Synthesis and Characterization of Poly(N-isopropylacrylamide)-graft-Poly(vinylidene fluoride) Copolymers and Temperature-Sensitive Membranes

Abstract

Molecular modification of ozone-pretreated poly(vinylidene fluoride) (PVDF) via thermally induced graft copolymerization with N-isopropylacrylamide (NIPAAM) in N-methyl-2-pyrrolidone solution was carried out (the NIPAAM-g-PVDF copolymer). The chemical structure and composition of the NIPAAM-g-PVDF copolymers were characterized by Fourier transform infrared spectroscopy, elemental analysis, and thermogravimetric analysis. In general, the graft concentration increased with the NIPAAM monomer concentration used for graft copolymerization. Microfiltration membranes were prepared from the NIPAAM-g-PVDF copolymers by the phase inversion method. The bulk and surface graft concentrations of the membranes were obtained by elemental analysis and X-ray photoelectron spectroscopy (XPS), respectively. XPS analyses of the copolymer membranes revealed a substantial surface enrichment of the grafted NIPAAM polymer. The pore sizes of the pristine PVDF and the NIPAAM-g-PVDF membranes were measured using a Coulter Porometer. The morphology of the membranes was studied by scanning electron microscopy. The membrane surface composition, mean pore size, and morphology varied with the temperature of the casting bath. For the copolymer membrane cast below the lower critical solution temperature (LCST) of the NIPAAM polymer (∼32 °C), the rate of water permeation increased substantially at a permeate temperature above 32 °C. A reverse permeate temperature dependence was observed for the flux of 2-propanol through the membrane cast above the LCST of the NIPAAM polymer.