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

Polyvinylidene fluoride (PVDF) is known for its piezoelectric properties. This material has different crystalline phases, alpha (alpha), beta (beta) and gamma (gamma), where the beta-phase, in particular, is related to the piezoelectric behavior of PVDF. While the transformation from the alpha-phase to beta-phase in PVDF is well-documented and widely studied, the transformation from gamma- to beta-phase has not yet been fully explored. However, when PVDF is produced by certain solution-based methods it can adopt its gamma-form, which is not as piezoelectric as the beta-phase. Hence, this study aims to bridge this gap by investigating the transformation from gamma- to beta-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 gamma- to beta-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 alpha-, beta- and gamma-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 beta + gamma phases, the majority of which is the gamma-phase. Poling of these samples resulted in the combination of the beta + gamma phases reaching 93% with the appearance of 40% of absolute fraction of the beta-phase. We then demonstrated that the fraction of beta-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 beta-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. This novel work provides a step-by-step procedure to enhance piezoelectricity by studying the previously unresearched PVDF's gamma- to beta-phase transformation.