1

e

15.3

3.3

[19]

MB

0.1

e

18.2

1.8

[19]

MB

0.1

e

22.2

3.4

[19]

MB

0.1

e

9.9

1.5

[19]

MB

0.1

e

16.1

0.8

[19]

flax (dr) flax (dr) flax (dr)

PLA PLA PLA

cooling at 10 cooling at 1

 C/min

 C/min

annealing at 50 C

C

for 72 h

EP

curing at 65

flax (dr)

UP

untreated

MB

0.1

e

14.2

0.4

[19]

flax (dr)

PLA

untreated

PO

0.02

e

12.6

4.4

[21]

flax (dr)

PLA

acetone

PO

0.02

e

10.3

4.8

[21]

flax (dr)

PLA

glycerol triacetate (8e10%v/v)

PO

0.02

e

14.6

1.7

[21]

flax (dr)

PHB

acetone

PO

0.02

e

10.5

4.8

[21]

flax (dr)

PHB

acetone þ thiodiphenol (10%v/v)

PO

0.02

e

16.7

4.8

[21]

flax (dr)

PHB

acetone þ hyperbranched polyester (10%v/v)

PO

0.02

e

8.3

2.0

[21]

flax (dr)

UP

untreated

MB

e

e

14.2

0.4

[22]

for 14 h

169

flax (dr)

A review on the interfacial characteristics of natural fibre

flax

Continued

170

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

MB

e

e

6.2

0.5

[22]

NaOH (10 g/L) þ acetylation

MB

e

e

16.1

0.8

[22]

UP

formic acid (99%)

MB

e

e

16.4

0.8

[22]

flax (dr)

PP

melt flow index ¼ 450

PO

5

e

14.9

5.1

[31]

flax (dr)

PP

melt flow index ¼ 2

PO

5

e

10.4

2.8

[31]

flax (dr)

PP

MAPP (0.6%)

PO

5

e

25.1

7.9

[31]

flax (dr)

PP

MAPP (8%e10%)

PO

5

e

13.8

4.6

[31]

flax (dr)

PP

melt flow index ¼ 450 þ propyltrimeth oxysilane

PO

5

e

10.5

3.1

[31]

flax (dr)

PP

melt flow index ¼ 450 þ phenylisocyanate (20 g/l)

PO

5

e

16.8

4.0

[31]

flax (dr)

PP

melt flow index ¼ 450 þ MAPP (0.1%)

PO

5

e

19.6

3.1

[31]

flax (dr)

PP

untreated

FM

1

1.19

12.8

e

[32]

FM

1

0.71

23.1

e

[32]

FM

1

0.71

23.1

e

[32]

FM

1

0.71

23.1

e

[32]

Matrix

Fibre/Matrix modificationb

Test

flax (dr)

UP

NaOH (10 g/L)

flax (dr)

UP

flax (dr)

flax (dr) flax (dr)

PP PP

crystallisation (50 mm) at 145

C

crystallisation (100 mm) at 145

C

C

c

flax (dr)

PP

crystallisation (50 mm) at 140

flax (dr)

PP

untreated

FM

1

1.19

12.8

e

[33]

flax (dr)

PP

acetylation (1 h)

FM

1

1.07

13.1

e

[33]

flax (dr)

PP

acetylation (2 h)

FM

1

1.07

13.0

e

[33]

flax (dr)

PP

acetylation (4 h)

FM

1

1.07

12.5

e

[33]

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

PP

stearic acid (12 h)

FM

1

1.19

12.0

e

[33]

flax (dr)

PP

stearic acid (36 h)

FM

1

1.07

13.4

e

[33]

flax (dr)

PP

stearic acid (90 h)

FM

1

1.52

6.4

e

[33]

flax (dr)

PP

untreated

PO

e

e

10.6

e

[34]

flax (dr)

HDPE

untreated

PO

e

e

9.1

e

[34]

flax (dr)

LDPE

untreated

PO

e

e

5.5

e

[34]

flax (dr)

PP

maleic anhydride (5%)

PO

e

e

11.4

e

[34]

flax (D)

PP

untreated

MB

1

e

7.5

2.1

[30]

flax (D)

PP

hot-cleaned

MB

1

e

6.6

1.3

[30]

flax (D)

PP

MAPP (5 wt%)

MB

1

e

7.2

2.2

[30]

flax (D)

UP

untreated

PO

e

e

11.7

e

[35]

flax (D)

EP

untreated

PO

e

e

23.2

e

[35]

flax (D)

LDPE

untreated

PO

e

e

4.4

e

[35]

flax (D)

HDPE

untreated

PO

e

e

10.1

e

[34]

flax (D)

LDPE

untreated

PO

e

e

6.2

e

[34]

hemp

PLA

untreated

MB

0.1

e

>20

e

[17]

hemp

PLA

untreated

PO

0.5

e

5.6

e

[36,37]

hemp

PLA

NaOH (5 wt%)

PO

0.5

e

11.4

e

[36,37]

hemp

PLA

acetylation

PO

0.5

e

6.3

e

[37]

hemp

PLA

maleic anhydride (5 wt%)

PO

0.5

e

5.3

e

[37]

hemp

PLA

silane (0.5 wt%)

PO

0.5

e

8.2

e

[37]

hemp

PLA

NaOH (5 wt%) þ silane (0.5 wt%)

PO

0.5

e

9.9

e

[37]

hemp

UP

untreated

PO

0.5

e

9.9

e

[37]

A review on the interfacial characteristics of natural fibre

flax (dr)

171 Continued

172

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

PO

0.5

e

11.7

e

[37]

acetylation

PO

0.5

e

12.6

e

[37]

UP

maleic anhydride (5 wt%)

PO

0.5

e

15.1

e

[37]

hemp

UP

silane (0.5 wt%)

PO

0.5

e

16.3

e

[37]

hemp

UP

NaOH (5 wt%) þ silane (0.5 wt%)

PO

0.5

e

20.3

e

[37]

hemp

PLA

untreated

PO

0.5

e

2.2

e

[38]

hemp

PLA

Na2SO3 (2 wt%) þ NaOH (5 wt%)

PO

0.5

e

2.8

e

[38]

hemp

PP

MAPP (2%)

FM

0.2

3.16

14.3

e

[13]

hemp

PP

untreated

MB

1

4.70

5.1

1.4

[20]

hemp

MaterBi

untreated

MB

1

7.26

3.0

0.9

[20]

hemp

PuraSorb

untreated

MB

1

2.45

11.3

3.4

[20]

hemp

PP

untreated

MB

e

1.8

3.4

0.4

[12]

hemp

PP

(SFO þ MA)PP

MB

e

2.81

5.4

0.6

[12]

hemp

PP

untreated

MB

0.5

e

4.9

e

[39]

hemp

PP

NaOH (0.5 wt%)

MB

0.5

e

5.3

e

[39]

hemp

PP

silane (0.5 wt%)

MB

0.5

e

5.2

e

[39]

hemp

PP

MAPP (1%)

MB

0.5

e

5.1

e

[39]

hemp

PP

MAPP (3%)

MB

0.5

e

5.9

e

[39]

hemp

PP

MAPP (5%)

MB

0.5

e

6.3

e

[39]

henequen

PP

untreated

MB

e

e

4.1

1.5

[44]

Matrix

Fibre/Matrix modificationb

Test

hemp

UP

NaOH (5 wt%)

hemp

UP

hemp

c

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

tap water (soaking)

MB

e

e

4.8

0.7

[44]

henequen

PP

tap water (ultrasonication)

MB

e

e

6.0

0.7

[44]

henequen

UP

untreated

MB

e

e

5.6

1.1

[44]

henequen

UP

tap water (soaking)

MB

e

e

7.5

0.2

[44]

henequen

UP

tap water (ultrasonication)

MB

e

e

8.7

0.7

[44]

henequen

HDPE

untreated

PO

0.02

e

2.5

e

[4]

henequen

HDPE

NaOH (2%w/v)

PO

0.02

e

4.2

e

[4]

henequen

HDPE

preimpreg (1.5%w/w HDPE/xylene)

PO

0.02

e

3.5

e

[4]

henequen

HDPE

NaOH (2%w/v) þ preimpreg

PO

0.02

e

4.0

e

[4]

henequen

HDPE

silane (1%)

PO

0.02

e

3.8

e

[4]

henequen

HDPE

NaOH (2%w/v) þ silane (1%)

PO

0.02

e

5.0

e

[4]

henequen

HDPE

untreated

FM

e

12.96

5.4

e

[4]

henequen

HDPE

NaOH (2%w/v)

FM

e

11.4

5.0

e

[4]

henequen

HDPE

preimpreg (1.5%w/w HDPE/xylene)

FM

e

9.25

6.0

e

[4]

henequen

HDPE

NaOH (2%w/v) þ preimpreg

FM

e

6

9.2

e

[4]

henequen

HDPE

silane (1%)

FM

e

5.45

11.9

e

[4]

henequen

HDPE

NaOH (2%w/v) þ silane (1%)

FM

e

3.5

16.0

e

[4]

henequen

HDPE

untreated

FM

0.5

e

20.2

1.1

[1]

henequen

HDPE

silane (0.005%w/w)

FM

0.5

e

21.8

1.3

[1]

henequen

HDPE

silane (0.01%w/w)

FM

0.5

e

31.2

2.6

[1]

henequen

HDPE

silane (0.05%w/w)

FM

0.5

e

27.8

1.8

[1]

henequen

HDPE

untreated

PO

1.2

e

2.4

0.5

[41]

henequen

HDPE

NaOH (2%w/v)

PO

1.2

e

3.4

1.0

[41]

173

PP

A review on the interfacial characteristics of natural fibre

henequen

Continued

174

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

PO

1.2

e

5.0

0.6

[41]

NaOH (2%w/v) þ preimpreg

PO

1.2

e

8.0

0.4

[41]

HDPE

NaOH (2%w/v) þ silane (1%) þ preimpreg

PO

1.2

e

9.0

e

[41]

henequen

HDPE

untreated

FM

2

e

4.4

1.2

[41]

henequen

HDPE

NaOH (2%w/v)

FM

2

e

6.3

1.3

[41]

henequen

HDPE

NaOH (2%w/v) þ silane (1%)

FM

2

e

16.0

1.8

[41]

henequen

HDPE

NaOH (2%w/v) þ preimpreg

FM

2

e

9.0

1.5

[41]

henequen

HDPE

NaOH (2%w/v) þ silane (1%) þ preimpreg

FM

2

e

20.0

e

[41]

jute

PP

untreated

FM

0.25

3.73

2.1

0.2

[7]

jute

PLA

no irradiation

MB

2

e

4.6

0.1

[42]

jute

PLA

irradiation (2 kGy)

MB

2

e

4.7

0.1

[42]

jute

PLA

irradiation (5 kGy)

MB

2

e

4.8

0.1

[42]

jute

PLA

irradiation (10 kGy)

MB

2

e

5.5

0.1

[42]

jute

PLA

irradiation (20 kGy)

MB

2

e

5.2

0.1

[42]

jute

PLA

irradiation (30 kGy)

MB

2

e

4.9

0.1

[42]

jute

PLA

irradiation (50 kGy)

MB

2

e

4.3

0.1

[42]

jute

PLA

irradiation (100 kGy)

MB

2

e

4.2

0.1

[42]

jute

PP

untreated

MB

0.5

e

5.4

0.9

[43]

Matrix

Fibre/Matrix modificationb

Test

henequen

HDPE

NaOH (2%w/v) þ silane (1%)

henequen

HDPE

henequen

c

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

boiling

MB

0.5

e

2.1

0.6

[43]

jute

PP

NaOH (0.5 wt%)

MB

0.5

e

6.3

1.0

[43]

jute

PP

NaOH (0.5 wt%) þ boiling

MB

0.5

e

2.4

0.8

[43]

jute

PP

silane (0.5 wt%)

MB

0.5

e

6.2

0.8

[43]

jute

PP

silane (0.5 wt%) þ boiling

MB

0.5

e

3.7

0.8

[43]

jute

PLA

untreated

MB

e

e

5.4

0.7

[44]

jute

PLA

tap water (soaking)

MB

e

e

9.7

1.2

[44]

jute

PLA

tap water (ultrasonication)

MB

e

e

13.3

1.4

[44]

jute

PP

untreated

MB

0.5

e

4.6

e

[39]

jute

PP

NaOH (0.5 wt%)

MB

0.5

e

5.3

e

[39]

jute

PP

silane (0.5 wt%)

MB

0.5

e

5.1

e

[39]

jute

PP

MAPP (1%)

MB

0.5

e

5.6

e

[39]

jute

PP

MAPP (3%)

MB

0.5

e

6.1

e

[39]

jute

PP

MAPP (5%)

MB

0.5

e

6.4

e

[39]

jute

LDPE

untreated

FM

e

7.83

3.6

e

[45]

jute

LDPE

bleached (0.7%NaClO2)

FM

e

11.56

1.7

e

[45]

jute

LDPE

carding

FM

e

5.28

3.8

e

[45]

jute

LDPE

bleached þ NaOH (2%)

FM

e

11.33

1.0

e

[45]

jute

PP

untreated

FM

e

3.19

12.0

e

[45]

jute

PP

bleached (0.7%NaClO2)

FM

e

4.64

5.8

e

[45]

jute

PP

carding

FM

e

2.75

11.0

e

[45]

jute

PP

bleached þ NaOH (2%)

FM

e

3.75

3.7

e

[45]

kenaf

PHB

untreated

PO

1

1.95

13.2

e

[15]

175

PP

A review on the interfacial characteristics of natural fibre

jute

Continued

176

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d Lc (mm)

IFSS (MPa)

S.D (MPa)

References

PO

1

0.73

25.7

10.6

[15]

untreated

FM

0.2

5.05

7.4

e

[15]

PP

MAPP (2%)

FM

0.2

1.12

17.2

37.8

[15]

kenaf

PLA

untreated

MB

e

e

5.4

2.2

[46,47]

kenaf

PLA

untreated

MB

e

e

10.6

0.5

[44]

kenaf

PLA

tap water (soaking)

MB

e

e

11.7

0.3

[44]

kenaf

PLA

tap water (ultrasonication)

MB

e

e

11.5

0.3

[44]

lyocell

PHB

untreated

PO

1

0.92

7.1

e

[15]

lyocell

PLA

untreated

PO

1

0.75

10.3

4.9

[15]

lyocell

PP

untreated

PO

1

1.31

6.3

4.0

[15]

lyocell

PP

MAPP (2%)

PO

1

0.94

8.8

5.7

[15]

lyocell

PP

untreated

FM

0.2

2.50

3.8

2.5

[15]

lyocell

PP

MAPP (2%)

FM

0.2

1.46

5.0

4.3

[15]

lyocell

PP

untreated

MB

0.05

1.20

5.3

1.0

[48]

lyocell

PP

maleic anhydride (2.5 wt%)

MB

0.05

0.60

9.2

0.8

[48]

lyocell

PP

MAPP (0.2 wt%)

MB

0.05

0.70

8.2

1.2

[48]

lyocell

EP

untreated

MB

0.05

0.40

14.8

1.3

[48]

lyocell

PP

untreated

MB

1

e

4.2

1.7

[49]

pineapple

PHBV

untreated

MB

0.2

e

8.2

1.5

[50]

ramie

PP

MAPP (0.2 wt%)

MB

0.05

1.10

5.9

1.1

[48]

Matrix

Fibre/Matrix modificationb

Test

kenaf

PLA

untreated

kenaf

PP

kenaf

c

Interfaces in Particle and Fibre Reinforced Composites

CHS (mm/ min)

Fibrea

EP

untreated

MB

0.05

0.20

21.2

3.9

[48]

ramie

PP

MAPP

FM

0.2

0.98

24.9

2.9

[18]

ramie

PP

untreated

MB

1

e

5.0

1.8

[49]

sisal

PLA

untreated

PO

e

e

2.4

e

[51]

sisal

PLA

NaOH (2 wt% þ 7.5 wt%)

PO

e

e

6.0

e

[51]

sisal

PLA

silane (2%v/v)

PO

e

e

5.3

e

[51]

sisal

PLA

NaOH þ silane

PO

e

e

5.8

e

[51]

sisal

PP

untreated

PO

e

e

6.1

0.7

[52]

sisal

PLA

untreated

PO

e

e

17.1

2.7

[52]

sisal

PVA

untreated

PO

e

e

17.3

0.9

[52]

sisal

PLA

untreated

MB

1

e

10.5

3.7

[53]

sisal

PLA

NaOH (6 wt%)

MB

1

e

15.3

6.0

[53]

sisal

PP

untreated

MB

1

8.05

4.6

1.8

[20]

sisal

MaterBi

untreated

MB

1

10.64

3.2

1.1

[20]

sisal

PuraSorb

untreated

MB

1

3.72

14.3

3.7

[20]

sisal

PE

untreated

FM

4

e

2.2

0.4

[54]

sisal

PE

stearic acid

FM

4

e

2.7

0.6

[54]

sisal

UP

NaOH (6 wt%)

MB

1

e

10.4

1.9

[55]

sisal

UP

untreated

MB

1

e

5.4

2.0

[55]

sisal

UP

NaOH (6 wt%)

MB

1

e

7.5

1.8

[55]

sisal

UP

untreated

MB

1

e

6.7

2.5

[55]

A review on the interfacial characteristics of natural fibre

ramie

177

Continued

178

Appendix 1 Comparison of interfacial bonding studies in various fibre/matrix systems.dcont’d CHS (mm/ min)

Lc (mm)

IFSS (MPa)

S.D (MPa)

References

FM

e

0.75

4.8

e

[56]

acetone extraction þ fibrillation

FM

e

0.58

6.6

e

[56]

water extraction þ fibrillation

FM

e

0.55

8.4

e

[56]

Fibrea

Matrix

Fibre/Matrix modificationb

Test

wheat straw pulp

LDPE

methylene chloride extraction þ fibrillation

wheat straw pulp

LDPE

wheat straw pulp

LDPE

c

Flax(dr) and flax(D) indicate dew-retted and Duralin flax fibre, respectively. MA refers to maleic anhydride, SFO indicates sunflower oil and VTMO is vinyl trimethoxysilane. FM, MB and PO in the fourth column refer to fragmentation, microbond and pull-out test, respectively.

a

b c

Interfaces in Particle and Fibre Reinforced Composites

A review on the interfacial characteristics of natural fibre

179

Cellulose fibers Bast/stem

Flax, hemp, jute, kenaf, ramie

Leaf

Abaca, banana, henequen, pineapple leaf, sisal

Grass fibers

Bamboo, elephant grass, switch grass

Seed

Fruit

Cotton, kapok, milkweed

Coir

Straw/stalk

Barley, corn, maize, oat, rice, rye, wheat

Wood

Soft and hard woods

Fig. 7.1 Classification of cellulosic natural fibres [57e59].

The common polymers used for composite systems include thermosets such as unsaturated polyester (UP) [8,11,19,22,24,27,34,35,37,44,55], epoxy (EP) [16,19,24,35,48] and vinyl ester (VE) [6,9,24,27]. UP is commonly used in marine industry, whereas EP is popular in aircraft industry [60]. As for VE, it is famous for its corrosion and water absorption resistance [61,62]. Thermoplastics are also popular as matrix in the polymer composites. The most common ones are polyethylene (PE) [1,4,34,35,41,45,54,56] and polypropylene (PP) [5,7,12e15, 18,20,23,25,26,29e31,34,35,39,43e45,48,49,52]. PP has the advantages of environmental friendly and low density [59]. Polylactic acid (PLA) is a biodegradable thermoplastics. It has advantages of lower production energy and reduced greenhouse gas generation [63]. PLA has been used as the matrix in several researches on interfacial characteristics [15,17,19,21,37,42,46,47,53,64,65]. Some other less common polymers used include poly (3-hydroxybutyrate) (PHB) [21,28], poly (hydroxybutyrate-co-valerate) (PHBV) [50], polyvinylidene-fluoride (PVDF) [5], polyvinyl alcohol (PVA) [52], MaterBi [20] and PuraSorb [20]. Among the abovementioned thermoplastics, PHB, MaterBi and PuraSorb are also recognised as biopolymers [20,21,28].

3. Interfacial characterisation methods The common methods to characterise the interfacial shear strength is microbond/ microdroplet (MB) [10,12,15e17,19,20,22,30,39,42e44,46e50,53,55], pull-out (PO) [4e6,8,9,11,15,21,23,29,31,34,35,37,38,41,51,52,65] and single fibre fragmentation (FM) [1,4,7,13e15,18,24,26,27,32,33,41,45,54,56] test. Fig. 7.2 describes the test configurations of the abovementioned methods [41,63]. MB and PO tests are similar, with fibre gripped at one end and the matrix being clamped at another end. The fibre end is loaded gradually until fibre is completely pulled out. As for FM test, a single fibre is placed parallel to the length of the dogbone specimen. The specimen is loaded until fragmentation is saturated. The final fragment length, which is also known as fibre critical length is measured and hence the IFSS value is calculated. It is worth to note that different test on the same composite system could lead to different interfacial shear strength (IFSS) values. For example, in the micromechanical

180

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b)

Fibre

Microvise

Matrix Microdrop 2r

Load, F

2r Load, F Fibre

L

L

(c) σ

σ

Fig. 7.2 Schematic diagrams for (a) microbond (MB), (b) pull-out (PO) and (c) single fibre fragmentation (FM) tests. Redrawn from A. Valadez-Gonzalez, J.M. Cervantes-Uc, R. Olayo, P.J. Herrera-Franco, Effect of fiber surface treatment on the fiberematrix bond strength of natural fiber reinforced composites, Compos. Part B Eng. 30 (1999) 309e320; P.J. Herrera-Franco, A. ValadezGonz
alez, Chapter 6: fiber-matrix adhesion in natural fiber composites, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, Boca Raton, CRC Press, 2005.

characterisation of the henequen/high density polyethylene (HDPE) composite with different types of fibre surface treatments, it was observed that the IFSS from FM was always higher than the one obtained through PO test [4]. The PO/FT ratio was between 0.31 and 0.58. However, a reversed trend was observed by Graupner et al. [15] in lyocell/PP, flax/PP, lyocell/MAPP and flax/MAPP composites. The PO/FM ratio was in the range of 1.5e1.8. At the current stage, the relationship between PO and FM tests remains unclear. Hence, in order to have direct comparison, the type of the test needs to be consistent. Some researchers have devised the testing method for better success rate of the experiment. For example, Khoo et al. [11] have proposed a newly designed jig for the micromechanical testing of bamboo bundle reinforced polyester composites through pull-out test. The schematic diagram of the jig is as illustrated in Fig. 7.3. The advantage of the jig is that none of the fibre bundle end is necessary to be clamped, thus avoided the initial fibre bundle failure due to stress concentration at the clamping region. In their tests, the free gauge length was always fixed at 5 mm. The fixed end was consistently fixed at 20 mm, which was at least twice of the embedded length. This ensured that the debonding always occurred at the loading end. The authors embedded the bamboo bundles at 3, 5, 7 and 10 mm lengths in unsaturated polyester (UP). In addition, the concentration of sodium hydroxide (NaOH) was varied from 0 (untreated), 1, 3, 5 to 7 wt%. Through this newly designed jig, the authors reported that the success rate of the pull-out tests was at least 90% of the total tests. The same jig has also been used in another publication with 10 wt% NaOH treatment [66].

A review on the interfacial characteristics of natural fibre

181

Loading

Jig

Matrix

Embedded length

Matrix

Gauge length

Fibre

Fixed end length

Fig. 7.3 Schematic diagram of the pull-out jig designed by Khoo et al. [11].

Yang and Thompson [17] developed a new microvice for MB and PO tests. The jig allows the shear stress transfer through the interface. In addition, it has an advantage of flexible control for fibre alignment. Through their results, it was found that even with the same jig, the type of test did influence the IFSS values. For both glass fibre (GF) reinforced homopolymer PP (PPh) and maleic anhydride grafted PP (MAPP), results by PO test were lower compared to MB test. The PO/MB ratio was 0.49 and 0.71 for PPh and PPm, respectively.

4. Effects of the types of fibres and matrix Different combinations of fibre and matrix would exhibit different interfacial adhesion behaviour. For example, Stamboulis et al. [35] compared different types of flax fibre in different resins (Fig. 7.4). For green flax fibre, reinforcing in polyester (UP) exhibited higher interfacial shear strength (IFSS ¼ 17.1 MPa) compared to low density 25 Green

IFSS (MPa)

20

Duralin

15 10 5 0

LDPE

Polyster

Epoxy

Resins

Fig. 7.4 Comparison of the IFSS values of green and Duralin flax fibres reinforced in different resins [35].

182

Interfaces in Particle and Fibre Reinforced Composites

12 Duralin

IFSS (MPa)

10

Dew-retted

8 6 4 2 0 LDPE

HDPE

PP

Resins

Fig. 7.5 Comparison of the IFSS values of Duralin and dew-retted flax fibres reinforced in various thermoplastics [34].

polyethylene (LDPE) (5.4 MPa). As for Duralin flax fibre, the IFSS were 4.4, 11.7 and 23.2 MPa, respectively when reinforced in LDPE, UP and epoxy (EP) resins. In another work, Stamboulis et al. [34] reported that both Duralin and dew-retted flax fibres showed higher IFSS in high density polyethylene (HDPE) (10.1 and 9.1 MPa, respectively) compared to LDPE (6.2 and 5.5 MPa, respectively). Nevertheless, dew-retted flax fibre/polypropylene (PP) exhibited a higher IFSS (10.6 MPa) compared to both LDPE and HDPE resins (Fig. 7.5). Similarly, Tripathy et al. [45] also discovered that the jute fibre in PP exhibited higher IFSS (12 MPa) compared to LDPE (3.6 MPa). In addition, Sawpan et al. [37] reported that the interfacial shear strength (IFSS) was higher when hemp fibre was reinforced in unsaturated polyester (UP) (9.9 MPa) compared to polylactic acid (PLA) (5.6 MPa). From the abovementioned comparison, it seems that thermosets are in general having a better adhesion compared to thermoplastics. In addition, among thermoplastics, PP is better than LDPE. In polylactic acid (PLA) resin, Graupner et al. [15] discovered that the IFSS of flax fibre was the highest (28.3 MPa), followed by kenaf (25.7 MPa) and lyocell (10.3 MPa). However, Park et al. [39] reported similar IFSS values when jute (4.6 MPa) and hemp (4.9 MPa) fibres were reinforced in PLA. Despite the values by Graupner et al. [15] and Park et al. [39] cannot be directly compared, their results highlight that different fibres exhibit different interfacial characteristics. As in PP resin, Graupner et al. [15] found that flax (17.9 MPa) exhibits higher IFSS than lyocell (6.3 MPa). Comparison among two different thermoplastics indicates that PLA exhibits better interfacial adhesion than PP. Based on the above discussion, the order of decreasing IFSS to reflect the degree of interfacial bonding is as follows PLA > PP > LDPE. The results by Graupner et al. [15] are displayed in Fig. 7.6. With reference to E-glass fibre (IFSS ¼ 5.8 MPa), ramie and lyocell exhibited lower IFSS by 14% and 28% in the same polypropylene (PP) resin [49]. Interestingly, in their later work, it was also reported that treating all three abovementioned fibres using 0.2 wt% polypropylene-graft-maleic anhydride (MAPP) exhibited different

A review on the interfacial characteristics of natural fibre

183

30

IFSS (MPa)

25

PLA

PP

20 15 10 5 0

Lyocell

Flax

Kenaf

Fibres

Fig. 7.6 Comparison of the IFSS values of lyocell, kenaf and flax fibres in two different thermoplastics [15]. 40 Untreated/PP MAPP/PP Untreated/EP

35 IFSS (MPa)

30 25 20 15 10 5 0

Glass

Ramie

Lyocell

Fibres

Fig. 7.7 Comparison among the IFSS of different fibres reinforced in various resins [48,49].

trend, where the IFSS was the highest in lyocell (8.2 MPa), followed by E-glass (6.7 MPa) and finally ramie (5.9 MPa) [48]. In addition, similar trend as the untreated fibres in PP was reported for ramie and lyocell fibres embedded in epoxy (EP) resin, where the IFSS was 42% and 59% lower compared to E-glass fibre (36.5 MPa). Comparing the results by Graupner et al. [15] and Adusumalli et al. [48,49], it is noticed that untreated lyocell fibre is generally having weaker bonding. Furthermore, when comparing the untreated fibres in both PP and EP resins, it was found that EP has a better interfacial bonding than PP. This again confirms the abovementioned observation, where thermosets give a stronger bonding compared to thermoplastics. Fig. 7.7 compares the IFSS values among different types of fibre and matrix by Adusumalli et al. [48,49]. Joffe et al. [27] reported that the IFSS of enzyme retted flax fibre was 18 MPa when embedded inside UP. A larger IFSS value was obtained (24 MPa) when vinyl ester (VE) resin was used. In their later work, Joffe et al. [24] further compared the same

184

Interfaces in Particle and Fibre Reinforced Composites

35 30 IFSS (MPa)

25 20 15 10 5 0

UP

VE

(Joffe et al., 2003)

UP

VE

EP

(Joffe et al., 2005)

Fig. 7.8 Comparison of the IFSS values of enzyme retted flax fibre reinforced in various thermosets [24,27].

fibre in UP, VE and EP resins. The IFSS values obtained were 13, 28 and 33 MPa, respectively. The difference in the IFSS values is not clear, however, it could be due to different batch of materials used at different time of fabrication. The comparison is displayed in Fig. 7.8 below. Based on the results by Stamboulis et al. [35] and Joffe et al. [24], it is noticed that EP has a stronger adhesion compared to UP.

5.

Effects of fibre extraction method

The extraction method of the natural fibres has also been observed to exhibit different interfacial behaviour. For example, Karlsson et al. [56] extracted wheat straw pulp using methylene chloride, acetone and water. The average fibre diameters obtained from each extraction method were 17.5, 18.5 and 22.3 mm, respectively. The fibres were subsequently embedded into low density polyethylene (LDPE) resin. Results showed that methylene chloride extraction provided the lowest IFSS (4.8 MPa), followed by acetone (6.6 MPa) and water (8.4 MPa). In Japanese bamboo reinforced vinyl ester (VE) composite, Kim et al. [6] discovered that chemical extraction was the best method (11.5 MPa), followed by steam explosion (10.8 MPa) and 1.5 N NaOH alkali extraction (6.4 MPa). Tokoro et al. [10] investigated the influence of bamboo fibre extraction method on the IFSS in polylactic (PLA) resin. They found that compared to the as received fibres (4.3 MPa), steamexploded technique was the best (12.6 MPa, which was approximately threefold higher). As for 1.5 N NaOH alkali treatment, it exhibited around 2.3 times higher in IFSS (9.8 MPa). Comparing green and Duralin flax fibres, Stamboulis et al. [35] reported that green flax fibre showed a higher IFSS as compared to Duralin flax fibres in both polyester (UP) and LDPE resins. In UP resin, the IFSS of green flax fibre was 17.1 MPa as compared to Duralin flax fibre which was 11.7 MPa. As for LDPE, the IFSS values were 5.4 and 4.4 MPa in green and Duralin flax fibres, respectively.

A review on the interfacial characteristics of natural fibre

185

18 16

IFSS (MPa)

14 12 10 8 6 4 2

UP

LDPE

lin ra

re wDe

Du

tte

d

lin ra

re w-

Du

tte

d

lin ra De

Du

G

re

en

lin ra Du

G

re

en

0

LDPE

HDPE

Fig. 7.9 Comparison of the IFSS values of the flax fibres using different extraction methods [34,35].

In another work published by the same authors, Stamboulis et al. [34] reported that Duralin flax fibre showed better interfacial bonding compared to dew-retted flax. In LDPE, their values were 6.2 and 5.5 MPa, respectively. When reinforced in HDPE, the values were 10.1 and 9.1 MPa, respectively. Fig. 7.9 compares the results by Stamboulis et al. [34,35]. Hence, for the interfacial adhesion of flax fibres, green > Duralin > dew-retted.

6. Effects of fibre embedded length The embedded length plays in an important role in the mechanical performance of the composite. A short fibre does not allow effective stress transfer between the fibre and the matrix. Pickering et al. [36] discovered that for both untreated and alkali treated hemp fibre reinforced polylactic acid (PLA) composite, the interfacial shear strength (IFSS) decreased with the embedded length. This trend was observed between the range of approximately 0.15e1.0 mm. Beyond that value, the IFSS value of both types of composite levelled off (Fig. 7.10). Similar trend was observed when hemp/PLA and hemp/unsaturated polyester (UP) composites were treated using different methods [37]. However, through the studies by Khoo et al. [11] on the bamboo/UP composite, it was concluded that within the embedded length range of 3e10 mm, the embedded length has negligible effect on the IFSS. Similar observation was reported by Yang and Thompson [17], where the MB and PO specimens were fabricated at different fibre embedded lengths. From all results at different embedded lengths, an average value was obtained for each case. It is thus believed that as long as the fibre length is sufficient for shear stress transfer, the embedded length will not influence the IFSS, which is an interface property of the corresponding composite system.

186

Interfaces in Particle and Fibre Reinforced Composites

18 PLA/untreated

16

PLA/alkali

IFSS (MPa)

14 12 10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Embedded length (mm)

Fig. 7.10 Variation of IFSS with fibre embedded length for untreated and treated hemp/PLA composites [36].

7.

Effects of fibre surface treatment

Due to the incompatibility between hydrophilic natural fibre and hydrophobic matrix, the fibre/matrix interface is generally recognised as the weakest link in the natural fibre composite system. Consequently, fibre/matrix debonding is usually found to be the earliest failure mechanism observed upon external loadings. Researchers have been trying to improve the fibre/matrix interface to enhance to interfacial shear strength (IFSS) of natural fibre reinforced polymer composites. For instance, a positive effect of all treatment methods henequen/high density polyethylene (HDPE) composite was reported by Valadez-Gonzalez et al. [41]. Fig. 7.11 compares the IFSS values 25 PO FM

IFSS (MPa)

20 15 10 5

e en

e

yl /x PE D

PE

H +

N

aO

H

+

N

Si

la

aO

H

ne

+

N

H

aO

D

H

U

+

/x

Si

yl

la

en

ne

H aO N

nt

re

at

ed

0

Treatment

Fig. 7.11 Comparison of the IFSS of henequen/HDPE composite by different treatments [41].

A review on the interfacial characteristics of natural fibre

187

18 PO

16

FM

IFSS (MPa)

14 12 10 8 6 4 2

ne

ne

la

la

Si aO N

D H

N

aO

H

+

H

H

PE

+

yl /x

yl /x PE D

U

Si

e en

e en

H aO N

nt

re

at

ed

0

Treatment

Fig. 7.12 Influence of treatment methods on henequen/HDPE composite [4].

subjected to various treatments. The largest improvement was seen in combined 2%w/ v sodium hydroxide (NaOH) with 1% silane and 1.5%w/w HDPE/xylene preimpregnation. Approximately fourfold augmentation was found for both pull-out (PO) and fragmentation (FM) tests. Subsequently, Herrera-Franco and Valadez-Gonzalez [4] further studied the effects of various treatment methods on henequen/HDPE composite. The treatments included 2%w/v NaOH, 1.5%w/w HDPE/xylene, combination of NaOH and HDPE/xylene, 1% silane and combination of NaOH and silane. Results are presented in Fig. 7.12. In general, all treatments have enhanced the interfacial characteristics except 2%w/v NaOH treatment using FM test. The influence of different treatment methods on hemp fibre is shown in Fig. 7.13 [37]. Improvement in IFSS was observed in all treatments except 5 wt% maleic anhydride (MA) treated hemp fibre in polylactic acid (PLA) resin. The largest increment in PLA resin was obtained using 5 wt% NaOH. As for unsaturated polyester (UP), 5 wt% of NaOH with 0.5 wt% silane (NaOH þ S) was found to be the best treatment method. For both methods, the IFSS was doubled. Fig. 7.14 displays the treatment effects IFSS of jute and hemp reinforced polypropylene (PP) composites by Park et al. [39]. It could be seen that maleic anhydride polypropylene (MAPP) provided significant improvement in both composites. The largest increment was seen with 5% of treatment (39% for jute and 28% for hemp). In their later work, Park et al. [43] reported that boiling has negative effect on the interfacial bonding of jute in PP. This included all untreated, 0.5 wt% NaOH and 0.5 wt% silane treated jute fibre (Fig. 7.15). Among the abovementioned fibre surface treatment methods, alkaline (NaOH) treatment is one of the most popular methods. Several other studies on NaOH effects are available from the literature. For example, Khoo et al. [11] studied the effects of

188

Interfaces in Particle and Fibre Reinforced Composites

25

IFSS (MPa)

20

UP PLA

15 10 5 0

U

NaOH

AA

MA

S

NaOH + S

Treatment

Fig. 7.13 Treatment effects on the IFSS of hemp/PLA composite [37]. U e untreated, NaOH e 5 wt% sodium hydroxide, AA e acetic anhydride (C4H6O3), MA e 5 wt% maleic anhydride, S e 0.5 wt% silane, NaOH with S e 5 wt% sodium hydroxide with 0.5 wt% silane. 7

IFSS (MPa)

6

Jute Hemp

5 4 3 2 1 0 Untreated NaOH 0.5 Silane 0.5 MAPP 1% MAPP 3% MAPP 5% wt% wt%

Treatment

Fig. 7.14 Influence of fibre surface treatment on jute and hemp/PP composites [39]. 7

IFSS (MPa)

6

Unboiled Boiled

5 4 3 2 1 0 Untreated

NaOH 0.5 wt%

Silane 0.5 wt%

Treatment

Fig. 7.15 Effects of boiling on the IFSS of jute/PP composite [43].

A review on the interfacial characteristics of natural fibre

189

7

IFSS (MPa)

6 5

5.31 4.76

4.62 4.33

4

1.96 1.81 1.38 1.67

2 1

3.78

3.72 3.55

3

2.87

Embedded length (mm) 3

5

7

10

0 –1

0

1

2

3

4

5

6

7

8

NaOH (wt%)

Fig. 7.16 Variation of the IFSS with NaOH (wt %) for bamboo/polyester composite [11].

NaOH treatment on the bamboo fibre reinforced in unsaturated polyester (UP) resin. Within the range of study (1e5 wt%), the authors reported that the optimised IFSS was achieved at 3 wt% (Fig. 7.16). By using 2 wt% sodium sulphite (Na2SO3) with 5 wt% NaOH, Islam et al. [38] reported 27% of improvement in hemp/PLA composite. In addition, twofold increment was obtained in 5 wt% NaOH treated hemp/PLA composite [36]. Furthermore, Prajer and Ansell [53] reported an approximate 50% of increment was achieved in 6 wt% NaOH treated sisal fibre reinforced in PLA resin. Positive effect was also reported for 6 wt% NaOH treated sisal fibre in UP resin [55]. An increment of 56% and 38% was found using density and microscopy method, respectively. In alkali extracted sisal reinforced in PLA resin, Orue et al. [51] obtained at least twofold increment when treating the sisal fibre using alkali (2 wt% followed by 7.5 wt%), silane (2% v/v) and combined (alkali and silane) treatments. In addition, Tripathy et al. [45] found that bleaching the jute fibre using 0.7% of sodium chlorite (NaClO2) improved the IFSS in both low density polyethylene (LDPE) and PP. In both resins, an increment of approximately 45% was attained. Bleaching þ 2% of NaOH treatment has also a positive effect towards both composite systems, however, a lower percentage of improvement was observed in PP resin (18%). In both resins, carding caused deterioration in the interfacial adhesion. As for silane treatment, Herrera-Franco and Valadez-Gonzalez [1] has investigated the influence of the percentage of silane treatment in henequen/high density polyethylene (HDPE). As could be seen in Fig. 7.17 below, in general, silane enhanced the interfacial bonding, with the maximum improvement of 54% at 0.01%w/w silane. Another common treatment method from previous discussion is MAPP. A positive effect of using MAPP has been reported by several researchers. For example, by using 1 wt% of MAPP to treat elementary and scrutched flax fibres reinforced in PP, Bos [26] found positive effect of around twofold and 50% increment, respectively. Graupner et al. [15] have investigated the effects of 2% MAPP in various different fibre/matrix composite systems. A 30%e40% increment in IFSS was found for lyocell/PP and flax/PP composites. Adusumalli et al. [48] reported that treatment using 0.2 wt% of

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Interfaces in Particle and Fibre Reinforced Composites 32

IFSS (MPa)

30 28 26 24 22 20

0

0.01

0.02

0.03

0.04

0.05

0.06

Percentage of silane treatment (% w/w)

Fig. 7.17 Effects of silane treatment on henequen/HDPE composite [1].

MAPP and MA managed to improve the average IFSS of lyocell/PP composite up to 55% and 74%, respectively. By using MA and sunflower oil treated PP, Czig
any [12] reported 64%, 56%, 98% and 12% increment in basalt, hemp, glass and carbon fibres, respectively. However, by using PP with melt flow index (MFI) 450 as resin, van de Velde and Kiekens [31] achieved different results by treating dew-retted flax fibre using propyltrimethoxysilane (decrement of 30%), phenylisocyanate (20 g/L) (increment of 13%) and 0.1% MAPP (increment of 32%). Biagiotti et al. [25] found that in flax/PP composite, both 5% and 10% MA has deteriorated the interface property for approximately 20%. As for MAPP (both 5% and 10%) and 2.5 wt% vinyltrimethoxysilane (VTMO) treatment, insignificant improvement (maximum of z 10%) was found. IFSS was found to be invariant when dewretted flax was treated by 5% MA and reinforced in PP [34]. Negligible effect was also found by Garkhail when PP was reinforced by green and Duralin flax fibres which were treated using 5 wt% of MAPP and hot-cleaned [30]. Fibre surface treatment using acid is also another common method. Joffe et al. [27] found that both low and high concentrations of acrylic acid (C3H4O2) exhibited similar improvement (22%) in enzyme-retted flax/UP composite. In vinyl ester (VE) resin, 58% and 75% increment was obtained for low and high concentrations, respectively. For VTMO at low concentration, a 25% of improvement was obtained. However, at high concentrated VTMO, negligible influence was found for in both UP and VE resins. In their later work [24], all C3H4O2, VTMO and MA exhibited negative or invariant effect on the flax fibre in both UP and VE resins. Zafeiropoulos et al. [33] used acetylation and stearic acid (CH3(CH2)16COOH) to treat green and dew-retted flax fibres. They found that both methods have increased the IFSS of green flax/isotactic PP film composite. The largest improvement was found at 4 h of acetylation (83%) and 36 h of stearic acid treatment at 105  C (50%). However, 90 h of acid immersion led to decrement in the IFSS by 28%. As for the dewretted flax fibre, both treatment methods did not have any influence in the interfacial property. Similar to green flax fibre, the IFSS of dew-retted flax fibre decreased by 50% under 90 h of acid treatment. In stearic acid (CH3(CH2)16COOH) treated sisal

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191

reinforced polyethylene (PE) composite, Torres and Cubillas [54] found negligible increment in IFSS. Through the above comparison, it is also noticed that treatment does not always guarantee enhancement in the interfacial adhesion. This included the findings by Arbelaiz et al. [23], where the authors did not find significant improvement when treating flax fibre using 20 wt% NaOH (8%), 2.5 wt% VTMO (3%) and 10% (MAPP) (15%). Decrement in 22% was even observed when the flax fibre was treated by 10% MA. In that publication, PP was used as the matrix. Not only that, Baley et al. [22] treated the flax fibre using 10 g/L NaOH and found 56% of decrement of IFSS when embedded in UP. However, marginal increment (approximately 15%) was found when treated using NaOH with pure acetic anhydride (C4H6O3) and 99% formic acid (CH2O2). On the other hand, Lee et al. [28] reported that acetic anhydride (C4H6O3) and plasma treated flax fibre exhibited better adhesion (17% and 41%, respectively) in poly (3hydroxybutyrate) (PHB) resin. Soaking and ultrasonication in tap water by Cho et al. [44] are two of the less common treatment methods. Nevertheless, significant improvement was obtained for jute/ PLA (80% and 150%, respectively), henequen/PP (17% and 46%, respectively) and henequen/UP (34% and 55%, respectively). However, the effect was found to be negligible in kenaf/PLA composite. Another less popular method was electron beam irradiation by Ji et al. [42] on jute/PLA composite. Fig. 7.18 depicts that irradiation was not promising method (maximum of 20% improvement at 10 kGy). Lastly, Le Duigou et al. [19] discovered that cooling rate also influence the interfacial bonding of dew-retted flax/poly (L-Lactic acid) PLLA composite. Cooling at 10  C/min and 1  C/min enhanced the IFSS by approximately 19% and 45% as compared to cooling in air. However, annealing at 50  C for 72 h led to deterioration of 35%. From the abovementioned findings, it could be seen that researchers have tried various treatment methods to enhance the interfacial adhesion. It is obvious that different treatment methods have different effects on different natural fibre/polymer interface. It seems that the optimum percentage of treatment for each method has not yet been available at the current stage of research. 5.6 5.4

IFSS (MPa)

5.2 5 4.8 4.6 4.4 4.2 4 0

20

40

60

80

100

120

Irradiation of electron beam (kGy)

Fig. 7.18 Variation of the IFSS of jute/PLA composite with the electron beam irradiation level [42].

192

8.

Interfaces in Particle and Fibre Reinforced Composites

Effects of strain rate

It is commonly recognised that polymer composites are susceptible to loading rate [67,68]. A significant amount of researches have been conducted to study the effects of the strain rate on the delamination between the composite plies. Some recent publications include [69e76]. However, the strain rate effect studies at the micromechanical level (fibre/matrix interface) is still very lacking. Awal et al. [18] compared the IFSS of flax/maleic anhydride polypropylene (MAPP) at the crosshead speeds of 0.1, 0.2, 0.3, 0.5 and 2 mm/min using FM. Results showed that the IFSS was independent of the speed. Using the value at 0.1 mm/min as reference, the maximum difference was 13%, which was attained at 0.2 mm/min. Nevertheless, as the range of speed was rather limited, it is difficult to conclude from the results. There is a possibility that when high speed loading is imposed, the IFSS would be different. Further works are needed to verify this hypothesis.

9.

Effects of environmental attack

Polymer composites are generally recognised to be susceptible to environmental (moisture and temperature) attack. For example, Maslinda et al. [77] studied the tap water immersion effects on the tensile and flexural properties of kenaf, jute, hemp, hybrid kenaf/jute and hybrid kenaf/hemp reinforced epoxy composites. Through their studies, it was found that both strength and modulus degraded due to fibre/matrix interface weakening. Similar observations were reported for the strength and modulus of hybrid P. purpureum/glass epoxy composite under tensile and flexural loadings [78]. At micromechanical level, Le Duigou et al. [16] studied the de-ionised water immersion effects on the flax fibre reinforced epoxy (EP) composite. Results showed that the interfacial shear strength (IFSS) decreased by 33% (from 22.5 to 15 MPa) after immersion of 15 min. This implied that the water molecules had a very rapid effect to weaken the fibre/matrix interface. When the immersion time reached 14 h, only half of the initial IFSS value was retained. At the end of the ageing period (135 h), the residual IFSS was 40% as compared to the unaged specimen. Fig. 7.19 plots the variation of the IFSS with respect to ageing period. Chen et al. [9] studied the tap water absorption effects on the interfacial characteristics of bamboo/vinyl ester (VE) composites. They discovered that from day 1 to day 9 of immersion, the IFSS decreased linearly (Fig. 7.20). Beyond that period, the IFSS levelled off up to 100 days of immersion. The authors also concluded that the degradation was due to the interface weakening effect. From the results by both Le Duigou et al. [16] and Chen et al. [9], it is noticed that the natural fibre/polymer interface is very sensitive to moisture attack. Hence, special attention is needed when using natural fibre/polymer composites in humid environment.

10.

Conclusions and future recommendations

This chapter reviews the major parameters that influence the interfacial characteristics of natural fibre/polymer composites. Comparison is made among types and effects of

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25 Wet Wet + drying

IFSS (MPa)

20

15

10

5 0

1

2

3

4

5

6

Immersion time (day)

Fig. 7.19 Variation of the IFSS with the ageing period of flax/epoxy composite in de-ionised water [16]. 13 12

IFSS (MPa)

11 10 9 8 7 6 0

20

40

60

80

100

120

Immersion time (day)

Fig. 7.20 Influence of tap water absorption on the IFSS of bamboo/vinyl ester composites [9].

fibre and matrix, interfacial bonding testing methods, effects of fibre extraction method, effects of fibre embedded length, effects of treatment method, effects of strain rate and effects of moisture absorption. Further works could be extended to obtain the optimum percentage of treatment using different types of chemicals. This would require comparatively extensive studies for each treatment methods. Statistical analysis might also be required for optimised interfacial shear strength. In addition, the strain rate dependence on the interfacial characteristics could be further explored. This enables the design of the short natural fibre composites to be used under dynamic loading. Considering the potential for outdoor applications, moisture absorption effects need to be investigated for various types of natural fibres as well. This is also due to the known fact that both natural fibres and polymers are highly susceptible to moisture attack. Not only that, temperature effects could also be considered. It is because the natural fibre/polymer composites could behave differently under different temperature operating conditions. As the loadings in real life applications are generally

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Interfaces in Particle and Fibre Reinforced Composites

not static, fatigue behaviour of natural fibre/polymer interface is also an area that is worth to be investigated. Enhancement in the interfacial characteristics of natural fibre/polymer composites would extend the range of their applications in various industries, which contributes to the sustainability of the environment.

Acknowledgements This work is supported by Universiti Teknologi Malaysia (UTM) through Tier 1 Research University Grant (RUG) No. 19H01.

References [1] P.J. Herrera-Franco, A. Valadez-Gonz
alez, Mechanical properties of continuous natural fibre-reinforced polymer composites, Compos. Part A 35 (2004) 339e345. [2] B.C. Suddell, W.J. Evans, Chapter 7: Natural fiber composites in automotive applications, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [3] B. Singh, M. Gupta, Chapter 8: Natural fiber composites for building applications, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [4] P.J. Herrera-Franco, A. Valadez-Gonz
alez, A study of the mechanical properties of short natural-fiber reinforced composites, Compos. Part B Eng. 36 (2005) 597e608. [5] C.A. Fuentes, L.Q.N. Tran, M. Van Hellemont, V. Janssens, C. Dupont-Gillain, A.W. Van Vuure, I. Verpoest, Effect of physical adhesion on mechanical behaviour of bamboo fibre reinforced thermoplastic composites, Colloids Surf. A 418 (2013) 7e15. [6] H. Kim, K. Okubo, T. Fujii, K. Takemura, Influence of fiber extraction and surface modification on mechanical properties of green composites with bamboo fiber, J. Adhes. Sci. Technol. 27 (2012) 1348e1358. [7] S. Nahar, R.A. Khan, K. Dey, B. Sarker, A.K. Das, S. Ghoshal, Comparative studies of mechanical and interfacial properties between jute and bamboo fiber-reinforced polypropylene-based composites, J. Thermoplast. Compos. Mater. 25 (2012) 15e32. [8] K.J. Wong, B.F. Yousif, K.O. Low, The effects of alkali treatment on the interfacial adhesion of bamboo fibres, P. I. Mech. Eng. L. J. Mat. 224 (2010) 139e148. [9] H. Chen, M. Miao, X. Ding, Influence of moisture absorption on the interfacial strength of bamboo/vinyl ester composites, Compos. Part A 40 (2009) 2013e2019. [10] R. Tokoro, D. Vu, K. Okubo, T. Tanaka, T. Fujii, T. Fujiura, How to improve mechanical properties of polylactic acid with bamboo fibers, J. Mater. Sci. 43 (2008) 775e787. [11] S.J. Khoo, M. Johar, K.O. Low, K.J. Wong, Interfacial shear strength characterisation of alkali treated bamboo bundle e polyester composites using an improved technique, Plast. Rubber Compos. 46 (2017) 450e457. [12] T. Czig
any, Special manufacturing and characteristics of basalt fiber reinforced hybrid polypropylene composites: mechanical properties and acoustic emission study, Compos. Sci. Technol. 66 (2006) 3210e3220. [13] T. Huber, J. M€ussig, Fibre matrix adhesion of natural fibres cotton, flax and hemp in polymeric matrices analyzed with the single fibre fragmentation test, Compos. Interfac. 15 (2008) 335e349.

A review on the interfacial characteristics of natural fibre

195

[14] J.M. Felix, P. Gatenholm, Effect of transcrystalline morphology on interfacial adhesion in cellulose/polypropylene composites, J. Mater. Sci. 29 (1994) 3043e3049. [15] N. Graupner, J. R€oßler, G. Ziegmann, J. M€ussig, Fibre/matrix adhesion of cellulose fibres in PLA, PP and MAPP: a critical review of pull-out test, microbond test and single fibre fragmentation test results, Compos. Part A 63 (2014) 133e148. [16] A. Le Duigou, P. Davies, C. Baley, Exploring durability of interfaces in flax fibre/epoxy micro-composites, Compos. Part A 48 (2013) 121e128. [17] L. Yang, J.L. Thomason, Development and application of micromechanical techniques for characterising interfacial shear strength in fibre-thermoplastic composites, Polym. Test. 31 (2012) 895e903. [18] A. Awal, G. Cescutti, S.B. Ghosh, J. M€ussig, Interfacial studies of natural fibre/polypropylene composites using single fibre fragmentation test (SFFT), Compos. Part A 42 (2011) 50e56. [19] A. Le Duigou, P. Davies, C. Baley, Interfacial bonding of Flax fibre/Poly(l-lactide) biocomposites, Compos. Sci. Technol. 70 (2010) 231e239. [20] T. Czig
any, B. Morlin, Z. Mezey, Interfacial adhesion in fully and partially biodegradable polymer composites examined with microdroplet test and acoustic emission, Compos. Interfac. 14 (2007) 869e878. [21] S. Wong, R.A. Shanks, A. Hodzic, Effect of additives on the interfacial strength of poly(llactic acid) and poly(3-hydroxy butyric acid)-flax fibre composites, Compos. Sci. Technol. 67 (2007) 2478e2484. [22] C. Baley, F. Busnel, Y. Grohens, O. Sire, Influence of chemical treatments on surface properties and adhesion of flax fibreepolyester resin, Compos. Part A 37 (2006) 1626e1637. [23] A. Arbelaiz, G. Cantero, B. Fern
andez, I. Mondragon, P. Ga~ n
an, J.M. Kenny, Flax fiber surface modifications: effects on fiber physico mechanical and flax/polypropylene interface properties, Polym. Compos. 26 (2005) 324e332. [24] R. Joffe, J. Andersons, L. Wallstr€om, Interfacial shear strength of flax fiber/thermoset polymers estimated by fiber fragmentation tests, J. Mater. Sci. 40 (2005) 2721e2722. [25] J. Biagiotti, D. Puglia, L. Torre, J.M. Kenny, A. Arbelaiz, G. Cantero, C. Marieta, R. Llano-Ponte, I. Mondragon, A systematic investigation on the influence of the chemical treatment of natural fibers on the properties of their polymer matrix composites, Polym. Compos. 25 (2004) 470e479. [26] H.L. Bos, The Potential of Flax Fibres as Reinforcement for Composite Materials (Ph.D.), Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2004. [27] R. Joffe, J. Andersons, L. Wallstr€om, Strength and adhesion characteristics of elementary flax fibres with different surface treatments, Compos. Part A 34 (2003) 603e612. [28] S.G. Lee, S.-S. Choi, W.H. Park, D. Cho, Characterization of surface modified flax fibers and their biocomposites with PHB, Macromol. Symp. 197 (2003) 089e100. [29] S.J. Eichhorn, C.A. Baillie, N. Zafeiropoulos, L.Y. Mwaikambo, M.P. Ansell, A. Dufresne, K.M. Entwistle, P.J. Herrera-Franco, G.C. Escamilla, L. Groom, M. Hughes, C. Hill, T.G. Rials, P.M. Wild, Review: current international research into cellulosic fibres and composites, J. Mater. Sci. 36 (2001) 2107e2131. [30] S.K. Garkhail, Composites Based on Natural Fibres and Thermoplastic Matrices (Ph.D.), Queen Mary College, University of London, London, United Kingdom, 2001. [31] K. van de Velde, P. Kiekens, Influence of fiber surface characteristics on the flax/polypropylene interface, J. Thermoplast. Compos. Mater. 14 (2001) 244e260.

196

Interfaces in Particle and Fibre Reinforced Composites

[32] N.E. Zafeiropoulos, C.A. Baillie, F.L. Matthews, A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials, Compos. Part A 32 (2001) 525e543. [33] N.E. Zafeiropoulos, C.A. Baillie, F.L. Matthews, Modification and Characterisation of the Interface in Flax/Polypropylene Composite Materials, 9th European Conference on Composite Materials, Brighton, United Kingdom, 2000. [34] A. Stamboulis, C. Baillie, E. Schulz, Interfacial characterisation of flax fibre-thermoplastic polymer composites by the pull-out test, Angew. Makromol. Chem. 272 (1999) 117e120. [35] A. Stamboulis, C. Baillie, M. Piacesi, E. Schulz, Interfaces in Flax Fibre-Polymer Composites. Proceedings of 12th International Conference on Composite Materials, France, Paris, July 1999. [36] K.L. Pickering, M.A. Sawpan, J. Jayaraman, A. Fernyhough, Influence of loading rate, alkali fibre treatment and crystallinity on fracture toughness of random short hemp fibre reinforced polylactide bio-composites, Compos. Part A 42 (2011) 1148e1156. [37] M.A. Sawpan, K.L. Pickering, A. Fernyhough, Effect of fibre treatments on interfacial shear strength of hemp fibre reinforced polylactide and unsaturated polyester composites, Compos. Part A 42 (2011) 1189e1196. [38] M.S. Islam, K.L. Pickering, N.J. Foreman, Influence of alkali treatment on the interfacial and physico-mechanical properties of industrial hemp fibre reinforced polylactic acid composites, Compos. Part A 41 (2010) 596e603. [39] J.-M. Park, S.T. Quang, B.-S. Hwang, K.L. DeVries, Interfacial evaluation of modified jute and hemp fibers/polypropylene (PP)-maleic anhydride polypropylene copolymers (PPMAPP) composites using micromechanical technique and nondestructive acoustic emission, Compos. Sci. Technol. 66 (2006) 2686e2699. [40] D. Cho, H.-J. Kim, L.T. Drzal, Surface treatment and characterization of natural fibers: effects on the properties of biocomposites, in: S. Thomas, K. Joseph, S.K. Malhotra, et al. (Eds.), Polym. Compos., Wiley-VCH Verlag GmbH & Co. KGaA, 2013, pp. 133e177. [41] A. Valadez-Gonzalez, J.M. Cervantes-Uc, R. Olayo, P.J. Herrera-Franco, Effect of fiber surface treatment on the fiberematrix bond strength of natural fiber reinforced composites, Compos. Part B Eng. 30 (1999) 309e320. [42] S. Ji, D. Cho, W. Park, B. Lee, Electron beam effect on the tensile properties and topology of jute fibers and the interfacial strength of jute-PLA green composites, Macromol. Res. 18 (2010) 919e922. [43] J.-M. Park, P.-G. Kim, J.-H. Jang, Z. Wang, B.-S. Hwang, K.L. DeVries, Interfacial evaluation and durability of modified Jute fibers/polypropylene (PP) composites using micromechanical test and acoustic emission, Compos. Part B Eng. 39 (2008) 1042e1061. [44] D. Cho, J.M. Seo, H.S. Lee, C.W. Cho, S.O. Han, W.H. Park, Property improvement of natural fiber-reinforced green composites by water treatment, Adv. Compos. Mater. 16 (2007) 299e314. [45] S.S. Tripathy, G. Levita, L. Di Landro, Interfacial adhesion in jute-polyolefin composites, Polym. Compos. 22 (2001) 815e822. [46] H. Anuar, A. Zuraida, B. Morlin, J.G. Kov
acs, Micromechanical property investigations of poly(lactic acid)ekenaf fiber biocomposites, J. Nat. Fibers 8 (2011) 14e26. [47] H. Anuar, A. Zuraida, B. Morlin, J.G. Kov
acs, Interfacial shear strength of polylactic acidkenaf fibre biocomposites, Key Eng. Mater. 471e472 (2011) 781e785. [48] R.-B. Adusumalli, M. Reifferscheid, H.K. Weber, T. Roeder, H. Sixta, W. Gindl, Shear strength of the lyocell fiber/polymer matrix interface evaluated with the microbond technique, J. Compos. Mater. 46 (2012) 359e367.

A review on the interfacial characteristics of natural fibre

197

[49] R.-B. Adusumali, M. Reifferscheid, H. Weber, T. Roeder, H. Sixta, W. Gindl, Mechanical properties of regenerated cellulose fibres for composites, Macromol. Symp. 244 (2006) 119e125. [50] S. Luo, A.N. Netravali, Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin, J. Mater. Sci. 34 (1999) 3709e3719. [51] A. Orue, A. Jauregi, C. Pe~na-Rodriguez, J. Labidi, A. Eceiza, A. Arbelaiz, The effect of surface modifications on sisal fiber properties and sisal/poly (lactic acid) interface adhesion, Compos. Part B Eng. 73 (2015) 132e138. [52] I.M. Taha, G. Ziegmann, Potential of sisal reinforced biodegradable polylactic acid and polyvinyl alcohol composites, Key Eng. Mater. 425 (2010) 167e178. [53] M. Prajer, M.P. Ansell, Thermomechanical Evaluation of Sisal-PLA Composites. Proceedings of 17th International Conference on Composite Materials; Edinburgh, United Kingdom, July 2009. [54] F.G. Torres, M.L. Cubillas, Study of the interfacial properties of natural fibre reinforced polyethylene, Polym. Test. 24 (2005) 694e698. [55] A.N. Towo, M.P. Ansell, M.-L. Pastor, D.E. Packham, Weibull analysis of microbond shear strength at sisal fibreepolyester resin interfaces, Compos. Interfac. 12 (2005) 77e93. [56] J.O. Karlsson, P. Gatenholm, J.F. Blachot, A. Peguy, Improvement of adhesion between polyethylene and regenerated cellulose fibers by surface fibrillation, Polym. Compos. 17 (1996) 300e304. [57] A.K. Mohanty, M. Misra, L.T. Drzal, S.E. Selke, B.R. Harte, G. Hinrichsen, Chapter 1: Natural fibers, biopolymers and biocomposites: an introduction, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [58] A. Bismarck, S. Mishra, T. Lampke, Chapter 2: Plant fibers as reinforcement for green composites, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [59] S. Godavarti, Chapter 10: Thermoplastic wood fiber composites, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [60] D. Ray, J. Rout, Chapter 9: Thermoset biocomposites, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [61] M. Lindgren, M. Wallin, M. Kakkonen, O. Saarela, J. Vuorinen, The influence of hightemperature sulfuric acid solution ageing on the properties of laminated vinyl-ester joints, Int. J. Adhesion Adhes. 68 (2016) 298e304. [62] A. Afshar, H.-T. Liao, F-p Chiang, C.S. Korach, Time-dependent changes in mechanical properties of carbon fiber vinyl ester composites exposed to marine environments, Compos. Struct. 144 (2016) 80e85. [63] P.J. Herrera-Franco, A. Valadez-Gonz
alez, Chapter 6: Fiber-matrix adhesion in natural fiber composites, in: A.K. Mohanty, M. Misra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, 2005. [64] M.R. Islam, M.D.H. Beg, Effects of reinforcing arrangement of kenaf fibres into unsaturated polyester for improved properties, J. Mech. Eng. Sci. 10 (2016) 2020e2030. [65] K.L. Pickering, Assessing fibre surface treatment to improve the mechanical properties of natural fibre composites, in: N. Zafeiropoulos (Ed.), Interface Engineering of Natural Fibre Composites for Maximum Performance, Woodhead Publishing, Philadelphia, USA, 2011.

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[66] J.W. King, J.K. Siu, O.L. Kean, Influence of alkali treatment on the interfacial properties of bamboo/polyester composites, Recent Pat. Mech. Eng. 9 (2016) 247e254. [67] W.J. Cantwell, M. Blyton, Influence of loading rate on the interlaminar fracture properties of high performance composites - a review, Appl. Mech. Rev. 52 (1999) 199e212. [68] M. May, Measuring the rate-dependent mode I fracture toughness of composites e a review, Compos. Part A 81 (2016) 1e12. [69] J.J.M. Machado, E.A.S. Marques, R. Campilho, L.F.M. da Silva, Mode I fracture toughness of CFRP as a function of temperature and strain rate, J. Compos. Mater. (2016), 0021998316682309. [70] J.J.M. Machado, E.A.S. Marques, R.D.S.G. Campilho, L.F.M. da Silva, Mode II fracture toughness of CFRP as a function of temperature and strain rate, Compos. Part B Eng. 114 (2017) 311e318. [71] H. Zabala, L. Aretxabaleta, G. Castillo, J. Aurrekoetxea, Loading rate dependency on mode I interlaminar fracture toughness of unidirectional and woven carbon fibre epoxy composites, Compos. Struct. 121 (2015) 75e82. [72] M. Colin de Verdiere, A.A. Skordos, M. May, A.C. Walton, Influence of loading rate on the delamination response of untufted and tufted carbon epoxy non crimp fabric composites: mode I, Eng. Fract. Mech. 96 (2012) 11e25. [73] M. Colin de Verdiere, A.A. Skordos, A.C. Walton, M. May, Influence of loading rate on the delamination response of untufted and tufted carbon epoxy non-crimp fabric composites/mode II, Eng. Fract. Mech. 96 (2012) 1e10. [74] P. Navarro, J. Aubry, F. Pascal, S. Marguet, J.F. Ferrero, O. Dorival, Influence of the stacking sequence and crack velocity on fracture toughness of woven composite laminates in mode I, Eng. Fract. Mech. 131 (2014) 340e348. [75] T. Lyashenko-Miller, J. Fitoussi, G. Marom, The loading rate effect on Mode II fracture toughness of composites interleaved with CNT, Nanocomposites 2 (2016) 1e7. [76] M. Yasaee, G. Mohamed, A. Pellegrino, N. Petrinic, S.R. Hallett, Strain rate dependence of mode II delamination resistance in through thickness reinforced laminated composites, Int. J. Impact Eng. 107 (2017) 1e11. [77] A.B. Maslinda, M.S. Abdul Majid, M.J.M. Ridzuan, M. Afendi, A.G. Gibson, Effect of water absorption on the mechanical properties of hybrid interwoven cellulosic-cellulosic fibre reinforced epoxy composites, Compos. Struct. 167 (2017) 227e237. [78] M.J.M. Ridzuan, M.S. Abdul Majid, M. Afendi, K. Azduwin, N.A.M. Amin, J.M. Zahri, A.G. Gibson, Moisture absorption and mechanical degradation of hybrid Pennisetum purpureum/glasseepoxy composites, Compos. Struct. 141 (2016) 110e116.

Interfaces in sugar palm fibres reinforced composites: A review

8

A. Atiqah a , M.T. Mastura c , M. Jawaid d , S.M. Sapuan e , M.N.M. Ansari a,b a Department of Mechanical Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia; bInstitute of Power Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malaysia; cFaculty of Mechanical and Manufacturing Engineering Technology, Universiti Teknikal Malaysia, Durian Tunggal, Melaka, Malaysia; dLaboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia UPM Serdang, Selangor, Malaysia; eDepartment of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia

1. Introduction The usage of plant bast fibres has increased in recent years to satisfy the natural fibre demand for composites. For example, sugar palm is now recognised a significant result of the establishment as a fibre crop around the world. Sugar palm fibres (SPF) have higher cellulose content, and therefore sugar palm fibres are biodegradable [1,2]. The incorporation of sugar palm fibres filler has an adverse effect on the strength of the composites [3e5]. The potential of sugar palm particles as a filler in polymer and hybrid composites has not been fully realised. The improvement on the mechanical properties of the sugar palm particles composites is needed to ensure the superior properties of the composites. However, sugar palm fibres has its limitation in terms of mechanical properties. The mechanical properties of sugar palm particle composites are substantially dependent upon the strength of the interfacial strength and the load transfer across the interface. Therefore, the understanding of the interface properties is crucial for the manufacturing of sugar palm composites, which will assist the usage of the interface in various applications. Previous researches have mostly focused on the interface of sugar palm [6e11] because a deep understanding of the interfacial characteristics is crucial in improving the properties of the composites. The interface plays an essential role in determining the efficiency, the load transfer, and hence the performance of the composites. The understanding of the interaction mechanism interaction between sugar palm fibres and polymer composites is still lacking. To overcome this problem, great understanding of the interfacial bonding of sugar palm particles is essential to enhance the properties of the sugar palm particle composites. A better understanding, focusing on the relationship between sugar palm particles and interphase characteristics of the composites is needed. The modification of the sugar palm fibres is an effective approach to improve the interphase attribution of the composites.

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00008-X Copyright © 2020 Elsevier Ltd. All rights reserved.

200

Interfaces in Particle and Fibre Reinforced Composites

Several review articles have been published on various aspects of sugar palm fibres and its composites [12e15]. Present article deals about sugar palm fibres as particles and its interfacial bonding with polymer for different applications. Altogether, these articles point to an increment in the successful utilisation of sugar palm fibres. However, review articles on the topic of sugar palm particles interface are still lacking. This chapter provides an overview of the recent advancement in the interface sugar palm particles composites.

2.

Classification and structures of sugar palm fibres

Sugar palm fibres are abundantly available in South East Countries, especially in Malaysia [15]. Some of the essential aspects of sugar palm fibres, such as their mechanical properties of different parts of sugar palm fibres, are listed in Table 8.1. A complete classification of various parts of sugar palm fibres is shown in Fig. 8.1. Sugar palm fibres have been used as reinforcing materials in combination with polymeric materials. Moreover, sugar palm fibres are used as composites because of the Table 8.1 Mechanical properties for different parts of sugar palm fibres tree [16]. Fibres

Sugar palm frond

Sugar palm bunch

Sugar palm trunk

Ijuk

Tensile strength (MPa)

4241.4

365.1

198.3

276.6

Tensile modulus (GPa)

10.4

8.6

3.1

5.9

Elongation at break (%)

9.8

12.5

29.7

22.3

Acc.V Spot Magn 20.0 kV 3.0 1000x

20 µm Det WD SE 13.6 EMUPM/025 1 jam

Fig. 8.1 Image of single sugar palm fibres from SEM [34].

Interfaces in sugar palm fibres reinforced composites: A review

201

low cost, low density, high toughness, ease of separation, enhance energy recovery, and a significant biodegradability. The strength and stiffness of the composites are rendered by the sugar palm fibres, which also avoid fractures on the fibres, as compared to synthetic fibres such as glass fibres.

2.1

Chemical compositions and properties of sugar palm fibres

The chemical composition of sugar palm fibres depends on the type and nature of the fibre. The overall properties of each fibre are influenced by the properties of each constituent. The variations of chemical compositions between plants, and within different parts of the same plant are quite obvious. The main constituents of all cell walls are sugar based polymers (cellulose and hemicellulose) [17]. The chemical compositions of sugar palm fibres are listed in Table 8.2. The primary component in sugar palm fibres are waxes, followed by cellulose (a-cellulose), lignin, pectin, and hemicellulose. The hemicellulose part existed in sugar palm fibres acted as a compatibiliser between lignin and cellulose.

2.2

Sugar palm particles reinforced composites

Researches on sugar palm fibres reinforced composites have generated increasing attention due to their excellent properties and ecological considerations. In general, the use of sugar palm fibres reinforced composites can help to generate income in both rural and urban areas. Moreover, a healthier environment can be provided by sugar palm fibres reinforced composites by reducing the wastage of using sugar palm fibres for several applications. The most significant development on sugar palm particles reinforced composites is their abundant availability to be used in various manufacturing processes, such as melt-mixing compounding and injection moulding. The main driving force of sugar palm particles utilisation in composites is their light weight and low cost, making them a competitive material in terms of composites performance. Table 8.2 The chemical compositions of sugar palm fibres [16]. Composition

Sugar palm frond

Sugar palm bunch

Ijuk

Sugar palm trunk

Cellulose (%)

66.49

61.76

52.29

40.56

Helocellulose (%)

81.22

71.78

65.62

61.1

Lignin (%)

18.89

23.48

31.52

46.44

Ash (%)

3.05

3.38

4.03

2.38

Moisture (%)

2.74

2.7

7.4

1.45

Extraction (%)

2.46

2.24

4.39

6.3

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Interfaces in Particle and Fibre Reinforced Composites

Table 8.3 The number of papers related to sugar palm particles composites published (2010e9). Matrix

2010e5

2015e9

Phenolics

e

[3,18e20]

Thermoplastic Polyurethane

e

[7,11,21e29]

Sugar Palm Starch

e

[30e32]

High Impact Polystyrene

[33e36]

e

Cassava starch

e

[37,38]

The types of matrices used in sugar palm particles reinforced composites can be divided into the following categories: • • •

Sugar palm particles-reinforced thermoset Sugar palm particles-thermoplastics Sugar palm particles-reinforced biopolymers

The number of papers published since 2010 are listed in Table 8.3. The most widely used matrix for sugar palm particle composites has evolved from thermoset to thermoplastics. Thermosetting matrices, such as polyester, epoxy and phenolic, have also been utilised for sugar palm particles reinforced composite.

2.2.1

Physical properties

The properties of the interface are vital in distinguishing the physical properties of the sugar palm particles composites. The most important criterion in the interfacial properties is the efficiency assessment of load transfer across the interface [39]. The subdivided topics highlighted how the properties affect the physical properties of the sugar palm particle composites. Enhancement in the physical properties of various composites in the literature is summarised in Table 8.4. Previous studies show that sugar palm particles are a promising potential as filler in the composites to improve the properties of sugar palm. The advantages of the sugar palm particle enhancement lead to lead to an improved physical properties such as higher density, water absorption and thickness swelling compared to neat polymer composites. In another study by Ref. [40] studied the moisture absorption and thickness swelling of sugar palm particle reinforced TPU composites in relation to the effect on fibre loading. As expected, the moisture absorption and thickness swelling recorded a gradual increase with fibre content reaching a maximum value of 50%. Conversely, the density gradually decreases as the fibre content approaching the maximum value. The TPU can be well-attached to the particles when the fibre content of sugar palm increases, leading to void formation in the interface of fibre and matrix. The higher density of sugar palm fiber reduces the density of the composites up to 1.26 g/cm3, compared to only 1.13 g/cm3 reduction of that by TPU matrix.

Physical properties Particle size

Matrix

Treatment

Density (g/ cm3)

Water absorption (%)

Thickness swelling (%)

References

125-250(mm)

Thermoplastic Polyurethane (TPU)

e

1.20

8.21 (168h)

e

[40]

125-250(mm)

Thermoplastic Polyurethane (TPU)

2% silane

1.17

6.83

6.84

[11]

130
30.23 nm

Sugar palm starch (SPS)

e

1.417e1.432

100.5e112.43

e

[32]

250(mm)

Thermoplastic Polyurethane (TPU)

6% NaOH, Microwave at 70  C

e

e

e

[29]

150e300 mm

cassava bagasse (CB)- sugar palm fiber (SPF) -cassava starch (CS)

e

123.32

22.13

[37]

2 mm

Seaweed/Sugar palm starch

e

1.34
0.01

58.09
1.52 (2h)

43.03
1.90 (2h)

[30]

250(mm)

Thermoplastic Polyurethane (TPU)

2e6% NaOH

e

e

e

[27]

Interfaces in sugar palm fibres reinforced composites: A review

Table 8.4 Previous studies on physical properties of sugar palm particle composites.

203

204

Interfaces in Particle and Fibre Reinforced Composites

In another study conducted by Ref. [11]; the density, the thickness swelling, and the moisture absorption of the treated silane decrease when the sugar palm particles were treated with 2% silane treatment for 3 h. The chemical modification of silane treatment after hydrolysis underwent bond formation stage and condensation that induces polysiloxane structures from the reaction of hydroxyl groups on the fibre. Thus, this phenomenon leads to the improvement of interfacial adhesion, which enables good encapsulating of the fibre and matrix.

2.2.2

Mechanical properties

2.2.2.1 Sugar palm particle reinforced thermoset matrices The most widely used thermosetting matrix reinforced by sugar particle palm is phenolic [3,18e20]. Compression moulding is the most widely used and easy method to fabricate these composites [41]. Several researchers studied the effects of fibre loading (0e40 vol%) of sugar palm particles on the mechanical properties of the composites concluding that 30 vol% of fibre loading recorded the optimum mechanical performance compared to the untreated sugar palm particles (Table 8.5). The fibre matrix bonding recorded good compatibility when treated with sea water, which agrees well with the mechanical properties.

2.2.2.2 Sugar palm particle reinforced thermoplastic matrices The sugar palm particle reinforced thermoplastics composites have gained much interest among researchers in recent years compared to thermosets due to their low cost and recyclable properties. Many papers have been published related to the study on the mechanical and thermal properties of these composites, rather than sugar palm particle reinforced thermosets. Mohammed et al. [26] investigated the effects of operating parameters, including the fibre particle sizes (160, 250, and 425 mm), the temperatures for extrusion (170e190  C), and the rotational velocities (30e50 rpm). Moreover, they also studied the effects of fibre sizes on the mechanical properties including tensile flexural and impact strength. From their findings, the optimum temperature for extrusion is 190  C with the optimum rotational velocity of 40 rpm. Meanwhile, the size of 250 mm is the optimum particle size which provides superior properties compared to other particle sizes of sugar palm.

2.2.3

Thermal properties

Atiqah et al. [25] studied the effect of glass fibre on the thermal properties of sugar palm particle reinforced thermoplastic polyurethane by using melt mixing and followed by hot pressing machine. From the study, 10 wt% of sugar palm particles recorded the highest storage modulus (E0 ), loss modulus (E’), and damping factor (tan d) of the hybrid composites. This phenomenon is due to good fibre-matrix interfaces leading to the improvement of the thermal properties of the hybrid composites.

Mechanical properties

Matrix

Tensile strength (MPa)

Flexural strength (MPa)

Impact strength (kJ/m2)

Compressive strength (MPa)

References

30

Phenolic

e

32.3

4.12

61.66

[18]

125e250 mm

30

Glass Fiber/Thermoplastic Polyurethane (TPU)

15e21.1

15e31.09

18.9e20.6

e

[42]

e

50

Poly Lactic Acid (PLA)

13.65

e

e

e

[43]

0e50 mesh

10e50

High Impact Polystyrene(HIPS)

19.3e29.92

e

e

e

[44]

125e250 mm

0e50

Thermoplastic Polyurethane (TPU)

6e17.22

4e13.96

7e15.47

e

[23]

130
30.23 nm

e

Sugar palm starch (SPS)

11.47

e

e

e

[32]

250(mm)

10

Thermoplastic Polyurethane (TPU)

18.42

e

e

e

[29]

2 mm

e

Seaweed/Sugar palm starch

17.74

31.24

5e7

e

[30]

Particle size

Fiber loading (%)

150 mm

Interfaces in sugar palm fibres reinforced composites: A review

Table 8.5 Previous studies on mechanical properties of sugar palm particle composites.

205

206

Interfaces in Particle and Fibre Reinforced Composites

Mohammed et al. [28] investigated the treatment of potassium permanganate (KMnO4) with various concentrations (0.033%, 0.066%, and 0.125%). The thermogravimetric analysis found that the lowest concentration of KMnO4 resulted in the highest thermal stability of the sugar palm particle reinforced thermoplastic polyurethane composites. Similar to the mechanical properties of the composites, the interface of the fibre and reinforcement are both improved [37]. studied the dynamical mechanical properties hybridization sugar palm particle with cassava bagasse increased the storage modulus (E’) than cassava starch. Nevertheless, in another study by Edhirej et al. [38], no significant effect was found for thermal properties when sugar palm particles were substituted with cassava fibre reinforced with cassava starch hybrid composites.

3.

Research methodology

The interface properties have been an interesting topic, considering the importance of determining the properties of the composites [3,18,19]. For instance, the mechanical properties of the sugar palm particle composites are greatly dependent upon the properties of the composites [23]. Hence, the improvement of the sugar palm fibres properties has been focused on the optimisation of the characteristics of the interface to enhance the performance of the composites. The study of the interface properties is essential in the manufacturing of sugar palm particles for various applications.

3.1

Fibre modification

The preparation of sugar palm composites material includes several processes starting with the extraction of raw fibre from its resources until its formation into a desired shape and type, such as fabric, yarn, particles, and mat [45]. Each of the processes would influence the final material properties of the composites material. Composites are unique materials where the properties depend on its constituents. Therefore, the process of the composite materials should be well-planned and performed in a systematic and structured method to obtain the required composite materials with adequate material properties. Moreover, in comparison with synthetic fibre composite, the process of bio-composites such as sugar palm particles composite is much easier and requires a lower cost. This is one of the advantages of bio-composites that attracts many manufacturers to employ this type of materials in their products. Prior to the mixing process with the matrix, the fibres should undergo a modification process to enhance its performance. Fibre modification is conducted by removing the impurities to improve the surface of the fibres. Consequently, the interfacial adhesion between fibre and matrix is improved. Modification of the fibres also affects the results of Thermogravimetric Analysis (TGA) [46]. Moreover, modification on the fibres is required to encounter the weaknesses of natural fibres, that are high polarity and hydrophilicity [47]. Hence, fibre modification is an initial preparation process for composite materials with desired materials properties.

Interfaces in sugar palm fibres reinforced composites: A review

207

Generally, fibre modification was studied experimentally as shown in Table 8.6. Fibre modifications include chemical modification by alkaline treatment, silane treatment, and seawater treatment. The raw fibres will be soaked in the solution with a certain amount of alkaline or silane or sea water concentration. The optimum time of treatment will be determined experimentally through several trials. Usually, several samples of fibres are prepared with the different amount of concentrations and duration of treatment. The optimum time, the amount of concentration and the suitable type of treatment are then determined based on the results. Every fibre would exhibit different results with no matching treatment for a specific natural fibre. However, improper treatment process would lead to degradation of the fibres. Therefore, it is necessary to study the optimum parameter of modification process experimentally, to avoid degraded fibres for the composite materials [46].

3.2 3.2.1

Chemical modification Alkaline treatments

There are several studies performed by past researchers on the alkaline treatment of sugar palm fibres in the preparation of the sugar palm composites. Sugar palm fibres consists of three major chemical compositions which are lignin, hemicellulose, and cellulose. These compositions are affected by the chemical reaction that occurs during alkaline treatment. The outer layer of the fibres will be removed during this treatment to allow fibre adhesion with the matrix. Generally, fibres will be soaked in the solution of alkaline or silane or seawater, for a particular duration. Seawater treatment requires higher soaking time compared to alkaline and silane treatment. The fibres will then rinsed with distilled water and dried naturally or in the oven to remove excessive moisture. Dried fibres will be grounded to the desired particles sizes and ready to be processed with the matrix for composites. Different methods would exhibit different effects on the fibres and their properties [48]. treated sugar palm fibres with sodium hydroxide (NaOH) solutions at two different concentrations (0.25 M and 0.5 M) and three different soaking times (1 h, 4 h, and 8 h). In comparison with the untreated fibre composite, the tensile modulus of the treated fibre composites is higher, and the treatment effectively improves the properties of sugar palm fibres reinforced epoxy composites. Moreover, the maximum flexural strength was improved by 24.41% from untreated fibre composite at the lowest soaking time and the lowest alkaline concentration. Meanwhile, the maximum flexural modulus improved by 148% from untreated composite in average soaking time and reasonable alkaline concentration solution [48]. The tensile strength of the sugar palm fibres is enhanced by immersing the fibre in 4% and 6% alkali solutions for 1 h which increased by 35% [33]. This finding is also supported by a study from Ref. [49]; where the sugar palm fibres was treated with NaOH in two different concentrations, which are 5% and 10% NaOH for 2 h to evaluate the modifications of the fibre’s characteristics. Fibre morphology, diameter, density, single fibre tensile testing, and fibre thermal stability investigated. The treated sugar palm fibres recorded an altered diameter range, density, single fibre tensile properties, and

208

Table 8.6 Previous studies on fibre modifications of sugar palm particles. Flexural strength (MPa)

Impact strength (kJ/m2)

References

Matrix

Treatments

250 mm

TPU

(2%e6%) Alkaline treatments

5.49

e

e

[27]

150e250 mm

TPU

2%Silane

6e17.22

4e13.96

12.47

[23]

150e250 mm

TPU

6% Alkaline 2% Silane, 6% Alkaline þ 2%Silane

21.15e24.46

16e24

20e25

[22]

250 mm

TPU

6% Alkaline þ Microwave

18.42

e

e

[29]

250 mm

TPU

(0.033%, 0.066%, and 0.125%) Potassium permanganate (KMnO4)

e

8.118

55.185

[28]

0e50 mesh

HIPS

(4 & 6%) Alkaline treatments

e

38.99

5.31

[34]

0e50 mesh

HIPS

(4 & 6%) Alkaline treatments

24.34e30.34

e

e

[33]

 150 mm

Phenolics

Seawater þ 0.5 M Alkaline

e

75.24e92.59

5.38e7.28

[3,19,41]

Interfaces in Particle and Fibre Reinforced Composites

Particle size

Tensile strength (MPa)

Interfaces in sugar palm fibres reinforced composites: A review

209

different thermogravimetric plots. Moreover [50], used 0.25 M NH4OH, 0.25 M NaOH, and 0.25 M KOH to treat sugar palm fibres for 1 h at room temperature. The fibre surface treatment affects the electrical properties of sugar palm fibres/epoxy composites. The increment of the resistivity of the composites is more significant when the stronger base or alkaline pH was used, while the value of the dielectric constant reduces [51]. treated sugar palm fibres with alkaline solutions to investigate the thermal stability of the fibres [52]. treated the sugar palm fibres through alkalinisation with 0.25 M NaOH solutions for 30 min to investigate the effect on the sugar palm reinforced polylactic acid composite properties. Hence, the quality of the fibre treated using alkalinisation would be different depending on the soaking duration and NaOH concentrations. The chemical reaction occurred during the treatment enhances the properties of the fibres and enhances adhesion properties of the fibres with the matrix. The needs for the treatment depend on the desired properties of the fibres for a particular application.

3.2.2

Silane treatment

Fibre modification using silane treatment has also been implemented for sugar palm fibres composites. Previous work done by other researchers [11,23e25] immersed sugar palm particles in 2% silane solution for 3 h. The preparation of silane treatment includes the mixing of 3-aminopropyl-tri ethoxy silane (APS) with methanol and water. The silane treatment was combined with alkaline treatment for better fibre-matrix interaction and mechanical properties of the hybrid sugar palm fibres composites [53]. used Vinyltrimethoxy silane with 98% concentration to be immersed with sugar palm fibres for 15 min. The treated sugar palm fibres composites recorded higher strength and Young’s modulus compared with the untreated ones. However, the elongation at break remained unchanged. More studies are needed to further investigate the effects of silane treatment towards a high quality of sugar palm fibres. Different silane concentrations and variable soaking times should be tested on the sugar palm fibres.

3.2.3

Seawater treatment

Other researchers used seawater treatment as an alternative for fibre modification [8]. immersed sugar palm fibres in sea water, pond water, and sewage water for 30 days and the single fibre pull out, and fibre-matrix interfacial adhesion was studied. In another study [54], soaked sugar palm fibres in the sea water for the same duration to find a suitable replacement for the chemical treatment of natural fibres for enhancement of fibre-matrix interfacial adhesion. Seawater treatment removes hemicellulose and pectin of the fibres and consequently improves the fibre-matrix interface. Therefore, impact and flexural strength of the treated sugar palm fibres composites are higher than the untreated fibre composites. Moreover [55], treated sugar palm fibres by soaking the fibres in different treatment mediums for 30 days. The seawater is one of the medium applied. The highest value obtained for fracture toughness and energy release rates are 1.248 MPa/m and 1.19 kJ/m2, respectively. These findings imply that the mechanical properties of sugar palm fibres composites have been improved. Similar to

210

Interfaces in Particle and Fibre Reinforced Composites

other studies, the interfacial adhesion of fibre-matrix has been improved from the analysis through a Scanning Electron Microscope (SEM). A comparison study has been performed by Ref. [19] between alkaline and seawater treatment. The study was conducted to compare the effects of both treatments in terms of the mechanical, thermal, and morphological properties. Different treatment for different duration influences the resultant properties of sugar palm fibres composites. According to Ref. [3]; several experiments were performed by various researches, where seawater treatment is proven able to improve the surface properties of the sugar palm fibres higher than alkaline treatment. In addition, several tensile tests were also proven able to improve the tensile properties of seawater treated sugar palm fibres composites by 67.26% [56]. Hence, seawater treatment is commonly found in any fibre modification process because of its ease of operation and fewer chemical materials required. This treatment can also change the chemical composition of the fibres and improve their mechanical properties.

3.3

Characterization methods

After the modification process, the fibres will structurally change because of the reaction. The structural changes of the fibres could be investigated using optical equipment such as Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), and X-Ray Photo Electron Spectroscope. Moreover, to determine the chemical, physical, and mechanical properties of the fibres, investigation on the surface morphology analysis should be performed. These methods will characterise the structure of the fibre by examining the morphology. It is necessary to understand the characteristics of the structure as it implies the mechanical and interfacial properties of the fibre composites.

3.4

Scanning electron microscope

Scanning Electron Microscope (SEM) has been used by many researchers to characterise the structure of the fibres in any condition. This equipment is flexible equipment as the operator can control the number of the electron beam, detector, and stage parameters. The operator should understand the fundamental of electron optical parameters for different basic imaging modes of SEM. The right parameters setting of the SEM help the operator to meet the needs of the analysis. Similar with other natural fibres, the surface of the sugar palm fibres is not smooth with holes and irregular stripes along the surfaces. It is expected that this type of surface is the medium for mechanical interlocking between fibre and polymer matrix in composite materials as shown in Fig. 8.1. From another observation, the water molecules have filled the cell wall freely through the micropores along the fibre surface. The excessive moisture found in the fibre is reduced by a few methods such as impregnation. After the treatment, the fibre was found to be covered with the impregnation agent for moisture absorption protection [56,57] also presented their finding through SEM, where fibre modification process removes the first layer of sugar palm fibres leading to better interfacial adhesion between the fibre and matrix. Consequently, high tensile

Interfaces in sugar palm fibres reinforced composites: A review

211

strength is obtained. Moreover, due to flexural and impact test, sugar palm fibres composites recorded lower flexural and impacted strength. From SEM, the fracture surface of the fibres contains holes which have been pulled out during fractures [58]. On the other hand [44], investigated the dispersion of sugar palm fibres in the matrix through SEM. They found that non-uniform dispersion occurred during lower fibre loadings, where the fibres were disoriented, and most of the fibres underwent agglomeration. In addition [18], found an excellent adhesion condition of sugar palm particles with the phenolic resin, where there are more fibre breakage than fibre pull out in the fracture specimens at 20 and 30 vol% of sugar palm particles. Hence, good compatibility between the fibre and matrix leads to higher mechanical properties. SEM is usually conducted by knowledgeable and experienced researchers, who are skilful in analysing the data and handling the preparation of the operation. The sample for SEM is limited to solid and acceptable size to fit inside the vacuum chamber. The preferred material of the sample is flexible and vacuum resistant to be easily characterised [59].

3.5

Atomic force microscope

Atomic Force Microscope (AFM) is one of the techniques that is well applied in material characterisation process, where its ability to measure surfaces with subnanometre resolution and high accuracy allows the researchers to study the properties of the material at the atomic scale [60]. conducted a study on bamboo fibres using AFM to investigate the effect of alkaline treatment on wettability and thermal stability. Moreover [61], studied the characterisation of the new natural fibre, by analysing the surface texture of the fibre through AFM. Thus, AFM is commonly used in the characterisation of natural fibre. Physical properties of the fibre could be identified through AFM as shown in Ref. [51] study. The sugar palm fibres was treated with a sulphuric acid solution, and the diameter of the fibre reduced from micro to nanometers. From the image shown in AFM, sugar palm fibres were characterised and analysed by particle length, diameter, and aspect ratio. On the other hand, the study on the characterisation of sugar palm particles using AFM was yet to be studied. Relatively slow scan time causing thermal drift on the sample is one of the reasons this topic has not been an interest among researchers. Other than that, the single scan image size is 150  150 mm, which is not suitable for some of the fibre samples compared to other microscopy equipment [59].

3.6

X-Ray photo electron spectroscope

X-Ray Photoelectron Spectroscope (XPS) has been used by the researchers to study the surface of materials, qualitatively and quantitatively. Through the photoemission from atomic levels with binding energy, this technique is able to analyse the elemental composition and characterisation of the associated chemical states [62]. The composition and thickness of the sample could be determined by measuring the kinetic energy and the number of electrons that escape from the sample surface where the sufficient

212

Interfaces in Particle and Fibre Reinforced Composites

Table 8.7 Application of sugar palm particle composites. Particle size

Matrix

Application

References

250 mm

Thermoplastic Polyurethane (TPU)

automotive

[11,22,23],

Sugar palm starch

packaging

[31]

100 mm

PLA

Honey comb structure

[68]

150e300 mm

cassava starch

packaging, automotive, and agro-industrial

[37]

150 mm

Phenolics

Friction composites

[3,18]

2 mm

Sugar palm starch

disposable tray, plate,

[30]

150 mm

Phenolics

tribo-materials for friction materials

[19,20]

energy releases the photoelectrons [63]. Therefore, some of the researchers employed the XPS technique to analyse the surface composition of natural fibres for treated and untreated conditions [64e67]. XPS has also been suggested by Ref. [43] to be applied in the advanced characterisation of sugar palm fibres other than basic mechanical, thermal, chemical, and physical characterisations.

3.7

Applications and challenges

The sugar palm particle composites can be adopted in various applications as in Table 8.7. For instance, the sugar palm particle composites have high potential in automotive applications such as automotive parts [11,23e25,40], automotive friction materials [20], food packaging [30,31] and many others. Table 8.5 shows the various applications, which adopted sugar palm particle in their composites.

4.

Conclusion

The tremendous development of sugar palm particle composites with improved properties has greatly stimulated the enthusiasm of research to create an optimum reinforced sugar palm particles. It is essential to understand the properties of the interface between the sugar palm particles and the polymer matrix, as the properties greatly influence the properties of interface interactions for various applications. There is still a need for the development of the techniques aimed at enabling the incorporation of the sugar palm particles composites. It is expected that the findings from the previous studies between interfacial properties and the composites could be employed to fill the gap in recent studies.

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Acknowledgements The authors are grateful for the financial support from Universiti Putra Malaysia through Putra grant no. GP-IPS/2015/9441501. The author would also like to thank the Ministry of Higher Education for the MyBrain15 scholarship.

References [1] M.L. Sanyang, S. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Recent developments in sugar palm (Arenga pinnata) based biocomposites and their potential industrial applications: a review, Renew. Sustain. Energy Rev. 54 (2016a) 533e549. [2] M. Ishak, S. Sapuan, Z. Leman, M. Rahman, U. Anwar, Characterization of sugar palm (Arenga pinnata) fibres, J. Therm. Anal. Calorim. 109 (2012) 981e989. [3] B. Rashid, Z. Leman, M. Jawaid, M. Ghazali, M. Ishak, Physicochemical and thermal properties of lignocellulosic fiber from sugar palm fibers: effect of treatment, Cellulose 23 (2016a) 2905e2916. [4] S. Sapuan, H. Lok, M.R. Ishak, S. Misri, Mechanical properties of hybrid glass/sugar palm fibre reinforced unsaturated polyester composites, Chin. J. Polym. Sci. 31 (2013) 1394e1403. [5] D. Bachtiar, S. Sapuan, E.S. Zainudin, A. Khalina, K. Dahlan, The Tensile Properties of Single Sugar Palm (Arenga Pinnata) Fibre. IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2010, p. 012012. [6] M.R. Ishak, Z. Leman, M.S. Salit, M.Z.A. Rahman, M.K. Anwar Uyup, R. Akhtar, IFSS, TG, FT-IR spectra of impregnated sugar palm (Arenga pinnata) fibres and mechanical properties of their composites, J. Therm. Anal. Calorim. 111 (2013a) 1375e1383. [7] A. Atiqah, M. Jawaid, M.R. Ishak, S.M. Sapuan, Effect of alkali and silane treatments on mechanical and interfacial bonding strength of sugar palm fibers with thermoplastic polyurethane, J. Nat. Fibers (2017b) 1e11. [8] Z. Leman, S.M. Sapuan, S. Suppiah, Sugar palm fibre-reinforced unsaturated polyester composite interface characterisation by pull-out test, Key Eng. Mater. 471 (2011) 1034e1039. [9] L.S. Khor, Z. Leman, C.H. Lee, Interfacial debonding force and shear strength of sugar palm (arenga pinnata) fiber reinforced composites by pull-out test, Adv. Mater. Res. 634 (2013) 1931e1936. [10] M. Arif Irfai, D. Wulandari, Sutriyono, E. Marsyahyo, Effect of NaOH treatment on bending strength of the polyester composite reinforce by sugar palm fibers, IOP Conf. Ser. Mater. Sci. Eng. 336 (1) (2018) 012008. [11] A. Atiqah, M. Jawaid, S.M. Sapuan, M.R. Ishak, Physical properties of silane-treated sugar palm fiber reinforced thermoplastic polyurethane composites, IOP Conf. Ser. Mater. Sci. Eng. 368 (1) (2018) 012047. [12] M.R. Ishak, S.M. Sapuan, Z. Leman, M.Z.A. Rahman, U.M.K. Anwar, J.P. Siregar, Sugar palm (Arenga pinnata): its fibres, polymers and composites, Carbohydr. Polym. 91 (2013c) 699e710. [13] I. Mukhtar, Z. Leman, M.R. Ishak, E.S. Zainudin, Sugar palm fibre and its composites: a review of recent developments, BioResources 11 (2016) 10756e10782. [14] M.L. Sanyang, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Recent developments in sugar palm (Arenga pinnata) based biocomposites and their potential industrial applications: a review, Renew. Sustain. Energy Rev. 54 (2016c) 533e549.

214

Interfaces in Particle and Fibre Reinforced Composites

[15] M.R.M. Huzaifah, S.M. Sapuan, Z. Leman, M.R. Ishak, M.A. Maleque, A review of sugar palm (Arenga pinnata): application, fibre characterisation and composites, Multidiscip. Model. Mater. Struct. 13 (2017) 678e698. [16] J. Sahari, S.M. Sapuan, Z.N. Ismarrubie, M.Z.A. Rahman, Physical and chemical properties of different morphological parts of sugar palm fibres, Fibres Text. East. Eur. 91 (2012) 21e24. [17] M. Masudul Hassan, M.H. Wagner, Surface modification of natural fibers for reinforced polymer composites: a critical review, Rev. Adhesion Adhesives 4 (2016) 1e46. [18] B. Rashid, Z. Leman, M. Jawaid, M.J. Ghazali, M.R. Ishak, The mechanical performance of sugar palm fibres (ijuk) reinforced phenolic composites, Int. J. Precis. Eng. Manuf. 17 (2016b) 1001e1008. [19] B. Rashid, Z. Leman, M. Jawaid, M. Ghazali, M. Ishak, Effect of treatments on the physical and morphological properties of SPF/phenolic composites, J. Nat. Fibers (2017a) 1e13. [20] B. Rashid, Z. Leman, M. Jawaid, M.J. Ghazali, M.R. Ishak, M.A. Abdelgnei, Dry sliding wear behavior of untreated and treated sugar palm fiber filled phenolic composites using factorial technique, Wear 380e381 (2017c) 26e35. [21] A. Atiqah, M. Jawaid, S. Sapuan, M. Ishak, Dynamic mechanical properties of sugar palm/ glass fiber reinforced thermoplastic polyurethane hybrid composites, Polym. Compos. (2018). [22] A. Afzaluddin, M. Jawaid, S. Sapuan, M.R. Ishak, Effect of surface treatment on the mechanical properties of sugar palm/glass fiber-reinforced thermoplastic polyurethane hybrid composites, BioResources 13 (2017) 1174e1188. [23] A. Atiqah, M. Jawaid, S. Sapuan, M. Ishak, Mechanical and thermal properties of sugar palm fiber reinforced thermoplastic polyurethane composites: effect of silane treatment and fiber loading, Renew. Mat. 6 (5) (August 2018). [24] A. Atiqah, M. Jawaid, S. Sapuan, M.R. Ishak, Effect of surface treatment on the mechanical properties of sugar palm/glass fiber-reinforced thermoplastic polyurethane hybrid composites, BioResources 13 (2017d) 1174e1188. [25] A. Atiqah, M. Jawaid, S. Sapuan, M. Ishak, O.Y. Alothman, Thermal properties of sugar palm/glass fiber reinforced thermoplastic polyurethane hybrid composites, Compos. Struct. 202 (15 October 2018) 954e958. [26] A.A.-S. Mohammed, D. Bachtiar, J.P. Siregar, M.R.B.M. Rejab, S.F. Hasany, Physicochemical study of eco-friendly sugar palm fiber thermoplastic polyurethane composites, BioResources 11 (2016a) 9438e9454. [27] A.A. Mohammed, D. Bachtiar, J.P. Siregar, M.R.M. Rejab, Effect of sodium hydroxide on the tensile properties of sugar palm fibre reinforced thermoplastic polyurethane composites, J. Mech. Eng. Sci. 10 (2016b) 1765e1777. [28] A.A. Mohammed, D. Bachtiar, M.R.M. Rejab, X.X. Jiang, F.O. Abas, R.U. Abass, S.F. Hasany, J.P. Siregar, Effects of KMnO4 treatment on the flexural, impact, and thermal properties of sugar palm fiber-reinforced thermoplastic polyurethane composites, J. Occup. Med. 70 (2018a) 1326e1330. [29] A.A. Mohammed, D. Bachtiar, M.R.M. Rejab, J.P. Siregar, Effect of microwave treatment on tensile properties of sugar palm fibre reinforced thermoplastic polyurethane composites, Def. Technol. 14 (2018b) 287e290. [30] R. Jumaidin, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Thermal, mechanical, and physical properties of seaweed/sugar palm fibre reinforced thermoplastic sugar palm Starch/Agar hybrid composites, Int. J. Biol. Macromol. 97 (2017) 606e615.

Interfaces in sugar palm fibres reinforced composites: A review

215

[31] M.L. Sanyang, R.A. Ilyas, S.M. Sapuan, R. Jumaidin, Sugar palm starch-based composites for packaging applications, Bionanocomposites for Packaging Applications (2018) 125e147. Springer, Cham. [32] R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin, Development and characterization of sugar palm nanocrystalline cellulose reinforced sugar palm starch bionanocomposites, Carbohydr. Polym. 202 (2018b) 186e202. [33] D. Bachtiar, M.S. Salit, E. Zainuddin, K. Abdan, K.Z.H.M. Dahlan, Effects of alkaline treatment and a compatibilizing agent on tensile properties of sugar palm fibre-reinforced high impact polystyrene composites, BioResources 6 (2011) 4815e4823. [34] D. Bachtiar, S. Sapuan, A. Khalina, E. Zainudin, K. Dahlan, Flexural and impact properties of chemically treated sugar palm fiber reinforced high impact polystyrene composites, Fibers Polym. 13 (2012a) 894e898. [35] D. Bachtiar, S. Sapuan, A. Khalina, E. Zainudin, K. Dahlan, The flexural, impact and thermal properties of untreated short sugar palm fibre reinforced high impact polystyrene (HIPS) composites, Polym. Polym. Compos. 20 (2012b) 493. [36] D. Bachtiar, M.S. Salit, E.S. Zainudin, K. Abdan, M. Dahlan, K. Zaman, Thermal properties of alkali-treated sugar palm fibre reinforced high impact polystyrene composites, Pertanika J. Sci. Technol. 21 (2013) 141e150. [37] A. Edhirej, S.M. Sapuan, M. Jawaid, N.I. Zahari, Tensile, barrier, dynamic mechanical, and biodegradation properties of cassava/sugar palm fiber reinforced cassava starch hybrid composites, BioResources 12 (2017b) 7145e7160. [38] A. Edhirej, S.M. Sapuan, M. Jawaid, N.I. Zahari, Cassava/sugar palm fiber reinforced cassava starch hybrid composites: physical, thermal and structural properties, Int. J. Biol. Macromol. 101 (2017a) 75e83. [39] S. Wu, Polymer Interface and Adhesion, 2017. [40] A. Atiqah, M. Jawaid, M. Ishak, S. Sapuan, Moisture absorption and thickness swelling behaviour of sugar palm fibre reinforced thermoplastic polyurethane, Proc. Eng. 184 (2017a) 581e586. [41] B. Rashid, Z. Leman, M. Jawaid, M.J. Ghazali, M.R. Ishak, Influence of treatments on the mechanical and thermal properties of sugar palm fibre reinforced phenolic composites, BioResources 12 (2017b) 1447e1462. [42] A. Afzaluddin, M. Jawaid, M.S. Salit, M.R. Ishak, Physical and mechanical properties of sugar palm/glass fiber reinforced thermoplastic polyurethane hybrid composites, J. Mat. Res. and Technol. 8 (1) (2019) 950e959. [43] M.L. Sanyang, S.M. Sapuan, M. Jawaid, M.R. Ishak, J. Sahari, Development and characterization of sugar palm starch and poly(lactic acid) bilayer films, Carbohydr. Polym. 146 (2016b) 36e45. [44] S.M. Sapuan, D. Bachtiar, Mechanical properties of sugar palm fibre reinforced high impact polystyrene composites, Proc. Chem. 4 (2012) 101e106. [45] A. Ticoalu, T. Aravinthan, F. Cardona, A study into the characteristics of gomuti (Arenga pinnata) fibre for usage as natural fibre composites, J. Reinf. Plast. Compos. 33 (2014) 179e192. [46] A. Ticoalu, T. Aravinthan, F. Cardona, A review on the characteristics of gomuti fibre and its composites with thermoset resins, J. Reinf. Plast. Compos. 32 (2013b) 124e136. [47] M. Przybylak, H. Maciejewski, A. Dutkiewicz, I. Dąbek, M. Nowicki, Fabrication of superhydrophobic cotton fabrics by a simple chemical modification, Cellulose 23 (2016) 2185e2197. [48] D. Bachtiar, S. Sapuan, M. Hamdan, The effect of alkaline treatment on tensile properties of sugar palm fibre reinforced epoxy composites, Mater. Des. 29 (2008) 1285e1290.

216

Interfaces in Particle and Fibre Reinforced Composites

[49] A. Ticoalu, T. Aravinthan, F. Cardona, Properties and behaviour of gomuti fibre composites under tensile and compressive load, in: Proceedings of the 22nd Australasian Conference on the Mechanics of Structures and Materials (ACMSM 22), Taylor & Francis (CRC Press)/Balkema, 2013a, pp. 531e535. [50] I. Santhiarsa, A. Sonief, E. Marsyahyo, Effects of alkali treatment and weight fraction on electrical properties of palm sugar fibre-epoxy composite, Contemp. Eng. Sci. 7 (2014) 907e914. [51] R. Ilyas, S. Sapuan, M. Ishak, Isolation and characterization of nanocrystalline cellulose from sugar palm fibres (Arenga Pinnata), Carbohydr. Polym. 181 (2018a) 1038e1051. [52] M. Chalid, I. Prabowo, The effects of alkalization to the mechanical properties of the ijuk fiber reinforced PLA biocomposites, International Journal of Chemical, Molecular, Nuclear, Material and Metallurgical Engineering Vol9. No2. (2015) 342e346. [53] W. Zahari, R. Badri, H. Ardyananta, D. Kurniawan, F. Nor, Mechanical properties and water absorption behavior of polypropylene/ijuk fiber composite by using silane treatment, Procedia Manufacturing 2 (2015) 573e578. [54] M.R. Ishak, Z. Leman, S.M. Sapuan, M.Y. Salleh, S. Misri, The effect of sea water treatment on the impact and flexural strength of sugar palm fibre reinforced epoxy composites, Int. J. Mech. Mater. Eng. 4 (2009b) 316e320. [55] P.A. Khudhur, O.S. Zaroog, B.A. Khidhir, Z.S. Radif, Fracture Toughness of Sugar Palm Fiber Reinforced Epoxy Composites, 2013. [56] Z. Leman, S.M. Sapuan, M. Azwan, M.M.H.M. Ahmad, M. Maleque, The effect of environmental treatments on fiber surface properties and tensile strength of sugar palm fiber-reinforced epoxy composites, Polym. Plast. Technol. Eng. 47 (2008) 606e612. [57] M.R. Ishak, Z. Leman, S.M. Sapuan, M.Z.A. Rahman, U.M.K. Anwar, Impregnation modification of sugar palm fibres with phenol formaldehyde and unsaturated polyester, Fibers Polym. 14 (2013b) 250e257. [58] M. Ishak, Z. Leman, S. Sapuan, M. Salleh, S. Misri, The effect of sea water treatment on the impact and flexural strength of sugar palm fibre reinforced epoxy composites, Int. J. Mech. Mater. Eng. 4 (2009a) 316e320. [59] Z. Karim, S. Afrin, Bacterial Cellulose: Preparation and Characterization. CelluloseReinforced Nanofibre Composites, Elsevier, 2017. [60] H. Chen, W. Zhang, X. Wang, H. Wang, Y. Wu, T. Zhong, B. Fei, Effect of alkali treatment on wettability and thermal stability of individual bamboo fibers, J. Wood Sci. (2018) 1e8. [61] P. Manimaran, P. Senthamaraikannan, M. Sanjay, M. Marichelvam, M. Jawaid, Study on characterization of furcraea foetida new natural fiber as composite reinforcement for lightweight applications, Carbohydr. Polym. 181 (2018) 650e658. [62] L. Matuana, J. Balatinecz, R. Sodhi, C. Park, Surface characterization of esterified cellulosic fibers by XPS and FTIR spectroscopy, Wood Sci. Technol. 35 (2001) 191e201. [63] Y. Song, Y. Huang, Y.-S. Lin, High-k Metal Gate Inline Measurement Technique Using XPS. Semiconductor Technology International Conference (CSTIC), 2017 China, IEEE, 2017, pp. 1e3. € Andiç-C¸akir, M. Sarikanat, H.B. T€ufekçi, C. Demirci, U.H. € [64] O. Erdo gan, Physical and mechanical properties of randomly oriented coir fiberecementitious composites, Compos. B Eng. 61 (2014) 49e54. [65] Q. Luo, S. Lu, L. Song, Y. Li, Fabrication of Sisal Fibers/epoxy Composites with Liquid Crystals Polymer Grafted on Sisal Fibers. IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2016, p. 012052.

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[66] N.M. Stark, L.M. Matuana, Characterization of weathered woodeplastic composite surfaces using FTIR spectroscopy, contact angle, and XPS, Polym. Degrad. Stab. 92 (2007) 1883e1890. [67] N. Zafeiropoulos, C. Baillie, J. Hodgkinson, Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part II. The effect of surface treatments on the interface, Compos. Appl. Sci. Manuf. 33 (2002) 1185e1190. [68] Z. Ansari, M.R.M. Rejab, D. Bachtiar, J. Siregar, Crushing response of green square honeycomb structure from sugar palm & PLA, Mater. Sci. Forum 909 (2017) 122e126.

Characterization studies of polymer-based composites related to functionalized filler-matrix interface

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A. Rajeswari, E. Jackcina Stobel Christy, Sreerag Gopi, K. Jayaraj, Anitha Pius Department of Chemistry, The Gandhigram Rural Institute e Deemed to be University, Dindigul, Tamil Nadu, India

1. Introduction The world around us is full of polymer products. Polymers are materials composed of long molecular chains that are well-accepted for a wide variety of applications. They are different from low molecular weight compounds like common salt. To contrast the difference, the molecular weight of common salt is lesser (only 58.5), while that of a polymer can be as high as several hundred thousands. These big molecules or macromolecules are made up of much smaller molecules. The small molecules, which combine to form a big molecule, can be of one or more chemical compounds. To illustrate, imagine a set of rings of the same material. When those rings are interlinked the chain formed can be considered as representing a polymer from molecules of the same compound. Alternatively, individual rings could be of different sizes and materials are interlinked to represent a polymer from molecules of different compounds. The two situations are shown in Fig. 9.1. Simply stated, a polymer is a long chain molecule that is composed of a large number of repeating units of identical structures. Polymers that are capable of high extension under ambient conditions find important applications as elastomers. In addition to natural rubber there are synthetic elastomers such as nitrile and butyl rubber. Other polymers may have characteristics that permit their formation into long fibers suitable for textile applications. The synthetic fibers principally nylon and polyesters are good substituent for the naturally occurring fibers such as cotton, wool and silk. In contrast to the usage of polymer, those commercial materials other than elastomers and fibers that are derived from synthetic polymers are called plastics. A typical commercial plastic resin may contain two or more polymers in addition to various additives and fillers. These are added to improve certain propertied such as the processability, thermal or environmental stability and mechanical properties of the final product [1].

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00009-1 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Same size rings

Different size rings

Interlinked rings

Fig. 9.1 Schematic representation of the formation of polymer molecule by chain linking.

1.1

Classification of polymers

Thousands of polymers have been synthesized and more likely to be produced in the future. Fortunately, all polymers that can be assigned to one of two groups based upon their processing characteristics or type of polymerization mechanism. Polymers can have different chemical structures, physical properties, mechanical behavior, thermal characteristics etc. and can be classified in different ways that are shown in Table 9.1 [1]. i. Origin (a) Natural polymers: They are available in nature. Ex: Natural rubber, natural silk, cellulose, starch, proteins etc (b) Semisynthetic polymers: They are chemically modified natural polymers. Ex: Hydrogenated or hydro-halogenated natural rubber, cellulosics, esters and ethers of cellulose, cellulose nitrate, methyl cellulose, etc. (c) Synthetic polymers: They are man-made polymers prepared synthetically. Ex: Polyethylene, polystyrene, polyvinyl chloride, polyesters and phenol-formaldehyde resins, etc. ii. Thermoplastic and thermosetting polymers ➢ Some polymers soften on heating and can be converted into any shape that they can retain on cooling. The process of heating, reshaping and retaining the same on cooling can be repeated several times. Such polymers that soften on heating and stiffen on cooling are termed “thermoplastics”. Polyethylene, PVC, nylon and sealing wax are examples of thermoplastic polymers. ➢ Some polymers undergo some chemical change on heating and convert themselves into an infusible mass. They are like the yolk of the egg, which on heating sets into a mass Table 9.1 Important classifications of polymers. Classification

Types of polymers

Origin

Natural, semisynthetic, synthetic

Thermal response

Thermoplastic, Thermosetting

Organic and inorganic

Glass, silicon rubber

Crystallinity

Non-crystalline (amorphous), semi-crystalline, crystalline

Characterization studies of polymer-based composites

221

and once set can’t be reshaped. Such polymers that become an infusible and insoluble mass on heating are called thermosetting polymers. iii. Organic and Inorganic polymers

A polymer whose backbone chain is essentially made up of carbon atoms is termed an organic polymer. The atoms attached to the side valencies of the backbone carbon atoms are usually those of hydrogen, oxygen, nitrogen etc. Majority of synthetic polymers are organic and they are very extensively studied. In fact, the number and variety of organic polymers are so large that when we refer to polymers, we normally mean organic polymers. The molecules of inorganic ‘polymers’, generally contain no carbon in their chain backbone. Glass and silicone rubber are examples of inorganic polymers.

2. Classifications based on application Another way of classifying polymers is in terms of their form or application, varying from additives to other bulk materials, coatings to products (e.g., paints), film and membranes to fibres (e.g., textiles) and bulk products such as pipe, containers and mouldings shown in Fig. 9.2. Important types of modified polymer systems include polymer composites, polymer - polymer blends, polymeric foams etc. [2]. Some of these materials are of course used as products in their own right, or manipulated further into finished products. This does not always happen, however, some polymers being a disposable intermediary in certain industrial processes. Thus photoresists are used to create the circuit patterns on semiconductor chips through controlled degradation, and are entirely absent in the final product. Among them composite materials are used for advanced applications such as airframes, space structures, race cars and sporting goods etc.

2.1

Polymer composite

Composites can be defined as materials that consist of two or more chemically and physically different phases separated by a distinct interface. The different systems are combined judiciously to achieve a system with more useful structural es Com p

bran

osi

tes

Additives Fo am s

Ad sor

ers

ben

Fill

ts

Mem

Pho

tore

sists

Polymer

Fil

ms

rs

aine

Fibers Cont

Fig. 9.2 Classification of polymers based on applications.

222

Interfaces in Particle and Fibre Reinforced Composites

or functional properties non attainable by any of the constituent alone. Composites, the wonder materials are becoming an essential part of today’s materials due the advantages such as low weight, corrosion resistance, high fatigue strength, and faster assembly. They are extensively used as materials in making aircraft structures, electronic packaging to medical equipment and space vehicle to home building. The basic difference between blends and composites is that the two main constituents in the composites remain recognizable while these may not be recognizable in blends. The predominant useful materials used in our day-to-day life are wood, concrete, ceramics and so on. Surprisingly, the most important polymeric composites are found in nature and these are known as natural composites. The connective tissues in mammals belong to the most advanced polymer composites known to mankind where the fibrous protein, collagen is the reinforcement. It functions both as soft and hard connective tissue. Composites are combinations of materials differing in composition, where the individual constituents retain their separate identities. These separate constituents act together to give necessary mechanical strength or stiffness to the composite part. Thus composite material is a material composed of two or more distinct phases (matrix phase and dispersed phase) and having bulk properties significantly different from those of any of the constituents. Matrix phase is the primary phase having a continuous character. Matrix is usually more ductile and less hard phase. It holds the dispersed phase and shares a load with it. Dispersed (reinforcing) phase is embedded in the matrix in a discontinuous form. This secondary phase is called the dispersed phase. Dispersed phase is usually stronger than the matrix, therefore, it is sometimes called reinforcing phase. Composites in structural applications have the following characteristics: • • •

They generally consist of two or more physically distinct and mechanically separable materials. They are made by mixing separate materials in such a way as to achieve controlled and uniform dispersion of the constituents. They have superior mechanical properties and in some cases uniquely different from the properties of their constituents.

Wood is a natural composite of cellulose fibers in a matrix of lignin. Most primitive man-made composite materials were straw and mud combined to form bricks for building construction. Most visible applications pave our roadways in the form of either steel and aggregate reinforced Portland cement or asphalt concrete. Reinforced concrete is another example of composite material. The steel and concrete retain their individual identities in the finished structure. However, because they work together, the steel carries the tension loads and concrete carries the compression loads. Most advanced examples perform routinely on spacecraft in demanding environments. Advanced composites have high-performance fiber reinforcements in a polymer matrix material such as epoxy. Examples are graphite/epoxy, Kevlar/ epoxy and boron/epoxy composites. Advanced composites are traditionally used in the aerospace industries, but these materials have now found applications in commercial industries as well.

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In general, parameters affecting the properties of polymer composites, whether continuous or discontinuous, include: • • • •

the properties of the additives (inherent properties, size, shape) composition the interaction of components at the phase boundaries, which is also associated with the existence of a thick interface, known also as the interphase; this is often considered as a separate phase, controlling adhesion between the components the method of fabrication

2.2

Types of polymer composites

Based on the types of reinforcement used, the composites are classified particulate, fiber, laminate and hybrid as depicted in Fig. 9.3.

2.2.1

Particulate reinforced composites

A composite whose reinforcement is a particle with all the dimensions roughly equal are called particulate reinforced composites. Particulate fillers are employed to improve high temperature performance, reduce friction, increase wear resistance and to reduce shrinkage [3]. The particles will also share the load with the matrix, but to a lesser extent than a fiber. A particulate reinforcement will therefore improve stiffness but will not generally strengthen.

2.2.2

Fiber reinforced composites

Fiber reinforced composites contain reinforcements having lengths higher than cross sectional dimension. Fibrous reinforcement represents physical rather than a chemical means of changing a material to suit various engineering applications [4]. These can be broadly classified as shown in Fig. 9.4. Reinforcing fiber in a single layer composite may be short or long based on its overall dimensions. Composites with long fibers are called continuous fiber reinforcement and composite in which short or staple fibers are embedded in the matrix are termed as discontinuous fiber reinforcement (short fiber composites). In continuous fiber composites fibers are oriented in one direction to produce enhanced strength properties.

Pa

rtic

ula

te

Fiber

Composites

Lamin Hy

Fig. 9.3 Classification of composites.

brid

ate

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Interfaces in Particle and Fibre Reinforced Composites

Continuous

Discontinuous

Single layer composites

Laminate

Hybrid

Multi layer composites

Fiber reinforced composites

Fig. 9.4 Classification of filler reinforced composites.

In short fiber composites, the length of short fiber is neither too high to allow individual fibers to entangle with each other nor too small for the fibers to loss their fibrous nature. The reinforcement is uniform in the case of composites containing well dispersed short fibers. There is a clear distinction between the behavior of short and long fiber composites.

2.2.3

Hybrid composites

Composite materials incorporated with two or more different types of fillers especially fibers in a single matrix are commonly known as hybrid composites. Hybridization is commonly used for improving the properties and for lowering the cost of conventional composites. There are different types of hybrid composites classified according to the way in which the component materials are incorporated. Hybrids are designated as (i) sandwich type (ii) interply (iii) intraply and (iv) intimately mixed [5]. In sandwich hybrids, one material is sandwiched between layers of another, whereas in interply, alternate layers of two or more materials are stacked in regular manner. Rows of two or more constituents are arranged in a regular or random manner in intraply hybrids while in intimately mixed type, these constituents are mixed as much as possible so that no concentration of either type is present in the composite material.

2.2.4

Laminates

A laminate is fabricated by stacking a number of laminae in the thickness direction. Generally three layers are arranged alternatively for better bonding between reinforcement and the polymer matrix, for example plywood and paper. These laminates can have unidirectional or bi-directional orientation of the fiber reinforcement according to the end use of the composite. A hybrid laminate can also be fabricated by the use of different constituent materials or of the same material with different reinforcing pattern. In most of the applications of laminated composite, manmade fibers are used due to their good combination of physical, mechanical and thermal behavior.

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2.3

225

Polymer nanocomposites

Over the past several years, research on nanoparticles (NPs) reinforced polymer composites or polymer nanocomposites (PNCs) have received enormous attention in both scientific and industrial communities due to their superior mechanical, physical, chemical, electrical, optical and barrier properties [6e10]. In principle, properties of nanocomposites depend on a set of parameters including nanoparticles size, shape, volume fraction, degree of dispersion, characteristics of polymer matrix, characteristics of nanoparticles, characteristics of polymer-filler inter-phase and interactions between filler and matrix at their interface. Experimental results, however, often exhibits different trends [11]. To elucidate quantitative relations between the process parameters and overall properties of nanocomposites, many researchers resort to molecular level modeling and simulations of polymer nanocomposites [12e17]. Adnan et al. modeled bucky ball reinforced polyethylene composites using molecular dynamics (MDs) simulations and found that the elastic properties of nanocomposites are significantly enhanced with the reduction of bucky-ball size [18]. Work by Toth et al. suggests that both size and shape alters the mechanical properties of nanocomposites [19]. It is, however, often argued that nanoparticles size or shape effects largely depend on their dispersion state. It is believed that good nanoparticles dispersion is the key to maximize nanoparticle-polymer interface interaction which, in turn, will enhance the overall properties of nanocomposites [20,21]. While the debate whether good nanoparticle dispersion is sufficient to ensure improved properties of nanocomposites is still ongoing, it is often argued that a good dispersion during the preparation phase and before nanocomposite processing does not necessarily guarantee a strong interaction between the constituents of the final product [22]. It is therefore, suggested [22] that both experimental and modeling investigations should consider the quantitative relation between good dispersion and polymer-nanoparticles interface interaction, and the relation between strong polymer-nanoparticles interface interaction and overall mechanical properties of nanocomposites. From classical mechanics of composites [23], it seems obvious that a strong interface between polymer and nanoparticles would yield better properties of nanocomposites. The same argument may not work for nanocomposites for two reasons: (1) In nanocomposites, polymer properties are not homogenous everywhere in the nanocomposites [18]; rather, properties of polymer dramatically vary near the nanoparticle. It is observed that the radius of gyration of polymer chains increases significantly, and morphologically, they become more ordered near nanoparticles surfaces (2) Creation of new material phase, known as “interphase”, near the polymer-nanoparticles interface [24e29]. Macroscopically, an interface is generally referred to the contact plane between polymer matrix and nanoparticles. However, defining interface for nanocomposite becomes very subtle due to the presence of “interphase” between the bulk polymer and the nanoparticles.

Nanocomposites are a new class of composites that are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range [30e32]. Nanometer is an atomic dimension and hence the properties of nanoclusters

226

Interfaces in Particle and Fibre Reinforced Composites

or particles are reflective of atoms rather than bulk materials. An example for nanocomposite in nature is the natural bone consisting of approximately 30% matrix material and 70% nanosized mineral. Here the matrix material is collagen fibers (polymer) and the mineral is hydroxyapatite crystals of 50 nm
25 nm
3 nm size (ceramic). The outstanding reinforcement of nanocomposite is primarily attributed to the large interfacial area per unit volume or weight of the dispersed phase. The nanolayers have much higher aspect ratio than typical microscopic aggregates [30,33,34]. The three major advantages that nanocomposites have over conventional composites are as follows. ❖ Lighter weight due to low filler loading ❖ Low cost due to fewer amount of filler use ❖ Improved properties such as mechanical, thermal, optical, electrical, barrier etc., compared with conventional composites at very low loading of filler.

Three types of nanocomposites can be distinguished depending upon the number of dimensions of the dispersed particles in the nanometer range [30] as follows. ❖ Nanocomposites that can be reinforced by isodimentional nanofillers which have three dimensions in the nanometer range. E.g.,: Spherical silica nanoparticles obtained by in-situ sol-gel methods [35,36] or by polymerization promoted directly from their surface [37]. ❖ Nanocomposites which can be reinforced by fillers which have only two dimensions in the nanometer scale. E.g.,: Carbon nanotube [38,39] or cellulose whiskers [40,41]. ❖ The reinforcing phase, in the shape of platelets, has only one dimension on a nano level. E.g.,: Clays and layered silicates.

Polymer based organic/inorganic nanocomposites have gained increasing attention in the field of materials science [42,43]. Effect of acrylic polymer and nanocomposite with nano-SiO2 on thermal degradation and fire resistance of ammonium polyphosphateedipentaerythritolemelamine (APPeDPEReMEL) coating was studied by Zhenyu and co workers [44]. Effect of microstructure of acrylic copolymer/ terpolymer on the properties of silica based nanocomposites prepared by solegel technique was studied by Patel et al. [45]. Bandyopadhyay et al. [46] studied the reaction parameters on the structure and properties of acrylic rubber/silica hybrid nanocomposites prepared by sol-gel technique.

3.

Effects of additives on composite

The primary reasons for using additives are: ➢ property modification or enhancement ➢ overall cost reduction ➢ improving and controlling of processing characteristics

Additives for polymer composites have been variously classified as, • •

Reinforcements Fillers or reinforcing fillers

Characterization studies of polymer-based composites

3.1

227

Reinforcements

Reinforcements, being much stiffer and stronger than the polymer, usually increase its modulus and strength. Although mechanical property modification may be considered as their primary function their presence may significantly affect thermal expansion, transparency, thermal stability, etc. For composites containing continuous reinforcements, mostly in thermosetting matrices, the long fibers or ribbons, when pre-arranged in certain geometric patterns, may become the major component of the composite (they can constitute as much as 70% by volume in oriented composites). For discontinuous composites, the directional reinforcing agents (short fibers or flakes) are arranged in the composite in different orientations and multiple geometric patterns, which are dictated by the selected processing and shaping methods, most often extrusion or injection molding. In this case, the content of the additive does not usually exceed 30%e40% by volume. It should be noted, however, that manufacturing methods for continuous oriented fiber thermoplastic composites are available that are amenable to much higher fiber contents, as used in high performance engineering polymers [6]. Fillers can also be used in the thermoset processes, often in the presence of primary continuous fiber reinforcement. The concentration and inherent properties of the additive, as well as its interaction with the matrix, are important parameters controlling the processability of the composite.

3.2

Fillers

The term filler is very broad and encompasses a very wide range of materials. In this chapter, we arbitrarily define as fillers a variety of solid particulate materials (inorganic, organic) that may be irregular, acicular, fibrous or plate-like in shape and which are used in reasonably large volume loadings in plastics. Pigments and elastomeric matrices are not normally included in this definition. There is significant diversity in the chemical structures, forms, shapes, sizes, and inherent properties of the various inorganic and organic compounds that are used as fillers. They are usually rigid materials, immiscible with the matrix in both the molten and solid states, and, as such, form distinct dispersed morphologies. Their common characteristic is that they are used at relatively high concentrations (>5% by volume), although some surface modifiers and processing aids are used at lower concentrations. Fillers may be classified as inorganic or organic substances and further subdivided according to chemical family (Table 9.2) or according to their shape and size or aspect ratio. In a recent review [47], Wypych reported more than 70 types of particulates or flakes and more than 15 types of fibers of natural or synthetic origin that have been used or evaluated as fillers in thermoplastics and thermosets. The most commonly used particulate fillers are industrial minerals such as talc, calcium carbonate, mica, kaolin, wollastonite, feldspar, and barite. As market penetration increased, people started to look for ways to reduce the cost of the plastic materials and for ways to extend the property spectrum, to allow plastics entry into new applications. Fillers were introduced and were readily accepted because they are easy to incorporate into plastics and offer myriad possibilities for product

228

Interfaces in Particle and Fibre Reinforced Composites

Table 9.2 Chemical families of fillers for plastics.

Inorganic Oxides

Glass (fibers, spheres, hollow spheres, flakes), MgO, SiO2, Sb2O3, Al2O3

Hydroxides

Al(OH)3, Mg(OH)2

Salts

CaCO3, BaSO4, CaSO4, phosphates

Silicates

Talc, mica, kaolin, wollastonite, montmorillonite, nanoclays, feldspar, asbestos

Metals

Boron, steel

Organic Carbon, graphite

Carbon fibers, graphite fibers and flakes, carbon nanotubes, carbon black

Natural polymers

Cellulose fibers, wood flour and fibers, flax, cotton, sisal, starch

Synthetic polymers

Polyamide, polyester, aramid, polyvinyl alcohol fibers

improvement and differentiation. The rather unglamorous term “filler” does not do justice to the essential role these additives play in tuning processability as well as mechanical, thermal, optical, electrical, and other key properties. Therefore, they are referred to as “functional fillers.” As we shall see, these unassuming additives are a vital addition to the arsenal of the plastics formulator. Each type of filler lends a unique property set to the host polymer. Fillers are an extremely diverse group of materials. They can be minerals, metals, ceramics, bio-based (e.g., plant matter), gases, liquids, or even other polymers. Minerals alone account for well over 4000 different distinct species. Any particulate material added to a plastic will behave like filler. For example, anti-block, pigments, impact modifiers, nucleating agents, antioxidant crystals, and numerous other additives will affect the mechanical and other properties of polymers in the same way that filler particles do. Despite the limitless array of potential filler types, the numbers that have achieved wide-scale commercial adoption is far more limited. A multi-billion-euro a year filler market is dominated by fewer than 10 fillers. Elastomers account for approximately 50% of filler usage followed by thermoplastics at 35% and thermosets with 15%. Examples of fillers are carbon black, natural CaCO3, silica, Al(OH)3, talc etc. Regarding the tensile behaviour of the composite, it is given by the shape, concentration and orientation of reinforcement. ❖ The shape of reinforcement particles can be considered approximately as a sphere (the powder form of reinforcement) or as a cylinder (fibres). Their size and distribution then determine the texture of the composite.

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229

❖ The concentration is a density of the reinforcing phase, expressed in terms of volume or the quantity of weight. It is one of the most important parameters that affect the properties of composite material. ❖ The orientation of the reinforcing phase affects the isotropy of the system. If the reinforcing particles have the same shape and dimensions in all directions (for example powders), the composite behaves basically as an isotropic material, therefore its properties are the same in all directions. On the contrary, systems reinforced with cylindrical reinforcement (fibres) show anisotropy of properties.

3.3 3.3.1

Different types of fillers Natural and renewable fillers

For several decades, natural fibers have been proposed as fillers for thermoplastics [48]. They provide reasonable properties and have low density compared to mineral fillers. Fibers include sisal, jute, coir, flax, and wood. Chemically they are composed of lignin and cellulose where the lignin is rather unstable toward heating and begins to decompose near 200  C in air as determined by loss in weight via thermogravimetric analysis (TGA). Moreover, the fiber may lose its strength at 160  C [48]. This instability limits the use of natural fibers. Other problems include high and variable water content plus the inconsistency associated with fibers from plants, which change depending upon the weather and season. If the use of such fibers had been compelling, then they would have enjoyed commercial adoption decades ago. Instead, the present interest in them is due to a perceived environmental advantage. In fact, it can be argued that using natural and renewable fibers may actually be harmful from an environmental perspective. Whereas unfilled or mineral filled thermoplastics can be recycled numerous times without degradation of properties. Natural fiber filled plastics cannot be easily recycled due to thermal degradation of the fibers during extrusion. Only life cycle analysis (LCA) can conclusively determine whether such fillers are actually good or bad for the environment. One area where natural fillers have attained commercial success is in plastic lumber for decking which is particularly popular in the USA. By filling polyethylene or polypropylene with wood flour, one can achieve the look of wood but without the need to maintain the product, for example by varnishing. Often such plastic lumber is optimized through use of recycled polymer and via foaming to reduce materials cost.

3.3.2

Zeolites

Zeolites, also known as molecular sieves, are inorganic substances with a nanoporous structure [49] such that molecules preferentially adsorb within the pores depending on their size and polarity. A well-known example is the use of 4 A molecular sieves to remove traces of water from solvents. More recently, they have been used commercially to adsorb bad odors or other unwanted volatile substances from plastic films and articles.

230

3.3.3

Interfaces in Particle and Fibre Reinforced Composites

Dense fillers

Dense fillers are used when heft, weight, or sound/vibration damping is required. Heft is the perception of quality associated with products that feel substantial in the hand. Dense formulations are used in washing machine counterweights to reduce vibration. Formulations include iron slag in polypropylene or epoxy. Dense fillers include barium sulfate (density 4.5 g cm3), magnetite (density 5.0 g cm3), icaceous iron oxide (density 5.0 g cm3) and metals (density 8.20 g cm3).

3.3.4

Expandable microspheres

These specialty fillers are comprised of a polymeric shell surrounding a core of lower molecular weight substance [50], typically a member of the alkane family. Upon heating to a temperature near the boiling point of the encapsulated substance, the particles expand dramatically due to the high vapor pressure as the boiling point is approached. By expanding such beads within a polymeric matrix, a syntactic foam is created.

3.3.5

Nano-fillers

Nanometer scale reinforcing particles have attracted considerable attention from polymer scientists. Due to their high aspect ratio (surface/area ratio) and low density, they may be used as substitutes for traditional fibers as fillers in polymer matrices. The most common reinforcements at nanoscale are inorganic clay minerals consisting of silicate layers [51e54]. The excitation that followed the discovery of the possibility to prepare multiwall carbon nanotubes (CNTs) and other carbon nano-structured materials via a catalyst-free process [55] has inspired scientists for a range of potential applications. The fundamentals as described here for traditional microcomposites all apply but allowance has to be made for the smaller particle size, added surface area and, in the case of nanoclays, the high aspect ratio.

3.3.5.1 Cellulose-based nanofillers Cellulose microfibrils and nanocrystalline celluloses or cellulose nanowhiskers (CNWs) are the two types of nano reinforcements obtained from cellulose [51]. Cellulose microfibrils consist of bundles of molecules that are elongated and stabilized through hydrogen bonding [52]. The typical dimensions of these nanofibrils are 2e20 nm in diameters, while the lengths are in micrometer range. Also, these fibrils consist of both amorphous and crystalline regions. The crystalline regions can be isolated by various techniques and resultant material is known as whiskers. These whiskers are also known as nanorods and nanocrystals. The lengths of these whiskers typically range from 500 nm to 1e2l mm in length and diameter in the range of 8e20 nm [53]. Also, it was found that cellulose crystals have a modulus of around 150 GPa and a strength of 10 GPa [54]. This is a very interesting data as it suggests that cellulose can replace single-walled carbon nanotubes (SWCNTs) in many

Characterization studies of polymer-based composites

231

applications. Acid hydrolysis is the most widely used method for extracting CNW, which removes the amorphous regions while crystalline regions remain intact [55].

3.3.5.2 Clay Layered silicates, also known as nanoclays, are most commonly utilized nanofillers in the synthesis of polymer layered silicate nanocomposites. Among these layered silicates, phyllosilicates (2:1) are extensively used in preparing clay-based nanocomposites. The crystal arrangement in the silicate layers is made up of two tetrahedrally coordinated atoms amalgamated to edge-shared octahedral sheets. The dispersibility of layered silicates into individual layers is governed by its own ability for surface modification via ion exchange reactions that can replace interlayer inorganic ions with organic cations. Renewable polyesters are mostly organophilic compounds, while the pristine silicate layers are miscible only with hydrophilic polymers. The silicate layers can be made miscible with hydrophobic polymer by introducing/exchanging interlayer cation galleries (Naþ, Ca2þ etc.) of layered silicates with organic compounds.

3.3.6

Molecular fillers

Polyhedral oligomeric silsesquioxanes are a family of molecules that consist of a silica-like core surrounded by a shell of organic groups. Conceptually, they can be considered to be the smallest possible particle of silica, which has been surface treated either with dispersant or coupling agent depending upon the type. The term “molicle” has been coined to describe these hybrid materials [56,57] which combine the solubility of organic molecules and the rigidity of inorganic particles. Other molecular fillers include the fullerenes such as C60, C70 and their derivatives. Like polyhedral oligomeric silsesquioxanes their commercial application is severely limited by high cost.

3.3.7

Functional fillers

Now-a-days, medical implants are very common in practice that utilizes a wide range of biocompatible materials such as metals, alloys, ceramics, polymers, and composites [58]. Among them, bio nanocomposites that are fabricated using the combination of biopolymers and various nanostructured inorganic/organic functional fillers receive extensive attention due to their diversified biomedical as well as biotechnological application [59]. Nanostructured fillers play an important role in biocomposite fabrication, since they bring various desired functionalities to the composites [59]. Functional nanofillers such as cellulose nanofibers, hydroxyapatite (HAp), layered double hydroxides (LDHs), silica nanoparticles, and polyhedral oligomeric silsesquioxanes (POSSs) are mostly investigated for this proposes [60]. Recently, HAp and LDH have received more attention due to their versatility in the fabrication of various nanocomposites for biomedical application [59] (Table 9.3).

232

Interfaces in Particle and Fibre Reinforced Composites

Table 9.3 Fillers, shape and applications. Filler

Shape

Application

Calcite CaCO3

Blocky

Mechanicals (PP,PE)

Talc Mg3(Si4O10)(OH)2

Platy

Mechanicals (PP,PE,nylons)

Mica KM(AlSi3O10)(OH)2

Platy

Mechanicals (PP,nylons)

Wollastonite CaSiO3

Acicular

Mechanicals (PP)

Kaolin Al2O3 2SiO2 2H2O

Platy

Mechanicals (PE,elastomers)

Dolomite CaCO3$MgCO3

Blocky

Mechanicals (PP,PE)

Glass fiber SiO2

Fibrous

Mechanicals (PP,nylons,PBT)

Carbon black

Variable

Processing (elastomers)

Barites BaSO4

Blocky

Sound

Magnetite Fe3O4

Blocky

Sound (PP, nylons)

Graphite

Platy

Conductivity (lubrication)

ATH Al(OH)3

Platy

Flame retardant (Elastomers, PE)

Magnesium Hydroxide Mg(OH)2

Platy

Flame retardant (PE,EVA,PP)

4.

Characterization of polymer composites for filler matrix interface

4.1

SEM and TEM analysis

In the introduction section we pointed out the importance of evaluating the degree of dispersion of fillers within the polymer matrix and how this can affect different properties of nanocomposites. SEM is a powerful tool as it enables us to study the dispersion at several dimensions, with resolution ranging from those obtained using high-magnification light microscopy up to resolution typically encountered in lowmagnification TEM. However, like most imaging techniques, care must be taking during imaging acquisition and with the interpretation because of intrinsic artifacts.

Characterization studies of polymer-based composites

233

The SEM technique that proved to be the most convenient to study dispersion relied on voltage contrast imaging. Electron microscopes use electrons for imaging, in a similar way that light microscopes use visible light. A Scanning Electron Microscope (SEM) is a tool for seeing the invisible worlds of microspace (1 mm ¼ 106 m) and nanospace (1 nm ¼ 109 m). SEM uses a specific set of coils to scan the beam in a raster-like pattern and use the electrons that are reflected or knocked off the near-surface region of a sample to form an image. Since the wavelength of electrons is much smaller than the wavelength of light, the resolution of SEM is superior to that of a light microscope. By using a focused beam of electrons, the SEM reveals details and complexity inaccessible by light microscopy. SEMs can magnify an object from about 10 times up to 300,000 times. A scale bar is often provided on a SEM image. From this the actual size of structures in the image can be calculated. Essentially, the way the scanning electron microscope “looks” at the surface can be compared to a person alone in a dark room using a fine beamed torch to scan for objects on a wall. By scanning systematically from side-to-side and gradually moving down the wall, an image of the objects in their memory. The SEM uses an electron beam instead of a torch, an electron detector instead of eyes, and a viewing screen and camera as memory can be built. The transmission electron microscope is a very powerful tool for material science. A high energy beam of electrons is shone through a very thin sample and the interactions between the electrons and the atoms can be used to observe features in the crystal structure like dislocations and grain boundaries. Chemical analysis can also be performed. TEM can be used to study the growth of layers, their composition and defects in semiconductors. High resolution can be used to analyze the quality, shape, size and density of quantum wells, wires and dots. By combining SEM techniques, detailed information of the nanocomposite morphology, conductivity and even of modes of fracture can be obtained at the nanometer scale. Mazhar Ul-Islam et al. [61] demonstrated the surface and cross section morphologies of (A, B) pure BC (Bacterial Cellulose); (C, D) BCeMMT1(montmorillolite); (E, F) BCeMMT2; and (G, H) BCeMMT3 by impregnation of BC sheets in respective concentrations (1%, 2% and 4%) of MMT suspensions at 150 rpm for 24 h in Fig. 9.5. M.A. Lopez Manchado et al. [62] have obtained a homogeneous dispersion of CNFs into an epoxy network as well as producing a strong interface between the fillers and polymer matrix by means of CNFs functionalization. Fig. 9.6 shows SEM images of 0.5% and 1% PPe SWNT composites. For the 1% concentration (Fig. 9.6(b)), we can see an aggregate in which we observe a large amount of SWNTs that are self-organized in bundles. In the 0.5% concentration sample (Fig. 9.6(a)) we notice a more uniform distribution of the bundles with a small quantity of aggregates. Fig. 9.7(a) shows TEM image of the original nanotubes. As can be seen in this figure, the carbon nanotubes have outside diameters of about 1.5e2 nm. TEM image of bare carbon black (CB) is shown on Fig. 9.7(b). From this image it is clear how CB are nanoparticles with a diameter of about 100 nm, which form aggregates. B.J. Ash et al. [63] have synthesized Alumina/polymethylmethacrylate (PMMA) nanocomposites using 39-nm sized nanoparticles and in situ freeradical polymerization. The PMMA matrix in both the neat PMMA and in nanocomposites has a molecular weight of approximately 1,50,000, average polydispersity of 1.5%, and 57%

234

Interfaces in Particle and Fibre Reinforced Composites

Fig. 9.5 FE-SEM micrographs of the surface and cross section morphology of the (a and b) pure BC; (c and d) BCeMMT1; (e and f) BCeMMT2; and (g and h) BCeMMT3. The BCeMMT composites (BCeMMT1, BCeMMT2 and BCeMMT3). Source: M. Ul-Islam, T. Khana, J.K. Parka, Nanoreinforced bacterial celluloseemontmorillonite composites for biomedical applications, Carbohydr. Polym. 89 (2012) 1189e1197. Reproduced with permission from Copyright © 2012 Elsevier Ltd.

syndiotactic at an average as previously reported [64]. The composites exhibit excellent dispersion as shown in the TEM micrograph of Fig. 9.8.

4.2

Atomic force microscopy (AFM)

Atomic force microscope is an useful technique to determine the surface roughness of polymer composites. Its advantages such as high resolution and non-destructivity offer a unique possibility for repetitive examinations. The force modulation mode

Fig. 9.6 SEM images of: (a) 0.5%, (b) 1% PPeSWNT composites. The scale bar indicates 500 nm. Source: M.A. Lopez Manchado, L. Valentini, J. Biagiotti, J.M. Kenny, Thermal and mechanical properties of single-walled carbon nanotubesepolypropylene composites prepared by melt processing, Carbon 43 (2005) 1499e1505. Copyright © 2008 Elsevier Ltd.

Fig. 9.7 TEM micrographs showing (a) bare Carbolex SWNT bundles and (b) bare carbon black. Source: M.A. Lopez Manchado, L. Valentini, J. Biagiotti, J.M. Kenny, Thermal and mechanical properties of single-walled carbon nanotubesepolypropylene composites prepared by melt processing, Carbon 43 (2005) 1499e1505. Copyright © 2008 Elsevier Ltd.

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 9.8 TEM micrograph showing the dispersion of as-received nanoparticles in the polymer matrix at 5 wt%. The scale bar represents 2 mm. Source: B.J. Ash, L.S. Schadler, R.W. Siegel, Glass transition behavior of alumina/polymethyl methacrylate nanocomposites, Mater. Lett. 55 (2002) 83e87. Copyright © 2002 Elsevier Science B.V.

gives a qualitative statement about the local sample surface elasticity using an oscillating cantilever tip, which indents into the sample surface. The amplitude of this deflection is measured as a function of tip position when cantilever tip indents cyclically into the surface. AFM utilizes much smaller forces between the tip and specimen typically 0.1 pNl and the smaller radius of curvature of the tip gives better spatial resolution. Ebru G€ unister et al. [65] used AFM to characterize the morphology of MMT/chitosan biocomposite systems. Fig. 9.9(a) and (b) shows, AFM image of the surface of 10 g/g (MMT/chitosan) and 50 g/g (MMT/chitosan), respectively. When a layered structure has been observed at Fig. 9.9(a), the clusters formed a layered structure by increasing the clay particles as in Fig. 9.9(b). B. Kumar et al. [66] have investigated the role of poly(lactic acid)/carbon nanotube (PLA/CNT) conductive biopolymer nanocomposites (CPC) and its crystallinity towards vapour molecule sensing. PLA/CNT CPC based sensor was prepared via spray layer by layer technique using pre-dispersed CNT in PLA solution and its novel potential as smart material for vapour sensing have been revealed. Interestingly, influences of vapour nature, filler concentration and crystallization have been envisaged. Nanoscale characteristics of surface morphology of CNT and PLA/CNT CPC have been visualized by atomic force microscopy (AFM). Primarily, surface morphology of pristine CNT, pristine PLA and PLA/3%CNT before and after annealing treatment at nanoscale have been investigated by AFM as shown in Figs 9.10 and 9.11. From Fig. 9.10(a), it is possible to measure CNT diameter (found to be close to 10 nm as claimed by the provider), whereas Fig. 9.10(b) showed dark dots remaining from

Characterization studies of polymer-based composites

(a)

2.00 μm

237

(b)

5.00 μm ¥ 5.00 μm

1.40

1.24

μm

μm

0 00

2.00 μm

5.00 μm ¥ 5.00 μm

0 00

Fig. 9.9 Atomic force microscopy image of (a) 10 g/g and (b) 50 g/g (MMT/chitosan) dispersions. € u, O. Atıcı, N. G€ Source: E. G€unister, D. Pestreli, C.H. Unl€ ung€ or, Synthesis and characterization of chitosan-MMT biocomposite systems, Carbohydr. Polym. 67 (2007) 358e365. Reproduced with permission from Copyright © 2006 Elsevier Ltd.

(a)

(b)

0.5 μm

0.5 μm

(d) Height (nm)

Height (nm)

(c)

Distance (μm)

Distance (μm)

Fig. 9.10 AFM images (Z scale e 40 nm) of pristine CNT (a) and PLA (b) with corresponding height graph (c and d). Source: B. Kumar, M. Castro, J.F. Feller, Poly (lactic acid)emulti-wall carbon nanotube conductive biopolymer nanocomposite vapour sensors, Sens. Actuators B 161 (2012) 621e628. Reproduced with permission from Copyright © 2011 Elsevier Ltd.

238

Interfaces in Particle and Fibre Reinforced Composites

Fig. 9.11 AFM height images (Z scale e 50 nm) of PLA/3%CNT: (a) before annealing, (b) after annealing. Source: B. Kumar, M. Castro, J.F. Feller, Poly (lactic acid)emulti-wall carbon nanotube conductive biopolymer nanocomposite vapour sensors, Sens. Actuators B 161 (2012) 621e628. Reproduced with permission from Copyright © 2011 Elsevier Ltd.

solvent evaporation during sample preparation. On the other hand, it is likely that these dots correspond to nanopores also present in PLA thin films. In fact CNT are mostly embedded in the PLA matrix creating conductive pathways and only emerging in some places as clearly seen by AFM in Fig. 9.11(a). AFM is also used to determine the interface thickness based on the load variations where it is being used as an input parameter for finite element analysis/simulations.

4.3

Thermal analysis

Thermal analysis is a sophisticated yet easy-to-use series of analytical techniques for characterizing composite materials. These techniques can provide information for materials development and selection process optimization, engineering design and prediction of end-use performance. They can also be used to test materials for consistency against specifications and to trouble shoot processing problems. Thermal analysis instruments measure changes in physical or reactive properties of materials as functions of time and temperature.

4.3.1

Differential scanning calorimetry (DSC)

This is the measurement of heat flow into or out of a sample providing data relating to curing rate, degree of cure, Tg, vitrification temperature, percentage crystallinity, melting points and specific heats. DSC can be used to study cross-linking reaction kinetics, as well as to determine resin/substrate uniformity. Pressure DSC can determine the oxidative stability of a material and analyse pressure-sensitive reactions.

239

Heat flow

Characterization studies of polymer-based composites

PLA/3%CNT PLA/3%CNT (annealed) 0

20

40

60

80

100

120

140

160

180

200

Temperature (°C)

Fig. 9.12 DSC curve of PLA/3%CNT and annealed PLA/3%CNT. Source: B. Kumar, M. Castro, J.F. Feller, Poly (lactic acid)emulti-wall carbon nanotube conductive biopolymer nanocomposite vapour sensors, Sens. Actuators B 161 (2012) 621e628, Fig. 6. Reproduced with permission from Copyright © 2011 Elsevier Ltd.

B. Kumar et al. [66] reported about modification induced by this thermal treatment on the crystallinity of PLA/CNT CPC as well as important changes in vapour sensing behavior which is shown in Fig. 9.12.

4.3.2

Thermogravimetric analysis (TGA)

By measuring the amount and rate of change in the weight of a sample as temperature is varied, definitive data for materials selection and product design may be obtained. TGA decomposition kinetics can provide stability information within hours, compared to the months required for oven ageing tests. E. Lester et al. [67] have tested samples of glass wool and sand (post-treatment), along with the treated and untreated samples using a PerkinElmer Pyris 1 to determine the composition on a weight basis and also on a mass number basis for comparison. Fig. 9.13(a) shows thermogravimetric analysis (TGA) curves for the carbon fibre composites-treated, untreated and virgin fibre. Fig. 9.13(b) shows the weight loss of the sand and glass wool during heating in the TGA.

4.4

X-ray diffraction (XRD)

XRD technique utilizes the X-ray scattering phenomenon to elucidate the crystal structure of crystalline/semicrystalline materials, with scattering of X-rays by periodic array of atoms giving rise to definite diffraction patterns that bestows a qualitative image of atomic arrangements within the crystal lattice. Powder X-Ray Diffraction (P-XRD) is one such characterization tool that offers the advantage

240

Interfaces in Particle and Fibre Reinforced Composites

(b)

(a)

95

85

85

Weight (%)

105

95 Weight (%)

105

75 65 55

75 65 55

45

45 0

200

400

600

800

Temperature (°C) Virgin fibre

Untreated fibre

0

200

400

600

800

1000

Temperature (°C)

Treated fibre Untreated fibres

Sand

Wool

Fig. 9.13 TGA results for (a) The carbon fibres pre and post-treatment (b) The sand, wool (post-treatment) and untreated fibre composite. Source: E. Lestera, S. Kingman, W. Kok Hoong, C. Rudd, S. Pickering, N. Hilal, Microwave heating as a means for carbon fibre recovery from polymer composites: a technical feasibility study, Mater. Res. Bull. 39 (2004) 1549e1556. Reproduced with permission from Copyright © 2004 Elsevier Ltd.

of simultaneous characterizing of both the precursor and end products with a detailed qualitative presentation of their micro-structural behaviours. It is the most sought after and convenient technique towards the characterization of polymer based nanocomposite in contrast to single crystal technique, which demands the sample in the form of individual/single/independent crystals. Furthermore, XRD is a versatile, non-destructive characterization technique that offers a thorough output of chemical composition and hence the crystallographic structure of materials [68e70]. XRD is the most commonly used method to investigate the structural features of nanocomposites due to its ease and availability. XRD is used to evaluate the crystalline structure, the ratio of crystalline to non-crystalline (amorphous) regions, crystal size, the arrangement pattern of crystals and the distance between the planes of the crystal [71]. This means that structural changes induced in a crystalline material by blending with other materials can be monitored using the XRD technique. Mazhar Ul-Islama et al. [61] have investigated the micro-structural changes using XRD in the BC sheets caused by the adsorption and penetration of MMT particles. The XRD patterns of BC (bacterial cellulose), MMT (montmorillolite), BCeMMT1, BCeMMT2 and BCe MMT3 are shown in Fig. 9.14. The BCeMMT composites (BCeMMT1, BCeMMT2 and BCeMMT3) were prepared by impregnation of BC sheets in respective concentrations (1, 2 and 4%) of MMT suspensions at 150 rpm for 24 h. The smaller angle (7e10 degrees) XRD patterns for all these samples have been shown in the upper left corner.

Characterization studies of polymer-based composites

241

Intensity (a.u)

MMT BC-MMT1 BC-MMT2 BC-MMT3 BC

10

20

30

40

50

60

70

80

2ϴ (degrees)

Fig. 9.14 XRD patterns of pure BC, pure MMT, BCeMMT1, BCeMMT2 and BCeMMT3. Source: M. Ul-Islam, T. Khana, J.K. Parka, Nanoreinforced bacterial celluloseemontmorillonite composites for biomedical applications, Carbohydr. Polym. 89 (2012) 1189e1197. Reproduced with permission from Copyright © 2012 Elsevier Ltd.

In general, the rate of crystallization and the degree of crystallinity are affected by the addition of fillers. M.A. Lopez Manchado et al. [62] have used single-walled nanotubes (SWNTs) as filler. However, no significant change in the polypropylene (PP) crystalline structure was observed in the presence of SWNTs, as shown in Fig. 9.15. The set of reflections proves unambiguously that isotactic polypropylene (iPP) remains primarily in monoclinic form (a-form) [72]. These changes in PP crystallization from the addition of carbon nanotubes are of crucial importance for interpreting the mechanical behaviour of the composites.

4.5

FTIR analysis

The FTIR spectra (vibrational spectra) of polymer composites can be used to identify bands associated with vibrational modes of functional groups of both polymer chains and filler particles. The analysis of vibrational spectra can provide information on interaction between the organic and inorganic phases, the state of intercalation and exfoliation in polymer composites containing layered silicates, filler dispersion and functionalization, the degree of orientation of both the polymer chains and the reinforcing anisometric particles. Based on the peak shifts, it could be postulated that the possible hydrogen bonding formation while appearance of new peaks could denote to the interaction between matrix and filler via other chemical interactions (covalent bonding, etc.). Ashutosh Tiwari et al. [73] have used FTIR spectra of acacia gum, silica sol and AgSiO2 composite were recorded on PerkinElmer PKe 1310 instrument by making

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6 PP

5

PP-CB

Intensity (u.a.)

PP-SWNTs

4

3

2

1

0

4

8

12

16

20

24

28

32

2q

Fig. 9.15 X-ray diffractograms of pure PP and its composites filled with 0.5 wt%. Source: M.A. Lopez Manchado, L. Valentini, J. Biagiotti, J.M. Kenny, Thermal and mechanical properties of single-walled carbon nanotubesepolypropylene composites prepared by melt processing, Carbon 43 (2005) 1499e1505. Reproduced with permission from Copyright © 2008 Elsevier Ltd.

pellet with dehydrated KBr in reflectance mode. Fig. 9.16 shows the FTIR spectra of Acacia gum (curve-A), silica sol (curve-B) and AgSiO2 composite (curve-C). Mazhar Ul-Islam et al. [61] have recorded FT-IR spectra of the dried pure BC (bacterial cellulose) and BCeMMT2 by using a PerkinElmer FTIR spectrophotometer. FT-IR analyses of the pure BC and the BCeMMT composites were carried out in order to verify the composition of BCeMMT composite by analyzing specific peak positions. The BCeMMT2 was prepared by impregnation of BC sheets in 2% MMT suspensions at 150 rpm for 24 h (Fig. 9.17).

4.6

XPS analysis

In X-ray photoelectron spectroscopy (XPS), a solid sample is irradiated with soft X-rays resulting in the emission of electrons (photoelectrons) from atoms close to the sample surface. The measured photoelectron kinetic energies are diagnostic of the specific atoms which constitute the sampled depth (250 C

ðNH4 PO3 Þ ƒƒƒ! ðHPO3 Þ þ C  ðH2 OÞ/ðCx Þ þ ðHPO3 Þ$H2 O
NH3

(12.6)

In the volatile phase, phosphorus-based flame retardants form active radicals of PO2 $ , PO$ and HPO$ . These radicals are released at temperature below the polymer decomposition temperature and acts as scavengers of H$ and OH$ radicals in the flame, which aid in terminating the combustion process [25,31]. Several researches on incorporation of APP into NFRC have reported effective improvement in flammability resistant, inclusive of UL94-V0 rating in ramie fibre

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reinforced PLA composite with 10.5 wt% of APP [76] and kenaf fibre reinforced PLA composite with greater than 20 wt% of APP [77], kenaf fibre filled PP composite with 20e30 phr of APP [78]; flax fibre filled unsaturated polyester resin with APP ranging from 25 to 186 phr [79] amongst others.

3.1.6

Halogenated flame retardant

Halogenated flame retardants such as compounds containing organobromine (polybrominated diphenyl ethers PBDE, hexabromocyclododecanes HBCD, and tetrabromobisphenol-A TBPA) and organochlorine (chlorinated paraffins, antimony trichloride, etc.) can readily releases highly reactive free radical species through thermal decomposition to form HCl and hydrogen bromide (HBr). These species react with the H$ and OH$ radicals that sustain combustion through a cascaded chain mechanism in the flame zone to yield inactive H2 and H2O along with evolution of Cl and Br radical, in a cascaded scheme similar to Eq. (12.7), hence discontinuing the chain decomposition and the combustion process [70]. R-X / R* þ X* X* þ RH / R* þ HX HX þ H* / H2 þ X* HX þ OH* / H2O þ X*

(12.7)

where R-X ¼ flame retardant and X* ¼ Cl or Br. Fluorinated and iodinated compounds are not used as flame retardants owing the high thermal stability of the fluorinated compound which does not release halogen radicals at common combustion temperature while the iodinated compounds are much less stable than most polymer materials which tends to release halogen radicals during processing [80]. It is worthwhile to note that many halogenated flame retardants contain aromatic rings, Fig. 12.3, that degrade into dioxin or dioxin-like toxic compounds and hence shall be used with caution.

3.1.7

Hybrid flame retardants with synergistic effect

In order to further improve the flame resistivity of conventional flame retardants without scarifying the mechanical properties of NFRC, several researchers have Br Br Br

Br

Br OH

HO

Br

Br TBBPA

Br

Br Br O

Br

Br

Br Br PBDE

Br

Br

Br

Br

O O

Br Br

Br

Br

Br HBCD

Br TBPA

Fig. 12.3 Common organobromine halogenated flame retardant with aromatic rings.

O

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Interfaces in Particle and Fibre Reinforced Composites

attempted hybridisation of flame retardants and there have been considerable success with the establishment of synergistic effects between the difference flame retardants. Khalili and co-workers reported the synergistic between APP and ATH flame retardants on enhance fire resistant of palm EFB fibre filled epoxy composite at the combination of 10 wt% APP with 5 wt% ATH, where self-extinguishing behaviour was observed in vertical Bunsen burner test [81]. In the event of fire, chemical reaction first occurred between the intumescent APP (NH4PO3) and the hydroxyl
OH groups of ATH at temperature below 400 C, resulting in the formation of aluminum phosphate, ammonia (NH3) and water compound. The creation of aluminum phosphate hinders evaporation of phosphorous volatility while the ATH can readily furnish proton exchange to provide a more extensive evolution of water and NH3 gas, which cool and dilute the developing flame. The resulting aluminum phosphate can repeatedly react with APP in the system to yield long chain aluminum metaphosphate [Al(PO3)3]n. At temperature above 800  C, chain scission of aluminum polyphosphate took place, forming aluminum orthophosphate (AlPO4) along with evolution of NH3 gas and water compound, Fig. 12.4 [81,82]. Recently many researches have been focused on the synergistic effect of the combination of EG and intumescent flame retardant (IFR). For instances, the effects of APP modified with 3-(methylacryloxyl) propyltrimethoxy silane (M-APP) [83] and O

O

O

O

P O P O P

Al(OH)3

ONH4 ONH4 ONH4

O

(APP)

O

O

O

O

ONH4 ONH4

Al(OH)2 Aluminium phosphate O

O

O

O

P O P O P

P O P O P

O

O

ONH4 ONH4

ONH4 ONH4

NH3

AlOH

Al(OH)2 ONH4 ONH4 ONH4

O

P O P O P

P O P O P

O

O

O

H 2O

NH3

P O P O P

O

H2O

–(mH2O+mNH3)

[Al(PO3)3])n

ONH4 ONH4

O

O

APP + ATH

(APP)

Reactions between ATH and APP O

O

O

O

Al(OH)3

P O P O P ONH4

ONH4

ONH4

ONH4

ONH4

HO

O

Al(OH)2

P OH

AlPO4

NH3

P O ONH4

(APP)

O

P O P O

Al(OH)2

ONH4

ONH4

(APP)

O

O

O

P O P O

H2O

ONH4

(APP)

Fig. 12.4 Synergistic reaction between APP and ATH at elevated temperature [82]. Copyright (2005), reprinted with permission from Elsevier.

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EG and the effects of APP, EG and char forming agent [84] on the flame retardancy and mechanical properties of wood flour PP composites have been investigated. In both cases, the tensile strength and flexural strength decreased obviously when EG was incorporated into wood flour PP composites. Similar results were reported by Ref. [65]. Comparison with the LOI of composites with individual filler only, EG was more effective than APP in improving the LOI of wood flour PP composites even though both composites could pass the UL-94 vertical tests of V-0 and V-1 rating. When both APP and EG were incorporated into the composites, the LOI increased with the EG content and the synergistic effect reached a maximum when a right ratio was used. The synergistic flame retardant properties could be explained as follow: With the addition of EG, the brittle char layers emerged during burning. When APP was added, the structure of char layer was much stronger than before, which isolated the air, hence the dual-flame retardant systems showed higher efficiency.

3.2

Nanoscale particulates flame retardant additives

Conventional flame retardants such as halogen-based flame retardants have been widely used and they continue to play a large part in flame retardancy. However, these compounds are persistent organic pollutants of global concern as they generate corrosive and toxic combustion products (tricyclic aromatic compounds), such as dioxins (polyhalogenated dibenzo-p-dioxins) and furans (polyhalogenated dibenzo-p-furans) [85]. The alternative methods involve halogen-free flame retardants such as ATH and magnesium hydroxide. However these additives require high loading (>60 wt%) [86,87] for a satisfactory flame-retarding benefits and this often has deleterious effects on the host polymer’s mechanical and electrical properties. Phosphorusand nitrogen-based flame retardants could be the options, but these materials have the same limitations as the mineral fillers where relatively large amount of addition of these materials would lower the mechanical properties of the composites [88]. By considering the environmental factors along with the mechanical and physical properties required for end applications as well as the processing difficulties, the window of flame retardants options becomes too narrow. In recent years, several researchers have begun to look into the incorporation of nanoscale particular additives such as carbon nanotube, graphene and nanoclays to produce intumescent flame retardant nanocomposites [89]. Polymer nanocomposites have attracted great attention in materials science because they often exhibit improved properties compared to conventional composites. A polymer nanocomposite in this context is defined as a two-phase material whose filler is dispersed throughout the polymer on a nanoscale. Nanoscale particulate additives offer several advantages, including higher aspect ratios. When these nanofillers are well dispersed in polymer matrix, the polymer nanocomposites show macro/micro/nanointerfaces, hence a significant improvement in mechanical properties compared to conventional fillers can be observed [86]. Furthermore, they can be used in smaller amount (2e10 wt%) which give a lower viscosity and density to the final nanocomposite materials. The advantages of lower filler requirements include high processability of the polymer and more importantly less embrittlement than conventional composites with equivalent

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Interfaces in Particle and Fibre Reinforced Composites

mechanical properties. The incorporation of nanofillers into polymers has improved not only the mechanical properties but also reduced the flammability of the composites. It has been recognized that the use of nanofillers as flame retardants in polymer composites has demonstrated a significantly reduced PHRR, a slower combustion process and a decrease in the speed at which flames spread throughout them [85]. The flame retardancy mechanism of nanocomposites that was proposed involves the formation of inorganicorganic protective residue layers on top of the burning material. This protective layer insulates the underlying materials from the heat flux of the flame, reduces the transport of heat to the pyrolysis front and the volatiles into the flame zone. To maximize the protection, the structure and surface of the residue layers have to be dense and closed. Therefore a good dispersion of nanoparticles in the polymer is the main keys for improving fire retardancy [90]. In this section, emphasis will be laid on the recent advances on the understanding of fire behaviour of polymer nanocomposites. Various kinds of nanofillers as flame retardants, for example, layer particles such as layered silicates, tubular particles such as carbon nanotubes and other carbon-family nanomaterials will be discussed.

3.2.1

Layered silicates

On recent years, the growing public awareness about toxic gases and smoke evolved during the burning of halogen-based flame retardants have been increased. Research interest has been shifting away from the use of conventional flame retardants and leading to worldwide adoption of halogen-free flame retardants. Layered silicate nanoparticles have been the alternative approach as the polymer nanocomposites precursors to improve the flame retarding properties of polymer for more than two decades [91]. These layered silicate nanoparticles are inexpensive, abundant and their intercalation chemistry has been explored extensively [92]. Most importantly, their large surface area provide sufficient interfacial regions in the nanocomposites, resulting in an improvement of thermal and mechanical properties even at a low loading into the polymer matrix [93]. The most common layered silicates used are montmorillonite (MMT), hectorite and saponite [93,94]. They belong to the same general family known as 2:1 phyllosilicates. The crystal structure of these layered silicates is made up of two-dimensional layers where a central octahedral sheet of either aluminium or magnesium hydroxide is sandwiched within two tetrahedrally coordinated silicon atoms. The layer thickness is around 1 nm with the lateral dimensions between 30 nm to a few microns or larger. These nanolayers stack together with a regular van der Waals gap in between them, which is called the interlayer or the gallery [94,95]. The face-to-face stacking of nanolayers makes the dispersion of the agglomerated tactoids into discrete monolayers very difficult. What is more, the dispersion of layered silicates in polymer matrices is further restricted by the intrinsic incompatibility of hydrophilic layered silicates and hydrophobic engineering polymers. However, the forces that hold the nanolayers together are relatively weak, so it is possible to intercalate small molecules between the nanolayers. In order to develop the high performance clay-polymer nanocomposites, it is important to organically modify the layered silicates by substituting the hydrated cations in the gallery with cationic surfactants such as alkylammonium or alkylphosphonium [92].

Advancement in flame retardancy of natural fibre reinforced composites

Layered silicates

Conventional nanocomposites

329

Polymer

Intercalated nanocomposites

Exfoliated nanocomposites

Fig. 12.5 Three types of nanocomposites arising from the interaction of layered silicates and polymer.

Generally, the dispersion of layered silicates in monomer or polymer matrix can result in the formation of three main types of nanocomposites, namely conventional, intercalated and exfoliated, as shown in Fig. 12.5. The conventional nanocomposites contain clay tactoids that are dispersed as a segregated phase in the polymer matrix. Intercalated nanocomposites are formed when one or more molecular layers of polymer are inserted between the nanolayers resulting in a well-arranged multilayer structure of alternating polymeric and clay layers. The last type is exfoliated clay-polymer nanocomposite where the layered silicates are uniformly dispersed in the continuous polymer matrix. This type of nanocomposite is desirable as it maximizes the interactions between silicates and polymer, giving huge interfacial area between silicates and polymer [91,96]. The flame retarding properties of nanocomposites containing nanoclay have been widely explored. In general, the nanoclay builds up a carbonaceous silicate char on the surface of nanocomposites during burning which creates a continuous protective barrier that shields the underlying material from the heat flux of the flame and acts as a mass transfer barrier, hence increases the flame resistance property of the nanocomposites [85,97,98]. However, to the best of our knowledge, research on introducing nanoclay particles into NFRC has not been extensively reported. Guo and co-workers [99] investigated the effect of nanoclay on flame retardancy of extruded metallocene polyethylene-wood fibre nanocomposites. By incorporating 5 wt% nanoclay into the wood fibre composites, it was found that char was formed during burning and hence retarding the flame and preventing the fire from spreading. The burning rate was lower than that for the composites without nanoclay. Another polymer, high density polyethylene (HDPE) has also been reported to form nanoclay-wood fibre nanocomposites via an intermeshing and corotating twin-screw extrusion. Maleic anhydride-graft-HDPE (MA-g-PE) was used as a compatibilizer for preparing HDPE nanocomposites [87]. X-ray diffraction has been used to study

330

Interfaces in Particle and Fibre Reinforced Composites

the spatial correlations of the layered nanoparticles and their interlayer spacing. HDPE nanocomposites without coupling agent (conventional) showed the same characteristic peak as that of pristine MMT nanoclay while for intercalated HDPE nanocomposites, the characteristic peak was shifted to lower diffraction angle, indicating the polymer chains were intercalated into the layered silicates. When the exfoliated structures were obtained, the characteristic peak of nanoclay could not be detected. In other words, the nanolayers were delaminated into nanoscale platelets and dispersed randomly in the polymer matrix. In term of the flame-retardant properties, the incorporation of wood fiber into HDPE matrix increased the flammability by 50% compared to the neat HDPE, yet the flammability did not change much with the addition of the coupling agent. With the addition of small loading of MMT nanoclay (0.1 wt%) into HDPE, the conventional and intercalated nanocomposites showed similar burning rate compared to the composite without clay while the exfoliated nanocomposites showed decreased flammability. As the nanoclay content increased, the burning rate for all the nanocomposites exhibiting a decreasing trend. Deka and Maji [100] prepared wood polymer nanocomposites by using polymer blend (consisted of HDPE, low-density polyethylene (LDPE), PP and PVC) and wood flour with the co-incorporation of nanoclay and titanium oxide (TiO2) nanopowder via solution blending method. Poly(ethylene-co-glyciidyl methacrylate) (PE-coGMA) was used as the compatibilizer. The disappearance of nanoclay characteristic peak indicated the formation of exfoliated structures in the nanocomposites. It was noticed that both the mechanical and thermal properties of the nanocomposites showed improvement after the addition of compatibilizer and wood fibre into the polymer blend. This could be attributed to the improvement of the interfacial adhesion between polymers and wood as well as the reinforcing effect provided by wood fibre as a load carrier. It was also found that the mechanical and thermal properties were further increased after the addition of fixed level of nanoclay (3 phr) and up to 3 phr of TiO2 nanopowder. Further increase of TiO2 loading resulted in agglomeration of the nanoparticles which led to the decrease in the properties. Similar trend was found in the LOI results where LOI values increased from polymer blend, followed by polymer blend with compatibilizer, polymer blend with wood fiber and a substantial improvement was observed after addition of nanoclay (3 phr) and TiO2 (3 phr). The nanoclay played a role in providing better barrier property to the oxygen and heat which slowed down the burning capacity of the nanocomposites while the TiO2 nanopowder improved the interaction between wood, nanoclay and polymer through its functional groups. A group of researchers in India [101] prepared nanoclay-banana fibre-PP nanocomposites by using melt-blending technique. Maleic anhydride-graft-polypropylene (MA-g-PP) was used as the compatibilizer and was fixed at 5 wt% in the nanocomposites. For the nanocomposites without banana fibre, it was evident that the addition of nanoclay and MA-g-PP into PP matrix increased the mechanical and flexural strength of the composites with the optimum strength achieved at 3 wt% of nanoclay. As expected, the increase of mechanical properties was mainly due to the good dispersion of nanoclay that allowed the insertion of the polymer chains between the nanolayers as well as the compatibilizer that facilitated the expansion of the gallery space of the reinforcing nanoclay. The inclusion of banana fibre into nanoclay-PP composites

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further improved the mechanical properties of the nanocomposites. The tensile and flexural strength showed linear increases with increasing fibre loading from 10 to 30 wt%. The interfacial adhesion between non-polar PP matrix and the polar fibres and nanoclay was balanced by the compatibilizer. Interestingly, the XRD diffractograms in the region 0e10 degrees revealed an intercalated structure for 5 wt% nanoclay-PP nanocomposites and an exfoliated structure for 5 wt% nanoclay 30 wt% banana fibre-PP nanocomposites. Furthermore, the flammability of PP, nanoclay-PP and nanoclay-banana fibre-PP nanocomposites were compared. It was interesting to find that those nanocomposites behaved differently in fire, depending on their respective thermal stabilities. Nanoclay-banana fibre-PP composites indicated a variation in reaction-to-fire parameters with an earlier time to ignition compared to PP and composites without fibre. The other parameters, such as HRR and mass loss rate (MLR) increased with increasing fibre content. Recently, Subasinghe and co-workers [102] reported the synergistic effect of halloysite nanotubes (HNTs) and MMT nanoclay on flame and mechanical properties in an intumescent flame retardant PP-kenaf nanocomposites. The delamination and dispersion of the clay nanolayers with a mixture of exfoliation and reduced intercalation was observed in TEM images, as shown in Fig. 12.6. Various tests were conducted to determine the flammability and thermal properties of the nanocomposites, such as UL-94 vertical burning test, cone calorimeter test and TGA. Compared to APP-kenaf-PP nanocomposites, the addition of HNT and MMT nanoparticles improved the flame-retardant performance by lowering the dripping time. The cone calorimetry results showed slightly delayed time to ignition with the

Fig. 12.6 The TEM image of nanoclay-ammonium polyphosphate (APP)-kenaf-PP nanocomposites [102]. Copyright (2016), reprinted with permission from Taylor & Francis.

332

Interfaces in Particle and Fibre Reinforced Composites

Fig. 12.7 The digital images of cone calorimeter char residues for (a) kenaf-PP, (b) APP-kenafPP, (c) HNT-APP-kenaf-PP and (d) MMT-APP-kenaf-PP nanocomposites [102]. Copyright (2016), reprinted with permission from Taylor & Francis.

incorporation of nanoparticles in the composites. Under constant heat thermooxidation decomposition, the MMT nanocomposites exhibited better flame retardation properties over others. Fig. 12.7 shows the digital images of the formed char residues after cone calorimeter test. It can be seen that the addition of kenaf into PP resulted in the formation of levoglucosan continuous lignocellulosic ash layer at the end of the sample burning. With the inclusion of APP into the kenaf-PP composites, a thin stable char layer was formed. After the addition of nanoparticles into APP-kenaf-PP composites, the structural density of formed char residue showed further increment.

3.2.2

Carbon-family nanomaterials

Carbon is one of the materials that provides the extraordinary opportunity to explore a material in all possible dimensionalities, from zero-dimensional fullerenes, one-dimensional carbon nanotubes (CNTs), two-dimensional graphene and threedimensional diamond and graphite [103]. CNT and graphene, among other carbonbased materials, have aroused a great attention due to their unique properties. CNT with diameters ranging from 1 to 100 nm and lengths up to millimeters [104], possess high aspect ratio, low mass density, high tensile moduli and strength with values

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greater than 1 TPa [105,106]. Graphene, discovered in 2004 [107], has a planar and hexagonal arrangement of carbon atoms which gives a large theoretical specific surface area (2630 m2g
1), high Young’s modulus (similar to CNT) [108], high thermal [109] and electrical conductivity and high intrinsic mobility [110,111]. The abovementioned fascinating properties make carbon-family materials ideal candidate for developing functional polymer composites with enhanced mechanical, thermal, conductivity, gas barrier and flame retardant performances. Different types of polymers like ethylene-vinyl acetate copolymer (EVA) [112,113], linear low density polyethylene (LLDPE) [114], PP [115], polystyrene (PS) [116,117] and nylon [118] have been reported with reduce fire hazards by adding a small loading of CNT. In addition, a lot of articles have been published on the flammability of expandable graphite [66,67,119] and graphene-polymer nanocomposites [89,120,121]. The factors that influence the flammability of polymer nanocomposites include the type of carbon nanomaterials, their loading and the dispersion state in polymer matrices. Yu and co-workers [122] investigated the flame retardancy of ramie-PLA composites with functionalized multi-walled carbon nanotube (MWCNT). MWCNTs were modified with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), which is a type of cyclic phosphate with a diphenyl structure. DOPOs exhibited excellent flame retarding properties in the materials by acting as a flame inhibitor [123]. The flame retardant mechanism involves the release of low molecular weight phosphorous containing species which are able to scavenge the H and OH radicals in the flame [124]. Ramie-PLA composites with 5 wt% MWCNT-DOPOs showed increased LOI value (26.4%) compared to pristine MWCNT (21.6%). The flammability was further assessed by vertical burning test, where the ramie-PLA composites with MWCNT-DOPOs exhibited little dripping and managed to achieve UL-94 V-0 standard. TGA results showed that at higher temperature, the weight of the char residue for ramie-PLA with MWCNT-DOPOs was increased compared to ramie-PLA and ramie-PLA with MWCNT. Based on the experimental results obtained, the flame retardancy mechanism was proposed as shown in Fig. 12.8. Pristine MWCNTs alone are not effective enough to provide the self-extinguishing properties to the composites. However, with the addition of MWCNT-DOPOs in the ramiePLA composites, uniformly dispersed MWCNTs formed a three-dimensional continuous network structure in polymer matrix to support the chars produced [98] and to hinder the movement of polymer chains during combustion. In addition, natural fibre containing high amount of polyhydric compound plays the role as a charring agent to form a flame retardant system.

4. Prospects of macro- to nano-particulate flame retardants in NFRC Natural fibre reinforced/filled polymeric composites have been widely utilised in engineering applications. However, their susceptibility to thermal decomposition and high flammability of the cellulose fibres, polymer and resulting composites remain one of the critical impediments for integration into more demanding applications such

334

Interfaces in Particle and Fibre Reinforced Composites

Combustion

Charring

Ramie/PLA

Combustion

Charring

Ramie/PLA/MWCNT

Combustion

Charring

Ramie/PLA/MWCNT-DOPO

PLA

Ramie

MWCNT

MWCNT-DOPO

Fig. 12.8 Proposed flame retardant mechanism [122]. Copyright (2014), reprinted with permission from Elsevier.

as the aerospace and load bearing structure in civil industries. In the current scenario, majority of the studies on reducing the flammability of NFRC have been centred on the incorporation of macro- and nano-size particulate flame-retardant additives with various level of successes. Most of the effective strategy with inclusion of macroscale flame retardant additives such as the mineral hydroxide, expandable graphite, intumescent ammonium polyphosphate, hydroxycarbonates and metal borates required high dosages loadings to realise appreciable improvement in fire resistivity. However, the high concentration of macro-scale flame retardant often leads to reduction in mechanical properties of the composite. Although nano-scale additives such as the layered silicates and those from the carbon-based CNT and graphene have attained pronounced enhancement in flame retardancy at much lower concentration, arriving without significant reduction in composite strength. However, their tendency to agglomerate during processing remain a great challenge to achieve homogenous dispersion and limit their widespread utilisation as flame retardant in NFRC. There have been few studies that successfully exploited the potential synergistic effects between ATH and halogen free intumescent APP but limited investigation into potential synergistic effects with hybrid macro- and nano-particulate flame retardants has been

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reported to date. Therefore, a balance between flame resistant behaviour, mechanical properties and manufacturing processes could be viable through more comprehensive and innovative research to explore the potential synergy between macro- and nanoparticulate flame retardants and their incorporation into NFRC.

References [1] S. Prashanth, K. Subbaya, K. Nithin, S. Sachhidananda, Fiber reinforced composites - a review, J. Mater. Sci. Eng. 6 (2017), https://doi.org/10.4172/2169-0022.1000341. [2] M. Sain, S.H. Park, F. Suhara, S. Law, Flame retardant and mechanical properties of natural fibre-PP composites containing magnesium hydroxide, Polym. Degrad. Stab. 83 (2004) 363e367, https://doi.org/10.1016/S0141-3910(03)00280-5. [3] P. Wambua, J. Ivens, I. Verpoest, Natural fibres: can they replace glass in fibre reinforced plastics? Compos. Sci. Technol. 63 (2003) 1259e1264, https://doi.org/10.1016/S02663538(03)00096-4. [4] H. yan Cheung, M. po Ho, K. tak Lau, F. Cardona, D. Hui, Natural fibre-reinforced composites for bioengineering and environmental engineering applications, Compos. Part B Eng. 40 (2009) 655e663, https://doi.org/10.1016/j.compositesb.2009.04.014. [5] K.L. Pickering, M.G.A. Efendy, T.M. Le, A review of recent developments in natural fibre composites and their mechanical performance, Compos. Part A Appl. Sci. Manuf. 83 (2016) 98e112, https://doi.org/10.1016/j.compositesa.2015.08.038. [6] N. Saba, M.T. Paridah, M. Jawaid, Mechanical properties of kenaf fibre reinforced polymer composite: a review, Constr. Build. Mater. 76 (2015) 87e96, https://doi.org/ 10.1016/j.conbuildmat.2014.11.043. [7] C. Miao, W.Y. Hamad, Cellulose reinforced polymer composites and nanocomposites: a critical review, Cellulose 20 (2013) 2221e2262. [8] K.Y. Lee, Y. Aitom€aki, L.A. Berglund, K. Oksman, A. Bismarck, On the use of nanocellulose as reinforcement in polymer matrix composites, Compos. Sci. Technol. 105 (2014) 15e27, https://doi.org/10.1016/j.compscitech.2014.08.032. [9] B. Szolnoki, K. Bocz, P.L. Soti, B. Bodzay, E. Zimonyi, A. Toldy, B. Morlin, K. Bujnowicz, M. Wladyka-Przybylak, G. Marosi, Development of natural fibre reinforced flame retarded epoxy resin composites, Polym. Degrad. Stab. 119 (2015) 68e76, https://doi.org/10.1016/j.polymdegradstab.2015.04.028. [10] K.F. Adekunle, Surface treatments of natural fibres d a review: part 1, Open J. Polym. Chem. 5 (2015) 41e46, https://doi.org/10.4236/ojpchem.2015.53005. [11] S. Mukhopadhyay, R. Fangueiro, Physical modification of natural fibers and thermoplastic films for composites - a review, J. Thermoplast. Compos. Mater. 22 (2009) 135e162, https://doi.org/10.1177/0892705708091860. [12] M.M. Kabir, H. Wang, K.T. Lau, F. Cardona, Chemical treatments on plant-based natural fibre reinforced polymer composites: an overview, Compos. Part B Eng. 43 (2012) 2883e2892, https://doi.org/10.1016/j.compositesb.2012.04.053. [13] V. Fiore, G. Di Bella, A. Valenza, The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites, Compos. Part B Eng. 68 (2015) 14e21, https://doi.org/10.1016/j.compositesb.2014.08.025. [14] T. Sullins, S. Pillay, A. Komus, H. Ning, Hemp fiber reinforced polypropylene composites: the effects of material treatments, Compos. Part B Eng. 114 (2017) 15e22, https://doi.org/10.1016/j.compositesb.2017.02.001.

336

Interfaces in Particle and Fibre Reinforced Composites

[15] M.N. Akhtar, A.B. Sulong, M.K.F. Radzi, N.F. Ismail, M.R. Raza, N. Muhamad, M.A. Khan, Influence of alkaline treatment and fiber loading on the physical and mechanical properties of kenaf/polypropylene composites for variety of applications, Prog. Nat. Sci. Mater. Int. 26 (2016) 657e664. [16] M. Abdelmouleh, S. Boufi, M.N. Belgacem, A. Dufresne, Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading, Compos. Sci. Technol. 67 (2007) 1627e1639, https://doi.org/10.1016/ j.compscitech.2006.07.003. [17] A. Orue, A. Jauregi, U. Unsuain, J. Labidi, A. Eceiza, A. Arbelaiz, The effect of alkaline and silane treatments on mechanical properties and breakage of sisal fibers and poly(lactic acid)/sisal fiber composites, Compos. Part A Appl. Sci. Manuf. 84 (2016) 186e195, https://doi.org/10.1016/j.compositesa.2016.01.021. [18] R. Sepe, F. Bollino, L. Boccarusso, F. Caputo, Influence of chemical treatments on mechanical properties of hemp fiber reinforced composites, Compos. Part B Eng. 133 (2018) 210e217, https://doi.org/10.1016/j.compositesb.2017.09.030. [19] A.K. Bledzki, A.A. Mamun, M. Lucka-Gabor, V.S. Gutowski, The effects of acetylation on properties of flax fibre and its polypropylene composites, Express Polym. Lett. 2 (2008) 413e422, https://doi.org/10.3144/expresspolymlett.2008.50. [20] K. Diharjo, A. Permana, R. Arsada, G. Asmoro, H.S. Budiono, Y. Firdaus, Effect of acetylation treatment and soaking time to bending strength of sugar palm fiber composite, AIP Conf. Proc. 1788 (2017), https://doi.org/10.1063/1.4968302. [21] T.J. Keener, R.K. Stuart, T.K. Brown, Maleated coupling agents for natural fibre composites, Compos. Part A Appl. Sci. Manuf. 35 (2004) 357e362, https://doi.org/10.1016/ j.compositesa.2003.09.014. [22] M.H.M. Hamdan, J.P. Siregar, D. Bachtiar, M.R.M. Rejab, M. Samykano, E.H. Agung, C. Tezara, J. Jaafar, Effect of alkaline treatment on mechanical properties of woven ramie reinforced thermoset composite, IOP Conf. Ser. Mater. Sci. Eng. 257 (2017) 012044, https://doi.org/10.1088/1757-899X/257/1/012044. [23] N. Saba, M. Jawaid, O.Y. Alothman, M.T. Paridah, A review on dynamic mechanical properties of natural fibre reinforced polymer composites, Constr. Build. Mater. 106 (2016) 149e159, https://doi.org/10.1016/j.conbuildmat.2015.12.075. [24] R. Kozłowskiy, M. Władyka-Przybylak, Flammability and fire resistance of composites reinforced by natural fibers, Polym. Adv. Technol. 19 (2008) 446e453. [25] S. Chapple, R. Anandjiwala, Flammability of natural fiber-reinforced composites and strategies for fire retardancy: a review, J. Thermoplast. Compos. Mater. 23 (2010) 871e893, https://doi.org/10.1177/0892705709356338. [26] A.G.W. Bradbury, Y. Sakai, F. Shafizadeh, A kinetic model for pyrolysis of cellulose, J. Appl. Polym. Sci. 23 (1979) 3271e3280. [27] A.R. Horrocks, An introduction to the burning behaviour of cellulosic fibres, J. Soc. Dye. Colour. 99 (1983) 191e197. [28] N.K. Kim, S. Dutta, D. Bhattacharyya, A review of flammability of natural fibre reinforced polymeric composites, Compos. Sci. Technol. 162 (2018) 64e78, https://doi.org/ 10.1016/j.compscitech.2018.04.016. [29] FAR25.853, FAA Fire Test Handbook, Federal Aviation Adminstration, 2018. [30] UL-94, Test for Flammability of Plastic Materials for Parts in Devices and Appliances, Underwriters Laboratories Inc., 2013. [31] F. Laoutid, L. Bonnaud, M. Alexandre, J.M. Lopez-Cuesta, P. Dubois, New prospects in flame retardant polymer materials: from fundamentals to nanocomposites, Mater. Sci. Eng. R Rep. 63 (2009) 100e125, https://doi.org/10.1016/j.mser.2008.09.002.

Advancement in flame retardancy of natural fibre reinforced composites

337

[32] A. Horner, Aircraft Materials Fire Test Handbook, National Technical Information Service (NTIS), Springfield, Virginia 22161, 2000. [33] IEC 60695-11-10, Fire Hazard Testing - Part 11-10: Test Flames - 50 W Horizontal and Vertical Flame Test Methods, International Electrotechnical Commission, 2013. [34] IEC 60695-11-20, Fire Hazard Testing - Part 11-20: Test Flames - 500 W Flame Test Method, International Electrotechnical Commission, 2015. [35] ISO 9772, Cellular Plastics – Determination of Horizontal Burning Characteristics of Small Specimens Subjected to a Small Flame, International Organization for Standardization, 2012. [36] ISO 9773, Plastics – Determination of Burning Behaviour of Thin Flexible Vertical Specimens in Contact with a Small-Flame Ignition Source, International Organization for Standardization, 1998. [37] ASTM D3801, Standard Test Method for Measuring the Comparative Burning Characteristics of Solid Plastics in a Vertical Position, American Society for Testing and Materials, 2000. [38] ASTM D5025, Standard Test Method for Measuring the Comparative Burning Characteristics and Resistance to Burn-Through of Solid Plastics Using a 125-mm Flame, American Society for Testing and Materials, 2014. [39] M. Suzanne, M.A. Delichatsios, J.P. Zhang, Flame extinction properties of solids obtained from limiting oxygen index tests, Combust. Flame 161 (2014) 288e294. [40] ISO 4589, Plastics e Determination of Burning Behaviour by Oxygen Index, International Organization for Standardization, 2017. [41] ASTM D2863, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), American Society for Testing and Materials, 2017. [42] A.R. Horrocks, B.K. Kandola, Flame resistant composites and nanocomposites, in: K.F. Selcen (Ed.), Handb. Fire Resist. Text., first ed., Woodhead Publishing, 2013, pp. 283e321. [43] B.K. Kandola, Flame retardancy design for textiles, in: C.A. Wilkie, A.B. Morgan (Eds.), Fire Retard. Polym. Mater., second ed., CRC Press, Taylor & Francis, 2009, pp. 725e762. [44] V. Babrauskas, Performance of the Ohio State University rate of heat release apparatus using polymethylmethacrylate and gaseous fuels, Fire Saf. J. 5 (1982) 9e20. https://doi. org/10.1016/0379-7112(82)90003-0. [45] V. Babrauskas, From Bunsen burner to heat release rate calorimeter, in: V. Babrauskas, S.J. Grayson (Eds.), Heat Release Fires, Taylor & Francis, 1990, pp. 7e29. [46] ASTM E906-98, Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products (Configuration A), American Society for Testing of Materials, West Conshohocken, PA, 1998. [47] Y. Tsuchiya, J.F. Mathieu, Measuring degrees of combustibility using an OSU apparatus and oxygen-depletion principle, Fire Saf. J. 17 (1991) 291e299. https://doi.org/10.1016/ 0379-7112(91)90024-S. [48] F. Moussan, J.-L. Delfau, C. Vovelle, C.P. Van Cang, G. Bosseboeuf, Heat release rate and effective heat of combustion measurements: a comparative study of thermal and oxygen consumption techniques, J. Fire Sci. 13 (1995) 482e499. [49] E.E. Smith, Heat release rate calorimetry, in: Fire Technol., 1996, pp. 333e347. [50] R. Filipczak, S. Crowley, R.E. Lyon, Heat release rate measurements of thin samples in the OSU apparatus and the cone calorimeter, Fire Saf. J. 40 (2005) 628e645, https:// doi.org/10.1016/j.firesaf.2005.05.009.

338

Interfaces in Particle and Fibre Reinforced Composites

[51] M. Sponton, J.C. Ronda, M. Galia, V. Cadiz, Cone calorimetry studies of benzoxazineepoxy systems flame retarded by chemically bonded phosphorus or silicon, Polym. Degrad. Stab. 94 (2009) 102e106, https://doi.org/10.1016/ j.polymdegradstab.2008.10.005. [52] ISO 5660, Reaction to Fire Tests e Heat Release, Smoke Production and Mass Loss Rate - Part 1: Heat Release Rate (Cone Calorimeter Method) and Smoke Production Rate (Dynamic Measurement), International Organization for Standardization, 2015. [53] ASTM E1354, Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter, American Society for Testing of Materials, 2017. [54] ASTM E662, Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials, American Society for Testing of Materials, 2017. [55] U. Almeida Pinto, L.L.Y. Visconte, J. Gallo, R.C.R. Nunes, Flame retardancy in thermoplastic polyurethane elastomers (TPU) with mica and aluminum trihydrate (ATH), Polym. Degrad. Stab. 69 (2000) 257e260, https://doi.org/10.1016/S0141-3910(00) 00047-1. [56] A.P. Mouritz, A.G. Gibson, Flame retardant composites, in: Fire Prop. Polym. Compos. Mater., 2006, pp. 237e286. [57] B.K. Kandola, E. Kandare, Composites having improved fire resistance, in: A.R. Horrocks, D. Price (Eds.), Advances in fire retardant materials, Woodhead Publishing, Cambridge, England, 2008, pp. 398e442, https://doi.org/10.1533/ 9781845694701.3.398. [58] L. Larrondo, R.S.J. Manley, Electrostatic fiber spinning from polymer melts. III. Electrostatic deformation of a pendant drop of polymer melt, J. Polym. Sci. Polym. Phys. Ed. 19 (1981) 933e940, https://doi.org/10.1002/pol.1981.180190603. [59] A. El-Sabbagh, L. Steuernagel, G. Ziegmann, Low combustible polypropylene/flax/ magnesium hydroxide composites: mechanical, flame retardation characterization and recycling effect, J. Reinf. Plast. Compos. 32 (2013) 1030e1043, https://doi.org/10.1177/ 0731684413480993. [60] T.D. Hapuarachchi, G. Ren, M. Fan, P.J. Hogg, T. Peijs, Fire retardancy of natural fibre reinforced sheet moulding compound, Appl. Compos. Mater. 14 (2007) 251e264, https:// doi.org/10.1007/s10443-007-9044-0. [61] E. Gallo, G. Sanchez-Olivares, B. Schartel, Flame retardancy of starch-based biocomposites - aluminum hydroxide-coconut fiber synergy, Polimery/Polymers 58 (2013) 395e402, https://doi.org/10.14314/polimery.2013.395. [62] K.Y. Tshai, H.J. Tan, P.S. Khiew, M.E. Hoque, C.H. Chia, Effects of aluminum trihydrate on the flame-retardant and smoke-suppressant properties of natural fibres filled composites, Polym. Res. J. 9 (2015) 181e195. [63] A. Musbah Redwan, K. Haji Badri, M.A. Tarawneh, The effect of aluminium hydroxide (ATH) on the mechanical properties and fire resistivity of palm-based fibreboard prepared by pre-polymerization method, Adv. Mater. Res. 1087 (2015) 287e292, https://doi.org/ 10.4028/www.scientific.net/AMR.1087.287. [64] X. Pang, X. Shi, X. Kang, M. Duan, M. Weng, Preparation of borate-modified expandable graphite and its flame retardancy on acrylonitrile-butadiene- styrene resin, Polym. Compos. 37 (2016) 2673e2683. https://doi.org/10.1002/pc.23461. [65] P. Khalili, K.Y. Tshai, I. Kong, Natural fiber reinforced expandable graphite filled composites: evaluation of the flame retardancy, thermal and mechanical performances, Compos. Part A Appl. Sci. Manuf. 100 (2017) 194e205, https://doi.org/10.1016/ j.compositesa.2017.05.015.

Advancement in flame retardancy of natural fibre reinforced composites

339

[66] X. Meng, L. Ye, X. Zhang, P. Tang, J. Tang, X. Ji, Z. Li, Effects of expandable graphite and modified ammonium polyphosphate on the flame-retardant and mechanical properties of wood flour-polypropylene composites, J. Appl. Polym. Sci. 114 (2009) 853e863, https://doi.org/10.1002/app. [67] M. Thirumal, D. Khastgir, N.K. Singha, B.S. Manjunath, Y.P. Naik, Effect of expandable graphite on the properties of intumescent flame-retardant polyurethane foam, J. Appl. Polym. Sci. 110 (2008) 2586e2594, https://doi.org/10.1002/app. [68] B. Schartel, U. Braun, U. Schwarz, S. Reinemann, Fire retardancy of polypropylene/flax blends, Polymer (Guildf.) 44 (2003) 6241e6250, https://doi.org/10.1016/S00323861(03)00692-X. [69] L.A. Hollingbery, T.R. Hull, The thermal decomposition of huntite and hydromagnesite d a review, Hermochim. Acta 509 (2010) 1e11, https://doi.org/ 10.1016/j.tca.2010.06.012. [70] M.N. Prabhakar, A.U.R. Shah, J.-I. Song, A review on the flammability and flame retardant properties of natural fibers and polymer matrix based composites, Compos. Res. 28 (2015) 29e39, https://doi.org/10.7234/composres.2015.28.2.029. [71] G. Camino, A. Maffezzoli, M. Braglia, M. De Lazzaro, M. Zammarano, Effect of hydroxides and hydroxycarbonate structure on fire retardant effectiveness and mechanical properties in ethylene-vinyl acetate copolymer, Polym. Degrad. Stab. 74 (2001) 457e464, https://doi.org/10.1016/S0141-3910(01)00167-7. [72] O. Sulaiman, R. Hashim, R.N. Kumar, P. Tamyez, R.J. Murphy, Z. Ali, Effect of incorporation of flame retardants on some of the properties of phenol formaldehyde medium density fiberboard, Int. J. Agric. Res. 3 (2008) 331e339. [73] R. Kozlowski, B. Mieleniak, M. Helwig, A. Przepiera, Flame resistant lignocellulosicmineral composite particleboards, Polym. Degrad. Stab. 64 (1999) 523e528, https:// doi.org/10.1016/S0141-3910(98)00145-1. [74] B. Garba, Effect of zinc borate as flame retardant formulation on some tropical woods, Polym. Degrad. Stab. 64 (1999) 517e522, https://doi.org/10.1016/S0141-3910(98) 00136-0. [75] S. Bhattacharjee, M.H. Sazzad, M.A. Islam, M.M. Ahtashom, Asaduzzaman, M.Y. Miah, Effects of fire retardants on jute fiber reinforced polyvinyl chloride/polypropylene hybrid composites, Int. J. Mater. Sci. Appl. 2 (2013) 162, https://doi.org/10.11648/ j.ijmsa.20130205.13. [76] L. Shumao, R. Jie, Y. Hua, Y. Tao, Y. Weizhong, Influence of ammonium polyphosphate on the flame retardancy and mechanical properties of ramie fiber-reinforced poly(lactic acid) biocomposites, Polym. Int. 59 (2010) 242e248, https://doi.org/10.1002/pi.2715. [77] D. Cho, Y. Woo, K.H. Lee, Flame retardant kenaf/pla biocomposites : effect of ammonium polyphosphate, in: 19Th Int. Conf. Compos. Mater., 2013. [78] A. Ismail, A. Hassan, A.A. Bakar, M. Jawaid, Flame retardancy and mechanical properties of kenaf filled polypropylene (PP) containing ammonium polyphosphate (APP), Sains Malays. 42 (2013) 429e434. [79] N.U. Loredo, J.S. Bermejo, Enhanced flame retardancy of flax bio-composites for the construction market, J. Facade Des. Eng. 4 (2016) 67e76, https://doi.org/10.3233/FDE160052. [80] A.B. Morgan, C.A. Wilkie, Flame Retardant Polymer Nanocomposites, Wiley, 2007. [81] P. Khalili, K.Y. Tshai, D. Hui, I. Kong, Synergistic of ammonium polyphosphate and alumina trihydrate as fire retardants for natural fiber reinforced epoxy composite, Compos. Part B Eng. 114 (2017) 101e110, https://doi.org/10.1016/j.compositesb.2017.01.049.

340

Interfaces in Particle and Fibre Reinforced Composites

[82] A. Castrovinci, G. Camino, C. Drevelle, S. Duquesne, C. Magniez, M. Vouters, Ammonium polyphosphate-aluminum trihydroxide antagonism in fire retarded butadiene-styrene block copolymer, Eur. Polym. J. 41 (2005) 2023e2033, https://doi.org/ 10.1016/j.eurpolymj.2005.03.010. [83] C. Guo, L. Zhou, J. Lv, Effects of expandable graphite and modified ammonium polyphosphate on the flame-retardant and mechanical properties of wood flour-polypropylene composites, Polym. Polym. Compos. 21 (2013) 449e456. [84] G. Bai, C. Guo, L. Li, Synergistic effect of intumescent flame retardant and expandable graphite on mechanical and flame-retardant properties of wood flour-polypropylene composites, Constr. Build. Mater. 50 (2014) 148e153, https://doi.org/10.1016/ j.conbuildmat.2013.09.028. [85] A. Dasari, Z.-Z. Yu, G.-P. Cai, Y.-W. Mai, Recent developments in the fire retardancy of polymeric materials, Prog. Polym. Sci. 38 (2013) 1357e1387. https://doi.org/10.1016/j. progpolymsci.2013.06.006. [86] Y. Arao, Flame retardancy of polymer nanocomposite, in: Flame Retard. Polym. Blends, Compos. Nanocomposites, 2015, pp. 15e44. [87] G. Guo, C.B. Park, Y.H. Lee, Y.S. Kim, M. Sain, Flame retarding effects of nanoclay on woodefiber composites, Polym. Eng. Sci. 47 (2007) 330e336, https://doi.org/10.1002/ pen.20712. [88] T.D. Hapuarachchi, T. Peijs, Multiwalled carbon nanotubes and sepiolite nanoclays as flame retardants for polylactide and its natural fibre reinforced composites, Compos. Part A Appl. Sci. Manuf. 41 (2010) 954e963. https://doi.org/10.1016/j.compositesa.2010.03. 004. [89] G. Huang, S. Wang, P. Song, C. Wu, S. Chen, X. Wang, Combination effect of carbon nanotubes with graphene on intumescent flame-retardant polypropylene nanocomposites, Compos. Part A Appl. Sci. Manuf. 59 (2014) 18e25, https://doi.org/10.1016/ j.compositesa.2013.12.010. [90] B. Dittrich, K.-A. Wartig, D. Hofmann, R. M€ulhaupt, B. Schartel, Flame retardancy through carbon nanomaterials: carbon black, multiwall nanotubes, expanded graphite, multi-layer graphene and graphene in polypropylene, Polym. Degrad. Stab. 98 (2013) 1495e1505. https://doi.org/10.1016/j.polymdegradstab.2013.04.009. [91] P.C. LeBaron, Z. Wang, T.J. Pinnavaia, Polymer-layered silicate nanocomposites: an overview, Appl. Clay Sci. 15 (1999) 11e29. https://doi.org/10.1016/S0169-1317(99) 00017-4. [92] S. Pavlidou, C.D. Papaspyrides, A review on polymerelayered silicate nanocomposites, Prog. Polym. Sci. 33 (2008) 1119e1198. [93] C.D. Delhom, L.A. White-Ghoorahoo, S.S. Pang, Development and characterization of cellulose/clay nanocomposites, Compos. Part B Eng. 41 (2010) 475e481. https://doi.org/ 10.1016/j.compositesb.2009.10.007. [94] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng. R Rep. 28 (2000) 1e63. https://doi.org/10.1016/S0927-796X(00)00012-7. [95] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (2003) 1539e1641. https://doi.org/10. 1016/j.progpolymsci.2003.08.002. [96] X. Fu, S. Qutubuddin, Polymereclay nanocomposites: exfoliation of organophilic montmorillonite nanolayers in polystyrene, Polymer (Guildf.) 42 (2001) 807e813. https://doi.org/10.1016/S0032-3861(00)00385-2.

Advancement in flame retardancy of natural fibre reinforced composites

341

[97] H. Qin, S. Zhang, C. Zhao, G. Hu, M. Yang, Flame retardant mechanism of polymer/clay nanocomposites based on polypropylene, Polymer (Guildf.) 46 (2005) 8386e8395. https://doi.org/10.1016/j.polymer.2005.07.019. [98] T. Kashiwagi, F. Du, J.F. Douglas, K.I. Winey, R.H.H. Jr, J.R. Shields, Nanoparticle networks reduce the flammability of polymer nanocomposites, Nat. Mater. 4 (2005) 928e933. [99] G. Guo, K.H. Wang, C.B. Park, Y.S. Kim, G. Li, Effects of nanoparticles on the density reduction and cell morphology of extruded metallocene polyethylene/wood fiber nanocomposites, J. Appl. Polym. Sci. 104 (2007) 1058e1063. [100] B.K. Deka, T.K. Maji, Effect of TiO2 and nanoclay on the properties of wood polymer nanocomposite, Compos. Part A Appl. Sci. Manuf. 42 (2011) 2117e2125. https://doi. org/10.1016/j.compositesa.2011.09.023. [101] M. Biswal, S. Mohanty, S.K. Nayak, Thermal stability and flammability of banana-fiberreinforced polypropylene nanocomposites, J. Appl. Polym. Sci. 125 (2012) E432eE443. [102] A. Subasinghe, R. Das, D. Bhattacharyya, Study of thermal, flammability and mechanical properties of intumescent flame retardant PP/kenaf nanocomposites, Int. J. Smart Nano Mater. 7 (2016) 202e220. [103] M.I. Katsnelson, Graphene: carbon in two dimensions, Mater. Today 10 (2007) 20e27. https://doi.org/10.1016/S1369-7021(06)71788-6. [104] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes, Science 306 (2004) 1362e1364. [105] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotubeepolymer composites, Carbon N.Y. 44 (2006) 1624e1652. https://doi.org/10.1016/j.carbon.2006.02.038. [106] E.W. Wong, P.E. Sheehan, C.M. Lieber, Nanobeam mechanics: elasticity, strength, and toughness of nanorods and nanotubes, Science 277 (1997) 1971e1975. [107] K.S. Novoselov, A.K. Geim1, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. [108] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385e388. [109] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902e907, https://doi.org/10.1021/nl0731872. [110] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906e3924. [111] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L. Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Commun. 146 (2008) 351e355. https://doi.org/10.1016/j.ssc.2008.02.024. [112] G. Beyer, Short communication: carbon nanotubes as flame retardants for polymers, Fire Mater. 26 (2003) 291e293. [113] S. Peeterbroeck, F. Laoutid, B. Swoboda, J.-M. Lopez-Cuesta, N. Moreau, J.B. Nagy, M. Alexandre, P. Dubois, How carbon nanotube crushing can improve flame retardant behaviour in polymer nanocomposites? Macromol. Rapid Commun. 28 (2007) 260e264. [114] S. Bocchini, A. Frache, G. Camino, M. Claes, Polyethylene thermal oxidative stabilisation in carbon nanotubes based nanocomposites, Eur. Polym. J. 43 (2007) 3222e3235. https://doi.org/10.1016/j.eurpolymj.2007.05.012.

342

Interfaces in Particle and Fibre Reinforced Composites

[115] T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad, J. Douglas, Thermal degradation and flammability properties of poly (propylene)/carbon nanotube composites, Macromol. Rapid Commun. 23 (2002) 761e765, https://doi.org/10.1002/15213927(20020901)23. [116] M.C. Costache, M.J. Heidecker, E. Manias, G. Camino, A. Frache, G. Beyer, R.K. Gupta, C.A. Wilkie, The influence of carbon nanotubes, organically modified montmorillonites and layered double hydroxides on the thermal degradation and fire retardancy of polyethylene, ethyleneevinyl acetate copolymer and polystyrene, Polymer (Guildf.) 48 (2007) 6532e6545. https://doi.org/10.1016/j.polymer.2007.08.059. [117] B.H. Cipiriano, T. Kashiwagi, S.R. Raghavan, Y. Yang, E.A. Grulke, K. Yamamoto, J.R. Shields, J.F. Douglas, Effects of aspect ratio of MWNT on the flammability properties of polymer nanocomposites, Polymer (Guildf.) 48 (2007) 6086e6096. https://doi. org/10.1016/j.polymer.2007.07.070. [118] B. Schartel, P. P€otschke, U. Knoll, M. Abdel-Goad, Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites, Eur. Polym. J. 41 (2005) 1061e1070. https://doi.org/10.1016/j.eurpolymj.2004.11.023. [119] R. Xie, B. Qu, Synergistic effects of expandable graphite with some halogen-free flame retardants in polyolefin blends, Polym. Degrad. Stab. 71 (2001) 375e380. https://doi.org/ 10.1016/S0141-3910(00)00188-9. [120] B. Yu, Y. Shi, B. Yuan, S. Qiu, W. Xing, W. Hu, L. Song, S. Lo, Y. Hu, Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites, J. Mater. Chem. A 3 (2015) 8034e8044, https://doi.org/10.1039/ c4ta06613h. [121] X. Wang, L. Song, H. Yang, H. Lu, Y. Hu, Synergistic effect of graphene on antidripping and fire resistance of intumescent flame retardant poly(butylene succinate) composites, Ind. Eng. Chem. Res. 50 (2011) 5376e5383, https://doi.org/10.1021/ie102566y. [122] T. Yu, N. Jiang, Y. Li, Functionalized multi-walled carbon nanotube for improving the flame retardancy of ramie/poly(lactic acid) composite, Compos. Sci. Technol. 104 (2014) 26e33, https://doi.org/10.1016/j.compscitech.2014.08.021. [123] B. Perret, B. Schartel, K. St€oß, M. Ciesielski, J. Diederichs, M. D€ oring, J. Kr€amer, V. Altst€adt, Novel DOPO-based flame retardants in high-performance carbon fibre epoxy composites for aviation, Eur. Polym. J. 47 (2011) 1081e1089, https://doi.org/10.1016/ j.eurpolymj.2011.02.008. [124] A. K€onig, E. Kroke, Methyl-DOPO-a new flame retardant for flexible polyurethane foam, Polym. Adv. Technol. 22 (2011) 5e13, https://doi.org/10.1002/pat.1728.

Current review on the utilization of nanoparticles for ceramic matrix reinforcement

13

M.M.M.G.P.G. Mantilaka a, b , W.P.S.L. Wijesinghe a , D.M.S.N. Dissanayake a , U.G. Mihiri Ekanayake a , Anoja Senthilnathan a a Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahewatta, Pitipana, Homagama, Sri Lanka; bPostgraduate Institute of Science, University of Peradeniya, Peradeniya, Peradeniya, Sri Lanka

1. Introduction Ceramic industry has received much more attention from the early as 24000 BCE to the current 21st century. The industry has been further developed in a drastic manner, in recent years, due to evolution of nanostructured ceramics in replacement of conventional ceramics [1]. Nanostructured ceramics are referred to inorganic materials that consist of structural units with a size scale of less than 100 nm at least in one dimension [2]. These nanoceramic materials have achieved greater research interest in its numerous unique properties in comparison coarser structural units. Ceramic matrices are filled with nanomaterials to synthesize ceramic nanocomposites in order to improve properties including greater mechanical strengths, high stiffness, stability, high fracture toughness and wear resistance. As a result of introducing “Ceramic Nanocomposites” by Niihara et al. in 1991, the researches related to nanotechnology and ceramic property enhancements, grew largely because of their extensive applications. With the recent advancements of ceramic reinforcement, advanced structural ceramic nano-composites [SD1] or nanoparticle dispersion-strengthened materials [SD2] play a huge role which influence many researches to invent new advanced ceramics with enhanced mechanical, tribological and thermal properties. According to Niihara, ceramic nanocomposites are divided into 3 categories as intra-granular, inter-granular and nano/nano-composites (Fig. 13.1) according to their dispersion and matrix. Also there are several other classifications are introduced as well. Intra-granular and inter-granular nanocomposites show 2e5 times higher toughness, hardness, creep and fatigue resistance at elevated temperatures, and thermal shock resistance than the monolithic ceramics, while nano/nano composites show advanced properties like machinability and super-plasticity. Normally, dispersion properties of such ceramic matrix nanocomposite depends on the following factors including particle size of the dispersion material, Thermal expansion mismatch (Da) of the dispersion and the matrix, elastic modulus of the dispersion and bonding & fracture toughness of the dispersion and matrix. These factors enhance Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00013-3 Copyright © 2020 Elsevier Ltd. All rights reserved.

346

(a)

(b) Inter type

Nano-nano type

Nano-micro type

Intra/inter type

Nano-nano type

Nano-fiber type

Nano-nanolayer type

Fig. 13.1 Classifications introduced by Kuntz et al. [130] and Niihara [131]. (a) Niihara’s classification. (b) New classification.

Interfaces in Particle and Fibre Reinforced Composites

Intra type

Current review on the utilization of nanoparticles for ceramic matrix reinforcement

347

the properties of composite such as density, hardness, corrosion or wear resistance, tribological properties, fracture resistant (either creep or fatigue at high temperatures), thermal shock resistance, advanced properties (machinability and super-plasticity). Interfaces between nanoparticles and ceramic matrices play a vital role in ceramic nanocomposites. Therefore this chapter describes role of various nanomaterials in reinforcement of ceramics and their interfaces.

2. Properties and applications of nanoparticles in reinforcement of ceramics Nanomaterials have shown outstanding properties due to their high surface area-tovolume ratio and small particle sizes which can fill small voids and fractures. Therefore, nanomaterials are widely used to reinforce various matrices in order to produce nanocomposites. Ceramic composites are produced by reinforcing and filling with various inorganic nanoparticles. These nanoparticles add various properties to ceramic matrices such as mechanical properties, chemical and thermal stability, antimicrobial properties, strength, anticorrosion, fire retardant property, hardness, self-healing property, elasticity and biocompatibility.

3. Common nanomaterials used in reinforcement of ceramics 3.1

Alumina based materials

Alumina (Al2O3) based nanomaterials have gained a significant demand because of aluminum oxide itself is a ceramic material which is very stable at high temperatures and resistance to abrasion. Alumina nanomaterials are used for enhancement of properties of ceramics such as density, hardness, Young’s modulus, fracture strength, fracture toughness, wear resistance and creep resistance. As a ceramic materials, alumina is mostly used in the matrix component in the ceramics in the micrometer scale. Nevertheless, in most nanocrystalline ceramics or nano/nano-ceramics, nanomaterials are used in the dispersed phase as well as in the matrix. In this section, we discuss about the utilization of alumina nanomaterials in the process of reinforcement of ceramics. Bhaduri et al. have introduced the nanocrystalline Al2O3eZrO2 nano/nano-ceramic nanocomposite using the technique called auto ignition [3]. This method consists of rapid heating and cooling steps to interpenetrate the two phases in nano-domain which resulted because of the enhanced grain boundary stability after soaking at 1200
C and hot isostatic pressing to 99% of the theoretical density. The average hardness and toughness values were improved than the conventional ceramic materials. Liao et al. introduced the concept of high pressure/low temperature sintering of nanocrystalline alumina using a toroidal-type high pressure apparatus [4]. The precursor, nanocrystalline g phase alumina transforms into a phase upon sintering. They have found that the grain growth is limited by the low sintering temperature and, a populated nucleation

348

Interfaces in Particle and Fibre Reinforced Composites

events in the parent g phase at very high pressure creates a nanoscale a grain size. Due to its high surface area, nano- Al2O3 powder acts as a good adsorptive material. Zhan et al. have invented a novel processing route to develop a dense nanocrystalline alumina matrix ( 4s) Before a metal (or electrolyte)-semiconductor contact is built, the Fermi level of a semiconductor is elevated than that of a metal and the Eredox at the initial condition. Therefore, in order to coordinate the Fermi levels, electrostatic potential of the semiconductor should be increased (i.e., electron energies should be lowered) in comparison to the metal (or electrolyte). For an n-type semiconductor, adjacent to the junction, a debilitation region is formed. When the positive charge of the uncompensated donor ions is equivalent to the negative charge of the metal or electrolyte ion within the depletion region then band bending or electric field bending occur in the depletion region similar to p-n junctions (Fig. 15.9). Case 2: (4m < 4s) Similarly for a p-type semiconductor where 4m < 4s, alignment of Fermi levels at equilibrium necessitates the metal side or the electrolyte ion to possess a positive charge and on the semiconductor side of the junction a negative charge. Here, the negative charge will be present in the depletion region in which acceptor ions are uncompensated by holes. A potential barrier (4s e 4m) is generated that resists

Fig. 15.9 Schottky barrier formation for n-type semiconductor.

408

Interfaces in Particle and Fibre Reinforced Composites

diffusion of holes from the semiconductor to the metal (or electrolyte) and accordingly the field can be increased or decreased by the application of a voltage across the junction. Thus, we see the reversal of the potential barrier for the positive charge to the negative charge energy diagram (Fig. 15.10). In effect of the above discussed two cases of metal (or electrolyte)-p/n semiconductor, non-rectifying contacts are formed. In further elaboration, we may add that Schottky barrier and Fermi level can be modulated by an external bias where cathodic voltage will cause a decrease in the extent of band-bending while a reversal is observed on applying anodic voltage. It is to be noted that if the recombination of electron-hole pairs is reduced by rapid electron-hole separation by ensuring steeper band-bending then a high-performing photocatalyst can be achieved. In addition, for an efficient transfer rate of charge-carriers with the electrolyte, a maximum overlap of specific redox systems with band-edge positions are to be accomplished. The treatment of the cases will now be discussed under Ohmic contacts.

3.2

Ohmic contacts

In an ideal metal-semiconductor when the alignment of Fermi levels is induced by the charge delivered by a greater number of carriers, Ohmic contacts result. For metal-ntype semiconductor, at equilibrium, Fermi level alignment is brought about by electrons being relocated from the metal to the semiconductor. As a result, the semiconductor electrostatic potential is reduced compared to that of the metal. Therefore, the barrier formed is small and can easily be prevailed over by applying a small voltage. Similarly, metal-p-type semiconductor allows the facile flow of holes across the junction. Unlike Schottky barriers, depletion region formation does not occur due to an assemblage of most of the carriers arising from the difference in electrostatic potential required for alignment of the Fermi levels. A general approach for Ohmic contacts formation is carried by a high addition of dopants in the contact area. Hence, the existence of a barrier at the interface results in a small width of the depletion region that permits carriers to channel through the barrier. Examples include Au/Sb alloyed on n-Si and Al alloyed on p-Si creating nþ and pþ layers at the semiconductor surfaces, respectively.

Fig. 15.10 Schottky barrier formation for a p-type semiconductor.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

409

Thus, by knowing the basic principles of junction theory, one can design suitable metal-semiconductors by altering the band gaps and positions according to the requirement for energy applications.

3.3

The basic principle of photocatalytic water-splitting and purification of toxic water/air systems

Overall water splitting is an ‘up-hill’ reaction with a high positive change in Gibbs free energy (DG > 0; DG0 ¼ þ237.2 kJ mol1, 2.46 eV per molecule). Thus, light or preferably sunlight (photons) can prevail over the high positive change in Gibbs free energy and hence, is the driving force for photoelectrochemical and photocatalytic water splitting reactions. Since decomposition of a water molecule involves two-electron transfer, semiconductor surface active sites are competent to absorb solar energy and subsequently generate electrons and holes which can reduce and oxidize the water molecules accumulated on the catalysts surface, respectively. Photocatalytic water splitting reaction comprises of five processes [262]. Light absorption by the photocatalyst causes excitation of electrons and generation of holes as expressed by the following reaction in TiO2 semiconductors: hy

TiO2 ! e TiO2 þ hþ TiO2

(15.4)

Charge-transfer reaction transpires between the photoexcited carriers and surface active sites. Mass diffusion of reactants and products by adsorption and desorption must take place jointly. Charge recombination of carriers occurs simultaneously with the reactions. Hence, for favorable water splitting, charge separation and surface redox reactions on a photocatalyst should advance with the lifespan of the photocarriers. The electronic band profile of a semiconductor plays an important role in semiconductor-based photocatalysis. In a semiconductor, the highest range of electron energies where usually electrons reside at absolute zero Kelvin temperature is the valence band (VB), while the least range of vacant electronic states makes the conduction band. The separation between the bottom of the conduction band (CB) and the top of the valence band energy levels is known as the energy gap (Eg). Unlike insulators, band gap energy less than 3 eV are semiconductors. Without any excitation, electrons are present only in the VB. When semiconductors are excited thermally or optically and if they absorb photon energy equal or higher than the band-gap then electrons are promoted to the CB level and holes are formed in the VB level. This is followed by the separation of charges and migration of the carrier pairs from the bulk of the semiconductor to the reactive sites of the photocatalyst surface. The photogenerated carriers can bring about oxidation and reduction reactions if the charge inclusion in the adsorbed reactant molecules on a semiconductor is thermodynamically favorable i.e., recombination of electron-hole pairs do not occur. The mechanism is expressed as: Oxidation: H2O þ 2hþ / 2Hþ þ 1/2 O2

(15.5)

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Interfaces in Particle and Fibre Reinforced Composites

Reduction: 2Hþ þ 2e / H2

(15.6)

Overall water-splitting reaction: H2O / H2 þ 1/2 O2

(15.7)

When reducing agents (alcohols, sulphides, EDTA, etc.) or oxidizing agents (persulphate, Agþ, etc.) also known as sacrificial agents are used then the reaction is not regarded as an overall water-splitting reaction, rather they are referred to as half reactions which govern whether a concern semiconductor agrees with the thermodynamic and kinetic aspects for H2 or O2 evolution [263e266]. In addition, the agents used can act as hole-scavengers to prevent photocorrosion and recombination of electrons and holes simultaneously. The photo-generated carries which do not experience any charge annihilation can induce surface redox secondary reactions of adsorbed materials. For example, superoxide and hydroperoxide radicals formed when photoexcited electrons in CB reacts with oxygen partake in the degradation process of toxic organic pollutants while holes in VB band oxidizes water or hydroxyl ion to hydroxyl radical which in effect oxidizes along with mineralizing the pollutants (Fig. 15.11). The equations are expressed as [267]: hþ þ H2O / OH þ Hþ

(15.8)

OH þ pollutant / H2O þ CO2

(15.9)

e  þ O 2 / O  2

(15.10)



þ  O 2 þ H / OOH

(15.11)



OOH þ OOH / H2O2 þ O2

(15.12)



O 2 þ pollutant / CO2 þ H2O

(15.13)



OOH þ pollutant / CO2 þ H2O

(15.14)

O2 CB

e–

e–

•O2–

VI

S

UV

h+

•O2– 1O 2

VB

h+

H2O/OH–

•OH

Fig. 15.11 Schematic representations of semiconductor photocatalysis. Reprinted with permission from S. Banerjee, S.C. Pillai, P. Falaras, K.E. O’Shea, J.A. Byrne, D.D. Dionysiou, New insights into the mechanism of visible light photocatalysis, J. Phys. Chem. Lett. 5 (2014) 2543e2554. Copyright 2014 American Chemical Society.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

3.4

411

Essentialities to be a photocatalyst

For a semiconductor to function as a photocatalyst following requirements must be satisfied in terms of electrochemical properties and band gap energy: Capacity to absorb solar visible light radiations and appropriate band edge potentials. Potential to isolate photoexcited electrons from highly responsive holes. To curtain the energy losses due to transport and recombination of charge carriers. Ability to resist corrosion and photocorrosion i.e., good chemical stability towards solid/ liquid interface reactions. 5. Kinetically satisfying electron migration properties from semiconductor to water junction. 1. 2. 3. 4.

A semiconductor can act as a photocatalyst for hydrogen generation if its conduction band is more negative than hydrogen production level (EH2 =H2 O ¼ 0 V vs. Normal hydrogen electrode (NHE); pH ¼ 10) and for oxygen generation the valence band must be more positive than oxygen production level (EO2 =H2 O ¼ 1.23 V vs. NHE; pH ¼ 0) (Fig. 15.12). Thus, theoretically, the band gap of 1.23 eV is sufficient to run the overall reaction. However, loss of energy due to thermodynamic conditions, carrier pair transport, etc., will increase the energy required greater than 1.23 eV limit in actual practice (2.0e2.2 eV) [269]. Semiconductors like KTaO3, SrTiO3, TiO2, ZnS, etc. [270], are common photocatalysts that satisfy the thermodynamic requirements i.e., the band structure potential of the semiconductor. CdS and SiC semiconductors are not ideal for a water-splitting reaction because they tend to cause photocorrosion. Over the years, TiO2 for the most part has been the commonly used photocatalyst due to its high chemical stability, good catalytical activities, relatively low-cost and long life-cycle of carrier pairs. At present, solar energy conversion efficiency to hydrogen from the water-splitting reaction by TiO2 photocatalyst still remains to be a practically low generation process owing to the following reasons: 1. Energy loss because of rapid recombination of photogenerated carriers. 2. According to Le-Chartier principle, the backward reaction of hydrogen and oxygen proceeds easily due to gain in energy in the forward reaction of H2O production. 3. TiO2 with band gap energy of 3.2 eV is not capable of utilizing visible light. It is imperative to harness visible light for effective conversion of solar energy to hydrogen because visible light accounts for a maximum of 53% of total solar energy.

As mentioned earlier, the development of photocatalysts with high quantum yield is a major limitation. The effectiveness of a photocatalyst relies on the ratio of the surface transfer rate of carriers to recombination carrier rate. As per literature reports, 90% of photogenerated electron-hole pairs quickly recombine after excitation [272,273]. This accounts for the moderately poor quantum yield of the majority of semiconductorbased photocatalysts.

3.5

Bang-gap engineering

The above-mentioned drawbacks have been resolved to a certain extent by a powerful technique referred to as bang-gap engineering whereby strategic control of semiconductor band profile efficiency of photocatalysts to visible light response is enhanced.

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Interfaces in Particle and Fibre Reinforced Composites

–2.0 GaP Cdse

2.0

2.3eV

H+/H2 Wo3 Fe2O3

2.8eV

1.0

MoS2 1.75eV

1.1eV

3.0eV

Si 2.25eV

1.7eV

TiO2

3.0eV

3.2eV

3.4eV

2.4eV

CdS

KTaO3 SrTiO3 0

SiC

ZnS

ZrO2

5.0eV

Potential / eV vs. NHE

–1.0

3.6eV

(a)

O2/H2O

3.0 4.0 Bias

hv

(b)

0 1 2 3

H2 H+

ECB H+/H2

Eg=3.2eV

Potential / eV vs. NHE

–1

(c)

H2O/O2

H 2O

EVB

hv Electron Hole

O2

TiO2

e–

e–

H+ H2

H 2O H++O2 TiO2

Membrane

Pt

Fig. 15.12 Correlation between band profile of semiconductor and redox potentials of water splitting (a) schematic of photocatalytic (b) photoelectrocatalytic, and (c) water splitting on TiO2 [271]. (a) Reproduced from A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253e278, with permission from The Royal Society of Chemistry.

3.5.1

Composite semiconductors (formation of semiconductor heterojunctions)

Coupling (composition) between two semiconductors will take place when conduction band electrons are introduced from one semiconductor having a narrowband gap to a semiconductor with a large band gap (Fig. 15.13). For example, CdS (band gap of 2.4 eV) can couple with SnO2 (band gap of 3.5 eV) to generate hydrogen from visible light irradiation [274]. Following conditions must be fulfilled for composite semiconductors to carry out water splitting i.e., for the production of hydrogen/oxygen: 1. The semiconductors should be free from photo-corrosion. 2. Electron injection must be an efficient rapid process. 3. The conduction band of the narrow band-gap semiconductor is more negative than large band gap semiconductor. 4. The conduction band of the wide band-gap semiconductor is more negative than EH2 =H2 O. 5. A narrow band-gap semiconductor is readily excitable by visible light energy.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

413

Semiconductor 1 Semiconductor 2 CB

e– e–

CB

E°(H+/H2) E°(O2/H2O)

Vis VB

UV h+ h+

VB

Fig. 15.13 Band structure profile of a composite photocatalyst with an improved dvisible-light response [282].

Doong et al. [275] and Kang et al. [276] showed in their experiments separately the improved photocatalytic activity of CdSeTiO2 composite semiconductor for 2chlorophenol degradation and 4-CP photodegradation, respectively, due to improved charge separation compared to individual CdS and TiO2 semiconducting materials. PANI/TiO2 composite has also exhibited high photocatalytic degradation of organic pollutants [277]. Further, So et al. [278] exhibited a higher rate of hydrogen production taking the same semiconductor composite material in comparison to an individual component. CdSeZnS, TiO2eSiO2, and RuS2/TiO2eSiO2 composite semiconductors have also been studied for the same application [279e281].

3.5.2

Cation/anion doping

It has been explored that doping can amplify a UV active photocatalyst response towards visible light irradiation. Using suitable transition metals as dopants have been a good approach. Examples include ZnS doped with Cu/Ni [283,284], C doped TiO2 [285], Ce doped TiO2 [286], Mn-doped ZnO [287], etc. Substitution of cations in the crystal lattice structure of a semiconductor creates impurity energy levels which aid the photocatalyst to absorb visible light. Even though the cation-dopant (Fig. 15.14) can generate the response to visible light, but these photocatalysts do

e–

CB

E°(H+/H2 )

E°(O2/H2O)

Vis

UV

Cation doping h+

VB

Fig. 15.14 Band structure profile of a cation-doped, large band-gap semiconductor photocatalyst [282].

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Interfaces in Particle and Fibre Reinforced Composites

not exhibit photoactivity since the dopants not only behave as light absorbing sites with the absorption coefficient relying on dopant density but furthermore as recombination centers between photogenerated electrons and holes [288]. Further, the impurity energy levels formed are generally discontinuous which would be unfavorable for the photogenerated holes to migrate [289]. Therefore, it is important to make minor adjustments in terms of dopant substitution by impregnation, precipitation or advanced ion implantation means [290e292]. Anion doping is also another approach to increase the responsive behavior of oxide based semiconductors towards visible light where N like anion dopants (S,C) replaces oxygen present in the oxide lattice (Fig. 15.15) [271,293e296]. Inter-mixing of p orbitals of the dopant and 2p orbitals of oxygen reduces the photocatalyst band gap energy by raising the valence band edge. Unlike in cation-dopant substitution, for anion replacement technique only free recombination sites are generated as a consequence they tend to exhibit better efficiency than the former case. However, the number of oxygen defects has to be inhibited or else the defects themselves can operate as a recombination center. Li et al. [297] demonstrated an interesting example of anion doping for purification of toxic air by studying N-F-co-doped TiO2 where the dopants exhibited a synergistic effect. N-dopant resulted in enhanced visible light response while F-dopant resulted in the formation of _OH and _O 2 radicals through oxygen vacancies.

3.5.3

Surface co-catalysts

Water adsorption on the surface of a photocatalyst and photocatalytic reactions may be amplified by utilizing noble metals or metal oxides (Pt, Rh, NiO, RuO2, etc.). The efficiency of a photocatalyst is improved because of the following reasons: 1. Co-catalysts help to capture the valence band holes and conduction band electrons [298]. As a consequence, the possibilities of generation of recombination centers are minimized. 2. Transport of electrons and holes to surface adsorbed water molecules is aided by using cocatalysts. This allows the activation energy barrier for the photocatalytic reaction to be reduced [299].

Examples like WO3 with CuO co-catalyst [300], Pt-TiO2/WO3 [301,302], etc., have been designed for environmental remediation.

Anion–doped semiconductor

E°(H+/H2)

E°(O2/H2O) O 2p+ N 2p, C 2p, or S 3p

CB

e–

Vis VB

Semiconductor CB e–

UV h+

VB h+

O 2p

Fig. 15.15 Band structure profile of an anion-doped, large band-gap semiconductor [282].

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

3.5.4

415

Semiconductor alloys

This approach involves solid solution formation between wide and narrow band gaps of similar crystal lattice structured semiconductors where formations of solid solution take place when the lattice sites are inter-dispersed in the components of solid-solution (Fig. 15.16). By varying the composition, the band gap levels can be customized. Examples include ZnSeCdS [303], GaNeZnO [274], (ZnS)x(CuInS2)1x [304], etc.

3.5.5

Nanodesign

The production and severance of carriers with a low rate of recombination is an important requisite for the functioning of a photocatalyst. The migration of the carriers will depend on the crystalline structure and their size, characteristics and number of crystal defects present and surface properties associated with the photocatalyst. To increase the rate by which the charge carrier reaches the surface, diffusion lengths of the carriers should be longer than the particle dimension i.e., as the particle size is decreased, the dynamics of the charge carriers will be enhanced [305]. As discussed previously, defects in the lattice of a semiconductor will engage in an important role in the transport of carriers because they can work as recombination centers for electron-holes pairs if present in the bulk/grain boundaries or they can assist the progress of a photocatalytic reaction by trapping charges if located at the surface. Thus, a photocatalyst with a high extent of particle crystallinity will decrease the defect density and migration of carriers will be unrestricted [306e308]. Further, crystal size also has a controlling effect on the electronic properties of a photocatalyst. Literature reports show that the band gap energy level of a crystalline based semiconductor is a function of the particle size of the crystal [309e311]. When the particle size is decreased to that of Bohr radius (first excitation state), the charge carriers are spatially confined and provide sizequantization effect. Due to this effect, the electrons and holes are restricted in a potential well and are not delocalized unlike in the bulk phase. This causes the increase in the band gap energy of a photocatalyst with a reduction in particle size from macro to nanometer scale [276]. As a consequence; the rate of photon absorption is decreased along with an alteration in the position of the flat-band which will ultimately modify

Semiconductor 1 CB

e–

Solid- solution semiconductor CB

e–

CB

E°(H+/H2) E°(O2/H2O)

VB

h+

VB

e–

Vis

Vis

UV

Semiconductor 1

h+

VB

h+

Fig. 15.16 Band structures of photocatalysts constructed from solid solutions of wide- and narrow-band-gap photocatalysts [282].

416

Interfaces in Particle and Fibre Reinforced Composites

the electron-hole pairs redox potential [312]. Additionally, nano-sized particles will give rise to new electronic states that can act as trapping sites and upgrade the electron-hole separation. Examples include CdS [313], HgSe, PbSe [314], ZnO [315] and TiO2 [316,317] with particle size varying between 3 and 15 nm. Table 15.7 presents a list of recently synthesized photocatalysts for water splitting reaction under visible light irradiation [283]. It is noteworthy to mention that metal chalcogenides are an emerging class of photocatalysts with complex morphological varieties, high activation and reaction abilities. In comparison to metal oxides, chalcogenide semiconductors often exhibit a small band gap which can help them to effectively utilize the maximum energy of the solar spectrum. To name a few very recent examples of metal chalcogenides as efficient photocatalysts for the water-splitting reaction are: ZnIn2S4 [318], WO3@ZnIn2S4 [319], CuGaS2 [320], CuS/TiO2 [321], etc. With regard to photocatalytic degradation of organic pollutants, catalysts such as CuSbSe2/TiO2 [322], SiO2/CdInSe-graphene [323], Ba2AsGaSe5 [324], Sn2SiS4 [325], a-EuZrS3 [326], and Cu7Te4 [327] have shown promising results. In recent times, 2D chalcogenides are drawing in great attention as active photocatalyst materials [328e330]. However, there are limitations to its performance in terms of swift recombination of electron-hole pairs [331], photocorrosion and poor accessibility of active site due to the restacking character [332]. To overcome such hurdles we have already presented the general

Table 15.7 Recently developed photocatalysts for water splitting reaction under visible light irradiation.

Photocatalyst

Cocatalyst

Activity (m mmol hL1gL1)

Sacrificial reagent

H2

O2

References

TiO2eN

Pt

CH3OH(0.1 M)/ AgNO3(0.05 M)

0

221

[253]

SrTiO3eCreTa

Pt

CH3OH (0.1 M)

140

e

[334]

Sm2Ti2S2O5

Pt

CH3OH(0.3 M)/ AgNO3(0.01 M)

40

16

[335]

TaON

Pt e

CH3OH(0.06 M) AgNO3(0.01 M)

50

3300

[280]

BiVO4

e

CH3OH(0.1 M)/ AgNO3(0.05 M)

0

421

[251]

CdS CdSeCdOeZnO

e

S2/SO2 3 (0.1 M/ 0.04 M)

9.8 11.6

e

[336]

ZnSeCu

e

SO2 3 (0.5 M)

450

e

[267]

/SO2 3

7666 4000

e

[276]

(CuAgIn)xZn2(1x)S2 (CuIn)xZn2(1x)S2

Ru

S

2

0.25 M)

(0.35 M/

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

417

strategies employed under the heading ‘3. The concept of band-gap engineering for the development of visible light active photocatalysts for energy harvesting applications’ where the approach has been to spatially integrate the metal and semiconductor to form a favorable interface of prime importance. Usually, larger the contact region on the interface higher will the adequate trapping and charge-transfer pathways are formed for effective separation of electron-hole pairs [333].

4. Probable mechanistic pathways at the interfaces of particle reinforced nanocomposites in energy storage devices As mentioned earlier on the importance of energy storage devices, we will now provide insight into their fundamentals. Depending on the character of materials and mechanism of charge-storage, super capacitors are broadly categorized as EDLC, the most prevalent device used for the commercial purpose [337], have conductive carbon-based materials with a high specific surface area as their constituent active material because of their low-priced tag and high electrochemical stability. The storage principle is based on the charge separation i.e., Helmholtz double layer (Fig. 15.17) at electrode-electrolyte interface i.e., excess conduction band electrons introduced in the electrode material are compensated by the cations/anions charge density on the electrolyte [338]. The absence of redox reactions in the active electrode materials of EDLC causes no limiting electrochemical kinetics through polarization resistance unlike in batteries. In addition, EDLCs can exhibit millions of charge-discharge cycles

Electrolyte

Collector

Solvated cations Separator

Polarized solvent molecular layer

Inner helmholtz plane (IHP)

Diffuse layer Outer helmholtz plane (OHP)

Fig. 15.17 Overall view of a double-layer of negatively charged ions in the electrode surface and solvated positive charged ions in the electrolyte media divided by a layer of polarized solvent molecules [340].

418

Interfaces in Particle and Fibre Reinforced Composites

without any lumps in the cycle curves in contrast to batteries. For example, exfoliation of bulk layered graphite gives 2D graphene, a promising EDLC material [339]. The second class of capacitors, known as pseudocapacitors is based on the rapidly reversible surface or near surface Faradaic redox reactions of active materials through the accumulation of charges on electrochemical active sites but not in the bulk unlike batteries (Fig. 15.18). Most commonly studied are metal oxides (RuO2, Fe3O4, MnO2, NiO, etc.) [32,33], conducting polymers [34] and functionalized porous carbons. Although metal oxides provide better performance in terms of specific capacitance, they show poor cyclic stability like batteries due to the occurrence of redox reactions that lowers the mechanical stability by swelling/shrinking of electrodes. The reversible surface adsorption reaction of electrolyte ions Cþ(Hþ, Naþ, Kþ) with an active electrode material (MnO2) is given as; MnO2 þ xCþ þ yHþ þ (x þ y)e 4 MnOOCxHy

(15.15)

The electronically conductive polymers such as polyaniline, polypyrrole etc. exhibits high volumetric and gravimetric capacitance in different organic electrolytes with a larger potential window. However, due to restricted stability during cycling process [302], research is oriented towards developing their role in hybrid systems. A pseudocapacitor is known as an intrinsic pseudocapacitor when materials show pseudocapacitive characteristics in a large domain of particle architecture and size. Extrinsic pseudocapacitance arises when the material in the nanoscale dimension exhibits pseudocapacitive performance under many conditions but if the same material exists in bulk phase then pseudocapacitive behavior will not be manifested [341].

Pseudocapacitance with specifically adsorbed ions Electrolyte

Collector

Solvated cations

Polarized solvent molecules Specifically adsorbed ions (redox ions) Separator Inner helmholtz plane

Fig. 15.18 Overall view of the Faradaic charge-transfer reaction of a pseudocapacitance [340].

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

419

Apart from the above mentioned classical category of capacitors, there is a diffusive intercalation pseudocapacitance mechanism which differs from redox pseudocapacitance in terms of its function at different kinetic systems (Fig. 15.19) [342,343]. The intercalation charge-storage mechanism is based on the intercalation of ions in the tunnels of layered materials like MXenes, transition metal dichalcogenides, etc., without involving any phase transitions unlike in batteries were phase changes occur [344]. Further, if the intercalation process is kinetically facile and occurs at the same redox reaction timescale then it is considered to be capacitive. This concept has been studied in MoO3 thin films where its nanostructure provided facile intercalation of Liþ ions into van der Waals gaps that contributed significantly to the pseudocapacitance [345]. Keeping this featured intercalation mechanism in mind one can design novel nanostructured active electrode materials for application as pseudocapacitors. It is possible to estimate the comparative contribution of intercalation mechanism and surface-confined redox pseudocapacitance to the total charge stored by the power-law equation expressed as current (i) - scan rate (v) relation [346]: i ¼ avb

(15.16)

or

(a)

RuOx(OH)y + δ H+ + δ e– ↔ RuOx–δ (OH)y+δ

(b)

H+ in electrolyte

Redox pseudocapacitance

Insertion host material

Current collector

Current collector

RuO2 nanocluster Hydrous grain boundary

Nb2O5 +xLi+ + xe– ↔ LixNb2O5

Li+ in electrolyte

Intercalation pseudocapacitance

Fig. 15.19 (a) Redox pseudocapacitance in RuO2 takes place in the near-surface area and (b) Intercalation pseudocapacitance in Nb2O5 (T-phase) is a bulk diffusion effect of Liþ insertion. Reproduced from V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for highrate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597e1614, with permission from The Royal Society of Chemistry.

420

Interfaces in Particle and Fibre Reinforced Composites

log i ¼ log a þ blog v

(15.17)

where, ‘a’ and ‘b’ are constants such that ‘b’ value can be obtained from the slope of log i versus log v plot. For b value ¼ 0.5, the kinetics of the system is considered to be diffusion-controlled and for b value ¼ 1 (or close to 1), the kinetics of the system is regarded to be capacitive controlled. For quantitative analysis, Dunn’s method [327,347,348] is followed and the equation is given as: i ¼ k1 v þ k2 v1=2

(15.18)

or i 1

v2

¼ k1 v1=2 þ k2

(15.19)

Where, k1 v and k2 v1=2 are the capacitive and diffusion current contributions, respectively. The slope and intercept of plot v1/2 vs. i(v)/v1/2 gives the values of k1 and k2 , respectively. At low scan rates due to sufficient time available for the electrolyte ions to diffuse easily in the interiors of the electrode active material, diffusion-controlled mechanism dominates whereas at high scan rates the ions can access only the surface thus the capacitive contribution becomes prevalent. We have presented our work on AueFe2O3 composite nanorod as a supercapacitor device (Fig. 15.20) as an example to explain Dunn’s equation for the study of charge storage mechanism where we obtained b values for anodic and cathodic peaks to be 0.90 and 0.87, respectively, suggesting a mixed contribution of capacitive and diffusive processes. Also, quantitatively we observed the capacitive contribution to be 78% at 1.5 mV s1. Thus, we see that depending on the origin of the charge storage mechanism, each energy storage system has its own limitations in distinct fields of applications in terms of energy and power density. Battery materials have high energy density and capacity but due to transitions at the bulk phase and slow solid-state diffusion, they tend to exhibit lower rate capability and low power density [350]. EDLCs, on the other hand, have long cycle life, high rate capability and high power density due to rapid charge storage physical process [351] on the surface but suffer from lower energy density than batteries. Therefore, the combination of EDLCs and pseudocapacitors or battery-type material can give rise to the latest kind of new-generation electrochemical capacitors known as hybrid capacitors which can possibly bridge the gap between conventional EDLCs and batteries and give an intermediate performance or even a better one than the constituent systems. The performance of a supercapacitor is controlled by the choice of electrode materials and electrolyte. We will briefly describe their importance in the smooth functioning of a supercapacitor device in subsequent sections.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

(a)

(b) Peak 1

0.5 mV 0.8 mV 1 mV 1.5 mV

2

3.5 3.4

1

Log i (Ag–1)

Current density (A/g)

3

0 –1 –2

Peak 1 2

b value 0.9 0.87

3.3 3.2

Peak 1 Peak 2 Linear fit of Peak 1 Linear fit of Peak 2

3.1

–3

Peak 2 –0.2

0.0

0.2

0.4

3.0 0.6

0.8

1.0

–0.3

–0.2

Potential V vs Ag/AgCl

–0.1

0.0

Log v (mV

(c)

0.1

0.2

s–1)

(d)

100

3 2 1 0 –1 –2 Diffisive

80 70 60 50 40 30 20 10

–3

0 –0.2

0.0

0.2

0.4

0.6

0.8

Potential (V vs Ag/AgCl)

Diffisive Capacitive

90

Capacitive

Capacitance (%)

Current density (A/g–1)

421

1.0

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Scan rate (mV sec–1)

Fig. 15.20 Cyclic voltammetric curves of the (a) AueFe2O3 at low scan rates (b) logarithmic plot for determination of b value, (c) capacitive and diffusive nature of the composite at 1.5 mV s1 scan rate, and (d) capacitance and diffusive current contributions to the overall capacitance at different scan rates. Adapted with permission from S. Rudra, A.K. Nayak, S. Koley, R. Chakraborty, P.K. Maji, M. Pradhan. Redox-mediated shape-transformation of Fe3O4 nanoflake to chemically stable AuFe2O3 composite nanorod for high-performance asymmetric solid-state supercapacitor device. ACS Sustain. Chem. Eng. 7 (2019) 724e733. Copyright 2018 American Chemical Society.

4.1 4.1.1

Electrode materials Carbon materials

Carbon-based materials are widely used as electrode materials due to their many qualities such as low cost, high availability and settled industrial process.

4.1.1.1 Activated carbon Activated carbon is the most preferred choice being cost-effective and having a large surface area. They are obtained by carbonization of natural precursors followed by activation which generates porosity formation in the bulk material. Table 15.8 lists a few carbon precursors and their BET surface area.

422

Interfaces in Particle and Fibre Reinforced Composites

Table 15.8 Carbon precursors and their respective BET surface area [352]. Carbon precursor

Activation method

Sbet (m2 gL1)

Coconut shell

KOH

1660

Bamboo

KOH

1290

Banana fiber

ZnCl2

1100

Sunflower seed shell

KOH

2510

Fish scale

e

2270

Rice husk

NaOH

1890

Nanopores can be segregated into three categories depending on their size, macropores (>450 nm), mesopores (2e50 nm) and micropores (1000 cycles)

References

Material

Electrolyte

Current density (A/g)

Ni3S2@ Ni(OH)2/ 3D graphene nanosheet

3 M KOH

5.1

1037.5

99.1

[405]

CuS nanohollow spheres

6 M KOH

1

948

90.0

[406]

MoS2/ graphene

1M Na2SO4

0.1

270

89.6

[407]

Bi2S3/MoS2

6 M KOH

1

3040

92.6

[408]

WS2/RGO

1M Na2SO4

(5 mV s1)

350

99.9

[409]

426

Interfaces in Particle and Fibre Reinforced Composites

show enhanced electrochemical performance which arises mainly from their high electronic conductivity along with good thermal and mechanical stability. Metal-selenides (Table 15.11) have also emerged as potential electrode materials with impressive redox chemistry. However, they are less reported compared to metal-sulfides.

4.2

Electrolyte

Electrolyte medium is a crucial character that regulates the activity of a supercapacitor. The criteria of an appropriate electrolyte are as follows: 1. High electrolyte concentration to prevent depletion that generally occurs during charging. 2. Temperature coefficient and conductivity. 3. High electrochemical stability, wide potential window, low solvated ionic radius, little viscosity, non-toxic, cost-effective and availability at pure state. 4. Non-corrosive to electrodes and current collectors.

4.2.1

Aqueous and organic electrolytes

Aqueous electrolytes characteristics are a suitable operating voltage window till 1 V (to avoid water decomposition around 1.23 V), conductivity of 1 S cm1, reduced pore size requirements and smaller ionic radius. Organic electrolytes can extend its potential window up to 2.7 V and even higher. The higher potential window of organic electrolytes produces nearly 50 times higher specific capacitance but lower power density [302]. Most widely used organic electrolytes are acetonitrile (ACN) and propylene carbonate (PC). ACN can dissolve a large amount of salts but is toxic in nature while in Table 15.11 Important metal selenide composites as electrochemical supercapacitors [404].

Capacitance (F/g)

Capacitance retention (>1000 cycles)

References

Material

Electrolyte

Current density (A/g)

NiSe2 single crystal

4 M KOH

3

1044

87.4

[410]

CuSe2/Cu

1 M NaOH

(0.25 mA/ cm2)

1037.5

104.3

[411]

MoSe2 nanosheet

6 M KOH

1

1114.3

104.7

[412]

Co0.85Se nanosheet

3 M KOH

1

1378

95.5

[413]

SnSe nanosheets

6 M KOH

0.5

228

e

[414]

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

427

comparison PC is environment-friendly, good electrical conductivity and broad operating temperature and voltage ranges. However, their expensive nature limits their practical applications.

4.2.2

Ionic liquids

Ionic liquids (ILs) are salts present in the liquid state. The two categories of ILs beneficial to supercapacitors are low-temperature ILs where the organic salts contain no solvent and have melting points below 100  C and room-temperature ILs (RTILs) where the liquid is maintained at room temperature. RTILs are regarded as a better choice of electrolyte over other conventional electrolytes due to them being highly resistant to heat, non-volatile and non-combustible nature and electrochemical stable potential range (4.5e6 V) [415]. Their peculiar yet convenient properties are controlled by their constituent components i.e., organic and counter ions. However, the main disadvantage lies in lower electrical conductivity (10 mS cm1) than aqueous electrolytes. Therefore, to overcome this drawback organic solvents are applied [374]. Even though they are considered as an effective treatment for improving supercapacitor performance yet their high generation cost and poor suitability with microporous carbons limits their requirements. Over the years, though maximum efforts have been carried out to know more about the fabrication of materials as supercapacitor devices, yet due to its vast potential and tremendous scope its research is still an ongoing process. Therefore, there is a limited concept of band-gap engineering in designing of the nanocomposites. However, we will now provide a brief idea about the usefulness of the technique in one of our work on tailored AueFe2O3 composite nanorod. From experimental observation (electrochemical impedance spectroscopy) we observed an increase in conductivity of AueFe2O3 (charge-transfer resistance ¼ 0.81 U) as compared to the mother component g-Fe2O3 (charge-transfer resistance ¼ 1.44 U). This implied that due to nanocomposite formation, the rate of electron transfer is reinforced in the designed nanocomposite. Further, to understand the mechanism behind the increase in conductivity we conducted theoretical analysis on AueFe2O3 and g-Fe2O3 (WIEN2K). Fig. 15.21(a) and (b) provide the crystal structural aspects of g-Fe2O3 and AueFe2O3 where we observed Au atoms to be occupying the interstices of the g-Fe2O3 matrix. Additionally, Fig. 15.21(c) shows that for g-Fe2O3 there is a distinct band gap energy separation between the valence and conduction bands whereas in Fig. 15.21(d), AueFe2O3, the band-gap discontinuities and the valence bands show crossing over the Fermi energy level which is an indication of heightened conductivity in AueFe2O3 than its mother component. Further, due to nanocomposite formation through bandgap engineering technique, increase in optical conductivity is also observed in AueFe2O3 (Fig. 15.22). Thus, we see that by band-gap engineering tool on designing metal-semiconductor nanocomposite, reinforcement of properties take place which enhances the overall functioning of a system.

(c)

(d)

0.04

0.04

0.02

0.02

Energy (eV)

(b)

Energy (eV)

(a)

0

–0.02

–0.02

–0.04

–0.04

Z

R

X

Z

R

X

(f)

(e) 0.6

0.6

xray spectra

0.5

0.5

0.4

0.4 xray spectra

xray spectra

0

0.3

0.3

0.2

0.2

0.1

0.1

0

0

0.5

1 eV

1.5

2

xray spectra

0

0

0.5

1 eV

1.5

2

Fig. 15.21 Theoretical crystal structure determination of (a) g-Fe2O3 and (b) AueFe2O3 (Fe3þ; green colour, O2 ;red colour and Au atoms; yellow colour); Calculated band structural profiles of (c) g-Fe2O3 and (d) AueFe2O3; Theoretically measured X-ray absorption spectra of (e) g-Fe2O3 and (f) AueFe2O3. Adapted with permission from S. Rudra, A.K. Nayak, S. Koley, R. Chakraborty, P.K. Maji, M. Pradhan. Redox-mediated shape-transformation of Fe3O4 nanoflake to chemically stable Au-Fe2O3 composite nanorod for high-performance asymmetric solid-state supercapacitor device. ACS Sustain. Chem. Eng. 7 (2019) 724e733. Copyright 2018 American Chemical Society.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

429

(b)

(a) 5

10

Optical conductivity

Optical conductivity

4.5 4

8

3.5 3

6

2.5 2

4

1.5 2

1 0.5 0

0

2

4

6

8

Energy (eV)

10

12

14

0

0

2

4

6

8

10

Energy (eV)

Fig. 15.22 Optical conductivity as a function of energy (eV) for (a) g-Fe2O3 and (b) AueFe2O3. Adapted with permission from S. Rudra, A.K. Nayak, S. Koley, R. Chakraborty, P.K. Maji, M. Pradhan. Redox-mediated shape-transformation of Fe3O4 nanoflake to chemically stable AuFe2O3 composite nanorod for high-performance asymmetric solid-state supercapacitor device. ACS Sustain. Chem. Eng. 7 (2019) 724e733. Copyright 2018 American Chemical Society.

5. Conclusions The semiconductor matrix reinforced nanocomposites with metals/metal oxides/metal chalcogenides have presented as a green, practical, versatile, and technology friendly material for energy-related applications. The first part of the chapter aims to provide the readers about the elementary concepts on energy harvesting and storage devices and their significance in today’s global crisis on environmental issues. In this regard, we see that scientific research has evolved in all dimensions and at the same time kept the fundamental objective intact in providing sustainable clean energy. Over the decade, many synthetic approaches have been proposed and followed as required for diverse energy-related purposes. We have discussed many such processes including commonly employed methods such as chemical and thermal methods for production of desired nanocomposites. With respect to photocatalysis, we have seen that conventional metal-oxides such as TiO2 respond most effectively under UV light which compromises only 4%e5% of the total solar spectrum while visible-light comprises 25%. In order to develop visible-light active photocatalysts, it is essential to narrow the band-gaps so that the longer-wavelength region can be harvested and in turn improve the charge separation of the photogenerated carriers. In case of materials designed for supercapacitor applications, low conductivity of current collectors and chemical stability have always been a graving concern. Again, one can tune the band-gaps by the construction of suitable nanocomposites. Keeping these aspects in mind, we carried our discussion on band-gap engineering technique focusing on the general mechanistic pathways involved in photocatalysis and in supercapacitors. The importance of Schottky barrier in metal (electrolyte)-semiconductor interfaces

430

Interfaces in Particle and Fibre Reinforced Composites

along with different processes by which band-gaps can be engineered such as ion doping or design of nanostructures for the fabrication and improvement of the overall performance of metal-semiconductor materials have also been emphasized. In view of this, many significant results have been highlighted along the way such as PANI/TiO2 and N-F-co-doped TiO2 have shown to be promising environmental remediation photocatalyst, ZnSeCu and TiO2eN exhibit water-splitting reaction and AueV2O5 and AueFe2O3 as high-performing supercapacitors. Although the unique and versatile properties of metal oxide semiconductors and their nanocomposites have shown significant effects on the performance of energy harvesting and storage devices efficiency, improvement in terms of scientific and technological domains are to be carried out with greater efforts. Further, future research should be focused on the development of new photocatalyst materials for effective solar energy utilization and efficient conversion to electrical or chemical energy resulting in a steady energy source along with improved environmental remediation materials. In addition, energy storage devices are to be better designed and fabricated so to achieve high energy density performance without compromising the power density of material thus ensuring a more sustainable and greener environment for future generations.

References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer, 1999. [2] J.O'M. Bockris, The origin of ideas on a hydrogen economy and its solution to the decay of the environment, Int. J. Hydrogen Energy 27 (2002) 731e740. [3] J.M. Ogden, Testimony to the Committee on Science, US House of Representatives, Washington, DC, 2003. [4] R.M. Navarro, M.A. PeC¸a, J.L.G. Fierro, Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass, Chem. Rev. 107 (2007) 3952e3991. [5] J.A. Armor, The multiple roles for catalysis in the production of H2, Appl. Catal. A Gen. 176 (1998) 159e176. [6] D.L. Trimm, Z.I. Onsan, Onboard fuel conversion for hydrogen-fuel-cell-driven vehicles, Catal. Rev. Sci. Eng. 43 (2001) 31e84. [7] J.L.G. Fierro, M.A. PeC¸a, in: M. Fernndez-Garca, J.A. Anderson (Eds.), Supported Metals in Catalysis, ICP, London, 2005. [8] J.R. Rostrup-Nielsen, J.S. Sehested, J.K. Nrskov, Hydrogen and synthesis gas by steamand CO2 reforming, Adv. Catal. 47 (2002) 65e139. [9] J.R. Rostrup-Nielsen, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Springer Verlag, Berlin, 1984. [10] N. Muradov, Hydrogen via methane decomposition: an application for decarbonization of fossil fuels, Int. J. Hydrogen Energy 26 (2001) 1165e1175. [11] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 15729e15735. [12] R.M. Navarro, M.C. Snchez-Snchez, M.C. Alvarez, F. del Valle, J.L.G. Fierro, Improving biodiesel fuel properties by modifying fatty ester composition, Energy Environ. Sci. 2 (2009) 759e766.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

431

[13] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253e278. [14] K. Maeda, K. Domen, Photocatalytic water splitting: recent progress and future challenges, J. Phys. Chem. Lett. 1 (2010) 2655e2661. [15] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446e6473. [16] J.M. Lehn, J.P. Sauvage, R. Ziessel, Photochemical water splitting continuous generation of hydrogen and oxygen by irradiation of aqueous suspensions of metal loaded strontiumtitanate, Nouv. J. Chim. (N. J. Chem.) 4 (1980) 623e627. [17] S. Sato, J.M. White, Photocatalytic production of hydrogen from water and Texas lignite by use of a platinized titania catalyst, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 542e544. [18] K. Domen, S. Naito, M. Soma, T. Onishi, K. Tamaru, Photocatalytic decomposition of water vapour on an NiO-SrTiO3 catalyst, J. Chem. Soc. Chem. Commun. 12 (1980) 543e544. [19] F.E. Osterloh, Inorganic materials as catalysts for photochemical splitting of water, Chem. Mater. 20 (2008) 35e54. [20] K. Maeda, K. Domen, New non-oxide photocatalysts designed for overall water splitting under visible light, J. Phys. Chem. C 111 (2007) 7851e7861. [21] R.M. Navarro, M.C. Snchez-Snchez, M.C. Alvarez-Galvan, F.D. Valle, J.L.G. Fierro, Hydrogen production from renewable sources: biomass and photocatalytic opportunities, Energy Environ. Sci. 2 (2009) 35e54. [22] N. Serpone, E. Pelizzetti, Photocatalysis, Wiley, New York, 1989. [23] W. Zhang, S.B. Park, E. Kim, The rise of renewable energy, Energy Fuels 295 (2006) 82e93. [24] K. Maeda, K. Teramura, K. Domen, Effect of post-calcination on photocatalytic activity of (Ga1 xZnx)(N1 xOx) solid solution for overall water splitting under visible light, J. Catal. 254 (2008) 198e204. [25] I. Paramasivam, H. Jha, N. Liu, P. Schmuki, A review of photocatalysis using selforganized TiO2 nanotubes and other ordered oxide nanostructures, Small 8 (2012) 3073e3103. [26] T. Maggos, P. Leva, J.B. Bartzis, C. Vasilakos, D. Kotzias, Document gas phase photocatalytic oxidation of VOC using TiO2-containing paint: influence of NO and relative humidity, WIT Trans. Ecol. Environ. 101 (2007) 585e593. [27] H. Irie, Y. Watanabe, K. Hashimoto, Carbon-doped anatase TiO2 powders as a visiblelight sensitive photocatalyst, Chem. Lett. 32 (2003) 772e773. [28] S. Yang, L. Gao, New method to prepare nitrogen-doped titanium dioxide and its photocatalytic activities irradiated by visible light, J. Am. Ceram. Soc. (2004) 1803e1805. [29] A. Fuerte, M.D. Hernandez-Alonso, A.J. Maria, A. Martinez-Arias, M. FernandezGarcia, J.C. Conesa, J. Soria, Visible light activated nanosized doped-TiO2 photocatalysts, Chem. Commun. (2001) 2718e2719. [30] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [31] R.J. Brodd, K.R. Bullock, R.A. Leising, R.L. Middaugh, J.R. Miller, E. Takeuchi, Batteries, 1977 to 2002, J. Electrochem. Soc. 151 (2004) K1eK11. [32] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [33] M. Armand, P. Johansson, Novel weakly coordinating heterocyclic anions for use in lithium batteries, J. Power Sources 178 (2008) 821e825.

432

Interfaces in Particle and Fibre Reinforced Composites

[34] A. Pandolfo, A. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (2006) 11e27. [35] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Sci. Mag. 321 (2008) 651e652. [36] T. Christen, M.W. Carlen, Theory of Ragone plots, J. Power Sources 91 (2000) 210e216. [37] H.I. Becker, U.S. Patent No. 2,800,616, U.S. Patent and Trademark Office, Washington, DC, 1957. [38] O. Barbieri, M. Hahn, A. Herzog, R. Ktz, Capacitance limits of high surface area activated carbons for double layer capacitors, Carbon 43 (2005) 1303e1310. [39] N.L. Wu, Nanocrystalline oxide supercapacitors, Mater. Chem. Phys. 75 (2002) 6e11. [40] T. Brousse, M. Toupin, R. Dugasa, L. Athouëla, O. Crosniera, D. Bélanger, Crystalline MnO2 as possible alternatives to amorphous compounds in electrochemical supercapacitors, J. Electrochem. Soc. 153 (2006) A2171eA2180. [41] A. Rudge, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Conducting polymers as active materials in electrochemical capacitors, J. Power Sources 47 (1994) 89e107. [42] L. Ji, P. Meduri, V. Agubra, X. Xiao, M. Alcoutlabi, Graphene-based nanocomposites for energy storage, Adv. Energy Mater. 6 (2016) 1502159. [43] V.A. Agubra, L. Zuniga, D. Flores, J. Villareal, M. Alcoutlabi, Composite nanofibers as advanced materials for Li-ion, Li-O2 and Li-S batteries, Electrochim. Acta 192 (2016) 529e550. [44] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682e2699. [45] C. Ma, C. Ma, J. Wang, H. Wang, J. Shi, Y. Song, Q. Guo, L. Liu, Exfoliated graphite as a flexible and conductive support for Si-based Li-ion battery anodes, Carbon 72 (2014) 38e46. [46] R. Roy, R.A. Roy, D.M. Roy, Alternative perspectives on “quasi-crystallinity”: nonuniformity and nanocomposites, Mater. Lett. 4 (1986) 323e328. [47] O. Kamigaito, What can be improved by nanometer composites? J. Jpn. Soc. Powder Metall. 38 (1991) 315e321. [48] D. Schmidt, D. Shah, E.P. Giannelis, New advances in polymer/layered silicate nanocomposites, Curr. Opin. Solid State Mater. Sci. 6 (2002) 205e212. [49] F. Wypych, N. Seefeld, I. Denicolo, Preparation of nanocomposites based on the encapsulation of conducting polymers into 2H-MoS2 and 1T-TiS2, Quim. Nova 20 (1997) 356e360. [50] S.T. Aruna, K.S. Rajam, Synthesis, characterisation and properties of Ni/PSZ and Ni/YSZ nanocomposites, Scr. Mater. 48 (2003) 507e512. [51] E.P. Giannelis, Polymer layered silicate nanocomposites, Adv. Mater. 8 (1996) 29e35. [52] F. Wypych, L.B. Adad, M.C. Grothe, Synthesis and characterisation of the nanocomposites K-0, K0,1(PEO)xMoS2(X ¼ 0,5; 1,2), Quim. Nova 21 (1998) 687e692. [53] M. Sternitzke, Review: structural ceramic nanocomposites, J. Eur. Ceram. Soc. 17 (1997) 1061e1082. [54] A. Peigney, C.H. Laurent, E. Flahaut, A. Rousset, Carbon nanotubes in novel ceramic matrix nanocomposites, Ceram. Int. 26 (2000) 677e683. [55] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng. R 28 (2000) 1e63. [56] R. Gangopadhyay, D. Amitabha, Conducting polymer nanocomposites: a brief overview, Chem. Mater. 12 (2000) 608e622.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

433

[57] E.T. Thostenson, Z. Ren, T.W. Chou, Advances in the science and technology of carbon nanotubes and their composites: a review, Compos. Sci. Technol. 61 (2001) 1899e1912. [58] V.V. Pokropivnyi, Two-dimensional nanocomposites: photonic crystals and nanomembranes (Review). Part 1. Types and preparation, Powder Metall. Met. Ceram. 41 (2002) 264e272. [59] V.V. Pokropivnyi, Two-dimensional nanocomposites: photonic crystals and nanomembranes (Review). Part 2: properties and applications, Powder Metall. Met. Ceram. 41 (2002) 369e381. [60] K. Gall, M.L. Dunn, Y. Liu, D. Finch, M. Lake, N.A. Munshi, Shape memory polymer nanocomposites, Acta Mater. 50 (2002) 5115e5126. [61] G. Kickelbick, Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale, Prog. Polym. Sci. 28 (2003) 83e114. [62] H. Fischer, Polymer nanocomposites: from fundamental research to specific applications, Mater. Sci. Eng. C 23 (2003) 763e772. [63] S.S. Ray, M. Okamoto, Polymer-layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (2003) 1539e1641. [64] R. Andrews, M.C. Weisenberger, Carbon nanotube polymer composites, Curr. Opin. Solid State Mater. Sci. 8 (2004) 31e37. [65] C. Wang, Z.X. Guo, S. Fu, W. Wu, D. Zhu, Polymers containing fullerene or carbon nanotube structures, Prog. Polym. Sci. 29 (2004) 1079e1141. [66] J.K. Pandey, K.R. Reddy, A.P. Kumar, R.P. Singh, An overview on the degradability of polymer nanocomposites, Polym. Degrad. Stab. 88 (2005) 234e250. [67] E.T. Thostenson, C. Li, T.W. Chou, Nanocomposites in context, Compos. Sci. Technol. 65 (2005) 491e516. [68] J. Jordan, K.I. Jacob, R. Tannenbaum, M.A. Sharaf, I. Jasiuk, Experimental trends in polymer nanocomposites: a review, Mater. Sci. Eng. A 393 (2005) 1e11. [69] S.M. Choi, H. Awaji, Nanocomposites: a new material design concept, Sci. Technol. Adv. Mater. 6 (2005) 2e10. [70] X.L. Xie, Y.W. Mai, X.P. Zhou, Dispersion and alignment of carbon nanotubes in polymer matrix: a review, Mater. Sci. Eng. R 49 (2005) 89e112. [71] S.S. Ray, M. Bousmina, Biodegradable polymers and their layered silicate nanocomposites: in greening the 21st century materials world, Prog. Mater. Sci. 50 (2005) 962e1079. [72] J.K. Pandey, A.P. Kumar, M. Misra, A.K. Mohanty, L.T. Drzal, R.P. Singh, Recent advances in biodegradable nanocomposites, J. Nanosci. Nanotechnol. 5 (2005) 497e526. [73] B.K.G. Theng, The Chemistry of Clay-Organic Reactions, Wiley, New York, 1974. [74] M. Ogawa, K. Kuroda, Preparation of inorganic composites through intercalation of organoammoniumions into layered silicates, Bull. Chem. Soc. Jpn. 70 (1997) 2593e2618. [75] M.W. Noh, D.C. Lee, Synthesis and characterization of ps-clay nanocomposite by emulsion polymerization, Polym. Bull. 42 (1999) 619e626. [76] A. Nakahira, K. Niihara, Structural ceramics-ceramic nanocomposites by sintering method: roles of nano-size particles, J. Ceram. Soc. Jpn. 100 (1992) 448e453. [77] L. Stearns, C. Zhao, J. Martin, P. Harmer, Processing and microstructure development in Al2O3-SiC ‘nanocomposites’, J. Eur. Ceram. Soc. 10 (1992) 473e477.

434

Interfaces in Particle and Fibre Reinforced Composites

[78] C.E. Borsa, R.J. Brook, Fabrication of Al2O3/SiC nanocomposites using a polymeric precursor for SiC, in: Proceedings of the International Conference of Ceramic Processing Science and Technology, Sept. 11e14, Friedrichshafen, The American Ceramic Society, Westerville, Germany, 1995, pp. 653e658. [79] R. Riedel, M. Seher, J. Mayer, D. Szabo, Polymer-derived Si-based bulk ceramics. Part I: preparation, processing and properties, J. Eur. Ceram. Soc. 15 (1995) 703e715. [80] J. Livage, Sol-gel processes, Curr. Opin. Solid State Mater. Sci. 2 (1997) 132e136. [81] K.A. Vorotilov, M.I. Yanovskaya, E.P. Turevskaya, A.S. Sigov, Sol-gel derived ferroelectric thin films: avenues for control of microstructural and electric properties, J. Sol. Gel Sci. Technol. 16 (1999) 109e118. [82] L.L. Hench, J.K. West, The sol-gel process, Chem. Rev. 90 (1990) 33e72. [83] S. Sen, R.N.P. Choudharya, P. Pramanik, Synthesis and characterization of nanostructured ferroelectric compounds, Mater. Lett. 58 (2004) 3486e3490. [84] N. Viart, M. Richard-Plouet, D. Muller, G. Pourroy, Synthesis and characterization of Co/ ZnO nanocomposites: towards new perspectives offered by metal/piezoelectric composite materials, Thin Solid Films 437 (2003) 1e9. [85] K. Baiju, C.P. Sibu, K. Rajesh, P.K. Pillai, P. Mukundan, K.G.K. Warrier, W. Wunderlich, An aqueous sol-gel route to synthesize nanosized lanthana doped titania having an increased anatase phase stability for photocatalytic application, Mater. Chem. Phys. 90 (2005) 123e127. [86] S. Ananthakumar, A.K. Prabhakaran, U.S. Hareesh, P. Manoharan, K.G.K. Warrier, Gel casting process for Al2O3-SiC nanocomposites and its creep characteristics, Mater. Chem. Phys. 85 (2004) 151e157. [87] S. Sivakumar, C.P. Sibu, P. Mukundan, P.K. Pillai, K.G.K. Warrier, Nanoporous titaniaalumina mixed oxides: an alkoxide free sol-gel synthesis, Mater. Lett. 58 (2004) 2664e2669. [88] W. Wunderlich, P. Padmaja, K.G.K. Warrier, TEM characterization of sol-gel-processed alumina-silica and alumina-titania nano-hybrid oxide catalysts, J. Eur. Ceram. Soc. 24 (2004) 313e317. [89] N.N. Ghosh, P. Pramanik, Aqueous sol-gel synthesis of nanosized ceramic composite powders with metal-formate precursors, Mater. Sci. Eng. C 16 (2001) 113e117. [90] J. Ning, J. Zhang, Y. Pan, J. Guo, Fabrication and mechanical properties of SiO2 matrix composites reinforced by carbon nanotube, Mater. Sci. Eng. A 357 (2003) 392e396. [91] R. Kamalakaran, F. Lupo, N. Grobert, D. Lozano-Castello, N.Y. Jin-Phillipp, M. Ruhle, In-situ formation of carbon nanotubes in an alumina-nanotube composite by spray pyrolysis, Carbon 41 (2003) 2737e2741. [92] W. Chaisan, R. Yimnirun, S. Ananta, Preparation and characterization of ceramic: nanocomposites in the PZTeBT system, Ceram. Int. 35 (2009) 121e124. [93] S.I. Cha, K.T. Kim, K. Lee, C.B. Mo, S.H. Hong, Strengthening and toughening of carbon nanotube reinforced alumina nanocomposite fabricated by molecular level mixing process, Scr. Mater. 53 (2005) 793e797. [94] D.D.L. Chung, Cement-matrix structural nanocomposites, Met. Mater. Int. 10 (2004) 55e67. [95] A. Peigney, E. Flahaut, C.H. Lautent, F. Chastel, A. Rousset, Aligned carbon nanotubes in ceramic-matrix nanocomposites prepared by high-temperature extrusion, Chem. Phys. Lett. 352 (2002) 20e25. [96] D.Y. Lee, M.H. Lee, K.J. Kim, S. Heo, B.Y. Kim, S.J. Lee, Effect of multiwalled carbon nanotube (M-CNT) loading on M-CNT distribution behavior and the related electromechanical properties of the M-CNT dispersed ionomeric nanocomposites, Surf. Coat. Technol. 200 (2005) 1920e1925.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

435

[97] X.T. Wang, N.P. Padture, H. Tanaka, Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites, Nat. Mater. 3 (2004) 539e544. [98] W. Chen, J. Zhang, W. Cai, Sonochemical preparation of Au, Ag, Pd/SiO2 mesoporous nanocomposites, Scr. Mater. 48 (2003) 1061e1066. [99] Y. Sakka, D.D. Bidinger, A. Aksay, Processing of silicon carbide-mullite-alumina nanocomposites, J. Am. Ceram. Soc. 78 (1995) 479e486. [100] M. Nawa, T. Sekino, K. Niihara, Fabrication and mechanical-behavior of Al2O3/Mo nanocomposites, J. Mater. Sci. 29 (1994) 3185e3192. [101] L.A. Timms, C.B. Ponton, M. Strangwood, Processing of Al2O3/SiC nanocompositespart 2: green body formation and sintering, J. Eur. Ceram. Soc. 22 (2002) 1569e1586. [102] G.D. Zhan, J.D. Kuntz, J. Wan, J.E. Garay, A.K. Mukherjee, Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes, Appl. Phys. Lett. 83 (2003) 1228e1230. [103] Z. Xia, L. Riester, W.A. Curtin, H. Li, B.W. Sheldon, J. Liang, B. Chang, J.M. Xu, Direct observation of toughening mechanisms in carbon nanotube ceramic matrix composites, Acta Mater. 52 (2004) 931e944. [104] P.H.C. Camargo, K.G. Satyanarayana, F. Wypych, Nanocomposites: synthesis, structure, properties and new application opportunities, Mater. Res. 12 (2009) 1e39. [105] C.S. Balazsi, Z. Shen, Z. Konya, Z.A. Kaztovszky, F. Wéber, Z. Vértesy, L.P. Bir o, I. Kiricsi, P. Arato, Processing of carbon nanotube reinforced silicon nitride composites by spark plasma sintering, Compos. Sci. Technol. 65 (2005) 727e733. [106] C.C. Anya, Microstructural nature of strengthening and toughening in Al2O3-SiC(P) nanocomposites, J. Mater. Sci. 34 (1999) 5557e5567. [107] C. Goujon, P. Goeuriot, Solid state sintering and high temperature compression properties of Al-alloy 5000/AlN nanocomposites, Mater. Sci. Eng. A 315 (2001) 180e188. [108] C. Baker, S. Ismat Shah, S.K. Hasanain, Magnetic behavior of iron and iron-oxide nanoparticle/polymer composites, J. Magn. Magn. Mater. 280 (2004) 412e418. [109] A. Contreras, E. Bedolla, Mg/TiC composites manufactured by pressureless melt infiltration, Scr. Mater. 51 (2004) 249e253. [110] D. Srinivasan, K. Chattopadhyay, Hardness of high strength nanocomposite AleXeZr (X¼Si,Cu,Ni) alloys, Mater. Sci. Eng. A 375e377 (2004) 1228e1234. [111] L. Xiaochun, Y. Yang, X. Cheng, Ultrasonic-assisted fabrication of metal matrix nanocomposites, J. Mater. Sci. 39 (2004) 3211e3212. [112] M.C. Joseph, C. Tsotsos, M.A. Baker, P.J. Kench, C. Rebholz, A. Matthews, A. Leyl, Characterisation and tribological evaluation of nitrogen-containing molybdenum-copper PVD metallic nanocomposite films, Surf. Coat. Technol. 190 (2005) 345e356. [113] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Recent advances in the liquid-phase syntheses of inorganic nanoparticles, Chem. Rev. 104 (2004) 3893e3946. [114] R. West, Y. Wang, T. Goodson, Nonlinear absorption properties in novel gold nanostructured topologies, J. Phys. Chem. B 107 (2003) 3419e3426. [115] E.E. Carpenter, A. Kumbhar, J.A. Wiemann, H.J. Srikanth Wiggins, W. Zhou, C.J. O’Connor, Synthesis and magnetic properties of goldeironegold nanocomposites, Mater. Sci. Eng. A 286 (2000) 81e86. [116] W.X. Chen, J.Y. Lee, Z. Liu, The nanocomposites of carbon nanotube with Sb and SnSb0.5 as Li-ion battery anodes, Carbon 41 (2003) 959e966. [117] J. Yang, R. Schaller, Mechanical spectroscopy of Mg reinforced with Al2O3short fibers and carbon nanotubes, Mater. Sci. Eng. A 370 (2004) 512e515. [118] Special Feature, Tiny tubes boost for metal matrix composites, Met. Powder Rep. 59 (2004) 40e43.

436

Interfaces in Particle and Fibre Reinforced Composites

[119] Y. Jang, S. Kim, S. Lee, D. Kim, M. Um, Fabrication of carbon nano-sized fiber reinforced copper composite using liquid infiltration process, Compos. Sci. Technol. 65 (2005) 781e802. [120] L.S. Schadler, S.C. Giannaris, P.M. Ajayan, Load transfer in carbon nanotube epoxy composites, Appl. Phys. Lett. 73 (1998) 3842e3844. [121] X. Chen, J. Xia, J. Peng, W. Li, S. Xie, Carbon-nanotube metal-matrix composites prepared by electroless plating, Compos. Sci. Technol. 60 (2000) 301e306. [122] C.L. Xu, B.Q. Wei, R.Z. Ma, J. Liang, X.K. Ma, D.H. Wu, Fabrication of aluminiumcarbon nanotube composites and their electrical properties, Carbon 37 (1999) 855e858. [123] T. Noguchi, A. Magario, S. Fuzukawa, S. Shimizu, Carbon nanotube aluminium composites with uniform dispersion, Mater. Trans. 45 (2004) 602e604. [124] Y.H. Choa, J.K. Yang, B.H. Kim, Y.K. Jeong, J.S. Lee, T. Nakayama, T. Sekino, K. Niihara, Preparation and characterization of metal: ceramic nanoporous nanocomposite powders, J. Magn. Magn. Mater. 266 (2003) 12e19. [125] F.A. Khalid, O. Beffort, U.E. Klotz, B.A. Keller, P. Gasser, S. Vaucher, Study of microstructure and interfaces in an aluminiumeC60 composite material, Acta Mater. 51 (2003) 4575e4582. [126] K.L. Choy, Chemical vapour deposition of coatings, Prog. Mater. Sci. 48 (2003) 57e170. [127] V. Bhattacharya, K. Chattopadhyay, Microstructure and tribological behaviour of nanoembedded al-alloys, Scr. Mater. 44 (2001) 1677e1682. [128] D.J. Branagan, Y. Tang, Developing extreme hardness (>15 GPa) in iron based nanocomposites, Compos. Part A 33 (2002) 855e859. [129] P.V. Kamat, M. Flumiani, A. Dawson, Metal-metal and metal-semiconductor composite nanoclusters, Colloids Surf. A 202 (2002) 269e279. [130] G. Jimenez, N. Ogata, H. Kawai, T. Ogihara, Structure and thermal/mechanical properties of poly (ε-caprolactone)-clay blend, J. Appl. Polym. Sci. 64 (1997) 2211e2220. [131] H.G. Jeon, H.T. Jung, S.W. Lee, S.D. Hudson, Morphology of polymer silicate nanocomposites, Polym. Bull. 41 (1998) 107e113. [132] A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, Synthesis of nylon-6-clay hybrid, J. Mater. Res. 8 (1993) 1179e1183. [133] K.J. Yao, M. Song, D.J. Hourston, D.Z. Luo, Polymer/layered clay nanocomposites: 2-polyurethane nanocomposites, Polymer 43 (2002) 1017e1020. [134] M. Okamoto, S. Morita, T. Kotaka, Dispersed structure and ionic conductivity of smectic clay/polymer nanocomposites, Polymer 42 (2001) 2685e2688. [135] R.A. Vaia, E.P. Giannelis, Lattice of polymer melt intercalation in organically modified layered silicates, Macromolecules 30 (1997) 7990e7999. [136] J.W. Gilmann, Flammability and thermal stability studies of polymer-layered-silicate (clay) nanocomposites, Appl. Clay Sci. 15 (1999) 31e49. [137] D.L. Tomasko, X. Han, D.H. Liu, W. Gao, Supercritical fluid applications in polymer nanocomposites, Curr. Opin. Solid State Mater. Sci. 7 (2003) 407e412. [138] K.A. Carrado, L.Q. Xu, In situ synthesis of polymer-clay nanocomposites from silicate gels, Chem. Mater. 10 (1998) 1440e1445. [139] Z.H. Mbhele, M.G. Salemane, C.G.C.E. Van Sittert, J.M. Nedeljkovic, V. Djokovic, A.S. Luyt, Fabrication and characterization of silver-polyvinyl alcohol nanocomposites, Chem. Mater. 15 (2003) 5019e5024. [140] V.M.F. Evora, A. Shukla, Fabrication, characterization, and dynamic behavior of polyester/TiO2 nanocomposites, Mater. Sci. Eng. A 361 (2003) 358e366. [141] S.S. Park, N. Bernet, S. De La Roche, H.T. Hanh, Processing of iron oxide-epoxy vinyl ester nanocomposites, J. Compos. Mater. 37 (2003), 465e465.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

437

[142] J. Liu, Y. Gao, F. Wang, D. Li, J. Xu, Preparation and characteristic of a new class of silica/polyimide nanocomposites, J. Mater. Sci. 37 (2002) 3085e3088. [143] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle, Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties, Polymer 40 (1999) 5967e5971. [144] Z. Roslaniec, G. Broza, K. Schulte, Nanocomposites based on multiblock polyester elastomers (PEE) and carbon nanotubes (CNT), Compos. Interfac. 10 (2003) 95e102. [145] M. Reza Gholipour, C.T. Dinh, F. Beland, T.O. Do, Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting, Nanoscale 7 (2015) 8187e8208. [146] M. Wang, J. Ioccozia, L. Sun, C. Lin, Z. Lin, Inorganic-modified semiconductor TiO2 nanotube arrays for photocatalysis, Energy Environ. Sci. 7 (2014) 2182e2202. [147] D.P. Dubal, O. Ayyad, V. Ruiz, P. Gomez-Romero, Hybrid energy storage: the merging of battery and supercapacitor chemistries, Chem. Soc. Rev. 44 (2015) 1777e1790. [148] Z. Liang, Q. Zhang, L. Jiang, G. Cao, ZnO cathode buffer layers for inverted polymer solar cells, Energy Environ. Sci. 8 (2015) 3442e3476. [149] K.C. Leung, S. Xuan, X. Zhu, D. Wang, C.P. Chak, S.F. Lee, W.K. Ho, B.C. Chung, Gold and iron oxide hybrid nanocomposite materials, Chem. Soc. Rev. 41 (2012) 1911e1928. [150] N.E. Motl, A.F. Smith, C.J. DeSantis, S.E. Skrabalak, Engineering plasmonic metal colloids through composition and structural design, Chem. Soc. Rev. 43 (2014) 3823e3834. [151] A.K. Sasmal, J. Pal, R. Sahoo, P. Kartikeya, S. Dutta, T. Pal, Superb dye adsorption and dye-sensitized change in Cu2OeAg crystal faces in the dark, J. Phys. Chem. C 120 (2016) 21580e21588. [152] A.K. Sasmal, S. Dutta, T. Pal, A ternary Cu2OeCueCuO nanocomposite: a catalyst with intriguing activity, Dalton Trans. 45 (2016) 3139e3150. [153] A.K. Sasmal, C. Mondal, A.K. Sinha, S.S. Gauri, J. Pal, T. Aditya, M. Ganguly, S. Dey, T. Pal, Fabrication of superhydrophobic copper surface on various substrates for roll-off, self-cleaning, and water/oil separation, ACS Appl. Mater. Interfaces 6 (2014) 22034e22043. [154] J. Pal, M. Ganguly, S. Dutta, C. Mondal, Y. Negishi, T. Pal, Hierarchical AueCuO nanocomposite from redox transformation reaction for surface enhanced Raman scattering and clock reaction, CrystEngComm 16 (2014) 883e893. [155] H. Zarrin, J. Fu, G. Jiang, S. Yoo, J. Lenos, M. Fowler, Z. Chen, Quaternized graphene oxide nanocomposites as fast hydroxide conductors, ACS Nano 9 (2015) 2028e2037. [156] M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Long-lived, visible-light-excited charge carriers of TiO2/BiVO4 nanocomposites and their unexpected photoactivity for water splitting, Adv. Energy Mater. 4 (2014) 1300995. [157] J. Ma, Z. Fang, Y. Yan, Z. Yang, L. Gu, Y.S. Hu, H. Li, Z. Wang, X. Huang, Popcorn inspired porous macrocellular carbon: rapid puffing fabrication from rice and its applications in lithiumesulfur batteries, Adv. Energy Mater. 8 (2015) 1500046. [158] M. Gou, X. Qu, W. Zhu, M. Xiang, J. Yang, K. Zhang, Y. Wei, S. Chen, Bio-inspired detoxification using 3D-printed hydrogel nanocomposites, Nat. Commun. 5 (2014) 3774. [159] L. Fu, K. Tang, H. Oh, K. Manickam, T. Brauniger, C.V. Chandran, A. Menzel, M. Hirscher, D. Samuelis, J. Maier, Job-sharing Storage of hydrogen in Ru/Li2O nanocomposites, Nano Lett. 15 (2015) 4170e4175.

438

Interfaces in Particle and Fibre Reinforced Composites

[160] X. Jin, K. Adpakpang, I. Young Kim, S. Mi Oh, N.S. Lee, S.J. Hwang, An effective way to optimize the functionality of graphene-based nanocomposite: use of the colloidal mixture of graphene and inorganic nanosheets, Sci. Rep. 5 (2015) 11057. [161] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911. [162] E.M. Zahran, N.M. Bedford, M.A. Nguyen, Y.J. Chang, B.S. Guiton, R.R. Naik, L.G. Bachas, M.R. Knecht, Gold nanocluster-catalyzed semihydrogenation: a unique activation pathway for terminal alkynes, J. Am. Chem. Soc. 32 (2014) 11347e11354. [163] Y. Yu, T. He, L. Guo, Y. Yang, L. Guo, Y. Tang, Y. Cao, Efficient visible-light photocatalytic degradation system assisted by conventional Pd catalysis, Sci. Rep. 5 (2015) 9561. [164] R. Costi, G. Cohen, A. Salant, E. Rabani, U. Banin, Electrostatic force microscopy study of single AuCdSe hybrid nanodumbbells: evidence for light-induced charge separation, Nano Lett. 9 (2009) 2031e2039. [165] A. Figuerola, A. Fiore, R. Di Corato, A. Falqui, C. Giannini, E. Micotti, A. Lascialfari, M. Corti, R. Cingolani, T. Pellegrino, P.D. Cozzoli, L. Manna, One-pot synthesis and characterization of size-controlled bimagnetic FePtiron oxide heterodimer nanocrystals, J. Am. Chem. Soc. 130 (2008) 1477e1487. [166] S.D. Standridge, G.C. Schatz, J.T. Hupp, Distance dependence of plasmon-enhanced photocurrent in dye-sensitized solar cells, J. Am. Chem. Soc. 131 (2009) 8407e8409. [167] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2014) 95e103. [168] P. Du, P. Jing, D. Li, Y. Cao, Z. Liu, Z. Sun, Plasmonic Ag@oxide nanoprisms for enhanced performance of organic solar cells, Small 11 (2015) 2454. [169] H. Liu, Y. Feng, D. Chen, C. Li, P. Cui, J. Yang, Noble metal-based composite nanomaterials fabricated via solution-based approaches, J. Mater. Chem. A 3 (2015) 3182e3223. [170] L. Sang, Y. Zhao, C. Burda, TiO2 nanoparticles as functional building blocks, Chem. Rev. 114 (2014) 9283e9318. [171] X.C. Ma, Y. Dai, L. Yu, B.B. Huang, Energy transfer in plasmonic photocatalytic composites, Light Sci. Appl. 5 (2016), e16017. [172] M.S. El-Deab, T. Ohsaka, Manganese oxide nanoparticles electrodeposited on platinum are superior to platinum for oxygen reduction, Angew. Chem. 118 (2006) 6109e6112. [173] B. Gilbert, F. Huang, H. Zhang, G.A. Waychunas, J.F. Banfield, Nanoparticles: strained and stiff, Science 305 (2004) 651e654. [174] W.J. Huang, R. Sun, J. Tao, L.D. Menard, R.G. Nuzzo, J.M. Zuo, Coordinationdependent surface atomic contraction in nanocrystals revealed by coherent diffraction, Nat. Mater. 7 (2008) 308e313. [175] H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379e382. [176] S.H. Oh, G.B. Hoflund, Infrared spectroscopic characterization of rotaxane molecular switch tunnel junction devices, J. Phys. Chem. A 110 (2006) 7609e7612. [177] K. Zorn, S. Giorgio, E. Halwax, C.R. Henry, H. Gronbeck, G. Rupprechter, CO oxidation on technological PdAl2O3 catalysts: oxidation state and activity, J. Phys. Chem. C 115 (2011) 1103e1111. [178] J. Guzman, S. Carrettin, J.C. Fierro-Gonzalez, Y.L. Hao, B.C. Gates, A. Corma, CO oxidation catalyzed by supported gold: cooperation between gold and nanocrystalline rare-earth supports forms reactive surface superoxide and peroxide species, Angew. Chem. 117 (2005) 4856e4859.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

439

[179] Y. Wang, S. Van de Vyver, K.K. Sharma, Y.R. Leshkov, Insights into the stability of gold nanoparticles supported on metal oxides for the base-free oxidation of glucose to gluconic acid, Green Chem. 16 (2014) 719e726. [180] C. Chen, C. Nan, D. Wang, Q. Su, H. Duan, X. Liu, L. Zhang, D. Chu, W. Song, Q. Peng, Y. Li, Mesoporous multicomponent nanocomposite colloidal spheres: ideal high-temperature stable model catalysts, Angew. Chem. 123 (2011), 3809-2813. [181] C.Y. Ma, Z. Mu, J.J. Li, Y.G. Jin, J. Cheng, G.Q. Lu, Z.P. Hao, S.Z. Qiao, Mesoporous Co3O4 and Au/Co3O4 Catalysts for low-temperature oxidation of trace ethylene, J. Am. Chem. Soc. 132 (2010) 2608e2613. [182] K. Zhao, B. Qiao, J. Wang, Y. Zhang, T. Zhang, A highly active and sintering-resistant Au/FeO xehydroxyapatite catalyst for CO oxidation, Chem. Commun. 47 (2011) 1779e1781. [183] X. Liu, A. Wang, X. Wang, C. Mou, T. Zhang, AueCu alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation, Chem. Commun. 27 (2008) 3187e3189. [184] J. Feng, C. Ma, P.J. Miedziak, J.K. Edwards, G.L. Brett, D. Li, Y. Du, D.J. Morgan, G.J. Hutchings, AuePd nanoalloys supported on MgeAl mixed metal oxides as a multifunctional catalyst for solvent-free oxidation of benzyl alcohol, Dalton Trans. 42 (2013) 14498e14508. [185] J. Qi, J. Chen, G. Li, S. Li, Y. Gao, Z. Tang, Facile synthesis of coreeshell Au@CeO2 nanocomposites with remarkably enhanced catalytic activity for CO oxidation, Energy Environ. Sci. 5 (2012) 8937e8941. [186] R.G. Chaudhuri, S. Paria, Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications, Chem. Rev. 112 (2012) 2373e2433. [187] Q. Zhang, I. Lee, J.B. Joo, F. Zaera, Y. Yin, Coreeshell nanostructured catalysts, Acc. Chem. Res. 46 (2013) 1816e1824. [188] H.C. Zeng, Integrated nanocatalysts, Acc. Chem. Res. 46 (2013) 226e235. [189] K. Mallik, M. Mandal, N. Pradhan, T. Pal, Seed mediated formation of bimetallic nanoparticles by UV irradiation: a photochemical approach for the preparation of “coreshell” type structures, Nano Lett. 1 (2001) 319e322. [190] J. Turkevich, P.C. Stevenson, J. Hillier, The formation of colloidal gold, J. Phys. Chem. 57 (1953) 670e673. [191] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Synthesis of thiolderivatised gold nanoparticles in a two-phase liquideliquid system, J. Chem. Soc. Chem. Commun. 7 (1994) 801e802. [192] S.T. Kochuveedu, J.H. Oh, Y.R. Do, D.H. Kim, Surface-plasmon-enhanced band emission of ZnO nanoflowers decorated with Au nanoparticles, Chemistry 18 (2012) 7467e7472. [193] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M.F. Chi, Y.D. Yin, A highly active titanium dioxide based visible-light photocatalyst with nonmetal doping and plasmonic metal decoration, Angew. Chem. 123 (2011) 7226e7230. [194] J.J. Chen, J.C.S. Wu, P.C. Wu, D.P. Tsai, Plasmonic photocatalyst for H2 evolution in photocatalytic water splitting, J. Phys. Chem. C 115 (2011) 210e216. [195] S.T. Kochuveedu, D.P. Kim, D.H. Kim, Surface-plasmon-induced visible light photocatalytic activity of TiO2 nanospheres decorated by Au nanoparticles with controlled configuration, J. Phys. Chem. C 116 (2012) 2500e2506. [196] N. Udawatte, M. Lee, J. Kim, D. Lee, Well-defined Au/ZnO nanoparticle composites exhibiting enhanced photocatalytic activities, ACS Appl. Mater. Interfaces 3 (2011) 4531e4538.

440

Interfaces in Particle and Fibre Reinforced Composites

[197] J. Lee, H.S. Shim, M. Lee, J.K. Song, D. Lee, Size-controlled electron transfer and photocatalytic activity of ZnOeAu nanoparticle composites, J. Phys. Chem. Lett. 2 (2011) 2840e2845. [198] D.B. Ingram, S. Linic, Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface, J. Am. Chem. Soc. 133 (2011) 5202e5205. [199] D.B. Ingram, P. Christopher, J.L. Bauer, S. Linic, Predictive model for the design of plasmonic metal/semiconductor composite photocatalysts, ACS Catal. 1 (2011) 1441e1447. [200] P. Christopher, D.B. Ingram, S. Linic, Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: photochemistry mediated by Ag surface plasmons, J. Phys. Chem. C 114 (2010) 9173e9177. [201] H. Liu, G. Shao, J. Zhao, Z. Zhang, Y. Zhang, J. Liang, X. Liu, H. Jia, B. Xu, Worm-like Ag/ZnO coreeshell heterostructural composites: fabrication, characterization, and photocatalysis, J. Phys. Chem. C 116 (2012) 16182e16190. [202] S. Anandan, M. Ashokkumar, Sonochemical synthesis of AueTiO2 nanoparticles for the sonophotocatalytic degradation of organic pollutants in aqueous environment, Ultrason. Sonochem. 16 (2009) 316e320. [203] S. Pande, A. Saha, S. Jana, S. Sarkar, M. Basu, M. Pradhan, A.K. Sinha, S. Saha, A. Pal, T. Pal, Resin-immobilized CuO and Cu nanocomposites for alcohol oxidation, Org. Lett. 10 (2008) 5179e5181. [204] S. Saha, A. Pal, S. Kundu, S. Basu, T. Pal, Photochemical green synthesis of calciumalginate-stabilized Ag and Au nanoparticles and their catalytic application to 4-nitrophenol reduction, Langmuir 26 (2010) 2885e2893. [205] D. Imbraguglio, A.M. Giovannozzi, A.M. Rossi, Nanometrology, Proceedings of the International School of Physics “Enrico Fermi” Course 185 “Metrology and Physical Constants”, vol. 185, 2013, pp. 193e220. [206] G. Wang, X. Wang, J. Liu, X. Sun, Mesoporous Au/TiO2 nanocomposite microspheres for visible-light photocatalysis, Chemistry 18 (2012) 5361e5366. [207] Y. Lu, H. Yu, S. Chen, X. Quan, H. Zhao, Integrating plasmonic nanoparticles with TiO2 photonic crystal for enhancement of visible-light-driven photocatalysis, Environ. Sci. Technol. 46 (2012) 1724e1730. [208] Y. Tang, Z. Jiang, Q. Tay, J. Deng, Y. Lai, D. Gong, Z. Dong, Z. Chen, Visible-light plasmonic photocatalyst anchored on titanate nanotubes: a novel nanohybrid with synergistic effects of adsorption and degradation, RSC Adv. 2 (2012) 9406e9414. [209] Solution phase methods based on hydrothermal synthesis, Nanoscale Materials, https:// www.texaspowerfulsmart.com/nanoscale-materials/solution-phase-methods-based-onhydrothermal-synthesis.html. [210] A.A. Peterson, F. Vogel, R.P. Lachance, M. Fr€oling, M.J. Antal Jr., J.W. Tester, Thermochemical biofuel production in hydrothermal media: a review of sub-and supercritical water technologies, Energy Environ. Sci. 1 (2008) 32e65. [211] R.I. Walton, Subcritical solvothermal synthesis of condensed inorganic materials, Chem. Soc. Rev. 31 (2002) 230e238. [212] M. Pradhan, A.K. Sinha, T. Pal, Redox transformation reaction for hierarchical hollow AueMnOOH flowers for high SERS activity, RSC Adv. 4 (2014) 30315e30324. [213] M. Roca, A.J. Haes, Silica-void-gold nanoparticles: temporally stable surface-enhanced Raman scattering substrates, J. Am. Chem. Soc. 130 (2008) 14273e14279. [214] X. Li, H. Hu, D. Li, Z. Shen, Q. Xiong, S. Li, H.J. Fan, Ordered array of gold semishells on TiO2 spheres: an ultrasensitive and recyclable SERS substrate, ACS Appl. Mater. Interfaces 4 (2012) 2180e2185.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

441

[215] Y. Wang, W. Song, W. Ruan, J. Yang, B. Zhao, J.R. Lombardi, SERS spectroscopy used to study an adsorbate on a nanoscale thin film of CuO coated with Ag, J. Phys. Chem. C 113 (2009) 8065e8069. [216] M. Pradhan, A.K. Sinha, T. Pal, Mn oxide-silver composite nanowires for improved thermal stability, SERS and electrical conductivity, Chemistry 20 (2014) 9111e9119. [217] A.K. Sinha, S. Jana, S. Pande, S. Sarkar, M. Pradhan, M. Basu, S. Saha, A. Pal, T. Pal, New hydrothermal process for hierarchical TiO2 nanostructures, CrystEngComm 11 (2009) 1210e1212. [218] H. Qi, D. Alexson, O. Glembocki, S.M. Prokes, Plasmonic coupling on dielectric nanowire core-metal sheath composites, Nanotechnology 21 (2010), 085705. [219] S. Rudra, A.K. Nayak, R. Chakraborty, P.K. Maji, M. Pradhan, Synthesis of Au-V2O5 composite nanowire through shape transformation of vanadium (III) metal complex for high- performance solid-state supercapacitor, Inorg. Chem. Front. 5 (2018) 1836e1843. [220] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X.B. Zhao, H.J. Fan, High-quality metal oxide core/shell nanowire arrays on conductive substrates for electrochemical energy storage, ACS Nano 6 (2012) 5531e5538. [221] Z. Zhian, L.Y. Qing, L. Jie, L.Y. Xiang, Effect of Ni-doping on electrochemical capacitance of MnO2 electrode materials, J. Cent. South Univ. Technol. 5 (2007) 638e642. [222] X. Pan, C. Li, J. Zheng, S. Liang, R. Huang, Z. Yi, A new defect pyrochlore oxide Sn1.06Nb2O5.59F0.97: synthesis, noble metal hybrids, and photocatalytic applications, Inorg. Chem. 57 (2018) 6641e6647. [223] L. Yi, F. Lan, J. Li, C. Zhao, Efficient noble-metal-free Co-NG/TiO2 photocatalyst for H2 evolution: synergistic effect between single-atom Co and N-doped graphene for enhanced photocatalytic activity, ACS Sustain. Chem. Eng. 6 (2018) 12766e12775. [224] Z. Zhong, J. Lin, S.P. Teh, J. Teo, F.M. Dautzenberg, A rapid and efficient method to deposit gold particles onto catalyst supports and its application for CO oxidation at low temperatures, Adv. Funct. Mater. 17 (2007) 1402e1408. [225] T.C. Damato, C.C.D. Oliveira, R.A. Ando, P.H. Camargo, A facile approach to TiO2 colloidal spheres decorated with Au nanoparticles displaying well-defined sizes and uniform dispersion, Langmuir 29 (2013) 1642e1649. [226] F. Moreau, G.C. Bond, A.O. Taylor, The influence of metal loading and pH during preparation on the CO oxidation activity of Au/TiO2 catalysts, Chem. Commun. 14 (2004) 1642e1643. [227] R. Zanella, Characterization and reactivity in CO oxidation of gold nanoparticles supported on TiO2 prepared by deposition-precipitation with NaOH and urea, J. Catal. 222 (2004) 357e367. [228] J. Radnik, L. Wilde, M. Schneider, M.M. Pohl, D. Herein, Influence of the precipitation agent in the depositionprecipitation on the formation and properties of Au nanoparticles supported on Al2O3, J. Phys. Chem. B 110 (2006) 23688e23693. [229] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, Lowtemperature oxidation of CO over gold supported on TiO2, a-Fe2O3, and Co3O4, J. Catal. 144 (1993) 175e192. [230] C.R. Chenthamarakshan, Y. Ming, K. Rajeshwar, Underpotential photocatalytic deposition: a new preparative route to composite semiconductors, Chem. Mater. 12 (2000) 3538e3540. [231] K. Wenderich, G. Mul, Methods, mechanism, and applications of photodeposition in photocatalysis: a review, Chem. Rev. 116 (2016) 14587e14619. [232] Y. Lee, E. Kim, Y. Park, J. Kim, W. Ryu, J. Rho, K. Kim, Photodeposited metalsemiconductor nanocomposites and their applications, J. Materiomics 4 (2018) 83e94.

442

Interfaces in Particle and Fibre Reinforced Composites

[233] J.G. Yu, L.F. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (2010) 13118e13125. [234] K.H. Leong, H.Y. Chu, S. Ibrahim, P. Saravanan, Palladium nanoparticles anchored to anatase TiO2 for enhanced surface plasmon resonance-stimulated, visible-light-driven photocatalytic activity, Beilstein J. Nanotechnol. 6 (2015) 428e437. [235] T. Wang, R. Lv, P. Zhang, C.J. Li, J.L. Gong, Au nanoparticle sensitized ZnO nanopencil arrays for photoelectrochemical water splitting, Nanoscale 7 (2015) 77e81. [236] Z. Zeng, C. Tan, X. Huang, S. Bao, H. Zhang, Growth of noble metal nanoparticles on single-layer TiS2 and TaS2 nanosheets for hydrogen evolution reaction, Energy Environ. Sci. 7 (2014) 797e803. [237] Y.X. Zhang, J.Q. Yan, X.P. Ren, L.Q. Pang, H. Chen, S. Liu, 2D WS2 nanosheet supported Pt nanoparticles for enhanced hydrogen evolution reaction, Int. J. Hydrogen Energy 42 (2017) 5472e5477. [238] A. Abiyasa, S. Yu, S. Lau, E.S.P. Leong, H. Yang, Enhancement of ultraviolet lasing from Ag-coated highly disordered ZnO films by surface-plasmon resonance, Appl. Phys. Lett. 90 (2007) 231106. [239] T. Torimoto, Y. Ohta, K. Enokida, D. Sugioka, T. Kameyama, T. Yamamoto, T. Shibayama, K. Yoshii, T. Tsuda, S. Kuwabata, Ultrathin oxide shell coating of metal nanoparticles using ionic liquid/metal sputtering, J. Mater. Chem. A 3 (2015) 6177e6186. [240] S. Dhara, P. Giri, On the origin of enhanced photoconduction and photoluminescence from Au and Ti nanoparticles decorated aligned ZnO nanowire heterostructures, J. Appl. Phys. 110 (2011) 124317. [241] I. Alessandri, M. Ferroni, Exploiting optothermal conversion for nanofabrication: siteselective generation of Au/TiO2 inverse opals, J. Mater. Chem. 19 (2009) 7990e7994. [242] H. Wang, T. You, W. Shi, J. Li, L. Guo, Au/TiO2/Au as a plasmonic coupling photocatalyst, J. Phys. Chem. C 116 (2012) 6490e6494. [243] A. Honciuc, M. Laurin, S. Albu, M. Sobota, P. Schmuki, J. Libuda, Controlling the adsorption kinetics via nanostructuring: Pd nanoparticles on TiO2 nanotubes, Langmuir 26 (2010) 14014e14023. [244] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Haruta, Deposition of gold nanoparticles on silica by CVD of gold acethylacetonate, Stud. Surf. Sci. Catal. 118 (1998) 277e284. [245] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, M. Haruta, Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2, Catal. Lett. 51 (1998) 53e58. [246] F. Tu, M. Drost, I. Szenti, J. Kiss, Z. Konya, H. Marbach, Localized growth of carbon nanotubes via lithographic fabrication of metallic deposits, Beilstein J. Nanotechnol. 8 (2017) 2592e2605. [247] N.K. Shrestha, P. Schmuki, Electrochemistry at TiO2 nanotubes and other semiconductor nanostructures, Electrochemistry 12 (2013) 87e131. [248] R. Beranek, H. Hildebrand, P. Schmuki, Self-organized porous titanium oxide prepared in H2SO 4/HF electrolytes, Electrochem. Solid State Lett. 6 (2003) B12eB14. [249] J.M. Macak, P. Schmuki, Anodic growth of self-organized anodic TiO2 nanotubes in viscous electrolytes, Electrochim. Acta 52 (2006) 1258e1264. [250] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Smooth anodic TiO2 nanotubes, Angew. Chem. Int. Ed. 44 (2005) 7463e7465.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

443

[251] S.P. Albu, A. Ghicov, J.M. Macak, P. Schmuki, 250 mm long anodic TiO2 nanotubes with hexagonal self-ordering, Phys. Status Solidi RRL 1 (2007) R65eR67. [252] K. Lee, D. Kim, P. Roy, I. Paramasivam, B. Birajdar, E. Spiecker, P. Schmuki, Anodic formation of thick anatase TiO2 mesosponge layers for high-efficiency photocatalysis, J. Am. Chem. Soc. 132 (2010) 1478. [253] B. Chen, J. Hou, K. Lu, Formation mechanism of TiO2 nanotubes and their applications in photoelectrochemical water splitting and supercapacitors, Langmuir 29 (2013) 5911e5919. [254] M. Salari, S.H. Aboutalebi, K. Konstantinov, H.K. Liu, A highly ordered titania nanotube array as a supercapacitor electrode, Phys. Chem. Chem. Phys. 13 (2011) 5038e5041. [255] J.V. Patil, S.S. Mali, J.S. Shaikh, C.K. Hong, P.S. Patil, Electrochemically anodized ultralong TiO2 nanotubes for supercapacitors, J. Electron. Mater. 48 (2018) 873e878. [256] N. Negishi, T. Iyoda, K. Hashimoto, A. Fujishima, Preparation of transparent TiO2 thin film photocatalyst and its photocatalytic activity, Chem. Lett. 24 (1995) 841e842. [257] Y. Hu, G. Martra, J. Zhang, S. Higashimoto, S. Coluccia, M. Anpo, Characterization of the local structures of Ti-MCM-41 and their photocatalytic reactivity for the decomposition of NO into N2 and O2, J. Phys. Chem. B 110 (2006) 1680e1685. [258] Y. Li, S. Zhang, Q. Yu, W. Yin, The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method, Appl. Surf. Sci. 253 (2007) 9254e9258. [259] J. Nowotny, C.C. Sorrell, T. Bak, L.R. Sheppard, Solar-hydrogen: unresolved problems in solid-state science, Sol. Energy 78 (2005) 593e602. [260] H.P. Maruska, A.K. Ghosh, Photocatalytic decomposition of water at semiconductor electrodes, Sol. Energy 20 (1978) 443e458. [261] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to hydrogen and oxygen, Acc. Chem. Res. 28 (1995) 141e145. [262] T. Hisatomi, T. Minegishi, K. Domen, Kinetic assessment and numerical modeling of photocatalytic water splitting toward efficient solar hydrogen production, Bull. Chem. Soc. Jpn. 85 (2012) 647e655. [263] J. Yoshimura, Y. Ebina, J. Kondo, K. Domen, A. Tanaka, Visible light-induced photocatalytic behavior of a layered perovskite-type rubidium lead niobate, RbPb2Nb3O10, J. Phys. Chem. 97 (1993) 1970e1973. [264] A. Kudo, K. Omori, H. Kato, A novel aqueous process for preparation of crystal formcontrolled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties, J. Am. Chem. Soc. 121 (1999) 11459e11467. [265] H. Kato, K. Asakura, A. Kudo, Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure, J. Am. Chem. Soc. 125 (2003) 3082e3089. [266] H.G. Kim, D.W. Hwang, J.S. Lee, An undoped, single-phase oxide photocatalyst working under visible light, J. Am. Chem. Soc. 126 (2004) 8912e8913. [267] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari, D.D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B Environ. 125 (2015) 331e349. [268] S. Banerjee, S.C. Pillai, P. Falaras, K.E. O’Shea, J.A. Byrne, D.D. Dionysiou, New insights into the mechanism of visible light photocatalysis, J. Phys. Chem. Lett. 5 (2014) 2543e2554. [269] J.R. Bolton, Solar photoproduction of hydrogen: a review, Sol. Energy 57 (1996) 37e50.

444

Interfaces in Particle and Fibre Reinforced Composites

[270] Y. Xu, M.A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral. 85 (2000) 543e556. [271] M. Ge, Q. Li, C. Cao, J. Huang, S. Li, S. Zhang, Z. Chen, K. Zhang, S.S. Al-Deyab, Y. Lai, One-dimensional TiO2 nanotube photocatalysts for solar water splitting, Adv. Sci. (2016) 1600152. [272] D.E. Skinner, D.P. Colombo Jr., J.J. Cavaleri, R.M. Bowman, Femtosecond investigation of electron trapping in semiconductor nanoclusters, J. Phys. Chem. 99 (1995) 7853e7856. [273] D.P. Colombo Jr., K.A. Roussel, J. Saeh, D.E. Skinner, J.J. Cavaleri, R.M. Bowman, Femtosecond study of the intensity dependence of electron-hole dynamics in TiO2 nanoclusters, Chem. Phys. Lett. 232 (1995) 207e214. [274] K. Gurunathan, P. Maruthamuthu, M.V.C. Sastri, Photocatalytic hydrogen production by dye-sensitized Pt/SnO2 and Pt/SnO2/RuO2 in aqueous methyl viologen solution, Int. J. Hydrogen Energy 22 (1997) 57e62. [275] R.A. Doong, C.H. Chen, R.A. Maithreepala, S.M. Chang, The influence of pH and cadmium sulfide on the photocatalytic degradation of 2-chlorophenol in titanium dioxide suspensions, Water Res. 35 (2001) 2873e2880. [276] M.G. Kang, H.E. Han, K.J. Kim, Enhanced photodecomposition of 4-chlorophenol in aqueous solution by deposition of CdS on TiO2, J. Photochem. Photobiol. A Chem. 125 (1999) 119e125. [277] Y. Lin, D. Li, J. Hu, G. Xiao, J. Wang, W. Li, X. Fu, Highly efficient photocatalytic degradation of organic pollutants by PANI-modified TiO2 composite, J. Phys. Chem. C 116 (2012) 5764e5772. [278] W.W. So, K.J. Kim, S.J. Moon, Photo-production of hydrogen over the CdSeTiO2 nanocomposite particulate films treated with TiCl4, Int. J. Hydrogen Energy 29 (2004) 229e234. [279] G.C. De, A.M. Roy, S.S. Bhattacharya, Effect of n-Si on the photocatalytic production of hydrogen by Pt-loaded CdS and CdSZnS catalyst, Int. J. Hydrogen Energy 21 (1996) 19e23. [280] A. Koca, M. S¸ahin, Photocatalytic hydrogen production by direct sun light from sulfide/ sulfite solution, Int. J. Hydrogen Energy 27 (2002) 363e367. [281] T.V. Nguyen, S. Kim, O.B. Yang, Water decomposition on TiO2eSiO2 and RuS2/ TiO2eSiO2 photocatalysts: the effect of electronic characteristics, Catal. Commun. 5 (2004) 59e62. [282] R.M.N. Yerga, M.l.G. Consuelo, F.d. Valle, J.A. V.d.l. Mano, J.L.G. Fierro, Water splitting on semiconductor catalysts under visible-light irradiation, ChemSusChem 2 (2009) 471e485. [283] A. Kudo, M. Sekizawa, Photocatalytic H2 evolution under visible light irradiation on Zn 1-x Cu x S solid solution, Catal. Lett. 58 (1999) 241e243. [284] A. Kudo, M. Sekizawa, Photocatalytic H2 evolution under visible light irradiation on Nidoped ZnS photocatalyst, Chem. Commun. 15 (2000) 1371e1372. [285] S.U. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243e2245. [286] A.W. Xu, Y. Gao, H.Q. Liu, The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO2 nanoparticles, J. Catal. 207 (2002) 151e157. [287] R. Ullah, J. Dutta, Photocatalytic degradation of organic dyes with manganese-doped ZnO nanoparticles, J. Hazard. Mater. 156 (2008) 194e200. [288] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669e13679.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

445

[289] G. Blasse, G.J. Dirksen, P.H.M. de Korte, Materials with cationic valence and conduction bands for photoelectrolysis of water, Mater. Res. Bull. 16 (1981) 991e998. [290] M. Anpo, Utilization of TiO2 photocatalysts in green chemistry, Pure Appl. Chem. 72 (2000) 1265e1270. [291] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue, M. Anpo, Degradation of propanol diluted in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts, J. Photochem. Photobiol. A Chem. 148 (2002) 257e261. [292] M. Matsumura, Y. Saho, H. Tsubomura, Photocatalytic hydrogen production from solutions of sulfite using platinized cadmium sulfide powder, J. Phys. Chem. 87 (1983) 3807e3808. [293] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269e271. [294] G. Hitoki, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, An oxynitride, TaON, as an efficient water oxidation photocatalyst under visible light irradiation (l 500 nm), Chem. Commun. 16 (2002) 1698e1699. [295] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Band gap narrowing of titanium dioxide by sulfur doping, Appl. Phys. Lett. 81 (2002) 454e456. [296] S. Liu, Xiaoyun Chen A visible light response TiO2 photocatalyst realized by cationic Sdoping and its application for phenol degradation, J. Hazard. Mater. 152 (2008) 48e55. [297] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596e2602. [298] P. Maruthamuthu, M. Ashokkumar, Hydrogen production with visible light using metal loaded-WO3 and MVþ 2 in aqueous medium, Int. J. Hydrogen Energy 14 (1989) 275e277. [299] P. Maruthamuthu, M. Ashokkumar, Hydrogen generation using Cu (II)/WO3 and oxalic acid by visible light, Int. J. Hydrogen Energy 13 (1988) 677e680. [300] T. Arai, M. Horiguchi, M. Yanagida, T. Gunji, H. Sugihara, K. Sayama, Reaction mechanism and activity of WO3-catalyzed photodegradation of organic substances promoted by a CuO cocatalyst, J. Phys. Chem. C 113 (2009) 6602e6609. [301] C.A. Linkous, G.J. Carter, D.B. Locuson, A.J. OUELLETTE, D.K. Slattery, L.A. Smitha, Photocatalytic inhibition of algae growth using TiO2, WO3, and cocatalyst modifications, Environ. Sci. Technol. 34 (2000) 4754e4758. [302] R. Abe, H. Takami, N. Murakami, B. Ohtani, Pristine simple oxides as visible light driven photocatalysts: highly efficient decomposition of organic compounds over platinumloaded tungsten oxide, J. Am. Chem. Soc. 130 (2008) 7780e7781. [303] I. Tsuji, H. Kato, A. Kudo, Visible-light-induced H2 evolution from an aqueous solution containing sulfide and sulfite over a ZnSeCuInS2eAgInS2 solid-solution photocatalyst, Angew. Chem. 117 (2005) 3631e3634. [304] C. Ye, M.D. Regulacio, S.H. Lim, Q.H. Xu, M.Y. Han, Alloyed (ZnS)x(CuInS2)1x semiconductor nanorods: synthesis, bandgap tuning and photocatalytic properties, Chem. Eur. J. 18 (2012) 11258e11263. [305] M. Ashokkumar, An overview on semiconductor particulate systems for photoproduction of hydrogen, Int. J. Hydrogen Energy 23 (1998) 427e438. [306] J.F. Reber, K. Meier, Photochemical production of hydrogen with zinc sulfide suspensions, J. Phys. Chem. 88 (1984) 5903e5913. [307] H. Kominami, T. Matsuura, K. Iwai, B. Ohtani, S.I. Nishimoto, Y. Kera, Ultra-highly active titanium (IV) oxide photocatalyst prepared by hydrothermal crystallization from titanium (IV) alkoxide in organic solvents, Chem. Lett. 24 (1995) 693e694.

446

Interfaces in Particle and Fibre Reinforced Composites

[308] S. Ikeda, A. Tanaka, K. Shinohara, M. Hara, J.N. Kondo, K.I. Maruya, K. Domen, Effect of the particle size for photocatalytic decomposition of water on Ni-loaded K4Nb6O17, Microporous Mater. 9 (1997) 253e258. [309] L.E. Brus, Electroneelectron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state, J. Chem. Phys. 80 (1984) 4403e4409. [310] L. Brus, Electronic wave functions in semiconductor clusters: experiment and theory, J. Phys. Chem. 90 (1986) 2555e2560. [311] J.P. Rino, N. Studart, Structural correlations in titanium dioxide, Phys. Rev. B 59 (1999) 6643. [312] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69e96. [313] M. Haase, H. Weller, A. Henglein, Photochemistry of colloidal semiconductors. 26. Photoelectron emission from cadmium sulfide particles and related chemical effects, J. Phys. Chem. 92 (1988) 4706e4712. [314] J.M. Nedeljkovic, M.T. Nenadovic, O.I. Micic, A.J. Nozik, Enhanced photoredox chemistry in quantized semiconductor colloids, J. Phys. Chem. 90 (1986) 12e13. [315] U. Koch, A. Fojtik, H. Weller, A. Henglein, Photochemistry of semiconductor colloids. Preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects, Chem. Phys. Lett. 122 (1985) 507e510. [316] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, Photocatalytic hydrogenation of propyne with water on small-particle titania: size quantization effects and reaction intermediates, J. Phys. Chem. 91 (1987) 4305e4310. [317] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Preparation and characterization of quantum-size titanium dioxide, J. Phys. Chem. 92 (1988) 5196e5201. [318] Y. Zhu, L. Wang, Y. Liu, L. Shao, X. Xia, In-situ hydrogenation engineering of ZnIn2S4 for promoted visible-light water splitting, Appl. Catal. B Environ. 241 (2019) 483e490. [319] L. Ye, Z. Wen, ZnIn2S4 nanosheets decorating WO3 nanorods core-shell hybrids for boosting visible-light photocatalysis hydrogen generation, Int. J. Hydrogen Energy 44 (2019) 3751e3759. [320] Z. Liu, J. Liu, Y. Huang, J. Li, Y. Yuan, H. Ye, D. Zhu, Z. Wang, A. Tang, From onedimensional to two-dimensional wurtzite CuGaS2 nanocrystals: non-injection synthesis and photocatalytic evolution, Nanoscale 11 (2019) 158e169. [321] M. Chandra, K. Bhunia, D. Pradhan, Controlled Synthesis of CuS/TiO2 heterostructured nanocomposites for enhanced photocatalytic hydrogen generation through water splitting, Inorg. Chem. 57 (2018) 4524e4533. [322] A.S. Kshirsagar, P.K. Khanna, CuSbSe2/TiO2: novel type-II heterojunction nanophotocatalyst, Mater. Chem. Front. (2019). Advance Article. [323] W.C. Oha, K.Y. Choc, C.H. Jungd, D.C.T. Nguyen, Photocatalytic activities using a nanocomposite of mesoporous SiO2 and CdInSe-graphene nanoparticles under visible light irradiation, J. Photochem. Photobiol. A Chem. 371 (2019), 271e28. [324] C. Li, X. Li, H. Huang, J. Yao, Y. Wu, Ba2AsGaSe5: a new quaternary selenide with the novel [AsGaSe5]4e cluster and interesting photocatalytic properties, Inorg. Chem. 54 (2015) 9785e9789. [325] C. Li, Z.H. Lin, L. Kang, Z.S. Lin, H.W. Huang, J.Y. Yao, Y.C. Wu, Sn2SiS4, synthesis, structure, optical and electronic properties, Opt. Mater. 47 (2015) 379e385. [326] S.P. Guo, Y. Chi, J.P. Zou, H.G. Xue, Crystal and electronic structures, and photoluminescence and photocatalytic properties of a-EuZrS3, New J. Chem. 40 (2016) 10219e10226.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

447

[327] A. Ghosh, M. Mitra, D. Banerjee, A. Mondal, Facile electrochemical deposition of Cu7Te4 thin films with visible-light driven photocatalytic activity and thermoelectric performance, RSC Adv. 6 (2016) 22803e22811. [328] Y. Fan, B. Yang, X. Song, X. Shao, M. Zhao, Direct Z-scheme photocatalytic overall water splitting on 2D CdS/InSe heterostructures, J. Phys. D Appl. Phys. 51 (2018) 39. [329] M. Qiao, J. Liu, Y. Wang, Y. Li, Z. Chen, PdSeO3 Monolayer: promising inorganic 2D photocatalyst for direct overall water splitting without using sacrificial reagents and cocatalysts, J. Am. Chem. Soc. 140 (2018) 12256e12262. [330] S. Wei, X. Cui, Y. Xu, B. Shang, Q. Zhang, L. Gu, X. Fan, L. Zheng, C. Hou, H. Huang, S. Wen, W. Zheng, Iridium-Triggered phase transition of MoS2 nanosheets boosts overall water splitting in alkaline media, ACS Energy Lett. 4 (2019) 368e374. [331] Y. Li, Y.L. Li, B. Sa, R. Ahuja, Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective, Catal. Sci. Technol. 7 (2017) 545e559. [332] S. Kouser, A. Thannikoth, U. Gupta, U.V. Waghmare, C. Rao, 2D-GaS as a photocatalyst for water splitting to produce H2, Small 11 (2015) 4723e4730. [333] Y.J. Yuan, D. Chen, J. Zhong, L.X. Yang, J. Wang, M.J. Liu, W.G. Tu, Z.T. Yu, Z.G. Zou, Interface engineering of a noble-metal-free 2De2D MoS2/Cu-ZnIn2S4 photocatalyst for enhanced photocatalytic H2 production, J. Mater. Chem. 5 (2017) 15771e15779. [334] T. Ishii, H. Kato, A. Kudo, H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation, J. Photochem. Photobiol. A Chem. 163 (2004) 181e186. [335] A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, Oxysulfide Sm2Ti2S2O5 as a stable photocatalyst for water oxidation and reduction under visible light irradiation (l  650 nm), J. Am. Chem. Soc. 124 (2002) 13547e13553. [336] R.M. Navarro, F. Del Valle, J.L.G. Fierro, Photocatalytic hydrogen evolution from CdSZnO-CdO systems under visible light irradiation: effect of thermal treatment and presence of Pt and Ru cocatalysts, Int. J. Hydrogen Energy 33 (2008) 4265e4273. [337] R. K€otz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochim. Acta 45 (2000) 2483e2498. [338] T. Momma, X. Liu, T. Osaka, Y. Ushio, Y. Sawada, Electrochemical modification of active carbon fiber electrode and its application to double-layer capacitor, J. Power Sources 60 (1996) 249e253. [339] W. Du, X. Jiang, L. Zhu, From graphite to graphene: direct liquid-phase exfoliation of graphite to produce single-and few-layered pristine graphene, J. Mater. Chem. A 1 (2013) 10592e10606. [340] Elcap, Own Work, Wikimedia Commons, April 9, 2013. [341] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597e1614. [342] J. Chen, J. Xu, S. Zhou, N. Zhao, C.P. Wong, Nitrogen-doped hierarchically porous carbon foam: a free-standing electrode and mechanical support for high-performance supercapacitors, Nano Energy 25 (2016) 193e202. [343] H.S. Kim, J.B. Cook, S.H. Tolbert, B. Dunn, The development of pseudocapacitive properties in nanosized-MoO2, J. Electrochem. Soc. 162 (2015) A5083eA5090. [344] X. Yu, S. Yun, J.S. Yeon, P. Bhattacharya, L. Wang, S.W. Lee, X. Hu, H.S. Park, Pseudocapacitance: emergent pseudocapacitance of 2D nanomaterials, Adv. Energy Mater. vol. 8 (2018) 1870058.

448

Interfaces in Particle and Fibre Reinforced Composites

[345] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Ordered mesoporous a-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors, Nat. Mater. 9 (2010) 146e151. [346] T. Brousse, D. Bélanger, J.W. Long, To be or not to be pseudocapacitive? J. Electrochem. Soc. 162 (2015) A5185eA5189. [347] D. Chao, C. Zhu, P. Yang, X. Xia, J. Liu, J. Wang, X. Fan, S.V. Savilov, J. Lin, H.J. Fan, Z.X. Shen, Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance, Nat. Commun. 7 (2016) 12122. [348] T.C. Liu, W.G. Pell, B.E. Conway, S.L. Roberson, Behavior of molybdenum nitrides as materials for electrochemical capacitors comparison with ruthenium oxide, J. Electrochem. Soc. 145 (1998) 1882e1888. [349] S. Rudra, A.K. Nayak, S. Koley, R. Chakraborty, P.K. Maji, M. Pradhan, Redoxmediated shape-transformation of Fe3O4 nanoflake to chemically stable Au-Fe2O3 composite nanorod for high-performance asymmetric solid-state supercapacitor device, ACS Sustain. Chem. Eng. 7 (2019) 724e733. [350] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31e35. [351] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbon-based supercapacitors produced by activation of graphene, Science 332 (2011) 1537e1541. [352] L. Wei, G. Yushin, Nanostructured activated carbons from natural precursors for electrical double layer capacitors, Nano Energy 1 (2012) 552e565. [353] K. Jurewicz, C. Vix-Guterl, E. Frackowiak, S. Saadallah, M. Reda, J. Parmentier, J. Patarin, F. Béguin, Capacitance properties of ordered porous carbon materials prepared by a templating procedure, J. Phys. Chem. Solids 65 (2004) 287e293. [354] J.A. Fernandez, T. Morishita, M. Toyoda, M. Inagaki, F. Stoeckli, T.A. Centeno, Performance of mesoporous carbons derived from poly (vinyl alcohol) in electrochemical capacitors, J. Power Sources 175 (2008) 675e679. [355] Z.G. Cambaz, G.N. Yushin, Y. Gogotsi, K.L. Vyshnyakova, L.N. Pereselentseva, Formation of carbide-derived carbon on b-silicon carbide whiskers, J. Am. Ceram. Soc. 89 (2006) 509e514. [356] D.A. Ersoy, M.J. McNallan, Y. Gogotsi, Carbon coatings produced by high temperature chlorination of silicon carbide ceramics, Mater. Res. Innov. 5 (2001) 55e62. [357] G. Yushin, Y. Gogotsi, A. Nikitin, in: Y. Gogotsi (Ed.), Nanomaterials Handbook, CRC Press, Boca Raton, 2005, pp. 237e280. [358] Y. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J.E. Fischer, B. Yi, H.C. Foley, M.W. Barsoum, Nanoporous carbide-derived carbon with tunable pore size, Nat. Mater. 2 (2003) 591e594. [359] P. Simon, A.F. Burke, Nanostructured carbons: double-layer capacitance and more, Electrochem. Soc. Interface 17 (2008) 38e44. [360] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Supercapacitor electrodes from multiwalled carbon nanotubes, Appl. Phys. Lett. 77 (2000) 2421e2423. [361] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, High power electrochemical capacitors based on carbon nanotube electrodes, Appl. Phys. Lett. 70 (1997) 1480e1482. [362] C. Emmenegger, P. Mauron, A. Z€uttel, C. N€utzenadel, A. Schneuwly, R. Gallay, L. Schlapbach, Carbon nanotube synthesized on metallic substrates, Appl. Surf. Sci. 162 (2000) 452e456.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

449

[363] Z.S. Wu, G. Zhou, L.C. Yin, W. Ren, F. Li, H.M. Cheng, Graphene/metal oxide composite electrode materials for energy storage, Nano Energy 1 (2012) 107e131. [364] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M. Katsnelson, I. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197e200. [365] H. Chen, M.B. M€uller, K.J. Gilmore, G.G. Wallace, D. Li, Mechanically strong, electrically conductive, and biocompatible graphene paper, Adv. Mater. 20 (2008) 3557e3561. [366] M. Pumera, Graphene-based nanomaterials and their electrochemistry, Chem. Soc. Rev. 39 (2010) 4146e4157. [367] S.R.C. Vivekchand, C.S. Rout, K.S. Subrahmanyam, A. Govindaraj, C.N.R. Rao, Graphene-based electrochemical supercapacitors, J. Chem. Sci. 120 (2008) 9e13. [368] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498e3502. [369] Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103e13107. [370] Y.S. He, D.W. Bai, X. Yang, J. Chen, X.Z. Liao, Z.F. Ma, A Co(OH)2 graphene nanosheets composite as a high performance anode material for rechargeable lithium batteries, Electrochem. Commun. 12 (2010) 570e573. [371] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A threedimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors, Adv. Mater. 22 (2010) 3723e3728. [372] H.K. Jeong, M. Jin, E.J. Ra, K.Y. Sheem, G.H. Han, S. Arepalli, Y.H. Lee, Enhanced electric double layer capacitance of graphite oxide intercalated by poly (sodium 4styrensulfonate) with high cycle stability, ACS Nano 4 (2010) 1162e1166. [373] J.K. Lee, K.B. Smith, C.M. Hayner, H.H. Kung, Silicon nanoparticlesegraphene paper composites for Li ion battery anodes, Chem. Commun. 46 (2010) 2025e2027. [374] L. Deng, J. Wang, G. Zhu, L. Kang, Z. Hao, Z. Lei, Z. Yang, Z.H. Liu, RuO2/graphene hybrid material for high performance electrochemical capacitor, J. Power Sources 248 (2014) 407e415. [375] Y. Song, J.L. Xu, X.X. Liu, Electrochemical anchoring of dual doping polypyrrole on graphene sheets partially exfoliated from graphite foil for high-performance supercapacitor electrode, J. Power Sources 249 (2014) 48e58. [376] D. Wang, Y. Min, Y. Yu, B. Peng, A general approach for fabrication of nitrogen-doped graphene sheets and its application in supercapacitors, J. Colloid Interface Sci. 417 (2014) 270e277. [377] D.Y. Lee, S.J. Yoon, N.K. Shrestha, S.H. Lee, H. Ahn, S.H. Han, Unusual energy storage and charge retention in Co-based metaleorganic-frameworks, Microporous Mesoporous Mater. 153 (2012) 163e165. [378] J. Yang, C. Zheng, P. Xiong, Y. Li, M.J. Wei, Zn-doped Ni-MOF material with a high supercapacitive performance, J. Mater. Chem. A 2 (2014) 19005e19010. [379] N. Campagnol, R. Romero-Vara, W. Deleu, L. Stappers, K. Binnemans, D.E. DeVos, J. Fransaer, A hybrid supercapacitor based on porous carbon and the metal-organic framework MIL-100(Fe), ChemElectroChem 1 (2014) 1182e1188. [380] Y. Gao, J. Wu, W. Zhang, Y. Tan, J. Zhao, B. Tang, The electrochemical performance of SnO2 quantum dots@zeolitic imidazolate frameworks-8 (ZIF-8) composite material for supercapacitors, Mater. Lett. 128 (2014) 208e211.

450

Interfaces in Particle and Fibre Reinforced Composites

[381] Y.Z. Zhang, Y. Wang, Y.L. Xie, T. Cheng, W.Y. Lai, H. Pang, W. Huang, Porous hollow Co₃O₄ with rhombic dodecahedral structures for high-performance supercapacitors, Nanoscale 6 (2014) 14354e14359. [382] W. Meng, W. Chen, L. Zhao, Y. Huang, M. Zhu, Y. Huang, Y. Fu, F. Geng, J. Yu, X. Chen, C. Zhi, Porous Fe3O4/carbon composite electrode material prepared from metalorganic framework template and effect of temperature on its capacitance, Nano Energy 8 (2014) 133e140. [383] S. Maiti, A. Pramanik, S. Mahanty, Extraordinarily high pseudocapacitance of metal organic framework derived nanostructured cerium oxide, Chem. Commun. 50 (2014) 11717e11720. [384] L. Wang, Y. Han, X. Feng, J. Zhou, P. Qi, B. Wang, Metaleorganic frameworks for energy storage: batteries and supercapacitors, Coord. Chem. Rev. 307 (2015) 361e381. [385] B. Liu, H. Shioyama, H. Jiang, X. Zhang, Q. Xu, Metaleorganic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor, Carbon 48 (2010) 456e463. [386] J. Hu, H. Wang, Q. Gao, H. Guo, Porous carbons prepared by using metaleorganic framework as the precursor for supercapacitors, Carbon 48 (2010) 3599e3606. [387] D. Yuan, J. Chen, S. Tan, N. Xia, Y. Liu, Worm-like mesoporous carbon synthesized from metaleorganic coordination polymers for supercapacitors, Electrochem. Commun. 11 (2009) 1191e1194. [388] X. Yan, X. Li, Z. Yan, S. Komarneni, Porous carbons prepared by direct carbonization of MOFs for supercapacitors, Appl. Surf. Sci. 308 (2014) 306e310. [389] P. Zhang, F. Sun, Z. Shen, D. Cao, ZIF-derived porous carbon: a promising supercapacitor electrode material, J. Mater. Chem. A 2 (2014) 12873e12880. [390] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828. [391] H. Lee, M.S. Cho, I.H. Kim, J. Do Nam, Y. Lee, RuOx/polypyrrole nanocomposite electrode for electrochemical capacitors, Synth. Met. 160 (2010) 1055e1059. [392] J.P. Zheng, P.J. Cygan, T.R. Jow, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors, J. Electrochem. Soc. 142 (1995) 2699e2703. [393] J.P. Zheng, T.R. Jow, High energy and high power density electrochemical capacitors, J. Power Sources 62 (1996) 155e159. [394] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [395] C.C. Hu, W.C. Chen, K.H. Chang, How to achieve maximum utilization of hydrous ruthenium oxide for supercapacitors, J. Electrochem. Soc. 151 (2004) A281eA290. [396] N.L. Wu, S.L. Kuo, M.H. Lee, Preparation and optimization of RuO2-impregnated SnO2 xerogel supercapacitor, J. Power Sources 104 (2002) 62e65. [397] Y. Takasu, T. Nakamura, H. Ohkawauchi, Y. Murakami, Dip-coated Ru-V oxide electrodes for electrochemical capacitors, J. Electrochem. Soc. 144 (1997) 2601e2606. [398] J.K. Chang, M.T. Lee, W.T. Tsai, In situ Mn K-edge X-ray absorption spectroscopic studies of anodically deposited manganese oxide with relevance to supercapacitor applications, J. Power Sources 166 (2007) 590e594. [399] J. Wei, N. Nagarajan, I. Zhitomirsky, Manganese oxide films for electrochemical supercapacitors, J. Mater. Process. Technol. 186 (2007) 356e361. [400] A. Zolfaghari, F. Ataherian, M. Ghaemi, A. Gholami, Capacitive behavior of nanostructured MnO2 prepared by sonochemistry method, Electrochim. Acta 52 (2007) 2806e2814.

Band-gap engineering using metal-semiconductor interfaces for photocatalysis

451

[401] C.C. Hu, C.C. Wang, Nanostructures and capacitive characteristics of hydrous manganese oxide prepared by electrochemical deposition, J. Electrochem. Soc. 150 (2003) A1079eA1084. [402] N. Nagarajan, H. Humadi, I. Zhitomirsky, Cathodic electrodeposition of MnOx films for electrochemical supercapacitors, Electrochim. Acta 51 (2006) 3039e3045. [403] T. Shinomiya, V. Gupta, N. Miura, Effects of electrochemical-deposition method and microstructure on the capacitive characteristics of nano-sized manganese oxide, Electrochim. Acta 51 (2006) 4412e4419. [404] J. Theerthagiri, K. Karuppasamy, G. Durai, A. ul H.S. Rana, P. Arunachalam, K. Sangeetha, P. Kuppusami, H.S. Kim, Recent advances in metal chalcogenides (MX; X ¼ S,Se) nanostructures for electrochemical supercapacitor applications: a brief review, Nanomaterials 8 (2018) 256. [405] W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H. Zhang, One-step synthesis of Ni3S2nanorod@Ni(OH)2 nanosheet core-shell nanostructures on a threedimensional graphene network for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2216e2221. [406] H. Heydari, S.E. Moosavifard, S. Elyasi, M. Shahraki, Nanoporous CuS nano-hollow spheres as advanced material for high-performance supercapacitors, Appl. Surf. Sci. 394 (2017) 425e430. [407] R. Thangappan, S. Kalaiselvam, A. Elayaperumal, R. Jayavel, M. Arivanandhan, R. Karthikeyan, Y. Hayakawa, Graphene decorated with MoS2 nanosheets: a synergetic energy storage composite electrode for supercapacitor applications, Dalton Trans. 45 (2016) 2637e2646. [408] L. Fang, Y. Qiu, T. Zhai, F. Wang, M. Lan, K. Huang, Q. Jing, Flower-like nanoarchitecture assembled from Bi2S3 nanorod/MoS2 nanosheet heterostructures for highperformance supercapacitor electrodes, Colloids Surf. A 535 (2017) 41e48. [409] S. Ratha, C.S. Rout, Supercapacitor electrodes based on layered tungsten disulfidereduced graphene oxide hybrids synthesized by a facile hydrothermal method, ACS Appl. Mater. Interfaces 5 (2013) 11427e11433. [410] S. Wang, W. Li, L. Xin, M. Wu, Y. Long, H. Huang, X. Lou, Facile synthesis of truncated cube-like NiSe2 single crystals for high-performance asymmetric supercapacitors, Chem. Eng. J. 330 (2017) 1334e1341. [411] P. Pazhamalai, K. Krishnamoorthy, S.J. Kim, Hierarchical copper selenide nanoneedles grown on copper foil as a binder free electrode for supercapacitors, Int. J. Hydrogen Energy 41 (2016) 14830e14835. [412] K.J. Huang, J.Z. Zhang, Y. Fan, Preparation of layered MoSe2 nanosheets on Ni-foam substrate with enhanced supercapacitor performance, Mater. Lett. 152 (2015) 244e247. [413] Y. Zhang, A. Pan, Y. Wang, X. Cao, Z. Zhou, T. Zhu, S. Liang, G. Cao, Self-templated synthesis of n-doped CoSe2/C double-shelled dodecahedra for high-performance supercapacitors, Energy Storage Mater. 8 (2017) 28e34. [414] C. Zhang, H. Yin, M. Han, Z. Dai, H. Pang, Y. Zheng, Y.Q. Lan, J. Bao, J. Zhu, Twodimensional tin selenide nanostructures for flexible all-solid-state supercapacitors, ACS Nano 8 (2014) 3761e3770. [415] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (2006) 5567e5580.

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

16

A. Pramanik a , A.K. Basak b , G. Littlefair c , A.R. Dixit d , S. Chattopadhyaya d a School of Civil and Mechanical Engineering, Curtin University, Bentley, WA, Australia; b Adelaide Microscopy, University of Adelaide, SA, Adelaide, Australia; cThe University of Auckland, Auckland, Auckland, New Zealand; dDepartment of Mechanical Engineering, Indian School of Mines, Dhanbad, Jharkhand, India

1. Introduction Metal matrix composites (MMCs), especially aluminium alloy with reinforced particles, are used widely as structural materials in high-tech fields, such as aerospace, defence, automotive and civil engineering [1,2]. Particle reinforced MMCs have potentials to offer excellent mechanical properties, for example higher specific strength and stiffness [3,4]. Ceramic materials are commonly used to reinforce aluminium matrix to include higher stiffness, strength, hardness and wear resistance while preserving a quasi-isotropic behaviour which facilitates conventional re-shaping process. Silicon carbide (SiC) is one of the sucg commonly used particles to reinforce aluminium matrix owing to their low price and availability of various grades [5]. Mechanical behaviours of particulate reinforced MMCs are controlled by the geometry [6,7], size [8], volume fraction [9] and distribution [10] of reinforcements as well as the properties of the reinforcements and matrix materials [11,12]. The interfaces between reinforcements and matrix materials influence the mechanical properties of the MMCs [13,14]. Therefore, load-carrying capacity of reinforced ceramic particles in metal matrix [15] and the morphology of particles significantly affect the overall mechanical properties of the MMCs [16e18]. The debonding of reinforcements from matrix significantly reduces the toughness, strength and ductility [19] of the composite. Cracks initiated when the stress surpasses the strength of the interface, usually at the locations where maximum stresses are generated such as, at the poles of spherical reinforcements and at the corners of triangular/rectangular particles. The damage propagates as the crack grows alongside the interface of matrix/particles and diminishes the extent of load that transmitted to reinforcement from matrix, i.e., the strengthening result of reinforcements. Lastly, cavities develop from the cracks in the interface and fracture takes place by amalgamation of interfacial cavities. These mechanisms are broadly recognized in polymer [20,21] and metal matrix based composite materials [22e24]. However, these facts have been accepted qualitatively. Accurate reckonable data on the sovereign influence of interface properties (toughness and strength) on

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00016-9 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Interfaces in Particle and Fibre Reinforced Composites

macroscopic behaviour and minuscular fracture process are not available in literature. The complexities related to the interface are chemical reactions, reinforcement degradation, deficiency in wettability with matrix materials etc. These issues vary from system to system. Hence, matrix-reinforcement interfaces are significantly affected regions at the time of manufacture as well as under service conditions [25]. Finite element simulations have increasingly turned into essential mean to investigate the mechanical behaviour considering micro-structure of particle-reinforced MMCs with the progress of computation power [26]. Additionally, numerical methods are frequently more operational compare to analytical modelling for such composites as the intricate arrangements of composites are inappropriate for closed-form theoretic investigation [27]. Having said these, this chapter investigates the stresses in the interface between matrix and reinforcement due to change of temperature and application of tensile load with respect to reinforcing particle content, size and shape. The results obtained from this investigation will be helpful for the researchers and professionals in this filed to understand the behaviour of MMCs in more details.

2.

Simulation facts

ANSYS, finite element software, has been utilized for generating the replicas to obtain the outcomes. Overall, thirty-six simulations were carried out for thermal as well as tensile loading conditions. The dimensions of MMC samples were constant in finite element models where reinforcements were distributed homogeneously. MMCs reinforced with 6, 12 and 24 mm diameter circular SiC particles were modelled where each particle size had 10%, 15% and 20% contents in finite element models. Therefore, nine models were developed for circular particle reinforced MMCs. Similar combinations were considered for triangle, square and rectangle shaped particles where area fraction of reinforced particles were unchanged (¼ circular reinforcement area) and the arms of equilateral square, triangle and rectangle shape reinforcements were designed in view of that. Hence, all four particle shapes had identical areas but dissimilar boundaries. SiC reinforcements were perfectly joined with 6061 aluminium alloy matrix material. The particles were assumed as linear isotropic material which follows the generalized Hook’s law. A multi-linear kinematic hardening material model (existing in ANSYS) in addition to related flow rule was considered for the matrix [28]. Fig. 16.1 shows stress-strain curves that were incorporated into the simulation for matrix and particle materials. Al 6061 aluminium alloys have higher thermal conductivity and SiC is very hard which also contains lower thermal expansion coefficient as well as higher strength at high temperature. Properties of matrix material and reinforcements are shown in Table 16.1. Eight-node solid elements Plane 183 and Plane 77 for 2D structural and thermal investigation were applied respectively for studying the stresses due to thermal loading. Size of the elements was optimized based on convergence of solution and correctness of the results. Melting point (582  C) of 6061 aluminium matrixes was

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

(b)

800

1000

600

750

Stress (MPa)

Stress (MPa)

(a)

457

400 200 0

500 250 0

0

20 40 Strain (%)

60

0

0.001 0.002 0.003 0.004 Strain (%)

Fig. 16.1 Stress-strain curves of (a) 6061 aluminium alloy and (b) SiC particles [29].

Table 16.1 Properties of SiC reinforcement and 6061 aluminium alloy matrix [30,34].

Material

Thermal conductivity (W/mK)

Heat capacity (J/kgK)

SiC

120

750

Al 6061

167

896

Density (kg/m3)

Coefficient of thermal expansion (1/K)

Modulus of elasticity (GPa)

Poisson’s ratio

3100

4.0  106

400

0.17

71.6

0.33

2700

25.2  10

6

applied as start temperature on finite element model with room temperature of 25  C. Due to cooling down of the models of different configurations at ambient temperature, thermal stresses were produced in composites. A fixed positive elongation of blocks was induced and the effect of tensile loading on interface was analysed. Structural solid Plane183 elements were employed in this two-dimensional study where plane strain condition was used to simplify the simulation process.

3. Thermal loading 3.1

Stress contour

The influence of shape of particles on spreading of thermal residual stress is explicitly shown in Fig. 16.2 using contour plot where the particle size and content were constant. Stress rises as approaching to interface and becomes maximum at matrixparticle interface then decreased suddenly inside the particle of circular shape. Thermal stress inside the circular reinforcement is dispersed evenly through the whole particle. The residual stresses concentrate at the corners for reinforcement shapes which have pointed edges for instance, square, rectangular and triangular reinforcements.

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 16.2 Effect of reinforcement geometry on stress distribution when area and content (15%) of particles are constant [31].

In case of rectangle particles, stress rises in the direction of matrix-particle interfaces. Nevertheless, stress rises sharply as it passed the interface between matrix and reinforcements which is reverse to that of circular reinforcements. Although the four corners of reinforcements get stress concentration, lower stress regions are noted at the top and bottom of reinforcements. The variations of stress spreading in square and rectangular shapes typically take place within the reinforcements. The least stress occurs at the centroid of square reinforcement whereas it arises at the mid position of short arms of rectangular inforcements. This could be because of the dissimilarity in aspect ratio of the shapes. Nevertheless, the difference in stress spreading in matrix near the reinforcements is relatively insignificant. Residual stress in matrix materials was not dependent on axial coordinate around the corners of particles and comparatively in short distance. The change of residual stress is not considerable through the interface between matrix and particle central area on the sides of triangle shaped particle, which is different from that of other shaped particles. Nonetheless, the difference in stress increases in the direction of sharp corners are in two folds. The stress at the pointed edges of triangular reinforcement is maximum amongst for all geometric shapes such as square, rectangle and circular. The concentrated stress in square shaped reinforcements is marginally greater than that of rectangular shaped reinforcement. It is remarkable that the results of finite element analysis furnished a topmost stress of more than 800 MPa after cooling to room temperature, this is higher than the ultimate strength of 6061 aluminium alloy. This is compressive and concerted hydrostatic stress in the thin layer of matrix material in the interface. Practically, it has been described that the interface has the minimum affinity to fracture. The interface is similar to the particles as that is very hard and brittle [32,33], which might be assumed as an addition of the reinforcement [28] due to very large compressive stress in that section.

3.2

Scattering of principal stress

Fig. 16.3 depicts the spreading of principal stresses in matrix and adjoining reinforcements of different shapes. The white, green and blue arrows represent first, second and

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

(a)

459

(c)

S1 = 0.2470 GPa

S1 = 0.272 GPa S3 = –0.458 GPa

S3 = –0.470 GPa

(b)

(d)

S1 = 0.353 GPa

S1 = 0.305 GPa

S3 = –0.611 GPa

S3 = –0.344 GPa

Fig. 16.3 Effect of reinforcements geometry on stress distribution for (a) circular, (b) rectangular, (c) square and (d) triangular reinforcements (15% reinforcement content) [31].

third principal stresses, respectively. The direction and size of the arrows denote the magnitudes and directions of respecting principal stresses. Dimension-based vector plots in Fig. 16.3 display that, irrespective of reinforcement shapes, stresses in reinforcements are generally compressive. The matrix in the interface amongst particles and adjoining matrix undergo tensile in addition to compressive stresses. The compressive stress is comparatively greater than tensile stress in this region. The values of tensile stress rise and compressive stress decreases as the distance rises from the interface. For that reason, MMCs are probable to fracture in matrix instead of at interface under common loading circumstances. Exclusive topographies in vector plots including directions and magnitude of stresses are formed for the particles of various shapes. The vector plot of circular reinforcements presents radial direction of stress extents steadily from the reinforcements. The stress in radial way is compressive through the interface in the reinforcement in addition to matrix. The tensile stress in matrix is perpendicular to that of compressive stress. Vector plot of stresses in rectangular reinforcements has the form of magnetic field as shown in Fig. 16.3(b). MMCs with square shaped particles experiences a cross (‘X’) shaped distribution of compressive stress in matrix

460

Interfaces in Particle and Fibre Reinforced Composites

through the corners, linking the sharp corners of surrounding reinforcements (Fig. 16.3(c)). In the case of triangle reinforcements, compressive stress confined within the reinforcements that acts towards the vertex from the centroid of the triangles (Fig. 16.3(c)). The compressive stress in this direction carried into the matrix through the nearby vertexes of the reinforcements.

3.3

Distribution of Von-Mises stress

Distribution and range of Von-Mises stress in matrix and reinforcements are depicted in Figs 16.4e16.7 for all circumstances simulated in this study. The contour plots given in the figures indicate that, stressed area in matrix rises with the increase in particle content. Greatly localised stress in the matrix is noticed in surrounding areas of reinforcements due to reduced inter-particle distance which gives increased average residual stress. At constant reinforcement content, reinforcement number increases as the reinforcement size reduces. This lessens the distance among the particles. As stated earlier, the intensity of stress generated by the particles declines in matrix as the distance from the reinforcements rises. Hence, it is expected that the stress in matrix spreads and rises more with the reduction of distance among particles. Above contour plots also showed that maximum stresses were produced in triangular particle reinforced MMCs and this stress decreased gradually in the order from rectangular to square and then to circular particle MMCs. The maximum stress is generated evenly in the interface around circular particles (Fig. 16.4) where the

Fig. 16.4 Effect of content (%) and size (diameter - mm) of circular reinforcements on Von-Mises stress in MMCs: (a) 10% and 24 mm (b) 15% and 24 mm (c) 20% and 24 mm, (d) 10% and 12 mm, (e) 15% and 12 mm, (f) 20% and 12 mm, (g) 10% and 6 mm, (h) 15% and 6 mm and (i) 20% and 6 mm [31].

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

461

Fig. 16.5 Effect of content (%) and size (length of an arm - mm) of square reinforcements onVon-Mises stress in MMCs: (a) 10% and 21.27 mm (b) 15% and 21.27 mm (c) 20% and 21.27 mm, (d) 10% and 10.63 mm, (e) 15% and 10.63 mm, (f) 20% and 10.63 mm, (g) 10% and 5.31 mm, (h) 15% and 5.31 mm and (i) 20% and 5.31 mm [31].

highest stress considerably rises with the reduction of particle size followed by marginal reduction with the additional diminution of reinforcement size. Extreme stresses were concentrated in the corner of square shaped particle as shown in Fig. 16.5 and declines with the reduction of reinforcement size for the range investigated in this study. For the specific arrangements of shape and size of reinforcements, stress distributions in circular and square reinforcement reinforced MMCs are alike as presented in Figs 16.4(e) and (h) and 16.5(e) and (h). The stress was also localised near the corners of triangle (Fig. 16.7) and rectangle (Fig. 16.6) reinforcements as the stress localized in square reinforcements. At the beginning, highest stress decrease when the size of reinforcement reduces for rectangular particle reinforced MMCs, then it upsurges with continued reduction of reinforcements size. Nonetheless, the highest stress in triangular particle reinforced MMCs diminishes when the size of reinforcements reduces.

4. Tensile loading MMCs undergo tensile loading in service conditions very often. This section of the chapter considers the influence of shape, size and content of reinforced particles on stress and strain at particle-matrix interface subjected to tensile loading.

462

Interfaces in Particle and Fibre Reinforced Composites

Fig. 16.6 Effect of content (%) and size (length of the big arm - mm) of rectangular reinforcements on Von-Mises stress in MMCs: (a) 10% and 12.28 mm (b) 15% and 12.28 mm (c) 20% and 12.28 mm, (d) 10% and 6.14 mm, (e) 15% and 6.14 mm, (f) 20% and 6.14 mm, (g) 10% and 3.07 mm, (h) 15% and 3.07 mm and (i) 20% and 3.07 mm [31].

4.1

Influence of particle content on stress distribution

The deviation of stresses for 10% and 20% reinforcement contents (one-fourth of 12 mm-diameter circular particle) MMCs are given in Fig. 16.8 which demonstrates that the third principal stress is greatly lesser than first principal stress. Regardless of reinforcement contents, stresses in the reinforcements are larger than that of matrix. Nevertheless, first and third principal stresses rises, when the content of particles increase, which suggests that the composites become stiffer with the increase of reinforcement content.

4.2

Influence of particle size on stress distribution

The influence of particles size on stress distribution is depicted in Fig. 16.9 where the MMCs are reinforced with 15% circular shaped particles. In this case, third principal stress is greatly lower than first principal stress where the stresses in matrix material are lower than that of particles, regardless of reinforcement size. The figures also show that the magnitudes of principal stress rise with the reduction of reinforcement size.

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

463

Fig. 16.7 Effect of content (%) and size (length of an arm - mm) of triangular reinforcements on Von-Mises stress in MMCs (a) 10% and 32.32 mm (b) 15% and 32.32 mm (c) 20% and 32.32 mm, (d) 10% and 16.16 mm, (e) 15% and 16.16 mm, (f) 20% and 16.16 mm, (g) 10% and 8.08 mm, (h) 15% and 8.08 mm and (i) 20% and 8.08 mm [31].

(a)

(b) S1: 0.25 e9 S3: 0.25 e8

S1: 0.21 e9 S3: 0.74 e8

Fig. 16.8 Effect of reinforcement content on stress vectors: (a) 10% and (b) 20% plot for 12 mm diameter circular reinforcements [29].

4.3

Influence of particles’ shape on stress distribution

The influence of reinforcements’ shape has been analysed for MMCs with intermediate particle size and 15% content. The stress vector direction and spreading are presented for three steps of total displacement such as, first, middle and last steps for every reinforcement shapes with part of particles (Figs 16.10e16.13).

464

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b) S1: 0.22 e9 S3: 0.59 e8

S1: 0.22 e9 S3: 0.30 e8

Fig. 16.9 Effect of reinforcements size on stress vectors: (a) 6 mm and (b) 24 mm for 15% circular particles [29].

(a)

(b)

(c)

S1: 0.59 e8 S3: 0.13 e8

S1: 0.22 e9

S1: 0.18 e9 S3: 0.71 e8

S3: 0.64 e8

Fig. 16.10 Effect of elongation on the stress vectors: (a) 0.08, (b) 3.6 and (c) 6% strain for one-fourth of 12 mm diameter and 15% circular reinforcements [29].

(a)

(b)

S1: 0.64 e8 S3: 0.15 e8

(c)

S1: 0.19 e9 S3: 0.92 e8

S1: 0.22 e9 S3: 0.66 e8

Fig. 16.11 Effect of elongation on stress vectors: (a) 0.08, (b) 3.5 and (c) 6% strain for the half 12 mm-15% triangle reinforcements [29].

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

(a)

(b)

S1: 0.53 e8 S3: 0.14 e8

465

(c)

S1: 0.20 e9 S3: 0.81 e8

S1: 0.16 e9 S3: 0.83 e8

Fig. 16.12 Effect of elongation on stress vectors: (a) 0.08, (b) 2.9 and (c) 6% strain for the one-fourth 10 mm-15% square reinforcement [29].

Fig. 16.10 presents that, stress directions remain constant from the start to end of tension displacement on MMCs with circular reinforcements. However, when the reinforcements are of triangular shape, first principal stresses along the direction of elongation at the beginning of loading (Fig. 16.11(a)). With the rise of elongation (Fig. 16.11(b)), stress directions changes and the values are augmented. The stress direction changes primarily nearby two edges of trianguler reinforcement along with matrix, just overhead the reinforcement. First principal stresses nearby the edges become parallel to the sides. The stresses in matrix above the reinforcements rotate away from the sides of the triangles. A comparable tendency is persistent with further rise of elongation (Fig. 16.11(c)). Fig. 16.12 presents the spread of stresses in MMCs with square shaped reinforcements at various elongated steps where stress directions remain almost constant from the start to the end of elongation. For MMCs with rectangular reinforcements (Fig. 16.13), a tendency of stress divergence is prominent in the matrix located at the top and bottom sides of the reinforcements. The spreading of stresses is comparable to the arrangement of magnetic field about a rectangle magnet. This tendency starts at the beginning of elongation and sustained up to the end of elongation process. After comparing all MMCs with diverse particle shapes, it was noted that the uppermost first principal stress generates in MMCs with rectangular reinforcements. Furthermore, the change of stresses from matrix to reinforcements is height for these MMCs.

(a)

(b)

S1: 0.63 e8 S3: 0.10 e8

S1: 0.15 e9 S3: 0.65 e8

(c)

S1: 0.19 e9 S3: 0.81 e8

Fig. 16.13 Effect of elongation on stress vectors: (a) 0.08, (b) 2.3 and (c) 6% strain for the half of 6 mm-15% rectangular reinforcement [29].

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 16.14 Effect of reinforcement content (a) 12 mm-10% (b) 12 mm-15% and (c) 12 mm-20% on Von Mises total strain for MMCs with 12 mm circular particles [29].

Figs 16.10e16.13 clearly show that, MMCs reinforced with rectangle particles have uppermost ability to transfer loads from the matrix to reinforcements, in direction loading. Nevertheless, triangle shaped reinforcement carries the loads across the direction of elongation over the bottom part of the reinforcement. A substantial amount of third principal stresses are present at this side of the reinforcements (Fig. 16.11). The square reinforcements transfer compressive stresses through the top and bottom (Fig. 16.12). The circular reinforcements also convey the compressive stresses from side to side of its middle part, transversely the direction of elongation (Fig. 16.10).

4.4

Effect of reinforcement content on strain field

Von Mises total strain in 12 mm-diametre circular particles reinforced MMCs of three different reinforcement contents is shown in Fig. 16.14 which indicates that Von Mises stress generates in matrix partly then again symmetrically about the reinforcements. With the increase of particle content, Von Mises strain increases and spreads through further spaces. When the content of reinforcement increases to 20%, the surroundings of reinforcements experience too much Von Mises strain which explains the greater stiffness of MMCs with higher amount of particle contents.

4.5

Effect of reinforcement size on strain

The Von Mises total strain distributions in MMCs with different reinforcement sizes are presented in Fig. 16.15 where the MMCs contain 15% circular reinforcements.

Fig. 16.15 Effect of reinforcement size (a) 24 mm-15% (b) 12 mm-15% and (c) 6 mm-15% on Von Mises total strain for MMCs with 15% circular reinforcements [29].

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

467

It is seen that, Von Mises stress generates in the matrix to some extent which is symmetric around the reinforcements. When the size of reinforcements reduces, Von Mises strain rises and spreads more areas over the matrix material. Nearly whole matrix material is evenly strained when the size of reinforcements decreases to 6 mm in diameter. In this case, strain bands are noticed in line with the reinforcements which are perpendicular to the direction of elongation. The reduced reinforcement size increases the total contact surface with the matrix which increases load transfer capability to stiff reinforcements. This explains the improved performance of MMCs with smaller reinforcements.

4.6

Effect of particles’ shape on strain field

The strain distribution of MMCs for different shapes of reinforcements is presented in Fig. 16.16 which depicts that Von Mises stress generates to some extent in matrix and it is symmetrical around nearby reinforcements in all circumstances. The range and quantity of strain depends on the shape of reinforcements. The broadly distributed strain occurs for the MMC with rectangle reinforcements which is trailed by square to triangle and to circle reinforcements, in the order of high to low strain scattering. In the above figure (Fig. 16.16), mainly two aspects control the behaviours of MMCs. The first aspect is that the active sides of the reinforcements restrict dislocation movements. This is not effective in circular reinforcements, where smaller strains are generated around the reinforcements. In triangular particles, the stress is localised around the sharp corners. Square and rectangular reinforcements have four active sides with lower strains around those side. The perimeter of rectangle reinforcements is larger; hence, it experiences lower strains compare to square shape reinforcements. This makes the MMCs stiffer under tensile loading. The second aspect is the spacing among the reinforcements. As the reinforcements are closely positioned to each other, matrix materials in between the particles becomes slimmer, which gives rise greater strain concentration in that region. Strain concentration reduces as the shape changes to circular or rectangular, which delivers smoother strain scatter among reinforcements. Above mentioned first aspect is more effective when MMCs with diverse reinforcement shapes are compared.

5. Conclusions Finite element analysis was performed on aluminium alloy matrix MMCs reinforced with four different geometries of SiC particles under three different contents and sizes. Altogether thirty-six study cases were considered for stress development for cooling as well as tensile loading. The above results and analysis can summarise as follows: (a) Maximum stress was generated at matrix-particle interfaces regardless of size, shape and content of particles during cooling. The influence of shape, size and content of particles on Von-Mises stress was significant with the increase of distance from the interface. An abrupt variation of stress took place at matrix/reinforcement interface where a decrease/increase of stress was influenced by the shape reinforcements.

468 Interfaces in Particle and Fibre Reinforced Composites

Fig. 16.16 Effect of geometry of reinforcement (a) circle (b) triangle (c) square and (d) rectangle on Von Mises total strain for MMCs with 15% medium size reinforcements [29].

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

469

(b) Compressive stresses were generated in the reinforcements and successively, tensile stresses were developed in the matrix during cooling because of lower thermal expansion coefficient of reinforcements. The stress in matrix is generally tension but there are areas of compressive stress next to the interface. Reinforcement shapes influence the stress vector directions and scattered as magnetic field. (c) The range of stresses in matrix material rises with the decrease of inter-particle distance. This occurs because of reinforcement content increase or increase particle size when content remains unchanged. Consequently, the areas of higher reinforcement contents are expected to be act as crack instigation spots. (d) Stresses due to cooling depend significantly on the geometry of reinforcements which affects the aspect ratio. Highest Von-Mises stress was noted in composites with triangle reinforcements then it declines in order from rectangle to square and then to circular reinforcements successively. (e) Under tensile loading, stiffness of MMCs depends on interface length, which assists load transmission from the matrix to reinforcements, along with matrix condition which is affected by the size, shape and content of particle. (f) There is an optimal arrangement of size, shape and content of particles at which the uppermost stiffness of composites can be attained under tensile loading.

References [1] F. Hakami, A. Pramanik, A.K. Basak, Tool wear and surface quality of metal matrix composites due to machining: a review, Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 231 (5) (2017) 739e752. [2] A. Pramanik, A.K. Basak, Effect of machining parameters on deformation behaviour of Albased metal matrix composites under tension, Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 232 (2) (2018) 217e225. [3] A. Pramanik, M. Islam, I. Davies, B. Boswell, Y. Dong, A. Basak, M. Uddin, A. Dixit, S. Chattopadhyaya, Contribution of machining to the fatigue behaviour of metal matrix composites (MMCs) of varying reinforcement size, Int. J. Fatigue 102 (2017) 9e17. [4] A. Pramanik, L. Zhang, Particle fracture and debonding during orthogonal machining of metal matrix composites, Adv. Manuf. 5 (1) (2017) 77e82. [5] J.M. Torralba, C.E. Da Costa, F. Velasco, P/M aluminum matrix composites: an overview, J. Mater. Process. Technol. 133 (1e2) (2003) 203e206. [6] D.F. Watt, X.Q. Xu, D.J. Lloyd, Effects of particle morphology and spacing on the strain fields in a plastically deforming matrix, Acta Mater. 44 (2) (1996) 789e799. [7] S. Qin, C. Chen, G. Zhang, W. Wang, Z. Wang, The effect of particle shape on ductility of SiCp reinforced 6061 Al matrix composites, Mater. Sci. Eng. A 272 (2) (1999) 363e370. [8] Y.W. Yan, L. Geng, A.B. Li, Experimental and numerical studies of the effect of particle size on the deformation behavior of the metal matrix composites, Mater. Sci. Eng. A 448 (1e2) (2007) 315e325. [9] M.T. Kiser, F.W. Zok, D.S. Wilkinson, Plastic flow and fracture of a particulate metal matrix composite, Acta Mater. 44 (9) (1996) 3465e3476. [10] J. Segurado, J. Llorca, Computational micromechanics of composites: the effect of particle spatial distribution, Mech. Mater. 38 (8e10) (2006) 873e883.

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Interfaces in Particle and Fibre Reinforced Composites

[11] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Particulate reinforced metal matrix composites - a review, J. Mater. Sci. 26 (5) (1991) 1137e1156. [12] J. Llorca, C. Gonzalez, Microstructural factors controlling the strength and ductility of particle-reinforced metal-matrix composites, J. Mech. Phys. Solids 46 (1) (1998) 1e28. [13] M.K. Surappa, Aluminium matrix composites: challenges and opportunities, Sadhana Acad. Proc. Eng. Sci. 28 (1e2) (2003) 319e334. [14] H. Ban, Y. Yao, S. Chen, D. Fang, A new constitutive model of micro-particle reinforced metal matrix composites with damage effects, Int. J. Mech. Sci. 152 (2019) 524e534. [15] N. Chawla, Y.L. Shen, Mechanical behavior of particle reinforced metal matrix composites, Adv. Eng. Mater. 3 (6) (2001) 357e370. [16] D.J. Lloyd, Particle reinforced aluminium and magnesium matrix composites, Int. Mater. Rev. 39 (1) (1994) 1e23. [17] L.H. Dai, Z. Ling, Y.L. Bai, Size-dependent inelastic behavior of particle-reinforced metalmatrix composites, Compos. Sci. Technol. 61 (8) (2001) 1057e1063. [18] Q. Wu, W. Xu, L. Zhang, Microstructure-based modelling of fracture of particulate reinforced metal matrix composites, Compos. Part B Eng. 163 (2019) 384e392. [19] J. Llorca, Void formation in metal matrix composites, Compr. Compos. Mater. 3 (2000) 91e115. [20] A. Moloney, H. Kausch, T. Kaiser, H. Beer, Parameters determining the strength and toughness of particulate filled epoxide resins, J. Mater. Sci. 22 (2) (1987) 381e393. [21] W. Cantwell, A. Roulin-Moloney, T. Kaiser, Fractography of unfilled and particulate-filled epoxy resins, J. Mater. Sci. 23 (5) (1988) 1615e1631. [22] A. Whitehouse, T. Clyne, Cavity formation during tensile straining of particulate and short fibre metal matrix composites, Acta Metall. Mater. 41 (6) (1993) 1701e1711. [23] N. Kanetake, M. Nomura, T. Choh, Continuous observation of microstructural degradation during tensile loading of particle reinforced aluminium matrix composites, Mater. Sci. Technol. 11 (12) (1995) 1246e1252. [24] L. Babout, E. Maire, J.-Y. Buffiere, R. Fougeres, Characterization by X-ray computed tomography of decohesion, porosity growth and coalescence in model metal matrix composites, Acta Mater. 49 (11) (2001) 2055e2063. [25] T. Rajan, R. Pillai, B. Pai, Reinforcement coatings and interfaces in aluminium metal matrix composites, J. Mater. Sci. 33 (14) (1998) 3491e3503. [26] N. Chawla, R.S. Sidhu, V.V. Ganesh, Three-dimensional visualization and microstructurebased modeling of deformation in particle-reinforced composites, Acta Mater. 54 (6) (2006) 1541e1548. [27] Q. Meng, Z. Wang, Prediction of interfacial strength and failure mechanisms in particlereinforced metal-matrix composites based on a micromechanical model, Eng. Fract. Mech. 142 (2015) 170e183. [28] A. Pramanik, L. Zhang, J. Arsecularatne, An FEM investigation into the behavior of metal matrix composites: tooleparticle interaction during orthogonal cutting, Int. J. Mach. Tool Manuf. 47 (10) (2007) 1497e1506. [29] A. Paknia, A. Pramanik, A. Dixit, S. Chattopadhyaya, Effect of size, content and shape of reinforcements on the behavior of metal matrix composites (MMCs) under tension, J. Mater. Eng. Perform. 25 (10) (2016) 4444e4459. [30] ASM, Aluminium 6061-T6, 2017. Retrieved from: http://asm.matweb.com/search/ SpecificMaterial.asp?bassnum¼ma6061t6. [31] C.S. Wong, A. Pramanik, A. Basak, Residual stress generation in metal matrix composites after cooling, Mater. Sci. Technol. 34 (11) (2018) 1388e1400.

Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading

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[32] T. Das, P. Munroe, S. Bandyopadhyay, T. Bell, M. Swain, Interfacial behaviour of 6061/ AI203 metal matrix composites, Mater. Sci. Technol. 13 (9) (1997) 778e784. [33] Y. Zhu, H. Kishawy, Influence of alumina particles on the mechanics of machining metal matrix composites, Int. J. Mach. Tool Manuf. 45 (4e5) (2005) 389e398. [34] Accuratus, Silicon Carbide, SiC Ceramic Properties, 2017. Retrieved from: http:// accuratus.com/silicar.html.

Interface tailoring and thermal conductivity enhancement in diamond particles reinforced metal matrix composites

17

Hailong Zhang a , Xitao Wang a , Jinguo Wang b , Moon J. Kim b a University of Science and Technology Beijing, Beijing, China; bUniversity of Texas at Dallas, Richardson, TX, United States

The miniaturization and integration of electronic devices are producing very high power density [1]. Thermal management materials having high thermal conductivity are urgently demanded to dissipate the heat rapidly in order to maintain the performance of the devices. Traditional thermal management materials like Cu/W, Cu/Mo, Al-Si, SiC and Al/SiC have moderate thermal conductivities and may no longer meet the demand. Diamond has the highest thermal conductivity of w2000 W/mK in nature [2], but diamond alone is difficult to act as thermal management materials due to its supreme hardness and poor machinability. Compared with other materials, the diamond exhibits unique advantages, especially the high thermal conductivity; while how to effectively avoid its shortcomings and have a role, is still the research hotspot. It is an effective way to select materials with better thermal conductivity and ductility for composite preparation. Diamond particles reinforced metal matrix (metal/diamond) composites could be a competitive candidate for electronic packaging applications by combining high thermal conductivity of diamond and easy processing of metal matrix composites. The interface plays a critical role in determining the thermal conductivity of the metal/diamond composites by affecting the interfacial thermal conductance. The interface structure can be tailored by chemical composition design or by processing parameter optimization to enhance the thermal conductivity of the metal/diamond composites. Al and Cu are currently adopted as the metal matrix [3,4] in the metal/diamond composites due to their common use in industry and the excellent thermal conductivity. In the field, various routes like vacuum hot pressing [5,6], spark plasma sintering [7,8], pressure infiltration [9,10], gas pressure infiltration [11,12], and high pressure high temperature method [4,13] have been applied to produce Al/diamond and Cu/diamond composites. Generally, the thermal conductivities are reported to be 500e700 W/mK [5,7,9,11] for Al/diamond composites and 500e900 W/mK [4,6,8,10,12,13] for Cu/diamond composites. Nevertheless, the interface structure characterization is rarely reported [14e16] and the underlying mechanisms for interface formation and thermal property enhancement still remain unclear in the literature. Our group in University of Science and Technology Beijing (USTB) and University of Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00017-0 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Interfaces in Particle and Fibre Reinforced Composites

Texas at Dallas (UTD) has concentrated on metal/diamond composites for years. With the formation of appropriate quantity of aluminum carbide at the Al/diamond interface by gas pressure infiltration, we have attained a high thermal conductivity of 710 W/mK in the Al/diamond composite [11]. By producing discrete carbides at the Cu/diamond interface by alloying Ti element into Cu matrix, we have attained a high thermal conductivity of 735 W/mK in the Cu-Ti/diamond composite [17]. By optimizing thickness of carbide layer at the Cu/diamond interface by coating Ti element onto diamond surface, we have attained a high thermal conductivity of 811 W/mK in the Cu/Tidiamond composite [18]. With thorough characterization by focused ion beam (FIB) and scanning transmission electron microscopy (STEM), we have clarified the formation mechanism of the interface structure and demonstrated the thermal conductivity enhancement. In this chapter, the Al/diamond, Cu-Ti/diamond, and Cu/Ti-diamond composites are adopted as three examples to outline the development in the metal/diamond composites, with an emphasis on the tailoring of interface structure and the improvement of thermal properties.

1.

Diamond particles reinforced Al matrix composites

Al has a low density and the supply is abundant on the earth. The Al/diamond composites could find important applications in aerospace technology as an attractive thermal management material. Aluminum can react with carbon to form aluminum carbide (Al4C3). The reaction ensures sound interfacial bonding between the diamond reinforcement and the Al matrix in the composites. In the Al/diamond composite community, the manipulation of the interfacial reaction between Al and diamond is of great interest [19e21]. But fine characterization of the Al/diamond interface is sparsely reported [3,15,22]. Some reports are inconsistent regarding the formation of Al4C3 on the diamond surfaces [3,20]. Here we prepare Al/diamond composites and characterize the interfacial structure using STEM. The results could be of assistance to identify the interface formation mechanism in Al/diamond composites.

1.1

Fabrication of Al/diamond composites

Several processing techniques have been reported to prepare Al/diamond composites, including solid-state process like vacuum hot pressing [5] and spark plasma sintering [7] as well as liquid-state process like pressure infiltration [9]. Here we use a gas pressure infiltration technique to prepare Al/diamond composites [11]. The schematic illustration of the technique is shown in Fig. 17.1. Al bulks (99.97 wt%) and synthetic single-crystalline diamond particles (MBD8, 150e180 mm) were used as the starting materials. The cubo-octahedron shaped diamond particles are shown in Fig. 17.2. To prepare Al/diamond composites, the diamond particles were installed into a cylinder-shaped graphite mold, with the Al bulks covered

Interface tailoring and thermal conductivity enhancement in diamond

475

Gas pressure Metal bulks

Ar

Heating

Diamond particles

Fig. 17.1 Schematic illustration of gas pressure infiltration.

(a)

(b)

400 mm

Fig. 17.2 Starting materials of diamond particles as the filler in the composites. (a) at low magnification and (b) at high magnification.

on top of the mold. The assembly was moved to a furnace and a vacuum of 99.98% procured from Ampal Inc. USA with scanning electron morphology (SEM; Carl Zeiss EVO 18) presented in Fig. 18.1(a) and (b) were employed as matrix precursor. In addition, the chemical compositions of aluminum alloy corresponding to the measured value are summarized in Table 18.1. Graphene with average X-Y dimensions w5e10 mm, 1200e1450 m2/g SSA procured from Angstron Materials, OH, SWCNT (w5e8 mm length, 500 m2/g SSA), MWCNT (2e10 mm length, 330 m2/g SSA) supplied by UNI Pvt, Ltd,. INDIA considered as reinforcement. Fig. 18.1(c)e(e) shows the transmission electron morphology (TEM; Tecnai, G2 20 Twin) of Graphene, SWCNT & MWCNT respectively which were initially subjected to ultrasonication for 45 min each separately (0.5 wt% each). AA powder particles stirred for 60 min and obtained the uniform slurry, then Graphene and CNT dispersions added dropwise on individual alloy powder slurry, filtered to remove the acetone and vacuum dried out for 2 h at 60  C. Thus achieved precursors are vacuum hot pressed with a holding time of 1 h at 200 MPa pressure to get Ø8mm
20 mm long billet. Finally, a billet preheated in the quartz tube at 500  C with continuous argon inert gas environment and extruded (extrusion ratio 8:1) to get the square bar (pin) and the complete fabrication process is schematized in Fig. 18.2 (See Table 18.2).

2.2

Characterization and testing

The square bars are finely polished to remove contaminants and microgrooves arisen on the surface. X-ray diffraction analysis carried out to infer the internal phase constituents on all samples with the step size of Ø ¼ 0.0250. Vickers hardness (HV) test was conducted with maximum indenting load of 500 gf. The densities of sintered and extruded nanocomposites were measured and quantified conferring to Archimedes

Particle reinforced nanocomposites, interfaces, strength and tribological properties

497

Fig. 18.1 SEM of (a) AA 6061 (b) AA 7075 alloy powder particle; TEM of (c) Graphene (d) SWCNT (e) MWCNT. Table 18.1 Chemical composition of AA 7075 and AA 6061 (wt%). Aluminum alloy

Zn

Cu

Mg

Si

Cr

Fe

Ti

Mn

Al

AA 7075

5.0

1.8

1.4

0.41

0.24

0.62

0.21

0.15

Bal.

AA 6061

0.25

0.9

0.23

0.6

0.35

0.73

0.15

0.34

Bal.

principle and compared with theoretical densities. Tribological test of extruded AA 6061 & AA7075 nanocomposites was performed through pin-on-disc wear and friction testing machine in dry sliding conditions according to ASTM G 99-95 standards at ambient temperature and 60%e65% humidity. Wear debris are collected to study morphology and structural changes of various reinforcement separated through

498

Interfaces in Particle and Fibre Reinforced Composites

Ultrasonic liquid dispersion

Aluminium alloy (AA) powder particles

Graphene (GR)

SWCNT (SW) MWCNT (MW) Vacuum drying of mixture

Ø8mm ^20mm

Vacuum hot press

Extrusion Preheating (Quartz tube at 500 °C)

AA+SW

AA+MW

Extrusion direction

AA+GR

Aluminum alloy/Graphene/SWCNT/MWCNT composite

Fig. 18.2 Schematics of fabrication method of nanocomposites. Table 18.2 Density of AA 7075, AA 6061 and all nanocomposites. Composition (series code)

rth (g/cc)

rexp (g/cc)

rexp/rth (%)

AA 6061

(6AA)

2.7

2.670

98.88

AA 6061- Graphene

(6GR)

2.69

2.653

98.62

AA 6061- SWCNT

(6SW)

2.67

2.629

98.46

AA 6061- MWCNT

(6MW)

2.63

2.588

98.40

AA 7075

(7AA)

2.7

2.671

98.92

AA 7075 - Graphene

(7GR)

2.69

2.654

98.66

AA 7075 - SWCNT

(7SW)

2.66

2.615

98.48

AA 7075 - MWCNT

(7MW)

2.65

2.609

98.47

Particle reinforced nanocomposites, interfaces, strength and tribological properties

499

vacuum setup. The 4 mm square pins prepared by machining of the composite were subjected to fine polishing with diamond paste (diamond of particle of 0.6e1.0 mm paste suspended in the aerosol) and were tested against the disc which was wellpolished with alumina paste to have a roughness value 0.3 mm (Ra) throughout the experiment. Sliding speed (0.05, 0.1, 0.15 m/s) and normal load (20, 40, 60 N) were considered to infer the mass loss (gms) and to investigate the effect of Graphene, SWCNT & MWCNT addition on the nanocomposites. Friction tests were conducted at a load of 15 N and a speed of 0.3 m/s conditions during testing with the help of frictional force sensor assembled in the machine. The tested samples yielded superior tribological performance at these conditions and tiniest modifications on the rubbing pin [19]. The tests were repeated four times for each experiment and the average outcomes are reported. Further, the significant changes on the wearing surface due to the incorporation of Graphene SWCNT & MWCNT characterized through SEM. Raman spectroscopy (Renishaw InVia - Raman spectroscopy) at 785 nm laser wavelength on the wear debris collected at extreme wear loss conditions in order to comprehend the wear mechanism at micrometer level.

3. Microstructure and hardness analysis The micrograph evidently indicates the microstructure of the fused powder. Although all three added nano-additives composite displayed fine grain structure, it could be inferred that the pure aluminum has a slightly coarser structure than the nanocomposites. This microstructure analysis of the nanocomposites reveals that recrystallization has taken place and refined grain structure formed from larger (w10 mm) grains to well refine grains due to an addition of Graphene/SWCNT/MWCNT during vacuum hot pressing. This can be ascribed to the fact that the added constituents with varying structures act as a grain growth inhibitor (pinning effect) [20]. Varying specific surface area (SSA) resulted in the formation of finer grains and grain coarsening took place at a constant diffusion rate during vacuum controlled pressing for a longer time of 1hr enhancing the microstructure. Further, finer grains are observed for the AA 7075 - Graphene nanocomposites compared to AA 6061 nanocomposites. However, the refined grains are relying on the temperature and time profile and it is dependent on initial particle size [21e23]. But such a transformation in the grain size can be due to the huge difference in alloying percentage where AA 7075 constitutes w9 wt% second phase elements compared to AA 6061 with w3.5 wt% (from Table 18.1) which will provide the number of nucleation regions. Further, major alloying element copper in AA 7075 to reduce the nucleation temperature. In addition Zn and Mn other two major alloying elements which will speed up the diffusion. These vacuum pressed samples were hot extruded and it is expected that all the grains are elongated along with bonded Graphene, SWCNT & MWCNT in the direction of metal flow (schematics are shown in Fig. 18.2) during extrusion. XRD analysis of extruded AA 6061 and AA 7075 with Graphene/SWCNT/ MWCNT nanocomposites presented in Fig. 18.3(a) and (b) respectively show the

500

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b) 6MW 6SW 6GR

27

30

33

C

Al Al Al

C

20

40 60 Diffraction angle 2Ø

Al

7MW 7SW 7GR Al

Intensity (a.u)

Intensity (a.u)

C

20

30

Al

C

80

20

Al

Al

40 60 Diffraction angle 2Ø

80

Fig. 18.3 XRD patterns of (a) AA 6061- Graphene/SWCNT/MWCNT (b) 7075- Graphene/ SWCNT/MWCNT nanocomposites.

Aluminum peak and other peaks due to unusual reactions. The enlarged view shows (inset) the region 2q from 10.0 to 30.0 degrees which confirms the presence of carbon peaks in the extruded nanocomposites. The extruded sample of AA 6061 with various reinforcements shows the strong Al peaks and no other noticeable reflections were observed for carbide formation due to the presence of low content carbon allotropes. Strong aluminum peaks with Al4C3 peaks were observed for AA7075 - Graphene & AA7075 - SWCNT nanocomposites which are having more interface surface area compared to MWCNT for the same 0.5 wt% fraction content along with higher Zn (5.0e6.2 wt%), Cr (0.18e0.28) & Cu (1.3e1.9 wt%) content of second phase nucleation sites elements which are observed to be much lower in AA6061. The microhardness data were recorded by micro-indenting on cross sections of the samples after vacuum pressed and extruded condition. Fig. 18.4(a) corresponds to the microhardness of pure AA6061 and AA 6061 e Graphene/SWCNT/MWCNT composites. With the same wt%, Al 6061 - Graphene observed to have higher hardness compared to the pressed condition, monolithic alloys, Al 6061- SWCNT and Al

(b) As pressed condition As extruded condition

160

120

80

40

Vickers hardness (HV)

Vickers hardness (HV)

(a)

As pressed condition As extruded condition

160

120

80

40

0 6AA

6GR

6SW

6MW

7AA

7GR

7SW

7MW

Fig. 18.4 Vickers hardness; (a) AA 6061 and (b) AA 7075 with Graphene/SWCNT/MWCNT nanocomposites.

Particle reinforced nanocomposites, interfaces, strength and tribological properties

501

6061- MWCNT under same processing conditions. The same trend of hardness (Fig. 18.4(b)) has been observed for AA7075- Graphene, which is higher than the Al 7075- SWCNT and Al 7075- MWCNT for the same extruder conditions, superior to raw alloy and pressed state owing to dispersion strengthening mechanism irrespective of the Al4C3 formations [24e29].

4. Effect of load on wear loss and friction coefficient The wear test was performed on extruded AA 6061 & AA 7075 and its composite reinforced with Graphene/SWCNT/MWCNT. The test had made it possible to validate the wear resistance of synthesized nanocomposites reinforced with different carbon constituents having varying physical forms and specific surface area. Many researchers have reported that wear loss is inversely related to hardness values of the nanocomposites or alloys [30]. It was also mentioned that the physical form and surface contact area are also essential parameters that influence the wear loss. Fig. 18.5 shows the plots

(a)

(b) 0.09 0.08

6AA 6GR

0.09

6SW 6MW

0.08 0.07 Wear loss (gm)

Wear loss (gm)

0.07 0.06 0.05 0.04 0.03

7AA 7GR

0.06 0.05 0.04 0.03 0.02

0.02

0.01

0.01 20

40

20

60

40

60

Load (N)

Load (N)

(c)

(d) 0.09

0.09 0.08

6AA 6GR

6SW 6MW

0.08

0.07

Friction coefficient

Friction coefficient

7SW 7MW

0.06 0.05 0.04 0.03

7AA 7GR

7SW 7MW

0.07 0.06 0.05 0.04 0.03 0.02

0.02

0.01

0.01 20

40 Load (N)

60

20

40

60

Load (N)

Fig. 18.5 Wear loss; (a) AA 6061 and (b) AA 7075 friction coefficient; (c) AA 6061 and (d) AA 7075 Graphene/SWCNT/MWCNT nanocomposites varying with the load (N).

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comprising the magnitude of weight loss (mg) and friction coefficient against load (N) with the sliding distance of 1000 m. Fig. 18.5(a) and (b) presents the plot between wear loss and load for AA 6061 and AA 7075 nanocomposites and it was found that wear rates increased dramatically for an increase in increasing load. This is due to increase in sliding forces which will plow up the wearing mechanism and increases the material removal rate. Further, AA 6061- Graphene nanocomposites are found to have lesser wear rate when compared to AA 6061 e CNTs nanocomposites and the base alloy which is attributed to a difference in the interfacial bonding. The same trend of wear behavior was observed for AA 7075 e Graphene/SWCNT/ MWCNT nanocomposites. Graphene nanocomposites exhibit more grain boundary pinning and more uniform distribution which prevent the crack propagation. Such a boundary obstructive leads to an increase in fracture toughness, thermal conductivity, heat and stress dissipation [31,32] which bring down the wear losses. Due to these favorable features, the nanocomposites are much suitable for tribological applications. Differences in wear loss observed due to differences in the reinforcement morphology are found to be consistent with the hardness values reported in this work. Worn surface of Graphene reinforced composite are observed to have minor delamination with minimal subsurface fracturing owing to 2D morphology which will adhere and spread into a wider plane in the matrix compared to the tubular structure of SWCNT/MWCNT. Fig. 18.5(c) and (d) shows the proportional increase in average friction coefficient with increasing load (20e60 N) at a constant sliding speed of 0.15 m/s for AA 6061 & AA 7075 respectively. Nanocomposites posed lesser wear loss and friction coefficient compared to base alloy materials due to the presence of nanomaterials in the nanocomposites. The significant differences in the friction coefficients due to high protective nature of Graphene will lower the sheer force and reduction in the material losses. Further, 2D Graphene stances high surface area and superoleophilic nature which leads to the drastic reduction in the coefficient of friction compared to CNTs hydrophilic nature [33]. The reduction in the friction and associated mechanism is explained by puckering influence on Graphene sheet and CNTs when strongly bonded to the matrix. Fig. 18.6(a) shows the puckering mechanism on the carbon sheet which mainly

(a)

(b)

Graphene

SWCNT

MWCNT

Fig. 18.6 (a) Puckering influence on hexagonal carbon sheet (b) specific surface area of different physical forms of carbon.

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depends on the specific surface area and physical forms (Fig. 18.8(b)) which influences the passivation coating to minimize the frictional forces [34]. Further, MWCNT nanocomposites are observed to experience more wear loss and friction coefficient followed by SWCNT and Graphene nanocomposites for both the alloys inconsistent with the hardness values reported. Fig. 18.7 shows the SEM micrographs of worn surfaces of AA 6061 nanocomposites which depict the effect of the interaction of Graphene/SWNT/MWCNT and its influence on the worn-out surfaces after various loading condition. Figs. 18.7(a) and 18.8(a) represents AA 6061 and AA 7075 base alloy in which more severe delamination of material and deep abrasive grooves are observed compared to nanocomposites. The difference in the delamination characteristics is observed where, AA 6061 alloy comprises more compared to AA 7075 alloy which is attributed to more fine grain refinement and the major copper alloying element which improves the toughness of the base alloy and also, it creates delamination membrane which will interlock the material from the pull out phenomenon. This observation is consistent with the difference in its hardness values. Analysis of the worn surface of AA 6061- Graphene and AA 7075- Graphene (Figs. 18.7(b) and 18.8(b)) shows the significant change in the wear mechanism from adhesion to abrasion. But this abrasion is arrested by the influence of plane puckering mechanism discussed earlier; no abrasive scars are inferred on

(a)

(b)

(c)

(d)

Fig. 18.7 SEM micrographs of worn surface interfaces (a) AA 6061 (b) AA 6061 e Graphene. (c) AA 6061 e SWCNT (d) AA 6061- MWCNT.

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(a)

(b)

(c)

(d)

Fig. 18.8 SEM micrographs of worn surface interfaces (a) AA 7075 (b) AA 7075 e Graphene. (c) AA 7075 e SWCNT (d) AA 7075- MWCNT.

the surface. Thus, Graphene in the composite (embedded or laminated on the surface; shown inset) will act as efficient wear resistant film thereby lowering the friction as well as wear losses (in dry wear conditions). This type of solid lubricating film will result in enhanced wear resistant platform upkeep of delamination of the Graphene layer. The marked spots in EDS analysis (by fixing the light elements) confirms the presence of higher carbon (w25 wt%) aluminum (w70 wt%) and other elements (w5 wt%) of the nanocomposites. Figs. 18.7(c) and 18.8(c) shows the SEM micrographs of AA 6061- SWCNT and AA 7075- SWCNT, similarly Figs. 18.7(d) and 18.8(d) shows the SEM micrographs of AA 6061- MWCNT and AA 7075- MWCNT which were tested under different loading conditions. Few kinds of literature are available on CNT based nanocomposites and its worn surface characterizations, but the current study, explain few other facts about the tubular structure of CNTs in the nanocomposites during wear test. Minor abrasion scars were noticed on the worn surfaces, but a majority of CNTs are positioned at squeezed out abrasive grooves valleys edges. This shows that tubular structure is more prominent to de-bonding behavior compared to planar and 2D Graphene at higher applied loads. However, the minimal differences in surface roughness compared with planar Graphene; CNTs nanocomposites are restricted with only line contacts and more cushioning whereas, Graphene establishes plane contact with

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(a)

(b)

(c)

(d)

505

Fig. 18.9 SEM micrographs of worn surface interfaces showing (a) laminate breakage (b) Clustering SWCNT at breakage site (c) De-bonding sites (d) De-bonded MWCNT.

the counter surfaces. Fig. 18.9(a) and (b) explains the surface lamination breakage at higher load due to the presence of SWCNT cluster. The cracks are initiated at these points and are subjected to surface delamination sites. Dispersion of SWCNTs is the important criteria of the developed composite which will index the wear resistant of the nanocomposites. De-bonding of MWCNT on the worn out surface is shown in Fig. 18.9(c), where MWCNT leaves an impression and gets completely separated from the matrix (Fig. 18.9(d)) which is attributed to spongy nature and hydrophobic nature. But for MWCNT, only outer carbon surface (constitute 40%e50% of overall surface area) is bonded with the matrix which makes more prone to de-bonding when subjected to compressive and shear forces during sliding wear.

5. Effect of sliding speed on wear loss and friction coefficient Fig. 18.10 presents the plots comprising the magnitude of weight loss (mg) and friction coefficient against sliding speed (m/s) with a constant sliding distance of 1000 m. Fig. 18.10(a) and (b) shows the wear loss of AA 6061 & AA 7075 respectively varying with sliding speeds at constant applied load (40 N). Decreasing trend of wear rate is observed for both the composite on increasing the sliding speeds. Base alloy

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(a)

(b) 0.09

0.09 6SW 6MW

6AA 6GR

0.08

0.07

Wear loss (mg)

Wear loss (mg)

0.08

0.06 0.05 0.04 0.03

0.07 0.06 0.05 0.04 0.03 0.02

0.02 0.01

0.01 0.05

0.10

0.15

0.05

Sliding speed (m/s)

0.15

(d)

0.09

0.09 6AA 6GR

6SW 6MW

0.08 Friction coefficient

Friction coefficient

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Sliding speed (m/s)

(c) 0.08

7SW 7MW

7AA 7GR

0.07 0.06 0.05 0.04 0.03 0.02

7SW 7MW

7AA 7GR

0.07 0.06 0.05 0.04 0.03 0.02 0.01

0.01 0.05

0.10

Sliding speed (m/s)

0.15

0.05

0.10

0.15

Sliding speed (m/s)

Fig. 18.10 Wear loss; (a) AA 6061 and (b) AA 7075, friction coefficient; (c) AA 6061 and (d) AA 7075 Graphene/SWCNT/MWCNT nanocomposites varying with the sliding distance.

experienced higher wear loss compared to carbon constituents nanocomposites which signify the effect of added nanomaterials and their influence on wear resistance mechanism. More heat generation is expected while increasing the sliding speeds, but the presence of high thermally conductive reinforcements (Graphene-5.3
103 Wm1K1, CNT -35 Wm1K1) in the nanocomposites will take a role in dissipating the heat energy through the specimen in the faster rate. Bruggeman model can be considered for thermal interactions and heat dissipation which is formulated for MMCs [35]. Further, nanoparticles which are accommodated in the valley regions of both disc and pin materials are bouncing to wear surface and are thrown away due to higher centrifugal force, thereby reducing the abrasion wear. Further, it will reduce the shear fatigue life of the reinforcements where, severe damages, pullout and structural damage of CNTs are observed before failure which is experimented for longer sliding durations. It implies that an increase in the sliding speed has less influence to wear rate for AA 6061 & AA 7075 nanocomposites compared to its base alloys.

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Fig. 18.10(c) and (d) represents the plots between the friction coefficient against sliding speed (m/s) of AA 6061 & AA 7075 respectively with the sliding distance of 1000 m with an applied load (40 N). It is a common inference that lower degree of friction coefficient is addressed at higher speeds [23]. Initially, it seems to be a higher but once a constant higher sliding speed is reached softening of both disc and pin material take place which results in lower friction values. More noise is observed for the nanocomposites with respect to the base alloy which indicates that the pin is trying slide. Further, rolling up of debris is seen at higher speeds resulting in damage and fatigue failure of CNTs. Also, embedded Graphene gives planar edges compared to a spherical or sharp edge (asperities) of the CNT in the nanocomposites which are prone to high stress cooperating sites.

6. Wear debris analysis Wear debris investigations focused on many features and the observations revealed different microscale wear features on the collected debris. Fig. 18.11(a) and (b) presents the wear debris of AA 6061 and AA 7075 respectively in submicron’s size and they exist in shreds and nodules form with wear cracks. The observation confirms that the deformation is pure delamination of ductile material on the surface at transition load. Fig. 18.11(c), (d) and (e) shows flake like wear debris collected from the nanocomposites, which are formed due to spalling of a matrix along with reinforcements at

(a)

(c)

(b)

(d)

(e)

Fig. 18.11 SEM of debris; (a and b) pure alloy; (c) with Graphene; (d) with SWCNT; (e) MWCNT of AA 6061 and AA 7075 nanocomposites respectively.

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G

Intensity (a.u)

D

c3 c2 c1

(c) G

6SW 7SW SW

D

c3 c2

G Intensity (a.u)

(b) GR 6GR 7GR

Intensity (a.u)

(a)

MW 6MV 7MV

D c3 c2 c1

c1 1000

2000

3000

Raman shift (cm–1)

1000

2000

3000

Raman shift (cm–1)

1000

2000

3000

Raman shift (cm–1)

Fig. 18.12 Raman spectra of wear debris comparison of composite containing (a) Graphene; (b) SWCNT; and (c) MWCNT.

high concentration of applied load by creating a subsurface. These are initiated at the crack or void points which become the agents for reinforcements pulling and finally fail at critical transition load act as flake type abrader. Flake debris wrapped with Graphene/CNT have least significant on friction and wear loss [36]; MWCNT are detached from the matrix and bristles morphology is found at its edges due to tearing of multiple carbon rings. Fig. 18.12(a) shows the comparison of pristine Graphene (GR(c1)) and wear debris (6GR (c2) & 7GR (c3) of nanocomposites analyzed through Raman spectroscopy. Spectrum clearly visualizes the D and G band with intensity ratio (ID\IG - 0.3 to 0.8) which signifies that the structural distortion on Graphene is negligible. Fig. 18.12(b) represents the comparison of pristine SWCNT (SW e c1) and wear debris (6-SW (c2) & 7SW (c3)) of nanocomposites; Fig. 18.12(c) presents the comparison of pristine MWCNT (MW e c1) and wear debris (6-MW (c2) & 7MW (c3)) of nanocomposites. The observations on the debris collected from CNT reinforced nanocomposites shows the increasing intensity of G band (SP2). It signifies the creation of aluminum carbide (Al4C3) in which the carbon outer surface react with matrix and have the significant influence on the hardness of the prepared nanocomposites [37]. Further, the position and peak shape of G eband, and D-band is not changed after all experiments denote the retain of structures.

7.

Conclusions

One of the most common engineering challenges in a material is to establish the ways to improve the resistance to the wear between any rubbing surfaces. In the present research work AA 6061 & AA 7075 with Graphene/SWCNT/MWCNT are processed through hot press followed by hot extrusion successfully. AA 7075 constitutes w10 wt% second phase elements compared to AA 6061 with w2.3 wt% which provides the number of nucleation’s regions. With the same 0.5 wt% reinforcement addition Al 6061 - Graphene and AA7075- Graphene nanocomposites are observed

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to have higher hardness values compared to monolithic alloys and MWCNT reinforced nanocomposites leading to uniform dispersal and higher dispersion reinforcement strengthening mechanism. It is also due to the presence of major alloying elements which speed up the diffusion at the grain boundary region and are observed to be more in AA 7075. Graphene exhibited superior grain refinement related to CNTs, which enhance the fracture toughness of the developed nanocomposites. This feature has decreased wear losses and hence Graphene nanocomposites are suitable for various tribological applications. AA 6061- Graphene and AA 7075 - Graphene nanocomposites found to experience lesser wear rate compared to CNT based nanocomposites. This is attributed to the difference in interfacial bonding due to varying specific surface area (SSA). Further, 2D Graphene stances high surface area and superoleophilic nature which leads to the drastic reduction in wear parameters compared to CNTs.

References [1] M. Bastwros, G.Y. Kim, C. Zhu, K. Zhang, S. Wang, X. Tang, X. Wang, Effect of ball milling on graphene reinforced Al6061 composite fabricated by semi-solid sintering, Compos. B Eng. 60 (2014) 111e118. [2] H.J. Choi, S.M. Lee, D.H. Bae, Wear characteristic of aluminum-based composites containing multi-walled carbon nanotubes, Wear 270 (1e2) (2010) 12e18. [3] C.B. Lin, Z.C. Chang, Y.H. Tung, Y.Y. Ko, Manufacturing and tribological properties of copper matrix/carbon nanotubes composites, Wear 270 (5e6) (2011) 382e394. [4] L.A. Mitchell, C. Osgood, A theory of friction and wear based on a new characterisation of asperity interactions, Wear 40 (2) (1976) 203e222. [5] H.P. Kumar, M.A. Xavior, Fatigue and wear behavior of Al6061egraphene composites synthesized by powder metallurgy, Trans. Indian Inst. Met. 69 (2) (2016) 415e419. [6] P.K. HG, M.A. Xavior, Graphene Reinforced Metal Matrix Composite (GRMMC): A Review, 2014. [7] F. Li, H.M. Cheng, S. Bai, G. Su, M.S. Dresselhaus, Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes, Appl. Phys. Lett. 77 (20) (2000) 3161e3163. [8] H. Nagai, Y. Okazaki, S.H. Chew, N. Misawa, Y. Yamashita, S. Akatsuka, T. Ishihara, K. Yamashita, Y. Yoshikawa, H. Yasui, L. Jiang, Diameter and rigidity of multiwalled carbon nanotubes are critical factors in mesothelial injury and carcinogenesis, Proc. Natl. Acad. Sci. U.S.A. 108 (49) (2011) E1330eE1338. [9] F. Mokdad, D.L. Chen, Z.Y. Liu, B.L. Xiao, D.R. Ni, Z.Y. Ma, Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite, Carbon 104 (2016) 64e77. [10] C.L. Xu, B.Q. Wei, R.Z. Ma, J. Liang, X.K. Ma, D.H. Wu, Fabrication of aluminume carbon nanotube composites and their electrical properties, Carbon 37 (5) (1999) 855e858. [11] X. Zhang, S. Li, B. Pan, D. Pan, S. Zhou, S. Yang, K. Kondoh, A novel strengthening effect of in-situ nano Al2O3w on CNTs reinforced aluminum matrix nanocomposites and the matched strengthening mechanisms, J. Alloy. Comp. 764 (2018) 279e288.

510

Interfaces in Particle and Fibre Reinforced Composites

[12] J. Stein, B. Lenczowski, N. Fréty, E. Anglaret, Mechanical reinforcement of a highperformance aluminium alloy AA5083 with homogeneously dispersed multi-walled carbon nanotubes, Carbon 50 (6) (2012) 2264e2272. [13] M.M. Bastwros, A.M. Esawi, A. Wifi, Friction and wear behavior of AleCNT composites, Wear 307 (1e2) (2013) 164e173. [14] A.M. Al-Qutub, A. Khalil, N. Saheb, A.S. Hakeem, Wear and friction behavior of Al6061 alloy reinforced with carbon nanotubes, Wear 297 (1e2) (2013) 752e761. [15] H.J. Hwang, S.L. Jung, K.H. Cho, Y.J. Kim, H. Jang, Tribological performance of brake friction materials containing carbon nanotubes, Wear 268 (3e4) (2010) 519e525. [16] R. Pérez-Bustamante, F. Pérez-Bustamante, W. Antunez-Flores, J.M. Herrera-Ramírez, I. Estrada-Guel, P. Amezaga-Madrid, M. Miki-Yoshida, R. Martínez-Sanchez, Al 2024CNTs composites by mechanical alloying, Microsc. Microanal. 16 (S2) (2010) 1256e1257. [17] X. Zeng, J. Yu, D. Fu, H. Zhang, J. Teng, Wear characteristics of hybrid aluminum-matrix composites reinforced with well-dispersed reduced graphene oxide nanosheets and silicon carbide particulates, Vacuum 155 (2018) 364e375. [18] X. Zeng, J. Teng, J.G. Yu, A.S. Tan, D.F. Fu, H. Zhang, Fabrication of homogeneously dispersed graphene/Al composites by solution mixing and powder metallurgy, Int. J. Miner. Metall. Mater. 25 (1) (2018) 102e109. [19] M. Narayan, M.K. Surappa, B.P. Bai, Dry sliding wear of Al alloy 2024-Al203 particle metal matrix composites, Wear 181 (1995) 563e570. [20] H. Kwon, H. Kurita, M. Leparoux, A. Kawasaki, Carbon nanofiber reinforced aluminum matrix composite fabricated by combined process of spark plasma sintering and hot extrusion, J. Nanosci. Nanotechnol. 11 (5) (2011) 4119e4126. [21] L. Lu, A.K. Dahle, D.H. StJohn, Grain refinement efficiency and mechanism of aluminium carbide in MgeAl alloys, Scr. Mater. 53 (5) (2005) 517e522. [22] K. Symeonidis, D. Apelian, M.M. Makhlouf, Controlled diffusion solidification: application to metal casting. la metallurgia italiana, 2009, p. 39. [23] J.F. Guo, H.C. Chen, C.N. Sun, G. Bi, Z. Sun, J. Wei, Friction stir welding of dissimilar materials between AA6061 and AA7075 Al alloys effects of process parameters, Mater. Des. (1980e2015) 56 (2014) 185e192. [24] R. George, K.T. Kashyap, R. Rahul, S. Yamdagni, Strengthening in carbon nanotube/ aluminium (CNT/Al) composites, Scr. Mater. 53 (10) (2005) 1159e1163. [25] I.Y. Kim, J.H. Lee, G.S. Lee, S.H. Baik, Y.J. Kim, Y.Z. Lee, Friction and wear characteristics of the carbon nanotubeealuminum composites with different manufacturing conditions, Wear 267 (1e4) (2009) 593e598. [26] L. Wang, H. Choi, J.M. Myoung, W. Lee, Mechanical alloying of multi-walled carbon nanotubes and aluminium powders for the preparation of carbon/metal composites, Carbon 47 (15) (2009) 3427e3433. [27] T. Tokunaga, K. Kaneko, Z. Horita, Production of aluminum-matrix carbon nanotube composite using high pressure torsion, Mater. Sci. Eng. A 490 (1e2) (2008) 300e304. [28] Y. Feng, H.L. Yuan, M. Zhang, Fabrication and properties of silver-matrix composites reinforced by carbon nanotubes, Mater. Char. 55 (3) (2005) 211e218. [29] R. Perez-Bustamante, I. Estrada-Guel, W. Antunez-Flores, M. Miki-Yoshida, P.J. Ferreira, R. Martínez-Sanchez, Novel Al-matrix nanocomposites reinforced with multi-walled carbon nanotubes, J. Alloy. Comp. 450 (1e2) (2008) 323e326. [30] A.M. Hassan, A.T. Mayyas, A. Alrashdan, M.T. Hayajneh, Wear behavior of AleCu and AleCu/SiC components produced by powder metallurgy, J. Mater. Sci. 43 (15) (2008) 5368e5375.

Particle reinforced nanocomposites, interfaces, strength and tribological properties

511

[31] W. Zhai, X. Shi, M. Wang, Z. Xu, J. Yao, S. Song, Y. Wang, Grain refinement: a mechanism for graphene nanoplatelets to reduce friction and wear of Ni3Al matrix selflubricating composites, Wear 310 (1e2) (2014) 33e40. [32] H.G. Prashantha Kumar, M. Anthony Xavior, Effect of graphene addition and tribological performance of Al 6061/graphene flake composite, Tribol. Mater. Surf. Interfaces 11 (2) (2017) 88e97. [33] M.J. Nine, M.A. Cole, D.N. Tran, D. Losic, Graphene: a multipurpose material for protective coatings, J. Mater. Chem. 3 (24) (2015) 12580e12602. [34] D. Berman, A. Erdemir, A.V. Sumant, Graphene: a new emerging lubricant, Mater. Today 17 (1) (2014) 31e42. [35] K. Pietrak, T.S. Wisniewski, A review of models for effective thermal conductivity of composite materials, J. Power Technol. 95 (1) (2014) 14e24. [36] K. Rajkumar, S. Aravindan, Tribological studies on microwave sintered copperecarbon nanotube composites, Wear 270 (9e10) (2011) 613e621. [37] J.G. Park, D.H. Keum, Y.H. Lee, Strengthening mechanisms in carbon nanotubereinforced aluminum composites, Carbon 95 (2015) 690e698.

Stress induced at the boneparticle-reinforced nanocomposite interface: a finite element approach

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Andy H. Choi Faculty of Science, University of Technology Sydney, Australia

1. Introduction The principal issues governing the success of any biomaterial in clinical applications are its biofunctionality and biocompatibility; both of these factors are directly related to the interactions at the tissue-implant interface. At present, this approach is being explored in the development of a new generation of nanobioceramics with a widened range of biomedical and dental applications [1]. The emphasis at the moment is on the production of new nanobioceramics that are relevant to a wide range of applications including: increased bioactivity for tissue regeneration and engineering; drug and gene delivery; treatment of bacterial and viral infections; implantable surface-modified medical devices for better hard- and softtissue attachment; delivery of oxygen to damaged tissues; cancer treatment; imaging; and materials for minimally invasive surgery. The term biomaterial can be defined as a non-drug substance that is ideal to be included in systems that enhances or replaces the intention and function of bodily tissues and organs. A century ago, synthetic devices constructed from materials as diverse as wood and gold were developed to a point where they could replace the numerous components of the human body. These materials were capable of being in contact with bodily fluids and tissues for prolonged periods, while causing little, if any, adverse reactions. The human tissues react towards these synthetic biomaterials in a number of ways when placed within the human body. At the nanoscale level, the mechanism of tissue interaction depends on the response to the implant surface, and consequently, three terms have been defined that describes a biomaterial with respect to the response of tissues, namely bioactive, bioinert, and bioresorbable. As soon as a material is placed within the human body, and if it has a minimal interaction with its surrounding tissue, it is called bioinert. A few examples of bioinert materials include titanium, stainless steel, and ultra-high-molecular weight polyethylene. The minute a material that, upon placement within the human body, begins to dissolve or to be resorbed and slowly replaced by the advancing tissues is classed

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00019-4 Copyright © 2020 Elsevier Ltd. All rights reserved.

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as bioresorbable. An example of bioresorbable material is bone. A material that is referred to as being bioactive will interact with the surrounding bone, and even soft tissue in some case, when placed within the human body. The development of bone tissue engineering in the past has been related directly to changes in nanotechnology and materials. Although the presence of requirements for materials is standard in the design process of engineered bone substitutes, it is also vital to include clinical requirements such that clinically relevant devices can be engineered. A number of clinical reasons arise from the development of bone tissue-engineering replacements, for example the need for orthopedic implants that are mechanically more well suited to their biological environment and a need for better filler materials when reconstructing large orthopedic defects. Traditionally, autografting and allografting are the biological approaches for the management of bone defects. With regard to autografting, the concerns that contribute to the drawbacks are donor-site morbidity and limited quantity of graft available for harvest. These reasons present a drawback for certain patient populations. With regard to allografting, the concern addresses the long term graft strength. So far, it is still debatable as to whether an allograft will loose strength over time. Bone mineral is made up of nanocrystals, or more correctly, nanoplatelets first defined as hydroxyapatite (HAp) and comparable to the mineral dahllite. It has now been widely accepted that bone apatite can be better thought of as carbonate HAp and approximated by the formula (Ca,Mg,Na)10(PO4CO3)6(OH)2. The composition of commercial carbonate HAp is similar to that of bone mineral apatite. Bone pore size range from 1 to 100 nm in normal cortical bone and from 150 to 400 mm in trabecular bone tissue, and the pores are interconnected. Most information published on HAp is categorized under calcium phosphate, to which HAp belongs. Consequently, the chemical properties will be observed from the perspective that HAp is a calcium phosphate, although it has different solubility and reactivity to other calcium phosphates within the physiological environment [2,3]. The process of biomimetics is based on the idea that, at the molecular level, biological systems process and store information, and this concept has been extended to the processing of nanocomposites for biomedical devices and tissue engineering, such as scaffolds for bone regeneration [4]. The synthesis of novel bone nanocomposites of hydroxyapatite (HAp) and collagen, gelatin, or chondroitin sulphate, through a self-assembly mechanism has been reported by a number of research groups. These self-assembled experimental bone nanocomposites have been reported to exhibit similarities to natural bone in both structures and physiological properties [5]. The process of bone regeneration is common to the repair of fractures. The combination of bone grafts, the skeletal homeostasis and the cascading sequence of biological events are often described as the remodeling cycle. During the past two decades, i.e., from 2000, a considerable amount of attention has been focussed on bioactive composite grafts consisting of bioactive ceramic filler in a polymeric matrix. The objective of these bioactive composite grafts is to achieve interfacial bonding between the host tissues and the graft. Notable examples of bioactive composite grafts include HAp/collagen, HAp/Ti-6Al-4V, and HAp/polyethylene [6,7].

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The manufacture of a nanocomposite can be accomplished by either physically mixing or introducing a new component into an existing nanosized material that permits for changes in properties of the nanostructured materials and may even offer new material functions. For instance, some synthetic and naturally found biopolymers and biomolecules, such as poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), polyamide, collagen, silk fibrin, chitosan, and alginate have been reported to mix into nano-HAp systems. The gel system is another type of nanocomposite that has been developed for biomedical applications. A gel system can be thought of as a three-dimensional network immersed in a fluid. Nanostructured materials can be entrapped within a gel such that the properties of the nanomaterials can be improved and tailored to suit the specific needs of certain biomedical devices. A nanogel is an example of a gel that can be employed in drug delivery carriers. By definition, it is a flexible hydrophilic polymer gel on the nanoscale [8]. Through ionic interactions, these nanogels can spontaneously bind and encapsulate any kind of negatively charged oligonucleotide drug. One major advantage of nanogels is its high “payload” of macromolecules of up to 50 wt%. Such a high value can be achieved by a nanogel, which cannot be attained typically with conventional nano-drug delivery systems [9]. The application of nanocomposite bone grafts for the reconstruction of bone tissue with biomechanics, biological, physio-chemical, and compositional features that imitate those of natural bone is an objective to be pursued. It is widely accepted that natural bone is made up of nanosized plate-like crystals of HAp grown in intimate contact with an organic matrix rich in collagen fibers. Using strategies found in nature, an innovative technique in the manufacture of nanocomposite bone grafts has received a lot of attention and seem to be beneficial compared to conventional methods. Numerous fabrication approaches have been utilized for the formation of collagencoated ceramics, ceramic-coated collagen matrices, collagen-HAp composite gels, films, and composite scaffolds for the repair of spine and hard tissue [10]. Stem cells, which are cells from an embryo, fetus, or adult that have the ability to reproduce for long periods, have been included into a range of bioceramics and when implanted, can combine with mineralized three-dimensional scaffolds to create highly vascularized bone tissue. These cultured cell-bioceramic nanocomposites can be utilized to treat critical sized defects in long bone shafts, therefore providing an excellent integration of the ceramic scaffold with bone, and hence a good functional recovery. Nanostructured materials and their modified forms offer some attractive possibilities in the fields of tissue and implant engineering for biomedical and dental applications, taking advantage of the combined use of living cells and 3-D scaffolds to deliver vital cells to the damaged site of the patient. The aim of this chapter is to provide a brief background on the current applications of finite element analysis (FEA) in bone tissue engineering and dentistry. The basic principles of finite element analysis will be examined followed by its applications in the design and analysis of nanocomposite bone grafts and tissue scaffolds. Traced how the technology of nano-fibre reinforced composites has evolved a bit more systematically.

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2.

Finite element approach

2.1

Overview

Introduced in 1956, the finite element method (FEM) was extensively used in the fields of civil and mechanical engineering during the initial stages and in 1970s in orthopedic biomechanics to evaluate stresses in human bones during functional loading. The application of finite element analysis (FEA) in implant design and analysis and in dentistry related to the deformations under functional loadings accelerated after the 1980s. Since then, this method has widely been accepted in engineering and biomedical systems and applied with increasing frequency for stress analyses of bone and bone-prosthesis structures, dental implants and devices (Fig. 19.1), fracture fixation devices for soft and hard tissues. The FEM has also been used in evaluating nanoindentation and nanomechanical testing to determine the biomechanical properties of nanocoatings such as hydroxyapatite on metallic implants and devices [1].

2.2

Some key issues

Among the most common families used for typical structural models are onedimensional (1-D) beam elements, two-dimensional (2-D) plane stress and plane strain elements, axisymmetric elements, and three-dimensional (3-D) shell and solid elements (Fig. 19.2) [1]. Beam elements are useful for modelling beam-like structures where length is much greater than other dimensions and the overall deflection and bending moments can be predicted. However, this type of model will not be able to predict the local stress concentrations at the point of application of a load or at joints. Plane stress elements are appropriate for thin 2-D structures, in which stresses out of the plane can be neglected. Plane strain elements simulate a special 3-D stress state, occurring when out-of-plane deformation is constrained (for example, in relatively thick plates). As an example, we can generate 2-D and 3-D models of the interface

Fig. 19.1 Schematic illustration of a dental implant. (a) Finite element mesh; (b) solid model.

Stress induced at the bone-particle-reinforced nanocomposite interface

1-D element Beam

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3-D elements Tetrahedral

Pyramid

Wedge

Hexahedral

2-D elements Triangular

Quadrilateral

Fig. 19.2 Types of FEA element.

between bone tissue and a particle or fiber-reinforced nanocomposite using beam elements to mimic fibers and quadrilateral and hexahedral elements to imitate bone and particles. Element topology refers to the general shape of the element, i.e., triangular or quadrilateral (Fig. 19.2). The topology also depends on the family of the element (for example, 2-D or 3-D). In general, quadrilateral elements may be considered more suitable than triangular in complex structural models, since the quadrilateral can match the true displacement function more accurately due to a higher number of degrees of freedom. In addition, the number of elements in meshes built from triangular elements tends to be larger. On the other hand, the simplicity of triangular elements makes them very attractive, for example, for automatic mesh generation. Triangular-shaped elements are easier to fit into geometrically complex structures. Combining different element topologies and element orders, such as triangular and parabolic, could increase the accuracy of a topologically lesser element. This is of vital significance when it comes to simulating the effects of stresses at the bone-particle-reinforced nanocomposite interface as the model will require a combination of various element topologies to simulate the different components and to provide a relatively accurate prediction of events during the loading process. Extra precaution also needs to be taken into consideration during the creation of the interface between an implant and bone tissue, for instance the thickness and the constituent of the interface such as the presence of callus tissues and other biological materials such as blood clots. In this chapter, these points are highlighted in brief. Dealing with the explanation, namely how the precautions are necessary or how to exercise the precautions, adequately leads to chapter that is estimated to take additional pages. However, the reader may refer to an earlier work for a similar discussion [1]. The other modeling option would be to assume the implant is “fused” to the bone tissue, which mimics the conditions of complete healing. In that scenario, stresses applied to the

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implant are directly transferred to the surrounding bone tissue. Furthermore, the other issue will be to replicate the distribution of the reinforcement phase within the matrix to a certain level of accuracy as it may also affect the outcome of the simulation. The assignment of proper material properties to a finite element model is a necessary step to ensure predictive accuracy. In recent years, there have been a number of studies examining the elastic properties of bone. This will be further discussed in the following section. One of the principal results of these investigations is that the elastic properties of bone vary with both the position and orientation of the specimen; that is, bone is both heterogeneous and anisotropic in its elastic properties [1]. The mechanical characteristics of bone vary with the types of bone, biological variables such as gender and age, its possible state of pathological degradation, levels of activity of the living bone, along with the preservation conditions of the specimen up until the time of experimentation, all of which contribute to the irregular distribution of its mechanical properties [11]. A suitable approximation of the distribution of bone material can be acquired through the use of computed tomography scans. This approach ensures in a more relevant physiological model on a subject specific basis [1].

3.

Structures of human bone

3.1

Overview

From a macroscopic view, human bone appears in two different forms and the most noticeable difference between these two types of bone is their relative densities or volume fraction of solids. The term cortical or compact is used to classify bone with a volume fraction of solids greater than 70%. On the other hand, bone with a volume fraction of solids less than 70% is termed cancellous or trabecular. In general, most bones within the human body possess both types: an outer shell comprised of a dense compact bone encapsulating a core of spongy cancellous bone [1,11]. On a smaller scale, bone consists of concentric cylinders referred to as a Haversian system, which is a matrix with layers of cells laminated between cylinders. The complete structure can be either loosely packed cancellous bone or densely packed cortical bone [1,11]. Both the cortical and cancellous bones are constructed using two-thirds inorganic material and one-third organic material. The inorganic material is mainly calcium phosphate and calcium carbonate, while the organic material consists of three different types of cells. The first type of cells is the osteoblasts, which are commonly thought of as the cells responsible for bone formation. The second type is the osteoclasts and they are responsible for the demineralization of bone and in the formation of the matrix. The last type is the fibroblasts, which are responsible for forming collagenous fibers. Both the osteoblasts and osteoclasts are confined within the ‘Haversian’ and surrounding network. It

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has been previously suggested that these cells might function as site-specific activators in addition to regulators governing the establishment of new bones [12,13]. Cortical bone can be regarded as a multi-component biological composite material consisting of an organic matrix (collagen and protein molecules), water, and mineral phases (crystalline and amorphous hydroxyapatite (HAp)) [14e16]. The organic phase mainly contains Type I collagen fibers that possess a number of the characteristics associated with polymeric materials [17]. The mineral phase is mainly made of carbonate apatite crystals (CHAP) which precipitate around the collagen fibers [18]. Furthermore, it has been revealed that CHAP displayed a behavior similar to that of ceramic materials [17]. The water phase, bound and unbound, enables the interactions between the organic and mineral phases [19,20]. Furthermore, cortical bone can be thought of as a hierarchical solid created from structural elements from a different perspective, which in themselves exhibit a discrete structure [16,21,22]. The structure of cortical bone can be classified as lamellar (lamellae), porous (osteocyte lacunae), fibrous (collagen fibers and osteons), and particulate (CHAP crystallites) based on their relative sizes [22]. The mechanical behavior of cortical bone is dependent on the distribution and sizes of the phases that begins at the elemental (less than 0.005 mm) and microstructural (between 1 and 10 mm) level [23] as demonstrated by scanning electron microscopy. At the next level in the hierarchy, which is from 10 to 50 mm, the mechanical behavior of cortical bone can be determined from microhardness testing [24], which measures the physical effects of small-scale changes in the mineral content of the bone [25]. More importantly, nanoindentation has been utilized to investigate the properties of hard tissues such as bone since the 1990s [26e32]. This approach permits the determination of mechanical properties such as the Young’s modulus and hardness at the surface of a material. The procedure for nanoindentation is much simpler in comparison to other conventional mechanical tests such as tensile testing. This is particularly the case for small complex-shaped samples such as dentine, cementum, and enamel [33]. Importantly, this technique enables the measurement of mechanical properties in a very small selected region within the specimen where the dimensions may be at a micrometer or even nanometer scale, which is essential when measuring local properties of non-homogeneous structures such as dental calcified tissues. Of vital importance are the constitutive properties of cancellous bone as it is this bone that is in direct contact with the implant or prosthesis. In simple terms, cancellous bone is a cellular material composed of a connected network of plates or rods. A network of plates produces closed cells, while a network of rods creates open cells. Its mechanical behavior is typical of a cellular material. The direction of the applied load will govern the symmetry of the cancellous bone structure. For example, in bones such as the vertebrae where the loading is mainly uniaxial, the trabeculae frequently develop a columnar structure with cylindrical symmetry. On the other hand, if the stress pattern in the cancellous bone is complex, then the structure of the trabeculae network will also become complex and highly asymmetric [34,35].

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Bone remodeling

The principle behind the bone remodeling sequence is to sustain the integrity of the skeleton has been a widely accepted fact. Bone remodeling is achieved through the concerted efforts of the constituent cells of bone, the osteoclasts and osteoblasts. The origin of osteoblast cells can be traced back to multipotent mesenchymal stem cells (MSCs), which possess the capability of separating into osteoblasts as well as into myoblasts, chondrocytes, adipocytes, and fibroblasts [36,37]. The responsibility of osteoblasts is the production of bone matrix constituents and they are discovered in clusters along the bone surface, “coating” the layer of bone matrix they are producing [37]. On the other hand, the osteoclasts are responsible for bone resorption. They are derived from hematopoietic cells of the mononuclear lineage and are giant multinucleated cells with a diameter of up to 100 nm [38]. The term basic multicellular unit is often used to describe the close collaboration between osteoblasts and osteoclasts in the remodeling process. Bone resorption and bone formation are balanced in a homeostatic equilibrium. On a cellular level, the simultaneous biological and mechanical actions play a governing role in the delicate equilibrium between bone growth, formation, and resorption. In addition, the combination of the osteoblasts and osteoclasts also contributes to the bone remodeling of defects such as micro-fractures [39]. The bone remodeling sequence is composed of three consecutive phases: resorption, reversal, and formation. The resorption phase initiates with the migration of partially differentiated mononuclear pre-osteoclasts to the bone surface for the formation of multinucleated osteoclasts. The appearance of mononuclear cells on the bone surface as well as the commencement of osteoclastic resorption marks the beginning of the reversal phase. Mononuclear cells provide signals for the differentiation and migration of osteoblasts and create surfaces for new osteoblasts to begin bone formation. The final stage is the formation phase and it occurs when the resorbed bone is replaced completely with the new bone deposited by osteoblasts. Flattened lining cells cover the entire surface when the formation phase is completed. A prolonged resting period will take place until the beginning of a new remodeling sequence [37]. The interface between bone tissue and biomedical implant is of vital importance to osseointegration as the utilization of implants may alter the mechanical environment of the implantation site such as the hip, knee, or mandible. This may also lead to the remodeling and adaptation of the surrounding cortical and cancellous bone tissues. During the last thirty years, the use of nanocomposite bone grafts and implants has received a significant amount of attention as it is possible to manipulate mechanical properties such as Young’s modulus of the composites closer to those of human bone tissues through the addition of secondary nanoparticles and nanofibers such as carbon nanotubes (CNTs) and bioactive glass. These bioactive composite grafts implants are essentially designed to achieve interfacial bonding between the host tissues and the graft. Consequently, it is vital that the influence of bone remodeling on the longevity of implants and devices be considered in an effort to enhance its efficacy [1,40,41]. During functional loadings such as walking or chewing, forces on the prosthesis (e.g., a hip joint) will be transferred to the implants and this will result in stresses being

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generated within the bone surrounding the implants. Bone remodeling occurs in response to the applied forces. The type of bone remodeling, i.e., constructive or destructive, taking place in the bone tissue surrounding the implant will be governed by the variations in the internal stress state. Stress shielding and bone resorption will occur when no load is being transferred to the supporting tissues, while abnormally high stress concentration can lead to implant failure. For these reasons, it is essential to consider the effect of bone remodeling on the performance of biomedical implants and prostheses in order to improve its efficiency. A number of factors are involved in the achievement of osseointegration and these include material composition, design and geometry of implant, adequate bone quality and interface interactions, and absence of overheating during site preparation and surgical technique.

4. Application of FEA in biomedical engineering and dentistry 4.1

Overview

The impact of new generation of nanocomposites can be found in a diverse area, none more so than in biomedical engineering. Presently, opportunities for nanocomposites in the biomedical field come from different applications and different functional requirements for those applications. For instance, using nanocomposite scaffolds in bone tissue engineering to achieve better mechanical and physiological properties.

4.2

Bone tissue engineering and scaffolds

Long ago, the focus of bone tissue engineering has been undeniably centered on the modifications in materials and nanotechnology. Even though the inclusion of material requirements is normal during the design and development of engineered bone substitutes, of vital importance is the inclusion of clinical prerequisites so that clinically relevant devices and prosthetics can be engineered. Currently, the emphasis of tissue engineering has changed by seizing the advantage of combining the utilization of living cells with three-dimensional scaffolds to transport vital cells to the damaged site of the patient. Several clinical opinions exist in relations to the advancement of bone tissue-engineering substitutes, including a need for better filler materials during the reconstruction of large bone defects, and for implants that are more mechanically suited to their biological surroundings. Exploiting nanocomposite bone grafts to reconstruct bone tissue with features such as physiochemical, biological, structural, biomechanical, and compositional which imitates those of natural bone is an objective worthy of pursuing. Furthermore, a composite scaffold that combines a polymer with a bioceramic would ideally unite the advantages of the two materials where the polymer would improve the toughness of the composite and the bioceramic would increase the bioactivity of the resulting composite. A recent study

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Interfaces in Particle and Fibre Reinforced Composites

attempted to synthesize a bioactive nanocomposite consisting of poly(lactic-co-glycolic acid) and TiO2 nanotube. Results from three-dimensional finite element analysis utilizing information of an adult femur revealed the addition of TiO2 nanotube to the polymeric matrix would give a more monotonous stress distribution in the bone and reduce the maximum stress on the surrounding bone tissues [42].

4.3

Dentistry

In dentistry, the utilization of composites has been widely accepted and applied as a substitution for metallic restorations. There has been a significant evolution in the composition of resin-based dental composite since its introduction. Resin composites are employed for several applications such as cement for tooth prostheses and pit and fissure sealants. Conditions in the oral environment have defined certain special requirements and challenges for the clinical applications of composites. Incompatibility from a biological perspective can arise from factors such as nanoparticles, short fibers, polymerization shrinkage, and residual monomers. Tissue compatibility is indirectly influenced by polymerization shrinkage. Several adverse biological reactions may be caused by residual monomers. Resin-based composites may also promote bacterial growth in an oral environment. This can be caused by a volume change leading to a marginal gap between tooth tissue and the restorations. Filler content can also play a part in cytotoxicity [43]. Over the past few decades, the development of nanotechnology has revolutionized the field of dentistry and has led to major improvements in materials and their clinical applications. Nanocomposites and other nanostructured materials such as nanocoatings have been thoroughly examined from its improved mechanical properties to the biological behavior and cytotoxicity by various researchers for different applications in dentistry. Worldwide, edentulism or toothlessness remain a primary public health issue which can have an impact on the psychosocial and physical well-being of patients in addition to the life quality, appearance, and function of mastication, even though there have been improvements in preventive dentistry and the increasing adoption of wearing removable dentures [44,45]. An option that is widely available to address the issue of replacing missing teeth in patients is to use partial or fixed dentures [46]. It has been hypothesized that a nanocomposite composed of alumina and ceria-stabilized zirconia could potentially be used in the production of partial denture frameworks. Threedimensional finite element analyses have been carried out to simulate cantilever tests to examine the stress distribution during loading until fracture [47]. In another study, the relationship between the stress distributions in fiber-reinforced resin composite adhesive fixed partial dentures and the mechanical properties of adhesive resin cements was investigated using FEA [48]. Their study concluded that stress analysis using FEA appeared to be a good approach for examining stresses in fiber-reinforced resin composite adhesive fixed partial dentures under simulated clinical conditions and taking into consideration the position of fiber framework placement and adhesive cement layer thickness. The reader is invited to refer to the original report [48] for a more elaborate discussion on the analysis addressing the suitability of FEA, in relation to experimental study and imaging results.

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Moreover, a growing interest has surrounded the development of a new composite that contains only nanoparticles referred to as “nanofill” composite. The addition of nanofillers has been described to be advantageous in enhancing the mechanical performances of dental composites. Selecting the most appropriate form and class of filler such as ceramic or polymeric nanotubes, nanoparticles, or nanofibers for that application is made possible through the use of FEA [49]. As a result of this innovative development, a new group of composites referred to as “nanohybrids” has been developed by altering the formulations of their microhybrids to include more nanoparticles [43]. In a recent study by Saen et al. [50], the flexural properties of dental resins and dental composites containing silanized silica nano-particles were examined using 3-point bending, 4-point bending and piston-on-three ball biaxial tests at different test speeds. Finite element analysis was also performed to estimate the effect of the test method on the flexural strength.

5. Conclusion Finite element analysis has become widely accepted and applied in all biomechanics, in particular for examining stresses and strains in dental implants and the surrounding bone structures as well as for normal bone remodeling. In addition to biological factors, issues such as biomechanics and the magnitude of the functionally applied multi-axial forces will govern the interactions between the implant and the surrounding bone tissues. Gaining an in-depth understanding into the biological systems can only be achieved with appropriate nanoscale mechanical properties of the bone structures as well as an insight into the effect of nanoloadings on the nanostructures of these biological systems. In recent times, tissue engineering has been directed toward seizing the advantage of combining three-dimensional scaffolds with living cells to deliver much-needed cells to damaged sites in the human body. However, the design of the scaffold is one of the most important challenge in tissue engineering at the moment. Ideally, the scaffold should possess a suitable environment for the cells to attach and multiply as well as adequate biological functions and structure. The integration of cultured cellbiomaterial nanocomposites into the healthy bone tissue is a significant prerequisite of any successful bone grafting with regards to both biological sealing and mechanical fixation. A noticeable increase of interest has been observed related to the use of nanomaterials and nanotechnology in advanced technologies. The application of nanotechnology has also revolutionized certain traditional areas of biomedical engineering and dentistry through the introduction of nanobiomaterials, tissue engineering nanocomposite scaffolds, nano-drug delivery systems, and dental nanocomposites. Despite being widely considered as exceptionally beneficial, considerations need to be taken concerning the impact of nanotechnology and the associated risks when using such structures.

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At present, nanocomposites are vital to the design and development of a wide variety of biomedical and dental devices and implants. The research and development in the field of new and unique nanocomposites are encouraging. They can be utilized for a wide spectrum of areas such as in tissue engineering, implantable devices, and in drug-delivery systems.

References [1] A.H. Choi, B. Ben-Nissan, Anatomy, Modeling and Biomaterial Fabrication for Dental and Maxillofacial Applications, Bentham Science Publishers, United Arab Emirates, 2018. [2] R.Z. LeGeros, Calcium Phosphates in Oral Biology and Medicine. Monographs in Oral Sciences, vol. 15, S. Karger, Basel, 1991. [3] R.Z. LeGeros, Biodegradation and bioresorption of calcium phosphate ceramics, Clin. Mater. 14 (1) (1993) 65e88. [4] M.C. Chang, C.C. Ko, W.H. Douglas, Conformational change of hydroxyapatite/gelatin nanocomposite by glutaraldehyde, Biomaterials 24 (18) (2003) 3087e3094. [5] W. Zhang, S.S. Liao, F.Z. Cui, Hierarchical self-assembly of nano-fibrils in mineralized collagen, Chem. Mater. 15 (16) (2003) 3221e3226. [6] R. Murugan, S. Ramakrishna, Development of nanocomposites for bone grafting, Compos. Sci. Technol. 65 (15e16) (2005) 2385e2406. [7] J.M. Oliveira, M.T. Rodrigues, S.S. Silva, P.B. Malafaya, M.E. Gomes, C.A. Viegas, Novel hydroxyapatite/chitosan bilayered scaffold for osteochondral tissue-engineering applications: scaffold design and its performance when seeded with goat bone marrow stromal cells, Biomaterials 27 (36) (2006) 6123e6137. [8] S.V. Vinogradov, T.K. Bronich, A.V. Kabanov, Nanosized cationic hydrogels for drug delivery: preparation, properties and interactions with cells, Adv. Drug Deliv. Rev. 54 (1) (2002) 135e147. [9] S.V. Vinogradov, E.V. Batrakova, A.V. Kabanov, Nanogels for oligonucleotide delivery to the brain, Bioconjug. Chem. 15 (1) (2004) 50e60. [10] D.A. Wahl, J.T. Czernuszka, Collagen-hydroxyapatite composites for hard tissue repair, Eur. Cells Mater. 11 (2006) 43e56. [11] I.V. Knets, G.O. Pfafrod, J.Z. Saulgozis, Deformation and fracture of hard biological tissue, Zinatne Riga (1980) 319. [12] G.A. Rodan, L.A. Bourret, A. Harvey, T. Mensi, Cyclic AMP and cyclic GMP: mediators of the mechanical effects on bone remodeling, Science 189 (4201) (1975) 467e469. [13] I. Binderman, Z. Shimshoni, D. Somjen, Biochemical pathways involved in the translation of physical stimulus into biological message, Calcif. Tissue Int. 36 (Suppl. 1) (1984) S82eS85. [14] J.L. Katz, Hard tissue as a composite material. I. Bounds on the elastic behavior, J. Biomech. 4 (5) (1971) 455e473. [15] K. Piekarski, Analysis of bone as a composite material, Int. J. Eng. Sci. 11 (6) (1973) 557e565. [16] J.L. Katz, Anisotropy of Young’s modulus of bone, Nature 283 (5742) (1980) 106e107. [17] J.L. Katz, The structure and biomechanics of bone, in: J.F.V. Vincent, J.D. Currey (Eds.), Mechanical Properties of Biological Materials, Cambridge University Press, Cambridge, 1980, pp. 137e168.

Stress induced at the bone-particle-reinforced nanocomposite interface

527

[18] L. Weiss, Histology: Cell and Tissue Biology, Elsevier Biomedical, New York, 1983, p. 43. [19] R.A. Robinson, Crystal, collagen, and water relationships in bone matrix, Clin. Orthop. 17 (1960) 69e76. [20] P.A. Timmins, J.C. Wall, Bone water, Calcif. Tissue Res. 23 (1) (1977) 1e5. [21] J.B. Park, Biomaterials: An Introduction, Plenum, New York, 1979. [22] R. Lakes, Materials with structural hierarchy, Nature 361 (6412) (1993) 511e515. [23] P. Frasca, R.A. Harper, J.L. Katz, Scanning electron microscopy studies of collagen, mineral and ground substance in human cortical bone, Scanning Electron Microsc. 3 (1981) 339e346. [24] R. Amprino, Investigations on some physical properties of bone tissue, Acta Anat. 34 (3) (1958) 161e186. [25] G.P. Evans, J.C. Behiri, J.D. Currey, W. Bonfield, Microhardness and Young’s modulus in cortical bone exhibiting a wide range of mineral volume fractions, and in a bone analogue, J. Mater. Sci. Mater. Med. 1 (1) (1990) 38e43. [26] J.Y. Rho, T.Y. Tsui, G.M. Pharr, Elastic properties of human cortical and trabecular lamellar bone measured by nanoindentation, Biomaterials 18 (20) (1997) 1325e1330. [27] M.E. Roy, J.Y. Rho, T.Y. Tsui, N.D. Evans, G.M. Pharr, Mechanical and morphological variation of the human lumbar vertebral cortical and trabecular bone, J. Biomed. Mater. Res. 44 (2) (1999) 191e197. [28] C.H. Turner, J. Rho, Y. Takano, T.Y. Tsui, G.M. Pharr, The elastic properties of trabecular and cortical bone tissues are similar: results from two microscopic measurement techniques, J. Biomech. 32 (4) (1999) 437e441. [29] P.K. Zysset, X.E. Guo, C.E. Hoffler, K.E. Moore, S.A. Goldstein, Elastic modulus and hardness of cortical and trabecular bone lamellae measured by nanoindentation in the human femur, J. Biomech. 32 (10) (1999) 1005e1012. [30] J.G. Swadener, J.Y. Rho, G.M. Pharr, Effects of anisotropy on elastic moduli measured by nanoindentation in human tibial cortical bone, J. Biomed. Mater. Res. 57 (1) (2001) 108e112. [31] Z. Fan, J.Y. Rho, Effects of viscoelasticity and time-dependent plasticity on nanoindentation measurements of human cortical bone, J. Biomed. Mater. Res. A 67 (1) (2003) 208e214. [32] D.G. Kim, Y.H. Jeong, E. Kosel, A.M. Agnew, D.W. McComb, K. Bodnyk, et al., Regional variation of bone tissue properties at the human mandibular condyle, Bone 77 (2015) 98e106. [33] N.E. Waters, Some mechanical physical properties of teeth, in: J.F.V. Vincent, J.D. Currey (Eds.), Mechanical Properties of Biological Materials, Cambridge University Press, Cambridge, 1980, pp. 99e134. [34] J.K. Weaver, J. Chalmers, Cancellous bone: its strength and changes with ageing and an evaluation of some methods for measuring content e I. Age changes in cancellous bone, J. Bone Jt. Surg. 48A (1966) 289e298. [35] W.J. Whitehouse, E.D. Dyson, C.K. Jackson, The scanning electron microscope in studies of trabecular bone from a human vertebral body, J. Anat. 108 (3) (1971) 481e496. [36] P. Bianco, M. Riminucci, S. Gronthos, P.G. Robey, Bone marrow stromal stem cells: nature, biology, and potential applications, Stem Cells 19 (3) (2001) 180e192. [37] D.J. Hadjidakis, I.I. Androulakis, Bone remodeling, Ann. N.Y. Acad. Sci. 1092 (2006) 385e396. [38] S.L. Teitelbaum, Bone resorption by osteoclasts, Science 289 (5484) (2000) 1504e1508.

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[39] A.H. Choi, R.C. Conway, B. Ben-Nissan, Finite-element modeling and analysis in nanomedicine and dentistry, Nanomedicine 9 (11) (2014) 1681e1695. [40] E. Tanaka, S. Yamamoto, T. Nishida, Y. Aoki, A mathematical model of bone remodeling under overload and its application to evaluation of bone resorption around dental implants, Acta Bioeng. Biomech. 1 (1999) 117e121. [41] A.H. Choi, B. Ben-Nissan, J.P. Matinlinna, R.C. Conway, Current perspectives: calcium phosphate nanocoatings and nanocomposite coatings in dentistry, J. Dent. Res. 92 (10) (2013) 853e859. [42] H. Eslami, H. Azimi Lisar, T.S. Jafarzadeh Kashi, M. Tahriri, M. Ansari, T. Rafiei, et al., Poly(lactic-co-glycolic acid)(PLGA)/TiO2 nanotube bioactive composite as a novel scaffold for bone tissue engineering: in vitro and in vivo studies, Biologicals 53 (2018) 51e62. [43] A.H. Choi, J.P. Matinlinna, G. Heness, B. Ben-Nissan, Nanocomposites for biomedical and dental applications, in: M. Aliofkhazraei (Ed.), Handbook of Functional Nanomaterials: Properties and Commercialization, Nova Science Publishing, New York, 2014, pp. 149e172. [44] N.P. Lang, H. De Bruyn, The rationale for the introduction of implant dentistry into the dental curriculum, Eur. J. Dent. Educ. 13 (Suppl. 1) (2009) 19e23. [45] E. Emami, R.F. de Souza, M. Kabawat, J.S. Feine, The impact of edentulism on oral and general health, Int. J. Dent. 2013 (2013) 498305. https://doi.org/10.1155/2013/498305. [46] L.F. Cooper, The current and future treatment of edentulism, J. Prosthodont. 18 (2) (2009) 116e122. [47] S. Urano, Y. Hotta, T. Miyazaki, K. Baba, Bending properties of Ce-TZP/A nanocomposite clasps for removable partial dentures, Int. J. Prosthodont. 28 (2) (2015) 191e197. [48] D. Yokoyama, A. Shinya, H. Gomi, P.K. Vallittu, A. Shinya, Effects of mechanical properties of adhesive resin cements on stress distribution in fiber-reinforced composite adhesive fixed partial dentures, Dent. Mater. J. 31 (2) (2012) 189e196. [49] X. Li, W. Liu, L. Sun, K.E. Aifantis, B. Yu, Y. Fan, et al., Resin composites reinforced by nanoscaled fibers or tubes for dental regeneration, BioMed Res. Int. 2014 (2014) 542958. [50] P. Saen, M. Atai, A. Nodehi, L. Solhi, Physical characterization of unfilled and nanofilled dental resins: static versus dynamic mechanical properties, Dent. Mater. 32 (8) (2016) e185ee197.

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Kheng Lim Goh a, b a Advanced Composites Research Group, Newcastle Research & Innovation Institute (NewRIIS), Singapore; bNewcastle University, Faculty of Science, Agriculture and Engineering, Newcastle Upon Tyne, United Kingdom

1. Introduction The intent of this final chapter is to review the current understanding of stress transfer at the interface of the collagen fibril/proteoglycan(PG)-rich extracellular matrix (ECM) in soft connective tissue. In contrast, Chapter 19 has addressed ECM in hard connective tissue. The common focus of both Chapter 19 and the current chapter is on matrices produced by cells to form the shape and size of the tissue, unlike the other chapters which have addressed engineering materials for the matrix material, such as polymers (Chapter 2 to 10), ceramics (Chapter 11 to 13) and metals (Chapter 14 to 18). The ECM of soft connective tissues, such as tendon and ligament, may be considered as a biological example of a fibre-reinforced composite in which the collagen fibrils (mainly type I collagen), which are strong and stiff in tension, provide reinforcement to the weak hydrated PG-rich matrix (Fig. 20.1). While mamalian collagen fibrils are extremely slender (ratio of length to diameter ranges 200 to 3500), they are tiny to the naked eye, with lengths ranging on the order of magnitude from 10
6 to 10
3 m whereas the diameter ranges 10
9 to 10
8 m [1,2]. These length scales are tiny compared to the dimensions of the connective tissue-to some extent the collagen fibrils may be regarded as resembling monofilaments or whiskers in the tissue for the purpose of this argument. However, the fibrils have high tensile strength of 20e600 MPa and high stiffness of 200e6000 MPa [4,1,2,3] the wide variability may be attributed to the test specimens derived from different tissue. These mechanical properties are very much higher in value compared to those of the hydrated PGrich matrix; for instance the stiffness of the hydrated PG-rich matrix is estimated to be of the order of magnitude 104e105 Pa [4,1]. The structure of ECM is regarded as a hierarchical architecture (Fig. 20.2), comprising 300 nm long triple-helical collagen molecules, which assemble into D-periodic axial structure (D w 67 nm) fibrils with a quasi-hexagonal lateral arrangement of 5-stranded microfibrils [5,6]. These fibrils in turn bundle into collagen fibres and these fibres aggregate into fascicles, as viewed from the lowest to the highest hierarchical level based on the Kastelic model [7,8]. Individual collagen fibrils are stabilized by intermolecular crosslinks involving the non-triple

Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00020-0 Copyright © 2020 Elsevier Ltd. All rights reserved.

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Fibrillar collagen

Laminin

Fibronectin

Proteoglycan

Fig. 20.1 Schematic of extracellular matrix (ECM) [11]. Reprinted from J.M. Aamodt, D.W. Grainger, Extracellular matrix-based biomaterial scaffolds and the host response, Biomaterials 86 (2016) 68e82, Copyright (2016), with permission from Elsevier.

Tendon Fascicle Collagen molecule

Cell

Fibril

1.5 nm 50–500 nm 50–500 µm

5–10 mm

Fig. 20.2 Schematic of a Kastelic model [23] of the hierarchical architecture of soft connective tissue as shown for a tendon [24]. Reprinted from S.E. Szczesny, D.M. Elliott, Incorporating plasticity of the interfibrillar matrix in shear lag models is necessary to replicate the multiscale mechanics of tendon fascicles, J. Mech. Behav. Biomed. Mater. 40 (2014b) 325e338, Copyright (2014), with permission from Elsevier.

helical terminal domains (telopeptides) of the collagen molecule [9]. In addition, water in the hydrated PG-rich matrix plays an important role in the lubricating mechanism which regulates stress transfer between the PG matrix and fibril [10]. Along with the Kastelic model, there is a great deal of interest in the detailed description of the mechanisms that regulate the deposition of collagen into fibrils and in the spatial organisation that forms the hierarchical architecture [12e15] There are important implications arising from the findings from these studies. From the perspective of mechanical engineering, how long are these fibrils in the tissue? The structural properties-as well as the material properties of collagen that make up the fibrils-of these collagen fibrils are important for understanding how fibrils provide reinforcement to the tissue. By an analogy to fibre-reinforced composites, this involves the stress transfer mechanism, whereby stress generated in the matrix is transferred to the fibre via the fibre/matrix interface. However, the effectiveness

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of the stress transfer depends on several factors, namely the nature of the fibril/PG matrix interface and the critical stress transfer length [16]. It was reported that fibrils in embryonic tendon were discrete segments of length 10 to 30 mm [13] but no segments (or visible ends) could be found in tendons from older individuals [14,17]. Of note, in the Svensson report, the tendon studied was relatively short by any standard [17]. There may be many more ends if longer tendons were studied; one of the authors here have certainly seen tapered tips in the dispersions of fibrils in mouse Achilles tendons (unpublished). Decrease in the number of fibril ends per unit volume can result from axial growth of fibrils and/or by fusion of tips to the shafts of neighboring fibrils [18] In fibril segments, isolated from embryonic vertebrate tendon, it was observed that the fibrils featured smoothly tapered tips [19,20]. This suggests that the fibrils could be quite short, with tapered ends, or very long. An understanding of this fibril tip issue is important for tissue engineers to guide development of viable methods for fabricating connective tissue-like materials for tissue repair [21]. Additionally, of current interests in tissue engineering is the construction of tissue-like materials by 3D printing [22]. However, this project is still in its early stages of development. Advancement in the basic science such as the fundamentals underpinning fibril reinforcement of tissue could inform the fabrication of 3D constructs that could closely mimick the hierarchical architecture, including the detailed structure/spatial organisation of collagenous components, of connective tissue. Clearly, as noted in previous paragraphs, a variety of insights into how the ECM functions have been gained over the years, derived from mathematical, computational and experimental studies. With regard to the mathematical and computational studies, models that relate structure to mechanical function in ECM are bound to be simplifications; different models to explaining the ECM will underpin different basic principles. Contradictions may even arise between the different models. This chapter focuses on the mechanics of stress transfer at the fibril-PG matrix interface. It is not intended to be comprehensive to begin, it is necessary to clarify the meaning of the interface between the collagen fibrils. Here, we shall refer to the interface in two ways, described simply as (1) the region between the fibrils and the surrounding hydrated PG-rich matrix (Fig. 20.1), (2) the matrix region in between adjacent fibrils (Fig. 20.1). This discussion is divided into two parts. Firstly, early studies on engineering fibre composites, as well as collagen fibril reinforcement in connective tissues, are discussed to introduce the reader to the concept of stress transfer. Secondly, descriptions of the key ECM studies (over the last 5 years) that sought to illuminate the detailed mechanism of stress transfer are discussed. In this chapter, the term ‘fibril’ is used to refer to collagen fibril. Where appropriate this is stated as ‘collagen fibril’ to avoid unambiguity. The term ‘fibre’ may refer to fibres that are used in engineering composites, in which case, this is further emphasized by the term ‘engineering’ in the sentence to avoid confusion with the term ‘collagen fibre’ which refer to a bundle of collagen fibrils. It is hoped that the chapter as a whole will paint a picture of the current perspectives of our understanding of how the interface at the collagen fibril/matrix facilitates stress transfer.

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2.

Early studies on interfacial stress transfer mechanisms

2.1

Basic concepts, from an engineering perspective

In the mid 50s, as the use of discontinuous fibre reinforcement of composites increased, Kelly [16], Outwater [25] and Riley [26] sought to rationalize the reinforcement capability of short fibres, as well as whiskers, in composites. The work that they have reported became the foundation for later studies on understanding how stress transfer occurs between the PG matrix and collagen fibrils at the interface. Originally, Kelly developed a simple model to predict the strength of such composites for short fibres (in engineering composites) reinforcing a ductile matrix. For definiteness, the short fibres have an aspect ratio q ¼ Lf/Df, where Df is fibre diameter Df and Lf is fibre length. According to this model, as the composite is loaded under an increasing applied tensile force, the shear stress in the matrix near the fibre ends increases and eventually this causes the matrix to yield. A constant interfacial shear stress (s) is generated in the matrix along the fibre ends. (Of note, in engineering polymeric materials, residual stresses are generated during curing and this may lead to the fibres experiencing compression as the matrix shrinks [25]. This compressive stress provides the frictional shear stress, which is identified with s, when the fibre is displaced relative to the matrix during stretching [25].) Eventually the s reaches a level (sY) that is sufficient to cause the PG matrix to yield. The axial stress (sz) uptake in the fibril is described by a simple first order differential equation of the following form, dsz = dZ ¼ 2sq

(20.1)

and when solved, has a distribution of the form in which the magnitude is uniform (where s ¼ 0 because no sliding action between the PG matrix and fibril is taking place) over the bulk of the fibre but this changes at a distance of one-half the critical length of the fibril (measured from the fibril end); because s is a non-zero constant over this distance, the axial stress decreases linearly with distance until it reaches zero at the fibre ends.

2.2

Early analysis of interfibrillar shear stress

The theory of stress transfer in Section 2.1 has yielded insight into the mechanical properties of soft connective tissues such as tendons and ligaments [27,28], cartilage [29,30]. For instance, Aspden has applied the theory to study the dramatic changes in the mechanical properties of the uterine cervix connective tissue at parturition [31]. About a decade earlier, Holmes and Chapman have shown that collagen fibrils grown (i.e., enzymically controlled) in vitro featured a non-uniform axial mass distribution which appeared to peak at the fibril centre and tapered to a minimum at the ends of the fibrils [32,33]. Subsequently, with the advent of more powerful microscopes, Holmes and co-workers observed that these in vitro fibrils possessed tapered ends which may be described as paraboloidal in shape [19,34,20,35]. At about the same

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period, John Trotter and his co-workers also observed tapered ends in collagen fibrils that were successfully isolated from some connective tissues [36]. Thereafter, John Trotter went on to suggest how synthetic collagen fibrils could be designed as a bottom-up approach to making ECM bioscaffolds [37]. Altogether these observations reported by Holmes and Trotter groups respectively are important because the findings (1) revealed detailed description about the structure of collagen fibrils, particularly the fact that fibrils possessed tapered ends when many researchers have assumed that fibrils were very long and uniform cylindrical in shape (because many researchers have yet to observe the ends of collagen fibrils in native tissues), (2) provided the structural biologists a way to model the reinforcing capability of collagen fibrils based on models of engineering discontinuous fibres reinforced composites (e.g., Eqs 20.1 and 20.3) provided new issues to be clarified, namely the nature of the fibril/matrix interface (including the ends) and how the interface facilitate stress transfer from the interfibrillar matrix to the fibrils. One of the early researchers who applied models of engineering discontinuous fibres reinforced composites to understand how collagen fibrils provide reinforcement to connective tissues was Richard Aspden [31,38,39]. Aspden observed that collagen in tendon deformed elastically up to a peak strain (z4%), beyond this peak strain, the deformation was plastic, and it failed at about 10% [38]. Defining the stress transfer parameter to be equal to the ratio of the applied interfacial shear stress (s) to the shear modulus (G), calculations of the maximum s/G showed that s/G was inversely proportional to the axial ratio. From this, it was argued that the s was equal to 20 MPa, scaled by the inverse of fibril aspect ratio (q). This begs the question of how exactly the applied interfacial shear stress was transferred to the fibril. If the PGs (namely fibromodulin and decorin)dattached to specific sites in each D-period along the collagen fibril surface [40]dcould contribute to this stress transfer mechanism, it would be feasible to calculate the molecular interaction force associated with the average shear stress. Assuming the collagen fibrils were uniform cylinders of diameter Df and length Lf, the force that could be applied over the cylindrical surface would be sS, where S (¼pDfLf) represented the area of the cylindrical surface [38]. The number of interaction sites (n) could be estimated conservatively by assuming that there would one interaction site for each Dperiod ¼ 67 nm repeating structure of a collagen fibril, giving n ¼ Lf/Dperiod [38]. Thus the force per interaction was given by F ¼ sS/n ¼ 4.2Df/q N [38]. Aspden showed that this force ranged from 8.4 to 84 pN, depending on the fibrillar length and diameter of the type of tissues (the calculations was performed for tendon and articular cartilage) [38]. For comparison, the force required to break a covalent bond such as the carboncarbon bond was determined to be about 6.1 nN [41,42]. In contrast, the force to disrupt van der Waal’s bonding was estimated at about 50 pN which was of same order of magnitude as the prediction reported by Aspden [38]. Thus, weak nonspecific interactions such as van der Waal’s bond were implication in the interfacial stress transfer [38]. The above analysis was established on the basis of plastic stress transfer mechanism where the applied interfacial shear stress assumes a uniform distribution throughout the interface. The other mechanism of stress transfer is known as the

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Interfaces in Particle and Fibre Reinforced Composites

elastic stress transfer mechanism [38,4,1]. When low forces are applied to stretch a tissue, stress will be transferred to the fibrils elastically [4,1]. The interfacial shear stress is non-linearly distributed throughout the interface, from a minimum value at the fibril centre, it increases non-linearly and smoothly to a maximum value at the fibril ends [43,4,1,44]. When the tissue is subjected to higher forces, stress will be transferred plastically [43,4,1]. A fibril which is longer than the critical fibril length (Lc) could be effective at taking up stress and the magnitude could increase until the point of fracture when subjected to very high forces [45,46]. A fibril which is shorter than Lc exhibit stress uptake that may not be sufficient to cause it to break [45,46]. Instead the fibril may be forced out, i.e., pulled out, of the matrix but how this occurs depends on the energy available for the pull-out mechanism. It is thought that this energy is sensitive to two parameters, namely s and Df [47]. When Holmes and co-workers showed that the fibrils in some tissues have tapered ends (Fig. 20.3) [36,19,34,20,35], the two mechanisms became the subject of intense focus in conjunction with the question of how tapered fibrils provide reinforcement to ECM [2,3]. Clearly, the tapered fibril requires less volume of collagen than a cylindrical fibril of the same length [45,47]. For both elastic and plastic stress transfer mechanisms, a taper leads to a more even distribution of axial tensile stress along the fibril than would be generated if it were cylindrical; in other words the stress is shared more evenly along its length [48,4,1]. This suggests that It is also less likely to fracture than a cylindrical fibril of the same length in a tissue subjected to the same force. The role of collagen fibrils in the presence of varying age have also been proposed. Based on observation of the fibril diameter frequency distribution, supported by a novel theoretical basis underpinning the interfibrillar shear, it was proposed that the age-related variation in the fibril diameter contributed respectively to the energy for resilience and fracture toughness energy [50]. Based on observation of collagen volume fraction, supported by a simple rule-of-mixture for the two dominant mesoscopic components, it is suggested that changes in the collagen volume fraction contributed near-linearly to the changes in the strength and stiffness of the tissue [45].

2.3

Insights derived from materials engineering and biologicalbased experiments

Glassmaker and co-workers [51] investigated how fibrillar structures can facilitate adhesion between surfaces analoguous to how lizards and some insects are able to use their limbs (which features fibrillar structures) to secure themselves on surfaces. They built models using sheets of poly(vinyl butyral) (PVB) and glass plates: the PVB sheets were bonded to the glass plate like a laminate. The PVB sheets featured a central fibrillar region surrounded by flat unstructured regions. Pull-out test was carried out to study how varying fibrillar geometry influences the ‘stickiness’. To ensure consistency, a crack initiator was introduced at one end; Glassmaker and co-workers were expecting (fibrillar) failure to occur by interfacial crack propagation when pull-off began. The results showed that fibrils failed at a critical stress as predicted from the theory on flaw insensitivity. A differential toughening effect

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between the fibrillar and non-fibrillar region was observed: the energy release rate resulting in the disruption of the interface in the former region was higher than that of the latter region by an order of magnitude. Thus the fibrillar region is tougher than the non-fibrillar region. The authors pointed out that the higher energy to fracture exhibited by the fibrillar region may be attributed to dissipation of strain energy in the fibrils; a part of the energy absorbed during work done was diverted to the disruption of the interfacial bonds. This implies that the interface at the fibrillar-surfaces region could provide effective stress transfer from the glass and the PVB to the fibrils during initial loading, before fracture occurred. Wagner and co-workers [52,53] studied the stress concentrations arising from fibre failure (in engineering composites) that could be attributed to fibre/fibre interaction in a composites [54]. The aims of the study were two-fold (1) to understand the failure mode that occurs in the composite and (2) to evaluate the toughness of the composite. Wagner and co-worker regarded composites with strong fibre/fibre interaction (high stress concentrations) as brittle while those with low fibre/fibre interaction as ductile. An experiment was carried out on graphite fibre/epoxy composites. Micro-Raman spectroscopy was carried out to evaluate the strain distribution along a fibre to assess the effect of fibre fracture on neighbouring fibres. In other words, when a fibre breaks, how does this affect the neighbouring fibres? Before we go on to answer this question, it is worth pointing out a recent study using a finite element method approach to investigate how the stress in a fibre is taken up during fibre/fibre interaction [54]. In this study, it was found that stress discontinuity (an abrupt step-wise drop in the stress along the fibre) occurs at the fibre; the extent of the discontinuity depends on the fibre-fibre lateral separation and the relative fibre to matrix stiffnesses. Now, back to the question of when a fibre breaks, how does this affect the neighbouring fibres. Wagner and co-worker approached the answer to this question by assessing the sensitivity of the stress concentration factor (SCF) and the radius of the zone of influence (RZI) to varying fibre/matrix interphase toughness and strength. The toughness of the interphase was found to play an important role in the SCF and RZI. The RZI suffered a reduction, i.e., from 30 fibre diameters to 20 fibre diameters, when a brittle interphase was present; the SCF decreased as inter-fibre distance/separation increased. Of all the values of SCF, it was found that the maximum value of SCF appeared to be insensitive to the interphase toughness. In general, the RZI and maximum SCF were sensitive to interfacial shear stress: it was found that decreasing magnitude interfacial shear stress corresponded to decreasing radius of the RZI and decreasing maximum SCF value. As pointed out in Section 2.2, decorin PGs, a member of the family of small leucine-rich proteoglycans (SLRPs), are present in ECM of connective tissues in the body such as tendon and ligament. Decorin may be involved in facilitating fibrilfibril interaction via the inter-fibrillar matrix [55]. Decorin PG core protein is bound to collagen molecule on the fibril; the GAG side chain of the PG interacts with neighbouring GAGs from decorin PG on adjacent fibril [56,57,58](Fig. 20.3). To show if this could be possible, Liu and co-workers contrived a novel experiment to determine the forces of interaction of decorin GAG chains using a laser tweezers/interferometer for carrying out mechanical testing of molecules. A bead-on-glass(cover-slip) setup (Fig. 20.4) was devised, comprising bovine decorin molecules ‘stuck’ (1) to the

536

Interfaces in Particle and Fibre Reinforced Composites

External load

External load Hydrate proteoglycan-rich gel Collagen fibril Decorin

Fig. 20.3 Schematic of extracellular matrix subjected to an applied load [1]. Here each collagen fibril is shown as having pointy ends. Reprinted from K.L. Goh, J.R. Meakin, R.M. Aspden, D.W.L. Hukins, Stress transfer in collagen fibrils reinforcing connective tissues: effects of collagen fibril slenderness and relative stiffness, J. Theor. Biol. 245 (2007) 305e311, Copyright (2007), with permission from Elsevier.

carboxyl-functional polystyrene beads by covalent bonding between its core protein and the bead surface and (2) to the glass cover-slips by covalent bonding between its core protein and the glass cover surface. The aim was to quantify the resistance force generated in the bond formed between decorin on the bead surface and that on the glass surface when a force was applied. It was found that the rupture force generated in the bonds between decorins (more specifically, arising from bonding between GAG chains) was 16.5  5.1 pN. This falls within order of magnitude for the weak non-specific forces, i.e., van Der Waals, and is also consistent with the prediction of Aspden [38] as described in Section 2.2.

3.

Later studies on interfacial stress transfer mechanisms

3.1

Engineering perspective: nanofibre-like particles reinforcing composites

This paragraph is concerned with a technical discussion on the study of interfacial shear stresses in slender fibre-like particles reinforcing polymer composites, such as carbon nanotube (CNT) reinforcing epoxy composites. The intent is to draw parallels to similarities between engineering models of fibre reinforced composites and collagen fibril reinforcing connective tissues, underpinning the stress transfer mechanisms. Engineering models of composites such as CNT reinforced epoxy composites are much simpler systems than connective tissues. However, the simplicity, or otherwise, of the comparative method of analysis is not a measure of the scientific worth of an investigation. Clearly any model has to make simplifying assumptions and a simple engineering composite system has been chosen to model connective tissue to

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(a)

r θ L

r

D C

B

(b)

F1

F2

F θ

Fig. 20.4 Schematic of the setup for direct measurement of the force to rupture the bonds between decorins glycosaminoglycans [59]. (a) The bead-molecule-coverslip setup for surface linkage. The circle represents the bean; the base represents the coverslip; the bold line labelled ‘L’ represents the molecular linkage. (b) A description of how the force was applied to the molecular linkage. ‘r’: radius of the bead; ‘q’: angle between the linkage and the vertical; F: the resultant force on the bead (F1 and F2 are the components of the F). Reprinted from X. Liu, M.-L. Yeh, J.L. Lewis, Z.-P. Luo, Direct measurement of the rupture force of single pair of decorin interactions, Biochem. Biophys. Res. Commun. 338 (2005) 1342e1345, Copyright (2005), with permission from Elsevier.

investigate how interfacial shear stresses in collagen fibril reinforcing the extracellular matrix of connective tissues. This approach is the simplest which enables this question to be answered. It is not good practice to base an investigation on an over-elaborate model. As pointed out previous paragraph, the engineering fibrous structures of concern here are CNTs which are essentially fibre-like whiskers. It is important to note that the dimensions of CNTs are somewhat similar to those of collagen fibrils. Let us consider the study reported by Wong and co-workers [60] who carried out an experiment to study CNT reinforcement in polymers of (1) polystyrene (PS) and (2) epoxy to understand the mechanics of bonding at the CNT-polymer interface. Examination of electron micrographs of CNT agglomerates pulled out from the respective CNT/PS and CNT/epoxy composite revealed that (1) polymer materials were ‘stuck’

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Interfaces in Particle and Fibre Reinforced Composites

Fig. 20.5 Schematic of a model of a carbon nanotube (CNT) in polystyrene (PS) matrix [60]. Here, the CNT is showed as partially pulled out from the matrix. A similar model was also developed for CNT in surrounded by epoxy molecules. Reprinted M. Wong, M. Paramsothy, X.J. Xu, Y. Ren, S. Li, K. Liao, Physical interactions at carbon nanotube-polymer interface, Polymer 44 (2003) 7757e7764, Copyright (2003), with permission from Elsevier.

onto the surface of individual CNTs-this indicated that the polymer was favorable to wetting onto the CNT surface, in other words, the surface energies were adequately high to enable good contact between CNT-polymer, (2) failure occurred within the polymer matrix but not at the interface of the CNT and polymer material. Molecular mechanics simulations showed (Fig. 20.5) that even when there was no chemical bonds between CNT and the polymer matrix, the presence of non-bond interactions (electrostatic forces and van der Waals forces) could generate high shear stress at the CNT-polymer interface. This shear stress was higher (by an order of magnitude) than what was found for conventional fibre reinforced composites . With regard to collagen fibril mechanics, to the best of the author’s knowledge, no papers have yet to be published on the topic of collagen fibril pull-out from connective tissues based on computer simulation study, as well as experimental study.In particular, if collagen fibril pullout experiments could be feasibly designed (e.g. using atomic force microscopy or optical tweezers), it would provide a direct measurement of the shear strength at the collagen fibril/matrix interface. The hierarchical architecture of soft connective tissues such as tendons and ligaments described how collagen fibrils are bundled into collagen fibres and how collagen fibres are bundled into fascicles (Section 1). How the higher order components such as collagen fibres interact with the PG matrix may be derived from insights derived from test on engineering fibre bundles, without loss of generality of course. As an example, we shall take a look at study of Zu and co-workers [61] who produced very long (‘continuous’) CNT fibres spun from a CNT carpet consisting of mainly

Current understanding of interfacial stress transfer mechanisms in connective tissue

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double- and triple-walled tubes. They then embedded these CNT fibres into an epoxy matrix to investigate the interfacial properties of CNT/epoxy. They did not test for single fibre but a bundle of CNT fibres. The Using microdroplet testing, they performed ‘pull-out’ tests on the fibre bundle treated to a drop of epoxy resin, with adequate time given for curing, to test the interfacial response. They observed that sliding occurred with the CNT fibre bundle because of weak interactions between CNT fibers in the bundle owing to reduced resin infiltration. The take-home-message for collagen fibrils in connective tissue is that if the forces of interaction between collagen fibrils are weaker than those between collagen fibres (bundles of fibrils), the deformation in ECM would be determined by collage fibrils sliding as the collagen phase takes up stress in response to an external load acting on the tissue. The presence of PGs in the matrix has led many researchers to speculate about their role in stress transfer. In particular, decorin PGs have been implicated in the stress transfer to the fibrils from the surrounding interfibrillar matrix material but this issue has been contentious [62]. Nevertheless, from an engineering perspective, it is attractive to have some kind of structural architecture for the interfibrillar matrix material. Dong and co-workers [63] have designed a novel CNT organization in the form of a continuous network, dispersed in the continuous carbon fibre reinforced epoxy composite. To prepare the fabric/CNTs preform the fibre fabric was immersed into a CNT aqueous solution (containing dispersant), followed by freeze drying to dry the fabric, and heat treatment (pyrolysis) to remove the PVP dispersant in the fabric. Addition of the epoxy resin to the preform yields the desired composite. Interlaminar shear strength (ILSS) testing showed that the novel composite material (1% by wt.) exhibited higher ILSS compared to one without the CNT network incorporated. However, higher weight fraction of CNT led to lower ILSS. This could be attributed to agglomeration of the CNTs during the processing stage; the CNTs in each agglomerate would be held together weakly and this weak interaction would break the agglomerate easily.

3.2

Evidence against proteoglycans regulating collagen fibril stress uptake

The previous section has highlighted how the nanotube dispersion in the matrix in between engineering fibres could help to facilitate stress transfer. Going by this analogy, it is still opined that there must be some kind of mechanically coupling to bind the fibrils in order to achieve high strength and toughness, by minimizing the sliding motion between fibrils. The concept of cross linking of the fibrils forms the basis of this assumption. However, the binding of hydrated PG-rich matrix to the collagen fibril is not well understood. In particular, observations from electron microscopy suggests that GAG dermatan sulfate (DS) and chondroitin sulfate (CS) side-chains of SLRPs act as molecular ‘mechanical linkages’ between adjacent collagen fibrils. At a more fundamental level, this structural organization suggests that GAGs might be involved in facilitating stress uptake in fibrils when the tissue is loaded in tension as pointed out by Scott and many others [64e68].

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A lot of the work reported so far by Fessel, Snedneker and co-workers have led to doubts about the hypothesis that PG-GAGs act as ‘mechanical linkages’ between collagen fibrils. The first work of these investigators was carried out on (rat tail) tendon fascicles by the method of enzymatic depletion of GAG complexes using chondroitinase ABC [62]. The mechanical test involved dynamic viscoelastic tensile tests to address the underlying viscoelasticity of the tissue; the working hypothesis was that the GAGs are responsible for regulating frictional action, through fluid flow through the ECM, while the tissue is deforming. Results from the dynamic viscoelastic tensile tests on fascicles showed that there was no appreciable change in storage modulus (which described the elastic behavior)-over a wide range of strain-rates (from 0.05% to 5% change in length per second)-in the respective linear or nonlinear regions of the stress-strain curve between fascicles which contained no GAGs as compared to the controls. However, there was a different in the loss modulus (which described the viscoelastic behavior) in the nonlinear region when tested at high strain-rate, between fascicles which contained no GAGs as compared to the controls but the researchers cautioned that this effect could be marginal (in that there was only 19% increase in loss modulus with a p value of 0.035). Thus, the overall study suggested that GAG chains of SLRPs do not appear to be involved in regulating the elastic and viscoelastic behaviour of tendon fascicles. Recently, the ‘mechanical linkage’ theory of PG/GAG was revisited by Haverkamp and co-workers [69]. They carried out a study to investigate changes in the collagen fibril alignment with regards to GAGs acting as cross links between fibrils. Bovine pericardium tissues were treated with (1) chondroitinase ABC to remove natural cross links or (2) glutaraldehyde to generate cross links [69]. To assess the collagen fibril alignment, the investigators employed three methods: (1) small angle X-ray scattering (SAXS, carried out on a synchrotron), (2) atomic force microscopy (AFM) and (3) histology [69]. The study revealed that collagen fibril alignment is affected by the treatment. While both untreated pericardium and chondroitinase ABC treated tissue yielded similar results, the glutaraldehyde treated tissue showed a lower degree of alignment [69]. This led them to suggest that cross links with glutaraldehyde resulted in a more pronounced (randomly oriented) network structure of fibrils than when the tissue was untreated. More importantly, since GAGs did not appear to influence the fibril alignment, this led them to suggest that GAG’s role in coupling the fibrils might not be appreciable. But to directly answer the question of whether the so-called ‘mechanical linkages’ (i.e., cross links) glycosaminoglycan (GAG) (found naturally in the tissue) and glutaraldehyde (introduced chemically into the tissue) help to facilitate stress transfer the Haverkamp group assessed the structural changes in the tissue subjected to external loads [55]. They observed that the proportions of aligned collagen fibrils in a tissue in the direction of the strain increased as the tissue was stretched [55]-a result which was very well-known for a long time. However, what was interesting was that they found that the proportion of fibrils that were recruited during stress

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was highest in the glutaraldehyde-treated tissue, followed by chondroitinase ABCtreated tissue and finally, native tissue. Thus, the proportion of fibril recruitment differed depending on the specific treatment to the tissue [55]. The differential increases in D-spacing between chondroitinase ABC-treated and native tissue were marginal (2.4% and 2.7%); the increase in D-spacing in the glutaraldehyde-treated tissue, at 3.2%, was somewhat higher than the other two cases, suggesting that the individual fibrils could develop the highest extensibility in the glutaraldehydetreated tissue [55]. These simple structural analyses were used to suggest that glutaraldehyde cross links could act as useful mechanical linkages by facilitating stress transfer between collagen fibrils; GAGs (as revealed by the native tissue versus the GAG-removed tissue) and that these glutaraldehyde cross links might be responsible for reducing the sliding action between fibrils [55]. In a more recent report, deeper analysis of results from a SAXS study revealed that stretching a tissue resulted in some interesting structural changes occurring simultaneously, namely (1) decrease in the intermolecular spacing between tropocollagen (by 10%, from 15.8 to 14.3 A), (2) decrease in the fibril diameter (from 400 to 375 A), (3) extension of fibrils (by 3.1%) by measuring the D-spacing [70]. If fibril orientation (at the fibrillar network level) could play an important role in these changes as fibrils sought to align in the direction of the external load to resist the load that was deforming the tissue [70], this could cascade down the length scale to the fibrillar level (involving fibril extension, fibril sliding) and the tropocollagen level (involving molecular packing) [70]. These findings appeared to deal a devastating blow to proponents of the GAG chains in fibril-associated PG (e.g., decorin PGs [58]) as key to understanding the mechanical behaviour of ECM in connective tissues. On this note, in the interim between the reports of Fessel [62] and Kayed [69], others have sought to establish more evidence that could provide confirmation as shown in the following reports which surprisingly revealed otherwise [71-74,].

3.3

Interfibrillar shear stress is responsible for collagen fibril stress uptake

If interfibrillar shear stress plays an important role in facilitating collagen fibril stress uptake [50,3], what is the mechanics underpinning this role? Early studies have addressed this question by computer models, complemented by analytical models, and these studies were reported in two papers published in 2005 and 2007 [4,1]. The findings from these studies suggest that the elastic mechanisms is responsible for generating interfibrillar shear stress when loads, such as those associated with normal physiological activities, are acting on the tissue. At higher loads, such as those associated with the point of fracturing the tissue, plastic mechanisms is responsible for generating interfibrillar shear stress. These conclusions, however, were not validated ̊ by experiments until about a decade later after these papers were published. ̊

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Unit cell

1 X P

P

τ(X) 2

L/2

Fig. 20.6 Model of collagen fibrils in hydrated proteoglycan-rich matrix for the so-called ‘shearlag’ approach, based on an approach that was first proposed by Cox [75]. Labels ‘1’ and ‘2’ refer to fibril 1 and 2; P represents the applied force; t represents the interfacial shear stress; L represents the length of a fibril [24]. Reprinted from S.E. Szczesny, D.M. Elliott, Incorporating plasticity of the interfibrillar matrix in shear lag models is necessary to replicate the multiscale mechanics of tendon fascicles, J. Mech. Behav. Biomed. Mater. 40 (2014b) 325e338, Copyright (2014), with permission from Elsevier.

To investigate how interfibrillar shear stress could partake in facilitating collagen fibril stress uptake, Szczesny and Elliott [21] sought to answer this question by carried out an experiment by (uniaxial) tensile testing on tendon fascicles using a unique horizontal-frame mechanical tester, mounted on an inverted confocal microscope. The testing protocol consisted of stretching the tendon at a strain rate of 1% s
1 to respectively 2%, 4%, 6%, 8% and 10% strain. Thereafter, the tendon was allowed to relax, until the stress remained stable (which was then recorded), at the predetermined strain point. A unique feature of this study was a multiscale analysis of the relationship between micrometer length scale deformation (under the optical microscope) and macroscopic length scale deformation of the tissue. During the relaxation process, microscale deformation was examined (using image stacks of photo-bleached lines spanned the tissue width); these images were acquired first at the start and then at the end of each relaxation period. The researchers defined a parameter known as tortuosity to quantify the extent of interfibrillar sliding; the tortuosity was determined by measuring the waviness of the photobleached lines. Since these images captured position data at the microscale, the data could be used to determine the spread of the shear strain experienced by the tissue at the microscale. Using the predetermined strain points and the associated stress level at equilibrium during stress relaxation, the stress-strain data were plotted. A plastic stress transfer (“shear lag”) model more sophisticated that the one described by Eq. (20.1) (Sections 2.1 and 2.2) was developed. The model described a periodic array of short fibrils, arranged in a staggered manner (Fig. 20.6); a key parameter of these fibrils is the critical fibrillar ‘length’, LLC, along the fibril over which the load is transferred to the fibril from the interfibrillar matrix, is

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independent of the strain applied to the tissue. (Of note, LLC may be regarded as a fibrillar property that depends on the material and geometric properties.) The model underpinned an assumption that the fibrillar micromechanics could be ‘scaled’ to the macroscopic stress-strain behaviour of the tissue. The model was used to evaluate the experimental data by fitting the model to the stress-strain points. The good fit between the model and the data confirmed that collagen fibrils of short length, characterized by LLC for stress transfer, contribute to taking up load during stress relaxation [21]. A novel scale parameter which quantifies the ratio of fibril to tissue strain was used for further analysis to relate macroscopic deformation and microscopic interfibrillar displacement. They found that there was good correlation between the experimental results and model predictions. Thus, this study suggests that during stress relaxation (rather than the direct loading) of the fascicle, plastic deformation of the interfibrillar matrix led to generating interfibrillar shear stress to enable load to be transferred between (discontinuous) collagen fibrils in the fascicles [21]. To what extent does plastic stress transfer contribute to the interfibrillar shear stress? Is contribution from two or more mechanisms possible? To answer these questions Szczesny and Elliot compared the predictions of four different models, namely (1) a model that featured long fibrils (where fibrils span from one end of the fascicle to the other) and three other (shear-lag related) models that described (2) an elastic, (3) plastic, and (4) elastoplastic interfibrillar matrix respectively, with the previously derived stress-relaxation experimental data [24]. The elastoplastic model, as the name suggests, combines the elastic and plastic interfibrillar matrix but accounts for these two by regarding the matrix as initially elastic at low applied loads but eventually behaves plastically (when it began to generate a critical shear stress) as the load increases. All four models were developed to relate the fibrillar micromechanics to the macroscopic mechanical behaviour of the tissue. All models were incorporated with a tissue post-yield argument, i.e., a critical point beyond which the interfibrillar matrix yielding begins. One finding that emerged from this comparative study was that there was less interfibrillar sliding in the plastic model than in the elastic model; the interfibrillar shear stress in the plastic model is larger than the maximum interfibrillar shear stress in the elastic model. (Of note an energy-related model developed by Goh and co-workers [50] has also arrived at predictions which are consistent with these underpinning arguments; more will be said about this in the final paragraph.) While the predictions (i.e., the stress-strain curve) of the plastic and elastoplastic (shear lag) models agreed with the experimental data well, the predictions from the continuous fibril model and the elastic shear lag (elastic stress transfer) model did not agree well with the experimental data. In particular, the elastoplastic model agreed very well with the experimental data at low applied tissue strains. Altogether, these findings suggested that the initial stress response was regulated by interfibrillar elasticity, but after the fibrils had yielded, plastic deformation of the interfibrillar matrix regulated the tendon fascicle mechanical behavior. To find out what components in ECM are responsible for the interfibrillar shear stress, Szczesny and co-workers worked out an elimination approach to remove

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the usual suspects in the tissue -namely decorin PGs, collagen XII (FACIT), tenascin C and COMP proteins-by trypsin digestion, followed by the stress relaxation test complemented by simultaneous multiscale structural displacement examination. As described in previous paragraphs, the stress relaxation test recorded the final stress at different predetermined strain points; the multiscale structural displacement measurement recorded the sliding mechanism and deformation at fibrillar level [76]. The plot of the stress versus strain profile revealed no significant difference between native and trypsin-treated tissue. The plot of fibril/tissue strain ratio versus tissue strain also revealed no significant difference between native and trypsin-treated tissue. However, the plot of interfibrillar sliding extent versus tissue strain yielded significant difference between native and trypsin-treated tissue. The researchers claimed that the interfibrillar sliding method may be sensitive to additional phenomena that could influence this measurement, such as the effects arising from the gripping of the tissue. (Nevertheless, it remains contentious as one could also argue that the fibril/tissue strain ratio parameter may not be sensitive to the trypsin treatment as compared to the interfibrillar sliding parameter.) Further analysis by the researchers revealed that the interfibrillar load transfer in tendon may be facilitated by the presence of small diameter fibrils [76], particularly because the fibrils with diameter of less than 150 nm (considered small) tended to wind around and weaved between the fibrils of larger diameters (i.e., greater than 150 nm). These closeness, proximity also included fusion between the small and large fibrils. Finally, measuring the gradients in the axial strain (and hence stress) across the tissue width and length of notch tensioned tissue, Szczesny and co-workers were able to determine the corresponding axial stress gradients, prior to failure at the cut/uncut tissue interface; from this information, they could estimate the interfibrillar shear stress which was found to have a value of 32 kPa [77]. To order of magnitude, we noted that this estimate was comparable to the interfibrillar shear stress predicted by our model of tendon fascicles which described how discontinuous fibrils transmit load through interfibrillar shear by plastic stress transfer mechanism [50]. According to a model of plastic stress transfer related structure-function relationship of tendon fascicles, namely the fibril structure versus fascicle resilience and fracture toughness, that we have developed in our previous work [50], the model showed that there were two interfacial shear stress parameters corresponding to two different stages of the loading process, namely (1) the yielding of the interfibrillar matrix and interfacial bonding and (2) the critical stress leading to fibril fracture or pull-out. The predicted value for the former was found to be 7.5 kPa; the value predicted for the latter was 100 kPa [50]. Overall, these values may be regarded as the lower and upper limits of the interfibrillar shear stress; the value predicted by reported by Szczesny and co-workers then falls somewhat between these limits [77]. Altogether, the take-home message is that these micro- to macromechanical studies have yielded insights into the mechanics of stress transfer (at low strains) and failure (at high strains) at two different length scales. In particular, these studies indicated that the interfibrillar matrix has a role in the stress transfer mechanism analoguous to the principles underpinning engineering fibre reinforced composites.

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4. Conclusion Collagen fibrils, the tiny numerous slender fibrillar particles that are embedded in extracellular matrix of soft connective tissues such as tendons and ligaments, provide mechanical reinforcement to the tissue during physiological activity and also functions to resist forces that attempt to break the tissue apart. The mechanics of reinforcement underpins stress transfer at the collagen fibril/PG matrix interface in connective tissue involving the following key aspects. • •

•

A plastic stress transfer mechanism, described by a model of a plastic PG matrix shearsliding over an elastic collagen fibril, when the tissue is subjected to an external applied load No consensus has yet to be established for the nature of the bridges linking the adjacent fibrils, particularly the molecular interactions at the fibril/PG matrix interface that contribute to the mechanical response of the tissue. It is instructive that nanotube network and particles in the fibre-fibre matrix in engineering fibre reinforced composites have demonstrated that they can enhance the mechanical properties of the composites. Simultaneous micromechanical/microscopy testing of tendon fascicles has enabled valuable insights to be drawn concerning the interaction of the hydrated PG-rich matrix and the discontinuous collagen fibrils, pointing to the importance of interfibrillar shear as underpinning the fascicle mechanical response to external loading.

References [1] K.L. Goh, J.R. Meakin, R.M. Aspden, D.W.L. Hukins, Stress transfer in collagen fibrils reinforcing connective tissues: effects of collagen fibril slenderness and relative stiffness, J. Theor. Biol. 245 (2007) 305e311. [2] K.L. Goh, D.F. Holmes, Collagenous extracellular matrix biomaterials for tissue engineering: lessons from the common sea urchin tissue, Int. J. Mol. Sci. 18 (2017). [3] K.L. Goh, A. Listrat, D. Bechet, Hierarchical mechanics of connective tissues: integrating insights from nano to macroscopic studies, J. Biomed. Nanotechnol. 10 (2014) 2464e2507. [4] K.L. Goh, J. Hiller, J.L. Haston, D.F. Holmes, K.E. Kadler, A. Murdoch, J.R. Meakin, T.J. Wess, Analysis of collagen fibril diameter distribution in connective tissues using small-angle X-ray scattering, Biochim. Biophys. Acta Gen. Subj. 1722 (2005) 183e188. [5] D.J.S. Hulmes, Building collagen molecules, fibrils, and suprafibrillar structures, J. Struct. Biol. 10 (2002) 2e10. [6] D.J.S. Hulmes, T.J. Wess, D.J. Prockop, P. Fratzl, Radial packing, order, and disorder in collagen fibrils, Biophys. J. 68 (1995) 1661e1670. [7] M. Fang, E.L. Goldstein, A. Simon Turner, C.M. Les, B.G. Orr, G.J. Fisher, K.B. Welch, E.D. Rothman, M.M. Banaszak Holl, Type I collagen D-spacing in fibril bundles of dermis, tendon, and bone: bridging between nano- and micro-level tissue hierarchy, ACS Nano 6 (2012) 9503e9514. [8] H.R.C. Screen, D.L. Bader, D.A. Lee, J.C. Shelton, Local strain measurement within tendon, Strain 40 (2004) 157e163.

546

Interfaces in Particle and Fibre Reinforced Composites

[9] D.R. Eyre, M.A. Weis, J.J. Wu, Advances in collagen cross-link analysis, Methods 45 (2008) 65e74. [10] S.J.R. Kelly, R. Weinkamer, L. Bertinetti, R.L. Edmonds, K.H. Sizeland, H.C. Wells, P. Fratzl, R.G. Haverkamp, Effect of collagen packing and moisture content on leather stiffness, J. Mech. Behav. Biomed. Mater. 90 (2019) 1e10. [11] J.M. Aamodt, D.W. Grainger, Extracellular matrix-based biomaterial scaffolds and the host response, Biomaterials 86 (2016) 68e82. [12] D.E. Birk, R.L. Trelstad, Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation, J. Cell Biol. 103 (1986) 231e240. [13] D.E. Birk, E.I. Zycband, D.A. Winkelmann, R.L. Trelstad, Collagen fibrillogenesis in situ: fibril segments are intermediates in matrix assembly, Proc. Natl. Acad. Sci. U.S.A. 86 (1989) 4549e4553. [14] D.E. Birk, E.I. Zycband, S. Woodruff, D.A. Winkelmann, R.L. Trelstad, Collagen fibrillogenesis in situ: fibril segments become long fibrils as the developing tendon matures, Dev. Dyn. 298 (1997) 291e298. [15] N.S. Kalson, Y. Lu, S.H. Taylor, T. Starborg, D.F. Holmes, K.E. Kadler, A structure-based extracellular matrix expansion mechanism of fibrous tissue growth, Elife 4 (2015). [16] A. Kelly, G.J. Davies, The principles of the fibre reinforcement of metals, Metall. Rev. 10 (1965) 1e77. [17] R.B. Svensson, H. Andreas, S. Tobias, L. Michael, K.E. Kadler, K. Qvortrup, S. Peter Magnusson, Evidence of structurally continuous collagen fibrils in tendons, Acta Biomater. 50 (2017) 293e301. [18] T. Starborg, Y. Lu, A. Huffman, D.F. Holmes, K.E. Kadler, Electron microscope 3D reconstruction of branched collagen fibrils in vivo, Scand. J. Med. Sci. Sport. 19 (2009) 547e552. [19] D.F. Holmes, J.A. Chapman, D.J. Prockop, K.E. Kadler, Growing tips of type I collagen fibrils formed in vitro are near-paraboloidal in shape, implying a reciprocal relationship between accretion and diameter, Proc. Natl. Acad. Sci. U.S.A. 89 (1992) 9855e9859. [20] D.F. Holmes, H.K. Graham, K.E. Kadler, Collagen fibrils forming in developing tendon show an early and abrupt limitation in diameter at the growing tips, J. Mol. Biol. 283 (1998) 1049e1058. [21] S.E. Szczesny, D.M. Elliott, Interfibrillar shear stress is the loading mechanism of collagen fibrils in tendon, Acta Biomater. 10 (2014a) 2582e2590. [22] X. Li, R. Cui, L. Sun, K.E. Aifantis, Y. Fan, Q. Feng, F. Cui, F. Watari, 3D-Printed biopolymers for tissue engineering application, Int. J. Polym. Sci. 2014 (2014). [23] J. Kastelic, A. Galeski, E. Baer, The multicomposite structure of tendon, Connect. Tissue Res. 6 (1978) 11e23. [24] S.E. Szczesny, D.M. Elliott, Incorporating plasticity of the interfibrillar matrix in shear lag models is necessary to replicate the multiscale mechanics of tendon fascicles, J. Mech. Behav. Biomed. Mater. 40 (2014b) 325e338. [25] J.O. Outwater, The mechanics of plastics reinforcement in tension, Mod. Plast. 33 (1956) 156e165. [26] V.R. Riley, Fibre/fibre interaction, J. Compos. Mater. 2 (1968) 436e446. [27] D.W.L. Hukins, M.C. Kirby, T.A. Sikoryn, R.M. Aspden, A.J. Cox, Comparison of structure, mechanical properties, and functions of lumbar spinal ligaments, Spine 15 (1990) 787e795. [28] D.W.L. Hukins, Biomechanical properties of collagen, in: Collagen in Health and Diseases, 1982.

Current understanding of interfacial stress transfer mechanisms in connective tissue

547

[29] D.W.L. Hukins, R.M. Aspden, Y.E. Yarker, Fibre reinforcement and mechanical stability in articular cartilage, Eng. Med. 13 (1984) 153e156. [30] D.W.L. Hukins, A simple model for the function of proteogylcans and collagen in the response to compression of the intervertebral disc, Proc. R. Soc. B 249 (1992) 281e285. [31] R.M. Aspden, The theory of fibre-reinforced composite materials applied to changes in the mechanical properties of the cervix during pregnancy, J. Theor. Biol. 130 (1988) 213e221. [32] K.L. Goh, D.F. Holmes, H.Y. Lu, S. Richardson, K.E. Kadler, P.P. Purslow, T.J. Wess, Ageing changes in the tensile properties of tendons: influence of collagen fibril volume, J. Biomech. Eng. 130 (2008), 021011. [33] D.F. Holmes, J.A. Chapman, Axial mass distributions of collagen fibrils grown in vitro: results for the end regions of early fibrils, Biochem. Biophys. Res. Commun. 87 (1979) 993e999. [34] D.F. Holmes, H.K. Graham, J.A. Trotter, K.E. Kadler, STEM/TEM studies of collagen fibril assembly, Micron 32 (2001) 273e285. [35] D.F. Holmes, R.B. Watson, J.A. Chapman, K.E. Kadler, Enzymic control of collagen fibril shape, J. Mol. Biol. 261 (1996) 93e97. [36] J.E. DeVente, G.E. Lester, J.A. Trotter, L.E. Dahners, Isolation of intact collagen fibrils from healing ligament, J. Electron. Microsc. 46 (1997) 353e356. [37] J.A. Trotter, J. Tipper, G. Lyons-Levy, K. Chino, A.H. Heuer, Z. Liu, M. Mrksich, C. Hodneland, W.S. Dillmore, T.J. Koob, M.M. Koob-Emunds, K. Kadler, D. Holmes, Towards a fibrous composite with dynamically controlled stiffness: lessons from echinoderms, Biotechnol. Extracell. Matrix (2000) 357e362. [38] R.M. Aspden, Fibre reinforcing by collagen in cartilage and soft connective tissues, Proc. R. Soc. Lond. B 258 (1994b) 195e200. [39] R.M. Aspden, Fibre stress and strain in fibre-reinforced composites, J. Mater. Sci. 29 (1994a) 1310e1318. [40] E. Hedbom, D. Heinegard, Binding of fibromodulin and decorin to separate sites on fibrillar collagens, J. Biol. Chem. 25 (1993) 27307e27312. [41] A.M.A. Huq, K.L. Goh, Z.R. Zhou, K. Liao, On defect interactions in axially loaded single-walled carbon nanotubes, J. Appl. Phys. 103 (2008). [42] A.M.A. Huq, A.K. Bhuiyan, K. Liao, K.L.L. Goh, Defectedefect interaction in singlewalled carbon nanotubes under torsional loading, Int. J. Mod. Phys. B 24 (2010) 1215e1226. [43] K.L. Goh, R.M. Aspden, D.W.L. Hukins, Review: finite element analysis of stress transfer in short-fibre composite materials, Compos. Sci. Technol. 64 (2004) 1091e1100. [44] K.L. Goh, J.R. Meakin, D.W.L. Hukins, Influence of fibre taper on the interfacial shear stress in fibre-reinforced composite materials during elastic stress transfer, Compos. Interfac. 17 (2010) 75e81. [45] K.L. Goh, A.M.A. Huq, R.M. Aspden, D.W.L. Hukins, Nano-fibre critical length depends on shape, Adv. Compos. Lett. 17 (2008) 131e133. [46] K.L. Goh, D.W.L. Hukins, R.M. Aspden, Critical length of collagen fibrils in extracellular matrix, J. Theor. Biol. 223 (2003) 259e261. [47] X.W. Ng, D.W.L. Hukins, K.L. Goh, Influence of fibre taper on the work of fibre pull-out in short fibre composite fracture, J. Mater. Sci. 45 (2010) 1086e1090. [48] K.L. Goh, R.M. Aspden, K.J. Mathias, D.W.L. Hukins, Effect of fibre shape on the stresses within fibres in fibre-reinforced composite materials, Proc. R. Soc. Lond. A 455 (1999) 3351e3361.

548

Interfaces in Particle and Fibre Reinforced Composites

[49] K.L. Goh, J.R. Meakin, R.M. Aspden, D.W.L. Hukins, Influence of fibril taper on the function of collagen to reinforce extracellular matrix, Proc. R. Soc. Lond. B 272 (2005) 1979e1983. [50] K.L. Goh, D.F. Holmes, Y. Lu, P.P. Purslow, K.E. Kadler, D. Bechet, T.J. Wess, Bimodal collagen fibril diameter distributions direct age-related variations in tendon resilience and resistance to rupture, J. Appl. Physiol. 113 (2012) 878e888. [51] N.J. Glassmaker, A. Jagota, C.Y. Hui, Adhesion enhancement in a biomimetic fibrillar interface, Acta Biomater. 1 (2005) 367e375. [52] Amer, S. Maher, L.S. Schadler, The effect of interphase toughness on fibre/fibre interaction in graphite/epoxy composites: an experimental and modelling study, J. Raman Spectrosc. 30 (1999) 919e928. [53] H.D. Wagner, M.S. Amer, L.S. Schadler, Fibre interactions in two-dimensional composites by micro-Raman spectroscopy, J. Mater. Sci. 31 (1996) 1165e1173. [54] V.K. Mohonee, K.L. Goh, Effects of fibreefibre interaction on stress uptake in discontinuous fibre reinforced composites, Compos. B Eng. 86 (2016) 221e228. [55] H.R. Kayed, N. Kirby, A. Hawley, T. Mudie, R.G. Haverkamp, Collagen fibril strain, recruitment and orientation for pericardium under tension and the effect of cross links, RSC Adv. 5 (2015) 103703e103712. [56] K.G. Danielson, H. Baribault, D.F. Holmes, H. Graham, K.E. Kadler, R.V. Iozzo, Targeted disruption of decorin leads to abnormal collagen fibril morphology and skin fragility, J. Cell Biol. 136 (1997) 729e743. [57] J.P.R.O. Orgel, A. Eid, O. Antipova, J. Bella, J.E. Scott, Decorin core protein (decoron) shape complements collagen fibril surface structure and mediates its binding, PLoS One 4 (2009) e7028. [58] G. Zhang, Y. Ezura, I. Chervoneva, P.S. Robinson, D.P. Beason, E.T. Carine, L.J. Soslowsky, R.V. Iozzo, D.E. Birk, Decorin regulates assembly of collagen fibrils and acquisition of biomechanical properties during tendon development, J. Cell. Biochem. 1449 (2006) 1436e1449. [59] X. Liu, M.-L. Yeh, J.L. Lewis, Z.-P. Luo, Direct measurement of the rupture force of single pair of decorin interactions, Biochem. Biophys. Res. Commun. 338 (2005) 1342e1345. [60] M. Wong, M. Paramsothy, X.J. Xu, Y. Ren, S. Li, K. Liao, Physical interactions at carbon nanotube-polymer interface, Polymer 44 (2003) 7757e7764. [61] M. Zu, Q. Li, Y. Zhu, M. Dey, G. Wang, W. Lu, J.M. Deitzel, J.W. Gillespie, J.-H. Byun, T.-W. Chou, The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test, Carbon 50 (2011) 1271e1279. [62] G. Fessel, J.G. Snedeker, Evidence against proteoglycan mediated collagen fibril load transmission and dynamic viscoelasticity in tendon, Matrix Biol. 28 (2009) 503e510. [63] L. Dong, F. Hou, L. Yang, L. Wang, H. Gao, Y. Tang, Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite, Compos. Part A 56 (2014) 248e255. [64] J.E. Scott, Proteoglycan: collagen interactions in connective tissues. Ultrastructural, biochemical, functional and evolutionary aspects, Int. J. Biol. Macrmol. 13 (1991) 157e161. [65] J.E. Scott, Extracellular matrix, supramolecular organisation and shape, J. Anat. 187 (1995) 259e269. [66] J.E. Scott, Structure and function in extracellular matrices depend on interactions between anionic, Pathol. Biol. 49 (2001) 284e289.

Current understanding of interfacial stress transfer mechanisms in connective tissue

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[67] J.E. Scott, R.O. Constance, Dermatan sulphate-rich proteoglycan associates with rat tailtendon collagen at the d band in the gap region, Biochem. J. 19 (1981) 213e216. [68] J.E. Scott, R.O. Constance, E.W. Hughes, Proteoglycan-collagen arrangements in developing rat tail tendon, Biochem. J. 195 (1981) 573e581. [69] H.R. Kayed, K.H. Sizeland, N. Kirby, A. Hawley, T. Mudie, R.G. Haverkamp, Collagen cross linking and fibril alignment in pericardium, RSC Adv. 5 (2014) 3611e3618. [70] H.C. Wells, K.H. Sizeland, N. Kirby, A. Hawley, S. Mudie, R.G. Haverkamp, Acellular dermal matrix collagen responds to strain by intermolecular spacing contraction with fibril extension and rearrangement, J. Mech. Behav. Biomed. Mater. 79 (2018) 1e8. [71] B.K. Connizzo, J.J. Sarver, D.E. Birk, L.J. Soslowsky, R.V. Iozzo, Effect of age and proteoglycan deficiency on collagen fiber re-alignment and mechanical properties in mouse supraspinatus tendon, J. Biomech. Eng. 135 (2013), 021019. [72] A.A. Dunkman, M.R. Buckley, M.J. Mienaltowski, S.M. Adams, S.J. Thomas, L. Satchell, A. Kumar, L. Pathmanathan, D.P. Beason, R.V. Iozzo, D.E. Birk, L.J. Soslowsky, Decorin expression is important for age-related changes in tendon structure and mechanical properties, Matrix Biol. 32 (2013) 3e13. [73] J.A. Gordon, B.R. Freedman, A. Zuskov, R.V. Iozzo, D.E. Birk, L.J. Soslowsky, Achilles tendons from decorin- and biglycan-null mouse models have inferior mechanical and structural properties predicted by an image-based empirical damage model, J. Biomech. 48 (2015) 2110e2115. [74] Y. Li, Y. Liu, W. Xia, D. Lei, J.J. Voorhees, G.J. Fisher, Age-dependent alterations of decorin glycosaminoglycans in human skin, Sci. Rep. 3 (2422) (2013) 1e8. [75] H.L. Cox, The elasticity and strength of paper and other fibrous materials, Br. J. Appl. Phys. 3 (1952) 72e79. [76] S.E. Szczesny, K.L. Fetchko, G.R. Dodge, D.M. Elliott, Evidence that interfibrillar load transfer in tendon is supported by small diameter fibrils and not extrafibrillar tissue components, J. Orthop. Res. 35 (2017) 2127e2134. [77] S.E. Szczesny, J.L. Caplan, P. Pedersen, D.M. Elliott, Quantification of interfibrillar shear stress in aligned soft collagenous tissues via notch tension testing, Sci. Rep. 5 (14649) (2015) 1e9.

Index Note: ‘Page numbers followed by “f ” refer to figures and “t” refers to tables’. A Activated carbon, 421e422 Al/diamond composites, 474e475, 478e479 Al/diamond interface, 476e478 Alkaline treatments, 252 Alumina based materials, 347e349 Animal-based fibres, 251e252 Anodization technique, 405e406 Aqueous electrolytes, 426e427 Atomic force microscopy (AFM), 10, 234e238 B Band-gap engineering, 406 bang-gap engineering, 411e417 essentialities to photocatalyst, 411 Ohmic contacts, 408e409 photocatalytic water-splitting, 409e410 purification of toxic water/air systems, 409e410 Schottky barrier, 407e408 Bang-gap engineering cation/anion doping, 413e414 composite semiconductors, 412e413 nanodesign, 415e417 semiconductor alloys, 415 surface co-catalysts, 414 Bioinert, 515e516 Biological-based experiments, 534e536 Biological materials and interface effects multiscale structure of timber, 142e143 nanostructures in nacre, 140e142 Biological matrix, 2 Biomedical engineering, FEA in, 523e525 Bionanocomposite fabrication, 10e11 Bioplastics, 277t Bone-particle-reinforced nanocomposite interface, 515

finite element approach, 518e520 structures of human bone, 520e523 Bone remodeling, 522e523 Bone tissue engineering and scaffolds, 523e524 Borates based flame retardant, 262e265 Bunsen burner test, 254e255 C Carbide-derived carbons (CDC), 422 Carbon based biopolymer composites, 318 Carbon based materials, 351 carbon fibres, 353e354 carbon nanotubes, 352e353 graphene, 354e355 Carbon-family nanomaterials, 332e333 Carbon fiber matrix interface, 378e379 Carbon materials, 421e423 Carbon nanotubes (CNTs), 49, 422, 536e537 Cation/anion doping, 413e414 Cellulose-based nanofillers, 230e231 Cellulose microfibrils, 298 Ceramic-ceramic composites, 373 Ceramic fiber matrix interface, 380e381 Ceramic material as particulate reinforcement, 371e372 Ceramic matrix reinforcement, 345, 347 alumina based materials, 347e349 carbon based materials, 351 carbon fibres, 353e354 carbon nanotubes, 352e353 graphene, 354e355 hydroxyapatite and phosphates, 357 metal particles, 357e358 surface modification of nanoparticles, 358 titanium dioxide, 349e351 zirconia and rare earth materials, 355e357 Ceramic-metal composites, 374

552

Ceramics matrix, 2 Ceramic whisker matrix interface, 381 Chemical vapor deposition (CVD), 405 Clay, 231 Collagen fibrils, 534 stress uptake, 539e544 Composite materials, fire safety of, 253e257 Composite microstructure, 475 characterization of, 480e481, 485 Composite nanomaterials, synthesis of, 396, 397te398t anodization technique, 405e406 chemical method, 399e400 chemical vapor deposition (CVD), 405 deposition-precipitation method, 403 photodeposition method, 403e404 sputtering method, 404e405 thermal method, 400e403 Composite semiconductors, 412e413 Contact angle, measurement of, 74e76 Continuous fiber reinforcement, 191 Continuous phase (matrix), 312 Conventional water retting process, 39e40 Cu/diamond composites fabrication of, 480, 485 thermal conductivity of, 482e483, 486e488 Cu/diamond interface characterization of, 481, 485e486 by diamond surface coating, 483e488 by metal matrix alloying, 480e483 D Dense fillers, 230 Dense membranes, permeation through, 221e226 Density, 299e300 Dentistry, 524e525 Deposition-precipitation method, 403 Derjaguin-Muller-Toropov (DMT), 11e12 Diamond particles, 473 reinforced Al matrix composites, 474e479 reinforced Cu matrix composites, 479e488 Diamond surface coating, Cu/diamond interface by, 483e488 Differential scanning calorimetry (DSC), 10, 238e239

Index

Discontinues phase (reinforcement/ dispersed), 312 3D printed poly(lactic acid)/halloysite nanocomposites filament diameter and quality, 206e212 fracture morphology, 207e209 fused deposition modelling (FDM), 251 impact tests, 206e207 interfacial interactions, 209 materials and methods, 200e206 PLA/HNT filament specimens, 207e210 3D printed specimens, 204 ‘Droplet aspect ratio method’, 74e76 Dual mode sorption model, 274e275 Dynamic mechanical analysis (DMA), 10 E Effective interfaces model, 146e149 Electrochemical double-layer capacitor (EDLC), 394e395 Empty palm fruit bunch fibres (EPFBFs), 76 Energy storage devices, 393e394 Epoxy resins, 30e31 Ethylene propylene dine monomer (EPDMs), 10 Expandable graphite (EG), 259 Expandable microspheres fillers, 230 Extracellular matrix (ECM), 529e530, 530f F Fiber-reinforced ceramics, 374e375 Fiber reinforced composites, 144e145, 186e191 Fibre, 1e3 adhesion on matrix composite, 49e50 loading on matrix composites, 50e51 modification, 127e132 reinforced polymer composites, 251 surface modification, 252 Field emission scanning electron microscopy (FESEM), 54 Filament diameter and quality, 206e212 Filler matrix interfaces, 375 carbon fiber matrix interface, 378e379 ceramic fiber matrix interface, 380e381 ceramic whisker matrix interface, 381

Index

glass fiber matrix interface, 379e380 non-oxide fiber matrix interface, 382 oxide fiber matrix interface, 382 particle-matrix interface, 377e378 of polymer composite, 314e321 atomic force microscopy (AFM), 234e238 FTIR analysis, 241e242 SEM and TEM analysis, 232e234 thermal analysis, 238e239 XPS analysis, 242e245 X-ray diffraction (XRD), 239e241 Fillers, 227e229 dense, 230 expandable microspheres, 230 functional, 231 molecular, 231 nano-fillers, 230e231 natural and renewable, 229 zeolites, 229 Finite element analysis (FEA), 518e520 in biomedical engineering, 523e525 Fire test techniques, 255 Bunsen burner test, 254e255 heat release and mass loss rate tests, 256e257 limiting oxygen index test, 255e256 smoke generation and toxicity tests, 257 Flexural modulus, 54 Flexural strength, 279e280 Fourier transform infrared (FTIR) analysis, 241e242 spectroscopy, 70e72 Fracture morphology, 207e209 Friction coefficient, 501e507 Functional fillers, 231 Functionalized fibre-matrix interface, 29e30 density, 299e300 flexural strength, 279e280 impact strength, 280 mechanical properties of nanocellulose, 276e279 microstructure of fibre, 274e275 nanostructure of fibre, 275 natural fibres, 272e275 physio-chemical characterisation of interphase, 278

553

processing of polymer composites, 276e278 scanning electron microscope (SEM), 49e52 tensile strength, 279e292 theories of adhesion and type of bonding, 276e277 thermal gravimetric analysis (TGA) on types of fibre and matrix composites, 52e54 thickness swelling, 294e298 water absorption, 292e294 Functionalized filler-matrix interface, 369 classification of composites matrix material, 370e371 reinforcement/filler phase, 371e372 filler matrix interfaces, 375 carbon fiber matrix interface, 378e379 ceramic fiber matrix interface, 380e381 ceramic whisker matrix interface, 381 glass fiber matrix interface, 379e380 non-oxide fiber matrix interface, 382 oxide fiber matrix interface, 382 particle-matrix interface, 377e378 make-up and characteristics of composite, 370 reinforced ceramic composites, 382e383 types of ceramic used in composites, 372e375 Fused deposition modelling (FDM), 251 G Gas separation membrane technology, 220e221 Glass fiber matrix interface, 379e380 Glass fibre reinforced polymer (GFRP), 49 Glassy polymers, 224 Global climate change, 391 Glycosaminoglycan (GAG), 540e541 Graphene, 422e423 Graphene nanoplatelet (GNP), 49 Graphite powder composites, 374 H Halogenated flame retardant, 325 Heat release rate (HRR), 256e257 Henry’s law, 274 Hierarchical fiber reinforced composites, 149e152

554

Hooke’s law, 54e55 Human bone, 520e523 Hybrid capacitors, 420 Hybrid composites, 192 Hybrid flame retardants with synergistic effect, 325e327 Hydroxyapatite, 357 Hydroxycarbonates flame retardant, 260e262 I Impact tests, 206e207 Inorganic/biopolymer composites, 311, 318e320 carbon based biopolymer composites, 318 filler-matrix interface of polymer composite, 314e321 metal based biopolymer composites, 314e321 metal oxide based biopolymer composites, 315e318 practical applications of, 320e321 Inorganic materials, 278e279, 279t Interfacial adhesion, 1e2 Interfacial interactions, 209 Interfacial stress transfer mechanisms for collagen fibril stress uptake, 541e544 early studies on, 532e536 extracellular matrix (ECM), 529e530 later studies on, 536e544 proteoglycans regulating collagen fibril stress uptake, 539e541 Interfibrillar shear, 532e534 Interlaminar shear strength (ILSS) testing, 539 Interphase elastic properties, 20e21 Intrinsic pseudocapacitor, 418 In-vivo fatigue microdamage, 2 Ionic liquids (ILs), 427 embedded ternary, 229 K Kastelic model, 530e531 L Laminates, 192 Layered silicate materials, 49e50 Layered silicates, 328e332 Lignocellulosic fibres, 269

Index

M Macroscale flame retardant particulate additives, 262e265 borates based flame retardant, 262e265 expandable graphite (EG), 259 halogenated flame retardant, 325 hybrid flame retardants with synergistic effect, 325e327 hydroxycarbonates flame retardant, 260e262 mineral hydroxide flame retardant, 257e258 phosphorous based flame retardant, 265 Macro-to nano-particulate flame retardants in NFRC, 333e335 Maleic anhydride grafted polypropylene (MAPP), 253 Mass loss rate tests, 256e257 Materials and fabrication, 496 Materials engineering, 534e536 Matrix, 1e3 adhesion on untreated fibre, 51e52 composites, 53e54 interfacial adhesion, 60e61 material, 370e371 ‘Mechanical linkages’, 540e541 Membrane selectivity, 221e222 Membrane synthesis materials, 276 inorganic materials, 278e279, 279t polymer, 223e224 glassy polymers, 224 rubbery polymers, 224 Metal based biopolymer composites, 314e321 Metal chalcogenides, 425e426 Metal matrix alloying, Cu/diamond interface by, 480e483 Metal matrix composites (MMCs), 455e456 simulation facts, 456e457 tensile loading, 461e467 thermal loading distribution of Von-Mises stress, 460e461 scattering of principal stress, 458e460 stress contour, 457e458 Metal-organic frameworks (MOFs), 423e424

Index

Metal oxide, 424e425 based biopolymer composites, 315e318 Metal particles, 357e358 Metals matrix, 2 Micro-Raman spectroscopy, 535 Microstructure of fibre, 274e275 Mineral hydroxide flame retardant, 257e258 Mixed matrix membranes (MMM), 219 concept of, 227 ionic liquid embedded ternary, 229 issues and challenges in, 229e231 membrane synthesis materials, 276 inorganic materials, 278e279, 279t polymer, 223e224 selected reports on, 280 ternary, 229 Modulus elasticity, 54 Molecular fillers, 231 Monomers, 311 Montmorillonite (MMT), 31e32, 49e50 Multi-walled carbon nanotube (MWCNT), 48e49 N Nanocellulose effect of, 60e61 mechanical properties of, 276e279 reinforced biopolymer, 62e67 in starch matrix, 61e62 Nanoclay filled Napier/epoxy composites, 36e37 fabrication and flexure test of, 37 flexural strength and modulus of, 39e40 Nanoclay filled reinforced epoxy composites, 40e42 Nanoclay/polymer interface, 146e149 Nanoclay reinforced composites, 33e36 Nanocomposite system, 9 Nanocrystalline metals, 153e157 Nanodesign, 415e417 Nanodiameter-based bamboo charcoals (NBCs) bionanocomposite fabrication, 10e11 interphase characterisation of, 16e19 interphase dimensions, 21e22 nanomechanical properties of, 14e16 particle size and elastic modulus of, 13e14 Nanoeingineered interfaces, 149e152

555

Nanofibre-like particles reinforcing composites, 536e539 Nano fibrillated cellulose, 76 Nano-fillers, 230e231 cellulose-based nanofillers, 230e231 clay, 231 Nanomechanical properties, 11 Nanoparticles, surface modification of, 358 Nanoscale particulate additives, 257e265 Nanoscale particulates flame retardant additives, 327 carbon-family nanomaterials, 332e333 layered silicates, 328e332 Nanostructure of fibre, 275 Nano-technology, 495e496 Napier/epoxy composites, 58e59 epoxy resins, 30e31 fabrication and flexure test of nanoclay filled, 37 flexural strength and modulus of nanoclay filled, 39e40 montmorillonite (MMT), 31e32 montmorillonite clay, 31e32 morphology of nanoclay filled, 40e42 nanoclay filled, 36e37 nanoclay reinforced composites, 33e36 Napier grass fibre, 33 natural fibre, 30e32, 30f, 34te35t natural fibre reinforced epoxy composites, 33 Napier grass fibre, 33 Natural fibre reinforced composite (NFRC), 251e252 macro-to nano-particulate flame retardants in, 333e335 nanoscale particulate additives in flame retardancy of, 257e265 Natural fibres, 30e32, 30f, 34te35t, 251e252, 272e275 reinforced epoxy composites, 33 Natural fillers, 229 Non-oxide fiber matrix interface, 382 Nuclear magnetic resonance (NMR) spectroscopy, 10, 73e74 O Ohmic contacts, 408e409 Organic electrolytes, 426e427

556

Oxide fiber matrix interface, 382 Oxygen index test, 255e256 P Particle, 1e3 content on stress distribution, 462 loading, effect of, 58e60 size effect of, 56e58 on stress distribution, 462 Particle-matrix interface, 377e378 Particulate-polymer biocomposites, 54e67 Particulate reinforced composites, 190e191 Particulate reinforcements, 371 Peak force quantitative nanomechanical mapping (PFQNM), 10 Peeling of platelets, 312 Pennisetum purpureum, 37 Phosphates, 357 Phosphorous based flame retardant, 265 Photocatalytic water-splitting, 409e410 Photodeposition method, 403e404 Physio-chemical characterisation of interphase, 278 PLA-based nanocomposite fllaments, 202e204 PLA/epoxy/HNT nanocomposites, 204 PLA/HNT filament specimens, 207e210 Plants-based fibres, 251e252 Plastics, 163 Plastic stress transfer mechanism, 545 Poly(vinyl butyral) (PVB), 534e535 Poly(vinyl) alcohol (PVA) bionanocomposites atomic force microscopy (AFM), 10 characterisation techniques, 11e12 materials, 10e12 nanocomposite system, 9 NBC bionanocomposite fabrication, 10e11 interphase characterisation of, 16e19 interphase dimensions, 21e22 nanomechanical properties of, 14e16 particle size and elastic modulus of, 13e14

Index

nuclear magnetic resonance (NMR) spectroscopy, 10 peak force quantitative nanomechanical mapping (PFQNM), 10 Polymer membranes diffusion in, 223e224 sorption in, 221e223 Polymers, 163 classification of, 163e179 composite, 181e184 fiber reinforced composites, 186e191 hybrid composites, 192 laminates, 192 particulate reinforced composites, 185 processing of, 276e278 composites for filler matrix interface atomic force microscopy (AFM), 234e238 FTIR analysis, 241e242 SEM and TEM analysis, 232e234 thermal analysis, 238e239 XPS analysis, 242e245 X-ray diffraction (XRD), 239e241 effects of additives on composite fillers, 227e229 reinforcements, 227 glassy polymers, 224 nanocomposites, 192e194 rubbery polymers, 224 Polymers matrix, 2 Probable mechanistic pathways, 417e427 electrode materials, 421e426 electrolyte, 426e427 Pseudocapacitors, 394e395, 418 R Raman spectroscopy (RS), 76e77 Rare earth materials, 355e357 Rectilinear pattern, 207 Reinforced ceramic composites, 382e383 Reinforcement/filler phase, 371e372 Reinforcement size on strain, 466e467 Reinforcing phase, 185 Renewable fillers, 229 Resin transfer moulding (RTM) method, 48 Rubbery polymers, 224

Index

S Scanning electron microscope (SEM), 49e52, 67e69 Scattering of principal stress, 458e460 Schottky barrier, 407e408 Seawater treatment, 209e210 Semiconductor alloys, 415 ‘Shear-lag’ approach, 542e543, 542f Silane coupling chemicals, 253 Silane treatment, 209 Smoke generation, 257 Solution diffusion model, 223e224 Sorption in polymer membranes, 221e223 Sputtering method, 404e405 Starch matrix, nanocellulose in, 61e62 Strain field particles’ shape on, 467 reinforcement content on, 466 Stress contour, 457e458 Stress distribution particle content on, 462 particle size on, 462 particles’ shape on, 463e466 Stress transfer, 532e534 Sugar palm fibres (SPF) Atomic Force Microscope (AFM), 211 characterization methods, 210 chemical modification, 132e133 classification and structures of, 117 fibre modification, 127e132 Scanning Electron Microscope (SEM), 210e211 X-Ray Photoelectron Spectroscope (XPS), 211e212 Sugar palm particle reinforced thermoplastic matrices, 125e127 Sugar palm particle reinforced thermoset matrices, 122e125 Supercapacitors, 394e395 Surface co-catalysts, 414 Synergistic effect, hybrid flame retardants with, 325e327 T Tensile strength, 279e292 Ternary mixed matrix membranes (MMM), 229

557

Theories of adhesion, 276e277 Thermal analysis, 238e239 Thermal gravimetric analysis (TGA), 52e54 Thermal management materials, 473 Thermal method, 400e403 Thermal properties, 125e127 Thermogravimetric analysis (TGA), 239 Thickness swelling, 294e298 Titanium dioxide, 349e351 Toxicity tests, 257 Toxic water/air systems, 409e410 Transport of gases, through membranes, 223f diffusion in polymer membranes, 223e224 permeation through dense membranes, 221e226 sorption in polymer membranes, 221e223 U Ultrafine grained (UFG) metals, grain boundaries of, 21e22 Ultrafine grained titanium, 157e158 V Vacuum infusion technique, 50 Von-Mises stress distribution, 460e461 W Water absorption, 292e294 Wear debris analysis, 507e508 X X-ray diffraction (XRD), 72e73, 239e241 X-Ray Photoelectron Spectroscope (XPS), 211e212 Y Young’s modulus and tensile strength, 54e56 Z Zeolites, 229 Zirconia, 355e357