Thin Solid Films

Gas transfer in supported films made by molecular self-assembly of ionic polymers

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Asymmetric membranes for gas separation were fabricated using the layer-by-layer adsorption process based on the spontaneous self-assembly of alternating layers of cationic and anionic polymers on porous and solid support membranes. The support membranes were first dipped in a dilute solution of poly(allylamine) (a polycation) followed by dipping in a dilute solution of poly(styrenesulfonate) (a polyanion). Repeating this process, 40 polymer layers were deposited on porous poly(propylene) membranes (Celgard 2400) and 100 layers on solid dimethyl silicone membranes. Gas-transfer experiments at several temperatures indicated reduced mass permeabilities due to the adsorbed films. Permeabilities of pure CO2 and N2 through coated Celgard samples were virtually equal at all temperatures indicating dominance of Knudsen diffusion through micropores in the film. Permeabilities in coated silicone membranes indicate higher CO2/N2 selectivity than the bare membrane at elevated temperatures. Microphotography indicated the presence of breaks in the polyion complex coating on the silicone membrane. Fabrication of gas transfer membranes via self-assembly of ionic polymers offers the possibility of designing highly selective membranes as well as the ability to control the thickness and architecture of films at the molecular level.