In Situ Enhancement in the β-Phase Content of a PVDF Film Achieved by Circumfluent Synergistic Blow Molding Technology

Abstract

High β-phase crystal content of PVDF is commonly achieved by a transition from the kinetically favorable α-phase via various treatments; however, directly enhancing β-phase content from melt crystallization still remains challenging. Especially, during traditional blown film processing, the large condensate depression and rapid cooling rate dominate the crystallization progress, greatly restraining the stretching effect caused by the draw ratio and blow-up ratio, which significantly hinders the α- to β-phase transition and restricts the development of PVDF blown films. Herein, this work introduces Circumfluent Synergistic Blow Molding Technology (CSBMT), which utilizes the rotation of a blow molding die to generate a circumferential shear flow field (CSFF). CSFF can induce molecular chain preorientation during the melt state in the die, which reduces the energy barrier for ordered chain arrangement. Then, the synergistic effect of stretching and CSFF facilitates conformational transitions, realizing an in situ enhancement of β-phase content, which has increased by 50%. Consequently, the PVDF film manifests benign dielectric constant (12 at 50 Hz), low dielectric loss (0.033 at 1 kHz), and high triboelectric voltage (105 V), which satisfies multiple electronic applications. Therefore, CSBMT establishes a paradigm for tailoring PVDF crystallization and broadens a pathway for the development of PVDF blown films.