Dynamics and charge transport in PVDF-HFP/protic ionic liquid (PIL) membranes: The effect of PIL concentration

The performance of high-temperature proton exchange membranes is critically determined by the interplay between polymer segmental dynamics and ionic transport. In this work, we investigate how the incorporation of a protic ionic liquid (PIL) modifies the molecular dynamics and charge transport behavior of PVDF-HFP-based composite membranes. The glass transition temperature exhibits a pronounced nonmonotonic dependence on PIL content. At low PIL loadings, polymer mobility is unexpectedly reduced, which can be ascribed to strong dipole-dipole interactions between the polymer matrix and the ionic species. Upon further increasing the PIL concentration, these interaction-related constraints are progressively released. This evolution is accompanied by a reduction in crystalline order and enhanced ionic dissociation, giving rise to a substantial increase in proton conductivity. Concomitantly, the dominant relaxation behavior shifts from localized dipolar motions toward cooperative segmental dynamics, reflecting systematic PIL-induced modifications of the polymer environment. In this context, the results clarify how polymer rigidity and ionic mobility are jointly regulated in such composite systems and provide a physically consistent basis for tailoring solid-state proton conductors for operation at elevated temperatures.