Miniemulsion Polymerization Technology

Miniemulsion Polymerization Technology Vikas Mittal BASF SE, Polymer Research, Germany Scrivener )WILEY This Page...

Author: Vikas Mittal

62 downloads 1528 Views 19MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!

Report copyright / DMCA form

DOWNLOAD PDF

Miniemulsion Polymerization Technology

Vikas Mittal

BASF SE, Polymer Research, Germany

Scrivener

)WILEY

This Page Intentionally Left Blank

Miniemulsion Polymerization Technology

Scrivener Publishing 3 Winter Street, Suite 3 Salem, MA 01970 Scrivener Publishing Collections Editors James E. R. Couper Richard Erdlac Pradip Khaladkar Norman Lieberman W. Kent Muhlbauer S. A. Sherif

Ken Dragoon Rafiq Islam Vitthal Kulkarni Peter Martin Andrew Y. C. Nee James G. Speight

Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Miniemulsion Polymerization Technology

Vikas Mittal

BASF SE, Polymer Research, Germany

Scrivener

)WILEY

Copyright © 2010 by Scrivener Publishing LLC. All rights reserved. Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., Ill River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. For more information about Scrivener products please visit www.scrivenerpublishing.com. Cover design by Russell Richardson. Library of Congress Cataloging-in-Publication ISBN 978-0-470-62596-5

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Data:

Contents Preface

1

2

3

Miniemulsion Polymerization: An Overview y. Mittal 1.1 Introduction to Polymerization Techniques 1.2 Emulsion and Miniemulsion Polymerization 1.3 Properties of Miniemulsion Polymerization 1.4 Controlled Miniemulsion Polymerization References Multi-Functional Stabilizers in Miniemulsion Polymerization Alain Durand 2.1 Introduction 2.2 Stability of Initial Monomer Droplets 2.3 Stabilizers and Polymerization Processes 2.3.1 Mass-Transfer Processes 2.3.2 Reactive Stabilizers 2.4 Conclusion References Structured Copolymer Particles by Miniemulsion Polymerization V. Mittal 3.1 Introduction 3.2 Styrene-Dodecyl Methacrylate/Stearyl Methacrylate 3.3 n-Butyl Methacrylate-Crosslinking Monomers 3.4 Vinyl Acetate-Butyl Acrylate

xiii

l 2 3 10 19 22

25 25 27 30 30 31 39 39

43 43 46 49 51 v

vi

CONTENTS

3.5

Butyl Acrylate-(2-Methacryloxy)ethyl)trimethyl Ammonium Chloride 3.6 Butyl Acrylate-Methyl Methacrylate-Vinyl Acetate 3.7 Styrene-Acrylic Acid or 2-Aminoethyl Methacrylate Hydrochloride (AEMH) 3.8 Styrene-Butyl Acrylate 3.9 Styrene-Butadiene Rubber 3.10 Fluoroacrylate—LaurylMethylacrylate—Methyl Methacrylate 3.11 Polyurethane-Block-Polystyrene 3.12 Alkyd-Acrylic 3.13 Oil-Acrylate 3.14 Urethane-Acrylic References Encapsulation of Inorganic Nanoparticles by Miniemulsion Polymerization Jacqueline Forcada and Jose Ramos 4.1 Introduction 4.2 Miniemulsion Polymerization in the Presence of Inorganic Nanoparticles 4.2.1 Hydrophobization of Inorganic Nanoparticles 4.2.2 Dispersion of Hydrophobized Inorganic Nanoparticles in Monomer Phase 4.2.3 Miniemulsification of the Lipophilic Dispersion in Water 4.2.4 Polymerization of Droplets 4.3 Encapsulation of Silica Nanoparticles 4.3.1 Miniemulsion Polymerization with Hydrophilic Silica Nanoparticles 4.3.2 Miniemulsion Polymerization with Surface-Modified Silica Nanoparticles 4.3.3 Miniemulsion Polymerization with Locally Surface-Modified Silica Nanoparticles 4.4 Encapsulation of Magnetite Nanoparticles 4.4.1 Encapsulation of Magnetite by a Single Miniemulsion Polymerization Process 4.4.2 Encapsulation of Magnetite by a Double Miniemulsion Polymerization Process

53 54 55 57 57 61 62 63 65 67 68 71 71 73 73 75 75 76 76 77 78 83 85 86 89

CONTENTS

4.5 4.6

Conclusions and Future Perspectives Acknowledgements References

Polymeric Nanocapsules by Interfacial Miniemulsion Polymerization Guo-Rong Shan and Zhi-Hai Cao 5.1 Introduction 5.2 Organic Nanocapsules by Inter facial Miniemulsion Polymerization 5.2.1 Thermodynamic Prediction for the Morphology of Organic Nanocapsules 5.2.2 Particles Morphology of the System without Added NIPAM and DVB 5.2.3 Particles Morphology of the System with DVB 5.2.4 Particle Morphology of the System with Added NIPAM and DVB 5.2.5 Particle Size and Size Distribution in the Process of Polymerization 5.2.6 Mechanism for the Formation of Organic Nanocapsules through Interfacial Miniemulsion Polymerization 5.2.7 Influences on the Formation of Organic Nanocapsules through Interfacial Miniemulsion Polymerization 5.3 Organic-Inorganic Hybrid Nanocapsules by Interfacial Miniemulsion Polymerization 5.3.1 Thermodynamic Analysis and Morphological Prediction 5.3.2 Synthesis of Organic-Inorganic Hybrid Nanocapsules under Neutral Conditions 5.3.3 Synthesis of Organic-Inorganic Hybrid Nanocapsules under Acidic or Basic Conditions 5.3.4 Mechanism Analysis of Organic-Inorganic Hybrid Nanocapsules Formation 5.4 Conclusions References

91 92 92 97 97 99 99 101 103 105 109 112

113 117 117 119

124 134 136 137

viii

CONTENTS

Miniemulsion Polymerization of Vegetable Oil Macromonomers Shelby F. Thames, James W. Razvlins, and Sharathkumar K. Mendon 6.1 Introduction and Background 6.2 Emulsion Polymerization of Alkyds and Vegetable Oils 6.3 (Meth)acrylated Vegetable Oil Derivatives 6.4 Vegetable Oil Macromonomers 6.5 The Potential for Emulsion Polymerization of Model Saturated Monomers 6.6 Nucleation Mechanisms 6.7 Design of Thermosetting Latex Polymers 6.8 Classifying Monomer Solubility for Macro and Miniemulsion Polymerization 6.9 Soybean Acrylated Monomer Synthesis 6.10 Miniemulsion Polymerization 6.11 Conclusions References Controlled/Living Radical Polymerization in Aqueous Miniemulsion Catherine Lefay, and Julien Nicolas 7.1 Introduction 7.2 Controlled/Living Radical Polymerization in Bulk/Solution: General Considerations 7.2.1 CLRP Based on Reversible Termination 7.2.1.1 Nitroxide-Mediated Polymerization (NMP) 176 7.2.1.2 Atom Transfer Radical Polymerization (ATRP) 177 7.2.2 CLRP Based on Degenerative Transfer 7.2.2.1 Reversible Addition-Fragmentation Chain Transfer (RAFT) 7.2.2.2 Iodine Transfer Polymerization (ITP) 7.3 Nitroxide-Mediated Miniemulsion Polymerization 7.3.1 Oil-Soluble Bicomponent Initiating System 7.3.2 Water-Soluble Bicomponent Initiating System

139

139 143 145 146 150 152 154 158 160 160 168 169 173 174 174 175

178 179 180 182 183 185

CONTENTS

7.3.3 7.3.4

Oil-Soluble Monocomponent Initiating System Water-Soluble Monocomponent Initiating System 7.4 Atom Transfer Radical Miniemulsion Polymerization 7.4.1 Direct ATRP 7.4.2 Reverse ATRP 7.4.3 Simultaneous Reverse and Normal Initiation (SR&NI) ATRP 7.4.4 Activators Generated by Electron Transfer (AGET) ATRP 7.5 Reversible Addition-Fragmentation Chain Transfer Miniemulsion Polymerization 7.5.1 Key-Steps for the Success of RAFT Miniemulsion Polymerization 7.5.1.1 Inhibition and Retardation 7.5.1.2 Colloidal Instability 7.5.1.3 Livingness and Controlled Polymerization 7.5.2 RAFT Miniemulsion Polymerization of Vinyl Acetate 7.5.3 Nanocapsules Synthesized by RAFT Miniemulsion Polymerization 7.6 Iodine Transfer Polymerization in Miniemulsion 7.7 Conclusion References Inverse Miniemulsion Polymerization of Unsaturated Monomers Ignác Capek 8.1 Introduction 8.2 General 8.3 Kinetic Studies 8.4 Traditional and Nonconventional Inverse Latexes 8.4.1 Water Soluble Monomers 8.4.2 Hydrophobie Monomers 8.5 Controlled Radical Miniemulsion Polymerization 8.6 Amphiphilic and Associating Copolymers 8.7 Conclusion

ix

186 188 188 190 190 192 192 193 194 194 196 198 199 200 201 202 203 211 211 215 218 221 221 230 232 237 240

x

CONTENTS

8.8

9

Acknowledgements Abbreviations References

Double Miniemulsion Preparation for Hybrid Latexes R.Y. Hong, G. Liu, B. Feng, and H.Z. Li 9.1 Introduction 9.2 Hybrids via Mini-Emulsion Polymerization 9.3 Double-Miniemulsion Formation 9.4 Stability 9.5 Characterization 9.6 Applications 9.6.1 Effects of Reaction Conditions 9.6.1.1 Initiator Dosage 9.6.1.2 MMA Monomer Concentration 9.6.2 Rheological Properties of Magnetic Emulsion 9.6.2.1 Viscosity Versus Time 9.6.2.2 Viscosity with/without Magnetic Field 9.6.2.3 Applications of Magnetic Polymer Microspheres 9.7 Summary 9.8 Acknowledgments References

10 Surfactant Effect in Miniemulsion Polymerization for Biodegradable Latexes V. Soldi, B.G. Zanetti-Ramos, and E. Minatti 10.1 Introduction 10.2 Miniemulsion Polymerization of Biodegradable Latexes 10.3 Mechanisms of Surfactant Protection of Colloidal Dispersions 10.3.1 General Behavior of a Surfactant Molecule at the Interface 10.3.2 Mechanism 1: Lowering the Interfacial Tension 10.3.3 Mechanism 2: Electrostatic Stabilization 10.3.4 Mechanism 3: Steric Stabilization

244 244 246 251 252 253 255 255 257 261 266 266 267 268 268 269 270 271 272 272 277 277 278 282 282 284 286 288

CONTENTS

4

5

Effect of Surfactants on Miniemulsion Polymerization 10.4.1 Effect of Surfactant Type on the Particle Size and Latex Yield 10.4.2 Effect of Surfactant Concentration on Particle Size and Latex Yield 10.4.3 Effect of Surfactant on the Stability Final Remarks References

xi

291 291 294 297 298 299 303

This Page Intentionally Left Blank

Preface Miniemulsion polymerization provides many distinct advantages over the conventional emulsion polymerization because the monomer droplets are directly polymerized, whereas in the case of emulsion polymerization, the monomer is polymerized in the micelles and needs to travel through the aqueous phase. The polymerization of very hydrophobic monomers is thus difficult in the case of emulsion polymerization owing to limited diffusion through the aqueous phase. The miniemulsion polymerization does not suffer from these limitations and can lead to the polymerization of very hydrophobic as well as very hydrophilic monomers. The system parameters in miniemulsion polymerization include the choice of surfactant, co-surfactant, agitation speed, and by careful control on these parameters one can achieve efficient control on the polymerization reaction. The miniemulsion polymerization is also well suited for the various controlled living polymerization reactions like nitroxide mediated polymerization, atom transfer radical polymerization, as well as radical addition fragmentation chain transfer. Apart from that, the miniemulsion polymerization is also beneficial for the synthesis of encapsulated particles as well as core-shell particles to generate advanced polymer nanoparticles. There have been a large number of texts on emulsion and other forms of polymerization methods, but miniemulsion polymerization, though it provides unique routes for polymer particle synthesis, has been neglected. This book summarizes the recent advances in miniemulsion polymerization technology including the advances on the selection of surfactants and co-surfactants, the expansion of miniemulsion technology in various polymers and co-polymer systems, and the use of miniemulsion polymerization for the synthesis of advanced polymer particle morphologies. Chapter 1 provides the background information on the kinetics and mechanism of xm

xiv

PREFACE

miniemulsion polymerization technology as well as its comparison with the conventional emulsion polymerization. The importance of the selection of co-surfactants, initiators and surfactants is drawn out. Chapter 2 summarizes the advances in the multi-functional surfactants, i.e. the stabilizers which can be involved in other aspects of polymerization in addition to their function as stabilizers. Chapter 3 details the synthesis of structured copolymer particles by using miniemulsion polymerization. A large number of polymer and copolymer systems are demonstrated. Chapter 4 deals with the synthesis of the organic-inorganic hybrid particles by the encapsulation of silica and magnetite nanoparticles by polymers. Chapter 5 describes the synthesis of polymeric nanocapsules by interfacial miniemulsion polymerization. Miniemulsion polymerization is well suited for the polymerization of hydrophobic monomers, especially vegetable oil macromonomers that are too hydrophobic to traverse the aqueous phase during conventional emulsion polymerization. Such a system is described in Chapter 6. General considerations about four main CLRP techniques, namely nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), and iodine transfer polymerization (ITP) are given along with the review of their application in aqueous miniemulsion in Chapter 7. Chapter 8 focuses on the inverse miniemulsion polymerization of unsaturated monomers. Chapter 9 describes the double miniemulsion processes for the synthesis of organic-organic hybrid latexes. Chapter 10 focuses in general on the synthesis of the biodegradable latexes and specifically on the surfactant effects in these latexes. I would like to take this opportunity to express my heartfelt thanks to Scrivener Publishing for agreeing to publish the book. The contribution of my family especially my mother is to be mentioned as the book would not have been possible without it. Many thanks to my wife Preeti for co-editing the book as well as for many ideas for the better presentation of the information contained in the book. Vikas MITTAL Ludwigshafen.

1 Miniemulsion Polymerization: An Overview* V. Mittal BASF SE, Polymer Research, 67056 Ludwigshafen, Germany

Abstract

Miniemulsion polymerization exhibits distinct advantages over the conventional emulsion polymerization. The polymerization of monomer droplets is directly achieved when the diffusion of the monomer from the droplets to polymer particles is not required. This helps to polymerize water insoluble monomers. It also allows the presence of various system components like initiators, costabilizers, etc. directly at the site of the polymerization thus allowing better control. Conventionally, volatile hydrophobes or costabilizers like cetyl alcohol or hexadecane have been used. A number of advances have been reported in recent years on the use of costabilizers which are more compatible to the polymerization system. Use of polymers as costabilizers was reported to be very effective even though the polymer forms a poor costabilizer. Similarly, the comonomers and initiator have also been used as costabilizers. The use of chain transfer agents as costabilizers also opens the opportunities for molecular weight control in the polymer particles. These advances ensure that the particles are free from any low molecular weight impurity or volatile components. The living polymerization techniques like nitroxide mediated polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer are also well suited for miniemulsion processes in order to generate specific morphologies in polymer particles and to control the molecular weight and its distribution in the particles. Keywords: diffusion, monomer droplet, costabilizer, surfactant, initiator, micelles, chain transfer, comonomer, miniemulsion, conversion, colloidal stability, controlled living polymerization.

•This review work was carried out at Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland. V. Mittal (ed.) Miniemulsion Polymerization Technology, (1-23) © Scrivener Publishing LLC

1

2

1.1

MINIEMULSION POLYMERIZATION TECHNOLOGY

Introduction to Polymerization Techniques

Free radical polymerization can be carried out by using a number of different techniques. The simplest of these techniques is the bulk polymerization. In this technique, monomer is in the liquid form and the generated polymer is in solid form. Though the reaction mixture is free from any unwanted impurities or contamination leading to clean polymer, however, the viscosity of the system increases significantly due to the generation of polymer chains during the course of polymerization and thus the mixing of the system becomes extremely difficult leading to very broad molecular weight distributions in the polymer chains. Additionally, the polymer chains do no diffuse freely in the highly viscous medium leading to the accumulation of radicals at particular sites causing the polymerization rate to increase exponentially. Solution polymerization is an alternative method in which a solvent is used in which the monomer and polymer are soluble. The use of solvents eliminates the problems of higher viscosity and heat dissipation associated with bulk polymerization and allows one to stir the reaction medium easily. However, the choice of solvents must be proper; otherwise extensive chain transfer to solvent can take place resulting in only low molecular weight polymer chains. Precipitation polymerization is another form of polymerization in which the polymer is not soluble in the monomer or the reaction medium and precipitates out soon from the solution. Thus, precipitation polymerization starts as homogenous polymerization, but is soon turns into a heterogeneous polymerization. Dispersion polymerization is also similar to precipitation polymerization that the polymer formed is not soluble in monomer or organic solvent. After the formation of polymer particles, these particles are stabilized by added particle stabilizer and the polymerization proceeds in the polymer particles by the absorption of monomer into the polymer particles. Suspension or bead polymerization is a method in which monomer droplets are directly polymerized to generate high molecular weight polymers. In this method, water insoluble monomer is suspended in water with the aid of suspension stabilizers. The initiator used is also water insoluble or monomer soluble. The size of monomer droplets can be controlled depending on the ratio of monomer to the dispersion medium, i.e., water, the speed of agitation to generate droplets as well as by the amount of stabilizing agents.

MINIEMULSION POLYMERIZATION

3

During the polymerization, the monomer droplets polymerize independently and each droplet can be visualized as a bulk polymerization happening inside the droplet. Inverse suspension polymerization is also possible in which a water soluble monomer is used and its droplets are generated in an organic solvent. Initiators used are also water soluble and the monomer droplets are stabilized similarly by using suspension stabilizers. Emulsion polymerization is the one of the most versatile technique to generate small particles. With this technique, water insoluble monomers are polymerized by suspending them in water in the form of emulsion droplets stabilized by surfactants. The initiators used are water soluble in contrast to suspension polymerization where water insoluble initiators are used, the most common being potassium persulphate (KPS). Polymerization of extremely low water soluble monomers is very difficult with conventional emulsion polymerization. The low solubility of the monomer would not allow its diffusion to the polymer particles through the aqueous medium. Miniemulsion polymerization has been developed for such purposes, in which the monomer droplets generated by using high shear in the presence of an ionic surfactant and a co-surfactant or hydrophobe like hexadecane, are directly polymerized. The droplets and hence resulting polymer particles are generally in the size range of 50-500 nm. There are other forms of polymerization techniques like microemulsion, melt polycondensation and solution polycondensation etc. Figure 1.1 lists the large number of polymerization techniques used to synthesize a variety of polymers [1].

1.2

Emulsion and Miniemulsion Polymerization

The common mode of particle synthesis in emulsion polymerization is achieved by micellar nucleation method, though there is also the presence of homogenous nucleation especially in water soluble monomers. The surfactants like sodium dodecyl sulphate are added which at a concentration higher than the critical micelle concentration in the aqueous phase form micelles. These micelles owing to their hydrophobic nature inside the inner space are an ideal site for the radical entry as well as propagation of polymerization. The structure of surfactant is generally amphiphilic, with one part hydrophobic and the other part hydrophilic. These

4

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Solid & gas phase polymerization Bulk polymerization

Solution polymerization

Inverse suspension polymerization . M, , E m u i s i o n \ / Dispersion . Suspension \Upolymer,zat,onj L o l mer¡zat¡ ».polymerization/1 v / v ' /

Melt . polycondensationi

Polymerization techniques

Solution vpolycondensationy Interfacial . polymerization y

Inverse emulsion polymerization,

Miniemulsion polymerization ,

fMicro-suspension polymerization

Microemulsion .polymerization,

Precipitation polymerization^

Figure 1.1. Various polymerization techniques to generate a wide spectrum of polymers. Reproduced with permission from Nova Science Publishers [1].

molecules thus arrange themselves in a way that their hydrophilic parts are in interface with water. Every surfactant has a different critical micelle concentration value and it should be considered carefully while using different kinds of surfactants. The micelles generally have a size of 10 nm and generally 100-200 surfactant molecules form a micelle [2,3]· It is generally known that the surface tension of the solution decreases with the addition of surfactant at critical micelle concentration. However, it is not only the surface tension that is affected by the surfactant, rather a host of other properties of the solution are affected at critical micelle concentration. Once the monomer is added to the system, a small amount of monomer enters the micelles and some gets dissolved in the aqueous phase owing to the partial solubility in water. However, the majority of the monomer is generally present in the form of monomer droplets. These droplets are stabilized by the adsorption of surfactant molecules on the surface. The number of micelles is much

MINIEMULSION POLYMERIZATION

5

larger than the number of droplets and the droplet size may fall in the range of tens of micrometers [2,4]· When the polymerization is initiated by the addition of the initiator and after achieving the required polymerization temperature, the radicals are generated in the aqueous phase. The generated radicals have two possibilities to propagate further: to enter either the micelles or the monomer droplets. However, the experimental studies report that it is very rare that the radicals enter the monomer droplets. This is because of very large number of micelles present in the system as well as the architecture of the micelles provides ideal conditions for the monomer polymerization. When the radicals enter the micelles and start polymerizing the monomer contained in these micelles, the polymer particles form. These growing polymer particles are then supplied by the monomer molecules from the monomer droplets by diffusion through the aqueous phase. The termination of the radicals is quite slow as at a particular time during polymerization, there is rarely more than one radical per particle. The conventional emulsion polymerization is thus divided into three intervals as shown in Figure 1.2. On addition to the aqueous phase, the monomer enters the micelles as well as forms the monomer droplets apart from dissolution in water to some extent based on the solubility of the monomer as shown in Figure 1.2a [2]. The first interval, also termed as particles formation phase, is then initiated. The radicals re generated in the aqueous phase due to the decomposition of initiator. These radicals enter the micelles and initiate monomer polymerization leading to the generation of polymer particles. The number of particles keeps on increasing in this interval which also results in the continuous enhancement in the polymerization rate. The system, as shown in Figure 1.2b, thus consists of polymer particles, monomer droplets, and the inactive micelles. The particles keep on increasing in size, thus requiring more and more surfactant to stabilize the increasing surface area. This leads to the adsorption of the dissolved surfactant in the aqueous phase on the surface of the particles and the surfactant concentration thus falls much below the critical micelle concentration. This results into the destabilization of the remaining micelles and they provide their surfactant to stabilize the growing particles. The number of particles generated from total micelles in the beginning is generally in the range of 0.1%. At the end of first interval, all of the micelles either are polymer particles or are destabilized to lose the surfactant. In the second interval, the particles keep on growing in size and no new particles are nucleated

6

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 1.2. Representation of various intervals of emulsion polymerization [1].

thus leading to the constant rate of polymerization. As the particles grow in size during the course of polymerization, they deplete the monomer content present in them. This depletion is continuously replenished by the absorption of more monomer from the water phase, which has been dissolved in it. The water phase in return absorbs more monomer from the monomer droplets resulting in the reduction of the size of the monomer droplets as shown in Figure 1.2c. After a certain conversion of the monomer is achieved, the monomer droplets also disappear which forms the transition period between the second and third interval. As shown in Figure 1.2d, the particles in this interval keep on polymerizing the monomer enriching them. Thus, concentration of the monomer in the particles decreases, and subsequently the polymerization rate also decreases in this interval. The number of particles thus also remains the same as the second interval and after the monomer has been completely depleted, the polymerization rate climbs down to zero.

MINIEMULSION POLYMERIZATION

7

Miniemulsion polymerization exhibits several advantages over conventional emulsion polymerization also called macroemulsion polymerization [5-8]. The most prominent advantage is the elimination of the need of the monomer to diffuse through the aqueous phase from the monomer droplets to the polymer particles during the course of polymerization. It is due to the reason that the monomer droplets are directly polymerized in this mode of polymerization. The monomer droplets are generated by shearing the system with high energy along with the addition of costabilizer (with the surfactant) which is needed 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 polymerization, it is of importance to avoid the micellar nucleation, therefore, the amount of surfactant is below the critical micelle concentration. Thus, miniemulsion polymerization differs from the macroemulsion polymerization significantly in the mechanism of particle nucleation. In fact, this difference also acts as an advantage of miniemulsion polymerization as micellar nucleation in conventional emulsion polymerization is extremely sensitive to a large number of factors such as amount of surfactant, amount of initiator, agitation speed, temperature of the polymerization reaction, mode of addition of the monomers, etc. The number of particles in miniemulsion polymerization is thus dependant on the shearing forces as well as the amount of surfactant and costabilizer, and is independent on the initiator amount. A significant advancement has been achieved in the living polymerization methods in miniemulsion polymerization. The colloidal stability is also much better in miniemulsion polymerization as compared to the conventional emulsion polymerization, which makes it a technique of choice. Figure 1.3 demonstrates the mechanism of miniemulsion polymerization [1]. The costabilizer and the surfactant are added along with monomer in the aqueous medium. The miniemulsion is then achieved by the action of shear. The application of shear breaks the bigger monomer droplets into the droplets of size range 10-500 nm which also forms the range of polymer particles generated by miniemulsion polymerization. However, as mentioned above, the size of the monomer droplets and hence polymer particles can be tuned by the amount of surfactant and costabilizer in combination with shearing forces. The surfactant is required in the system to eliminate the droplet coalescence by the action of Brownian motion or settling, whereas the costabilizer prevents the

8

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 1.3. Process of miniemulsion polymerization. The molecules with open and filled circles represent the surfactant and costabilizer, respectively.

Ostwald ripening [5]. When the emulsion is subjected to shear by sonicator or mechanical homogenizer, the generation of small droplets is achieved in the liquid medium. As the droplets have a distribution is the sizes, the monomer tends to diffuse from the smaller droplets into the large ones if the monomer is even slightly soluble in the continuous phase. The surface area of the monomer droplets is quite high and most of the surfactant is adsorbed on the particle surface. As no micelles exist in the system therefore the particle nucleation takes place by radical entry into the droplets. The initiators used for the miniemulsion polymerization can be both water soluble as well as monomer soluble. Figure 1.4 is another representation of the comparison between the emulsion and miniemulsion polymerization processes.

MINIEMULSION POLYMERIZATION

9

Figure 1.4. Comparison of (a) emulsion and (b) miniemulsion polymerization.

Owing to the differences in the mechanism of particle nucleation as well as propagation, the rates of polymerization in both macroemulsion and miniemulsion polymerization are also different. As described above, the conventional emulsion polymerization has initiation of polymer particles in micelles followed by the diffusion of monomer from the monomer droplets to the polymer particles though the aqueous phase. The polymerization rate, therefore, first grows till the micelles are present in the system owing to the increase in the number of the particles. Subsequently, the rate of polymerization becomes constant as the polymer particles grow only in size and not in number owing to the use of all the surfactant to stabilize the polymer particles. The monomer in this interval is diffusing continuously to the polymer particles and is, therefore, getting depleted in the monomer droplets. When the droplets cease to exist, the rate starts to fall and becomes zero when all the monomer enriching the polymer particles is also consumed. However, in the case of miniemulsion polymerization, there is no diffusion of the monomer through the aqueous phase owing to direct polymerization of monomer droplets, therefore, there is no constant polymerization rate period in this mode of polymerization. This is, however, only true if the monomer is not diffusing from the small droplets to the large ones even in small extents. The rate initially grows owing to the increasing number of polymer particles by the entry of radicals in the droplets, and then drops down as the monomer in the droplets is depleted. One limitation of miniemulsion polymerization is

10

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

(A)

(B)

Figure 1.5. SEM micrographs of polystyrene latex generated by miniemulsion polymerization.

the use of hydrophobe which can be volatile in nature, the use of which thus the limits the applications of the polymer particles generated by miniemulsion. Figure 1.5 shows the representative scanning electron microscopy (SEM) micrographs of polystyrene particles synthesized with miniemulsion polymerization. The miniemulsion polymerization of hydrophobic monomers in the aqueous phase is achieved owing to no or insignificant dissolution of monomer in the aqueous phase which leads to the droplet stability in the system. However, if the monomers are water soluble, one can use inverse miniemulsion polymerization. Here, a hydrophobic reaction solvent or dispersion medium like cyclohexane is used instead of water, and the process is exactly similar to miniemulsion polymerization. Instead of a hydrophobe as a costabilizer, one must use a lipophobe such as sodium chloride, and the stabilizer is also different.

1.3

Properties of Miniemulsion Polymerization

If the Ostwald ripening is allowed to occur continuously, the monomer from the smaller particles would diffuse into the larger particles and extensive creaming would result. Costabilizers are therefore added as they help to stop the Ostwald ripening by stopping the diffusion of the monomers from monomer droplets. Therefore, they

MINIEMULSION POLYMERIZATION

11

should be very hydrophobic in nature and should be soluble in the monomers. As the costabilizers generally have a little water solubility therefore the Ostwald ripening still occurs but the time required for the destabilization of the miniemulsion runs in the range of months which thus allows enough time to achieve polymerization using stable miniemulsions. As mentioned earlier, the miniemulsions are generated by the combination of the high shear to break the bigger monomer droplets into the sub-micron monomer droplets in the presence of costabilizer to stop the diffusion of monomer from these particles. One must be clear that the addition of a costabilizer stops the conversion of a miniemulsion into a conventional emulsion; however, the addition of a costabilizer to conventional emulsion does not automatically convert it into a miniemulsion. It is only after addition of high shearing energy that it becomes a stable miniemulsion. The mechanical shear is generated by stirring, ultra-turrax or by ultrasonication. The mechanism of ultrasonication is primarily cavitation. Sonication is an attractive method for the laboratory scale miniemulsion generation, however, it is not suitable for the large scale processes and more efficient sharing devices are needed. In most of the reported studies over the use of miniemulsion polymerization, the use of anionic surfactant is most common. Sodium dodecyl sulphate is one the most commonly used anionic surfactant used for emulsion polymerizations. Therefore, it has also been automatically used for the majority of the miniemulsion polymerization reactions. These anionic surfactants are also attractive choices owing to their compatibility with the majority of the monomers and the initiators. However, the use of anionic surfactants is not suitable in controlled living polymerization achieved by the atom transfer radical polymerization and in such cases, non-ionic surfactants are mostly used. However, some studies have also reported the use of cationic surfactants like cetyltrimethylammonium bromide and dodecyltrimethylammonium chloride [9,10] and the resulting particles were reported to be similar to the particles achieved by using anionic surfactants. Some studies have also reported the use of nonionic surfactants. Wang et al. reported the use of poly (vinyl alcohol) as stabilizer with hexadecane as co-stabilizer [11]. The authors also reported that the use of hexadecane costabilizer was important as the use of poly(vinyl alcohol) was not sufficient to control the polymerization reaction. The costabilizer is conventionally required to be monomer soluble, water insoluble and with a low molecular weight. The insolubility

12

MINIEMULSION POLYMERIZATION TECHNOLOGY

in water leads to the elimination of the diffusion of monomer into the aqueous phase from the monomer droplets. The low molecular weight, on the other hand, allows to increase the weight ratio of costabilizer molecules as compared to monomer molecules in the monomer droplets. In the early studies cetyl alcohol and hexadecane have been used as costabilizers as they fit well to the characteristics required from an ideal costabilizer. However, these costabilizers are volatile in nature and their presence in the product may not be desirable for a number of applications. To circumvent these limitations, a number of studies have reported the use of various costabilizers which are not volatile in nature and help to achieve better acceptability of the polymer particles. Reimers et al. [12] reported the use of polymer as a costabilizer. The authors reported that using the polymer which is soluble in its own monomer would also fulfill the requirements of the costabilizer. The high molecular weight of the polymer was reported to make the polymer as poor costabilizer, but still the use of polymer as costabilizer was reported to reduce the diffusion of the monomer from the monomer droplets owing to high water insolubility of the polymer. The miniemulsion in such cases were reported to be thermodynamically unstable, but kinetically stable which still allowed the miniemulsions to be stable for the time scale suitable for the polymerization. The miniemulsions generated by using polymer as costabilizer were observed to be not true miniemulsion as they did get destabilized after a period of time, however, the polymer particles generated from these systems were similar to the systems where cetyl alcohol or hexadecane were used as costabilizers. This, therefore, completely eliminates the use of volatile hexadecane or other low molecular weight costabilizers in these miniemulsion polymerizations. The low molecular weight components are not desirable in the final latex, as these can easily migrate to other materials owing to their low molecular weight, thus causing health and safety concerns. Reimers et al. [12-13] reported on the effect of the amount of polymeric hydrophobe and its molecular weight on the generated size and size distribution of the monomer droplets. It was reported that the droplets diameters could range between 19.5 nm to 141.2 nm using polymeric costabilizer. These values were reported to be similar when the miniemulsion had hexadecane as co-stabilizer. The size of the droplets was generally observed to decrease on increasing the concentration of hexadecane, such phenomenon was also observed when polymer stabilizers were used. Though polymeric

MINIEMULSION POLYMERIZATION

13

costabilizer acted only as a poor costabilizer, the generated latexes with polymeric costabilizer were observed to have much lower polydispersity of 1.006 as compared to the 1.049 for the emulsion polymerization and 1.037 for the alkane stabilized miniemulsion. As the solubility parameter of PMMA is 19.3 MPa 1/2 , which is very similar to 18.9 MPa 1/2 value for the MMA monomer, therefore, indicating, using polymers as co-stabilizers in the polymerizations of its monomers ensures its solubility with monomer and elimination of diffusional degradation of monomer droplets owing to the water insolubility. Similarly, other monomer/polymer systems were reported to be effective. The polymeric costabilizers were also reported to be stable against the presence of small amount of inhibitors, retarders or other monomer impurities, which is generally not the case for low molecular weight co-stabilizers. Other similar studies have also been reported [14]. It was also reported that the used of a comonomer can also be employed to act as costabilizer. The comonomer can also subsequently get polymerized along with the monomer during the course of polymerization. [15]. Vinyl hexanoate, p-methyl styrene, vinyl stéarate etc. were used as comonomers with MMA and the reported system had stable miniemulsions with droplets diameters between 150 and 230 nm. The advantage of the comonomer as stabilizer is that it would reinforce the polymer chains, thus removing any concerns regarding the diffusion of low molecular weight costabilizer molecules out of the particles. Dodecyl methacrylate was also reported as a comonomer as well as costabilizer in the miniemulsion polymerization of styrene [16]. Figure 1.6 demonstrates the conversion as well as diameter evolution in the miniemulsion particles as a function of time as well as the concentration of costabilizer. Low water solubility of the costabilizer and better solubility with styrene monomer helped to achieve stable miniemulsions. Use of imitator has also been similarly reported to act also as costabilizer. Schork et al. [17] reported the use of lauroyl peroxide as initiator as well as costabilizer. The molecules first act as costabilizers helping to stabilize the monomer droplets and subsequently during the course of polymerization yield radicals by the thermal decomposition. The droplet sizes were also reported to be in the similar range as observed in miniemulsion polymerization and the ratio of polymer particles to the monomer droplets was also observed to be near unity thus confirming the efficiency of lauroyl peroxide molecules to act both as initiator as well as monomer costabilizer.

14

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 1.6. Use of comonomer as a costabilizer [15]. Monomer conversion and (b) average particle size as a function of the reaction time: [DMA]. (Δ) 5; (O) 20; (*) 40 mM. Reproduced from reference 16 with permission from Elsevier.

In this process, similar to the use of comonomer as costabilizer, the initiator moieties are chemically incorporated in the polymer chains, thus leaving no low molecular weight residue in the polymer particles.

MINIEMULSION POLYMERIZATION

15

The use of chain transfer agent as costabilizer was reported to open opportunities for molecular weight control [5]. The chain transfer agent is generally difficult to transport to the polymer particles from the monomer droplets in the conventional emulsion polymerization, however, when the chain transfer agent is present at the site of polymerization as is possible in the case of miniemulsion polymerization, various possibilities of control of polymer reaction can be achieved. Dodecylmercaptan was reported to be used as a chain transfer agent as well as costabilizer for the miniemulsion polymerization of methyl methacrylate [18]. Sodium lauryl sulfate was used as surfactant and potassium persulphate as initiator. Stable monomer droplets were observed to form and the nucleation of the polymer particles proceeded by droplet polymerization owing to the presence of surfactant below the critical micelle concentration. When the surfactant concentration was held constant, the size of the droplets was observed to decrease on increasing the concentration of dodecylmercaptan, which is similar to the behavior as observed in the miniemulsion polymerization using hexadecane as costabilizer. The rate of polymerization was observed to be dependant upon factors such as the concentration of surfactant, co-surfactant as well as initiator. The authors reported that at low concentrations of initiator and surfactant (below CMC), the ratio of number of particles to number of droplets was 0.80 indicating that all the droplets were not nucleated. On the other hand, at higher concentrations of surfactant and initiator, the ratio was 13.56 indicting that nucleation of the polymer particles occurred not only by droplet nucleation but also by micellar nucleation. The surfactant concentration below CMC and initiator concentration at intermediate levels was observed to result a ratio near to unity indicating that the droplet nucleation resulted in the formation of polymer particles. It was observed by the authors that the value of chain transfer coefficient for the system of dodecylmercaptan as chain transfer agent and methyl methacrylate as monomer was in the range 0.6-0.8, which indicated that the chain transfer agent reacted only slightly more than the monomer thus ensuring its presence and effect throughout the course of polymerization. On the other hand, in the system with dodecylmercaptan as chain transfer agent and styrene as monomer, the chain transfer coefficient lies in the range of 15-20, indicating that it would be consumed very early in the polymerization reaction. Thus, its role as costabilizer would end very shortly in the polymerization

16

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

reaction leading to the Ostwald ripening of the smaller droplets into the larger ones subsequently resulting in the collapse of miniemulsion. Figure 1.7 lists various costabilizers reported in literature for miniemulsion polymerization [1]. Majority of the miniemulsion polymerizations have used the water soluble initiators. However, some studies also reported the use of oil soluble initiators for the polymerization of monomer droplets. Alduncin et al. [19,20] reported the use of lauroyl peroxide (LPO), benzoyl peroxide (BPO) and azobis(isobutyronitrile) (AIBN) initiators for the miniemulsion polymerization of styrene. Two different types of polymerization reactions were carried out. In the first case, o

o

CH 3 — (CH2)10 — C — O — O — C —(CH 2 ) 1 0 —CH 3 CH3 CH3 —(CH 2 ) 1 5 —OH

I

CH2=CH

Cl6H34

O

II

CH2 = C H — O ^ C — ( C H 2 ) 4 ^ C H 3

H 3 C—sr / o \

^s¡—CH 3 \ o /

H3C—Si^0/Si—CH3

-CH 2 -

OHo

Orlo

i ;

i

COOCHH3 COOCHH3

UH-i

i COOCHH3

— CH 2 — CH-[-CH 2 — CH j - C H 2 — CH —

) 6" ό

Figure 1.7. Various co-stabilizers used in the miniemulsion polymerization process. Reproduced with permission from Nova Science Publishers.

MINIEMULSION POLYMERIZATION

17

the initiator also acted as costabilizer so that no additional costabilizer was added to the polymerization medium. The second case was more conventional miniemulsion polymerization where hexadecane was used. Figure 1.8 demonstrates the time evolution of the droplet diameters. It was observed that the miniemulsions containing LPO had droplet sizes similar to those of the classical miniemulsion polymerizations and the size of these miniemulsions remained roughly constant for more than two hours after the sonication. The average size of the miniemulsions containing BPO and AIBN was, on the other hand, very large suggesting that these miniemulsions suffered a quick partial degradation after sonication. In the presence of hexadecane, however, the size distributions were almost similar irrespective of any initiator used. BPO and AIBN, on their own, are not water insoluble enough to avoid the Ostwald ripening of small particles. Figure 1.9 also represents the mechanism by which the three initiator systems work in the absence and presence of hexadecane. Figure 1.10 also represents the time evolution of conversion of the monomer when oil soluble initiator lauroyl peroxide was used [20]. As can be seen, the polymerization rate was not affected by the presence of hexadecane also confirmed by the particle size analysis. The conventional emulsion polymerization was also carried out for

Figure 1.8. Time evolution of the droplet diameter for the different miniemulsions. (O) LPO, (·) LPO+HD, (Δ) BPO, (A) BPO+HD, (D) AIBN, (■) AIBN+HD. Reproduced from reference 19 with permission from American Chemical Society.

18

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Figure 1.9. Schematic of various processes occurring during the miniemulsion polymerization with different oil soluble initiators. Reproduced from reference 19 with permission from American Chemical Society.

Figure 1.10. Time evolution of the conversion in the polymerizations initiated by LPO: (O) LPO, (·) LPO+HD and (Δ) conventional emulsion polymerization. Reproduced from reference 20 with permission from Elsevier.

MINIEMULSION POLYMERIZATION

19

comparison and it was reported that the polymerization could not reach full conversion owing to the extensive coagulation.

1.4

Controlled Miniemulsion Polymerization

Miniemulsion polymerization in the recent years has also been performed in the controlled living conditions in order to synthesize particles with specific morphologies or in order to control the molecular weight and its distribution [21-30]. Various living polymerization methods like nitroxide mediated polymerization, atom transfer radical polymerization as well as radical addition fragmentation chain transfer have been applied successfully to polymerization processes in miniemulsion. The schematic of nitroxide mediated polymerization is shown in Scheme 1.1 (Relation 1). The technique is based on the reversible termination of the radicals where the radicals are made inactive by reaction with other radicals but only temporarily. The species end capped by the nitroxide, as shown in Scheme 1.1, are termed as dormant species and they exhibit reversible dormant and active

—Pn-X

,——

-~Pn°

+ X°

(1)

Scheme 1.1. Representation of controlled polymerization. Reproduced from reference 31 with permission from American Chemical Society.

20

MINIEMULSION POLYMERIZATION TECHNOLOGY

behavior throughout the course of polymerization. Termination reactions do take place, but the extent of termination is greatly reduced. The emulsion polymerization required the diffusion of the nitroxodes from the aqueous phase to the polymer particles, which is not straight forward. However, in the case of miniemulsion polymerization, the diffusion of the control radicals is not required as these can be directly obtained in the monomer droplets, i.e. at the site of polymerization thus simplifying the process. Different nitroxodes have been reported in literature for different monomer systems like TEMPO and SGI as shown in equations 1 and 2 respectively. Apart from that, alkoxyamines and nitroxide terminated oligomers are also used as unimolecular systems, where the molecule acts both as nitroxide as well as initiator. Atom transfer radical polymerization (ATRP) is also based on reversible termination approach to achieve living polymerization.

M I N I E M U L S I O N POLYMERIZATION

21

The process involves an organic halide which reversibly terminates the polymer chains and generates reversibly active chains and the redox process is catalyzed by a transition metal compound such as cuprous chloride or bromide complexed with a ligand. Scheme 1.1 (Relation 2) shows the schematic of ATRP process. The limitation of ATRP process is the presence of transition metal compounds in the end product. Another limitation of this method is the possible interaction of copper compounds with the emulsifiers used in emulsion polymerization. The polymerization in emulsion phase can though work when no surfactant is used in the system or non ionic surfactants are used. Reversible addition fragmentation chain transfer (RAFT) is another form of controlled polymerization (Scheme 1.1, Relation 3)

Scheme 1.2. Systematic approached to synthesize hybrid polymers. Reproduced from reference 31 with permission from American Chemical Society.

22

MINIEMULSION POLYMERIZATION TECHNOLOGY

which operates by reversible transfer mode thus not similar to the reversible termination performed in ATRP or nitroxide mediated polymerization. The core of this process is a RAFT agent which contains dithioester groups. The living polymerization takes place as the transferred end group in the polymeric dithioester is as labile as the dithioester group in the starting RAFT agent. The disadvantage of the techniques operating by reversible termination is the partitioning of the deactivating species in the aqueous phase as well as organic phase. It complicates the concentration of active and dormant species in the polymer particles. However, the techniques based on reversible transfer do not suffer from these disadvantages, as the number of free propagating radicals in these polymerization methodologies practically remains unchanged. The initiator for the polymerization can be the conventional initiators like AIBN or benzoyl peroxide. Important advantage of this technique is the possibility of polymerization reaction to be carried out at lower temperatures. However it also suffers from the presence of excess or remainder RAFT agent which owing to the presence of sulphur also leads to color and odors to the product. Synthesis of hybrid polymers can also be achieved by using these methods as depicted in Scheme 1.2 [31].

References 1. V. Mittal, Advances in Polymer Latex Technology, New York, Nova Science Publishers, 2009. 2. G. Odian, Principles of Polymerization, Fourth Edition, New Jersey, John Wiley & Sons, Inc., 2004. 3. P.C. Hiemenz and R. Rajagopalan, Principles of Colloid and Surface Chemistry, New York, Marcel Dekker, Inc., 1997. 4. V.R. Gowariker, N. V. Viswanathan and J. Sreedhar Polymer Science, New Delhi, John Wiley & Sons, Wiley Eastern Limited, 1986. 5. F.J. Schork, Y. Luo, W Smulders, J.P. Russum, A. Butté, and K. Fontenot, Advances in Polymer Science, Vol. 175, p. 129,2005. 6. K. Landfester, Macromolecular Rapid Communications, Vol. 22, p. 896,2001. 7. P.A. Lovell and M.S. El-Aasser Emulsion Polymerization and Emulsion Polymers, Eds., England, John Wiley and Sons Limited, 1997. 8. K. Matyjaszewski and T.P. Davis, Eds., Handbook of Radical Polymerization, New Jersey, John Wiley & Sons, Inc., 2002. 9. K. Landfester, N. Bechtold, F. Tiarks, and M. Antonietti, Macromolecules, Vol. 32, p. 2679,1999. 10. M. Bradley, and F. Grieser, Journal of Colloid and Interface Science, Vol. 251, p. 1,2002.

MINIEMULSION POLYMERIZATION

23

11. S. Wang, and FJ. Schork, Journal of Applied Polymer Science, Vol. 54, p. 2157, 1994. 12. J.L. Reimers, and F.J. Schork, Journal of Applied Polymer Science, Vol. 60, p. 251, 1996. 13. J. Reimers, and F.J. Schork, Journal of Applied Polymer Science, Vol. 59, p. 1833, 1996. 14. H. Dong, J.W. Gooch, and F.J. Schork, Journal of Applied Polymer Science, Vol. 76, p. 105, 2000. 15. J.L. Reimers, and F,J. Schork, Polymer Reaction Engineering, Vol. 4, p. 135,1996. 16. C.S. Chern, and Y.C. Liou, Polymer, Vol. 40, p. 3763,1999. 17. J.L. Reimers, and F.J. Schork, Industrial & Engineering Chemistry Research, Vol. 36, p. 1085,1997. 18. D. Mouran, J. Reimers, and F. J. Schork (1996). Journal of Polymer Science, Part A: Polymer Chemistry, 34,1073-1081. 19. J. Asua, J. Alduncin, and J. Forcada, Macromolecules, Vol. 27, p. 2256,1994. 20. J.A. Alduncin, and J.A. Asua, Polymer, Vol. 35, p. 3758,1994. 21. G. Pan, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Macromolecules, Vol. 35, p. 6915,2002. 22. M.N. Alam, P.B. Zetterlund, and M. Okubo, Macromolecular Chemistry and Physics, Vol. 207, p. 1732,2006. 23. J.W. Ma, J.A. Smith, K.B. McAuley, M.F.Cunningham, B.Keoshkerian, and M.K. Georges, Chemical Engineering Science, Vol. 58, p. 1163, 2003. 24. J. Nicolas, B. Charleux, O. Guerret, and S. Magnet, Macromolecules, Vol. 37, p. 4453,2004. 25. M. Li, and K. Matyajaszewski, Macromolecules, Vol. 36, p. 6028,2003. 26. M. Li, K. Min, and K. Matyajaszewski, Macromolecules, Vol. 37, p. 2106,2004. 27. K. Min, H. Gao, and K. Matyajaszewski, Journal of American Chemical Society, Vol. 127, p. 3825,2005. 28. J.K. Oh, C. Tang, H. Gao, N.V. Tsarevsky, and K. Matyajaszewski, Journal of American Chemical Society, Vol. 128, p. 5578,2006. 29. G. Qi, C.W. Jones, and F.J. Schork, Macromolecular Rapid Communications, Vol. 28, p. 1010,2007. 30. M.F. Cunningham, Progress in Polymer Science, Vol. 33, p. 365, 2008. 31. J. Pyun, and K. Matyajaszewski, Chemistry of Materials, Vol. 13, p. 3436,2001.

This Page Intentionally Left Blank

2

Multi-Functional Stabilizers in Miniemulsion Polymerization Alain Durand Alain Durand Laboratoire de Chimie Physique Macromoléculaire, UMR 7568 CNRS - Nancy-University ENSIC, 1 rue Grandville, BP 20451, F-54001 Nancy cedex, France

Abstract Multi-functional surfactants in miniemulsion polymerization are defined as stabilizers which can be involved in other aspects of polymerization. For reactive surfactants, inisurfs, surfmers and transurfs have been reported in miniemulsion polymerization. Their applications in the preparation of latexes with well-controlled properties (chemical structure, particle morphology and colloidal stability) are overviewed. In addition, it is showed that some polymeric stabilizers can also suppress the need for ultrahydrophobes (acting against Ostwald ripening). Key words: Miniemulsion, polymerization, surfactants, inisurf, surfmers, transurfs

2.1

Introduction

In this chapter, we will define a stabilizer as a (macro)molecular species able to accumulate at liquid/liquid interfaces and which contributes to increase the kinetic stability of dispersions (like monomer emulsions or suspensions of polymeric particles) as well as to facilitate their formation. Stabilizers involved in miniemulsion polymerization processes have been the object of many studies because they are at the very heart of the process: they are involved in the stabilization of the droplets/particles, they are in direct contact with the reactants (monomers, initiators), they are present in

V. Mittal (ed.) Miniemulsion Polymerization Technology, (25-42) © Scrivener Publishing LLC

25

26

MINIEMULSION POLYMERIZATION TECHNOLOGY

rather high amounts when comparing to other polymerization processes in disperse media like emulsion or suspension and they are often retained in the final product. For all these reasons, stabilizers became rapidly an important topic in the literature on miniemulsion polymerization [1-4]. One major interest of miniemulsion process is the ability to produce a final dispersion of polymeric particles whose size distribution is an (almost) exact copy of that of the initial monomer emulsion. Another specificity of miniemulsion polymerization is the predominance of direct nucleation in previously stabilized droplets. This fact may promote some chemical processes taking place at interface (as we will see in the case of reactive surfactants). To approach that situation, several challenges need to be overcome in which the stabilizer is involved. First, each monomer droplet must be nucleated so that the whole population of the initial miniemulsion is converted into polymeric particles. Second, any homogeneous nucleation event should be avoided so that no supplementary particle could be produced. Third, droplet/particle identity should be preserved over the time needed for polymerization. A great deal of work has been carried out on how the structure of stabilizers should be selected according to the nature of continuous and disperse phase, the volume fraction of disperse phase and the conditions of polymerization, so as to conform the preceding requirements. A wide variety of stabilizers has been described for miniemulsion polymerization in scientific literature. A complete review of that topic would be above the scope of this chapter and the interested reader is referred to relevant monographs [3]. Briefly we could distinguish three different types of miniemulsion polymerization systems: oil-in-water (i.e. direct), water-in-oil (i.e. inverse) and non-aqueous. Molecular surfactants are particularly efficient for direct systems (especially ionic surfactants). Macromolecular surfactants are required for non-aqueous systems since steric repulsion is the main contribution to the colloidal stability of non-aqueous miniemulsions [5]. In the past 10 years, a certain number of publications about miniemulsion polymerization showed that the use of multi-functional stabilizers could be a promising approach. The general idea was to design molecular or macromolecular surfactants that could combine other functions to that of stabilizing the dispersed reaction medium. There were many interests in that strategy. First by the use of multi-functional surfactants it was possible to limit the number of components of a miniemulsion polymerization medium

MULTI-FUNCTIONAL STABILIZERS

27

and particularly to avoid the use of some labile compounds that could diffuse out of the final material. This latter process could be a problem by itself but could also modify end-use properties, particularly for coatings. In addition, a careful design of the stabilizer allows reducing the number of steps required for preparing surfacefunctionalized particles for targeted applications like drug delivery. The functional properties related to the surface of the particles are thus brought by the stabilizer used in the preparation of the particles. Finally, using reactive surfactants for confining polymerization processes at interface was a reported strategy for the control of particle morphology. Polymer chemistry was useful to design well-controlled macromolecular surfactants with reactive end-groups or to prepare polymeric stabilizers from more specific polymers. Controlled radical polymerization was particularly employed for the synthesis of specifically designed block copolymer surfactants. In most of cases, reactive groups were located at the end of the hydrophobic sequences. Chemical modification of structurally complex macromolecules, like polysaccharides, was another approach. Macromolecular surfactants were obtained following two strategies: either by the attachment of hydrocarbon groups on native polysaccharides or by growing graft chains from polysaccharide backbone using a controlled polymerization (grafting from strategy) [6-8]. In what follows, we will focus on the use of multi-functional stabilizers in miniemulsion polymerization, in other words of miniemulsion stabilizers which have another function (called the "supplementary function"). For the sake of clarity, we will distinguish the supplementary functions related to the stability of the initial monomer emulsion (mostly encountered with non-reactive surfactants) and those related to the polymerization reaction itself (essentially reactive surfactants).

2.2 Stability of Initial Monomer Droplets In any miniemulsion process, the initial emulsion of monomer may degrade either by coalescence or by Ostwald ripening (molecular diffusion) [9]. The generally followed strategy for reaching a shelf life consistent with the time needed for polymerization involves the combined uses of a surfactant acting against coalescence and a co-stabilizer preventing Ostwald ripening [3]. While the surfactant is expected to

28

MINIEMULSION POLYMERIZATION TECHNOLOGY

adsorb at oil/water interface for preventing coalescence events, the co-stabilizer is dissolved in the disperse phase and prevents molecular diffusion by an osmotic effect which has no relation with any interfacial activity. This is the reason why the word "cosurfactant" although widely employed is often misleading. The simplest co-stabilizer is a molecule which is readily soluble in the disperse phase while having an ultra low solubility in the continuous phase. For direct miniemulsions, a great deal of work has been carried out about the use of functional hydrophobes that could be involved either in the polymerization itself (monomer, chain transfer agent, initiator) or in the final properties of the latex (elastomers) [10-16]. Nevertheless, a limited number of papers examined the possibility of combining the two stabilizing functions (against coalescence and Ostwald ripening) into a single constituent of the miniemulsion recipe. This was achieved with some polymeric surfactants which were reported to stabilize direct miniemulsions without the need of adding any hydrophobe (like hexadecane) in the monomer phase (Table 2.1). To the best of our knowledge, no molecular surfactant has been reported to exhibit that property. The corresponding polymeric surfactants were anionic or cationic polyelectrolytes with long hydrophobic sequences or repeat units carrying long hydrocarbon tails (acrylic esters or quaternized amines). No direct comparison of the average diameter of the initial monomer emulsion and of the final latex is available. Nevertheless, with the cationic polymeric surfactants, no significant difference in the average particle diameter could be evidenced with or without the presence of hexadecane dissolved in the monomer [17]. With anionic polymers, latex particles with average diameters below 250 nm were obtained without adding any molecular hydrophobe in Table 2.1. Macromolecular surfactants suppressing the need for hydrophobe in miniemulsion polymerizations.

a

Stabilizer3

Monomer

(S)x-(VBC)y quaternized by C12 or C16 fatty amine

S

[17]

P(ODMA-co-MAA) andP(ODMA-co-AA)

S

[18]

C4H9S-CS-S-(BA)x-(AA)-C(CH3)-COOH C4H9S-CS-S-(S)x-(AA)y-à(CH3)-COOH

S,BA

[25]

For abbreviations, see footnote of Table 2.4.

Reference

MULTI-FUNCTIONAL STABILIZERS

29

the monomer [18]. Two alternative explanations can be proposed depending on the partition of the polymeric surfactant between the continuous and disperse phases. Oil-soluble polymeric surfactants containing octadecyl ester units were dissolved in styrene. Since these polymers were reported to act both as surfactants and as costabilizers, it can be assumed that the polymeric stabilizer dissolved in the droplets had an osmotic effect which slowed down Ostwald ripening. In addition, the presence of sodium carboxylate units led to a significant adsorption at the styrene/water interface preventing droplet coalescence by electrostatic repulsions. It must be noted that rather high amounts of polymeric surfactant were used (1.7-10 wt% relative to monomer) [18]. Similarly, carboxylated polyurethanes (PU) soluble in styrene were used as co-stabilizers in combination with SDS which was added in the aqueous phase as a surfactant (contrary to other studies in which the amphiphilic polymer was the sole surfactant) [19, 20]. The authors noticed that decreasing the molar mass of carboxylated PU was in favour of the shelf life of the monomer emulsion. Increasing the pH of the aqueous phase provided the same beneficial effect. These results were interpreted by considering both the adsorption of polymers at interface and the effect on Ostwald ripening. On the other hand, water-soluble comblike copolymers were also reported to suppress the need for a hydrophobic co-stabilizer in styrene miniemulsion polymerization. The authors showed that the kinetics of styrene conversion was unchanged by the presence or the absence of hexadecane dissolved in the monomer [17]. This strongly supports the idea that droplet ageing is very slow in both situations since droplet size has a significant effect on the overall rate of polymerization. Such results cannot be explained by an osmotic effect of the macromolecules. These copolymers carried long hydrocarbon chains (C12 or C16) ensuring a strong adsorption onto droplet surface. Their effect upon the rate of Ostwald ripening can be explained by a modification of interfacial elasticity. It was theoretically suggested by several authors that strongly adsorbed polymeric surfactants could slow down Ostwald ripening [8,21-23]. The almost irreversible adsorption of some sequences of polymeric surfactants could lead to an increase of the elastic modulus of surface layer as well as to a decrease of interfacial tension because of the reduction of droplet area upon shrinking. These combined effects could compensate the difference of chemical potential of the oil and consequently stop aging (or at least slow it down sufficiently for droplet nucleation to occur. This phenomenon was quantitatively treated and first

30

MINIEMULSION POLYMERIZATION TECHNOLOGY

experimental evidences were reported with oil-in-water emulsions stabilized with polymeric surfactants [8, 24]. Nevertheless, theoretical considerations stressed the need for minimal thickness of the covering layers in order to have a significant impact on the rate of Ostwald ripening. To the best of our knowledge, this aspect has not been experimentally investigated up to now. These considerations support the idea that a convenient design of polymeric stabilizers could suppress the need for hydrophobic co-stabilizers, taking the opportunity of elastic behaviour of the layer of polymeric surfactant at the surface of monomer droplets, which cannot be obtained with molecular surfactants. A direct comparison between cationic comblike copolymers and a cationic surfactant confirmed the strong difference in shelf-life of miniemulsions and the longer stability observed with the polymeric stabilizer [17]. Anionic diblock copolymers were also shown to act as co-stabilizers [25]. This result can be related to their strong adsorption at interfaces. Indeed these block copolymers contained long hydrophobic blocks and short anionic sequences (the ratio of the number of hydrophobic units to that of hydrophilic units is 4). This composition is very different from that of other gradient copolymers with the same repeat units but much more hydrophilic units (70 mole %) for which the presence of hydrophobe was essential for maintaining the colloidal stability during the reaction. At present, it is not possible to extract guidelines for a rational design of stabilizers that would also act as hydrophobes in miniemulsion polymerization. Nevertheless, several examples are available in the case of polymeric surfactants and give interesting tracks for further investigations.

2.3

Stabilizers and Polymerization Processes

2.3.1 Mass-Transfer Processes In miniemulsion polymerization, when chain polymerization is carried out, the initiator may be dissolved either in the disperse phase or in the continuous one. The high interfacial area allows an efficient droplet nucleation even if the initiator is initially dissolved in the continuous phase. The stabilizer forms a molecular (at least) layer covering the droplets and ensuring colloidal stability. When the initiator is dissolved in the continuous phase, one could wonder whether the stabilizing layer may interfere with the nucleation process and

MULTI-FUNCTIONAL STABILIZERS

31

particularly with the transfer processes. In case the answer is positive, this would imply that the stabilizer could have a direct influence on the reaction kinetics. Alkali-soluble resins were shown to limit the rate of BA/MMA miniemulsion polymerization when KPS was used as the initiator. This effect was attributed to the electrosteric barrier formed by the ASR. Such limitation was not observed when using a low-molar mass surfactant [26]. An example of steric repulsions was reported in the case of the ring-opening metathesis polymerization of norbornene carried out in miniemulsion [27, 28]. PEO-based ruthenium carbenes were designed and applied in the miniemulsion metathesis polymerization of norbornene. The simultaneous use of bulky initiator and neutral polymeric surfactant PS-b-EO at the surface of the droplets limited the diffusion of the initiator into monomers droplets by the presence of a steric barrier. This limitation had two consequences: the nucleation period increased with the molar mass of the initiator while the efficiency of initiation decreased (the experimental molar masses were higher than the expected ones).

2.3.2

Reactive Stabilizers

The stabilizer can be involved directly in some reactive steps of the polymerization reaction. One important consequence is that the stabilizers are covalently linked to the latex particles once miniemulsion polymerization has proceeded. Alternatively, the use of reactive stabilizer may be a way to confine reaction processes at the interface so as to obtain nanocapsules instead of bulk nanoparticles. Reactive stabilizers must comprise some functional groups in one or several parts of their chemical structure. To the best of our knowledge, in the large majority of reported examples, the reaction was carried out in direct miniemulsion polymerization. Consequently the functional groups were often located in the hydrophobic sequences of the molecular or macromolecular surfactants. The only exceptions were for molecular surfactants acting as initiators for ionic polymerizations through strong acid or basic functions. The chemical processes in which multi-functional stabilizers were involved belong to three categories: initiation, propagation and transfer (Table 2.2 to 2.4). The corresponding terms inisurf, surfmer and transurf, respectively, which were widely employed for emulsion polymerization will be used here for miniemulsion polymerization [29-31]. In a lot of examples, multi-functional stabilizers

S,DVB

TD

P(S-co-MA)-TEMPO

'For abbreviations, see footnote of Table 2.4.

BMA, EGDMA, DSDMA

HD

PE0114-PBMA49-C1

CRP

CRP

CRP

BA

HD

PEO120-PS15-Br, PEO120-PS29-Br, PEO^-PS^-Br, PEO^-Br

CRP

[43]

[42]

[41]

[39]

[40]

[32]

[37]

CRP RP

[35],[36]

CP

[38]

[33],[34]

AP

CRP

Reference

Polymerizationb

CRP

4-VP, DVB

S

MMA

S

p-MS

PGE, D4

Monomer

S, BA, BMA

-

PEO m -Br, PNIPAM89-Br HD

HD

[CH 2 =CH-Ph-CH 2 -0-(EO)-CO-CH 2 -CH 2 C(CH3)2-N=]2

CH3-0-(EO)nl-CO-C(CH3)2-(S)33-Br

HD

Oligostyrene

Br-C(CH 3 ) 2 -COO-(CH 2 ) ir N (CH 3 ) 3 Br

+

-

CH 3 -(CH 2 ) n -Ph-S0 3 H

CH 3 -(CH 2 ) n -Ph-S0 3 Na

-

Co-stabilizer

CH 3 -(CH 2 ) 11 (Ph-CH 2 )(CHy 2 NOH(CH3(CH2)11)2(CH3)2N+OH-

Inisurfa

Table 2.2. Molecular and macromolecular inisurfs used in miniemulsion polymerizations.

MULTI-FUNCTIONAL STABILIZERS

33

Table 2.3. Molecular and macromolecular surfmers used in miniemulsion chain polymerizations and step polymerization. Surfmera

Monomer

(CH 2 =CH-CH 2 -0-CH 2 -) 3 C-CH 2 -0(PO)x-(EO)v-R R = H or phosphate

S,BA

[49]

CH 2 =CH-CO-0-CH 2 -CH(CH 3 )(EO)x-(PO) -(EO)x-CH(CH3)-CH20-CO-CH=CH 2

S

[55]

CH3-(CH2)15-N+(CH3)2-(CH2)2-OOCC(CH3)=CH2, B r

S

[45]

CH2===CH-Ph-CH2-0-(DEA)x-(DMA)y partly quaternized

S

[46]

s

[56]

[CH 2 =CH-Ph-CH 2 -0-(EO) -CO-CH 2 CH 2 -C(CH 3 ) 2 -N=] 2

s

[32]

CF 3 -(CF 2 ) 7 -0-CO-CH=CH-COOH

S, BMA

[50]

S,MMA Na -0 3 SO-(CH 2 ) n -OOC-C(CH 3 )=CH 2 (CH3)3N+-(CH2)n-OOC-C(CH3)=CH2,Br

[44]

((DMA) 39 -0-CH 2 ) 2 -C(C 2 H.)-CH 2 -0CH 2 -CH=CH 2

+

a

Reference

Na + -OOC-(CH 2 ) 9 -OOC-CH=CH 2

AC8CBb, DACTPll b

[57]

CH 3 -(CH 2 ) 8 -Ph-CH=CH-CH 3 O-(EO)20-SO3NH4 CH 3 -(CH 2 ) n -CH-CH 2 -0-CH 2 -CH= CH 2 (n = 8,10) O-(EO)10-SO3NH4

S

[47]

Bu x -succinimide-(NH-CH 2 -CH 2 ) 4 -NH 2

HMDA, TDI

[48]

For abbreviations, see footnote of Table 2.4. AC8CB: polymerizable liquid crystal mesogen, DACTP11: diacrylate crosslinker.

b

were polymeric surfactants and more particularly amphiphilic block copolymers. The same trend has been evidenced for reactive surfactants used in emulsion polymerization. The use of controlled polymerizations allowed preparing amphiphilic copolymers with functional groups located either at the end of the chains or between different blocks (Figure 2.1 to 2.3). A few examples can be found where one stabilizer was involved in two auxiliary functions apart from that of providing colloidal stability to the emulsion. In one work, the transurf also replaced the hydrophobe to slow down

34

MINIEMULSION POLYMERIZATION TECHNOLOGY

Table 2.4. Molecular a n d macromolecular transurfs used in miniemulsion RAFT polymerizations Transurf 3

Monomer

Type of particles 0

Reference

S,MMA

BN

[44]

S,BA

BN

[25]

Ph-CS-S-C(CH3)(CN)-(CH2)2CO-NH-(CH 2 ) 2 -(EO) x -0-CH 3

S

BN

[58]

Ph-CH2-CS-S-(S)x-(MA)v-Ph

S

NC d

[59]

NC

[60]

Ph-CS-S-C(CH 3 )(CN)-(CH 2 ) 2 -COOH C4H9S-CS-S-(BA)x-(AA)yC(CH 3 )-COOH C4H9S-CS-S-(S)x-(AA) C(CH 3 )-COOH

C12H2.S-CS-S-(S)2-(AA)2CH(CH 3 )-COOH

s

HOOC-(CH 2 ) 2 -C(CH 3 )(CN)(DMA) -S-CS-CH.

MMA

BN

[61]

Ph-CH2-CS-S-(S)x-(MA)y-Ph

S

NC

[62]

NIPAM

NC

[63]

S,ND

NC

[64]

x

6

0

CH3-(EO)17-OOC-C(CH3)2-SSCS-C12H25 HOOC-(CH 2 ) 2 -C(CH 3 )(CN)(MAA)y-(S)y-SSC-S-CI2H25

"Abbreviations: AA, Acrylic acid; BA, Butyl acrylate ;BMA, Butyl methacrylate; Bu, Butylène; D4, Octamethylcyclotetrasiloxane; DEA, 2-(diethylamino)ethyl methacrylate; DMA, 2-(dimethylamino)ethyl methacrylate; DSDMA, bis(2-methacryloyloxyethyl) disulfide; DVB, Divinylbenzene; EGDMA, Ethylene glycol dimethacrylate; EO, Ethylene oxide; HD, Hexadecane; HMDA, 1,6-Diaminohexane; MA, Methyl acrylate; MAA, Methacrylic acid; MMA, Methyl methacrylate; ND, n-nonadecane; NIPAM, N-isopropylacrylamide; ODMA, Octadecyl methacrylate; PGE, Phenylglycidylether; p-MS, para-Methoxystyrene; PO, Propylene oxide; S, Styrene (with an index, otherwise sulfur atom); TD, tetradecane; TDI, Toluene 2,4-diisocyanate; TEMPO, 2,2,6,6-Tetramethyl-l-piperidinyloxyl; VBC, 4-Vinylbenzyl chloride; 4-VP, 4-Vinylpyridine. b Abbreviations: AP, Anionic polymerization; CP, Cationic polymerization; CRP, Controlled radical polymerization; RP, classical radical polymerization. C BN, bulk nanoparticles; NC, nanocapsules. d The final latex contained both nanocapsules and pure PS nanoparticles.

Ostwald ripening process [25]. This is, as far as we are aware, the only example of a reactive stabilizer acting as a hydrophobe. In another study, the macromolecular stabilizer was both inisurf and surfmer because it comprised both carbon double bonds at both

MULTI-FUNCTIONAL STABILIZERS

35

Figure 2.1. Schematic chemical structures of inisurfs used in miniemulsion. X is an halogen (bold line: hydrophilic block, circle: ionizable group).

Figure 2.2. Schematic chemical structures of surfmers used in miniemulsion. R : H or CH3 (bold line: hydrophilic block, circle: ionizable group).

Figure 2.3. Schematic chemical structures of transurfs used in miniemulsion. T is a functional group acting in chain transfer (dithioester...). (bold line: hydrophilic block, circle: ionizable group).

ends and azo bonds in the middle of the chain [32]. In all other studies, the stabilizer was involved in one supplementary function through one specific functional group. Inisurfs have been used in miniemulsion for various chain polymerization mechanisms: anionic, cationic as well as free-radical (Table 2.2). For ionic polymerizations, inisurfs were molecular surfactants in which the polar head was a strong acid or base [33-36]. Thus the

36

MINIEMULSION POLYMERIZATION TECHNOLOGY

counter ions (proton or hydroxide) were involved in the initiation of polymerization. During the anionic polymerization of phenyl glycidyl ether in miniemulsion, hydroxide ions (counter ions of the cationic surfactants) initiated the polymerization at the interface [34]. Then the anionic active centers formed complexes with ammonium groups with enough stability to allow a few propagation events to occur before termination with water molecules. The authors used surfactants mixtures combining cationic surfactant (with hydroxide counter ion) with nonionic ones. Increasing the nonionic surfactant content decreased the polymerization rate which was attributed to the reduction of the surface concentration of hydroxide ions. No hydrophobe was added into the monomer which, as noted by the authors, did not lead to emulsions with a correct stability. Consequently, the polymerization was carried out in conditions that may not be those of completely rigorous miniemulsion polymerization. Cationic polymerization of p-methoxystyrene was initiated by the use of dodecylbenzenesulfonic acid which acted as an inisurf [35,36]. The authors demonstrated that the addition of a hydrophobe could be avoided. Nevertheless, they observed that, in those conditions, the particle size was continuously decreasing while polymerization progressed. On the contrary, in the presence of hexadecane dissolved in monomer droplets, particle size was significantly lower and remained unchanged up to total monomer consumption. The proposed interpretation was that cationic polymerization started during the sonication step thus rapidly producing macromolecules in the droplets. These hydrophobic macromolecules were supposed to act as hydrophobe and prevent Ostwald ripening. As for anionic polymerization of PGE, the interfacial concentration of inisurf was shown to influence the rate of polymerization. Indeed this concentration directly impacts the amounts of protons at interface, which is the real initiator. In addition, a maximum number-average molar mass (around 1000 g / mol) was reached which is a common feature of ionic polymerizations carried out in miniemulsion. This fact is attributed to the low surface activity of oligomers having higher degrees of polymerization. Since these oligomers cannot maintain at interface, they are rapidly buried in the particles and a termination process occurs. Inisurfs have been employed for free radical miniemulsion polymerization mainly in the case of controlled radical polymerization (CRP). To the best of our knowledge, there is one example of study involving an inisurf for a miniemulsion polymerization following a classical mechanism [32]. The inisurf was used as the sole stabilizer and initiator for the preparation of polystyrene nanoparticles.

MULTI-FUNCTIONAL STABILIZERS

37

In addition, the inisurf carried double bonds at both ends and, thus, acted also as a monomer. The length of the poly(ethylene oxide) sequences was shown to have a great influence on the colloidal stability of the particles during polymerization. The nanoparticles were expected to be covered by loops of PEO chains covalently attached at both ends, which is obviously consistent with the previous results concerning the influence of inisurf structure on colloidal stability. Sodium dodecylbenzenesulfonate (SDBS) was shown to greatly influence the rate of TEMPO-mediated styrene miniemulsion polymerization [37]. Other sulfonated surfactants did not exhibit such effect. The authors attributed those results to a participation of SDBS in reactions generating radicals. Nevertheless, the exact processes were not identified. Several studies involved molecular or macromolecular inisurfs in controlled radical polymerization [38-43]. To the best of our knowledge, only one work reported the use of a molecular inisurf for controlled radical polymerization [38]. Block copolymers prepared by ATRP were used in most cases. It was showed that the control of polymerization was obtained together with convenient colloidal stability. As underlined by the authors, with such systems, the molar masses obtained are closely linked to the average particle diameter. The only reported way to overcome this restriction is the addition of a hydrophobic co-initiator. It is noticeable that bromineterminated PEO were reported to be efficient reactive surfactants for ATRP and AGET ATRP [41]. Thus, the presence of a hydrophobic block in the initial macroinitiator was not required. Molecular and macromolecular surfmers (Table 2.3) were used in miniemulsion polymerization essentially with the aim of obtaining an irreversible anchoring of stabilizer at the surface of the final particles. According to the reactivity ratios, molecular surfmers can either be readily incorporated into the growing chains or homopolymerize. Thus, a transition may occur between molecular surfactants to macromolecular surfactant while polymerization proceeds [44]. Several studies compared the kinetics of polymerization either in the presence of surfmers or with non-reactive analogues. Results are available for both molecular and macromolecular surfmers. Comparing anionic and cationic molecular surfmers to non-reactive surfactants, Matahwa et al. concluded that the rate of polymerization, the molar mass distribution and the particle size were similar with cationic species with both styrene and methyl

38

MINIEMULSION POLYMERIZATION TECHNOLOGY

methacrylate [44]. With anionic surf mer, some differences were noticed. In another study about styrene miniemulsion polymerization stabilized with molecular cationic surfactants (surfmer or non-reactive), Cao et al. found similar particle sizes but faster polymerization kinetics with the surfmer [45]. With cationic polymeric surfactants used as stabilizers in styrene miniemulsion polymerization, Houillot et al. reported that particle sizes fell in the same range with the macromonomer and with its non-reactive analogue. Nevertheless, they also noticed that the kinetics of polymerization was slightly faster with the macromonomer despite similar droplet sizes [46]. Braganza et al. compared the incorporation of several surfmers in the final latex particles using either miniemulsion polymerization or emulsion polymerization [47]. They found that the overall incorporation was similar for the two processes but the fraction of surfmer buried into particle core was higher in the case of emulsion polymerization. To the best of our knowledge, there is only one example of surfmer used for step-polymerization in miniemulsion [48]. Polyurea nanocapsules were prepared using a surfmer carrying amine groups, in inverse minemulsion. The surfmer was involved in the formation of the polymeric shell and modified its permeability. Covalent anchoring of stabilizers onto particle surface provided excellent stability upon dialysis to the final latexes [46,49, 50]. Transurfs with controlled chemical structure were mainly applied for RAFT polymerization in miniemulsion (Table 2.4). One general idea was to use the transurf to confine radicals at the interface and thus prepare nanocapsules. The efficiency of such strategy was demonstrated in several examples. Poly(vinyl alcohol) (PVA) can be also considered as a transurf since it has been widely used as a polymeric stabilizer for emulsion and miniemulsion radical polymerization and it is involved in grafting reactions occurring through hydrogen abstraction followed by propagation. This latter process leads to a covalent attachment of PVA chains at the surface of the formed particles. Several studies have showed that miniemulsion process leads to a better accessibility of PVA chains as compared to emulsion polymerization. As a result, a higher fraction of attached PVA chains is obtained by the miniemulsion process [51]. Colloidal stability of the latexes was also reported to be improved by the miniemulsion process but this result was not directly linked to the amount of grafted PVA but to the chemical structure of the grafted chains [52-54].

MULTI-FUNCTIONAL STABILIZERS

39

2.4 Conclusion This chapter illustrated the concept of multifunctional stabilizer in miniemulsion polymerization, which has been particularly investigated over the last ten years. This concept is for a large part the application of that of reactive surfactant, which is well-known in emulsion polymerization. Nevertheless, the specificities of miniemulsion process allowed applications in controlling the morphology of the formed particles as well as their surface characteristics. In addition, stabilizers acting as hydrophobes were also reported which is specific to miniemulsion polymerization.

References 1. F.J. Schork, Y. Luo, W. Smulders, J.P. Russum, A. Butté, and K. Fontenot, Advances in Polymer Science, Vol. 175, p. 129,2005. 2. J.M. Asua, Progress in Polymer Science, Vol. 27. p. 1283,2002. 3. M. Antonietti, and K. Landfester, Progress in Polymer Science, Vol. 27, p. 689, 2002. 4. N. Bechthold, F. Tiarks, M. Willert, K. Landfester, and M. Antonietti, Macromolecular Symposia, Vol. 151, p. 549, 2000. 5. K. Landfester, M. Willert, and M. Antonietti, Macromolecules, Vol. 33, p. 2370, 2000. 6. A. Durand, and E. Marie, Advances in Colloid and Interface Science, Vol. 150, p. 90,2009. 7. H. Cramail, E. Cloutet, and K. Radhakrishnan, Macromolecular Engineering, Vol. 4, p. 2181, 2007. 8. T.F. Tadros TF, Advances in Colloid and Interface Science, Vol. 147-148, p. 281,2009. 9. I. Capek, Advances in Colloid and Interface Science, Vol. 107, p. 125,2004. 10. C.S. Chern, and C.-H. Lin, Journal of Applied Polymer Science, Vol. 92, p. 1961, 2004. 11. C.S Chern, Y.C. Liou, and T.J. Chen, Macromolecular Chemistry and Physics, Vol. 199, p. 1315,1998. 12. C.S. Chern, and J.-C. Sheu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 38, p. 3188,2000. 13. C.S. Chern, and T.J. Chen, Colloids and Surfaces A, Vol. 138, p. 65,1998. 14. J.A. Alduncin, J. Forcada, and J.M. Asua, Macromolecules, Vol. 27,p. 2256, 1994. 15. G. Jia, N. Cai, Y. Xu, C. Liu, and X. Tan, European Polymer Journal, Vol. 43, p. 4453,2007. 16. Z.-Q. Yu, P.-H. Ni, J.-A. Li, and X.-L. Zhu, Colloids and Surfaces A,Vol. 242, p. 9,2004. 17. M. Manguian, M. Save, C. Chassenieux, and B. Charleux, Colloid and Polymer Science, Vol. 284, p. 142,2005.

40

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

18. G. Baskar, K. Landfester, and M. Antonietti, Macromolecules, Vol. 33, p. 9228, 2000. 19. Z.-Q. Yu, D.-Y. Lee, I.-W. Cheong, J.-S. Shin, Y-J. Park, and J.-H. Kim, Journal of Applied Polymer Science, Vol. 87, p. 1941, 2003. 20. Z.-Q. Yu, D.-Y. Lee, I.-W. Cheong, J.-S. Shin, Y-J. Park, and J.-H. Kim, Journal of Applied Polymer Science, Vol. 87, p. 1933,2003. 21. M.B.J. Meinders, W. Kloek, and T. van Vliet, Langmuir, Vol. 17, p. 3923, 2001. 22. M.B.J. Meinders, and T. van Vliet, Advances in Colloid and Interface Science, Vol. 108-109, p. 119,2004. 23. A.S. Kabal'nov, Journal of Dispersion Science and Technology, Vol. 22, p. 1, 2001. 24. S. Mun, and D.J. McClements, Langmuir, Vol. 22, p. 1551, 2006. 25. B.T.T. Pham, D. Nguyen, C.J. Ferguson, B.S. Hawkett, A.K. Serelis, and C.H. Such, Macromolecules, Vol. 36, p. 8907, 2003. 26. A.N.F. Peck, and J.M. Asua, Macromolecules, Vol. 41, p. 7928, 2008. 27. D. Quémener, V. Héroguez, and Y. Gnanou, Macromolecules, Vol. 38, p. 7977, 2005. 28. D. Quémener, V. Héroguez, and Y. Gnanou, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 44. p. 2784, 2006. 29. A. Guyot, Macromolecular Symposia, Vol. 179. p. 105, 2002. 30. A Guyot, Recent Research Development in Polymer Science, Vol. 8. p. 207,2004. 31. A Guyot, and K. Tauer, Advances in Polymer Science, Vol. I l l , p. 42,1994. 32. U. Yildiz, and K. Landfester, Polymer, Vol. 49, p. 4930, 2008. 33. C. Maitre, F. Ganachaud, O. Ferreira, J.-F. Lutz, Y. Paintoux, and P. Hémery, Macromolecules, Vol. 33, p. 7730,2000. 34. A. De Gunzbourg, S. Maisonnier, J.-C. Favier, C. Maitre, M. Masure, and P. Hémery, Macromolecular Symposia, Vol. 132, p. 359,1998. 35. S. Cauvin, R. Dos Santos, and F. Ganachaud, e-Polymers, 050, 2003 36. S. Cauvin, A. Sadoun, R. Dos Santos, J. Belleney, F. Ganachaud, and P. Hémery, Macromolecules, Vol. 35, p. 7919,2002. 37. M. Lin, J.C.C. Hsu, M.F. Cunningham, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 44, p. 5974,2006. 38. F. Stoffelbach, N. Griffete, C. Bui, and B. Charleux, Chemical Communications, p. 4807,2008. 39. F. Stoffelbach, B. Belardi, J.M.R.C.A. Santos, L. Tessier, K. Matyjaszewski, and B. Charleux, Macromolecules, Vol. 40, p. 8813,2007. 40. D. Xiong, Z. Li, R. Ma, Y. An, and L. Shi, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 47, p. 1651,2009. 41. W. Li, K. Min, K. Matyjaszewski, F. Stoffelbach, and B. Charleux,Macromolecules, Vol. 41, p. 6387, 2008. 42. W. Li, K. Matyjaszewski, K. Albrecht, and M. Möller, Macromolecules, Vol. 42, p. 8228,2009. 43. M.N. Alam, RB. Zetterlund, and M. Okubo, Polymer, Vol. 50, p. 1632,2009. 44. H. Matahwa, J.B. McLeary, R.D. Sanderson, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 44, p. 427,2006. 45. N. Cao, X. Wang, L. Song, and Z.C. Zhang, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 45, p. 5800,2007. 46. L. Houillot, J. Nicolas, M. Save, B. Charleux, Y Li, and S.P Armes, Langmuir, Vol. 21, p. 6726, 2005.

MULTI-FUNCTIONAL STABILIZERS

41

47. S.N. Braganza, E.S. Daniels, E.D. Sudol, V.L. Dimonie, A. Klein, and M.S. El-Aasser, PMSE Preprints, Vol. 99, p. 316,2008. 48. E.-M. Rosenbauer, K. Landfester, and A. Musyanovych, Langmuir, Vol. 25, p. 12084, 2009. 49. D. Crespy, A. Musyanovych, and K. Landfester, Colloid and Polymer Science, Vol. 284, p. 780,2006. 50. A. Pich, S. Datta, A. Musyanovych, H.-J.P. Adler, and L. Engelbrecht, Polymer, Vol. 46, p. 1323,2005. 51. S.J. Bohorquez, and J.M. Asua, Macromolecules, Vol. 41, p. 8597,2008. 52. N. Kim, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Macromolecules, Vol. 36, p. 5573,2003. 53. N. Kim, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Macromolecules, Vol. 37, p. 2427,2004. 54. N. Kim, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Macromolecules, Vol. 37, p. 3180,2004. 55. J.-W. Kim, J.-Y. Ko, J.-G. Park, J.-B. Jun, I.-S. Chang, and K.-D. Suh, Journal of Applied Polymer Science, Vol. 85, p. 328,2002. 56. P. Ni, M. Zhang, L. Ma, and S. Fu, Langmuir, Vol. 22, p. 6016,2006. 57. C M . Spillmann, J. Naciri, K.J. Wahl, YH. Garner III, M.-S. Chen, and B.R. Ratna, Langmuir, Vol. 25, p. 2419,2009. 58. A. Martins dos Santos, T. Le Bris, F. D'Agosto, and M. Lansalot, Macromolecules, Vol. 42, p. 946, 2009. 59. Y. Luo, and H. Gu, Macromolecular Rapid Communications, Vol. 27, p. 21,2006. 60. F. Lu, Y. Luo, and B. Li, Macromolecular Rapid Communications, Vol. 28, p. 868, 2007. 61. Q. Xiong, P. Ni, F. Zhang, and Z. Yu, Polymer Bulletin, Vol. 53, p. 1,2004. 62. Y. Luo, and H. Gu, Polymer, Vol. 48, p. 3262,2007. 63. F. Lu, Y Luo, B. Li, Q. Zhao, and F.J. Schork, Macromolecules, Vol. 43, p. 568, 2010. 64. F. Lu, Y Luo, and B. Li, Industrial and Engineering Chemistry Research, Industrial and Engineering Chemistry Research, Vol. 49, p. 2206,2010.

This Page Intentionally Left Blank

3

Structured Copolymer Particles by Miniemulsion Polymerization* V. Mittal BASF SE, Polymer Research, 67056 Ludwigshafen, Germany

Abstract

Miniemulsion polymerization has been used for the synthesis of a large number of copolymer particle systems. It has been successful in the incorporation of the very hydrophobic monomers in the polymer particles owing to the polymerization of monomer droplets and no requirement of the diffusion of the monomers from the aqueous phase. The hydrophobic monomer in such cases also acts as a costabilizer by granting stability to the monomer droplets against Ostwald ripening. Both batch as well as semicontinuous modes of copolymerization processes has been reported, which have significant impact on the monomer compositions of the copolymer chains. Various ionic and non-ionic surfactants have been employed as well as the use of various water soluble or oil soluble initiators. Redox initiator pairs have also been used in order to polymerize the hydrophobichydrophilic monomer pairs. The copolymerization reactions of not only liquid monomers, but the monomers in the gaseous form have also been copolymerized using miniemulsion processes. Keywords: miniemulsion, stability, Ostwald ripening, surfactant, costabilizer, hydrophobicity, monomer droplets, conversion, size distribution, redox initiation, initiator, hybrid.

3.1

Introduction

Copolymer particles are synthesized in order to combine the good properties of the individual polymer materials. Thus, it is more the copolymer or composite particles which find wider "This review work was carried out at Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland. V. Mittal (ed.) Miniemulsion Polymerization Technology, (43-70) © Scrivener Publishing LLC

43

44

MINIEMULSION POLYMERIZATION TECHNOLOGY

commercial applications owing to the broader range of different properties, than the homopolymer particles. The kinetics of copolymerization is more complex in nature owing to the different reactivity ratios of the monomers. Scheme 3.1 represents the multitude of various copolymerization reaction steps when only two monomers are copolymerized. The polymerization involving three or four monomers leads to much more complex kinetics. The feeding methodology of monomers during the course of polymerization as well as reactivity ratios of the monomers are required to be considered in order to achieve copolymer with uniform compositions of the monomer in the particles. Batch polymerization of the monomers, which differ significantly in their reactivity ratios, would lead to the generation of gradient copolymer particles. In some cases, it is beneficial to achieve gradient in the concentration of the monomers to achieve special functionalities in the polymer particles, but in other cases, it is important to control the

Scheme 3.1. Representation of the copolymerization process. Reproduced from Nova Science Publishers with permission.

STRUCTURED COPOLYMER PARTICLES

45

composition of the polymer chains through out the particles. In the gradient copolymer particles, monomer concentration changes along the radius of the particles, i.e. the more reactive monomer has higher concentration at the core of the particles owing to its faster reaction, thus leaving the less reactive monomer to lag behind which subsequently is present mostly in the shell of the polymer particles. This phenomenon is also termed as compositional drift and it leads to compositions of the monomers in the polymer chains much different from the initial monomer feed ratios. Differential partitioning of the monomers in the aqueous and organic phases also leads to the compositional drift. Semibatch addition of the monomers is preferred over the batch addition, as this mode of monomer feeding allows better control over the chemical composition of the copolymer particles. It thus allows the composition of the polymer chains in the particles to have similar monomer compositions as in the monomer feeds. Semibatch mode of addition is also performed by two different methodologies: flooded addition and starved addition of monomers. Flooded addition of monomers corresponds to monomer addition protocol whereby the monomer addition rate is faster than the polymerization rate of the monomers. But the limitation of this method is the generation of significant extent of secondary nucleation. Secondary nucleation may be beneficial in some cases, but most of the times, it is not required. This limitation can be overcome by following the second methodology of monomer addition in starved addition mode, in which the monomer addition rate is slower than the monomer polymerization rate, and this allows the chemical composition of the polymer chains to be equal to the ratio of monomers in the feed. By the variation in the monomer feeds, the particles with specific characteristics of core and surface can be synthesized. The semibatch methods though allow to generate polymer particles with controlled composition, however, they also lead to the broadening of the molecular weight distribution of the polymer chains in the particles owing to the chain transfer to the polymer during the course of polymerization. Thus, apart from the method of polymerization i.e. emulsion or miniemulsion polymerization, the process by which the polymerization is carried out also determines the morphology as well as properties of the particles. The following paragraphs review the various copolymer particle systems in miniemulsion as reported in the literature.

46

MINIEMULSION POLYMERIZATION TECHNOLOGY

3.2 Styrene-Dodecyl Methacrylate/Stearyl Methacrylate Chern et al. [1-4] reported extensively on the copolymerization of dodecyl methacrylate and stearyl methacrylate with styrene in miniemulsion. Dodecyl methacrylate and stearyl methacrylate were also used as reactive costabilizers in these polymerizations, therefore, their amount was much lower as compared to styrene. Nonylphenol polyethoxylate with an average of 40 ethylene oxides per molecule (NP-40) and sodium dodecyl sulphate (SDS) were used as surfactants. Figure 3.1 demonstrates the cube of monomer droplet diameter as a function of time when the concentration of the comonomer is increased. In first case, SDS is used as surfactant whereas NP-40 is used in the second case. By the time derivative of the cube of droplet diameter, one can attain the quantitative information on the Ostwald ripening of the particles. On comparison it was observed that stearyl methacrylate was more effective in retarding the Ostwald ripening than dodecyl methacrylate. Figure 3.2 also demonstrates the conversion as well as size evolution of the polymer particles as a function of time. The behavior of two comonomers of dodecyl methacrylate and stearyl methacrylate was compared. As stearyl methacrylate was better in retarding the Ostwald ripening of the monomer droplets, the polymerization rate is faster in this case as the number of polymerization sites is also much higher than in the case of dodecyl methacrylate. The lower size of droplets in the case of stearyl methacrylate also leads to lower particles sizes after miniemulsion polymerization as represented in Figure 3.2b. Different amounts of hydrophilic monomers like 2-hydroxyalkyl methacrylates were also added to the styrene miniemulsion copolymerization with dodecyl methacrylate or stearyl methacrylate [4], which acted also as costabilizers, as mentioned above. The generation of hydroxyl groups on the surface of the particles owing to the incorporation of 2-hydroxyalkyl methacrylates leads to specific surface properties in the polymer particles which are of immense importance in defining their applications. The addition of such hydrophilic monomers leads to the generation of polymer nuclei in the aqueous phase thus leading to the homogenous mode of particle nucleation as compared to the droplet nucleation mode. As mentioned above that the use of dodecyl methacrylate comonomer also as costabilizer led to the generation of higher Ostwald ripening

STRUCTURED COPOLYMER PARTICLES

47

Figure 3.1. Cube of droplet diameter as a function of time by using two different surfactants and comonomers. Reproduced from reference 1 with permission from Elsevier.

48

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 3.2. Monomer conversion as well as particle diameters as a function of time using two different comonomers or costabilizers. Reproduced from Elsevier with permission from reference 3.

than the stearyl methacrylate, therefore the particles generated in this system on the addition of 2.hydroxyalkyl methacrylates were predominately formed by homogenous nucleation. The use of stearyl methacrylate on the other hand does not lead to diffusional

STRUCTURED COPOLYMER PARTICLES

49

degradation of the monomer droplets thus helping to stabilize the monomer droplets better. The particles formed thus in this system after the incorporation of 2-hydroxyalkyl methacrylate were formed by droplet nucleation mode. Overall, in both the cases, increasing the amount of 2-hydroxyalkyl methacrylate in the system led to faster monomer conversion owing to the increase in the number of reaction nuclei. The overall particles size thus also dropped on increasing the extent of hydrophilic monomer. The size of the particles in the case of stearyl methacrylate comonomer and costabilizer was lower than the size of the particles in dodecyl methacrylate system owing to the lower extent of Ostwald ripening. The extent of monomer droplet nucleation was also increased on increasing the amount of 2-hydroxyalkyl methacrylate during the polymerization owing to the increased probability of capturing the waterborn particle nuclei by monomer droplets exhibiting a very large oil-water interfacial area. Two different kinds of 2-hydroxyalkyl methacrylate monomers (2-hydroxyethyl methacrylate (HEMA), and 2-hydroxypropyl methacrylate (HPMA)) were also added to the miniemulsion polymerization of styrene, but the effect of these monomers was similar.

3.3 n-Butyl Methacrylate-Crosslinking Monomers Ghazaly et al. [5] reported the miniemulsion copolymerization of n-butyl methacrylate with the various crosslinking monomers. These monomers included (a) macromonomer crosslinker (Mac) with a number average molecular weight of 3480 g / m o l and polydispersity index of 1-1 to 1.2, (b) ethylene glycol dimethacrylate (EDGMA) and (c) aliphatic urethane aery la te macromonomer (AU A). As the crosslinking monomers have very low water solubility thus limiting their diffusion through the aqueous phase, therefore miniemulsion polymerization was preferred over the emulsion polymerization. Hexadecane was used as a costabilizer and sodium dodecyl sulphate was used a surfactant in the copolymerization reactions. Both oil soluble and water soluble initiators were used. Potassium persulphate was the water soluble initiator while 2,2'-azobis(2-methy lbutyronitrile) (AMBN) was oil soluble initiator. In the case of oil soluble initiator, the initiator was added along with the monomers during the preparation of miniemulsions, whereas when water soluble initiator was used, it was added later to the miniemulsion.

50

MINIEMULSION POLYMERIZATION TECHNOLOGY

Stable miniemulsions were observed in all the three crosslinking co-monomers with n-butyl methacrylate and the resulting latexes were also stable. The development of gel content in the crosslinked latexes using oil soluble initiator as a function of n-butyl methacrylate was different for all the three crosslinking monomers. Faster rates of gel formation were reported for the copolymer with EDGMA and AUA, the fastest being EDGMA. The behavior was also overall similar when water soluble initiator was used. As the reactions in the system involved copolymerization, it was therefore also remarked that the differences in the reactivity ratios of the monomers leads to the compositional drift in the polymer chains, with more reactive monomers tending to polymerize first, followed by the polymerization of less reactive monomers. The reactivity ratio of the n-butyl methacrylate was much higher from Mac indicating that the propagating chains of poly(n-butyl methacrylate) would prefer to react with its own monomer than Mac. Mac monomer was also very hydrophobic, therefore, it was opined that it may be present mostly in the core of the particles and more of n-butyl methacrylate may then be found on the surface of the particles. The authors also postulated from the experimental data of the homo-polymerization of Mac using water and oil soluble initiators that the in the copolymer particles, the Mac chains may orient in a way that the less hydrophobic portion of these chains is oriented towards the surfaces of the particles that are more hydrophilic in nature. The use of water and oil soluble initiators also affected the kinetics of the system significantly. Owing to the different initiators, the radicals were generated at different sites. When a water soluble initiator was used, it produced radicals in the aqueous phase and reacted with the butyl methacrylate molecules dissolved in water. If the reactive end groups of Mac comonomer were present away in the core of the growing particles, it would then delay their polymerization and the n-butyl methacrylate polymerization would take place predominately. From the particle size distributions, it was concluded that when the copolymers of n-butyl methacrylate were synthesized with crosslinker of larger molecular weight, the polydispersity index in the size distribution of particles increased. It was attributed to the possibility of attainment of additional stability to the droplets against Ostwald ripening in the presence of long chain crosslinker molecules. The swelling behavior of the three copolymer particles was also analyzed and it was

STRUCTURED COPOLYMER PARTICLES

51

observed that the morphology of the copolymer particles was different from each other.

3.4

Vinyl Acetate-Butyl Acrylate

Copolymerization of vinyl acetate and butyl acrylate was reported by using the batch miniemulsion and macroemulsion polymerization methods, and the emulsion stability, polymerization kinetics, copolymer composition and particle properties were compared owing to these two different polymerization methods [6]. Sodium hexadecyl sulphate was used as surfactant and hexadecane was used as cosurfactant in the miniemulsion polymerization. Sodium hexadecyl sulphate was synthesized by the reaction of hexadecanol with chlorosulfonic acid followed by neutralization of alkyl sulfuric cid with sodium hydroxide in butanol. Miniemulsions were generated in two different ways. In the first case, hexadecane was mixed with the organic phase, i.e., monomer mixture, which was then mixed with the aqueous phase containing surfactant. In the second case, hexadecane was added to the aqueous phase containing surfactant first and the system was sonified to generate a miniemulsion. The monomer mixture was subsequently added to this miniemulsion. It was observed that the amount of sodium hexadecyl sulphate adsorption onto the mixed monomer droplets was significantly affected by hexadecane. Sodium hexadecyl sulphate concentrations of 5,10 and 20 mM were used and the presence of a small amount of hexadecane led to a drastic increase in the adsorption of surfactant in all the three concentrations. However, the adsorption leveled off at a hexadecane/sodium hexadecyl sulphate ratio of 3. As the amount of the surfactant adsorbed on the surface of droplets is relatable to the size of the droplets, therefore, at a given concentration of surfactant, the addition of hexadecane leads to the decrease in the monomer droplet size leading to the extension of surface area available for adsorption, which justifies the sharp increase in the amount of adsorption of the surfactant. Further increase in hexadecane amount leads to the droplet size to the minimum. It was thus observed that as hexadecane to sodium hexadecyl sulphate ratio and initial concentration of sodium hexadecyl sulphate was increased, the droplet size of monomer mixture was reduced and the emulsion stability was enhanced. Different recipes were produced by varying

52

MINIEMULSION POLYMERIZATION TECHNOLOGY

the amounts of surfactant and hexadecane. Induction period was observed to be present in all the miniemulsion polymerization trials. The presence of stable oxygen- containing air bubbles formed during the extensive emulsification process was reported to be a reason for such induction periods. It was also confirmed to the case as there was no induction period when no hexadecane was used for the emulsion polymerization. An increased surface resistivity to the entry of the radicals generated in the aqueous phase was also reported to be an alternate explanation, which occurs due to the presence of a thick layer of hexadecane-sodium hexadecyl sulphate at the surface of monomer droplets. The amount of coagulum generated in the polymerization reactions was also different, when hexadecane was used, only a small extent of coagulum was observed, on the other hand, the coagulum was significant in the trials without the use of hexadecane. The particle sizes were significantly different with and without the use of hexadecane. For latexes synthesized with 10 mM sodium hexadecyl sulphate, the particle diameter was observed to increase from 122 nm to 195 nm after the addition of hexadecane. The chemical compositions of the copolymer chains generated by conventional emulsion and miniemulsion polymerization were also significantly different. The polymer chains synthesized by miniemulsion process had lower contents of vinyl acetate units up to 70% conversion compared to the chains formed in emulsion polymerization process, thus confirming that the copolymerization achieved by the two methods have different kinetic behavior and leads to the generation of chemically different copolymer chains. Two different glass transition temperatures were observed in the latexes owing to the butyl acrylate and vinyl acetate rich phases. For the latex with miniemulsion polymerization, the glass transition temperatures were -29°C (butyl acrylate) and 52°C (vinyl acetate), whereas the temperatures were -20°C and 50°C respectively for the emulsion polymerization without hexadecane. The shift of 9°C in the miniemulsion latex was attributed to fewer amounts of vinyl acetate monomer units in the polymer chains during the first 70% conversion. It was concluded by the authors that hexadecane helped in the formation of stable emulsions with very small droplet size which became the locus of polymerization. The authors also opined that hexadecane may also act to hinder the rate of interparticle monomer transport during the polymerization and it was

STRUCTURED COPOLYMER PARTICLES

53

also observed to act as swelling promoter in the monomer-polymer system present in the particles.

3.5 Butyl Acrylate-(2-Methacryloxy)ethyl)trimethyl Ammonium Chloride Luo et al. reported the copolymerization of butyl acrylate with cationic monomer using the interfacial redox initiator system [7]. Butyl acrylate and cationic monomer 2-(methacryloyoxy)ethyl trimethyl ammonium chloride (MAETAC) form a hydrophobic and hydrophilic monomer pair and macroemulsion and miniemulsion polymerization reactions were carried out using cumene hydroperoxide and tetraethylenepentamine as initiators, hexadecane as cosurfactant and Triton X-405 as surfactant. The copolymerization of hydrophobic and hydrophilic monomers by emulsion polymerization is a challenge due to the residing of the hydrophilic monomers extensively in the aqueous phase. In this study, similarly, as butyl acrylate monomer is hydrophobic, it therefore stayed in the droplets, whereas owing to hydrophilicity, 2-(methacryloyoxy)ethyl trimethyl ammonium chloride significantly partitions in the aqueous phase. Gilbert et al. proposed that the section of initiator pair like cumene hydroperoxide and tetraethylenepentamine provides opportunities to graft hydrophilic monomer onto a hydrophobic polymer [8]. This redox initiator system forms a hydrophobic and hydrophilic pair system. Cumene hydroperoxide, owing to its hydrophobicity, remains significantly in the monomer droplets, whereas hydrophilic tetraethylenepentamine is significantly present in the aqueous phase. The authors underlined the ambiguity prevailing over the site of the radical generation from the oil soluble initiators like AIBN. Some studies have reported the generation of radicals in the aqueous phase even though AIBN is water insoluble, on the other hand, other studies reported the generation of the free radicals in the particles. The authors carried out the conventional emulsion polymerization of the system without hexadecane and using a water soluble initiator. It was observed that only MAETAC monomer was polymerized and butyl acrylate was not polymerized indicating that the free radicals could not enter the micelles to initiate the polymerization of hydrophobic monomer when a surfactant

54

MINIEMULSION POLYMERIZATION TECHNOLOGY

with a long hydrophobic tail was used thus requiring a modification in the components of the polymerization system. However, for the surfactant Triton X-405 used in the study, which has a long hydrophilic tail, it was suggested that the particles covered by this would have a viscous corona through which the radicals would penetrate to initiate the polymerization of the organic phase. Also, it was observed that the use of the redox initiator pair used in this study did not have the problem of initiation as mentioned above. Using this initiator pair, the radicals were generated at the aqueous organic interface and, thus, the radicals need not penetrate through the surfactant layer in order to initiate the polymer particles. It was also suggested that, owing to the water solubility of MAETAC monomer, it has the possibility to polymerize at two sites. The first is the polymerization in the aqueous phase and as the amount of butyl acrylate is not much in the aqueous phase owing to its water insolubility, therefore the generated chains would be either homopoly(MAETAC) or very rich in poly(MAETAC). The second possibility of the polymerization of MAETAC is at the interface of organic phase with aqueous phase. In such scenario, the butyl acrylate chains would also be included in the polymer chains. It was also reported that the more MAETAC was consumed at the very beginning of the macroemulsion polymerization than in the miniemulsion polymerization where no micelles exist. It was also observed that although more amount of MAETAC was consumed in the very beginning in the case of macroemulsion polymerization, the total amount of homopoly(MAETAC) was the same for both the macroemulsion and miniemulsion polymerizations. It was also observed that, in the case of miniemulsion polymerization, the homogenous nucleation occurred at low MAETAC concentration, but this was not the case at higher concentrations of MAETAC. Findings showed that although in both cases the kinetics of polymerization was significantly different, 18% of the MAETAC was polymerized in the aqueous phase in the final resulting latexes in both cases.

3.6 Butyl Acrylate-Methyl Methacrylate-Vinyl Acetate Unzue et al. [9] studied the terpolymerization of butyl acrylate, methyl methacrylate and vinyl acetate in the semicontinuous mode. The effect of operating conditions like solids content, initiator concentration, feed flow rate, amount of cosurfactant etc. on the kinetics

STRUCTURED COPOLYMER PARTICLES

55

of terpolymerization reaction was studied. Hexadecane was used as cosurfactant and potassium persulphate was used as initiator. The polymerization reactions were carried out following two set of experiments. In the first set, 10% of the miniemulsion was heated to 80°C and polymerized shortly for 15 minutes in batch mode. Subsequently, the remaining miniemulsion was fed to the reactor at constant feed rate. After the feeding operation was completed, the polymerization was run for further 1-2 hours. In the second set of experiments, the effect of operating conditions like partitioning of the monomer between the initial charge and the feed, type and concentration of surfactant, feed flow rate and sonication on the feasibility to obtain a 65 wt% solid content latex. The authors observed that in the beginning of the polymerization, the terpolymer was richer in methyl methacrylate and butyl acrylate and it contained almost no vinyl acetate. This effect was attributed to very different reactivity ratios of the monomers. Methyl methacrylate was the most reactive of the three followed by butyl acrylate and vinyl acetate. It was also observed that the homogeneity of the terpolymer improved when the instantaneous conversion increased as a result of the increase in the initiator concentration. The polymerization rate in the miniemulsion polymerization decreased as the concentration of the hexadecane cosurfactant increased. At the beginning of the process when the monomers accumulate in the reactor, it was observed that the instantaneous conversion was sensitive to the polymerization conditions, but the sensitivity was observed to significantly decrease as the polymerization proceeded further. It was concluded that the combination of anionic as well as nonionic emulsifiers was required to achieve stability in the system, and the anionic surfactant on its own was not sufficient to impart colloidal stability. The latex with high solid content of 65 wt% was also achieved and it was observed that the amount of coagulum increased when the anionic emulsified was used alone. Also, in the absence of sonication, the amount of coagulum was observed to increase.

3.7 Styrene-Acrylic Acid or 2-Aminoethyl Methacrylate Hydrochloride (AEMH) Musyanovych et al. [10] reported the synthesis of carboxyl and amino functionalized copolymer particles with styrene using the miniemulsion copolymerization reactions of styrene with acrylic acid or 2-aminoethyl methacrylate hydrochloride (AEMH). 2,2'-azobis

56

MINIEMULSION POLYMERIZATION TECHNOLOGY

(2-methylbutyronitrile) was used as oil soluble initiator. Lutensol AT-50 was used as non-ionic surfactant, whereas sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTMA-C1) were employed as ionic surfactants. The effect of the various comonomers and the surfactant types on the particles size as well as its distribution was investigated. The copolymer latexes polyStpolyAA and polySt-polyAEMH generated by using non-ionic surfactant were observed to show higher tendency to coagulate. The tendency to coagulate was more pronounced during the synthesis of polySt-polyAA latexes in comparison to the pure polystyrene or poly St-polyAEMH latexes. At 3 wt% amount of acrylic acid corresponding to total monomer content, the latex after 30 min of polymerization became viscous, and phase separation was observed in the end of polymerization. It was also observed that the diameter of the polymer particles sharply decreased with increasing acrylic acid content. The amino-functionalized copolymer particles were also reported to decreases sharply in size with the increase of the functional monomer till 3 wt%. The decrease was slow for the subsequent addition of AEMH until 20 wt% and an average particle size of about 120 nm was observed. For the generation of the latexes with ionic surfactants, polySt-polyAA latex copolymers particles was achieved by using anionic surfactant SDS, whereas polyStpolyAEMH copolymer latexes were synthesized by using cationic surfactant (CTMA-C1). The latexes generated with ionic surfactants were observed to have very low extent of coagulum generation. In contrast to the polymerization with non-ionic surfactant, polyStpolyAA latex particles generated by using the ionic surfactants were particularly stable even with 20 wt% of acrylic acid. The size of the particles was also observed to be constant until 10 wt% of added acrylic acid, which is again markedly different from the case of nonionic surfactants. At higher amounts of acrylic acid, the diameter was observed to sharply increase thus reaching an average value of 140 nm. The authors attributed the increase in particle size on increasing the content of acrylic acid to the formation of a "hairy" layer around the particles composed of the hydrophilic poly(acrylic acid) units. Interesting differences were also observed in the particle size distributions of the copolymer latexes with two different types of surfactants. The carboxyl and amino-functionalized particles generated by using non-ionic surfactants were observed to have high extent of polydispersity index and a bimodal size distribution of particles. The particles size distributions, on the other

STRUCTURED COPOLYMER PARTICLES

57

hand, in both the carboxyl and amino-functionalized particles synthesized by using ionic surfactant was monomodal, and the polydispersity index was not greater than 0.2. Figure 3.3 demonstrates carboxyl- and amino-functionalized polystyrene particles stabilized with non-ionic surfactant with 1 wt% of acrylic acid and with 3 wt% of AEMH. The results thus confirmed that it is not the functional comonomers themselves, but the combination of functional comonomer with non-ionic surfactant which leads to the generation of bimodality in the particle size. Figure 3.4 shows the mechanism of the bimodal particle generation in the presence of non-ionic surfactant suggested by the authors. By using the fluorescent dye molecules in miniemulsion polymerizations, they concluded that bimodal size distribution of the final particles may be a result of a budding-like effect, i.e., splitting of the miniemulsion droplets before or during the early stage of polymerization.

3.8

Styrene-Butyl Acrylate

Roberge et al. [11] reported the batch copolymerization of styrene and butyl acrylate for the synthesis of pressure sensitive adhesives. For the generation of miniemulsion system, organic components styrene, butyl acrylate and octadecyl acrylate were mixed separately from the Triton X-405 and SDS surfactants in water. Both solutions were then mixed followed by stirring and sonication to generate the miniemulsion. The reactions were carried out at 80°C and potassium persulphate was used as an initiator. Figure 3.5 demonstrates the conversion of the monomers as a function of reaction time for the various feed compositions. It was observed that the fast reaction rates were observed for all the different composition runs. Increasing the content of butyl acrylate in the system also enhanced the rate of polymerization as the conversion was enhanced as a function of time. Also, the polymerizations were observed to reach full conversions in all the different runs. Figure 3.6 also shows the impact of monomer feed composition on cumulative copolymer composition.

3.9

Styrene-Butadiene Rubber

Styrene butadiene copolymer latexes were synthesized by miniemulsion polymerization [12]. Various oil soluble initiators like

58

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 3.3. TEM images (left) and particle size distribution analysis (right) of carboxyl- and amino-functionalized polystyrene particles stabilized with nonionic surfactant: (a) no functionality, (b) with 1 wt% of acrylic acid, (c) with 3 wt% of AEMH. Reproduced from reference 10 with permission from American Chemical Society.

STRUCTURED COPOLYMER PARTICLES

59

Figure 3.4. Mechanism of bimodal particle size distribution generation by using non-ionic surfactant in carboxyl and amino-functionalized particles. Reproduced from reference 10 with permission from American Chemical Society.

Figure 3.5. Conversion vs. reaction time for the miniemulsion copolymerization of styrene and butyl acrylate. Run 1: Styrene 5, butyl acrylate 95, Triton X-405 0.5, SDS 0.03 and octadecyl acrylate 0.5; run 4: Styrene 10, butyl acrylate 90, Triton X-405 0.5, SDS 0.03 and octadecyl acrylate 0.5; run 7: Styrene 15, butyl acrylate 85, Triton X-405 0.5, SDS 0.03 and octadecyl acrylate 0.5. Reproduced from reference 11 with permission from Elsevier.

60

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 3.6. Cumulative feed composition for various feed compositions. Reproduced from reference 11 with permission from Elsevier.

2,2'-azobis(2-methylbutyronitrile) (V59), 2,2'-azobis(4-methoxy-2,4dimethyl valeronitrile) (V70), 2 / 2'-azobis(2,4-dimethyl valeronitrile) (V65) were used. N-dodecyl mercaptane (n-DM), and tert-dodecyl mercaptane (t-DM) were employed as chain transfer agents. Hexadecane was used as cosurfactant whereas SDS and potassium oléate were chosen as surfactants. The miniemulsion copolymerization of this system was special in nature owing to the gaseous form of butadiene, whereas most of the reported studied used the liquid monomers. To obtain miniemulsion, initiator, hexadecane and chain transfer agent were first dissolved in styrene. The solution was then dispersed in water by stirring at 2000 rpm for 1 hour. The suspension was then sonicated to achieve styrene based miniemulsion. The required amount of butadiene was condensed into a pre-cooled reaction vessel. The styrene miniemulsion was subsequently added to the frozen butadiene followed by homogenization of the reaction mixture by vigorous stirring. The polymerization of the monomers was carried out for 14 hours. The weight ratios of butadiene to styrene of 75:25 were used, which are also similar to the ratios commonly employed in the industrial polymerizations. The miniemulsions observed to be suitable for generating high solid content latexes up to 60 wt%. The particles also showed a narrow and monomodal size distribution and were stable and for several months. No coagulum was formed during this time. The polymerization reaction was performed at 72°C, when V59 was used as initiator. The average

STRUCTURED COPOLYMER PARTICLES

61

particle size of the samples was observed to be around 130 nm and a small polydispersity index values. The initiator V70 was used at 30°C for the miniemulsion copolymerization. The resulting particles were stable and narrowly dispersed, however, the conversion was too slow indicating that the initiator may not ne suitable for achieving high extent of monomer conversion in the time period of interest. The use of V65 initiator at 50°C also resulted in stable and narrowly dispersed latexes and the conversion was also high. Average particles size of 100 nm was observed. The authors also reported that by the use of chain transfer agents, the molecular weight distribution in the chains as well as amount of gel could be reduced. t-DM chain transfer agent was observed to be more efficient than n-DM agent. The gel content in the particles using the n-DM chain transfer agent was 28% whereas it was significantly reduced to 6% when t-DM was employed as chain transfer agent. The two surfactants of SDS and potassium oléate were not very different in behavior. The average particle size of the latexes using SDS as surfactant had somewhat smaller size as compared to the oléate stabilized latexes, however, the particles in both the latexes had low polydispersity.

3.10

Fluoroacrylate-Lauryl Methylacrylate-Methyl Methacrylate

Zhang et al. [13] reported the miniemulsion copolymerization of fluoroacrylate, lauryl methylacrylate, and methyl methacrylate monomers. It is difficult to achieve the polymerization of fluoroacrylate in the conventional emulsion polymerization owing to its hydrophobicity which retards its diffusion in the aqueous phase. Fluoroacrylate also acted as a cosurfactant apart from being a comonomer owing to its extremely hydrophobic nature. Oil soluble 2,2'-azobisisobutyronitrile (AIBN) was used as initiator and nonylphenol polyethoxylate with an average of 40 ethylene oxide units per molecule (NP-40) and cetyltrimethylammonium (CTAB), were used as surfactants. It was observed that the use of fluoroacrylate retarded the diffusional degradation of monomer droplets by Ostwald ripening and allowed the production of stable miniemulsions. The composition of the monomers in copolymer chains synthesized in miniemulsion polymerization was observed to be in good agreement with the feed ratios used. The chains, on the other hand, had significant differences in the calculated and theoretical

62

MINIEMULSION POLYMERIZATION TECHNOLOGY

values of monomer compositions, even at low feed ratio of fluoroacrylate, when the polymerization was carried out in emulsion. The size of the particles was similar to the size of the monomer droplets in the case of miniemulsion polymerization indicating efficient monomer droplet mechanism. The particle size distributions were broader than the particles synthesized conventional emulsion polymerization. The delayed nucleation of the monomer droplets is the result of the broad size distributions as the monomer droplets have much lower radical capture efficiency. On the other hand, in emulsion polymerization, the micelles are very quickly turned into growing polymer particles and thus the particles size distribution is also narrow. Reaction parameters were observed to greatly influence the morphology of the resulting copolymer latex. Polymerization rate and monomer conversion was reported to increase with increasing concentrations of surfactants and initiator. The particles generated in miniemulsion polymerization had a variety of internal structures, such as core—shell morphology or multiblob structures.

3.11

Polyurethane-Block-Polystyrene

Koenig et al. [14] reported the synthesis of water borne polyurethane-block-polystyrene latexes in miniemulsion polymerization. To obtain such functional latexes, miniemulsion copolymerization to obtain copolymer chains with specific blocks was performed by using macroazoinitiator. Styrene, isophorone diisocyanate (IPDI), 2,4-diethyl-l,5-pentanediol and a diol-functionalized azoinitiator were dispersed in water. As a first step, the polyaddition reaction was performed at room temperature to attain polyurethane block. Catalyst mixtures dibutyltindilaurate (DBTDL) and dimethyldodecylamine (DMDA) was used for this purpose. This led to the synthesis of a PU macroazoinitiator. Subsequently, the miniemulsion was heated to 72°C to initiate the free radical polymerization of styrene. Hexadecane was used as cosurfactant whereas sodium dodecyl sulphate was used as surfactant. It was observed that the copolymer could be successfully formed and it was not a mixture of two homopolymers. It was observed from the gel permeation chromatography, differential scanning calorimetry and nuclear magnetic resonance studies that 45% of a (linear) copolymer consisting of PU and PS was obtained. 28 wt% of homo-polystyrene was

STRUCTURED COPOLYMER PARTICLES

63

also observed owing to the presence of free azoinitiator molecules, which have not been incorporated in the PU macroazoinitiator. Transmission electron microscopy studies of the copolymer particles also exhibited homogeneous structure inside the particles.

3.12

Alkyd-Acrylic

Nabuurs et al. [15] reported the synthesis of alkyd-acrylic hybrid. Though the system was reported as an emulsion polymerization, but the authors subjected the emulsions to high shear before polymerization. Although no costabilizer was additionally added, but the alkyd is presumed to acts also a costabilizer. The polymerization reactions were initiated with potassium persulfate (KPS), and anionic or mixed anionic/non-ionic surfactants were used for stabilization. Nonylphenol-10 ethylene oxide sodium sulfate was used as a surfactant in most of the polymerization trials. To synthesize the hybrid, the alkyd was first dissolved in the acrylic monomers followed by the addition of surfactant and water. The emulsions after high shear homogenization were polymerized either in batch or in semi continuous mode. It was observed that the polymerization rates were slow and polymerization efficiency was also low owing to the presence unsaturated fatty acids in alkyd. The authors used various acrylic monomers, but methyl methacrylate was observed to generate the most finely dispersed pre-emulsion monomer droplets. When the polarity of the monomers was increased, i.e. when the monomer became too water soluble, the alkyd/acrylic ratio in the droplets was increased. This resulted in the more viscous system and in a less efficient particle size reduction. Figure 3.7 shows the conversion as well as polymerization rate of the hybrid polymerization reactions using the alkyds with varying concentrations of unsaturated aliphatic groups. It was observed that the increasing concentration of unsaturated fatty acids in the alkyd resulted in the enhanced extent of retardation of the polymerization rate. The conversion in the case of alkyds containing higher concentration of unsaturated groups did not reach to completion. The hybrid with fully saturated fatty acids was observed to reach a conversion of 100% after less than 1 h, whereas the hybrid with the highest concentration of unsaturated fatty acids had a monomer conversion of 93% after about 3 h. The enhancement of the properties of the hybrids over the individual components was observed as shown

64

MINIEMULSION POLYMERIZATION TECHNOLOGY

in Figure 3.8 for the hardness of the coatings generated from the latex particles. The coatings generated form the hybrid particles were much harder as compared to acrylic or alkyd coatings and the enhancement was also observed to retain as a function of time.

STRUCTURED COPOLYMER PARTICLES

3.13

65

Oil-Acrylate

Similar to the above reported, alkyd - acrylic system, Van Hamersveld et al. [16,17] also reported the use of oxidized triglycérides (e.g. sunflower oil) as initiator for the hybrid miniemulsion polymerization with methyl methacrylate. Unsaturated triglycérides were treated in the study with molecular oxygen to generate fatty-acid hydroperoxide groups. Fatty-acid hydroperoxides were then used as initiators for the hybrid miniemulsion polymerization reactions. Fatty-acid hydroperoxide Fe2 ethylenediaminetetraacetic acid sodium formaldehyde sulfoxylate (SFO-HP/Fe 2 /EDTA/SFS) formed the redox initiation system. The mini-emulsion system was stabilized with dodecyl sulphate as surfactant and hexadecane as hydrophobe. To generate miniemulsion, hydroperoxidized sunflower oil (SFO-HP) was added with monomer and hexadecane. The oil-monomer solution was then dispersed into a SDS-water solution. The miniemulsion was generated by sonication of the emulsion. Figure 3.9 shows the number average size distribution of the monomer droplets after homogenization. The miniemulsion was then transferred into a polymerization reactor and a concentrated solution of SFS was added. The polymerization was initiated by the adding of a catalytic amount of complexed Fe2 solution. The cryo transmission electron

66

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Figure 3.9. Number average size distribution of the sunflower oil - MMA monomer droplets after homogenization. Reproduced from reference 16 with permission from Elsevier.

microscopy analysis (Figure 3.10) revealed that the hybrid particles generated by initiation with the fatty-acid hydroperoxides did not show intraparticle heterogeneity, whereas the particles generated by initiation with t-butyl hydroperoxide has heterogeneous morphology. The particles contained light and dark colored phases, the lighter phases was present either on the side or in the centre of the particles. The light colored component of the particles was not observed to protrude out of the particles. Therefore, the morphology of these particles was described as core-shell, but the core was not always present at the centre of the particles. The presence of no such heterogeneity in the particles initiated by SFO-HP indicated that the use of fatty-acid hydroperoxides resulted in the formation of oilacrylate copolymer which acted as a compatibilizer for the copolymer particles. The effect of reaction parameters on the conversion was also recorded. When SFO-HP was used as hydroperoxide, the rate of polymerization did not depend on the concentration of SFS

STRUCTURED COPOLYMER PARTICLES

67

Figure 3.10. Cryo-TEM images of sunflower oil-poly(methyl methacrylate) hybrid latexes initiated by (a) t-butyl hydroperoxide and (b) SFO-HP. Reproduced from reference 17 with permission from Elsevier.

as shown in Figure 3.11a. The concentration of Fe2 also did not have a significant effect on the conversion or rate of polymerization as exhibited in Figure 3.11b. The increase in polymerization temperature as well as hydroperoxide concentration led to the enhancement of the polymerization rate as shown in Figure 3.11c and lid.

3.14

Urethane-Acrylic

Li et al. [18] reported the synthesis of urethane/acrylic hybrid miniemulsion latex nanoparticles. The miniemulsion polymerization of n-butyl methacrylate was performed both in the presence and absence of the urethane prepoylmers and the kinetics and rates of polymerization were compared. The behavior of conventional emulsion polymerization was also compared with the miniemulsion reaction trials. Hexadecane was used as costabilizer and sodium dodecyl sulphate was used as surfactant. Redox initiator pair of hydrogen peroxide and ascorbic acid was used for the initiation of miniemulsions. The homopolymerization of n-butyl methacrylate was observed to faster in miniemulsion as compared to conventional emulsion polymerization where no cosurfactant and sonication was used. It was further observed that the rate of

68

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Figure 3.11. Conversion vs. time curves for (a) various concentrations of SFS, (b) various concentrations of Fe, (c) various temperatures; and (d) various hydroperoxide concentrations. Reproduced from reference 16 with permission from Elsevier.

miniemulsion polymerization rate of n-butyl methacrylate was much faster when urethane prepolymer was added to the system. The authors attributed this behavior to the difference in the number of initial miniemulsion droplets. As the urethane prepolymer was more hydrophilic than n-butyl methacrylate, the initial size of the monomer droplets was much smaller than droplets containing pure n-butyl methacrylate. This led to the generation of the large number of monomer droplets in the system resulting in the creation of large number of polymerization loci.

References 1. 2. 3. 4.

C.S. Chern, and T.J. Chen, Colloids Surfaces A, Vol. 138, p. 65,1998. C.S. Chern, and T.J. Chen, Colloid and Polymer Science, Vol. 275, p. 1060,1997. C.S. Chern, and Y.C. Liou, Polymer, Vol. 40, p. 3763,1999. C.-S. Chern, and J.-C. Sheu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 38, p. 3188, 2000.

STRUCTURED COPOLYMER PARTICLES

69

5. H.M. Ghazaly, E.S. Daniels, V.L. Dimonie, A. Klein, and M.S. El-Aasser, Journal of Applied Polymer Science, Vol. 81, p. 1721,2001. 6. J. Delgado, M.S. El-Aasser, and J. W. Vanderhoff, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 24, p. 861,1986. 7. Y. Luo, and F.J. Schork, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 39, p. 2696,2001. 8. R.G. Gilbert, J.F. Anstey, N. Subramaniam, and M.J. Monteiro, American Chemical Society, Division of Polymer Chemistry, Vol. 40, p. 102,1999. 9. M.J. Unzue, and J.M. Asua, Journal of Applied Polymer Science, Vol. 49, p. 81,1993. 10. A. Musyanovych, R. Rossmanith, C. Tontsch, and K. Landfester, Langmuir, Vol. 23, p. 5367,2007. 11. S. Roberge, and M.A. Dube, Polymer, Vol. 47, p. 799, 2006. 12. M.-V. Kohnle, U. Ziener, and K. Landfester, Colloid and Polymer Science, Vol. 287, p. 259, 2009. 13. Q. Zhang, X. Zhan, and F. Chen, Journal of Applied Polymer Science, Vol. 104, p. 641, 2007. 14. A. Koenig, U. Ziener, A. Schaz, and K. Landfester, Macromolecular Chemistn/ and Physics, Vol. 208, p. 155,2007. 15. T. Nabuurs, R.A. Baijards, and A.-L. German, Progress in Organic Coatings, Vol. 27, p. 163,1996. 16. E.M.S. van Hamersveld, J.J.G.S. van Es, and F.P. Cuperus, Colloids and Surfaces A: Physiochemical and Engineering Aspects, Vol. 153, p. 285,1999. 17. E.M.S. van Hamersveld, J.J.G.S. van Es, A.L. German, F.R Cuperus, R Weissenborn, and A.-C. Hellgren, Progress in Organic Coatings, Vol. 35, p. 235,1999. 18. M. Li, E.S. Daniels, V.L. Dimonie, E.D. Sudol, and M.S. El-Aasser, Polymeric Materials: Science & Engineering, Vol. 85, p. 258,2001.

This Page Intentionally Left Blank

4

Encapsulation of Inorganic Nanoparticles by Miniemulsion Polymerization Jacqueline Forcada2 and Jose Ramos1 1

Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain institute for Polymer Materials POLYMÄT and Grupo de Ingeniería Química, Facultad de Ciencias Químicas, Universidad del País Vasco/EHU, Apdo. 1072, Donostia-San Sebastián 20080, Spain

Abstract

Miniemulsion polymerization is a powerful technique to encapsulate inorganic nanoparticles into a polymer shell. However, prior to carrying out the miniemulsion process, the surface of the inorganic nanoparticles must be converted into a more hydrophobic one by using surface modifiers or coupling agents. The type of surface modifier used depends strongly on the type of inorganic nanoparticle to encapsulate. In this chapter, special attention will be paid to the encapsulation of silica and magnetite nanoparticles by miniemulsion polymerization using different surface modifiers. The use of the right combination of surface modifiers together with the adequate miniemulsion process (single or double) are the key factors for obtaining a high encapsulation degree of the inorganic nanoparticles together with a well-controlled morphology of the hybrid nanocolloids. Keywords: miniemulsion polymerization; inorganic nanoparticles; encapsulation

4.1

Introduction

Nanotechnology is one of the key technologies of the21 5t century, which is making great steps forward in the improvement of existing materials and the production of advanced and innovative materials in the V. Mittal (ed.) Miniemulsion Polymerization Technology, (71-96) © Scrivener Publishing LLC

71

72

MINIEMULSION POLYMERIZATION TECHNOLOGY

colloidal range based on both inorganic and polymeric materials as well as nanocomposites consisting of a mixture of them. In this way, considerable efforts have been devoted in the development of new polymer encapsulation techniques because polymer-encapsulated inorganic nanoparticles exhibit enhanced even novel properties (e.g., mechanical, chemical, electrical, rheological, magnetic and optical) and offer very interesting actual and potential applications in different fields, such as optics, catalysis, microelectronics, coating, cosmetics, inks, agriculture, drug release systems, diagnostics, and so on [1-5]. Among the different methods for the encapsulation of inorganic nanoparticles, heterophase polymerization is by far the most frequently used technique [6,7]. In this case, composite nanoparticles are prepared by carrying out the polymerization in the presence of inorganic nanoparticles via emulsion or miniemulsion polymerization. In the case of an emulsion polymerization, it is well known that homogeneous and micellar nucleations are the main mechanisms for particle formation. In the presence of inorganic particles dispersed in the aqueous phase, particles surface can be an additional site for particle nucleation. Thus, the control of the morphology of the composite nanoparticles can become complicated due to the competition among these nucleation mechanisms. However, in miniemulsion polymerization, particle nucleation occurs mainly inside submicrometer-sized monomer droplets (ranging from 50 to 500 nm in diameter). Because of their small size, the large overall surface area of the droplets can effectively compete for radical capture. If inorganic particles can be dispersed in the monomer phase followed by miniemulsification, then each miniemulsion droplet can indeed be treated as a small nanoreactor, which produces composite particles with adequate morphology control and a high encapsulation degree of inorganic particles. One of the most important issues during the preparation of inorganic/polymeric composite particles is the formation of specific interactions at the interface of the organic and inorganic phases. Therefore, surface modification of inorganic particles is a prerequisite step before performing the polymerization reaction. With this surface modification, two important goals can be achieved: introduction of different reactive groups useful in the subsequent polymerization reaction and enhancement of the surface hydrophobicity of the inorganic particles.

ENCAPSULATION OF INORGANIC NANOPARTICLES

73

The aim of this chapter is to present a review on the encapsulation of different kinds of inorganic nanoparticles by miniemulsion polymerization of hydrophobic monomers. Special attention will be paid to the encapsulation of silica and magnetite nanoparticles by miniemulsion polymerization using different surface modifiers or coupling agents, because the morphology of the hybrid nanocolloids is strongly affected by them.

4.2 Miniemulsion Polymerization in the Presence of Inorganic Nanoparticles In the last ten years, several reviews highlighted miniemulsion polymerization over other heterophase processes as a powerful technique to encapsulate inorganic solids successfully [6-15]. For the encapsulation of inorganic nanoparticles by miniemulsion, the inorganic/polymer interface as well as the polymer/water interface has to be carefully adjusted in order to obtain encapsulation as a thermodynamically favored state. The design of the interfaces is mainly dictated by the use of two surfactant systems, which govern the interfacial tensions, as well as by the use of appropriate functional comonomers, initiators, or co-stabilizers. The sum of all interfaces has to be minimized. In general, the most useful process appeared in literature to encapsulate hydrophilic inorganic nanoparticles by miniemulsion polymerization is shown in Figure 4.1 and consists of four steps: - S t e p 1: Hydrophobization of the hydrophilic inorganic nanoparticles. - Step 2: Dispersion and stabilization of hydrophobized inorganic nanoparticles in monomer phase. - Step 3: Miniemulsification of the lipophilic dispersion in water. - Step 4: Polymerization of droplets. 4.2.1

H y d r o p h o b i z a t i o n of I n o r g a n i c N a n o p a r t i c l e s

For the encapsulation of hydrophilic inorganic nanoparticles, the surfaces have to be converted into more hydrophobic by functionalization with a surfactant having a low HLB (hydrophilic/

74

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 4.1. Encapsulation of inorganic nanoparticles by miniemulsion polymerization.

lipophilic balance) value, with coupling agents or surface modifiers. Apart from the enhancement of the surface hydrophobicity of the inorganic particles, in some cases different reactive groups are introduced for polymerization with the hydrophobic monomer. The type of surface modifier or coupling agent used depends on the type of inorganic nanoparticle to encapsulate. For example, calcium carbonate (CaC0 3 ) can be successfully encapsulated into polymer particles using stearic acid as hydrophobizing agent [16]. Erdem et al. [17-20] used OLOA 370, a polybutene succinimide pentamide, for a successful dispersion of titanium dioxide (Ti02) nanoparticles into styrene and cyclohexane. However, when the organic phase was a mixture of styrene and n-butylacrylate, Al-Ghamdi et al. [21] found that Solsperse 32000, a polyamine/polyester comb polymer with several anchor and several tails connected together in one molecule, was the best stabilizer for Ti0 2 . In the case of yttrium oxysulfide (Y202S) phosphorescent nanoparticles, the polymeric dispersant Solsperse 24000 improved Y 2 0 2 S dispersion and stability in styrene and methyl methacrylate monomers [22]. Fluorescent CdS/ZnS-coated CdSe or CdS quantum dots could be functionalized with a trialkylphosphine modified with an atom transfer radical polymerization (ATRP) initiator (i.e., 2-chloropropionate) [23], trioctylphosphine oxide (TOPO) [24,25], and 4-mercaptovinylbenzene [25] to be dispersed and stabilized in the organic phase previous to carry out the miniemulsion process. Zinc oxide (ZnO) nanoparticles were coated with 3-(trimethoxysilyl)propylmethacrylate (TPM) [26]

ENCAPSULATION OF INORGANIC NANOPARTICLES

75

and 3-aminopropyltriethoxysilane (APTS) [27] before dispersing them in styrene and styrene/n-butylacrylate, respectively. Silver (Ag) nanoparticles were made hydrophobic by dodecanethiol [28], and nano-alumina (A1203) with oleic acid (OA) [29]. On the other hand, in literature the surface modification of silica (Si02) and magnetite (Fe304) nanoparticles is studied in detail and several coupling agents are proposed for their hydrophobization. In the case of Si0 2 , silane coupling agents, such as TPM, 3-aminopropyltriethoxysilane (APTS), 3-glycidoxypropyltrimothoxysilane (GPM), or 3-mercaptopropyltrimethoxysilane (MPM) are commonly used [30-33], whereas for Fe 3 0 4 , fatty acids, such as oleic acid (OA), oleoyl sarkosine acid (OSA) or stearic acid are extensively used [34-37].

4.2.2 Dispersion of Hydrophobized Inorganic Nanoparticles in Monomer Phase In this step, a stable dispersion of inorganic nanoparticles in the organic phase is required. For that purpose, the previously hydrophobized inorganic nanoparticles are mixed with the monomer and the hydrophobe to polymerize and to suppress the Ostwald ripening effect, respectively. The stability of inorganic nanoparticles can be described by the well-known DLVO theory of electrostatic stabilization [38,39]. However, the primary minimum of the inorganic nanoparticle stabilization in nonaqueous media means steric stabilization. This arises from the adsoption of the anchoring groups of a stabilizer molecule onto the surface of each particle while the rest of the molecule, solvated by the medium, provides steric stabilization [40]. Thus, the key factor in this step is the nature and surface density of the surface modifier or coupling agent used.

4.2.3

Miniemulsification of the Lipophilic Dispersion in Water

The stable dispersion of inorganic nanoparticles in the monomer is miniemulsified in the water phase, employing a surfactant with high HLB which has a higher tendency to stabilize the monomer (polymer)/water interface. For that, the mixture is sonicated up to a stable miniemulsion is achieved. The stability of the miniemulsion droplets arises from the use of an ionic surfactant coupled with a low molecular weight, highly water-insoluble co-stabilizer (hydrophobe),

76

MINIEMULSION POLYMERIZATION TECHNOLOGY

which substantially retards diffusion of monomer out of the miniemulsion droplets (Otswald ripening effect) [9,10]. However, a successful encapsulation of inorganic nanoparticles via miniemulsion polymerization requires these nanoparticles to be substantially smaller than the monomer droplets generated [18,19].

4.2.4 Polymerization of Droplets The main difficulty in the process of miniemulsion polymerization is to avoid micellar nucleation and reduce homogeneous nucleation in order to ensure monomer droplet nucleation. In the case of encapsulation of inorganic nanoparticles, micellar and homogeneous nucleation will result in the formation of pure polymer particles decreasing encapsulation degree. Once a stable miniemulsion containing inorganic nanoparticles inside monomer droplets is obtained, adding the initiator and raising the temperature then start the polymerization. It is well known that in miniemulsion polymerization the nucleation occurs mainly inside monomer droplets. When inorganic nanoparticles are hydrophobic and dispersed directly in monomer droplets before polymerization, they will be encapsulated directly and do not need to diffuse into micelles; thus, the mass transfer of such large amount (in volume) of inorganic nanoparticles is avoided [9,10].

4.3

Encapsulation of Silica Nanoparticles

Monodisperse colloidal silica particles having diameters in the range 50-700 nm can be conveniently prepared by hydrolysis of tetra alkyl orthosilicates according to Stöber 's method [41]. Silica can be used in a wide range of colloidal products, ranging from paints and magnetic fluids to high-quality paper coatings because it is chemically inert and optically transparent. In addition, silica nanoparticles have also great potentials in biomédical applications due to good compatibility, resistance to decomposition in vivo and the presence of surface silanol groups. These groups can easily react with alcohols and silane coupling agents to produce dispersions that are not only stable in non-aqueous solvents but also provide ideal anchorage for covalent bonding of specific ligands [42-44]. Therefore, the encapsulation of silica nanoparticles by hydrophobic polymers allows the preparation of well-defined organic/inorganic

ENCAPSULATION OF INORGANIC NANOPARTICLES

77

hybrids nanomaterials with properties suitable for use in a variety of sensing or biomédical applications in addition to preparing materials suitable for reinforcing organic polymers.

4.3.1.

M i n i e m u l s i o n Polymerization with Hydrophilic Silica Nanoparticles

In a first approach, Tiarks et al. [45] carried out the polymerization of styrene (St) by a miniemulsion process in the presence of a hydrophobe (hexadecane), a coupling monomer (4-vinylpyridine) and bare silica nanoparticles of 22 nm, using 2,2'-azobis isobutironitrile (AIBN) as initiator. Depending on the reaction conditions and the surfactants employed (nonionic, anionic or cationic surfactant), different hybrid morphologies were obtained, comprising a raspberrylike morphology where the silica surrounds the latex particles and provides stabilization even without any low molecular weight surfactant. This morphology was obtained because under alkaline conditions 4-vinylpyridine provides a strong acid-base interaction with silica. At other compositions, non-coupled structures as well as completely encapsulation of silica nanoparticles in the polymer particle were obtained. The addition of the nonionic surfactant Lutensol AT50, due to the specific interaction of silica surfaces with poly(ethylene glycol), was counterproductive and worsens the particle size distribution and hybrid definition. The anionic surfactant sodium dodecyl sulfate (SDS), when applied under basic conditions, assisted the colloidal stability but did not really improve incorporation of silica nanoparticles, which was presumably due to the negative charge of both species. The cationic surfactant cetyltrimethylammonium chloride (CTMA) led to an improvement of the coupling between latex and silica nanoparticles. Using very large amounts of CTMA to hydrophobize the complete silica surface led to the encapsulation of silica within the latex particles. However, the non-homogeneity of both particles size and silica loading observed in this morphology, suggests that the colloidal stabilization of the silica nanoparticles within the monomer droplets is still to be improved. In more recent papers [46,47], raspberry-like composite microspheres with polystyrene (PSt) as cores and silica nanoparticles of 20 nm as shell were prepared through miniemulsion polymerization by using SDS as surfactant and 1-vinylimidazole [46] or 2-(methacryloyl)ethyltrimethylammonium chloride [47] as auxiliary cationic monomer. The strong acid-base interaction between acidic

78

MINIEMULSION POLYMERIZATION TECHNOLOGY

hydroxyl groups of the silica surface and basic amino groups of the cationic monomer promote the formation long-term stable PSt/Si0 2 nanocomposites with raspberry-like morphology, but no encapsulation was observed.

4.3.2

Miniemulsion Polymerization with Surface-Modified Silica Nanoparticles

It was found in literature that the most common silane coupling agent used to modify the surface of silica nanoparticles previous to encapsulate them by miniemulsion polymerization is TPM [48-54]. The surface silylation with TPM provides silica nanoparticles with an organic layer, thus achieving a considerable organophilation and compatibility with hydrophobic monomers. In addition, TPM bears a polymerizable double bound, acting as a comonomer in the polymerization of vinyl monomers. Zhang et al. [48] studied the encapsulation of TPM-modified silica nanoparticles by the miniemulsion polymerization of St using SDS as surfactant, hexadecane as hydrophobe and potasium persulfate (KPS) as initiator. They found that the size and morphology of the nanocomposite particles could be tuned by adjusting the silica particle size and surfactant concentration. For 45 nm silica nanoparticles, the size of the nanocomposite decreased from 200 to 80 nm with increasing surfactant concentration from 20 to 40 mM, and the number of silica nanoparticles encapsulated into each polymer particle gradually decreased and finally formed core-shell morphology (see TEM microphotographs in Figure 4.2). For 90 nm silica nanoparticles, the size of the nanocomposite particles also decreased from 180 to 130 nm with increasing surfactant concentration from 20 to 40 mM, but the core-shell morphology was kept unchanged. For 200 nm silica nanoparticles, some "raspberry-like" morphology was observed (see TEM microphotographs in Figure 4.3). In addition, they studied the effect of surface chemistry of the silica nanoparticles on the size and morphology of the nanocomposites particles by using two types of hydrofobized silica having 3.6 and 8.7 TPM molecules per square nanometer. The graft density of TPM on silica surface had almost no influence on the size and morphology of the nanocomposite nanoparticles. This behavior was completely different from that of an emulsion polymerization due to the different nucleation mechanism. In emulsion polymerization, the surface chemistry of the silica nanoparticles, especially the density

ENCAPSULATION OF INORGANIC NANOPARTICLES

79

Figure 4.2. TEM microphotographs of Si0 2 /polystyrene composite particles obtained using different SDS concentrations in the presence of 45 nm silica particles: (a) 20 mM; (b, c) 30 mM; (d) 40 mM. (Reprinted from Ref. 48 with permission from the American Chemical Society).

of TPM grafted on the silica surface, strongly influenced the final morphology of the nanocomposite particles [55]. Zhou et al. [49] found that the use of a soft monomer such as n-butyl acrylate (BA) also affected the encapsulation of silica nanoparticles by miniemulsion polymerization. The incorporation of BA together with St into the polymer was propitious for causing a multicore-shell morphology in comparison with only having a hard monomer (St) in the polymer, other parameters being equal. This multicore-shell structure was formed because BA and its polymer have much lower surface free energies than St and its polymer,

80

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 4.3. TEM microphotographs of Si0 2 /polystyrene composite particles obtained using 20 mM SDS in the presence of silica particles with different sizes: (a) 45 nm; (b) 90 nm; (c) 200 nm. (Reprinted from Ref. 48 with permission from the American Chemical Society).

incorporating BA monomer should decrease the surface energy of the miniemulsion, increasing the droplet size of the miniemulsion. In addition, the TPM-modified silica nanoparticles had a surface structure similar to that of the BA molecules, which also helped in lodging more silica nanoparticles in each droplet. On the other hand, increasing the particle size of the silica or decreasing the surfactant content tended to form a normal core-shell or even raspberry-like structure. Furthermore, the encapsulation of TPM-modified silica nanoparticles by miniemulsion polymerization was also successful using a mixture of methyl methacrylate (MMA) and BA [51] and a mixture of St and 2-hydroxyethylmethacrylate (HEMA), or

ENCAPSULATION OF INORGANIC NANOPARTICLES

81

styrene sulfonic acid (SSA), or aminoethyl methacrylate hydrochloride (AEMH) [53]. Recently, Costoyas et al. [54] synthesized core-shell hybrid nanoparticles having narrow particle size distributions (PSDs) as well as a high degree of silica encapsulation by miniemulsion polymerization of St using SDS as surfactant, hexadecane as hydrophobe, and KPS as initiator. In this work, to enhance the hydrophobicity of the silica nanoparticles, apart from using TPM, another surface modifier was added simultaneously, i.e. oleic acid (OA). OA can be bonded to the silanol groups present at surface of silica nanoparticles by a simple hydrogen bond, and the double bonds are able to polymerize with a vinyl monomer. A synergistic effect was observed using TPM together with OA in the compatibilization step between the organic phase (monomer) and silica nanoparticles. In addition, to optimize the production of monodisperse hybrid particles, they studied the effect of the size of silica nanoparticles, the ratio styrene/silica, the surfactant concentration, and the presence of ethanol in the reaction. Increasing the size of the silica nanoparticles (66,107, and 202 nm) larger particle sizes of hybrid latexes (89,95, and 155 nm) were obtained. However, only the size of the latex with the smallest silica showed fully encapsulation. The decrease in St concentration a n d / or the increase in surfactant concentration caused smaller particle diameters and lower encapsulation degree. In addition, when the miniemulsion polymerization was carried out without ethanol a larger particle size and a higher conversion were achieved than those when ethanol was used. Figure 4.4 shows the TEM microphotographs of the latex obtained with ethanol (MP8), and without ethanol (MP11). As can be seen, latex particles prepared in the presence of alcohol (MP8, Figure 4.4.a) had a smaller particle diameter than those obtained without alcohol (MP11, Figure 4.4.b). The most of the hybrid particles of latex MP11 had one silica nucleus, finding some hybrid particles with 2 or 3 nuclei (Figure 4.4.c). In addition, the cycles of centrifugation and re-dispersion were effective and no pure polymer particles were observed in latex MP11H. A more challenging encapsulation of silica nanoparticles by miniemulsion polymerization used silane-coupling agents, which are able to initiate a controlled /living radical polymerization of hydrophobic monomer droplets. Alkoxamine initiators based on N-tert-butyl-1diethylphosphono-2,2-dimethylpropyl nitroxide (DEPN) and carrying a terminal functional group were synthesized in situ and grafted to silica surface by Bailly et al. [56]. The resulting grafted alkoxamines

82

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 4.4. TEM microphotographs of latexes MP8 (a); MP11 (b); and hybrid particles of latex MP11 (after various cycles of centrifugation-redispersion) MP11H (c). (Reprinted from Ref. 54 with permission from Wiley).

were employed to initiate, through nitroxide-mediated polymerization (NMP), the growth of polystyrene chains from the silica surface. Then, the PSt-grafted silica nanoparticles were entrapped inside latex particles via miniemulsion polymerization forming silica-polystyrene core-shell particles. On the other hand, Bombalski et al. [57] reported the efficient synthesis of hybrid organic/inorganic nanoparticles using silica nanoparticles with surface tethered initiators an activators generated by electron transfer (AGET) ATRP miniemulsion process of BA. The surface-modified silica was prepared by reacting l-(chlorodimethylsilyl)propyl 2-bromoisobutyrate with hydroxyl groups on silica particle surface. In comparison to the bulk polymerization, using the same stoichiometry, miniemulsion allowed the preparation of hybrid materials with a higher yield, i.e., higher monomer conversion, and a higher polymerization rate without macroscopic gelation. Direct visualization by AFM provided additional evidence for the formation of wellcontrolled hybrids.

ENCAPSULATION OF INORGANIC NANOPARTICLES

83

4.3.3 Miniemulsion Polymerization with Locally Surface-Modified Silica Nanoparticles In the last years, some works presented the synthesis of asymmetric nanocomposites particles of polystyrene and silica via conventional (with a chemical initiator) [58,59] or radiation miniemulsion polymerization [60]. The key to obtain the asymmetric nanocomposite particle pairs was the combination of miniemulsion polymerization and the local surface modification of silica nanoparticles. Because of localized surface modification on the silica surface by using silane-coupling agents, such as n-octadecyltrimethoxysilane (ODMS) [58,59] or TPM [60], the nucleation and formation of the polymer nodule in miniemulsion polymerization took place only in the modified area on silica surface, thus ensuring the asymmetry morphology. In addition, controlling the monomer/silica weight ratio, different morphological anisotropic PSt/silica hybrid particles were prepared (see Figure 4.5). The morphology varied from mushroom-like, via hollow egg-like, to bowl-like structures with increasing monomer/silica weight ratio. This method offers an effective and feasible way to synthesize morphologically controllable polymer/inorganic anisotropic hybrid nanoparticles, which because to the existence of different surface functionalities, could have a wide range of potential applications in bioscience and molecular recognition. Polystyrene/silica hybrid asymmetric dimer particles were also synthesized by miniemulsion polymerization in one step [61]. In this case, a solution of St monomer with dissolved AIBN was added to tetraethyl orthosilicate (TEOS), which is the precursor of silica nanoparticles, TPM, and hexadecane to form an oil phase. On the other hand, cetyltrimethylammonium bromide (CTAB) dissolved in water was employed as the aqueous phase. The mixture of oil and water phase was sonicated and miniemulsion was carried out. After the polymerization of styrene, ammonia was added to the dispersion to form silica nanoparticles. Phase separation of the organic moiety and the inorganic moiety resulted from temperature raising and polymerization, and it was enhanced after the formation of polystyrene and silica due to the hydrophobicity of polystyrene and hydrophilicity of silica. In addition, the size of polystyrene particles in asymmetric dimer particles was adjusted easily either by changing the weight ratio of St/TEOS or by altering the sonication power during the miniemulsion preparation.

84

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 4.5. TEM (a-e) and SEM (a'-e') images of anisotropic PS/silica hybrid particles synthesized in the presence of locally surface-modified silica nanoparticles. The weight ratio of monomer/silica was increased from a to e and a'to e'; (a) and (a'): 28 : 1, (b) and (b'): 60 : 1, (c) and (c'): 72 : 1 , (d) and (d'): 80 : 1 , and (e) and (e'): 100 :1.). (Reprinted from Ref. 60 with permission from the Royal Society of Chemistry).

ENCAPSULATION OF INORGANIC NANOPARTICLES

85

4.4 Encapsulation of Magnetite Nanoparticles Magnetic fluids or ferrofluids can be classified according to the hydrophilicity-hydrophobicity of the stabilizers used as hydrophilic and hydrophobic ferrofluids. They are stable dispersions of ultrafine magnetic particles in an organic or aqueous carrier medium. The stabilization of these ultrafine magnetic particles can be achieved by adsorption of stabilizers to hinder particles flocculation and sedimentation. Magnetite, Fe 3 0 4 , is the most common magnetic material used, and it is usually prepared by chemical co-precipitation of an aqueous of Fe 3+ /Fe 2+ solution (Fe3+: Fe2+ = 2:l(mol)) with a base in the presence of a stabilizer or surface modifier, such as oleic acid (OA), the most common used one. Magnetic polymer nanoparticles (MPNPs) have extensive applications in biomédical [62,63], bioengineering and biotechnology fields, such as cell separation [64], immunoassays [65], nucleic acid purification [66], DNA separation [67], enzyme immobilization [68], magnetic resonance imaging [69], and hyperthermia [70]. MPNPs exhibit high magnetic susceptibility to an external magnetic field and in addition, they are easily further functionalized and surface-modified by the attachment of various bioactive molecules [71]. In all the cases, MPNPs should fulfill certain criteria to fit further biotechnological applications [72], such as: no sedimentation, uniform size and size distribution, high and uniform magnetic content, superparamagnetic behavior, no toxicity, and no iron leaking. In general, however, the polymerization process for the encapsulation of magnetite may produce three possible types of particles in the resulting magnetic latex, i.e., magnetic polymeric nanoparticles (MPNPs; with magnetite encapsulated inside), pure polymer nanoparticles (PPNPs; without magnetite inside), and bare (free) magnetite nanoparticles (BMNPs; without polymer coating). The existence of PPNPs is not desirable because the magnetic properties of the composite particles will be decreased, and the existence of BMNPs is also not desired because they cannot be further functionalized due to their bare surfaces without polymer surrounding. In addition, particle size distribution (PSD) should be as narrow as possible so that MPNPs can respond to an external magnetic field as uniformly as possible.

86

MINIEMULSION POLYMERIZATION TECHNOLOGY

4.4.1 Encapsulation of Magnetite by a Single Miniemulsion Polymerization Process Most of the works found in literature devoted to encapsulate magnetite nanoparticles [73-98] proposed the single miniemulsion process consisting in four steps shown in Figure 4.1. As aforementioned, the successful incorporation of hydrophilic magnetite nanoparticles into hydrophobic polymer particles by miniemulsion polymerization relies on their surface modification to make magnetite/polymer compatible. Different surface modifiers have been used to obtain stable and hydrophobic magnetic nanoparticles, being oleic acid (OA) the most common one [75-84,87,92-97]. The carboxylate head of OA is able to anchor on the surface of iron oxide nanoparticles, while its hydrophobic tail ensures steric stabilization as well as compatibility with the solvent [34-37]. Oleoyl sarcosine acid (OSA) [73,95], bis(2-ethylhexyl) sulfosuccinate (AOT) [74], TPM [85,95], sorbitan oléate (Span-80) [86,95,96], alkylolammonium salts of low molecular weight polycarboxylic acid polymer (Disperbik-106, Disperbik-108, and Disperbik-111) [87,88], Sipomer PAM200, which is a phosphate-based poly(propylene glycol) methacrylate [89], lauric acid [90,91], stearic acid [95], and 12-hexanoyloxy-9-octadecenoic acid (HOA) [98] have also been used. However, most of them caused (i) inhomogeneous distribution of the magnetic nanoparticles inside and among the particles and / o r (ii) PPNPs a n d / o r (iii) BMNPs or magnetic aggregates in the aqueous phase a n d / o r (iv) broad PSD, a n d / o r (v) limited loading of the particles with magnetic material. With respect to the monomers, St has been the most common one used [73,74,76,78,80-98], but acrylates [75,77,79,8591,96] such as MMA and BA have also been successfully used. In addition, various surfactants, and hydrophobes have also been tested, being the most common ones SDS and hexadecane, respectively. The use of water-soluble or oil-soluble initiator affects greatly the morphology of the final composite. Mori et al. [84] observed that the size of monomer droplets/polymer particles increased from 60 to 300 nm during polymerization, keeping magnetite in core when potassium persulfate (KPS) or ammonium persulfate (APS) was used as the sole water-soluble initiator. In contrast, when 2,2'-azobis isobutyronitrile (AIBN) was used as the oil-soluble initiator, the size of the droplets/particles was retained to be 90 nm at the most and magnetite nanoparticles located at the surface of polystyrene particles.

ENCAPSULATION OF INORGANIC NANOPARTICLES

87

The effect of initiator on particle size in persulfate system was likely originated from the decrease of pH value and the increase of ionic strength, which induced the fusion of droplets/particles containing magnetite. When both AIBN and KPS were used, magnetite distributed randomly in polystyrene particles and the size of particles was around 200 nm. Mixed initiator systems employed to improve conversion and encapsulation of magnetite resulted in the formation with middle characteristics between those of sole initiator systems. On the other hand, recently ultrasound initiated miniemulsion polymerization was employed to prepare stable MPNPs [95-98] due to its many advantages, such as high polymerization rate, free chemical initiator, low surfactant concentration and low reaction temperature. It is well known that the challenges in almost all the strategies involving a single miniemulsion polymerization process to prepare MPNPs are to minimize, even eliminate, the formation of PPNPs and BMNPs during the preparation stage with an even greater challenge in obtaining MPNPs with very narrow PSD. However, to date these important issues have not been analyzed and studied in detail in literature. With the aim of optimizing the encapsulation of magnetite, Lu and Forcada [82] prepared MPNPs by miniemulsion polymerization in the presence of oil-based St ferrofluid with hexadecane as hydrophobe, AIBN as initiator and SDS as surfactant. Methacrylic acid (MAA) was used as comonomer, and hydroxyethyl cellulose (HEC) and polyvinylpyrrolidone (PVP) were used as aid stabilizers subsequently. Reducing the amount of SDS improved the Fe 3 0 4 encapsulation degree, narrowed the PSD, and reduced the number of BMNPs and PPNPs at the same time. The optimum percentage of SDS, based on the total amount of St and Fe 3 0 4 , was 2-3%. Increasing the amount of hexadecane improved the Fe 3 0 4 encapsulation degree, sharpened the PSD, and reduced the number of BMNPs and PPNPs. Increasing the amount of Fe 3 0 4 increased the difficulty of the encapsulation of Fe 3 0 4 . The number of BMNPs increased, and the PSD became much broader when the concentration of Fe 3 0 4 increased. The optimum concentration of Fe 3 0 4 was 10%. The use of MAA as a comonomer facilitated the encapsulation of Fe 3 0 4 , but at the cost of broadening the PSD. The particle size of the MPNPs was larger than that of the reactions carried out without MAA. HEC in the recipe assisted the encapsulation of Fe 3 0 4 and also improved the PSD of MPNPs. The most suitable concentration of HEC for use as a stabilizer aid was 2% together with 15% of

88

MINIEMULSION POLYMERIZATION TECHNOLOGY

hexadecane. PVP improved the encapsulation degree of Fe 3 0 4 and reduced the polydispersity of MPNPs. However, in the next work of Lu et al. [83] fairly monodisperse selfstabilized magnetic polymeric composite nanoparticles (SS-MPCPs) were prepared by surfactant-free miniemulsion polymerization using St as main monomer, sodium p-styrene sulfonate (NaSS) as ionic comonomer, hexadecane as hydrophobe, and AIBN as initiator in the presence of oleic acid-coated magnetite particles. TEM microphotographs of the SS-MPCPs synthesized with different amounts of NaSS are shown in Figure 4.6. As can be seen, the amount of NaSS had a notable effect on the magnetite encapsulation degree, particle size, and PSDs of the SS-MPCPs. From 5% to 15% of NaSS the encapsulation degree was increased, and some BMNPs but no PPNPs were observed. At 20% NaSS, the encapsulation of magnetite was successful, and neither BMNPs nor PPNPs were observed. The absence of PPNPs formed during the polymerization process indicated that monomer droplet nucleation was achieved entirely by using an emulsifier-free miniemulsion polymerization technique. Micellar nucleation was avoided completely in the absence of emulsifier, and homogeneous nucleation was also prevented. However, when the concentration of NaSS was 25% a significant number of PPNPs were observed. At this concentration, the amount of oligomers formed in the aqueous phase was enough to stabilize the particles generated by homogeneous nucleation and PPNPs were produced. On the other hand, the distribution of magnetite inside composite particles is an important factor, which can influence the magnetic characteristics of composite particles. In all the SS-MPCPs synthesized the magnetite particles are mainly in the core of the composite particles. Due to the ionic character of NaSS, the copolymerization of St with NaSS increases the hydrophihcity of the polymer formed and enhances the hydrophobicity of oleic acidcoated magnetite particles. Thus, magnetite particles are mainly in the core of SS-MPCPs. In addition, a few large composite particles in the magnetic latexes were observed. The distribution of magnetite particles inside the largest composite particles was completely different to that of the smallest ones. This means that the formation of this kind of particles was due to aggregation between some of the particles formed. Besides, it can be seen that the number of magnetite particles encapsulated inside each magnetic particle was not the same.

ENCAPSULATION OF INORGANIC NANOPARTICLES

89

Figure 4.6. TEM microphotographs of SS-MPCPs prepared with different amounts of NaSS (Reprinted from Ref. 83 with permission from the American Chemical Society).

4.4.2 Encapsulation of Magnetite by a Double Miniemulsion Polymerization Process Using the single miniemulsion polymerization process detailed above an inhomogeneous distribution of the magnetite in the polymer nanoparticles together with a limited magnetite content in the

90

MINIEMULSION POLYMERIZATION TECHNOLOGY

polymer matrix (15-20%) is obtained. In order to increase the magnetic content in the polymer particles another process was developed by Landfester et al. [99-101]. They encapsulated high amounts of magnetite particles into polystyrene particles by a three-step preparation route including two miniemulsion processes. In the first step, oleic acid coated magnetic nanoparticles in octane were prepared. In the second step, a dispersion of magnetite in octane was miniemulsified in water by using SDS as surfactant. After evaporation of the octane, the magnetite aggregates, which were covered by an OA/SDS bilayer, were mixed with a St miniemulsion and in the third step of the synthesis route; an ad-miniemulsification process was used to obtain final and full encapsulation. Here, a fusion/fission process induced by ultrasound was just effective for the monomer droplets, whereas the monomer coated magnetite aggregates stayed intact. That way, all monomer droplets were split and hetero-nucleated onto the magnetite aggregates to form a monomer film. After polymerization, polymer encapsulated magnetite aggregates were obtained, and they found that up to 40% magnetite could be encapsulated, resulting in particles with a high homogeneity of the magnetite content. On the other hand, magnetite/polystyrene latexes with narrow size distribution and high magnetite content were also prepared by a hybrid miniemulsion polymerization process containing binary droplets [102-104]. First of all, magnetite nanoparticles modified with oleic acid were synthesized and dispersed in octane. This ferrofluid was added to an aqueous solution with SDS as surfactant and treated ultrasonically to obtain miniemulsion droplets composed of magnetite nanoparticles aggregations with a diameter of 100-200 nm (Mag-droplets). Another miniemulsion made of St monomer droplets with a diameter of 3-4 mm (St-droplets) was prepared by membrane emulsification equipment and mixed with Mag-droplets to obtain a double-miniemulsion system, which contained microsized St droplets and nanosized magnetite aggregation droplets. With extremely low surfactants concentration, the nucleated loci were selectively controlled in the Mag-droplets, as the result of smaller droplet size and larger surface ratio. Both water soluble KPS and oil-soluble AIBN was adopted to initiate the polymerization. When AIBN was used, the obtained magnetic polystyrene latex (MPL) exhibited spherical shape. Nevertheless, microphase separation still occurred in the interior of MPL. However, in the presence of KPS, MPLs with particle size of 60-200 nm, narrow size distribution, and

ENCAPSULATION OF INORGANIC NANOPARTICLES

91

Figure 4.7 TEM images, at different magnifications, of MPLs obtained using KPS as initiator. (Reprinted from Ref. 103 with permission from Wiley).

high magnetite content (86 wt%) were attained successfully (see Figure 4.7). The magnetite content of as-synthesized MPLs depends on the volume ratio of Mag-droplets and St-droplets. In addition, without hexadecane in the St-droplets, the achieved MPLs have wide size distribution because of fast monomer diffusion during the polymerization process.

4.5

Conclusions and Future Perspectives

In this chapter, the incorporation of inorganic nanoparticles into miniemulsion polymerizations to give hybrid nanoparticles is revised. It can be concluded that the use of the right combination of surface modifiers or coupling agents together with the adequate miniemulsion process (single or double) are the key factors for attaining a high encapsulation degree of the inorganic nanoparticles together with a well-controlled morphology of the hybrid nanocolloids. Up to now, most of reports present the synthesis of polymerinorganic nanoparticles using only one type of inorganic nanoparticle. However, the use of two populations of inorganic nanoparticles opens new routes to the production of multilayer or multimodal hybrid nanoparticles [50,52,102,105]. Recently, van Berkel et al. [105] presented a convenient and highly modular co-encapsulation method by miniemulsion polymerization for the preparation of multimodal composite nanoparticles, consisting of a spherical,

92

MINIEMULSION POLYMERIZATION TECHNOLOGY

cross-linked poly(diviny lbenzene) matrix imbibed with two different types of inorganic core materials (MnFe 2 0 4 and Au nanoparticles). These particles exhibit a combination of properties characteristic of the incorporated inorganic material, in this case, magnetism due to MnFe 2 0 4 nanoparticles and UV-Vis absorption due to the surface plasmon resonance of Au nanoparticles. First, MnFe 2 0 4 and Au nanoparticles grafted with short polystyrene ligands were dispersed in the monomer, divinylbenzene. This monomer/nanoparticle solution was then emulsified with an aqueous solution of CTAB surfactant, via ultrasonication, to generate a miniemulsion of submicrometer monomer droplets containing the inorganic nanoparticles. The free radical polymerization of these droplets (using 2,2'-azobis(2-amidinopropane) dihydrochloride as initiator) yields the co-encapsulation of MnFe 2 0 4 and Au nanoparticles.

4.6

Acknowledgements

This work has been supported by the Spanish Programa Nacional de Materiales (MAT 2009-13155-C04-01). J. Ramos acknowledges the financial support by the Ministerio de Ciencia e Innovación: Subprograma Juan de la Cierva (JCI-2008-2217).

References 1. G. Schottner, Chemistry of Materials, Vol. 13, p. 3422, 2001. 2. C. Sanchez, G.J.deA.A. Soler-Illia, F. Ribot, T. Lalot, C.R. Mayer, and V. Cabuil, Chemistry of Materials, Vol. 13, p. 3061, 2001. 3. F Caruso, Advanced Materials, Vol. 13, p. 11,2001. 4. V.N. Manoharan, M.T. Elsesser, and D.J. Pine, Science, Vol. 301, p. 483, 2003. 5. Z. Yang, Z. Niu, Y. Lu, Z. Hu, and C.C. Han, Angewandte Chemie, International Edition, Vol. 42, p. 1943,2003. 6. E. Bourgeat-Lami, Journal of Nanoscience and Nanotechnology, Vol. 2, p. 1, 2002. 7. V. Castelvetro, and C. DeVita, Advances in Colloid and Interface Science, Vol. 108109, p. 167,2004. 8. K. Landfester, Advanced Materials, Vol. 13, p. 765,2001. 9. K. Landfester, Macromolecular Rapid Communications, Vol. 22, p. 896,2001. 10. M. Antonietti, and K. Landfester, Progress in Polymer Science, Vol. 27, p. 689, 2002. 11. J.M. Asua, Progress in Polymer Science, Vol. 27, p. 1283,2002. 12. K. Landfester, Topics in Current Chemistry, Vol. 227, p. 75, 2003.

ENCAPSULATION OF INORGANIC NANOPARTICLES

93

13. M.S. El-Aasser, and E.D. Sudol, Journal of Coatings Technology and Research, Vol. 1, p. 20, 2004. 14. K. Landfester, Annual Reviews in Materials Research, Vol. 36, p. 231,2006. 15. K. Landfester, Angewandte Chemie, International Edition, Vol. 48, p. 4488,2009. 16. N. Betchthold, F. Tiarks, M. Willert, K. Landfester, and M. Antonietti, Macromolecular Symposia, Vol. 151, p. 549, 2000. 17. B. Erdem, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Macromolecular Symposia, Vol. 155, p. 181,2000. 18. B. Erdem, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 38, p. 4419, 2000. 19. B. Erdem, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 38, p. 4431, 2000. 20. B. Erdem, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 38, p. 4441,2000. 21. G.H. Al-Ghamdi, E.D. Sudol, V.L. Dimonie, and M.S. El-Aasser, Journal of Applied Polymer Science, Vol. 101, p. 3479,2006. 22. H. Kim, E.S. Daniels, S. Li, V.K. Mokkapati, and K. Kardos, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 45, p. 1038,2007. 23. A.C.C. Esteves, L. Bombalski, B. Cusick,A. Barros-Timmons, K. Matyjaszewski, and T. Trindade, Polymer Preprints, Vol. 46, p. 134,2005. 24. A.C.C. Esteves, A. Barros-Timmons, T. Monteiro, and T. Trindade, Journal of Nanoscience and Nanotechnology, Vol. 5, p. 766,2005. 25. N. Jourmaa, M. Lansalot, A. Théretz, and A. Elaissari, Langmuir, Vol. 22, p. 1810,2006. 26. J.J. Zhang, G. Gao, M. Zhang, D. Zhang, C.L. Wang, D.C. Zhao, and F.Q. Liu, Journal of Colloid Interface Science, Vol. 301, p. 78,2006. 27. E. Tang, and S. Dong, Colloid and Polymer Science, Vol. 287, p. 1025,2009. 28. H.D. Koh, M. Changez, J.P. Lee, and J.S. Lee, Macromolecular Rapid Communications, Vol. 30, p. 1583,2009. 29. A.R. Mahdavian, Y. Sarrafi, and M. Shabankareh, Polymer Bulletin, Vol. 63, p. 329, 2009. 30. A.R Philipse, and A. Vrij, Journal of Colloid and Interface Science, Vol. 128, p. 121.1989. 31. J.W. Goodwin, R.S. Harbron, and P.A. Reynols, Colloid and Polymer Science, Vol. 268, p. 766,1990. 32. R.D. Badley, W.T. Ford, FJ. McEnroe, and R.A. Assink, Langmuir, Vol. 6, p. 792.1990. 33. A. Van Blaaderen, and A. Vrij, Journal of Colloid and Interface Science, Vol. 156, p. 1,1993. 34. A. Wooding, M. Kilner, and D.B. Lambrick, Journal of Colloid and Interface Science, Vol. 144, p. 236,1991. 35. L. Shen, P.E. Laibinis, and T.A. Hatton, Langmuir, Vol. 15, p. 447,1999. 36. S. Sun, and H. Zeng, Journal of the American Chemical Society, Vol. 124, p. 8204, 2002. 37. M. Lattuada, and T.A. Hatton, Langmuir, Vol. 23, p. 2158,2007. 38. R.J. Hunter, Foundations of Colloid Science, Oxford, Clarendon Press, 1987. 39. S. Ross and I.D. Morrison, Colloidal Systems and Interfaces, New York, Wiley, 1988.

94

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

40. D.H. Napper, Polymeric Stabilization of Colloidal Dispersions, New York, Academic, 1983. 41. W. Stöber, A. Fink, and E. Bohn, Journal of Colloid and Interface Science, Vol. 26, p. 62,1968. 42. Z.F. Li, and E. Ruckenstein, Nano Letters, Vol. 4, p. 1463,2004. 43. S. Santra, H. Yang, D. Dutta, J.T. Stanley, P.H. Holloway, W. Tan, B.M. Moudgil, and R.A. Mericle, Chemical Communications, p. 2810,2004. 44. L.M. Rossi, L. Shi, EH. Quina, and Z. Rosenzweig, Langmuir, Vol. 21, p. 4277, 2005. 45. F. Tiarks, K. Landfester, and M. Antonietti, Langmuir, Vol. 17, p. 5775, 2001. 46. Y. Zhang, H. Chen, and Q. Zou, Colloid and Polymer Science, Vol. 287, p. 1221, 2009. 47. Y. Zhang, H. Chen, X. Shu, Q. Zou, and M. Chen, Colloid and Surfaces A: Physicochemical and Engineering Aspects, Vol. 350, p. 26,2009. 48. S.W. Zhang, S.X. Zhou, Y.M. Weng, and L.M. Wu, Langmuir, Vol. 21, p. 2124, 2005. 49. J. Zhou, S.W. Zhang, X.G. Qiao, X.Q. Li, and L.M. Wu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 44, p. 3202, 2006. 50. J. Zhou, M. Chen, X.G. Qiao, and L.M. Wu, Langmuir, Vol. 22, pp. 1017510179,2006. 51. D.M. Qi, Y.Z. Bao, Z.X. Weng, and Z.M. Huang, Polymer, Vol. 47, p. 4622, 2006. 52. X.G. Qiao, M. Chen, J. Zhou, and L.M. Wu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 45, p. 1028, 2007. 53. O. Töpfer, and G. Schmidt-Naake, Macromolecular Symposia, Vol. 248, p. 239, 2007. 54. A. Costoyas, J. Ramos, and J. Forcada, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 47, p. 935, 2009. 55. S. Reculusa, C. Mingotaud, E. Bourgeat-Lami, E. Duguet, and S. Ravaine, Nano Letters, Vol. 4, p. 1677,2004. 56. B. Bailly. A.C. Donnenwirth, C. Bartholome, E. Beyou, and E. Bourgeat-Lami, Journal ofNanomaterials, Vol. 2006, p. 1,2006. 57. L. Bombalski, K. Min, H. Dong, C. Tang, and K. Matyjaszewski,Macromolecules, Vol. 40, p. 7429, 2007. 58. W. Qiang, Y Wang, P. He, H. Xu, H. Gu, and D. Shi, Langmuir, Vol. 24, p. 606, 2008. 59. Y. Wang, H. Xu, W. Qiang, H. Gu, and D. Shi, Journal of Nanomaterials, Vol. 2009, p. 1,2009. 60. X. Ge, M. Wang, Q. Yuan, and X. Ge, Chemical Communications, p. 2765,2009. 61. W. Lu, M. Chen, and L. Wu, Journal of Colloid and Interface Science, Vol. 328, p. 98,2008. 62. J. Ugelstad, P. Stenstad, L. Kilaas, W.S. Prestvik, R. Herje, A. Bererge, and E. Homes, Blood Purification, Vol. 11, p. 349,1993. 63. A. Elaissari, R. Veyret, B. Mandrand, and J. Chatterjee, Surfactant Science Series, Colloidal Biomolecules, Biomaterials, and Biomédical Applications, Vol. 116, p. 1,2004. 64. M. Kuhara, H. Takeyama, T. Tanaka, and T. Matsunaga, Analytical Chemistry, Vol. 76, p. 6207, 2004. 65. Y Namba, M. Usami, and O. Suzuki, Analytical Sciences, Vol. 15, p. 1087,1999.

ENCAPSULATION OF INORGANIC NANOPARTICLES

95

66. A. Elaïssari, M. Rodrigue, F. Meunier, and C. Hervé, Journal of Magnetism and Magnetic Materials, Vol. 225, p. 127, 2001. 67. Oster, J. Parker, and L. A. Brassard, Journal of Magnetics and Magnetic Materials, Vol. 225, p. 145,2001. 68. S. Akgol, K. Yasemin, A. Denizli, and M.Y. Anca, Food Chemistry, Vol. 74, p. 281,2001. 69. F.Y. Cheng, C.H. Su, Y.S. Yang, C.S. Yeh, C.Y. Tsai, C.L. Wu, M.T. Wu, and D.B. Shieh, Biomaterials, Vol. 26, p. 729,2005. 70. J. Chatterjee, M. Bettge, Y. Haik, and CJ. Chen, Journal of Magnetics and Magnetic Materials, Vol. 293, p. 303, 2005. 71. X.Q. Liu, Y.P. Guan, Z.Y Ma, and H.Z. Liu, Langmuir, Vol. 20, p. 10278, 2004. 72. K. Landfester, and L.P. Ramirez, Journal of Physics: Condensed Matter, Vol. 15, p. S1345, 2003. 73. D. Hoffmann, K. Landfester, and M. Antonietti, Magnetohydrodynamics, Vol. 37, p. 217, 2001. 74. R. Betancourt-Galindo, R. Saldivar-Guerrero, O.S. Rodríguez-Fernández, L.A. García-Cerda, and J. Matutes-Aquino, Journal of Alloys and Compounds, Vol. 369, p. 87, 2004. 75. X. Liu, Y Guan, Z. Ma, and H. Liu, Langmuir, Vol. 20, p. 10278,2004. 76. I. Csetneli, M.K. Faix, A. Szilágyi, A.L. Kovács, Z. Németh, and M. Zrinyi, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 42, p. 4802,2004. 77. X. Liu, Y. Guan, H. Liu, Z. Ma, Y Yang, and X. Wu, Journal of Magnetics and Magnetic Materials, Vol. 293, p. I l l , 2005. 78. W. Zheng, F Gao, and H. Gu, Journal of Magnetics and Magnetic Materials, Vol. 293, p. 199,2005. 79. J.S. Nunes, C.L. de Vasconcelos, F.A.O. Cabrai, J.H. de Araújo, M.R. Pereira, and J.L.C. Fonseca, Polymer, Vol. 47, p. 7646, 2006. 80. R. Faridi-Majidi, N. Sharifi-Sanjani, and F. Agend, Thin Solid Films, Vol. 515, p. 368.2006. 81. R. Faridi-Majidi, N. Sharifi-Sanjani, and F Agend, Journal of Magnetics and Magnetic Materials, Vol. 311, p. 55,2007. 82. S. Lu, and J. Forcada, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 44, p. 4187,2006. 83. S. Lu, J. Ramos, and J. Forcada, Langmuir, Vol. 23, p. 12893, 2007. 84. Y Mori, and H. Kawaguchi, Colloids and Surfaces B - Biointerfaces, Vol. 56, p. 246.2007. 85. Z. Chen, K. Peng, and Y Mi, Journal of Applied Polymer Science, Vol. 103, p. 3660, 2007. 86. Y Sun, B. Wang, H. Wang, and J. Jiang, Journal of Colloid and Interface Science, Vol. 308, p. 332, 2007. 87. Q. Zhang, H. Zhang, G. Xie, and J. Zhang, Journal of Magnetics and Magnetic Materials, Vol. 311, p. 140, 2007. 88. Q. Zhang, G. Xie, H. Zhang, J. Zhang, and M. He, Journal of Applied Polymer Science, Vol. 105, p. 3525,2007. 89. N. Joumaa, P. Toussay, M. Lansalot, and A. Elaissari, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 46, p. 327,2008. 90. Y.D. Luo, C.A. Dai, and W.Y. Chiu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 46, p. 1014,2008.

96

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

91. YD. Luo, C.A. Dai, and W.Y Chiu, Journal of Applied Polymer Science, Vol. 112, p. 975, 2009. 92. A.R. Mahdavian, Y. Sehri, and H. Salehi-Mobarakeh, European Polymer Journal, Vol. 44, p. 2482,2008. 93. S. Lu, J. Ramos, and J. Forcada, Macromolecular Symposia, Vol. 281,p. 89,2009. 94. S. Lu, R. Qu, and J. Forcada, Materials Letters, Vol. 63, p. 2539, 2009. 95. G. Qiu, Q. Wang, C. Wang, W. Lau, and Y Guo, Polymer International, Vol. 55, p. 265, 2006. 96. A.R. Mahdavian, M. Ashjari, and H.S. Mobarakeh, Journal of Applied Polymer Science, Vol. 110, p. 1242, 2008. 97. B.M. Teo, F. Chen, T.A. Hatton, F. Grieser, and M. Ashokkumar, Langmuir, Vol. 25, p. 2593, 2009. 98. Z. Qian, Z. Zhang, and Y Chen, Journal of Colloid and Interface Science, Vol. 327, p. 354,2008. 99. L.P Ramirez, and K. Landfester, Macromolecular Chemistry and Physics, Vol. 204, p. 22, 2003. 100. K. Landfester, and L.P. Ramirez, Journal of Physics: Condensed Matter, Vol. 15, p. SI 345, 2003. 101. V Holzapfel, M. Lorenz, C.K. Weiss, H. Schrezenmeier, K. Landfester, and V Mailander, Journal of Physics: Condensed Matter, Vol. 18, p. S2581, 2006. 102. H. Xu, L. Cui, N. Tong, and H. Gu, Journal of the American Chemical Society, Vol. 128, p. 15582, 2006. 103. L. Cui, H. Xu, P. He, K. Sumitomo, Y Yamaguchi, and H. Gu, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 45, p. 5285,2007. 104. T. Gong, D. Yang, J. Hu, W. Yang, C. Wang, and J.Q. Lu, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 339, p. 232,2009. 105. K.Y. van Berkel, A.M. Piekarski, P.H. Kierstead, E.D. Pressly, PC. Ray, and C.J. Hawker, Macromolecules, Vol. 42, p. 1425,2009.

5

Polymeric Nanocapsules by Interfacial Miniemulsion Polymerization Guo-Rong Shan and Zhi-Hai Cao State Key Laboratory of Chemical Engineering, Institute of Polymerization and Polymer Engineering, Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, China

Abstract A water-soluble co-monomer, N-isopropylacrylamide (NIPAM), and an oil-soluble crosslinker, divinylbenzene (DVB), and alkoxysilane monomer, γ-methacryloxypropyltrimethoxysilane (MPS), have been combined in a system for the synthesis of organic and organic-inorganic hybrid nanocapsules with crosslinked shells through interfacial miniemulsion polymerization by encapsulating a liquid non-sol vating hydrocarbon. Oligomers of poly(N-isopropylacrylamide) (PNIPAM) were dehydrated and separated from the aqueous phase and were adsorbed by the nanodroplets or latex particles and then anchored at their interfaces by means of a crosslinking reaction with DVB a n d / o r MPS, and the hydrolysis and condensation reactions of MPS. Nanocapsules were formed through encapsulation of the hydrocarbon by the newly produced polymers at the interfaces of the droplets. The crosslinked structure gradually grew to stabilize the shell morphology. Keywords: miniemulsion, nanocapsules, interfacial, nanodroplets, encapsulation, oligomer, morphology, crosslinker.

5.1

Introduction

Nanoparticles w i t h void h a v e s h o w n a great potential for the e n c a p sulation, e.g. catalysts, e n z y m e s , D N A s , medicines, or p i g m e n t s [1-3]. A large n u m b e r of innovative techniques h a v e been designed to elaborate hollow particles [4-10]. Recently, to directly encapsulate V. Mittal (ed.) Miniemulsion Polymerization Technology, (97-138) © Scrivener Publishing LLC

97

98

MINIEMULSION POLYMERIZATION TECHNOLOGY

liquid substances via a synthetic method in the emulsion, microemulsion, or miniemulsion has showed to be promising to synthesize nanocapsules [11-28], The formation of nanocapsules with an oily core in most literatures followed the mechanism of polymer phase separation in the oil droplets during polymerization. This process is controlled by the thermodynamic and kinetic factors. The thermodynamic aspect referring to the interfacial tensions of different phases can be influenced by the nature of oils, polymers, surfactants, and initiators. The kinetic factors included the viscosity in the dispersed phase and the mobility of polymer chains. The formulations and techniques for synthesizing the product with a high number fraction of nanocapsules need to be precisely designed. In addition, another limitation of the polymer phase separation in oil droplets is that the targeted nanocapsules cannot be obtained in the case that the morphology of nanocapsules is not the thermodynamically favorable state, because the diffusion of polymer chains to the interfaces of latex particles is no longer a spontaneous process. Interfacial polymerization has shown to be a robust alternative in the elaboration of nanocapsules through confining the processes of polymerization and polymer phase separation on the interfaces of latex particles. The polyurethane, polythiourea and polyurea nanocapsules with a hydrophilic core were synthesized in an inverse miniemulsion system [19-20], while a hydrophobic liquid core was encapsulated in a direct miniemulsion system [21] by interfacial polycondensation. Scott et al. reported an analogical method to the interfacial polycondensation for synthesizing nanocapsules but via alternating free radical copolymerization of hydrophobic maléate and hydrophilic vinyl ether monomers on the interfaces of the oil droplets in miniemulsion [22-23]. In addition, Luo et al. reported that the targeted nanocapsules could be synthesized via the interfacially confined controlled/living radical miniemulsion polymerization by using an amphiphilic RAFT agent [24-26]. Thermo-sensitive polymers such as poly(N-isopropylacrylamide) (PNIPAM) can undergo volume phase transition to separate out of the aqueous phase above their low critical solution temperature (LCST). The separated PNIPAM cannot dissolve into either the aqueous or the oil phases, and thus they are inclined to locate at the interfaces of the oil/water. Using the volume phase transition of PNIPAM, the thermo-sensitive capsules have been synthesized via the interfacial polymerization in an inverse emulsion system [27].

POLYMERIC NANOCAPSULES

99

The morphology of polystyrene nanocapsules was not the thermodynamically favorable state in the system containing surfactant, but the product with a high number fraction of the nanocapsules could be synthesized through the control of kinetic factors to perform interfacial miniemulsion polymerization by the synergic interaction of N-isopropylacrylamide (NIPAM) and divinylbenzene (DVB) [28]. The incorporation of functional groups into latex particles is of major interest in the field of colloidal science. Recently, the hydroxyl functionalized hybrid latex particles have been conveniently obtained via the incorporation of alkoxysilane units into the copolymer chains in a dispersed medium [29-31]. The synthesis of hybrid nanocapsules via miniemulsion (co)polymerization of γ-me thacryloxypropyltrimethoxysilane (MPS) and styrene (St) [18] was reported. In that case, the formation mechanism of nanocapsules mainly followed the polymer phase separation in oil droplets, and thus the products with high number fraction of nanocapsules only could be obtained in a limited range.

5.2

Organic Nanocapsules by Interfacial Miniemulsion Polymerization

5.2.1 Thermodynamic Prediction for the Morphology of Organic Nanocapsules Torza and Mason [32] discussed the equilibrium configurations of two immiscible liquid drops, designated as phase-1 and phase-3, suspended in a third immiscible liquid, phase-2, from the interfacial tensions and spreading coefficients. The relationship between spreading coefficients and interfacial tensions is given, s

.-=7,*-fy+7»)

where, y;. is the interfacial tension of i-phase and j-phase, S; is the spreading coefficient. If γ12 > γ23, the equilibrium configurations of two immiscible liquid drops can be judged by the spreading coefficient. Phase-1 is completely engulfed in phase-3 to form core-shell structure (Figure 5.1a) when S3 < 0, S2 < 0 and S3 > 0. Phase-1 is partially engulfed in phase-3

100

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

(A) Core-shell

(B) Hemispheres

(C) Individual particles

Figure 5.1. Schematic representation of the morphological prediction based on the the spreading coefficients, (a) S, < 0, S2 < 0 and S3 > 0 (b) Sx < 0, S2 < 0 and S3 < 0 (c) S, < 0, S2 > 0 and S3 < 0, O Phase-1 phase-2.

— Phase-3 Medium-

to form hemisphere structure (Figure 5.1b) when S2 < 0, S2 < 0 and S3 < 0. Phase-1 and phase-3 are completely separated (Figure 5.1c) when S1 < 0, S2 > 0 and S3 < 0. From Table 5.1, the interfacial tension of octane/water is greater than that of polystyrene/water in the system without any surfactant. Morphology predictions were conducted using the methods of Torza and Mason. Subscript assignment, 1, 2 and 3, accounts for the octane, water and polystyrene phase, respectively. According to the spreading coefficients result (S1 < 0, S2 < 0 and S3 > 0), core-shell structure is the thermodynamics stable state. And phase-1 is octane phase, so the octane core is engulfed in the polystyrene shell, which means capsule structure. The interfacial tensions of polystyrene/SDS aqueous solution and octane/SDS aqueous solution at different SDS concentrations are listed in Table 5.1. Upon the introduction of a surfactant, the interfacial tensions of both the polystyrene/SDS aqueous solution and octane/SDS aqueous solution decreased, but the latter showed a more pronounced response to the change in surfactant concentration. According to the result above, we could predict that the morphology of the system containing SDS is still core-shell structure, when phase-1 and phase-3 account for polystyrene and octane phase, respectively. Although the morphology of the final product is still a core-shell structure, the equilibrated morphology will be a polymer core with an oil shell in the system containing SDS based on the results of spreading coefficients in Table 5.1.

POLYMERIC NANOCAPSULES

101

Table 5.1. Interfacial tension between different phases, spreading coefficients, and morphology prediction. Reproduced from reference 28 with permission from Wiley. SDS 'Octane/W 'PS/W 'Octane/PS concentration (mN/m) (mN/m) (mN/m) (mmol/L) 0 1.25 2.5 5

30.1 25.8 21.5 16.8

49.7 11.3 4.6 4.8

7.2 7.2 7.2 7.2

[18] [18] [18] [18]

s, -27 -22 -24 -19

s 2 s3

Morphology Prediction

-73 12 Core-shell 3 -30 7 Core-shell b -19 10 Core-shell b -14 5 Core-shell b

"oil core and polymer shell; bpolymer core and oil shell.

5.2.2 Particles Morphology of the System without Added NIPAM and DVB The ratio of styrene/hydrocarbon was set at about 3:7. The surfactant concentration used throughout in this work was 5 mmol/L, and therefore the morphology of the nanocapsules should not be the thermodynamically favorable state according to the results of Table 5.1. Figure 5.2 shows that there is only little amount of 100 nm incomplete nanocapsules. Meanwhile, most of the particles are small solid balls. This result is in agreement with the thermodynamic prediction (Table 5.1). Landfester and coworkers [13] also found that polymerization initiated by KPS leaded to mostly solid spherical polystyrene particles, but not to the formation of nanocapsules. The evolution of the morphology and polymerization is shown in Figure 5.3. In this system, KPS was used as initiator, and therefore initiator radicals reacted with styrene dissolved in aqueous solution. Styrene oligomer propagated to the critical length (about 3 [33]), became insolubility and adsorbed on the surface of the droplets or latexes, while there are a lot of droplets and latexes in the miniemulsion system which provides huge surface area for adsorption. Because of the hydrophilic of the initiator fragment (S04~) and the interaction of electronegative charge of the initiator fragment (S04~) with SDS, the oligomer radicals are expected to remain at the oil/water interface temporarily and propagate with monomers. In the system, the polymer radicals will dissolve in oil droplets with the propagation of polymer chains (with the lipophilicity increasing).

102

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 5.2. TEM image of latex particles of the system without added NIPAM and DVB (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, KPS 0.30 g, water 150 g, T = 75°C).

The polymers will separate from the oil phase as polymer concentration increases and the monomer is consumed. Moreover, as mentioned above, although the nanocapsules represent the thermodynamically favored state in the system without the addition of surfactant, the prevailing thermodynamic conditions in the system with surfactant are not expected to be favorable for the formation of nanocapsules because of the decrease in interfacial tension of polystyrene/SDS solution and octane/SDS solution when compared with pure water. Therefore, polymer chains separated at the interface can diffuse into the interior of the latex particles, and finally the droplets changed into solid particles. But along with polymerization, the viscosity of droplet increases, the diffusion of separated polymer chains became rather difficult, and some of the polymer radicals stayed at the oil/water interface, then a few irregular nanocapsules are formed. It is believed that these small solid particles were composed of a polymer core and an oil shell structure, and that their small size may have been due to the evaporation of octane during the process of TEM sample preparation and observation. Also, some visible octane droplets were found during the experiment.

POLYMERIC NANOCAPSULES

103

Figure 5.3. Schematic representation of the formation of latex particles in the system without added NIPAM and DVB.

5.2.3 Particles Morphology of the System with DVB Both of the thermodynamic prediction and experiment confirm that the styrene miniemulsion system, octane as template and SDS as emulsifier, will not form thermodynamic stable nanocapsules. Therefore, some kinetic aspects are presented to control the polymerization location and morphology. If polymer radical chains could be anchored and propagate at the interface of the latex particles, it should be possible to obtain nanocapsules. In order to confirm this hypothesis, a crosslinking agent, DVB, was introduced into the system. The result was shown in Figure 5.4. Although there were many solid particles, the number of regular nanocapsules increased remarkably. The formation and adsorption of radicals process was the same with previous ones. But DVB could limit the oligomer radicals at the droplet/water or latex/water interface. Therefore, oligomer radicals could react at the surface of droplet/water or latex/water. At the same time, monomer diffuses from the inside of droplet or latex to the interface and form crosslinking shell. Otherwise, the crosslinking shell could also slow down the oligomer radicals entering into the droplet or latex, and it made more oligomer radicals react at the interface to form nanocapsules. Actually, there were still many solid particles in the system initiated by KPS. This confirms that the hydrophilic of the initiator fragment (S04")

104

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Figure 5.4. TEM image of latex particles of the system with DVB but without NIPAM (SDS 0.20 g, octane 8.33 g, HD 0.60 g / St 3.33 g, DVB 0.40 g, KPS 0.30 g, water 150 g, T=75 °C).

and the interaction of electronegative charge of the initiator fragment (S04") with SDS were not enough to anchor most of the oligomer radicals at the oil/water interface to attack DVB. Most of the oligomer radicals would enter into the droplets or latex and react there. This would lead to polymer-core and oil-shell structure. The evolution of the reaction with DVB was shown in Figure 5.5. The locus of radical formation and the property of oligomer radicals will be decided by the type and property of the initiator. Both of these two aspects will influence the final morphology of particles. The oligomer radicals initiated by KPS have the initiator fragment (S04~), which could be adsorbed at the surface of droplets or latexes and drove down the interface tension between oil and water. Therefore, oligomer radicals could temporarily rest on the interface of droplets or latexes. AIBN, instead of KPS, was used to compare the influence of initiator type and property on the particle morphology, while other conditions kept changeless. AIBN is an oil-soluble initiator and dissolve in the oil droplet or latex. It could decompose into nonionic/oil-soluble radicals and initiate the monomer in the oil droplet or latex, which means the radicals

POLYMERIC NANOCAPSULES

105

Figure 5.5. Schematic representation of the formation of latex particles in the system with added DVB.

have no anchor effect at the surface of droplet or latex. Some of the nonionic/oil-soluble oligomer radicals can go out from or enter into the particles directly without any anchor effect, leading to solid particles (see Figure 5.6). van Zyl and coworkers[34] could not obtain nanocapsules with AIBN initiator system either. Comparing with Figure 5.4 and Figure 5.6, it can be found that there is more nanocapsules when using KPS as initiator rather than that of AIBN. That means KPS fragment oligomer radicals make it easier form nanocapsules.

5.2.4

Particle Morphology of the System with Added NIPAM and DVB

Previous analysis show that the precondition of interfacial miniemulsion polymerization is the diffusion rate of oligomer radicals and polymer chains being slow enough to be crosslinked by DVB on the interface of the latex particles. In other words, although the system with surfactant is thermodynamic unstable, nanocapsules could still be produced by controlling kinetic aspect. The above result shows that the anchor effect of oligomer radicals with KPS fragment should be enhanced. PNIPAM is the most familiar temperature-sensitive polymer. Its low critical solution temperature (LCST) is about 32 oç[35,36] Therefore, PNIPAM can undergo phase separation at the

106

MINIEMULSION POLYMERIZATION TECHNOLOGY

Figure 5.6. TEM image of latex particles of the system initiated by AIBN (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0.40 g, AIBN 0.30 g, water 150 g, T=75 °C). Reproduced from reference 28 with permission from Wiley.

reaction temperature (75 °C), and the separated PNIPAM oligomer is expected to be locate at the droplet/water or latex/water interface, because it can not dissolve either aqueous or oil phase at the reaction temperature. One should be illuminated is that oligomer formed from ordinary water-soluble monomer (such as AA, MA) has so water-soluble that it may lead to aqueous phase nucleation. But PNIPAM oligomer radical could separate from aqueous phase and be adsorbed at the surface of droplet or latex, which reduce the probability of aqueous phase nucleation. In order to confirm the previous analysis, NIPAM was added into the system with DVB. In contrast with Figure 5.2 and Figure 5.4, the morphology of latex particles, shown in Figure 5.7, indicated that most of these latex were in the form of nanocapsules when some amount of NIPAM was introduced in the system. The numberaverage diameter of the nanocapsules and the average shell thickness estimated by TEM results was about 79 nm and 13.5 nm, respectively. The average void fraction of the capsule was about 28.4%, which is lower than the calculated value for the feed mixture composition. However, it should be borne in mind that this value is

POLYMERIC NANOCAPSULES

107

Figure 5.7. TEM image of nanocapsules of the system with added NIPAM and DVB (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0.40 g, NIPAM 0.40 g, KPS 0.30 g, water 150 g, T = 75 °C). Reproduced from reference 28 with permission from Wiley.

only approximate and is lower than the exact value because of the existence of large and deformed capsules as well as a volume contraction because of the evaporation of octane in vacuum condition during the TEM sample preparation and observation. This estimation is also confirmed by the DLS analysis, because the number-average diameter of capsules obtained by DLS is 87.6 nm (Figure 5.8). To verify whether NIPAM participates in the copolymerization and into the shell, FTIR and solid-state 13C-NMR measurements were performed to characterize the compositions of the purified shellcopolymersofthenanocapsules.ComparingtheFTIRspectra shown in Figure 5.9, it can be seen that a sharp band at 1650 cm -1 , characteristic of the amide carbonyl groups of PNIPAM, as well as the specific bands of the aromatic ring of polystyrene at 1600, 1492, 906, 756 and 696 cm"1 are observed in the spectrum of the nanocapsules. The peak marked with an asterisk in the solid-state 13 C-NMR spectrum in Figure 5.10 is attributable to the carbonyl groups of PNIPAM. Thus, both the FTIR and solid-state 13 C-NMR spectra indicated that NIPAM molecules had been

108

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Figure 5.8. Particle size and size distribution of nanocapsules of the system with added NIPAM and DVB (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0 .40 g, NIPAM 0.40 g, KPS 0.30 g, water 150 g, T=75 °C).

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Figure 5.9. Fourier-transform infrared spectra of pure PSt and the copolymers of the nanocapsules. Reproduced from reference 28 with permission from Wiley.

copolymerized into the shell of the nanocapsules. In fact, that the morphology of latex particles relies on the NIPAM amounts (This will be shown in the following section) can also serve as an important evidence for the participation of NIPAM in the copolymerization.

POLYMERIC NANOCAPSULES LD

in

Tl-

CD

|CM -~

109

h-C*5 0 3 f ^ CO

LOO

ι-σ> CM

Ti-Tf CO CM CM

— i — ' — i — ' — i — ' — i — ■ — i —

180

160

140

120

100

80

60

40

20ppm

13

Figure 5.10. Solid-state C-NMR spectrum of the copolymers of the nanocapsules. Reproduced from reference 28 with permission from Wiley.

5.2.5

Particle Size and Size Distribution in the Process of Polymerization

The droplet and particle sizes during the polymerization were measured by DLS, the Z-average size and volume-average size of the particles was used to evaluate the size evolution and size distribution, respectively. The results in Figure 5.11(a) indicate a rapid decrease in the particle size during the initial stage of the polymerization, and thereafter (about 30min) the particle size remains constant. That the polydispersity index (PDI) of the particles decreasing distinctly elucidates the particles size distribution narrowing. This phenomenon could have been easily ascribed to secondary nucleation. However, the fact that nearly all of the latex particles were capsules did not support the occurrence of secondary nucleation. Droplet nucleation is a prerequisite for obtaining nanocapsules, whereas secondary nucleation leads to the formation of solid particles. As shown in Figure 5.11(b), the volume distribution of the initial droplets was broad, and some large droplets were presented in the initial miniemulsion. However, as the polymerization proceeded, the number of large droplets decreased. Generally, in miniemulsion polymerization, if the droplet size distribution is narrow and nucleation time is short, the monomer transportation

110

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY 170 160

0

N 150

'€ 140 to

a.

130 120

(A)

. \

1.0

V □

X

-a— Particle size -o-PDI

CO

300 min

- 0.6

120 min 0.4 .a 0.2

0.0 50 100 150 200 250 300 Polymerization time (min)

o

Q.

0 (B)

200 400 600 Volume particle size (nm)

800

Figure 5.11. Evolution of particle size (a) and volume distribution (b) during the process of interfacial miniemulsion polymerization with added NIPAM and DVB (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0.40 g, NIPAM 0.40 g, KPS 0.30 g, water 150 g, T = 75°C). Reproduced from reference 28 with permission from Wiley.

between the droplets a n d / o r latex can be negligible. But in a broad size distribution system, small droplets have more opportunities to nucleate and capture radicals than larger ones because of their greater overall surface area. Once polymerization starts in small latex particles, monomers were consumed in these nucleated latex particles, resulting in the difference between the monomer concentration in small nucleated latex particles and large non-nucleated droplets. To maintain equilibrium, monomers would diffuse from the nonnucleated large droplets to the smaller nucleated latex particles. The droplets were a mixture of octane and monomer in this system, and therefore the concentration of octane in the large droplets would increase with the loss of monomer. Kim and Burgess [37] found that the interfacial tension of oil/ water interface is related to the oil phase constitute. The interfacial tension of the styrene/water interface is increased with decreasing styrene weight content in the oil phase, as can be seen in Figure 5.12. As a result, with the loss of monomers in large droplets, the interfacial tension of the oil/water interface was increased. It resulted in colloidal instability, and subsequently most of the non-nucleated large droplets coalesced to form macro-droplets. In addition, the coalescence of large droplets was consistent with the relatively low hydrocarbon encapsulation efficiency. The rapid decrease in the Z-average particle size during the

POLYMERIC NANOCAPSULES

111

60 ■

50

>

40 -

Ό-^.

CO

Ό co t

"~~~Ό-^

-/",

C

S Έ 30 ■

20

1

.

1

1

,

20 40 60 St weight content (wt%)

1

80

100

Figure 5.12. Influence of St weight content on the interfacial tension of oil mixture/water. Reproduced from reference 28 with permission from Wiley.

0 (A)

50 100 150 200 250 300 Polymerization time (min)

0

200 400 600 Volume particle size (nm) (B)

800

Figure 5.13. Evolution of particle size (a) and volume distribution (b) during the process of miniemulsion polymerization without added NIPAM (SDS 0.20 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0.40 g, KPS 0.30 g, water 150 g, T=75°C). Reproduced from reference 28 with permission from Wiley.

initial phase of polymerization could also be reasonably ascribed to the simultaneous coalescence of large droplets. The particle size and size distribution of the system to which no NIPAM had been added, shown in Figure 5.13, demonstrate similar behavior to that of the system with NIPAM. As in the earlier analysis, the fast decrease in particle size during the initial phase of polymerization could again be ascribed to the disappearance of

112

MINIEMULSION POLYMERIZATION TECHNOLOGY

large droplets. Although TEM results showed that a large number of small solid particles were presented in the final product, the final sizes of the products with or without NIPAM were quite similar. It was further confirmed that particles with an inverted morphology, that is, with a solid polymer core and an oil shell, were produced.

5.2.6

Mechanism for the Formation of Organic Nanocapsules through Interfacial Miniemulsion Polymerization

The schematic representation in Figure 5.14 illustrates the mechanism for the formation of nanocapsules through interfacial polymerization. At first, the oil phase was dispersed into nanodroplets, and NIPAM molecules were mainly distributed in the aqueous phase and at the interface of the droplets, while only a small amount of styrene was dissolved in the aqueous phase because of its poor solubility in water. The water-soluble initiator, KPS, decomposed to form primary radicals, which mainly propagated with NIPAM to generate oligomer radicals of PNIPAM. The polymerization temperature used (75 °C) was much higher than the LCST of PNIPAM. As a result, PNIPAM oligomer radicals beyond the critical chain length separated from the aqueous phase.

Figure 5.14. Schematic representation of the process of the formation of nanocapsules through interfacial copolymerization in miniemulsion. Reproduced from reference 28 with permission from Wiley.

POLYMERIC NANOCAPSULES

113

Because of the tremendous interface areas of the droplets and latex particles, these were able to adsorb most of the phase separated PNIPAM oligomers. Chain ends of the oligomer radicals bearing S04~ groups were difficult to bring close to the interface of the latex particles because of the electrostatic repulsion between the initiator fragments and the surfactants. From the thermodynamic point of view, the adsorption of PNIPAM at the interface of droplets or latex particles was a favorable state, because PNIPAM, which is less hydrophobic than the hydrocarbon or polystyrene, lowered the interfacial Gibbs energy. In addition, the PNIPAM oligomers can not dissolve in both the aqueous and oil phase at the reaction temperature. The diffusion rate of oligomer radicals containing NIPAM units becomes slow enough to be cross linked by DVB, and then be anchored at the interfaces of the latex particles due to the decrease of mobility as the result of the formation of cross linked structure, leading to the interfaces to be the main loci of polymerization. Monomers diffused from the interiors of the latex particles as a supply for further polymerization. The crosslinked polymer shell could also slow down the diffusion rate of newly formed radicals and make it easier to crosslink. Consequently, nanocapsules with a crosslinked shell could be obtained through the encapsulation of the hydrocarbon by the accumulation of crosslinked polymers at the interfaces of latex particles.

5.2.7

Influences on the Formation of Organic Nanocapsules through Interfacial M i n i e m u l s i o n Polymerization

1) Amount of NIPAM Regular and high void fraction nanocapsules could be obtained by interfacial miniemulsion polymerization with both the contribution of NIPAM and DVB. NIPAM can slow down the oligomer radicals' ingoing rate, delay their residence time at the interface of the latex particles. The anchored radicals polymerize on the interface to ensure the interfacial polymerization. Once a crosslinked shell is formed at the interface of the latex particles, access by other radicals will be impeded. Under these circumstances, radicals without a NIPAM unit may also be crosslinked and anchored at the interface. Without adding NIPAM, only a small number of nanocapsules could be obtained in Figure 5.4. The number of nanocapsules increased markedly upon the addition of NIPAM, even a small

114

(A)

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

(B)

Figure 5.15. TEM images of nanocapsules and particles of the systems with different amounts of NIPAAM (SDS 0.20 g, water 150 g, octane 8.33 g, HD 0.60 g, St 3.33 g, DVB 0.40 g, KPS 0.30 g, T=75°C). (a) 0.2 g NIPAM (b) 1.0 g NIPAM [28].

amount (0.2g), as can be seen in Figure 5.15(a). Almost all of the droplets evolve into nanocapsules, as more NIPAM (0.4g) was introduced into the system (Figure 5.7). Although the nanocapsules still dominated in the final product, they were inclined to stick together in the larger amount (lg) NIPAM system and the contrast of the shell boundary is not clear. It is due to more NIPAM is incorporated into the shell. From the dependence of the number of capsules on the amount of NIPAM, it may be stated that for this system, 0.4g of NIPAM was enough to make nearly all droplets evolve to form nanocapsules. However, no evident aqueous phase nucleation could be observed. Therefore, PNIPAM is in the shell part and the rapid separation of PNIPAM oligomer could depress the aqueous phase nucleation. 2) Amount of DVB The mobility of (co)polymer chains can be conveniently decreased by adding crosslink agent to the system. However, addition of crosslink agent is not suitable for nanocapsules' formation in the system ruled by the inner phase separation mechanism. Unexpectedly, for the systems without or with a low amount of DVB, it is difficult to get nanocapsules (Figure 5.16(a) and Figure 5.16(b)). Although NIPAM was added in each runs and separated PNIPAM oligomer radicals were adsorbed by the droplets or latex, polymer chains would still propagate into hydrophobe and enter the oil phase

POLYMERIC NANOCAPSULES

(A)

115

(B)

Figure 5.16. TEM images of nanocapsules and particles of the systems with different amounts of DVB (SDS 0.20 g, water 150 g, octane 8.33 g, HD 0.60 g, St 3.33 g, NIPAM 0.40 g, KPS 0.30 g, T=75°C). Reproduced from reference 28 with permission from Wiley, (a) Og DVB (b) 0.20g DVB.

without the anchor effect of DVB. Another aspect is also adverse, in the system containing SDS, nanocapsule is not a thermodynamic stable state. The polymer chains separated on the interface will be driven into the interior of the latex by the thermodynamic force. The hypothesis was confirmed by Figure 5.16(a). However, addition of NIPAM in the system without DVB improved the fraction of regular nanocapsules (Figure 5.16(a) and Figure 4.2). For the systems with a low amount of DVB, the crosslinking reactions are so insufficient to anchor most of the oligomer radicals on the interface that many oligomer radicals could enter the droplets. The number fraction of nanocapsules slightly increased with the addition of 0.2 g DVB, but solid particles are primary. As the DVB amount increased to 0.4 g, the morphology of nanocapsules dominated in the latex particles. Lower chain mobility can be obtained by using a larger amount of DVB. Most of droplets undergo the interfacial polymerization. Newly formed radicals will probably be intercepted by the crosslinked shell and propagate, even if no crosslinking reactions happen on the interface. The different morphologies of the products obtained with different amounts of DVB provide strong evidence that the formation of nanocapsules follows the mechanism of interfacial polymerization in these systems with both NIPAM and DVB.

116

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

3) Weight content of monomer in the oil phase Higher weight content of monomer in the oil phase enhances the solubility of the polymer resulting in higher droplet viscosity, which is not favorable for polymer chain diffusion. Thus, formation of nanocapsules mainly followed an inner phase separation mechanism, which was strongly influenced by the internal viscosity of droplets, and this, in turn, was dependent on the solubility of the polymer in the oil mixture. In our previous research [18], nearly all droplets evolved to form solid particles when the weight content of monomer was 50%. The formation of nanocapsules follows an interfacial polymerization mechanism, and therefore the influence of the weight content of the monomer in the oil phase on the formation of nanocapsules can be analyzed from two aspects. Firstly, with increasing monomer content of the oil phase, the solubility of polymer chains in the oil droplets will increase. Once the radicals enter into the oil droplets, the crosslinked structure of the polymer chains will decrease the chain mobility, which is favorable for the formation of solid particles. Secondly, the higher viscosity also makes it hard for radicals transport to the central part of the droplet. It is favorable for nanocapsule structure. Figure 5.17 shows that the number of nanocapsules is still

Figure 5.17. TEM image of nanocapsules of the system with 50% monomer weight content in oil phase (SDS 0.29 g, water 150 g, octane 8.33 g, St 8.80 g, DVB 0.40 g, HD 0.87 g, NIPAM 0.58 g, KPS 0.74 g, T=75 °C).

MINIEMULSION POLYMERIZATION

117

more than that of solid particles at 50% weight content of monomer in the oil phase. However, a small increasing number of solid particles can also be observed in contrast to 30% weight content of the oil phase. It indicates that the solubility of oligomer radicals in the oil droplets will influence the morphology; however, the fact that most of the particles are nanocapsules supports the interfacial polymerization mechanism. Compared with low monomer weight content system, higher monomer weight content results in more regular nanocapsules.

5.3

Organic-Inorganic Hybrid Nanocapsules by Interfacial Miniemulsion Polymerization

5.3.1 Thermodynamic Analysis and Morphological Prediction The morphologies of ternary system of poly(MPS-co-St)/HD/ SDS solution were predicted again with the method of Torza and Mason [32]. According to the interfacial tensions of the polymer/ water, the polymer/SDS solutions, the oil/water and the oil/SDS solutions, and the spreading coefficients listed in Table 5.2, 5.3 and 5.4, and 5.5, the morphology of nanocapsules (oil core and polymer shell) is the most favorable state only in the case that water is used as the continuous phase. Upon the introduction of surfactant, both interfacial tensions of the poly(MPS-co-St)/SDS solutions and HD/SDS solutions decreased, but the latter showed a much more pronounced response to the change in surfactant concentration, resulting in γ /w > yo/w (seen in supporting information). Combined Table 5.2. Influences of monomer weight content on the oil/SDS solution interfacial tension. SDS concentration (mmol/L)

IHD/W

(mN/m)

γΟΛν (mN/m) (M% = 50 wt%) a

0

51.2

12.3

1.25

14.0

4.0

-5

7.5

1.8

5

6.2

1.8

2

a

M% represents the monomer weight content in the oil mixture of HD and monomers.

118

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Table 5.3. Influences of MPS weight contents on the interfacial tension of oil/SDS solution 3 . SDS concentration (mmol/L)

γΟΑν (mN/m) (MPS = 20.0 wt%)

y 0 / w (mN/m) (MPS = 33.3 wt%)

Ύο/w (mN/m) (MPS = 50.0 wt%)

0

12.26

11.12

11.43

1.25

4.00

4.11

3.51

2.5

1.83

1.67

1.68

5

1.83

1.65

1.63

The monomer weight content was 50%. Table 5.4. Surface tension, dispersion force and polar force of poly (MPS-co-St) and the interfacial tension of HD and poly(MPS-co-St)a. ys (mN/m)

■f (mN/m)

■f (mN/m)

Yo/p(mN/m)

poly(MPS-co-PS) (MPS=20.0 wt%)

28.43

24.38

4.05

4.67

poly(MPS-co-PS) (MPS=33.3 wt%)

32.53

28.03

4.50

8.22

poly(MPS-co-PS) (MPS=50.0 wt%)

27.45

22.67

7.34

9.17

HD

27.47

27.47

0

-

"γ., ~f, and ψ represent surface tension, dispersion force and polar force of poly(MPS-co-St) respectively; γο/ represents the interfacial tension of HD and poly(MPS-co-St). Table 5.5. Spreading coefficients of the ternary system of poly (MPS-co-St)-HD-water. p(MPS-co-St) p(MPS-co-St) SDS p(MPS-co-St) concentration (MPS = 20.0 wt%) (MPS = 33.3 wt%) (MPS = 50.0 wt%) (mmol/L) s, s 2 S3 s, s 2 S3 s, s 2 S3 0 1.25 5

-29.5 -72.9 20.2 -40.1 -62.4 23.6 -42.2 -22.7 23.9 -10.5 -29.1

1.2

-10.9 -25.9 -2.0 -14.0 -23.3 -4.37

-13.4 -16.4 4.04 -12.5 -11.9 -0.4 -13.7 -15.2 -4L63

POLYMERIC NANOCAPSULES

119

with the spreading coefficients listed in Table 5.5, the morphologies of final product may be an inverted core-shell (oil shell and polymer core) or a hemisphere in ternary system of poly(MPS-co-St)/ HD/SDS solution, depending on the SDS concentrations and the copolymer compositions.

5.3.2 Synthesis of Organic-Inorganic Hybrid Nanocapsules under Neutral Conditions 1) Role of cross-linking reaction in the formation of organic-inorganic hybrid nanocapsules Alkoxysilane, MPS, was introduced to synthesize nanocapsules with an organic-inorganic hybrid shell. Afore-mentioned thermodynamic analyses have shown that the morphology of nanocapsules is not the thermodynamically favorable state for a ternary system of poly(MPSco-St)/HD/SDS solutions. Therefore, it was necessary to synthesize the hybrid nanocapsules by performing the interfacial miniemulsion copolymerization through anchoring the oligomeric radicals on the interfaces of latex particles/water. It is well known that MPS can undergo the hydrolysis and condensation reactions in an aqueous system, e.g. emulsion or miniemulsion polymerization [29-31]. The hydrolysis and condensation rate of MPS depends greatly on the suspension pH. The FTIR spectrum (line a in Figure 5.18) of the hybrid copolymers synthesized under the neutral condition showed that a sharp absorbance peak at 821 and 1087 cm-1 belonging to Si-O-C supported that most of MPS did not participate in the hydrolysis. The spectrum of solid-state 29Si-NMR (Figure 5.19) showed that T° species dominated in the hybrid copolymers and only a small part of silicon species condensed (According to the conventional Tn nomenclature where n designates the number of Si-O-Si bonds, the signal at -43, -48, -58 and -68ppm corresponding to T° (CH30)3SiR), T1 (CH30)2Si(OSi) R), T2(CH30)Si(OSi)2R) and T3 species (Si(OSi)3R) respectively). Both FTIR and solid-state 29Si-NMR spectra definitely indicated that the hydrolysis and condensation of MPS remained at a low level under the neutral condition. Nevertheless, upon the introduction of MPS, the formation of cross-linking structure through condensation reactions taking place on the oil/water interfaces could help to anchor a part of oligomeric radicals on the interfaces of the latex particles /water. Therefore, some droplets could undergo interfacial copolymerization to form nanocapsules in a system without added DVB under the

120

MINIEMULSION POLYMERIZATION

TECHNOLOGY

Figure 5.18. FTIR spectra of shell copolymers synthesized via miniemulsion copolymerization by using HD as template under different pH conditions (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS0.15g,T = 70°C).

-42

T1

T2

h-48 -58

ΙΛ 66

100

50

0

_1_ -50

T3

_l_ -100

J_ -150

J -200

Chemical shift (ppm) 29

Figure 5.19. Solid-state Si-NMR spectrum of shell copolymers synthesized via miniemulsion copolymerization by using HD as template under the neutral condition (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS 0.15 g, T = 70°C).

POLYMERIC NANOCAPSULES

121

Figure 5.20. Influence of DVB weight contents on the morphology of latxe particles synthesized by miniemulsion copolymerization by using HD as template with 5 mmol/L SDS (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS 0.15 g, T =70°C) (aland bl are the typical TEM images of sample; a2 and b2 are the TEM images at the boundary of particle aggregates), (al) DVB = 0 g (a2) DVB = 0 g (bl) DVB = 0.4 g (b2) DVB = 0.4 g.

neutral condition. The typical TEM image shown in Figure 5.20(al) indicated that the number fraction of nanocapsules was close to half of the overall latex particles, while the TEM image at the boundary of particle aggregates seen in Figure 5.20(a2) showed that the solid particles dominated in this area. Without DVB, the radicals were produced in the aqueous phase, and the primary radicals would mainly propagate with NIPAM and

122

MINIEMULSION POLYMERIZATION TECHNOLOGY

MPS because their relatively larger water solubility than St. It was believed that the oligomeric radicals were inclined to locate at the interfaces of droplet/water at first because of their relative hydrophilicity and poor solubility in the oil phase. However, they would propagate with oil monomers (St and unhydrolyzed MPS) at the interfaces of the latex particles/water, and thus the hydrophobicity of the polymer chains increased. In this system, most oligomeric radicals and the polymer chains would diffuse into the interior of droplets under the drive of the thermodynamic force due to the insufficient formation of the crosslinked structure. Consequently, more than half of latex particles with the morphology of solid particles appeared in the final product. The cross-linking reaction can be conveniently improved by adding some cross-linkers (e.g. DVB). The formation of crosslinked structure as the introduction of DVB can decrease the mobility of polymer chains, and thus the crosslinked polymer chains were anchored at the interfaces of latex particles/water. The number fraction of hybrid nanocapsules increased obviously, seen in Figure 5.20(bl) and 5.20(b2) as the introduction of DVB. This clearly indicated that the formation of solid particles without DVB mainly relied on the insufficient cross-linking reactions to anchor oligomeric radicals and polymer chains on the interfaces. These results could be regarded as evidence for that the formation of nanocapsules followed the mechanism of interfacial copolymerization. 2) SDS concentration A series of experiments varied DVB weight content with 10 mmol/L SDS have been performed to investigate the SDS concentration on the morphology of the particles. Only very few nanocapsules could be found in the product of the system absent of DVB seen in Figure 5.21(a). As the introduction of DVB, the number fraction of nanocapsules increased, but more than half of particles were solid particles in the system with 0.4 g DVB shown in Figure 5.21(b). The number fraction of nanocapsules increased as the increase of the weight content of DVB, and the nanocapsules dominated in the product of the system with 0.8 g DVB seen in Figure 5.21(c). Through comparing the morphologies of products with different SDS concentrations (Figure 5.20 and 5.21), it could be found that the amount of solid particles in the system with 10 mmol/1 SDS was obviously more than that of the system with 5 mmol/L SDS, especially for systems with a low level of DVB. The higher the SDS concentration,

POLYMERIC NANOCAPSULES

123

Figure 5.21. Influence of DVB weight contents on the morphology of latxe particles synthesized by miniemulsion copolymerization by using HD as template with 10 mmol/L SDS (SDS 0.432 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS 0.15 g, T = 70°C). (a) DVB = 0 g (b) DVB = 0.4 g (c) DVB = 0.8 g.

the larger is the thermodynamic driving force for the formation of non-nanocapsules. A similar dependency of the number fraction of nanocapsules on the SDS concentration has been reported by several other literatures in which the formation of nanocapsules followed the mechanism of polymer phase separation in oil droplets. Landfester et al. [13] found that the nanocapsules could be obtained in the system with 0.5 wt% SDS (with respect to the organic phase), but not in the system with 4 wt% SDS. Luo et al. [15] reported that most of particles had a half-moon structure in the case with a high level of surfactant.

124

MINIEMULSION POLYMERIZATION TECHNOLOGY

Ni et al. [18] also found that the solid particles dominated in the final product with a high level of surfactant concentration. Under the neutral condition, only a small part of MPS molecules participate in condensation reactions, and thus most of oligomeric radicals could not be anchored at the interfaces of oil/water in the system absent of DVB. The formation of the final morphology in the system with a low level of cross-linking reactions is thermodynamically controlled, similar to that of the system following the mechanism of polymer phase separation in oil droplets. However, the formation of nanocapsules can still be obtained controlling the kinetic factors by introducing some amount of DVB. This has been attested by the fact that most of droplets evolved to form nanocapsules in the system with 0.8 g DVB and 10 m m o l / L SDS.

5.3.3

Synthesis of Organic-Inorganic Hybrid Nanocapsules under Acidic or Basic Conditions

For the miniemulsion copolymerization of MPS and St, the hydrolysis and condensation of MPS are significantly influenced by the suspension pH, and both of them will be accelerated under the basic or acidic condition. In the previous section, since the hydrolysis and condensation of MPS remained at a low level under the neutral condition, an additional amount of DVB was required to improve the anchoring effect of oligomeric radicals for synthesizing products with a high number fraction of nanocapsules. In this section, it is tried to elaborate products with a high number fraction of nanocapsules to improve the anchoring effect of oligomeric radicals via the increase of the condensation degree through performing reactions under the acidic or basic conditions. 1) Suspension pH values In order to synthesize products with good colloidal and storage stability, the reactions under weak acidic or basic conditions are conducted. The pH values of the initial miniemulsions and the final latexes were summarized in Table 5.6. According to the FTTR spectra shown in Figure 5.18, all the characteristic absorbance peaks belonging to the units of St, NIPAM and MPS could be found in the resulting copolymers of hybrid shells. It indicated that all monomers participated in the shell formation of nanocapsules.

POLYMERIC NANOCAPSULES

125

Table 5.6. p H values of the initial miniemulsion and the final latexes under different pH conditions. Types of buffers

None

ADMP and DNHP

Sodium bicarbonate

Sodium tetraborate decahydrate

Initial pH

7.1

6.8

8.5

9.3

Final pH

2.6

6.5

8.9

9.1

The number fraction of nanocapsules was less than 50% of the overall number of latex particles in the product synthesized under the buffered neutral system in the absence of DVB (Figure 5.20(al) and Figure 5.20(a2)). The decomposition of KPS will decrease the pH value of the system, but for a buffered system, the suspension pH only decreased from 6.8 to 6.5. The hydrolysis and condensation rate remains at a low level attested by the results of solid-state 29 Si-NMR and FTIR seen in Figure 5.18 and Figure 5.19. Without added DVB, the anchoring effect only depending on the condensation reactions was insufficient to perform the interfacial copolymerization in the most of droplets, and thus the product with a high number fraction of nanocapsules could not be obtained under the buffered neutral system. The pH value of the unbuffered neutral system decreased since the polymerization started as a result of the decomposition of KPS, and the pH value of the product was 2.6. The decrease of pH value from 7 dramatically promoted the hydrolysis and condensation of MPS. The spectrum of solid-state 29Si-NMR of the hybrid copolymers shown in Figure 5.22(a) indicated that all MPS underwent the condensation reactions to form condensed silicone species. This was supported by the fact that the characteristic absorbance peak at 821 cm"1 disappeared in the spectrum of FTIR (Line b in Figure 5.18). Although the main silicone species was the incomplete condensed species (T2), the cross-linking degree under this condition was significantly higher than that of the buffered neutral system. Consequently, the number fraction of nanocapsules obviously increased compared to the buffered neutral system. The TEM image shown in Figure 5.23(al) indicated that nearly all latex particles were nanocapsules in the typical area of the sample. However, it was clearly seen that some solid particles still could be found at the boundary of particle aggregates (Figure 5.23(a2)). The solid

126

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

-58 T2

-66

? T°

1

T -48

-66

f , t

100 (A)

i

50

r

i

>

T3

-57 T2

V

V

-A,\

i

T3

i

i

0 -50 -100 -150 -200 Chemical s hift (ppm)

/

_ YfV"Vf 100 (B)

50

1

,

lV

,

I

.

A

0 -50 -100 -150 -200 Chemical shift (ppm)

Figure 5.22. Solid-state 29Si-NMR spectra of shell copolymers synthesized via miniemulsion copolymerization by using HD as template under different pH conditions (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 ¡ KPS 0.15 g, T = 70°C).(a) pH = 7.1, unbuffered (b) pH=8.5.

particles may be produced by homogenous nucleation or the diffusion of copolymer chains into the interior of particles under the thermodynamic force. Further detailed discussion about the formation of these solid particles will be found in the following sections. The FTIR spectrum of Line c in Figure 5.18 showed that the absorbance peak of Si-O-C at 821 cm-1 decreased obviously for the product synthesized at pH 8.5. This indicated that the hydrolysis of MPS has been promoted at pH 8.5. Moreover, most resultants of hydrolysis have condensed to form the complete condensed species (T3) seen in Figure 5.22(b). Compared to that of the buffered neutral system, the degree of hydrolysis and condensation of MPS has been significantly promoted. As the increase in the level of the cross-linking reactions at pH 8.5, the oligomeric radicals were more inclined to be crosslinked and anchored on the latex particles/water interfaces to perform interfacial copolymerization. As a result, the product with the high number fraction of nanocapsules was obtained at pH 8.5, as shown in Figure 5.23(bl). But some solid particles still appeared at the boundary of the particle aggregates (Figure 5.23(b2)). First effort to decrease the amount of solid particles at the boundary of particle aggregates was increasing the pH value to 9.3. However, the number fraction of nanocapsules did not improve obviously. This indicated that the degree of hydrolysis and condensation of MPS still could not fulfill the requirement of the formation of nanocapsules under this condition.

POLYMERIC NANOCAPSULES

127

Figure 5.23. TEM images of nanocapsules synthesized via miniemulsion copolymerization by using HD as template under different pH conditions (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS 0.15 g, T = 70°C). (a) pH = 7.1, unbuffered; (b) pH = 8.5; (c) pH = 9.3. (al, bl and cl are the typical TEM images of sample; a2, b2 and c2 are the TEM images at the boundary of particle aggregates).

128

MINIEMULSION POLYMERIZATION TECHNOLOGY

2) MPS weight content at pH 8.5 The formation of solid particles in the product synthesized with 20 wt% MPS at pH 8.5 may be induced by the insufficient anchoring effect of oligomeric radicals due to the small amount of MPS. Thus, the second effort to decrease the amount of solid particles has been made by increasing the MPS weight content, because the level of hydrolysis and condensation reactions is anticipated to be improved through the increase of MPS weight content which in turn results in the increase of the anchoring effect of oligomeric radicals at the interfaces. The contributions to the increase of the anchoring effect of oligomeric radicals at the interfaces of oil/water as the increase in the level of hydrolysis and condensation reactions originate from two aspects. First, the increase of the hydrophilicity of the oligomeric radicals due to the incorporation of more hydrolyzed MPS units can decrease the diffusion rate of oligomeric radicals into the interior of droplets or latex particles, and thus the possibility to be crosslinked through condensation of oligomeric radicals at the interfaces increases. Secondly, as the increase of MPS amount participating into the hydrolysis and condensation reaction, more oligomeric radicals undergo condensation reactions and are anchored at the interfaces of droplets/water and latex particles/ water to perform interfacial copolymerization. The solid-state 29Si-NMR spectrum of the hybrid copolymer with 50 wt% MPS shown in Figure 5.24 was similar to that of the system with 20% MPS (Figure 5.22(b)). Considering the larger MPS amount used in the present system than that of the system with 20 wt% MPS and the similar portion of uncondensed and condensed species as shown in the 29Si-NMR spectrum, the amount of condensation resultants were expected to be more in the present system with 50 wt% MPS. In contrast to the product with 20 wt% MPS, the number fraction of nanocapsules obviously increased in the product synthesized with 33.3 wt% MPS, seen in Figure 5.25(al) and Figure 5.25(a2). Only very few solid particles could be found at the boundary of particle aggregates. As the MPS weight content increased to 50 wt%, the latex particles with the nanocapsules morphology dominated in both the typical area and the boundary of particle aggregates too (Figure 5.25(bl) and Figure 5.25(b2)). These results definitely showed that more droplets underwent interfacial copolymerization due to the increase of the anchoring effect of oligomeric radicals as a result of the increase in the level of hydrolysis and condensation reactions.

POLYMERIC NANOCAPSULES

129

57 T2

V,SV/>A/%>WWV/ÍVV'

100

50

0 -50 -100 Chemical shift (ppm)

-150

-200

Figure 5.24. Solid-state 29Si-NMR spectrum of shell copolymers synthesized by miniemulsion copolymerization by using HD as template with 50.0 wt% MPS relative to overall monomers at pH = 8.5 (SDS 0.216 g, water 150 g, HD 10.8 g, St 5.40 g, NIPAM 0.8 g, MPS 5.40 g, KPS 0.15 g, T=70 °C).

3) DVB weight contents at pH 8.5 The number of solid particles at the boundary of particle aggregates was reduced through increasing the MPS weight content as presented in the previous section. The anchoring effect of oligomeric radicals on the interfaces of latex particles was improved due to the increase of the cross-linking reaction and the hydrophilicity of oligomeric radicals with the increase of MPS weight content. But it is still impossible to clearly distinguish the respective role of cross-linking reactions or the hydrophilicity of oligomeric radicals in the formation of nanocapsules. The hydrophilicity of oligomeric radicals is independent of the introduction of DVB, while the crosslinking reactions greatly rely on the amount of DVB. Therefore, the third effort to decrease the amount of solid content was made by changing the amount of DVB used in the system. The differences in the number fraction of nanocapsules of the products with different DVB weight contents can be completely ascribed to the different levels of the cross-linking reaction. The TEM images of the products with different DVB weight contents synthesized under pH 8.5 were shown in Figure 5.26. The results indicated that in the product with 0.2 g DVB nearly all the

130

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

(A1)

(A2)

(B1)

(B2)

Figure 5.25. TEM images of nanocapsules synthesized via miniemulsion copolymerization by using HD as template with different MPS weight contents at pH = 8.5CSDS 0.216 g, water 150 g, HD 10.8 g, NIPAM 0.8 g, St+MPS 10.80 g, KPS 0.15 g, T = 70°C). (a) MPS = 33.3 wt%, (b) MPS = 50.0 wt%. (al and bl are the typical TEM images of sample; a2 and b2 are the TEM images at the boundary of particle aggregates).

latex particles in the typical areas were nanocapsules (Figure 5.26(31)), and the number of solid particles at the boundary of particle aggregates obviously decreased, seen in Figure 5.26(a2). Only very few solid particles could be found in the final product with 0.4 g DVB. These results clearly indicated that the formation of most solid particles in the product without DVB was due to the insufficient crosslinking reactions to anchor the oligomeric radicals at the interfaces of oil/water, but not the hydrophilicity of the oligomeric radicals.

POLYMERIC NANOCAPSULES

(A1)

(A2)

(B1)

(B2)

131

Figure 5.26. TEM images of nanocapsules synthesized via miniemulsion copolymerization by using HD as template with different DVB weight contents at pH 8.5 (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, MPS 2.16 g, KPS 0.15 g, T = 70°C). (a) DVB= 0.2 g, (b) DVB = 0.4 g. (al and bl are the typical TEM images of sample; a2 and b2 are the TEM images at the boundary of particle aggregates).

4) Monomer weight content at pH 8.5 The last effort to decrease the amount of solid particles was made by increasing the monomer weight content used in the system. The TEM images shown in Figure 5.27 for the product with 66.7 wt% monomer weight content synthesized at pH 8.5 indicated that the nanocapsules dominated in the typical area of sample (Figure 5.27(al)), but the latex particles were mainly composed of solid particles at the boundary of particle aggregates (Figure 5.27(a2)). Compared to the product with

132

(A1)

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

(A2)

Figure 5.27. TEM images of nanocapsules synthesized via miniemulsion copolymerization by using HD as template with 66.7% monomer weight content synthesized at 8.5 (SDS 0.216 g, water 150 g, HD 7.2 g, St 11.52 g, NIPAM 0.8 g, MPS 2.88 g, KPS 0.15 g, T=70 °C). (al is the typical TEM images of sample; a2 is the TEM images at the boundary of particle aggregates).

50 wt% monomer weight content (Figure 5.23(al ) and Figure 5.23(a2)), the number of solid particles slightly increased in this system. As the increase of overall amount of monomers, the amount of MPS increases too, leading to the increase in the level of hydrolysis and condensation reactions under the basic condition. This is believed to be favorable to perform interfacial copolymerization in more droplets by enhancing the anchoring effect of oligomeric radicals. However the solubility of oligomeric radicals in the oil phase will increase as the increase of the monomer weight content too, resulting in the dissolution of more oligomeric radicals in the oil phase which is unfavorable to form nanocapsules through interracial copolymerization. Consequently, under the control of these two factors, the number fraction of nanocapsules did not increase for the system with 66.7 wt% monomer weight content, compared to the system with 50 wt%. It is worth noting that the nanocapsules synthesized with high monomer weight content were well-defined and with a more uniform size. 5) Morphological evolution in the process of miniemulsion copolymerization The TEM images of the particle morphology at different conversions were shown in the Figure 5.28. At the stage of low conversion

POLYMERIC NANOCAPSULES

(A)

(B)

(C)

(D)

133

(E)

Figure 5.28. Morphological evolution in the process of miniemulsion copolymerization of MPS and St by using HD as template at pH 8.5 (SDS 0.216 g, water 150 g, HD 10.8 g, St 8.64 g, NIPAM 0.8 g, DVB 0.4 g / MPS 2.16 g, KPS 0.15 g, T = 70°C).

134

MINIEMULSION POLYMERIZATION TECHNOLOGY

(conversion=12.7 %), the latex particles with a low contrast could be found in the product (Figure 5.28(a)). The morphology of latex particles is not stable enough to resist deformation induced by the oil evaporation in the process of the preparation and the observation of TEM sample. It is believed these particles are nanocapsules but with a thin polymer shell because of the low conversion. As the polymerization proceeded, more polymers were produced in the system (conversion=36.5 %), and the contrast of the latex particles increased correspondingly (Figure 5.28(b)). At this stage, the shell of nanocapsules was still thin but clearly distinguishable. The shell stability of nanocapsules further increased as the increase of conversion to 56.8% (Figure 5.28(c)). As the conversion increased to 75.5%, the resultant nanocapsules are elliptical (Figure 5.28(d)). For the product with the high conversion, the well-defined spherical nanocapsules dominated in the final product (Figure 5.28(e)). It indicated that with the increase of the amount and the cross-linking degree of copolymers in the shell, the morphology of nanocapsules became stable enough to keep spherical even with the loss of HD. The morphological evolution of latex particles definitely indicated that the morphology of nanocapsules has been formed at the initial stage of polymerization. The polymerization in the rest of reaction time contributed to increase the shell thickness and the cross-linking degree of copolymers, resulting in the increase of shell stability. These results again indicated that the formation of nanocapsules mainly follows the mechanism of interfacial copolymerization in present system. 5.3.4

M e c h a n i s m A n a l y s i s of O r g a n i c - I n o r g a n i c Hybrid Nanocapsules Formation

1) Anchoring effects The prerequisite to perform interfacial miniemulsion copolymerization is that the separated oligomeric radicals are adsorbed by the nanodroplets and then anchored at the oil/water interfaces. The anchoring effect of oligomeric radicals is governed by thermodynamic (the hydrophilicity and the oil solubility of oligomeric radicals, the thermodynamically favorable state) and kinetic (the formation of crosslinked structures) aspects concurrently. For the present systems, from the thermodynamic point of view, the hydrophilicity of oligomeric radicals can be increased via the incorporation of hydrolyzed MPS units in the oligomeric radicals;

POLYMERIC NANOCAPSULES

135

In addition, at the reaction temperature (70 °C), the PNIPAM oligomeric radicals cannot dissolve into either the aqueous phase or the oil phase. Both of them are favorable to locate the oligomeric radicals on the interfaces of oil/water to perform the interfacial copolymerization. From the kinetic point of view, the anchoring effects of oligomeric radicals can be reinforced by the introduction of crosslinker, and the condensation of MPS. 2) Process analysis In the pre-treating process, the oil phase was homogenized to form nanodroplets, resulting in the drastic increase of the interfacial area of oil/water. Under the neutral conditions, a small part of MPS participates in the hydrolysis at the interfaces of oil /water. The monomers in the aqueous phase were consisted of NIPAM and hydrolyzed MPS, while the un-hydrolyzed MPS, St and DVB were in the oil phase. For the basic or acidic system, MPS molecules not only took part in the hydrolysis reactions, but also in the condensation reactions. The monomers in the aqueous phase were consisted of NIPAM, hydrolyzed MPS and the polysiloxane oligomers below the critical chain length, while the unhydrolyzed MPS and St were in the oil phase. Under the neutral condition, upon the introduction of KPS, the oligomeric radicals containing the NIPAM and hydrolyzed MPS units were produced in the aqueous phase. At the reaction temperature, these oligomeric radicals could separate out of the aqueous phase, and then be adsorbed by oil droplets. The adsorbed oligomeric radicals were inclined to locate at the interfaces of oil/ water due to their hydrophilicity and the oil insolubility. The mobility of polymer chains was reduced through the reactions with DVB as a result of the formation of the crosslinked structures. Therefore, the oligomeric radicals were anchored, and then polymerized at the interfaces of oil/water. For the acidic or basic system, the propagation of oligomeric radicals to the critical chain length could be through the free radical copolymerization, the condensation reaction, or the combination of them. In another word, the separation of oligomeric radicals out of the aqueous phase could be promoted by the condensation. The separated oligomeric radicals could be adsorbed by the nanodroplets or latex particles. These oligomeric radicals containing silanol groups were anticipated to reduce the interfacial tension of droplets/water or latex particles /water, and therefore were inclined to locate at the interface of droplets or

136

MINIEMULSION POLYMERIZATION TECHNOLOGY

latex particles. The oligomeric radicals could be immobilized on the interfaces of latex particles/water via the condensation reactions and then propagate there. The monomer diffuses from the interiors of latex particles to the interfaces as a supply for the monomer consumption, and the newly-produced copolymers separated at the interfaces. Although the morphology of nanocapsules is not the thermodynamically favorable state for the system containing surfactant, the crosslinked copolymer chains cannot diffuse into the interiors of latex particles due to their low mobility. Consequently, the nanocapsules are obtained by the encapsulation of HD template via the accumulation of the copolymers at the interfaces.

5.4

Conclusions

The thermody namic prediction for the ternary system of PS /octane / SDS aqueous solution and poly(MPS-co-PS)/HD/SDS aqueous solution based on the interfacial tensions showed that the morphology of nanocapsules was not the most favorable state. However, the product with a high number fraction of well-defined organic and organic-inorganic hybrid nanocapsules still could be synthesized via interfacial miniemulsion polymerization of St, NIPAM, DVB, a n d / o r MPS. NIPAM and DVB played key roles in inducing the interfaces of the nanodroplets to be the loci of polymerization. In a well-designed system, although the droplet size distribution was broad and a rapid decrease in particle size was observed during the initial stage of the polymerization, the fact that nearly all droplets evolved to form organic nanocapsules indicated the exclusion of secondary nucleation. An investigation of the influence of the amount of DVB on the formation of nanocapsules in the presence of NIPAM has further confirmed that the organic nanocapsules are formed by an interfacial miniemulsion polymerization mechanism. Under the neutral condition, the results showed that the hydrolysis and condensation reactions of MPS remained at a low level. Therefore, an additional amount of DVB was required to improve the anchoring effects of oligomeric radicals on the interfaces of oil/water to promote more nanodroplets to perform the interfacial copolymerization. An investigation of the influence of

POLYMERIC NANOCAPSULES

137

the amount of DVB on the formation of organic-inorganic hybrid nanocapsules in the presence of NIPAM has further confirmed that the nanocapsules are formed by an interfacial miniemulsion polymerization mechanism. The number of solid particles increased as the increase of the SDS concentration, especially in the system with the insufficient cross-linking reactions. The morphology of latex particles could be controlled by the hydrolysis and condensation reactions of MPS via changing the pH value of suspension. Increasing or decreasing the pH value from 7, the number fraction of organic-inorganic hybrid nanocapsules increased as the result of the increase of the condensation reactions attested by the results of 29Si-NMR spectra and TEM. However, some solid particles still could be found at the boundary of particle aggregates of the product with 20 wt% MPS synthesized under the acidic and basic conditions. The number fraction of organic-inorganic hybrid nanocapsules could be further improved through increasing the MPS weight content or introducing a small amount of DVB at pH 8.5. The morphological evolution in the process of copolymerization indicated that the formation of organic-inorganic hybrid nanocapsules following the interfacial miniemulsion polymerization.

References 1. A.V. Korobko, W. Jesse, and J.R.C. van der Maarel, Langmuir, Vol. 21, p. 34, 2005. 2. K. Bouchemal, S. Briançon, E. Perrier, H. Fessi, I. Bonnet, and N. Zydowicz, International journal of Pharmaceutics, Vol. 269, p. 89, 2004. 3. A.V. Jovanovic, J.A. Flint, M. Varshney, T.E. Morey, D.M. Dennis, and R.S. Duran, Biomacromolecules, Vol. 7, p. 945,2006. 4. F. Caruso, R.A. Caruso, and H. Möhwald, Science, Vol. 282, p. 1111,1998 5. F. Caruso, M. Spasova, A. Susha, M. Giersig, and R.A. Caruso, Chemistry of Materials, Vol. 13, p. 109,2001. 6. Q. Zhang, E.E. Remsen, and K.L. Wooley, Journal of the American Chemical Society, Vol. 122, p. 3642, 2000. 7. X.Y. Liu, M. Jiang, S.L. Yang, M.Q. Chen, D.Y. Chen, C. Yang, and K. Wu, Angewandte Chemie International Edition, Vol. 41, p. 2950,2002. 8. L. Sun, R.M. Crooks, and V. Chechik, Chemical Communications, p. 359, 2001 9. T.K. Mandai, M.S. Fleming, and D.R. Walt, Chemistry of Materials, Vol. 12, p. 3481, 2000. 10. M. Chen, L. Wu, S. Zhou, and B. You, Advanced Materials, Vol. 18, p. 801,2006. 11. C.J. McDonald, K.J. Bouck, A.B. Chaput, and C.J. Stevens, Macromolecules, Vol. 33, p. 1593, 2000.

138

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

12. J. Jang, and K. Lee, Chemical Communications, p. 1098,2002. 13. F. Tiarks, K. Landfester, and M. Antonietti, Langmuir, Vol. 17, p. 908,2001. 14. A.J.P. van Zyl, R.D. Sanderson, D. de Wet-Roos, and B. Klumperman, Macromolecules, Vol. 36, p. 8621,2003. 15. Y.W. Luo, and X.D. Zhou, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 42, p. 2145,2004. 16. H.D. Koh, and J.S. Lee, Macromolecular Rapid Communications, Vol. 28, p. 315, 2007. 17. Y. Chen, H.R. Liu, Z.C. Zhang, and S.J. Wang, European Polymer Journal, Vol. 43, p. 2848,2007. 18. K.F. Ni, G.R. Shan, and Z.X. Weng, Macromolecules, Vol. 39, p. 2529,2006 19. D. Crespy, M. Stark, C. Hoffmann-Richter, U. Ziener, and K. Landfester, Macromolecules, Vol. 40, p. 3122,2007. 20. N. Jagielski, S. Sharma, V. Hombach, V. Mailander, V. Rasche, and K. Landfester, Macromolecular Chemistry and Physics, Vol. 208, p. 2229,2007. 21. L. Torini, J.F. Argillier, and N. Zydowicz, Macromolecules, Vol. 38, p. 3225, 2005. 22. C. Scott, D. Wu, C.C. Ho, and C.C. Co, Journal of the American Chemical Society, Vol. 127, p. 4160,2005. 23. D. Wu, C. Scott, C.C. Ho, and C.C. Co, Macromolecules, Vol. 39, p. 5848,2006. 24. Y.W. Luo, and H.Y. Gu, Macromolecular Rapid Communications, Vol. 27, p. 21, 2006. 25. Y.W. Luo, and H.Y. Gu, Polymer, Vol. 48, p. 3262,2007 26. F.J. Lu, Y.W. Luo, and B.G. Li, Macromolecular Rapid Communications, Vol. 28, p. 2868, 2007. 27. Q.H. Sun, and YL. Deng, Journal of the American Chemical Society, Vol. 127, p. 8274, 2005. 28. Z.H. Cao, and G.R. Shan, Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 47, p. 1522, 2009. 29. S.W. Zhang, S.X. Zhou, Y.M. Weng, and L.M. Wu, Langmuir, Vol. 22, p. 4674, 2006. 30. E. Bourgeat-Lami, I. Tissot, and F. Lefebvre, Macromolecules, Vol. 35, p. 6185, 2002. 31. Z.H. Cao, G.R. Shan, G. Fevotte, N. Sheibat-Othman, and E. Bourgeat-Lami, Macromolecules, Vol. 41, p. 5166,2008. 32. S. Torza, and S.G. Mason, Journal of Colloid and Interface Science, Vol. 33, p. 67, 1970. 33. F.K. Hansen, and J. Ugelstad, Journal of Polymer Science, Polymer Chemistry, Vol. 16, p. 1953,1978. 34. A.J.P. van Zyl, R.F.P. Bosch, J.B. McLeary, R.D. Sanderson, and B. Klumperman, Polymer, Vol. 46, p. 3607,2005 35. M. Heskins, and J.E. Guillet, Journal of Macromolecular Science Chemistry, Vol. 2, p. 1441,1969. 36. X.C. Xiao, L.Y. Chu, WM. Chen, S. Wang, and R. Xie, Langmuir, Vol. 20, p. 5247, 2004. 37. H. Kim, and D.J. Burgess, Journal of Colloid and Interface Science, Vol. 241, p. 509, 2001.

6 Miniemulsion Polymerization of Vegetable Oil Macromonomers Shelby F. Thames*, James W. Rawlins, and Sharathkumar K. Mendon S F Thames, Polymer Science Research Center, The University of Southern Mississippi, Thames-Rawlins Research Group, Hattiesburg, USA

Abstract

Miniemulsion polymerization is well suited for the polymerization of hydrophobic monomers, especially VOMMs that are too hydrophobic to traverse the aqueous phase during conventional emulsion polymerization. Since VOMMs possess some inherent hydrophobicity due to their fatty acid content, they are intrinsically suited for miniemulsion polymerizations, and the resulting polymers are crosslinkable at ambient conditions. While other miniemulsions can be made to crosslink only via in situ grafting, crosslinking in VOMM miniemulsions occurs after film formation is complete, thereby enabling improved flow and leveling during film formation with subsequent oxygen uptake for ambient crosslinking and polymer characteristic changes. Unlike many other vegetable oil derivatives, VOMMs effectively stabilize monomer droplets for miniemulsion polymerization. Novel soybean oil-based VOMMs were synthesized and copolymerized with conventional acrylic and (meth)acrylate monomers at various weight percents ranging from 5 to 70%. Gel content studies confirm retention of the VOMM unsaturation during the polymerization and consequently they cure to solid, high performance polymers via ambient auto-oxidative polymerization, after they are applied to a chosen substrate. Keywords: vegetable oil, acrylic acid, auto-oxidative polymerization, soybean oil, in-situ, gel content, unsaturation, ambient crosslinking, vegetable oil macromonomers, VOMMs, miniemulsions, and emulsion polymerization.

6.1 Introduction and Background Vegetable Oil M a c r o m o n o m e r s (VOMMs) are derived from vegetable oils, i.e., triglycéride esters of fatty acids (Figure 6.1). The chemical V. Mittal (ed.) Miniemulsion Polymerization Technology, (139-172) © Scrivener Publishing LLC

139

140

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY O

II

CH2—O—C

R

CH

R2

I

°

0—C

CH2—0

n

R3

O Ri,R2, and R3 are fatty acid residues

Figure 6.1. Vegetable Oil.

Table 6.1. Common vegetable oil fatty acids. Stearic acid

CH3-(CH2)16-COOH

Oleic acid

CH3-(CH2)7-CH=CH-(CH2)7-COOH

Linoleic acid

CH 3 -(CH 2 ) 4 -CH=CH-CH 2 -CH=CH-(CH 2 ) 7 -COOH

Linolenic acid

CH 3 -CH 2 -CH=CH-CH 2 -CH=CH-CH 2 -CH= CH-(CH2)7-COOH

Eleostearic acid

CH 3 -(CH 2 ) 3 -CH=CH-CH=CH-CH=CH(CH2)7-COOH

Ricinoleic acid

CH3-(CH2)5-CH(OH)-CH2-CH=CH-(CH2)7-COOH

composition of the fatty acids and the proportions in which they are present determine the general properties of the vegetable oils. Furthermore, the fatty acid composition of vegetable oils is affected by environmental conditions such as soil type, growing location, and climatic changes such as humidity and temperature [1]. Most fatty acids contain 18 carbon atoms and are characterized by varying degrees of unsaturation (conjugated or unconjugated) (Table 6.1). Fatty acid chains inherently impart flexibility and hydrophobicity to vegetable oils and their derivatives. The propensity of unsaturated fatty acid chains to react with atmospheric oxygen and polymerize is reflected in the "drying" characteristics of vegetable oils. Drying, in terms of vegetable oil chemistry, defines the oxygen initiated polymerization of various fatty acid chains with subsequent conversion to high molecular weight semi-solid or solid materials. In this respect, conjugated double bonds are more reactive than unconjugated double bonds.

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

141

All vegetable oils contain a proportion of saturated acids such as palmitic acid and stearic acid, and the monounsaturated acid, i.e., oleic acid, that make little or no contribution to the drying process. Consequently, vegetable oils are distinguished from one another by their "drying" ability and are segregated via their iodine value (IV) into non-drying oils with IVs of 120 (coconut, cottonseed), semidrying oils with IVs of 120-150 (safflower, sunflower, soybean), and as drying oils with IVs > 150 (linseed oil, tung oil). The fatty acid compositions of various vegetable oils are listed in Table 6.2 [2]. Vegetable oils are a key component of oil-modified polyesters (alkyd resins) synthesized via esterification of vegetable oils (or vegetable oil derived acids and esters) with a variety of acids or anhydrides such as phthalic anhydride, isophthalic acid, or trimellitic anhydride, and polyols such as glycerol, trimethylol propane, or pentaerythritol. Despite their excellent gloss, flexibility, and favorable cost-performance ratio, alkyd resins possess poor hydrolytic resistance due to their ester functionality and tend to yellow during exterior exposure as further auto-oxidation increases visible light absorption through chromophore formation. On the other hand, acrylic derived polymers are easily synthesized via a number of polymerization techniques, including emulsion polymerization, and offer excellent color and color retention on exterior weathering, but coatings based on

Table 6.2. Fatty acid composition of common vegetable oils. Saturated

Oleic

Linoleic

Linolenic

Iodine value

Cottonseed

26

20

54

-

108

Linseed

11

21

18

50

180

Safflower

10

12

78

-

145

Soybean

15

22

55

8

130

Sunflower

12

26

62

-

135

a

5

8

4

3

165

3

3

4

-

85

Oil

Tung

Castor*

Contains 80% eleostearic acid. "Contains 90% ricinoleic acid.

142

MINIEMULSION POLYMERIZATION TECHNOLOGY

petroleum-based monomers sometimes possess limited flexibility. The complementary features of these two major resin types have resulted in significant efforts to produce high quality, waterborne alkyd-acrylic hybrids combining the advantages of both polymer types. Monomer hydrophobicity plays an important and controlling role in emulsion polymerizations. For instance, monomers must possess sufficient water solubility to diffuse through the water phase into the surfactant stabilized micelle where polymerization occurs. Monomers unable to diffuse into micelles will reside in monomer droplets, often unpolymerized, and over time will attempt to coalesce in order to minimize their surface energy. Thus, the hydrophobic nature of each monomer is vastly important both singularly and in relation to each other for efficient and random copolymerization, not only in particle nucleation but also in particle growth throughout the feeding stage. Monomers such as butyl acrylate (BA) and methyl methacrylate (MMA) are used extensively in emulsion polymerization chemistry and despite their general hydrophobicity, possess sufficient water solubility for efficient emulsion polymerization. Table 6.3 lists the water solubility of a number of monomers commonly used in emulsion polymerizations [3]. It can be seen that the low water solubility (<0.001 wt%) of monomers with >12 carbons limits their use in emulsion polymerizations. This Table 6.3. Monomer solubility in water. Monomer

Solubility in water (wt%), 20°C

Methyl acrylate

5.2

Vinyl acetate

2.5

Methyl methacrylate

1.5

Butyl acrylate

0.16

Styrene

0.03

2-Ethylhexyl acrylate

0.01

Vinyl neo-pentanoate

0.08

Vinyl 2-ethylhexanoate

<0.01

Vinyl neo-nonanoate

<0.001

Vinyl neo-undecanoate

< 0.001

POLYMERIZATION OF VEGETABLE OIL MACROMONOMERS

143

is especially true when the polymerization recipe includes monomers with widely varying water solubilities wherein some monomers readily diffuse into micelles and diffusion of others are inhibited by their higher hydrophobicity.

6.2 Emulsion Polymerization of Alkyds and Vegetable Oils Emulsions containing vegetable oil derivatives were designed to combine the advantages of solvent-soluble polyester polymers with the environmentally responsible, human health friendly, "Green" features of waterborne polymers. Unlike traditional latexes, vegetable oil latexes contain vegetable oil/alkyd particles stabilized as oil-in-water emulsions. Choosing the appropriate surfactant and emulsification protocol are critical to ensure colloidal stability of alkyd/vegetable oil derived emulsions. Nabuurs et al. investigated emulsion polymerization of acrylate monomers in the presence of alkyds and noted that acrylate monomer polymerization was severely retarded by alkyd unsaturation reacted in their chosen conditions [5]. However, the alkyd-acrylic hybrid systems were much better than the alkyd-acrylic blends in terms of film formation. For instance, the minimum filming temperature (MFT) of the hybrids was 5°C whereas the MFT of the alkyd-acrylic blend with the same overall composition was 21 °C, which coincided with the glass transition temperature (T ) of the acrylic resin. Studying the same system via mmiemulsion polymerization, Wang et al. were able to achieve copolymerization of the alkyd and acrylate [6]. Only 20-30% of the alkyd double bonds were consumed or lost during the polymerization process under their chosen conditions. The fraction of polyacrylate grafted onto the alkyd increased with alkyd content and ranged from 22-70%. Wu et al. investigated alkyds as a co-stabilizer in miniemulsion polymerization of butyl acrylate and methyl methacrylate and verified the formation of poly(acrylate-graftalkyd) as the predominant polymeric structure [7]. At least 20% alkyd was necessary to ensure stabilization. Only 17-28% of the alkyd double bonds were consumed during the polymerization, van Hamersveld et al. conducted miniemulsion copolymerization of an alkyd (oil length 83%) with MMA, and claimed lack of intraparticle heterogeneity via cryogenic transmission electron microscopy (cryo-TEM) analysis. The authors contended that the use of partly hydroperoxidized sunflower

144

MINIEMULSION POLYMERIZATION TECHNOLOGY

oil as the oil soluble initiator resulted in the formation of triglycéride modified polyacrylate molecules acting as a compatibilizer between the alkyd and PMMA phases [8]. However, in a subsequent study, van Hamersveld et al. reported that an AFM study of the film formation process of the same oil/alky d-acrylic hybrid latexes showed phase separation between the oil and the acrylic phases upon drying. The films consisted of deformed acrylic particles embedded in a continous matrix of oil [9]. Tsavalas et al. noted that hybrid miniemulsion polymerization of an MMA-alkyd system gave incomplete conversion while full conversion was achieved for an identical recipe without the alkyd [10]. The limiting conversion was attributed to the presence of relatively inactive alkyl allyl macroradicals formed due to chain transfer (retardive chain transfer mechanism). A physical explanation was also postulated proposing the partitioning of the monomer-alkydpolymer mixture and the formation of a hard polyacrylate shell that prevented initiator penetration. In a follow-up study, Hudda et al. developed mathematical models to describe the kinetics of an alkydacrylate system, i.e., a homogenous model based on retardive chain transfer kinetics and a heterogeneous model based on an alkyd core and acrylic shell [11]. The authors concluded that retardive chain transfer alone could not explain the limited conversion but the coreshell model in which polymerization occurred in an acrylic-rich shell while the alkyd-rich core served as a reservoir for the acrylic monomer and alkyd did result in correct simulation of the limiting conversion. To circumvent the immiscibility between the alkyds and the growing acrylate chains, Guo and Schork investigated hybrid miniemulsion polymerization of butyl acrylate and methyl methacrylate in the presence of a lower molecular weight analog of alkyds, i.e., linoleic acid and safflower seed oil [12]. The authors reported that hybrid particles from the acrylate/linoleic acid system exhibited homogeneous morphology with degrees of grafting near 100% and a single T while hybrid particles from acrylate/safflower seed oil exhibited lower degree of grafting and a core-shell structure with two T s. Tsavalas et al. studied the morphology of latexes synthesized via miniemulsion copolymerization of a medium oil length alkyd with MMA and BA by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and nuclear magnetic resonance (NMR) spectroscopy [13]. Hybrid alkyd-MMA systems were shown as a raspberry morphology with alkyd core and PMMA shell. BA/MMA copolymer-alkyd polymers exhibited typical core-shell morphology with alkyd core

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

145

and smooth acrylic shell. In a BA-alkyd system, however, a continuous particle phase of PBA with small internally dispersed island domains of alkyd was observed.

6.3 (Meth)acrylated Vegetable Oil Derivatives Bunker and Wool synthesized acrylated methyl oléate (AMO) by epoxidizing methyl oléate with a peroxy acid and reacting the product with acrylic acid [14]. Despite using 15 wt% sodium dioctyl sulfosuccinate as the surfactant for emulsion polymerization, the low water solubility of this macromonomer (approximately 10~7 M) gave only 91% conversion after 18 hours of reaction. Branching was attributed to chain transfer to polymer via hydrogen abstraction from a tertiary backbone C-H bond. In a second study, Bunker et al. adopted miniemulsion polymerization to synthesize pressure sensitive adhesives (PSAs) by copolymerizing MMA with AMO and reported complete conversion within 1 hour using only 2% surfactant by weight [15]. While the AMO and petroleum-based PSAs had comparable tack, shear strength and elasticity, the AMO-based polymers had lower peel strength. The authors proposed that the molecular weight of the AMO polymer needed to be increased via appropriate crosslinking to achieve the desired balance of properties. Spagnola et al. pursued the synthesis of functionalized fatty acid derivatives by adding a free radical polymerizable double bond to a fatty alcohol [16]. Specifically, oleyl alcohol and a mixture of oleyl and linoleyl alcohols were esterified with methacrylic anhydride to yield the respective methacrylated esters, i.e., oleyl methacrylate (OM) and a mixture of linoleyl and oleyl methacrylates (LOM). The comonomer, 2-ethylhexyl methacrylate (EHMA), was chosen for its low T (-10.15°C) and lack of hydrogen abstraction, which can lead to a grafting or crosslinking reactions during polymerization. The OM and LOM were each copolymerized with EHMA via miniemulsion batch copolymerization at 0, 5, 20, and 40 wt% based on monomer phase using a thermal initiator. The authors noted a slight conversion decrease for the EHMA with increasing amounts of OM incorporation and a large drop in conversion for EHMA with increasing concentrations of LOM in the system. These data are consistent with Tsavalas et al.'s results discussed above. Oleyl hydrocarbon chains possess fewer unsaturated sites than linoleyl chains and are therefore less susceptible to hydrogen abstraction. The improved conversion noted with increased reaction times for the LOM system

146

MINIEMULSION POLYMERIZATION TECHNOLOGY

was attributed to retardation effects from hydrogen abstraction being overcome by generation of new radicals from the thermal initiator. Although the radicals formed by chain transfer were sufficiently less reactive and did not reduce the reaction rate, they participated in polymerization reactions and formed a high degree of insoluble polymer (75±2% and 77.6+1% for 3% and 28.2% OM, respectively, and 88.3±1.1% and 86±3.3% for 3% and 28.2% LOM, respectively, after 24 hours). To reduce the amount of hydrogen abstraction and the reactivity of the radicals formed, the polymerizations were repeated at ambient with a redox initiator. The redox-initiated copolymerizations proceeded to completion much faster than the thermally initiated systems and offer significantly lower fractions of insoluble polymer (4.7±3.3% and 12.2±2.4% for 3% and 28.2% OM, respectively, and 39.8+1.6% and 51.1±1.2% for 3% and 28.2% LOM, respectively, after 3 hours). The authors attributed these results to reductions in hydrogen abstraction a n d / or that the fatty acid radical was less reactive in the ambient temperature redox-initiated system.

6.4

Vegetable Oil Macromonomers

In the early 90s, Thames et al. developed a series of vegetable oil derivatives, termed vegetable oil macromonomers (VOMMs), functionalized for efficient incorporation into emulsions [17]. VOMMs are a viable biobased alternative to obviate the need for coalescing solvents in waterborne coatings. Although waterborne coatings are regarded as the closest and environmentally favored alternative to solvent-based coatings, they are often formulated with significant amounts of solvents, i.e. coalescing aids, to facilitate good film formation of high T polymers. The disadvantages of this technology are clear; the volatile organic compounds (VOCs) formulated into the coatings recipe enter the atmosphere during the drying process as pollutants and constitute real and confirmed adverse health effects. VOCs are an environmental concern as they react with atmospheric nitrogen oxides to form ozone. Environmental Protection Agency (EPA) findings have linked ground level ozone to increased asthmatic and respiratory conditions in humans [18]. Even short-term exposure to very low levels of ozone can cause chest pain, coughing, nausea, throat irritation, congestion, and reduced lung capacity. In addition, ozone can exacerbate cardiac and lung conditions such as bronchitis, asthma, pneumonia, emphysema, and heart disease.

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

147

In view of the detrimental effect of ozone, the EPA imposes restrictions on the maximum VOC content permissible in coatings. VOMMs have three distinct structural and design characteristics favoring their use for high performance environmentally-responsible emulsions: a. VOMMS, by virtue of their large monomer size, are excellent coalescing aids (plasticizing monomers) and facilitate smooth film formation at zero VOC, b. VOMMs copolymerize with other monomers via their acrylate and (meth)acrylate functionality, and consequently are retained in the final film where they become an integral part of the applied coating, and c. VOMMs, when forming part of a polymer system, crosslink to higher molecular weight polymers during ambient cure. Thus, VOMM-derived polymers function as thermoplastic polymers during application and thermosetting polymers as auto-oxidative polymerization progresses during ambient cure. The overall objective of the polymer design process is to achieve and optimize control of the MFT and final T (after auto-oxidation) of VOMM emulsion polymers. This can be accomplished by choosing appropriate amounts and type of VOMMs for incorporation into the latex polymer composition. Emulsion polymers synthesized in this way represent the synergism of two technologies, i.e., alkyds and acrylics, where the advantages of each are retained and the disadvantages minimized. As evident from Bunker et al.'s work discussed above, achieving the appropriate degree of auto-oxidative crosslinking is critical if one is to control and utilize the inherent plasticization efficacy of the VOMM fatty acid chains. In general, unmodified vegetable oils lack reactive functional groups with the exception of a few oils such as castor and lesquerella oils (which are hydroxyl functional), while vemonia oil offers epoxide functionality. The hydroxyl moiety is a convenient site to incorporate polymerizable acrylate and methacrylate functionalities. Consequently, acrylated castor oil methyl esters (Figure 6.2) and acrylated castor oil (Figure 6.3) were among the first VOMMs to be synthesized and studied. The synthesis process has since been expanded to include other vegetable oils and reactants to produce a variety of VOMMs. Triglycérides are fatty acid esters, and compositionally, the presence of ester groups, either from the glyceryl backbone or introduced

148

M I N I E M U L S I O N POLYMERIZATION T E C H N O L O G Y CH 3 —0—C—(CH 2 ) 7 —CH = CH —CH 2 —CH —(CH 2 ) 5 —CH 3 O

O

Figure 6.2. Acrylated castor oil methyl ester. O O

II

¡I

H 2 C=CH — C — O

I

-O — C — (CH2)7 — CH = CH—CH 2 — CH — (CH 2 ) 5 —CH 3 O O

lí

II

H 2 C=CH — C — O

I

HC—O —C—(CH 2 ) 7 —CH = CH —CH 2 —CH —(CH 2 ) 5 —CH 3 O O

II

H2C=CH— C— O

I

H 2 C—O —C—(CH 2 ) 7 —CH = CH—CH 2 —CH —(CH 2 ) 5 —CH 3

Figure 6.3. Idealized acrylated castor oil (drawn with all three fatty acids as ricinoleic acid derivatives, typically the oil is limited to about 80% ricinoleic acid).

during derivatization steps, reduce long-term stability. Furthermore, the likeliness of chain transfer to allylic double bonds during free radical polymerization posed significant challenges, especially with VOMMs based on vegetable oils containing higher amounts of unsaturation. To overcome these hurdles, several VOMMs were designed and synthesized by modifying the composition, connectivity and length of polymerizable acrylate or methacrylate moieties linked with the fatty acid chain. A few such VOMMs are listed in Figure 6.4 [19,20,21,22]. The novel VOMMs were studied to determine the positive attributes for incorporation in the polymer backbone via emulsion polymerization, and achieving the desired final film properties. The areas of study include: 1. 2. 3. 4. 5.

Solubility parameter, Water solubility/ water partitioning, Functionality, Molecular weight, Polymer design for retention of allylic unsaturation (triene content minimization), 6. Color, 7. Propensity for degradative chain transfer,

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

149

8. Emulsion polymer and particle stability, 9. Pre-cured and post-cured polymer T changes as a function of polymer composition, and 10. VOMM plasticization efficacy for film formation.

O

O

I!

H

il

H

I

O

! ü

H 2 C—CH —C—OCH 2 CH 2 —O —C — N — R — N — C — O R ' — O H OCN —R — NCO ¡s a diisocyanate (A)

HO — R'—OH is a castor oil ora vegetable oil monoglyceride (diol) O

H

¡I

H

I

O

HO—R" -O — C — N — R - -N — C- OR'—X OCN — R —NCO is a diisocyanate HO — R'—X is hydroxy vinyl ether or hydroxy (meth)acrylate (B)

HO — R"—OH is a castor oil or a vegetable oil monoglyceride (diol) O

II

-R —N —C—OR'—OH - C — OR'H2C = C H — R — NCO is an acrylate isocyanate (C)

HO — R"—OH is a castor oil or a vegetable oil monoglyceride (diol) O

I!

O H

II

I

H O

I

II

O H

II

I

H O

I II

H,C=HC—C—OCHpCH, - O - C —N —R —N —C—OR'—OC—N—R — N — C - O X X —OH is a fluorinated alcohol OCN — R — NCO is a diisocyanate (D)

HO — R'—OH is a castor oil or a vegetable oil monoglyceride (diol)

(E)

XisHorCH,

Figure 6.4. VOMM Range. (Continued)

150

MINIEMULSION POLYMERIZATION TECHNOLOGY

CH, OH

CHo—O — C — R , (F)

N-CH?-CHp-0-C-C=CH? II C(CH3 O

o CH 2 ~- 0 - - C — \ ^ - v

v / ~ \ ^

o

CH-- 0 - - C - R ,

í

\

r

0=J

o il

OH

CH 2 - - 0 - - C — R 2 (G)

Ri

V=0

v

- ^ ' ^ ^

OH

0 X II I 0 - C H 2 - C H - C H 2 -- 0 - C - C = C H 2

XisHorCH3 and R2 are fatty acid chains

OH

CH 2 —O—C—R.

^ \ / ^

N-CH2-CH=CH2

I

H

(H)

R, and R2 are fatty acid chains

CH—0—C—R,

I

o

CH 2 — O — C — R (I)

R-0-CH=CHP

R, and R2 are fatty acid chains, HO-R-0-CH=CH2 is a vinyl ether

Figure 6.4. V O M M Range.

6.5 The Potential for Emulsion Polymerization of Model Saturated Monomers VOMMs are based on unsaturated fatty acids containing eighteen carbon atoms and intrinsically possess very low solubility in water. Moreover, when the fatty acid or vegetable oil derivative, the loss

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

151

of a functional group capable of hydrogen bonding further reduces its aqueous affinity. Figure 6.5 demonstrates that saturated carbons are the source of unfavorable interactions with the water phase. Ruelle and Kesselring quantified that addition of two méthylènes to a parent functional molecule (alcohols, acids, alkanes, methyl esters, or amines) reduced the molecules water solubility by an order of magnitude [23]. Ostwald ripening rates, i.e., the diffusion rate of monomer from smaller to larger droplets brought about by a difference in droplet pressures, are determined by measuring the droplet particle size over time. Under these conditions, the diffusion rate of the most hydrophobic monomer is proportional to the rate of droplet growth, allowing their diffusion rate to be determined using the Lifshitz-Slyozov-Wagner relationship. Alkanes bearing more than ten carbons have the capability to kinetically stabilize emulsions by retarding monomer flux from smaller to larger particles. After homogenizing the dispersion with a microfluidizer or sonicator, each monomer droplet must maintain an equal concentration of hydrophobic material. Ripening readily occurs when alkanes with less than ten carbons are employed, leading to phase separation within hours. On the contrary, hexadecane and octadecane imparts stability for days or months. Gilbert concluded

Figure 6.5. Water solubility versus number of carbons for acids, acrylates, alcohols, alkanes, and methyl esters.

152

MINIEMULSION POLYMERIZATION TECHNOLOGY

that homogeneous nucleation does not occur when a C12 acrylate is emulsion polymerized due to lack of monomer in the aqueous phase. Furthermore, Chern and Lin investigated lauryl methacrylate, a C12 methacrylate, as a stabilizer in miniemulsions and initiated ca. 30% of the monomer droplets [24]. Matsumoto et al. observed a 4.2 hour and 2.3 hour induction period of a C12 and C10 methacrylate homopolymerization, respectively, using conventional macroemulsion techniques [25]. On the contrary, an induction period was not noted when MMA and hexyl methacrylate were polymerized under similar conditions, and they concluded the induction periods resulted from the hydrophobic monomers inability to form oligomeric radicals (z-mers) that are generally thought to initiate polymerization. Therefore, these findings indicate diffusional limitations are encountered in macroemulsion when monomers have less than 10"4 wt% water solubility. The diffusion rates of lauric (C12), palmitic (C16), and stearic (C18) fatty acids have been measured to be ca. 10 7 cm 2 /sec indicating that the acid group does not supplement the polarity sufficiently to offset the hydrophobic effect [26]. Moreover, extremely nonpolar hexadecane yields diffusivity ca. 10®cm2/sec. In an emulsion polymerization, the monomer's water solubility chiefly controls its diffusion rates and surfactant has been shown to have minor effects on these rates. Anionic surfactants were observed to slightly increase Ostwald ripening rates upon increasing its concentration thirty times [27]. Nonionic surfactants enhance diffusion to a similar degree except when added to a pre-dispersed emulsion. When homogenizing oil droplets in the presence of a nonionic surfactant, the emulsion degrades by molecular diffusion of oil through the continuous phase, and micellar diffusion is enhanced when a large concentration of nonionic surfactant is added to a pre-formed emulsion. In the latter scenario, the mass transport mechanism has been shown to be a function of particle fusion-fission phenomenon and molecular diffusion [28]. Therefore, only monomers having moderately different water solubilities can be influenced by the choice of surfactant and processing conditions.

6.6 Nucleation Mechanisms Several mechanisms could be active in the particle formation stage of conventional, macro- and miniemulsion polymerization including micellar nucleation [29,30], homogeneous nucleation [31], and droplet initiation [32]. The dominant mechanism is chiefly

POLYMERIZATION OF VEGETABLE OIL MACROMONOMERS

153

determined by the monomer(s) hydrophilicity, surfactant concentration, and droplet's particle size. For example, micellar nucleation dominates when slightly water soluble monomers are polymerized in the presence of copious surfactant and large monomer droplets (>1μπ\). Homogeneous nucleation dominates when very hydrophilic monomers are copolymerized with surfactant levels below the CMC (or surfactant-free). Finally, monomer droplet nucleation can be produced using very hydrophobic monomers, a [S] < CMC, and intense homogenization to reduce the droplets size. Hansen and Uglestead first recognized droplet nucleation to be a dominant mechanism in miniemulsions, and they demonstrated that a very hydrophobic "cosurfactant" or hydrophobe sufficiently retards diffusional degradation to allow radical penetration into the droplets [32]. Typically, cosurfactants are fugitive, lipophilic substances, e.g., hexadecane; however, reactive cosurfactants are also effective in reducing Ostwald ripening. Chern and Sheu utilized stearyl methacrylate (SMA) at low concentrations (24mM) to demonstrate the effect of methacrylic acid on particle nucleation [33], They reported slightly enhanced droplet nucleation upon addition of acrylic acid or methacrylic acid to a miniemulsion polymerization. While miniemulsion has the advantage of copolymerizing diffusionally limited monomers, secondary nucleated particles can be formed and compete with the droplets as polymerization sites. This phenomenon is favored when the [S] > CMC and slightly water soluble monomers are employed, and miniemulsion polymerizations employ water insoluble monomers with the surfactant concentrations at or below the CMC to avoid micellar nucleation. However, commercial latexes are usually manufactured with surfactant levels higher than the CMC to impart shear stability and reduce coagulum. Higher concentrations of a reactive hydrophobe, SMA or stearyl acrylate (SA), were than what is routinely employed were investigated, and a slightly water soluble monomer, MMA, was copolymerized with the reactive hydrophobe at [S] » CMC. The crystalline character of SA and SMA emulsion copolymers allowed probing its distribution amongst the polymer chains. Jordan et al. explored the reactivity and thermal transitions of SA and methyl methacrylate (MMA) solution copolymers, and found a minimum of forty monomer wt% SA was required to produce crystalline domains in SA/MMA solution copolymer [34]. This research formulated SA and SMA at lower concentrations, ten monomer wt%, in macro- and "pseudo" miniemulsion conditions. The broadness and

154

MINIEMULSION POLYMERIZATION TECHNOLOGY

area of the melting point was correlated to the extent of copolymerization occurring inside monomer droplets. As previously mentioned, SA was emulsion copolymerized at 30 wt% with BA using a phase transfer catalyst, ß-cyclodextrin, to carry SA into the micelles [35]. This research showed random incorporation of SA in emulsion copolymers decreases its T and produced amorphous domains at these levels. However, without the catalyst, no T decrease was observed and crystalline melting peaks were observed, indicating SA was locally concentrated in distinct domains formed by two polymerization sites.

6.7 Design of Thermosetting Latex Polymers While there are a number of variables that must be considered in the design of thermosetting latexes, secondary particle nucleation or homopolymerization of crosslinker in the aqueous phase or droplet is detrimental to network formation. The difficulties encountered in forming effective network formation result from kinetic or thermodynamic implications or a combination of the two, and the major issues are listed below: 1. Crosslinking before complete coalescence 2. Low crosslinker concentration near the particle interface 3. Homopolymerization of very water soluble crosslinking comonomer during synthesis 4. Homopolymerization of water insoluble crosslinking comonomer during synthesis 5. Competitive particle nucleation mechanisms Bufkin and Grawe first reported crosslinked films by introducing functional monomers in a two-stage monomer addition method that increases the probability of having functional groups near the surface [36]. Complete network formation is dependent on competitive crosslinking reaction rates and polymer diffusion coefficients [37-39]. Maintaining a balance between these two competing phenomena is vital to the design of ambient curable latexes. This issue is not as critical when auto-oxidative crosslinking mechanisms are employed due to their considerably reduced reaction rates as compared to other ambient cured polymers. During macroemulsion copolymerization, monomers exhibiting large differences in both

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

155

aqueous phase partitioning and reactivity ratios have increased liveliness of creating non-uniform monomer-particle distributions. Comonomers exhibiting similar reactivity ratios and water solubilities produce polymers and films with more evenly distributed crosslinking sites throughout the particles. Semi-batch feeding is capable of altering the comonomer distribution of latexes synthesized with comonomers possessing moderate differences in reactivities or water solubilities. However, this processing method cannot improve the copolymerization when two significantly different solubility (water solubility/hydrophobicity) comonomers are macroemulsion copolymerized [40]. Distinctly different copolymer compositions are generated when copolymerizing water soluble functional monomers, e.g., acrylic acid, with other slightly water soluble comonomers, e.g., glycidyl methacrylate, to produce a crosslinked film [38,41]. A significant fraction of the water-soluble functional comonomer will homopolymerize in the aqueous phase yielding a broad comonomer composition distribution or a blocky like copolymer [42]. For example, when BA is copolymerized with sodium acrylate, homopolymerization primarily occurs in both organic and aqueous phases. Very little copolymerization occurs between BA and sodium acrylate. On the other hand, when MMA and sodium acrylate are copolymerized, more homogeneous copolymers are formed because MMA's water solubility is an order of magnitude greater than that of BA [43]. Random copolymerization of insoluble and slightly soluble monomers requires reduced monomer feed rates to create monomer starve feed conditions. When a functional monomer homopolymerizes, complete network formation cannot occur, leading to ineffective network formation [44]. For instance, acrylic acid and methacrylic acid (MAA) are thermodynamically driven to the particle/water interface due to their increased polarity and hydrogen bonding. However, at pH > 6, MAA fully partitions into the aqueous phase [45]. Utilizing semibatch methods at a reduced pH permit acid functional groups to be present on the particle's surface for interparticle crosslinking. In a batch polymerization, the comonomers' reactivity ratios and water solubilities chiefly determine the copolymers microstructure and distribution throughout the particles using the macroemulsion technique. Comonomers with similar polymerizable moieties, e.g., acrylic esters, display similar reactivity ratios and the resulting polymers chemical and particle compositional distribution is chiefly controlled by the comonomers partioning behavior between the

156

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

aqueous phase, polymeric particles, and monomer droplets. Very water soluble or water insoluble comonomers (Figure 6.6) that do not diffuse into the polymerization locus before free radical polymerization occurs, broadens the chemical composition distribution and in extreme circumstances, generates two distinct polymers in separate particle populations. Figure 6.6 displays water solubility values of monomers commonly used in latexes, along with two fatty acids for comparison [46,47,48,49]. Fatty acids have negligible water solubility due to the presence of eighteen nonpolar carbons incapable of hydrogen bonding. Since the fatty structures are the fundamental building blocks for VOMMs, their hydrophobicity will be carried over, and the miniemulsion technique must be used to copolymerize the macromonomers with typical acrylic monomers. Furthermore, GPC analysis (Figure 6.7) did not indicate the presence of residual fatty acid, and the product's reduced retention

Figure 6.6. Monomer water solubility chart where DMF= Dimethylformamide, MAA = Methacrylic Acid, HEA = Hydroxyethyl Acrylate, HEMA = Hydroxyethyl Methacrylate, 2-EHMA = 2-Ethylhexyl Methacrylate, f-Bu Styrene = ferf-Butyl Styrene, and Ricin. Acid = Ricinoleic Acid.

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS 240.00

J i PEGMM /< I J

LLPEGMM

220.00

157

' ill

200.00

' i!« I 'ί'

180.00 160.00

i

140.00 120.00

1

100.00

i

80.00

/

i

V ,1 y III

60.00

77 ü

40.00

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 Minutes

Figure 6.7. GPC overlay of LLPEGMM and PEGMM.

times indicate the ethoxylated methacrylate was esterified. The number of peaks in the PEGMM and LLPEGMM chromatograms reveals the distribution of ethoxylated repeat units. Both analyses have more than six distinct n values most likely ranging from 3 to 9. Literature data on similar ethoxylated molecules correlated well with the measured distribution. As the average number of ethoxylated units increases, broader distributions are obtained. Our analysis indicates thirteen distinct n values of PEGMM having an average of ten repeat units. Ferguson et al. also reported larger distributions of repeat units as the number of ethoxylated repeat units with increasing n values. RP-HPLC chromatography is a powerful tool for analyzing reaction conversion while providing estimates for each of the analyte's partition coefficient. Figure 6.8 shows RP-HPLC chromatograms comparing stearyl acrylate (very hydrophobic), LLPEGMM, and common monomers used in emulsions. The four monomers with the shortest retention times do not have diffusional limitations, and analyte water solubility decreases with increasing retention time. Comparing the retention times of stearyl acrylate to the ethoxylated VOMM, shows ethylene oxide's hydrophilic contributions. Combining the six possible ethoxylated methacrylates having different n values with the four fatty acids in soybean oil produces 24 different ethoxylated VOMMs and explains the broad spectrum

158

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY -20% Non-reacted Ethoxylate Methacrylate 250

Λ

200

> CD O)

c

o

Q. C/> CU

Œ

150 50 H

Ethoxylated VOMM

!.>"""

0

% -100 ω ω -150-1 £

2-EHA

100

-50 H

Stearyl Acrylate

MMAI

! BA

-200-250-3006

8

10 Retention Time (min)

12

14

Figure 6.8. RP-HPLC of typical emulsion monomers, LLPEGMM, and SA.

observed in Figure 6.5. Most of the monomer is more hydrophobic than an eight-carbon acrylate, i.e., 2-EHA, and the mean fraction of LLPEGMM has water solubility comparable to a ten-carbon acrylate. Most of the monomer fractions are more hydrophobic than a twelve-carbon acrylate; while larger n values and unsaturated fatty acids produce more water soluble VOMMs. The hydrophobicity of a twelve-carbon acrylate was shown to stabilize monomer droplets; however, using the semi-continuous feed method can minimize these effects.

6.8

Classifying Monomer Solubility for Macro and Miniemulsion Polymerization

The type of emulsion polymerization employed is determined by the comonomer water solubilities, concentration of surfactant(s), ([S]), and desired polymer microstructure. As stated previously, semibatch processing renders slightly more homogeneous copolymerization of comonomers varying in water solubilities by less than one or two orders of magnitude. Warson and Fitch divided latex monomers into four categories based on aqueous affinity.

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

159

1. Very water insoluble monomers, e.g., styrene and vinyl stéarate 2. Appreciably water soluble monomers producing water insoluble polymers, e.g., methyl methacrylate and acrylonitrile 3. Appreciably water soluble monomers producing slightly hydrophilic polymers, e.g., vinyl acetate 4. Highly water soluble monomers producing very hydrophilic polymers, e.g., MAA and acrylic acid Under special conditions, comonomers of group 4 can be macroemulsion copolymerized with comonomers of groups 2 and 3. Group 1 monomers are not readily incorporated using traditional macroemulsion polymerization and when they are copolymerized with group 4 monomers homopolymerization will occur in separate phases [50,51]. Only monomers possessing water solubility greater than 10"7wt% are capable of diffusing through the aqueous phase on a reasonable timescale. Other monomers cannot diffuse out of the monomer droplet sufficiently quick to match the reactions timescale and ultimately become concentrated in hydrophobic monomer droplets as the more water soluble monomers are moving across the aqueous phase to growing particles, creating two distinct particle populations. On the contrary, miniemulsion polymerization methods evenly distribute hydrophobic monomers into micelles and minimize or eliminate the necessity of macroemulsion phase transfer from larger monomer droplets. Monomer flux between droplets and particles is inhibited to maintain equal hydrophobe concentration throughout the particles. A reduction in monomer diffusion combined with a low free surfactant concentration promotes monomer droplet nucleation while decreasing the probability of homogeneous and micellar nucleation. Moreover, homogeneous nucleation is further suppressed by polymerizing water insoluble monomers. The inherent hydrophobicity of oils and oil-based derivatives limit the ability of VOMMs to migrate from the monomer droplets to the site of polymerization through the aqueous phase. Therefore, it was necessary to functionalize the VOMM to increase its hydrophilic character and enhance its ability to be used as a copolymerizable hydrophobe for miniemulsion polymerization. Unlike other hydrophobes, the acrylate and methacrylate functionalities improve covalent bonding to form polymers while the VOMM sites of allylic unsaturation retain their potential for crosslinking during and after film formation.

160

6.9

M I N I E M U L S I O N POLYMERIZATION TECHNOLOGY

Soybean Acrylated Monomer Synthesis

Soybean oil (500 g, 0.56 mole) and 83.5 mg of 2-mercaptobenzothiazole were heated to 100°C under a nitrogen purge of 1 hour to remove the oxygen before adding 111.8 g (1.12 moles) maleic anhydride (MA) to the blend. The reaction temperature was increased to 215°C and maintained for 2.5 hours to yield maleinized soybean oil. Next, hydroxyethyl acrylate (26.95 g), phenothiazine (0.153 g), and 1-methyl imidazole (0.29 g) were added to the reactor. The reaction was heated to 115°C and maintained at that temperature for 2.5 hours to yield soybean acrylated monomer (SAM) (Figure 6.9). The FTIR spectrum of SAM shows absorptions at 1785 and 1853 cm"1 that arise from the stretch vibrations of the residual maleic anhydride (Figure 6.10). The absorption at 3010 cm'1 and 1650 cm"1 result from the C-H and C=C stretch vibrations, respectively, of the fatty acid double bond. Acrylate (-CH=CH 2 ) absorptions are noted at 919 cm'1 and 984 cm 1 -while maleic acid carboxylic group is seen at 1408 cm"1 and 1750 cm 1 , although the latter is partially overlapped by the ester carbonyl group. The wide and low absorptions at 2500 cm'1 and 3400 cm 1 are attributed to -OH vibration.

6.10

Miniemulsion Polymerization

Unlike conventional emulsion polymerization, miniemulsion polymerization occurs inside monomer droplets. Energetic homogenization is essential to achieve the desired submicron droplet size, and the droplets are protected against diffusional degradation and

'^/wwvvvvvvvvwvv

- fatty acid chain

Figure 6.9. Soybean acrylate monomer (SAM).

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

161

8-

6-

1

4000

1

3500

1

1

3000

Figure 6.10. FTIR spectra of S A M .

1

1

2500

1

1

2000

1

1

1500

Wavenumber (cm-1)

1

1

1000

1

1—

500

droplet coagulation by using a hydrophobe/costabilizer and an appropriate surfactant. Homogenization is typically achieved via the use of rotor-stator systems, sonifiers, or high-pressure homogenizers. Hydrophobie materials employed in miniemulsions include low molecular weight molecules [52], pigments [53], and oligomers or polymers [54-56]. The low water solubility of the hydrophobes prevents their diffusion through the water phase and keeps all other comonomers inside the droplet, thus minimizing the system's free energy. Since most hydrophobes are not covalently bonded to the polymer, they are prone to leaching from the coating or phase separation, prompting research efforts directed at developing reactive hydrophobes [57-59]. Typically, the surfactant is dissolved in water while the costabilizer is dissolved in the monomers, and the mixture is subject to high energy homogenization. For instance, when cetyl alcohol is used as a costabilizer, it can be dissolved in the surfactant solution by heating beyond its melting point, at which time the mixture is cooled and sonicated to break the gel phase before adding monomer and proceeding with homogenization to form droplets. With SAM, monomer droplets were generated by inverting a water-in-oil (w/o) emulsion, a process referred to as catastrophic phase inversion. SAM, ammonium bicarbonate, anionic surfactant

162

MINIEMULSION POLYMERIZATION TECHNOLOGY

Rhodapex® CO-436 [ammonium salt of nonylphenol-4-ethoxy sulfate (58 wt%) in water (27 wt%) and ethanol (15 wt%)], and deionized water were added to a reaction kettle, and placed in a 65°C water bath (Table 6.4). The contents were stirred at 100 rpm for 10 minutes to form a water-in-oil emulsion at which time phase inversion to an oil-in-water emulsion was affected with the continued addition of water. Samples were subjected to particle size analysis and stability evaluations. A temperature of 80°C was chosen for the polymerization. BA and MMA were added continuously over 20 minutes to swell the already formed VOMM droplets, and the system was allowed to equilibrate for an additional 30 minutes at which time a sample was collected for particle size analysis to verify droplet swelling. The miniemulsion was polymerized in a semi-continuous mode by adding ammonium persulfate over a two-hour period. For comparison, acrylic controls [control 1 (BA/ MMA), and control 2 (BA/MMA/Acrylic acid)] were synthesized via conventional emulsion polymerization (Table 6.5). Monomers and initiator were added over a two-hour period. Particle size distribution analysis was used to follow the nucleation mechanism and particle growth of the polymerization process. Figure 6.11 (0 min, immediately before initiator addition) confirms the presence of large monomer droplets (200-2200 nm). Figure 6.12 (6 min) shows generation of small particles (50 nm) by micellar nucleation as predicted for emulsion polymerization when the surfactant concentration is above the critical micelle

Table 6.4. Miniemulsion formulation. Materials

Weight (g)

SAM

16.95

Ammonium bicarbonate

0.42

Rhodapex CO-436

1.30

Deionized water

112.24

Butyl acrylate

16.50

Methyl methacrylate

12.65

Ammonium persulfate

0.83

POLYMERIZATION OF VEGETABLE O I L MACROMONOMERS

163

Table 6.5. Conventional emulsion formulation. Materials

Weight (g)

Rhodapex CO-436

13.10

Ammonium bicarbonate

1.65

Water

307.70

Butyl acrylate

165.00

Methyl methacrylate

126.50

Acrylic acid Ammonium persulfate

0.00/10.00 2.42

Figure 6.11. Particle size analysis of monomer droplets. concentration