The effect of pressure on some thermophysical characteristics of liquid-crystalline copolyester based on p-hydroxybenzoic acid and poly(ethylene terephthalate)

The specific volume of LC copolymer based on p-hydroxybenzoic acid and poly(ethylene terephthalate) was Studied as a function of temperature in the range from 20 to 300 degrees C upon compression to 700 MPa. For the copolymer, two transition temperatures were detected, which are related to the activation of mobility in blocks enriched with PETP and p-hydroxybenzoic acid, respectively. The phase diagram of the copolymer versus temperature and applied pressure was constructed. It was found that the temperature and baric dependences of the coefficients of thermal expansion and compressibility increase sharply in the range of the transition temperatures and appreciably decrease with the growing pressure, especially in the region corresponding to the LC state of the copolymer. The compressibility of the LC copolymer, as well as PETP, is the increasing function of the fraction of free volume which varies with temperature or pressure. It was shown that the dependence of the specific volume of the copolymer on pressure at temperatures above 50 degrees C may be described to a good approximation by the modified van der Waals equation.