Crystalline structure control of poly(vinylidene fluoride) films with the antisolvent addition method

Poly(vinylidene fluoride) (PVDF) is a well-known ferroelectric polymer possessing superior electrical characteristics such as piezoelectricity and pyroelectricity. These electrical characteristics are caused by the polarized structure between hydrogen (δ+) and fluorine (δ−). The crystalline structures of PVDF may be present in at least four types: α, β, γ and δ phases. PVDF β phase (form I) is an all trans planar zigzag conformation (TTTT), PVDF α phase (form II) is a trans-gauche twist conformation (TGTG′), PVDF δ phase (form IV) is a different packing structure of PVDF α phase and PVDF γ phase (form III) is an intermediate conformation of PVDF β and α phases (TTTGTTTG′).^{1, 2} The calculated peak differences of PVDF α and δ phases in x-ray diffraction (XRD) and infrared spectroscopy (IR) are significant only with respect to the intensity, and the experimental patterns and spectra are similar. The differences in the PVDF α and δ phases are paraelectric and ferroelectric. PVDF β phase possesses superior ferroelectric properties because of the well-oriented polarized structure of the all trans planar zigzag conformation. However, the crystalline structure of PVDF obtained by the melting and solvent evaporation method is generally PVDF α phase. The intramolecular interaction energy of PVDF β phase is −0.48 kcal mol⁻¹ molecular unit (m.u.), whereas that of PVDF α phase is −1.46 kcal mol⁻¹ m.u.¹ The intermolecular interaction energy of the PVDF β phase is −5.25 kcal mol⁻¹ m.u., whereas that of the PVDF α phase is −4.57 kcal mol⁻¹ m.u.¹ On the basis of these data, the total potential energy of the PVDF β phase is −5.73 kcal mol⁻¹ m.u., and that of the PVDF α phase is −6.03 kcal mol⁻¹ m.u.¹ Therefore, the PVDF α phase is more stable than the PVDF β phase in terms of potential energy. The PVDF β phase is typically produced by a poling process while stretching a PVDF α phase film. The control of PVDF crystalline structures has attracted much attention, with many researchers reporting on the production of the PVDF β phase.^{3, 4, 5, 6}

Previously, it was reported that PVDF crystalline structures were prepared separately by the solvent-casting method using different solvents.⁷ Hexamethylphosphoramide (HMPA), acetone and dimethylacetamide (DMAc) were utilized in solvent-casting to prepare the PVDF β, α and γ phase crystalline structures, respectively. We focused on the difference in solvent-boiling points and demonstrated that the crystalline structures of the β, α and γ phases can be prepared separately using only a single solvent by controlling the evaporation conditions.⁸ The crystalline structures of PVDF β, α and γ phases were formed with an evaporation rate of less than 0.0001 g min⁻¹, more than 0.2 g min⁻¹, and between 0.03 and 0.00058 g min⁻¹, respectively. The PVDF β phase was obtained using the spin-coating method by evaporating the solvent for ~2 months.

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