Thermal behavior of polyhydrazides and poly-1,3,4-oxadiazoles

The thermal properties of a series of polyhydrazides and corresponding poly-1,3,4-oxadiazoles have been investigated using differential thermometric and thermogravimetric techniques. In addition, the conversion rates of the polyhydrazides to the corresponding oxadiazole structures have also been measured. In general, weight loss in polyhydrazides occurs in three distinct stages corresponding, respectively, to loss of adsorbed water, cyclodehydration, and decomposition of the poly-1,3,4-oxadiazole formed in situ. The polyoxadiazoles generally exhibit only a single weight loss step (below 700°C.) corresponding to decomposition and volatilization of the substrate itself. These data clearly indicate that the presence of aliphatic carbon–carbon linkages in the polymer backbone reduces the temperature at which decomposition of the polyoxadiazoles is noted. Corresponding differential thermometry patterns give additional data on phase transitions in the polymers and indicate, in general, that the poly-1,3,4-oxadiazoles formed in situ decompose before reaching their intrinsic melting points (under the specific conditions of these experiments). Comparison of the poly-1,3,4-oxadiazoles prepared under the (rapid) dynamic conditions of the differential thermometry and thermogravimetry techniques employed (30°C./min.) with isothermal rate data clearly suggests subtle structural differences in the polyoxadiazoles prepared by the two different modes. In general, ΔE? for the hydrazide–oxadiazole conversion amounts to about 49–53 Kcal./mole while ΔS? amounts to 14–17 e.u. However, both ΔE? and ΔS? for the polymer containing exclusively 1,3-phenylene linkages between incipient oxadiazole groups appear to be considerably higher (ΔE? = 62 Kcal./mole, ΔS? = 34 e.u.). This striking difference is explained in terms of the increased order resulting from the higher degree of crystallinity in the polyhydrazide.