Surface Modification of Polymers: Methods and Applications 9783527819218, 3527819215

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Surface Modification of Polymers

Surface Modification of Polymers Methods and Applications

Edited by Jean Pinson and Damien Thiry

Editors Prof. Jean Pinson

Universitéde Paris ITODYS, CNRS, UMR 7086 15 rue J-A de Baïf F-75013 Paris France

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Dr. Damien Thiry

University of Mons Chimie des Interactions Plasma-Surface 1, Avenue Copernic Parc Initialis 7000 Mons Belgium

Library of Congress Card No.:

applied for British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34541-0 ePDF ISBN: 978-3-527-81921-8 ePub ISBN: 978-3-527-81923-2 oBook ISBN: 978-3-527-81924-9 Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

v

Contents Introduction xiii 1

The Surface of Polymers 1 Rosica Mincheva and Jean-Marie Raquez

1.1 1.2 1.2.1 1.2.2 1.2.2.1 1.2.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.2.1 1.3.2.2 1.4

Introduction 1 The Surface of Polymers 2 Definition of a Polymer Surface 2 Factors Determining a Polymer Surface 3 Internal Factors 3 External Factors 4 The Polymer Surface at a Microscopic Level 11 Properties of Polymer Surfaces at Interfaces 12 Surface Wettability 13 Surface Thermal Properties 15 Surface T g 15 Surface Crystallization 17 Experimental Methods for Investigating Polymer Surfaces at Interfaces 21 Conclusions 21 References 21

1.5

Part I

Gas Phase Methods 31

2

Surface Treatment of Polymers by Plasma 33 Pieter Cools, Laura Astoreca, Parinaz Saadat Esbah Tabaei, Monica Thukkaram, Herbert De Smet, Rino Morent, and Nathalie De Geyter

2.1 2.1.1 2.1.2 2.1.3 2.1.4

Plasma: An Introduction 33 Definition 33 Thermal Versus Nonthermal Plasma 34 The Formation of Nonthermal Plasma 35 Plasma Generation and Operating Conditions 37

vi

Contents

2.1.4.1 2.1.4.2 2.1.4.3 2.1.4.4 2.1.4.5 2.1.4.6 2.1.4.7 2.1.5 2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.3 2.2.3.1 2.2.3.2 2.3 2.4 2.5

Different Methods of Plasma Generation 37 DC Discharges 38 DC Pulsed Discharges 38 RF and MW Discharges 38 Dielectric Barrier Discharge (DBD) 39 Atmospheric Pressure Plasma Jet (APPJ) 40 Gliding Arc 41 Nonthermal Plasma for Polymer Surface Treatment 41 Applications of Plasma Surface Activation of Polymers 43 Adhesion Improvement 43 Packaging and Textile Applications 47 Printability Enhancement 47 Dyeability Improvement 47 Mass Transfer Changes 49 Biomedical Applications 50 Inert Synthetic Polymers 50 Biodegradable Polymers 53 Plasma Grafting 56 Hydrophobic Recovery 59 Conclusion 61 References 61

3

A Joint Mechanistic Description of Plasma Polymers Synthesized at Low and Atmospheric Pressure 67 Damien Thiry, François Reniers, and Rony Snyders

3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.4

Introduction 67 Plasma Polymerization 69 Plasma Fundamentals 70 Growth Mechanism 72 Probing the Plasma Chemistry 83 Optical Emission Spectroscopy 84 Mass Spectrometry 87 Conclusions 96 References 97

4

Organic Surface Functionalization by Initiated CVD (iCVD) 107 Karen K. Gleason

4.1 4.2 4.3

Introduction 107 Mechanistic Principles of iCVD 108 Functional, Surface Reactive, and Responsive Organic Films Prepared by iCVD 113 Interfacial Engineering with iCVD: Adhesion and Grafting 127 Reactors for Synthesizing Organic Films by iCVD 128 Summary 129 References 130

4.4 4.5 4.6

Contents

5

Atomic Layer Deposition and Vapor Phase Infiltration 135 Mark D. Losego and Qing Peng

5.1 5.2 5.2.1 5.2.2

Atomic Layer Deposition Versus Vapor Phase Infiltration 135 Atomic Layer Deposition (ALD) on Polymers 138 Chemical Mechanisms of ALD 138 ALD on Polymers with Dense –OH Groups: Cellulose and Poly(vinyl alcohol) 140 ALD onto “Unreactive” Polymer Substrates 141 Applications of ALD Coated Polymers 143 ALD Coated Cotton Fibers 143 Applications for ALD Coatings on Other Polymers 144 Vapor Phase Infiltration of Polymers 145 Processing Thermodynamics and Kinetics of VPI 145 Thermodynamics of Vapor-Phase Precursor Sorption into Polymers 145 Kinetics of Precursor Diffusion During VPI 147 VPI Processes Incorporating Both Penetrant Diffusion and Reaction 148 Measuring the Thermodynamics and Kinetics of a VPI Process 149 Applications of Vapor Phase Infiltrated Polymers 150 Altering Mechanical Performance 150 Contrasting Agent for Multi-phase Polymer Imaging 152 Improved Chemical Resistance 152 Patterning for Microsystems 153 Vapor Diffusion Barriers 154 Conducting Polymers and Hybrid Photovoltaic Cells 154 Other Application Spaces 155 Summary and Future Outlook for ALD and VPI on Polymers 156 References 156

5.2.3 5.2.4 5.2.4.1 5.2.4.2 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.1.3 5.3.1.4 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.2.5 5.3.2.6 5.3.2.7 5.4

Part II

UV and Related Methods 161

163

6

Photoinduced Functionalization on Polymer Surfaces Kazuhiko Ishihara

6.1 6.2

Introduction 163 Improving the Surface Properties of Polymeric Materials by Photoirradiation 165 Photoreaction of Polymers with Other Polymers 166 Photoinduced Chemical Reaction Between Polymers 166 Photoinduced Grafting at the Polymer Surface 168 Preparation of High-functionality Surface by Photoinduced Graft Polymerization 169 Application of Photoinduced Grafting Process to Artificial Organs 172 Self-initiated Photoinduced Graft Polymerization 174 Poly(ether ketone) as Photoinitiator for Graft Polymerization 174

6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.4 6.4.1

vii

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Contents

6.4.2 6.5

Effects of Inorganic Salts on Photoinduced Graft Polymerization in an Aqueous System 178 Conclusion and Future Perspective 180 References 181

7

𝜸-Rays and Ions Irradiation 185 Alejandro Ramos-Ballesteros, Victor H. Pino-Ramos, Felipe López-Saucedo, Guadalupe G. Flores-Rojas, and Emilio Bucio

7.1 7.2 7.3 7.3.1 7.3.1.1 7.3.1.2 7.3.1.3 7.3.1.4 7.3.2 7.3.2.1 7.3.2.2 7.3.2.3 7.4 7.4.1 7.5

𝛾-Rays and Ions Irradiation 185 Ionizing Radiation Sources 186 𝛾-Ray-Induced Modifications 186 Grafting Modifications 186 Radiation-induced Grafting Methods 188 Ionic Grafting 192 RAFT-graft Polymerization 193 Applications 194 Cross-linking 197 𝛾-Ray Cross-linking Modifications 199 Cross-linking with Additives 200 Industrial Applications 201 Heavy Ion-Induced Modifications 202 Polymers 204 Conclusions 205 Acknowledgments 206 References 206

Part III

Chemical Methods 211

8

Functionalization of Polymers by Hydrolysis, Aminolysis, Reduction, Oxidation, and Some Related Reactions 213 Dardan Hetemi and Jean Pinson

8.1 8.1.1 8.1.2 8.1.3 8.1.4 8.1.5 8.1.6 8.1.7 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5

Hydrolysis and Aminolysis 213 PLA and Polyesters 213 Hydrolysis 214 Aminolysis 214 PCL 215 PET 216 PMMA 216 Cellulose 217 Chemical Reduction 220 PEEK 220 PET 225 PMMA 227 PC 227 PTFE 229

Contents

8.3 8.4 8.5

Chemical Oxidation 231 Non-covalent Surface Modification 234 Conclusion 235 References 236

9

Functionalization of Polymers by Reaction of Radicals, Nitrenes, and Carbenes 241 Jean Pinson

9.1 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5 9.1.6 9.1.6.1 9.1.6.2 9.1.6.3

Functionalization of Polymers by Reaction of Radicals 241 Peroxides as Radical Initiators 241 Hydrogen Peroxides as Radical Initiator 244 Persulfates as Radical Initiators 246 Oxygen as Radical Initiator 248 Azo Compounds as Radical Initiator 249 Diazonium Salts as Radical Initiator 250 Polypyrrole 251 Polyaniline 251 Poly(3,4-ethylenedioxythiophene)–Poly(styrenesulfonate) (PEDOT:PSS) 253 Polymethylmethacrylate (PMMA) 254 Polypropylene (PP) 255 Polyvinyl Chloride 255 Cyclic Olefin Copolymers (COC) 256 Polyetheretherketone (PEEK) 256 PET (Polyethylene Terephthalate) 257 Polysulfone Membranes 258 Cation Exchange Membranes 258 Fluoro Polymers 259 Natural Polymers 260 Alkyl Halides as Radical Initiator 260 Surface Modification of Polymers with Carbenes and Nitrenes 260 Carbenes 261 Nitrenes 264 Conclusion 267 References 268

9.1.6.4 9.1.6.5 9.1.6.6 9.1.6.7 9.1.6.8 9.1.6.9 9.1.6.10 9.1.6.11 9.1.6.12 9.1.6.13 9.1.7 9.2 9.2.1 9.2.2 9.3

10

Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed Macromolecular Grafts 273 Fatima Mousli, Youssef Snoussi, Ahmed M. Khalil, Khouloud Jlassi, Ahmed Mekki, and Mohamed M. Chehimi

10.1 10.1.1 10.1.2 10.2

Introduction 273 Context 273 Scope of the Chapter 274 Surface-confined Radical Photopolymerization of Insulating Vinylic and Other Monomers 274 Type I and Type II Photoinitiation Systems 275

10.2.1

ix

x

Contents

10.2.2 10.2.3 10.2.4 10.2.5 10.3 10.3.1 10.3.1.1 10.3.1.2 10.3.2 10.3.2.1 10.3.2.2 10.4 10.4.1 10.4.2 10.4.2.1 10.4.2.2 10.4.3 10.5

Simultaneous Photoinduced Electron Transfer and Free Radical Polymerization Confined to Surfaces 282 Surface-initiated Photoiniferter 284 “Brushing Up from Anywhere” Using Polydopamine Thin Adhesive Coatings 284 Recent Trends in Surface-confined Photopolymerization (CRP) 287 Surface-confined Photopolymerization of Conjugated Monomers 289 Polypyrrole 290 Mechanisms of Photopolymerization of Pyrrole 290 Substrates for in Situ Photoinduced Polymerization of Pyrrole and Potential Applications 291 Polyaniline 294 Mechanisms of Photopolymerization of Aniline 294 Substrates for in Situ Photoinduced Polymerization of Aniline 298 Surface-confined Sonochemical Polymerization of Conjugated and Vinylic Monomers 298 Insights into Sonochemistry: Origin of the Phenomenon and Mechanism of Polymer Synthesis 298 Ultrasound-assisted Polymerization or Polymer Deposition over Organic Polymeric Substrates 303 Sonopolymerization 303 Ultrasonic Spray 303 Sonopolymerization over Miscellaneous Types of Surface: Inorganic Polymeric Substrates 305 Conclusion 306 Acknowledgments 307 References 307

Part IV

Applications 317

11

Surface Modification of Nanoparticles: Methods and Applications 319 Gopikrishna Moku, Vijayagopal Raman Gopalsamuthiram, Thomas R. Hoye, and Jayanth Panyam

11.1 11.2 11.3 11.3.1 11.3.2 11.3.2.1 11.3.3 11.3.4 11.3.5

Introduction 319 Polymers Used in the Preparation of Nanoparticles 320 Common Biodegradable Polymers for Nanoparticle Fabrication 320 Albumin 320 Alginate 320 Chitosan 321 Gelatin 322 Poly(lactide-co-glycolide) (PLGA) and Polylactide (PLA) 322 Poly-ε-caprolactone (PCL) 323

Contents

11.4 11.5

Fabrication of Nanoparticles 323 Linker Chemistry for Attaching Ligands on Polymeric Nanoparticles 324 11.5.1 Hydrazone Bond Formation 327 11.5.2 Non-covalent Attachment 328 11.6 Surface-functionalized Polymeric Nanoparticles for Drug Delivery Applications 328 11.6.1 Polysaccharides 329 11.6.2 Lipids 329 11.6.3 Aptamers 332 11.6.4 Antibodies 332 11.6.5 Peptides 333 11.6.5.1 Polyethylene Glycol (PEG) 334 11.7 Characterization of Surface-modified Nanoparticles 336 11.7.1 Particle Size 336 11.7.2 Dynamic Light Scattering (DLS) 337 11.7.3 Scanning Electron Microscopy (SEM) 337 11.7.4 Transmission Electron Microscopy (TEM) 339 11.7.5 Surface Charge 339 11.7.6 Surface Hydrophobicity 340 11.7.7 Fourier Transform IR (FTIR) Spectroscopy 341 11.8 Summary/Conclusion 342 References 342 12

Surface Modification of Polymers for Food Science 347 Valentina Siracusa

12.1 12.2 12.2.1 12.2.1.1 12.2.1.2 12.2.2 12.2.2.1 12.2.2.2 12.2.3 12.2.4 12.3 12.4 12.5 12.5.1 12.5.1.1 12.5.1.2 12.5.1.3 12.5.2 12.6

Introduction 347 Physical and Chemical Methods 348 Gas Phase and Radiation 349 Gas Phase 349 Radiation 350 Liquid and Bulk Phase Methods 352 Adsorption Methods 352 Desorption Method 352 Interfacial Adhesion of Polymers 353 Grafting and Polymerization 354 Mechanical Method 354 Biological Method 354 Surface Modification of Polymer for Food Packaging Applications 355 Surface Sterilization 355 Printing 355 Mass Transfer 356 Polymers 356 Conclusion 358 References 359

355

xi

xii

Contents

13

Surface Modification of Water Purification Membranes 363 Anthony Szymczyk, Bart van der Bruggen, and Mathias Ulbricht

13.1 13.2 13.2.1 13.2.2 13.2.3 13.3 13.3.1 13.3.2 13.4 13.4.1 13.4.2 13.4.2.1 13.4.2.2 13.4.2.3 13.4.2.4 13.4.3 13.5

Introduction 363 Irradiation-Based Direct Polymer Modification 365 Plasma Treatment 365 UV Irradiation 366 Irradiation with High Energy Sources 368 Coatings 369 Coatings from Gas Phase 369 Coatings from Wet Phase 371 Grafting Methods 378 Grafting-to 378 Grafting-from 381 Plasma-Induced Graft Polymerization 381 UV-Induced Grafting 383 Grafting Induced by High Energy Radiations 385 Grafting Initiated by Chemical/Electrochemical Means 385 Controlled Grafting-from 389 Conclusion 392 References 394

14

Surface Modification of Polymer Substrates for Biomedical Applications 399 P. Slepiˇcka, N. Slepiˇcková Kasálková, Z. Kolská, and V. Švorˇcík

14.1 14.2 14.3 14.3.1 14.3.2 14.4

Introduction 399 Plasma Treatment 400 Laser Modification 411 Interaction with Cells 411 Sensor Construction 412 Conclusion 416 Acknowledgments 417 References 417 Index 427

xiii

Introduction Since the Hermann Staudinger’s proposal about the structure of large molecules published in 1920, during the course of his studies on the chemistry of rubber, and its Nobel Prize in 1953, polymers have undergone a fantastic development. At the present time they are present everywhere not only in our daily life but also in technological applications, for example, in automobile, avionics, and paints, and also in biomedical applications, for example, drug delivery system, biosensor devices, tissue engineering, cosmetics, etc. Nowadays, there are very few objects that do not include the presence of polymer(s). Research and industry have steadily increased the structures and properties of polymers order to adapt their properties to their uses. However, for a given polymer the chemical composition of the surface and therefore its properties are dictated by the chemical formula and structure of this polymer. In order to disconnect the properties of the surface from that of the bulk polymer, the surface must be modified. Modification or functionalization of polymers refers to the introduction of different chemical groups onto its surfaces without changing its bulk properties. That is, the polymer retains its mechanical properties but gains novel surface properties. For example, it is possible that the surface of textiles are changed from partly hydrophilic to hydrophobic, and it is also possible, in the biomedical field, to attach new chemical functions that permit the adhesion and development of cells. Other applications include filtration membranes, the delivery of drugs, and packaging processes. The surface of most polymers is quite unreactive, and activation by chemical or physical methods is required. This is the topic of this book. The editors and authors have tried to give an overall account of the very different and quite numerous methods that permit to modify and functionalize the surface of polymers. Chapter 1 gives a description of “The Surface of Polymers” and discusses the different internal and external factors such as surrounding environment and chemical nature of polymers that influence surface properties. The first part includes the different gas phase methods. Chapter 2 “Surface Treatment of Polymers by Plasma” defines the different kinds of plasmas, discusses their composition and the way they are produced. In a second part of the chapter, the numerous different applications of plasmas for modifying the surface of polymers are described: adhesion improvements, packaging and textile applications, biomedical applications, and plasma grafting.

xiv

Introduction

Chapter 3 “A Joint Mechanistic Description of Plasma Polymers Synthesized at Low and Atmospheric Pressure” focuses on the plasma polymerization technique that provides polymeric-like thin films on the surface of the polymeric substrate. The conditions that permit the formation of these plasma polymers are examined altogether with the characterization of the plasma chemistry. Chapter 4 is dedicated to “Organic Surface Functionalization by Initiated CVD (iCVD),” enabling the formation of polymers on the surfaces of the polymeric substrate. In this technique, vapor phase monomers are introduced along with initiators inside reactor chambers held at modest vacuum level and organic thin films deposit on a cooled surface. The different films that have been prepared are described including surfaces especially attractive for biomedical and sensing applications. Chapter 5 describes “Atomic Layer Deposition and Vapor Phase Infiltration” methods. Atomic layer deposition (ALD) is a form of chemical vapor processing, in which vapor phase precursors are delivered sequentially to a substrate. When chemistries are properly selected, these precursors undergo self-limited surface reactions that deposit a conformal coating onto a substrate. Vapor phase infiltration (VPI) is based on precursor sorption and diffusion into the polymeric material, embedding inorganic constituents into the subsurface of the polymer. Application of these techniques are numerous: improvement of mechanical performances and chemical resistance, creation of coated cotton fibers, contrasting agent vapor diffusion barriers, and hybrid photovoltaic cells. The second part is devoted to UV and Related Methods. Chapter 6 is devoted to the “Photoinduced Functionalization on Polymer Surfaces.” UV irradiation induces the functionalization of the surface of materials enabling a reaction between two polymers or to grow polymers from another one by self-initiated photoinduced graft polymerization. Among others, application of photoinduced grafting process to artificial organs is provided. Chapter 7 describes the “Surface Modification of Polymers,” by “𝛾-Rays and Ions Irradiation.” These methods permit to graft polymeric chains to the surface of the treated polymer according to different procedures and chemical reactions (e.g. grafting-from, grafting-to). Different applications are described. The third part examines the Chemical Methods that are used for the surface modification of polymers. The particularity of these reactions is the possibility of post-functionalization to provide complex chemical structures attached to the surface of the polymer. Chapter 8 is devoted to the “Functionalization of Polymers by Hydrolysis, Aminolysis, Reduction, Oxidation, and Some Related Reactions.” The conditions of these reactions on different polymers are examined altogether with some applications. Chapter 9 relates on the “Functionalization of Polymers by Reaction of Radicals, Nitrenes, and Carbenes.” These very reactive species are very efficient for the surface modification of polymers and can be applied to many polymers more or less irrespective of their chemical structure. The methods used to create these species are reported as well as some applications. Chapter 10 includes the “Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed Macromolecular Grafts.” It includes

Introduction

not only the different surface-confined radical photopolymerization of vinylic monomers and conjugated polymers but also surface-confined sonochemical polymerization of conjugated and vinylic monomers. The fourth part is devoted to the different possible applications of these surface modifications. Chapter 11 is dedicated to “Surface Modification of Nanoparticles: Methods and Applications.” It focusses on the use of polymeric carriers for which the diameter is in the order of ∼100 nm for drug delivery. It describes the different polymers that can be used as well as the methods employed to obtain nanoparticles and their use for drug delivery. Chapter 12 describes “Surface Modification of Polymers for Food Science.” These modifications are mostly performed by plasmas-based approach. The different polymers used in the food industry as well as their modifications are reported. Chapter 13 reports on the “Surface Modification of Water Purification Membranes.” These membranes are mainly polymeric and can be modified by the different methods that permit functionalization, coating, or grafting. Chapter 14 is dedicated to “Surface Modification of Polymer Substrates for Biomedical Applications.” These modifications can be performed by plasma or laser treatments, and the interaction of these surfaces with different cells is reported. Careful characterization of the functionalized surfaces is mandatory after the different reactions described in this book; the authors have included many different methods that permit to ascertain the success of the modification and the structure of the modified films, many examples are reported.

xv

1

1 The Surface of Polymers Rosica Mincheva and Jean-Marie Raquez University of Mons (UMons), Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), 20, Place du Parc, 7000 Mons, Belgium

1.1 Introduction Surface properties of any polymers have an imminent influence over key properties such as wetting, adhesion, friction, and biocompatibility, therefore affecting the applicability of a polymer material [1]. It is nowadays well accepted that surface properties differ from bulk in many aspects and a multitude of scientific works has been done for the last 70 years in an attempt to highlight what actually constitutes the surface, including the interphase, and how far into the material its surface goes [2–10]. Moreover, while classical surface model will consider a surface as rigid, immobile, and at equilibrium, which is more likely to be true for rigid solids, the surface of a polymer material is highly depending on time and temperature due to its viscoelastic behavior and is therefore thermodynamically and kinetically dependent [11]. From this viewpoint , the polymer surface can continuously restructure and reorient in response to different external factors such as atmosphere, solvent, and so on and might be inherently a nonequilibrium dynamic system. The guiding force for these structural changes is that the surface tends to decrease its free energy in a continuous way. In other terms, surface chemistry, reactivity, and aspect vary in function of environmental and processing conditions, influencing any desired modification and/or application of the related material even when bulk properties are considered [12]. In order to understand the application-related modification of a polymer surface, one should first learn what the polymer surface is actually, how its properties are generally influenced, and what analytical methods are the most appropriate to study and understand. This chapter aims at providing a summary of experimental and theoretical concepts describing polymer surfaces near interfaces. It discusses the role of the different factors such as the surrounding environment in the surface properties and shows the multitude of analytical tools under different situations involving surfaces and interfaces.

Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

2

1 The Surface of Polymers

1.2 The Surface of Polymers 1.2.1

Definition of a Polymer Surface

The word “surface” in its most general use includes the outermost or the uppermost layer/boundary of a physical object or space/area (http://www .wordreference.com/definition/surface). From the materials science viewpoint, the surface, defined as the frontier between two different media, is characterized by a certain thickness, reflecting a gradient of properties. With this respect, surface ever differs from the bulk of any material in terms of density, composition, or structure, and, even if it is present at very small fraction (by comparison to bulk), the surface governs any polymer properties, as being the first contact sets on. This statement remains true whatever the macroscopic material, including polycrystalline solids or polymers. However, for polymer surfaces, the molecular length scale goes well above the angstrom scale (e.g. a typical end-to-end distance is about 10−6 m for a polymer of 10 000 monomer units and considering the random-coil conformation [13]), and the term “small fraction” is broadly true. Herein, the connectivity, the entanglements and the interactions between polymer chains at the surface are built up for a surface thickness varying from several nanometers (for a layer in direct contact with other medium) up to several micrometers (for a crystalline morphology) [12]. Even though the interactions decrease upon increasing the distance, they remain the source of cohesion and determine the surface properties such as friction, adhesion, surface tension, and biological activity. Moreover, polymer chains have high degree of freedom (side-group [methyl, hydroxyl, carbonyl, etc.] C–C rotation, segmental α-process, and overall chain dynamics) and actual time and temperature-dependent local or long-range motions, making surfaces dynamic objects thereof [14] undergoing rearrangements upon changes into the surrounding phase(s): gas(es), liquid(s), or solid(s). Additionally, for a polymer macromolecule in the bulk, the interactions are similar in terms of type and force in all three directions, while for a macromolecule at the surface, they are unbalanced, leading to an excess of surface/interface free energy [15]. All these characteristics create a thermodynamic force (configurational entropy) – the guiding force determining an equilibrium state of minimal free energy or of maximum entropy by transferring end-groups, functional groups, or additives to the surface, which on the other hand causes segregation of polymer chains and/or their parts [5, 12]. The phenomena are known since the 70th of the twentieth century [16] and are emphasized even today [17]. Examples can be found for gels (presenting low or high contact angle in contact with water or air, respectively [18]), grafted polymers (where the grafted chains are found to be hidden in the bulk or exposed on the surface depending on the treatment conditions [19]), or even segmented polymers [16]. Consequently, a polymer surface is a dynamic surface having temperature and environmental responses – a place where phenomena provoking major evolutions influence the polymer properties and lifetime (Figure 1.1) [12, 20].

1.2 The Surface of Polymers Solvents s

le cu

ole

ed

m

er

lym

Po

rg

a Ch

Factors determining a polymer surface

s

Molar ma ss an d

a on ositi mp co

Gase

Stru ctu re

d Soli

s

s

Chemistry

nd

External

y rsit pe dis

Internal Amorphous

Crystalline

Non-wetting Wettability

Crystallinity

λ Hard Soft

λ Brilliance

Hardness

Figure 1.1 Schematic representation of factors determining a polymer surface and its related properties.

1.2.2

Factors Determining a Polymer Surface

The previously driven consideration suggests that the polymer surface will be determined by a multitude of factors within complex relationship without a sharp discrepancy between them. A very general classification, however, can be done based on factors’ origin: (i) internal – related to the polymer itself and (ii) external – environment related. 1.2.2.1

Internal Factors

Among the internal factors, the polymer chemistry, composition and structure, and molar mass and dispersity can be listed: Polymer Chemistry It is generally considered that aliphatic C—C or C—O bonds with non-bulky substituents are quite mobile and flexible, which make them able to adopt any infinite number of configurations (in the ideal case) with quasi-equal energy and to have a maximum entropy at thermodynamic equilibrium. In this case, the substituents will be exposed to the surface or not depending on the environment as shown by Cimatu et al. [21] for substituted (in terms of ethyl/methyl groups) polymethacrylates with hydroxyl, chloro, or phenoxy moieties. On the other hand, cyclic aliphatic or aromatic structures, branches, or cross-linking points, as bulky substituents have a marked stiffening effect that forces polymer chains to adopt a certain configuration that will reduce system entropy and increase free energy [22]. Bulky substituents will therefore be segregated at the surface [23], as illustrated by the studies of Hirai et al. [24] on polymethacrylates with “side crystalline” chains. Restrictive chain mobility and conformation are also related with the presence of functionalities allowing attractive hydrogen, dipole, or electrostatic interactions. Such functionalities

3

4

1 The Surface of Polymers

will force adopting a certain conformation out of thermodynamic equilibrium. In other words, chemistry determines mobility and the properties of a dynamic polymer surface such as surface topography (or roughness) from atomic to macroscopic scale and surface morphology in terms of crystallinity and crystal structure [12, 13, 25]. Together, polymer chemistry, mobility, and conformation will influence the surface chemistry and thus the surface topography and wetting. Therefore, they all will play on surface mechanical properties, adhesion, friction, etc. Polymer Composition and Structure Additives in polymers, especially of low molar

mass (e.g. plasticizers), are often excluded from polymer bulk and migrate to the surface, changing its properties and composition [2]. Moreover, the composition will change with thickness and form a gradient. This is similar in the case of immiscible polymer blends, where the blend component with lower surface free energy will migrate to the polymer surface in order to reduce the total free energy of the system and place it in a thermodynamic equilibrium. Surface excess and concentration gradient can be calculated based on the mean field arguments [26, 27]. A similar effect is observed with block copolymers or comb-type copolymers where one sequence enriches the surface depending on miscibility and composition. In some particular cases, such segregation may even lead to the formation of lamellar structure normal to the surface [2]. Molar Mass and Dispersity Other important factors influencing polymer surfaces

are polymer molar mass and dispersity [22]. A low molar mass polymer or a polymer with large dispersity, for example, is expected to present a greater number of chain ends at the polymer surface, when comparing with a high molar mass polymer or a polymer of lower dispersity. As chain ends are less restrained, they provide the polymer surface with greater mobility at low temperature, yielding it to expand. Greater mobility, as discussed earlier, will decrease surface free energy and affect topography and morphology. Higher molar mass and lower dispersity, on the other hand, will cause entanglements and even some crystallization at the polymer surface. These will reduce chain mobility and increase surface free energy, causing irreversible topography and morphology changes. 1.2.2.2

External Factors

Understanding the interactions of polymer surfaces with external factors is important for selecting the right application. A multitude of examples can be found in areas of wetting and dewetting, crystallization, or “smart” materials for optoelectronics, as summarized in a very recent review [6]. Here again, the surface properties are thermodynamically driven in such a way to decrease the energy between different interacting components. This thermodynamically driven decrease in surface free energy is directly related to the dispersion forces, providing principal contribution to molecular interactions across polymer surfaces and allowing adsorption of substances from the environment (gas, liquid, or solid low- or high-molar mass molecules) to the polymer surface [12, 28, 29].

1.2 The Surface of Polymers

As shown by several reviews and experimental papers, these substances can also penetrate the surface and cause swelling or plasticizing at the near-surface regions [6, 8, 12, 29–31]. Gas Molecules Experiments on the physical adsorption of gases are one source of

information about dispersion forces at interfaces [28]. They date back to the late 50–60th of the twentieth century and usually show a fast way to reach the adsorption equilibrium without any hysteresis, suggesting a lack of gas molecules penetration into the near-polymer surface range for energetically homogenic polymer surfaces [32]. Gas molecules penetration into the near-polymer surface range was found highly heterogeneous in terms of energetics polymer surfaces [29, 33], and it evidenced the contribution of the dispersion forces to the surface free energy. In these cases, a first gas monolayer is formed over the high energy portion of the adsorbent, followed by a second monolayer formation over the polymer surface [29]. More recent studies have shown that adsorption of gas molecules expanses polymer surfaces and causes plasticization of the near surface regions [12]. Charged Molecules The role of the dispersion forces should also be considered in

the specific adsorption of ionic molecules onto polymer surfaces as it influences phenomena as conformation and solubility of electrolytes, flocculation, micelle formation, etc. [34]. For example, charged molecules such as salts and ionic surfactants, can form charged polymer surfaces from polymers that lack surface potential [8, 35]. The surface potential will be different from zero whether ions of the same sign are preferably adsorbed or equal to zero in the case of charge nonspecific adsorption. This is due to the formation of an electrical double layer at the interface with a polymer surface of zero “native” surface potential as shown by the studies of Jacobasch et al. [35] on technical poly(ether ether ketone) (PEEK, Victrex 450G, Victrex GmbH, Germany) and a fluorocarbon polymer (provided by the Institute for Applied Polymer Research, Teltow, Germany). Indeed, these two polymers do have a zero “native” surface potential in aqueous solutions, because they do not contain dissociable surface groups. However, direct force and zeta potential experiments show a negative electrostatic surface potential due to an excess adsorption of anions from the electrolyte solution. Therefore, an electrical double layer is formed next to the solid polymer surface [35]. Such formation of an electrical double layer can be explained by considering the contribution of dispersion interactions acting on ions in the theory of colloid science. Specific ion adsorption due to dispersion interactions can be dominant as well as in the case of charged interfaces at high salt concentrations. The effect is shown to be at the same level of approximation as, and precisely equivalent to, the Onsager limiting law for the interfacial tension change related with dissolved salt at a single interface, i.e. to linearization of the Poisson–Boltzmann distribution, and restriction to electrostatic potentials as the sole determinant of adsorption excesses [8, 35]. Solvents Contact with solvents strongly influences polymer surface formation and restructuring, in terms of segregation, composition, and morphology [6, 31]. The comprehension of the mechanisms again lies on a consideration of

5

6

1 The Surface of Polymers

thermodynamics. Polymers in contact with solvents tend to change conformation and configuration in order to reduce surface energy through the formation of temporary and favorable polymer–solvent interactions [6]. This ability of polymer surfaces to change their performance (actuate and interact) with the environment without requiring an external intervention conveys them as a form of a “smart” behavior that attracts a lot of attention. The first studies in this aspect were based on conformational changes of polymer brushes induced by the environment [36]. Furthermore, they were translated to the migration of substituents and related to surface entropy and free energy [21, 22]. Hydrogen bonding, van der Waals interactions, or electrostatics were found to be responsible for solvent–polymer interactions. Complexity arises if solvent also lowers its free energy in contact with polymers. Therefore, solvent–polymer, solvent–solvent, and polymer–polymer interactions must be considered to understand how solvent influences polymer surface. Solvent effects on polymer surfaces have been pioneered by Thomas and O’Malley [37]. The authors investigated the role of solvent–polymer interactions onto the surface chemical composition of thin poly(styrene)-b-poly(ethylene oxide) films. Results with different solvents showed that polymer surface enriches in the component allowing lowering the surface energy depending on solvent polarity, as expected from thermodynamics and energetics [37]. Additionally, and as expected from solubility considerations, a good solvent yielded less surface segregation of the soluble component, while a mutual solvent resulted in most pronounced composition-depth gradient. In other words, good solvent brings higher mobility of the polymer chains and reduces surface segregation due to preferential solubility and higher mobility [37]. On the opposite, a bad solvent contracts the polymer chains, restricts their motion, and favors surface segregation. The picture becomes more complicated by introducing solvent–solvent to solvent–polymer interactions. For mixed solvents with preferential solubility to both components, highly segregated surfaces have to be obtained [38]. In this case, the effect of solvent–solvent interactions on polymer surface segregation was found predictable based on the Hildebrand solvent parameter and was considered relevant to solvent volatility [31]. Indeed, high solvent volatility decreases segregation by allowing less time for the process to occur. Concerning polymer blends (polymer–polymer interactions) and the Hildebrand and Hansen solubility parameters, hydrogen (or electrostatic) bonding might be concluded to influence the phase behavior of polymer blends [39]. For miscibility based on hydrogen bonding polymers blends, a solvent of strong hydrogen bonding ability will reduce miscibility and induce segregation of polymer chains, based on competitive phenomena – solvation phenomenon (according to Horn [40]). This will be different from the behavior of immiscible polymer blends, where the surface organization is dominated by the equilibrium thermodynamics [31]. As already discussed, equilibrium thermodynamics favors complete phase separation and surface segregation of the component with lower surface energy [5, 12].

1.2 The Surface of Polymers

Lowering surface free energy might have undesired effect on polymer surface stability and gives rise to solvent absorption into the polymer and/or the dissolution of polymer into the solvent. These considerations have been investigated by Ruckenstein and Gourisankar [30], by a simple contact angle procedure for the case of polymeric surface water interface. The experiments were performed on three types of polymeric surfaces presenting high (Teflon FEP), medium (sputtered Teflon), and low (etched-sputtered Teflon) interfacial free energies with water as a result of surface restructuring in aqueous environment. The results from contact angle and dynamics of solid surface restructuring investigations under water evidenced the instability of polymer surface in the case of very small water–polymer interfacial free energy (the etched-sputtered solid surface), allowing penetration of water molecules from the surface to the bulk of the polymer material and possible dissolution of the solid in water. These contact angle results were well supported by the physicochemical characteristics of the different polymer surfaces used in this study [30]. The experimental works have been supported by a growing body of theoretical studies with the interest in switchable wettability and adhesion, mainly for polymer brushes in different geometries [41–46]. Polymers Another very intriguing (in view of applications) subject concerns the

changes of a polymer surface when placed in contact with another polymer surface. Indeed, the idea of using polymer blends in order to broaden material applications is quite natural but turns out to be quite difficult. The reason for this comes from the competition between entropy and enthalpy, often guiding a phase separation of the pure polymers. The studies here lie on the Flory–Huggins theory of mixing and consider four potential sources of errors coming from the following assumptions: • The long-range chain statistics of polymer molecules are ideal random coils, while in reality they might collapse in contact with another polymer [47]. • Large composition fluctuation should be neglected, although in the case of diluted regime or of low molar mass polymers, this mean-field theory faces a failure [48]. In the case of the dilute regime, the failure will be determined by the chain connectivity, while for low molar mass polymers, it will be induced by the absence of screening effects [4]. • There is no volume change (extra space creation) when two polymers are mixed. However, if two polymers are facing strong unfavorable interactions, such as repulsive forces, the system will tend to slightly lower density, reduce the number of possible interaction points, and gain some extra translational entropy in order to reduce energy. These all will create “vacancies” or extra space [4]. • There is now influence of the local structure and packing. Yet, it was already shown that bulky side groups or local structure of monomer units restrict the number of available conformations and thus provoke changes in entropy of mixing [4, 21, 23]. A large number of studies have gone into achieving universal theory, but with no success [4, 10, 49–51], and despite all its shortcomings, the Flory–Huggins

7

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1 The Surface of Polymers

theory (Eq. (1.1)) remains up to date the universal framework for considering polymer blends. 𝜒 = z(2𝜀AB − 𝜀AA − 𝜀BB )∕(kT)

(1.1)

where 𝜒 is the Flory–Huggins free energy of mixing, z – the z spatial dimension, 𝜀AB , 𝜀AA , and 𝜀BB – the energies associated to the interactions between neighboring segments: an A next to a B, an A next to an A, and B next to a B, respectively; k – the Boltzmann’s constant (1.380 649 × 10−23 J/K), and T – temperature (K). According to this theory, polymer blends might be divided into three general classes: • Miscible polymer blends. Occurring only in few cases, where the energy difference (𝜒, Flory–Huggins free energy of mixing) is negative, so intimate mixing between the components can be obtained [4]. Moreover, 𝜒 will be negative only for a certain temperature range, as shown for the case of deuterated poly(styrene)/hydrogenated poly(vinyl methyl ether) blends [52], and only within this temperature range, both polymers will be miscible for all molecular weights. • Similar polymer pairs. Nearly identical in structure (e.g. differing only in isotropic substitution), where very small positive values of 𝜒 can be obtained. The classical example in this case is blends made of hydrogenated and deuterated components of equivalent structure [53]. Although the technological interest of such blends is irrelevant, they remain important for purely scientific studies of miscibility. • Immiscible polymer blends. Comprising almost all other polymer blends lacking chemical similarity or specific interactions, where 𝜒 is taking a positive sign in the range 0.001–0.1 and both components will be miscible only for lowest molar masses [4]. Following the discussion, one may conclude that the important length scale for polymer–polymer interfaces is the overall size of the polymer chain, which broadens polymer/polymer interfaces compared with other liquids. Indeed, the width of polymer/polymer interface depends on polymer miscibility, and for miscible polymers the interface will broaden with time by the process of interdiffusion of polymer chains and result into a single phase at equilibrium [4]. This interdiffusion will be slow due to chain entanglements but will control the strength of the final material at equilibrium. On the opposite, immiscible polymer blends will tend to demix with time and will form coarsen domains at equilibrium (Figure 1.2). The predominant feature here being segregation with the lowest free energy component situated at the near-interface polymer surface [4]. Although, the knowledge on how polymer molecules orient and interact with the molecules of another polymer at the interface, the subject continuously evokes interest as shown in the significant number of scientific papers, reviews, book chapters, and books on polymer blends in the very recent years [55–73]. Solid Surfaces As was shown earlier, the composition of a polymer surface in contact with a small molecule or with another polymer significantly differs from bulk. The same applies to the polymer surface in contact with a solid and is of

1.2 The Surface of Polymers

3.5 (d) (c) 3.0

Log domain area (pixels)

(b)

2.5

(a) 2.0

1.5

= kt 2n

1.0

0.5 –1.0

0.0

1.0 2.0 Log time (min)

3.0

4.0

Figure 1.2 Domain coarsening. Temporal domain area for three runs at a defined composition and temperature. “k” – rate of growth of the mean radius, “n” – the exponent in the growth low and “t” – growth time. Solid lines represent linear fits and (a) – (d) are optimal parameters obtained from Gaussian fits. Inset: snapshots of domain patterns at different stages of coarsening (from the top plot); the black panels correspond to 150 μm (left) and to 240 μm (right). Source: Seul et al. 1994 [54]. Reused with permission from Scientific Publishing and Remittance Integration Services.

importance in a number of problems of practical interest as polymer processing, lubrication, permeation, separation, and adsorption processes [74–77]. A very practical example can be found in colloid and surface science, where the polymer surface will respond to a solid colloid particle by either preventing or enhancing its aggregation, depending on polymer-induced phenomena, known as adsorption or depletion [77]. Basically, both these terms refer again to the surface segregation or enrichment in the species of low surface energy at contact. As already discussed for other factors, large degrees of adsorption or segregation can be obtained even when the interaction driving the segregation is small; offset against any gain in energy obtained on adsorption is a loss of translational entropy of the molecule in the bulk, but for a large macromolecule this entropy is rather small. The length scale characterizing the distance from the surface over which the composition is perturbed is generally much larger than that for

9

1 The Surface of Polymers

small molecules, either liquid or solid, because this length scale is itself set by the size of the molecules themselves [4]. The difficulty to understand the behavior of polymer chains at solid interfaces comes from the fact that there are inaccessible (buried). According to the literature, the polymer/solid interface structure is determined by the molecular interactions at such buried interfaces [78–82]. Therefore, elucidation of interfacial structures at the buried polymer/solid interface leads to the detailed understanding of the buried interfacial interactions, which can be used to rationally design interfacial structures with improved properties (e.g. adhesion). According to the chemistry of the solid, polymer/solid interfaces might be divided into three main groups: polymer/metal, polymer/metal oxide and polymer/polymer interfaces. The later has been shortly discussed earlier, so this section will focus on the behavior of polymer chains in contact with metal and metal oxide surfaces. • Polymer/metal interfaces have been studied [78] due to their importance in microelectronics or anticorrosion coating applications, with one of the largely investigated model systems being the poly(methyl methacrylate) (PMMA)/silver (Ag) interface. Experiments have shown that at this buried interface, in addition to the surface dominating ester methyl groups of PMMA, the methylene groups and the alpha methyl groups are also present. Orientation analysis indicated that the side ester methyl groups at the PMMA/Ag interface tilted toward the polymer/metal interface [83]. Using a self-assembled monolayer of methyl 3-mercaptopropionate (MMP) on Ag as a control, the absolute orientation of ester methyl groups of PMMA at the buried PMMA/Ag interface was deduced to be tilting away from the Ag surface (Figure 1.3) [84]. Last experiments studied the interfacial molecular structures of an adhesion promoter, polybutadiene modified epoxy (PBME) rubber or polystyrene (PS), with gold (Au) [85, 86]. Buried surfaces of perfluorosulfonated ionomers (such as Nafion), used in fuel cells and electrodes (such as Pt) have also been investigated [87]. The obtained results suggested that Nafion molecules interact with the Pt electrode surface via side-chain sulfonate terminals. Besides the evolution of polymer chemical functionality in contact with metal surfaces, the conformation evolution of the macromolecules was also investigated [80]. For PS chains at the surface of a spin-coated film in a temperature-ramping mode as well as under isothermal annealing, the conformation of the surface chains was IR

Sum

Air

CH

3

Visible

O—

10

CH 3

O—

PMMA Silver

Figure 1.3 Molecular structure of the buried PMMA/Ag interface studied by Sum frequency generation (SFG) vibrational spectroscopy showing side ester methyl groups tilting toward the polymer–metal interface. Source: Adapted with permission from Lu et al. [83]. Copyright 2018, American Chemical Society.

1.2 The Surface of Polymers

found to be in a nonequilibrium state. Based on these studies, the relaxation of surface nonequilibrium chains was stated to be induced by the enhanced surface mobility, while the whole chain motion (such as reptation) was suggested to be a key factor in determining the time scale for equilibrating the surface chain conformation [80]. • The importance of polymer/metal oxide interfaces can be found in the use of metal oxides (such as SiO2 ) as (nano)fillers in polymer (nano)composite materials to enhance the mechanical strength of the final material. The multitude of experimental and theoretical (modeling) studies have pointed out the importance of polymer/filler interactions (such as hydrogen bonding or CH–𝜋) and the preparation method (spin- or solvent-casting) onto the chemical and conformational structure of the polymer chains ate the interface [78, 88, 89]. More detailed information on the conformation of the polymer chains near interfaces can be found in the late literature, summarized in the book of Jones et al. [4] 1.2.3

The Polymer Surface at a Microscopic Level

An elegant physical argument about the effect of a surface on polymer configurations was made by Silberberg in 1982, who argued the conformational modifications by considering an imaginary plane cutting a polymer melt in an exact mirror image [90]. In particular, each configuration that crosses the plane has an image configuration crossing in the other direction. Moreover, if cutting each chain that crosses the plane and joining up the free end with the cut end of the mirror conformation, there remain no bonds crossing the plane, but the same number of bonds are still present. From this, these chains whose centers of gravity lie less than about two radii of gyration from the surface will have their configurations perturbed; in particular the radius of gyration perpendicular to the interface will tend to be reduced, with rather less change occurring in the directions in the plane of the surface [90]. This picture of chains near the surface adopting rather flattened configurations has been confirmed by lattice Monte Carlo study in computer simulations, considering the position of the chains, the chain-monomer profile (as the average number of monomers in a layer a given distance z from the surface that belong to the same chain), and the location of the chain ends [91]. Simulations herein confirmed the hypothesis (based on physical grounds) that a polymer chain at or near an interface suffers from a smaller loss of entropy (by virtue of the surface) than does a chain with a middle segment at the surface, so that the density of chain ends at the surface should be somewhat enhanced. The knowledge over polymer interactions and chain conformation in contact with surfaces is extremely important in the construction of materials for healthcare applications [92]. Studies in this field consider mainly protein orientation (in terms of chemical functions and chain conformation) at the contact with other surfaces. Despite any difference, they might be translated to any other polymer chain and shed much light on the subject of what a polymer surface is.

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1 The Surface of Polymers

Denatured

Albumin Fibrinogen

Native

(A) (a)

(b)

(B)

Figure 1.4 Schematic representation demonstrating control of conformation and orientation by surface curvature (A) (Source: Reproduced with permission from Giamblanco et al. [97]. Copyright 2018, American Chemical Society) and (B) laminin adsorbed on a GaP nanowire substrate (Source: Reproduced with permission from Fortes [99]. Copyright 1983, Springer Nature). Confocal three-dimensional stack image of a 143 × 143 μm2 area with vertical 90 μm diameter, 3.2 μm long GaP nanowires (a). Single plane image (7.3 nm optical slice) of the same sample (b). Scale bar 3 nm [99].

Based on a considerable body of experimental and theoretical work, the orientation of a protein molecule on the surface can be characterized as “side on” or “end-on” depending on the axis (long or short, respectively) interacting with the surface [93–98]. Such interactions have been found to strongly influence the protein secondary structure. Another important factor is the surface curvature. High surface curvature (as in the case of carbon nanotubes or silica nanoparticles) promotes globular structure [97], while flat surfaces (such as gallium phosphate) is more suitable for rod-like macromolecules (Figure 1.4) [99].

1.3 Properties of Polymer Surfaces at Interfaces The dynamic nature of the structure and morphology of polymer surfaces near interfaces influences surface properties and fascinates scientists since more than half a century. Among all, surface wettability, density, adhesion, thermal

1.3 Properties of Polymer Surfaces at Interfaces

(surface glass-transition temperature and crystallinity), optical (brilliance), and mechanical (surface modulus, flexibility) properties are of primary interest in terms of applications. 1.3.1

Surface Wettability

Historically speaking, the first surface property to be studied is the surface wettability, defined as the ability of a material to remain solvated in the presence of other molecules. The more common situation in wetting involves a solid (s), interacting with a liquid (l), at the environment of a second immiscible fluid (v) [100], each of them characterized by its interfacial tension under equilibrium conditions at a certain temperature (𝛾 sv , 𝛾 sl , and 𝛾 lv , respectively). From a physical point of view, the phenomenon is explained by the cohesive forces holding liquid molecules together determining surface tension. As polymer macromolecules at the surface are also held together by the action of cohesive forces, they undoubtedly exhibit surface tension. Detailed consideration and understanding of this phenomenon with polymers are present in the books of Fortes [99] and Jones et al. [4] and recently reviewed by Hall and Geoghegan [6], and therefore this part of the chapter will only aim on providing a picture of the influence over the internal and external factors discussed earlier on surface wettability in terms of surface tension. Surface tension is calculated from contact angle (𝜃, ∘ ) measurements and compared to water. With this respect surfaces are divided into hydrophilic (where the interfacial energy at the water–polymer surface contact is below the free surface energy of the polymer) and hydrophobic (where the polymer surface is of lower free energy compared with the interfacial energy at the water–polymer interface). If 𝜃 is constant with time and the surface is in equilibrium, it will be expressed by the Young’s equation (1.2) ) ( 𝛾sv − 𝛾sl (1.2) 𝜃 = arccos 𝛾lv where the surface free energies are assumed to have the same value at all points in each interface [100, 101]. This equation is only true for the case of ideal surface (unreactive and chemically homogeneous, insoluble, completely flat, rigid, and static) [6]. From the already exposed definition, however, polymer surfaces are dynamically changing objects, responsive to internal and external stimuli. This dynamic nature creates instability in the contact angles, described by a contact angle hysteresis – a change in a droplet shape and volume provoked by spreading and/or adsorption. In practice, the dynamic phenomenon of contact angle is limited between the advancing (maximal) contact angle (𝜃 adv ) and the receding (minimal) contact angle (𝜃 rec ), the first associated to wetting and the second to dewetting of a surface [102]. The picture might be visualized according to Figure 1.5. It is then natural to consider that internal factors such as polymer chemistry (presence of reactive/functional groups along the chains, lateral, or chain ends), composition (presence of additives), and dispersity (presence of low molar mass chains) will influence surface reactivity, chemical homogeneity, and

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1 The Surface of Polymers

Figure 1.5 Schematic representation of the wetting and dewetting of a surface (a) and according to the contact angle value (b). “𝜃” - contact angle.

θ rec

θ adv Wetting

Dewetting

(a)

θ:

90° Non-wetting

>150° Dewetting

(b)

(a)

θ* α1

1

θ

θ

2

2

α2

Figure 1.6 Schematic representation of (a) the “beer bottle cap” concept and (b) of a profile of rough surface showing the expected critical positions of the contact line for the advancing (1) and receiving (2) modes. The line of contact jumps (arrows) between the positions indicated as the volume of the drop is made to change. The horizontal line represents the mean surface. Source: Fortes et al. 1983 [99]. Adapted with permission of Springer Nature.

1 (b)

solubility [100]. External factors such as gases, solvents, polymer, and solids will change the hydrophilic/hydrophobic balance due to the thermodynamically driven reorganization of the functional groups exposed at a polymer surface [6, 31]. This creates a multiplicity of equilibrium configurations of polymer chains near the surface and consequently modifies surface wettability. This multiplicity of equilibrium configurations is expected to give rise to a waved contact lines between polymer surface and a liquid drop and to a liquid interface with convolutions near the polymer surface as described by the “beer bottle cap” concept (Figure 1.6a) [100]. Reconstructuring due to conformational changes (chain motions as displacement and reptation) will induce local variations in surface rugosity and rigidity that will create a droplet spreading and/or adsorption. It is obvious that modifications in surface roughness also induces the previously discussed multiplicity of equilibrium configurations for polymer chains near the surface and thus influences surface wettability. The effect can

1.3 Properties of Polymer Surfaces at Interfaces

be explained with Figure 1.6b, where the observed contact angle, measured in relation to the mean surface (𝜃*) is actually the sum of the real 𝜃 and 𝛼 1 at point 1 or the difference between 𝜃 and 𝛼 2 at point 2 [100]. Practically, surface wetting is an indicator of surface fouling and adsorption for example in protein fouling. Here, two cases can be observed: – In the case of hydrophilic polymer surfaces, the strong surface wettability creates a high-density layer of adsorbed water molecules that shields the polymer surface and prevents it from interactions with proteins [92]. In this case, surfaces are characterized with limited (if any) protein fouling as very limited (if any) polymer–protein interactions are possible and proteins preserve their secondary structure. – On the opposite, proteins undergo partial unfolding and spreading onto hydrophobic surfaces, where polymer–protein interactions are highly possible. Decreasing fouling of such hydrophobic surfaces is then possible with introducing specific nanocharges, influencing the morphology and the rugosity of the polymer surface at the nano-level [17, 103]. Similar results are obtained with polymer surfaces when wetting with nonaqueous solvents. Thus, the surface reorganization upon wetting is an extremely important factor to be taken under consideration when choosing the right application of a polymer material.

1.3.2

Surface Thermal Properties

If it is intuitive to think that polymer chains near the surface would have higher degree of freedom, and thus higher mobility in comparison to bulk ones [12], it is also intuitive to consider that the thermal properties – glass (T g ) and melting (T m ) transition temperatures and crystallinity – will also differ from bulk. These hypotheses are confirmed by experimental works showing the importance of the environment (external factors) on chain mobility and thus on chain orientation and rearrangement [8]. Experiments also revealed gradient evolution of polymer thermal properties from surface to a bulk [104]. In general, there are three factors to be considered to determine mobility and consequently thermal properties of polymers: chain flexibility, interchain interactions, and polymer regularity [22], all of them determined by internal (polymer chemistry, composition, and molar mass) or external (contact with gas, solvent, or solid) factors. The way that they influence bulk transition temperatures and crystallinity was recently described by Gilbert [22] and can easily be transferred to the polymer chains near the surface. 1.3.2.1

Surface T g

For a polymer surface below the glass transition temperature (T g ), where molecular rotation about a single (C—C) bond becomes restricted and where the backbone dynamic is completely frozen, only small side-groups (methyl, hydroxyl, carbonyl or phenyl substituents) can move [105]. It is then clear how polymer chemistry, composition, and molar mass will influence T g .

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1 The Surface of Polymers

• In terms of polymer chemistry, the presence of more bulky substituents will restrict local motions from rotation to torsional processes, flips, or only oscillations and thus increase surface T g . The presence of polar or hydrogen bonding groups will also tend to increase T g , as they might create bonding between adjacent groups and thus increase the energy required for side-groups movement. • In terms of composition, branching with long side-chains or cross-linking will cause entanglements and again rise the T g . • A low molar mass polymer will have higher amount of chain ends tending to migrate to the surface and increase its mobility, which reduces T g . The relation between T g and the number average molar mass of (Mn ) a polymer is given by the Flory–Fox equation (1.3): ) ( K ∞ Tg = Tg − (1.3) Mn where Tg∞ is the glass transition temperature of a sample containing polymer chains of infinite molar mass and K is a positive constant [106]. Above T g , segmental relaxation (α-relaxation) process, based on cooperative motion of chain segments, completes side-group dynamics. Here, the rotation of one part of the macromolecule about another one (over a C—C or C—O bond) will also be impeded by the presence of more bulky groups (hydrocarbon groups, phenyl groups, etc.) along the polymer chain backbone and T g will increase again [22]. The presence of unsaturations as double (C=C) or triple bonds (C≡C) will stiffen the chain at the point of inclusion but might increase the flexibility of adjacent bonds, thus reducing the overall glass transition temperature. If polymer composition is concerned, random copolymers will have a T g between those of the corresponding copolymers according to the Fox equation (1.4): ∑ 𝜔i 1 ≈ (1.4) mix Tg,i Tg i where Tgmix and T g,i are the glass transition temperature of the mixture and of the component i and 𝜔i is the mass fraction of component i [107]. It must be pointed out that since random copolymerization tends to promote disorder and reduce molecular packing and interchain interactions, the T g of random copolymers is often lower (although sometimes also higher) than predicted by the linear relationship. In the case of block copolymers, two types of effect have been observed depending on blocks miscibility/compatibility. For incompatible sequences, a transition corresponding to each block is observable, while for compatible blocks a single transition is observed. This last one is also usually close as predicted by the linear relationship [22]. Both below and above bulk T g , external factors as the presence of gases, liquids, or solids will generally cause chain separation and increase mobility [22, 105]. This effect is known as plasticization and results in a shift of the surface T g toward lower temperatures with respect to bulk T g . As an example, the system poly(methyl phenyl siloxane) (PMPS)/organophilized silicate might be cited [105]. With this system, a very fast process was found to dominate the

1.3 Properties of Polymer Surfaces at Interfaces

dielectric spectra with a very weak, almost Arrhenius temperature dependence. Additionally, at high temperatures its dynamics appeared to overlay the dynamics of the bulk PMPS, whereas near and even below the bulk T g , it was almost 6 orders of magnitude faster than the bulk segmental process. The attempt to analyze the relaxation times of this process resulted in a Vogel temperature T 0 of about 93 ± 2 K, i.e. almost 100 K lower than that in the bulk. The fragility parameter, D = B/T 0 also increases about five times for the confined chains with comparison to the PMPS, indicating that the confined PMPS behaves as a stronger glass (more Arrhenius-like), i.e. confinement influences a conformational rearrangement. At high temperatures, a slow process was evident with a weak dielectric strength and its relaxation times coinciding with the ones of the bulk polymer; likely, this was due to the segmental motion of PMPS, which occurred outside the galleries of the nanocomposites. This is because the relaxation times of the polymer chains at the surface are much shorter than those of the bulk and follow almost Arrhenius temperature dependence [105]. Alternatively, the enhanced dynamics of surface polymer chains might be explained by an enhanced monomeric mobility due to preferential parallel organization of the polymer chains in contact with another surface that increases supercooling [108]. 1.3.2.2

Surface Crystallization

In the case of semicrystalline polymers, their crystallization starts with the formation of lamellae [109, 110], due to chain folding effects. The lamellae grow at different rates (with the fastest rate dominating the crystallization) until spherulites are formed. Given this, crystallization is considered as a kinetic phenomenon, although some considerations of a spinodal process also exist [111]. Here again, among the internal factors, polymer chemistry plays an important role in crystallization as it determines chain regularity [22]. For bulk polymers, it is generally considered that regular polymers (with long linear chains) will be able to crystallize, while branched or cross-linked polymers will form amorphous structures. This somehow implies that polymer chains near the surface, where the concentration of branches and chain ends is much higher the bulk, impeding crystallinity. However, the higher mobility of polymer chains in the presence of branches and chain ends facilitates rearrangement and would eventually improve crystallization. Moreover, long branches can bring chain entanglements and even crystallization. Due to all these differences, the surface formed by polymer crystals might have different perfection (size and shape) and therefore, different melting/crystallization temperatures. If not the most important, the most discussed external factor influencing the crystallization of polymer chains at surfaces is the solid surface related with nucleation [6, 22]. The role of a solid surface in polymer crystallization (epitaxy) is important, particularly because it is accepted that polymer crystallization proceeds by nucleation. In this case, the solid surface takes the role of a nucleating site for crystallization. This would mean that crystallization proceeds at a higher temperature on cooling or at a lower temperature on heating (if cold crystallization is involved) [6]. These hypotheses are confirmed by experimental

17

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1 The Surface of Polymers

results, mostly including poly(ethylene terephthalate) (PET) [112–114]. Results have shown that PET at a solid surface will start its surface crystallization at a temperature close to its glass transition temperature, i.e. below the bulk T c occurs. Because of this faster crystallization at surfaces, the process is considered to be confinement controlled, what basically means that the thickness of the sample in contact with a solid surface will be another important factor in crystallization of surface polymer chains [114]. In any case, considering the overall thermal properties of a polymer surface is of crucial importance as it imminently influences surface optical and mechanical properties. Surface Optical Properties Among the polymer material optical properties, the

transparence and the brilliance are the most often sought in a multitude of industrial application areas such as painting, automobile, etc. Between them, the brilliance is the surface property that scientists and industry often focused on. The brilliance translates the ability of a surface to reflect light, or its reflectance (R). This last one being defined as the ratio between the amount of reflected (Φre ) and incident (Φie ) light in a gas or fluid. Ideally, in air and on a plain surface R will be only dependent on the incident light angle and the refractive index of the material, which is determined by internal factors as surface chemistry (for polymer surfaces, surface chemistry). However, reaching this ideal case of plain surfaces is not always easy, and for the most common rough surfaces, the light is not only reflected but also diffused in the environment just above the surface (Eq. (1.5)): 4π𝜎 cos 𝜃 𝜆

R = R0

(1.5)

where 𝜎 is the surface root mean squared roughness, 𝜃 is the incident light angle, and 𝜆 is the incident light wavenumber [115]. Surface Mechanical Properties Last, but not least, the mechanical properties of a

polymer surface raise significant interest as they represent a direct measurement of the surface resistivity toward mechanical aggressions as abrasion. As surface thermal properties (T g and crystallinity) were found to differ from bulk with a gradient evolution from the surface to the polymer bulk [8, 104, 105], the surface mechanical properties are also suggested to differ from these of the bulk [116]. Indeed, experimental studies have shown that surface dynamic storage modulus (E′ ) and surface loss tangent (tan 𝛿) vary as a function of polymer surface chemistry, polymer surface chains molar mass, segregation, and composition (enrichment in chain-end groups) [116]. Surface Young’s modulus (E) and surface hardness (H) on the scale of 0.05–1 μm were also found to be related to polymer homogeneity at a surface [117], which is affected by the complex action of both internal and external factors. With similar experiments, the E variation with depth is also confirmed [118]. Despite the fact that the experiments provide direct information on the surface properties of polymers, their scientific knowledge requires the use of numerical simulation techniques [119].

Table 1.1 Analytical methods and general information for the characterization of polymer surfaces [2, 6, 11, 78, 119]. Method

Radiation In

Out

Dept Resolution

Profiling

Information Chemistry

morphology

Surface

Typical

Contact angle (CA)

1, while good organic polymeric TEGs have ZT in the 0.1–0.5 range. Thermoelectric sheets were mounted in series as displayed in Figure 10.17; the upper panel shows how the TEGs were mounted and the lower panel schematically illustrates the mechanism of thermoelectric behavior of the sheets coated with PPyAg. Series connection of PPy–Ag films

Equivalent circuit

(a)

5 mm

3 mV

300 kΩ

300 kΩ

3 mV

Cold (copper block) Load resistance current

Current (10 nA)

6 mV

TEG (PPy–Ag connected in series)

Mica

PPy–Ag films

Hot (Pt wire heater)

rs

rrie

arg

no

Ch

Ag

In

Phonon

ch

Ag

PP y

ec arr ier

Ag

Ag

arg

ain

Ag Ch

Cold end

ce

Ag

te rfa

n

Ph o

ai

Hot end

ch

Ag

n

y

PP

a ec

s

Ag

Ag

Ag (b)

Figure 10.17 (a) Schematic showing the practical demonstration of a set of PPy–Ag film thermoelectric power generators along with its equivalent circuit. PPy–Ag films are connected electrically in series and thermally in parallel. Inset shows the photograph of the polymer films connected in series. (b) Schematic showing the role of PPy/Ag interface in facilitating the charge transport but scattering phonons. Source: Bharti et al. 2017 [74]. Reproduced with permission of Elsevier.

293

294

10 Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed

In the studies conducted by Singh et al., it was found that the initial concentration of AgNO3 plays an important role in designing gas sensors [73] and TEGs [74]; a too high concentration (0.5 mol/l AgNO3 for 0.5 mol/l pyrrole and 120 minutes photopolymerization time) that yield highly conductive PPyAg on PET will not undergo significant change of resistance upon interaction with gases, and rather lower concentration (0.1 mol/l AgNO3 for 0.5 mol/l pyrrole and 120 minutes photopolymerization time) is needed, so that any specific interaction leading to molecular recognition of the gas induces significant change in resistance. In contrast, the figure of merit ZT implies that electrical conductivity is required to obtain a good TEG, and in this case high initial AgNO3 concentration (1.2–1.5 mol/l for 0.5 mol/l pyrrole) was found to boost up the final conductivity of the PPyAg-coated silanized PET (∼12 S/cm), a low thermal conductivity and highest ZT figure of merit for similar materials (ZT = 7.4 × 10−3 at 335 K). In another study, interdigitated gold electrode-coated flexible PET sheet was modified by TiO2 /PPyAg hybrid composite thin layer and served as humidity sensor. Photopolymerization using AgNO3 photosensitizer was conducted for 20 minutes under UV light (254 nm). To improve flexibility, the pre-polymer mixture was mixed with poly[3-(methacrylamino)propyl] trimethyl ammonium chloride (PMAPTAC) [76]. Other authors have taken advantage of flexible resins to design interdigitated PPy humidity sensors in PDMS molds [77]. Flexibility was imparted to the photopolymerized poly(ethylene glycol diacrylate) (PEGDA) or epoxy resin by 1,6-hexanediol diglycidyl ether (Araldite DY-H/CH, HDGE). The formulation consisted of the resin, pyrrole, and photoinitiator (4-methylphenyl) [4-(2-methylpropyl)phenyl]–iodonium; hexafluorophosphate (Irgacure 250), a liquid cationic photoinitiator; and 2hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173, D-1173), a radical photoinitiator in liquid state. The formulation can be cured by UV light; Irgacure 250 serves to initiate the photopolymerization of pyrrole and HDGE, whereas Darocur 1173 photoinitiates the polymerization of PEGDMA. The proposed mechanism of pyrrole photopolymerization is depicted in Figure 10.18. Table 10.2 gathers experimental conditions for the design of conductive polypyrrole coatings on polymeric and other substrates by means of photopolymerization. Besides corrosion control, all other potential applications are within the domain of electronics. 10.3.2 10.3.2.1

Polyaniline Mechanisms of Photopolymerization of Aniline

Polyaniline, like PPy, is one of the most studied conductive polymer; it has received particular attention due to its remarkable physicochemical properties (stability, color change upon doping/dedoping, high conductivity) and its variant fields of application (corrosion resistance, energy storage, molecular recognition of pollutants). It can be synthesized in several shapes by different methods including the photopolymerization in the presence of an initiator.

10.3 Surface-confined Photopolymerization of Conjugated Monomers

hν

Ph2I PF6

+

PhI PF6

+ PhI

PhI

+

N H

N H H N

2 N H

H N

N H

H N

PhI

N H

H N

–2H

N H

H N

Ph

H N

N H

N H

N H

H N

H N

and/or

N H

N H

H N hν

and/or

n H N

Ph2I PF6

N H

Figure 10.18 Proposed mechanism of UV-induced step-growth polymerization of pyrrole initiated by radical cations generated from the photodecomposition of the diaryliodonium salt. Source: Razza et al. 2017 [77]. Adapted with permission of John Wiley & Sons.

Barros et al. [91] have proposed a reaction mechanism of photopolymerization of the aniline monomer in the presence of the Ag+ cation (Figure 10.19). According to them, the light causes the excitation of the monomer, which behaves as an electron donor, while the silver cation acts as an acceptor of electron. Photopolymerization of aniline has been reported but at a lesser extent compared with that of pyrrole and concerned essentially bulk powder. As far as photochemically prepared PANI (polyaniline) thin films are concerned, they were achieved on rare occasions on metallic (Ti, W) and semiconductor (ITO) substrates [90, 92]. The process could be adapted to plastic and biosourced substrates. Nanoparticles of PANI was also synthesized by photopolymerization of the aniline monomer in the presence of an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6 ), it is the most used in photopolymerization processes [93]. The irradiation leads to the photoexcitation of the ionic liquid and produces a cation radical [bmim]2 + . ; these radicals react with the monomer and cause its oxidation and the formation of dimers by head-to-tail coupling. Quinoid units can be formed by the oxidation of dimers, the protonation of these quinoids leads to the formation of nitrenium ions; they can also react with another monomer to produce trimers and finally the polymer (Figure 10.20).

295

Table 10.2 Summary of synthesis conditions, physicochemical properties, and potential applications of selected photochemically prepared polypyrrole-coated organic soft substrates and other materials. Polymeric substrate

Compounds, 𝝀 (nm)

Final hybrid material

Potential application

References

PET-Py

Py, AgNO3

PPyAg

Flexible H2 S gas sensor

[73]

PET

Py, AgNO3

PPy NT/Ag

Thermoelectric power generator

[74]

Wool fabric

Py, AgNO3

PPy/Ag

—

[75]

Glass, PET

Py, AgNO3

PPy/Ag

—

[78]

F-SnO2 /TiO2 MP

Py, methyl viologen dichloride, Na2 SO4

TiO2 –PPy

—

[79]

Si wafer

Py, SDS

PPy-SDS

—

[80]

TiO2 electrode

Py, Na2 SO4

PPy Au/TiO2

—

[81] [68]

PVC

Py, Cp2 Fe

PVC/Ppy

—

PET

Py, AgNO3 , TiO2 NPs, PMAPTAC, 254 nm

TiO2 /PPy/PMAPTAC

Humidity sensors

[76]

PDMS mold

Py, I-250, PEGMA, D-1173; or Py, HDGE, I-250/UV light 70 mW/cm2

Interpenetrated PPyPF6 /PPEGMA or PPyPF6 /Epoxy

UV printable humidity sensor

[77]

PI, silicone, PET

Py, AgNO3

PPyAg

Microwiring of flexible electronic devices

[82]

Glossy premium

Py, AgNO3

PPy/Ag

—

[83]

Nanotubular TiO2 electrode

Py, SDS

TiO2 /PPy

—

[84]

FTO

Py, KCl

PPy/TiO2

—

[85]

Ta electrode

Py, LiClO4 , MeCN

Ta2 O5 /PPy

Corrosion protection

[86]

Single-crystal TiO2 flat sheet

Py, TEAPTS

TiO2 /PPy

—

[87]

Al

Py, TiO2 , AgNO3

TiO2 /PPy

Humidity sensors

[88]

Glass

Py, Ru(bpy)3 2+

PPyRu

—

[89]

Ti foil

Py, SDS

TiO2 /Ppy

—

[90]

W foil

Py SDS

WO2 /PPy

—

[90]

FTO, fluorine doped tin oxide glass; MP, mesoporous; NT, nanotube; PET-Py, PET silanized with pyrrole silane; Py, pyrrole; SDS, sodium dodecyl sulfate; PVC, Poly(vinylchloride); PMAPTAC, poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride; TEAPTS, tetra ethyl ammonium p-toluene sulfonate.

10.3 Surface-confined Photopolymerization of Conjugated Monomers

* + NH+3 + (Ag)

+

NH+3 + Ag + hν

* NH+3 + (Ag+) – H

+– NH2

2

+–

NH2 + Ag0

H

H

N

N + nAg0 H

(a) hν

e–

LUMO

Polymer

e–

Acceptor

Monomer h+

HOMO

Sensitizer (b)

Figure 10.19 Proposed mechanism of photopolymerization of aniline in the presence of silver nitrate photosensitizer (a) and schematic representation of photopolymerization of conductive polymer in the presence of a sensitizer (b). Source: de Barros et al. 2003 [91]. Reproduced with permission of Elsevier.

NH2 + bmim

2+

+

2 H N

NH2 + 2bmim H N +

NH

2+

+

NH2

+

H N

NH2

+

NH2

+

NH3 + bmim+ H N H N

+

NH2 + 2 H+ +

NH2 + 2bmim+

H N

NH

H N

H N

+

+ H+ NH2 + H+

Figure 10.20 Mechanism of photoinduced polymerization of aniline in [bmim]PF6. Source: Zhou et al. 2009 [93]. Adapted with permission of Springer.

297

298

10 Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed

10.3.2.2

Substrates for in Situ Photoinduced Polymerization of Aniline

There are a few examples of plastic surface modification by photochemically prepared PANI. The reason lies in the fact that PANI synthesis is easier by oxidative polymerization or by electrochemistry on electrode materials. So, we are left here with one case study of PANI photochemically prepared on PET using AgNO3 photosensitizer [94]. Adhesion was ensured by silanization of the underlying PET; simply aminopropylsilane (APS) was used to tightly anchor the growing chains to the substrate. Figure 10.21a shows the photochemical reaction that provides PANI and silver NPs and the possible chemical attachment of the growing PANI chains to APS-modified PET (Figure 10.21b). The photopolymerization process results in flexible green sheet due to the conductive form of PANI (Figure 10.21c). These sheets are selective sensors for H2 S with optimized sensing for an initial concentration of AgNO3 of 2 mol/l (Figure 10.21d). Interestingly, Singh and coworkers [95] have evaluated the effect of electron beam radiation on similar PANI–Ag coated silanized PET sheets and found improvement of adhesion for doses above 30 kGy that resulted in cross-linking of the PANI chains (Figure 10.21e–f ). The authors have also found a positive effect on the sensing of H2 S for higher irradiation doses (75 and 100 kGy) with quasi linear response for 100 kGy (Figure 10.21g).

10.4 Surface-confined Sonochemical Polymerization of Conjugated and Vinylic Monomers 10.4.1 Insights into Sonochemistry: Origin of the Phenomenon and Mechanism of Polymer Synthesis Historically, the first application of ultrasound dates back to 1927 when Richards and Loomis [96] discovered the effects of high frequency sound waves on the promotion of the reaction rates. Technically, an ultrasound is a sound with a frequency greater than 20 kHz, which is the upper audible limit that human can hear, and can reach 10 MHz [97]. The ultrasound supplies a sufficient energy to trigger both chemical and physical processes [98], a typical high-intensity ultrasonic device is schematically illustrated in Figure 10.22a. In aqueous media, ultrasonic irradiation induces the water sonolysis and the formation of the free radicals H• and OH• [100]. These radicals may recombine to get back to their primary form or also to produce H2 , H2 O2 , or even HO2 , all of them are strong oxidants and reductants [99]. The alternating rarefactive and compressive acoustic waves results in the bubbles formation and cavities and provokes their oscillation in the irradiated liquid [99]. The oscillating cavities get expanded by the absorption of vapor and gas from the solution until a maximum volume, and then they collapse to very shortly release stored energy inside the bubble (cooling and heating rates > 1010 K/s) as illustrated in Figure 10.22b [97] and produce a very high local and transient heating and pressure reaching ∼5000 K and ∼1000 bar, respectively [99, 101].

(a)

*

hν 365 nm

Protonated monomer

NH3

+ Ag

NH3

Excited monomer

H N

0

+Ag

Radical cation

NH2 (CH2)3

(c)

N

N H

(H2C)3 Si

Si O

600

Response (%)

450

O

O

OH

O 150

NH2

NH2 (CH2)3 Si

O O

PANI–Ag (2 M)

20

40

60 80 Time (min)

O NH2

O O

+ HNO3

PANI–Ag (0.5 M)

BOPET

0 0

O OH

(CH2)3 Si

300

+ n Ag

n Polymerized aniline

Growth direction

NH3 + Ag

100

120

140

BOPET AgNO3 + UV (365 nm)

(b)

Figure 10.21 Synthesis of PANI–Ag thin films on silanized PET: (a) mechanism of photo-polymerization of aniline in presence of AgNO3 photo-initiators and UV light (365 nm). (b) Scheme showing the synthesis of PANI–Ag nanocomposite films on the APS-modified PET substrate by photopolymerization. The color code distinguishes aniline (red) from APS (black) N atoms. (c) Digital photograph of the green-colored silanized PET sheet coated with PANI–Ag top layer. (d) H2 S sensing experiments with green sheets shown in (c). Panels (e) and (f ) are digital photographs of PANI–Ag coated PET after irradiation with electron beam the energy of which was 10 and 75 kGy (kilo Gray), respectively. (g) Effect of electron beam radiation energy on the response of the gas sensor to H2 S. Source: (a)–(d) Mekki et al. 2014 [94]. Adapted with permission of Elsevier and (e)–(g) Chaudhary et al. 2018 [95]. Adapted with permission of Elsevier.

70

Response (%)

60 50 40

Pristine 10 kGy 30 kGy 50 kGy 75 kGy 100 kGy

30 20 10

2 mm

2 mm (e)

Figure 10.21 (Continued)

(f)

0

(g)

5

10

15

H2S concentration (ppm)

20

10.4 Surface-confined Sonochemical Polymerization of Conjugated and Vinylic Monomers

Power supply

Piezoelectric transducer

Titanium Hom Collar and O-rings Gas inlet/outlet

Cooling bath

Glass cell Reaction solution

(a)

Acoustic pressure

+

Liquid density

–

Implosion 100

th

Grow

Formation

Bubble radius (μm)

150

50

Shockwave

Hot spot

0 0 (b)

100

200

300 Time (μs)

Rapid quenching

400

Figure 10.22 (a) A typical ultrasonic spray setup, (b) life cycle of an acoustic cavitation. Source: Bang and Suslick 2010 [99]. Reproduced with permission of John Wiley & Sons.

The coincidence of such harsh conditions can engender a light emission, a fascinating physical phenomenon that was first observed by Frenzel and Schultes in 1934 [102], the so-called sonoluminescence. In polymer science, the as-mentioned conditions serve to promote polymerization. The ultrasound provides a highly efficient and particular initiator facilitating the chemical bonds breaking as shown in Figure 10.23 [97]. It replaces the chemical radical initiator or they can be used together [104]. The irradiation of a solution containing vinyl monomers is an alternative strategy of macromolecules synthesis [105].

301

302

10 Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed S

S 1

+

S

S

R

S

1R

R2

H2O

S

R2

n

(a) Acoustic frequency

20 kHz

Physical/ mechanical

(b)

Dominant effect

Chemical

R2

R2

H2O

Pn

HO

+ H

500 kHz

R3

R3

OH

S

S

R

Z R2 R2 Pm

M

S

R

R3

Kp

Pn Kt

+

S

Z

Pm +

Pn

S

S

Pn

+R

R3

S

S

Z

Pm

S

S

Pn +

Z

Z

Pm Kt

Main RAFT equilibrium

(c)

Figure 10.23 (i) Schematic illustration of sono-assisted reversible addition-fragmentation chain transfer (RAFT) polymerization, (ii) force types depending on the ultrasonic frequencies, and (iii) mechanism proposition of the sono-induced RAFT polymerization. Source: McKenzie et al. 2017 [103]. Adapted with permission of John Wiley & Sons.

It has been demonstrated that sono-induced polymerization is not limited to water medium, but it can take place in organic solvent. Weissler et al. [106] have first reported the breakdown of acetonitrile under ultrasonic treatment into nitrogen, methane, and hydrogen. Since then, ultrasound-assisted polymerization in organic medium has been thoroughly investigated [105]. Several studies revealed the high efficiency of the sono-assisted route for the preparation of acrylic hydrogels [107], polystyrene [108], poly(methyl methacrylate) [109], and poly(n-butyl acrylate) [110] to name but a few. Price and coworkers [104, 109, 111, 112] have extensively studied the ultrasound-initiated polymerization and detailed very important findings: (i) high molecular weight polymers could be synthesized at early stages of the vinylic monomers polymerization, (ii) a prolonged ultrasonic irradiation may degrade the fabricated polymer, and (iii) the polymer formation increases the

10.4 Surface-confined Sonochemical Polymerization of Conjugated and Vinylic Monomers

liquid viscosity, the bubbles formation decreases as well and induces a decrease of the monomers conversion to polymer [113]. Kruus et al. [114, 115], when studying the sonopolymerization mechanism of methyl methacrylate, showed that under sono-treatment, pyrolysis of the monomer could occur and then insoluble chars could be formed as well as linear polymers. In this section of the chapter, we intend to highlight how the versatile sonochemistry served to the coating and the modification of synthetic- and biopolymeric surfaces for various applications. 10.4.2 Ultrasound-assisted Polymerization or Polymer Deposition over Organic Polymeric Substrates 10.4.2.1

Sonopolymerization

The ultrasound-assisted route for polymer deposition figures as a “green” and impressive process among the available synthetic ones. It made possible modifying polymer surfaces using generated free radicals [116]. It increases the reaction yield for a short time process. Recently, Sanaeishoar et al. [116] introduced a sono-based method (using an ultrasound probe) to graft polyacrylamide onto nano-fibrillated cellulose (NFC) under mild conditions. Such material has particular physical and chemical properties motivating its application in some fields like the separation and the electronic devices. They demonstrated the high efficiency of sonication in promoting the “grafting-from” of polyacrylamide on the backbone of a commercial NFC in the presence of potassium persulfate (K2 S2 O8 ), used as initiator (see also Chapter 11). The energy generated from the ultrasound waves facilitates sulfate ion radicals’ formation (SO4 • − ). Then, these species attack the hydroxyl groups on the organic substrate (NFC) to form alkoxy radicals. Both of these reactions constitute the initiation phase of the copolymerization process (Figure 10.24). Hamouma et al. [117] proposed a new strategy to coat surface-modified cellulosic paper electrodes with polypyrrole. For that purpose, an ultrasonic tank was used. The preparation strategy is schematically represented in Figure 10.25. The paper surface was first modified by a layer of diazonium modified-multiwalled carbon nanotubes (CNTs). The diazonium modification of CNTs ensures their adhesion to the paper surface and favors coating with polypyrrole. This layer provided a very suitable platform for the in situ sonopolymerization of pyrrole. The sonochemical deposition of the polypyrrole film took only one hour, it showed a better electrochemical behavior than other chemical routes and produced a nanocomposite with a particular physicochemical properties. 10.4.2.2

Ultrasonic Spray

Polymer deposition or polymer coating could be performed via the ultrasonic spray. Fine droplets of the coating solution (polymer solution) are formed using a spraying nozzle and then deposited over the substrate to coalesce and to form the coating film, the setup is illustrated in Figure 10.26. The breakdown of the liquid into droplets, the atomization, is ultrasonic [119]. The ultrasonic spray is economical and provides certain important advantages like simplicity, uniformity, reproducibility and it permits to coat

303

304

10 Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed

Initiation S4O82–

2SO4

OH

+ SO4

O

– + HSO4

NFC

Propagation H2C + H2N

O

O O

CH2 CH2

O

H2N

+

n

CH2CH2

O

H2C O

O

CH2 HC

O

H2N

n+1

O

H2N

H2N

Termination 2

O

CH2 HC

O

n+1

O

CH2 HC

H2N

O O

2n+2

H2N

Figure 10.24 Ultrasound assisted preparation of polyacrylamide modified NFC. Source: Sanaeishoar et al. 2018 [116]. Reproduced with permission of Elsevier. (1) (a)

SO3– N N

(b)

SEM

(2) (a)

(b)

SEM

R

R = SO3–, NH2

Figure 10.25 General strategy for fabricating the modified cellulosic paper (1) SEM images of CNTs modified paper electrodes (2) SEM images of the nanocomposite after the sono-coating with polypyrrole; (a): low magnification, and (b): high magnification. Source: Hamouma et al. 2018 [117]. Adapted with permission of Elsevier.

three-dimensional micro-structured materials with planar and nonplanar surfaces [119–121]. Several parameters are involved for the achievement of the spraying process, namely, the polymer concentration, the used solvent, the type of the substrate, the total amount that was sprayed, the spraying flow rate, the speed with which the nozzle is moved, the air pressure used to vehiculate fine droplets, and the nozzle-substrate distance [119].

10.4 Surface-confined Sonochemical Polymerization of Conjugated and Vinylic Monomers

(b)

Liquid inlet

Solution inlet

Deagglomeration

Power connector Ultrasonic generator

Ultrasonic nozzle

Piezoelectric transducers

Nitrogen inlet Atomizing surface Spray

Nitrogen shroud

Shroud

Atomized solution

(a)

Vibrating ultrasonic nozzle tip

Substrate (A)

Ultrasonic atomization at the atomizing surface

(B)

Droplets form

Figure 10.26 (A) Principle of the polymer ultrasonic spray deposition [116]. (B) (a) The ultrasonic atomization process, and (b) droplets formation using an ultrasonic nozzle [118]. Source: (B) Stryckers et al. 2016 [118]. Adapted with permission of John Wiley & Sons.

Selgers et al. [122] reported an ultrasonic spray-based protocol to coat selective laser sintered surfaces with a hydrophobic layer of 5% polyvinylidene fluoride in acetone. The surfaces consist of flat tiles made of nylon 12. They showed how the ultrasonic spray coating reduces the surface roughness of the layer-upon-layer prepared materials (additive manufactured pieces), and how it guarantees a better filling of the porous surface of the substrates. Selgers et al. [122] insisted on three crucial aspects to achieve a “perfect” coating: the optimization of the whole process (influencing parameters), the ink optimization, and the substrate preparation and pretreatment. Gilissen et al. [123] deposited a light-emitting layer of super yellow (poly(phenylene vinylene) copolymer) on poly(3,4-ethylenedioxythiophene) : poly(4-styrenesulfonate) (PEDOT:PSS) surface using the ultrasonic spray. The later was employed as a wet solution process to make a polymer light-emitting diodes. After evaporating the dissolving solvent, a coating with low roughness and notable uniformity was obtained without altering the light emission properties. 10.4.3 Sonopolymerization over Miscellaneous Types of Surface: Inorganic Polymeric Substrates Snoussi et al. [113] have recently introduced a new ultrafast strategy to sono-prepare a magnetic ternary nanocomposite, a Fe3 O4 core nanostructured material for catalytic applications. The upper layer, the catalytically active one,

305

(b)

0.8

t0

0.6 0.4

15s 30s

0.2

60s 90s 120s 150s 180s

0.0

200 300 400 500 600 700 800 Wavelength (nm)

3 Absorbance (a.u.)

(a)

Absorbance (a.u.)

10 Surface Modification of Polymeric Substrates with Photo- and Sonochemically Designed

EDX elemental mapping of the ternary nanocomposite

306

2 1 0 200

t0 120s 180s 300s 600s

600 400 Wavelength (nm)

Figure 10.27 (a) Strategy of making Fe3 O4 @NH2 -mesoporous silica@polypyrrole/Pd; (b) (1) monitoring of the methyl orange degradation over the nanocatalyst and (2) monitoring of the p-nitrophenol reduction over the same nanocatalyst. Source: Snoussi et al. 2018 [113]. Adapted with permission of Elsevier.

consisting of palladium-doped polypyrrole was ultrasonically deposited over amino modified-mesoporous silica surface (the intermediate layer). Compared the conventional methods, the proposed ultrasound-assisted route that was performed using an ultrasonic processor (ultrasonic probe) permitted to drastically save time and energy. The deposition took one hour to be achieved. The nanocatalyst exhibited high catalytic activities. The general process description is shown in Figure 10.27. Barkade et al. [124] have implemented a method based on the ultrasound assistance to realize the in situ mini-emulsion polymerization of pyrrole around ZnO nanoparticles. The polypyrrole constitutes the outer coating layer. It is worth noting that also the spherical ZnO particles have been sonochemically synthesized. Besides the uniform dispersion of ZnO nanoparticles in the polypyrrole sono-formed matrix, the ultrasonic treatment helped in the size control of the hybrid functional colloidal particles and in the enhancement of synergy between ZnO and PPy. The final product denoted by PPy/ZnO was used for the sensing of liquefied petroleum gas. It revealed remarkable sensing ability and notable stability.

10.5 Conclusion In this chapter we have laid out a fistful of strategies to modify polymeric substrates by surface-initiated polymerization method triggered by light or sonowaves. Classical Norrish Type I and Type II photoinitaitors continue to receive much attention by material scientists owing to their versatility and ease

References

in designing polymers and polymer grafts. CRP methods have dramatically progressed, and much is done to trigger CRP methods in bulk or at surface via photoinduced electron transfer. In this regard, photocatalysts, metal complexes, dyes, or even Type I and Type II photoinitiators proved efficient to conduct in a new way ATRP and other CRP methods. The interest of radical photopolymerization lies in the spatiotemporal feature as it is convenient to pattern polymer surfaces. With a large series of acrylic monomers in hand, much can be done to tune the surface properties of plastics by surface-initiated photopolymerization. We have provided some interesting applications such as improved filtration, anti-inflammatory implant, protein imprinted polymer beads, or anti-icing surfaces. As far as conjugated polymers are concerned, the choice of monomer is restricted to pyrrole and aniline. However, by making nanocomposite coating on plastic or paper substrates, it is possible to achieve ultimate performances for future electronic devices such as TEGs, paper electrodes, supercapacitors, and gas sensors to name but a few. Both photo- and sono-induced surface-initiated oxidative polymerizations of conjugated monomers permit to provide accelerated polymerization and unusual micro-nanostructures of conductive polymer coatings and conductive polymer nanocomposites. We anticipate remarkable progress of surface-confined radiation-induced polymerization to impart unique and unprecedented properties to plastic and paper or cellulosic objects. With the rapidly growing domain of flexible electronics and “elastronics” (stretchable electronics), clearly there is room for future innovative surface confined polymerization and the development of 3D patterned surfaces.

Acknowledgments The authors would like to thank Campus France for the provision of PROFAS B+ fellowship to FM, North Atlantic Treaty Organization (NATO) for financial support (CATALTEX project No. 984842), and Qatar National Research Fund for NPRP award No 8-878-1-172.

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11 Surface Modification of Nanoparticles: Methods and Applications Gopikrishna Moku 1,# , Vijayagopal Raman Gopalsamuthiram 2,# , Thomas R. Hoye 2 , and Jayanth Panyam 1 1 University of Minnesota, Department of Pharmaceutics, College of Pharmacy, 308 Harvard St SE, Minneapolis, MN 55455, USA 2 University of Minnesota, Department of Chemistry, College of Science and Engineering, 207 Plesant St SE, Minneapolis, MN 55455, USA

11.1 Introduction The therapeutic potential of a drug molecule is dependent on its availability at the target site at the requisite amount and for the required duration. In addition, it is important to minimize the exposure of the drug to non-target tissues to avoid potential side effects. It is estimated that greater than 70% of newly discovered small molecules are hydrophobic and have poor aqueous solubility, limiting their ability to be transported by blood and other body fluids [1, 2]. In addition, some drugs undergo rapid clearance and thus have short half-life and residence time. Delivering a drug to the right place, at the right concentration, and for the right period of time is, thus, a challenge. Incorporation of the drug in a suitable delivery system can overcome some of these challenges. Drug delivery systems improve drug efficacy and safety by modifying the pharmacokinetic properties (distribution, absorption, distribution, and elimination) of the drug [3]. Over the past two decades, there has been an intense focus on the use of carriers that are on the order of ∼100 nm in diameter for drug delivery. This particle size range enables systemic administration because the smallest blood capillaries are 10–20 μm in diameter. Further, carriers in this size range could be used for targeted delivery of different types of therapeutic payloads to specific organs and tissues [4]. Nano-delivery systems with different architectures have been developed, including lipid nanoparticles, micelles, dendrimers, polymeric conjugates, solid-lipid nanoparticles, and inorganic nanoparticles [5]. Such systems have shown promising pre-clinical activity in various diseases such as AIDS, cancer, malaria, diabetes, and tuberculosis [6–10], and some of these have, in fact, been approved for human use [11]. This chapter will discuss the various polymers used in the formulation of nanoparticles, fabrication and #

Equal contribution.

Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

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11 Surface Modification of Nanoparticles: Methods and Applications

characterization techniques, surface modification methods, and the different types of targeting ligands that have been evaluated.

11.2 Polymers Used in the Preparation of Nanoparticles Various materials are available for the fabrication of nanoparticles including polymers, lipids, inorganic compounds (silica, silicate), and metals (gold, silver, iron). Nature has also designed nanosize particles, specifically viruses, that have been co-opted for tissue-specific gene delivery [12]. Due to their stability, drug loading capacity, and tunable properties, polymers have specific advantages as a drug carrier. Biodegradable polymers are more advantageous than nonbiodegradable materials for drug delivery applications because of the need for easy removal of the carrier after drug release [13]. The selection of polymer for fabricating nanoparticles depends on the desired size and surface characteristics of the particle as well as the nature of the drug or active ingredient. Physicochemical properties of the polymer determine the fabrication process to be employed.

11.3 Common Biodegradable Polymers for Nanoparticle Fabrication Table 11.1 lists some of the polymers commonly used in the fabrication of nanoparticles. Natural polymers tend to be hydrophilic, while synthetic polymers can be hydrophobic or hydrophilic. A brief description of each of these polymers is provided in the following text. Structural features of these polymers are shown in Figure 11.1. 11.3.1

Albumin

Albumin is a natural transport protein that delivers vitamins, minerals, and other hydrophobic compounds such as steroids to various tissues. This natural transport function, its ability to internalize into different cell types, and multiple drug binding sites provide the rationale for its use in drug delivery. Importantly, albumin is constituted by a single polypeptide chain of 585 amino acids and contains a low amount of methionine and tryptophan and a large amount of glutamic acid, cysteine, lysine, aspartic acid, and arginine. Another major advantage of albumin in drug delivery is that the therapeutic drug of interest can be easily attached covalently or non-covalently. Albumin is an endogenous protein and hence is highly biocompatible. In addition, it has functional groups that can be used to bind different ligands and complex drugs (e.g. paclitaxel in Abraxane , insulin detemir Levemir , GLP-1 in Victoza ) [14, 15].

®

11.3.2

®

®

Alginate

Alginate, a naturally occurring anionic polysaccharide of α-l-guluronic acid and β-d-mannuronic acid repeating units linked by a 1 → 4 linkage, is widely used for pharmaceutical applications. It is biodegradable, nontoxic, inexpensive,

11.3 Common Biodegradable Polymers for Nanoparticle Fabrication

Table 11.1 Examples of polymers used for preparation of nanoparticles in drug delivery. Nature of the polymer

Name of the polymer

Advantages

Disadvantages

Natural

Albumin Alginate Chitosan Gelatin

Easily available Generally nontoxic Biodegradable

Structural complexity Batch-to-batch variations

Synthetic

Polylactide (PLA) Poly(lactide-co-glycolide) (PLGA) Poly(ε-caprolactone) (PCL) Poly(malic acid) (PMLA) Polyacrylamide (PAM) Poly(isobutyl cyanoacrylate) (PIBCA) Poly(isohexyl cyanoacrylate) (PIHCA) Poly(n-butyl cyanoacrylate) (PBCA) Poly(acrylate) and poly(methacrylate) (Eudragits) Poly(vinyl alcohol) (PVA) Polyethylene glycol (PEG) Poly(lactide)–polyethylene glycol (PLA-PEG) Poly(ε-caprolactone)–polyethylene glycol (PCL-PEG) Poly(lactide-co-glycolide)–polyethylene glycol (PLGA-PEG) Tween 20 and Tween 80 Dextran

Biocompatibility

Generally more expensive than natural polymers

Possibility of sustained release Large diversity in functional groups Properties can be tuned

Can be polydispersed, depending on the synthetic method

and readily available and has been found to be a mucoadhesive, biocompatible, and non-immunogenic material. Specifically, the simple aqueous-based gel formulation of alginate in the presence of divalent cations such as Ca2+ has been used in the preparation of alginate delivery systems [16]. 11.3.2.1

Chitosan

Chitosan (CS) is a modified natural cationic polysaccharide prepared by chemical deacetylation of chitin, the second most abundant natural biopolymer after cellulose, and is derived from crustacean shells [17]. The primary amino groups in the polymer backbone of CS provide positive charge to the polymer. Because of its mucoadhesivity, CS has been regarded as a potential carrier for oral drug delivery. Another important feature of CS is its metabolic degradation in the body. While small molecular weight CS can be eliminated renally, large molecular weight CS can be degraded by endogenous enzymes. The rate and extent of degradation depends on the molecular weight and degree of acetylation of the polymer. CS has many potential applications including for drug delivery through the oral, nasal, transdermal, parenteral, vaginal, cervical, and rectal routes [18].

321

322

11 Surface Modification of Nanoparticles: Methods and Applications O

O O

O

HO O

O

mO

n

PLA

H

H

O

OH

n

H

O

n

PLGA

O OH n

n

PEG

OH

OH

PGA

n

PVA

PAA

O O

O O

O

O

O

n

O

O

PVP

S O O

n

PSF CN N

O

(b)

n

m

PBAT

H n

H

O

O

PHBV N

O

O

O

n

PCL

(a)

O

O

n R O

PCA [Polycyanoacrylates]

O

O n

O O

n

14

PHDCA

O

O

PVMMA

Figure 11.1 Some of the polymers most commonly used in nanoparticle preparation. (a) Commercially available. (b) Non-commercial.

11.3.3

Gelatin

Gelatin is a natural, biocompatible, biodegradable, and multifunctional protein for use in controlled drug release. It is a polyampholyte having both cationic and anionic groups as well as hydrophobic groups, and it can be obtained from acid/alkaline/enzymatic hydrolysis of collagen. The gelatin molecule chain contains ∼13% lysine and arginine (imparts positive charges); ∼12% glutamic and aspartic acid (provides negative charges); ∼11% leucine, isoleucine, methionine, and valine (imparts hydrophobicity) amino acids; and ∼64% glycine, proline, and hydroxyproline amino acids. Commercially, gelatin is available as both cationic (gelatin type A, isoelectric point (pI) 7–9) or anionic (gelatin type B, pI 4.8–5) protein without the necessity for additional functionalization [19]. 11.3.4

Poly(lactide-co-glycolide) (PLGA) and Polylactide (PLA)

Poly(lactide-co-glycolide) (PLGA) and polylactide (PLA) polyester polymers have been extensively studied for drug delivery. They are widely used because they undergo hydrolysis in the body and produce biologically compatible monomers lactic acid and glycolic acid, which can be further metabolized via the citric acid cycle. The degradation of PLGA and PLA is an autocatalytic process, in which acidic degradation products generated in the interior of the carrier accelerate the degradation reaction. The drug release from PLGA and PLA matrices depends on both drug diffusion through the polymer matrix and the polymer degradation rate. A wide spectrum of PLGA polymers with different molecular weights and

11.4 Fabrication of Nanoparticles

lactide-to-glycolide weight ratios, which determine the biodegradation and drug release rates, are available commercially. Polymers with higher molecular weight usually exhibit lower degradation rates compared with lower molecular weight polymers. The Food and Drug Administration (FDA) and European Medicine Agency (EMA) have approved the use of PLGA and PLA in humans [20]. 11.3.5

Poly-𝛆-caprolactone (PCL)

Poly-ε-caprolactone (PCL) is another synthetic aliphatic polyester that has been investigated extensively for use in controlled drug delivery systems [21, 22]. It is a biocompatible, biodegradable, and hydrophobic polymer suitable for drug delivery applications. It can form compatible blends with other polymers. Owing to its slow biodegradation, it is ideally suited for long-term delivery, extending over a period of more than one year.

11.4 Fabrication of Nanoparticles Techniques used for the fabrication of nanoparticles can be broadly classified based on whether a preformed polymer is used or the polymer is produced in situ with the concomitant formation of nanoparticles. This approach has been extensively studied for preparation of nanoparticles of various sizes. Some common techniques that utilize preformed polymer include: (i) (ii) (iii) (iv) (v) (vi)

Emulsification–solvent evaporation [23] Emulsification–solvent diffusion [24] Salting out [24] Nanoprecipitation [25] Dialysis [26] Supercritical fluid technology [27]

Surfactants are used to stabilize nanoparticles, and various surfactants can be used for this purpose. Surfactants serve as a stabilizer during the manufacturing process and during nanoparticle use by offering a protective coating around the nanoparticle and preventing aggregation. The most common surfactants used in this process (Figure 11.2) include polysorbate 20, sodium cholate, cetyltrimethylammonium bromide (CTAB), didodecyldimethylammonium bromide (DDAB), and polyvinyl alcohol (PVA). In some instances, amphiphilic block copolymers such as poly(lactide-co-glycolide)–polyethylene glycol (PLGA-PEG) or PEG-PLA have been used based on their surface activity. This is achieved through polymerization of monomers into a polymer that encapsulates a drug during the fabrication process. This is typically done via two approaches: (i) emulsion polymerization [28] or (ii) interfacial polymerization [29]. A key limitation of these methods is that the monomers used are often nonbiodegradable and can result in non-biocompatible by-products. Extensive purification is often needed to remove monomers or initiators to afford an

323

324

11 Surface Modification of Nanoparticles: Methods and Applications O O w O

HO

O

O z

O OH

Br

OH x

N OH n

y

CTAB

PVA

Polysorbate 20

OH O

Stearic acid O

OH

OH ONa H

H

Brij-35

O

H H OH H Sodium cholate

HO

n

N DDAB O

S O O Sodium dodecyl sulfate (SDS)

O H

O

O

O n

O O

H

O

O x

TPGS

O y

OH z

Pluronic F-68

Figure 11.2 Surfactants commonly used in nanoparticle preparation.

acceptable pharmaceutical formulation. Recently, some newer methods have been reported for the preparation of nanoparticles. These include high frequency thermal plasma methods [30], spray and microemulsion methods, laser ablation method [31], flash creation and mechanochemical bonding methods, and spray pyrolysis [32].

11.5 Linker Chemistry for Attaching Ligands on Polymeric Nanoparticles To develop an effective polymer-based targeted drug delivery system, it is critical to immobilize one or more functionalities/ligands on the surface of nanoparticles (Figure 11.3). To achieve this, it is necessary to chemically modify the surface of nanoparticles with a suitable linker to introduce targeting moieties. In addition, the targeting ligand must have a functional group that can be used for conjugation. A wide range of conjugation approaches have been studied, and the specific method used depends on the nanoparticle preparation technique, surface chemistry, and the functional group available in the ligands. Polymeric nanoparticle surface modification methods can be broadly divided in two categories.

11.5 Linker Chemistry for Attaching Ligands on Polymeric Nanoparticles Antibody PEG

H

O

n

Drug-loaded polymer nanoparticle

Surface functionality

O

Mannose

ar

h cc

H

ide

a lys Po

id

Drug-loaded polymer nanoparticle

Lip

Ap

tam e

r

Biotin

e tid Pe p

Est ra

dio l

Folic acid

Figure 11.3 Graphical representation of surface-functionalized polymeric nanoparticles loaded with drugs (incorporated within the matrix of the polymer), targeting molecules (e.g. antibodies, peptides, aptamers, and small molecular ligands) for targeted drug delivery. Left: drug-loaded nanoparticle; right: drug-loaded nanoparticle with surface functionalization.

Chemical conjugation allows for stable incorporation of the targeting ligand on the surface of nanoparticles. However, this requires the presence of a functional group on the surface that can subsequently be used to attach the targeting ligand. Various functional groups have been immobilized onto the surface of polymeric nanoparticles including carboxylic acids, amines, thiols, alcohols, maleimides, azides, aldehydes, and alkynes. Some of these covalent conjugation methods are summarized in Table 11.2. Formation of an amide bond is a widely used method to conjugate targeting ligands onto the surface of polymeric nanoparticles. Formation of the amide takes place in two steps. In the first, carboxylic acid group(s) present on either the polymer nanoparticles or the conjugating ligands are activated using 1-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC) hydrochloride or dicyclohexylcarbodiimide (DCC). Since EDC is readily soluble in water, it is often preferred over DCC. Frequently, N-hydroxysuccinimide (NHS) or sulfo-NHS is used along with EDC to stabilize the highly active intermediate generated via EDC and increase the efficiency of the coupling reaction. In the following step, primary amine(s) present in either the conjugating ligand or the polymeric nanoparticles react with the activated ester to form stable amide bonds between the conjugating ligand and nanoparticles. The main advantage with this conjugation reaction is that it does not require any initial modification of the targeting ligand, which can result in loss of activity. In a recent study, Feng et al. modified polymeric nanoparticles by trans-activator of transcription (TAT) peptide or anti-EpCAM antibody using EDC-NHS reaction [33]. Bare particles had greater negative ζ potential due to the higher number of carboxylic acid groups compared with TAT/antibody conjugated particles. Moreover, bare nanoparticles showed greater mobility in gel compared with TAT/antibody conjugated nanoparticles. In another study, polymeric nanoparticles with carboxylic acid surface groups were modified with

325

326

11 Surface Modification of Nanoparticles: Methods and Applications

Table 11.2 Schematic representation of covalent conjugation methods. Type of covalent conjugation

Acid/amine

Linkage

Amide bond

Stability under physiological conditions

Reaction scheme O + H2N

Thio–ether bond

L

EDC

L

DCC

Maleimide/ thiol

NH

NP

O NP

Disulfide

S

HS

L

NP

Hydrazide/ aldehyde

Hydrazone

Azide/alkyne (click chemistry)

O NP

Triazole ring

NP

N H

L

N O

SH +

NP

Highly stable

O +

N O

Thiol/thiol

Highly stable

O OH

NP

NP

L

HS

+ H

+ N N+

L

N–

Cleavable under reducing conditions

L

O

O

NH2

S S

L

NP

NP

NH

Highly stable

N L

N N N

Highly stable L

NP, polymeric nanoparticle; L, ligand.

a targeting antibody (anti-CD63) using EDC/NHS coupling. Anti-CD63 antibodies were detected on the nanoparticle surface by a secondary anti-IgG1 antibody against the fragment crystallizable (Fc) region of the targeting antibody [34]. Thio–ether bond formation is another popular covalent conjugation method in which a maleimide group reacts with a thiol to form a stable thio–ether bond. Typically, amine groups in the targeting ligands are converted into free thiol/activated sulfhydryl group using Traut’s reagent (2-iminothiolane), N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP), or N-succinimidyl-Sacetylthioacetate (SATA), followed by reduction with the reducing agent dithiothreitol (DTT)/tris(2-carboxyethyl)phosphine (TCEP) hydrochloride. In the second step, the thiol group in the targeting ligand is reacted with maleimide groups on the surface of nanoparticles. This reaction is quick and highly efficient and occurs in aqueous solutions under mild conditions. This bond is stable for 24 hours even under reducing conditions [35]. The main disadvantage with this bond is that conversion of amines in the targeting ligand into free thiols may lead to loss in activity. In addition, side reactions such as intermolecular rearrangement or formation of disulfides makes this reaction less selective toward the thio–ether bond formation in aqueous solutions. Our group successfully conjugated imino-thiolated anti-CD133 antibody to maleimide functionalized PLGA nanoparticles. First, amines in the anti-CD133 antibody were converted into free thiols using 2-iminothiolane. In the next step, iminothiolated antibody was added to maleimide functionalized PLGA nanoparticles. The conjugation

11.5 Linker Chemistry for Attaching Ligands on Polymeric Nanoparticles

of anti-CD133 antibody was confirmed by observing bands corresponding to heavy and light chains in Western blotting [36]. The disulfide bond formation occurs by the conjugation of thiol groups present on nanoparticles and thiol groups in the targeting ligands. In the case of antibodies, free thiol groups can be formed by reducing disulfide linkages with DTT. Free thiols can be introduced in targeting ligands by allowing it to react with Traut’s reagent/SPDP/SATA. The main disadvantage with this bond is its instability in presence of reducing agents, which hinders its use for in vivo applications. In addition, competing disulfide formation between two of the same thiol (homo-coupling) can lower the selectivity of the desired cross-coupling reaction. Due to its high conjugation efficiency, click reactions represent a promising conjugation strategy for the immobilization of biological ligands on to the surface of polymer nanoparticles. A hallmark of a click reaction is that it is highly specific in the presence of many other types of common functionality and in aqueous solutions and the products are stable. In 2002, the Sharpless and Meldal groups independently developed the copper(Cu-I)-catalyzed alkyne-azide cycloaddition (CuAAC). For some drug delivery applications, copper-free cycloaddition reactions are desired over copper-catalyzed reactions. Copper ions interact with biomolecules and can cause toxicity. Recently, Layek et al. applied click chemistry in vivo to deliver dibenzocyclooctyne (DBCO) functionalized, paclitaxel-loaded PLGA nanoparticles for cancer therapy [37]. They developed a two-step tumor targeting strategy based on mesenchymal stem cells (MSCs), which actively traffic tumors. First, they expressed non-natural azide groups on the surface of MSCs using glycoengineering protocols. These glycoengineered MSCs were intravenously administered into lung tumor bearing mice. In response to inflammatory signals, these glycoengineered MSCs actively trafficked to tumor sites. In the second step, intravenous administration of DBCO functionalized PLGA nanoparticles resulted in their accumulation in tumor tissues that were pre-targeted with MSCs.

11.5.1

Hydrazone Bond Formation

Acid cleavable, hydrazone bonds are readily formed between a hydrazide group and an aldehyde group. In general, targeting ligands do not contain aldehyde functional groups. The hydroxyl groups present in targeting ligands can be oxidized to aldehydes in the presence of various oxidizing agents [38]. In a recent study, Liu et al. developed a dual pH-responsive multifunctional polymeric nanoparticle system based on poly(l-histidine) (PHIS) and hyaluronic acid (HA) [39]. They first prepared resiquimod (R848, a TLR7/8 agonist) loaded PHIS (PHIS/R848) nanocores. Next, a polymeric prodrug HA-doxorubicin (DOX) was prepared through hydrazone bond linkage. Finally, the polymer prodrug HA-DOX was coated on the surface of PHIS/R848 nanocores to form HA-DOX/ PHIS/R848 nanoparticles. These multifunctional nanoparticles significantly inhibited tumor growth by regulating both tumor immunity and killing tumor cells in a 4T1 murine tumor model.

327

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11 Surface Modification of Nanoparticles: Methods and Applications

11.5.2

Non-covalent Attachment

Non-covalent interactions are also used to attach targeting ligands to polymeric nanoparticles. In general, this technique does not require any pre-modification of targeting ligand or nanoparticle because the immobilization of ligand onto the surface of the nanoparticles occurs through simple physical contact. Non-covalent immobilization depends on attractive forces between the surface of nanoparticles and the targeting ligand, including electrostatic interactions, hydrogen bonding, hydrophobic interactions, and van der Waal’s interactions. For example, tetrameric glycoprotein avidin (67 kDa) and streptavidin (60 kDa) form a strong, non-covalent complex with the small molecule biotin (244 Da), with a dissociation constant K d around 10−14 M [40]. Due to strong irreversible binding, this noncovalent interaction has been used to functionalize nanoparticles with targeting ligands. Recently, Angsantikul et al. developed gastric epithelial cell membrane-coated PLGA nanoparticles loaded with clarithromycin antibiotic by simply mixing PLGA nanoparticles and the cell membrane [41]. These biomimetic nanoparticles retained the original surface antigens of the source cells and showed preferential adhesion and retention with Helicobacter pylori bacteria. These membrane-coated nanoparticles have shown increased therapeutic efficacy compared to free drug in a mouse model of H. pylori infection. On the other hand, physical interactions are not specific, more labile, and less reproducible compared to covalent conjugation methods. Importantly, it is difficult to control the orientation of the targeting ligand, which in turn can affect activity. For example, electrostatic attractions mainly depend on the surface charge of nanoparticles and the targeting ligands. In the case of antibodies, their orientation on the surface of nanoparticles relies on the position of charged groups in the tertiary structure of the proteins. It also depends on the pH of solution and the isoelectric point (pI) of the antibody and nanoparticle. If the pH of the solution changes, the antibody may detach from the surface of the nanoparticles. In addition to electrostatic interactions, hydrophobic interactions can also cause a change in the tertiary structure of the protein, resulting in loss of activity [42]. An interesting approach that seeks to eliminate the inefficiency and complexity associated with the previously mentioned modification processes is based on a dopamine polymerization technique. The product polydopamine (PDA) has a unique ability to deposit on polymeric nanoparticles upon simple immersion of nanoparticles in an aqueous dopamine solution buffered to pH 8–8.5. Subsequently, amine or thiol-containing molecules can be conjugated onto the PDA layer via Michael addition or Schiff-base reactions [43].

11.6 Surface-functionalized Polymeric Nanoparticles for Drug Delivery Applications Polymeric nanoparticles offer an attractive platform for the delivery of therapeutic agents due to their unique physical properties that stem from a size range

11.6 Surface-functionalized Polymeric Nanoparticles for Drug Delivery Applications

that is comparable to and compatible with biomolecular and cellular systems. However, the nature of the nanoparticle surface determines its interaction with the biological environment. Thus, the goal of nanoparticle surface functionalization is to affect these interactions so as to increase the circulation half-life and minimize off-target drug exposure. This section discusses some examples of nanoparticle surface functionalization and the important criteria to consider during the fabrication process. In addition, the importance of surface chemistry and potential drug delivery applications are described. Surface functionalization strategies can be categorized based on the nature of the ligand that is being conjugated: (i) Functionalization with macromolecules such as polysaccharides, lipids, peptides, antibodies, and nucleic acids (aptamers). (ii) Functionalization with small molecule ligands such as mono or oligosaccharides, steroids, and vitamins. (iii) Functionalization with hydrophilic polymers. Owing to their specific affinity for target sites, macromolecules such as polysaccharides, lipids, peptides, and nucleic acids (aptamers) are widely used as recognition ligands. In general, biomolecules can be coated on polymeric nanoparticles via chemical bond formation between biomolecules and the substrate surface either directly, through a spacer, or by noncovalent physisorption. 11.6.1

Polysaccharides

Polysaccharides have the advantages of biocompatibility, ease of availability, and well-established modification schemes [44]. Polysaccharides such as dextran, CS, HA, and heparin can provide steric protection against protein adsorption and macrophage uptake and are recognized as stealth-coating materials [35]. Additionally, several studies have demonstrated the active targeting properties of polysaccharides such as HA [45], CS [46], and chondroitin sulfate [44]. Some examples of polysaccharide-coated polymeric nanoparticles and their applications are summarized in Table 11.3. 11.6.2

Lipids

Lipid-coated polymeric nanoparticles provide a range of advantages in drug delivery, including a broad range of flexible strategies and ease of surface engineering, extended circulation half-life, reduced cytotoxicity, and better target specificity [66]. Natural phospholipids such as phosphatidylglycerol, phosphatidylcholine, phosphatidylinositol, phosphatidylserine, phosphatidic acid, and phosphatidylethanolamine and their synthetic counterparts {e.g. N-[1(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate (DOTAP), 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), 1,2-distearoylphosphatidylethanolamine (DSPE), N-(methylpolyoxyethylene oxycarbonyl)-1,2distearoyl-sn-glycero-3-phosphoethanolamine (DSPE-PEG), 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)} are often used to coat the surface of polymeric nanoparticles [67].

329

Table 11.3 Recent examples of biomolecule conjugated polymeric nanoparticles and their therapeutic uses.

Class

Nanoparticle

Polysaccharides PLGA

Lipids

Aptamers

Functionalization

Conjugation chemistry

Drug

Therapeutic use

References

Dextran

Amine/acid

Ifosfamide

Osteosarcoma

[47]

PLGA

Chitosan/alginate

Electrostatic interactions

Doxorubicin

Cancer therapy

[48]

Chitosan

Alginate

Dual cross-linker

Naringenin

Diabetes mellitus

[49]

Poly(l-histidine)

Hyaluronic acid

Hydrazone

R848/ doxorubicin

Cancer therapy

[39]

PLGA

Chitosan

Electrostatic interactions

Tobramycin

Cystic fibrosis

[50]

PLGA

Soybean lecithin

Self-assembly

Silymarin

Nonalcoholic fatty liver disease (NAFLD)

[51]

PLGA

DSPE-PEG2000

Self-assembly

Docetaxel

Cancer therapy

[52]

PCL

DSPE-PEG2000-NH-Fucose

Self-assembly

Methotrexate/ aceclofenac

Cancer therapy

[9]

PLGA

Gastric epithelial cell membrane

Physical adsorption

Clarithromycin

Helicobacter pylori

[41]

PLGA

DOPC/DOTAP/DSPE-PEG

Self-assembly

Sirolimus

Restenosis

[53]

PLGA-PEG-COOH

CD133

Acid/amine

Propranolol

Hemangioma

[54]

BSA

AS1411

Acid/amine

Doxorubicin

Cancer therapy

[55]

PLGA/Chitosan

5TR1

Electrostatic interactions

Epirubicin

Cancer therapy

[56]

PLGA-PEG-COOH

A10

Acid/amine

Docetaxel

Cancer therapy

[57]

PLGA-COOH

A10-3.2

Acid/amine

Paclitaxel

Cancer therapy

[58]

Peptides

Antibodies

PLGA

Tet1

Acid/amine

Nattokinase

Alzheimer’s disease

[59]

PLGA-PEG-COOH

cNGR

Acid/amine

Docetaxel

Cancer therapy

[60]

PLGA-PEG-Maleimide

iRGD

Thio–ether bond

Rosiglitazone/ PEGE2

Obesity

[61]

PVDF

CBO-P11

Click reaction

NA

Angiogenesis

[62]

PFT-PS/PEG-COOH

TAT

Acid/amine

Fluorescent dye

Cancer cell imaging/ photodynamic therapy

[33]

Polystyrene

Anti-CD63

Acid/amine and physical adsorption

NA

NA

[34]

PEGylated P(HDCA-co-MePEGCA)

Anti-Aβ1-42

Biotin-streptavidin

Fluorescent dye

Alzheimer’s disease

[63]

PLGA/PLA-PEG-Maleimide

Anti-CD133

Thio–ether bond

Paclitaxel

Cancer therapy

[36]

PLGA

KIM-1

Acid/amine

Resveratrol

Chronic kidney disease

[64]

Multi-block copolymer

Trastuzumab/folic acid

Acid/amine

Doxorubicin

Cancer therapy

[65]

PFT-PS/PEG-COOH

Anti-EpCAM

Acid/amine

Fluorescent dye

Cancer cell imaging/ photodynamic therapy

[33]

332

11 Surface Modification of Nanoparticles: Methods and Applications

Due to the amphiphilic nature of the phospholipids, they can form membrane mimetic structures on nanoparticles. Hydrophobic interactions and electrostatic attraction are the major chemical forces responsible for the lipid self-assembly process on polymeric nanoparticle surfaces [67]. A variation of this approach is the use of membranes from various cells including red blood cells, neutrophils, and T cells to coat the nanoparticle surface. For example, Zhang et al. prepared neutrophil membrane-coated nanoparticles by fusing neutrophil membrane onto polymeric nanoparticle cores. These nanoparticles inherit the antigenic exterior and associated membrane functions of the source cells. In addition, these nanoparticles neutralized proinflammatory cytokines, suppressed synovial inflammation, targeted deep into the cartilage matrix, and provided strong chondroprotection against joint damage [68]. 11.6.3

Aptamers

Aptamers are single-stranded (ss) short oligonucleotides (6–30 kDa) that can interact with cellular targets such as nucleic acids, transmembrane proteins, or sugars with high affinity and selectivity by acquiring defined three-dimensional secondary and tertiary structures [69]. Aptamers are synthetically prepared from an initial library containing 1013 –1016 random ssDNA or ssRNA sequences through an in vitro selection process termed systematic evolution of ligands by exponential enrichment (SELEX) [70]. In this cell-based method, first a library of oligonucleotides is incubated with the target of interest, and the ones that have higher affinity are enriched and purified in the following steps. Aptamers have distinct advantages over traditional antibodies, including ease of isolation, smaller size, higher ratio of target accumulation, lack of immunogenicity, and higher in vivo stability [70]. Importantly, aptamers do not have an Fc region, which interacts with soluble Fc receptors or Fc receptors expressed on immune cells and other certain type of cells. This obviates undesired interactions following systemic administration, which may result in immune stimulation or other unforeseen side effects. Also, for the treatment of solid tumors, an antibody’s high molecular weight hinders its ability to penetrate deep into a tumor. The molecular weight of an aptamer is usually in the range of 6–30 kDa, much smaller than that of an antibody (∼150 kDa), which often leads to better tumor uptake kinetics. In addition, an aptamer has greater stability than a protein in biological fluids and lower production costs. However, the lack of an Fc region limits their circulation half-life compared with that of an antibody, although this is not a major impediment when using the aptamer as a targeting ligand. 11.6.4

Antibodies

Antibodies are attractive as targeting ligands because of their excellent target specificity and affinity. The conjugation of polymer nanoparticles with antibodies can impart high target recognition capability to nanoparticles. However, stable conjugation of the antibody to a nanoparticle surface in the correct orientation while avoiding aggregate formation is crucial for successful functionalization. To conjugate the antibody covalently to nanoparticles, a suitable functional

11.6 Surface-functionalized Polymeric Nanoparticles for Drug Delivery Applications

group must be introduced onto the nanoparticle surface and into the antibody molecule. Amino (lysine), carboxy (glutamic acid and aspartic acid), and sulfhydryl (cysteine) are the most widely used functional groups in antibodies. Physical adsorption of antibodies on the surface of nanoparticles has been investigated as well. In a recent study, Tonigold et al. demonstrated that the pre-adsorption of antibodies on the surface of polymer nanoparticles resulted in efficient targeting of nanocarrier compared with nanoparticles that were chemically attached to the antibody [34]. In that study, binding affinity of the antibody chemically conjugated to nanoparticles was significantly affected by protein corona formed when nanoparticles were introduced in a biological fluid. Physically adsorbed antibodies, on the other hand, remained functional and were not affected by the protein corona. Monoclonal antibodies (mAb) targeting various markers and receptors present on cancer cells including human epidermal growth factor receptor-2 (HER2), αvβ3 integrin, prostate-specific membrane antigen (PSMA), and CD20 antigen on B-cell lymphomas have been used for nanoparticle functionalization. However, the presence of full-length mAbs (∼150 kDa) can affect the tumor penetration of the nanoparticle and also result in increased macrophage uptake via Fc receptor (FcR) identification. To minimize these issues, antibody fragments such as single-chain variable fragment (scFv) and antigen-binding fragment (Fab) have been investigated as targeting ligands. 11.6.5

Peptides

Short tissue-homing peptides offer the advantages of high stability, reduced immunogenicity, and ease of synthesis and conjugation to nanoparticles. However, the design of a small peptide molecule that fits into a usually shallow and hydrophobic binding pocket on the target can be challenging. These homing peptides are typically identified by phage display technology [71], which is a screening tool that allows the selection of peptide sequences against specific target tissues. Various peptides have been used as targeting ligands and can be divided into two groups: cell-penetrating peptides (CPPs) and cell-targeting peptides (CTPs). CPPs have the ability to enter cells, and they have been employed in cellular delivery of biologically active therapeutics [72]. Among the various CPPs, TAT peptide (∼1.5 kDa) is a well-known cationic CPP sequence derived from the human immunodeficiency virus 1 (HIV-1) protein [14]. It enhances cellular uptake of nanoparticles by interacting with proteoglycans of the cell membrane. Unlike CPPs, CTPs interact in a cell- or tissue-specific manner. Arginine–glycine–aspartic acid (RGD)-containing peptides (435–784 Da), which interact specifically with αvβ3 integrin overexpressed on cancer cells and tumor microvasculature endothelial cells, is an example of a CTP [73]. Because small molecule ligands can recognize certain markers or receptors present on the surface of target cells, a wide variety of ligands have been incorporated onto polymeric nanoparticle surfaces, allowing them to be used in intracellular drug delivery [74]. Functionalization of nanoparticles with a small molecule ligand has been shown to increase cellular uptake of nanoparticles via receptor-mediated endocytosis, resulting in enhanced drug delivery and therapeutic efficacy [75]. The major advantages of using a small molecule as

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11 Surface Modification of Nanoparticles: Methods and Applications

the targeting ligand over macromolecules is their stability, ease of conjugation with nanoparticles, and low cost. The drawbacks associated with the use of small molecules as targeting ligands include the lack of an efficient approach to identify/develop such ligands, and their lower specificity and affinity for surface receptors on the target cells. In general, rapidly dividing cancer cells require large amounts of certain vitamins, such as vitamin B9 (folic acid), vitamin B12, and vitamin H (biotin); therefore, the receptors involved in vitamin uptake are overexpressed on the surfaces of cancer cells. Folic acid has been investigated extensively as a targeting ligand for clinical applications. Folic acid is a high affinity ligand of endogenous folate receptor, which is frequently overexpressed in many types of human cancer cells. Several reports have been demonstrated that folic acid-functionalized polymeric nanoparticles can be actively internalized via receptor-mediated endocytosis resulting in enhanced therapeutic efficacy in various tumor models [76]. Our group designed and developed folate and biotin functionalized PLGA/PLA-PEG nanoparticles for tumor targeted drug and nucleic acid delivery [77]. Oligosaccharide–lectin interactions have been exploited for the targeting of carbohydrate ligands to site-specific target receptors. Due to the low affinity of mono/oligosaccharides with cell surface receptors, multiple carbohydrate ligands are often required to achieve the required binding strength. One well-known example uses galactose or galactose-mimics (e.g. N-acetylgalactosamine (NAcGal)) as ligands to asialoglycoprotein receptor (ASGPR), an endocytic cell surface lectin receptor primarily expressed on the sinusoidal surface of hepatocytes [78]. The mannose receptor (CD206 or MR) is a 175 kDa C-type lectin receptor present on cell surface of macrophages and immature dendritic cells. Carbohydrates such as mannose, fucose, and N-acetyl glucosamine were used to enhance the binding and uptake of the polymeric nanoparticles by macrophages or dendritic cells [79]. Estradiol is a steroid hormone that can specifically bind to estrogen receptors (ER) overexpressed on a variety of cancer types including ovarian, breast, and endometrial cancers. Being an endogenous molecule, estradiol is also biocompatible and non-immunogenic, which makes it a promising ligand for targeted cancer therapy. Estradiol-functionalized polymeric nanoparticles have been developed for the efficient tumor targeting and improved therapeutic outcomes [80]. Some selected recent examples that demonstrate the utilization of small molecule ligands for targeted delivery of polymeric nanoparticles are summarized in Table 11.4. 11.6.5.1

Polyethylene Glycol (PEG)

An important characteristic of an effective targeted delivery system is its ability to stay in systemic circulation for prolonged periods of time. Phagocytic uptake by macrophages is the primary mechanism of particle clearance from systemic circulation. Surface functionalization of nanoparticles with hydrophilic polymers such as PEG (2–20 kDa) can increase the circulation half-life of polymeric nanoparticles [88]. Surface modification with hydrophilic polymers

Table 11.4 Recent examples of small molecule ligand-functionalized polymeric nanoparticles and their therapeutic uses.

Nanoparticle

Functionalization

Conjugation chemistry

Poly(methacrylic acid-co-histidine/ doxorubicin/biotin

Biotin

Drug

Therapeutic use

References

Acid/amine

Doxorubicin/ imiquimod

Cancer therapy

[81]

[78]

PLGA/polydopamine

Folic acid/RGD

Michael addition

Doxorubicin

Cancer therapy

PLGA-PEG-Dopamine

Dopamine

Acid/amine

FK506

Type 1 Diabetes

[82]

Chitosan

Folic acid

Acid/amine

Temozolomide (TMZ)

Cancer therapy

[83]

PLGA-PEG-estradiol

17β-estradiol

Acid/amine

Docetaxel (DTX)

Cancer therapy

[80]

Chitosan

Mannose

Reductive amination

Curcumin

Visceral leishmaniosis

[84]

PLGA

Monosaccharides

2-(2-Aminoethoxy) ethanol linker

NA

NA

[79]

Poly-(ε-caprolactone)/ PLGA-PEG-COOH

DCL (pseudomimetic dipeptide)

Acid/amine

Epillgallocathechin3-gallate (EGCG)

Cancer therapy

[85]

PLGA/PLA-PEG-DBCO

DBCO

Click reaction

Paclitaxel

Cancer therapy

[37]

Chitosan

Cysteine

Acid/amine

Amoxicillin

Helicobacter pylori

[86]

PLMB

Carborane

Acid/alcohol

NA

Cancer therapy

[87]

Chitosan-TPE

Galactose/dopamine

Michael addition

Tetraphenylethene

Cancer therapy

[78]

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11 Surface Modification of Nanoparticles: Methods and Applications

creates a steric barrier and delays plasma protein adsorption or “opsonization,” which is a critical first step in macrophage-mediated uptake of nanoparticles [88]. This suppression of nonspecific interactions with the blood components leads to reduced blood clearance of polymeric nanoparticles (“stealth effect”). Furthermore, PEG increases the stability of polymeric nanoparticles during storage and in aqueous dispersions by reducing the tendency of particles to aggregate (steric stabilization) [89]. Although PEGylation is effective in prolonging polymeric nanoparticle blood circulation time after intravenous administration, its presence on the surface of a nanoparticle can reduce the nanoparticle interaction with target cells. This is referred to as the “PEG dilemma.” [90] This issue can be overcome by functionalizing the nanoparticles with cell- or receptor-specific ligands (in addition to PEG), which further increase the nanoparticle interactions with target cells. The presence of PEG can also result in the generation of anti-PEG antibodies and in hypersensitivity reactions in some patients. In addition to PEG, other synthetic hydrophilic polymers such as polyvinyl pyrrolidone (PVP), polyaminoacids, poloxamers, and PVA have been employed as stealth coatings [88].

11.7 Characterization of Surface-modified Nanoparticles Characterization of nanoparticles for size, morphology, and surface charge has been done using various microscopic techniques including scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These techniques have also been used to understand the variation in the physical properties of particles after surface modification. The average particle diameter, their size distribution, and charge affect the physical stability and their in vivo distribution. Electron microscopy techniques are very useful in ascertaining the overall shape of polymeric nanoparticles, parameters that influence their safety and biodistribution. For example, disc-shaped particles distribute more to the lungs than to the liver. The surface charge of nanoparticles affects the physical stability and redispersibility of the polymer dispersion as well as their in vivo performance. Many techniques have been used to study the surface modification of nanoparticles. 11.7.1

Particle Size

Particle size distribution is an important parameter that affects the biological performance of nanoparticles. Further, particle size affects the drug release rate, which is a key performance parameter. Smaller particles offer larger surface area for a given volume. On the other hand, smaller particles also tend to aggregate during storage. Hence, there is often an optimal size that balances biological performance with stability [91]. Polymer degradation can also be affected by the particle size. There are several tools for determining nanoparticle size as discussed in the following text.

11.7 Characterization of Surface-modified Nanoparticles

11.7.2

Dynamic Light Scattering (DLS)

The most popular and convenient method of determining particle size is photoncorrelation spectroscopy (PCS) or dynamic light scattering (DLS). DLS has been widely used to determine the size of Brownian nanoparticles in colloidal suspensions in the nano- and submicron ranges. Wang et al. reported the preparation of a zwitterionic polymer capsuled protein-based nanogel by in situ free radical polymerization on the surface of an acryloylated bovine serum albumin (BSA) protein with 2-methacryloxyethyl phosphorylcholine using N,N ′ -methylene bis(acrylamide) (BIS) and N,N,N ′ ,N ′ -tetramethylene ethylenediamine (Scheme 11.1) [92]. Ammonium persulfate was used to initiate the radical polymerization. Dialysis was then used to obtain the BSA nanogel, which was surface modified by using EDC and N-hydroxy succinimide to enable conjugation to the TAT peptide (Scheme 11.1). The size of the resulting nanoparticles was observed to increase after surface modification using TAT peptide from 5.24 nm for blank BSA to 19.46 nm for acrylic BSA to 22 nm for TAT-5-BSA (Figure 11.4). Varying levels of conjugation was achieved by varying the equivalents of TAT peptide used in the conjugation step. O O

O N

O

BIS MPC + AA BSA O

O O

MPC

O

P

O

N+

O– O

AA

H N

BIS OH

O

H N O

GRKKRRQRRRPP-CH3O

Scheme 11.1 Synthesis of a zwitterionic polymer capsuled protein nanogel. Source: Wang et al. 2017 [92]. https://creativecommons.org/licenses/by/3.0/.

11.7.3

Scanning Electron Microscopy (SEM)

In a typical scanning electron microscope, a focused electron beam is scanned over a surface to create an image. The electrons in the beam interact with the sample, producing signals that are often used to assess surface topography and composition. Thus, SEM allows morphological examination with direct visualization of the surface.

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11 Surface Modification of Nanoparticles: Methods and Applications

Native BSA TBSA nBSA Acrylic nBSA TAT-1-nBSA TAT-5-nBSA TAT-10-nBSA

30

Intensity (%)

338

(5.24 ± 1.00 nm) (7.45 ± 0.78 nm) (18.33 ± 6.80 nm) (19.46 ± 5.35 nm) (21.29 ± 4.66 nm) (22.00 ± 3.83 nm) (24.31 ± 4.42 nm)

15

0 1

10

100

1000

Hydrodynamic diameter (nm)

Figure 11.4 DLS image and sizes of BSA, TBSA, nBSA, acrylic nBSA TAT-1-nBSA, TAT-5-nBSA, and TAT-10-nBSA nanoparticles. Source: Wang et al. 2017 [92]. https://creativecommons.org/licenses/by/3.0/

Prior to analysis by SEM, dry nanoparticles are mounted on a sample holder, followed by coating with a conductive metal, such as gold, using a sputter coater. The sample is then scanned with a focused fine beam of electrons and, upon emission of secondary electrons from the sample surface, certain surface characteristics of the sample are revealed. The limitations of this method are that nanoparticles must be able to withstand vacuum and the electron beam-induced damage. Often, the mean particle size obtained by SEM is compared with that from DLS measurements, thereby providing additional experimental evidence for evaluation of that important parameter. As an example, surface-modified biodegradable polymeric nanoparticles from PEG-PLGA was used to deliver GSE24.2 peptide intracellularly to cells for the treatment of dyskeratosis congenita and other telomerase disorders by Egusquiaguirre et al. [93] The GSE24.2 peptide was produced from Rosetta 2-gami cells by treatment with pGATEVGSE24.2 and lysates. PLGA and PEG-PLGA nanoparticles were surface modified with GSE24.2 peptide by a water-in-oil-in-water double emulsion solvent evaporation method. A variety of CPPs were prepared by sequential deprotection, coupling, and deprotection using an Fmoc/Boc/Alloc solid-phase approach on a MBHA resin [94]. Cationic polymeric nanoparticles were prepared from PEG-PLGA nanoparticles containing the GSE24.2 peptide either by treatment with dextran in the aqueous phase or with polyethyleneimine in the organic phase. The CPPs were then conjugated with the cationic polymeric nanoparticles containing the GSE24.2 peptide using EDC/sulfo NHS chemistry. SEM analysis of the particles before and after conjugation with CPPs showed uniform spherical morphology (Figure 11.5).

11.7 Characterization of Surface-modified Nanoparticles

(a)

(b)

(c)

Figure 11.5 SEM images of (a) blank PEG PLGA NPs, (b) PEG PLGA NPs with GSE24.2 (c) surface-modified PEG PLGA NPs containing GSE 24.2 with CPP. Source: Egusquiaguirre et al. 2015 [93]. Reproduced with permission of Elsevier.

11.7.4

Transmission Electron Microscopy (TEM)

TEM is complementary to SEM; similar insight to nanoparticle properties can be ascertained, although the principle is slightly different from that of SEM. Because the key requirement in TEM is for the sample to be ultrathin to enable electron transmittance, the sample preparation time and procedure can be longer. Usually, the nanoparticle dispersion is typically deposited onto support grids or films and either stained using a negative staining agent such as phosphotungstic acid or uranyl acetate or by plastic embedding onto the surface to enable the dispersion to withstand vacuum. Another approach is to flash freeze an aqueous dispersion of nanoparticles, which results in embedding of the sample in vitrified water. The surface properties of the sample are obtained when a beam of electrons is transmitted through the ultrathin sample, interacting with the sample as it passes through. An example application of TEM is its use to characterize quercetin PLGA nanoparticles coated with BSA-bound Adriamycin (ADR) [95]. In this case, the PLGA nanoparticles with quercetin were prepared by an emulsion solvent evaporation method. ADR bound to BSA (BSA-ADR) was independently prepared. PLGA nanoparticles were then added to the BSA-ADR complex and sonicated. Incubation and ultracentrifugation resulted in the formation of BSA-ADR-coated nanoparticles. TEM showed the presence of three distinct layers, interpreted as one being the drug in the core of nanoparticles, the second being the polymer encapsulating the core, and the third being the BSA-ADR coating on the surface (Figure 11.6). 11.7.5

Surface Charge

The intensity and nature of the surface charge of nanoparticles determines their interaction with the biological environment. The colloidal stability of nanoparticles is determined by their ζ potential [96], which can be defined as the potential difference that exists between the bulk solvent and the stationary layer of solvent attached to the particle. The measurement of ζ potential enables one to assess the storage stability of colloidal dispersion and predict the surface hydrophobicity. High ζ potential values, either positive or negative, increases stability by minimizing aggregation of particles. ζ Potential can also provide information regarding the nature of material encapsulated within the particles or

339

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11 Surface Modification of Nanoparticles: Methods and Applications

Figure 11.6 Transmission electron microscope image of a nanoformulation showing polymer capsule, encapsulated quercetin, and surface modification.

200 nm CRNN (CU)

coated onto the surface. For example, polyquercetin (pQCT) nanoparticles were synthesized by oxidative self-polymerization of quercetin under basic conditions using sodium periodate [97]. PEGylation of the nanoparticles was done in bicine buffer at pH 9 and using mPEG5K-NH2 . pQCT@PEG NPs were then loaded with DOX as a model drug and as a fluorescent probe. The ζ potential of the nanoparticles was measured in PBS at pH 7.4. QCT NPs exhibited a negative potential of −14.9 ± 3.0 mV. This value significantly decreased upon PEGylation and after encapsulation of DOX, providing further analytical evidence to substantiate surface modification of pQCT NPs with PEG after DOX incorporation. 11.7.6

Surface Hydrophobicity

Techniques used to understand surface hydrophobicity include methods such as hydrophobic interaction chromatography, biphasic partitioning, adsorption of probes, and contact angle measurements. Amongst these, X-ray photon correlation spectroscopy (XPS or XPCS) is unique in that it permits identification of specific functional groups on the surface of nanoparticles [98]. A random diffraction or “speckle” pattern is produced when the coherent light beam scatters light upon interaction with a disordered system. The observed pattern is related to the precise spatial arrangement of the disordered scatterers and can be interpreted as the instantaneous diffraction pattern of the disordered system. If one uses a noncoherent light beam, instead of the speckle pattern, an average over “many” speckle patterns is observed. In an XPCS experiment, a “movie” is built by consecutive collection of a sufficient quantity of those instantaneous images. The quercetin nanoparticles discussed earlier [95] were also characterized by XPS spectra. The C/O ratio decreased from polyquercetin nanoparticles upon PEGylation, which further decreased upon encapsulation of DOX (Table 11.5). High resolution XPS spectra also showed increased nitrogen (N1s) signals compared with the unmodified nanoparticles, indicating the surface functionalization of the nanoparticles with mPEG-NH2 (Figure 11.7).

11.7 Characterization of Surface-modified Nanoparticles

Table 11.5 Elemental composition of unmodified and modified polyquercetin nanoparticles from X-ray photon correlation spectroscopy high resolution scans of C1s, O1s, and N1s regions. NP

C (%)

O (%)

N (%)

C/O

pQCT

68.0 ± 3.3

31.6 ± 3.4

0.3 ± 0.2

2.15

pQCT@PEG

64.2 ± 2.9

34.7 ± 2.9

1.2 ± 0.1

1.85

pQCT@PEG@DOX

61.0 ± 0.4

37.4 ± 0.8

1.6 ± 0.4

1.63

Source: Sunoqrot et al. 2018 [97]. Reproduced with permission of RSC.

Intensity (cps)

5 × 105

pQCT@PEG@DOX

4 × 105 3 × 105

395

405

400

395

0 405

400

395

pQCT

0 1200 1100 1000 900 800 700 600 500 400 300 200 100

(a)

400

pQCT@PEG

2 × 105 1 × 105

405

Binding energy (eV)

(b)

Figure 11.7 (a) X-ray photon correlation spectroscopy for polyquercetin, PEGylated nanoparticles of polyquercetin and nanoparticles loaded with doxorubicin. (b) N1s scans for each sample showing increase in % N content. Source: Sunoqrot et al. 2018 [97]. Reproduced with permission of RSC.

11.7.7

Fourier Transform IR (FTIR) Spectroscopy

FTIR spectroscopy has also been used as a tool to understand surface modification of nanoparticles. An example of IR as a tool to examine nanoparticle surface modification was done by Nivedh et al. [99] CS-PEG-PLGA nanoparticles were surface modified with rabies viral antigen. For this purpose, the PEG-PLGA polymer was first synthesized and the rabies viral antigen was incorporated into the particles by emulsion solvent evaporation method. CS coating was added by mixing CS with nanoparticles in the presence of acetic acid. The resultant CS-PEG-PLGA rabies viral antigen nanoparticles were functionalized with biocompatible materials such as starch, acacia, and ovalbumin. To confirm the surface functionalization, the infrared spectrum was recorded for blank CS-PEG-PLGA nanoparticles and compared with those from the polymer-treated particles. The changes in the peak positions for nanoparticles after surface modification with rabies viral antigen with respect to the parent unmodified CS-PEG-PLGA nanoparticles (CS NP) were noted. Significant peak broadening across the entire regions in the case of CS-PEG-PLGA rabies attenuated viral antigen nanoparticles with various biocompatible materials in

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comparison to the controls suggested the presence of viral antigen on the surface of the nanoparticles.

11.8 Summary/Conclusion Significant advances continue to be made for functionalizing the surface of nanoparticles for specific therapeutic applications. Critical to the success of surface functionalization are the methods used to functionalize the surface, the choice of the linking agent, and the analytical methods used to characterize such nanoparticles. Ease of synthesis and industrial scalability are significant challenges and further research is needed to address these limitations.

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2: S25.

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12 Surface Modification of Polymers for Food Science Valentina Siracusa University of Catania, Department of Chemical Science (DSC), Viale A. Doria 6, 95125 Catania, Italy

12.1 Introduction Synthetic polymers as well as biodegradable and biobased polymers are currently the largest class of materials extensively studied and used for different application, thanks to their unique and peculiar properties. They provide greater flexibility, transparency, chemical inertness, low specific weight, and low cost in respect to traditional materials such as glass, metal, and paper. In addition, viscosity, electric and mechanical performances, thermal behavior as well as gas and water vapor barrier properties, biocompatibility, and/or biodegradability are some of other characteristics that could be properly modulated in order to render such materials adaptable for different applications (engineering, medicine, food packaging). Their bulk properties are greatly influenced by their surface properties such as hydrophilicity and hydrophobicity, adsorption and/or desorption of molecules, adhesion, permeation, chemical or biological reaction, pH, roughness and friction, wettability, and so on. Any alteration of the surface properties can consequently impact the final performance of the polymer, influencing their applications. Vice versa, in most instances, a surface polymer modification is required in order to obtain polymers with the desired properties. Surface functionalization, cleaning/etching, and deposition are the most studied surface treatments for polymer used for packaging application [1]. Surface functionalization serves to introduce specific functional groups on to the polymer surface layer in order to improve properties such as wettability, sealability, printability, resistance to glazing, and adhesion to other polymers or materials, to enhance the barrier characteristic, and to impart antimicrobial properties, without influencing the polymer bulk properties, as reported by several authors [2–5]. Wettability by hydrophobic and/or hydrophilic substances is the most crucial property for practical application and is related to the surface roughness, adhesion, adsorption, and chemical composition of the polymer surface. The polymer wettability is expressed in terms of polymer surface water contact angle (WCA), by the determination of the water drop shape on the polymer surface. The WCA could vary from 0∘ to 150∘ . A value of WCA below 90∘ is attributed to a hydrophilic-nature polymer, while above 90∘ is related Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

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to a hydrophobic-nature polymer. Most of the polymers present in commerce have a hydrophobic nature and are not wetted by polar fluid. Just few polymers are hydrophilic, and the water can spread on the surface and adsorbed into the polymer bulk. The adhesion property, the ability of a polymer to attach with another material such as metals, ceramics, and chemical molecules, is also crucial for the final polymer performance. In general, due to the fact that polymers are hydrophobic, while the other materials are hydrophilic, mixing these materials would be difficult. In order to overcome this obstacle, one of the materials must be surface-modified to acquire a hydrophilic or hydrophobic nature, by addition of chemical chain or chemical groups with specific reactivity. The key role in this case is played by polymer surface properties such as charge or pH, adsorption, permeation, chemical reaction, roughness, and polymer chemical composition. By working on those surface properties, it is possible, for example, to add a hydrophilic chain on the hydrophobic polymer surface. As an example, when polymers are used for medical application as for implantation of medical devices, the biocompatibility between the polymers and the body is of crucial importance. In this case, the cell adhesion that involves the cell growth, migration, and differentiation processes is strongly influenced by the polymer surface roughness, wettability, and chemical structure. The other surface treatments, such as surface cleaning/etching and deposition, could be used to remove unwanted materials and contaminants from polymers surfaces and for the deposition of thin layers of coating materials and sterilization. These modification techniques are divided into three main groups: physicochemical, mechanical, and biological methods, in the gas phase and in solution. Physical methods are preferred, thanks to their greater precision, easy process control, and environmental aspect. Physicochemical techniques are used to enhance the surface polymer hydrophilicity by use of coupling agents with hydrophilic chain. Further, desorption processes could be used to remove surface impurities while maintaining the ions present on the surface and susceptible of attaching by coupling agents. The mechanical techniques are mainly used to improve the surface roughness and friction, by altering the surface topography, improving meanwhile the polymer wettability and adhesion. Biological methods are used to improve the interaction between the biomolecules and polymers, especially for medical application. For each method, the mechanisms as well as the possible applications focused on food packaging are discussed.

12.2 Physical and Chemical Methods Different techniques could be used to perform chemical and/or physical surface modification of polymers. The most commons are summarized on Figure 12.1. Coating, polishing, grit blasting, etching, vapor deposition, and self-assembly techniques are some of the surface methods involved in the polymer surface modification. With most of them being new molecules, atoms or ions can be attached

12.2 Physical and Chemical Methods

Use of gas with active species such as radicals, electrons, ions, and molecules Gas phase and radiation method Electromagnetic radiation such as visible light, UV, and γ-rays

Polymer substrate Liquid and bulk phase method

Sorption and desorption of atoms, molecules, or ions from or through the bulk to the surface of the polymer

Grafting and polymerization

Figure 12.1 Schematic representation of the chemical and physical polymer surface modification.

to the surface by hydrogen bonding, hydrophobic interactions, or by Van der Waals forces. In addition, radiations such as visible light, UV, γ-rays, and plasma are used. 12.2.1 12.2.1.1

Gas Phase and Radiation Gas Phase

Gas containing active species such as free radicals, electrons, ions, and molecules can be used as surface modifiers for polymers materials. The most common method that use gas as active substance is the chemical vapor deposition (CVD), first developed by Gleason (see Chapter 5) [6, 7]. In this process three steps are involved: initiation, propagation, and termination, performed on a cooled polymer substrate, giving rise to a thin polymer film layer. The first step requires an initiator and a monomer. The initiator, in the form of active free radical, attacks a bond of the monomer, producing a highly reactive free radical monomer (initiation step). Then, during the propagation step, the free radical monomer attacks another monomer, giving rise to larger propagating monomer radicals. When two of propagating monomer radicals react with each other, or a propagating monomer radical react with an initial radical or when two initial radicals react with each other, the termination step occurs. Deposition of a thin film, with chemical characteristics related to the used monomer, is observed on the polymer surface. This process takes place into a hot chamber (200–300 ∘ C) containing the material to be coated, under ultrahigh vacuum conditions; chemical reactions occur on the hot surface. During the cooling step, a polymer thin film will be formed on the substrate. The most interesting advantages of this method are as follows: (i) the film formed on the polymer surface is quite uniform and can be adapted also on elaborated shapes, (ii) the film completely

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fills holes and cracks, and (iii) different layers of different materials be deposited, with good reproducibility. The most important disadvantage is the necessity to have a volatile initiator, that is, most of the time is toxic, explosive, and corrosive; further, the by-product produced during the deposition process and the unreacted gases can be dangerous, the high temperature necessary for the CVD deposition limits the substrates to be coated to those thermally resistant, and that, lastly, the mechanical characteristics of the deposited film and of the substrate must be similar in order to avoid instabilities. 12.2.1.2

Radiation

Radiation is a form of energy that can be used to modify the surface of polymers. Radiation used in this case is produced from atoms when electrons drop from higher to lower energy levels. In an electrode, ions and/or electrons are generated by a radiofrequency power source, microwave, and dielectric barrier discharge. Depending on the used source, radiations can be categorized as ionizing radiation (high-energy) and non-ionizing (low-energy) radiation. High-energy radiation such as X-ray, γ, α, β, and neutron is able to remove electrons from atoms and molecules, forming ions (see Chapter 8). Low-energy electromagnetic radiations such as UV (see Chapter 7), infrared, visible light, microwaves, radio waves, thermal radiation, low pressure plasma, laser treatment, and in general low frequency radiations do not have sufficient energy for removing electrons and form ions during collision but can form radicals. In order to avoid the modification of the bulk properties and characteristics of polymers, low energy radiations are preferred; γ-ray, electron beam, and UV radiation are mostly used for surface modification of polymers. The first two types are used for sterilization and cross-linking. Gamma radiation is used to enhance heat stability, processability, and biocompatibility of polymers materials. By γ-radiation, Yoshii et al. [8] enhanced the heat resistance and improved the processability of polycaprolactone (PCL), which is an aliphatic biodegradable polyester used for food applications. Also, Cottam et al. [9] improved the mechanical and thermal performance, as well as the ability of cells to attach and grow on PCL surface used for medical application, by γ-irradiation (dose 2.5 Mrad). Due to the fact that the electron beam is less invasive, with a limited penetration depth, it can be used for medicine as well as for packaging application. As another example, this radiation was used by Leonard et al. to induce degradation in polylactic acid (PLA) and polylactic-co-glycolic acid (PLGA) used in medicine [10]. Samples of PLA and PLGA were exposed to 50 kGy of e-beam radiation from a 1.5 MeV accelerator, and an improvement of their performance was detected by the determination of the molecular weight nearest to the irradiated surface. UV is used, thanks to its cross-linking and photo-oxidative surface effects. For PLA, it was used to modify the hydrophobicity into hydrophilicity (reducing the WCA) and to increase its dyeability; this is obtained by breaking the ester linkage, reducing the molecular weight, and increasing the oxygen content on the surface [11]. As reported by Azevedo et al. [12], UV and γ-radiation were used to improve the mechanical performance such as hardness and elastic modulus of polyurethane (PU), thanks to the cross-linking and oxidation of the polymer surface. Plasma treatment is probably the most versatile method. Plasma, considered as the fourth state of matter, is a partially ionized gas, with a net neutral charge.

12.2 Physical and Chemical Methods

The plasma contains electrons, photons, positive or negative ions, atoms, and small/medium molecular fragments. This technique does not require any use of solvent making the plasma an environmental-friendly method. In general, plasma treatment could be divided in thermal plasma and cold plasma. Thermal plasma is characterized by the use of electrons and particles at high temperature and consequently cannot be employed for the surface modification of heat sensitive surfaces, such as synthetic and biodegradable polymers. Cold plasma instead is composed of low temperature particles (chemical species) and relatively high temperature electrons that do not cause any thermal material damage. Thanks to these properties, any damage produced on surface exposed to the plasma is avoided, and consequently this method can be used for heat-sensitive materials, such as biodegradable polymers [13]. However, the surface modification is not permanent and this effect is called “aging,” it is related to the temperature, crystallinity, humidity, and to the medium used [14]. Several results are reported by Pankaj et al. [1], on the application of cold plasma for the treatment of polymers such as polyethylene (PE), polypropylene (PP), and poly(ethylene terephthalate) (PET) used for food packaging application. Generally, the hydrophilicity/hydrophobicity ratio is modified, influencing the wettability of the polymer surface. Plasma can be used to remove the surface organic contamination (surface cleaning), to remove a weakly bonded layers, to increase the surface area (surface ablation and etching), to strengthen the surface layer (surface cross-linking), and to chemically modify the polymer surface (chemical modification). Depending on the chemical nature of the plasma, several reactions could take place at the polymer surface: by surface and interfacial reactions, by dry etching, and by plasma polymerization and deposition [15]. The first method uses gas plasmas such as argon, ammonia, carbon monoxide, carbon dioxide, fluorine, hydrogen, nitrogen, nitrogen dioxide, oxygen, and water to introduce surface functional groups on the polymer and to produce cross-linking in the polymer. The second method removes volatiles and contaminants materials from the polymer surfaces through plasma-induced chemical reactions and physical/chemical etching, induced by oxygen and oxygen–fluorine plasma. The third method deposits or grows a thin film of material on the polymer surface through polymerization of organic monomers or molecules (coming from hydroand fluorocarbons), providing unique chemical and physical properties [13, 16] (see Chapter 3). The polymers formed by plasma have a chemical structure and composition different from that of polymers obtained by conventional polymerization procedure. In general, they are highly cross-linked, insoluble, thermally stable, chemically inert, and mechanically tough (see Chapter 3). As a consequence, these three plasma methods offer an interesting freedom in the type of radiation that can be used and in the creation of modified surface films. As reported by Kowalonek et al. [17], several surface modification methods are used for food packaging. The most common plasma source is the capacity coupled plasma (CCP), used for applications such as deposition and cleaning [18]. The energy is supplied by a single radiofrequency power, typically at 13.56 MHz. The negative aspect is that the electrodes are placed inside the chamber and are exposed, as the polymer surface, to the plasma and to the reactive chemical species formed. If the plasma source is an inductive coupled plasma (ICP), the energy is supplied by electrical currents, produced by electromagnetic induction.

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The electrodes are outside the reaction chamber and consequently free of contamination. By electron cyclotron resonance (ECR), the electrons produced by a radiofrequency power, gain sufficient energy to cause ionization. The electron density is more than one order of magnitude higher in respect to CCP and ICP and consequently, being more efficient, could be used for food packaging surface functionalization, for surface cleaning, etching, and chemical deposition [19]. In general, plasma surface treatments are not permanent; consequently the surface tends to recover its original characteristics such as hydrophobicity. 12.2.2

Liquid and Bulk Phase Methods

In general, the surface of polymers is different from the bulk: impurities and contaminates can be absorbed by the surface from the environment or/and can come from the polymer bulk and be desorbed from the bulk phase to the polymer surface. Conversely, atoms, molecules, or ions could adhere to the polymer surface by adsorption and move to the bulk. Further, oxidation and/or hydrolysis (see Chapters 9 and 10) of chemicals groups could also change the surface polymers properties. All these adsorption and/or desorption processes change the polymer characteristics and consequently could be used for the modification of the polymer surface. 12.2.2.1

Adsorption Methods

In order to improve the dispersion of chemical species (atoms, molecules, ions) into the polymer and on the polymer surface (flocculation processes), modifications of the polymer surface have to be performed, and adsorption methods can be used as a mean to change the interaction at the polymer surface. For this purpose, a solution containing the desired chemicals is brought in contact with the polymer surface (and its concentration depletion can be used to measure the rate of interaction). In general, adsorption on polymers takes place through different steps (diffusion, attachment, and rearrangement) and is performed with the desired solvents (such as acetonitrile, dimethylformamide, chloroform, and toluene organic solvents). Conversely, by diffusion, chemicals are transported from the bulk to the polymer surface followed by attachment to the surface, thanks to hydrophobic and/or electrostatic interaction, polymer segments stabilize by rearrangement, on surface sites of the polymer. Several factors can influence the adsorption process. The type of solvent is the most important because in general stronger adsorption takes place when the interactions between the solvent and the polymer are not too strong. Further, polymers with high molecular weight, high density, and with a large surface to volume ratio are preferred. Finally, low temperature and high pressure are preferred to reach the maximum adsorption. 12.2.2.2

Desorption Method

By this method, desorption of surface active compounds from the bulk to the polymer surface can be forced through the addition of surface-active agents (surfactants). Thanks to their ability to reduce the surface tension as well as the interfacial tension of the polymer and they permit the dissolution of the polymer

12.2 Physical and Chemical Methods

in the liquid phase as well as the desorption from the surface. Surfactants can improve the polymer wettability, thanks to their hydrophilic moiety (anionic, cationic, or polar group) and hydrophobic moiety (long hydrocarbon chain), altering the polymer solubility. The hydrophobic part of the surfactant interacts with the hydrophobic part of the polymer, through hydrophobic interactions, lowering the surface and interfacial tension, improving at the same time the wettability of the polymer. The most used surfactants are sodium lauryl sulfate (SLS), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and zwitterionic ones [20]. 12.2.3

Interfacial Adhesion of Polymers

The use of silane coupling agents is one of the most common methods used to improve the interfacial adhesion of polymers with other biomaterials. These organic compounds have two kinds of reactive groups bonded to the silicon atom. The hydrophilic parts are hydrolysable alkoxy groups that react with water in order to form silanol groups; the hydrophobic part contains vinyl, amino, epoxy, mercapto, glycidoxy, and methacryloxy functional groups used to form oligomers through self-condensation. Thanks to this double nature, silane-coupling agents can be bound to hydrophobic polymers, through the hydrophobic part, and to hydrophilic fillers through the hydrophilic moiety. Further, adhesion between polymers and fillers can be improved by the producing of a smoother surface due to a lower viscosity during mixing. At the same time, improvement in the adhesion enhances the thermal and mechanical properties as well as the biocompatibility of the materials. For example, not only the adhesion between the PLA and fillers such as cellulose and clay was improved by silane coupling agents [15, 21] but also the adhesion to PCL, polybutylene succinate (PBS), polyvinyl alcohol (PVA), and PLGA polymers used for packaging [7, 22]. This procedure improves the thermal stability and increase the mechanical performances of such polymers. Among the silane-coupling agents, the most studied in literature is the 3-aminopropyltriethoxysilane (APS). As reported in Figure 12.2, it has three functional groups C2 H5 O— that can be hydrolyzed to give HO-groups and a C3 H7 NH2 that can be further reacted. APS must be activated before adding to the polymer. In this case, the APS is mixed in hot water, in order to form 3-aminopropylsilanetriol through hydrolysis, liberating ethanol. Then, it undergoes a self-condensation process, in order to form —Si—O—Si— and —Si—O—C— bonds (see scheme reported by Shimpi [7]). There are two possibilities of interaction: (1) Between the carboxylate group of the polymer and the amine group of the silane agent, forming a stable amide covalent bond. Figure 12.2 Schematic representation of 3-aminopropyltriethoxysilane (APS) coupling agent.

O H3C H3C

O Si O

CH3 NH2

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(2) Between –OH groups of the polymer and that of the hydrolyzed silane, by the formation of hydrogen bonds, converted to covalent bonds (C—O—Si bonds) during heating, liberating water. 12.2.4

Grafting and Polymerization

Grafting permits to bind various functional groups to the surface of polymers through covalent bonds. The use of ionizing radiation (see Chapter 8), UV (see Chapter 7), and ozone (see Chapter 9) as well as plasma (see Chapter 2, 3) is required for the initial modification of the surface with reactive groups. Generally the grafted polymer formed has the same chemical composition as the polymers obtained by conventional polymerization process (the monomer used for the polymerization process are the same) but improved reactivity, thanks to the activated superficial groups introduced by grafting [13]. These polymers are thermally stable, chemically inert, and mechanically tough.

12.3 Mechanical Method The surface of the polymers can be modified by mechanical methods, such as roughening and micromanipulation, in the absence of chemicals. By roughening, the polymer surface is changed from microrough to porous, while by micromanipulation, the polymer surface is modified under a microscope. The polymer surface can also be physically modified at micro- or nanolevel, with high precision, by a scanning tunneling microscope or an atomic force microscope. However, the cost of this procedure is still high and can be used only for polymers with high conductive surface (STM).

12.4 Biological Method Biomolecules such as enzymes, antibodies, proteins, cells, and drugs can be used for the biological modification of the polymer surface. In this case biomolecules can be “immobilized” temporarily or permanently on or within a polymer through physical adsorption or cross-linking, physical entrapment, and chemical attachment by covalent bonds [7, 23]. The biological modification is used principally in medical application, thanks not only to the absence of immunogenic responses but also to the lower cost and higher purity products by comparison to physicochemical and mechanical methods. The disadvantage is the resistance in mass transfer between the polymer substrate and the product and the possibility to inhibit the biomolecules. Physical adsorption is the easiest method, thanks to the possibility of treating the polymer surface with a solution containing the biomolecules. Due to the interactions such as Van der Waals, hydrogen bonds, electrostatic forces, and hydrophobic interaction, the biomolecules attach to the polymer surface, with no additional use of coupling agents or chemical modification. However, the

12.5 Surface Modification of Polymer for Food Packaging

interaction between the biomolecules and the polymer substrate are weak, thus creating a material with short-term stability. By chemical interaction, through covalent bonds, a more stable polymerbiomolecule material can be obtained. In this case, biomolecules functional groups as amines, thiols, carboxylic acids, and alcohols can react with the polymer surface by hydrogen, electrostatic, or hydrophobic interactions and be converted into covalent bonds upon heating. Consequently, the biomolecule is strongly immobilized on the polymer surface by chemical linking.

12.5 Surface Modification of Polymer for Food Packaging In many cases, the application of polymers to food packaging necessitates a functionalization of their surface, introducing new surface properties without compromising those of the bulk. 12.5.1

Applications

Actually, the surface modifications are used for three principal purposes: – For packaging sterilization. – For improving the printing process. – For influencing the mass transfer through the materials in order to tune the gas barrier properties as well as the low-molecular weight migration characteristics. 12.5.1.1

Surface Sterilization

Packaging plays a key role in order to preserve the food quality along the distribution chain and storage, to avoid food deterioration, damage, and contamination. The contamination of food can be further worsened if the material is not properly sterilized, adding health risks and economic losses. Beyond the traditional sterilization methods such as dry heat, steam, UV light, chemical treatments with ethylene oxide, and hydrogen peroxide, new technologies are now under investigation. Cold plasma is one of them because, as previously reported, it is a chemical free treatment, safe, and fast, without production of any residues. It can be applied to all packaging materials. The only problem associated with the application in the food industry is the sterilization time, in general extending to minutes, too long for the food industry. It is also used in the field of antimicrobial and active packaging, in order to introduce antimicrobial substances like lysozyme, nicin, vanillin, peptides, chitosan, silver, triclosan, and others [1]. 12.5.1.2

Printing

This process is generally used for printing the material surface the best-before date, the European Article Number, and all the information regarding the packed food. It is important that the imprints are safe for the health of consumers and secure against abrasion from the package. Plasma treatment could be used for

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this purpose, by modifying the surface materials wettability, the parameter closely related to the ink adhesion. The wetting of the material surface is closely related to the surface energy, the energy of adhesion between the solid and the liquid to the surface tension of the liquid [24, 25]. 12.5.1.3

Mass Transfer

Gas, vapor, and aroma compounds can permeate through the polymer wall, influencing the barrier behavior. Further, migration of low-molecular weight substances such as monomers, plasticizers, additives, solvents, and so on can modify the properties of the packed food. In order to control such mechanisms and consequently the food shelf life, different technologies could be employed that modify the polymer surface used for food packaging application. Plasma treatment is one of them and could be used to change the wettability of the material surface and consequently the interaction with the environment. As an example, by improving the hydrophobicity, the water permeability can be reduced. Of course, the percentage of reduction is related to the chemical surface polymers structure, as, for example, the content of polar groups. Plasma-enhanced chemical vapor deposition (PECVD) technology can be employed to improve the gas barrier behavior by the deposition of a thin layer of an organic substance such as, for example, thin layer of SiOx [26, 27], without compromising the film transparency. Regarding the migration process, it is most important because it can be considered a potential risk for the consumers’ health. The use of plasma technology for surface sterilization, adhesion and printability enhancement, and so on could promote the migration of low molecular weight substances formed when plasma is applied. The effect on migration is still not well investigated. Audic et al. reported the modification by cold plasma of polyvinyl chloride (PVC) flexible films in regard to the migration of two different plasticizers used during film production [28]. They found that after treatment with argon plasma, the migration showed a significant reduction. 12.5.2

Polymers

The most common synthetic polymers used in the food industry are PE, PP, and PET, which cover more than 80% of the total food packaging polymers. Besides these materials, biodegradable polymers coming from natural resources are currently playing an important role, not only as food packaging materials but also in engineering application and biomedicine such as medical devices and tissues. The most common are biodegradables aliphatic polyesters such as PLA, polyglycolic acid (PGA), PLGA, PCL, polyhydroxyalkanoates (PHA), and PBS. Both synthetic and natural polymers have been extensively investigated including their properties, and in order to improve their surface functionality and efficiency, technological methods have been developed. PE is the most common polymer used for food packaging application. Optimization of its performance can be achieved by varying its density and/or modulating its characteristics such as water vapor transmission rate, gas transmission rate, tensile strength, and heat sealability. Due to its high hydrophobicity and low surface energy, PE surface has often to be modified; this can be achieved

12.5 Surface Modification of Polymer for Food Packaging

through cold plasma. The proposed mechanism, by CO2 plasma, was described by Pankaj et al. and by Médard et al. [1, 29, 30], together with the results on the final properties such as WCA, crystallinity, and roughness after plasma treatment, in different conditions. PP is used as packaging material, thanks to its interesting properties such as low density, high melting point, good sealability, inert nature, and low cost. To improve the printing, coating, and lamination, a surface treatment has to be employed. In particular, plasma treatment permits to modify different properties such as WCA, adhesion, roughness, total surface energy, ink adhesion, and polarity, as carefully reported by Pankaj et al. in their review [1]. PET shows interesting properties for food packaging application such as good strength, rigidity, transparency, thermal stability, gas barrier property, chemical resistance, formability, and so on. However, like other synthetic polymers, due to the low surface energy and high hydrophobicity, a surface modification is required in order to improve properties such as adhesion, printing, and dyeing. The result of treatment by oxygen, carbon dioxide, nitrogen, and helium cold plasma has been reported by different authors [31, 32], and the results were summarized by Pankaj et al. in their review [1]. PLA is the most studied biodegradable polyesters, used for food applications. Hirotsu et al. [33], in 1997, first treated this polymer with oxygen, nitrogen, and helium plasma, recording an etching effect and an increase of surface hydrophilicity (by using scanning electron microscopy (SEM) and weight loss measurements). Further, they analyzed the effect of plasma on poly-l-lactic acid (PLLA) wettability [34]. A strong decrease of the WCA was recorded within 30 seconds of oxygen and helium plasma treatments, as a result of chemical changes on the polymer surface. Recently, also other researchers analyzed the effect of oxygen, air, nitrogen, helium, and argon plasmas on PLLA polyester [13]. They analyzed the effect on the chemical surface modification by X-ray photoelectron spectroscopy (XPS), in order to highlight differences in chemical composition between untreated and plasma-treated surfaces. They observed a significant increase in C—O and O—C=O bonds and a decrease of the C—H and C=O functional groups. Besides the chemical properties of the plasma-treated PLA films, the surface morphology was investigated by atomic force microscopy (AFM), highlighting the etching effect of plasma treatment and the presence of oxygen atoms on the polymer surface [35]. Similar behavior was also detected for PLGA polymer. Beside the oxygen plasma technology, several modification methods were also used such as chemical methods (sulfuric acid, chloric acid, sodium hydroxide) and physical methods (atmospheric pressure air discharge). Among all, chloric acid treatment and plasma were the most effective in incorporation oxygen on the plasma-modified films [13]. Others have applied air, ammonia, and CO2 plasmas for the surface modification of PLGA, showing an increase of the nitrogen and oxygen functional groups and an increase of the surface hydrophilicity [36–38]. PCL, a biodegradable aliphatic polyester, was surface modified by the use of oxygen, helium, and air plasmas, with similar effect as PLA and PLGA such as increased surface hydrophilicity and higher oxygen content. Lee et al. [39] studied the influence of several gas plasmas such as argon, argon + hydrogen,

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argon + nitrogen, and argon + oxygen. They found that when argon + nitrogen was used as plasma, an increase of the WCA, an increase of nitrogen content, and a decrease of the oxygen content were detected. These concentrations were determined by XPS measurements. Regarding the PHA polymers, the most studied was the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), treated with oxygen plasma [13]. By introduction of oxygen functional groups on the PHBV surface, a decrease of the WCA was recorded. In further studies, conducted by Wang et al. [40], the effect of oxygen and nitrogen plasma was considered on the surface wettability as well as on the chemical composition. While the effect of such gas plasma was similar on the WCAs, the effect on chemical composition, analyzed by XPS, shows some differences: C—O and C—C bonds were equally cleaved from both oxygen and nitrogen plasma, but the formation of COOH was promoted by oxygen, while the formation of C—N, C=N and amide bonds was promoted by the use of nitrogen plasma. On PBS, surface treatments were conducted in order to understand the effect of etching [41]. In this case, the most interesting effect was detected by the use of pulsed plasmas, generated by changing the ratio of the duty time, to obtain different on–off ratios. The etching effect was practically suppressed, especially when oxygen was used as plasma. The hydrophilicity was increased but the effect on the increased wettability was not attributed to a particular chemical group.

12.6 Conclusion In conclusion, synthetic polymers as well as biodegradable and biobased polymers are without doubt useful for different applications, from engineering to medical as well as food industries. However, their surface properties are correlated to their bulk properties and further, most of the time, the polymer surface is inert under ambient condition. In order to meet the specific properties required for special applications, most often a surface modification is required, involving mainly alterations of the surface wettability, roughness, adhesion, and adsorption of biomolecules. Generally, the principal objective is to modify the polymer surface in order to enhance the adhesion and the adsorption of materials and/or chemicals, without compromising its structure and bulk property. As a result, different techniques could be followed for the surface modification of polymers, in order to increase their reactivity. Consequently, inclusion of different chemical groups on the surface has gained even more interest in fields such as medicine, electronics, packaging and packaging processes, protective coatings, textile, and so on. Chemical surface modification techniques have been extensively studied and used, but researchers are now very active in trying to avoid the use of hazardous solvents. Consequently, solvent-free processes are preferred. Plasma treatment, as well as plasma grafting and plasma polymerization, is at the moment the most interesting method to perform a surface polymer modification, especially for food packaging application. Uniformity, reproducibility, short reaction time and environmental respect are the most interesting advantages of

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13 Surface Modification of Water Purification Membranes Anthony Szymczyk 1 , Bart van der Bruggen 2 , and Mathias Ulbricht 3 1 Univ Rennes, CNRS, ISCR, Institut des Sciences Chimiques de Rennes, UMR6226 35000 Rennes, France 2 Katholieke Universiteit Leuven, Department of Chemical Engineering, Celestijnenlaan 200F, B-3001 Leuven, Belgium 3 Universität Duisburg-Essen, Lehrstuhl für Technische Chemie II, 45117 Essen, Germany

13.1 Introduction Although it covers around 70% of the Earth’s surface, only 3% of the Earth’s water is freshwater, and more than 99% of this is locked in polar ice and groundwater so that only ∼0.01% of the global water resources is usable for people, mainly in the form of water for personal consumption, irrigation, and industry [1]. The availability of potable water has become nowadays a worldwide problem due to the continuous growth in water demand not balanced by an adequate recharge. Moreover, more and more often water sources are suffering from a worsening of their quality due to the indiscriminate discharge of both domestic and industrial effluents without adequate treatments [2]. The only methods to increase water supply beyond what is available from the hydrological cycle are desalination and water reuse [3]. In this respect, membrane processes are among the most effective technologies for water treatment, especially for unconventional water resources such as brackish water and seawater, as well as wastewater [4]. All membranes can be described as selective filters. However, depending on the intended separation and the utilized driving force for transport through the membrane, a range of different barrier structures can be utilized. Of particular relevance for water purification are micro- and ultrafiltration (UF) membranes with a porous barrier enabling separation of (colloidal) particles mainly based on size, and nanofiltration and reverse osmosis membranes with a dense barrier layer where transport and selectivity are mainly based on solubility and diffusivity in/through the barrier. Important examples for entirely different membrane processes are electrodialysis, where dense ion-exchange membranes are used for the removal of salt from water by an electrical potential gradient, or membrane distillation where water vapor can be transferred from one to another aqueous stream separated by a porous hydrophobic membrane driven by a temperature gradient.

Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Organic membranes are the most widely used membranes in industrial processes due to their ease of processability at large scale and low manufacturing cost compared with inorganic membranes. Hydrophobic polymers, like poly(vinylidene difluoride) (PVDF), poly(ether sulfone) (PES), and aromatic polyamide (PA), are frequently used for organic membrane preparation as they are intrinsically more chemically stable in water than their hydrophilic counterparts, for instance, cellulose [5]. However, hydrophobic membranes exhibit a limited pressure-driven water flux and are more prone to fouling by organic substances (e.g. proteins, humic acids,…) and microorganisms (biofouling); this represents the Achilles heel of membrane processes (fouling results in a progressive permeate flux reduction during operation and shortens the membrane lifespan, thus increasing the operational cost [6]). In this respect, less hydrophobic membranes are desirable due to the expected weaker interaction between the foulant(s) and the membrane surface. Owing to the aforementioned issues, it appears that a single polymer matrix is unable to offer all the required properties for specific applications in aqueous phase and fruitful modification routes of polymer membranes have to be implemented [7, 8]. When dealing with membrane separations carried out in the aqueous phase, the main intentions of membrane modification are (i) enhancing the membrane permeability at preserved separation selectivity, (ii) decreasing the fouling propensity by increasing the hydrophilicity of the membrane and/or decreasing its roughness, (iii) improving the membrane chemical stability, and (iv) modifying the membrane surface charge density in order to improve the rejection of charged solutes and/or ion selectivity. Membrane modification can be achieved by either pre-modification or postmodification methods. Pre-modification refers to the incorporation of additives into the monomer or polymer solutions before the membrane is formed so that these additives are embedded in the polymer matrix. Such pre-modification methods include blending with hydrophilic polymers (e.g. polyvinylpyrrolidone (PVP) [9], poly(ethylene glycol) (PEG) [10]) and the incorporation of fillers such as carbon nanotubes [11, 12], metals (e.g. Ag) and metal oxides (TiO2 , ZrO2 , Fe3 O4 ,…) nanoparticles (NPs) [13, 14], metal organic frameworks [15, 16], aquaporins [17], polymerosomes [18], polyrhodanine NPs [19], carbon quantum dots [20], etc. These pre-modification techniques are, however, out the scope of this chapter, which exclusively deals with post-modification of membranes. This latter refers to modification techniques that are applied after the membrane has been formed. A post-modification strategy allows for an alteration of the surface morphology and composition of polymer membranes, while the bulk structure of the base membrane can be essentially preserved. Importantly, methods and conditions must be identified where the barrier properties of the membrane are at least preserved from damage or, preferably, improved with respect to the usually observed trade-off between permeability and selectivity [21]. The main physical and chemical post-modification methods that have been developed to improve the performance of polymer membranes intended to separations in the aqueous phase (drinking water production, wastewater treatment,…) are presented and discussed in the following sections.

13.2 Irradiation-Based Direct Polymer Modification

13.2 Irradiation-Based Direct Polymer Modification 13.2.1

Plasma Treatment

Low temperature plasma techniques can be used for tailoring the surface properties of polymer membranes. The energized species in the plasma include ions, electrons, radicals, metastables, and photons [22]. An appropriate selection of the plasma source and operating parameters allows introducing various functional groups onto membrane surfaces. For instance, O2 plasma leads to the incorporation of oxygen in the polymer membrane surface in the form of hydroxyl, carbonyl, and carboxyl groups, while nitrogen containing plasmas can yield amine, imine, amide, and nitrile surface groups [23]. Such functional groups can induce changes in properties of membranes such as surface energy, wettability, and surface roughness [22, 24, 25]. For instance, Figure 13.1 illustrates the decreases in the contact angle of water on polysulfone (PSf) UF membranes after various water vapor plasma treatment times. Similarly, treatments with He, N2 , and O2 plasma were found to increase the surface hydrophilicity of UF membranes made of poly(vinyl chloride) (PVC) but with an impact of the treatment time on the surface morphology (pore enlargement) [27]. However, for optimized He/H2 O plasma treatment or He plasma treatment followed by exposure to air, polyacrylonitrile (PAN) UF membranes could be efficiently hydrophilized without detrimental effects on barrier properties; this had been interpreted to be caused by parallel oxidation and stabilization (via cyclization) of the membrane polymer [28]. Plasma treatments can also be used to increase the membrane surface hydrophobicity instead. For instance, Wu et al. observed a significant increase of the water contact angle from 47∘ to more than 130∘ after treatment of PAN membranes with a CF4 plasma as a result of the insertion of fluorine atoms, as confirmed by FTIR spectroscopy (Figure 13.2) [29]. The chemically reactive functional groups generated on the membrane surface after plasma treatment (e.g. free radicals, peroxides, …) can serve as 100 90

50 40

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Figure 13.1 Water contact angle as a function of time (age of the drop) for PSf ultrafiltration membranes treated by water vapor plasma for various times; (a) immediately after plasma treatment and (b) after two months aging under ambient laboratory conditions. Source: Reprinted with permission from Pegalajar-Jurado et al. [26]. Copyright 2016, John Wiley & Sons.

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Figure 13.2 FTIR spectra of the untreated (PAN) and CF4 -plasma-treated (PAN-C) membranes (C1, C2, and C3 refer to glow discharges initiated at radio frequency powers of 80, 120, and 160 W, respectively). Source: Reprinted with permission from Wu et al. [29]. Copyright 2018, Elsevier.

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promoters for polymer chain cross-linking or further surface functionalization (see plasma-induced grafting in Section 13.4.1). Although plasma treatment has a lot of advantages (it is a fast, solvent-free, and surface selective technique), its main drawback is its nonspecific nature that results in difficulties to obtain a monofunctional surface with only one kind of functional groups [22]. Moreover, a reported disadvantage of plasma modification is the time dependency of the induced changes [24]; notably polymer chain migration can result in a gradual loss of the surface properties [23] (see also Figure 13.1). 13.2.2

UV Irradiation

Photo-irradiation using UV light is another technique that can be used for polymer membrane post-modification, notably for surface layer cross-linking in order to improve the chemical and mechanical properties of polymer membranes. The UV-initiated cross-linking reaction can occur if the polymer membrane contains photoreactive groups (Figure 13.3a) or by addition of a photoreactive agent (Figure 13.3b). In both cases the presence of a hydrogen donor in an adjacent polymer chain is required. The photoreactive group is first excited by UV irradiation, followed by the abstraction of hydrogen from the donor. Two radicals are then produced, which can further recombine, thus leading to polymer chain cross-linking. In the case of polymers containing double bonds, the addition of a photoreactive agent can also produce polymer-bound double bonds (Figure 13.3c). In general, the degree of membrane cross-linking increases with time of exposure to UV irradiation, but this latter should be optimized as it has been reported that prolonged UV treatment may increase the brittleness of polymer membranes [30]. It has also been observed that UV light irradiation is likely to produce other phenomena such as polymer chain densification without formation of covalent bonds [31]. More details, in particular examples for the

13.2 Irradiation-Based Direct Polymer Modification

C C

H

C

hv

O

OH

C

Photoreactive group

(a)

OH C

C

H

C

O hv

OH

C +

2 H

C

C

C Added photoreactive agent

(b)

C

hv

O H

(c)

Added photoreactive

Double bond

Figure 13.3 Photo-initiated cross-linking reaction (with photoreactive benzophenone as example) and resulting network structure. (a) photoreactive polymer, (b) polymer with added photoreactive agent, and (c) polymer containing double bonds and added photo-initiator system. Source: Reprinted with permission from He et al. [30]. Copyright 2009, Elsevier.

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different approaches shown in Figure 13.3, can be found in a specialized review on that topic [30]. 13.2.3

Irradiation with High Energy Sources

High energy irradiations, such as electron or ion beams, have also been considered to modify membrane surface properties. Li et al. used high temperature electron-beam irradiation to cross-link PES films [32]. It was shown that the high energy of electrons caused PES-chain scission and that the cross-linking structure was formed through recombination of most of the radicals produced by chain scission. Chennamsetty et al. irradiated sulfonated PES membranes by 25 keV H+ ions with various irradiation fluences [33]. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analyses revealed that some of the sulfonic and C—H bonds were broken while new C—S bonds were formed after irradiation. Although both the pore size and hydrophobicity of the membranes were found almost unaffected after irradiation, a higher flux along with a weaker fouling propensity was obtained with irradiated membranes, which was attributed to the decrease in the membrane surface roughness after ion beam irradiation, as highlighted by atomic force microscopy (AFM) analyses (Figure 13.4). Indeed, the surface roughness decreased by 41%, 48%, and 53%

1

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Virgin membrane: roughness: 5.632 nm ± 1.323 nm

5 μm 1

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1 × 1014 ions/cm2: roughness: 2.673 nm ± 0.683 nm

Figure 13.4 AFM images of sulfonated PES membranes before and after irradiation by 25 keV H+ ions with various irradiation fluences. Source: Reprinted with permission from Chennamsetty et al. [33]. Copyright 2006, Elsevier.

13.3 Coatings

after irradiation with ion fluences of 1 × 1013 ions/cm2 , 5 × 1013 ions/cm2 , and 1 × 1014 ions/cm2 , respectively. It should be noted, however, that excitation with high energy irradiation has a low selectivity and bond scissions in the volume of the membrane cannot be avoided [34, 35].

13.3 Coatings Membrane coating refers to a variety of post-modification techniques leading to the formation of one of several layer(s) on top of an existing membrane surface without the creation of covalent bonds (intermolecular interactions are mainly responsible for adhesion between a coating and the underlying membrane). 13.3.1

Coatings from Gas Phase

Physical vapor deposition (PVD) is the general term that encompasses a variety of deposition methods allowing coating by thin inorganic films. Vapor deposition processes include multiple steps such as heating, evaporation, sputtering of vaporized materials, and deposition [22]. A few studies reported on polymer membrane coating via PVD. For instance, catalytic hybrid metal–polymer membranes have been recently fabricated by magnetron sputtering [36]. A schematic representation of the different fabrication stages is shown in Figure 13.5. A dense

Argon plasma

Etchant

Fe/Pd deposition

Dealloying (b)

(a) Bare membrane

80 at.%Fe/20 at.%pd dense Fe/Pd film

(c) 20 at.%Fe/80 at.%pd Fe/Pd nanoporous film

Figure 13.5 Fabrication of metallic thin film composite membranes by magnetron sputtering. Source: Reprinted with permission from Detisch et al. [36]. Copyright 2018, ACS.

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13 Surface Modification of Water Purification Membranes

layer of Fe/Pd alloy with atomic composition 80% Fe/20% Pd was first formed onto the surface of both PSf UF membranes and PVDF microfiltration membranes. The Fe/Pd film was further etched with sulfuric acid, which removed most of the iron in the coating, thus creating a nanoporous film with atomic composition 20% Fe/80% Pd. The resulting catalytic membranes exhibited promising results for dechlorination of toxic chlorinated organic compounds under H2 pressurization. This could be of relevance for advanced water purification. Some attempts have also been made to modify the surface of polymer membranes by chemical vapor deposition (CVD) techniques. These latter gather a series of deposition methods in which a substrate is exposed to vaporized precursors that can decompose or react onto the substrate surface, thus forming a thin coating layer. For instance, CVD was recently used to cover a PES membrane with graphene layers that were further exposed to pulsed oxygen plasma etching in order to obtain atomically thin nanoporous skin layer [37]. Unlike pressure-driven membrane processes for which hydrophilization of the membrane surface is usually sought for, membrane distillation requires using hydrophobic membranes (see Section 13.1). Initiated chemical vapor deposition (iCVD) was used to coat poly(trimethyl hexamethylene terephthalamide) hollow fiber membranes with a layer of hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate), thus demonstrating the possibility to construct membranes suitable for membrane distillation applications even from intrinsically hydrophilic materials after surface modification (hydrophilic polymers can be more easily processed by electrospinning than their hydrophobic counterparts and are therefore better suited to the fabrication of high surface-area-to-volume ratio fibrous membranes) [38]. Atomic layer deposition (ALD) is a sequential CVD technique based on the use of different precursors that react with the substrate surface one at a time in a sequential, self-limiting manner. Figure 13.6 shows a scheme of the ALD process. The vaporized precursor A first adsorbs onto the surface. The excess

Initial substrate

Precursor A

Purge

Purge

Precursor B by-product

Figure 13.6 Schematic representation of the ALD process. Source: Reprinted with permission from Feng et al. [39]. Copyright 2018, Elsevier.

13.3 Coatings

160

Water contact angles (°)

140 120 100 80 60 40 20 0

0

100

300 400 200 Number of ALD cycles

500

Figure 13.7 Water contact angles measured on Al2 O3 -modifed PTFE membranes as a function of the number of ALD cycles. Source: Reprinted with permission from Xu et al. [42]. Copyright 2012, Elsevier.

of A is subsequently removed by flushing the surface with a purge gas. Then, the vaporized precursor B reacts with A, which produces a uniform coating onto the substrate. Another purge step is performed to remove the excess of B and several ALD cycles can be repeated. ALD on polymers was not performed until recently because most polymers decompose at the temperatures required for many ALD systems. However, low temperature ALD has been successfully applied for surface modification of some organic polymers [40]. Notably, ALD has been demonstrated to be an effective tool for surface modification of polytetrafluoroethylene (PTFE) with controlled surface deposition of TiO2 (using titanium isopropylate and water as precursors) [41] and Al2 O3 (using trimethylaluminum and water as precursors) [42] with a significant increase in the membrane surface hydrophilicity (Figure 13.7). 13.3.2

Coatings from Wet Phase

Coating of membranes by polymer layer(s), which can be achieved by simple techniques, such as dip-coating or spin-coating, has been extensively considered in the literature. Such a simple surface modification strategy has also been considered for some commercial reverse osmosis membranes. Reverse osmosis membranes used for seawater or brackish water desalination are mainly aromatic PA membranes, which are synthesized by interfacial polymerization from polyfunctional acyl chlorides and amines onto a macroporous support. The PA skin layer formed by the interfacial polymerization process is hydrophobic and typically exhibits a ridge-and-valley topology with a roughness of the order of 100 nm, which makes this kind of membranes prone to fouling. As poly(vinyl alcohol) (PVA) is a highly hydrophilic polymer with good chemical and thermal stability, it has been considered as coating layer to reduce the fouling of polymer membranes

371

372

13 Surface Modification of Water Purification Membranes

Membrane

PA

Resin

PS Coating

Resin

PA

200 nm (a)

PS

Ridge and valley structure

Ridge and valley structure 200 nm

Membrane

(b)

Figure 13.8 Transmission electron microscopy (TEM) micrographs of (a) uncoated and (b) coated PA membranes. Source: Reprinted with permission from Tang et al. [44]. Copyright 2007, Elsevier.

[43] by increasing the membrane surface hydrophilicity and filling the valleys of the rough skin layers of PA reverse osmosis membranes (Figure 13.8). Very thin PVA coating layers have been highlighted at the surface of some commercial reverse osmosis membranes by characterization techniques such as X-ray photoelectron spectroscopy (XPS) (see Table 13.1; the membranes coated with an oxygen-rich polymer layer exhibits an oxygen-to-nitrogen ratio higher than the theoretical maximum for PA) or 𝜁 potential determination. Indeed, as shown in Figure 13.9, the 𝜁 potential of the uncoated PA membrane (ESPA4) first decreases as pH increases and then reaches an almost constant value for pH > 8, which indicates deprotonation of all carboxylic acid groups at the membrane surface. On the other hand, for the coated membranes (SW30HR and BW30), an inflection point is observed at pH c. 8.5, which indicates the start of the deprotonation of other functional groups (belonging to the coating layer) with very weak acid properties (pK a in the range 10–11, which is consistent with the pK a of the hydroxyl groups of PVA [46]). Tang et al. used UV-curing to produce cross-linked PVA UF membranes suited for the separation of oil/water emulsions [43]. While PVA cross-linking from the standard chemical method using glutaraldehyde can last several hours, an efficient and water-resistant cross-linked surface layer was obtained by exposure of UV-reactive PVA (PVA was first modified by addition of acryloyl chloride) to UV irradiation at 365 nm for 20 seconds at room temperature. Recently, mussel-inspired polydopamine (PDA) coatings have received wide interest due to the advantages of eco-friendliness, adhesion nature, and filmforming feasibility [47]. Indeed, a tightly adhesive hydrophilic PDA coating layer can be formed via treatment of a surface with a tris(hydroxymethyl) aminomethane hydrochloride buffered dopamine solution (Figure 13.10), regardless of the substrate material [49–52]. PDA coating onto PA reverse osmosis membranes was shown to reduce adhesion of oil to the membrane

13.3 Coatings

Table 13.1 Elemental compositions and oxygen to nitrogen (O/N) ratios of commercial reverse osmosis membranes from XPS analysis. XPS surface elemental analysis Membrane

O (%)

N (%)

C (%)

O/N ratio

XLE

12.6 ± 0.5

13.2 ± 0.7

74.3 ± 0.5

1.0

LE

13.1 ± 0.9

12.1 ± 0.6

74.8 + 0.6

1.1

ESPA3

12.8 ± 0.8

12.9 ± 1.0

74.3 ± 1.0

1.0

SWC4

13.2 ± 0.2

11.2 ± 0.8

75.5 ± 0.8

1.2

lfc1

22.2 ± 1.5

7.4 ± 0.8

70.4 ± 1.0

3.0

LFC3

22.4 ± 1.1

7.3 ± 0.8

70.3 ± 1.7

3.1

BW30

29.0 ± 1.5

3.1 ± 1.0

67.9 ± 0.6

9.3

SW30HR

4.9

27.0 ± 7.4

5.5 ± 4.9

67.5 ± 2.9

SW30HR (N rich)

20.3 ± 0.9

10.0 ± 1.1

69.8 ± 0.9

2.0

SW30HR (N lean)

33.8 ± 2.5

l.0 ± 1.2

65.2 ± 2.2

35

Fully cross-linked

12.5

12.5

75.0

1.0

Fully linear

19.1

9.5

71.4

2.0

Theoretical values

The theoretical elemental compositions were computed based on a PA layer formed by trimesoyl chloride and 1,3-benzenediamine. Source: Reprinted with permission from Tang et al. [44]. Copyright 2007, Elsevier.

20 10 0 ζ (mV)

0

2

4

6

8

10

pH

–10 –20 –30 –40

SW30HR BW30 ESPA4

–50

Figure 13.9 pH dependence of the 𝜁 potential of various commercial uncoated (ESPA4) and coated (SW30HR and BW30) PA membranes. Source: Adapted with permission from Idil Mouhoumed et al. [45]. Copyright 2014, Elsevier.

surface [53]. Analogous beneficial effects had been achieved for UF membranes from PES or PVDF [54, 55]. Despite a variety of advantages in PDA-assisted coating, self-polymerization of dopamine assisted by air oxidation under alkaline condition is time-consuming and dopamine easily aggregates via non-covalent interactions such as hydrogen bond and π–π stacking [56]. The inevitable aggregation of PDA during the

373

374

13 Surface Modification of Water Purification Membranes HO

HO

NH2

NH2

COOH HO

M2+

Low concentration

DOPA

HO

HO Dopamine

High concentration tris buffer

HO COOH N H

HO DHICA

HO

PVA PVA, H2O2, glycation

DHI

COOH N H

HO

DHICA-based melanin

n

Free radical scavenger, acidic groups, excited state deactivation

Antioxidant

Watersoluble melanins

UVabsorbing materials

Photoprotection

Metal chelation

Amine-rich melanin

HO

Zeolite HO

HO

Hybrids

Optoelectronics

OH

H2N

N H

Visible absorption, semiconductor behavior

Nanomedicine

n

Adhesion, basic groups

N H DHI-based melanin

OH

n

Surface coating, adhesion

Nanoparticles, drug delivery Organic electronics

Figure 13.10 Unifying tailoring strategy for PDA and Eumelanin synthesis. Source: Reprinted with permission from d’Ischia et al. [48]. Copyright 2014, ACS.

self-polymerization process of dopamine leads to an uneven coating layer, which may limit the performance of PDA-coated membranes such as their anti-fouling properties [57]. Several strategies have been proposed for eliminating PDA aggregation and expediting deposition rate. For instance, the addition of polyethylenimine (PEI) was shown to reduce self-aggregation of PDA through covalent connections between amino and catechol groups [58]. Moreover, the covalent cross-linking between PDA and PEI led to a better stability in alkaline environment [59]. Zhang et al. [60, 61] reported a defect-free PDA coating enabled by a Cu2 SO4 /H2 O2 system, in which a large amount of reactive oxygen species (ROS) is generated to promote dopamine polymerization. Recently, Zhu et al. reported a PDA deposition protocol onto hydrolyzed PAN membranes triggered by a FeCl3 /H2 O2 system under acidic conditions [47]. Similar to a Fenton system, Fe3+ catalyzes the formation of oxygen-containing radicals by H2 O2 , which was shown to considerably shorten the deposition time of PDA while improving the coating stability due to chelation of Fe ions with the PDA functional groups [47]. The resulting PDA-coated hydrolyzed-PAN membranes showed an improved organic fouling resistance, characterized by an irreversible fouling ratio of 6.6% after filtration of a bovine serum albumin protein solution, compared with 22.7% for the uncoated hydrolyzed-PAN membrane. Another strategy for membrane coating in wet phase is the layer-by-layer (LbL) adsorption of polyelectrolytes based on supramolecular assembly [62]. This self-assembly technique consists of the sequential deposition of oppositely

13.3 Coatings

charged polymers by alternating immersion of a substrate in baths containing positively and negatively charged polyelectrolyte aqueous solutions [63]. It enables the construction of ultrathin films with well-defined thickness, composition, and chemical functionalities [64]. Although the layers are not ideally ordered, the building principle enables the compensation of defects in surface coverage [35]. Various LbL modified membranes have recently been developed for different specific purposes by selecting various polyelectrolyte species, adjusting fabrication conditions and altering surface functionalities [65–68]. For instance, Malaisamy and Bruening used the LbL technique to convert polymer UF membranes into nanofiltration membranes exhibiting a pure water flux twice as large as that of commercial nanofiltration membranes [65]. The LbL approach has also been applied to the modification of ion-exchange membranes. For example, protonated poly(allylamine)/poly(4-styrenesulfonate) multilayer films were formed on Nafion 115 membranes to generate a permselective layer, with a tremendous enhancement of the monovalent/divalent cation selectivity [69]. The LbL approach has been recently extended to the electro-deposition of polyelectrolyte layers onto ion-exchange membranes with a benefit in terms of stability of the multilayer coated on the membrane [4, 70]. For the development of the LbL technology toward real applications, it is relevant that the preparation of such coated membranes can be done in membrane modules and that it can also be facilitated by flux through the module and membrane. Thus, LbL deposition can easily be applied to capillary UF membranes as support, leading to another nanofiltration membrane format [71]. It has also been demonstrated that such high-flux nanofiltration membranes can be cleaned by backflushing [72]. In addition, the removal of the barrier layer and its subsequent redeposition is an interesting option with respect to long-term application of such advanced nanofiltration membranes [73]. During the LbL assembly, electrostatic attraction, hydrogen bonding, and/or chemical bonding could be involved in the attachment of the polyelectrolyte layers onto the membrane surface. However, the electrostatic attraction is the most common force exploited [74]. Electrostatic attraction has also been explored to attach NPs onto the surface of polymer membranes. Notably, positively charged PEI encapsulated Ag NPs [75] or Cu NPs [76] have been deposited onto the surface of negatively charged membrane in order to enhance their antimicrobial and antifouling properties. Ben-Sasson et al. modified the surface of negatively charged a PA reverse osmosis membrane with positively charged PEI-encapsulated Cu NPs via a simple dip-coating procedure and further investigated the biocidal properties of the modified membranes. The Cu NPs functionalized membrane exhibited a significant antibacterial activity, leading to an 80−95% reduction in the number of attached live bacteria for three different model bacterial strains (Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus) [76]. Membranes with photocatalytic properties have been developed by incorporation of TiO2 NPs onto the surface of polymer membranes containing –COOH, –SO3 H or sulfone groups through coordination and H-bonding interactions [74]. High-flux

375

376

13 Surface Modification of Water Purification Membranes

superhydrophobic membranes with potential application in membrane distillation have been developed by electrospraying of hydrophobic functionalized TiO2 NPs onto the surface of poly(vinylidene fluoride-co-hexafluoropropylene) hollow-fiber membranes [77]. Graphene oxide (GO) nanosheets, which contain carboxyl, hydroxyl, and epoxide functional groups, are two-dimensional nanomaterials that have emerged as attractive modification materials to obtain uniform, stable, and multifunctional nanocomposite membranes with high chemical stability, strong hydrophilicity, and excellent antifouling properties [78–81]. The interest in such modification is often driven by the strong antibacterial properties of GO against a wide variety of microorganisms, including Gram-positive/negative bacteria, phytopathogens, and biofilm-forming microorganisms [82, 83]. Porous crystalline materials such as metal–organic frameworks (MOFs) have also drawn widespread attention because of their exceptionally high surface area, large pore volume, high degree of crystallinity, and alterable pore functionalities [84–86]. Of specific interest for surface modification are hybrid nanocomposites based on GO and MOFs [87]. For example, Hu et al. used two-dimensional ZIF-8/GO hybrid nanosheets as seeds to prepare defect-free ultrathin molecular sieving membranes for gas separation [88, 89]. Huang et al. developed a novel bicontinuous ZIF-8@GO membrane through LbL deposition of a GO suspension on a semicontinuous ZIF-8 layer; the ZIF-8@GO membranes show a high hydrogen selectivity [90]. Petit and Bandosz synthesized MOF–GO nanocomposites with different ratios of GO and MOF for gas adsorption [91]. Kumar et al. prepared GO@ZIF-8 hybrid nanocomposites with tunable morphology and porosity in addition to significant CO2 uptake capability [92]. Conventional antibacterial agents like metal ions (e.g. silver or copper) or NPs, titanium dioxide, GOs, chitosan, and enzymes have often been suggested for the preparation of antibacterial membranes. However, all of these traditional antibacterial agents are restricted by some drawbacks such as low stability, activity, and selectivity [93–97]. An improvement is the development of ZIF-8/GO hybrid nanosheets through facile in situ growth; ZIF-8 can be synthesized at room temperature on GO and may carry antibacterial properties [98]. The easiest approach for modification of a given membrane is by creating an additional layer via interfacial polymerization. When using nanomaterials directly in a modification layer, aggregates may be formed in the dispersion and membranes, ultimately negatively impacting the role of the nanomaterials in the eventual membranes, even when sonication is applied [99]. For example, the in situ growth of ZIF-8 onto GO surface can largely obliterate the agglomeration of ZIF-8 in the membrane due to the coordination between Zn2+ and the carboxyl groups present on GO, which can disperse Zn2+ uniformly on the GO surface. The release of zinc ions will also give rise to the antibacterial activity of ZIF-8 because of the natural antibacterial property of metal ions [100, 101]. MOFs can act as a reservoir of metal ions, providing their gradual release and resulting in a sustained antibacterial action similar to the effect of metal/metal oxide NPs. Therefore, a synergistic effect involving GO and ZIF-8 is a possible pathway to enhance the antibacterial activity of thin film nanocomposite (TFN) membranes. The antibacterial mechanism of ZIF-8/GO hybrid nanosheets-functionalized TFN membranes (TFN-ZG) is presented in

13.3 Coatings

GO was covered by

polymide TFN-GO membrane

(a) Synergistic antimicrobial effect of ZIF-8 and GO Release of Zn2+ by ZIF-8

Dead cell ROS

Live cell

Plasma membrane disorganization

Cellular internalization

Oxidative stress by GO Depletion of cellular antioxidants

ROS

ZIF-8/GO hybrid nan osh

eets

(b)

TFN-ZG membrane

Figure 13.11 Schematic representation of the structure and antibacterial effect of (a) TFN–GO membranes and (b) TFN–ZG membranes. Source: Reprinted with permission from Wang et al. [87]. Copyright 2016, ACS.

Figure 13.11. Oxidative stress is thought to be the primary antibacterial mechanism when GO is not in suspension. The oxidative stress of GO is triggered by the direct formation of ROS or by the depletion of cellular antioxidants, which can lead to the oxidation of lipids and can ultimately result in membrane damage and cell death [82]. Overall, it was found that surface modification with ZIF-8/GO hybrid nanosheets conferred effective antibacterial activity and high hydrophilicity to TFN nanofiltration membranes [87]. Recently, organosilica layers, formed by the sol–gel condensation of the precursor 1,2-bis-(triethoxysilyl)ethane, were successfully deposited onto the surface of sulfonated PES membranes [102], resulting in hybrid membranes with promising desalination performance (97.5–99% rejection for 2000 ppm NaCl at an operating transmembrane pressure of 30 bar) and higher chlorine resistance compared with the benchmark PA membranes used in RO [103]. A major concern associated with physical coatings of membranes is the depletion of the deposited materials during the filtration process. The loss of coating materials will gradually deplete the desired functionality of the composite membranes and may also release coating materials to water streams

377

378

13 Surface Modification of Water Purification Membranes

leading to potential risks to humans [74]. To overcome these shortfalls, surface modification involving stronger interactions such as covalent bonding is desirable (see Section 13.4).

13.4 Grafting Methods Grafting is a surface modification technique involving the chemical attachment of compounds to the membrane surface. In contrast to physical surface coatings, long-term chemical stability of grafted membranes is offered by covalent attachment of the grafted entities [104]. Grafting allows decorating the membrane surface with small molecular units or macromolecular species. Two main approaches can be considered, namely “grafting-to” or “grafting-from.” The “grafting-to” approach involves covalent bonding of the pre-fabricated species to be grafted, e.g. polymer chains, to the existing membrane surface while in the “grafting-from” approach polymer chains are grown directly from the membrane surface at initiator sites [105]. 13.4.1

Grafting-to

An example of the “grafting-to” approach is shown in Figure 13.12, which illustrates grafting of PEI onto a brominated tetra-methyl PES UF membrane. Grafting occurs through the Menshutkin reaction between tertiary amines and benzyl bromides, as confirmed by the appearance of a characteristic peak of –N+ R3 groups on the XPS spectrum of the grafted membrane (Figure 13.13) [106].

Br O

O

O S O

x

N

H2N

NH2 N H

N

N

NH2 H N

NH2

n

N

H2N

Brominated TM-PES

N H

NH2

PEI

H2N

N

NH2 N H H2N

O

N

N H N – N+Br

NH2 H N

NH2

n NH2

O

O S O

x

PEI grafted TM-PES

Figure 13.12 The reaction between PEI and brominated tetra-methyl PES. Source: Reprinted with permission from Lin et al. [106]. Copyright 2018, Elsevier.

13.4 Grafting Methods

C1s Brominated TM-PES (upper spectrum) PEI-grafted TM-PES (lower spectrum) –N+R3 O1s 404

N1s

402 400 398 396 Binding energy (eV)

Br3p3/2

550

500

450

400

350 300 250 200 Binding energy (eV)

150

Br3d

100

50

Figure 13.13 XPS spectra of the brominated tetra-methyl PES TM-PES membranes before (brominated TM-PES) and after (PEI-grafted TM-PES) PEI grafting. The inset shows high resolution spectra of the N1s region. Source: Reprinted with permission from Lin et al. [106]. Copyright 2018, Elsevier.

As mentioned in Section 13.3.2, PDA has received increasing attention in recent years for membrane surface modification owing to its outstanding adhesion properties. The functional groups (amine, catechol, and imine) derived from PDA matrices (see Figure 13.10) also enable post-modifications through covalent bonding. For instance, superwetting composite membranes were obtained by covalent grafting of poly(ethylene glycol) methyl ether thiol onto a PDA layer that was first coated onto the surface of a PVDF membrane [107]. Shevate et al. grafted zwitterionic l-cysteine on a PDA-coated polystyrene-b-poly(4-vinylpyridine) block copolymer membrane by conducting a Michael addition reaction and obtained a significant improvement of the antifouling ability toward foulants such as humic acid and sodium alginate [108]. Covalent grafting is also an attractive way to ensure long-term stability of membranes modified with NPs. Biocidal Ag NPs were effectively attached to the surface of a PA thin film composite membrane via covalent bonding with cysteamine as a bridging agent [109]. The surface of the PA membrane was first thiolderivatized by reacting with NH2 –(CH2 )2 –SH in ethanol solution, and then Ag NPs were attached onto the membrane surface via Ag–S bonding. Similarly, Ag NPs were covalently immobilized onto the surface of PVDF membranes by means of a thiol-end functional amphiphilic block copolymer linker [110]. Figure 13.14, which shows the release of Ag from membranes modified with (Ag/PVDF) and without (Ag/PVDF) thiolated linker, highlights the benefit of forming covalent bonds between the NPs and the membrane surface. Indeed, the amount of Ag released from the membrane surface (effective membrane area: 13.4 cm2 ) during continuous filtration using 2 l pure water was found to be approximately 52% for the membrane modified without thiolated linker

379

13 Surface Modification of Water Purification Membranes

100 80

Ag/PVDF 47.9

52.3

Ag/PVDF 60

52.3

43.6

0.05

40

0.05

0.005 0.005

0.00

0

0.5 1.0 1.5 2.0 0.5 1.0 Permeation volume (l)

0

Total released amount of Ag (%)

Released Ag amount Total percentage of Ag (%)

0.10 Released Ag amount (mg)

380

0.0 1.5 2.0

Figure 13.14 (a) Amount of Ag released from PVDF membranes modified by covalently bounded (Ag/PVDF) and non-covalently bounded (Ag/PVDF) Ag NPs. Source: Reprinted with permission from Park et al. [110]. Copyright 2013, ACS.

(no covalent bonds between Ag NPs and the membrane surface), while no detectable loss of Ag was observed for chemically grafted membrane during the entire water filtration process. As discussed in Section 13.2.2, irradiation with UV light is likely to generate chemical reactions provided that photoreactive groups are present in the system and then UV irradiation can be used for covalent post-modification of polymer membranes. According to the location of these photoreactive moieties, two routes can be envisaged for photo-functionalization of polymer membranes, namely, via photoreactive membrane or via photoreactive functionalization agents (Figure 13.15) [30]. Using electron-beam activation, the group of Schulze had in the recent years established a versatile platform for surface functionalization of a variety of different membranes (from different base polymers and having different barrier structures). The easy-to-perform modification is based on contacting the membrane with a solution of a surface modifying agent (this can be a small or larger molecule and does not need to contain a particular reactive group such as a C=C double bond) and then triggering the grafting by irradiation with e-beam [111, 112]. The grafting reaction is based on a hydrogen abstraction reaction, yielding to a radical pair between modifying agent and membrane polymer that will quickly combine, leading to the decoration of the membrane surface with the functionalization agent. The scope of the method is illustrated in Figure 13.16. All modified membranes had been much more hydrophilic compared with the unmodified one, but the protein adsorption was in some cases also increased since ionic binding sites had been introduced by the grafting. Because the e-beam has a large penetration depth into organic materials, the modification inside pores or even of membranes in modules has been shown to be feasible. Only possible e-beam-induced degradation reactions in the bulk of the membrane polymer

13.4 Grafting Methods

Nonporous membrane

X XX XXX X X X X X X XX X X X XXX X XX X X X X X X XX X X XX X X X X X X X X X

Porous membrane

X XX X X X X X XXX

XX X X X X X X X XX

RH hν

R RR R R R R R R R RR R R R RR R R RR R R R R R RR R RR R R R R RR R RRR R R R R R RRR

RRR R R R R R RR R

RX hν

hν

X

hν

X

X ... photoreactive group

RH ... small molecule

... macromolecule

Figure 13.15 Depiction of principal mechanisms for photo-functionalization of polymer membranes: via photoreactive membrane polymer (from left) and via photoreactive functionalization agents (from right). Source: Reprinted with permission from He et al. [30]. Copyright 2009, Elsevier.

could be a concern, but this can be addressed by the selecting appropriate e-beam energy and dose. A potential limitation of the “grafting-to” approach, when applied to grafting of macromolecular functionalization agents, is the rather limited degree of grafting that can be achieved because of steric hindrance. Such a disadvantage can be circumvented by the “grafting-from” approach (see Section 13.4.2) in which polymer chains are progressively grown from initiator sites located at the membrane surface. 13.4.2

Grafting-from

Due to the previously mentioned limitation of the “grafting-to” approach with respect to grafting density, “grafting-from” methods have received particular attention for covalent modification of polymer membranes. 13.4.2.1

Plasma-Induced Graft Polymerization

When polymers are exposed to plasma irradiation, highly reactive species such as free radicals are created in the polymer chains (see Section 13.2.1). If monomers are present in the vicinity of the polymer surface, these radicals initiate polymerization reactions, which results in growing of the grafted copolymers chains onto the activated surface. Combining plasma treatment with graft polymerization allows avoiding the progressive loss of surface characteristics after plasma modification. Moreover, plasma-induced graft polymerization has the advantage that the graft polymer is not chemically altered by the plasma [113].

381

13 Surface Modification of Water Purification Membranes

O

O S O

O

HO

n

O

O HO

OH

1

O

O

OH Carboxylic acids

OH 2

PES

3 SO3H

20 μm

O OH S O 5

SO3H 4

O

6

NH2

8

O

O N+

OH 13

NH2

O Amines, alcohols

HO OH OH 12

11

H2N

Phosphonic acids

OH OH OH

HO

N 10

O P O O NH2

7 OH

9

2 μm

O OH P OH

O P OH OH

HO

Sulfonic acids

O OH O O– Zwitterionic/ S P O OH H2N biomimetic O compounds 14 15

(a) 160% Albumin: 100% = 5.94 + 0.6 μg/cm2

Rel. protein adsorption (%)

140%

Myoglobin: 100% = 7.66 + 1.8 μg/cm2

120% 100% 80% 60% 40% 20%

)

(7)

(13

)

(6)

(14

(8)

)

/A )B

(12

(15

B/A

A/B

(4)

B/B

(2)

A/A

(1)

B/A

(5)

(3)

(9)

/A

/B

)A (11

)A

(10

mo dif ied

me

mb

ran

e

0%

Un

382

(b)

Figure 13.16 (a) Structure of PES membrane and functional molecules used for membrane surface modification by e-beam induced grafting; (b) protein adsorption after modification of the PES membrane with the respective substances. Source: Reprinted with permission from Schulze et al. [111]. Copyright 2010, John Wiley & Sons.

Membrane surface modification by plasma-induced graft polymerization has been successfully used to functionalize membranes made of various polymers such as PSf [113], polycarbonate (PC) [114], PVDF [115], or polypropylene (PP) [116]. For instance, Zhao et al. modified PP membranes by Ar plasma excitation followed by exposure of the activated membrane to hydrophilic acrylic acid monomers [116]. The grafted membrane exhibited an improved permeation flux (Figure 13.17) and antifouling properties compared with the unmodified membranes.

13.4 Grafting Methods

1800 20 W 40 W

Pure water flux/(kg/(m2 h ))

1600

60 W 1400 1200 1000 800 600

0

50

150 200 100 Irradiation time (s)

250

300

Figure 13.17 Effect of Ar plasma power and irradiation time on pure water flux of PP membranes grafted with acrylic acid monomers. Source: Reprinted with permission from Zhao et al. [116]. Copyright 2013, Elsevier.

13.4.2.2

UV-Induced Grafting

UV-induced grafting is an attractive technique for the surface functionalization of polymer membranes owing to its significant advantages such as low cost and mild reaction conditions [117]. In this method, radicals are first created on the membrane surface by UV irradiation. In the presence of monomers, free radical graft polymerization occurs, forming polymer chains that are covalently bonded to the membrane surface. UV-induced grafting has the advantage that the wavelength can be adjusted selectively to the reaction to be initiated, and, hence, undesired side reactions can be avoided or at least reduced very much [35]. In most cases the addition of a photoinitiator is required. For instance, UV grafting of vinyl monomers on PAN membrane was achieved by covering the membrane surface with benzophenone, upon which UV grafting could take place [118]. On the other hand, PSf and PES, which are widely used to make filtration membranes, are intrinsically photoactive, undergoing bond cleavage upon UV irradiation to produce free radicals without the use of photoinitiators [119]. The mechanism for the photochemical modification of PES membranes with vinyl monomers is shown in Figure 13.18. For non-intrinsically photoactive polymers, two preparation processes can be used, (i) UV irradiation in presence of photoinitiator and monomer (see preceding text), or (ii) a sequential, two-step approach [30]. For the latter, the bare membrane is first immersed into a solution containing the photoinitiator and irradiated with UV light in order to produce a radical pair and graft the initiator group to the membrane surface. Then the pretreated membrane is transferred into the monomer solution (with potential additives such as cross-linkers) and exposed to the UV light again in order to graft the monomers onto the membrane surface and then grow the polymer chains via radical polymerization.

383

384

13 Surface Modification of Water Purification Membranes CH3 O

O O

C

hv

S O

CH3

Sulfonyl radical

254 nm 112 kcal/mol

n

Aryl radical •

SO2 +

O

O

•

Vinyl monomer •

SO2

O

X

O

H2 H SO2 C C

n

X

•

–SO2 Vinyl monomer O

•

X

O

H2 H C C X

n

•

Figure 13.18 Mechanism of UV-induced grafting of PES membrane by vinyl monomer and subsequent radical polymerization. Source: Reprinted with permission from Van der Bruggen [23]. Copyright 2009, John Wiley & Sons.

For photografting of intrinsically photoactive polymers, similar to what had been mentioned earlier, two different methods have been reported, namely, the “immersion method” and the “dip method.” In the “immersion method,” the membrane is immersed in the monomer solution and the entire system is UV irradiated, while in the two-stage “dip method,” the membrane is first irradiated by UV light and is thereafter dipped in the monomer solution. For instance, hydrophilic nanofiltration membranes were obtained by UV irradiation of (photoactive) cardo poly(ether ketone) (PEK-C) UF membranes in the presence of acrylic acid [120]. The immersion technique typically requires longer reaction times than the dip method to obtain the same degree of grafting because UV radiation has to penetrate through the monomer solution that shields the membrane [23]. On the other hand, since the monomer solution is likely to absorb a substantial amount of the emitted energy, the membrane is protected from intense irradiation, which limits membrane structural modification [121]. Nevertheless, it had been demonstrated that the wavelength selective UV irradiation of PES UF membranes in the presence of hydrophilic (PEG-containing or zwitterionic) monomer solutions can be used very efficiently to tailor-make a range of thin film hydrogel composite UF membranes with tunable size selectivity and very pronounced anti-fouling properties [122] or with additional stimuli-responsivity [123]. Furthermore, by using vinyl sulfonic acid in combination with N,N-methylene bisacrylamide, PES UF membranes had been turned into versatile nanofiltration membranes [124].

13.4 Grafting Methods

13.4.2.3

Grafting Induced by High Energy Radiations

High energy irradiation sources (electrons [125, 126], ions [127], and γ-rays [39, 128–130]) have also been used to activate the surface of polymer membranes prior to grafting via radical polymerization onto the membrane surface. For instance, Keszler et al. modified PES membranes with electron-induced grafting, in a solution of acrylic acid and acrylamide [131]. Hidzir et al. grafted acrylic acid and itaconic acid on PTFE membranes by first dipping the membranes in the monomer solutions and further irradiating the system by 60 Co γ-rays, which resulted in a significant reduction of the membrane surface hydrophobicity [132]. It should be stressed, however, that high energy irradiation is likely to lead to a modification of the polymer microstructure as well as the morphology of the membrane surface [23]. 13.4.2.4

Grafting Initiated by Chemical/Electrochemical Means

Radicals can be easily generated under mild conditions by means of redox initiators [133]. An advantage of redox initiation is that it is not limited by temperature, so that it can be used to conduct polymerizations at room temperature. Most commonly used oxidant–reductant initiator pairs for polymer membrane functionalization involve K2 S2 O8 or Na2 S2 O8 as oxidant and Na2 S2 O5 , K2 S2 O5 , Na2 SO3 , or K2 SO3 as reductant [134–138]. In redox-initiated membrane grafting, the membrane surface is first brought into contact with a solution containing the monomer to be grafted. The oxidant–reductant initiator pair is further introduced in the solution in order to form the radicals that will initiate the polymerization (Figure 13.19). Recently, a novel two-component initiator system had been established for highly surface selective grafting of surface-anchored hydrogel layers via in situ cross-linking copolymerization, and its versatility had been proven for the site selective polyzwitterionic coating of the lumen surface of capillary PES UF membranes [139]. The system consists of a conventional water-soluble redox initiator and a macromolecular co-initiator, which accelerates the decomposition of the redox initiator. The macromolecular redox co-initiator, poly((2-dimethylamino)ethyl methacrylate-co-butyl methacrylate) (poly(DMAEMA-co-BMA)), contains hydrophobic “anchor” segments (BMA), which ensure tight adsorptive binding to the hydrophobic membrane surface, and “functional” segments (DMAEMA) with side groups that can serve as a co-initiator. In analogy to the reactivity of similar well-known low molecular weight tertiary amine systems [140], the side groups of DMAEMA segments are transformed in a bimolecular reaction with the other component of the initiator system, ammonium persulfate (APS), into starter radicals for a surface-initiated graft copolymerization (Figure 13.20). The facile two-step modification process consists of (i) adsorption of poly(DMAEMA-co-BMA) from an aqueous/organic solution on the lumen-side surface of the membrane (when the molecular weight is high enough, it will not permeate through the membrane barrier layer thus leading to site selectivity), and thereafter equilibrating the membrane with water, and (ii) flushing the lumen of the membrane with an aqueous solution comprising

385

386

13 Surface Modification of Water Purification Membranes Na2S2O8/Na2S2O5

: –COOH R R″ C H R′

–

•SO4 , •S2O5

–

+ H2O

H2 C C

COOH 〈MA〉

•OH

〈Raw membrane〉 R •

R″ C •

•

CH2 C

R′

R R″ C R′

〈MA membrane〉

CH3

CH3 COOH

COOH

•

•

CH2 C

CH3

...

COOH

COOH

... COOH

Growth of polymer chains

n

〈MA membrane〉

Figure 13.19 Schematic of membrane grafting by redox-initiated polymerization of methacrylic acid (MA) by means of Na2 S2 O8 /Na2 S2 O5 initiators. Source: Reprinted with permission from Kim et al. [138]. Copyright 2008, Elsevier.

O

O

H3C CH2

O

CH3 O SO3– N + –O3S O CH2

H3 C CH2

•

N

CH2

CH2

+

•O

HO

SO3– SO3–

O

N (a)

(b)

Figure 13.20 (a) Chemical structure of poly((2-dimethylamino)ethyl methacrylate-co-butyl methacrylate) (poly(DMAEMA-co-BMA)); (b) overall scheme describing the reaction between side groups of the surface-adsorbed poly(DMAEMA-co-BMA) and persulfate anion (from APS; ammonium counter ions omitted) to yield a surface-adsorbed and mobile starter radical for a surface-initiated radical polymerization. Source: Reprinted with permission from Lieu Le et al. [141]. Copyright 2017, Elsevier.

13.4 Grafting Methods

monomers and the complementary redox initiator APS. Upon contact of the two components of the initiator system at the membrane surface, in situ cross-linking copolymerization starts locally, and the degree of functionalization can be adjusted by reaction time and monomer solution composition. In two related studies, it had been demonstrated that the method enabled also the anti-fouling coating of the lumen side of polyetherimide membranes for pressure retarded osmosis [141] and that in a direct comparison the polyzwitterionic hydrogel antifouling coating was more stable under oxidative cleaning conditions than the PEG-grafted PDA coating (see Section 13.4.1) [142]. Another strategy is based on surface modification with aryl diazonium salt reduction, which allows the introduction of a large diversity of chemical groups so that many interfacial properties may be tuned [143]. The use of aryl diazonium salt for covalent modification of surfaces was first proposed by Pinson and coworkers who used electrochemical reduction of diazonium salts to covalently graft organic layers onto carbon electrodes [144]. Diazonium-induced grafting has been further extended to insulating materials by using additional reducing agents, thus removing the requirement of electrochemical assistance [145]. This approach has been successfully applied to the covalent modification of ion-exchange [146] and UF membranes [147]. Diazonium-induced grafting offers a versatile approach for membrane surface post-modification since diazonium salts can be easily prepared from a wide range of anilines [148]. For instance, Picot et al. modified the surface of PES membranes with various aryl diazonium salts bearing different functional groups (Figure 13.21a) resulting in membrane surfaces with different hydrophilic/ hydrophobic features and surface charge properties [147]. Aryl diazonium salts were generated in situ from the parent anilines by addition of sodium nitrite under acidic conditions and were further reduced by addition of hypophosphorous acid in order to form aryl radicals. The PES membrane was then dipped into the solution and left to react overnight. The proposed mechanism of PES grafting by chemical reduction of aryl diazonium salt is shown in Figure 13.21b–d. Ozone-induced grafting has also been reported for the surface modification of polymer membranes. For instance, Wang et al. [149] oxidized a PP microfiltration membrane by means of ozone, which resulted in the introduction of peroxide groups onto the membrane surface. The ozone-treated membrane was then dipped into a hydroxyethyl methacrylate (HEMA) monomer solution and the grafting reaction was initiated by the addition of FeCl2 through redox decomposition of peroxides (Figure 13.22). The HEMA grafting made the surface of the PP membrane more hydrophilic and less prone to protein adsorption. It should be noted, however, that the ozone treatment time has to be optimized in order to keep the membrane mechanical strength since excessive ozone treatment was found to make the membrane brittle [149]. Some species, such as azo compounds and peroxides, can also produce free radicals on heating [150]. For instance, Wei et al. succeeded in developing reverse osmosis membranes with improved chlorine resistance and anti-(bio)fouling property by modification of a commercial aromatic PA reverse osmosis membrane by free radical graft polymerization of 3-allyl-5,5-dimethylhydantoin

387

13 Surface Modification of Water Purification Membranes

+ P

N+ – Cl N

1-N2+

(a)

NH2

R

NaNO2 + HCl

CN

N+ – N Cl

N+ – N Cl

N+ – Cl N

3-N2+

N2+ Cl–

R

0 °C

COOH

NO2

2-N2+

1–NH2 R = CH2P+Ph3 2–NH2 R = NO2 3–NH2 R = CN 4–NH2 R = COOH

4-N2+

H3PO2

+ N2

R

0 °C to R.T. Aryl radical

Aryldiazonium salt R

(b) O S O

O

R

(c)

O S O

O n

PES

n

R

R

R

R R

O

R

O S O

O S O

O n

n

(d)

Figure 13.21 (a) Chemical structure of in situ-generated aryl diazonium salts. (b–d) Proposed mechanism for PES membrane grafting through aryl diazonium salt chemical reduction. Source: Reprinted with permission from Picot et al. [147]. Copyright 2012, Elsevier. CH3 Membrane

388

O•

OOH

O

CH2

C

m

OH

COCH2CH2OH

Fe2+ Fe3+

OOH Redox decomposition

CH2 O•

CH3

O

C

CH3

COCH2CH2OH

O

CH2

C

m

OH

O

COCH2CH2OH

HEMA

O

Figure 13.22 Redox decomposition of peroxide groups generated on the surface of a PP membrane and subsequent polymerization of hydroxyethyl methacrylate (HEMA) onto the membrane surface. Source: Reprinted with permission from Wang et al. [149]. Copyright 2000, Elsevier.

13.4 Grafting Methods

(ADMH) initiated from thermal decomposition of 2,2′ -azobis(isobutyramidine) dihydrochloride [151]. 13.4.3

Controlled Grafting-from

In order to overcome some limitations of conventional chain growth (radical) polymerizations, which are typically used for “grafting-from” functionalization (see Section 13.4.2), especially the polydispersity of obtained grafted polymers, so-called controlled polymerization methods had been developed. With view on methods where the very versatile (meth)acrylate or styrene monomers, which are available with a multitude of functional side groups, can be used, atom transfer radical polymerization (ATRP) [152] or reversible addition fragmentation chain transfer (RAFT) [153] are especially relevant. Both methods have in common that during polymer chain growth, the reactive end group of the macromolecule is preserved and unwanted termination reactions are prevented. When combined with a polymer surface containing suited initiator groups in predetermined density, it is in principle possible to prepare grafted functional polymer layers with predetermined grafting density and macromolecular chain lengths at low polydispersity and high nanoscale homogeneity. However, for membranes to be used in major water purification processes, the added benefit of such controlled grafted macromolecular architectures (compared to surfaces obtained by conventional “grafting-from”) has until now only rarely been utilized. In many cases, this is probably not necessary, for instance, when the main focus is on changing the wetting properties of the membrane surface. And it should be kept in mind that the implementation of surface-initiated, truly controlled graft copolymerization is challenging. The main reasons are that it is usually necessary to introduce a specific polymerization initiator on the membrane surface (that will potentially lead to damage of the membrane barrier) and that the interference of the membrane pore structure with the controlled growth of the polymer should be considered. The latter issue has until now been either ignored or considered only in a rather qualitative manner. A very recent paper demonstrates in a model system that the mechanism of surface-initiated (SI) ATRP changes with the degree of confinement of the grafting/grafted polymer in the micrometer and sub-micrometer range [154]. With stepwise decreasing dimension of the spatial confinement (studied down to 400 nm), the growth rate and the polydispersity increased systematically. On the other hand, pore sizes that are relevant for water purification membranes are in the range of 0.5 μm and (much) smaller. Among the frequently used membrane polymers, cellulose is unique because it is not only very hydrophilic but also has plenty of reactive hydroxyl groups. Those groups can easily be activated and thus also be used for immobilization of various initiators for polymerizations, including ATRP initiators (usually activated alkylhalide groups) [155]. Consequently, one of the early studies devoted to the functionalization of UF membranes via SI-ATRP has been performed with cellulose membranes having molecular weight cut-off (MWCO) of 100, 300, and 1000 kDa [156]. It has been demonstrated that with increasing SI-ATRP time to synthesize grafted hydrophilic poly((polyethyleneglycol) methacrylate), the water

389

390

13 Surface Modification of Water Purification Membranes

flux decreased monotonically, and the dextran rejection curves shifted to lower MWCO values. While in that and similar works, the growth of grafted polymer occurred on both the outer and the inner membrane surfaces, it had recently been demonstrated that a site-selective immobilization of the ATRP initiator only on the outer surface of cellulose UF membranes is possible. During initiator immobilization via esterification of hydroxyl groups, the membrane pores were blocked with a solvent such as glycerol, which is highly viscous and reactive (toward the ATRP initiator acyl halide derivative, thus deactivating it) [105]. After using optimized conditions for initiator immobilization and SI-ATRP, well-defined changes in MWCO at minimized loss of permeability had been achieved; this success could be related to the fact that pore blocking by grafted polymer inside barrier pores had been avoided. In some other special cases, it is even possible to use functional groups that are intrinsic to the membrane polymer itself as ATRP initiator; this had been demonstrated for poly(vinylidene fluoride-co-trifluoroethylene) that contains plenty of activated alkylhalide groups [157]. Usually, however, a pre-modification is necessary. Chloromethylation of polyarylsulfones is a very common method. This method has also been used by Yue et al. [158] to synthesize a modified PSf as membrane polymer, prepare UF membranes from that polymer and subsequently functionalize the membrane surface by SI-ATRP (see Figure 13.23). An elegant combination of surface modification of commercial PES ultrafiltration membranes with help of atmospheric plasma treatment (c.f. Section 13.2.1) in order to subsequently immobilize ATRP initiator groups on the membrane O

O S O

O

PSf

n

SnCl4/(CH3)3SiCl paraformaldehyde/CHCl3 O S O

O

O

O w

CH2Cl

O S O

Phase inversion

Cl R

CH2Cl

O

z

PSf-Cl

m

Cl R

z

CH2Cl SBMA CuCl/PMDETA PSf-g-PSBMA membrane

PSf-Cl membrance O R=

O

N+

O S O– O

Figure 13.23 Preparation of a polyzwitterion-grafted PSf membrane via polymer-analogous modification of PSf, membrane preparation by phase inversion and SI-ATRP of a zwitterionic methacrylate monomer. Source: Reprinted with permission from Yue et al. [158]. Copyright 2013, Elsevier.

13.4 Grafting Methods

1st step

UV

H

*C

C

OH

OH

C O Substrate membrane Surface initiator Grafting chain 1 UV

O 2st step

C

OH +

OH Acrylic acid S S S Dibenzyltrithiocarbonate Grafting chain 1 Grafting chain 2

Grafting chain 1 Heating/AIBN 3rd step O NH2 Acrylamide

Figure 13.24 Preparation of graft copolymer-functionalized PP membranes via UV-activated immobilization of a radical initiator (1st step), UV-initiated RAFT graft copolymerization (2nd step), and thermally reinitiated RAFT toward a grafted diblock copolymer (optional 3rd step). Source: Reprinted with permission from Zhou et al. [160]. Copyright 2010, Elsevier.

surface and to ultimately perform various “grafting-from” functionalizations by SI-ATRP has been demonstrated by the group of Belfort and coworkers [159]. A UV irradiation-based RAFT functionalization of PP membranes was based on the UV-activated immobilization of photoreactive benzopinacol (by grafting of benzophenone to PP via hydrogen abstraction and radical recombination; cf. Section 13.4.2.2), followed by an UV-initiated grafting step in contact with monomer and dibenzyltrithiocarbamate as RAFT agent [160]. The preservation of the reactive (RAFT) chain ends has been demonstrated by the preparation of a grafted diblock copolymer in an optional third step (Figure 13.24). However, the true benefit of well-defined grafted macromolecular architectures with tailored spacing between and well-defined lengths of the grafted chains can only be utilized in more specialized cases that are not (yet) relevant for practical water purification. For the preparation of macroporous flow-through membrane adsorbers, the controlled grafting is highly beneficial because maximum binding capacity in three-dimensional grafted layers at minimized loss

391

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13 Surface Modification of Water Purification Membranes

in permeability can be realized. SI-ATRP has already been used in academic studies toward such goal [161, 162], and it is expected that analogous membrane adsorbers for industrial bioseparations may in the future also be prepared by controlled polymerization instead of conventional radical processes. Other emerging examples are stimuli-responsive membranes, where the large potential of SI-ATRP for controlling grafted chain length or preserving the end group functionality, in order to synthesize grafted diblock copolymers [163, 164] or to attach functional NPs selectively to the chain ends [165], respectively, has already been demonstrated.

13.5 Conclusion As the global demand for clean water increases due to a growing population and depleting freshwater sources, much effort has been devoted to the development of efficient membrane materials. Indeed, membrane processes are recognized to be among the most effective technologies to address issues related to freshwater supply for human consumption, agriculture, and industry. In this respect, polymer membranes occupy the vast majority of the market owing to their advantages in terms of cost and processability. However, some intrinsic drawbacks of commercial polymer membranes may necessitate their post-modification to optimize their antifouling property, chemical stability, or separation efficiency. The great interest of post-modification techniques is that they are mainly surface selective, i.e. it is possible to alter the membrane surface properties while keeping almost intact the bulk and barrier structure. Post-modification of polymer membranes can be carried out by a variety of techniques, including irradiation-based modifications, physical coatings, and grafting. Irradiation-based methods (plasma, UV, electrons, ions, or γ-rays), which produce highly reactive species, such free radicals and peroxides, on the membrane surface can be used for direct membrane modifications (change in surface hydrophilicity, cross-linking reactions, …) or as a pretreatment step before a subsequent modification (e.g. irradiation-induced grafting). However, optimized treatment conditions (radiation energy, treatment time) have to be selected in order to limit modification of the bulk structure of the membrane, which may, for example, weaken its mechanical strength. Applying physical coatings has become a popular way to modify polymer membranes, especially polymer coatings from wet phase as these latter can be easily implemented. It is worth mentioning that such a simple surface modification strategy has already been considered by some membrane manufacturers and that large surface area membrane elements are available on the market (notably for seawater and brackish water desalination). The main concern associated with physical coatings is the absence of (strong) covalent bonds between the coating material and the existing membrane surface, which may result in the progressive

13.5 Conclusion

loss of the deposited material in long-term operation with a gradual loss of the desired functionality as well as potential water contamination. These potential concerns have been very well addressed by using the bioinspired PDA as tightly anchored functional layer or as interlayer for further grafting. Tailored LbL modifications rely on multiple non-covalent bonds between membrane surface and functional polyelectrolytes as well as within the polyelectrolyte layers. Depending on the application, the stability of the LbL layer may be tuned to the actual conditions or an easy regeneration by removal or reapplication may be possible. More stable membrane surface modifications can be achieved by graftingbased methods, which consist in the chemical attachment of compounds to the membrane surface by means of covalent bonds. Covalent grafting of membrane surfaces can be conducted by either “grafting-to” or “grafting-from” approach. The “grafting-to” approach is based on covalent bonding of pre-fabricated species with reactive sites of the membrane surface, which may limit the degree of grafting when bulky species, such as polymer chains, are to be grafted (due to steric hindrance). This intrinsic issue associated with “grafting-to” can be circumvented by the use of “grafting-from” methods, in which polymer chains are progressively grown from initiator sites located at the membrane surface. However, since most grafting-based membrane post-modifications are based on free radical graft polymerizations, both “grafting-to” and “grafting-from” approaches suffer from prevalent termination reactions between radicals in contrast to more sophisticated controlled radical polymerization techniques, such as ATRP and RAFT methods, which allow significant control over the polymerization reaction. However, although the benefit of well-defined grafted macromolecular architectures would be substantial for some niche applications (e.g. membrane adsorbers for bioseparations), it may be limited for membranes to be used in water purification processes. Ultimately, when selecting a method for post-modification, the interactions between the actual barrier layer of the membrane and the newly introduced functionality must be considered [35]. Under certain conditions, it might be possible to alter the wetting properties of the membrane surface and preserve the barrier properties, either based on solubility and diffusivity within a dense polymer film or on pore flow and size selectivity, at the same time. However, even this is often not the case. When larger entities such as functional groups, polymer chains, or coatings are added to the membrane surface, those will definitely interfere with the mass transfer through the barrier layer to a smaller or larger extent (and this may be desired, i.e. for increasing selectivity, or undesired when permeability is reduced). In general, the additional resistance toward mass transfer through the membrane will be smaller for membranes with a dense barrier layer (e.g. RO or NF membranes), while a precise tuning of the interplay between desired modification effects and pore blocking is crucial for membranes with porous barrier (e.g. UF and MF membranes). These aspects must and can be considered by the selection and adaptation of a modification method that fits to the membrane polymer, the detailed membrane barrier structure and the intended functionality that shall be achieved by the surface modification.

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13 Surface Modification of Water Purification Membranes

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14 Surface Modification of Polymer Substrates for Biomedical Applications P. Slepiˇcka 1 , N. Slepiˇcková Kasálková 1 , Z. Kolská 2 , and V. Švorˇcík 1 1 Department of Solid State Engineering, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic 2 Faculty of Science, J. E. Purkyne University, 400 96, Ceske mladeze 8, Ústí nad Labem, Czech Republic

14.1 Introduction The surface engineering of materials including the grafting of specific molecules, nanoparticles, and biologically active species in order to specifically drive cell function is still an emerging field, where almost every day one can find some new advancement or application. The biological studies involve biomimetic modification to alter a material’s compatibility with a biological system or construction of innovative substrates with altered surface and enhanced specific properties aimed on particular cell or antimicrobial response. Biopolymers such as cellulose (and their derivatives) or PLLA may be used, but it is also possible to enhance surface of polymers with lower degradability, such as PEN or PEEK. The control of cell behavior in contact with biomaterials critically depends on the material providing the optimal chemical and physical attributes that may consequently be applied to the cells with an appropriate response. Cell adhesion and proliferation may be controlled by incorporating at the interface of the material targeted chemical groups and/or tailoring the surface morphology. Recently, the concept of smart materials, which are considered as materials that may stimulate tissue formation, on the basis of “clever” response becomes a significant part in this field, which involves numerous studies including all classes of materials, such as metals, ceramics, polymers, and composites. Materials have a surface composition intended to interact with biological pathways and cellular functions. Biological functionality is also based on the material structures that are grafted onto material surfaces. New materials with enhanced design to mimic biological structures or functions give rise to the field of new biological materials. Also materials based on strictly biological basis, such as cellulose, collagen, fibrin, and hyaluronic acid polymers or polymers like poly-l-lactic acid, play an important role and their properties are sometimes very hard to mimic or duplicate by grafting or modification procedures. Surface structures including chemical composition and topography can be engineered to guide and direct a desirable biological response. Nano- or Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

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micropatterned material surfaces have also been developed (i) to contribute to the fundamental understanding of how surface geometries and physical properties affect cellular morphology and biology and (ii) to define properties of patterned or treated surface that optimally lead to cell proliferation and tissue reconstruction based on implanted materials. Advanced properties such as protein synthesis or cell differentiation on such substrates may be altered by changing the nuclear shape of cells present within the substrate morphology and surface shape. Surface modification of materials is an effective tool to change the biological interactions to a particular material and offers a number of advantages in biomedical fabrication. By changing only the composition at the outermost surface of a biomaterial, the mechanical properties and fabrication method of an implant can remain unaffected while modulating the surface chemistry and properties that directly affect the biological reactions. Additionally, since surface modification techniques require only short time for modifying the surface properties of a device or implant, expensive materials can be economically used in commercially competitive biomedical challenge. Surface modification can provide accessible, chemically reactive sites that can be used to immobilize drugs, enzymes, antibodies, or other biologically active species. Typical reasons for modifying the surface of an existing biomaterial include the following: to improve cytocompatibility [1], improve blood compatibility [2], reduce (or increase) tissue adhesion [3], increase (or decrease) the wettability of a surface [4], add biologically active substances to the surface layer [5], alter the protein adsorption characteristics [6], e.g. act as a rate-limiting membrane [7], and many others. Some common approaches for surface modification of materials are non-covalent coatings on the original surface [8], wet chemistry approach [9], Langmuir–Blodgett film deposition [10], surface active additives [11], vapor deposition (carbons, metals) [12], covalently attached coatings [13], photo or plasma grafting and initialization of surface reactions [14], plasma-based coatings [15], ion beam sputter deposition and/or ion beam etching or plasma etching [16], functional group modifications [17], laser evaporation [18, 19] and laser treatment [20]. Since the plasma and laser modification techniques may be used as primary modification for almost all previously introduced techniques with high versatility, we will focus mainly on these techniques in this chapter.

14.2 Plasma Treatment Plasma treatment (see Chapter 2 and 3) is used for tailoring of surface physical and chemical properties without affecting the bulk properties (especially mechanical ones) of the base material, which is advantageous in the design, development, and manufacturing of polymers. Surface treatment with aim of biocompatibility enhancement should provide a highly complex environment with many different chemical, physical/mechanical, and geometrical stimuli that affect cell phenotype, differentiation, and development. Researchers are focused on surface chemistry [21], growth factors, and mechanical forces [22], more specifically stem cell differentiation, surface nanotopography, and its effects on cell behavior have been studied intensively especially in the last decade

14.2 Plasma Treatment

0 –5 –10 –15 –20

ζ potential (mV)

–25 –30 –35 –40 –45 –50 –55 –60 –65 –70 –75 –80

ld

ld

8W

240

s, o

, old

s, o

10 s

240 3W

8W

h

, old

fres

3W

10 s

sh

0 s, , 24

8W

esh

, fre

10 s

8W

s, fr

, fre

240 3W

10 s

3W

pris

tine

Figure 14.1 Zeta potential values of pristine PEN and samples treated by Ar plasma (exposure powers 3 and 8 W, times 10 and 240 s). The ζ potential was determined for as-modified and aged samples (aging time 28 ˇ et al. 2016 days). Source: Nedela [27]. Reproduced with permission of Elsevier.

sh

[1, 23]. Surface topography may vary with different tissues and also within the tissue microenvironment ranging from macro (i.e. bone or ligament) to micro (i.e. shapes of neighboring cells) and to nano (i.e. protein folding and collagen binding) [24]. Cells that respond to shape opened new directions in the emerging field of cell biology and, later, tissue engineering. The plasma treatment offers significant advantages including its cheapness, effectiveness, and also a high flexibility for tailoring the physicochemical properties of the treated polymer [25–28]. It may change the surface chemical composition and surface roughness or induce the appearance of particular nano/micropattern such as globular and/or wrinkle-like structures on the surface, which will be discussed in subsequent paragraphs. The change of surface charge due to the cleavage of original polymer chains and creation of free radicals and double bonds on surface may take place. On the other hand the longer exposure time may induce to the creation of positively charged surface due to the presence of polar groups on surface, which can be confirmed by ζ-potential determination (Figure 14.1) [27]. The plasma treatment of polymers leads to the grafting of new chemical groups at the treated interface [29], cross-linking, and branching of the macromolecules and formation of low-molecular weight oxidized structures [30], the rates of these processes being a function of the plasma reactivity. The decrease of surface charge due to the cleavage of original

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polymer chains and creation of free radicals and double bonds on surface may also take place. Due to the potential occurrence of ablation phenomenon favored for high power conditions and long treatment time, the surface topography of the polymer could also be affected [31]. Depending on the chemical composition of the working gas (i.e. O2 , Ar, N2 , or NH3 ) oxygen and nitrogen containing functionalities are introduced onto the polymer surface [32, 33]. The presence of these polar groups may induce the creation of a positively charged surface. Several types of polymer foils, e.g. polyethylene terephthalate [34], polystyrene (PS) [35], polyimide [36], and biopolymers [32, 37] underwent plasma treatment with the aim of both chemical and morphological changes, in view of improving their performance for a given application. Surface activation of polymers was used for a variety of practical applications important both in the fields of electronics and development of materials with enhanced biocompatible properties [23]. A plasma-treated ultrathin polystyrene film surface was explored as a simple, robust, and low-cost surface chemistry system for protein biosensing applications. This surface could dramatically improve the binding efficiency of the protein–protein interactions, which is defined as the binding signal per immobilized ligand [35]. Grafted plasma pretreated polymers can also be used to enhance our understanding of cell behavior. It is well known that cells respond to a variety of chemical and morphological cues in their extracellular environment. Polymer patterning can be used to recreate the complexity of the cellular microenvironment to better control and understand the cellular response to specific cues [38, 39]. Activated plasma treated surfaces were also used as a base for consecutive grafting procedures. For sake of example, Au nanoparticles grafted on plasma-modified PE resulting in an increase of the roughness enhanced the polymer surface cytocompatibility [40]. Recently the surface of polyethylenenaphtalate (PEN) was extensively studied [41] as a promising material for flexible electronics and biosensor application or as a substrate for cell arrays combined with sensor electronics and transistors [42]. PEN has a potential to be used as a substitute for the standard used for tissue engineering applications, polystyrene (tissue polystyrene), which can be used as microfiber scaffold [43]. PEN, on contrary, as an aromatic polymer containing oxygen, possesses excellent mechanical properties and flexibility in connection with advantages arising from its chemical structure with oxygen incorporated within. The chemical stability and ability to create carboxyl and carbonyl groups can be of great benefit for some cell lines to be grown on this substrate. Surface treated polyesters were also used for osteoblast and endothelial cell improvement [44]. The biointerface or cytocompatible surface refers to the relationship between synthetic or natural solid-state material polymer or other and its interaction with tissue, microorganism, cell, virus, or biomolecule [45]. The surface treated PEN has a great potential as an alternative for use as a carrier for cell growth and consequent replication onto patient’s body or can serve as a basis for biosensors based on cell responding to various analytes of simple grafted substances [46]. The plasma modification of PEN [47] has a positive effect on the vascular smooth muscle cell (VSMC) adhesion and proliferation as illustrated in Figure 14.2. For a treatment duration of 10 seconds (both at 3 and a power of 8 W), the PEN exhibited significantly better proliferation of VSMC on

14.2 Plasma Treatment

1st day

3rd day

7th day

PEN

PEN 3 W/10

Figure 14.2 Photographs of adhered and proliferated VSMC cells on the 1st, 3rd, and 7th days from seeding on pristine PEN and PEN modified with 3 W and 10 s (PEN 3 W/10). Source: ˇ et al. 2016 [27]. Reproduced with permission of Elsevier. Nedela

the third day, which was even comparable to tissue culture polystyrene (TCPS) standard. For a treatment of 240 seconds and for both plasma powers applied, nonsignificant increase in cell number was observed after three days. However, after seven days from seeding, the cell proliferation was restored with even better results than that on tissue polystyrene [27]. On the other hand, polymers may also be used in a wide range of medical devices including restorative dentistry, soft-tissue reconstruction, orthopaedic implants, and vascular structures. The most commonly used inert medical polymers include polyethylene (low-density polyethylene [LDPE], high-density polyethylene [HDPE], and ultrahigh molecular weight polyethylene [UHMWPE]), polymethylmethacrylate (PMMA), fluoropolymers such as e-PTFE, polypropylenes (PP), polyesters, polyamides (nylons), polyurethane (PU), siloxanes (silicone), and polyetheretherketone (PEEK). Nondegradable polymers are used where long-term structural stability and biocompatibility are needed and are utilized in applications such as bearing surfaces in hip, knee, or shoulder implants; vascular grafts or catheters; fillings and resins for teeth; nose, chin, and cheek implants; and ocular implants. Polymers that are porous by design can facilitate tissue ingrowth and enable long-term stability. PEEK is a semi-crystalline linear polycyclic aromatic thermoplast and is used mainly in the electrical (cable, insulation), healthcare [48, 49], automotive (bearings, piston skirts) [50], chemical (compressor valve plates, pH meters), or aerospace structural components). PEEK is a member of the polyaryletherketone (PAEK) polymer family that has been increasingly employed as biomaterials for trauma [51], orthopedic [52], and spinal implants [53] since the 1980s. This group of polymers is inert and biocompatible [54]. The advantage of using PEEK for medical also used in orthopedic, dental [55], and traumatic applications [54]. This polymer was candidate as a matrix material for replacing metal implant components; commonly the polymer was applied in vertebral surgery as a material of the interbody fusion cage [56]. Bioactivity of PEEK might be improved

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14 Surface Modification of Polymer Substrates for Biomedical Applications

by preparation of a composite structure or PEEK surface modification by, for example, physical treatment (laser and plasma modifications) [57]. PEEK exhibits a hydrophobic bioinert surface characteristics that are not favorable for protein adsorption and cell adhesion [58]. Therefore, the surface of PEEK needs to be modified in order to enhance cells attachment. PEEK surface was treated with Ar plasma with aim at its chemical and physical properties improvement in order to provide a suitable and cytocompatible support for cell culture [57]. The treatment resulted in enhanced cell adhesion and proliferation of two model cell lines (L929 and U-2 OS), when compared to pristine PEEK. Scanning electron microscopy (SEM) analysis supported the observations by revealing pronounced number of filopodia in L929 cells growing on plasma treated PEEK (especially by 480 and 120 seconds treatment). (Figure 14.3). The plasma treatment was proved to be an attractive approach for increasing cytocompatibility

PEEK

PEEK / 120 s

PEEK / 240 s

Figure 14.3 Scanning electron microscopy (SEM) images of L929 cells cultivated (72 h) on PEEK and plasma treated (120 s and 240 s) PEEK matrices. SEM images of three different magnifications.

14.2 Plasma Treatment

of PEEK polymer and that it significantly improves its properties compared to pristine PEEK. The introduction of several functional groups by the plasma treatment may also be used for binding of specific biomolecules such as of DNA or proteins, as shown, e.g. for polymers with sulfonate and phosphonate surface groups [59, 60]. It was also reported that proteins and peptides can be covalently immobilized onto aldehyde and epoxide plasma-modified polymers [61]. Surface modification by selecting appropriate modification routes including plasma tailored for specific biomedical applications is limited not on polymers, but may also be applied to stainless steel for update of its biomedical applications [62]. Proteins may be also effectively grafted directly onto nanostructures such as gold nanoparticles [63], where consecutively such structures may be further attached onto solid substrates for construction of biologically active surface. These materials can be effectively used as DNA nanobiosensors [64]. For a scaffold construction, the knowledge of nanotechnology is essential, but for colonization of a scaffold by a cell population, it is necessary to understand cell and molecular biology and tissue physiology [65, 66], which can be also modified with high power plasmas. One of the recently applied polymer suitable for biocompatibility enhancement is poly-4-methyl-1-pentene (PMP), lightweight, transparent, rigid, tough, thermally, and chemically resistant polymer with excellent electrical insulating properties and high gas permeability [67]. These qualities allow its use in a wide range of industries. The applications include, for example, medical equipment, chemical tubes, syringes, laboratory equipment, microwave components, optical components such as lenses and windows, and many others. Low cytocompatibility hampers the application of PMP in tissue engineering, therefore an increase in its cytocompatibility remains a challenge before the benefits of using PMP in this field can be met [23]. Crucial factors for substrates intended for tissue engineering applications involve cell adhesion, wettability, biocompatibility, surface roughness, and morphology as well as the surface chemistry and “fair” electrical conductivity [68]. The listed properties are generally insufficient and can be modified by surface treatment [69–71]. The modification of the polymer surface upon plasma treated surface can also be modulated by the choice of working gas [72]. For example, oxygen is popular for an increase of polymer hydrophilicity and creation of functional reactive groups [73]. On the other hand, the most commonly used one is argon gas (Ar), which causes etching and netting of the material. It results in variation of its roughness and morphology, causing an improvement of the adhesion of cells or metals [74]. It should be noted that well-controlled replication of nanostructures with features with sizes as small as 50 nm on the surface of the PMP substrates were also achieved using injection molding as reported recently [75]. The PMP separator (membrane that enables, e.g. ion transport) exhibited a much improved thermal dimensional stability compared to a commercial polyolefine separator, and the cells with the PMP separators displayed stable cycling performance and good rate capabilities [76]. PMP was also used to study the properties of phantom tissue-like films in the frequency range 20–70 MHz [77] (“phantom” is specially designed object that is scanned or imaged in the field of

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medical imaging to evaluate, analyze, and tune the performance of various imaging devices). Synthetic polymers can significantly contribute in the field of clinical medicine [77]. They have further advantages as their physical, chemical, and biological properties are tunable over a wide range, and this can be used to match the requirements of specific applications. They comprise not only bulk materials but also coatings and pharmaceutical nanocarriers for drugs. They can also be used as materials for oxygenator membranes [78]. The advantage of PMP is in its mechanical properties and nontoxicity. However, this material has very rarely been used as a cell carrier, as far as is known. The optimization of PMP properties, testing of pristine PMP as cell carrier, and enhancement of its cytocompatibility by plasma treatment, which also influenced the surface morphology (Figure 14.4) and roughness was studied in [79]. The treatment was confirmed to be a successful tool for roughening the surface. The higher plasma power deepened the profile of the surface, which was associated with increased roughness. The proliferation of NIH 3T3 cells was 5–10 times higher on plasma treated PMP when compared with the growth on pristine PMP. The positive effect of a high plasma power treatment was unambiguously demonstrated (Figure 14.5): the cells growing on treated PMP were elongated and exhibited long and abundant protrusions as well as intercellular connections. In contrast, the cells lacked almost any adhesion sites to the pristine PMP. [79]. Spontaneous formation of complex structures offers an interesting alternative for the preparation of surfaces with specific properties. The structures formed by self-organization currently have a wide variety of applications, e.g. optical devices [80], the flexible electronics [81], or systems for separating particles [82]. (a)

(b)

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Figure 14.4 The surface morphology of PMP samples obtained from three-dimensional atom force microscopy (AFM) scans: (a) plasma treatment (100 W, 240 s, O2/Ar), (b) plasma treatment (200 W, 240 s, Ar), (c) pristine PMP, (d) plasma treatment (200 W, 240 s, O2/Ar), and (e) plasma treatment (50 W, 240 s, Ar). Source: Michaljaniˇcová et al. 2016 [79]. Reproduced with permission of RSC.

1 2

Figure 14.5 Immunofluorescence microscopy images of NIH 3T3 cells growing on pristine polymer and plasma treated samples – (1) (100 W, 240 s, O2/Ar), (2) (200 W, 240 s, O2/Ar) for 6, 24, and 72 h (from left). F-actin of cell cytoskeleton in green (phalloidin-Atto 488), nucleus in blue (DAPI), and talin 1 was visualized by antibody labeled with Atto 647. The scale bar is equal to 20 μm. Source: Michaljaniˇcová et al. 2016 [79]. Reproduced with permission of RSC.

Pristine

14.2 Plasma Treatment

One of the self-organizing mechanisms is wrinkling instability. By this bottom-up method, it is possible to prepare various surface structures with patterns in a wide range of length scales. Wrinkles are characterized as smooth and shallow surface undulations as a result of the uneven expansion of a surface consisting of materials with different mechanical properties [83]. The wrinkles appear, when the residual stresses exceed a critical value [83], which can be induced by heating [84], solvent swelling [85], mechanical stretching or compression, or others. The effect of wrinkling on a thin bilayer film (polymeric film metalized with a noble metal) was described in the following reference [86]. Spontaneous creation of complex structures in simple systems is very interesting and potentially useful phenomenon used in many applications aimed on biotechnology, biology, and tissue engineering. This capability to spontaneously create number of surface patterns over large surface areas can be used in optical devices [80], micro lenses arrays [87], flexible electronics [81], solvent-responsive microfluidic channels [88], or measurement of physical attributes of nanoscale thin films [89]. Formation of ripple-like structures was also observed on polymer films treated by radiation [90] and on annealed plasma pretreated films [91]. Wrinkle wavelength and amplitude are determined by physical properties of layers, their thickness, and type of treatment as a result of the minimization of the total energy in a film [92]. In case of a bilayer, formation of wrinkles relaxes the compressive strain in the thin hard layer that can be deposited using a plasma-based method as, for instance, the magnetron sputtering technique, thus reducing the elastic strain energy. At the same time, it bends the thin layer that results in an increase of bending energy [83]. Thermally induced wrinkling can be observed in two cases: (i) by heating a thin glassy polymer film with deposited metal [93] or (ii) by cooling a bilayer of a thin hard layer deposited on a suitable thick underlayer [94]. Thermal stress accumulated in order to create wrinkles can be caused by (i) internal stress rising from atomic interactions in sputter-deposited films, for example, the sputtered gas atoms

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As-sputtered

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Figure 14.6 AFM scans of as-sputtered and annealed PLLA samples deposited with gold nanolayer. As-sputtered samples with 5 and 40 nm Au nanolayers (left column) and annealed samples with the same Au nanolayers are shown in right column. Ra corresponds to surface roughness in nanometer. Source: Juˇrík et al. 2014 [96]. Reproduced with permission of Elsevier.

impinge on the growing film, generating mechanical stress in the film [95] or (ii) the difference in the thermal expansion coefficient between thin hard layer and the soft underlayer confined by the rigid substrate [94] or both. Chemical, morphological, electrical, and biological properties of poly-l-lactic acid (PLLA) thin films covered with gold nanolayers by the magnetron sputtering method were studied in [96]. The samples were examined as-sputtered and annealed at glass transition temperature. Morphological changes of poly-l-lactic films introduced by annealing were studied by means of atomic force microscopy, which showed formation of oriented ripple-like structures on the surface of the film (Figure 14.6). Combined data suggested that these ripple-like structures were formed by gold lines with insulating polymer gaps in between. These lines show preferential orientation over large areas and under proper conditions offer simple way for electrically anisotropic material on large scale. The cytocompatibility was also studied showing increased cell adhesion and proliferation of mouse embryonic fibroblasts offering another use for these easily formed structures. Mouse embryonic fibroblasts, NIH 3T3 adhesion (six hours after inoculation) and NIH 3T3 proliferation (24, 60, 120 hour after inoculation), were affected by the used modification [96]. While annealing PLLA without Au nanolayer seemed to slightly reduce cell proliferation in comparison with pristine PLLA, different thickness of gold layers yielded different results depending on the gold thicknesses, when gold layer of 3 nm thickness resulted in significantly increased cell proliferation at the beginning (24 hours) (Figure 14.7). It was observed an increased cell adhesion and proliferation for annealed 3 and 10 nm

14.2 Plasma Treatment

PS

Pristine PLLA

PLLA 60 °C

Au 3 nm/60 °C

Au 5 nm/60 °C

Au 10 nm 60 °C

Figure 14.7 Fluorescence microscopy images of mouse embryonic fibroblasts (NIH 3T3 cells) 24 h after seeding on polymer scaffolds in the following order: polystyrene Petri dish (PS, serving as a mock), pristine PLLA, annealed PLLA, PLLA deposited with 3, 5, and 10 nm gold nanolayer and then annealed (Au 3 nm/60 ∘ C; Au 5 nm/60 ∘ C; Au 10 nm/60 ∘ C). F-actin was stained with phalloidin-TRITC (pseudocolored in green) and cell nuclei with DAPI (pseudocolored in red). The scale bar corresponds to 50 μm. Source: Juˇrík et al. 2014 [96]. Reproduced with permission of Elsevier.

thick Au layers in comparison with PS standard. While adhesion and proliferation had not changed significantly in comparison with pristine and annealed PLLA, samples with 3 and 10 nm gold layer showed growth of physiologically looking cells with normal cell connections [96]. Cell–substrate interactions are crucial features for materials application in medical sciences, and mainly for tissue engineering. The substrate can be modified by surface grafting procedures [23]. These interactions play an important role in determining the cell growth, differentiation, and organization [97]. A key factor in cell/material or cell-to-cell communication is extracellular matrix (ECM). ECM provides biochemical and mostly structural support to surrounding cells. ECM is secreted naturally by cells and is made by a collection of extracellular molecules. This collection of molecules (e.g. various proteins and nonprotein substances, metabolites, or ions) is unique in composition for different types of cellular structures [98]. In tissue engineering, cells adhere and proliferate onto artificial scaffolds. Materials used for such purposes are called synthetic ECM, because they provide the necessary structural support [99]. A wide variety of polymers that are of natural or synthetic origin can be used for medical applications (e.g. tissue engineering). For such applications the materials need to be not only nontoxic but they also must exhibit certain characteristics depending on the target application – e.g. nerve tissue [100], artificial skin [101], bone grafts [102], cartilage [103], etc. The desired characteristics may include

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14 Surface Modification of Polymer Substrates for Biomedical Applications

mechanical or electrical properties that are an essential part of the materials that are to perform a given function. To enhance their biological performance, those materials need to be further adapted. One of the possibilities is reinforcing the polymers with nanoparticles [104]. Carbon-based nanomaterials, including carbon nanotubes, graphene, nanodiamond, and carbon nanoparticles (CNPs), have emerged as potential candidates for a wide variety of applications because of their unusual electrical, mechanical, thermal, and optical properties [105]. However, carbon nanostructures have a tendency to aggregate. To improve their limited solubility and aforementioned tendency to aggregate, they can be functionalized with hydrophilic groups [106]. Such hydrophilization can be done by introducing different amine groups onto the surface of CNPs [107]. Successful polyethylene surface modification caused either by plasma treatment or by subsequent grafting of amine modified CNPs onto the polymeric surface was presented in [108]. Prepared surfaces show higher hydrophilicity and fair surface morphology that are essential factors in cell cultivation. Such surfaces have shown an increase number of cultivated VSMC cells as well as an improved cell viability. Cultivated cells are homogeneously spread on the grafted surfaces and manifested optimal physiological morphology. Only slight differences between individual modifications of LDPE and HDPE in all studied surface properties were detected. Obtained results confirmed improvement in cytocompatibility of both tested polyethylene samples (Figures 14.8 and 14.9) after all individual modification steps. In any case, we believe that these results provide a good basis for determination of the potential application of polymeric materials grafted with amine-modified CNPs in tissue engineering [108].

110 100 Contact angle (°)

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Figure 14.8 Water contact angle measurements of different HDPE and LDPE samples: pristine (HDPE; LDPE), plasma treated (/plasma), etched in water solution of hydrochloric acid (/etched), and CNPs grafted (/CNPs). Source: Žáková et al. 2017 [108]. Reproduced with permission of Elsevier.

14.3 Laser Modification

HDPE pristine

1st day

HDPE/CNPs

HDPE pristine

6th day

HDPE/CNPs

Figure 14.9 Fluorescence microscopy images of VSMCs adhered (1st day) and proliferated (6th day) on pristine (HDPE) and CNPs grafted (/CNPs). Plasma exposure time for all modified samples was 120 s. Source: Žáková et al. 2017 [108]. Reproduced with permission of Elsevier.

14.3 Laser Modification 14.3.1

Interaction with Cells

As aforementioned, modification of polymer substrates can essentially change the properties of material, and thereby it allows their usage in attractive fields of material research. One of these fields is tissue engineering, which aims at healing the damaged tissues by using a compatible biomaterial alone or cellularized [109]. Therefore, the substrate for tissue engineering should be biocompatible material, which means that the material cannot cause unwanted reaction of a tissue and that provides an appropriate response of a host organism by special application [110]. For some applications, cell adhesion and proliferation on the substrate is required, but sometimes these properties are undesirable. Degeneration is possible only if it is required and degradation products have to be biocompatible as well. The stability of mechanical and physical properties and the possibility of sterilization of the material are fundamental. More about tissue engineering can be found in the review [111]. Widely used substrate for tissue engineering is polyhyhroxybutyrate (PHB) from the family of polyhydroxyalkanoates. PHB is an important polymer because of its special properties: biodegradability and biocompatibility. In 1926, it was isolated from bacteria Bacillus megaterium by Lemoigne [112], and now it is commercially produced by microorganisms via fermentation. There are tendencies to produce this material using transgenic plants to reduce costs for production. Because of the cost, it is rarely used for packaging although it is more environmentally friendly than usual polymers. PHB is also one of the materials used in medical applications, such as scaffolds

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and drug delivery system [111]. PHB homopolymers are highly crystalline, extremely fragile, and relatively hydrophobic, and they degrade near its melting point, which in fact complicates preparation. Because of that, the copolymers of PHB with polyhydroxyvalerate (PHV), which are less crystalline and more readily processible [113], are more frequently used. The latest research provided a good approach to evaluate the biocompatibility and adhesivity of an implanted material by studying the structural modifications. In vitro tests have shown that PHB is biocompatible to various cell lines, including osteoblastic, epithelial cell, and ovine chondrocytes [114]. It was also found out that cell cultures of various origins, including fibroblasts, endothelium cells, and isolated hepatocytes, exhibit high levels of cell adhesion in direct contact with PHB [115]. A functional nanoscale scaffold for tissue engineering can be an ideal template for cells to grow and function [116]. Scaffold provides mechanical and functional support for the cells. Different types of cell lines have different preferences on substrate properties, e.g. wettability and roughness. The laser treatment method appears as an attractive method to modify these properties and to fine-tune the interaction of the material with cells on demand. The laser treatment of polymer foil was confirmed to be an excellent tool for cell growth delimitation [117]. It was proved that the laser treatment contributes as major factor for wettability/contact angle change. The higher repetition rate of laser beam caused a significant increase of mass loss induced by surface ablation. This increase is even more pronounced if the higher number of laser pulses and thus higher dose of transferred laser energy is applied. Due to increase of laser fluence (number of laser pulses maintained), the significant increase of the surface roughness is observed on the PHB surface (pristine 31.9 nm → treated 270.0 nm) with significant change of surface morphology (Figure 14.10). The combination of optimal laser fluence and pulse number was therefore used for construction of updated surface with anti-cellular response. Due to the simplicity of the laser treatment, one can construct such kind of surface on the biopolymer and thus easily delimit the area of cell growth precisely. For the U-2 OS cells, the treatment of the surface leads to a decrease of cell number on substrate, even if this effect is more pronounced for laser fluence of only 30 mJ/cm2 (Figure 14.11). This effect may as a consequence guide the cell growth, i.e. the lines/areas may be induced by this type of treatment that significantly decrease the number of cell on such surface and exhibit a natural barrier for cell growth [117]. The laser treatment can be also used for support of cell growth, by general change of both surface chemistry and morphology [1, 118] or by construction of ripple pattern that guides the cell growth in particular direction [119, 120]. 14.3.2

Sensor Construction

Because of the low penetration depth of the UV light, excimer laser treatment causes morphological and chemical changes only on the surface layer of a substrate while leaving the bulk of the material unchanged [33]. Laser treatment of the surface is commonly utilized in micro processing of both organic and inorganic materials as the resulting structural changes in the surface layer can, for example, facilitate metallization of polymers by improving the adhesion of

14.3 Laser Modification

Figure 14.10 SEM images of pristine and laser treated PHB. Samples of PHB were treated with KrF laser (fluence 30 mJ/cm2 , number of pulses, 3000 and 6000 [250 × 250 μm2 scans are introduced in the left column and 30 × 30 μm2 are introduced in the right column]). White line represents 125 μm and blue line represents 15 μm. Source: Slepiˇcka et al. 2017 [117]. Reproduced with permission of Elsevier.

PHB

30 mJ/cm2 3000p

30 mJ/cm2 6000p

125 μm

Figure 14.11 Scanning electron microscopy images of U-2 OS cells cultivated for 48 h on pristine PHB, control glass and PHB treated with 9 mJ/cm2 and 1000 or 6000 pulses. Left column represents 100 μm, right column detail on cell 30 μm. White line represents 50 μm and blue line represents 15 μm. Source: Slepiˇcka et al. 2017 [117]. Reproduced with permission of Elsevier.

Pristine PHB

9 mJ/cm2 1000p

9 mJ/cm2 6000p

15 μm

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14 Surface Modification of Polymer Substrates for Biomedical Applications

metals to the polymer surface or allow for a better adhesion of living cells to the laser treated surface [121]. Laser nanopatterning through a lithographic contact mask of the PEN and polystyrene has been studied recently; both of these polymers contain aromatic rings and can be used for biosensor application [122]. PEN is an aromatic polyester, structurally very similar to polyethylene terephthalate. Because of the presence of the naphthalene rings in PEN (rather than the benzene rings in terephthalate), the polymer becomes more rigid [123], which leads to an enhancement of its mechanical properties, as well as thermal and chemical ones [124]. PEN furthermore exhibits excellent barrier properties for various gases and good chemical resistances [125]. PS is a widely utilized thermoplastic material, popular in various technological applications for its hardness, heat and electrical insulation properties, and ease of thermal processing [126]. It is also frequently used for lab-on-chip-type devices and as a substrate for cell cultivation [23]. Laser treatment can, under the optimal conditions, cause periodic structures to form on the surface of the substrate. On polymers, a pattern of ripples is the most common shape of the structures constructed upon irradiation with a linearly polarized laser beam [127]. The critical property of surface structures is their period, in respect to which, two types of ripples have been observed: (i) structures with a low spatial frequency, where the period is comparable with the wavelength of the laser radiation (Λ ≈ 𝜆), and (ii) structures with a high spatial frequency, where the period is much smaller than the wavelength of the laser radiation (Λ ≪ 𝜆) [128]. Whether low spatial frequency ripples or high spatial frequency ripples form on the treated surface of a substrate depends both on the material and on the conditions of the laser treatment [129]. Changes in surface morphology of PEN and PS induced by KrF excimer laser treatment through a lithographic contact mask with circular slits were studied for subsequent applications for sensor construction [122]. At first glance, both polymer substrates seemed to exhibit similar behavior, forming a relatively uniform ripple pattern in the central area of the treated region and a concentric interference pattern outside this central area, which becomes increasingly more pronounced toward the edge of the laser treated surface. On both samples, the exposed circular area where surface modulation has taken place was raised slightly in respect to the surrounding shielded surface. However, while the ripples on PEN are similar in their dimensions to those formed on a sample treated under similar conditions but without a contact mask, the ripples found on PS exhibit an unexpectedly large period, similar to a period measured on PS treated under the angle of incidence of 22.5∘ . Furthermore, the morphology of the two distinct types of surface forming the interference pattern is different on both polymers. On PEN, strips with a developed ripple pattern alternate with those where no surface modulation has taken place, while on PS, strips with a homogeneous ripple pattern alternate with strips where a ripple pattern has partially melted and lost its uniformity (Figure 14.12) [122]. Optical sensing strategies include numerous approaches and can be applied in many fields such as medicine, pharmaceuticals, toxicology, environmental monitoring, and others [130]. In clinical diagnostics, proteins are one of the most frequently detected components in body fluids [131]. For their optical detection, label-based immunoassays, which can also be classified as optical biosensors

14.3 Laser Modification

(a)

(b)

160 μm 30 μm

Figure 14.12 Modulated surface of polystyrene with a concentric interference pattern, (a) image taken by a confocal microscope in optical mode and (b) the same image taken by a ˇ et al. 2017 [122]. Reproduced with confocal microscope in confocal mode. Source: Nedela permission of John Wiley & Sons.

[132] are used as the most common qualitative and quantitative detection methods [133]. Singleplex formats (detection of single target molecules in analyte) of affinity-based immunoassays such as ELISAs (enzyme-linked immunosorbent assays) [134] have been the basic diagnostic systems for a long period of time. Recent progress in the development of fabrication techniques in microtechnology has facilitated the miniaturization of the immunoassays. These miniaturized analytical systems, the so-called protein microchips or microarrays, have been used as singleplex but also as multiplex detection formats (detection of multiple target molecules in analyte) [135]. Their basic components are micropatterns, often designed as microspots, with immobilized recognition elements. They provide affinity-based optical analysis of the measured samples, and their main advantages, in comparison with macrosystems, are (i) increased sensitivity, described and explained by Ekins in 1990s [136], (ii) multiplex samples detection experiment, and (iii) low consumption of the analyzed samples. Progress in the miniaturization also facilitated the development of microfluidic immunoassays and their integration into more complex Lab-on-Chip microdevices [137]. The crucial factor in the development of robust immunoassays is the quality of the immobilization process preserving important affinity-based properties of the immobilized proteins. If proper orientation of the proteins on the immobilization surface is necessary, a bio affinity method performed with avidin–biotin system can be considered a suitable strategy [138]. Because immobilization is usually performed on solid surfaces, their chemical and physical properties are important for the overall performance of the detection system. From the fabrication point of view, immobilization techniques for generation of micropatterns on 2D surfaces have been intensively studied in the last decades. Several different types of contact and noncontact techniques have been developed [139], and some of them can be performed in a high throughput automatized arrangement. As the most common are glass; silicon; thin metal films; polymers such as polystyrene, polycarbonate (PC), PMMA, and cyclic olefin copolymers (COCs); and 3D matrixes such as hydrogels, nylon, cellulose, and others, reviewed in [140]. Recently, thin and flexible polymeric materials have been used for the development of Lab-on-a-Chip systems, the so-called Lab-on-a-Foil [141]. These

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polymeric foils become materials for fabrication of micro electro mechanical systems (MEMS) sensor devices [142], and in life science applications for simple or more complex microfluidic solutions [143], filtration and separation devices for molecules [144], biosensors [145], or flexible electrode arrays [146]. Polymeric foils are good candidate materials for a low-cost and a high-volume production of commercial microsystems due to their relatively straightforward physical or chemical modification [28], natural flexibility, easy way of micro- or nanostructuring, and their potential to be processed by hot embossing [147]. Such properties are necessary for the device mass production, for a low-cost, and a high-volume production of commercial microsystems. Moreover, their specific modifications by roll-to-roll processing enable effective immobilization of bio affinity molecules in manufactured microdevices [148]. Laser material processing is a widely used manufacturing technique in industrial applications [149]. It can be used for bulk material and also for material surface processing. Laser processing can also be used as a serial micro fabrication technology. It has been shown that laser-induced periodic surface structures (LIPSS) can be induced by exposing the surface of polymeric material to a focused excimer laser beam with an intensity below the ablation threshold. With this method, permanent morphological changes in the form of regular micro- to nanopatterns. Such physical changes can be performed on various polymeric substrates [150], where periodic pattern or significant roughness increase may be induced [117]. Moreover, polymer foil treatment with laser beam can induce chemical changes on its surface [151]. Chemical surface modification of polymer with a laser beam leads to surface oxidation with a high number of available carbonyl groups. Such activated surface of polymers can be exploited in biosensing application for immobilization of various bioactive molecules to the polymer surfaces [46]. A methodology for micropatterning PEN foils by laser beam has been developed for designing a sensor with immobilized biotin micropatterns, and this sensor is suited for affinity-based optical biosensing [152]. Chemical changes on a PEN foil in contact with a lithography mask were achieved by laser beam, followed by covalent immobilization of biotin (Figure 14.13). The advantage of the designed process is that it can be developed for high throughput fabrication and roll-to-roll device production. The modified PEN foil sensing surface was adapted into a well-based format detection system with a low volume of chemical compounds. As a proof-of-concept, affinity-based fluorescence detection system for human serum albumin (HSA) was tested. A specific binding protein with natural affinity to HSA, the so-called albumin-binding protein domain, genetically fused with streptavidin (SA-ABDwt), was immobilized on the biotinylated surface of the PEN foil. The presented format of the sensing system can be developed into a fully automated fluorescence detection and data evaluation system, and depending on the required HSA detection range, it can be adopted for clinical use. [152]

14.4 Conclusion The chapter summarizes different approaches for surface modification of polymers aimed on their application in tissue engineering, for cytocompatibility

x,y movement of laser or foil

PEN foil

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(a) Foil patterning with laser

(d) Biotinylation of micropatterns 1

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Glass microwells on foil (c) with adhesive interlayer

(e) Immunoassay with 8 μl per well

Figure 14.13 Fabrication of biotinylated micropatterns. The surface of PEN foil was exposed to laser beam through a contact mask by positioning of the laser beam above the transparent parts of the photo mask in a serial manner (a). In a specified position of the laser above the mask, a certain number of laser pulses with a defined output were applied, and the laser was moved to the next position. By this process, micropatterned areas were fabricated on the PEN foil (b). Then, glass slides with cut holes with laminated adhesive were put into contact with the patterned foil and a well-based format of detection system was created (c). Coupling of biotin to the micropatterned foil surface followed (d) and the biotinylated micropatterns on the PEN foil were ready for affinity-based assays (e) with 12 testing wells. Source: Semerádtová et al. [152]. Reproduced with permission of Elsevier.

improvement, and for the development of biosensors. Different approaches for surface modification and functionalization are presented. The main focus is on plasma and laser surface modification tools. The main advantages of both techniques are explained, and each section contains numerous examples about the advantage of using this technique in the biological field.

Acknowledgments The work was supported by the GACR under the project No. 17-00885S.

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bioapplications induced by laser treatment. Biotechnol. Adv. 36: 839–855. 2 Xiang, T., Xie, Y., Wang, R. et al. (2014). Facile chemical modification of

polysulfone membrane with improved hydrophilicity and blood compatibility. Mater. Lett. 137: 192–195.

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3 Wu, W., Cheng, R., das Neves, J. et al. (2017). Advances in biomaterials for

preventing tissue adhesion. J. Controlled Release 261: 318–336. 4 Zhan, S., Pan, Y., Gao, Z.F. et al. (2018). Biological and chemical sensing

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the art and new developments. Proc. Inst. Mech. Eng., Part B: J. Eng. Manuf. 222: 107–116. Metz, S., Trautmann, C., Bertsch, A., and Renaud, P. (2002). Flexible microchannels with integrated nanoporous membranes for filtration and separation of molecules and particles. Technical Digest. MEMS 2002 IEEE International Conference. 15th IEEE International Conference on Micro Electro Mechanical Systems (Cat. No. 02CH37266). Las Vegas, NV, USA, USA: IEEE. p. 81–84. Gamby, J., Lazerges, M., Girault, H.H. et al. (2008). Electroacoustic polymer microchip as an alternative to quartz crystal microbalance for biosensor development. Anal. Chem. 80: 8900–8907. Rubehn, B., Bosman, C., Oostenveld, R. et al. (2009). A MEMS-based flexible multichannel ECoG-electrode array. J. Neural Eng. 6: 036003. Becker, H. and Heim, U. (2000). Hot embossing as a method for the fabrication of polymer high aspect ratio structures. Sens. Actuators, A 83: 130–135. Feyssa, B., Liedert, C., Kivimaki, L. et al. (2013). Patterned immobilization of antibodies within roll-to-roll hot embossed polymeric microfluidic channels. PLoS One 8: e68918. Dutta Majumdar, J. and Manna, I. (2011). Laser material processing. Int. Mater. Rev. 56: 341–388. Castillejo, M., Ezquerra, T.A., Martín, M. et al. (2012). Laser nanostructuring of polymers: ripples and applications. AIP Conf. Proc 1464: 372–380. Slepiˇcka, P., Michaljaniˇcová, I., Sajdl, P. et al. (2013). Surface ablation of PLLA induced by KrF excimer laser. Appl. Surf. Sci. 283: 438–444. Semerádtová, A., Štofík, M., Nedˇela, O. et al. (2018). A simple approach for fabrication of optical affinity-based bioanalytical microsystem on polymeric PEN foils. Colloids Surf., B 165: 28–36.

427

Index a Achilles heel of membrane processes 364 acrylamide (AAm) 190, 230, 243, 245, 247, 248, 385 acrylamide-co-N,N-methylenebisacrylamide 284 acrylic acid (AA) 45, 57, 81, 87, 89–91, 93, 94, 110, 114, 188, 224, 230, 231, 242, 244, 250, 257, 260, 284, 382–385 acrylic hydrogels 302 actinometry 86 activation energy for diffusion (ED ) 148, 150 activation enthalpy for diffusion (ΔHD ) 145 “activation-initiation” grafting mechanism 58 adhesion improvement 43–47 adhesion property 68, 348, 379 adsorption methods 352 adsorption theory 44 aging 59, 60, 69, 77, 123, 351, 365, 401 AIBN-bearing silanes 275 AIBN type-functionalized quaternary ammonium 275 albumin 320, 321 alginate 320–321 3-allyl-5,5-dimethylhydantoin (ADMH) 387, 389 alternating copolymers 108, 123 amide bond 325, 326, 358 aminolysis PET 216

PMMA 216–217 poly(ϵ-caprolactone) (PCL) 215–216 polylactic acid 213–214 aminopropylsilane (APS) 298, 353 3-aminopropyltriethoxysilane (APS) 353 Amonton’s first law 173 amphiphilic block copolymers 323, 379 anthraquinone 166, 251, 253, 256, 260, 277 anthraquinone-2-sulfonate sodium salt 277 antibacterial drug releasing materials 262 antibiofouling behaviour 124, 125 antibodies 332–333 anti-CD63 antibodies 326 anti-inflammatory and antibacterial polymeric implants 273 aptamers 260, 329, 332 Aquala technology 174 aramid powder 250 aromatic polyamide (PA) 265, 364 aromatic polyamide membranes 265 Ar plasma-treated nanofibers 55 Arrhenius plot of diffusivity 150 asialoglycoprotein receptor (ASGPR) 334 atmospheric pressure plasma jet (APPJ) 40–41, 52 atomic layer deposition (ALD) 370 cellulose 140–141 chemical mechanisms 138–139

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Surface Modification of Polymers: Methods and Applications, First Edition. Edited by Jean Pinson and Damien Thiry. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.

428

Index

coated cotton fibers 143–145 electrically conductive coatings 144 polyvinyl alchohol 140 protecting polymers from degradation 144 semiconductor manufacturing 138 unreactive polymer substrates 141–143 vapor barriers 144 vs. vapor phase infiltration 135–156 avalanche townsend discharges 35 4-azidophenylcarbonyloxyethyl-2bromoisobutyrate (AzEBI) 171 2,2′ -azobis[2-(2-imidazolyl-N-2-yl) propane] 250 azobisisobutyronitrile (AIBN) 275 azo compounds 241, 249–250 azoisobutyrylnitrile (AIBN) 250

biological polymer surface modification 354–355 biopolymers 303, 321, 402, 412 3,5-bistrifluorobenzylamine 226 3,5-bis-trifluoromethybenzene diazonium cation 254 β(1→4) linked d-glucose units 217 block co-polymers (BCPs) 4, 16, 123, 152, 153, 190, 196, 323, 379, 391, 392 Boltzmann constant 33 2-bromoisobutyrate compounds 171 Brownian nanoparticles 337 Brunauer-Emmett-Teller (BET) adsorption isotherm 110 butylmethacrylate (BMA) 248 n-butylmethacrylate (BMA) 248 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6 295

b

c

batch deposition technique 128 “beer bottle cap” concept 14 benzoin 166, 168, 169, 230, 275, 277 benzoin dianion 230 benzoin-functionalized multiwalled carbon nanotubes (MWCNTs) 275 benzoin-functionalized polymers 275 benzophenone (BP) 127, 166, 172, 230, 261, 274, 277, 278, 280, 391 biaxially oriented polypropylene (BOPP) 45, 248, 282 bifunctional N,N ′ -methylenebis (acrylamide) 248 binary graft copolymers 191 biodegradable and biobased polymers 347, 358 biodegradable polymers 49, 53–56, 213, 235, 320–323, 351, 356 biodegradables aliphatic polyesters 356 biofoulant 124 bio-inspired polydopamine (PDA) coating 235

camphorquinone (CQ) 279, 281 capacity coupled plasma (CCP) 351 carbenes 167, 241–268 carbonaceous PTFE 231 carbon based nanomaterials 410 carbon-fiber 174, 175 carbon nanotubes 12, 68, 118, 119, 122, 123, 127, 275, 303, 364, 410 carbon quantum dots 364 4-carboxy benzenediazonium salt 254 carboxylated CNCs 219 cardo polyetherketone 224 catalytic hybrid metal-polymer membranes 369 cation exchange membranes 258 cationic grafting 192, 193 cationic polymeric nanoparticles 338 cellobiose (β-1,4-glucosidic dimer) 219 cell-penetrating peptides (CPP’s) 333, 338 cell-targeting peptides (CTP’s) 333 cellulose 136, 140, 217–220 cellulose nanocrystals (CNC) 219

atomic layer deposition (ALD) (contd.)

Index

cetyltrimethylammonium bromide (CTAB) 323, 353 (CH2 )2 C6 F13 262 chain transfer agent (CTA) 193 charged molecules 5 chemical bonding 44, 46, 56, 71, 72, 74, 83, 96, 108, 127, 164, 178, 188, 301, 329, 375 chemical mechanisms 138 of ALD 138–139 chemical oxidation natural polymers 234 PET 234 PMMA 233 polyethylene 231–233 polypropylene 231 polystyrene 233 polyurethane 233–234 chemical reaction theory 44 chemical reduction PET 225–227 PMMA 227 polyaryletheretherketone 220–224 polycarbonates 227–229 polytetrafluoroethylene 229–231 chemical vapor deposition (CVD) techniques 370 chitosan 51, 53, 55, 57, 58, 187, 196, 216, 218, 234, 235, 244, 321, 330, 335, 355, 376 chitosan (CS) nanofibers 55 chitosan/PEO nanofibers 55 chloramine 224 click reaction 121, 327, 331, 335 60 Co 186, 196, 206, 385 coated cotton fibers 143 cold low-pressure plasmas 85 cold plasmas 70, 214, 351, 355–357 competitive ablation and polymerization (CAP) 74 completely ionized plasma 34 conducting polymers 154–155, 250–252, 287 configurational entropy 2 confined photocatalytic oxidation (CPO) 247, 248 conformality for iCVD coatings 112

conjugated monomers origin of the phenomenon and mechanism of polymer synthesis 298 polyaniline 294–298 polypyrrole 290–294 contact angle hysteresis 13, 120 copolymerization 16, 110, 111, 120, 123, 126, 224, 303, 385, 387, 389, 391 copolymers of maleic anhydride 166 copper(Cu-I)-catalyzed alkyne-azide cycloaddition (CuAAC) 327 cotton 140 gauzes 196 Coulomb-type collisions 34 covalent conjugation 325, 326, 328 crosslinking 198, 374 grafting 130, 266, 379, 393 crosslinked hydrogels 199 crosslinked poly(vinyl alcohol) (PVA) ultrafiltration membranes 372 crosslinkers 116, 119, 123, 124, 126, 201, 266, 330, 383 crosslinking 43 with additives 200–201 gamma ray modifications 199–200 industrial applications 201–202 vulcanization 197–198 crystalline morphology 2 137 Cs 186 Cu NPs 375 CVD polymer surfaces 107 cyclic olefin copolymers (COC) 45, 245, 256, 415

d DC non-thermal plasma discharges 38 DC pulsed discharges 38 DEAAm 114, 126 density functional theory 93 desorption method 352–353 dextran 266, 267, 321, 329, 330, 338, 390 diacyl peroxides (di-benzoylperoxide [C6 H4 C(=O)—O]2 241

429

430

Index

1,4-diaminobutane 216 1,12-diaminododecane 216 diarylcarbenes 261, 262 diazonium-induced grafting 387 diazonium-modified biaxially oriented PET (BOPET) sheets 257, 258, 284 diazonium modified-multiwalled carbon nanotubes (CNTs) 303 diazonium salt 241, 250–261, 279, 284, 387, 388 di-cumylperoxide [C6 H4 —C (CH3 )2 —O]2 241 dielectric barrier discharges (DBD) 39–40, 70, 79, 350 diene polymer 166 diethylaminosulfurtrifluoride (DAST, Et2 NSF3 ) 221 diffusion coefficient 147–149 diffusion theory 44 difluorotriazine 226 N,N-dimethylaminoethylmethacrylate (DMAEMA) 248 2-(N,N-dimethylaminoethyl) methacrylate (DMAEMA) 172, 196 dip method 384 disulfide bond 327 di-t-butylperoxide t-Bu—O—O—t-Bu 241 dithiol AIBN derivative 275 divinyl benzene (DVB) 120, 123 DMAAm 114, 126 DMEAMA 115, 126 DNA nanobiosensors 405 dyeability of textile materials 47 dynamic light scattering (DLS) 337, 338 Dyneon THVTM 259

e effective diffusion coefficient 148, 149 EGDA and EGDMA 124 electrically conductive coatings 144 electron avalanche 35, 36

electron density 33, 34, 41, 78, 81, 86, 113, 352 electrons energy distribution function (EEDF) 40, 70, 71, 86, 88, 95 electron temperature 33–35, 70 electrospun biosourced polymers 277 electrospun PCL scaffolds 54 electrospun silk fibroin (SF) nanofibers 55 electrostatic theory 44 energy-deficient domain 78 enthalpy of sorption (ΔHs ) 145–147, 149 enzyme-coated PE film 167 enzyme-linked immunosorbent assays (ELISAs) 415 equilibrium discharges 34, 35 equilibrium plasma 34 estradiol 334, 335 etched-sputtered Teflon 7 ethylene glycol diacrylate (EGDA) 115, 201 ethyleneglycol dimethacrylate (EGDMA) 115, 119, 120, 124, 201 ex situ surface modification 127

f floating potential 71–72, 88 fluorocarbon polymer 5 fluoropolymers 107, 119, 188, 229, 259 fluoro polymers 259–260 fluoropolymers 403 Fmoc/Boc/Alloc solid-phase approach 338 Fourier transform IR (FTIR) spectroscopy 341–342 frictional force 174 friction coefficient 172–174 fully-dense inorganic substrates 135 functionalized PPF 69, 76, 83, 87 functional polyperoxide (FPP) 243, 244 functional, surface reactive, and responsive organic films 113–127

Index

g

h

galactose/galactose-mimics 334 gamma radiation 350 gamma ray-induced modifications grafting modifications 186–197 gas molecules 5 gastric epithelial cell membranecoated PLGA nanoparticles 328, 330 gelatin 53, 166, 262, 321, 322 glass transition temperature 13, 15, 16, 18, 49, 113, 220, 408 gliding arc 41 glow discharges 35, 38, 40, 44, 68, 88, 232, 366 glycerol 188, 195, 196, 390 GO membrane 376, 377 grafted hydrophilic poly((polyethyleneglycol) methacrylate) 389 grafted plasma pre-treated polymers 402 grafted polymers 2, 57, 187, 232, 276, 285, 354, 389, 390 grafting initiated by chemical/electrochemical means 385–389 grafting methods controlled grafting-from 389–392 “grafting-to” approach 378–381 methods 381–389 grafting modifications applications 194–197 ionic grafting 192–193 limitations 187 natural polymers 187 propagation mechanism 187 radiation-induced grafting methods 188–192 RAFT 193 grafting, plasma 56–59 “grafting-to” approach 378–381 graft polymerization 163, 168–180, 187, 188, 244, 276, 381–382, 387, 393 graphene oxide (GO) nanosheets 376

hammering effect 205 heavy ion-induced modifications 202–205 high density polyethylene (HDPE) 243, 403 high energy irradiations 368, 369, 385 high-energy irradiation sources 385 high-flux superhydrophobic membranes 375–376 High Internal Phase Emulsion (HIPE) 216 highly crosslinked organic networks 107 highly cross-linked PPF 68 highly hydroxylated CNCs 219 high purity CVD polymers 107 Hildebrand and Hansen solubility parameters 6 Hildebrand solvent parameter 6 homopolymer(s) 108, 110, 113, 116, 119, 120, 123, 124, 130, 188–191, 242, 412 homopolymer formation 189–191 hybrid photovoltaic cells 154–155 hydrazone bonds 327 hydrogen passivated silicon 128 hydrogen peroxide 244–246 hydrolysis PET 216 PMMA 216–217 poly(ϵ-caprolactone) (PCL) 215 polylactic acid 213 hydrophilic nanofiltration membranes 224, 384 hydrophilic polymer-grafted UHMWPE 172 hydrophilic polymer surfaces 15 hydrophilic/water-soluble polymers 169 hydrophobic iCVD P(PFDA-co-DVB) 120 hydrophobic polymers 250, 323, 348, 353, 364 hydrophobic recovery of plasma-modified polymers 59

431

432

Index

4-hydroxybenzoic acid 249 2-hydroxyethyl methacrylate (HEMA) 195, 279, 387, 388 hydroxylated polyethylenes 243

i iCVD PDVB homopolymers 123 iCVD PPFM reactive layer 126 iCVD PV3D3 122, 124 imidazole 247 immersion method 384 immiscible polymer blends 4, 6, 8 induced plasma polymerization 74 inductive coupled plasma (ICP) 351 inert synthetic polmers 50–53 infiltrated precursors (penetrants) 148 infiltration 135–138 iniferter (initiator-transfer-terminator) 284 initiated chemical vapor deposition (iCVD) 370 adhesion and grafting 127–128 dewetting effects 108 functional, surface reactive, and responsive organic films 113–127 mechanistic principles of 108–113 reactors for organic films 128–129 initiators 109 inorganic polymeric substrates 305–306 inorganic salts 178–180 in situ graft co-polymerization 224 in situ polymerization stringed assembly (SPSA) 281 in situ surface modification 127 insulating vinylic and other monomers polydopamine (PDA) bioinspired adhesive 284 simultaneous photoinduced electron transfer and free radical polymerization 282–284 surface-initiated photoiniferter 284 type I and type II photoinitiation systems 275–282 interdigitated gold electrode-coated flexible PET sheet 294

interdigitated PPy humidity sensors 294 interfacial adhesion of polymers 353–354 ion-activated growth model (AGM) 74 ion energy distribution function (IEDF) 88, 94, 95 ion hammering effect 205 ionic grafting 192–193 ionic surfactants 5 ion imprinted clay-polymer nanocomposite 279 ionizing radiation sources 186 irradiation-based direct polymer modification high energy irradiations 368–369 plasma treatment 365–366 UV irradiation 366–368

j jute fibers 234 jute yarns 243, 245, 246

k 175 kDa C-type lectin receptor 334 266 Kevlar kinetic gas theory 70 kinetics of precursor diffusion 147–148

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l Lab-on-a-Foil 415 Lab-on-Chip microdevices 415 Large Hadron Collider 186 laser-induced periodic surface structures (LIPSS) 416 laser modification interaction with cells 411–412 sensor construction 412–416 laser nanopatterning 414 lauryl peroxide [C12 H25 —(C=O)—O]2 241 LbL modified membranes 375 light responsive iCVD layers 125 linear accelerators 186 linear energy transfer (LET) 203

Index

linear low density polyethylene (LLDPE) 243 line intensity ratio 86 linker-free grafted crosslinked PDVB layers 128 linker-free grafting 127 lipids 119, 329–332 living free-radical polymerization 193 local electrostatic potential 33 local thermodynamic equilibrium (LTE) conditions 33 low density polyethylene (LDPE) 45, 49, 243, 247, 403, 410 low molar mass polymer 4, 7, 16 low pressure plasma polymerization (LP-PP) 69, 79, 81, 83, 84, 94, 995 low temperature plasma techniques 365

m macroporous flow-through membrane adsorbers 391 manganese meso-tetra-2,6-dichlorophenylporphyrin acetate Mn(TDCPP)OAc) 247 mass spectrometry (MS) 69, 84, 87–96, 147 mass transfer changes 49–50 MATLAB software program 79 Maxwell-Boltzmann distribution 33, 70 mechanical interlocking theory 44 mechanical polymer surface modification 354 mechanical properties 4, 18, 76, 113, 150, 174, 198–200, 206, 213, 220, 242, 256, 353, 366, 400, 402, 406, 407, 414 membrane coating from gas phase 369–371 from wet phase 371–378 membrane modification 258, 364, 379, 392 mercury lamps 164, 165 metal-organic ALD precursors 140

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metal-organic frameworks (MOFs) 142, 364, 376 metal-organic precursor molecules 135 metal oxide ALD 140, 142, 144 (meth)acrylate/styrene monomers 389 methacrylic acid (MAAc) 110, 114, 167, 195, 216, 335, 386 2-(methacryloylethyl) phosphoric acid (MPA) 172 2-methacryloyloxy-benzoic acid 195 2-methacryloyloxyethyl phosphorylcholine (MPC) 171–172 methyl 3-mercaptopropionate (MMP) 10 miscible polymer blends 8 molecularly imprinted polymer (MIP) 284, 285 molecular recognition ability 273 momentum density 83 monoclonal antibodies (mAb) 333 monolith 216 monomer(s) 109, 116 monomer-deficient regime 79 multiple pulse infiltration (MPI) 138 multivinyl monomers 124 mussel-inspired polydopamine (PDA) coatings 372 MWCNT-polystyrene (MWCNT-PS) nanocomposites 275

n N-acetylgalactosamine (NAcGal) 334 Nafion 10 Nafion 115 membranes 375 nano-or micro-patterned material surfaces 399 nanoparticles albumin 320 alginate 320–322 chitosan 321 covalent conjugation 325–326 drug delivery applications 328–336 dynamic light scattering 337 fabricated using preformed polymer 323

433

434

Index

nanoparticles (contd.) Fourier transform IR (FTIR) spectroscopy 341–342 gelatin 322 non-covalent interactions 328 particle size 336 PLA 322–323 PLGA 322–323 poly-ϵ-caprolactone 323 polymers used in preparation of 320 scanning electron microscopy 337–339 in situ preparation of 323 surface charge 339–340 surface hydrophobicity 340–341 transmission electron microscopy 339 nanostructure 46, 69, 138, 153, 289, 290, 305, 307, 405, 410 native nylon 6 capillary-channeled polymer fibers 247 natural biodegradable polymers 55–56 natural polymers 187, 199, 234, 243, 260, 289, 291, 320, 321, 356 [Ni(Me4 Phen)3 ](BPh4 )2 243 (N-isopropylacrylamide) (NIPAAm) 114, 126, 190, 195 nitrenes 241–268 N-methyl-2-benzoyl-β-naphthiazoline 166 N,N-dimethylamino-functionalized diazonium cation 279 N,N-dimethylamino-functionalized thiol 279 N,N-dimethyl aminosilane 279 N,N ′ -methylene bis(acrylamide) (BIS) 337 N,N ′ -methylene-bis-acrylamide 243, 337 N,N,N ′ ,N ′ -tetramethylene ethylenediamine 337 noncovalent surface modification 234–235 nondegradable polymers 403 non-equilibrium plasmas 34, 35, 70 non-imprinted polymer-coated beads (NIP) 284

non-intrinsically photoactive polymers 383 non-polymerizable photoinduced surface reactions 166 non-thermal equilibrium 70 non-thermal plasma 34–38, 41–43, 47, 53, 54, 59–61, 67, 359 non-thermal plasma for polymer surface treatment 41–43 Norish type II photoinitiator 281 Norrish initiators 275 N-vinylcaprolactam (NVCL) 114, 191, 195, 196 N-vinylimidazole (NVIm) 191, 196 nylon 123, 136, 202, 247, 305, 403, 415

o oligomeric ions 93, 94 oligo(ethylene glycol) monomethacrylate (OEGMA) 172 oligosaccharide-lectin interactions 334 optical emission spectroscopy (OES) 69, 84–87 optical properties 18, 204, 410 optical sensing strategies 414 organic films 108, 113–128 organic-inorganic BCPs 153 organic membranes 364 organosilicon and organosilazanes 121 oxidizing plasmas 141 oxygen 248–249 ozonation 231–233, 244 ozone 39, 165, 231–233, 244, 354, 387 ozone-induced grafting 387

p P(4VP-co-EGDA) 124 palladium-doped polypyrrole 306 particle-decorated PET fibers 282 patterning for microsystems 153 PCL nanofibers 54 PEK-COOH ultrafiltration membranes 224 penetrant diffusion and reaction 148–149 peptides 333–336

Index

peptide-decorated silk fibroin coatings 223 perfluoroalkyl functional groups 119 perfluorosulfonated ionomers 10 peroxides 241–244 persulfates 241, 243, 246–248, 303, 337, 385, 386 PET aminolysis 216 chemical oxidation 234 chemical reduction 225–227 hydrolysis 216 phenylazides 167, 171, 264, 266 phenyldiazirine 167 photochemically prepared PANI (polyaniline) thin films 295, 298 photochemical surface modifications 163, 164 photoinduced chemical reaction between polymers 166–167 photoinduced grafting at the polymer surface 168–169 photoinduced graft polymerization artificial organs 172–174 high-functionality materials 169–172 inorganic salts 178–180 poly (aryl ether ketone) 174–178 photoinduced reactions on polymer substrates 165 photoinduced self-initiated polymerization 175, 176 photo-iniferter 284, 285 photoinitiated graft polymerization 168, 172 photoirradiation energy 164 photopolymerized poly(ethylene glycol diacrylate) (PEGDA) 294 photo-reactive benzopinacol 391 photoreactive polymers 167, 367 photosensitizers 125, 166, 168, 170, 180, 255, 257, 281, 287–291, 294, 297, 298 pH responsive iCVD films 126

physical crosslinking 198 physical vapor deposition (PVD) 369 physicochemical polymer surface modification 349 piezopolymer blend 254 pinhole-free iCVD homopolymer layers 124 Planck’s constant 84, 164 plasma 70 adhesion improvement 43–47 atmospheric pressure plasma jet 40–41 biomedical applications 50–56 DC non-thermal plasma discharges 38 DC pulsed discharges 38 definition 33–34 dielectric barrier discharge 39–40 formation of non-thermal plasma 35–37 gliding arc 41 grafting 56–59 hydrophobic recovery 59–60 methods of plasma generation 37–41 packaging & textile applications 47 RF and MW discharges 38–39 thermal vs. non-thermal 34–35 plasma ablation/etching 42 plasma activation 42–43 plasma assisted ALD 142 plasma enhanced CVD (PECVD) 113, 123, 356 plasma functionalization 42–43 plasma-induced graft polymerization 381–382 plasma passivation 42, 43 plasma polymer films (PPF) biotechnology 69 cross-linking density 67 mass spectrometry 87–96 optical emission spectroscopy 84–87 physico-chemical properties 68 plasma polymerization 69–83 precursor 67 synthesis 69

435

436

Index

plasma polymerization fundamentals 70–72 growth mechanism 72–83 plasma polymerized polyethylene glycol (PEG) 81 plasma polymerized propylisobutyrate (PiB) 81 plasma-polymerized styrene film 68 plasma polymer nanoparticles 73 plasma-state polymerization 74 plasma-treated CS/PEO nanofibers 55 plasma treatment 365–366, 400–411 plasticization 5, 16 plastic wastes 213, 273 PMMA aminolysis 216–217 chemical oxidation 233 chemical reduction 227 hydrolysis 216–217 poly((2-dimethylamino)ethyl methacrylate-co-butyl methacrylate) (poly(DMAEMA-co-BMA)) 385, 386 poly(2-hydroxyethyl methacrylate) (PHEMA) 279 poly(3-hexylthiophene-2,S-diyl) (P3HT) 141 poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) 358, 359 poly(3-sulfopropyl methacrylate) (PSPMA) 285, 287 poly(4-styrenesulfonate) (PEDOT : PSS) 305 poly(4-vinylpyridine) 195, 379 poly(acrylamide-co-bisacrylamide) 284 poly (aryl ether ketone) (PAEK) 174–178 poly(dimethylsiloxane) 250 poly(ϵ-caprolactone) (PCL) 213 aminolysis 215–216 hydrolysis 215–216 poly (ether ether ketone) (PEEK) 5, 45, 174 poly(ether sulfone) (PES) 364

poly(ethylene terephthalate) (PET) 18, 351 poly(glycolic acid) (PGA) 213, 214, 356 poly(lactic-co-glycolic acid) (PLGA) 214, 322–323, 356 poly(methyl methacrylate) 10, 144, 287, 302 poly(n-butyl acrylate) 302, 321 poly(N-isopropylacrylamide) (PNIPAAm) 190, 254, 264 poly(N-vinylpyrrolidone) 166 poly(oligo(ethylene glycol) methyl ether methacrylate) (OEGMA) 287 poly(pentafluorophenyl methacrylate) (PFMA) 287 poly(vinyl alcohol) (PVA) 140–141, 321, 371 poly(vinylidene difluoride) (PVDF) 45, 58, 112, 230, 254, 331, 364, 370, 373, 379, 380, 382 poly(vinylidene fluoride-co-trifluoroethylene) 390 polyacrylamide (poly(AAm)) 166 polyacrylicacid (PAA) 227, 235, 257, 258, 284 polyacrylic acid-co-maleic acid (PAA-co-PMA) 257 polyacrylonitrile (PAN) ultrafiltration (UF) membranes 365 6,6-polyamide 242 polyamide (PA) 49, 58, 166, 202, 242, 264–266, 364, 403 polyaniline 251–252 mechansims of photopolymerization 294–297 substrates for in situ photo-induced polymerization 298 polyaramide fibers 144 polyaryletheretherketone (PEEK), chemical reduction 220–224 polybutadiene modified epoxy (PBME) 10 polybutylene succinate (PBS) 340, 353, 356, 358, 359 polybutylene terephthalate (PBT) 202 polycaprolactone (PCL) 350, 353, 356

Index

polycarbonate compact discs (CD) 229 polycarbonates (PC) 45, 126, 166, 227–229, 382, 415 polycationic chitosan polymer 234 polydimethylsiloxane (PDMS) 141 polydopamine (PDA) 328 bioinspired adhesive 284 poly-ϵ-caprolactone (PCL) 323 polyester fabric 47 polyetheretherketone (PEEK) 256–257, 403 polyethersulfone membranes 277 polyethersulfone (PES) membranes 258, 277 polyethylene (PE) 165, 202, 243 chemical oxidation 231–233 food industry 356 polyethylene glycol (PEG) 81, 82, 114, 244, 265, 321, 323, 334–336 polyethyleneimine (PEI) 235, 258, 338 polyethylene naphtalate (PEN) 402, 414 polyethylene terephthalate (PET) 45, 165, 255, 257–258, 282, 402, 414 fibers 282 film 165 food industry 356 polyglycidal methacrylate (PGMA) 113, 118, 119 polyglycolic acid (PGA) 356 poly(MPC)-grafted PEEK 176–179 poly(MPC)-grafted-UHMWPE 173, 174 polyhydroxyalkanoates (PHA) 58, 356, 411 polyhydroxyethylacrylate (PHEMA) 120 polyhyhroxybutyrate (PHB) 411 polyimide 45, 144, 154, 402 polyimide films 154 polylactic acid (PLA) 350, 356 aminolysis 213–214 films 60 hydrolysis 213–214 polylactic-co-glycolic acid (PLGA) 350 polylactide PLA 321–323

poly(MPC) layer 173, 175, 178–180 poly-l-lactic acid thin films 408 polymer(s) 204 biological modification 354–355 blends 4, 6–8, 152 brushes 6, 7, 169, 276, 289 chemistry 3–4, 13, 15–17, 21, 68, 139, 151, 152 food packaging 355–358 gas phase 349–350 grafting and polymerization 354 interfacial adhesion 353–354 liquid and bulk phase methods 352 mechanical methods 354 patterning 402 radiation 350–352 polymer-ceramic and polymer-metal systems 46 polymer chains 2, 3, 6, 8, 10, 11, 14–18, 43, 60, 110, 123, 127, 141, 142, 165, 169–172, 177, 179, 180, 193, 199, 201, 214, 266, 282, 366, 378, 381, 383, 389, 393, 401, 402 polymeric foils 416 polymeric micelles 195 “polymer-like” films 113 polymer/metal interfaces 10 polymer-polymer interactions 6 polymer-polymer TENGs 156 polymer substrates selective adsorbents 273 polymer surfaces application-related modification 1 charged molecules 5 definition 2 experimental methods 21 Flory-Huggins theory 7, 8 gas molecules 5 glass transition temperature 15–16 guiding force 1 immiscible polymer blends 8 large composition fluctuation 7 long-range chain statistics of polymer molecules 7 mechanical properties 18 microscopic level 11–12 miscible polymer blends 8

437

438

Index

polymer surfaces (contd.) optical properties 18 polymer chemistry 3–4 protein fouling 15 similar polymer pairs 8 solid surface 8–10 solvents 5–7 surface crystallization 17 surface properties 1 surface wettability 13–15 treatment 41–43 poly[3-(methacrylamino)propyl] trimethyl ammonium chloride (PMAPTAC) 294, 296 polymethylmethacrylate (PMMA) 50, 216–217, 227, 232, 254–255, 403 poly-4-methyl-1-pentene (PMP) 405 poly(dimethyl siloxane) (PDMS) microfluidic channels 277 poly(ethylene glycol) monomethacrylate 171 poly(acrylic acid) nanobrushes 195 polyNIPAm 254 polyolefin 163, 166, 168, 169, 171, 405 polyolefine substrates 169, 171 poly(methyl phenyl siloxane) (PMPS)/organophilized silicate 16 poly(4-vinylbenzyl chloride-co-styrene) particles 284 poly(D,l-lactide-co-glycolide) PLGA 321–323 poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT : PSS) 253–254, 305 poly(ethylene 2,6-naphthalate) (PEN) polymers 144 poly(lactide-co-glycolide) (PLGA) polymers 322, 353, 357 poly[poly(ethylene glycol) methacrylate] (PPEGMA) monomers 282 (poly p-phenyleneterephthalamid) 250 polypropylene (PP) 165, 202, 255 chemical oxidation 231–233 food industry 356 sheet 245 polypyrrole 251

photopolymerization mechanism 290 substrates for in situ photo-induced polymerization 291–294 polyquercetin (pQCT) nanoparticles 340 polysaccharides 199, 216, 217, 266, 320, 321, 329, 330 poly(methyl methacrylate) (PMMA)/silver (Ag) interface 10 polystyrene 302, 402 chemical oxidation 233 polystyrene (tissue polystyrene) 402 polysulfone (PSf ) ultrafiltration membranes 365 poly(ethylene 2,6-naphthalate) surface 247 polytetrafluoroethylene (PTFE) 285, 371 chemical reduction 229, 231 polyurethane chemical oxidation 233–234 polyvinyl alcohol (PVA) 136, 140–141, 219, 323, 353 polyvinylchloride 255 potable water 363 PP/modified rubber powder composite 243 p(HEMA-co-PEGMA500) polymer 263 PPyAg-coated PET sheets 293 PPyAg/PPyAu binary nanocomposites 291 precursor-polymer couple 136 pre-irradiation method 189–190 pre-irradiation oxidative method 190–191, 193, 194 pro-drug polymers 195 1,3 propane sultone (PS) 124 propanethiol PPF 83 protein fouling 15 protein microchips or microarrays 415 protonated poly(allylamine)/ poly(4-styrenesulfonate) multilayer films 375 pseudo-ALD coatings 141

Index

P4VP (pH-sensitive polymer) 195 pyrrolyl-functionalized silane 292

q quadrupole analyzer 88 Quartz Crystal Microbalance (QCM) 19, 149, 262

r radiation-induced grafting methods and methods 191–192 pre-irradiation method 189–190 pre-irradiation oxidative method 190 simultaneous/direct method 190–191 radical initiators alkyl halides 260 azo compounds 249–250 diazonium salt 250 hydrogen peroxide 244–246 oxygen 248–249 peroxides 241–244 persulfates 246–248 random copolymers 16, 108, 120, 248 rapid step growth polymerization (RSGP) 72, 73 reaction enthalpy (ΔH rxn ) 145 redox-initiated membrane grafting 385 reverse osmosis membranes 124, 127, 265, 266, 363, 371–373, 375, 387 Reversible Addition-Fragmentation chain Transfer (RAFT) 188, 193, 302, 389 RF and MW discharges 38–39 rod-like macromolecules 12 ruthenium dye [Ru(bpy)3 ]Cl2 257

s salts 5, 178–180 scanning electron microscopy (SEM) 20, 152, 337–339, 357, 404, 413 scavenger effect 74 secondary electron avalanches 35, 36 secondary electron multiplier (SEM) detector 88

Seebeck coefficient 293 segmental relaxation (α-relaxation) process 16 self-assembled BCPs 153, 154 self-initiated photografting and photopolymerization (SIPGP) protocol 287 self-initiated photoinduced graft polymerization 174–180 self-organized block co-polymers (BCPs) 153 semibenzopinacol radical 174 sequential infiltration synthesis (SIS) 138 sequential vapor infiltration (SVI) 138 sheath 72, 75, 76, 94, 95 “side crystalline” chains 3 silica nanoparticles 12 silicone rubber 187, 195, 196 similar polymer pairs 8 simultaneous photoinduced electron transfer and free radical polymerization 282–284 small fraction 2 small molecule ligands 329, 333–335 smart materials 4, 399 “smart” materials 4, 399 sodium dodecyl sulfate (SDS) 353 sodium lauryl sulfate (SLS) 353 sodium styrene sulfonate (SSS) 189 solid surface 7, 8, 17, 18, 174, 415 solubility coefficient 146 solvation phenomenon 6 solvent(s) 5 solvent-polymer interaction 6 solvent-solvent interaction 6 sono-induced polymerization 302 sonoluminescence 301 sonopolymerization 273, 274, 303, 305–306 sputtered Teflon 7 stable hydrocarbon based molecules 90 sticking probability 75, 112 streamer discharges 35 streptavidin 328, 331, 416 styrenic monomers 122

439

440

Index

SU-8 151 substrate-C(Phenyl)2 -OH 277 sulfonated polystyrene 166 surface charge 328, 336, 339–340, 364, 387, 401 surface-confined photopolymerisation polyaniline 294–298 polypyrrole 290–294 surface-confined radical photopolymerisation polydopamine (PDA) bioinspired adhesive 284–287 recent trends 287–289 simultaneous photoinduced electron transfer and free radical polymerization 282–284 surface-initiated photoiniferter 284 type I and type II photoinitiation systems 275–282 surface-confined sonochemical polymerization inorganic polymeric substrates 305–306 origin of the phenomenon and mechanism of polymer synthesis 298–303 sonopolymerization 303 ultrasonic spray 303–305 surface crystallization 17 surface free energies 4, 5, 7, 13, 42, 43, 50–52, 59 surface functionalization 141, 189, 191, 206, 241, 242, 276, 340, 342, 347, 352, 380, 383 surface hydrophobicity 188, 339–341, 365, 385 surface-initiated ATRP (SI-ATRP) 171 surface-initiated photoiniferter 284 surface modification of polymers carbenes 261–264 nitrenes 264–267 surface modification technology 163 surface properties of polymeric materials by photoirradiation 165–166 surface tension 2, 13, 54, 108, 112, 156, 352, 356 surface treated polyesters 402

surface wettability 12–15, 52, 55, 214, 358 surfactants 5, 291, 323, 324, 352, 353 swift heavy ions (SHI) 202 irradiation 186 synchrotrons 186 synthetic biodegradable polymers 54–55 synthetic polymers 54, 55, 163, 169, 187, 188, 275, 276, 320, 347, 356–358, 406

t taurine 214, 215 technical poly(ether ether ketone) 5 229 Teflon Teflon FEP 7 tertiary amines 114, 115, 124–126, 170, 378, 385 tetrameric glycoprotein avidin 328 thermal equlibrium 70 thermal plasma 34, 40, 324, 351 thermal responsive hydrogels 126 thermal vs. non-thermal plasma 34–35 thermoplastic elastomers (TPE) 202 thermoplastic polyurethane membrane 244 thermo-sensitive polymer 195, 254 thin film nanocomposite (TFN) membranes 376 thin film transistors (TFTs) 122, 124 thin poly(styrene)-b-poly(ethylene oxide) films 6 thio-ether bond 326, 331 thiol-ene systems 170 3D-carbene surface 262 TiO2 /PPyAg hybrid composite thin layer 294 transmission electron microscopy (TEM) 20, 136, 152, 336, 339, 372 triallyl isocyanurate (TAIC) 201 triazole ring formation 326 triboelectric nanogenerators (TENGs) 155, 156 trifluorotriazine 226 tritiated lysine 226 two-dimensional ZIF-8/GO hybrid nanosheets 376 TM

Index

type I and type II photoinitiation systems 275–282 type II surface-confined photopolymerization initiation 279

u ultra-high molecular weight PE (UHMWPE) 52, 53, 58, 172–174, 180, 231, 403 ultrasonic spray 301, 303–305 ultrasound-assisted polymerization 302–305 ultrathin PEGDMA layers 124 ultrathin polystyrene (PS) 402 ultrathin (6 nm) PV3D3 layers 122 unreactive polymer substrates 141–143 unstretched PET film 165, 166 untreated SF nanofiber matrices 55 UV-induced grafting 383–384 UV irradiation 224, 230, 231, 247, 256, 276, 366–368, 380, 383, 384, 391

v Van de Graaff, cyclotrons 186, 202 Van’t Hoff equation 149 Van’t Hoff relationship 146 vapor barriers 135, 144, 154, 347 vapor diffusion barriers 154 vapor phase infiltration (VPI) altering mechanical performance 150–151 application spaces 155–156 vs. atomic layer deposition (ALD) 135–138 conducting polymers and hybrid photovoltaic cells 154–155 contrasting agent for imaging block co-polymer 152 fundamental steps 145 improved chemical resistance 152–153 kinetics of precursor diffusion 147–148 molecular precursor thermodynamics 145 patterning for microsystems 153

thermodynamics and kinetics 149–150 vapor diffusion barriers 154 vapor phase molecular precursor 145 vapor phase precursor solubilizing 145 vertically aligned core/shell-like polyaniline-wrapped ZnO nanorod composite films 257 vinyl ether 166 viscoelastic UHMWPE substrate 174 vulcanization 197, 198

w waste tire powder recycling 242 water contact angle (WCA) 51, 53, 60, 120, 173, 214, 216, 217, 225, 226, 230–234, 242–245, 247, 249, 253–255, 260, 265, 285, 347, 365, 371, 410 water vapor plasmas 141, 365 water vapour transfer rates (WVTRs) 154 wettability 347 wetting properties 243, 273, 389, 393

y Yasuda parameter

78, 79

z zero “native” surface potential 5 ZIF-8/GO hybrid nanosheets 376, 377 ZIF-8/GO hybrid nanosheets functionalized TFN membranes (TFN-ZG) 376 ZIF-8 hybrid nanocomposites 376 ZnO nanoparticles 154, 306 ZnO nanorod/polyaniline composite film 257 zwitterionic [3-(methacryloylamino) propyl]dimethyl(3-sulfopropyl) ammonium hydroxide 248 zwitterionic-modified surfaces 124 zwitterionic polymer capsuled protein based nanogel 337 zwitterionic polysulfobetaine hydrogel 248

441