Molecular design and characterization of new polyimides based on binaphthyl-ether diamines for gas separation

•A new series of binaphthyl-ether based polyimides were prepared.•Binaphthyl-ether unit and substituted group play crucial role on membrane properties.•Four designed PIs show good solubility, excellent mechanical and thermal properties.•High gas selectivity of CO2/CH4 (up to 69.7) and CO2/N2 (up to 37.6) were achieved. In this study, a series of new binaphthyl-ether diamine monomers were designed, synthesized, and then incorporated into the polymeric backbone through polycondensation reaction with 6FDA to obtain polyimides (PIs). Experimental results demonstrated that the corresponding binaphthyl-ether based PIs possessed high molecule weights (Mn = 5.5 × 104–1.2 × 105 g/mol), excellent thermal stability (Td,5% = 435–522 °C), and good solubility in not only high-boiling point polar aprotic solvents (e.g. NMP, DMAc, and DMF) but also low boiling-point solvents (e.g.THF, CHCl3). Furthermore, the effects of varying polymer backbone geometry and substituent property on polymeric fractional free volumes (FFV) and gas transport properties were systematically investigated. As a result, all the new PIs showed excellent ideal gas selectivity values of CO2/CH4 (up to 69.7) and CO2/N2 (up to 37.6). The CF3 substituting binaphthyl-ether PI membrane exhibited the highest gas permeability which could be attributed to the loose chain packing via the incorporation of contorted binaphthyl-ether unit and the bulky CF3 substituting groups. This work provides a new insight for binaphthyl unit-containing PIs and also shows a potential application in gas separation.