Synergistic Design of Enhanced π–π Interaction and Decarboxylation Cross-Linking of Polyimide Membranes for Natural Gas Separation

Conventional gas separation membranes made of glassy polymers often exhibit increased segmental motion and reduced selectivity when exposed to high-pressure condensable gases. Decarboxylation cross-linking can effectively improve the plasticization resistance and gas permeability of polymer membranes, but this usually comes at the expense of the gas selectivity. In this work, enhanced π–π interactions and decarboxylation cross-linking are synergistically designed among polymer chains by introducing benzimidazole units and carboxyl side groups into the 6FDA-PABZ:DABA polyimide (PI-Im-COOH). After the thermal treatment, a cross-linked structure was formed through the decarboxylation reaction as confirmed by FTIR spectroscopy and thermal analysis. The π–π interactions between benzimidazole moieties were simultaneously enhanced as proven by WAXD, UV–vis, and fluorescence spectroscopy. The gas separation performance and plasticization resistance of the decarboxylated PI-Im-COOH membranes were enhanced significantly. In particular, the decarboxylated PI-Im-COOH membranes exhibit higher gas selectivities for CO2/CH4 and CO2/N2 gas pairs than the previously reported decarboxylated membranes. The PI-Im-COOH membrane that was thermally treated at 450 °C for 2 h showed outstanding CO2/CH4 separation performance with a CO2 permeability of 685.1 barrer and a CO2/CH4 selectivity of 38.1, surpassing the 2008 Robeson upper bound. The synergistic design of enhanced π–π interactions and decarboxylation cross-linking proves to be a facile strategy to regulate interchain interactions and distances to achieve a high performance for natural gas separation.