Polyvinylpyrrolidone/Ionic Liquid Mixture with Loop-type Phase Behavior: Experiments and Molecular Dynamics Simulations

Abstract

Dual thermoresponsive systems provide greater adaptability for advanced applications than their single-responsive counterparts. Among them, polymer/ionic liquid (IL) mixtures have shown exceptional thermal responsiveness. In this study, we systematically investigate the phase behavior of polyvinylpyrrolidone (PVP) in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). PVP/BMIMBF₄ films were prepared and analyzed using differential scanning calorimetry (DSC) and light scattering, with a focus on temperature and polymer concentration effects. The system exhibits an unusual loop-type phase behavior, characterized by the presence of both a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST). To elucidate the molecular mechanism underlying this behavior, molecular dynamics (MD) simulations were performed using the OPLS5 all-atom force field. The simulations revealed a thermally induced conformational transition of PVP chains in BMIMBF₄, progressing from gradual collapse to re-extension. Structural parameters and phase-separation cluster visualizations confirmed this transition, while free volume analysis further supported the loop-type phase behavior. These findings offer new insights into the rational design and development of advanced smart materials with tunable properties.