Energy‐induced polymorphic changes in poly(vinylidene fluoride): How ultrasonication results in polymer with predominantly γ phase

ABSTRACT Self‐polarized poly(vinylidene fluoride) (PVDF) films were prepared via solution crystallization technique wherein the polymorphism of the films was controlled from α phase (>85%) to γ phase (>90%) by varying the time of ultrasonication. On increasing ultrasonication time up to 60 min, γ phase crystallites were found to be self‐aligned in the matrix while an equal proportion of α and γ phases coexist in the PVDF films ultrasonicated for 120 min. The phase conversion as well as inversion was evident from Fourier transform infrared, X‐ray diffraction, and differential scanning calorimetry analyses. Microscopic images of films ultrasonicated for 60 min showed a scrolled lamellar morphology while those sonicated for 120 min showed mixture of scrolled lamellar and spherulitic morphology. With the help of computational studies, it is explained that a large amount of energy is required for transforming trans‐gauche‐trans‐gauche into trans‐trans‐trans‐gauche conformation which is provided by ultrasonication. The mechanism of γ phase formation is proposed based on the experimental and theoretical approaches. Our studies show that just by tuning the time of ultrasonication, PVDF films with various morphologies can be processed; either one with predominantly electroactive γ phase with superior electrical properties or one with equal proportion of α and γ phases with superior mechanical properties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 40–50 Ultrasonication of Poly(vinylidene fluoride) (PVDF) up to 60 min produces a mixture of α and γ phases, whereas more than 60 min provides predominantly γ phase. The PVDF film ultrasonicated for 120 min with equal proportion of polar and non‐polar phases showed superior mechanical properties, whereas the one ultrasonicated for 60 min had improved electrical properties. The changes in morphology due to different extent of ultrasonication are evident in field emission scanning electron microscopy and atomic force microscopy images.