High-resolution thermogravimetric kinetics of thermotropic liquid crystalline dioxynaphthalene copolymers

Four types of thermotropic liquid crystalline aromatic copolymers containing oxybenzoate, ethylene terephthalate or 2,6-naphthalate, 2,7-dioxynaphthalene or 1,5-dioxynaphthalene terephthalate units were subjected to high-resolution thermogravimetry at a variable heating rate in nitrogen and air. The thermogravimetry curves, the derivative thermogravimetry curves, the second derivative thermogravimetry curves and heating rate curves for the copolymers are discussed. The thermal degradation temperature and kinetic parameters for the copolymers were presented and compared with those obtained with traditional thermogravimetry at a constant heating rate. The degradation process of the copolymers was considered. Three copolymers with 2,7-dioxynaphthalene and 1,5-dioxynaphthalene of 5 mol% exhibit the same initial degradation temperature of 454 degrees C and the same temperatures of 458 degrees C, 470 degrees C and 520 degrees C for the maximum weight-loss rates of 9.8%/min, 10%/min, and 4.7%/min in nitrogen but slight differences in the degradation temperatures and the maximum weight-loss rates with variation of the copolymer composition in air. The thermostability of the 2,7-dioxynaphthalene copolymers is slightly higher in nitrogen than in air. The copolymers show degradation activation energies of 208-308 kJ/mol in nitrogen and 158-258 kJ/mol in air. (C) 1999 Elsevier Science Ltd, All rights reserved.