The effect of purge environment on thermal rearrangement of ortho-functional polyamide and polyimide

The effects of nitrogen and air purge during thermal rearrangement of an ortho-functional polyamide (o-PA) and an ortho-functional polyimide (o-PI) towards a polybenzoxazole (PBO) structure have been investigated in terms of physicochemical changes and gas transport properties. The o-PA polymer was prepared from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluropropane (BisAPAF) and 4,4′-biphenyl-dicarbonyl chloride (BPDC) while the o-PI polymer was derived from 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3′-dihydroxybenzidine (HAB). Experimental results show that the purge environment for the conditions used does not affect the thermal rearrangement of the o-PA film but significantly affects the thermal conversion of the o-PI film. Nearly identical chemical structures and pure gas permeability values are observed for o-PA films thermally treated at 300 °C under air or N ₂. These properties become different in the o-PA films treated at 425 °C, which is presumed to be attributed to the influence of oxygen on the thermal stability of the derived PBO and probably the various degrees of thermal crosslinking reaction induced at a high temperature. The o-PI film was thermally rearranged at 425 °C because its thermal conversion takes place at a higher temperature range of 300 °C-450 °C. The o-PI film thermally rearranged in air exhibits improved gas permeation properties but significantly deteriorated mechanical properties. The air purge interrupts the thermal conversion of the ortho-functional imide to benzoxazole by oxidatively degrading the imide structure and forming the imine structure. As a result, both polymer structure and film properties change.