Craze microstructure and molecular entanglements in polystyrene-poly(phenylene oxide) blends

Polystyrene and poly(phenylene oxide) are miscible over the entire range of compositions. Thin films of five blends of high molecular weight polystyrene (PS) with high molecular weight poly(phenylene oxide) (PPO), and four blends of low molecular weight PS (whose molecular weight lies below its entanglement molecular weight M_e) with the same PPO have been prepared. Following bonding of these films to copper grids, crazes were grown by uniaxial straining in air. Suitable crazes were then observed by transmission electron microscopy. From microdensitometry of the image plates it is possible to measure the extension ratio λ_craze within crazes in the nine blends. These measured values are compared with predicted values of λ_max, computed from λ_max = I_ed, where I_e is the chain contour length between entanglements and d is the root mean square end-to-end distance for a chain of molecular weight M_e. For the high molecular weight PS blends λ_max depends on the entanglement properties of both PS and PPO chains. For the low molecular weight PS blends, the PS chains cannot form part of the entanglement network and the correct value of λ_max is obtained from appropriate scaling of the pure PPO value. Comparison of λ_craze and λ_max for both types of blends shows excellent agreement, demonstrating the importance of the entanglement network in determining craze parameters and hence the toughness of a given polymer.     

Crosslinked structure of pristine poly(p-phenylene vinylene) thin-films synthesized by chemical vapor deposition polymerization

The pristine conjugated polymers including poly(p-phenylene vinylene) (PPV) are often insoluble in organic solvents. In fact, PPV thin films prepared by chemical vapor deposition polymerization in this study were insoluble in common organic solvents as well. Although linear pristine conjugated polymers with saturated bonds and/or aromatics are often insoluble, the insolubility of CVDP-prepared PPV has never been studied. This study showed that the PPV film intentionally brominated to reduce the rigidity of polymer by saturating the vinylene groups in PPV was still insoluble indicating the crosslinking reaction occurred during polymerization. Insolubility of the precursor polymer was further confirmed and the swelling of the crosslinked polymers in various solvents was studied by optical microscopy. The presence of the diradical form of the activated monomer in the gas phase is proposed to explain the crosslinked structure.     

聚苯硫醚纤维热定型过程中的热收缩机理

采用声速仪、偏光显微镜、差示扫描量热仪(DSC)和动态热机械分析仪(DMA)对热定型过程中聚苯硫醚纤维的热收缩进行研究,探索了此过程中聚苯硫醚纤维的收缩机理。实验结果表明,纤维热收缩速率在其玻璃化转变温度附近达到最大值,其收缩过程在冷结晶温度之前基本完成;温度高于冷结晶温度之后,纤维体系中发生晶粒生长和晶体完善等晶区结构变化,这在一定程度上限制了纤维的进一步收缩。