Properties of reverse osmosis membranes prepared from poly(4-vinyl pyridine)-acrylonitrile copolymer and the partially quaternized copolymer

Abstract

Reverse osmosis membranes were prepared from 4-vinyl pyridine(4VP)-acrylonitrile (AN) copolymers and those partially quaternized. Their salt rejection ( S) and hydraulic water permeability ( K 1) were investigated with regard, to co-polymer composition, the membrane profile by SEM observation, cross-linking quaternization with diiodobutane (DIB) and the method of membrane preparation. S decreased with increasing AN content, but K 1 was closely dependent on water content which had a minimum at about 65% 4VP. The increase in K 1 and water content in a high AN range was caused by the change in the membrane structure from dense to porous one. Quaternization with DIB to a small extent notably improved K 1, up to ten times without significant change in S, and most effectively improved both of the membrane structure and the performance of AN rich copolymer membranes which were the highest in tensile strength among the copolymers. It was found by IR and elemental analysis that DIB charged was nearly completely reacted. The DIB modified membranes were anion exchangeable and more than two-thirds of iodide were exchanged for chloride. The modification reduced the membrane strength, to the least extent in the AN rich copolymers, with increasing DIB due to an increase in water content. The quaternized membranes prepared by surface-treating with DIB gave an essentially similar performance and had a composite structure which held unreacted layer. Monofunctional quaternizing agents merely brought about a poor membrane performance. Water transport through the membranes followed solution-diffusion mechanism for the membranes of volume fraction of water smaller than 0.41. The membranes modified with DIB to a small degree were regarded as tight ionic membranes and able to be treated as a nonionic ones, although they behaved ionically showing a decrease in S with the increasing feed concentration.