Nanopore size tuning of polymeric membranes using the RAFT-mediated radical polymerization

Poly(acrylic acid) (PAA) was grafted into the nanochannel walls of track-etched β-PVDF membranes in a controlled manner by RAFT polymerization. PAA-g-PVDF copolymers with various degrees of grafting from 5% to 63% were characterized by ATR-FTIR, X-ray photoelectron spectroscopy and atomic force microscopy (AFM). The controlled fashion of RAFT mediated grafting was demonstrated by size exclusion chromatography (SEC) and AFM. It was observed that the pore diameter decreases steadily with the degree of grafting (DOG) and pores start to be filled by the grafted PAA beyond ~40 wt% DOG, based on AFM measurements and ~15 wt% DOG, based on electrochemical analysis. The synthesized nanoporous membranes were later transformed into highly sensitive functionalized membrane electrodes (FMEs) by deposition of a thin gold (~50 nm) layer onto the membrane surfaces without blocking the nanochannels. The synthesized FMEs have been found to be sensitive to sub-ppb concentrations of Pb2+ in square-wave anodic stripping voltammetry (SW-ASV) measurements. The sensitivities of RAFT mediated FMEs compared to those synthesized by conventional free-radical polymerization were found to be almost three times higher at sub-ppb concentrations of Pb2+ in SW-ASV analysis.