Block Copolymers in Solution · 2013-07-23 · Contents Preface xi 1. Introduction 1 References 5...

Block Copolymers in Solution:Fundamentals and Applications

Block Copolymers inSolution: Fundamentalsand Applications

IAN HAMLEYUniversity of Reading, Reading, UK

John Wiley & Sons, Ltd

Copyright © 2005 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

Telephone (+44) 1243 779777

Email (for orders and customer service enquiries): [email protected] our Home Page on www.wileyeurope.com or www.wiley.com

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system ortransmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanningor otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under theterms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, LondonW1T 4LP, UK, without the permission in writing of the Publisher. Requests to the Publisher shouldbe addressed to the Permissions Department, John Wiley & Sons Ltd, The Atrium, Southern Gate,Chichester, West Sussex PO19 8SQ, England, or emailed to [email protected], or faxed to(+44) 1243 770620.

Designations used by companies to distinguish their products are often claimed as trademarks. Allbrand names and product names used in this book are trade names, service marks, trademarksor registered trademarks of their respective owners. The Publisher is not associated with any productor vendor mentioned in this book.

This publication is designed to provide accurate and authoritative information in regard to the subjectmatter covered. It is sold on the understanding that the Publisher is not engaged in rendering professionalservices. If professional advice or other expert assistance is required, the services of a competentprofessional should be sought.

Other Wiley Editorial Offices

John Wiley & Sons Inc., Ill River Street, Hoboken, NJ 07030, USA

Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA

Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany

John Wiley & Sons Australia Ltd, 42, McDougall Street, Milton, Queensland 4064, Australia

John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809

John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print maynot be availabe in electronic books.

Library of Congress Cataloging-in-Publication Data

Hamley, Ian W.Block copolymers in solution : fundamentals and applications/Ian Hamley

P. cm.Includes bibliographical references and index

ISBN-13: 978-0-470-01557-5 (acid-free paper)ISBN-10: 0-470-01557-8 (acid-free paper)

1. Block copolymers. I. TitleQD382.B5H355 2005547'.84-dc22 2005005799

British Library Cataloguing in Publication Data

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

ISBN: 0-470-01557-8 (HB); 9-78-0-470-01557-5 (HB)

Typeset in 10/12pt Times by Thomson Press (India) Limited, New DelhiPrinted and bound in Great Britain by TJ International Ltd., Padstow, CornwallThis book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.

www.wileyeurope.com

www.wiley.com

To Valeria and Lucas

Contents

Preface xi

1. Introduction 1

References 5

2. Neutral Block Copolymers in Dilute Solution 7

2.1 Introduction 72.2 Techniques for Studying Micellization 7

2.2.1 Cryo-TEM 72.2.2 Differential Scanning Calorimetry 82.2.3 Dynamic Light Scattering 82.2.4 Ellipsometry 102.2.5 Fluorescence Probe Experiments 102.2.6 Nuclear Magnetic Resonance 102.2.7 Rheology 112.2.8 Scanning Probe Microscopy 112.2.9 Small-angle X-ray and Neutron Scattering 122.2.10 Static Light Scattering 142.2.11 Surface Pressure-Area Isotherms 162.2.12 Surface Tensiometry 162.2.13 Viscometry 172.2.14 X-ray and Neutron Reflectivity 17

2.3 Micellization in PEO-based Block Copolymers 182.4 Micellization in Styrenic Block Copolymers 202.5 Determination of cmc 202.6 Thermodynamics of Micellization 22

2.6.1 Chain Length Dependence of Micellization 252.6.2 Effect of Architecture 272.6.3 Effect of Solvents and Salts on Micellization 32

2.7 Micellization and Micelle Dimensions: Theoryand Simulation 332.7.1 Scaling Models 332.7.2 The Brush Model 372.7.3 The Self-consistent Mean Field Theory 402.7.4 The Model of Nagarajan and Ganesh 432.7.5 Computer Simulations 44

viii Contents

2.7.6 Theory: ABC Triblock Micelles 452.8 Micelle Dimensions: Comparison Between

Experiment and Theory 472.9 Interaction between Micelles 512.10 Dynamics of Micellization 522.11 Dynamic Modes 562.12 Specific Types of Micelles 60

2.12.1 Micelles from Telechelics 602.12.2 Micelles from ABC Triblocks 622.12.3 Micelles from Rod-Coil Copolymers 662.12.4 Cross-linked Micelles 682.12.5 Janus Micelles 712.12.6 Nonspherical Micelles 712.12.7 Micelles Formed due to Specific Interactions 74

2.13 Micellization in Mixed Solvents 752.14 Mixed Micelles 752.15 Block Copolymer/Surfactant Complexes 762.16 Complex Morphologies 792.17 Vesicles 832.18 Crystallization in Micelles 90References 91

3. Concentrated Solutions 105

3.1 Understanding Phase Diagrams 1053.2 Phase Behaviour of PEO-containing Block Copolymers 1113.3 Gelation 117

3.3.1 Rheology 1173.3.2 Structure - Packing of Micelles 1243.3.3 Thermodynamics of Gelation and Micellization in

Concentrated Solution 1263.3.4 Effect of Added Homopolymer, Salt or Surfactant 1273.3.5 Influence of Architecture 129

3.4 Order-Disorder Phase Transition 1323.5 Order-Order Phase Transitions 135

3.5.1 Structural Aspects 1353.5.2 Ordering Kinetics 139

3.6 Domain Spacing Scaling, and Solvent DistributionProfiles 140

3.7 Semidilute Block Copolymer Solution Theory 1433.8 Theoretical Understanding of Phase Diagrams 1463.9 Flow Alignment 149

3.9.1 Lamellar Phase 1493.9.2 Hexagonal Phase 151

Contents ix

3.9.3 Cubic Micellar Phases 1523.10 Dynamics 159

3.10.1 Dynamic Modes 1593.10.2 Dynamics of Gelation 160

References 164

4. Polyelectrolyte Block Copolymers 173

4.1 Micellization 1734.1.1 General Remarks 1734.1.2 Micellization in Block Copolymers Containing

Anionic Blocks 1754.1.3 Micellization in Block Copolymers Containing

Cationic Blocks 1794.1.4 Micellization of Polyampholyte Block Copolymers 1824.1.5 Micellization of Polyelectrolyte-containing

ABC triblocks 1824.1.6 Micellization of Block Copolymers Containing

Grafted Polyelectrolytes 1834.1.7 Micellization in Block Copolymers Containing

Sulfonated Polyisoprene 1834.2 Chain Conformation 1844.3 Theory 1884.4 Polyion Complexes 1954.5 Copolymer-Surfactant Complexes 1984.6 Complexation with other Molecules 1994.7 Gelation 2004.8 Hierarchical Order in Peptide Block Copolyelectrolyte Solutions 200

4.8.1 a Helix Structures 2024.8.2 B Sheet Structures 2044.8.3 Hydrogels 2064.8.4 Polypeptide Block Copolymer-based Complexes 207

References 208

5. Adsorption 215

5.1 Introduction 2155.2 Adsorption at the Air-Water Interface 215

5.2.1 Adsorption of Neutral Block Copolymers 2155.2.2 Adsorption of Polyelectrolyte Block Copolymers 221

5.3 Adsorption on Solid Substrates 2225.3.1 Adsorption of Neutral Block Copolymers 2225.3.2 Adsorption of Polyelectrolyte Block Copolymers 225

x Contents

5.3.3 Surface Micelles 2265.4 Surface Forces Experiments 2315.5 Modelling Adsorption 234References 236

6. Applications 241

6.1 Surfactancy/Detergency 2416.2 Solubilization, Emulsification and Stabilization 241

6.2.1 Solubilization 2416.2.2 Emulsification and Stabilization 245

6.3 Drug Delivery 2476.4 Biodegradable Block Copolymer Micelles 2536.5 Thermoresponsive Micellar Systems 2546.6 Metal-containing Copolymer Micelles and Nanoreactors 2556.7 Vesicles 2616.8 Separation Media 2686.9 Templating 2686.10 Membranes 2746.11 Other Applications 275References 276

Index 285

Preface

I was inspired to write this book by developments in the field of block copolymerself-assembly in solution which have not been discussed and summarized in theform of a single convenient text. Aspects of the subject have been discussed in myprevious book,1 in that by Hadjichristidis et al.,2 and in several chapters of a recentedited text.3

Recent advances have been stimulated in part by new synthetic methodologies(living polymerizations in particular) that have enabled the preparation of newmaterials with novel self-assembling structures, functionality and responsiveness.The present text covers the principles of self-assembly in both dilute andconcentrated solution (micellization, mesophase formation, etc.) in Chapters 2and 3, respectively. Chapter 4 covers polyelectrolyte block copolymers-thesematerials are just beginning to attract significant attention from researchers and asolid basis for understanding their physical chemistry is emerging, and this isdiscussed. Chapter 5 discusses adsorption of block copolymers from solution atliquid and solid interfaces. Chapter 6 concludes with a discussion of selectedapplications, focusing on several important new concepts rather than providing anaccount of commercial applications, which can be found elsewhere.

I wish to thank several colleagues and collaborators for support and for helpfulcomments on several chapters: Colin Booth for Chapters 2 and 3, Steve Armes forChapter 4, Harm-Anton Klok for Chapters 4 and 6. Tom Waigh also providedparticularly insightful comments on Chapter 4. As usual I bear full responsibilityfor any errors and omissions, of which I would be grateful to be informed.

I wish to thank Jenny Cossham for her continued support and attention in editingthis book. I am also grateful to the Leverhulme Trust who provided a LeverhulmeResearch Fellowship which freed up time from some of my usual academic duties,enabling this book to be completed.

REFERENCES

(1) Hamley, I. W. The Physics of Block Copolymers. Oxford University Press: Oxford, 1998.(2) Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers. Synthetic Strategies,

Physical Properties and Applications. John Wiley & Sons: New York, 2003.(3) Hamley, I. W. (Ed.) Developments in Block Copolymer Science and Technology.

John Wiley & Sons, Ltd: Chichester, 2004.

1 Introduction

This book is concerned with the numerous aspects of the self-assembly of blockcopolymers in solution, and the diverse applications of this. Block copolymers inthe melt, or in blends are not considered, and information on this can be foundelsewhere.l

An early review of micellization in block copolymers was presented by Tuzarand Kratochvfl,2 and these authors provided a further review of the literature up to1992.3 Micellar properties of block copolymers were reviewed earlier by Price.4 Adiscussion of micellization was included in the general reviews on block copoly-mers by Riess et a/.5 and Brown et a/.6 Riess has recently published a very nicereview specifically dedicated to micellization in block copolymers.7 Excellentreviews focused on the solution properties of a particular class of copolymer, i.e.copolymers of poly(oxyethylene) with poly(oxypropylene), have been presented byseveral groups.8-13 Micellization and micellar association in related poly(oxyethy-lene)/poly(oxybutylene) copolymers has been summarized by Booth et a/.14-16

The micellar properties of block copolymers in dilute solution, the properties ofadsorbed block copolymers and ordered mesophase (lyotropic liquid crystal phase)formation in more concentrated solutions have been comprehensively discussed.1

Reviews on structure/rheology relationships in block copolymer gels,17 and onshear-alignment of ordered mesophases18'19 (the latter review incorporates work onblock copolymer melts also) have also been provided.

Liu and Armes20, Liu et a/.21 and Forster22'23 have reviewed the self-assembly ofamphiphilic block copolymers, and the numerous applications of the resultingnanostructures.

Applications of block copolymer surfactants have been the subject of a numberof reviews by researchers from Dow in the United States.24-26 The texts edited byNace27 and by Alexandridis and Lindman28 cover many aspects of the behaviourand properties of PEO-based amphiphilic block copolymers, with several chaptersdevoted to applications.

A standard notation for block copolymers is becoming accepted whereby, forexample, PX-b-PY denotes a diblock copolymer of polymer X and polymer Y.29

This convention is used here. In the case that a specific polymer with defined chainlengths is considered, the molecule is denoted Xm-b-Yn, where m and n are degreesof polymerization. This notation is somewhat more cumbersome than alternatives.For example, Booth and coworkers use single letters to indicate blocks in

Block Copolymers in Solution: Fundamentals and Applications I. W. Hamley© 2005 John Wiley & Sons, Ltd.

2 Block Copolymers in Solution: Fundamentals and Applications

PEO-based copolymers (E for poly(ethylene oxide), etc.), however this systembreaks down when considering large numbers of distinct materials, as is the casehere. Table 1.1 summarizes the abbreviations used. Note that throughout this bookwe have used the terms PEG and PEO according to the notation used in the originalresearch-we have not attempted to distinguish carefully between them (PEGdiffers from PEO by hydroxyl termination as opposed to methyl termination).

Table 1.1 Abbreviations used for polymers

Abbreviation Polymer/systematic name (where used alternatively)

OEGMA(see also PEGMA)

PAPCsAPNaAPAAPAIPAMPAMSPAspPBPBAPBLGPBMAPBOPBzMAPCEMAPCLPDAMAPDEAPDESCBPDLLPDMAPDMSPEBPEEPEGMAPEHAPEIPEMAPEO; PEG

PEPPE4VPPFMAPFP

Oligo(ethylene glycol) methacrylate

Poly(acrylate)Poly(caesium acrylate)Poly(sodium acrylate)Poly(acrylic acid)Poly[5- (N, N, N-diethyImethylammonium)]isoprenePoly(acrylamide)Poly(a-methyl styrene)Poly(a,b-L-aspartic acid)Poly(butadiene)Poly(butyl acrylate)Poly(7-benzyl L-glutamate)Poly(butyl methacrylate)/poly(n-butyl methacrylate)Poly(butylene oxide)/poly(oxybutylene)Poly(benzyl methacrylate)Poly(2-cinnamoyloxyethyl methacrylate)Poly(e-caprolactone)Poly[A^-(A^,A^-dicarboxymethylaminopropyl)methacrylamide]Poly [(2-diethylamino)ethyl methacrylate]Poly(diethylsilacyclobutane)Poly(D,L-lactide)Poly[(2-dimethylamino)ethyl methacrylate]Poly(dimethylsiloxane)poly(ethylene-co-butylene)Poly(ethylethylene)Poly(ethylene glycol) methacrylatePoly(ethylhexyl acrylate)Poly(ethyleneimine)Poly(2-phenylethyl methacrylate)Poly(ethylene oxide)/poly(oxyethylene);

Poly(ethylene glycol)Poly(ethylene-co-propylene)Poly(/V-ethyl-4-vinylpyridinium)Poly(perfluorohexylethyl methacrylate)Poly(ferrocenylphenyl phosphine)

Introduction

Table 1.1 (Continued)


PFPOPFSP4FSPGMAPHEMAPhIPHICPHOVEPHPMAPIsPIPIBVEPLGAPLLAPLMAPLysPMAPCsMAPNaMAPMAAPMDPS

PMEPMEMAPMMAPMOVEPMOXAPMPCPMPSP4MSPMTDPMTEGVEPMVEPNBVEPNIPAMPPhOVEPPO; PPG

PPQPPSPSPSMAPSOPSS

Poly(perfluoropropylene oxide)Poly(ferrocenylphenyl silane)Poly(4-fluorostyrene)Poly(glyceryl monomethacrylate)Poly(hydroxyethyl methacrylate)Poly(hydrogenated isoprene)Poly(hexyl isocyanate)Poly(2-hydroxyethyl vinyl ether)Poly[/V-(2-hydroxypropyl)methacrylamide]PolyisopreneSulfonated polyisoprenePoly(isobutyl vinyl ether)Poly(D,L-lactic acid-co-glycolic acid)Poly(L-lactic acid)Poly(lauryl methacrylate)Poly(L-lysine)Poly(methacrylate)Poly(caesium methacrylate)Poly(sodium methacrylate)Poly(methacrylic acid)Poly {3-[AK2-methacroyloylethyl)-A^W-dimethylammonio]-

propane sulfonate}Poly(methylene)Poly [2-(Ar-morpholino)ethyl methacrylate]Poly(methyl methacrylate)Poly(2-methoxyethyl vinyl ether)Poly(2-methyloxazoline)Poly(2-methacryloyloxy phosphorylcholine)Poly(methylphenyl silane)Poly(4-methyl styrene)Poly(methyltetracyclododecane)Poly[methyl tri(ethylene glycol) vinyl ether]Poly(methyl vinyl ether)Poly(n-butyl vinyl ether)Poly(N-isopropylacrylamide)Poly(2-phenoxyethyl vinyl ether)Poly(propylene oxide)/poly(oxypropylene); poly(propylene

glycol)Poly(phenylquinoline)Poly(propylene sulfide)PolystyrenePoly(solketal methacrylate)Poly(styrene oxide)/poly(oxyphenylethylene)Poly(styrene sulfonate)

(Continue)

3


Table 1.1 (Continued)


PNaSS Poly(sodium styrene sulfonate)PSSA Poly(styrene sulfonic acid)PTHF Poly(tetrahydrofuran)PtEA Poly(tert-butyl acrylate)PtBS Poly(tert-butyl styrene)PTMEMS Poly(trimethylammonium ethylacrylate methyl sulfate)PVA Poly(vinyl alcohol)PVBA Poly[(4-vinyl)benzoic acid]PVP Poly(vinyl pyridine) (position of substitution not stated)PVPh Poly(vinyl phenol)P2VP Poly(2-vinyl pyridine)P4VP Poly(4-vinyl pyridine)qP4VP Quaternized P4VPPVPEA Poly(vinylphenylethyl alcohol)PVSO Poly(phenylvinyl sulfoxide)

Abbreviations used for some common solvents and surfactants are listed inTable 1.2. Some technical terms are also abbreviated, but these can be cross-referenced using the index.

Certain topics are omitted from the present text. Associative polymers which maybe 'blocky' copolymers but are often random copolymers are generally notconsidered, although some aspects of the self-assembly of telechelic chains isdiscussed. Texts on this subject are available elsewhere.30-32 It should be noted thata telechelic polymer is defined by IUPAC as a 'prepolymer capable of entering intofurther polymerization via its reactive endgroups'.33 We follow common usagehere, and use telechelic to refer to an ABA triblock with short endblocks that canundergo physical as well as chemical cross-linking, for example due to associationof hydrophobes. The behaviour of block copolymers in blends with homopolymer'solvent' is also not considered (good reviews on this can be found elsewhere1).

Table 1.2 Abbreviations used for solvents

CPC1 Cetyl pyridinium chlorideCTAB Cetyl trimethylammonium bromideDBP Di-n-butyl phthalateDEP Di-n-ethyl phthalateDMF DimethylformamideDMP Di-tt-methyl phthalateDOP Di-n-octyl phthalateDTAB Dodecyl trimethylammonium bromideSDS Sodium dodecyl sulfateTHF Tetrahydrofuran

Introduction 5

Here we consider self-assembly of block copolymers in low molecular weightsolvents. The behaviour of block copolymer melts and nanostructure formation inthin films are also outside the scope of the present volume.

REFERENCES

1. Hamley, I. W. The Physics of Block Copolymers. Oxford University Press: Oxford, 1998.2. Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6, 201.3. Tuzar, Z.; Kratochvil, P. Micelles of Block and Graft Copolymers in Solutions. In Surface

Colloid Science; Matijevic, E., Ed. Plenum: New York, 1993; Vol. 15; pp 1.4. Price, C. Colloidal Properties of Block Copolymers. In Developments in Block Copoly-

mers; Goodman, I., Ed. Applied Science: London, 1982; Vol. 1; p 39.5. Riess, G.; Hurtrez, G.; Bahadur, P. Block Copolymers. In Encyclopedia of Polymer

Science and Engineering; Mark, H. E, Kroschwitz, J. I., Eds. Wiley: New York, 1985; Vol.2; p 324.

6. Brown, R. A.; Masters, A. J.; Price, C.; Yuan, X. F. Chain Segregation in BlockCopolymers. In Comprehensive Polymer Science; Booth, C., Price, C., Eds. Pergamon:Oxford, 1989; Vol. 2; p 155.

7. Riess, G. Prog. Polym.Sci. 2004, 28, 1107.8. Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2.9. Alexandridis, P. A.; Hatton, T. A. Coll. Surf. A 1995, 96, 1.

10. Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1997, 2, 478.11. Chu, B. Langmuir 1995, 11, 414.12. Chu, B.; Zhou, Z. Physical Chemistry of Polyoxyalkylene Block Copolymer Surfactants.

In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Nace, V M., Ed. MarcelDekker: New York, 1996; Vol. 60.

13. Mortensen, K. Coll. Surf. A 2001, 183-185, 277.14. Booth, C.; Yu, G.-E.; Nace, V. M. Block Copolymers of Ethylene Oxide and 1,2-Butylene

Oxide. In Amphiphilic Block Copolymers: Self-Assembly and Applications; Alexandridis,P., Lindman, B., Eds. Elsevier: Amsterdam, 2000; p 57.

15. Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501.16. Hamley, I. W.; Mai, S.-M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. Phys. Chem., Chem.

Phys. 2001, 3, 2972.17. Hamley, I. W. Phil. Trans. R. Soc. Lond. 2001, 359, 1017.18. Hamley, I. W. Curr. Opin. Colloid Interface Sci. 2000, 5, 342.19. Hamley, I. W. J. Phys.: Condens. Matter 2001, 13, R643.20. Liu, S.; Armes, S. P. Curr. Opin. Colloid Interface Sci. 2001, 6, 249.21. Liu, T; Burger, C.; Chu, B. Prog. Polym.Sci. 2003, 28, 5.22. Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195.23. Forster, S.; Plantenberg, T. Angew. Chem., Int. Ed. Engl. 2002, 41, 688.24. Nace, V. N. Properties of Polyoxyalkylene Block Copolymers. In Nonionic Surfactants.

Polyoxyalkylene Block Copolymers; Nace, V. N., Ed. Marcel Dekker: New York, 1996;Vol. 60; p 145.

25. Edens, M. W. Applications of Polyoxyethylene Block Copolymer Surfactants. In NonionicSurfactants. Polyoxyalkylene Block Copolymers; Nace, V. N., Ed. Marcel Dekker:New York, 1996; Vol. 60; p 185.


26. Edens, M. W.; Whitmarsh, R. H. Applications of Block Copolymer Surfactants. InDevelopments in Block Copolymer Science and Technology, Hamley, I. W., Ed. JohnWiley & Sons, Ltd: Chichester, 2004; p 325.

27. Nace, V. N. (Ed.) Nonionic Surfactants. Polyoxyalkylene Block Copolymers. MarcelDekker: New York, 1996; Vol. 60.

28. Alexandridis, P.; Lindman, B. (Eds) Amphiphilic Block Copolymers: Self-assembly andApplications. Elsevier: Amsterdam, 2000.

29. Hamley, I. W. (Ed.) Introduction to Block Copolymers. In Developments in BlockCopolymer Science and Technology. John Wiley & Sons, Ltd: Chichester, 2004.

30. Glass, J. E. (Ed.) Polymers in Aqueous Media: Performance Through Association.American Chemical Society: Washington, DC, 1989; Vol. 223.

31. Shalaby, S. W.; McCormick, C. L.; Butler, G. B. (Eds) Water-soluble Polymers. Synthesis,Solution Properties and Applications. American Chemical Society: Washington, DC,1991; Vol. 467.

32. Schulz, D. N.; Glass, D. E. (Eds) Polymers as Rheology Modifiers. American ChemicalSociety: Washington, DC, 1991; Vol. 462.

33. Odian, G. Principles of Polymerization. John Wiley & Sons, Ltd: New York, 2004.

2 Neutral Block Copolymers in DiluteSolution

2.1 INTRODUCTION

Block copolymers in a dilute solution of a solvent selective for one block usuallytend to form spherical micelles. This is now established for so many copolymersystems that to attempt to discuss every publication on this would be foolish. In thefollowing, the salient features are highlighted.

2.2 TECHNIQUES FOR STUDYING MICELLIZATION

In the following, the main techniques that are used to characterize block copoly-mers in solution are discussed - these include methods for characterizing lyotropicmesophases and transitions between them (the subject of Chapter 3) as well asclassical methods for studying micelle dimensions and the influence of micelliza-tion on solution properties. Characterization methods for adsoption are alsointroduced, in anticipation of the discussion of this in Chapter 5. The followingare listed in alphabetical order, not order of importance.

2.2.1 CRYO-TEM

Cryo-TEM is an abbreviation for cryogenic transmission electron microscopy. It isa technique where transmission electron microscopy (TEM) is used to imagecryogenically cooled samples. Rapid cooling into cryogenic liquids is intended to'trap' structures formed in solution, by vitrifying the sample and avoiding crystal-lization in the solvent.

TEM relies on electron density contrast within a thin film of a sample to providean image due to spatial variations in transmission of the electron beam. In the caseof block copolymer solutions, the sample is usually prepared by coating directlyonto a carbon-coated TEM grid (by spin or dip coating). Figure 2.1 shows arepresentative cryo-TEM image from an array of PSO-b-PEO diblock micelles.

Block Copolymers in Solution: Fundamentals and Applications I. W. Hamley© 2005 John Wiley & Sons, Ltd.

Block Copolymers in Solution: Fundamentals and Applications

Figure 2.1 Cryo-TEM image of micelles formed by a PSO-b-PEO diblock in aqueoussolution.429 Reproduced by permission of Springer Verlag.

Cryo-TEM as applied to imaging micellar structures is discussed in reviews byTalmon and coworkers.1,2 An excellent account of TEM is provided by Brydson andHammond.3

2.2.2 DIFFERENTIAL SCANNING CALORIMETRY

As its name suggests, this technique involves measuring the differential powernecessary to maintain a given temperature for two pans containing the polymer anda reference sample. Single pan differential scanning calorimetry (DSC) instrumentsare also available in which the reference sample is run prior to the sample to bestudied. In DSC, a phase transition is indicated by a sharp endotherm or exothermwhich causes changes in the differential power supplied to the sample. It is used toinvestigate the enthalpy of micellization (Section 2.6) and to detect the criticalmicelle concentration (cmc). It can also be used to detect gelation, as describedfurther in Section 3.3.3. Since the enthalpy associated with these transitions(especially the latter) can be small, high sensitivity instrumentation is sometimesrequired. The technique is discussed in more detail elsewhere.4

2.2.3 DYNAMIC LIGHT SCATTERING

Dynamic light scattering (DLS) is also known as photon correlation spectroscopy(PCS) or quasi-elastic light scattering (QELS). It involves measuring the temporalfluctuations of the intensity of scattered light. The number of photons entering a

8

Neutral Block Copolymers in Dilute Solution 9

detector are recorded and analysed by a digital correlator. The correlation betweencounts measured at angle 9 over an interval t is computed:

Laplace transformation of Equation (2.1) (often using the CONTIN program5)yields the distribution of relaxation times, A(t). The decay rates of the relaxationmodes provide translational diffusion coefficients.

The measured intensity correlation function is related to the field correlationfunction, g(1)(o,t) , by the Siegert relationship:6

where A2 is the second virial coefficient, Mw is the weight-average molar mass, andV is the partial specific volume, which is generally small compared with the othertwo terms on the right-hand side of Equation (2.4). The first term in this equationaccounts for thermodynamic interactions, and kf accounts for hydrodynamicinteractions.

Here c is an experimental constant proportional to the ratio between the coherencearea and the detector area.

In polymer solutions, DLS is used to determine the hydrodynamic radius of theconstituent particles using the Stokes-Einstein equation:

where kB is the Boltzmann constant, T is the absolute temperature, 77 is the solventviscosity and D is the diffusion coefficient. DLS has also been exploited to studydiffusion in polymer solutions, and details of experimental work are provided inSections 2.11 and 3.10. Because the intensity of scattered light is z-weighted(z a cMw, where c is mass concentration and Mw is mass-average molar mass),DLS is sensitive to low levels of high molar mass solutes.

The concentration dependence of the mutual diffusion coefficient, D, in binarysolution can be expressed as:

Here DO is the infinite dilution diffusion coefficient, kd is the concentrationcoefficient and c is the concentration. The concentration coefficient is given by:7


There is a substantial body of work using DLS to probe the hydrodynamicproperties of block copolymers containing PEO in aqueous solution, as discussed

oelsewhere.

The technique of DLS is the subject of the book by Berne and Pecora.6

2.2.4 ELLIPSOMETRY

This technique has been used to measure the thickness of adsorbed polymer films,and hence the adsorption isotherm. It relies on measurements on the angulardependence of the intensity of reflected s- and p-polarized light. The data aremodelled based on the thickness and refractive index of the layer. Further details onthe technique can be found elsewhere.9

Surface plasmon resonance has also been used to measure adsorbed layerthicknesses. Surface plasmons are electromagnetic surface waves propagating atthe interface between a metal and a dielectric material. The angular dependenceof the reflected p-polarized light exhibits a minimum at a resonance condition foran evanescent wave established in the electron gas in the metal near the interface.The position of the resonance depends on the dielectric properties of the mediumwhich can be modelled using formalisms from optics. The experiments arenormally performed using the so-called Kretschmann configuration where thelight is incident through an index-matched prism placed over a gold-plated slideonto which the polymer is adsorbed. The method is described in detail in a thoroughreview.10 Further details on the application of the technique to block copolymeradsorbed films are available.11-13

2.2.5 FLUORESCENCE PROBE EXPERIMENTS

This method relies on changes in the fluorescence of free probe molecules or probestagged to copolymer chains. In the former case, the fluorescence changes dependingon the environment of the probe. For example, for the commonly used probe pyrenethe intensity of the first and third vibronic peaks changes depending on the localpolarity. Pyrene is used due to its low solubility in water, its long fluorescencelifetime and its sensitivity to the polarity of its environment. Fluorescencequenchers are sometimes used as an alternative (donor-acceptor systems). Thetechnique of time-resolved fluorescence quenching is used to study kineticprocesses. An excellent review provides more detailed information on all aspectsof fluorescence probe experiments on block copolymer solutions.14

2.2.6 NUCLEAR MAGNETIC RESONANCE

Nuclear magnetic resonance (NMR) has been widely used to probe micelle struc-ture. Proton NMR on copolymers in D2O is employed to monitor the presence or


absence of micellization. For example, Wanka et al. used this technique to locatethe critical micelle temperatures of Pluronic block copolymers.15 The fine structureassociated with PO units present for molecularly dissolved unimers disappearsabove the critical micelle temperature (cmt) as the mobility of the PO units isreduced in the hydrophobic micellar core. Armes et al. have used NMR extensivelyto probe micellization in their tertiary amine methacrylate block copolymers (seeSection 4.1.3).

Pulsed field gradient NMR can be used to measure self-diffusion coefficients inpolymeric systems, and has been employed to determine this quantity for severaltypes of poly(oxyethylene)-based block copolymer in aqueous solution.16-18 Adifference in self-diffusion coefficients (and hydrodynamic radii derived fromthese) in H2O and D2O was noted for solutions of Pluronic F88.18 The methodhas also been used to examine diffusion in micellar solutions of PS-b-PEB-£-PS inthe midblock selective sovent, n-heptane.19 The technique has been used to probegelation, for example in PEO/PBO block copolymers in aqueous solution.20

2.2.7 RHEOLOGY

The flow properties of block copolymer solutions depend on the state of order in thesystem, and this has been exploited to locate sol - gel transitions in concentratedblock copolymer solutions. Gels exhibit a finite yield stress (i.e. they are Binghamfluids), which can be measured in steady shear experiments. Details of the linearand nonlinear viscoelasticity are provided in Section 3.3.1.

Experimentally, the dynamic shear moduli are usually measured by applyingsinusoidal oscillatory shear using constant stress or constant strain rheometers. Thiscan be in parallel plate, cone-and-plate or concentric cylinder (Couette) geometries.An excellent monograph on rheology, including its application to polymers, isavailable.21

The related technique of viscometry is discussed in Section 2.2.13.

2.2.8 SCANNING PROBE MICROSCOPY

Scanning probe microscopy (SPM) is a general term for methods where thedeflection of a scanning probe is used to build up an image of the sample surface.As applied to polymers, the SPM method usually used is often termed atomic forcemicroscopy (AFM). This is a technique for imaging surfaces to near 1 A resolution.The method depends on the interaction force between a sharp tip (often made fromsilicon nitride) and the substrate. The deflection of a cantilever to which the tip isattached due to the force it experiences as it approaches the surface is measuredusing a reflected laser beam or the interference pattern of a light beam from anoptical fibre. For polymeric systems, the SPM experiment is usually conducted in anoncontact 'tapping mode', where the tip oscillates in proximity to the sample


surface. This avoids damage to the sample surface. The sample or tip is then movedso that the tip rasters over the surface to build up an image. This image containsinformation on surface topography and phase contrast, which measures thedissipation of energy in regions of the surface with different stiffness. A furthervariant of this is lateral force microscopy, in which the displacement of thecantilever is resolved in-plane as well as perpendicular to the substrate. Manytexts on nanotechnology describe the principles of AFM in more detail.22,23

To date, AFM has largely been used to image dried films of block copolymersurfaces. In recent work, the technique has been applied to investigate adsorption ofblock copolymer micelles in situ. The AFM tip is placed directly into a cell contain-ing the liquid covering the substrate onto which adsorption occurs. A representativeimage is shown in Figure 2.2. Further details can be found in Section 5.2.3.

Figure 2.2 AFM topography image of micelles of a PPO-b-PEO diblock adsorbed from a1% aqueous solution onto silica.430 Reproduced by permission of American ChemicalSociety.

2.2.9 SMALL-ANGLE X-RAY AND NEUTRON SCATTERING

Small-angle scattering is a powerful technique to determine micelle dimensions andvia suitable models can provide detailed information on intra-micellar structure.Small-angle scattering methods are well suited to investigate the structure ofmicelles because their size is typically ~5-100 nm, which leads to scattering atsmall angles. Both small-angle X-ray scattering (SAXS) and small-angle neutronscattering (SANS) may be employed. In very dilute solution, it is possible tomeasure only intra-micellar scattering, the so-called form factor. However, in mostcases the inter-micellar scattering contributes to the intensity, especially at low


wave vector q, and to a greater extent as the concentration increases. For lyotropicmesophases, the relative positions of a sufficient number of reflections arising frommicrostructural periodicities enable unambiguous identification of morphology.Further information can be obtained by preparing oriented specimens, and obtain-ing diffraction patterns for different orientations.8,24,25

Scattering data are presented as a function of the scattering vector q or itsmagnitude, where:

Here 29 is the scattering angle and A is the wavelength. SAXS is appropriate wherethe electron density contrast (between micelle and solvent, for example) is sufficientfor the system to diffract X-rays.26 This is often possible with an intense source ofX-rays, such as a rotating anode generator or a synchrotron source. SANS isvaluable for studies of polymer structure27 because of the opportunity for contrastvariation via isotope labelling. Typically, hydrogen atoms are selectively replacedby deuterium. This changes the scattering contrast and can be used to obtain localinformation on chain conformation or intra-micellar structure, for example. Neutronscattering has also been extensively used to enhance the scattering contrast of thesolvent and/or the block copolymer.

The radius of gyration, Rg, of block copolymer micelles in dilute solution can beobtained from SAXS and SANS using the Guinier equation:28

This is valid for small scattering angles, qRg <<C 1. If the intensity is put on anabsolute scale, the particle molar mass M can be obtained from I(0) (by extrapola-tion of the data to q = 0) using Kratky's equation29 if the sample concentration,thickness and density and the incident intensity are known. SAXS is the subject ofseveral books.28,30,31

Expressions for the form and structure factors corresponding to different modelsfor spherical micelles are summarized in several reviews.32-37 Forster and Burgerhave provided analytical expressions for the form factor for particles (spheres,cylinders, lamellae) with multiple shell structures, which in general can be writtenin terms of hypergeometric functions.38 Theory was developed for the case of shellswith uniform density or with algebraically decaying density profiles. A very usefulform factor is that introduced by Pedersen and Gerstenberg for a micelle modelledas a homogeneous spherical core with attached Gaussian chains.39,40 Figure 2.3shows representative SAXS data showing the form factor of a PEO-b-PBO diblocktogether with a fit using this model. The dimensions of the micelle core and theradius of gyration of the corona chains are obtained from this model, along withthe relative electron density difference and the association number. The totalmicelle radius can also be computed.


Figure 2.3 SAXS data for 2 wt % PEO87-/?-PBO18 in water at 75 °C.213 The full line is a fitusing the Pedersen-Gerstenberg form factor for a spherical micelle with hard core andattached Gaussian chains and the hard sphere structure factor. Reproduced by permission ofAmerican Institute of Physics.

The structure factor is often taken to be that for hard spheres.32,35,41 Forinteracting micelles, use of the structure factor from Baxter's sticky hard spheremodel42 may be more appropriate. Expressions for Baxter's sticky hard spherestructure factor have been used by several groups in their analysis of SANS data forPluronic43 and reverse Pluronic44 block copolymers.

2.2.10 STATIC LIGHT SCATTERING

Here, the intensity of elastically scattered light is measured as a function ofscattering angle. Many micelles, particularly those of water-soluble block copoly-mer surfactants are small compared with the wavelength of light and act as pointscatterers, hence the scattered intensity does not depend on scattering angle 9 (whencorrected for geometrical effects). Micelles in organic solvents (and some in water)are larger, and intensity is sensitive to 0. In this case extrapolation to zero anglegives values of weight-average molecular weight (Mw) corrected for intramolecularinterference, and the angular dependence gives values of the z-average radius ofgyration (Rg)z. Alternatively, scattering is measured at small angle (small-anglelight scattering, SALS) to obtain Mw directly. In all cases, the variation of scatteringintensity with concentration gives information about the interactions of the micellesin solution (e.g. via the second virial coefficient) and so the excluded volume of onemicelle for another, which in turn relates to the effective volume fraction of


micelles in solution. In the analysis of light scattering data, only the scattering fromsolute is required. The contribution from local solvent concentration fluctuations isaccounted for by using the excess intensity of scattered light / — 7S, where / is thetotal intensity and IS is that of the solvent background. This can be used to calculatethe Rayleigh ratio, Ro, which for vertically polarized light is defined asRg = Ivd

2//v,0. Here 7V is the intensity of scattered light, Iv,o is the incidentintensity, and d is the distance from the particle to the detector.

The usual procedure for obtaining Mw and A2 involves Debye plots. At smallangles, the inverse scattered intensity is given by:45,46

where c is the concentration of polymer, K is an optical constant depending onrefractive index, wavelength and polarization of the light:47

Here A is the wavelength of the light, ns and 7?s are the refractive index andRayleigh ratio, respectively, of the solvent background and dn/dc is the refractiveindex increment.

In practice, plots of Kc/Rg often cannot be fitted to the Debye equation taken tothe second term, since they are strongly curved - see for example Figure 2.4. This

Figure 2.4 Debye plots of light scattering data for PEO-b-PSO-b-PEO triblock copolymerin aqueous solution at two temperatures.74 The curves are fits to Equation (2.10). Reproducedby permission of American Chemical Society.


indicates interactions between micelles (beyond a second virial coefficient descrip-tion). The data can be fitted using a procedure due to Vrij,48 who used theCarnahan-Starling equation49 (equivalent to the hard sphere structure factortaken to the seventh term) to obtain the following expression:

2.2.12 SURFACE TENSIOMETRY

The critical micelle concentration (cmc) in block copolymer solutions can bedetermined by measurement of the surface tension (7) as a function of concentra-tion. The method detects completion of the Gibbs monolayer at the air-waterinterface, and is a secondary indicator of the onset of micellization. The cmc forsolutions of monodisperse polymers is indicated by a fairly sharp decrease in 7versus log(c) (this plot being known as the Gibbs adsorption isotherm). The mostcommon methods used to measure surface tension of surfactant solutions usingcommercial instruments involve Du Noiiy ring and Wilhelmy plate techniques. Inthe former, the force necessary to detach a ring or wire loop from a liquid surface ismeasured (using a balance). This detachment force is proportional to surfacetension via Young's equation. The Wilhelmy plate method works similarly, in thedetachment mode. Here, a glass plate or slide is pulled from the surface. The weight

Here the inverse structure is given by:

with 0e the volume fraction of equivalent uniform spheres, which may be used tocompute the swollen volume, knowing the 'dry' volume calculated from theintercept (which provides Mw).47

Light scattering from block copolymer micelles has been reviewed.45

2.2.11 SURFACE PRESSURE-AREA ISOTHERMS

A Langmuir trough is used to measure the surface pressure (TT) of an adsorbedinsoluble copolymer layer as it is compressed using a barrier. Compression reducesthe surface area per molecule, A, and n—A isotherms are determined. The copolymeris spread as a film from a volatile organic solvent. A Wilhelmy plate (see followingsection) can be used to measure the surface force and hence pressure. Thecompression (or decompression) rate needs to be controlled to eliminate possiblekinetic effects. A description of Langmuir film techniques can be found in manyphysical chemistry textbooks.

Block Copolymers in Solution · 2013-07-23 · Contents Preface xi 1. Introduction 1 References 5...

Documents

Transcript of Block Copolymers in Solution · 2013-07-23 · Contents Preface xi 1. Introduction 1 References 5...