Chemical valves based on poly(4-vinylpyridine)-filled microporous membranes

Microporous membranes containing poly(4-vinylpyridine) anchored within the pores exhibit very large chemical valve effects with pressure-driven permeability changing by more than three orders of magnitude as a function of pH. The factors affecting the magnitude of this valve have been examined with a series of well characterized, poly(4-vinylpyridine)-modified, microporous polypropylene membranes. The permeability of these membranes to HCl/H2O at different pHs was measured as a function of pore size of the starting base membrane and the amount of poly(4-vinylpyridine) anchored within the pores. An analysis of these results shows that the magnitude of the chemical valve is largely determined by the permeability of the membranes in their open-valve states, i.e., high pH. While the magnitude of the chemical valve effect exhibited by the membranes varied, the pH at which the valve closed was in each case found to be similar and independent of pore size of the starting membrane or amount of poly(4-vinylpyridine) anchored within the pores. The permeability of the membranes at low pH (when the anchored poly(4-vinylpyridine) is ionized) was examined using two existing models of hydrodynamic permeability, namely, the pressure-driven flow through a right-cylinder pore partly obscured by a graft layer of a hydrodynamic thickness LH (brush model) and the hydrodynamic flow through supported hydrogels (pore-filled model). The theory of polyelectrolytes in the semi-dilute region was used to estimate the chain parameters of the incorporated polyelectrolyte. The results obtained show that the applicability of each model depends on the pore size of the substrate membranes and the molecular weight of the polyelectrolyte.