KINETICS OF MESOPHASE TRANSITIONS IN THERMOTROPIC COPOLYESTERS .5. A COPOLYESTER CONTAINING TPA PHQ HBA

The copolyester consisting of 90 mol % phenylhydroquinone terephthalate (TPA/PHQ) and 10 mol % hydroxybenzoic acid (HBA) has been studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized light microscopy (PLM). Upon cooling from the liquid crystal phase, two exothermic processes are observed. Neither of these processes can be by-passed when the sample is quenched. Results from nonisothermal DSC studies demonstrate that the exotherm occurring at higher temperatures corresponds to a nematic phase transition, and this transition is monotropic. The exotherm occurring at lower temperatures corresponds to a mesophase transition, resulting in a packing close to pseudohexagonal. Crystallization can take place using prolonged annealing times. Two kinds of crystals with different stabilities can be formed depending upon crystallization conditions. WAXD fiber patterns show that the crystals formed are monoclinic. Isothermal DSC experiments illustrate that the annealing kinetics at different temperatures are characterized by fractional Avrami exponents. Heat capacity data provide evidence that the glass transition temperature of TPA/PHQ/HBA (45/45/10) is around 460 +/- 10 K. The changes of the glass transition and crystal melting behavior are dependent on different thermal histories. The effect of the crystal on the interfacial structure is also discussed.