Gas permeation and mechanical properties of thermally rearranged (TR) copolyimides

Abstract

Gas transport and mechanical properties are reported for a series of copolyimides and their corresponding thermally rearranged (TR) analogs. Random copolyimides of APAF/HAB-6FDA with various compositions were synthesized via chemical imidization from 2,2′-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF), 3,3′-dihydroxy-4,4′-diaminobiphenyl (HAB), and 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA). These copolyimides were thermally treated at temperatures between 350 °C and 450 °C to prepare TR polybenzoxazoles (PBOs). Pure gas permeabilities of H2, CH4, N2, O2, and CO2 were measured at 35 °C with upstream pressures ranging from 3 to 17 atm. In general, gas permeability increased and selectivity decreased as TR conversion increased. Gas permeability also increased with increasing APAF content in the polyimides as a result of higher fractional free volume (FFV) imparted by the extra hexafluoroisopropylidene group in APAF. Stress–strain relationships were recorded for the polyimides and their TR analogs. As polyimides underwent thermal rearrangement, samples became more rigid, and tensile stress and elongation at break decreased. A similar trend in mechanical properties was also observed as APAF content increased, which was attributed to the lower molecular weight of APAF-containing polymers, resulting from the lower reactivity of APAF relative to that of HAB.