Double-decker-shaped phenyl-substituted silsesquioxane (DDSQ)-based nanocomposite polyimide membranes with tunable gas permeability and good aging resistance

Membrane-based gas separation technique shows great potential in CO₂ elimination from natural gas and N₂ enrichment from compressed air. In this study, a double-decker-shaped phenyl-substituted silsesquioxane (DDSQ)-based diamine monomer, that is, DDSQ-diamine, was synthesized and subsequently copolymerized with 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) in combination with 4,4′-oxydianiline (ODA) to fabricate a series of DDSQ-based polyimide membranes with intrinsic microporosity (PIM-PIs). The resulting membranes showcase desirable mechanical robustness, satisfactory heat resistance and good organo solubility. Owing to the inherent internal nanometer-sized cavity of DDSQ cage, DDSQ-based membranes are more nanoporous than DDSQ-free sample, accordingly exhibiting decreasing membrane density and increasing gas permeability. Among all membranes, DDSQ-25 and DDSQ-50 show more desirable overall gas separation performance, which almost achieve the 1991 Robeson upper bound. Additionally, DDSQ-25 demonstrates good plasticization resistance and CO₂/CH₄ mixed-gas separation properties when measured with 50:50 CO₂/CH₄ binary gas mixture at upstream pressure up to 20 bar. Despite physical aging at ambient conditions for 300 days, the resultant DDSQ-based polyimide membranes still have stable gas permeability and almost constant selectivity, showcasing satisfactory anti-aging characteristic.