Gas Permeation through Micropores of Carbon Molecular Sieve Membranes Derived from Kapton Polyimide

The evolution of micropores and the gas permeation properties have been investigated on carbon molecular sieve dense membranes prepared by pyrolysis of Kapton polyimide films under several conditions. With decreasing the pore sizes upon pyrolization under vacuum, the gas permeability decreased, whereas the permselectivity increased. The change in the heating rate was found to affect the permeation properties to a lesser extent; however, the pyrolysis atmosphere (vacuum or inert purge pyrolysis) did not appreciably influence the properties within the experimental conditions studied. The highest permselectivities attained by a membrane were H2/N2 4700, He/N2 2800, CO2/N2 122, and O2/N2 36, respectively, at 308 K. The permeabilities of the selected gases were shown to be in the order H2 > He > CO2 > O2 > N2 for almost all the membranes, whose order was not exactly in accordance with the order of kinetic gas diameters. From the temperature dependencies of permeability, diffusivity, and sorptivity of the membrane, the anomalous behavior that H2, with a larger kinetic diameter, permeated faster than the smaller He was explained to originate in the larger sorptivity of H2 than that of He.