Acid/base properties of poly(4-vinylpyridine) anchored within microporous membranes

Membranes consisting of poly(4-vinylpyridine) anchored within the pores of microporous polypropylene and polyethylene membranes exhibit a very large, fully reversible change in permeability over a very narrow pH range (pH val ve). A detailed examination of the acid/base properties of the incorporated poly(4-vinylpyridine) has been undertaken in order to understand the facto rs affecting the position (pH) at which this valve operates. It was shown t hat the position and magnitude of the valve is the same when either HCl, H3 PO4, or CH3COOH are used to adjust the acidity of the feed solution, indica ting that pH of the aqueous phase is the major determining factor controlli ng the valve operation with these acids. However, the valve behavior of the membrane with H2SO4 was found to be completely different than with the oth er acids in that the valve both closed at a substantially higher pH than wi th the other acids and then fully re-opened when the pH was decreased below 3. Potentiometric titrations of membranes containing poly(4-vinylpyridine) and control experiments involving solutions/suspensions of the homopolymer in water were undertaken. It was found that there are substantial differen ces in the protonation of poly(4-vinylpyridine) both in terms of its enviro nment (membrane bound or in solution) as well as with the acid used. The di fferences in the pK observed between H2SO4 and the other acids are discusse d in terms of conformational changes of poly(4-vinylpyridine) which are ind uced by both protonation and the counter-ion (anion) present. The results o f potentiometric titrations parallel the valve behavior of the membranes. T he conformational changes underlying the pH valve effects in different acid s were visualized by atomic force microscopy and followed by thickness chan ges in the membranes. (C) 1999 Elsevier Science B.V. All rights reserved.