Studies on the transformation process of PVDF from α to β phase by stretching

Poly(vinylidene fluoride) (PVDF) has excellent pyro- and piezoelectric properties which are related to the beta-crystal of PVDF. Most of the beta-crystal of PVDF can be transformed from the alpha-crystal by mechanical stretching. The effects of stretching conditions on the transformation from alpha to beta phase of PVDF were studied in this paper. In addition, changes of crystalline structure of PVDF during the stretching process were observed in situ under an optical tensile stress microscopy tester, and the samples after being stretched were investigated using a 3D digital microscope and an infrared microscope. The results show that stretching temperature (T-s) and drawing ratio (lambda) are critical to the transformation from alpha-crystal to beta-crystal of PVDF. The T-s around 100 degrees C and lambda above 3 is recommended. The in situ observation indicates that the deformation of crystalline structure began from the middle of alpha-spherulite and extended to another spherulite, and caused the large-scale transformation. Scanning results of infrared microspectroscopic and 3D digital microscopy revealed that alpha phase of PVDF would change into beta phase with the deformation of the spherulite, and the distribution of the different crystalline phase in PVDF after being stretched was also given. It is believed that this work is instructive and meaningful to studies on phase transformation and production of the piezoelectric film of PVDF.