Multi-stage crystallization mechanism of electroactive phase polyvinylidene fluoride induced by thermal and supercritical carbon dioxide processing

Abstract

Polyvinylidene fluoride (PVDF) is garnering interest in many applications ranging from sensors to high-frequency transducers to energy harvesters due to its piezoelectric properties. It is also an inexpensive, conformable, and more environmentally friendly alternative to polycrystalline ferroelectric ceramic lead zirconate titanate. This paper decouples the thermal and supercritical carbon dioxide (ScCO₂) processing of PVDF proposed in our previous study into individual processing steps to elucidate the multi-stage crystallization mechanism of electroactive phase PVDF. Differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy were employed to analyze the processed PVDF samples. As a result, the processing-to-structure relationships with respect to each crystal polymorph (i.e., α, β and γ phases) in different phases during the multi-stage mechanism were successfully identified. The γ phase content in PVDF was predominantly governed by isothermal and non-isothermal crystallization before the ScCO₂ injection as well as ScCO₂-assisted isothermal crystallization before foam expansion. In contrast, the β phase content in PVDF was governed by the foaming behaviours of PVDF during the ScCO₂ foaming phase. It was also observed that foam morphologies, including both cell population density and average cell size, governed the amount of local strain in PVDF matrices and thereby the α to β phase transformation during the foam expansion stage. The highest fraction of the PVDF electroactive phase achieved by the thermal and ScCO₂ processing (i.e., 72.2%) was shown to be slightly better than that yielded by mechanically stretching the PVDF film (i.e., 68%).