The permeation of gases through symmetric and asymmetric (Loeb-Type) cellulose acetate membranes

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Abstract
Significant differences have been observed in the steady-state permeation of gases through symmetric and asymmetric (Loeb-type) cellulose acetate membranes. The studies were made with O2, N2, Ar, Kr, Xe, and CO2 in the temperature range from -5 to 85°C and at subatmospheric pressures. The differences in permeation behavior may reflect structural differences between the symmetric membranes and the dense surface layer (“skin”) of the asymmetric membranes. The overall mechanism of gas permeation through the symmetric membranes appears to be one of “solution-diffusion,” similar to that observed with many other nonporous polymeric membranes. In the case of the asymmetric membranes, this mechanism is probably modified by the presence of micropores or other imperfections in the dense surface layer. Cellulose acetate exhibits two second-order transitions in the presence of the penetrant gases, one between 60 and 70°C and the other near 15°C. The transitions were observed with both types of membranes.

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