Effect of tensile strain rate and elongation on crystalline structure and piezoelectric properties of PVDF thin films

In this study, blown films of PVDF films were produced. The effect of biaxial orientation and deformation rate on the structure of stretched poly (vinylidene fluoride) samples was investigated by means of Fourier transform infrared spectroscopy (FTIR) and confirming observations by wide angle X-ray diffraction (WAXD). FTIR spectroscopy is a well-known technique for detecting changes in the content of β-phase in PVDF films. Polarized biaxially oriented PVDF films have balanced piezoelectric activities in the film plane as compared to uniaxially oriented films, which have larger piezoelectric coefficients. It was shown that stretching at a rate in the range of 10-50 cm/min results in an additional transition of non-polar α phase into polar β crystallites. β-phase formation is more apparent in stretched films by a transformation mechanism of α- to β-phase via the formation of a necking region during stretching. The analysis of the intensive absorption bands at 763 and 840 cm−1 of α and β phase indicates that the β crystallite content is affected by the direction of orientation. The highest content of well oriented β-phase, 86.5%, by using FTIR techniques, was achieved during drawing at 50 mm/min. This was accomplished during stretching at 87 °C and for a stretch ratio of 6.5. At this stretch ratio, a maximum stress piezoelectric coefficient of d33=33 pC/N was obtained. The diffraction patterns of the stretched films showed that diffractions at 2θ=17.6°, 18.3°, 19.9° and 26.5° related to the α-phase transformed to new peaks at 2θ=20.57° and 2θ=36.3° which are the characteristics of the β-phase.