Toughening of polystyrene by natural rubber-based composite particles: Part II Influence of the internal structure of PMMA and PS-grafted core-shell particles

This work was focused on the influence of the internal structure of natural rubber (NR)-based core-shell particles on the toughness of polystyrene (PS). Several emulsion polymerization processes were used to control the degree of grafting of the NR phase and the site of polymerization, which determines the final morphology of the prepared composite NR-based particles. PS subinclusions were introduced into the NR core in order to determine their influence on the deformation behaviour of PS. A continuous extrusion process was adapted for the direct feeding of the wet NR-based latexes into the molten PS matrix. Impact testing indicated that core-shell particles based on NR containing a large number of small crosslinked PS subinclusions toughened PS most effectively. A very effective toughening agent is obtained if a hard shell of 25 wt% crosslinked PMMA surrounds the composite rubber particle. Grafting of NR chains during the subinclusion synthesis has to be avoided since a high rubber particle modulus is detrimental for craze nucleation in PS. From the fracture surface morphology the craze nucleating and stabilizing efficiency of composite NR particles having different morphologies or grafting degrees could be deduced. This revised version was published online in November 2006 with corrections to the Cover Date.     

Toughening of polystyrene by natural rubber-based composite particles: Part III Fracture mechanisms

Impact testing has allowed the toughness of PS blends to be correlated with the morphology of the dispersed rubber phase, which was a natural rubber (NR) in particle form, coated with a shell of polystyrene (PS) or polymethylmethacrylate (PMMA). PS subinclusions were also introduced into the NR core. The impact resistance of the prepared PS blends began to rise steeply at a particle content of about 18 wt %. Transmission electron microscopy (TEM) in combination with osmium tetroxide staining techniques, allowed direct analysis of the crazing and cavitation processes in the composite natural rubber particle-toughened PS blends. Bulk samples were studied at high and slow deformation speeds. Different deformation mechanisms were effective, depending on the location of the observed stress-whitened zone relative to the notch tip. The apparent fracture mechanisms in rubber-toughened PS blends were also studied by scanning electron microscopy. PS blends containing polydisperse natural rubber-based particles or monodisperse poly(n-butylacrylate)-based particles, and commercial high-impact polystyrene, were compared. This revised version was published online in November 2006 with corrections to the Cover Date.     

Internal structure of rubber particles and craze break-down in high-impact polystyrene (HIPS)

Polystyrene can be substantially toughened by the addition of rubber particles, their role being to act as craze initiators permitting substantial plastic deformation to occur prior to fracture. The internal structure of these particles is variable: typically the smaller (1 m) particles are solid rubber and the larger particles contain sub-inclusions of polystyrene. Thin films of a toughened high-impact polystyrene (HIPS) suitable for optical and transmission electron microscopy (TEM) have been prepared, and the interplay between the internal structure of the particles and the crazes they generate has been examined by TEM. It is found that as crazes form around the solid rubber particles, significant lateral contraction occurs accompanying their elongation in the tensile direction. As this contraction proceeds, decohesion occurs just beneath the particlecraze interface, resulting in the formation of a void. This void will grow under increasing stress, leading to premature failure of the craze. In contrast to this behaviour, occluded particles can accommodate the displacements due to crazing by local fibrillation of the rubber shell which surrounds each sub-inclusion, without the formation of large voids. Consequently, the occluded particles do not act as sites for early craze break-down. These results suggest that the optimum morphology for rubber particles in HIPS will consist of a large number of small PS occlusions, each surrounded by a thin layer of rubber, in which case the size of the inherent flaws introduced during crazing will be minimized.     

核壳式纳米Al2O3/PS复合粒子的表征及增韧选区激光烧结聚苯乙烯的研究

运用TEM,FTIR对乳液聚合方法制备纳米Al2O3/PS复合粒子结构进行了表征,结果表明,制备出的复合粒子具备以纳米氧化铝为核、以聚苯乙烯为壳的核壳式结构;并将核壳式复合粒子用来增韧选区激光烧结聚苯乙烯,结果发现,其缺口冲击强度达到12.1kJ/m2,较纯聚苯乙烯提高了50%左右,比添加未经任何改性处理纳米氧化铝粒子的复合材料提高了30%;利用FE-SEM对试件的冲击断面进行了微观结构分析,结果表明:核壳式纳米Al2O3/PS复合粒子改善了纳米粒子与基体表面极性的差异,增强了其与聚合物基体之间的界面相容性,从而改性了选区激光烧结制备聚苯乙烯基复合材料,并很好地起到增韧的效果.     

亚微米核壳橡胶粒子改性聚苯乙烯 ——接枝度对形态结构及力学性能的影响

通过乳液接枝聚合技术合成不同聚丁二烯(PB)含量的聚丁二烯接枝聚苯乙烯(PB-g-PS)共聚物, 共混改性聚苯乙烯(PS), 使PB含量控制在20wt％。通过对接枝共聚物PB-g-PS接枝度的测定, 分析了不同接枝度对橡胶粒子分散状态以及橡胶粒子内部结构的影响, 考察了不同接枝度对改性聚苯乙烯力学性能的影响。通常认为橡胶粒子只有在1~3μm时才能对聚苯乙烯进行有效增韧, 然而实验结果表明, 使用粒径约为300nm的核壳橡胶粒子改性聚苯乙烯冲击韧性提高很大。此时, 接枝共聚物具有最佳核壳比PB/PS为70/30, 橡胶粒子呈均匀分散状态, 且最大冲击强度为124.9J/m, 相当于纯聚苯乙烯冲击强度的10倍。      

炭化聚甲基丙烯酸甲酯／聚丙烯腈核壳聚合物制备炭纳米空心球

采用炭化聚甲基丙烯酸甲酯(PMMA)/聚丙烯腈(PAN)核壳聚合物的方法制备了炭纳米空心球.以两步无皂乳液聚合法制备了PMMA/PAN核壳粒子:首先以间歇无皂乳聚合法制备出直径约200 nm的PMMA粒子乳液,再以其作为种子乳液,以饥饿滴定法在PMMA外表而聚合一层厚度约30 nm的PAN外壳.将制备的PMMA/PAN乳液冷冻干燥后,分别经过250℃预氧化及1000℃炭化工艺,制备了炭纳米空心球.透射电镜结果显示所有核壳粒子均炭化成空心球并呈现交联状态.     

Microdeformation mechanisms in rubber toughened PMMA and PMMA-based copolymers

The classical method of toughening polymethylmethacrylate (PMMA) is to incorporate composite rubber particles into the homopolymer matrix. This approach has been extended in the present work by (i) combining rubber toughening with chemical modification of the matrix or (ii) introduction of the rubber modifier via PMMA-b-polybutylacrylate (PBA)-b-PMMA triblock copolymers. Significant improvements in fracture toughness at low speeds were observed in notched compact tension tests when the ductility of the rubber toughened PMMA matrix was improved by copolymerization, and comparable levels of toughness were achieved in the block copolymers. However, both types of material showed a transition to more brittle behavior at impact speeds, associated with increased localization of the crack tip deformation. Physical interpretations for this behavior and the scope for further optimization of the fracture response are discussed.     

Impact modification of SAN using NR-g-SAN copolymers

The paper demonstrates the efficacy of natural rubber-g-poly (styrene-co-acrylonitrile) (NR-g-SAN) copolymers as impact modifier for SAN. The impact behaviour of SAN /NR-g-SAN blends were studied as a function of cross-link density of NR, percent grafting, rubber content in the blend and the AN content in the grafted chain. The cross-link density of NR had no significant effect on impact strength of the blends whereas increase in percent grafting (PG) of the rubber significantly improved the impact strength. Thus the impact strength increased four times when the PG of the rubber was increased from 34.2 to 65%. Further increase in PG decreased the impact strength. Similar trend was observed on increase in the rubber content, the maximum impact strength was observed at 20% rubber. The impact strength also depended on the acrylonitrile content of the grafted chain. Tensile and flexural strength and modulus of these blends were not influenced by PG and AN content in the grafted chain whereas these properties decreased with the increase in the rubber content. Scanning electron microscopic studies of the impact fractured surfaces showed cavitation of rubber particles and craze induced matrix deformation. Dynamic mechanical studies confirmed the two-phase structure of these blends.     

Correlation between craze formation and mechanical behaviour of amorphous polymers

The formation, growth and fracture of crazes have been studied for several amorphous polymers (PS, PMMA, SAN, NEC, PC). From bulk polymeric materials 0.5 to 5m thick sections were prepared and investigated after uniaxial deformation or duringin situ deformation in a 1000 kV high voltage electron microscope (HVEM). The use of the HVEM also allows one to study irradiation-sensitive polymers (PMMA and PC). The size of crazes (length and thickness), the shape (opening angle at the craze tip, craze thickness profile), the thickness of a pre-craze zone, the structure of the material inside the craze (fibrillar or more homogeneous), and the degree of deformation were measured. Correlations have been found between the type and the size of crazes and their mechanical properties, particularly fracture toughness and elongation at break. There are notable differences between unannealed and annealed samples (SAN and PC) as well as in the craze formation and in the fracture toughness.     

Mechanisms of cavitation over a range of temperatures in rubber-toughened PSAN modified with three-stage core-shell particles

In situ straining on a transmission electron microscope (TEM) stage has been used to study deformation mechanisms in a blend of poly(styrene-co-acrylonitrile) (PSAN) with PMMA/acrylate-rubber/PMMA core-shell particles, over a range of temperatures. Thin sections were tested at -20, 23 and 60°C at a constant tensile strain rate of 0.05% s-1. Cavitation was observed at all three temperatures. At -20°C, the main deformation mechanisms were crazing of the PSAN matrix and fibrillation of the acrylate rubber. At 23°C, crazing and shear yielding of the PSAN occurred simultaneously, with more extensive fibrillation of the rubber particles and drawing of material from the PMMA cores. This disruption of thermoplastic core material indicates that high stresses are generated within the modifier particles. At 60°C, crazing could no longer be detected: shear yielding of the matrix and cavitation of the rubber particles were the main mechanisms of deformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

用核壳型聚合物粒子增韧改性环氧树脂

介绍了环氧树脂增韧改性的新方法,即用橡胶弹性体、热塑性树脂、刚性粒子和核壳型结构聚合物来增韧环氧树脂。采用种子乳液聚合法制备出了聚丙烯酸丁酯/聚甲基丙烯酸甲酯(PBA/PMMA)核壳型聚合物粒子,并对其表观形貌及结构进行了SEM和FTIR分析。将所制备的核壳型聚合物粒子增韧改性环氧树脂,当用量仅为环氧树脂用量2％时,冲击强度有明显提高。     

Preparation of polystyrene–polyisoprene core–shell nanoparticles for reinforcement of elastomers

Latex-formed core–shell nanoparticles composed of cross-linked polystyrene (PS) core and polyisoprene (PI) shell were successfully synthesized by means of a two-stage emulsion polymerization. The PS core possessed a Z-average diameter of 50.3 nm, and the PS–PI particles took a spherical shape with a Z-average size of 50–70 nm in diameter. Shell thickness was controlled by varying isoprene loading. Necessary interphase interactions between the core and shell domains were also achieved by grafting and swelling polymerization. Latex compounding method was employed to prepare the filled elastomer compounds. As expected, the PS–PI core–shell nanoparticles exhibited excellent reinforcement to elastomeric matrix, enhancing the tensile strength of the styrene–butadiene rubber by approximately 400%. The lower density, better interfacial interactions, and latex compounding process would benefit the PS–PI nanoparticles reinforced elastomer nanocomposites in energy saving.     

高岭土/白炭黑并用填充天然橡胶复合材料的性能

以高岭土和白炭黑作为增强剂,采用熔融共混法制备了一系列的天然橡胶(NR)复合材料,并对其微观结构、气体阻隔性能和力学性能进行了表征.结果表明,改性高岭土(MK)以片层结构,白炭黑(PS)以球状结构,均匀分散于NR基体之中;MK与PS单独填充NR时,随着MK填充量的增加,NR复合材料的气体阻隔性能逐渐提高,而且MK的增强...     

脆性塑料改性PVC体系中相界面的作用

将ＰＶＣ体系分别与ＰＶＣ／ＰＳ、ＰＶＣ／ＰＭＭＡ体系相对比，观察到两类不同体系的拉伸及冲击性能具有相当的一致性。认为在用脆性塑料改性ＰＶＣ时，分散相对基体间的界面起很重要的作用，实验表明具有核／壳结构的分散相粒子对ＰＶＣ的影响主要取决于分散相壳层与基体间界面的性质。     

乳液聚合法制备TiO2/PS复合粒子及其性能研究

采用乳液聚合法制备了以纳米二氧化钛(TiO2)为核,聚苯乙烯(PS)为壳的复合粒子.利用正交实验研究了引发剂用量、单体用量、表面活性剂浓度对复合粒子包覆效率的影响;通过透光率和亲水亲油实验,检验复合粒子的分散性,并采用透射电镜(TEM),红外光谱(FTIR)表征了复合粒子的形貌和结构.     

A study of polymer fracture surface features and their relationship to toughness

The aim of this study was to research fracture surface features for polymers of different toughness and type. The materials chosen provided for an interesting comparison of fracture surfaces. Two brittle amorphous thermoplastics (SAN & PMMA) of the same toughness had very different fracture surfaces. An amorphous thermoplastic (PC) exhibited similar features as both SAN and PMMA but had a higher toughness. Two semi-crystalline thermoplastics (PE1 & PE2) had similar fracture surface features but one was twice the toughness of the other. A rubber toughened polymer (ABS) showed a very different fracture surface to SAN (the host material) and all the other polymers studied. A particular interest was to use the comparison of the fracture surfaces of the above materials to investigate the toughening effects of rubber particles in ABS.     

The effect of dispersed paint particles on the mechanical properties of rubber toughened polypropylene composites

The influence of dispersed paint particles on the mechanical properties of rubber toughened PP was investigated. The matrix was basically a hybrid of PP, rubber and talc. Model systems with spherical glass bead filled matrix were also studied to examine the effect of filler shape and size. Properties like tensile strength, strain at break, impact strength, and fracture toughness were influenced by the dispersed inclusions. Tensile strength at yield decreased linearly according to Piggott and Leinder's equation. Strain at break decreased more drastically with paint particles than glass beads, revealing that irregularly shaped particles offered greater stress concentrations. The tensile strength and strain at break were less influenced by the size of paint particles whereas a slight decrease in the modulus values was observed with decreasing particle size. Impact strength and fracture toughness also decreased with increasing filler fraction. Lack of stress transfer between filler and matrix aided in reduction of impact strength. Decrease in fracture toughness was influenced by volume replacement and constraints posed by fillers. The size of paint particles had little effect on the impact strength and fracture properties at the filler concentration levels used in this investigation.     

Toughened carbon/epoxy composites made by using core/shell particles

Toughened epoxy resin composites have been prepared by resin-transfer moulding by using a range of toughening agents. Two types of epoxy-functional preformed toughening particles were investigated and have a three-layer morphology in which the inner core is crosslinked poly(methyl methacrylate), the intermediate layer is crosslinked poly(butyl acrylate) rubber and the outer layer is a poly[(methyl methacrylate)-co-(ethyl acrylate)-co-(glycidyl methacrylate)]. The presence of glycidyl groups in the outer layer facilitates chemical reaction with the matrix epoxy resin during curing. Comparisons were made with acrylic toughening particles that have a similar structure, but which do not have the epoxy functionality in the outer shell, and with a conventional carboxy-terminated butadiene acrylonitrile (CTBN) liquid rubber toughening agent. The composites were characterised by using tensile, compression and impact testing. The fracture surfaces and sections through the moulded composites were examined by means of optical and scanning electron microscopy. Short-beam shear tests and fragmentation tests were used to investigate the interfacial properties of the composites. In general, use of the epoxy-functionalised toughening particles gave rise to superior properties compared with both the non-functionalised acrylic toughening particles and CTBN.     

Elastic properties of rubber particles in toughened PMMA: ultrasonic and micromechanical evaluation

Ultrasonic measurements and micromechanical models are used to evaluate elastic properties of rubber particles dispersed in toughened polymers. Ultrasonic phase velocities and attenuation spectra of rubber-toughened poly(methyl methacrylate) (PMMA) with different rubber particle fractions are measured for longitudinal as well as transverse waves. The ultrasonic properties of rubber-toughened PMMA are found to depend markedly on the rubber particle fraction. The bulk and shear moduli determined from the measured velocities are in turn used to estimate those moduli of the particles based on existing micromechanics models, namely the three-phase model and the Hashin–Shtrikman upper and lower bounds. The bulk modulus of the particle estimated by the three-phase model is found to be in close agreement with the result of previous investigators. Implications of the Hashin–Shtrikman bounds for the particle moduli are also examined.     

Design of colored multilayered electrophoretic particles for electronic inks

The preparation of multilayered latex particles with surface functional groups suitable for use as electrophoretic particles in electronic inks has been studied. The particles are formed by dispersion polymerization and have a polystyrene core, slightly cross-linked with divinylbenzene (DVB), and a poly(methyl methacrylate) (PMMA) or a poly(acrylic acid) (PAA) shell. After grafting alkyl chains on their surface, the particles are negatively or positively charged and sterically stabilized against aggregation in nonpolar solvent. The particles were dyed by incorporation of Nigrosin during polymerization or by swelling in supercritical CO(2) in the presence of a dye. Particle size, morphology, incorporated dye content and zeta potential were determined. A dual-particle electronic ink based on a mixture of the colored multilayered particles and white hybrid TiO(2)-polymer particles was prepared and electro-optically tested.