ION-BEAM MODIFICATION OF POLYIMIDE MEMBRANES FOR GAS PERMEATION

The irradiation of 6FDA (hexafluorodianhydrid) polyimide films, produc ed for gas separation, with ion beams leads to dramatic modifications of their permeability to hydrogen and methane. The irradiation of the PI membranes with 2 MeV alpha particles induces a permeability increas e for both gases, whereas a reduction of the permeability to CH4 is ob tained for a bombardment of the films with 170 keV N+ at fluences larg er than 10(15) cm(-2). The modification of the diffusion through the m embranes has been interpreted using a multilayer model, which enabled the calculation of the intrinsic permeabilities of the irradiated laye rs. The second irradiation regime induces a significant selectivity en hancement. The improvement of the selective properties of the irradiat ed membranes is interpreted in terms of modification of the elementary free volumes involving the reduction of the mean size.