Preparation of High-Performance Polyvinylidene Fluoride Films by the Combination of Simultaneous Biaxial Stretching and Solid-State Shear Milling Technologies

Different from the traditional uniaxial stretching films, biaxially stretched films have excellent properties and broad application prospects. In this paper, by adopting simultaneous biaxial stretching (BO) and solid-state shear milling (S3M) technologies, polyvinylidene fluoride (PVDF) films with more β phases and homogeneous structures as well as better comprehensive properties were prepared. The powerful squeezing and three-dimensional shearing forces of S3M equipment promoted the rearrangement of PVDF macromolecular chains, so this endowed PVDF milled powders with the β phase of 91.1%. On this basis, the melt-extruded precursor sheets with a relatively high β-phase content were obtained, and even some phase reconversion still occurred during the melt extrusion. When suffering subsequent simultaneous biaxial tensile stress, the arrangement of PVDF molecular chains and the conversion of α-phase PVDF to the β phase were further promoted, leading to the increased β-phase content and crystallinity of the stretch films as well as a high dielectric constant, e.g., for the film with a 3.5 × 3.5 draw ratio, the β phase, crystallinity, and dielectric constant at the frequency of 1000 Hz were 53.9%, 64.3%, and 7.4, approximately 7.8%, 9.1%, and 1.6 times higher, respectively, than those of the film only directly obtained via BO technology.