Novel copolyimide membranes for gas separation

A series novel fluorinated copolyimides were synthesized with various diamine compositions by chemical imidization in a two-pot procedure. The gas permeation behavior of 2,2′-bis(3,4′-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)–2,4,6-trimethyl-1,3-phenylenediamine (TMPDA)/2,6-diamino toluene (DAT) polyimides was investigated. The physical properties of the copolyimides were characterized by IR, DSC and TGA. All the copolyimides were soluble in most of the common solvents. The gas permeabilities and diffusion coefficients decreased with increasing DAT content; however, the permselectivity of gas pairs such as H2/N2, O2/N2, CO2/CH4 and CO2/N2 was enhanced with the incorporation of DAT moiety. The permeability coefficients of H2, O2, N2, CO2 and CH4 were found to decrease with the increasing order of kinetic diameters of the penetrants gases. Moreover, all of the copolyimides studied in this work exhibited performance near, lying on or above the existing upper bound trade-off line between permselectivity and permeability.