ADVANCEDPOLYMERNANOPARTICLESSynthesis and SurfaceModificationsCRC Press is an imprint of theTaylor & Francis Group, an informa businessBoca Raton London New YorkADVANCEDPOLYMERNANOPARTICLESSynthesis and SurfaceModificationsEdited byVikas MittalCRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742 2011 by Taylor and Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group, an Informa businessNo claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1International Standard Book Number: 978-1-4398-1443-7 (Hardback)Thisbookcontainsinformationobtainedfromauthenticandhighlyregardedsources.Reasonableefforts havebeenmadetopublishreliabledataandinformation,buttheauthorandpublishercannotassume responsibility for the validity of all materials or the consequences of their use. 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For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.Library of Congress Cataloging-in-Publication DataAdvanced polymer nanoparticles : synthesis and surface modifications / [edited by] Vikas Mittal.p. cm.A CRC title.Includes bibliographical references and index.ISBN 978-1-4398-1443-7 (hardcover : alk. paper)1.Polymerization. 2.Nanoparticles. 3.Polymers--Surfaces.I. Mittal, Vikas. II. Title.TP156.P6A38 2011620.192--dc222010020564Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.com vContentsPreface .................................................................................................................... viiEditor........................................................................................................................ixContributors ............................................................................................................xi1.Polymer Latex Technology: An Overview ................................................. 1V. Mittal2.Synthesis of Polymer Particles with Core-Shell Morphologies .......... 29Claudia Sayer and Pedro Henrique Hermes de Arajo3.Advanced Polymer Nanoparticles with Nonspherical Morphologies ................................................................................................. 61Yongxing Hu, Jianping Ge, James Goebl, and Yadong Yin4.Block, Graft, Star, and Gradient Copolymer Particles .......................... 97H. Matahwa, E. T. A. van den Dungen, J. B. McLeary, and B. Klumperman5.Polymer Nanoparticles by Reversible Addition-Fragmentation Chain Transfer Microemulsion Polymerization .................................. 133J. ODonnell and E. Kaler6.pH-Responsive Polymer Nanoparticles ................................................. 169Jonathan V. M. Weaver7.Smart Thermo-Responsive Nanoparticles ............................................ 197Peng Tian and Qinglin Wu8.Surface Tailoring of Polymer Nanoparticles with Living Polymerization Methods ........................................................................... 223Koji Ishizu and Dong Hoon Lee9.Effects of Nano-Sized Polymerization Locus on the Kinetics of Controlled/Living Radical Polymerization ........................................... 263Hidetaka Tobita 10.Functional Polymer Particles by Emulsifer-Free Polymerization ... 307V. MittalviContents 11.Polymer Nanoparticles with Surface Active Initiators and Polymer Initiators ....................................................................................... 329Klaus TauerIndex ..................................................................................................................... 361viiPrefacePolymerlatexparticlesareaveryimportantclassofpolymericmateri-als, which are used for a large number of commercial applications. These particlesaresynthesizedintheaqueousdispersionphasebynumerous synthesis methodologies such as emulsion, miniemulsion, microemulsion, dispersion,suspension,inverseemulsion(inorganicphase),polymeriza-tion, etc. Over the years, signifcant enhancement in the techniques deal-ing with the synthesis and surface tailoring of polymer particles has been achieved, which has also resulted in the widening of the application spec-trum of these particles. These advances include use of advanced controlled polymerizationmeanssuchasnitroxide-mediatedpolymerization,atom transferradicalpolymerization,radicaladditionfragmentationtransfer polymerization, etc., as well as use of advanced stabilizers, surface modi-fers,etc.Theseadvanceshavemadeitpossibletoachievepolymerpar-ticles with specifc sizes consisting of polymer chains of specifc molecular weightsandtailorablechemicalcompositionsorpropertiesaccordingto the requirement.Because the advanced synthesis techniques are the key to achieve new func-tional properties in the polymer nanoparticles, and the surface modifcations of these particles are required to ensure their use for specifc applications, it is of immense importance to bring readers up-to-date on recent advances in these felds. This information will enable readers to design the required par-ticle systems. This book thus serves the purpose of summarizing the devel-opmentsinthesynthesisandsurfacemodifcationtechniquestogenerate advancedpolymerparticles,andthecontentshavebeenaccordinglyorga-nized. Chapter 1 introduces polymer latex technology with an overview of the various conventional and recent synthesis methodologies. Synthesis and characterization of particles with core-shell morphol ogies have been focused on in Chapter 2. Chapter 3 reports the generation of nonspherical polymer particlesbyfollowingdifferentsyntheticroutes.Thegenerationofspecifc architectures such as block, star, graft, and gradient copolymer particles has been detailed in Chapter 4. Microemulsion polymerization using reversible addition-fragmentationchaintransfercontrolledradicalpolymerizationis thesubjectofChapter5.InChapter6,pH-responsivenanoparticleshave been described, whereas the synthesis of smart thermally responsive parti-cles has been reported in Chapter 7. Surface tailoring of various organic and inorganic nanoparticles by polymers is the subject of Chapter 8. Theoretical studiesonthekineticsofcontrolledradicalpolymerizationtechniques have been explained in Chapter 9. Chapter 10 reports the synthesis of func-tionalnanoparticlesbyusingthe surfactant-freeemulsionpolymerization viiiPrefaceapproach.Chapter11describesvarioussurface-activeinitiatorsaswellas polymeric stabilizers developed for polymer nanoparticles in recent years.Atthisjuncture,IwouldliketoexpressmyheartfeltthankstoTaylor& Francis Group for their kind support during the project. I am equally thankful to Professor Massimo Morbidelli at the Swiss Federal Institute of Technology, Zurich, Switzerland, who has been my guide in polymer latex technology. I am indebted to my family, especially my mother, whose continuous support and motivation have made this work feasible. I dedicate this book to my dear wifePreeti,forhervaluablehelpincoeditingthebookaswellasforher efforts in improving the quality of the book.Vikas MittalLudwigshafen, GermanyixEditorDr. Vikas Mittal studied chemical engineering at Punjab Technical Univer-sity in Punjab, India. He later obtained his master of technology in polymer science and engineering from the Indian Institute of Technology, Delhi, India. Subsequently, he joined Professor U. W. Suters polymer chemistry group at theDepartmentofMaterialsattheSwissFederalInstituteofTechnology, Zurich, Switzerland, where he worked for his doctoral degree with a focus on the subjects of surface chemistry and polymer nanocomposites. He also jointly worked with Professor M. Morbidelli at the Department of Chemistry andAppliedBiosciencesonthesynthesisoffunctionalpolymerlatexpar-ticles with thermally reversible behaviors.Aftercompletionofhisdoctoralresearch,Dr.MittaljoinedtheActive and Intelligent Coatings section of Sun Chemical Group Europe in London. Heworkedforthedevelopmentofwater-andsolvent-basedcoatingsfor food-packagingapplications.HelaterjoinedBASFPolymerResearchin Ludwigshafen, Germany, as a polymer engineer, where he is currently work-ing as a laboratory manager responsible for the physical analysis of organic and inorganic colloids.Hisresearchinterestsincludeorganicinorganicnanocomposites,novel fller surface modifcations, thermal stability enhancements, polymer latexes withfunctionalizedsurfaces,etc.Hehasauthoredmorethan40scientifc publications, book chapters, and patents on these subjects.xiContributorsPedro Henrique Hermes de ArajoDepartment of Chemical EngineeringFederal University of Santa CatarinaFlorianpolis, BrazilJianping GeDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, CaliforniaJames GoeblDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, CaliforniaYongxing HuDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, CaliforniaKoji IshizuDepartment of Organic Materials and MacromoleculesTokyo Institute of TechnologyTokyo, JapanE. KalerStony Brook UniversityStony Brook, New YorkB. KlumpermanDepartment of Chemistry and Polymer ScienceUniversity of StellenboschMatieland, South AfricaandLab of Polymer ChemistryEindhoven University of TechnologyEindhoven, the NetherlandsDong Hoon LeeDepartment of Organic Materials and MacromoleculesTokyo Institute of TechnologyTokyo, JapanH. MatahwaDepartment of Chemistry and Polymer ScienceUniversity of StellenboschMatieland, South AfricaJ. B. McLearyPlascon Research CentreUniversity of StellenboschMatieland, South AfricaV. MittalPolymer ResearchBASF SELudwigshafen, GermanyandDepartment of Chemistry and Applied BiosciencesInstitute of Chemical and BioengineeringETH ZurichZurich, SwitzerlandJ. ODonnellIowa State UniversityAmes, IowaClaudia SayerDepartment of Chemical EngineeringFederal University of Santa CatarinaFlorianpolis, BrazilxiiContributorsKlaus TauerDepartment of Colloid ChemistryMax Planck Institute of Colloids and InterfacesGolm, GermanyPeng TianSchool of Renewable Natural ResourcesLouisiana State UniversityBaton Rouge, LouisianaHidetaka TobitaDepartment of Materials Science and EngineeringUniversity of FukuiFukui, JapanE. T. A. van den DungenDepartment of Chemistry and Polymer ScienceUniversity of StellenboschMatieland, South AfricaJonathan V. M. WeaverDepartment of ChemistryUniversity of LiverpoolLiverpool, United KingdomQinglin WuSchool of Renewable Natural ResourcesLouisiana State UniversityBaton Rouge, LouisianaYadong YinDepartment of ChemistryUniversity of CaliforniaRiversideRiverside, California11Polymer Latex Technology: An Overview*V. Mittal1.1 IntroductionPolymernanoparticlesfnduseinanumberofapplicationslikecoatings, adhesives, paints, etc. The applications of these nanoparticles are signifcantly affectedbytheirphysicalpropertiesaswellassurfacemorphology,which canbecontrolledbythesynthesisprocessusedtogeneratesuchparticles. Emulsion poly mer i za tion and its modifed methodologies are the most com-monly used techniques to achieve poly mer nanoparticles. These techniques also allow the generation or surface functionalization of the particles either in situ or by following separate specifc steps. Polymerization of monomer by emulsion poly mer i za tion offers signifcant advantages in the whole poly mer-i za tionprocessascomparedtobulkandsolutionpoly mer i za tionmethods. It allows better control of the heat and viscosity of the system, and emulsion poly mer i za tion allows the achievement of an increase in molecular weight of the poly mer chains without negatively impacting the rate of poly mer i za tion [1].Inemulsionpoly mer i za tion,mostofthemonomerispresentasmono-mer droplets in the aqueous phase, which diffuses to the polymerizing par-ticles during the course of poly mer i za tion. The diffusion of the monomer is * TheworkwascarriedoutatInstituteofChemicalandBioengineering,Departmentof Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.CONTENTS1.1Introduction .................................................................................................... 11.2Emulsion Polymerization ............................................................................. 21.3Controlled Polymerization and its Use in Emulsion Polymerization Processes ............................................................................. 91.4Conventional and Controlled Miniemulsion Polymerization ............... 161.5Generation of Copolymer or Core-Shell Particles ................................... 20References ............................................................................................................... 252Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationspossible when the monomer is partially water soluble. Thus, emulsion poly-mer i za tion is not very effective with extremely hydrophobic and extremely hydrophilic monomers. The extremely hydrophobic monomers would always stayinthemonomerdroplets,leadingtonopoly mer i za tion,whereasthe hydrophilic monomers would poly mer ize mainly by homogenous poly mer-i za tionandnotmicellarpoly mer i za tion.Tocircumventthesediffculties, miniemulsionpoly mer i za tionisused[2,3].Inthistechnique,thediffusion of the monomer molecules through the aqueous phase is not required, as the monomer droplets are directly poly mer ized. Therefore, such a technique has no problem in achieving the poly mer i za tion of even extremely hydrophobic monomers.Topoly mer izeveryhydrophilicmonomers,inverseminiemul-sioncanbeused.Combinationofcontrolledpoly mer i za tionmethodslike nitroxide-mediatedpoly mer i za tion,atomtransferradicalpoly mer i za tion, and reversible addition fragmentation chain transfer poly mer i za tion with the emulsion and miniemulsion poly mer i za tion methods has further enhanced the possibilities of achieving functional poly mer particles [4]. By using these techniques, synthesis of functional block copolymer or graft copolymer par-ticles can be achieved, which is not possible by using conventional emulsion poly mer i za tion techniques owing to the very short lifetime of the radicals. Thesurfacemorphologiesoftheparticlescanalsobeeffcientlycontrolled or tuned by using such controlled poly mer i za tion methods, which expands the spectrum of application of these particles. This chapter aims to provide an overview of the conventional emulsion poly mer i za tion methods and the more advanced methods of synthesizing poly mer particles.1.2Emulsion PolymerizationEmulsion poly mer i za tion is a heterogeneous poly mer i za tion technique that uses water as dispersion medium for the poly mer i za tion of water-insoluble monomers in the form of suspended particles. Styrene, methyl methacrylate, butyl acrylate, etc. are examples of the most commonly used monomers for the generation of polymers by emulsion poly mer i za tion. The surfactants are generally used to provide colloidal stability to the system. The surfactant can be cationic, anionic, or nonionic, and its amount exceeds the critical micelle concentrationsignifcantly.Thesurfactantsformmicellesinthesystemin which the poly mer i za tion takes place. Thus, this process can be visualized as a bulk poly mer i za tion in each of the suspended particles. Polymerization bythismodehelpstocircumventtheproblemsofheatandviscositycon-trol generally associated with bulk poly mer i za tion. By changing the amount of surfactant, the molecular weight of the poly mer chains can be increased withoutdecreasingthepoly mer i za tionrate,whichisnotpossibleinother modes of poly mer i za tion. The presence of a signifcant amount of surfactant Polymer Latex Technology: An Overview3in the system can lead to certain disadvantages; however, many applications ofparticlesarenotaffectedbythepresenceofsurfactants.Surfactant-free poly mer i za tioncanalsobeusedtogeneratepoly merparticlesinorderto circumventtheproblemsassociatedwiththeuseofemulsifer,butinthis case, the mode of poly mer nucleation is completely different.Asmentionedpreviously,theamountofthesurfactantexceedsthecriti-cal micelle concentration in the emulsifed emulsion poly mer i za tion process. The micelles formed as a result of this excess amount have a size in the range of 10 nm, and one micelle generally consists of 100200 surfactant molecules [1]. Surface tension of the solution decreases with the addition of surfactant at critical micelle concentration. A host of other solution properties are also affectedatcriticalmicelleconcentrationofthesurfactant,whichinclude conductivity, turbidity, osmotic pressure, etc. [5]. However, in the emulsion poly mer i za tion process, it is the reduction in the surface tension of the aque-ous phase that is of prime importance. Because surfactants are amphiphilic molecules containing one hydrophobic part and one hydrophilic part, in the micelle they orient themselves in a way so that the hydrophobic part forms the inner part of the micelle and the hydrophilic part radiates away from this inner part of the micelle into the aqueous phase. The resulting hydrophobic space inside the micelle owing to the self-assembly of the surfactant molecules is an ideal place for the hydrophobic monomer to reside and also provides an ideal environment for the radicals to enter the micelle. Figure 1.1 shows the representationoftheassociationofthesurfactantmoleculesafterthecriti-calmicelleconcentrationofthesurfactantisreached[6].Whentheinverse emulsion poly mer i za tion is used, then the hydrophilic part of the surfactant forms the inner part of the micelles and the hydrophilic chains intermix with the organic dispersion phase.(a) (b)Figure 1.1Organization of the surfactant molecules (a) below and (b) above the critical micelle concentra-tion of the surfactant in the aqueous solution.4Advanced Polymer Nanoparticles: Synthesis and Surface ModifcationsThe monomers used for emulsion poly mer i za tion are water insoluble (water soluble for inverse emulsion poly mer i za tion). However, the monomer should have some extent of water solubility in order to diffuse through the aqueous phase as required during the course of poly mer i za tion. When the monomer is added to the system, a part of the monomer enters the micelles and a part is dissolved in the aqueous phase owing to partial water solubility. However, the majority of the monomer is present in the form of monomer droplets. The size of the monomer droplets is much larger than that of micelles; however, their number is much lower as compared to the micelles. Water-soluble initi-ators are generally used to initiate the poly mer i za tion reaction. The initiator generates the radicals in the aqueous phase owing to thermal dissociation. The generated radicals have the possibility of entering either the micelles or the monomer droplets. However, experimental evidence proves the absence of droplet poly mer i za tion. The radicals do not enter the monomer droplets, as the radical entities are hydrophilic in nature whereas the monomer drop-lets are hydrophobic. Also, because the number of monomer droplets is much smaller than the number of micelles, it is micelles that capture the majority of the radicals. Also, the unique architecture of the micelles provides attrac-tive conditions for the radicals to enter. Figure 1.2a shows the mechanism of the micellar nucleation for the generation of poly mer particles. This mode of nucleation is also termed heterogeneous nucleation. The homogenous mode ofparticlenucleationisalsopossiblewhen(a)theamountofsurfactantis below its critical micelle concentration, (b) no surfactant is used during the poly mer i za tion, or (c) the monomer is signifcantly water soluble.Inthismodeofnucleation,thegeneratedradicalsintheaqueousphase startreactingwiththedissolvedmonomermolecules.However,afteradd-ing a few monomer units in the chains, these chains no longer remain water soluble and come out of the solution. These chains are not stable on their own andkeepcollapsingwitheachotherinordertoattainstability.Theyalso adsorb a certain amount of surfactant from either the micelles or the aqueous phaseitself.Partialstabilityisalsoprovidedbythenegativechargesfrom theinitiatormoieties.Inthecaseofthesurfactant-freepoly mer i za tion,the initiatorchargesaretheonlysourceofcolloidalstabilityoftheparticles. Figure 1.2b shows the process of homogenous nucleation.The emulsion poly mer i za tion process is generally divided into three inter-vals. The frst is the particle formation interval. The radicals are generated in the aqueous phase after the thermal dissociation of the initiator. These radicals start entering the micelles and initiate poly mer i za tion. These active micelles where the poly mer i za tion starts to take place are then referred to as poly mer particles.Thenumberofparticlesinthisintervalkeepsincreasingowing tothecontinuousentryofthegeneratedradicalsinthemicelles.Thisalso leadstocontinuousincreaseintherateofthepoly mer i za tion.Asthepoly-mer i za tion of the monomer in the particles proceeds, the size of the particles keeps increasing and the amount of monomer in the particles keeps depleting. However, this depletion of the monomer is replenished by the absorption of Polymer Latex Technology: An Overview5themonomerfromtheaqueousphase.Theaqueousphaseinturnabsorbs more monomer from the monomer droplets. Therefore, a mass transfer from the monomer droplets to the poly mer articles keeps taking place during the course of poly mer i za tion. For this diffusion process to take place, the mono-merisrequiredtobepartiallysolubleinwater.Asthepoly merparticles become bigger in size and their surface area increases as a function of time or (a)(b)Figure 1.2(a) Representation of micellar nucleation mechanism for the generation of poly mer particles. (b) Homogenous nucleation mechanism for the synthesis of poly mer particles.6Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsmonomer conversion, they require more amount of surfactant to remain sta-ble. The surfactant dissolved in the aqueous phase is continuously adsorbed on the surface of the poly mer particles, leading to the reduction of the surfac-tantamountinthesolutiontolowerthanthecriticalmicelleconcentration. This in turn destabilizes the remaining micelles and these micelles disappear, providing their surfactant for the stabilization of the poly mer particles. Thus at the end of the frst interval, no micelle is left and most of the surfactant is used to stabilize the poly mer particles. It has to be noted that the fnal number of poly mer particles is much lower than the original number of micelles. Also, roughly 15% of the monomer is poly mer ized by the end of the frst interval [1]. Figure 1.3 represents the various intervals of emulsion poly mer i za tion.Once excess surfactant is no longer present in the system, no new par-ticles nucleate. This marks the beginning of the second interval of emul-sion poly mer i za tion. Because no new particles nucleate, the amount of the particlesremainsalmostconstant;thisalsoleadstoanalmostconstant poly mer i za tionrateinthisinterval.Thesizeofthepoly merparticles, however,keepsincreasingasafunctionofconversion.Themonomer presentinthemonomerdropletscontinuestoreplenishthemonomerin theaqueousphaseaswellasmonomer-swollenpoly merparticles.After a certain extent of conversion, the monomer droplets also disappear. This alsosignalsthestartofthefnalintervaloftheemulsionpoly mer i za-tion process. The concentration of the monomer in the poly mer particles keeps decreasing, and as a result the rate of poly mer i za tion also steadily decreases. Because the monomer is almost consumed, the poly mer i za tion ratevirtuallyfallstozero.Figure1.4showstheevolutionoftheparticle size as a function of conversion.Figure 1.3Schematic of various intervals of the emulsion poly mer i za tion process.Polymer Latex Technology: An Overview7Emulsion poly mer i za tion can also lead to the generation of various differ-ent surface morphologies of the poly mer particles as well as particle sizes or families.Figure1.5showstheexamplesofmonomodal,bimodal,ormulti-modal poly mer particles along with morphologies like planar, orange-peel, strawberry or surface craters, etc.Polymerssynthesizedwithemulsionpoly mer i za tionarenotalways homopolymers, but most of the time are copolymers. When more than one monomerispoly mer izedtogether,thereactivityofthemonomersdefnes the resulting morphology of the particles. Different reactivities of the mono-mer lead to totally different copolymer composition in the poly mer particles, leading to a gradient in the concentration of the monomers with radius. This occursbecauseofthefasterpoly mer i za tionofthereactivemonomerthus accumulating near the center of the particles, followed by the poly mer i za tion of the lesser reactive monomer, which then is present in a majority near the (a)200 nm(b)300 nm(c)400 nmFigure 1.4(ac) Increase of the size of the particles as a function of conversion.8Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsouter surface of the particles. Differences in the water solubilities of the mono-mers can also lead to the generation of specifc morphologies of the particles. As an example, in Figure 1.6 are shown the copolymer particles of styrene-co-N-isopropylacrylamide synthesized by the surfactant-free approach, that is,bythehomogenousnucleationmethod.N-isopropylacrylamidebeing hydrophilicinnaturestartstopoly mer izefrst,followedbythepoly mer i-za tionofmorehydrophobicstyrene.Butbecausethepoly merchainsfrom styrene are also hydrophobic in nature, they push the hydrophilic chains of poly(N-isopropylacrylamide) away to the surface, leading to the morphology as shown in Figure 1.6.(a) (b)300 nm 200 nm250 nm300 nm(c) (d)Figure 1.5(ad) Various morphologies of particles achieved with emulsion poly mer i za tion.Polymer Latex Technology: An Overview91.3 Controlled Polymerization and its Use in Emulsion Polymerization ProcessesThe conventional radical poly mer i za tion is limited as a technique in that the control in the molecular weight or its distribution is diffcult to achieve. It is also not easy to achieve well-defned morphologies in the particles like block copolymers because the life of the radical is too short, and uncontrolled termi-nation reactions take place very fast. Controlled living poly mer i za tion tech-niques,ontheotherhand,cancircumventtheaforementionedlimitations in the emulsion poly mer i za tion process [4]. In these techniques, the chains (a)250 nm(c)300 nm(b)300 nmFigure 1.6(ac) Scanning electron microscopy (SEM) micrographs of copolymer particles of styrene-co-N-isopropylacrylamide.10Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsare terminated but only irreversibly, and after a short period of time become active again to propagate the poly mer chains. In such processes, the termina-tion reactions are effectively eliminated, and the controlled molecular weight distributions as well as advanced morphologies in the poly mer particles can be achieved. There have been many techniques developed in the last years to achieve controlled poly mer i za tion, and these techniques are generally clas-sifedintotwocategories:thosebasedonreversibleterminationandthose basedonreversibletransfer.Figure1.7istherepresentationofthevarious controlled poly mer i za tion techniques. In the category of reversible termina-tion,nitroxide-mediatedpoly mer i za tion(NMP)andatomtransferradical poly mer i za tion (ATRP) are the most studied approaches. ATRP has also been further modifed into techniques like reverse atom transfer radical poly mer-i za tion,activatorgeneratedbyelectrontransferATRP,etc.Inthecategory ofreversibletransfer,techniqueslikereversibleaddition- fragmentation chain transfer (RAFT) poly mer i za tion and degenerative transfer are mostly reported. During the poly mer i za tion, the concentration of dormant species continues to increase as compared to the active chains. At the end of poly-mer i za tion, the dormant species may be present in amounts six times higher thantheactivechains.Thiseffectivelyleadstoeliminationoftermination and allows much longer lifetimes for the radicals.LivingpolymerizationDegenerativetransferReverse atomtransfer radicalpolymerization(ATRP)Activator generatedby elecron transfer (AGET)atom transferradical polymerization(ATRP)Activator regeneratedby elecron transfer (ARGET)atom transferradical polymerization(ATRP)Atom transferradical polymerization(ATRP)TEMPOmediated nitroxidepolymerization(NMP)SG1mediatednitroxidepolymerization(NMP)Reversible addition-fragmentationchain transfer (RAFT)polymerizationFigure 1.7Representation of various living poly mer i za tion techniques. (Reprinted from V. Mittal, Advances in Polymer Latex Technology, New York: Nova Science Publishers, 2009. With permission.)Polymer Latex Technology: An Overview11NMP,wherenitroxidesareusedtoirreversiblyterminatethepoly mer chains,hasbeenusedintwodifferentways.Inonecase,aconventional freeradicalinitiatorandaseparatelyaddednitroxideareaddedtocontrol thepoly mer i za tion.Thetwomostcommonlyusednitroxidesforthispur-poseare2,2,6,6-tetramethyl-1-piperidinoxyl(TEMPO)andN-ter-butyl-1-diethylphosphono-2,2-dimethylpropyl(SG1).Thenitroxideswereinitially developedforthepoly mer i za tionofstyrene;however,anumberofother nitroxides have been developed that are also suitable for the poly mer i za tion ofacrylates.Intheothercase,analkoxyamineisused,thedecomposition ofwhichleadstothegenerationoftworadicals:onereactiveandonesta-ble.Thisradicalpairthencontrolsthepoly mer i za tionandthusdoesnot require the addition of conventional free radical initiator. Figures 1.8 and 1.9 representthepoly mer i za tionoflaurylmethacrylateandstyrenebyusing nitroxidesandalkoxyamines,respectively.Theonlydisadvantageofthe nitroxide-mediatedstablefreeradicalpoly mer i za tionwastherequirement of a high temperature for the poly mer i za tion reaction, which is sometimes notfeasibleforthermallysensitivesystems;however,variousnitroxides have been developed that also allow use at lower reaction temperatures. The initiallycarried-outreactionswithstyreneinemulsionledtopoorcolloi-dal stability, which resulted in a large amount of coagulum generated in the poly mer i za tion reactions. It was claimed that the particle nucleation as well aspoly mer i za tionindropletswereafewreasons,amongothers,forthis behavior. The seed method has also been described for emulsion poly mer-i za tion with SG1 as nitroxide [7]. In this case, a seed is generated frst with low solid content, and the seed particles are then swollen in monomer and followed by subsequent poly mer i za tion of these seed particles. This helps to avoid the generation of monomer droplets and thus poly mer i za tion in drop-lets.Itwasalsopossibletoachievethepreviouslydescribedreactionasa single step. After the synthesis of seed as before, a certain amount of mono-mer is added without cooling the seed latex and the reaction is run until high conversion is achieved. The formed latexes were very stable in nature and no coagulum was generated. It is also important to monitor the progress of the reactionespeciallyathighconversion,asatveryhighconversions,chains starttoterminateeachotherandthepolydispersityinthechainlengthas well as molecular weight increases. Therefore, it is always benefcial to stop thepoly mer i za tionreactionalittlebelowthefullconversion.Itwasalso confrmed that the alkoxyamines based on SG1 are more optimally operat-able for achieving controlled poly mer i za tion of a wide range of monomers as compared to TEMPO nitroxide.ATRPrepresentsanothertechniquebasedontheprincipleofreversible termination,andinthisprocess,anorganichalideisusedtoirreversibly terminatethepropagatingchains.Thistechniquehasbeenverysuccess-fulforthecontrolledpoly mer i za tionofstyrenicsaswellasacrylatesand methacrylates. It also does not require very high temperatures as compared 12Advanced Polymer Nanoparticles: Synthesis and Surface ModifcationstoNMP,andinmanycasescanalsobeundertakenatroomtemperature. Figure 1.10a shows the schematic of the ATRP process. Cuprous salt forms a complexwithligand,L(aminesofdifferentchemicalarchitectures),which makesitmoresolubleinthesolvents[1].Initiationofthereactiontakes placebythedissociationofthehalideatomfromtheinitiatorandleading tothegenerationofafreereactiveradical.Thebromideatomiscaptured by cuprous halide ligand complex and it forms CuBr2 ligand complex. This compound is very stable and hence is called deactivator. The generation of this compound thus leads to reduction in the concentration of the free radi-calspeciesinthesystem.Thegrowingradicalcontinuestoaddmonomer units to the poly mer chain, and at some point it comes in contact with CuBr2 COO CH2CH2CH CHC OOC12H25C OOC12H25O NCHCH3POC2H5OOC2H5COO O COCH2CHC OOC12H25++O NCH3CH3H3CCH3CH3H3CCH POC2H5OC2H5CH3H3CCH3H3CCH3OCOOCH2CHC OOC12H25CH2CHC OOC12H25CH2CHC OOC12H25Figure 1.8Nitroxide-mediatedcontrolledradicalpoly mer i za tionoflaurylmethacrylatewithSG1 nitroxide.Polymer Latex Technology: An Overview13ligand complex and is temporarily terminated by the formation of RMn+1-Br compound. It is also possible to carry out the reverse ATRP process similarly (Figure 1.10b). In this process, a conventional free radical initiator like AIBN orbenzoylperoxideisusedtoinitiatethepoly mer i za tionreaction,which is controlled by the addition of CuBr2 ligand complex. The radicals add few monomer units, and during this process come in contact with this complex to form the dormant species. One limitation of such a technique is the pres-ence of transition metal in the fnal particles, which though possible to wash off adds another processing step in the synthesis process. Another limitation CHH3CO NCH3H3CCH3H3CCH2CHCHH3CCHH3CCH2+CHH3C+O NCH3H3CCH3H3CCHH3CCH2CH O NCH3H3CCH3H3CCH2CHCH2CHnnCHH3CCH2CH O NCH3H3CCH3H3CCHO NCH3H3CCH3H3CFigure 1.9Nitroxide-mediated free radical poly mer i za tion of styrene by using alkoxyamine as nitroxide as well as initiator.14Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsis the reaction of the copper compounds withthe other constituents of the system. One example is the reaction of these compounds with the emulsifer used in the poly mer i za tion system, leading to the poisoning of the initiator, which subsequently results in no or little poly mer i za tion. Therefore, it is pos-sible to work in the emulsion with the ATRP when there is no surfactant or surfactants with no interaction with the initiator are chosen. ATRP in emul-sion processes also faced problems similar to those in NMP. In one reported study, ethyl 2-bromoisobutyrate was used as an ATRP initiator, and copper bromide was complexed with 4,4-dinonyl-2,2-bipyridyl to form the catalyst system [8]. Nonionic surfactant Tween 85 was used. The poly mer i za tion was achieved byfrst mixing together copper salts with 4,4-dinonyl-2,2-bipyri-dyl, to which the monomer was added. The solution was allowed to mix and was added with surfactant. To this solution, water was added under vigorous P Br CuBr/L PPMPMn+1RMn+1 Br CuBr/L +MnMCuBr2/L + +(a)C OO OO C OOBr C CuBr2/L+ CuBr2/L+ CuBr/L+C OOMMnMC OOMn+1C OOMn+1Br(b)Figure 1.10Schematicof(a)ATRPand(b)reverseATRPprocessesforcontrolledlivingpoly mer i za tion. (AdaptedfromG.Odian,PrinciplesofPolymerization,Hoboken,NJ:JohnWiley&Sons,2004; and V. Mittal, Advances in Polymer Latex Technology, New York: Nova Science Publishers, 2009.)Polymer Latex Technology: An Overview15stirring to form the emulsion. To this emulsion was then added the initiator toinitiatethepoly mer i za tionreaction.Thereactionconditionsneededto be very accurately controlled, and the reaction was overall very sensitive to minor changes in the reaction parameters. Seeded poly mer i za tion similar to that used with NMP was also used in this case. The use of cationic surfac-tants was also reported for the ATRP processes in emulsion. Dodecyl trim-ethyl ammonium bromide and myristyl trimethyl ammonium bromide were used as cationic surfactants, and their effect on the latex stability, amount of coagulum,andthepolydispersityinthemolecularweightwasquantifed. The frst surfactant though gave a good control of polydispersity; however, the whole system was observed to coagulate after the initiation of poly mer i-za tion. In the case of the second surfactant, the latex stability was better, but the polydispersity in the molecular weight or chain lengths was very high.RAFT is a controlled poly mer i za tion technique based on the principle of reversibletransfer.ThecoreofthisprocessisaRAFTagentthatcontains dithioester groups. The living poly mer i za tion takes place because the trans-ferred end group in the polymeric dithioester is as labile as the dithioester group in the starting RAFT agent. The initiator for the poly mer i za tion can betheconventionalinitiatorslikeAIBNorbenzoylperoxide.Figure1.11 explains the principle of RAFT poly mer i za tion. Though it is one of the ver-satile techniques for the poly mer i za tion of a large number of monomers, it also has its own limitations, such as the presence of remainder RAFT agent and the commercial unavailability of the RAFT agents. Similar to NMP and ATRP,initialtrialswithRAFTalsowerefacedwithdiffcultiesofcoagu-lum generation. The RAFT agent was diffcult to be transported to the poly-mer particles through the aqueous phase. RAFT poly mer i za tion of styrene in emulsion was reported by Szkurhan et al. [9]. The process named nano-precipitation was carried out by forming nano-sized particles by precipita-tion of the acetone solution of macro RAFT agent in the aqueous poly(vinyl alcohol) solution. The macro RAFT agent was prepared by conventional free SMn+ +R2SR2R1MnSSR2R1MnS SR1SMm+ +MnSMnR1MmSSMnR1MmS SR1Figure 1.11Mechanismofreversibletransferprocessesusedinreversibleaddition-fragmentationchain transferprocesses.(AdaptedfromG.Odian,PrinciplesofPolymerization,Hoboken,NJ:John Wiley&Sons,2004;andV.Mittal,AdvancesinPolymerLatexTechnology,NewYork:Nova Science Publishers, 2009.)16Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsradical poly mer i za tion. The formed nanoparticles were subsequently swol-len with monomer and were poly mer ized in the living manner. This nano-precipitation method was also named seeded poly mer i za tion because, in this case,thenano-sizedparticlesformedbyprecipitationactasseedstoform poly mer particles. Both water- and oil-soluble initiators were used. When the initiator used was oil soluble, it was premixed with the RAFT agent, whereas the water-soluble initiator was dissolved in PVA solution. With both water-soluble and oil-soluble initiators, the rate of poly mer i za tion was quite slow, andincreasingthereactiontemperaturewasnothelpfulinincreasingthe rateofpoly mer i za tion.Anotherstudyreportedthesynthesisofpoly mer particles in emulsion by RAFT without the problems of loss of colloidal sta-bilityandthemolecularweightcontrol[10].TrithiocarbonateRAFTagents were used in the study to form short stabilizing blocks from a hydrophilic monomer,fromwhichdiblockswerecreatedbythesubsequentpoly mer-i za tionofahydrophobicmonomer.Thesediblocksself-assembledtoform micelles,andsubsequentpoly mer i za tioncouldbecarriedout.Figure1.12 demonstrates the particles generated by this technique.1.4 Conventional and Controlled Miniemulsion PolymerizationIn surfactant-aided emulsion poly mer i za tion, the goal is to achieve the micel-lar nucleation and to avoid the droplet nucleation as much as possible. But the poly mer i za tion of extremely hydrophobic monomers by conventional emul-sion poly mer i za tion is not possible because of their inability to diffuse from the monomer droplets through the aqueous phase to the poly mer particles. To achieve poly mer i za tion of such systems, miniemulsion poly mer i za tion has proved to be a versatile method [2,3]. The mode of poly mer i za tion is based on the droplet poly mer i za tion principle. The monomer droplets are generated by shearing the system with high energy along with the addition of costabilizer (withthesurfactant),whichneedstobehydrophobicinordertoavoidthe collapse of the monomer droplets by Ostwald ripening when the shearing of the system is stopped. Thus in this mode of poly mer i za tion, it is important to avoid the micellar nucleation; therefore, the amount of surfactant is below thecriticalmicelleconcentration.Theparticlesinthegeneralsizerangeof 50500 nm can be synthesized by using miniemulsion poly mer i za tion. These are similar in size to the monomer droplets in the beginning of the poly mer-i za tion. The initiators used for the poly mer i za tion are water soluble as in the case of emulsion poly mer i za tion. The initiator on dissociation generates radi-cals in the aqueous phase, and these radicals enter the droplets and initiate poly mer i za tion.Inconventionalemulsionpoly mer i za tion,alsocalledmac-roemulsionpoly mer i za tion,themicellarnucleationisverysensitiveandis affected by a large number of factors like surfactant amount, initiator amount, Polymer Latex Technology: An Overview17Conventional core-shell:dierent chains inthe core and shellTriblock polymer:the same chains extendfrom the shell into the core(a)0.20 mX 19000(b)Figure 1.12(a) Schematic representation of RAFT-based triblock copolymer chains forming the core and shelloftheparticlesandtheircomparisonwithconventionalcore-shellparticles.(b)TEM micrographofpoly(acrylicacid)-b-poly(butylacrylate)-b-polystyreneparticles.(Reprinted from C. J. Ferguson, R. J. Hughes, D. Nguyen, T. T. Pham, R. G. Gilbert, A. K. Serelis, C. H. Such, andB.S.Hawkett,Macromolecules38:21912204,2005.WithpermissionfromtheAmerican Chemical Society.)18Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsagitation, reaction temperature, etc. However, that is not the case in miniemul-sion poly mer i za tion. Figure 1.13 also represents the scheme of poly mer i za tion in miniemulsion. The monomer is added to the surfactant and costabilizer in theaqueousphasefollowedbyhomogenizationunderhighsheartobreak the bigger monomer droplets into droplets with the size range of 10500 nm [2,3,11]. The amount of the surfactants and the shearing translate into the size of the monomer droplets. As mentioned earlier, in emulsion poly mer i za tion, thepoly mer i za tionisdrivenbythediffusionofthemonomerthroughthe aqueous phase. In this process, monomer droplets disappear and the micelles convert into the poly mer particles, whereas in miniemulsion poly mer i za tion, themonomerdropletsdirectlytranslateintopoly merparticles.Asaresult, the rate of poly mer i za tion is also different for these two processes. The rate ofpoly mer i za tionintheemulsionpoly mer i za tionfrstincreasesowingto the generation of the particles and reaches a constant phase after the disap-pearanceofthemicelles.Theratethendecreasesowingtothedepletionof themonomerintheparticles.Asthereisnodiffusionofmonomerinthe miniemulsion poly mer i za tion, the constant rate period is absent. The rate frst increasesowingtothenucleationoftheparticlesandthendecreaseswhen themonomerisconsumedintheparticles.Notonlyhydrophobicmono-mers,butalsoextremelyhydrophilicmonomerscanbepoly mer izedusing theminiemulsionpoly mer i za tionmethod.Inthiscase,onehastousethe inverseminiemulsionpoly mer i za tion.Alsointhiscase,hydrophobicreac-tion medium is used along with a lipophobe used as costabilizer.One must be clear that the addition of costabilizer stops the conversion of a miniemulsion into a conventional emulsion; however, the addition of a costa-bilizer to conventional emulsion does not automatically convert it into a mini-emulsion. It is only after the addition of high shearing energy that it becomes astableminiemulsion.Thecostabilizersshouldbehydrophobic,solublein monomer, and have a low molecular weight. However, the use of conventional costabilizers like cetyl alcohol and hexadecane pose a potential hazard owing to their volatility, and the presence of these even in the minor amounts in the poly mer particles may not be acceptable for many applications. Therefore, a lot of research effort has been focused in the direction of generation of more com-patible costabilizers. Polymeric stabilizers from the poly mer of the monomer Figure 1.13Schematicoftheminiemulsionpoly mer i za tionprocess.Themoleculeswithblackandgray color represent the surfactant and costabilizer, respectively.Polymer Latex Technology: An Overview19tobepoly mer izedhavebeenusedinsomeofthereportedstudies[3,12,13]. Thesepolymersaresolubleintheirownmonomersandthusallowbetter intermixing at the interphase. They also eliminate the use of volatile costabi-lizers, providing more acceptability to the system for the commercial applica-tions. Monomeric costabilizers have also been developed in recent years that can copoly mer ize with the monomer under poly mer i za tion [14]. These costa-bilizers form copolymer chains that are bound inside the particles, and thus the possibility of the diffusion of low molecular weight components out of the particlesiseliminated.Thepotentialdiffusionofthelowmolecularweight components, especially substances like cetyl alcohol or hexadecane, can pose health hazards when the poly mer particles are used in application with food contact.Similarly,theothercomponentsofthepoly mer i za tionsystemlike initiator and chain transfer agent have also been used as costabilizers [15,16]. Therefore, the materials act as dual-role components. They not only perform their function as initiator or chain transfer agent, respectively, but also help to achieve the stability for the monomer droplets and poly mer particles.Miniemulsion poly mer i za tion has also been proved to be advantageous in living poly mer i za tion systems. The various living poly mer i za tion methods like NMP, ATRP, and RAFT have been shown to be benefcial in miniemul-sion poly mer i za tion to generate specialty polymers or polymers with special architecture like block copolymers. Colloidal stability and ease of poly mer-i za tionprocessarealsobetterinthecaseofminiemulsionpoly mer i za tion than conventional emulsion poly mer i za tion.ForNMP,nitroxide-cappedpoly merchainshavebeenusedasinitiator aswellasnitroxide.Theuseofsuchnitroxide-cappedpoly merchainsfor the initiation and reaction control allows one to properly estimate the num-ber of starting chains in the system, helping to achieve better control of the molecular weight. The use of nitroxide-capped poly mer also helps to parti-tionthe nitroxide solely inthe organic phase owing to the hydrophobicity. InsuchreportedstudieswithpolystyreneterminatedwithTEMPOasini-tiatoraswellasnitroxide,hexadecaneascostabilizer,andDOWFAX8390 assurfactant,itwasreportedthatchangingtheamountofsurfactantled tothegenerationofdifferentparticlesizes,buttherateofpoly mer i za tion was not affected, which is different from the behavior seen in conventional emulsionpoly mer i za tion[17,18].Figure1.14demonstratesthetransmission electron microscopy (TEM) images of the polystyrene particles generated in nitroxide-mediatedminiemulsionpoly mer i za tionusingdifferentamounts of TEMPO-terminated oligomers of polystyrene as macroinitiator.A great deal of process development has been reported for ATRP in mini-emulsion. A number of studies have been reported that apply the direct or forwardATRPinminiemulsion,butreverseATRP,inwhichconventional freeradicalpoly mer i za tioninitiatorlikeAIBNcanbeusedwiththetran-sitionmetalcompoundinitshigheroxidationstate,wasobservedtobe moresuitableforminiemulsionpoly mer i za tion.Thiseliminatestheuseof air-sensitive Cu(I) species and requires only the use of Cu(II) species, which 20Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsis more stable in air. A narrow molecular weight distribution as well as linear increase in the molecular weight as a function of conversion was reported, and the fnal latexes were stable over a period of time. In one such study on reverse ATRP processes [19], Brij 98 surfactant and CuBr2/dNbpy (4,4-di[5-nonyl]-4,4-bipyridine)complexwereusedalongwithhexadecanecostabi-lizer.Bothwater-solubleaswellasoil-solubleinitiatorswereusedforthe poly mer i za tion. It was observed that the poly mer i za tion rate was indepen-dent of the size and number of particles and the amount of surfactant. The shear forces were able to infuence only the size of the particles and not the poly mer i za tionrate.Inthecaseofoil-solubleinitiator,thepoly mer i za tion was observed to proceed by droplet nucleation mode because the monomer-solubleinitiatorwasalreadypresentinthedropletduringtheminiemul-sion; whereas when water-soluble initiator was used, both micellar as well as droplet nucleation were reported to take place. Similarly RAFT has also been used successfully in miniemulsion for the poly mer i za tion of styrene as well as water-soluble monomers like acrylamide [11,2022].1.5 Generation of Copolymer or Core-Shell ParticlesThe generation of well-defned copolymer morphologies like block copoly mer particles is diffcult to achieve by the use of conventional emulsion poly mer-i za tion because of the uncontrolled free radical poly mer i za tion and the short life of the radicals. To achieve certain control on the copoly mer i za tion, mono-mer reactivity ratios and monomer-feeding methodology must be considered. (a) (b)Figure 1.14TEM images of the polystyrene particles prepared by nitroxide-mediated miniemulsion poly-mer i za tion using different amounts of TEMPO-terminated oligomers of polystyrene (TTOPS) asmacroinitiator.(a)5%TTOPSand(b)20%TTOPS(100nm).(ReprintedfromG.Pan,E.D. Sudol,V.L.Dimonie,andM.S.El-Aasser,Macromolecules34:48188,2001.Withpermission from the American Chemical Society.)Polymer Latex Technology: An Overview21Themonomershavedifferentreactivityratios;therefore,ifthemonomers areaddedtogether,themorereactivemonomerstartstopoly mer izefrst followed by the poly mer i za tion of the less reactive monomer. This creates a gradientofconcentrationofthemonomerunitsinthepoly merparticlesas a function of radius. Figure 1.15 provides some examples of various copoly-mer particles that can be achieved by emulsion poly mer i za tion like core-shell graftedparticles,core-shellparticleswithhydrophilicshellandhydropho-bic core, copolymer particles with different surface morphologies, etc. Apart from reactivity ratios, mode of addition of the monomers during the course of poly mer i za tion is also of utmost importance to achieve control on the particle characteristics. Batch addition of the monomers does not lead togeneration ofthestructuredlatexes;therefore,semibatchadditionofthemonomersis generally preferred. This mode of addition can be achieved by either fooded (a) (b)300 nm300 nm300 nm300 nm(c) (d)Figure 1.15(ad) Different morphologies of copolymer particles generated by conventional emulsion poly-mer i za tion.22Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsaddition or starved addition of the monomers. In fooded addition, monomers are added at a rate higher than their rate of consumption. This mode of addi-tionleadstobuildupofthemonomersinthepoly mer i za tionreactionand may lead to the generation of secondary nucleation of the particles. Starved addition of the monomers, on the other hand, is the addition of monomers at a rate slower than their poly mer i za tion rate, and this allows one to retain the chemical composition of the poly mer chains equal to the monomer ratios in thefeedoraccordingtotherequirement.Thestarvedconditionseliminate the possibility of secondary nucleation, though one has to be careful about the control of the amount of the surfactant in the system as well as charges on the surface. As an example, if copoly mer i za tion of a hydrophilic monomer and a hydrophobic monomer is considered, initially the surface may be hydropho-bic, but as the chains rich in hydrophilic monomer content get pushed out to the surface of the particles during the course of poly mer i za tion, the surface becomes hydrophilic. This would lead to a change in the surface properties of the particles and would allow the release of the surfactant from the surface of the particles owing to the hydrophili city. This also results in the nonentry ofthehydrophobicmonomerintothepoly merparticles,causingmonomer concentration drift in particles or secondary nucleation.Interesting studies have been reported for the generation of copolymer par-ticle latexes by emulsion poly mer i za tion. Batch poly mer i za tion of copoly mer particles of polystyrene and poly(methyl methacrylate) were reported without the use of initiators [23]. These copolymer particles of poly(methyl methacrylate-co-styrene) were prepared by thermally initiated emulsion copoly mer i za tion. It was observed that totally different particle morphologies like hemispherical, sandwich-like, core-shell, inverted core-shell particle morphologies, etc. were obtaineddependingonthepoly mer i za tionconditions.Itwasreportedthat the incorporation of the initiator fragments to one end of the chains allows the polystyrene chains to become more hydrophilic, changing the surface nature ofthepoly merparticles.Inanotherstudytogeneratecopolymerparticles, poly(methyl methacrylate) seed was used to generate the copolymer particles ofpoly(methylmethacrylate)withpolystyrene[24].Itwasobservedthatby using the oil-soluble initiators, an inverted core-shell morphology of the par-ticles was obtained, in which the polystyrene chains were present in the core of the particles and the poly(methyl methacrylate) covered the particles owing to its hydrophilicity. In the case of water-soluble initiator, the morphology was less affected by the hydrophilicity of the polymers, but was more affected by the initiator concentration and poly mer i za tion temperature.Controlled living poly mer i za tion methods provide much better possibili-tiestogeneratethestructuredlatexparticleswithdifferentmorphologies ofchemistriesowingtotheprolongingoftheradicalageeitherbyrevers-ibleterminationorbyreversibletransfer.Inonesuchstudytogenerate triblockcopolymersusingemulsionpoly mer i za tion,water-solubleSG1-basedbifunctionalalkoxyamine(Figure1.16;sodiumsaltofalkoxyamine was used owing to water solubility) and Dowfax 8390 surfactant were used Polymer Latex Technology: An Overview23[11,25]. When a bifunctional alkoxyamine was used, two functional ends of thisalkoxyaminecouldbeusedtogeneratetriblockcopolymers.Thus,in ordertogeneratepolystyrene-b-poly(butylacrylate)-b-polystyrenetriblock copolymer particles, a seed was frst generated from butyl acrylate particles. The seed was further swollen with butyl acrylate to form central poly(butyl acrylate) block in the emulsion particles. The particles were then added with styrenetoformtwoblocksofstyrenearoundthecentralpoly(butylacry-late) block to form the triblock copolymer. In another study using ATRP, the seeded-poly mer i za tion approach was used to synthesize block copolymers ofpoly(i-butylmethacrylate)andpolystyreneusingethyl2-bromoisobu-tyrateasinitiatorandCuBr/4,4-dinonyl-2,2-dipyridinylascatalystligand OHOOCt-butanolT = 80 100CNP OOOEmulsionT = 112C(1) NaOHRR = Ph, COOBu(2)N OOOOPOHOOC COOHO3OO NP OOOCH2CH2OOO O3OCH2CH2N OORxOOPOCOO

+Na Na+

OOCO3OO NP OOOCH2CH2ORxFigure 1.16SynthesisanduseofSG1-basedwater-solublebifunctionalalkoxyamine.(Reprintedfrom J. Nicolas, B. Charleux, O. Guerret, and S. Magnet, Macromolecules 38: 996373, 2005. With per-mission from the American Chemical Society.)24Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationscomplex [26]. Tween 80 (polyoxyethylene sorbitan monooleate) was used as surfactant. First a seed of poly(i-butyl methacrylate) end-capped with ATRP initiator was prepared to which a batch of styrene then was added to form a block copolymer. Thermally responsive poly mer particles were also reported by the use of ATRP [2729]. The process included the synthesis of seed parti-cles, functionalization of seed particles by ATRP initiator, and the grafting of the poly(N-isopropylacrylamide) chains from the surface. Figure 1.17 shows thesefunctionalparticles[28].InanotherstudyusingRAFTpoly mer i za-tion,core-shellfunctionalparticleswereachievedbyusingo- ethylxanthyl (a) (b)300 nm 800 nm(c)100 nm(d)250 nmFigure 1.17(a, b) SEM and TEM micrographs of latex particles functionalized with an ATRP initiator. (c, d) The grafted brushes of thermally responsive poly mer poly(N-isopropylacrylamide) from the surface of the ATRP initiator functionalized particles.Polymer Latex Technology: An Overview25ethyl propionate as RAFT agent [30]. The poly mer i za tion reactions were car-ried out in the presence of poly(methyl methacrylate) seed particles of pre-determinednumberandsizedistribution.Theseedwasaddedfrstwith styrenemonomertoformpolystyreneblock,whichwasthenaddedwith butylacrylate.Styrenewasaddedinbatchconditions,whereasbutylacry-latewasaddedslowlytotheemulsionsystemsoastoavoidthebuildup ofhighconcentrationofmonomerinthissystem.Similarly,core-shellpar-ticlesconsistingofblockcopolymerofpolystyrene-b-poly/butylacrylate)/poly(acetoacetoxy ethyl methacrylate) were also prepared by using xanthates asRAFTagents.Figure1.18showstheTEMimageofsuchcore-shellpar-ticles [31]. Miniemulsion poly mer i za tion has also been extensively used for the synthesis of functional latex particles [3238].References1.Odian, G. 2004. Principles of polymeri za tion. Hoboken, NJ: John Wiley & Sons.2.Landfester,K.2001.Polyreactionsinminiemulsions.MacromolecularRapid Communications 22:896936.Observation at 150CpTA/RuO4 cryoFigure 1.18Core-shellparticlesofpolystyrene-b-poly(butylacrylate)/poly(acetoacetoxyethylmethacry-late).Theblackcorerepresentspolystyrenewhereasthesoftshellispoly(butylacrylate)/poly(acetoacetoxyethylmethacrylate)component.(ReprintedfromM.J.Monteiroand J. de Barbeyrac, Macromolecules 34: 441623, 2001. With permission from the American Chemical Society.)26Advanced Polymer Nanoparticles: Synthesis and Surface Modifcations3.Schork,F.J.,Luo,Y.,Smulders,W.,Russum,J.P.,Butt, A.,andK.Fontenot. 2005. Miniemulsion poly mer i za tion. Advances in Polymer Science 175:129255.4.Matyjaszewski,K.,andT.P.Davis.2002.Handbookofradicalpoly mer i za tion. Hoboken, NJ: John Wiley & Sons.5.Hiemenz, P. C., and R. Rajagopalan. 1997. Principles of colloid and surface chemistry. 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Swelling deswell-ingbehaviorofPS-PNIPAAMcopolymerparticlesandPNIPAAMbrushes graftedfrompolystyreneparticles&monoliths.MacromolecularMaterialsand Engineering 293:491502.30.Smulders,W.,andM.J.Monteiro.2004.Seededemulsionpolymer i za tionof blockcopolymercore-shellnanoparticleswithcontrolledparticlesizeand molecularweightdistributionusingxanthate-basedRAFTpoly mer i za tion. Macromolecules 37:447483.31.Monteiro,M.J.,andJ.deBarbeyrac.2001.Free-radicalpoly mer i za tionofsty-rene in emulsion using a reversible addition-fragmentation chain transfer agent with a low transfer constant: Effect on rate, particle size, and molecular weight. Macromolecules 34:441623.32.Farcet, C., and B. Charleux. 2002. Nitroxide-mediated miniemulsion polymer i-za tion of n-butyl acrylate: Synthesis of controlled homopolymers and gradient copolymers with styrene. Macromolecular Symposia 182:24960.33.Tortosa,K.,Smith,J.-A.,andM.F.Cunningham.2001.Synthesisof polystyrene-block-poly(butylacrylate)copolymersusingnitroxide-mediated living radical poly meri za tion in miniemulsion. Macromolecular Rapid Communications 22:95761.34.Keoshkerian,B.,MacLeod,P.J.,andM.K.Georges.2001.Blockcopoly-mersynthesisbyaminiemulsionstablefreeradicalpoly mer i za tionprocess. Macromolecules 34:359499.35.Li, M., Jahed, N. M., Min, K., and K. Matyjaszewski. 2004. Preparation of linear and star-shaped block copolymers by ATRP using simultaneous reverse and nor-mal initiation process in bulk and miniemulsion. Macromolecules 37:243441.28Advanced Polymer Nanoparticles: Synthesis and Surface Modifcations36.Min,K.,Li,M.,andK.Matyjaszewski.2005.Preparationofgradientcopoly-mersviaATRPusingasimultaneousreverseandnormalinitiationprocess. I.Spontaneousgradient.JournalofPolymerScience,PartA:PolymerChemistry 43:361622.37.Min,K.,Gao,H.,andK.Matyjaszewski.2005.Preparationofhomopolymers and block copolymers in miniemulsion by ATRP using activators generated by electron transfer (AGET). Journal of the American Chemical Society 127:382530.38.Luo,Y.,andX.Liu.2004.Reversibleaddition-fragmentationtransfer(RAFT) copolymer i za tion of methyl methacrylate and styrene in miniemulsion. Journal of Polymer Science, Part A: Polymer Chemistry 42:624858.292Synthesis of Polymer Particles with Core-Shell MorphologiesClaudia Sayer and Pedro Henrique Hermes de Arajo2.1 IntroductionCore-shellpoly merparticleshavereceivedagreatdealofindustrialand academic interest in the last decades. These multicomponent particles with controlledmorphologycreateaversatileclassofmaterialsinwhichthe fnal properties depend not only on the composition of each poly mer phase but also on the morphology of these particles. This characteristic opens the CONTENTS2.1Introduction .................................................................................................. 292.2Equilibrium and Nonequilibrium Morphologies ................................... 302.2.1Equilibrium Morphologies ............................................................. 312.2.2Nonequilibrium Morphologies ...................................................... 332.3Synthesis of Core-Shell Particles ............................................................... 352.3.1Emulsion Polymerization ............................................................... 352.3.1.1Synthesis of Core-Shell Particles (CS) ............................ 372.3.1.2Synthesis of Inverted Core-Shell Particles (ICS) ........... 412.3.2Miniemulsion Polymerization ....................................................... 432.3.3Microemulsion Polymerization ..................................................... 472.3.4Dispersion Polymerization ............................................................. 472.3.5Suspension Polymerization ............................................................ 482.3.6Other Techniques ............................................................................. 492.4Characterization of Core-Shell Particles ................................................... 522.4.1Transmission Electron Microscopy ............................................... 522.4.2Scanning Electron Microscopy ...................................................... 542.4.3Atomic Force Microscopy ............................................................... 542.4.4Additional Techniques Used for Particle Characterization ....... 54References ............................................................................................................... 5530Advanced Polymer Nanoparticles: Synthesis and Surface Modifcations possibilityfortailor-madepropertiesforeachapplicationas,forinstance, softcorehardshellresultsinparticlessuitabletoactasimpactmodifers, and hard coresoft shell latex particles result in paints with low flm forma-tiontemperature.Inaddition,viacore-shellpoly mer i za tion,itisalsopos-sible to get incompatible polymers into one particle or to add functionality either into the core or into the shell (Koskinen and Wiln 2009).Structured particles can be obtained with different morphologies: well-defnedcore-shellstructure,invertedcore-shell,interfacewithagradi-entofbothcoreandshell,interfacewithmicroclusters,andmultipleor irregularly shaped shells (Sundberg and Durant 2003). The fnal morphol-ogydependsonboththermodynamicandkineticaspects,asquitefre-quently the equilibrium morphology may not be achieved due to kinetic controlofthemorphologydevelopment.Thepoly mer i za tiontechniques playamajorroleintheparticlesizeaswellasinthekineticcontrolof thepoly mer i za tion.Severalheterogeneouspoly mer i za tiontechniques suchasemulsion,miniemulsion,microemulsion,dispersion,andsus-pension poly mer i za tionscouldbeemployedtoobtainpoly merparticles withcore-shellstructures.Thefrstthreetechniquesleadtotheforma-tionofsubmicrometricparticles(10800nm),whereasthetwolastare used,respectively,topreparesmall(130m)andlarge(501500m) micro metric particles. The end use properties of the structured particles dependonthedesignandcontrolofparticlemorphology;therefore,it isnecessarytounderstandhowthismorphologycanbecontrolledand which are the main features of each poly mer i za tion technique related to particle morphology control.Thepurposeofthischapteristodescribethefactorsthatwillleadtoa certain particle morphology and to discuss the heterogeneous poly mer i za-tion techniques that could be employed to obtain those particles. Section 2.2 introducesthebasicsofequilibriumandnonequilibriummorphologies. Section2.3dealswiththedifferentheterogeneouspoly mer i za tiontech-niquesandthemainfeaturesrelatedtomorphologycontrol.Section2.4 discussesbriefythecharacterizationtechniques,asanyworkinthisarea relies on the need to adequately characterize particle morphology.2.2 Equilibrium and Nonequilibrium MorphologiesThe formation of core-shell particles is a challenging issue of poly mer reaction engineering in dispersed media. In principle, the most stable particle mor-phology is determined by thermodynamics according to the minimum inter-facial energy (Gonzlez-Ortiz and Asua 1995), as given by Equation (2.1):Synthesis of Polymer Particles with Core-Shell Morphologies31 == = 121313aij ijj j i i(2.1)where aij and ij are, respectively, the interfacial area and the interfacial ten-sion between phases i and j.Nevertheless, frequently the equilibrium morphology is not achieved since particlemorphologydependsontheinterplaybetweenthermodynamics, which establishes the equilibrium morphology, and kinetics. If kinetic control prevails,nonequilibrium-type(metastableorkineticallystable)structures may be formed. In the following paragraphs, equilibrium morphologies and themainfactorsthatestablishthesemorphologieswillbediscussed,fol-lowed by nonequilibrium ones.2.2.1 equilibrium MorphologiesFigure2.1showsfourdifferentequilibriummorphologiesforatwo- componentsystembasedonthepoly mer-poly merandpoly mer-aqueous phase interfacial tensions, which determine the interfacial energy and, con-sequently, also the equilibrium morphology for a given system. The equilib-rium morphologies are:Coreshell (CS), in which the second-stage poly mer 2 () forms a con-tinuous shell around the seed poly mer 1 () dispersed in the aque-ous phase 3. This equilibrium morphology might be achieved when either (a) poly mer 1 is more hydrophobic than poly mer 2 (13 > 23) and poly mer 2 has more affnity with poly mer 1 than with the aque-ous phase (12 < 23) or (b) poly mer 1 has more affnity with poly mer 2thanwiththeaqueousphase(12 23). Inverted coreshell (ICS), in which the seed poly mer 1 forms a continu-ous shell around the second-stage poly mer 2. This equilibrium mor-phologymightbeobtainedwhenpoly mer2ismorehydrophobic than poly mer 1 (23 > 13) and poly mer 2 has more affnity with poly-mer 1 than with the aqueous phase (12 < 23). Hemisphere,snowman-like,Janus,half-moon,occluded,partially engulfed, depending on the different degrees of protrusion and on thedifferentcurvaturesofthepoly mer/poly merinterfaceandthe coverage of one poly mer upon the other (Sundberg and Durant 2003). Thisequilibriummorphologymightbeachievedunderdifferent conditions: (a) when poly mer 2 has similar affnities with poly mer 1 and with the aqueous phase (12 23); (b) when poly mer 2 has more 32Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsaffnity with poly mer 1 than with the aqueous phase (12 < 23) and bothpolymershavesimilaraffnitieswiththeaqueousphase (23 12 13); (c) when poly mer 2 has less affnity with poly mer 1 thanwiththeaqueousphase(12>23)andpoly mer1hassimilar affnities with poly mer 2 and the aqueous phase (23 12 13). Separateparticlesofthedifferentpolymers.Thisequilibriummor-phologymightbeobtainedwhenbothpolymershavemoreaffnity with the aqueous phase than with each other (13 < 12) and (23 < 12).AsshowninFigure2.1,equilibriummorphologiesoftwo-component systemsareestablishedbasicallybythreeinterfacialtensionvalues. Notwithstanding,severalfactorsinfuencethesethreeinterfacialtensions andmay,therefore,beusedforparticlemorphologycontrolpurposes. Besidespoly mertypes,whichdeterminetheinterfacialtensionbetween thepolymersandaffecttheinterfacialtensionbetweeneachpoly merand theaqueousphase,poly mer-poly merinterfacialtensioncanbeinfuenced substantially by compatibilizing agents, and the interfacial tensions between eachpoly merandtheaqueousphasearealsoaffectedbythetypesand amounts of surfactant and initiator. In addition, it has been shown that the equilibriummorphologymayalsobeaffectedbythelevelofcross-linking of poly mer 1, which infuences the free energy (Sundberg and Durant 2003), and by the molecular weights of the polymers, since the interfacial tension betweenthepoly merphases,whichismuchlowerthanthatbetweenthe poly mer and the aqueous phase, depends on the molecular weight (Tanaka 0.1 1 101223(23 12)131010.1Figure 2.1Equilibrium morphologies for a two-component system: poly mer 1 (seed), poly mer 2 (pro-ducedbythepoly mer i za tionofthesecond-stagemonomer).12:interfacialtensionbetween polymers 1 and 2; 13: interfacial tension between poly mer 1 and aqueous phase; 23: interfacial tension between poly mer 2 and aqueous phase. (Reprinted from V. Herrera, R. Pirri, J. R. Leiza, and J. M. Asua, Macromolecules 39: 696974, 2006. With permission.)Synthesis of Polymer Particles with Core-Shell Morphologies33et al. 2008). Finally, the ratio between polymers 1 and 2 may affect the cur-vatureofthepoly mer-poly merinterfaceinthehemispheremorphology (Sundberg and Durant 2003), and the CS and ICS morphologies may not be achieved if the amount of the shell forming poly mer (poly mer 2) in CS and poly mer 1 in ICS is not enough to form a continuous shell with a minimum thickness.Ifcopolymersaretobeconsidered,theanalysisbecomesmore complexsincethesurfaceandinterfacialtensionsdependonthecopoly-mer compositions.2.2.2Nonequilibrium MorphologiesAs mentioned at the beginning of this section, quite frequently the equilib-rium morphology is not achieved due to kinetic control of the morphology development.Inthiscase,nonequilibrium-typestructuresmaybeformed. ThreemainprocesseshavebeenusedbyGonzlez-OrtizandAsua(1995, 1996a, 1996b) to describe the morphology development:1.Theformationofpoly merchainsoccursatagivenpositioninthe poly mer particle.2.Incompatible poly mer chains cause phase separation leading to the formation of clusters.3.Clustersmigratetowardtheequilibriummorphologyinorderto minimizetheGibbsfreeenergy.Duringthismigrationthesize of the clusters may increase by (i) poly mer i za tion of monomer inside thecluster,(ii)diffusionofpoly merchainsintothecluster,and (iii) coagulation with other clusters. The rates of process (ii) and (iii) depend strongly on the particle viscosity.TheSundberggrouphasstudiedtheeffectofseveralfactorsonthe morphologydevelopmentduringtwo-stageemulsionpoly mer i za tions, especially those involving the less hydrophobic copolymer of methyl meth-acrylateandmethylacrylateasseedpoly merandpolystyreneasthemore hydrophobicsecond-stagepoly mer.Durantetal.(1997)verifedtheinfu-enceofdifferentamountsofcross-linkingmonomer(EGDMA)duringthe synthesesofthePMMAcoresonthemorphologyofPMMA/PSparticles. Itwasobservedthat0.015wt%EGDMAwasenoughtoshifttheparticle morphologyfromICS(second-stagePSinthecore)towardCS.At0.2wt% EGDMA, the particles was essentially of the CS morphology. Cross-linking duringthesecondstage,ontheotherhand,wasobservedbyStubbsand Sundberg(2006)tohaveverylittle,ifany,effectonmorphology,thoughit enhances the mechanical stability of the shell. The effect of the feed rate of the second-stage more hydrophobic monomer (styrene) when less hydropho-bic high-Tg seed polymers (poly[methyl methacrylate]) are used was studied byStubbsetal.(1999).Fastsecond-stagemonomeradditionresultedinCS 34Advanced Polymer Nanoparticles: Synthesis and Surface Modifcationsparticles, whereas slower addition increased the number of occlusions of the hydrophobic second-stage poly mer. When less hydrophobic low-Tg seed poly-mer (poly[methyl acrylate]) was used, ICS was obtained independently of the second-stagemonomerfeedrate.Ivarssonetal.(2000)andKarlssonetal. (2003) verifed that it is possible to keep the more hydrophobic second-stage poly mer (styrene) at the shell of the particles if the reaction temperature is less than 15C above Tg of the seed copolymer (poly[methyl methacrylate]/poly[methylacrylate]).StubbsandSundberg(2004)observedthat,though ionic initiators that are able to anchor the chains of the second-stage poly mer to the particle surface make it more likely to obtain CS morphologies with poly(methyl methacrylate)/poly(methyl acrylate) core and polystyrene shell under some conditions, this effect is not dominant under most conditions.Basedontheseresults,StubbsandSundberg(2008)proposedthedeci-sion fowchart shown in Figure 2.2 to be used for morphology prediction of latex particles obtained by emulsion poly mer i za tion. Stubbs and Sundberg (2008) considered three main questions for the prediction of the morphology of composite particles: the frst one is whether radicals may penetrate dur-ing the second stage of the synthesis, the second is about phase separation, and the last is related to phase consolidation. The theory of radical penetra-tion considers that in emulsion poly mer i za tion, radicals are typically created in the water phase, and thus enter latex particles at the outer particle surface. LargeocclusionsCore-shellCore-shell163Tg2 < TreactionLobedparticleYesYesNoNoEquilibriummorphology713 > 23Penetrationpossible?Phaseseparationpossible?Gradient ormixed phasePhaseconsolidationpossible?Extent largeor small?NoYesYesYesNoSmallocclusionsSmallLargeNo425Figure 2.2Adaptationofthedecisiontreefowchartforpredictingmorphologydevelopmentinmulti-phase particles proposed by Stubbs and Sundberg (2008). 13: interfacial tension between poly-mer 1 and aqueous phase; 23: interfacial tension between poly mer 2 and aqueous phase; Tg2: glass transition temperature of poly mer 2. (Reprinted from J. M. Stubbs and D. C. Sundberg, Progress in Organic Coatings 61: 15665, 2008. With permission.).Synthesis of Polymer Particles with Core-Shell Morphologies35The extent of radical penetration will depend on the effective Tg of the seed poly mer. The effective Tg considers that the particle will be partially swollen with second-stage monomer during the poly mer i za tion and this will lower its glass transition temperature below that of the pure poly mer. When pen-etration is possible, a second-stage poly mer chain will fnd itself inside the particle and fully entangled with the seed poly mer chains. Phase separation thenrequireschaindiffusioninordertogetmultiplechainstogether,and this process may be so slow that phase separation is not possible. The driving force for morphology rearrangement is the minimization of interfacial free energy,andthesystemwillevolvetowardtheequilibriummorphologyif given suffcient time. However, the process of phase consolidation requires an increased extent of poly mer mobility compared to the previous two pro-cesses of oligomeric radical penetration and poly mer phase separation.2.3Synthesis of Core-Shell ParticlesParticles with core-shell morphologies may be synthesized by a number of heterogeneous poly mer i za tion techniques such as emulsion, miniemulsion, microemulsion, dispersion, and suspension poly mer i za tions. The frst three techniquesleadtotheformationofsubmicrometricparticles(10800nm), whereas the two last are used, respectively, to prepare small (130 m) and large(501500m)micrometricparticles.Nevertheless,itmustbekeptin mind that in any of these techniques, the application of a two-stage strategy to build up a shell of the second-stage poly mer onto the core of the frst-stage poly mer core will not necessarily lead to the formation of particles with core-shell morphology (Rajatapiti et al. 1997).In the next sections, the synthesis of particles with core-shell structure by these different techniques will be described. A detailed description of these poly mer i za tiontechniquescanbefoundinseveralexcellentbooksinvolv-ing emulsion poly mer i za tion (Piirma 1982; Gilbert 1995; Lovell and El-Aasser 1997;VanHerk2005),aswellasinrecentbookchaptersaboutemulsion (de la Cal et al. 2005; Nomura et al. 2005), miniemulsion (Schork et al. 2005), microemulsion (Chow and Gan 2005), dispersion (Kawagushi and Ito 2005), suspension (Brooks 2005), and heterogeneous (Van Herk and Monteiro 2002) poly mer i za tion techniques.2.3.1 emulsion PolymerizationEmulsionpoly mer i za tionisaheterogeneouspoly mer i za tionsystemcom-posed of water, an initiator (usually water soluble), surfactant (usually above the critical micelle concentration [CMC]), and monomer with low water solu-bility, which under stirring forms droplets with diameters ranging from 1 to 36Advanced Polymer Nanoparticles: Synthesis and Surface Modifcations10m.Themainlocusofpoly mer i za tionisnotinthesemonomerdrop-lets,butwithinthesubmicrometricmonomer-swollenpoly merparticles (60800 nm). In ab initio emulsion poly mer i za tions, these poly mer particles are formed at the beginning of the poly mer i za tion by the entry of radicals into micelles,ifthesurfactantconcentrationisabovetheCMC(micellarnucle-ationmechanism)and/orbytheprecipitationofgrowingoligomersinthe aqueous phase (homogeneous nucleation mechanism). Due to the relatively large size of the monomer droplets compared to the size of monomer-swol-len micelles (1020 nm), the surface area of the monomer droplets is orders of magnitude smaller than that of the micelles and, consequently, the radi-calentryintomonomerdroplets(dropletnucleation)isinsignifcant.Since monomer droplet nucleation is insignifcant and monomer-swollen poly mer particles, instead of monomer droplets, are the main poly mer i za tion locus, monomerdropletsactasmonomerreservoirsandthemonomermustbe transportedfromthesedropletsbydiffusionthroughtheaqueousphase toallowthegrowthofthepoly merparticlesbypoly mer i za tion.Inseeded emulsion poly mer i za tions, poly mer particles (seeds) are added at the begin-ning of the poly mer i za tion and, usually, these reactions are conducted in the absence of micelles to avoid secondary particle nucleation.Seeded emulsion poly mer i za tion is by far the most applied technique for the synthesisofstructuredpoly merparticleswithcore-shellmorphology.And two operation modes, batch and semicontinuous, with or without preswelling of the seed particles, are commonly used in these seeded emulsion poly mer i-za tions.WhendirectCSparticleswithafrst-stagepoly mercoreandasecond-stagepoly mershellaretobeobtained,usuallythesecond-stagemonomer is continuously fed at a prespecifed rate in order to allow the second-stage poly mer to build up upon the surface of the seed particles, forming a uniform and continuous shell. In this case, reaction conditions that avoid or minimize phase consolidation are often required, especially in those cases in which the CS morphology is not the equilibrium morphology.When, on the other hand, ICS particles with a second-stage poly mer core and a frst-stage poly mer shell are to be obtained, usually seed particles are swelled by the second-stage monomer prior to either batch or fooded semi-continuouspoly mer i za tionofthesecondmonomer,sinceconditionsthat allowphaseconsolidationarerequiredforphaseinversionleadingtothe equilibriumICSmorphology.Thechoiceofthebestoperationmoderelies strongly on the poly mer types and whether the core-shell morphology is to be achieved directly (CS) or through phase inversion (ICS).In the following, the syntheses of CS and ICS particles via emulsion poly-mer i za tionwillbediscussed.Thisalsoincludessomeworksinwhichthe frst-stagepoly merseedsweresynthesizedviaminiemulsionpoly mer i-za tionandthesecond-stagemonomerpoly mer i za tionwasperformedvia emulsion poly mer i za tion.Synthesis of Polymer Particles with Core-Shell Morphologies372.3.1.1 Synthesis of Core-Shell Particles (CS)In this strategy the second-stage poly mer should build up upon the surface of the seed particles, forming a uniform and continuous and, consequently, concentricshell.Asaconsequence,CSparticlesmaybeformedindepen-dently of this being the equilibrium morphology. If CS is not the equilibrium morphology, reaction conditions must be carefully adjusted to avoid and/or minimize phase consolidation. Several conditions may favor the formation of CS particles:High superfcial area of the frst-stage poly mer. This can be achievedby using small seed particles and/or high seed contents.Reaction temperature below or close to the glass transition tempera- ture of the poly mer particles. If the Tg of the seed poly mer is not too low, this might be achieved by starved second-stage monomer feed and/orhighinitiationratetokeepsecond-stagemonomerconcen-tration, and its plasticizing effect, low in the particles.Cross-linkedcorepoly mertoreducediffusionofthesecond-stagemonomer and, especially, of the radicals into seed particles.Enhancedhydrophobicityofseedpoly merthroughitssynthesisusing an oil-soluble initiator and/or copoly mer i za tion with a hydro-phobic monomer.Aqueous phase initiator that is able to anchor the second-stage radi- cals at the particle surface.If the incompatibility between the frst- and second-stage polymersis too high to allow the formation of particles with a CS structure, the use of a compatibilizing agent is advised to reduce the poly mer- poly-mer interfacial tension. This compatibilizing agent can be produced in situ by the incorporation of a proper macromonomer (Nelliappan etal.1996),comonomer(ShermanandFord2005),orCRPagent (Herrera et al. 2006) during the synthesis of the seed poly mer.Though this strategy might seem the most straightforward for the synthesis of CS particles with hydrophobic cores and hydrophilic shells, in some cases it may result in considerable secondary nucleation instead of the formation of CS particles, since several points listed previously, as for instance high aqueous phase initiator concentration, may lead to homogenous particle nucleation.AquitecompletestudywaspresentedbyFergusonetal.(2002),whosuc-ceededinsynthesizingPS/PVAccore-shelllatexeswithsmallcores.Inthis casethesmallsizeofthePSseedparticles(unswollendiameterof88nm) resulted in a high enough superfcial area to capture PVAc radicals formed in the aqueous phase. When, on the other hand, PS seed particles with unswollen diameter of 400 nm were used, excessive new particle formation occurred and 38Advanced Polymer Nanoparticles: Synthesis and Surface ModifcationsnoPVAcshellscouldbedetected.Numerousstrategiesforovercomingthis were evaluated through simulations with a simplifed nucleation model and/or implemented experimentally, as for instance using an organic-phase initiator intheseededpoly mer i za tiontoavoidhomogeneousnucleationoraddition of surfactant during the seeded poly mer i za tion in order to maintain the sur-face charge density and keep the surfactant concentration below the CMC, but always either extensive secondary nucleation occurred or the system became colloidallyunstable.SimilarresultswereobtainedbyShermanandFord (2005). The authors used a cross-linked 80/20 PS/PMMA seed 70 nm in diam-eter to form cationic PS/PMMA CS particles with acentric cores with 530 nm inthreestepsofPMMAgrowth,usingstarvedsemicontinuousadditionof MMA to avoid secondary nucleation and a single initial addition of aqueous phase initiator to provide fast radical generation. The increase of the diameter of the core latex, on the other hand, led to secondary particle nucleation.DuetotheincompatibilityofPSandPMMA,theequilibriummorphol-ogyofthePS/PMMAsystemisasadouble-ballstructure.Consequently, whenPShomopolymerseedsareusedwithoutanyproceduretoreduce the interfacial tensionbetween both polymers, the CS morphology usually isnotattained.Agoodillustrationoftheeffectoftheoperationmodeon themorphologyofPS/