Synthesis and characterization of poly(acrylic acid)-graft-poly(vinylidene fluoride) copolymers and pH-sensitive membranes

Molecular modification of ozone-pretreated poly(vinylidene fluoride) (PVDF) via thermally induced graft copolymerization with acrylic acid (AAc) in N-methyl-2-pyrrolidone (NMP) solution was carried out (the AAc-g-PVDF copolymer). The microstructure and composition of the AAc-g-PVDF copolymers were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), elemental analysis, and thermogravimetric (TG) analysis. In general, the graft concentration increased with the AAc monomer concentration used for graft copolymerization. Microfiltration (MF) membranes were prepared from the AAc-g-PVDF copolymers by the phase inversion method. The bulk and the surface compositions of the membranes were determined by elemental analysis and XPS, respectively. XPS analyses of the copolymer membranes revealed a substantial surface enrichment of the hydrophilic AAc polymer graft. The pore sizes of the pristine PVDF and the AAc-g-PVDF membranes were measured using a Coulter porometer. The morphology of the membranes was studied by scanning electron microscopy (SEM). The rate of permeation through the AAc-g-PVDF MF membranes changed reversibly in response to pH variation of the aqueous solution, with the most drastic change in permeation rate occurring between pH 2 and 4.