Transport properties of anion exchange membranes prepared by the reaction of crosslinked membranes having chloromethyl groups with 4-vinylpyridine and trimethylamine

Abstract

The electrodialytic transport properties of new anion exchange membranes were evaluated that included the transport numbers of various anions, sulfate, bromide, fluoride, and nitrate ions, relative to chloride ions and current efficiency. The anion exchange membranes were prepared by the reaction of copolymer membranes crosslinked to different extents having chloromethyl groups with 4-vinylpyridine to form a ladder-like polymer in the membranes and then with trimethylamine to convert the remaining chloromethyl groups to benzyl trimethylammonium groups. The transport numbers of the sulfate and fluoride ions relative to the chloride ions were markedly less for the membranes that had been reacted with 4-vinylpyridine and then with trimethylamine compared with those of the membranes that had been reacted only with trimethylamine. On the other hand, the selective permeation of nitrate and bromide ions through the membranes was enhanced by the reaction with 4-vinylpyridine although the membranes became tighter by the reaction. The decrease in permeation of the sulfate ions was attributed to a synergistic effect involving the decrease in sulfate ions ion-exchanged with the membranes and the decrease in mobility of the sulfate ions in the membranes with a low degree of crosslinking. Though the ion-exchanged sulfate ion content was the lowest in the highly crosslinked membranes, the mobility ratio between the sulfate ions and chloride ions did not decrease in the membranes. However, the increase in the permeation of nitrate ions was based on the increase in the ion-exchanged amount of nitrate ions with the membrane, and not the change in the mobility ratio between the nitrate and chloride ions. The formation of the ladder-like polymer in the membrane matrix brought on a decrease in the hydrophilicity of the membranes due to pyridine groups and an increase in their tightness. The current efficiency of all membranes was greater than 99% during the electrodialysis of 0.50 N salt solutions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1773–1785, 1999