Unleashing the piezoelectric potential of PVDF: a study on phase transformation from gamma (γ) to beta (ß) phase through thermal contact poling.

Polyvinylidene fluoride (PVDF) is known for its piezoelectric properties. This material has different crystalline phases, alpha (α), beta (ß) and gamma (γ), where the ß-phase, in particular, is related to the piezoelectric behavior of PVDF. While the transformation from the α-phase to ß-phase in PVDF is well-documented and widely studied, the transformation from γ- to ß-phase has not yet been fully explored. However, when PVDF is produced by certain solution-based methods it can adopt its γ-form, which is not as piezoelectric as the ß-phase. Hence, this study aims to bridge this gap by investigating the transformation from γ- to ß-phase in PVDF nanocomposites films obtained from solution-based techniques. Our PVDF nanocomposite is made by solvent evaporation-assisted 3D printing of PVDF's nanocomposite with barium-titanate nanoparticles (BTO). To achieve the γ- to ß-phase transformation, we first highlight the importance of annealing in the successful poling of PVDF samples. We then perform an in-depth analysis of the α-, ß- and γ-crystallographic phases of PVDF-BTO using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). We observed that after annealing but before poling, the PVDF-BTO nanocomposite contains 76% of ß + γ phases, the majority of which is the γ-phase. Poling of these samples resulted in the combination of the ß + γ phases reaching 93% with the appearance of 40% of absolute fraction of the ß-phase. We then demonstrated that the fraction of ß-phase in the nanocomposite - as indicated by the 1275 cm-1 peak in PVDF's FTIR spectra - is not uniform on the surface area of the film. Additionally, the value of the absolute ß-phase content also depends on the poling field's direction. Our work reveals that while considering PVDF's piezoelectric behavior, it is critical to be aware of these nuances and this article offers essential insights on how to address them. Overall, this study provides a step-by-step guideline to enhance the piezoelectricity of PVDF-based nanocomposites for sensing applications.