Preparation\nof Chemically-Tailored Copolymer Membranes\nwith Tunable Ion Transport Properties

Abstract

Membranes derived from copolymer\nmaterials are a promising platform\ndue to their straightforward fabrication and small yet tunable pore\nstructures. However, most current applications of these membranes\nare limited to the size-selective filtration of solutes. In this study,\nto advance the utility of copolymer membranes beyond size-selective\nfiltrations, a poly­(acrylonitrile-<i>r</i>-oligo­(ethylene\nglycol) methyl ether methacrylate-<i>r</i>-glycidyl methacrylate)\n(P­(AN-<i>r</i>-OEGMA-<i>r</i>-GMA)) copolymer\nis used to fabricate membranes that can be chemically modified via\nstraightforward schemes. The P­(AN-<i>r</i>-OEGMA-<i>r</i>-GMA) copolymer is cast into asymmetric membranes using\na nonsolvent induced phase separation technique. Then, the surface\ncharge of the membrane is modified to tailor its performance for nanofiltration\napplications. The oxirane groups of the glycidyl methacrylate (GMA)\nmoiety that line the pore walls of the membrane allows for both positively\ncharged and negatively charged moieties to be introduced directly\nwithout any prior activation. Notably, the highly size-selective nanostructure\nof the copolymer materials is retained throughout the functionalization\nprocesses. Specifically, amine moieties are attached to the pore walls\nusing the aminolysis of the oxirane groups. The resulting amine-functionalized\nmembrane is positively charged and rejects up to 87% of the salt dissolved\nin a 10 mM magnesium chloride feed solution. Further modification\nof the amine-functionalized membrane with 4-sulfophenyl isothiocyanate\nresults in pore walls lined by sulfonic acid moieties. These negatively\ncharged membranes reject up to 90% of a 10 mM sodium sulfate feed\nsolution. Throughout the modification scheme, the membrane permeability\nremains equal to 1.5 L m^–2 h^–1 bar^–1 and the rejection of neutral solutes (i.e.,\nsucrose and poly­(ethylene oxide)) is consistent with the membrane\nhaving a single well-defined pore diameter of ∼5 nm. The performance\nof the membrane as a function of ion valence number, solution pH,\nand ionic strength is investigated.