Crystal orientation effect on electric energy storage in poly(vinylidene fluoride-co-hexafluoropropylene) copolymer films

By using different preparation and processing methods, poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] films with different crystal orientations were fabricated. Anisotropic dielectric properties and different electric energy storages were observed in these films. When the PVDF crystals in a film oriented with their c-axes perpendicular to the applied electric field, they exhibited large polarizability, because the CF₂ dipole moments were randomly distributed in a plane parallel to the electric field. As a result, high dielectric constant and high electric energy density were achieved. On the contrary, when the crystal c-axes in a film oriented parallel to the electric field (or the CF₂ dipole moments perpendicular to the electric field), polarization became difficult. Consequently, low dielectric constant and low electric energy density were resulted. The anisotropic polarizability was also displayed at high electric fields as evidenced by the difference in the remnant/maximum polarization and the dipole switching field for different crystal orientations. These results provide us a guidance to achieve optimal crystalline morphology in PVDF random copolymer films for high electric energy storage applications.