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

## TL;DR

This study shows that side-chain dispersity in graft polymer electrolytes enhances lithium-ion transport, but only when the polymer backbone is not too flexible.

## Contribution

The novel finding is that structural dispersity of side chains, not polymer dynamics, improves ionic conductivity in graft polymer electrolytes.

## Key findings

- Structurally polydisperse P(OEG)MAs show higher ionic conductivity with increased side-chain heterogeneity.
- Salt dissociation and conductivity improve at high salt contents in polydisperse P(OEG)MAs.
- The effect of dispersity is lost when using more flexible polyacrylate backbones.

## Abstract

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.