Gas separation performance of solid-state in-situ thermally crosslinked 6FDA-based polyimides

2,2-Bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA)-derived polyimides containing polar functional groups have been extensively studied for membrane-based gas separations and used as precursors for various solid-state thermal modifications such as thermal crosslinking of carboxyl-groups or thermal rearrangement of hydroxyl groups to form polybenzoxazoles. In this work, we studied four 6FDA-derived polyimides without polar functional groups to investigate in-situ thermal crosslinking involving partial decomposition of the hexafluoroisopropylidene (–C(CF3)2–) mainchain group. A thorough examination of the 6FDA-based polyimides via thermal gravimetric analysis coupled with mass spectroscopy (TGA-MS) and energy dispersive X-ray spectroscopy (EDX) under isothermal treatment at 450 °C for up to 3 h revealed that thermal treatment triggered continuous evolution of fluorine from (–C(CF3)2–) groups. The 6FDA-based polyimides were insoluble after thermal treatment for only 30 min and maintained good mechanical strength with favorable changes in gas separation properties. For example, after 3 h isothermal heat treatment, the CO2 permeability of crosslinked 6FDA-pPDA noticeably increased threefold from 39 to 115 barrer coupled with only a small drop in CO2/CH4 selectivity from 54 to 46. The crosslinked polyimides demonstrated excellent plasticization resistance when tested under high-pressure binary CO2/CH4 mixed-gas conditions.