Thermal degradation kinetics of thermotropic copoly (p-oxybenzoate-ethylene terephthalate-vanillate) by a high-resolution thermogravimetry

Thermotropic liquid crystalline terpolymers consisting of three units of p-oxybenzoate (B), ethylene terephthalate (E), and vanillate (V), were studied through a high-resolution thermogravimetry to ascertain their thermostability and kinetics parameters of thermal decomposition in nitrogen and air. Overall activation energy data of the major decomposition have been calculated through four calculating techniques. The thermal degradation occurs in three steps in nitrogen, but in four steps in air due to an additional thermo-oxidative step. The thermal degradation temperatures are higher than 436 degrees C in nitrogen and 424 degrees C in air and increase with increasing B-unit content at a fixed V-unit content of 5 mol%. The temperatures at the first maximum weight-loss rate are higher than 444 degrees C in nitrogen and 431 degrees C in air and increase slightly with an increase in B-unit content. The first, second, and third maximum weight-loss rates almost maintain at 10-11, 10-11, and 3.6-5.3%/min regardless of copolymer composition and testing atmosphere. The char yields at 500 degrees C in both nitrogen and air are larger than 40 wt% and increases with increasing B-unit content. But the char yields at 800 degrees C in nitrogen and air are quite different, i.e., 18-25 wt% in nitrogen and 0 wt% in air. The activation energy and Ln (pre-exponential factor) for the major decomposition are higher in nitrogen than in air and decrease slightly with an increase in B-unit content at a given V-unit content 5 mol%. There is no regular variation in the decomposition order with the variation of copolymer composition and testing atmosphere. It is found that the most V-unit-containing terpolymer exhibited the lowest degradation temperature, lowest activation energy, and lowest Ln (pre-exponential factor). The activation energy, decomposition order, and Ln (pre-exponential factor) of the thermal degradation for the terpolymers, are situated in the ranges of 121-248 kJ/mol, 1.5-2.8, 19-38 min(-1), respectively. These results indicate that the terpolymers exhibit high thermostability. The isothermal decomposition kinetics of the terpolymer at 450 degrees C have also been discussed and compared with the results obtained based non-isothermal high-resolution thermogravimetry.