Relationships between the chemical structures and the solubility, diffusivity, and permselectivity of propylene and propane in 6FDA-based polyimides

The solubility, diffusivity, and permselectivity of propylene and propane i n 40 different polyimides synthesized from 2,2-bis(3,4-decarboxyphenyl)hexa fluoropropane dianhydride (6FDA) were determined at 298 K. The influence of the chemical structures on the physical and gas permeation properties of t he 6FDA-based polyimides was studied. The solubility of propylene in an unr elaxed volume of a polymer matrix mainly contributes to the total solubilit y of propylene for various 6FDA-based polyimides. The diffusivity, the perm eability of propylene, and the permselectivity in the propylene/propane mix ed-gas system depend on the solubility of propylene. This is thought to be associated with the penetrant-induced plasticization effect. 6FDA-based pol yimides, which have a high glass-transition temperature and a large fractio nal free volume, exhibit a high permeability with a relatively low permsele ctivity. Changing the number of -CH3 substituents in the phenylene linkage and changing the connectivity in the main chain are good ways of controllin g the solubility of propylene and the corresponding permselectivity in the propylene/propane mixed-gas system. Some 6FDA-based polyimides restrict the solubility of propylene through the introduction of a -CONH- linkage betwe en the phenylene linkage; the -Cl substituent in the phenylene linkage at t he diamine moiety exhibits a high separation performance in the mixed-gas s ystem. The polyimides are potentially useful membrane materials for the sep aration of propylene and propane in the petrochemical industry. (C) 2000 Jo hn Wiley & Sons, Inc.