Gas Transport Properties of Ion-irradiated Membrane with Nano-channel

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In this research, the gas permeance and selectivity of the asymmetric polyimide membrane irradiated by ions were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76cmHg and 35°C. The asymmetric polyimide membranes were prepared from a dry-wet phase inversion process, and the surface skin layer on the membrane was irradiated by ions in the fluence range from lx1012 to 3x1015(He+/cm2) at 50keV. We demonstrated that the gas selectivity of the He+-irradiated membrane increased with an increase in the ion fluence and that the depth profile of the energy loss for the irradiating ions in the skin layer had a significant influence on the gas permeability and selectivity of the asymmetric membrane.The gas permeances for the membrane showed an increase at a low ion fluence and a decrease at a high ion fluence. On the other hand, the gas selectivity increased with an increase in the ion fluence. One possibility may be that the gas permeation space, which can provide a high degree of size and shape discrimination between the gas molecules, was formed on the ion-irradiated skin surface and suppressed the permeability of N_2 and CH_4 rather than that of O_2 and CO_2. Another interesting conclusion is that the thickness of the ion-irradiated skin layer had a significant influence on the gas permeability and selectivity. The polyimide structure in the ion-irradiated surface layer would be closely correlated to the amount of ion energy loss so that the structure may determine the free volume and the free volume distribution formed in the membrane.It is desirable to prepare a polymer membrane with both a high gas permeability and selectivity for gas separation. The ion-irradiated asymmetric polyimide membrane prepared in this study showed interesting gas permeation results, and we consider that the ion irradiation would be one of the important membrane fabrication techniques for realizing a high gas permeability and selectivity.