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 valve). A detailed examination of the acid/base properties of the incorporated poly(4-vinylpyridine) has been undertaken in order to understand the factors affecting the position (pH) at which this valve operates. It was shown that the position and magnitude of the valve is the same when either HCl, H₃PO₄, or CH₃COOH are used to adjust the acidity of the feed solution, indicating that pH of the aqueous phase is the major determining factor controlling the valve operation with these acids. However, the valve behavior of the membrane with H₂SO₄ was found to be completely different than with the other acids in that the valve both closed at a substantially higher pH than with 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 differences in the protonation of poly(4-vinylpyridine) both in terms of its environment (membrane bound or in solution) as well as with the acid used. The differences in the pK observed between H₂SO₄ and the other acids are discussed in terms of conformational changes of poly(4-vinylpyridine) which are induced by both protonation and the counter-ion (anion) present. The results of potentiometric titrations parallel the valve behavior of the membranes. The conformational changes underlying the pH valve effects in different acids were visualized by atomic force microscopy and followed by thickness changes in the membranes.