Dual-Functional Nanofiltration Membranes Exhibit Multifaceted Ion Rejection and Antifouling Performance.

Abstract

Charged functional groups are often incorporated onto the surface of nanofiltration (NF) membranes to facilitate the selective rejection of multivalent ions over monovalent ions. However, since fouling-resistant surfaces tend to be electrically neutral, the incorporation of charged functionality exacerbates membrane fouling. Multifunctional Janus membrane architectures, which incorporate chemically distinct domains over their cross section, provide a strategy for balancing the competing demands associated with making fouling-resistant, ion rejecting NF membranes. Here, through the controlled exposure of poly(trifluoroethyl methacrylate-co-oligo-(ethylene glycol) methyl ether methacrylate-co-(3-azido-2-hydroxypropyl methacrylate)) copolymer substrates to a series of reactive solutions containing alkyne-terminated molecules, the process for creating dual-functional membranes by using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction was analyzed. Under the appropriate conditions, the CuAAC reaction propagates into the copolymer substrate as a front. This phenomenon results in a process for creating layered domains of distinct functionality whereby the distribution of antifouling zwitterionic moieties and ion rejecting sulfonate moieties can be modified by manipulating the exposure time. The ion rejection and fouling propensity for a family of dual-functional membranes was examined. For short initial reaction times, which introduced a thin antifouling layer on top of an ion rejection layer, the rejection of 1 mM K2SO4, 87%, was comparable to the value for full charge control membranes, 90%. Moreover, when exposed to a fouling solution containing bovine serum albumin (BSA), these dual-functional membranes exhibited an 18% decline in normalized flux and recovered 99% of their flux upon rinsing with water. In comparison, the full charge membranes exhibited a 44% decline in normalized flux and recovered 65% of their flux upon washing. As such, the results demonstrate that the controlled functionalization process reported here is capable of balancing antifouling and ion rejection capabilities. Furthermore, the versatile nature of the click chemistry mechanism at the center of this process offers a means by which to design and fabricate multifunctional membranes for numerous future applications.