Effect of Annealing on Crystalline Phase and Piezoelectric Properties of PVDF/[BuPy]PF₆ Composites

In this work, the effect of N-Butylpyridinium hexafluorophosphate ([BuPy]PF₆) on the crystalline phase and phase transition of poly(vinylidene fluoride) (PVDF) was investigated in detail. The results show that [BuPy]PF₆ can induce the crystallization of β-PVDF crystals from the melt. With an increase in [BuPy]PF₆ content from 0% to 20%, the content of β-PVDF crystals rises from 16.7% to 65.6%. Taking the composites containing 4% and 20% [BuPy]PF₆ as examples, the in situ annealing process at 160 °C were tracked with the application of FT-IR spectra. It has been confirmed that the annealing process leads to a polar phase transition from α to γ, particularly evident in samples with a 4% [BuPy]PF₆ content, where the polar phase content rises from 48.1% to 61.4%. Furthermore, the α→γ phase transition improves the piezoelectric properties of PVDF films. After annealing, the piezoelectric coefficient d₃₃ of samples with a 4% [BuPy]PF₆ content increases from 4.2 to 5.5 pm/V.