Study of nanocrystals within lamellar structures of polyvinylidene fluoride using phase plate scanning transmission electron microscopy

Polyvinylidene fluoride (PVDF) is a polymer with excellent piezoelectric properties. The close relationship between structure and properties necessitates structural investigation. The PVDF structure has been investigated using techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, and polarized light microscopy. However, these methods only provide average structure information about the sample as a whole. To obtain local structure information, transmission electron microscopy should be used. Herein, the nanocrystals constituting the lamellar structure of heat-elongated PVDF were investigated using phase plate scanning transmission electron microscopy. Notably, the length of the nanocrystals increased with the increase in the elongation ratio, with no substantial change in its periodicity, and the nanocrystals exhibited a tendency to align with the lamellar structure orientation. This suggests that (1) the molecular chains within the nanocrystals elongate and tilt owing to the shear stress applied on them during elongation and (2) the entire lamellar structure rotates. To our knowledge, this is the first detailed study of nanocrystals in PVDF, which may further develop functional polymers.