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.