Molecular Dynamics Study on the Loop-type Phase Behavior of Polyvinylpyrrolidone/Ionic Liquid Mixtures

Conventional aqueous and organic solvents impose inherent limitations, whereas polymer/ionic liquid (IL) mixtures offer a broader potential for smart materials. In this study, molecular dynamics (MD) simulations using the OPLS-5 all-atom force field elucidate the microscopic origins of temperature- and structure-dependent phase behavior in polyvinylpyrrolidone (PVP)/imidazolium-based IL mixtures. A thermally induced conformational transition of PVP in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) occurred, evolving from gradual collapse to re-extension, consistent with loop-type phase behavior. Structural parameters, cluster visualizations, and free volume analysis collectively confirmed this transition. Furthermore, comparison among BMIMBF₄, HMIMBF₄, OMIMBF₄, and DMIMBF₄ revealed that longer alkyl chains strengthen hydrophobic interactions and promote phase separation. Radial distribution functions (RDF) further uncover the polymer–solvent interactions governing these phenomena, providing insights for designing tunable, stimuli-responsive materials.

Correspondence: jhcho@dankook.ac.kr