Responsive Pore Size Properties of Composite NF Membranes Based on PVDF Graft Copolymers

Abstract

Uncharged nanofiltration (NF) membranes can potentially perform size-based separations of small molecules. Thin film composite (TFC) membranes, prepared by coating a porous support membrane with a thin layer of poly(vinylidene fluoride)-graft-poly(oxyethylene) methacrylate (PVDF-g-POEM), were shown previously to have such capability, as well as exceptional fouling resistance and high pure water permeability (Macromolecules 2004, 37, 7663–7668). The selectivity and permeability of these membranes arises from the microphase separation of the PVDF backbone and polyethylene oxide (PEO) side chains into bicontinuous hydrophobic and hydrophilic phases, the PVDF domains providing mechanical integrity, and the PEO domains acting as “nanochannels” whose dimensions control molecular transport through the membrane. This study explores the influence of process and solution parameters on the permeability and selectivity of PVDF-g-POEM TFC NF membranes. By controlling the degree of swelling of the hydrated PEO chains filling the channels, the effective pore size of the membrane is shown to be tunable. Membrane permeability to water and to organic dye molecules of approximately 1 nm diameter is found to increase when the solvent quality of the feed for the PEO chains is reduced by raising the temperature, pressure, or ionic strength, or by the addition of ethanol. This property holds promise for applications in the biochemical, pharmaceutical, and food industries for low-cost, high-throughput fractionation of molecules.