Poly(vinylidene fluoride) with grafted 4-vinylpyridine polymer side chains for pH-sensitive microfiltration membranes

Abstract

The thermally induced molecular graft copolymerization of 4-vinylpyridine (4VP) with ozone-preactivated poly(vinylidene fluoride) (PVDF) was carried out in N-methyl-2-pyrrolidone (NMP) solution to produce 4VP-g-PVDF copolymers. The chemical composition and structure of the 4VP-g-PVDF copolymers were characterized by element analysis, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). In general, the graft concentration of the 4VP polymer in the 4VP-g-PVDF copolymer increased with the [4VP]/[PVDF] molar feed ratio used for the graft copolymerization. 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 4VP side chains was observed. The surface morphology of the MF membranes was investigated by scanning electron microscopy (SEM). The pore size and pore size distribution of the 4VP-g-PVDF MF membranes were measured using a Coulter Porometer II apparatus. The flux of the aqueous solutions through the 4VP-g-PVDF MF membranes exhibited a pH-dependent behavior, due to the interactions of the pyridine groups of the grafted 4VP chains with the aqueous solutions through hydrogen bonding and protonation.