Thermal decomposition of liquid crystalline random copoly(p-oxybenzoate-ethylene/phenylene terephthalate) by high-resolution thermogravimetry

Thermotropic liquid crystalline copolyesters consisting of three units of p-oxybenzoate (B), ethylene terephthalate (E), and phenylene terephthalate (P), were studied by a high-resolution thermogravimetry from ambient temperature to 800 degrees C to ascertain their thermostability and kinetics parameters of thermal decomposition in both nitrogen and air. Overall activation energy data of the major decomposition had been evaluated through three calculating techniques. The thermal degradation takes place in three steps in nitrogen and air. The degradation temperatures are higher than 450"C in nitrogen and 450 degrees C in air and increase with increasing B-unit content at a fixed P-unit content of 5 mol%. The temperatures at the first (and second) maximum weight-loss rate are higher than 456 degrees C (and 524 degrees C) in nitrogen and 451 degrees C (and 520 degrees C) in air and also increase with an increase in B-unit content. The first and second maximum weight-loss rates range from 10.9 to 12.3 %/min in nitrogen and from 4.2 to 5.5 %/min in air, and also increase slightly with increasing B-unit content. The char yields at 500 degrees C in both nitrogen and air range from 42.1 to 51.1 wt% and increase with increasing B-unit content. But the char yields at 800 degrees C in nitrogen are ranging from 16.0 to 22.4 wt% that is much higher than those in air. The activation energy and Ln (pre-exponential factor) for the major decomposition are usually larger in nitrogen than in air and decrease in nitrogen but increase slightly in air with an increase in B-unit content at a given P-unit content 5 mol%. The activation energy, decomposition order, and Ln (pre-exponential factor) of the thermal degradation for the terpolyesters, are situated in the ranges of 206-258 kJ mol(-1), 1.7-2.6, 32-40 min(-1), respectively. These results have also been compared with the results obtained by traditional thermogravimetry at a constant heating rate. (C) 1999 Elsevier Science Ltd. All rights reserved.