ENHANCEMENT OF THE MONOVALENT CATION PERM-SELECTIVITY OF NAFION BY PLASMA-INDUCED SURFACE MODIFICATION

In this paper, we focus on improvement of the monovalent cation perm-s electivity of a perfluorinated cation-exchange membrane, Nafion 117, b y depositing an anion-exchange layer using a plasma surface modificati on process. The anion-exchange layer was deposited from 4-vinylpyridin e monomer vapor followed by quaternization with 1-bromopropane. The tr ansference number of divalent cation (Fe2+) through the membrane, t(Fe ), decreased with increasing thickness of the plasma polymer layer at the expense of enhanced membrane resistance. A large interfacial resis tance was observed between Nafion and the plasma polymer layer which w as ascribed to the implantation of cationic species containing nitroge n. To avoid the formation of an interfacial layer, a novel method of p lasma-induced surface modification was devised. After a Nafion 117 she et was placed on an RF (radio-frequency) electrode and sputtered with an oxygen or argon plasma in order to produce active sites on the Nafi on, 4-vinylpyridine or 3-(2-aminoethyl)aminopropyltrimethoxysilane vap or was introduced into the reactor to react with radical sites. t(Fe) decreased with increasing RF power. t(Fe) through Nafion modified with 3-(2-aminoethyl)aminopropyltrimethoxysilane was lower than that for N afion modified with 4-vinylpyridine, probably due to its weak Si-C bon d. Nafion treated by the plasma surface modification method exhibited a very high monovalent cation perm-selectivity compared with Nafion tr eated by the plasma polymerization method.