Transesterification and Mechanical Properties of Blends of a Model Thermotropic Polyester and Polycarbonate

Transesterification between a model thermotropic polyester (TTP) and polycarbonate (PC) was investigated during melt blending in a twin-screw mini-compounder at temperatures below and above the clearing temperature ( T cl ) of the TTP. For the investigation, a model TTP, poly[(phenylsulfonyl)- p -phenylene hexamethylene-bis(4-oxybenzoate)] (PSHQ6), was synthesized that has a T cl (206 °C) much lower than the thermal degradation temperature (ca. 350 °C). PSHQ6 was melt-blended with PC at temperatures below and above the T cl of PSHQ6. In the preparation of the blends, temperature and residence time in the extruder were varied. It was observed from 13 C nuclear magnetic resonance (NMR) spectroscopy that negligible transesterification occurred when a blend was prepared at temperatures below the T cl of PSHQ6, while the extent of transesterification increased steadily as the melt blending temperature increased above the T cl of PSHQ6. The tensile strength of injection-molded specimens of PC/PSHQ6 blends prepared under various processing conditions was also investigated, showing that it first increased with increasing melt blending temperature, going through a maximum at a temperature above the T cl of PSHQ6, and then decreased rapidly as the melt blending temperature increased further. The experimental observations indicate that there exists an optimum degree of transesterification that gives rise to the highest tensile and impact strengths of PC/PSHQ6 blends. That is, the effectiveness of PSHQ6 as a reinforcing agent for an improvement of the mechanical properties of injection-molded PC specimens begins to diminish when the extent of transesterification exceeds a certain critical level. This is attributed to the fact that the inherent liquid crystalline characteristics of PSHQ6 were lost when the extent of transesterification exceeded a certain critical level.