Bifunctionalized Intrinsically Microporous Polyimides with Simultaneously Enhanced Gas Permeability and Selectivity

Two novel intrinsically microporous copolyimides synthesized by condensation reaction of 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3,3′,3′‐tetramethyl‐1,1′‐spirobisindane‐5,5′‐diamino‐6,6′‐diol, and 3,5‐diaminobenzoic acid with diamine ratios of 80/20 (Co‐80/20) and 50/50 (Co‐50/50) are reported. Unexpectedly, the Co‐80/20 not only demonstrates higher microporosity (300 m 2 g −1 ) than the PIM‐6FDA‐OH homopolymer (190 m 2 g −1 ) but also exhibits simultaneously enhanced CO 2 permeability (from 119 to 171 Barrer) and CO 2 /CH 4 selectivity (from 35 to 41) after thermal annealing at 250 °C. This higher permeability originates from enhanced diffusivity ( D CO2 ) and the higher selectivity results from its increased diffusion selectivity ( D CO2 / D CH4 ). After crosslinking at 300 °C, the Co‐80/20 exhibits an even higher CO 2 permeability (261 Barrer) and almost unchanged CO 2 /CH 4 selectivity.