In this work, a simple and efficient approach has been developed for preparing γ-PVDF films with a wide range of thickness based on the spontaneous diffusion of ionic liquids (ILs) into the α-PVDF matrix. It involves three steps: (i) preparation of α-PVDF films, (ii) surface coating of the resultant α-PVDF films with ionic liquids or quaternary ammonium solution, and (iii) annealing the surface modified PVDF thin film at a high temperature. In this way, the original α-PVDF film is then transferred into a γ-rich one through the well-known α−γ′ phase transition. The α−γ′ transition is found to be much faster and more efficient for high-temperature prepared samples. It has been further confirmed that the α−γ′ phase transition includes the induction and slow phase transition stages for low-temperature fabricated samples as well as the rapid and then slow transition stages for the samples prepared at 120 and 150 °C.

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The polar phase of poly(vinylidene fluoride) (PVDF) is attractive for fabricating electroactive devices. However, the nonpolar α form is thermodynamically most accessible during the crystallization process. Here three types of aliphatic polyesters are chosen for blending with PVDF to explore their effect on inducing polar γ phase crystallization of PVDF. It is found that the ability of γ phase nucleation of PVDF enhances as the blend ratio of aliphatic polyesters and isothermal crystallization temperature increase. Poly(butylene succinate) (PBS) with highest melting temperature shows the highest ability for inducing γ phase crystallization. For example, the pure γ phase has been obtained under an appropriate blend ratio and isothermal crystallization temperature in PVDF/PBS blends. This is attributed to the hydrogen bonding interaction of H atoms in PVDF with carbonyls in PBS and the slow diffusion rate of PBS chains in PVDF. The addition of PBS lowers the folding surface free energy of the γ phase. This work offers a reference for selecting a suitable blending system to obtain pure γ phase PVDF and contribute to a better understanding of the relationship between intermolecular interaction and polymorphic phase selection.

The high dielectric constant of PVDF is very important for its high-performance energy storage dielectrics, but it is more critical to strike a proper balance between high dielectric constant and low dielectric loss. In this work, deep eutectic solvent (DES) and montmorillonite (MMT) have been used together to prepare γ-phase PVDF composite films with excellent dielectric properties. It is found that DES significantly induces crystallization of γ-phase and increases the dielectric constant of the composite due to the incremental ion motion. Meanwhile, DES greatly promotes the stripping and dispersion of MMT layers, which in turn inhibits the long-range ion migration and therefore suppresses the dielectric loss of the composite to a very low level. The dielectric constant of PVDF/MMT/DES composite films can increase to 32, while the dielectric loss remains as low as 0.06 at 1 kHz. Moreover, the synergistic effect of MMT/DES also contributes to enhancing the toughness and optical transmittance of composite films.

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