GAS-PERMEABILITY AND PERMSELECTIVITY IN POLYIMIDE FILMS

Gas permeability and permselectivity of commercially available polyimides were investigated at 35-degrees-C and 100-degrees-C and compared with those of other glassy polymers. The polyimide prepared from 3,3', 4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and two different diamines, 4,4'-methylenedianiline (MDA) and 2,6-diaminotoluene (BTDA-M/T, commercially available as PI-2080), and the polyimide from 2,3,5-tricarboxycyclopentyl acetic acid dianhydride (TCDA) and 4,4'-oxydianiline (ODA) had excellent permeability and permselectivity for H-2/CO and H-2/CH4 systems (P(H2) = 1 x 10(-9) cm3(STP) cm-1 s-1cmHg-1, P(H2)/P(co) = 100, P(H2)/P(CH4) = 500 at 35-degrees-C). The polyimide prepared from BTDA and 3,3'-diaminobenzophenone (commercially available as LARC-TPI) had lower permeabilty and much higher permselectivity (P(H2) = 2 x 10(-10) cm3(STP) cm-1 s-1 cmHg-1, P(H2)/P(CO) = 350, P(H2)/P(CH4) = 1400 at 35-degrees-C). Three polyimides containing two ether linkages in the diamine moiety had poor permeability and permselectivity as compared with the above-mentioned polyimides. These results were discussed in terms of packing density and local mobility. All polyimides investigated were not attractive as membrane materials for CO2/CH4 separation because of low permeability for CO2. In a BTDA-M/T film, 2% residual dimethylacetamide solvent caused about 30% reductions in permselectivities for H-2/CO and H-2/CH4 without an appreciable increase in P(H2) as a result of the plasticization effect.