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

Abstract

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.