Structure and temperature behavior of thermotropic liquid-crystalline Ultrax copolyesters

Phase and relaxational transitions in the commercial thermotropic liquid-crystalline copolyesters Ultrax 4002 and Ultrax 4003 were studied by X-ray diffraction, differential scanning calorimetry, and mechanical analysis. In spite of only a slight difference in the compositions of the copolyesters, considerable differences in their structure and temperature behavior were observed. In particular, it was shown that the degree of crystallinity differs by more than a factor of 2 for the as-spun fibers upon annealing above the glass-transition temperature. The type of crystalline structure as well as the ability to crystallize are also different. The glass-transition temperature of both copolyesters is determined as the temperature of the appearance of mobility of the most rigid comonomer units and does not depend on the composition of the materials. Annealed copolyesters are semicrystalline materials with a two-phase structure, where the crystalline phase coexists with a liquid-crystalline one. The structure of the latter is characterized by aperiodic positions of smecticlike layers in space. On heating the materials the crystalline phase is transformed into a pseudohexagonal mesophase indicating features of a second-order phase transition.