NOVEL WHOLLY AROMATIC POLYAMIDE HYDRAZIDES .5. STRUCTURE MOLECULAR-WEIGHT-THERMAL-STABILITY RELATIONSHIPS

The influences of structural variations and molecular weight on the th ermal stability and degradation behaviour of several wholly aromatic p olyamide-hydrazides have been investigated using differential scanning calorimetry, thermal gravimetric analysis, infrared spectrophotometry and elemental analysis. The structural differences of the polymers we re achieved by varying the content of the meta- and para-substituted p henylene moieties incorporated into their chains. Polyamide-hydrazides having different molecular weights of completely para-substituted phe nylene type units, were also examined. All the polymers investigated w ere synthesized by a low temperature solution polycondensation reactio n. The results clearly revealed that these polymers possess high resis tance to elevated temperatures. Their weight loss occurred in three st eps. The first relatively small step was attributed to evaporation of adsorbed surface water. The second was considerable and was assigned t o a cyclodehydration reaction of the polymer hydrazide groups into 1,3 ,4-oxadiazole rings with loss of water. This is not a true degradation , but rather a chemical transformation reaction. The third was severe and steep, particularly in air, and corresponded to the decomposition of the polymers. The thermal stability is improved remarkably by incre asing the para-phenylene unit content of the polymer, in air as well a s in nitrogen. Moreover, it was found that the molecular weight of the polymers has no effect on their stability.