Thermal, Viscoelastic, and Dielectric Properties of Vinylimidazolium-Based Poly(ionic liquid)/Ionic Liquid Mixtures

Abstract Poly(ionic liquid)/ionic liquid mixtures are promising polymer electrolyte materials, yet their ion transport mechanisms remain unclear. Here, we investigate the thermal, viscoelastic, and dielectric properties of mixtures of poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (PC4-TFSI) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI). Differential scanning calorimetry reveals two glass transitions at intermediate compositions, indicating local compositional heterogeneity. Linear viscoelastic measurements show that Bmim-TFSI acts as a conventional plasticizer, modifying polymer dynamics while following typical scaling behavior for polymer solutions. Dielectric analysis demonstrates that ion transport becomes increasingly decoupled from structural relaxation with increasing polymer fraction, with a maximum at intermediate compositions. Comparison of dielectric and viscoelastic relaxation spectra reveals that ion transport is governed by structural relaxation in homogeneous systems, but becomes decoupled in heterogeneous mixtures, where ions preferentially migrate through regions associated with the lower glass transition. These results highlight the key role of local heterogeneity in ion transport and provide insights for designing high-performance polymer electrolytes.