Preparation and transport properties of anion-exchange membranes containing viologen moieties as anion-exchange groups in the presence or absence of photoirradiation

Abstract

Anion-exchange membranes containing viologen groups have been prepared from chloromethylstyrene and divinylbenzene copolymer membranes and 4,4′-bipyridine. The viologen moieties were reduced by photoirradiation to form radical monocations and biradicals in pure water and various salt solutions. As the viologen moiety was changed from a divalent to a monovalent cation and to an uncharged group upon photoirradiation, the transport numbers of sulfate, bromide, nitrate and fluoride ions, decreased relative to chloride ions. After oxidation of the reduced viologen group of the membranes with air, the transport properties of anions through the membrane became the same as before photoirradiation. The transport properties of the membranes changed reversibly between photoirradiation (reduced form) and oxidation of the reduced form. The decrease in the transport numbers of the various anions relative to chloride ions is due to a sieving effect because of the shrinkage of the polymer network due to a decrease in the charge density of the anion-exchange membranes. The decrease in the pore size of the membrane upon photoirradiation was also confirmed by the decrease in the permeability coefficient of a neutral molecule, urea.