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

Thermotropic liquid-crystalline copoly(ester-amide)s consisting of three un its of p-oxybenzoate (B), ethylene terephthalate (E) and p-benzamide (A) we re studied by proton nuclear magnetic resonance at 200 and 400 MHz, wide-an gle X-ray diffraction, and high-resolution thermogravimetry to ascertain th eir molecular and supermolecular structures, thermostability and kinetics p arameters of thermal decomposition in both nitrogen and air. The assignment s of all resonance peaks of [H-1]NMR spectra for the copoly(ester-amide)s a re given and the characteristics of X-ray equatorial and meridional scans a re discussed. Overall activation energy data of the first major decompositi on have been evaluated through three calculating techniques. The thermal de gradation occurs in three steps in nitrogen and air. The degradation temper atures are higher than 447 degrees C in nitrogen and 440 degrees C in air a nd increase with increasing B-unit content at a fixed A-unit content of 5 m ol%. The temperatures at the first maximum weight-loss rate are higher than 455 degrees C in nitrogen and 445 degrees C in air and also increase with an increase in B-unit content. The first maximum weight-loss rates range be tween 11.1 and 14.5%min(-1) in nitrogen and between 11.9 and 13.5%min(-1) i n air. The char yields at 500 degrees C in both nitrogen and air range from 45.8 to 54.3wt% and increase with increasing B-unit content. But the char yields at 800 degrees C in nitrogen and air are quite irregular with the va riation of copolymer composition and testing atmosphere. The activation ene rgy and Ln (pre-exponential factor) for the first major decomposition are u sually higher in nitrogen than in air and increase slightly with an increas e in B-unit content at a given A-unit content of 5mol%. The activation ener gy, decomposition order, and Ln (pre-exponential factor) of the thermal deg radation for the copoly(esteramide)s in two testing atmospheres, are situat ed 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(esteramide)s obtained by high-resolution thermogravimetry at a varia ble heating rate are almost the same as those by traditional thermogravimet ry at constant heating rate, suggesting good applicability of kinetic metho ds developed for constant heating rate to the variable heating-rate method. These results indicate that the copoly(ester-amide)s exhibit high thermost ability. The isothermal decomposition kinetics of the copoly(ester-amide) s at 450 and 420 degrees C are also discussed and compared with the results obtained based on non-isothermal high-resolution thermogravimetry. (C) 1999 Society of Chemical Industry.