Thermal transition of a wholly aromatic thermotropic liquid crystalline copolyester

A copolyester was prepared from p-hydroxybenzoic acid (HBA), 2,6-naphthalene dicaboxylic acid (NDA), and hydroquinone (HQ). Thermal transition behavior and the crystal structure of this copolyester were investigated by using polarized light microscopy (PLM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) after annealing at solid-phase polymerization conditions. A glass transition or newly ordered structure in the 270-290 degrees C range was observed on annealing at 260 degrees C, which increased with annealing time, attributed to mobility and reactive rearrangement in amorphous regions. Broad and unclear WAXD profiles and multimelting behaviors were found on annealing at 280 degrees C, and explained by hexagonal and orthorhombic lattice formation and transformation. A large increase in melting temperature was observed only on annealing at a temperature (320 degrees C) near the crystal-nematic transition, suggesting annealing temperatures near the melting point are required for sufficient mobility to afford crystalline rearrangement via transesterification. (C) 1999 John Wiley & Sons, Inc.