TEMPERATURE EVOLUTION OF THE STRUCTURE OF A THERMOTROPIC MAIN-CHAIN LIQUID-CRYSTALLINE COPOLYESTER

X-ray diffraction, differential scanning calorimetry and dynamic mechanical analysis are used to study the temperature evolution of the structure of the thermotropic copolyester poly[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenyl-hydroquinone and p-hydroxybenzoic acid (molar ratio 45/45/10). Needle-like crystals are developed in the quenched as-spun fibers despite the random sequence of the monomers of the copolyester. Moreover, two other crystalline phases are observed at elevated temperatures, which may be explained by a phase separation process. X-ray diffraction experiments show a significant temperature dependence of structural features, indicating some conformational disorder and mobility in this class of ''rigid-rod' liquid crystalline prolymers. Changes in the X-ray pattern are observed on heating, starting immediately above the glass transition temperature of the copolyester. Melting of PES reveals a two-step character with transitions at 300-degrees-C and 320-degrees-C. The material becomes isotropic at 400-degrees-C or higher.