BINARY BLENDS OF POLY(ETHYLENE-TEREPHTHALATE) WITH THERMOTROPIC LIQUID-CRYSTALLINE COPOLYESTERS - THERMAL, MORPHOLOGICAL AND MECHANICAL STUDIES

Binary mixtures of a thermoplastic engineering poly(ethylene terephthalate) (PETP) with a thermotropic liquid-crystalline random copolyester (PCL) were prepared by melt blending the polymers in a screw extruder or by precipitating them from a solution in a common solvent. Differential scanning calorimetry and polarizing optical microscopy were used to study their thermal and morphological properties as a function of temperature and composition. At high temperature, the blends rich in PETP are isotropic liquids, those rich in PCL are nematic fluids, and those of intermediate compositions are formed of two phases in equilibrium, one liquid-isotropic and one nematic. At low temperature, the blends are generally biphasic, with the two component polymers crystallized. The presence of crystallized PCL proved not to act as a nucleating agent towards the crystallization of PETP from the melt. A schematic phase diagram was drawn to summarize the thermal and morphological behaviour of the blends. Scanning electron microscopy studies of injection moulded samples revealed the presence of highly oriented fibrils when the composition exceeded roughly 50 wt.-% in PCL. The bending elastic modulus of the injected blends with 75 wt.-% of PCL surpasses that of the pure PCL, probably due to the presence of oriented high-molecular-weight PETP molecules. The particular dependence of the mechanical rigidity upon the concentration of PCL permits easily to determine conditions for the thermoforming of PETP.