Cation Valency and Size Govern Thermodynamic Interactions and Nanostructure in Single-Ion Conducting Polymer Blends

Abstract Understanding how cation valency and ion identity govern the structure and thermodynamics of single-ion conducting polymer blends remains a central challenge in the design of polymer electrolytes. Here, we investigate blends of deuterated poly(ethylene oxide) (dPEO) with poly[3-(methacryloyloxy)propylsulfonyl-1-(trifluoromethanesulfonyl)imide] containing Na+, K+, Mg2+, and Ca2+ counterions using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). The SANS profiles are quantitatively analyzed using a combination of random phase approximation (RPA) and Lorentzian models to quantify concentration fluctuations and charge correlations. We show that cation valency dictates the dominant structural and thermodynamic behavior: monovalent systems (Na+ and K+) exhibit pronounced concentration fluctuations and systematic composition-dependent structural evolution, whereas divalent systems (Mg2+ and Ca2+) show suppressed concentration fluctuations and more short-ranged charge correlations. At higher compositions of charged polymer, divalent systems further exhibit signatures of phase separation, as evidenced by the emergence of distinct scattering features and multiple glass transition temperatures. These results demonstrate that cation valency primarily governs phase behavior and nanoscale structure, while ion size provides secondary modulation through differences in ion-polymer coordination strength, establishing a direct relationship between cation characteristics, thermodynamic interactions, and nanostructure in charged-neutral polymer blends. Our findings are in agreement with previously developed theory which found that the resulting phase behavior of charged neutral-polymer blends is highly dependent on blend properties such as blend composition, charge concentration and electrostatic strength.