CO₂ separation performances of composite membranes of 6FDA-based polyimides with a polar group

2,2-bis(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-based polyimides with a polar group of hydroxyl or carboxyl such as 6FDA-BAPAF, 6FDA-DAP, and 6FDA-DABA were synthesized by the thermal imidization method. The corresponding composite membranes were then prepared by the dip-coating technique using a poly (ether sulfone) (PES) membrane as a supporting layer. Some alcohols and glycol ethers were used as coating solvents during the membrane preparation. The solubility of the polyimides synthesized in this study in these solvents depended on the nature of polymers and solvents. CO₂ permeances for these composite membranes were measured in comparison with those for other gases such as H₂, O₂, N₂, and CH₄. The membrane performances were affected considerably by the preparation conditions such as the kinds of diamine moiety, coating solvent, and coating polymer concentration. It was expected that these composite membranes could be applied usefully to the CO₂ separation, considering the CO₂ permeances in the range of 20-38 gas permeation unit (GPU) and the selectivities for CO₂/N₂ and CO₂/CH₄ equal to or even higher than those of other dense or asymmetric membranes of 6FDA-based polyimides reported in the literature.