Validity of –CF2 Band Shift in Poly(vinylidene fluoride) as an Indicator for the Ion–Dipole Interaction

Many works have characterized the ion–dipole interaction in poly(vinylidene fluoride) (PVDF) through the shift of the –CF2 band but ignoring the influence of crystal form on its shift. We were devoted to investigating the origins of the band shift for the –CF2 group in this work. The isothermal crystallization processes of the PVDF/cetyltrimethyl ammonium bromide blend film and the pure PVDF film have been studied and compared. It is found that the band of –CF2 is located at 879 cm–1 in both films at the melt state (200–160 °C). During the isothermal crystallization at 160 °C, the ion–dipole interactions induce crystallization of pure γ-phase in the blend film, and the position of the –CF2 band is unchanged throughout the crystallization process, while the pure PVDF film crystallizes the kinetically favored α-phase, and the –CF2 band shifts from 879 to 877 cm–1 with the content of α-phase increasing, indicating the shift of the –CF2 band resulting from the crystal modification of PVDF. On the other hand, in the nonisothermal crystallization process, the virgin and blend films exhibit almost the same crystals, and the position of the –CF2 band is unchanged at the beginning and then gradually shifted to a higher wavenumber of 881 cm–1 with the formation and increasement of β-phase PVDF. These results clearly reveal that the shift of the –CF2 band is a result of the modification of PVDF crystals rather than the ion–dipole interaction.