Crystalline Phase Formation of Poly(vinylidene fluoride) from Tetrahydrofuran/N,N‐dimethylformamide Mixed Solutions

The present work focused on the effect of the interactions between poly(vinylidene fluoride) (PVDF) chains and solvent molecules on the structure and crystallization behavior of PVDF in films obtained by solution casting. In a single solvent system, the film cast from the good solvent of N,N‐dimethylformamide (DMF), showed dominantly β‐phase crystals with the highest PVDF crystallinity (50.6%) and the largest spherulite size, about 4 μm, at the top surface. The samples deposited from good swelling agents, such as tetrahydrofuran (THF) and methyl ethyl ketone (MEK), exhibited mainly the original α phase with some amount of β‐phase crystals; the crystallization behavior and the morphology of the surface were similar to the original PVDF resin, because of the only partially dissolved PVDF chains in these two solvents. In a mixed solvent system (THF/DMF), the β phase formation linearly increased as the DMF component increased, determined by Fourier transform infrared spectroscopy (FTIR) techniques, owing to increased interactions between PVDF chains and DMF molecules. The film surface consisted of β spherulites with average size of about 3 μm, which were smaller than those grown from pure DMF, because of the increased crystallization rate in the mixed solvent.