Uniform nonspherical latex particles as model interpenetrating polymer networks

Polystyrene/polystyrene latex interpenetrating polymer networks (IPNs) were prepared by seeded emulsion polymerization of styrene–divinylbenzene mixtures in crosslinked monodisperse polystyrene particles. The resulting latexes comprised uniform nonspherical particles, e.g., ellipsodal and egg-like singlets, symmetry and asymmetric doublets, and ice cream cone-like and popcorn-like multiplets. The nonspherical particles, which were formed by separation of the second-stage monomer from the crosslinked seed network during swelling and polymerization, are excellent models for studying phase separation in IPN's. The degree of phase separation increased with increasing degree of crosslinking of the seed particles, monomer/polymer swelling ratio, polymerization temperature, and seed particle size, and with decreasing divinylbenzene concentration in the swelling monomer. The results were consistent with a thermodynamic analysis based on the elastic-retractile force of the polymer network, the monomer/polymer mixing force, and interfacial tension force.     

Phase separation in polystyrene latex interpenetrating polymer networks

Polystyrene/polystyrene latex interpenetrating polymer networks (IPNs) were prepared by seeded emulsion polymerzation of styrene–divinylbenzene mixtures in crosslinked monodisperse polystyrene seed latexes. The resulting latexes comprised uniform nonspherical particles, which were formed by separation of the second-stage monomer from the crosslinked seed network during swelling and polymerization. The kinetics of phase separation were investigated by examining the changes in particle morphology using optical microscopy, which revealed that the phase separation was induced by the relaxation of the polymer chains before polymerization began and was enhanced by increased conversion. The thermodynamics of phase separation were investigated by analysis of the free-energy changes during swelling and polymerization, and the phase separation was described by a nucleation-and-growth mechanism. The results of this study have been applied to the design and synthesis of a series of uniform nonspherical particles of different morphology.     

Surface heterogeneity of polystyrene latex particles determined by dynamic force microscopy

Atomic force microscopy (AFM) was employed to characterize the surface chemistry distribution on individual polystyrene latex particles. The particles were obtained by surfactant-free emulsion polymerization and contained hydrophilic quaternary ammonium chloride, sodium sulfonate, or hydroxyethyl groups. The phase shift in dynamic force mode AFM is sensitive to charge/chemical interactions between an oscillating atomic force microscope tip and a sample surface. In this work, the phase imaging technique distinguished phase domains of 50-100 nm on the surfaces of dried latex particles in ambient air. The domains are attributed to the separation of ion-rich and ion-poor components of the polymer on the particle surface.     

Effect of second monomer and initiator type, mixing method, and stabilizer content on the shape of the particles produced by seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this work, the preparation of micron-sized polymer particles with nonspherical shapes via seeded dispersion polymerization of 2-ethylhexyl methacrylate with polystyrene seed particles in the presence of hydrocarbon droplets and evaporation of hydrocarbon after the polymerization under various polymerization conditions was discussed. The effect of second monomer and initiator type, mixing method, and stabilizer content on the shape of the obtained particles was investigated. It was observed that particles with more nonspherical shapes were obtained with increasing the alkyl chain length of ester group of the methacrylate of the second monomer which is because of increasing the absorption amounts of hydrocarbon by second polymer domains. Moreover, the experimental results showed that shape of the particles which was produced by shaking is more nonspherical than the shape of the particles which was obtained from tumbling. Furthermore, particles with different nonspherical shapes were prepared by changing the initiator type and stabilizer content.     

Preparation of novel and unique nonspherical particles with almond-shell-like shape via dual-seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this study, the preparation of micron-sized polymer particles having a novel and unique nonspherical shape which we called almond-shell-like by dual-seeded dispersion polymerization (DSDP) of 2-ethylhexyl methacrylate (EHMA) with polystyrene (PS) and poly(methyl methacrylate) (PMMA) seed particles in the presence of decane droplets and evaporation of decane after the polymerization was discussed. The experimental results showed that mushroom-like morphology which is a precursor of the almond-shell-like shape was obtained from DSDP of EHMA. It was found that with changing the PS/PMMA seed particles' weight ratio, the size of the dents on the surface of the particles can be controlled. Furthermore, it was observed that various nonspherical particles can be produced using different methacrylic seed particles and initiators.     

Preparation of micron-sized nonspherical polystyrene/poly(styrene-<Emphasis Type="Italic">co</Emphasis>-ethyleneglycol dimethacrylate) particles by seeded soap-free emulsion polymerization

Micron-sized nonspherical polymer particles having different morphologies were synthesized by seeded soap-free emulsion polymerization of styrene(St) and ethyleneglycol dimethacrylate(EGDMA,used as a crosslinker) on spherical, linear polystyrene(PS) seed particles.The morphology of the resulting PS/poly(St-co-EGDMA) particles was dependent on the crosslinker concentration and polymerization temperature.     

Preparation of micron-sized, monodisperse polymeric nonspherical particles with tunable shapes by micromolding–polymerization

In this work, we present a novel synthesis method for the preparation of micron-sized, monodisperse polymeric nonspherical particles without changing the shape of a precursor. In this technique, which we called micromolding–polymerization, nonspherical particles of second polymer were formed directly on the surface of micromolds (solvent-swollen polymeric particles). The experimental results showed that the affinity between the second polymer and micromolds, and size and concentration of the oligoradicals specify the shape of the particles.     

Thermotropic liquid-crystal/polymer microcapsules prepared by in situ suspension polymerization

In this study, thermotropic liquid-crystal/polymer microcapsules were produced via in situ suspension polymerization. The phase separation between cholesteryl liquid crystal (CLC) and poly(methyl methacrylate) (PMMA) in the droplets was induced by polymerization, resulting consequently in uniform liquid-crystal-containing polymer microcapsules. The phase behavior of the microcapsules was dependent on the loading amount of the liquid crystals and the degree of cross-linking of the polymer phase. Above 30% loading amount of CLC, the liquid crystals started to appear clearly. It was found that the spherical morphology of the microcapsules was achieved within a slight degree of cross-linking of the PMMA phase. At a high degree of cross-linking, nonspherical particles with a rough surface and deeper dents were obtained, which was due to the elastic-retractive force of the cross-linked network. The liquid-crystal/polymer microcapsules produced in this study could find great applicability in pharmaceutics and electronics as a smart drug carrier.     

Preparation of nonspherical particles via dual-seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this study, the effect of various polymerization conditions on the shape of the particles produced by dual-seeded dispersion polymerization of a second monomer with polystyrene (PS) and poly(methyl methacrylate) (PMMA) seed particles in the presence of saturated hydrocarbon droplets in a polar media was discussed. It was observed that with changing the affinity between the hydrocarbon and PS seed particles, second monomer type, polarity, and alcohol type of the medium nonspherical particles with a variety of shapes can be produced. Furthermore, we suggested that the presence of PMMA seed particles in the medium affects the distribution of the second polymer domains on the surface of the PS seed particles in addition to the absorbed amount of the hydrocarbon by PS particles and second polymer domains and the distribution of the hydrocarbon between them. Moreover, the experimental results showed that almond shell-like PS particles can be prepared under certain conditions.     

Surface active properties of polyoxyethylene macromonomers and their role in radical polymerization in disperse systems

Conventional dispersion polymerization and copolymerization of low-molecular weight (conventional) unsaturated monomers allows preparation of monodisperse and micronsize polymer particles. A similar behavior can be found in the surfactant-free dispersion polymerization of non-traditional vinyl monomers, unsaturated macromonomers. The latter systems allow preparation of random, comb-like, star-like and graft copolymers as well. An interesting alternative arises with the use of amphiphilic reactive macromonomers that contain a polymerizable group and aggregate into an organized structure -- a micelle. Under such conditions the high rate of polymerization and ultrafine (microparticles) polymer dispersions are generated. Thus, the surface-active macromonomers promote the formation of micelles and polymer growth within the main reaction loci -- polymer particles. Furthermore, the surface-active compounds can be formed during the copolymerization of hydrophilic macromonomer and hydrophobic low-molecular weight comonomer. The reactive surface-active oligomeric radicals are incorporated into the polymer matrix or the particle surface layer, which prevents them from subsequent migration. Besides, the covalently bound surface-active groups at the particle surface strongly increase the colloidal stability of final polymer dispersion. This article presents a review of the current literature in the field of the surfactant-free dispersion polymerization of the polyoxyethylene unsaturated macromonomers. Besides a short introduction into some kinetic aspects of radical polymerization of traditional monomers in homogeneous and disperse systems, we mainly focus on the organized aggregation of amphiphilic polyoxyethylene macromonomers, the characterization of amphiphilic graft copolymers and their aggregation properties, and radical copolymerization of polyoxyethylene macromonomers. We discuss the birth and growth of chains, the transfer of reaction loci from the continuous phase to polymer particles, the diffusion-controlled termination, association of amphiphilic reaction by-products, the particle growth by agglomeration, the particle nucleation, the deactivation of polymer chain growth and the colloidal stability. Effects of initiator type and concentration, the surface activity of macromonomer, the macromonomer type and concentration, temperature, additives and the type of continuous phase on the kinetics of polymerization, and colloidal parameters of the reaction system are also evaluated. Variation of the polymer coil density, the polymer-polymer interaction, and polymer-solvent interaction with the molecular weight, diluent and method (light scattering, the size exclusion, etc.) are discussed. Polymerization of macromonomers provides regularly branched polymers with varied branching density. Since both the degree of polymerization and the length of branches may be varied, polymeric materials with specific properties can be prepared.     

PSt种子与“花瓣”形PSt/PAN复合颗粒的制备

以过硫酸钾为引发剂,在乙醇/水的混合介质中使苯乙烯进行无皂乳液聚合,得到了单分散亚微米级聚苯乙烯(PSt)微球.用扫描电子显微镜研究了引发剂浓度、单体浓度、反应温度和溶剂组成对PSt微球粒径的影响.结果表明,改变上述条件能明显影响其粒径.以所得单分散聚苯乙烯微球为种子,在丙烯酸单封端聚乙二醇大分子单体存在的条件下,使丙烯腈和少量苯乙烯进行新的无皂种子乳液聚合,在合适的条件下制得到了“花瓣”形的聚合物复合颗粒,为深入探讨这类特殊形态聚合物颗粒的形成机理提供了新的佐证.     

Facile one-pot synthesis and morphological control of asymmetric superparamagnetic composite nanoparticles

A one-pot colloidal reaction strategy was developed to synthesize asymmetric magnetic composite particles (AMCPs) with high anisotropy and magnetic content. The asymmetricity was established by having two portions of the particle composed of both polystyrene and silica, among which the latter was conjugated with magnetite nanoparticles. Phase separation was found to be critical in developing the asymmetric structure between the inorganic and polymer components during miniemulsion polymerization and the sol-gel reaction. Morphological control of the magnetic composite particles was achieved in a straightforward fashion by adjusting the processing parameters. These asymmetric nanomaterials with superparamagnetic and amphiphilic properties will have significant potential in biomedical applications.     

Crosslinked polystyrene latexes prepared with 12-(o-styryloxy)dodecyltrimethylammonium bromide

Crosslinked polystyrene latexes with high surface charge densities have been prepared with 1, 3, and 5 mol% divinylbenzene and 0.5–5.0 mol% of 12-(o-styryloxy)dodecyltrimethylammonium bromide ( 1 ). Within experimental error all of surfactant 1 was incorporated into the particles. Analysis of the latex particles by transmission electron microscopy showed nonspherical shapes, aggregates, and number average diameters ranging from 22 to 95 nm. The particle size decreases and the polydispersity increases with increasing amounts of surfactant 1 and of divinylbenzene in the monomer mixture. A mechanism is proposed for particle formation by precipitation of crosslinked polymer from monomer droplets to form primary particles, coagulation of the primary particles to colloidally stable aggregates, and growth by further polymerization.     

种子聚合法制备非球形聚合物颗粒的研究进展

十年前,光子晶体的需求促进了非球形颗粒的研究热潮。非球形颗粒由于其对称性下降,带来了新的性能和应用前景,成为当前材料领域研究的热点,得到了较广泛研究,目前其制备方法包括种子聚合法、自组装法、粒子聚集法、微流体法和机械拉伸法等。本文简述了种子聚合法的发展历程,介绍了种子聚合法制备非球形聚合物颗粒的相关机理,以种子微球材质分类综述了种子聚合法制备非球形聚合物颗粒的形貌和结构控制技术进展,概述了其潜在的应用领域,展望了可能的发展趋势。     

Model filled polymers. VIII. Synthesis of crosslinked polymeric beads by seed polymerization

Monodisperse crosslinked polystyrene (PS) and polymethacrylate (PMA) beads of sizes greater than 1 μm in diameter are prepared by particle nucleation onto pre-existing polymer seeds in a multistage emulsion polymerization, in the absence of emulsifier. An adequate seed number concentration, which decreases with increasing seed size, is necessary to achieve monodisperse beads. Monodisperse multicomposition beads are prepared by polymerizing styrene onto PMA seeds, but not by polymerizing methyl methacrylate onto PS seeds. Phase separation in growing seed particles or surface polymerization following free radical capture may lead to the formation of asymmetric shaped particles.     

Preparation and thermodynamic stability of micron-sized, monodisperse composite polymer particles of disc-like shapes by seeded dispersion polymerization

Micron-sized, monodisperse composite polymer particles having "disc-like" and "polyhedral" shapes were prepared by seeded dispersion polymerization of 2-ethylhexylmethacrylate (EHMA) with 2.67-mum-sized polystyrene (PS) seed particles in methanol/water media in the presence of droplets of various saturated hydrocarbons and evaporation of the hydrocarbon after the polymerization. Such nonspherical shapes were based on the volume reduction due to the evaporation. The primary factors influencing the particle shape seemed to be the absorption rate of the hydrocarbon into the resulting PS/poly(EHMA)/hydrocarbon composite particles during the polymerization, which affected the viscosities and the volumes of the PS and poly(EHMA) phases. It was found that the morphological development during the polymerization was retarded at "hamburger-like" morphology, which is a precursor of the disc-like particle, although this morphology is a thermodynamically metastable state.     

Effect of monomer and hydrocarbon content and polarity of the medium on the preparation of nonspherical particles via seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this work, the preparation of micron-sized polymer particles with nonspherical shapes via seeded dispersion polymerization of 2-ethylhexyl methacrylate (EHMA) with polystyrene (PS) seed particles in the presence of decane droplets and evaporation of decane after the polymerization under various polymerization conditions was discussed. The effect of monomer and decane content and polarity of the medium on the shape of the obtained particles was investigated. The experimental results showed that decreasing the amount of monomer and hydrocarbon volume of PEHMA/hydrocarbon domains, the particles decreased but they grew symmetrically, resulting in symmetric shapes. Furthermore, it was suggested that because of changing the solubility of the oligoradicals and hydrocarbon in the medium, the shape of the particles changed with changing the polarity of the medium. With decreasing the polarity of the medium, solubility of the oligoradicals in the medium increases and bigger polymer domains form on the surface of PS particles which can absorb higher amounts of decane. All of these can contribute to an increase in volume reduction after extraction of PEHMA/decane, resulting in various particle shapes. Further decreasing in polarity of the medium leads to an increase in the solubility of decane in the medium and decreasing the absorbed amount of decane by PS particles and PEHMA domains, resulting in lower volume reduction after evaporation of decane.     

Monodisperse porous polymer particles: Formation of the porous structure

Monodisperse porous styrene-divinylbenzene copolymer particles were prepared via seeded emulsion polymerization using a mixture of linear polymer (polystyrene seed) and non-solvent as inert diluent. Experimental evidence was presented to describe the mechanism of formation of porous polymer particles during the copolymerization and solvent extraction stages, in which porosity was a consequence of phase separation in the presence of diluents. Pore structure formation was investigated by changes in copolymerization kinetics, gel content, crosslinking density, particle morphology, surface area, pore volume, and pore size distribution. The process of copolymerization was presented, based on the concepts of production, agglomeration, and fixation of the interior gel microspheres of polymer particles. A portion of linear polymer used as diluent was found to participate in the network structure while the porous matrix was built-up. The influence of the removal of the linear polymer from the matrix pores during the solvent extraction process on the porous structure was also discussed.     

Highly magnetic latexes from submicrometer oil in water ferrofluid emulsions

The synthesis of functionalized submicrometer magnetic latex particles is described as obtained from a preformed magnetic emulsion composed of organic ferrofluid droplets dispersed in water. Composite (polystyrene/γ‐Fe₂O₃) particles were prepared according to a two‐step procedure including the swelling of ferrofluid droplets with styrene and a crosslinking agent (divinyl benzene) followed by seeded emulsion polymerization with either an oil‐soluble [2,2′‐azobis(2‐isobutyronitrile)] or water‐soluble (potassium persulfate) initiator. Depending on the polymerization conditions, various particle morphologies were obtained, ranging from asymmetric structures, for which the polymer phase was separated from the inorganic magnetic phase, to regular core–shell morphologies showing a homogeneous encapsulation of the magnetic pigment by a crosslinked polymeric shell. The magnetic latexes were extensively characterized to determine their colloidal and magnetic properties. The desired core–shell structure was efficiently achieved with a given styrene/divinyl benzene ratio, potassium persulfate as the initiator, and an amphiphilic functional copolymer as the ferrofluid droplet stabilizer. Under these conditions, ferrofluid droplets were successfully turned into superparamagnetic polystyrene latex particles, about 200 nm in size, containing a large amount of iron oxide (60 wt %) and bearing carboxylic surface charges. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2642–2656, 2006     

Synthesis of anisotropic nanoparticles by seeded emulsion polymerization

Anisotropic polystyrene nanoparticles of diameters below 0.5 microm were prepared by coating the surface of cross-linked polystyrene latex particles with a thin hydrophilic polymer layer prior to swelling the particles with styrene and then initiating second-stage free-radical polymerization. Conditions were found so that all particles had uniform asymmetry. The effect of surface chemistry on the development of particle anisotropy during seeded emulsion polymerization of sub-0.5 microm diameter particles was studied. The extent and uniformity of the anisotropy of the final particles depended strongly on the presence of the hydrophilic surface coating. Systematic variation of the degree of hydrophilicity of the surface coating provided qualitative insight into the mechanism responsible for anisotropy. Conditions were chosen so that the surface free energy favored the extrusion of a hydrophobic bulge of monomer on the hydrophilic surface of the particle during the swelling phase: the presence of a hydrophilic layer on the particle surface causes this asymmetry to be favored above uniform wetting of the particle surface by the monomer. Kinetic effects, arising from the finite time required for the seed to swell with the monomer, also play a role.