Structured latex particles as impact modifiers for poly(styrene–co-acrylonitrile) blends

This work was focused on the influence of the morphology of composite natural rubber (NR)-based particles on the toughness of poly(styrene–co-acrylonitrile) (PSAN) blends. In order to be suitable for the reinforcement of PSAN blends, the NR-based particles were coated with a shell of crosslinked poly(methylmethacrylate) (PMMA). Furthermore, polystyrene (PS) subinclusions were introduced into the NR rubber core. PSAN blends were prepared by adding the wet latex directly into a twin screw-extruder. This new method allowed even tacky pure rubber particles to be dispersed as shown by transmission electron photomicrographs which confirmed the integrity of the soft particles after mixing. Solid NR particles or NR-based latex particles containing rigid PS subinclusions and no hard shell did not offer any impact improvement to PSAN. Only NR-based core–shell particles containing at least 25% PMMA in the shell toughened the brittle matrix. Prevulcanized NR-based latex particles which do not cavitate easily were less effective. Core–shell particles containing PS subinclusions within a natural rubber core allowed more effective use of the rubber phase. From the fracture surface morphology the failure mechanisms of PSAN blends containing the different composite NR particles could be deduced. Monodisperse poly(n-butylacrylate)-based core–shell particles were too small to toughen PSAN. However, a similar dependence of the fracture mechanisms on the morphology of the incorporated toughening agent could be established by scanning electron microscopy.     

Morphology and grafting reactions in core/shell latexes

The particle morphology and percent grafting were investigated as a function of the crosslink density of the seed latex in two systems of core/shell latexes of polybutadiene/polymethyl methacrylate (PB/PMMA) and styrene–butadiene rubber/polymethyl methacrylate (SBR/PMMA) prepared by seeded emulsion polymerization at 50°C. The thin layer chromatography/flame ionization detection (TLC/FID) technique was used to characterize the grafting efficiency of the core/shell latexes. The percent grafting of the shell polymer was found to decrease with increasing the crosslink density of the core material. The particle morphology and precent grafting were also investigated as a function of composition and structure of the core material in four core/shell latex systems: polybutadiene/styrene–acrylonitrile copolymer (PB/SAN), (styrene-butadiene) random copolymer/styrene acrylonitrile copolymer (S:B/SAN), polystyrene : polybutadiene/styrene-acrylonitrile copolymer (PS:PB/SAN) and Kraton/styrene-acrylonitrile copolymer (Kraton/SAN), which were prepared by direct emulsification for the seed followed by emulsion polymerization at 70°C for the shell polymer. Grafting and crosslinking of the core material were found to be competitive reactions depending on the microstructure of the seed latex.     

Effects of methyl methacrylate grafting and in situ silica particle formation on the morphology and mechanical properties of natural rubber composite films

The effects of methyl methacrylate (MMA) grafting and in situ formation of silica particles on the morphology and mechanical properties of natural rubber latex (NRL) were investigated. MMA grafting on NRL was carried out using cumyl hydroxy peroxide/tetraethylene pentamine (CHPO/TEPA) as a redox initiator couple. The grafting efficiency of the grafted NR was determined by solvent extractions and the grafted NRL was then mixed with tetraethoxysilane (TEOS), a precursor of silica, coated by adherence to a glass surface to form a film and cured at 80°C. The resultant products were characterized by FT‐IR and transmission electron microscopy. The influence of varying the MMA monomer weight ratio on the surface morphology of the composites was investigated by scanning electron and atomic force microscopy. The PMMA (poly MMA) grafted NRL particles were obtained as a core/shell structure from which the NR particles were the core seed and PMMA was a shell layer. The silane was converted into silica particles by a sol–gel process which was induced during film drying at 80°C. The silica particles were fairly evenly distributed in the ungrafted NR matrix but were agglomerated in the grafted NR matrix. The root‐mean‐square roughness increased with an increasing weight ratio of MMA in the rubber. The in situ silica particles in the grafted NR matrix slightly increased both the modulus and the tear strength of the composite film. Copyright © 2008 John Wiley & Sons, Ltd.     

Relationship between structure and rheological properties in the melt of polymers containing spherical inclusions

The linear viscoelastic behavior of model rubbertoughened polymer melts has been studied. The most significant influence of the dispersed crosslinked rubber phase on the melt rheology of the blends is the existence of a secondary plateau for the storage modulus G′ at low frequencies. This behavior was ascribed to a percolation phenomenon, leading to the formation of a threedimensional network of inclusions, and contributing to the elasticity at low frequencies of the blend. Two different systems were investigated: (a) a polystyrene matrix with crosslinked and structured latex particles and (b) silicon oil matrices with homogeneous crosslinked PMMA particles. An initial shearing history was found to influence the dynamic mechanical properties of the molten blends and in particular to lower the lowfrequency plateau value for G′. During a subsequent annealing, the plateau modulus increases again. These results are in agreement with the assumption of a particle network.     

Latex interpenetrating polymer networks: From structure to properties

Latex interpenetrating and semi-interpenetrating polymer networks (LIPNs and semi-LIPNs) combine the morphological characteristics of bulk-polymerized IPNs with the characteristics of polymers produced by emulsion polymerization; there are IPN structures within the latex particles. These LIPNs can be injection-molded using standard thermoplastic methods and machinery. A dual thermoset—thermoplastic nature characterizing the LIPN manifests itself in the mechanical and rheological behavior reflecting unique morphologies. These morphologies result from a sequential two-stage latex (TSL) polymerization and include core—shell, domain, interpenetrating polymer networks and various other combinations. Elastomeric TSL with crosslinked polyacrylates (xPA) as the first stage and crosslinked polystyrene (xPS) as the second, each stage lightly crosslinked, yield IPN-nano-domain structural particles. Upon molding, the particles become interconnected by joint PS nanodomains, introducing a particle—particle strength-forming mechanism. The intraparticle glassy PS nanodomains reinforce the soft elastomeric particles enhancing their modulus. Glassy “all-styrene” semi-LIPNs made of PS and xPS show improved mechanical performance compared to PS, while exhibiting good transparency. Volumetric crazing in these PS/xPS materials develops in tension-improving elongation and strength. The presence of xPS particles, denser and thus stiffer than the PS matrix, renders a higher modulus. Essentially xPS highly filled blends are achieved along with significant particle—matrix interactions. The ability to generate a controlled plethora of morphologies offers a wealth of potential applications, from reinforced elastomers to high impact plastics. Poly(acrylonitrile—butadiene—styrene), a semi-LIPN, is a commodity plastic, clearly demonstrating the utilization potential of the TSL procedure for generating very fine multiphase materials of scientific and technological merits.     

Composite natural rubber–polychloroprene latex particles produced by the heterocoagulation technique

Composite natural rubber (NR) based latex particles were prepared using the heterocoagulation technique. A nonionic surfactant (Tween 80) whose molecules bear poly(ethylene oxide) (PEO) was adsorbed on polychloroprene (CR) latex particles and allowed to form complexes between PEO and indigenous surfactant (protein–lipid) on the NR particle surface. The heterocoagulated NR/CR–Tween particles produced were characterised by particle size, zeta-potential and glass-transition temperature measurements and the data indicated the presence of CR–Tween on the outer layer of the composite polymer particles. The results agreed well with the better oil resistance of films cast from heterocoagulated latex when compared with that of the NR film. Received: 22 August 2000 Accepted: 8 January 2001     

Effects of surfactants on the morphology of composite polymer particles produced by two‐stage seeded emulsion polymerization

Poly(methyl methacrylate) (PMMA)–polystyrene (PS) composite polymer particles were synthesized in the presence of a surfactant by two‐stage seeded emulsion polymerization. The first stage was the synthesis of PMMA particles by soapless emulsion polymerization; the second stage was the synthesis of the PMMA–PS composite polymer particles with the PMMA particles as seeds. In the second stage of the reaction, three kinds of surfactants—sodium laurate sulfate (SLS), polyoxyethylene (POE) sorbitan monolaurate (Tween 20), and sorbitan monolaurate (Span 20)—were used to synthesize the PMMA–PS composite particles. Both the properties and concentrations of the surfactants influenced the morphology of the composite particles significantly. Core–shell composite particles, with PS as the shell and PMMA as the core, were synthesized in the presence of a low concentration of the hydrophilic surfactant SLS. This result was the same as that in the absence of the surfactant. However, a low concentration of Tween 20 led to composite particles with a core/strawberry‐like shell morphology; the core region was a PS phase, and the strawberry‐like shell was a PS phase dispersed in a PMMA phase. With an increase in the concentration of SLS, the morphology of the composite particles changed from core (PMMA)–shell (PS) to core (PS)–shell (PMMA). Moreover, the effects of a high concentration of Tween 20 or Span 20 on the morphology of the PMMA–PS composite particles were investigated in this study. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2224–2236, 2005     

Morphology and mechanical and viscoelastic properties of natural rubber and styrene butadiene rubber latex blends

The morphology and mechanical and viscoelastic properties of a series of blends of natural rubber (NR) and styrene butadiene rubber (SBR) latex blends were studied in the uncrosslinked and crosslinked state. The morphology of the NR/SBR blends was analyzed using a scanning electron microscope. The morphology of the blends indicated a two phase structure in which SBR is dispersed as domains in the continuous NR matrix when its content is less than 50%. A cocontinuous morphology was obtained at a 50/50 NR/SBR ratio and phase inversion was seen beyond 50% SBR when NR formed the dispersed phase. The mechanical properties of the blends were studied with special reference to the effect of the blend ratio, surface active agents, vulcanizing system, and time for prevulcanization. As the NR content and time of prevulcanization increased, the mechanical properties such as the tensile strength, modulus, elongation at break, and hardness increased. This was due to the increased degree of crosslinking that leads to the strengthening of the 3‐dimensional network. In most cases the tear strength values increased as the prevulcanization time increased. The mechanical data were compared with theoretical predictions. The effects of the blend ratio and prevulcanization on the dynamic mechanical properties of the blends were investigated at different temperatures and frequencies. All the blends showed two distinct glass‐transition temperatures, indicating that the system is immiscible. It was also found that the glass‐transition temperatures of vulcanized blends are higher than those of unvulcanized blends. The time–temperature superposition and Cole–Cole analysis were made to understand the phase behavior of the blends. The tensile and tear fracture surfaces were examined by a scanning electron microscope to gain an insight into the failure mechanism. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2189–2211, 2000     

Blending of Natural Rubber/Recycled Ethylene-Propylene-diene Rubber: Promoting the Interfacial Adhesion Between Phases by Natural Rubber Latex

This article deals with blends based on natural rubber (NR) and recycled ethylene-propylene-diene rubber (R-EPDM). Natural rubber latex (NRL) was introduced into the blends to enhance interfacial adhesion between NR and R-EPDM. A new route of compounding was also suggested. The blends were prepared by mixing R-EPDM and other additives in NRL before blending with natural rubber on a two-roll mill. By applying this method, the homogeneity of the blends and cross-linking distribution are significantly improved. The blends exhibited superior state of cure, swelling resistance, mechanical properties and dynamic mechanical properties. The degree of entanglement between NR and R-EPDM also increased after NRL modification.     

Surface modification of polystyrene latex particles via atom transfer radical polymerization

The atom transfer radical polymerization (ATRP) technique using the copper halide/ N,N′,N′,N″,N″‐pentamethyldiethylenetriamine complex was applied to the graft polymerization of methyl methacrylate and methyl acrylate on the uniform polystyrene (PS) seed particles and formed novel core‐shell particles. The core was submicron crosslinked PS particles that were prepared via emulsifier‐free emulsion polymerization. The crosslinked PS particles obtained were transferred into the organic phase (tetrahydrofuran), and surface modification using the chloromethylation method was performed. Then, the modified seed PS particles were used to initiate ATRP to prepare a controlled poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA) shell. The final core‐shell particles were characterized using Fourier transform infrared spectroscopy, nuclear magnetic resonance, scanning electron microscopy, thermogravimetric analysis, and elementary analysis. The grafting polymerization was conducted successfully on the surface of modified crosslinked PS particles, and the shell thickness and weight ratio (PMMA and PMA) of the particles were calculated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 892–900, 2002; DOI 10.1002/pola.10160     

Synthesis of styrene‐g‐polyisoprene nanoparticles by emulsion polymerization and its effect on properties of polyisoprene composites

The graft copolymerization of styrene onto nanosized polyisoprene (PIP) was carried out by using cumene hydroperoxide and tetraethylene pentamine as redox initiators. The high conversion and high degree of grafting could be achieved when a small particle was used as the core polymer. The grafting efficiency and monomer conversion increased with increasing reaction temperature and monomer concentration. Transmission electron microscopy indicated that the small PIP nanoparticles were completely coated with polystyrene (PS) by grafting resulting in a core shell morphology of nanosized graft PIP. Nanosized PIP and nanosized PS‐g‐PIP could be used as compatibilizers for vulcanized rubber latex. The addition of nanosized PIP and PS‐g‐PIP strongly influenced the mechanical properties of the natural rubber (NR)‐based compound. Incorporation of nanosized PIP and PS‐g‐PIP resulted in an improvement of the resistance of the compounds to heat aging. Copyright © 2011 John Wiley & Sons, Ltd.     

Radiation-induced grafting polymerization of MMA onto polybutadiene rubber latex

The grafting of methyl methacrylate (MMA) onto polybutadiene rubber latex by the direct radiation method was carried out. The effects of monomer concentration, absorbed dose and dose rate of gamma rays on the grafting yield were investigated. The graft copolymers were characterized by transmission electron microscopy (TEM), FTIR spectroscopy, and differential scanning calorimetry. TEM photographs revealed that the core–shell structures of latex particles are formed at low MMA content, and with the increasing of MMA content, the semi-IPN-like structure with core–shell could be developed due to the high gel fraction of polybutadiene (PBD) seed particles. In addition, infrared analysis confirmed that MMA could be grafted onto PBD molecular chains effectively under appropriate irradiation conditions. The interfacial adhesion between PBD rubber (core) and PMMA (shell) phases could be enhanced with the increase of MMA concentration.     

Semiconductor nanoparticle/polystyrene latex composite materials

Cadmium sulfide and cadmium selenide/cadmium sulfide core/shell nanoparticles stabilized with poly(cysteine acrylamide) have been bound to polystyrene (PS) latexes by three methods. First, anionic 5 nm diameter CdS particles were electrostatically attached to 130 nm surfactant-free cationic PS latexes to form stable dispersions when the amount of CdS particles was less than 10% of the amount required to form a monolayer on the surface of the PS particles or when the amount of CdS particles exceeded the amount required to form a monolayer on the PS particles. Transmission electron microscopy (TEM) showed nanoparticles on the surface of the latex particles. Fluorescence spectra showed unchanged emission from the nanoparticles. Second, anionic, surfactant-free PS latexes were synthesized in the presence of CdS and CdSe/CdS nanoparticles. TEM showed monodisperse latex particles with trapped nanoparticles. Third, surfactant-stabilized latexes were synthesized by copolymerization of styrene with vinylbenzyl(trimethyl)ammonium chloride electrostatically bound to the CdSe/CdS nanoparticle surface. Brownian motion of the submicroscopic composite particles in water was detected by fluorescence microscopy.     

悬浮-乳液复合聚合聚苯乙烯-聚甲基丙烯酸甲酯复合粒子的颗粒特性和成粒机理

采用在苯乙烯 (St)悬浮聚合过程中滴加甲基丙烯酸甲酯 (MMA)乳液聚合组分的悬浮 乳液复合聚合方法 ,制备大粒径聚苯乙烯 聚甲基丙烯酸甲酯 (PS PMMA)复合粒子 .研究聚合物粒径分布和颗粒形态的变化发现 ,在St悬浮反应中期滴加MMA乳液聚合组分后 ,聚合体系逐渐由悬浮粒子与乳胶粒子并存向形成单峰分布复合粒子转变 ,最终形成核 壳结构完整的大粒径PS PMMA复合粒子 ;在St悬浮反应初期滴加MMA乳液聚合组分 ,St与MMA一起分散成更小液滴 ,反应后期凝并成非核 壳结构复合粒子 ;在St悬浮反应后期滴加MMA乳液聚合组分 ,PMMA乳胶粒子与PS悬浮粒子基本独立存在 .根据以上结果 ,提出了St MMA悬浮 乳液复合聚合的成粒机理 .     

Designs of latex particles' morphologies for toughening polymer matrices

The performance of the designed structured core/shell latex particles in toughening polycarbonate (PC) matrix was examined. The influence of parameters related to the core/shell latex particles on toughening PC such as: the particle size and levels of crosslinking of the core rubber particles, composition and molecular-weight of the shell polymer, the weight ratio of shell and core polymers as well as the particle morphology was evaluated. Core/shell structured latex particles with thinner shells of higher molecular-weight polymers were found to improve the impact resistance of PC. The role of chain entanglements in increased adhesion between the discrete rubbery phase and the continuous glass matrix are discussed.     

Prevulcanisation of skim latex: morphology and its use in natural rubber based composite material

The prevulcanisation of skim latex, a by-product of field natural rubber (NR) latex concentrated by centrifugation, using sulphur and peroxide systems was investigated. Compared to the peroxide prevulcanisation, the lower swelling ratio of film casted from sulphur-prevulcanised skim (SPVS) latex was observed. The latter was then employed for preparation of NR/SPVS core–shell particles by using heterocoagulation technique whose driving force was an interpolymer complex between poly(ethylene oxide) (PEO) moieties of a non-ionic surfactant (Nonidet) adsorbed on small SPVS particles and the indigenous surfactant (protein–lipid) on a large NR particle. The value of zeta potential of heterocoagulated particle and the better oil resistance of films casted from the composite latex when compared to that of the NR film revealed the NR/SPVS core–shell structure.     

反应性复合乳液的合成、表征及其交联反应

利用种子半连续乳液聚合方法合成了核层或壳层带有环氧基以及壳层带有羧基的3种不同核/壳结构的乳胶粒子,通过物理共混带环氧基和羧基的乳胶粒子,得到了两种反应性复合乳液.利用透射电镜和激光动态光散射对乳胶粒子进行了表征,其粒径分布较窄,粒径分布的多分散系数为0.062,平均粒径约76 nm,乳胶粒子具有明显的核/壳结构.通过胶膜的凝胶率和膨胀率的测定和红外光谱分析对反应性复合乳液中乳胶粒子的扩散及交联反应进行了研究,并探讨了不同核壳结构复合乳液对涂膜机械性能的影响.研究表明,当反应性复合乳液中的环氧基和羧基分别分布在乳胶粒子的核层和壳层时,有利于聚合物分子链的充分扩散和化学交联反应的进行,从而提高涂膜的物理化学性能,当甲基丙烯酸缩水甘油酯(GMA)含量为10 wt%时,涂膜的拉伸强度达20.3 MPa.     

Effect of introduction mode of hydroxyl functionality on morphology and film properties of cycloaliphatic diepoxide crosslinkable core‐shell latex

Three series of core‐shell hydroxyl‐functionalized latexes were synthesized and then crosslinked with a cycloaliphatic diepoxide. The same amount of hydroxyl functional monomer was added during the core stage, shell stage, or partitioned equally between the core and the shell. The morphology of the latexes was examined with transmission electron microscopy and contact‐angle measurement. The stress‐strain behavior, viscoelastic properties, and water adsorption were evaluated for the latex films as a function of hydroxyl location. The location of hydroxyl groups within latex particles appeared to be dependent on the introduction mode of hydroxyl functional monomers. The introduction of hydroxyl groups during the shell polymerization resulted in a higher crosslinking density but a lower Tukon hardness and tensile properties. Not surprisingly, distribution of hydroxyl groups in both core and shell polymerization provided the lowest water adsorption and impact resistance as well as the highest tensile elongation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4256–4265, 2002     

Rheological,mechanical and morphological properties of thermoplastic vulcanizates based on NR‐g‐PMMA/PMMA blends

Graft copolymer of natural rubber and poly(methyl methacrylate) (NR‐g‐PMMA) was prepared using semi‐batch emulsion polymerization technique via bipolar redox initiation system. It was found that the grafted PMMA increased with the increase of methyl methacrylate (MMA) concentration used in the graft copolymerization. The NR‐g‐PMMA was later used to prepare thermoplastic vulcanizates (TPVs) by blending with PMMA through dynamic vulcanization technique. Conventional vulcanization (CV) and efficient sulphur vulcanization (EV) systems were studied. It was found that the CV system provided polymer melt with lower shear stress and viscosity at a given shear rate. This causes ease of processability of the TPVs via extrusion and injection molding processes. Furthermore, the TPVs with the CV system showed higher ultimate tensile strength and elongation. The results correspond to the morphological properties of the TPVs. That is, finer dispersion of the small vulcanized rubber particles were observed in the PMMA matrix. Various blend ratios of the NR‐g‐PMMA/PMMA blends using various types of NR‐g‐PMMA (i.e. prepared using various percentage molar ratios of NR and MMA) were later studied via dynamic vulcanization by a conventional sulphur vulcanization system. It was found that increasing the level of PMMA caused increasing trend of the tensile strength and hardness properties but decreasing level of elongation properties. Increasing level of the grafted PMMA in NR molecules showed the same trend of mechanical properties as in the case of increasing concentration of PMMA used as a blend component. From morphological studies, two phase morphologies were observed with a continuous PMMA phase and dispersed elastomeric phase. It was also found that more finely dispersed elastomeric phase was obtained with increasing the grafted PMMA in the NR molecules. Copyright © 2005 John Wiley & Sons, Ltd.     

Reactive blends of thermoplastics and latex particles

Monodisperse homogeneous and core–shell latex particles of various sized between 200 and 600 nm were synthesized by emulsion copolymerization. Some of the core–shell particles were functionalized with epoxy groups at their peripheries upon introduction of glycidyl methacrylate (GMA) during the synthesis. The core consisted of crosslinked polybutylacrylate and the shell polymethylmethacrylate. Synthesis conditions at high and low temperatures were optimized to obtain coreshell particles with a well-defined morphology. The particles were characterized by quasi-elastic light scattering, scanning electron microscopy and transmission electron microscopy. The latex particles functionalized with GMA were then dispersed into a reactive matrix (styrene and maleic anhydride copolymer) using a batch mixer to obtain blends with well-defined and stabilized morphology. 4 Dimethylaminopyridine was used as a catalyst. The reaction between the epoxy groups at the particle surface and the maleic anhydride or diacid groups of the matrix was evaluated by torque and extraction techniques. A small amount of conversion generates sufficient amounts of grafted species at the matrix and particle interfaces to ensure a good interfacial adhesion.