MODIFICATION OF AN ION-EXCHANGE MEMBRANE-SURFACE BY PLASMA PROCESS .3. INTERFACIAL RESISTANCE OF MONOVALENT CATION PERM-SELECTIVE MEMBRANE FROM NAFION

The monovalent cation perm-selectivity of Nafion® 117, perfluoro-sulfonate cation-exchange membrane, was enhanced by depositing on its surface an ultrathin anion-exchange layer prepared by plasma polymerization. The anion exchanger was deposited from a 4-vinylpyridine monomer vapor followed by quaternariza-tion with 1-bromopropane. The resulting anion-exchange layers were found to be pinhole-free and of uniform thickness. The monovalent cation perm-selectivity of the plasmamodified Nafion membrane was determined by measuring the Fe 2+ ion transference number, tFe, in a Li+-Fe2+ system. The influence of the thickness of the plasma layers on the tFe and the membrane resistance was investigated. The tFe decreased with increasing thickness of the plasma polymer layer, but at the cost of enhanced membrane resistance. The resistance of treated Nafion was separated into bulk resistance and interfacial resistance. A large interfacial resistance was observed between Nafion and the plasma polymer layer, and was ascribed to the implantation of cation species. © 1990 The Chemical Society of Japan.