Functional group configuration influences salt transport in desalination membrane materials

Abstract

Five methacrylate-based co-polymers, which have different ratios of 2-hydroxyethyl methacrylate (HEMA) and glycerol methacrylate (GMAOH) co-monomers, were prepared to investigate the influence of functional group configuration on salt transport properties. The HEMA and GMAOH co-monomers were selected because of the differences in position and number of hydroxyl groups in the two molecules. Co-polymer composition was varied systematically from a vicinal diol-rich (GMAOH-rich) configuration to a distributed hydroxyl group (HEMA-rich) configuration. To decouple the impact of functional group configuration and of changing water content on transport properties, all of the materials were prepared to have statistically equivalent water content. Salt sorption was lower and salt diffusion was slower in the HEMA-rich configuration compared to the GMAOH-rich configuration. Microwave dielectric relaxation spectroscopy revealed lower relative permittivity (i.e., dielectric constant) for the HEMA-rich co-polymers, which is consistent with suppressed salt sorption in those materials compared to the GMAOH-rich co-polymers. This observation was complemented by state of water analysis that suggested freezable (i.e., bulk-like) water content decreased as HEMA content increased. Both results suggest that stronger interactions between water molecules and the polymer are favored when hydrophilic functional groups are distributed more evenly throughout the material. These results suggest that functional group position in hydrated polymers influences salt transport properties, and engineering polymers that have a distributed functional group configuration may suppress salt transport properties, which could result in favorable materials for desalination membranes.