advanced nano-particles

ADVANCED POLYMER NANOPARTICLES - Synthesis and Surface Modifications

ADVANCEDPOLYMER

NANOPARTICLESSynthesis and Surface

Modifications

CRC Press is an imprint of theTaylor & Francis Group, an informa business

Boca Raton London New York

ADVANCEDPOLYMER

NANOPARTICLESSynthesis and Surface

Modifications

Edited by

Vikas Mittal

CRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742

2011 by Taylor and Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group, an Informa business

No claim to original U.S. Government works

Printed in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1

International Standard Book Number: 978-1-4398-1443-7 (Hardback)

This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint.

Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmit-ted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers.

For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.

Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data

Advanced polymer nanoparticles : synthesis and surface modifications / [edited by] Vikas Mittal.

p. cm.A CRC title.Includes bibliographical references and index.ISBN 978-1-4398-1443-7 (hardcover : alk. paper)1. Polymerization. 2. Nanoparticles. 3. Polymers--Surfaces. I. Mittal, Vikas. II. Title.

TP156.P6A38 2011620.192--dc22 2010020564

Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.com

vContents

Preface .................................................................................................................... viiEditor........................................................................................................................ixContributors ............................................................................................................xi

1. Polymer Latex Technology: An Overview .................................................1V. Mittal

2. Synthesis of Polymer Particles with Core-Shell Morphologies .......... 29Claudia Sayer and Pedro Henrique Hermes de Arajo

3. Advanced Polymer Nanoparticles with Nonspherical Morphologies ................................................................................................. 61Yongxing Hu, Jianping Ge, James Goebl, and Yadong Yin

4. Block, Graft, Star, and Gradient Copolymer Particles .......................... 97H. Matahwa, E. T. A. van den Dungen, J. B. McLeary, and B. Klumperman

5. Polymer Nanoparticles by Reversible Addition-Fragmentation Chain Transfer Microemulsion Polymerization .................................. 133J. ODonnell and E. Kaler

6. pH-Responsive Polymer Nanoparticles ................................................. 169Jonathan V. M. Weaver

7. Smart Thermo-Responsive Nanoparticles ............................................ 197Peng Tian and Qinglin Wu

8. Surface Tailoring of Polymer Nanoparticles with Living Polymerization Methods ...........................................................................223Koji Ishizu and Dong Hoon Lee

9. Effects of Nano-Sized Polymerization Locus on the Kinetics of Controlled/Living Radical Polymerization ........................................... 263Hidetaka Tobita

10. Functional Polymer Particles by Emulsifier-Free Polymerization ... 307V. Mittal

vi Contents

11. Polymer Nanoparticles with Surface Active Initiators and Polymer Initiators ....................................................................................... 329Klaus Tauer

Index ..................................................................................................................... 361

vii

Preface

Polymer latex particles are a very important class of polymeric materi-als, which are used for a large number of commercial applications. These particles are synthesized in the aqueous dispersion phase by numerous synthesis methodologies such as emulsion, miniemulsion, microemulsion, dispersion, suspension, inverse emulsion (in organic phase), polymeriza-tion, etc. Over the years, significant enhancement in the techniques deal-ing with the synthesis and surface tailoring of polymer particles has been achieved, which has also resulted in the widening of the application spec-trum of these particles. These advances include use of advanced controlled polymerization means such as nitroxide-mediated polymerization, atom transfer radical polymerization, radical addition fragmentation transfer polymerization, etc., as well as use of advanced stabilizers, surface modi-fiers, etc. These advances have made it possible to achieve polymer par-ticles with specific sizes consisting of polymer chains of specific molecular weights and tailorable chemical compositions or properties according to the requirement.

Because the advanced synthesis techniques are the key to achieve new func-tional properties in the polymer nanoparticles, and the surface modifications of these particles are required to ensure their use for specific applications, it is of immense importance to bring readers up-to-date on recent advances in these fields. This information will enable readers to design the required par-ticle systems. This book thus serves the purpose of summarizing the devel-opments in the synthesis and surface modification techniques to generate advanced polymer particles, and the contents have been accordingly orga-nized. Chapter 1 introduces polymer latex technology with an overview of the various conventional and recent synthesis methodologies. Synthesis and characterization of particles with core-shell morphol ogies have been focused on in Chapter 2. Chapter 3 reports the generation of nonspherical polymer particles by following different synthetic routes. The generation of specific architectures such as block, star, graft, and gradient copolymer particles has been detailed in Chapter 4. Microemulsion polymerization using reversible addition-fragmentation chain transfer controlled radical polymerization is the subject of Chapter 5. In Chapter 6, pH-responsive nanoparticles have been described, whereas the synthesis of smart thermally responsive parti-cles has been reported in Chapter 7. Surface tailoring of various organic and inorganic nanoparticles by polymers is the subject of Chapter 8. Theoretical studies on the kinetics of controlled radical polymerization techniques have been explained in Chapter 9. Chapter 10 reports the synthesis of func-tional nanoparticles by using the surfactant-free emulsion polymerization

viii Preface

approach. Chapter 11 describes various surface-active initiators as well as polymeric stabilizers developed for polymer nanoparticles in recent years.

At this juncture, I would like to express my heartfelt thanks to Taylor & Francis Group for their kind support during the project. I am equally thankful to Professor Massimo Morbidelli at the Swiss Federal Institute of Technology, Zurich, Switzerland, who has been my guide in polymer latex technology. I am indebted to my family, especially my mother, whose continuous support and motivation have made this work feasible. I dedicate this book to my dear wife Preeti, for her valuable help in coediting the book as well as for her efforts in improving the quality of the book.

Vikas MittalLudwigshafen, Germany

ix

Editor

Dr. Vikas Mittal studied chemical engineering at Punjab Technical Univer-sity in Punjab, India. He later obtained his master of technology in polymer science and engineering from the Indian Institute of Technology, Delhi, India. Subsequently, he joined Professor U. W. Suters polymer chemistry group at the Department of Materials at the Swiss Federal Institute of Technology, Zurich, Switzerland, where he worked for his doctoral degree with a focus on the subjects of surface chemistry and polymer nanocomposites. He also jointly worked with Professor M. Morbidelli at the Department of Chemistry and Applied Biosciences on the synthesis of functional polymer latex par-ticles with thermally reversible behaviors.

After completion of his doctoral research, Dr. Mittal joined the Active and Intelligent Coatings section of Sun Chemical Group Europe in London. He worked for the development of water- and solvent-based coatings for food-packaging applications. He later joined BASF Polymer Research in Ludwigshafen, Germany, as a polymer engineer, where he is currently work-ing as a laboratory manager responsible for the physical analysis of organic and inorganic colloids.

His research interests include organicinorganic nanocomposites, novel filler surface modifications, thermal stability enhancements, polymer latexes with functionalized surfaces, etc. He has authored more than 40 scientific publications, book chapters, and patents on these subjects.

xi

Contributors

Pedro Henrique Hermes de ArajoDepartment of Chemical

EngineeringFederal University of Santa CatarinaFlorianpolis, Brazil

Jianping GeDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, California

James GoeblDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, California

Yongxing HuDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, California

Koji IshizuDepartment of Organic Materials

and MacromoleculesTokyo Institute of TechnologyTokyo, Japan

E. KalerStony Brook UniversityStony Brook, New York

B. KlumpermanDepartment of Chemistry and

Polymer ScienceUniversity of StellenboschMatieland, South AfricaandLab of Polymer ChemistryEindhoven University of TechnologyEindhoven, the Netherlands

Dong Hoon LeeDepartment of Organic Materials

and MacromoleculesTokyo Institute of TechnologyTokyo, Japan

H. MatahwaDepartment of Chemistry and

Polymer ScienceUniversity of StellenboschMatieland, South Africa

J. B. McLearyPlascon Research CentreUniversity of StellenboschMatieland, South Africa

V. MittalPolymer ResearchBASF SELudwigshafen, GermanyandDepartment of Chemistry and

Applied BiosciencesInstitute of Chemical and

BioengineeringETH ZurichZurich, Switzerland

J. ODonnellIowa State UniversityAmes, Iowa

Claudia SayerDepartment of Chemical

EngineeringFederal University of Santa

CatarinaFlorianpolis, Brazil

xii Contributors

Klaus TauerDepartment of Colloid ChemistryMax Planck Institute of Colloids and

InterfacesGolm, Germany

Peng TianSchool of Renewable Natural

ResourcesLouisiana State UniversityBaton Rouge, Louisiana

Hidetaka TobitaDepartment of Materials Science

and EngineeringUniversity of FukuiFukui, Japan

E. T. A. van den DungenDepartment of Chemistry and

Polymer ScienceUniversity of StellenboschMatieland, South Africa

Jonathan V. M. WeaverDepartment of ChemistryUniversity of LiverpoolLiverpool, United Kingdom

Qinglin WuSchool of Renewable Natural

ResourcesLouisiana State UniversityBaton Rouge, Louisiana

Yadong YinDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, California

11Polymer Latex Technology: An Overview*

V. Mittal

1.1 Introduction

Polymer nanoparticles find use in a number of applications like coatings, adhesives, paints, etc. The applications of these nanoparticles are significantly affected by their physical properties as well as surface morphology, which can be controlled by the synthesis process used to generate such particles. Emulsion poly mer i za tion and its modified methodologies are the most com-monly used techniques to achieve poly mer nanoparticles. These techniques also allow the generation or surface functionalization of the particles either in situ or by following separate specific steps. Polymerization of monomer by emulsion poly mer i za tion offers significant advantages in the whole poly mer-i za tion process as compared to bulk and solution poly mer i za tion methods. It allows better control of the heat and viscosity of the system, and emulsion poly mer i za tion allows the achievement of an increase in molecular weight of the poly mer chains without negatively impacting the rate of poly mer i za tion [1]. In emulsion poly mer i za tion, most of the monomer is present as mono-mer droplets in the aqueous phase, which diffuses to the polymerizing par-ticles during the course of poly mer i za tion. The diffusion of the monomer is

* The work was carried out at Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.

CONTENTS

1.1 Introduction ....................................................................................................11.2 Emulsion Polymerization .............................................................................21.3 Controlled Polymerization and its Use in Emulsion

Polymerization Processes .............................................................................91.4 Conventional and Controlled Miniemulsion Polymerization ............... 161.5 Generation of Copolymer or Core-Shell Particles ................................... 20References ...............................................................................................................25

2 Advanced Polymer Nanoparticles: Synthesis and Surface Modifications

possible when the monomer is partially water soluble. Thus, emulsion poly-mer i za tion is not very effective with extremely hydrophobic and extremely hydrophilic monomers. The extremely hydrophobic monomers would always stay in the monomer droplets, leading to no poly mer i za tion, whereas the hydrophilic monomers would poly mer ize mainly by homogenous poly mer-i za tion and not micellar poly mer i za tion. To circumvent these difficulties, miniemulsion poly mer i za tion is used [2,3]. In this technique, the diffusion of the monomer molecules through the aqueous phase is not required, as the monomer droplets are directly poly mer ized. Therefore, such a technique has no problem in achieving the poly mer i za tion of even extremely hydrophobic monomers. To poly mer ize very hydrophilic monomers, inverse miniemul-sion can be used. Combination of controlled poly mer i za tion methods like nitroxide-mediated poly mer i za tion, atom transfer radical poly mer i za tion, and reversible addition fragmentation chain transfer poly mer i za tion with the emulsion and miniemulsion poly mer i za tion methods has further enhanced the possibilities of achieving functional poly mer particles [4]. By using these techniques, synthesis of functional block copolymer or graft copolymer par-ticles can be achieved, which is not possible by using conventional emulsion poly mer i za tion techniques owing to the very short lifetime of the radicals. The surface morphologies of the particles can also be efficiently controlled or tuned by using such controlled poly mer i za tion methods, which expands the spectrum of application of these particles. This chapter aims to provide an overview of the conventional emulsion poly mer i za tion methods and the more advanced methods of synthesizing poly mer particles.

1.2 Emulsion Polymerization

Emulsion poly mer i za tion is a heterogeneous poly mer i za tion technique that uses water as dispersion medium for the poly mer i za tion of water-insoluble monomers in the form of suspended particles. Styrene, methyl methacrylate, butyl acrylate, etc. are examples of the most commonly used monomers for the generation of polymers by emulsion poly mer i za tion. The surfactants are generally used to provide colloidal stability to the system. The surfactant can be cationic, anionic, or nonionic, and its amount exceeds the critical micelle concentration significantly. The surfactants form micelles in the system in which the poly mer i za tion takes place. Thus, this process can be visualized as a bulk poly mer i za tion in each of the suspended particles. Polymerization by this mode helps to circumvent the problems of heat and viscosity con-trol generally associated with bulk poly mer i za tion. By changing the amount of surfactant, the molecular weight of the poly mer chains can be increased without decreasing the poly mer i za tion rate, which is not possible in other modes of poly mer i za tion. The presence of a significant amount of surfactant

Polymer Latex Technology: An Overview 3

in the system can lead to certain disadvantages; however, many applications of particles are not affected by the presence of surfactants. Surfactant-free poly mer i za tion can also be used to generate poly mer particles in order to circumvent the problems associated with the use of emulsifier, but in this case, the mode of poly mer nucleation is completely different.

As mentioned previously, the amount of the surfactant exceeds the criti-cal micelle concentration in the emulsified emulsion poly mer i za tion process. The micelles formed as a result of this excess amount have a size in the range of 10 nm, and one micelle generally consists of 100200 surfactant molecules [1]. Surface tension of the solution decreases with the addition of surfactant at critical micelle concentration. A host of other solution properties are also affected at critical micelle concentration of the surfactant, which include conductivity, turbidity, osmotic pressure, etc. [5]. However, in the emulsion poly mer i za tion process, it is the reduction in the surface tension of the aque-ous phase that is of prime importance. Because surfactants are amphiphilic molecules containing one hydrophobic part and one hydrophilic part, in the micelle they orient themselves in a way so that the hydrophobic part forms the inner part of the micelle and the hydrophilic part radiates away from this inner part of the micelle into the aqueous phase. The resulting hydrophobic space inside the micelle owing to the self-assembly of the surfactant molecules is an ideal place for the hydrophobic monomer to reside and also provides an ideal environment for the radicals to enter the micelle. Figure 1.1 shows the representation of the association of the surfactant molecules after the criti-cal micelle concentration of the surfactant is reached [6]. When the inverse emulsion poly mer i za tion is used, then the hydrophilic part of the surfactant forms the inner part of the micelles and the hydrophilic chains intermix with the organic dispersion phase.

(a) (b)

Figure 1.1Organization of the surfactant molecules (a) below and (b) above the critical micelle concentra-tion of the surfactant in the aqueous solution.


The monomers used for emulsion poly mer i za tion are water insoluble (water soluble for inverse emulsion poly mer i za tion). However, the monomer should have some extent of water solubility in order to diffuse through the aqueous phase as required during the course of poly mer i za tion. When the monomer is added to the system, a part of the monomer enters the micelles and a part is dissolved in the aqueous phase owing to partial water solubility. However, the majority of the monomer is present in the form of monomer droplets. The size of the monomer droplets is much larger than that of micelles; however, their number is much lower as compared to the micelles. Water-soluble initi-ators are generally used to initiate the poly mer i za tion reaction. The initiator generates the radicals in the aqueous phase owing to thermal dissociation. The generated radicals have the possibility of entering either the micelles or the monomer droplets. However, experimental evidence proves the absence of droplet poly mer i za tion. The radicals do not enter the monomer droplets, as the radical entities are hydrophilic in nature whereas the monomer drop-lets are hydrophobic. Also, because the number of monomer droplets is much smaller than the number of micelles, it is micelles that capture the majority of the radicals. Also, the unique architecture of the micelles provides attrac-tive conditions for the radicals to enter. Figure 1.2a shows the mechanism of the micellar nucleation for the generation of poly mer particles. This mode of nucleation is also termed heterogeneous nucleation. The homogenous mode of particle nucleation is also possible when (a) the amount of surfactant is below its critical micelle concentration, (b) no surfactant is used during the poly mer i za tion, or (c) the monomer is significantly water soluble.

In this mode of nucleation, the generated radicals in the aqueous phase start reacting with the dissolved monomer molecules. However, after add-ing a few monomer units in the chains, these chains no longer remain water soluble and come out of the solution. These chains are not stable on their own and keep collapsing with each other in order to attain stability. They also adsorb a certain amount of surfactant from either the micelles or the aqueous phase itself. Partial stability is also provided by the negative charges from the initiator moieties. In the case of the surfactant-free poly mer i za tion, the initiator charges are the only source of colloidal stability of the particles. Figure 1.2b shows the process of homogenous nucleation.

The emulsion poly mer i za tion process is generally divided into three inter-vals. The first is the particle formation interval. The radicals are generated in the aqueous phase after the thermal dissociation of the initiator. These radicals start entering the micelles and initiate poly mer i za tion. These active micelles where the poly mer i za tion starts to take place are then referred to as poly mer particles. The number of particles in this interval keeps increasing owing to the continuous entry of the generated radicals in the micelles. This also leads to continuous increase in the rate of the poly mer i za tion. As the poly-mer i za tion of the monomer in the particles proceeds, the size of the particles keeps increasing and the amount of monomer in the particles keeps depleting. However, this depletion of the monomer is replenished by the absorption of


the monomer from the aqueous phase. The aqueous phase in turn absorbs more monomer from the monomer droplets. Therefore, a mass transfer from the monomer droplets to the poly mer articles keeps taking place during the course of poly mer i za tion. For this diffusion process to take place, the mono-mer is required to be partially soluble in water. As the poly mer particles become bigger in size and their surface area increases as a function of time or

(a)

(b)

Figure 1.2(a) Representation of micellar nucleation mechanism for the generation of poly mer particles. (b) Homogenous nucleation mechanism for the synthesis of poly mer particles.


monomer conversion, they require more amount of surfactant to remain sta-ble. The surfactant dissolved in the aqueous phase is continuously adsorbed on the surface of the poly mer particles, leading to the reduction of the surfac-tant amount in the solution to lower than the critical micelle concentration. This in turn destabilizes the remaining micelles and these micelles disappear, providing their surfactant for the stabilization of the poly mer particles. Thus at the end of the first interval, no micelle is left and most of the surfactant is used to stabilize the poly mer particles. It has to be noted that the final number of poly mer particles is much lower than the original number of micelles. Also, roughly 15% of the monomer is poly mer ized by the end of the first interval [1]. Figure 1.3 represents the various intervals of emulsion poly mer i za tion.

Once excess surfactant is no longer present in the system, no new par-ticles nucleate. This marks the beginning of the second interval of emul-sion poly mer i za tion. Because no new particles nucleate, the amount of the particles remains almost constant; this also leads to an almost constant poly mer i za tion rate in this interval. The size of the poly mer particles, however, keeps increasing as a function of conversion. The monomer present in the monomer droplets continues to replenish the monomer in the aqueous phase as well as monomer-swollen poly mer particles. After a certain extent of conversion, the monomer droplets also disappear. This also signals the start of the final interval of the emulsion poly mer i za-tion process. The concentration of the monomer in the poly mer particles keeps decreasing, and as a result the rate of poly mer i za tion also steadily decreases. Because the monomer is almost consumed, the poly mer i za tion rate virtually falls to zero. Figure 1.4 shows the evolution of the particle size as a function of conversion.

Figure 1.3Schematic of various intervals of the emulsion poly mer i za tion process.


Emulsion poly mer i za tion can also lead to the generation of various differ-ent surface morphologies of the poly mer particles as well as particle sizes or families. Figure 1.5 shows the examples of monomodal, bimodal, or multi-modal poly mer particles along with morphologies like planar, orange-peel, strawberry or surface craters, etc.

Polymers synthesized with emulsion poly mer i za tion are not always homopolymers, but most of the time are copolymers. When more than one monomer is poly mer ized together, the reactivity of the monomers defines the resulting morphology of the particles. Different reactivities of the mono-mer lead to totally different copolymer composition in the poly mer particles, leading to a gradient in the concentration of the monomers with radius. This occurs because of the faster poly mer i za tion of the reactive monomer thus accumulating near the center of the particles, followed by the poly mer i za tion of the lesser reactive monomer, which then is present in a majority near the

(a)

200 nm

(b)

300 nm

(c)

400 nm

Figure 1.4(ac) Increase of the size of the particles as a function of conversion.


outer surface of the particles. Differences in the water solubilities of the mono-mers can also lead to the generation of specific morphologies of the particles. As an example, in Figure 1.6 are shown the copolymer particles of styrene-co-N-isopropylacrylamide synthesized by the surfactant-free approach, that is, by the homogenous nucleation method. N-isopropylacrylamide being hydrophilic in nature starts to poly mer ize first, followed by the poly mer i-za tion of more hydrophobic styrene. But because the poly mer chains from styrene are also hydrophobic in nature, they push the hydrophilic chains of poly(N-isopropylacrylamide) away to the surface, leading to the morphology as shown in Figure 1.6.

(a) (b)

300 nm 200 nm

250 nm300 nm

(c) (d)

Figure 1.5(ad) Various morphologies of particles achieved with emulsion poly mer i za tion.


1.3 Controlled Polymerization and its Use in Emulsion Polymerization Processes

The conventional radical poly mer i za tion is limited as a technique in that the control in the molecular weight or its distribution is difficult to achieve. It is also not easy to achieve well-defined morphologies in the particles like block copolymers because the life of the radical is too short, and uncontrolled termi-nation reactions take place very fast. Controlled living poly mer i za tion tech-niques, on the other hand, can circumvent the aforementioned limitations in the emulsion poly mer i za tion process [4]. In these techniques, the chains

(a)

250 nm

(c)

300 nm

(b)

300 nm

Figure 1.6(ac) Scanning electron microscopy (SEM) micrographs of copolymer particles of styrene-co-N-isopropylacrylamide.


are terminated but only irreversibly, and after a short period of time become active again to propagate the poly mer chains. In such processes, the termina-tion reactions are effectively eliminated, and the controlled molecular weight distributions as well as advanced morphologies in the poly mer particles can be achieved. There have been many techniques developed in the last years to achieve controlled poly mer i za tion, and these techniques are generally clas-sified into two categories: those based on reversible termination and those based on reversible transfer. Figure 1.7 is the representation of the various controlled poly mer i za tion techniques. In the category of reversible termina-tion, nitroxide-mediated poly mer i za tion (NMP) and atom transfer radical poly mer i za tion (ATRP) are the most studied approaches. ATRP has also been further modified into techniques like reverse atom transfer radical poly mer-i za tion, activator generated by electron transfer ATRP, etc. In the category of reversible transfer, techniques like reversible addition- fragmentation chain transfer (RAFT) poly mer i za tion and degenerative transfer are mostly reported. During the poly mer i za tion, the concentration of dormant species continues to increase as compared to the active chains. At the end of poly-mer i za tion, the dormant species may be present in amounts six times higher than the active chains. This effectively leads to elimination of termination and allows much longer lifetimes for the radicals.

Livingpolymerization

Degenerativetransfer

Reverse atomtransfer radicalpolymerization

(ATRP)Activator generatedby elecron transfer (AGET)atom transfer

radical polymerization(ATRP)

Activator regeneratedby elecron transfer (ARGET)

atom transferradical polymerization

(ATRP)

Atom transferradical polymerization

(ATRP)

TEMPOmediated nitroxide

polymerization(NMP)

SG1mediated nitroxide

polymerization(NMP)

Reversible addition-fragmentation

chain transfer (RAFT)polymerization

Figure 1.7Representation of various living poly mer i za tion techniques. (Reprinted from V. Mittal, Advances in Polymer Latex Technology, New York: Nova Science Publishers, 2009. With permission.)


NMP, where nitroxides are used to irreversibly terminate the poly mer chains, has been used in two different ways. In one case, a conventional free radical initiator and a separately added nitroxide are added to control the poly mer i za tion. The two most commonly used nitroxides for this pur-pose are 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO) and N-ter-butyl-1-diethylphosphono-2,2-dimethylpropyl (SG1). The nitroxides were initially developed for the poly mer i za tion of styrene; however, a number of other nitroxides have been developed that are also suitable for the poly mer i za tion of acrylates. In the other case, an alkoxyamine is used, the decomposition of which leads to the generation of two radicals: one reactive and one sta-ble. This radical pair then controls the poly mer i za tion and thus does not require the addition of conventional free radical initiator. Figures 1.8 and 1.9 represent the poly mer i za tion of lauryl methacrylate and styrene by using nitroxides and alkoxyamines, respectively. The only disadvantage of the nitroxide-mediated stable free radical poly mer i za tion was the requirement of a high temperature for the poly mer i za tion reaction, which is sometimes not feasible for thermally sensitive systems; however, various nitroxides have been developed that also allow use at lower reaction temperatures. The initially carried-out reactions with styrene in emulsion led to poor colloi-dal stability, which resulted in a large amount of coagulum generated in the poly mer i za tion reactions. It was claimed that the particle nucleation as well as poly mer i za tion in droplets were a few reasons, among others, for this behavior. The seed method has also been described for emulsion poly mer-i za tion with SG1 as nitroxide [7]. In this case, a seed is generated first with low solid content, and the seed particles are then swollen in monomer and followed by subsequent poly mer i za tion of these seed particles. This helps to avoid the generation of monomer droplets and thus poly mer i za tion in drop-lets. It was also possible to achieve the previously described reaction as a single step. After the synthesis of seed as before, a certain amount of mono-mer is added without cooling the seed latex and the reaction is run until high conversion is achieved. The formed latexes were very stable in nature and no coagulum was generated. It is also important to monitor the progress of the reaction especially at high conversion, as at very high conversions, chains start to terminate each other and the polydispersity in the chain length as well as molecular weight increases. Therefore, it is always beneficial to stop the poly mer i za tion reaction a little below the full conversion. It was also confirmed that the alkoxyamines based on SG1 are more optimally operat-able for achieving controlled poly mer i za tion of a wide range of monomers as compared to TEMPO nitroxide.

ATRP represents another technique based on the principle of reversible termination, and in this process, an organic halide is used to irreversibly terminate the propagating chains. This technique has been very success-ful for the controlled poly mer i za tion of styrenics as well as acrylates and methacrylates. It also does not require very high temperatures as compared


to NMP, and in many cases can also be undertaken at room temperature. Figure 1.10a shows the schematic of the ATRP process. Cuprous salt forms a complex with ligand, L (amines of different chemical architectures), which makes it more soluble in the solvents [1]. Initiation of the reaction takes place by the dissociation of the halide atom from the initiator and leading to the generation of a free reactive radical. The bromide atom is captured by cuprous halide ligand complex and it forms CuBr2 ligand complex. This compound is very stable and hence is called deactivator. The generation of this compound thus leads to reduction in the concentration of the free radi-cal species in the system. The growing radical continues to add monomer units to the poly mer chain, and at some point it comes in contact with CuBr2

C

O

O CH2CH2 CHCH

C O

OC12H25

C O

OC12H25

O N CH

CH3

P

OC2H5

O

OC2H5

C

O

O O C

OCH2 CH

C O

OC12H25

+

+

O N

CH3

CH3H3C

CH3

CH3

H3C

CH P

OC2H5OC2H5

CH3H3C

CH3H3C

CH3

O

C

O

O CH2 CH

C O

OC12H25

CH2 CH

C O

OC12H25

CH2 CH

C O

OC12H25

Figure 1.8Nitroxide-mediated controlled radical poly mer i za tion of lauryl methacrylate with SG1 nitroxide.


ligand complex and is temporarily terminated by the formation of RMn+1-Br compound. It is also possible to carry out the reverse ATRP process similarly (Figure 1.10b). In this process, a conventional free radical initiator like AIBN or benzoyl peroxide is used to initiate the poly mer i za tion reaction, which is controlled by the addition of CuBr2 ligand complex. The radicals add few monomer units, and during this process come in contact with this complex to form the dormant species. One limitation of such a technique is the pres-ence of transition metal in the final particles, which though possible to wash off adds another processing step in the synthesis process. Another limitation

CH

H3C

O N

CH3H3C

CH3H3C

CH2 CH

CH

H3C

CH

H3C

CH2

+

CH

H3C

+

O N

CH3H3C

CH3H3C

CH

H3C

CH2 CH O N

CH3H3C

CH3H3C

CH2 CH

CH2 CH

n

n

CH

H3C

CH2 CH O N

CH3H3C

CH3H3C

CH

O N

CH3H3C

CH3H3C

Figure 1.9Nitroxide-mediated free radical poly mer i za tion of styrene by using alkoxyamine as nitroxide as well as initiator.


is the reaction of the copper compounds with the other constituents of the system. One example is the reaction of these compounds with the emulsifier used in the poly mer i za tion system, leading to the poisoning of the initiator, which subsequently results in no or little poly mer i za tion. Therefore, it is pos-sible to work in the emulsion with the ATRP when there is no surfactant or surfactants with no interaction with the initiator are chosen. ATRP in emul-sion processes also faced problems similar to those in NMP. In one reported study, ethyl 2-bromoisobutyrate was used as an ATRP initiator, and copper bromide was complexed with 4,4-dinonyl-2,2-bipyridyl to form the catalyst system [8]. Nonionic surfactant Tween 85 was used. The poly mer i za tion was achieved by first mixing together copper salts with 4,4-dinonyl-2,2-bipyri-dyl, to which the monomer was added. The solution was allowed to mix and was added with surfactant. To this solution, water was added under vigorous

P Br CuBr/L P

PM

PMn+1

RMn+1 Br CuBr/L+

M

nM

CuBr2/L++

(a)

C O

O O

O C O

O

BrC CuBr2/L

+ CuBr2/L

+ CuBr/L

+

C O

O

M

M

nM C O

O

Mn+1

C O

O

Mn+1 Br

(b)

Figure 1.10Schematic of (a) ATRP and (b) reverse ATRP processes for controlled living poly mer i za tion. (Adapted from G. Odian, Principles of Polymerization, Hoboken, NJ: John Wiley & Sons, 2004; and V. Mittal, Advances in Polymer Latex Technology, New York: Nova Science Publishers, 2009.)


stirring to form the emulsion. To this emulsion was then added the initiator to initiate the poly mer i za tion reaction. The reaction conditions needed to be very accurately controlled, and the reaction was overall very sensitive to minor changes in the reaction parameters. Seeded poly mer i za tion similar to that used with NMP was also used in this case. The use of cationic surfac-tants was also reported for the ATRP processes in emulsion. Dodecyl trim-ethyl ammonium bromide and myristyl trimethyl ammonium bromide were used as cationic surfactants, and their effect on the latex stability, amount of coagulum, and the polydispersity in the molecular weight was quantified. The first surfactant though gave a good control of polydispersity; however, the whole system was observed to coagulate after the initiation of poly mer i-za tion. In the case of the second surfactant, the latex stability was better, but the polydispersity in the molecular weight or chain lengths was very high.

RAFT is a controlled poly mer i za tion technique based on the principle of reversible transfer. The core of this process is a RAFT agent that contains dithioester groups. The living poly mer i za tion takes place because the trans-ferred end group in the polymeric dithioester is as labile as the dithioester group in the starting RAFT agent. The initiator for the poly mer i za tion can be the conventional initiators like AIBN or benzoyl peroxide. Figure 1.11 explains the principle of RAFT poly mer i za tion. Though it is one of the ver-satile techniques for the poly mer i za tion of a large number of monomers, it also has its own limitations, such as the presence of remainder RAFT agent and the commercial unavailability of the RAFT agents. Similar to NMP and ATRP, initial trials with RAFT also were faced with difficulties of coagu-lum generation. The RAFT agent was difficult to be transported to the poly-mer particles through the aqueous phase. RAFT poly mer i za tion of styrene in emulsion was reported by Szkurhan et al. [9]. The process named nano-precipitation was carried out by forming nano-sized particles by precipita-tion of the acetone solution of macro RAFT agent in the aqueous poly(vinyl alcohol) solution. The macro RAFT agent was prepared by conventional free

S

Mn + + R2

SR2

R1

MnS

SR2

R1

MnS S

R1

S

Mm + + Mn

SMn

R1

MmS

SMn

R1

MmS S

R1

Figure 1.11Mechanism of reversible transfer processes used in reversible addition-fragmentation chain transfer processes. (Adapted from G. Odian, Principles of Polymerization, Hoboken, NJ: John Wiley & Sons, 2004; and V. Mittal, Advances in Polymer Latex Technology, New York: Nova Science Publishers, 2009.)


radical poly mer i za tion. The formed nanoparticles were subsequently swol-len with monomer and were poly mer ized in the living manner. This nano-precipitation method was also named seeded poly mer i za tion because, in this case, the nano-sized particles formed by precipitation act as seeds to form poly mer particles. Both water- and oil-soluble initiators were used. When the initiator used was oil soluble, it was premixed with the RAFT agent, whereas the water-soluble initiator was dissolved in PVA solution. With both water-soluble and oil-soluble initiators, the rate of poly mer i za tion was quite slow, and increasing the reaction temperature was not helpful in increasing the rate of poly mer i za tion. Another study reported the synthesis of poly mer particles in emulsion by RAFT without the problems of loss of colloidal sta-bility and the molecular weight control [10]. Trithiocarbonate RAFT agents were used in the study to form short stabilizing blocks from a hydrophilic monomer, from which diblocks were created by the subsequent poly mer-i za tion of a hydrophobic monomer. These diblocks self-assembled to form micelles, and subsequent poly mer i za tion could be carried out. Figure 1.12 demonstrates the particles generated by this technique.

1.4 Conventional and Controlled Miniemulsion Polymerization

In surfactant-aided emulsion poly mer i za tion, the goal is to achieve the micel-lar nucleation and to avoid the droplet nucleation as much as possible. But the poly mer i za tion of extremely hydrophobic monomers by conventional emul-sion poly mer i za tion is not possible because of their inability to diffuse from the monomer droplets through the aqueous phase to the poly mer particles. To achieve poly mer i za tion of such systems, miniemulsion poly mer i za tion has proved to be a versatile method [2,3]. The mode of poly mer i za tion is based on the droplet poly mer i za tion principle. The monomer droplets are generated by shearing the system with high energy along with the addition of costabilizer (with the surfactant), which needs to be hydrophobic in order to avoid the collapse of the monomer droplets by Ostwald ripening when the shearing of the system is stopped. Thus in this mode of poly mer i za tion, it is important to avoid the micellar nucleation; therefore, the amount of surfactant is below the critical micelle concentration. The particles in the general size range of 50500 nm can be synthesized by using miniemulsion poly mer i za tion. These are similar in size to the monomer droplets in the beginning of the poly mer-i za tion. The initiators used for the poly mer i za tion are water soluble as in the case of emulsion poly mer i za tion. The initiator on dissociation generates radi-cals in the aqueous phase, and these radicals enter the droplets and initiate poly mer i za tion. In conventional emulsion poly mer i za tion, also called mac-roemulsion poly mer i za tion, the micellar nucleation is very sensitive and is affected by a large number of factors like surfactant amount, initiator amount,


Conventional core-shell:dierent chains inthe core and shell

Triblock polymer:the same chains extend

from the shell into the core

(a)

0.20 m

X 19000

(b)

Figure 1.12(a) Schematic representation of RAFT-based triblock copolymer chains forming the core and shell of the particles and their comparison with conventional core-shell particles. (b) TEM micrograph of poly(acrylic acid)-b-poly(butyl acrylate)-b-polystyrene particles. (Reprinted from C. J. Ferguson, R. J. Hughes, D. Nguyen, T. T. Pham, R. G. Gilbert, A. K. Serelis, C. H. Such, and B. S. Hawkett, Macromolecules 38: 21912204, 2005. With permission from the American Chemical Society.)


agitation, reaction temperature, etc. However, that is not the case in miniemul-sion poly mer i za tion. Figure 1.13 also represents the scheme of poly mer i za tion in miniemulsion. The monomer is added to the surfactant and costabilizer in the aqueous phase followed by homogenization under high shear to break the bigger monomer droplets into droplets with the size range of 10500 nm [2,3,11]. The amount of the surfactants and the shearing translate into the size of the monomer droplets. As mentioned earlier, in emulsion poly mer i za tion, the poly mer i za tion is driven by the diffusion of the monomer through the aqueous phase. In this process, monomer droplets disappear and the micelles convert into the poly mer particles, whereas in miniemulsion poly mer i za tion, the monomer droplets directly translate into poly mer particles. As a result, the rate of poly mer i za tion is also different for these two processes. The rate of poly mer i za tion in the emulsion poly mer i za tion first increases owing to the generation of the particles and reaches a constant phase after the disap-pearance of the micelles. The rate then decreases owing to the depletion of the monomer in the particles. As there is no diffusion of monomer in the miniemulsion poly mer i za tion, the constant rate period is absent. The rate first increases owing to the nucleation of the particles and then decreases when the monomer is consumed in the particles. Not only hydrophobic mono-mers, but also extremely hydrophilic monomers can be poly mer ized using the miniemulsion poly mer i za tion method. In this case, one has to use the inverse miniemulsion poly mer i za tion. Also in this case, hydrophobic reac-tion medium is used along with a lipophobe used as costabilizer.

One must be clear that the addition of costabilizer stops the conversion of a miniemulsion into a conventional emulsion; however, the addition of a costa-bilizer to conventional emulsion does not automatically convert it into a mini-emulsion. It is only after the addition of high shearing energy that it becomes a stable miniemulsion. The costabilizers should be hydrophobic, soluble in monomer, and have a low molecular weight. However, the use of conventional costabilizers like cetyl alcohol and hexadecane pose a potential hazard owing to their volatility, and the presence of these even in the minor amounts in the poly mer particles may not be acceptable for many applications. Therefore, a lot of research effort has been focused in the direction of generation of more com-patible costabilizers. Polymeric stabilizers from the poly mer of the monomer

Figure 1.13Schematic of the miniemulsion poly mer i za tion process. The molecules with black and gray color represent the surfactant and costabilizer, respectively.


to be poly mer ized have been used in some of the reported studies [3,12,13]. These polymers are soluble in their own monomers and thus allow better intermixing at the interphase. They also eliminate the use of volatile costabi-lizers, providing more acceptability to the system for the commercial applica-tions. Monomeric costabilizers have also been developed in recent years that can copoly mer ize with the monomer under poly mer i za tion [14]. These costa-bilizers form copolymer chains that are bound inside the particles, and thus the possibility of the diffusion of low molecular weight components out of the particles is eliminated. The potential diffusion of the low molecular weight components, especially substances like cetyl alcohol or hexadecane, can pose health hazards when the poly mer particles are used in application with food contact. Similarly, the other components of the poly mer i za tion system like initiator and chain transfer agent have also been used as costabilizers [15,16]. Therefore, the materials act as dual-role components. They not only perform their function as initiator or chain transfer agent, respectively, but also help to achieve the stability for the monomer droplets and poly mer particles.

Miniemulsion poly mer i za tion has also been proved to be advantageous in living poly mer i za tion systems. The various living poly mer i za tion methods like NMP, ATRP, and RAFT have been shown to be beneficial in miniemul-sion poly mer i za tion to generate specialty polymers or polymers with special architecture like block copolymers. Colloidal stability and ease of poly mer-i za tion process are also better in the case of miniemulsion poly mer i za tion than conventional emulsion poly mer i za tion.

For NMP, nitroxide-capped poly mer chains have been used as initiator as well as nitroxide. The use of such nitroxide-capped poly mer chains for the initiation and reaction control allows one to properly estimate the num-ber of starting chains in the system, helping to achieve better control of the molecular weight. The use of nitroxide-capped poly mer also helps to parti-tion the nitroxide solely in the organic phase owing to the hydrophobicity. In such reported studies with polystyrene terminated with TEMPO as ini-tiator as well as nitroxide, hexadecane as costabilizer, and DOWFAX 8390 as surfactant, it was reported that changing the amount of surfactant led to the generation of different particle sizes, but the rate of poly mer i za tion was not affected, which is different from the behavior seen in conventional emulsion poly mer i za tion [17,18]. Figure 1.14 demonstrates the transmission electron microscopy (TEM) images of the polystyrene particles generated in nitroxide-mediated miniemulsion poly mer i za tion using different amounts of TEMPO-terminated oligomers of polystyrene as macroinitiator.

A great deal of process development has been reported for ATRP in mini-emulsion. A number of studies have been reported that apply the direct or forward ATRP in miniemulsion, but reverse ATRP, in which conventional free radical poly mer i za tion initiator like AIBN can be used with the tran-sition metal compound in its higher oxidation state, was observed to be more suitable for miniemulsion poly mer i za tion. This eliminates the use of air-sensitive Cu(I) species and requires only the use of Cu(II) species, which


is more stable in air. A narrow molecular weight distribution as well as linear increase in the molecular weight as a function of conversion was reported, and the final latexes were stable over a period of time. In one such study on reverse ATRP processes [19], Brij 98 surfactant and CuBr2/dNbpy (4,4-di[5-nonyl]-4,4-bipyridine) complex were used along with hexadecane costabi-lizer. Both water-soluble as well as oil-soluble initiators were used for the poly mer i za tion. It was observed that the poly mer i za tion rate was indepen-dent of the size and number of particles and the amount of surfactant. The shear forces were able to influence only the size of the particles and not the poly mer i za tion rate. In the case of oil-soluble initiator, the poly mer i za tion was observed to proceed by droplet nucleation mode because the monomer-soluble initiator was already present in the droplet during the miniemul-sion; whereas when water-soluble initiator was used, both micellar as well as droplet nucleation were reported to take place. Similarly RAFT has also been used successfully in miniemulsion for the poly mer i za tion of styrene as well as water-soluble monomers like acrylamide [11,2022].

1.5 Generation of Copolymer or Core-Shell Particles

The generation of well-defined copolymer morphologies like block copoly mer particles is difficult to achieve by the use of conventional emulsion poly mer-i za tion because of the uncontrolled free radical poly mer i za tion and the short life of the radicals. To achieve certain control on the copoly mer i za tion, mono-mer reactivity ratios and monomer-feeding methodology must be considered.

(a) (b)

Figure 1.14TEM images of the polystyrene particles prepared by nitroxide-mediated miniemulsion poly-mer i za tion using different amounts of TEMPO-terminated oligomers of polystyrene (TTOPS) as macroinitiator. (a) 5% TTOPS and (b) 20% TTOPS (100 nm). (Reprinted from G. Pan, E. D. Sudol, V. L. Dimonie, and M. S. El-Aasser, Macromolecules 34: 48188, 2001. With permission from the American Chemical Society.)


The monomers have different reactivity ratios; therefore, if the monomers are added together, the more reactive monomer starts to poly mer ize first followed by the poly mer i za tion of the less reactive monomer. This creates a gradient of concentration of the monomer units in the poly mer particles as a function of radius. Figure 1.15 provides some examples of various copoly-mer particles that can be achieved by emulsion poly mer i za tion like core-shell grafted particles, core-shell particles with hydrophilic shell and hydropho-bic core, copolymer particles with different surface morphologies, etc. Apart from reactivity ratios, mode of addition of the monomers during the course of poly mer i za tion is also of utmost importance to achieve control on the particle characteristics. Batch addition of the monomers does not lead to generation of the structured latexes; therefore, semibatch addition of the monomers is generally preferred. This mode of addition can be achieved by either flooded

(a) (b)

300 nm300 nm

300 nm 300 nm

(c) (d)

Figure 1.15(ad) Different morphologies of copolymer particles generated by conventional emulsion poly-mer i za tion.


addition or starved addition of the monomers. In flooded addition, monomers are added at a rate higher than their rate of consumption. This mode of addi-tion leads to buildup of the monomers in the poly mer i za tion reaction and may lead to the generation of secondary nucleation of the particles. Starved addition of the monomers, on the other hand, is the addition of monomers at a rate slower than their poly mer i za tion rate, and this allows one to retain the chemical composition of the poly mer chains equal to the monomer ratios in the feed or according to the requirement. The starved conditions eliminate the possibility of secondary nucleation, though one has to be careful about the control of the amount of the surfactant in the system as well as charges on the surface. As an example, if copoly mer i za tion of a hydrophilic monomer and a hydrophobic monomer is considered, initially the surface may be hydropho-bic, but as the chains rich in hydrophilic monomer content get pushed out to the surface of the particles during the course of poly mer i za tion, the surface becomes hydrophilic. This would lead to a change in the surface properties of the particles and would allow the release of the surfactant from the surface of the particles owing to the hydrophili city. This also results in the nonentry of the hydrophobic monomer into the poly mer particles, causing monomer concentration drift in particles or secondary nucleation.

Interesting studies have been reported for the generation of copolymer par-ticle latexes by emulsion poly mer i za tion. Batch poly mer i za tion of copoly mer particles of polystyrene and poly(methyl methacrylate) were reported without the use of initiators [23]. These copolymer particles of poly(methyl methacrylate-co-styrene) were prepared by thermally initiated emulsion copoly mer i za tion. It was observed that totally different particle morphologies like hemispherical, sandwich-like, core-shell, inverted core-shell particle morphologies, etc. were obtained depending on the poly mer i za tion conditions. It was reported that the incorporation of the initiator fragments to one end of the chains allows the polystyrene chains to become more hydrophilic, changing the surface nature of the poly mer particles. In another study to generate copolymer particles, poly(methyl methacrylate) seed was used to generate the copolymer particles of poly(methyl methacrylate) with polystyrene [24]. It was observed that by using the oil-soluble initiators, an inverted core-shell morphology of the par-ticles was obtained, in which the polystyrene chains were present in the core of the particles and the poly(methyl methacrylate) covered the particles owing to its hydrophilicity. In the case of water-soluble initiator, the morphology was less affected by the hydrophilicity of the polymers, but was more affected by the initiator concentration and poly mer i za tion temperature.

Controlled living poly mer i za tion methods provide much better possibili-ties to generate the structured latex particles with different morphologies of chemistries owing to the prolonging of the radical age either by revers-ible termination or by reversible transfer. In one such study to generate triblock copolymers using emulsion poly mer i za tion, water-soluble SG1-based bifunctional alkoxyamine (Figure 1.16; sodium salt of alkoxyamine was used owing to water solubility) and Dowfax 8390 surfactant were used


[11,25]. When a bifunctional alkoxyamine was used, two functional ends of this alkoxyamine could be used to generate triblock copolymers. Thus, in order to generate polystyrene-b-poly(butyl acrylate)-b-polystyrene triblock copolymer particles, a seed was first generated from butyl acrylate particles. The seed was further swollen with butyl acrylate to form central poly(butyl acrylate) block in the emulsion particles. The particles were then added with styrene to form two blocks of styrene around the central poly(butyl acry-late) block to form the triblock copolymer. In another study using ATRP, the seeded-poly mer i za tion approach was used to synthesize block copolymers of poly(i-butyl methacrylate) and polystyrene using ethyl 2-bromoisobu-tyrate as initiator and CuBr/4,4-dinonyl-2,2-dipyridinyl as catalyst ligand

OHOOC

t-butanolT = 80 100C

NP O

O

O

EmulsionT = 112C

(1) NaOH

RR = Ph, COOBu(2)

N OOO

OP

O

HOOC COOH

O3

OO N

P OO

O

CH2 CH2

O

O

O O3

O

CH2 CH2

N OO

R

x

OO

P

O

COO +NaNa+ OOC

O3

OO N

P OO

O

CH2 CH2

O

R

x

Figure 1.16Synthesis and use of SG1-based water-soluble bifunctional alkoxyamine. (Reprinted from J. Nicolas, B. Charleux, O. Guerret, and S. Magnet, Macromolecules 38: 996373, 2005. With per-mission from the American Chemical Society.)


complex [26]. Tween 80 (polyoxyethylene sorbitan monooleate) was used as surfactant. First a seed of poly(i-butyl methacrylate) end-capped with ATRP initiator was prepared to which a batch of styrene then was added to form a block copolymer. Thermally responsive poly mer particles were also reported by the use of ATRP [2729]. The process included the synthesis of seed parti-cles, functionalization of seed particles by ATRP initiator, and the grafting of the poly(N-isopropylacrylamide) chains from the surface. Figure 1.17 shows these functional particles [28]. In another study using RAFT poly mer i za-tion, core-shell functional particles were achieved by using o- ethylxanthyl

(a) (b)

300 nm 800 nm

(c)

100 nm

(d)

250 nm

Figure 1.17(a, b) SEM and TEM micrographs of latex particles functionalized with an ATRP initiator. (c, d) The grafted brushes of thermally responsive poly mer poly(N-isopropylacrylamide) from the surface of the ATRP initiator functionalized particles.


ethyl propionate as RAFT agent [30]. The poly mer i za tion reactions were car-ried out in the presence of poly(methyl methacrylate) seed particles of pre-determined number and size distribution. The seed was added first with styrene monomer to form polystyrene block, which was then added with butyl acrylate. Styrene was added in batch conditions, whereas butyl acry-late was added slowly to the emulsion system so as to avoid the buildup of high concentration of monomer in this system. Similarly, core-shell par-ticles consisting of block copolymer of polystyrene-b-poly/butyl acrylate)/poly(acetoacetoxy ethyl methacrylate) were also prepared by using xanthates as RAFT agents. Figure 1.18 shows the TEM image of such core-shell par-ticles [31]. Miniemulsion poly mer i za tion has also been extensively used for the synthesis of functional latex particles [3238].

References

1. Odian, G. 2004. Principles of poly mer i za tion. Hoboken, NJ: John Wiley & Sons. 2. Landfester, K. 2001. Polyreactions in miniemulsions. Macromolecular Rapid

Communications 22:896936.

Observation at 150C

pTA/RuO4 cryo

Figure 1.18Core-shell particles of polystyrene-b-poly(butyl acrylate)/poly(acetoacetoxy ethyl methacry-late). The black core represents polystyrene whereas the soft shell is poly(butyl acrylate)/poly(acetoacetoxy ethyl methacrylate) component. (Reprinted from M. J. Monteiro and J. de Barbeyrac, Macromolecules 34: 441623, 2001. With permission from the American Chemical Society.)


3. Schork, F. J., Luo, Y., Smulders, W., Russum, J. P., Butt, A., and K. Fontenot. 2005. Miniemulsion poly mer i za tion. Advances in Polymer Science 175:129255.

4. Matyjaszewski, K., and T. P. Davis. 2002. Handbook of radical poly mer i za tion. Hoboken, NJ: John Wiley & Sons.

5. Hiemenz, P. C., and R. Rajagopalan. 1997. Principles of colloid and surface chemistry. New York: Marcel Dekker.

6. Mittal, V. 2009. Advances in poly mer latex technology. New York: Nova Science Publishers.

7. Nicolas, J., Charleux, B., and S. Magnet. 2006. Multistep and semibatch nitrox-ide-mediated controlled free-radical emulsion poly mer i za tion: A significant step toward conceivable industrial processes. Journal of Polymer Science, Part A: Polymer Chemistry 44:414253.

8. Eslami, H., and S. Zhu. 2005. Emulsion atom transfer radical poly mer i za tion of 2-ethylhexyl methacrylate. Polymer 46:548493.

9. Szkurhan, A. R., Kasahara, T., and M. K. Georges. 2006. Reversible-addition frag-mentation chain transfer radical emulsion poly mer i za tion by a nanoprecipita-tion process. Journal of Polymer Science, Part A: Polymer Chemistry 44:570818.

10. Ferguson, C. J., Hughes, R. J., Nguyen, D., Pham, B. T. T., Gilbert, R. G., Serelis, A. K., Such, C. H., and B. S. Hawkett. 2005. Ab initio emulsion poly mer i za tion by RAFT-controlled self-assembly. Macromolecules 38:21912204.

11. Cunningham, M. F. 2008. Controlled/living radical poly mer i za tion in aqueous dispersed systems. Progress in Polymer Science 33:36598.

12. Reimers, J. L., and F. J. Schork. 1996. Predominant droplet nucleation in emul-sion poly mer i za tion. Journal of Applied Polymer Science 60:25162.

13. Reimers, J., and F. J. Schork. 1996. Robust nucleation in poly mer-stabilized mini-emulsion poly mer i za tion. Journal of Applied Polymer Science 59:183341.

14. Reimers, J. L., and F. J. Schork. 1996. Miniemulsion copoly mer i za tion using water-insoluble comonomers as cosurfactants. Polymer Reaction Engineering 4:13552.

15. Reimers, J. L., and F. J. Schork. 1997. Lauroyl peroxide as cosurfactant in mini-emulsion poly mer i za tion. Industrial Engineering Research 36:108587.

16. Mouran, D., Reimers, J., and F. J. Schork. 1996. Miniemulsion poly mer i za tion of methyl methacrylate with dodecyl mercaptan as cosurfactant. Journal of Polymer Science, Part A: Polymer Chemistry 34:107381.

17. Pan, G., Sudol, E. D., Dimonie, V. L., and M. S. El-Aasser. 2002. Surfactant con-centration effects on nitroxide-mediated living free radical miniemulsion poly-mer i za tion of styrene. Macromolecules 35:691519.

18. Pan, G., Sudol, E. D., Dimonie, V. L., and M. S. El-Aasser. 2001. Nitroxide-mediated living free radical miniemulsion poly mer i za tion of styrene. Macromolecules 34:48188.

19. Matyajaszewski, K., Qiu, J., Tsarevsky, N. V., and B. Charleux. 2000. Atom trans-fer radical poly mer i za tion of n-butyl methacrylate in an aqueous dispersed system: A miniemulsion approach. Journal of Polymer Science, Part A: Polymer Chemistry 38:472434.

20. Moad, G., Chiefari, J., Chong, Y. K., Krstina, J., Mayadunne, R. T. A., Postma, A., Rizzardo, E., and S. H. Thang. 2002. Living free radical poly mer i za tion with reversible addition-fragmentation chain transfer (the life of RAFT). Polymer International 49:9931001.


21. Butt, A., Storti, G., and M. Morbidelli. 2001. Miniemulsion living free radical poly mer i za tion by RAFT. Macromolecules 34:588596.

22. Qi, G., Jones, C. W., and F. J. Schork. 2007. RAFT inverse miniemulsion poly mer-i za tion of acrylamide. Macromolecular Rapid Communications 28:101016.

23. Du, Y. Z., Ma, G. H., Ni, H. M., Nagai, M., and S. Omi. 2002. Morphological stud-ies in thermally initiated emulsion (co)poly mer i za tion without conventional initiators. Journal of Applied Polymer Science 84:173748.

24. Cho, I., and K. W. Lee. 1985. Morphology of latex particles formed by poly(methyl methacrylate)-seeded emulsion poly mer i za tion of styrene. Journal of Applied Polymer Science 30:190326.

25. Nicolas, J., Charleux, B., Guerret, O., and S. Magnet. 2005. Nitroxide-mediated controlled free-radical emulsion poly mer i za tion using a difunctional water- soluble alkoxyamine initiator. Toward the control of particle size, particle size distribution, and the synthesis of triblock copolymers. Macromolecules 38:996373.

26. Okubo, M., Minami, H., and J. Zhou. 2004. Preparation of block copolymer by atom transfer radical seeded emulsion poly mer i za tion. Colloid and Polymer Science 282:74752.

27. Mittal, V., Matsko, N. B., Butt, A., and M. Morbidelli. 2007. Functionalized polystyrene latex particles as substrates for ATRP: Surface and colloidal charac-terization. Polymer 48:280617.

28. Mittal, V., Matsko, N. B., Butt, A., and M. Morbidelli. 2007. Synthesis of tem-perature responsive poly mer brushes from polystyrene latex particles function-alized with ATRP initiator. European Polymer Journal 43:486881.

29. Mittal, V., Matsko, N. B., Butt, A., and M. Morbidelli. 2008. Swelling deswell-ing behavior of PS-PNIPAAM copolymer particles and PNIPAAM brushes grafted from polystyrene particles & monoliths. Macromolecular Materials and Engineering 293:491502.

30. Smulders, W., and M. J. Monteiro. 2004. Seeded emulsion poly mer i za tion of block copolymer core-shell nanoparticles with controlled particle size and molecular weight distribution using xanthate-based RAFT poly mer i za tion. Macromolecules 37:447483.

31. Monteiro, M. J., and J. de Barbeyrac. 2001. Free-radical poly mer i za tion of sty-rene in emulsion using a reversible addition-fragmentation chain transfer agent with a low transfer constant: Effect on rate, particle size, and molecular weight. Macromolecules 34:441623.

32. Farcet, C., and B. Charleux. 2002. Nitroxide-mediated miniemulsion poly mer i-za tion of n-butyl acrylate: Synthesis of controlled homopolymers and gradient copolymers with styrene. Macromolecular Symposia 182:24960.

33. Tortosa, K., Smith, J.-A., and M. F. Cunningham. 2001. Synthesis of polystyrene-block-poly(butyl acrylate) copolymers using nitroxide- mediated living radical poly mer i za tion in miniemulsion. Macromolecular Rapid Communications 22:95761.

34. Keoshkerian, B., MacLeod, P. J., and M. K. Georges. 2001. Block copoly-mer synthesis by a miniemulsion stable free radical poly mer i za tion process. Macromolecules 34:359499.

35. Li, M., Jahed, N. M., Min, K., and K. Matyjaszewski. 2004. Preparation of linear and star-shaped block copolymers by ATRP using simultaneous reverse and nor-mal initiation process in bulk and miniemulsion. Macromolecules 37:243441.


36. Min, K., Li, M., and K. Matyjaszewski. 2005. Preparation of gradient copoly-mers via ATRP using a simultaneous reverse and normal initiation process. I. Spontaneous gradient. Journal of Polymer Science, Part A: Polymer Chemistry 43:361622.

37. Min, K., Gao, H., and K. Matyjaszewski. 2005. Preparation of homopolymers and block copolymers in miniemulsion by ATRP using activators generated by electron transfer (AGET). Journal of the American Chemical Society 127:382530.

38. Luo, Y., and X. Liu. 2004. Reversible addition-fragmentation transfer (RAFT) copoly mer i za tion of methyl methacrylate and styrene in miniemulsion. Journal of Polymer Science, Part A: Polymer Chemistry 42:624858.

29

2Synthesis of Polymer Particles with Core-Shell Morphologies

Claudia Sayer and Pedro Henrique Hermes de Arajo

2.1 Introduction

Core-shell poly mer particles have received a great deal of industrial and academic interest in the last decades. These multicomponent particles with controlled morphology create a versatile class of materials in which the final properties depend not only on the composition of each poly mer phase but also on the morphology of these particles. This characteristic opens the

CONTENTS

2.1 Introduction .................................................................................................. 292.2 Equilibrium and Nonequilibrium Morphologies ...................................30

2.2.1 Equilibrium Morphologies ............................................................. 312.2.2 Nonequilibrium Morphologies ......................................................33

2.3 Synthesis of Core-Shell Particles ...............................................................352.3.1 Emulsion Polymerization ...............................................................35

2.3.1.1 Synthesis of Core-Shell Particles (CS) ............................ 372.3.1.2 Synthesis of Inverted Core-Shell Particles (ICS) ........... 41

2.3.2 Miniemulsion Polymerization .......................................................432.3.3 Microemulsion Polymerization ..................................................... 472.3.4 Dispersion Polymerization ............................................................. 472.3.5 Suspension Polymerization ............................................................482.3.6 Other Techniques ............................................................................. 49

2.4 Characterization of Core-Shell Particles ................................................... 522.4.1 Transmission Electron Microscopy ............................................... 522.4.2 Scanning Electron Microscopy ......................................................542.4.3 Atomic Force Microscopy ...............................................................542.4.4 Additional Techniques Used for Particle Characterization .......54

References ...............................................................................................................55


possibility for tailor-made properties for each application as, for instance, soft corehard shell results in particles suitable to act as impact modifiers, and hard coresoft shell latex particles result in paints with low film forma-tion temperature. In addition, via core-shell poly mer i za tion, it is also pos-sible to get incompatible polymers into one particle or to add functionality either into the core or into the shell (Koskinen and Wiln 2009).

Structured particles can be obtained with different morphologies: well-defined core-shell structure, inverted core-shell, interface with a gradi-ent of both core and shell, interface with microclusters, and multiple or irregularly shaped shells (Sundberg and Durant 2003). The final morphol-ogy depends on both thermodynamic and kinetic aspects, as quite fre-quently the equilibrium morphology may not be achieved due to kinetic control of the morphology development. The poly mer i za tion techniques play a major role in the particle size as well as in the kinetic control of the poly mer i za tion. Several heterogeneous poly mer i za tion techniques such as emulsion, miniemulsion, microemulsion, dispersion, and sus-pension poly mer i za tions could be employed to obtain poly mer particles with core-shell structures. The first three techniques lead to the forma-tion of submicrometric particles (10800 nm), whereas the two last are used, respectively, to prepare small (130 m) and large (501500 m) micro metric particles. The end use properties of the structured particles depend on the design and control of particle morphology; therefore, it is necessary to understand how this morphology can be controlled and which are the main features of each poly mer i za tion technique related to particle morphology control.

The purpose of this chapter is to describe the factors that will lead to a certain particle morphology and to discuss the heterogeneous poly mer i za-tion techniques that could be employed to obtain those particles. Section 2.2 introduces the basics of equilibrium and nonequilibrium morphologies. Section 2.3 deals with the different heterogeneous poly mer i za tion tech-niques and the main features related to morphology control. Section 2.4 discusses briefly the characterization techniques, as any work in this area relies on the need to adequately characterize particle morphology.

2.2 Equilibrium and Nonequilibrium Morphologies

The formation of core-shell particles is a challenging issue of poly mer reaction engineering in dispersed media. In principle, the most stable particle mor-phology is determined by thermodynamics according to the minimum inter-facial energy (Gonzlez-Ortiz and Asua 1995), as given by Equation (2.1):

Synthesis of Polymer Particles with Core-Shell Morphologies 31

== =12

1

3

1

3

aij ijj j ii

(2.1)

where aij and ij are, respectively, the interfacial area and the interfacial ten-sion between phases i and j.

Nevertheless, frequently the equilibrium morphology is not achieved since particle morphology depends on the interplay between thermodynamics, which establishes the equilibrium morphology, and kinetics. If kinetic control prevails, nonequilibrium-type (metastable or kinetically stable) structures may be formed. In the following paragraphs, equilibrium morphologies and the main factors that establish these morphologies will be discussed, fol-lowed by nonequilibrium ones.

2.2.1 equilibrium Morphologies

Figure 2.1 shows four different equilibrium morphologies for a two- component system based on the poly mer-poly mer and poly mer-aqueous phase interfacial tensions, which determine the interfacial energy and, con-sequently, also the equilibrium morphology for a given system. The equilib-rium morphologies are:

Coreshell (CS), in which the second-stage poly mer 2 () forms a con-tinuous shell around the seed poly mer 1 () dispersed in the aque-ous phase 3. This equilibrium morphology might be achieved when either (a) poly mer 1 is more hydrophobic than poly mer 2 (13 > 23) and poly mer 2 has more affinity with poly mer 1 than with the aque-ous phase (12 < 23) or (b) poly mer 1 has more affinity with poly mer 2 than with the aqueous phase (12 < 13) and poly mer 2 has less affinity with poly mer 1 than with the aqueous phase (12 > 23).

Inverted coreshell (ICS), in which the seed poly mer 1 forms a continu-ous shell around the second-stage poly mer 2. This equilibrium mor-phology might be obtained when poly mer 2 is more hydrophobic than poly mer 1 (23 > 13) and poly mer 2 has more affinity with poly-mer 1 than with the aqueous phase (12 < 23).

Hemisphere, snowman-like, Janus, half-moon, occluded, partially engulfed, depending on the different degrees of protrusion and on the different curvatures of the poly mer/poly mer interface and the coverage of one poly mer upon the other (Sundberg and Durant 2003). This equilibrium morphology might be achieved under different conditions: (a) when poly mer 2 has similar affinities with poly mer 1 and with the aqueous phase (12 23); (b) when poly mer 2 has more


affinity with poly mer 1 than with the aqueous phase (12 < 23) and both polymers have similar affinities with the aqueous phase (23 12 13); (c) when poly mer 2 has less affinity with poly mer 1 than with the aqueous phase (12 > 23) and poly mer 1 has similar affinities with poly mer 2 and the aqueous phase (23 12 13).

Separate particles of the different polymers. This equilibrium mor-phology might be obtained when both polymers have more affinity with the aqueous phase than with each other (13 < 12) and (23 < 12).

As shown in Figure 2.1, equilibrium morphologies of two-component systems are established basically by three interfacial tension values. Notwithstanding, several factors influence these three interfacial tensions and may, therefore, be used for particle morphology control purposes. Besides poly mer types, which determine the interfacial tension between the polymers and affect the interfacial tension between each poly mer and the aqueous phase, poly mer-poly mer interfacial tension can be influenced substantially by compatibilizing agents, and the interfacial tensions between each poly mer and the aqueous phase are also affected by the types and amounts of surfactant and initiator. In addition, it has been shown that the equilibrium morphology may also be affected by the level of cross-linking of poly mer 1, which influences the free energy (Sundberg and Durant 2003), and by the molecular weights of the polymers, since the interfacial tension between the poly mer phases, which is much lower than that between the poly mer and the aqueous phase, depends on the molecular weight (Tanaka

0.1 1 101223

(23 12)13

10

1

0.1

Figure 2.1Equilibrium morphologies for a two-component system: poly mer 1 (seed), poly mer 2 (pro-duced by the poly mer i za tion of the second-stage monomer). 12: interfacial tension between polymers 1 and 2; 13: interfacial tension between poly mer 1 and aqueous phase; 23: interfacial tension between poly mer 2 and aqueous phase. (Reprinted from V. Herrera, R. Pirri, J. R. Leiza, and J. M. Asua, Macromolecules 39: 696974, 2006. With permission.)


et al. 2008). Finally, the ratio between polymers 1 and 2 may affect the cur-vature of the poly mer-poly mer interface in the hemisphere morphology (Sundberg and Durant 2003), and the CS and ICS morphologies may not be achieved if the amount of the shell forming poly mer (poly mer 2) in CS and poly mer 1 in ICS is not enough to form a continuous shell with a minimum thickness. If copolymers are to be considered, the analysis becomes more complex since the surface and interfacial tensions depend on the copoly-mer compositions.

2.2.2 Nonequilibrium Morphologies

As mentioned at the beginning of this section, quite frequently the equilib-rium morphology is not achieved due to kinetic control of the morphology development. In this case, nonequilibrium-type structures may be formed. Three main processes have been used by Gonzlez-Ortiz and Asua (1995, 1996a, 1996b) to describe the morphology development:

1. The formation of poly mer chains occurs at a given position in the poly mer particle.

2. Incompatible poly mer chains cause phase separation leading to the formation of clusters.

3. Clusters migrate toward the equilibrium morphology in order to minimize the Gibbs free energy. During this migration the size of the clusters may increase by (i) poly mer i za tion of monomer inside the cluster, (ii) diffusion of poly mer chains into the cluster, and (iii) coagulation with other clusters. The rates of process (ii) and (iii) depend strongly on the particle viscosity.

The Sundberg group has studied the effect of several factors on the morphology development during two-stage emulsion poly mer i za tions, especially those involving the less hydrophobic copolymer of methyl meth-acrylate and methyl acrylate as seed poly mer and polystyrene as the more hydrophobic second-stage poly mer. Durant et al. (1997) verified the influ-ence of different amounts of cross-linking monomer (EGDMA) during the syntheses of the PMMA cores on the morphology of PMMA/PS particles. It was observed that 0.015 wt% EGDMA was enough to shift the particle morphology from ICS (second-stage PS in the core) toward CS. At 0.2 wt% EGDMA, the particles was essentially of the CS morphology. Cross-linking during the second stage, on the other hand, was observed by Stubbs and Sundberg (2006) to have very little, if any, effect on morphology, though it enhances the mechanical stability of the shell. The effect of the feed rate of the second-stage more hydrophobic monomer (styrene) when less hydropho-bic high-Tg seed polymers (poly[methyl methacrylate]) are used was studied by Stubbs et al. (1999). Fast second-stage monomer addition resulted in CS


particles, whereas slower addition increased the number of occlusions of the hydrophobic second-stage poly mer. When less hydrophobic low-Tg seed poly-mer (poly[methyl acrylate]) was used, ICS was obtained independently of the second-stage monomer feed rate. Ivarsson et al. (2000) and Karlsson et al. (2003) verified that it is possible to keep the more hydrophobic second-stage poly mer (styrene) at the shell of the particles if the reaction temperature is less than 15C above Tg of the seed copolymer (poly[methyl methacrylate]/poly[methyl acrylate]). Stubbs and Sundberg (2004) observed that, though ionic initiators that are able to anchor the chains of the second-stage poly mer to the particle surface make it more likely to obtain CS morphologies with poly(methyl methacrylate)/poly(methyl acrylate) core and polystyrene shell under some conditions, this effect is not dominant under most conditions.

Based on these results, Stubbs and Sundberg (2008) proposed the deci-sion flowchart shown in Figure 2.2 to be used for morphology prediction of latex particles obtained by emulsion poly mer i za tion. Stubbs and Sundberg (2008) considered three main questions for the prediction of the morphology of composite particles: the first one is whether radicals may penetrate dur-ing the second stage of the synthesis, the second is about phase separation, and the last is related to phase consolidation. The theory of radical penetra-tion considers that in emulsion poly mer i za tion, radicals are typically created in the water phase, and thus enter latex particles at the outer particle surface.

Largeocclusions

Core-shell

Core-shell

1

6

3

Tg2 < TreactionLobed

particleYes

Yes

No

No

Equilibriummorphology 7

13 > 23Penetration

possible?

Phaseseparationpossible? Gradient or

mixed phase

Phaseconsolidation

possible?

Extent largeor small?

NoYes

Yes

Yes

No

Smallocclusions

Small

Large

No

4

2

5

Figure 2.2Adaptation of the decision tree flowchart for predicting morphology development in multi-phase particles proposed by Stubbs and Sundberg (2008). 13: interfacial tension between poly-mer 1 and aqueous phase; 23: interfacial tension between poly mer 2 and aqueous phase; Tg2: glass transition temperature of poly mer 2. (Reprinted from J. M. Stubbs and D. C. Sundberg, Progress in Organic Coatings 61: 15665, 2008. With permission.).


The extent of radical penetration will depend on the effective Tg of the seed poly mer. The effective Tg considers that the particle will be partially swollen with second-stage monomer during the poly mer i za tion and this will lower its glass transition temperature below that of the pure poly mer. When pen-etration is possible, a second-stage poly mer chain will find itself inside the particle and fully entangled with the seed poly mer chains. Phase separation then requires chain diffusion in order to get multiple chains together, and this process may be so slow that phase separation is not possible. The driving force for morphology rearrangement is the minimization of interfacial free energy, and the system will evolve toward the equilibrium morphology if given sufficient time. However, the process of phase consolidation requires an increased extent of poly mer mobility compared to the previous two pro-cesses of oligomeric radical penetration and poly mer phase separation.

2.3 Synthesis of Core-Shell Particles

Particles with core-shell morphologies may be synthesized by a number of heterogeneous poly mer i za tion techniques such as emulsion, miniemulsion, microemulsion, dispersion, and suspension poly mer i za tions. The first three techniques lead to the formation of submicrometric particles (10800 nm), whereas the two last are used, respectively, to prepare small (130 m) and large (501500 m) micrometric particles. Nevertheless, it must be kept in mind that in any of these techniques, the application of a two-stage strategy to build up a shell of the second-stage poly mer onto the core of the first-stage poly mer core will not necessarily lead to the formation of particles with core-shell morphology (Rajatapiti et al. 1997).

In the next sections, the synthesis of particles with core-shell structure by these different techniques will be described. A detailed description of these poly mer i za tion techniques can be found in several excellent books involv-ing emulsion poly mer i za tion (Piirma 1982; Gilbert 1995; Lovell and El-Aasser 1997; Van Herk 2005), as well as in recent book chapters about emulsion (de la Cal et al. 2005; Nomura et al. 2005), miniemulsion (Schork et al. 2005), microemulsion (Chow and Gan 2005), dispersion (Kawagushi and Ito 2005), suspension (Brooks 2005), and heterogeneous (Van Herk and Monteiro 2002) poly mer i za tion techniques.

2.3.1 emulsion Polymerization

Emulsi

advanced nano-particles

Education

scanning electron

transmission

nuclear magnetic

john wiley

swiss federal

heterophase

ical poly

dt poly mer