Abstract
The structures and phase transitions of thermotropic liquid-crystalline copolymers composed of 4- hydroxybenzoic acid (HBA), terephthalic acid (TPA) and biphenol (BP) (HBA/TPA/BP copolymer) with five different copolymer compositions were investigated using X-ray diffraction and thermal analyses. The HBA/TPA/BP copolymers exhibited nematic liquid-crystalline characteristics at elevated temperatures, and complicated fine structures formed in the solid state when cooled from the anisotropic nematic melt. The crystal to smectic E (SE) type and SE type to hexagonal phase transitions inevitably occurred at approximately 100°C and 300°C, respectively, in the samples of varied copolymer compositions. Particularly, the molecular chain of almost the same monomer fraction (HBA/TPA/BP = 35/32.5/32.5) showed sharp endothermic peaks on the differential scanning calorimetry curve; thereby, the crystallite size was considered to be comparatively uniform. The SE-type structure was frozen in almost all the quenched samples. Highly ordered crystallites were formed in the samples when they were annealed above 300°C where the hexagonal interchain packing order was formed.
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