# Tunable Nanoscale Structure via Divalent Ion Identity in Charged-Neutral Polymer Blends

## TL;DR

This paper shows how different divalent ions affect the nanoscale structure of polymer blends used in battery electrolytes.

## Contribution

The study experimentally confirms that counterion identity can control nanoscale morphology in charged-neutral polymer blends.

## Key findings

- Mg²⁺ ions induce microphase separation and nanostructures with short-ranged ordering.
- Ca²⁺ ions lead to more homogeneous blends with a single glass transition temperature.
- Counterion identity is a design parameter for controlling nanoscale morphology in polymer blends.

## Abstract

Charged-neutral polymer blends, wherein an ion-containing
polymer
is blended with a neutral polymer, are potential candidates for battery
electrolytes due to their improved ion transport properties and electrochemical
stability. Though electrostatic interactions in charged polymer blends
can theoretically stabilize ordered nanostructures analogous to those
observed in neutral block copolymers, direct experimental evidence
remains limited. Here, we investigate the effects of divalent cation
identity on the nanoscale morphology of charged-neutral polymer blends
composed of poly­(ethylene oxide) (PEO) and Mg2+ or Ca2+ ion-containing polymers, poly­[3-(methylacryloxy)­propylsulfonyl-1-(trifluoromethanesulfonylimide)]
(P­(Mg­(MTFSI)2) or P­(Ca­(MTFSI)2). By tuning the
size of the divalent counterion, we are able to precisely tune the
ion solvation between the free cation and PEO, which acts as a solvent
in this system. Differential scanning calorimetry (DSC) and small-angle
X-ray scattering (SAXS) measurements reveal that Mg2+ and
Ca2+ ions induce distinct structural behavior. In both
systems, the blends become more miscible as the concentration of ion-containing
polymer is increased indicated by increased suppression of PEO crystallinity.
At the highest concentrations of P­(Mg­(MTFSI)2), the blends
undergo microphase separation and generate nanostructures with short-ranged
ordering. In contrast, calcium ions, which are more readily solvated
by PEO, produce more homogeneous blends characterized by a single
glass-transition temperature and featureless SAXS data. The results
demonstrate the novel experimental confirmation that charged-neutral
polymer blends can undergo microphase separation and show that counterion
identity can be exploited as a design parameter to control nanoscale
morphology.