Structural Dispersity as a Determinant of Li-Ion Transport in Ethylene-Oxide-Based Graft Polymer Electrolytes

Graft polymers with oligo-(ethylene glycol) (OEG) side chains and poly-(meth)-acrylate backbones have been commonly studied as polymer electrolytes (PEs) owing to the ability of oligoether segments to coordinate Li⁺ ions. However, when poly-[oligo-(ethylene glycol) methyl ether methacrylate]-s (P-(OEG)-MAs) are synthesized from commercial macromonomers, these are structurally polydisperse, as OEG segments feature a broad distribution of lengths. Herein, we investigate the influence of side-chain heterogeneity on Li-ion transport by comparing structurally polydisperse P-(OEG)-MAs with analogous graft polymers with homogeneous architecture, generated from discrete macromonomer feeds obtained through flash chromatography. Ionic conductivity was found to increase with increasing side-chain dispersity. For structurally polydisperse P-(OEG)-MAs, enhancing side-chain heterogeneity resulted in greater salt dissociation and higher ionic conductivity at relatively high salt contents. These trends are uncorrelated with differences in thermal properties, rheology, and polymer diffusivity, indicating that ion transport is not governed by overall polymer dynamics. Dispersity of side chains thus emerges as a determinant for Li-ion transport in PEs based on P-(OEG)-MAs. However, this effect is lost when backbone flexibility increases, i.e., when polymethacrylates are substituted with more flexible polyacrylate counterparts. By elucidating how side-chain heterogeneity and backbone flexibility affect ion transport, this work provides guidance for the rational design of graft PEs.