Anomalous Phase Behavior of Polymer-Ionic Liquid Mixtures

Polymer-ionic liquid (IL) mixtures are promising materials for applications ranging from battery electrolytes and gas separation membranes to responsive actuators and sensors, owing to their unique combination of mechanical robustness and ionic conductivity. These systems often exhibit lower critical solution temperature (LCST) behavior and more symmetric phase envelopes than anticipated from the classical Flory-Huggins theory. In this work, we investigate the molecular origins of this anomalous behavior by systematically incorporating clustering, polymer-cation binding and electrostatic correlations into a simple thermodynamic framework. Our analysis reveals that the formation of clusters contributes to demixing at low temperatures, contrary to both experimental observations and prior theoretical predictions. In contrast, electrostatic interactions introduce persistent asymmetries in the free energy landscape, yielding different chain-length scaling in the critical composition and interaction parameter than predicted by the Flory-Huggins theory. Based on these insights, we propose a minimal, physically grounded model incorporating only mixing entropy, temperature-dependent Flory-Huggins parameter, and electrostatic correlation. This model successfully reproduces key experimental features in systems such as PEO + [EMIM][BF₄], using a small set of interpretable parameters. These results offer a mechanistic understanding of the phase behavior of polymer-IL mixtures and provide a practical modeling framework for their design and optimization.