Visualization of the lamellar structure of polyvinylidene fluoride using phase-plate scanning transmission electron microscopy

Polyvinylidene fluoride (PVDF) is a crystalline polymer well-known for its excellent piezoelectric properties, flexibility, chemical resistance, heat resistance, and mechanical strength. PVDF exhibits crystal polymorphisms, with five reported crystal structures. Among these, the β-type structure has attracted much interest due to its superior piezoelectric properties; however, the details of its piezoelectric mechanism remain unclear. It is essential to evaluate the structure of PVDF at the nanoscale to elucidate its piezoelectric mechanism. In this study, as a first step toward elucidating its piezoelectric properties, we employed phase-plate scanning transmission electron microscopy (P-STEM) to observe heat-elongated PVDF. P-STEM is particularly effective in examining materials composed of light elements. We successfully visualized the lamellar structure, which was characterized by a layered arrangement of crystalline and amorphous regions. The period of the lamellar structure was approximately 7 nm, which was in good agreement with the results of small-angle X-ray scattering studies. In addition, high-magnification P-STEM images revealed that bundle-like structures oriented in the elongation direction were likely crystalline regions. This result indicates that P-STEM provides detailed local information regarding the orientation of the PVDF polymer chains.