Intrinsically Microporous Polyimides Based on a Rigid–Soft Structure for Hydrogen Separation

Polyimide (PI)-based gas separation membranes are of great interest in the field of H2 purification owing to their good thermal stability, chemical stability, and mechanical properties. Among polyimide-based membranes, intrinsically microporous polyimides are easily soluble in common organic solvents, showing great potential for fabricating hollow fiber gas separation membranes. However, based on the solution-diffusion model, improving the free volume or the movability of polymer chains can improve gas permeability, but would result in poor thermal stability. Herein, we develop a carbazole-alkyl-based diamine monomer that endows PI chains with a "rigid-soft" structure to balance the trade-off between them. Soft units enhance the movability of polymer chains during the film-forming process, ensuring that rigid units achieve tight chain packing and strong intermolecular interactions. Meanwhile, bulky carbazole groups could further restrict the motion of soft units in the solid state. On the one hand, it restricts the movability of the polymer chains below Tg, enhancing the small gas selectivity for H2 and He. On the other hand, it ensures good thermal stability. Moreover, extending the length of the alkyl chains helps improve the free volume and intermolecular interactions simultaneously, thereby further optimizing the gas permeability/selectivity trade-off. As a result, the as-prepared PI shows H2 permeability of 89.61 Barrer, H2/CH4 selectivity of 87.85, and H2/N2 selectivity of 45.03 in contrast to the reference FPI TFMB-6FDA exhibiting H2 permeability of 92.95 Barrer, H2/CH4 selectivity of 72.62, and H2/N2 selectivity of 38.57. Meanwhile, a high Tg value of 334 °C is also achieved.