Permeability, diffusivity, and solubility of benzene vapor and water vapor in high free volume silicon- or fluorine-containing polymer membranes

The diffusivity, solubility, and permeability of benzene and water vapors, as well as nitrogen (i.e., major component of air) were systematically investigated for high free volume silicon-containing polymers, including, poly(dimethylsiloxane) (PDMS), poly(trimethylsilylmethylmethacrylate) (PTMSMMA), poly(1-trimethylsilyl-1-propyne) (PTMSP), fluorine-containing 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-based polyimide. The infinite-dilution permeability coefficient, P0, of benzene vapor and water vapor was diffusion controlled and did not depend on either the mobility of the polymer segments, the space between polymer segments, or cohesive energy density of the polymer but was dependent on the balance among them. In ideal permselectivity, which was determined from pure gas experiments, all silicon-containing polymers used in this study showed benzene vapor-permselective behavior while all fluorine-containing polymers had the opposite property (i.e., nitrogen-permselective). All polymers used in this study showed water vapor-permselective behavior. The benzene vapor/nitrogen permselectivity depended on diffusivity selectivity for fluorine-containing polymers and solubility selectivity for silicon-containing polymers. Additionally, the water vapor/nitrogen permselectivity depended on solubility selectivity for PTMSP and some 6FDA-based polyimides with methyl substituent, and both selectivity balance for PDMS, PTMSMMA, and some a 6FDA-based polyimide without methyl substituent.