Preparation of ion-permeable membranes having an azobenzene moiety and their transport properties in electrodialysis

Ion-permeable membranes having an azobenzene moiety were prepared by the reaction of a copolymer membrane composed of chloromethylstyrene and divinylbenzene with p-aminoazobenzene. Another piece of the membrane with an aminoazobenzene moiety was reacted with trimethylamine further: The membrane having only benzyl trimethylammonium groups was also prepared by the reaction of the copolymer membrane with trimethylamine without the reaction with p-aminoazobenzene. Though ion exchange capacity, water content and electrical resistance of the membrane with benzyl trimethylammonium groups did not change with irradiation of UV light (330-380 nm), those of the membranes with an azobenzene moiety changed by the UV irradiation. Then transport numbers of various anions, sulfate, fluoride, bromide and nitrate ions, relative to chloride ions of the membrane reacted with p-aminoazobenzene in electrodialysis increased by the UV irradiation. This is due to the conversion of the apolar trans form of the azobenzene moiety into more polar cis form, which caused water content and pore size of the membrane to increase. On the other hand, transport numbers of various anions relative to chloride ions of the membrane having both the azobenzene and benzyl trimethylammonium moieties decreased with the UV irradiation, because the pore size and water content of the membrane decreased with the UV irradiation. The change in pore size with the UV irradiation was confirmed with the measurement of permeability coefficient of urea (neutral molecule) through the membranes. Reversible photoisomerization of the azobenzene moiety bonded to the cross-linked membrane was confirmed with the reversible change in transport number of sulfate ions relative to chloride ions in electrodialysis. (C) 2000 Elsevier Science B.V. All rights reserved.