The kinetics of decomposition of thermotropic liquid crystalline poly(p-oxybenzoate-co-ethylene terephthalate) (poly(B-co-E), BE polymer) with di β erent monomer ratios in both nitrogen and air were studied by dynamic thermogravimetry (TG) from ambient temperature to 800Β‘C. The kinetic parameters, including the activation energy E@, the reaction order n, and the pre-exponential factor Z, of the degradation of the BE polymers were evaluated by the single heating rate methods of Friedman, FreemanΓCarroll and Chang. The BE polymers which degraded in two distinct stages in nitrogen and air, were stable under nitrogen, while almost completely burned in air. The weight losses in the Γrst stage in nitrogen and air were dominated by the thermal degradation of both B and E segments, but the weight losses in the second stage were governed by the thermal degradation of B segment in nitrogen and by the oxidative degradation of both B and E segments in air. The maximum rate of weight loss increased linearly with the increase of E content and heating rate, but as E content increased, the char yield at 800Β‘C in nitrogen decreased linearly. The E@, n and ln Z values of the BE polymers in the Γrst stage of thermal pyrolysis are higher in nitrogen than in air, indicating that the degradation rate is slower in an inert atmosphere. The E@ value increased with increasing heating rate but varied irregularly with the variations of B/E ratios and molecular weight. The n and ln Z values for the BE polymers in nitrogen were found to be in the wide ranges 1Γ5Γ5Γ6 and 12Γ48 min~1, respectively, suggesting a complex degradation process. The estimated lifetimes of the BE polymers at 250Β‘C were calculated to be at least 33 days in nitrogen and 3 h in air.
1998 SCI.