Engineering the Water/Salt Sorption Selectivity of Polymers for Desalination Applications

In this paper, we report two synthetic strategies to engineer the water/salt sorption selectivity of polymers: tethering polar functional groups to the polymer backbone and increasing the degree of cross-linking. For the first strategy, we found that at a given water content, the dielectric constant of hydrated methacrylate-based polymers functionalized with hydroxyethyl (i.e., two carbon) side chains (XL - p(HEMA-co-GMA) is less than that of hydrated methacrylate-based polymers with hydroxypropyl (i.e., three carbon) side chains (XL - p(HPMA-co-GMA), which contributes to suppressing salt sorption to increase the water/salt sorption selectivity. For the second strategy, we found that forming densely cross-linked polymers that contained only dimethacrylate-based monomers (XLPEGDMA) relative to less densely cross-linked copolymers containing both methacrylate- and dimethacrylate-based comonomers (XL - p(HEMA)) reduced the network mesh size at a given water content, which also suppressed salt sorption and increased the water/salt sorption selectivity. These structure-property results inform the design of advanced materials for desalination membrane applications.