Facilitated phase transformation of PVDF in its composite with an ionic liquid

Abstract

The phase transition of poly(vinylidene fluoride) (PVDF) from α-to γ′-phase is hardly accessible, especially for the low-temperature crystallized α-phase, owing to the diminished thermal energy available for transformation. In this study, the phase transition ability of α-PVDF is greatly improved in its composites with a typical imidazole ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). The enhanced phase transition is achieved by two thermal steps, including the crystallization of α-phase and subsequent annealing at a higher temperature. The transformed lamellar morphology is dependent on the crystallization temperature of α-phase, while the phase transition efficiency mainly rests with the annealing temperature. The phase transition is always accompanied by the thermal degradation with elimination of HF, which is significantly accelerated by [BMIM][PF6]. The introduction of molecular defects and carbon-carbon double bonds probably increase the fraction of planar chain conformation, therefore, enhancing the phase transition efficiency.