PHASE AND RELAXATION TRANSITIONS IN A THE RMOTROPIC LIQUID-CRYSTALLINE VECTRA COPOLYESTER

Phase and relaxation transitions in the commercial thermotropic LC cop olyester Vectra A900 were studied by means of X-ray scattering, DSC, a nd mechanical relaxation. The copolymer, which contains 73 mol % of 4- hydroxybenzoic acid and 27 mol % of 6-hydroxy-2-naphthoic acid, was ex amined as a highly oriented fiber, either freshly spun or subjected to prolonged annealing at elevated temperatures. It was found that the s tructure of copolyester was biphase at any temperature up to the tempe rature of transition to nematic LC state (315 degrees C). One of the p hase components was a crystalline phase. Its Low-temperature form is a n orthorhombic system, whereas the high-temperature modification shows hexagonal packing. Depending on the thermal history of the sample, th e degree of crystallinity of the copolymer varies from 5 - 10% for a q uenched sample to 25 - 30% for an annealed sample. The rest of the mat erial is in the smectic LC state, whose structure is peculiar and char acterized by aperiodic arrangement of smectic layers in space. Heating of the copolymer above T-g is accompanied by reversible polymorphic t ransition of orthorhombic crystalline phase to hexagonal phase and sub sequent melting of the latter and of the smectic LC phase, with format ion of a single-phase nematic LC state.