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 in 40 different polyimides synthesized from 2,2‐bis(3,4‐decarboxyphenyl)hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of the chemical structures on the physical and gas permeation properties of the 6FDA‐based polyimides was studied. The solubility of propylene in an unrelaxed volume of a polymer matrix mainly contributes to the total solubility of propylene for various 6FDA‐based polyimides. The diffusivity, the permeability of propylene, and the permselectivity in the propylene/propane mixed‐gas system depend on the solubility of propylene. This is thought to be associated with the penetrant‐induced plasticization effect. 6FDA‐based polyimides, which have a high glass‐transition temperature and a large fractional free volume, exhibit a high permeability with a relatively low permselectivity. Changing the number of  CH3 substituents in the phenylene linkage and changing the connectivity in the main chain are good ways of controlling 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 between the phenylene linkage; the  Cl substituent in the phenylene linkage at the diamine moiety exhibits a high separation performance in the mixed‐gas system. The polyimides are potentially useful membrane materials for the separation of propylene and propane in the petrochemical industry. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2525–2536, 2000