PHASE AND RELAXATION TRANSITIONS IN A THERMOTROPIC LIQUID-CRYSTALLINE VECTRA COPOLYESTER

Phase and relaxation transitions in the commercial thermotropic LC copolyester Vectra A900 were studied by means of X-ray scattering, DSC, and mechanical relaxation. The copolymer, which contains 73 mol % of 4-hydroxybenzoic acid and 27 mol % of 6-hydroxy-2-naphthoic acid, was examined as a highly oriented fiber, either freshly spun or subjected to prolonged annealing at elevated temperatures. It was found that the structure of copolyester was biphase at any temperature up to the temperature of transition to nematic LC state (315 degrees C). One of the phase components was a crystalline phase. Its Low-temperature form is an orthorhombic system, whereas the high-temperature modification shows hexagonal packing. Depending on the thermal history of the sample, the degree of crystallinity of the copolymer varies from 5 - 10% for a quenched sample to 25 - 30% for an annealed sample. The rest of the material is in the smectic LC state, whose structure is peculiar and characterized by aperiodic arrangement of smectic layers in space. Heating of the copolymer above T-g is accompanied by reversible polymorphic transition of orthorhombic crystalline phase to hexagonal phase and subsequent melting of the latter and of the smectic LC phase, with formation of a single-phase nematic LC state.