A method to quantify composition, purity, and cross-link density of the active polyamide layer in reverse osmosis composite membranes using 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy

A method for harvesting and purifying the thin polyamide (PA) active layer from thin-film composite (TFC) reverse osmosis (RO) membranes was developed, enabling quantitative nuclear magnetic resonance (NMR) measurements of the composition and cross-linking in the PA layer that can be directly related to membrane performance. Using our chemical separation process, we report on four trimesoyl chloride (TMC)/isophthaloyl chloride (IPC)/m-phenylene diamine (MPD)-based TFC membranes in which the cross-link density was intentionally reduced by replacing trifunctional cross-linking TMC monomers with their linear IPC difunctional analog. While the NMR results show a two-fold decrease in cross-linking that causes a 30% increase in salt passage, the addition of the difunctional analog leads to increased polar amine groups that reduce water permeance due to tighter binding of water in the PA membrane. Our results demonstrate that 13C cross polarization magic angle spinning (CPMAS) is a powerful method for quantitatively monitoring the purity, cross-linking, and chemical composition in PA membranes and will be an essential tool in ascertaining atomistic models of PA structure.