Poly(vinylidene fluoride) with grafted zwitterionic polymer side chains for electrolyte-responsive microfiltration membranes

Thermally induced molecular graft copolymerization of the zwitterionic monomer, N,N'-dimethyl-(methylmethacryloyl ethyl)ammonium propanesulfonate (DMAPS), with the ozone-preactivated poly(vinylene fluoride) (PVDF) was carried out in a mixed solvent of N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). The chemical composition of the resulting PVDF with grafted DMAPS polymer (PDMAPS) side chains, or the PDMAPS-g-PVDF copolymers, was analyzed by elemental analysis. An increase in the [DAMPS]/[-CH2CF2-] molar feed ratio used for graft polymerization gave rise to an increase in the graft concentration of the DMAPS polymer in the copolymer. Microfiltration (MF) membranes were prepared from the DMSO solutions of the copolymers by phase inversion in aqueous media of different ionic strength and temperature. The surface composition and morphology of the MF membranes were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. The mean pore size of the membrane decreased with the increase in graft concentration of the PDMAPS-g-PVDF copolymer. However, it increased with an increase in the ionic strength of the casting bath. Due to the anti-polyelectrolyte effect, the permeability of aqueous solutions through the PDMAPS-g-PVDF MF membranes exhibited a dependence on electrolyte concentration. The flow rate was observed to decrease as the electrolyte concentration of the permeate was increased.