Influence of uniaxial stretching rate and electric poling on crystalline phase transitions in poly(vinylidene fluoride) films

In this work, the phase transition from the paraelectric α phase to the ferroelectric β phase during uniaxial stretching of PVDF films at rates between 1 and 1000 mm/min was investigated. In our experiments, the final stretching ratio and the temperature during stretching were 4.5 and 110°C, respectively. The comparison of Infra Red (IR) spectra obtained on unstretched and stretched films shows that the β phase content is strongly determined by stretching rates, i.e. increases during stretching at high rates. An almost pure β PVDF film was achieved by stretching at rates above 600 mm/min. X-Ray Diffraction (XRD) in stretched PVDF films reveals that the size of the β-phase crystallites is smaller after faster stretching. Increasingly, the a-phase crystallites are destroyed and can no longer be found after very fast stretching at 1000 mm/min. An electric poling at electric fields of up to 140 MV/m also results in a phase transition from the α to the β phase in stretched PVDF films according to the observed XRD spectra. Poling leads to a further growth of the β phase crystallites and, consequently, to an increase of the coercive field in ferroelectric-hysteresis measurements.