Polymorphic State and Crystallinity of Ambient-Dried, Solution-Cast PVDF

Abstract

PPoly(vinylidene fluoride) (PVDF) is a technologically important semicrystalline polymer whose electroactive behavior is governed by its crystalline polymorphism. Among its most common phases, the nonpolar α phase and the polar β and γ phases exhibit distinct molecular conformations and electrical properties, making control over polymorph selection a key challenge in the development of PVDF-based piezoelectric, dielectric, sensing, and energy-harvesting materials. Here, we demonstrate that the drying and post-processing history of solution-cast PVDF provides an effective route to tailor its polymorphic state and its response to cold compression. Films were prepared from a fixed 10 wt% PVDF/N,N-dimethylformamide formulation using prolonged ambient evaporation, intermittent-vacuum desiccator drying, vacuum drying under different thermal conditions, post-drying quenching, and sonication-assisted processing. ATR-FTIR, Raman spectroscopy, and XRD were used to identify the crystalline phases, while DSC provided information on crystallinity and thermal transitions. Prolonged ambient evaporation and intermittent-vacuum drying produced γ-dominant films with markedly different first-heating crystallinities (37.1% and 52.7%, respectively), whereas sonication-assisted processing enabled a faster route to γ-rich films with intermediate crystallinity. In contrast, high-temperature vacuum drying promoted the formation of α-dominant material, which remained α-dominant after post-drying quenching. Remarkably, cold compression induced a strongly phase-dependent structural response without measurable loss of crystallinity: α-dominant films underwent pronounced β-like/electroactive enrichment, whereas γ-rich films largely retained their γ character and exhibited only weak β-like enrichment. These results demonstrate that processing history can be used not only to control PVDF polymorphism and crystallinity during solution casting, but also to modulate its subsequent response to mechanical processing, providing a versatile strategy for designing solution-processed PVDF films and composites with tailored electroactive properties.

Keywords: poly(vinylidene fluoride); γ-phase; solution casting