Studies on phase transformations and crystallinity changes of PVDF thin films via hot-pressing treatment

Abstract

The precise regulation of mechanical, chemical, and electrical properties in poly(vinylidene fluoride) PVDF thin films is imperatively required for each specialized application such as chemical and electrical insulators, and ferroelectric energy applications. Thus, extensive research endeavors have been dedicated to the manipulation and control of crystalline phases (specifically α, β, and γ) and overall crystallinity, which are the fundamental determinants of PVDF thin film functionality. In this study, we examined the phase transformation and changes in the degree of crystallinity of PVDF thin films subjected to hot-pressing treatment. The PVDF thin films were initially fabricated using three distinct methods: casting, spin-coating, and electrospinning techniques. Following hot-pressing, the β-phase content generally increased, accompanied by a decrease in the α and γ phases in PVDF thin films. This enhancement is attributed to the stretching and alignment of PVDF macromolecular chains under elevated pressure during the hot-pressing process, which promotes β-phase formation. Additionally, it was elucidated that PVDF thin films with high microscopic porosity prior to hot-pressing exhibited the most significant transformation of α and γ phases to β-phase after hot-pressing. This finding implies that, under high-pressure conditions, the inherent low density of PVDF thin films offers adequate morphology for the stretching of polymer chains. Such structural properties favor the formation of the β-phase crystalline structure.