Blends of poly(p-oxybenzoate-co-p-phenylene isophthalate) and polycarbonate: Miscibility and free volume behaviors

Blends of a thermotropic liquid crystalline polymer poly(p-oxybenzoate-co-p-phenylene isophthalate)] (HIQ45) and a polycarbonate (PC) are studied with regard to their miscibility and free volume behavior. The glass transition temperatures (T_gs) of the blends are found to be uncharacteristically higher than those of the pure components, even increasing with the addition of the low-T_g component, HIQ45. Based on the non-appearance of the T_g of HIQ45 in the tanδ spectra of the blends and the observed homogeneous morphology of the blend, mixtures of HIQ45 and PC are concluded to be a miscible blend system. The free volume properties determined by positron annihilation lifetime spectroscopy show that close packing does not occur in these miscible blends as indicated by positive deviations in size and number concentration of free volume cavities from average values as a function of blend composition. The increase in free volume is attributed to the loss of liquid crystalline alignment of HIQ45 and disturbed packing of PC molecules due to intimate miscibility between different molecules. The free volume fraction index of the blend correlates well with measured blend density.