GAS PERMEATION OF AROMATIC POLYIMIDES .1. RELATIONSHIP BETWEEN GAS PERMEABILITIES AND DIELECTRIC-CONSTANTS

The permeability of 17 different 6FDA polyimides membranes to CO2 and CH4 has been determined at 25-degrees-C and at an applied pressure of 4.87 atm. Dielectric constants epsilon of polyimides were measured with 1 MHz frequency at 23-degrees-C and at 55% rh (relative humidity). Fractional free volumes and dielectric constants were also obtained by theoretical calculation for comparison. Good correlation between dielectric constant and gas permeability was observed. Since the dielectric constant is a function of molar polarization and molar volume, polymer structure can be much better characterized by using epsilon than by using the fractional free volume. Good relationships between gas permeability and the solubility parameter and the cohesive energy density were also found. It was found that the gas permeability of the polymer can be estimated by measuring the dielectric constant, due to the correlation between epsilon and the gas permeability in common commercially available polymers. The dielectric constant can be applied to identify the structural factors that will lead to the preparation of polymer membranes that exhibit both a high permeability and permselectivity toward specific gases.