Carbazole-Based Polyimide Membranes with Hydrogen-Bonding Interactions for Gas Separation

Cross-linked polymers for gas separation have significant advantages in increasing gas selectivity and separation stability. However, the cross-linking strategies unavoidably form permanent interchain covalent bonds and alter the polymer packing state, which largely decrease polymer solubility, static toughness, and reprocessability. Herein, a secondary-amine-containing diamine (HCBDA) derived from carbazole is synthesized and polymerized with 6FDA to furnish a gas-permeable polyimide (HCB-PI) with a pseudo-cross-linked hydrogen-bonding network and a strengthened charge-transfer complex (CTC) effect. Compared with the hydrogen-bonding free sample (CB-PI), HCB-PI displays a more homogeneous micropore distribution and denser chain packing, as is proven by positron annihilation lifetime spectroscopy, which results in enhanced selectivity for O2/N2 and CO2/CH4 gas pairs and antiplasticization property. Owing to the stronger interaction between the HCB-PI skeleton and molecular oxygen and thus the competitive adsorption mechanism, HCB-PI exhibits more enhanced O2/N2 selectivity in mixed-gas measurements (7.54) than in pure-gas measurements (6.58), with the overall separation property approaching the 2008 Robeson upper bound. Additionally, HCB-PI is heat-resistant and mechanically robust, exhibiting static toughness up to 108 MJ·m–3. Our designing concept for HCB-PI has been demonstrated to be efficacious for oxygen enrichment from air.