Poly(2-vinylpyridine)- and poly(4-vinylpyridine)-graft-poly(vinylidene fluoride) copolymers and their pH-sensitive microfiltration membranes

Thermally-induced molecular graft copolymerization of 2-vinylpyridine (2VP) and 4-vinylpyridine (4VP) with the ozone-preactivated poly(vinylidene fluoride) (PVDF) was carried out in N-methyl-2-pyrrolidone (NMP) solution to produce, respectively, the 2VP-g-PVDF and 4VP-g-PVDF copolymer. In general, the graft concentration of both the 2VP-g-PVDF and the 4VP-g-PVDF copolymers increased with the respective monomer feed ratio used for graft copolymerization, with 4VP exhibiting a higher graft copolymerization efficiency than 2VP. Microfiltration (MF) membranes were prepared by phase inversion in aqueous solutions of different pH values and from copolymers of different graft concentrations. The surface composition of the copolymer MF membranes was investigated by X-ray photoelectron spectroscopy (XPS). A significant surface enrichment of the more hydrophilic vinylpyridine polymer side chains was observed. However, the surface graft concentration of the MF membranes decreased with the decrease in pH value of the casting bath. The mean pore size of the 4VP-g-PVDF and 2VP-g-PVDF MF membranes generally increased with the decrease in concentration of the casting solution, the decrease in pH value of the casting bath, and the increase in graft concentration. At a fixed graft concentration, the flux of the aqueous solution through the 4VP-g-PVDF MF membrane exhibited a higher pH-sensitivity than the flux through the corresponding 2VP-g-PVDF MF membrane, as a result of a higher degree of protonation of the former by the permeate.