PERMEABILITY AND PERMSELECTIVITY OF GASES IN FLUORINATED AND NON-FLUORINATED POLYIMIDES

Solubility and permeability coefficients for gases in a series of fluorinated and non-fluorinated polyimides were measured to investigate the effects of chemical structure on permeability and permselectivity. Polyimides with higher concentration of fluorine had lower chain packing density and higher permeability. Polyimides with -C(CF3)2- linkages had higher permeability with relatively higher permselectivity. 2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis(4-aminophenyl)hexafluoropropane (6FDA-BAHF) polyimide with two -C(CF3)2- linkages in the polymer repeat unit displayed excellent performance for CO2/CH4 and O2/N2 separations; its permeabilities for O2 and CO2 were 3.7 and 3.1 times, respectively, larger than those of 6FDA-pp'ODA (4,4'-oxydianiline), a conventional 6FDA-based polyimide, while the permselectivities for O2/N2 and CO2/CH4 were 13% and 22%, respectively, smaller. On the other hand, polyimides having two -C(CF3)2-linkages together with two ether ones in the repeat unit had lower permeability with a similar level of permselectivity as compared with 6FDA-BAHF polyimide. For H2/CH4 and H2/CO separations, polyimides prepared from 6FDA and meta-oriented non-fluorinated diamines were preferable. These separation properties were discussed in connection with packing density and local segmental mobility.