Phase and relaxation transitions of a multicomponent thermotropic liquid-crystalline copolyester

Temperature-induced evolution in the structure of oriented fibers of two thermotropic random LC copolyesters was studied by X-ray diffraction and DSC. The copolymers contain four or five monomer components, one of which is chlorohydroquinone and the rest are the residues of hydrobenzoic, naphthalenedicarboxylic, terephthalic, and isophthalic acids. It was shown that no transition from an orthorhombic crystalline phase to a pseudohexagonal mesophase, which is observed upon heating of the other polymers of this class, such as Vectra, Xydar, Ultrax, etc., takes place. To put it more precisely, the transition exhibits a virtual character. This implies that the phase transition could occur at 350°C if the melting and isotropization of a material proceed much below this temperature.