Structure and high-resolution thermogravimetry of liquid-crystalline copoly(p-oxybenzoate-ethylene terephthalate-p-benzamide)

Abstract

Thermotropic liquid-crystalline copoly(ester-amide)s consisting of three units of p-oxybenzoate (B), ethylene terephthalate (E) and p-benzamide (A) were studied by proton nuclear magnetic resonance at 200 and 400 MHz, wide-angle X-ray diffraction, and high-resolution thermogravimetry to ascertain their molecular and supermolecular structures, thermostability and kinetics parameters of thermal decomposition in both nitrogen and air. The assignments of all resonance peaks of [1H]NMR spectra for the copoly(ester-amide)s are given and the characteristics of X-ray equatorial and meridional scans are discussed. Overall activation energy data of the first major decomposition have been evaluated through three calculating techniques. The thermal degradation occurs in three steps in nitrogen and air. The degradation temperatures are higher than 447 °C in nitrogen and 440 °C in air and increase with increasing B-unit content at a fixed A-unit content of 5 mol%. The temperatures at the first maximum weight-loss rate are higher than 455 °C in nitrogen and 445 °C in air and also increase with an increase in B-unit content. The first maximum weight-loss rates range between 11.1 and 14.5%min−1 in nitrogen and between 11.9 and 13.5%min−1 in air. The char yields at 500 °C in both nitrogen and air range from 45.8 to 54.3 wt% and increase with increasing B-unit content. But the char yields at 800 °C in nitrogen and air are quite irregular with the variation of copolymer composition and testing atmosphere. The activation energy and Ln (pre-exponential factor) for the first major decomposition are usually higher in nitrogen than in air and increase slightly with an increase in B-unit content at a given A-unit content of 5 mol%. The activation energy, decomposition order, and Ln (pre-exponential factor) of the thermal degradation for the copoly(ester-amide)s in two testing atmospheres, are situated in the ranges of 210–292 kJmol−1, 2.0–2.8, 33–46 min−1, respectively. The three kinetic parameters of the thermal degradation for the aromatic copoly(ester-amide)s obtained by high-resolution thermogravimetry at a variable heating rate are almost the same as those by traditional thermogravimetry at constant heating rate, suggesting good applicability of kinetic methods developed for constant heating rate to the variable heating-rate method. These results indicate that the copoly(ester-amide)s exhibit high thermostability. The isothermal decomposition kinetics of the copoly(ester-amide)s at 450 and 420 °C are also discussed and compared with the results obtained based on non-isothermal high-resolution thermogravimetry. © 1999 Society of Chemical Industry