Effect of stretching orientation on the crystalline structure and energy storage properties of poly(vinylidene fluoride) films

Poly(vinylidene fluoride) (PVDF) with a high content of beta phase shows great potential for applications in the pulse energy storage field because of its high dielectric constant and breakdown strength. The stretching process can significantly induce the crystal phase transformation and change the structure of the aggregated state. To deeply investigate the influence mechanism of the fabrication process, solution casting (SC), melt stretching (MS), static biaxial orientation (SBO), and continuous biaxial orientation (CBO) were used to fabricate PVDF films. The essential difference between the four technological routes lies in the different forms of tensile orientation imposed. Stretching perpendicular to the machine direction promotes the transition of the original alpha phase crystals to the beta phase. Stretching orientation can regulate the crystalline phase structure of the film and its applicability. The relative fraction of beta phase crystals in the PVDF-CBO film reaches 84.1 %, resulting in a much higher dielectric constant (12.22 @1 kHz) than that of the PVDF-SC film (7.16). The continuous biaxially oriented PVDF exhibited excellent insulation properties with a breakdown strength of 759.21 kV/mm. Meanwhile, the CBO film obtained an energy storage density of 16.76 J/cm3 with a charge-discharge efficiency of 80.13 % at 675 kV/mm. This work is of great scientific significance for the in-depth study of the influence of different filmforming processes on the internal structure of films, ranging from laboratory fabrication to industrialized production.