Novel wholly aromatic polyamide-hydrazides. Part V: Structure-molecular-weight-thermal-stability relationships

Abstract

The influences of structural variations and molecular weight on the thermal stability and degradation behaviour of several wholly aromatic polyamide-hydrazides have been investigated using differential scanning calorimetry, thermal gravimetric analysis, infrared spectrophotometry and elemental ánalysis. The structural differences of the polymers were achieved by varying the content of the meta- and para-substituted phenylene moieties incorporated into their chains. Polyamide-hydrazides having different molecular weights of completely para-substituted phenylene type units, were also examined. All the polymers investigated were synthesized by a low temperature solution polycondensation reaction. The results clearly revealed that these polymers possess high resistance to elevated temperatures. Their weight loss occurred in three steps. The first relatively small step was atributed to evaporation of adsorbed surface water. The second was considerable and was assigned to 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 increasing the para-phenylene unit content of the polymer, in air as well as in nitrogen. Moreover, it was found that the molecular weight of the polymers has no effect on their stability.