Increasing salt size selectivity in low water content polymers via polymer backbone dynamics

Two low water content copolymers, rubbery 2-hydroxyethyl acrylate-co-ethyl acrylate (HEA-co-EA) and glassy 2-hydroxyethyl methacrylate-co-methyl methacrylate (HEMA-co-MMA), were crosslinked with poly(ethylene glycol) diacrylate and studied to investigate the influence of polymer backbone segmental dynamics on salt transport size selectivity. These copolymers were chosen for chemical similarity and because HEA-co-EA is rubbery while HEMA-co-MMA is glassy at room temperature. Thermal analysis indicated that the copolymers are relatively homogeneous as a single glass transition temperature was observed for each material. The copolymers were prepared to have low water content (approximately 8% by mass) similar to the reported water content of many commercially available desalination membrane polymers. At low water content, most of the water in the polymer is expected to interact either with the polymer backbone or sorbed ions, suggesting that the influence of backbone segmental dynamics on the transport properties of these low water content materials may be different from the situation in more highly hydrated polymers. Salt sorption similarly increased with water content for the two copolymers due to their chemical similarity. Size selectivity was quantified as ratios of the salt permeability and diffusion coefficients to the corresponding values for a reference salt, and salt size was quantified using the salt diffusivity in bulk aqueous solution at infinite dilution to take into account both ion size and hydration properties. At comparable water content, the salt permeability and diffusion coefficients of the rigid HEMA-co-MMA copolymer decreased to a greater extent as a function of salt size compared to the more flexible HEA-co-EA copolymer. These results suggest that more rigid polymer backbones may drive increases in size-based permeation and diffusion selectivity when polymer water content is sufficiently low.