Porous Membranes of Polysulfone-graft-poly(tert-butyl acrylate) and Polysulfone-graft-poly(acrylic acid): Morphology, pH-Gated Water Flow, Size Selectivity, and Ion Selectivity

Abstract

Membranes were prepared from two samples of polysulfone-graft-poly(tert-butyl acrylate), PSf-g-PtBA, by immersion into water of solution films of the polymers first solvated in 1-methyl-2-pyrrolidinone (MP) and propionic acid (PA). Polysulfone-graft-poly(acrylic acid), PSf-g-PAA, membranes were obtained after soaking the PSf-g-PtBA membranes in trifluoroacetic and formic acid to hydrolyze PtBA. Both PSf-g-PtBA and PSf-g-PAA membranes had high water permeability. An electron microscopic study revealed that the membranes were porous and asymmetric. In the skin layer, the polymer formed a nodular phase consisting of fused polymer lumps surrounded by void channels. The PAA chains and presumably also the PtBA chains were concentrated at the lump/lump or lump/void interfaces. At a constant pressure, the water flux across PSf-g-PAA membranes decreased with increasing solution pH probably due to the extension of the PAA chains into the channels at high pH. The PSf-g-PAA membranes also showed permselectivity toward anions of lower valence and may be used to separate NaCl from Na2SO4, for example. A permeation test using poly(ethylene glycol) samples of different molar masses showed that membranes of a PSf-g-PAA sample had a size of ∼2 nm at constrictions of the nanochannels.