Thermal cyclodehydration of aromatic precursor polymers to polybenzoxazole and polyimide; their thermal and mechanical properties

Two precursor polymers, polyhydroxyamide (PHA) and poly(amic acid) (PAA) for polybenzoxazole (PBO) and polyimide (PI), respectively, showed large endothermic enthalpies (Delta H) in DSC thermograms. The values of enthalpy become smaller, and then unobservable with increasing cyclodehydration. The cyclization onset temperature (T-1) of the PHA and PAA occurred at 255-309 degrees C and 175-234 degrees C, respectively. Of the two precursor polymers, T-1 showed an increase with increasing cyclodehydration. The initial decomposition temperature (T-2) showed no significant change dependent on cyclodehydration. The tensile properties of the two precursors were enhanced drastically with increasing degree of cyclodehydration. PHA was shown to have excellent solubility only in strong acids at room temperature. In contrast, PAA was soluble in all the testing solvents at room temperature.