pH-sensitive fluorinated polyimides with grafted acid and base side chains

Abstract

Molecular modification of the ozone-pretreated fluorinated polyimide (FPI) via thermally induced graft copolymerization with either acrylic acid (AAc) or 4-vinylpyridine (4VP) in an N-methyl-2-pyrrolidone solution was carried out. The resulting FPI copolymers with grafted AAc and 4VP side chains (the PAAc-g-FPI and P4VP-g-FPI copolymers, respectively) were characterized by Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, and differential scanning calorimetry. In general, the graft concentration increased with the monomer concentration used for graft copolymerization. Microfiltration (MF) membranes were prepared from the PAAc-g-FPI or P4VP-g-FPI copolymers by phase inversion in aqueous media with pH values ranging from 1.0 to 6.4. The surface composition of the membranes was characterized by X-ray photoelectron spectroscopy. A substantial surface enrichment of the grafted AAc and 4VP polymer was observed for the copolymer membranes. The morphology of the MF membranes was studied by scanning electron microscopy. The pore sizes of the MF membranes were measured using a Coulter porometer. The flux of aqueous solutions through the PAAc-g-FPI and P4VP-g-FPI MF membranes exhibited a pH-dependent behavior but in an opposite manner, with the most drastic change in the permeation rate being observed at solution pH values between 1 and 4.