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STRUCTURE AND TEMPERATURE BEHAVIOR OF THERMOTROPIC TERNARY LIQUID-CRYSTALLINE COPOLYESTER

A complicated pattern of phase transitions in highly oriented fibers of completely aromatic LC copolyester was studied in a wide temperature range by means of X-ray analysis and DSC. According to synthesis conditions, ternary copolymer based on terephthalic acid, phenylhydroquinone, and p-hydroxybenzoic acid was characterized by random structure of macromolecules. Even though the polymer chain had irregular structure, the copolymer showed the ability to crystallize, and its crystallization was accompanied by microphase separation and formation of two rhombic modifications with somewhat different parameters of the unit cells. The total crystallinity of the system was estimated to be equal to 35%, being higher than that of the other representatives of this class of polymers. In addition to the ability to produce a structure with a three-dimensional long-range order, the system studied showed an increased tendency toward mesomorphism. A peculiar conformational mesogen composed of two monomer units of the same length was shown to be the main reason for the development of layer (liquid-crystalline smectic) structure, which was characteristic of the as-spun fibers. Annealing the test samples at temperatures above the glass transition point (180 o C) resulted in crystallization and transformation of the metastable smectic phase to nematic liquid crystal. In a temperature range close to melting temperatures of the two crystalline phases, macromolecular imperfection became so high that the system lost its conformational order. As a result of structural changes, the transition from crystalline to condis-crystalline structure was observed. Melting of two condis-crystalline modifications was shown to be accompanied by transition to a single-phase state, that is, liquid-crystalline nematic state