Phase and relaxation transitions in thermotropic liquid-crystalline copolyesters based on p-hydroxybenzoic acid and ethylene terephthalate

Phase and relaxation transitions in thermotropic LC copolyesters based on n-hydroxybenzoic acid and ethylene terephthalate were studied by means of X-ray analysis, differential scanning calorimetry, and dynamic mechanical relaxation. Copolymers with the ratio between components of 80:20, 60:40, and 50:50 mol % were examined both in isotropic state and as highly oriented fibers. In the latter case, the samples were tested immediately after spinning and annealing of the as-spun fibers for several hours at elevated temperatures. Glass transition temperature of copolyesters, which is controlled by the mobility of most rigid structural units, was found to be independent of the composition and was equal to similar to 70 degrees C. At temperatures below the temperature of transition to viscous LC state, the above copolyester was characterized by a well-pronounced two-phase structure. One of the phase components was provided by crystalline phase. At temperatures below the glass transition temperature, this phase showed an orthorhombic crystalline structure, whereas at higher temperatures, it assumed hexagonal packing. As a result of annealing at optimal temperatures, the attainment of maximal crystallinity was observed, and the degree of crystallinity was estimated to be as high as 15%. The second phase was found to exist in the LC smectic state with an aperiodic arrangement of smectic layers.