Engineering Lithium–Magnesium Selectivity in Hydrated Polymer Membranes through Polymer Backbone Rigidity

Using computer simulations and experiments, we demonstrate that polymer backbone rigidity can be used to tune selectivities and permeabilities of lithium over magnesium in hydrated polymer membranes. Coarse-grained molecular dynamics (CGMD) simulations suggest a strong dependence of cation diffusion coefficients on polymer segmental dynamics and cation-solvent coordination strength, with water content and backbone dynamics having distinct effects on transport properties. Experimentally, we synthesized 2-hydroxyethyl acrylate-<i>co</i>-ethyl acrylate (HEA-<i>co</i>-EA) and 2-hydroxyethyl methacrylate-<i>co</i>-methyl methacrylate (HEMA-<i>co</i>-MMA) membranes. These polymers have different levels of backbone flexibility while maintaining similar chemistry. LiCl and MgCl₂ salt permeabilities and sorption coefficients were measured for membranes with varying water content. Magnesium chloride permeability and diffusion coefficients show a stronger dependence on backbone dynamics than lithium chloride, whereas backbone dynamics has a minor impact on salt sorption. Overall, these factors allow permeability and selectivity of LiCl relative to MgCl₂ to be increased simultaneously by increasing both water content and backbone rigidity.