Thermogravimetric kinetics of thermotropic copolyesters containingp-oxybenzoate unit by multiple heating-rate methods

Abstract

Two series of thermotropic liquid crystalline copolyesters containing mainly the p-oxybenzoate unit were studied by thermogravimetry to ascertain the kinetic parameters of their thermal degradation by six multiple heating-rate techniques for the first time. The two copolyesters are (1) poly(p-oxybenzoate-co-ethylene terephthalate-co-vanillate) and (2) poly(p-oxybenzoate-co-2,6-oxynaphthoate). The effect of copolymer composition, degradation stage, and test atmosphere on the three kinetic parameters of the thermal degradation in the weight loss range from 5 to 70% is discussed. Comparison of the multiple heating-rate techniques with single heating-rate techniques for calculating the kinetic parameters of thermal degradation was made. The respective activation energy, order, and natural logarithm of the frequency factor of the thermal degradation in nitrogen for the poly(p-oxybenzoate-co-ethylene terephthalate-co-vanillate)s are between 180 and 230 kJ/mol, between 2.0 and 5.0, and between 28 and 38 min−1 for the first degradation step and between 250 and 390 kJ/mol, between 6.4 and 7.6, and between 38 and 64 min−1 for the second degradation step of the poly(p-oxybenzoate-co-ethylene terephthalate-co-vanillate)s with the unit-B content in the range of 70–75 mol %. The respective activation energy, order, and natural logarithm of frequency factor of the first degradation stage for the poly(p-oxybenzoate-co-2,6-oxynaphthoate) (Vectra) are between 380 and 570 kJ/mol, between 2.0 and 3.1, and between 55 and 68 min−1 in nitrogen and between 160 and 210 kJ/mol, between 0.8 and 1.8, and between 25 and 32 min−1 in air. The best methods of calculating the kinetic parameters of the thermal degradation for the copolymers are suggested. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2016–2028, 1999