Finely tuning the microporosity in dual thermally crosslinked polyimide membranes for plasticization resistance gas separations

Thermally induced chemical crosslinking has attracted substantial attention for fabricating plasticization resistant membranes due to the facile structure tunability that enables the construction of robust and well-defined architecture for gas separation. In this study, we report a new series of dual thermally crosslinkable polyimides derived from 4,4′-diamino-2,2′-biphenyldicarboxylic acid (DCB) containing two carboxyl groups, and a systematic investigation of the thermal treatment above and below T_g demonstrated the decarboxylation-induced crosslinking. The dual thermally crosslinked membranes were insoluble in common organic solvents and maintained excellent mechanical properties. Due to the evolution of CO₂ and collapse of chain segments during thermal treatment, the crosslinked membranes exhibited hierarchical microcavity size distribution featuring ultra-micropore size in the range of 2.0–6.0 Å and micropore size in the range of 6.5–10.0 Å. Gas transport properties of the crosslinked membranes were feasibly tuned through the chemical compositions and thermal treatment procedures. For instance, the CO₂ permeability of crosslinked 6FDA-DAM_0.7-TFMB_0.1-DCB_0.2 increased almost three-fold with only a slight decrease in CO₂/CH₄ selectivity. The crosslinked membranes also demonstrated superior plasticization resistance with mixed-gas feed pressure up to 40 bar and excellent low-temperature gas separation performance at −30 °C, making them attractive for aggressive gas separations.