Morphology,thermal property,and mechanical property of core–shell latex polymers. I. Effect of heating and pressuring on PBA/PS linear composite polymer

In this work, butyl acrylate and styrene were used as monomers in the first stage and second stage of polymerization, respectively, and potassium persulfate (K₂S₂O₈) was used as the initiator to synthesize the poly(butyl acrylate)–polystyrene (PBA/PS) composite latex by the method of two-stage soapless emulsion polymerization. The morphology of the latex particles was observed by transmission electron microscopy (TEM), which showed that the composite latex particles had a core–shell structure. The particle-size distribution of the composite latex was very uniform. A thin layer of a PBA-graft-PS copolymer was formed in between the core (PBA) and shell (PS) regions, which thus increased the compatibility between the PBA and PS phases. The process of heating and pressuring influenced the morphology, mechanical properties, and thermal properties of the PBA/PS composite polymer. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 13–23, 1998     

Synthesis,characterization, and properties of interpenetrating polymer networks containing functionalized latex particles

Three-component interpenetrating polymer networks (IPNs) comprising polyurethane (PUR), poly(n-butyl methacrylate) (PBMA), and polystyrene (PS) latex particles were prepared in a modified one-shot synthesis. The hydroxy-functionlized and unmodified polystyrene latex particles were synthesized via a seeded emulsion polymerization. The incorporation of hydroxyethyl methacrylate into the latex particles was confirmed via diffuse reflectance infrared analysis and modulated-temperature differential scanning calorimetry. The IPNs were characterized by dynamic mechanical thermal analysis, tensile testing, hardness measurements, and transmission electron microscopy. The three-component materials exhibited higher values for the Young's modulus and the Shore A hardness and for the dynamic storage modulus in the higher temperature range from 80 to 140°C than did the PUR/PBMA IPN alone. The latex particles with the hydroxyl functionality exhibited a better miscibility with the microheterogeneous PUR/PBMA IPN than did unfunctionalized PS latex particles, and, therefore, resulted in materials with better damping properties in the temperature range between 80 and 140°C. Transmission electron micrographs confirmed the imporved miscibility of the functionalized latex particles. The latex particles were not, however, dispersed on an individual level but formed agglomerates of between 2 and 20 μm. © 1996 John Wiley & Sons, Inc.     

Preparation of polystyrene/poly(vinyl acetate) nanocomposites with a core–shell structure via emulsifier‐free emulsion polymerization

Polystyrene/poly(vinyl acetate) latex nanoparticles with a core–shell morphology in an emulsifier‐free emulsion polymerization system were prepared with purified styrene and vinyl acetate (VAc) as monomers and 2,2′‐azo bis(2‐amino propane) dihydrochloride (ABA,2HCl) as the initiator and emulsifier. The optimized conditions of polymerization of VAc, on top of the already‐formed polystyrene as a core polymer, with a core–shell morphology were obtained using various parameters such as volume ratio of the first and second stages, type of process, and reaction time. The morphologic structure of the nanoparticles was studied by scanning electron microscopy and transmission electron microscopy. The latex nanoparticles and polymers were characterized by differential scanning calorimetry. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2409–2414, 2006     

Surface characterization of nanoparticles carrying pH-responsive polymer hair

The surface of near-monodisperse colloidal polystyrene (PS) latex particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) polymer chains as a steric stabilizer has been extensively characterized in dry and wet states. X-ray photoelectron spectroscopy and contact angle measurement studies confirm that PDEA chains are located on the PS latex particle surface. Transmission electron microscopy (TEM) studies reveal that the sterically stabilized PS particles have a core-shell morphology and wet atomic force microscopy data confirm that the pH-responsive PDEA component covers the PS latex core and the thickness of the protonated steric polymer is estimated to be approximately 15 nm. The PS latex particles can be easily dispersed in acidic aqueous media (pH 3.0) whilst they flocculate in basic aqueous media (pH 10.0) due to the pH-responsive PDEA hair. This dispersion-flocculation cycle of the PS latex is fully reversible.     

The characteristic properties of poly(methyl methacrylate)/polystyrene core‐shell composite polymer latex

In this study, the poly(methyl methacrylate/polystyrene (PMMA/PS) core‐shell composite latex was synthesized by the method of soapless seeded emulsion polymerization. The morphology of the PMMA/PS composite latex was core‐shell structure, with PMMA as the core and PS as the shell. The core‐shell morphology of the composite polymer latex was found to be thermally unstable. Under the effect of thermal annealing, the PS shell region first dispersed into the PMMA core region, and later separated out to the outside of the PMMA core region. This was explained on the basis of lowing interfacial tension between the PMMA and PS phases owing to the interpenetration layer. The interpenetration layer, which was located at the interface of the core and shell region, contained graft copolymer and entangled polymer chains. Both the graft copolymer and entangled polymer chains had the ability to lower the interfacial tension between the PMMA and PS phases. Also, the effect of thermal annealing on the morphology of commercial polymer/composite latex polymer blends was examined. The result showed that the core‐shell composite latex had the ability to enhance the compatibility of the components of polymer blends. The compatibilizing ability of the core‐shell composite latex was better than that of a random copolymer. Moreover, the effect of the amount of core‐shell composite latex on the morphology of the polymer blend was investigated. The polymer blends, which contained composite latex above 50% wt, showed the morphology of a double sea‐island structure. In addition, the composite latex was completely dissolved in solvent to destroy the core‐shell structure and release the entangled polymer chains, and then dried to form the entangled free composite polymer. The entangled free composite polymer had the ability to enhance the compatibility of the components of the polymer blend as usual. The weight ratio 3/7 commercial polymer/entangled free composite polymer blend showed the morphology of the phase inversion structure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 312–321, 2003     

Morphology development in polychloroprene–polystyrene latex interpenetrating polymer networks

Latex interpenetrating polymer networks (LIPNs) have been prepared using a crosslinked polychloroprene latex as the seed emulsion, followed by the in situ polymerization of styrene, typically with a 10% divinyl benzene crosslinker. Polychloroprene–crosslinked polystyrene (XPS) ratios ranging from 70/30 to 40/60 were used, with the second monomer being added as a single aliquot rather than by “starvation” routes. The majority of the work has been conducted using the water‐soluble persulfate initiator method, which entails lengthy (∼ 6 h) polymerizations. To follow the development of microstructure, polymerizations were also stopped at 0.5, 1, and thence hourly intervals up to 6 h, so that any effect of time on shell and domains could be seen by transmission electron microscopy (TEM). Parallel studies using azo‐bis(isobutyronitrile) (AIBN) as initiator at the same temperature were conducted. Products were also studied, after staining, by TEM. For the persulfate initiator, domain structures predominated for the 70/30 ratio, but polystyrene‐rich shells are found in all cases, with increasing thickness as the chloroprene/styrene ratio was reduced. The styrene‐rich products (i.e., 40/60 Neoprene/XPS ratio) appear to have larger unstained domains suggesting phase separation. For the AIBN‐initiated styrene polymerization, shells are less evident, and where they exist, are both thinner and less continuously developed. Domain sizes are somewhat larger. This relatively hydrophobic initiator has caused polymerization predominately in the interior of each latex particle. The particle size distribution of the seed neoprene latex is broad and bimodal. As the LIPNs form, the larger diameter component increases and little evidence for fresh nucleation, in the form of small diameter particles, is seen. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 629–638, 1999     

Water-based radiation-curable latexes

Film-forming polystyrene/poly(n-butyl acrylate-co-glycidyl methacrylate) [PS/P(BA-co-GMA)] core–shell latex particles were prepared via a two-stage emulsion polymerization procedure using a polystyrene latex seed. Delayed addition of GMA was used in order to locate functional epoxy groups close to the particle surfaces. It was found that a temperature of 25°C at the second-stage polymerization, in combination with a redox initiator system, was essential for the formation of a uniform shell of BA–GMA copolymer around the PS core. The latex particle morphology was investigated by transmission electron microscopy (TEM). Reactive double bonds were introduced into the particle shells in order to produce a film-forming latex system that could be cured by ultraviolet (UV)-radiation without any need to use reactive multifunctional monomers or oligomers as crosslinkers. The surface-bound epoxy groups were used as grafting sites for amine or carboxyl functional unsaturated monomers, respectively. The grafting was demonstrated by Fourier transform infrared (FTIR) spectroscopy. Films prepared from modified and unmodified latexes were exposed to UV radiation in the presence of a photoinitiator. Crosslinking was tested by thermal mechanical analysis (TMA) and by determination of swelling and gel content of exposed films. It was demonstrated that films from the modified latexes after irradiation had significantly higher stiffness and gel content and showed lower swelling than unmodified ones. © 1996 John Wiley & Sons, Inc.     

Elastomeric latex domain-interpenetrating polymer networks: Physical and rheological properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from a two-stage latex (TSL) polymerization include core/shell, domain, interpenetrating polymer networks (IPN), and various combinations thereof. The sequence of polymerization, crosslinking, grafting, and composition are among the significant parameters that determine the particle morphology. Elastomeric TSL with soft polyacrylates (PA) as the seed particles and polystyrene (PS) as the second stage, with each stage lightly crosslinked, may yield IPN-microdomain particles. The particle morphology has been elucidated through a combination of microscopy and mechanical property analyses. The significant modulus of elastomeric latex interpenetrating polymer networks (LIPN) results from reinforcement by PS intra-particle microdomains and their significant tensile strength from a strength forming mechanism of PS inter-particle microdomains. The increase in the PA seed crosslinking increases the crosslinked PS (xPS) level of molecular mixing with, and grafting via residual unsaturation to, the crosslinked PA (xPA) network and decreases particle deformnability. At higher xPS concentrations the formation of an xPS-rich shell enhances xPS continuity in the molded material through the partial coalescence of the shells, diminishing the PA continuity, and yielding more PS-like properties. The submicron lightly crosslinked latex particles with these different morphologies flow as a pseudoplastie material through a particle slippage flow mechanism exhibiting neither a Newtonian plateau nor a yield stress at low shear rates. The deformable lightly crosslinked particles with interchangeable PS ties which disintegrate at elevated temperatures retain their identity and regain their shape at the cessation of shear. The LIPN can be processed using standard thermoplastic methods and machinery, with power law constants and shear insensitive flow activation energies that are similar to those of thermoplastics at high levels of shear. Uncrosslinked PS shells around crosslinked PA seed particles, on the other hand, completely coalesce upon molding to form a continuous thermoplastic PS matrix that may essentially flow through molecular deformation.     

Modification of aqueous polyurethanes via latex AB crosslinked polymers

A series of Latex AB crosslinked polymers have been synthesized from polyurethane (PU) (polymer A) and polystyrene (PS) (polymer B). The effect of PU/PS composition, crosslinking density in the PS domain, as well as in PU has been studied in terms of dispersion size, TEM morphology, mechanical, dynamic mechanical properties, in addition to swellability in water and toluene of the dispersion cast film. An inverted core (PS)‐shell (PU) morphology with very fine (tens of nanometers) dispersion was obtained, and the film properties were well controlled by the Latex composition and crosslinking density of both phases. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1315–1322, 2001     

Modification of natural rubber latex. III. Natural rubber–polystyrene composite latexes synthesized using azobisisobutyronitrile as initiator

Materials which may be classified as interpenetrating polymer networks have been prepared by the in situ polymerization of styrene in natural rubber latex using azobisisobutyronitrile (AIBN) as the initiator. The resulting materials have been characterized by electron microscopy, Soxhlet extraction coupled with proton nuclear magnetic resonance spectroscopy, dynamic mechanical analysis, and stress-strain analysis. The styrene polymerizes within the natural rubber latex particles to give a relatively fine phase-separated morphology with some evidence for a limited degree of segmental mixing. Moreover, it is clear that AIBN, despite what is stated in the literature, does lead to some grafting of polystyrene, and, in addition, causes significant degradation of the natural rubber molecules when the styrene content is low.     

Effect of the composition of structured rubber particles on the toughness of poly(methylmethacrylate)

Composite natural rubber (NR) and monodisperse poly(n-butylacrylate) (PBuA) based latex particles were tested as possible impact modifiers for a poly(methylmethacrylate) (PMMA) matrix. A continuous extrusion process was used for the incorporation of wet latexes directly into a twin-screw extruder. All latexes had been coated by a PMMA shell. Furthermore, polystyrene (PS) subinclusions were introduced into the NR core. The impact resistance of the prepared PMMA blends can be most effectively improved by NR particles containing a large weight fraction of compatibilising PMMA in the shell. The degree of crosslinking of the shell polymer has to be restricted. PBuA based latex particles of 180 nm in size are ineffective to toughen the PMMA matrix. The degree of grafting of the NR phase in core–shell particles containing PS subinclusions is not crucial. Scanning electron microscopy was used to analyse the failure processes in composite rubber particle toughened PMMA blends at fast (impact conditions) and slow (tensile testing) deformation speeds.     

Polystyrene–fluorohectorite nanocomposites prepared by melt mixing with and without latex precompounding: Structure and mechanical properties

Sodium fluorohectorite (FH) was dispersed in polystyrene (PS) by direct melt blending with and without a master batch composed of PS and FH and produced by latex compounding. FH was not intercalated by PS when it was prepared by direct melt compounding. In contrast, FH was well dispersed (mostly intercalated) in PS via the PS‐latex‐mediated predispersion of FH following the master‐batch route. The dispersion of FH was studied with transmission and scanning electron microscopy and X‐ray diffraction techniques and discussed. The nanocomposites produced by the master‐batch technique outperformed the directly melt‐compounded microcomposites with respect to stiffness, strength, and ductility according to dynamic mechanical analysis and static tensile tests. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Simultaneous Interpenetrating Polymer Networks of Polyurethane from Pentaerythritol–Modified Castor Oil and Polystyrene: Structure–Property Relationships

Castor oil was transesterified with pentaerythritol thereby introducing reactive hydroxyl groups. The method of matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy was used to determine the components of the castor oil pentaerythritol alcoholysis products. Simultaneous interpenetrating polymer networks of pentaerythritol modified castor oil and diphenyl-methane 4-4′ diisocyanate polyurethane (P1-CO-PU) and polystyrene (PS) were synthesized using benzoyl peroxide as the initiator and divinyl benzene as the crosslinker. The effect of PU/PS composition on the morphology, miscibility and physical properties of the resulting IPNs was investigated by scanning electron microscopy, dynamic mechanical thermal analysis, measurements of mechanical properties and resistance to chemical reagents. The patterns of the extent of the phase separation and the characteristics of the interpenetration reached as a function of the components were deduced.     

Preparation and characterization of poly(butadiene-stat-styrene)/poly(styrene-stat-acrylonitrile) structured latex particles

In rubber toughening of thermoplastics, core/shell polymers have been used extensively. This work introduces the synthesis and characterization of polybutadiene based core/shell latex particles with controlled particle size and crosslinking density of the core. A lithium soap recipe was employed to prepare a series of poly(butadiene-stat-styrene) (90/10 by wt) core particles by conventional emulsion polymerization through a batch process. The shell polymer, poly(styrene-stat-acrylonitrile) (72/28 by wt), was polymerized by a semicontinuous process in the presence of the core particles to form a core/shell morphology. The effects of initiator concentration, monomer feeding rate, core/shell ratio, and gel-fraction of the core on the core/shell particle morphology were studied. The degree of grafting of the shell polymer on the core particles was determined as well. The morphology and glass transitions of these particles were characterized by transmission electron microscopy, differential scanning calorimetry, and dynamic mechanical spectroscopy. These latex particles can be used specifically in toughening polycarbonate. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1123–1134, 1997     

Preparation and morphological characterization of two- and three-component natural rubber-based latex particles

Different emulsion polymerization processes allowed variation in the microstructure of composite natural rubber (NR)-based latex particles. A prevulcanized and a not-crosslinked natural rubber latex were coated with a shell of crosslinked poly(methyl methacrylate) (PMMA) or polystyrene (PS). The bipolar redox initiating system tert-butyl hydroperoxide/tetraethylene pentamine promoted a core–shell arrangement. Furthermore, PS subinclusions were introduced into the NR core. The initiators used for the subinclusion synthesis were azobisisobutyronitrile at high temperature and a redox initiation system consisting of tert-butyl hydroperoxide/dimethylaniline at low temperature. The morphology of the resulting latex interpenetrating networks (IPN) was characterized by transmission electron micros-copy (TEM) and scanning electron microscopy (SEM). Different staining methods allowed us to increase the contrast between the NR phase and the secondary polymers in the composite latex particles. A semicontinuous feeding process decreased the PS subinclusions size by a factor of 6 in comparison with a batch reaction. Depending on the NR/styrene swelling ratio, the crosslinking degree, and the polymerization temperature used, distinct differences of the phase arrangement of polymers in the latex particles were revealed. © 1996 John Wiley & Sons, Inc.     

表面含磺酸基的核-壳型乳胶粒子的制备与表征

为了合成表面含磺酸基的双亲型核-壳聚合物,本文采用两步乳液聚合法,第1步合成PS种子乳液;第2步用氧化还原引发体系在PS种子乳胶粒外包覆1层交联的聚苯乙烯磺酸酯,得到表面含磺酸酯的核-壳型聚苯乙烯(CPS)乳胶粒子。通过调节壳层单体的加入量,可以控制外壳层聚合物的质量分数(相对核层)在10%~30%之间。将外壳层的磺酸酯基水解转化成磺酸基得目标产物。用透射电子显微镜(TEM)、红外光谱(IR)、差热分析(DSC)和热失重分析(TGA)、X射线光电能谱(XPS)等方法对制备的乳胶粒子进行了测试和表征。     

STUDIES ON SYNTHESIS OF SILICA SOL-ORGANIC POLYMER COMPOSITE EMULSION

An emulsion of polystyrene/poly (butylacrylate-methyl methacrylate acrylic acid) core/shell latex particles (PS/P (BA-MMA-AA)) has been prepared by use of three synthetic methods. The effects of synthetic methods on the distribution of carboxyl groups in latex particles were studied. The results show that the seed emulsion polymerization in which the pre-emulsified monomers were added by dropping method to the second stage is the best technique for obtaining the optimum distribution of carboxyl groups on the surface of the latex particles. Furthermore, by using PS/P (BA-MMA-AA), a type of novel composite emulsion of silica sol-PS/P (BA-MMA-AA) was synthesized with the above method. By observation through transmission EM, the morphology of the latex particles obtained shows that a composite structure has been formed between silica sol particles and organic polymer particles.     

Carboxylic acid-functionalized, core-shell polystyrene@polypyrrole microspheres as platforms for the attachment of CdS nanoparticles

Polypyrrole-coated polystyrene latex particles bearing N-carboxyl functional groups (PS@PPyCOOH) were prepared by the in-situ copolymerization of pyrrole (Py) and the active carboxyl-functionalized pyrrole (PyCOOH) in the presence of 390 nm diameter-sized polystyrene (PS) latex particles. Uncoated PS particles were prepared by emulsion polymerization of styrene. The initial comonomer fractions (in mol%) were 25/75, 50/50, 75/25 and 100/0 for pyrrole and PyCOOH, respectively. The PS@PPyCOOH_x particles, where x stands for the initial molar fraction of PyCOOH (x = 0, 25, 50 or 75%), were characterized in terms of particle size, surface morphology, chemical composition and electrochemical redox activity using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), FTIR, TGA and cyclic voltammetry respectively. TEM showed an increase of the latex particle diameter after coating by the conducting polymer layer, from 390 nm for uncoated PS to 430 nm for PS@PPyCOOH₅₀ particles, allowing an estimation of the PPyCOOH shell thickness to 20 nm. FTIR and XPS detected PyCOOH repeat units at the surface of the latex particles, indicating that this monomer had indeed copolymerized with pyrrole. The core-shell structure of the PS@PPyCOOH_x particles was confirmed by etching the polystyrene core in THF, leading to the formation of hollow conducting polymer capsules. Positively charged CdS nanoparticles were electrostatically assembled onto the surface of PS@PPyCOOH₅₀ particles, as a function of pH. It was found that, contrarily to unfunctionalized PPy-coated latex particles, PS@PPyCOOH₅₀ particles could be evenly decorated with stabilized CdS nanoparticles, at pH 5.The films of the PS@PPyCOOH@CdS-coated ITO electrodes are shown to be electroactive and electrochemically stable.     

Monodisperse ellipsoidal polystyrene latex particles: Preparation and characterisation

An improved version of a novel method first employed by Nagy & Keller for the preparation of monodisperse ellipsoidal polystyrene latex particles is described. The method involves embedding monodisperse spherical polystyrene latex particles as starting material in a deformable polymer matrix such as poly(vinyl alcohol) and deforming these mechanically to various predetermined macroscopic draw ratios to give ellipsoids of various axial ratios. Ellipsoids with lengths ranging from about 350 to 12250nm were prepared. Stable aqueous dispersions of these were recovered and characterised with respect to particle size and axial ratio distributions and surface morphology using electron microscopy. Axial ratios ranging from 2.0 to 5.65 were obtained for the resulting ellipsoids. Various factors influencing the monodispersity of the resulting ellipsoids in the preparation method are discussed.     

Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology

Conductive multiwall carbon nanotube/polystyrene (MWCNT/PS) composites are prepared based on latex technology. MWCNTs are first dispersed in aqueous solution of sodium dodecyl sulfate (SDS) driven by sonication and then mixed with different amounts of PS latex. From these mixtures MWCNT/PS composites were prepared by freeze-drying and compression molding. The dispersion of MWCNTs in aqueous SDS solution and in the PS matrix is monitored by UV–vis, transmission electron microscopy, electron tomography and scanning electron microscopy. When applying adequate preparation conditions, MWCNTs are well dispersed and homogeneously incorporated in the PS matrix. The percolation threshold for conduction is about 1.5 wt% of MWCNTs in the composites, and a maximum conductivity of about 1 S m⁻¹ can be achieved. The approach presented can be adapted to other MWCNT/polymer latex systems.