Synthesis and characterization of flexible polyoxadiazole films through cyclodehydration of polyhydrazides

Aiming to produce flexible films of poly(4,4′-diphenylether-1,3,4-oxadiazole), POD-DPE, the synthesis of POD-DPE through thermal cyclodehydration of polyhydrazide in solutions of poly(phosphoric acid), PPA, and in film form is studied here. It was observed that POD-DPE polymer samples obtained when the cyclodehydration of polyhydrazide is performed in solutions of PPA are insoluble in the solvents normally used for film preparation. On the other hand, the cyclodehydration of polyhydrazide in film form can lead to brittle POD-DPE films due to the high temperatures required for reaction to occur. In order to obtain flexible POD-DPE films with high conversion through cyclodehydration of polyhydrazide films, it is shown here that it is very important to adjust the cyclodehydration temperature (T_c) as a function of the glass transition temperature (T_g) and the weight average molecular weight (M_w) of the polyhydrazide. In all cases analyzed, polyhydrazides were synthesized by low temperature solution polycondensation reactions and were characterized by intrinsic viscosity measurements, size exclusion chromatograpy, nuclear magnetic resonance, infrared spectroscopy and thermal analysis. Polyhydrazides with weight average molecular weights up to 74,100 g/mol were obtained. The POD-DPE films obtained were characterized by infrared spectroscopy and nuclear magnetic resonance, showing that the production of POD-DPE films is indeed possible from cyclodehydration of polyhydrazide films.