Characterization and piezoelectric activity, of stretched and poled poly(vinylidene fluoride). Part I: Effect of draw ratio and poling conditions

A number of investigators have reported on the high degree of piezoelectricity manifested by oriented films of poly(vinylidene fluoride) (PVF2). To develop applications for this piezoelectric effect, our laboratory is involved in a systematic investigation of the factors responsible for this remarkable behavior of PVF2. In a unique high‐speed process, commercial PVF2 film was uniaxially stretched to a series of draw ratios ranging up to 7/1. The resulting films were characterized by techniques involving infrared spectroscopy, density, birefringence, sonic modulus, X‐ray diffraction, and dynamic mechanical response. The films were then poled at various electric field strengths, temperatures, and times. Correlations have been made between draw ratio, physical properties, poling conditions, and piezoelectric activity of the films. It was found that the piezoelectric activity increased to limiting values with draw ratio, poling voltage, poling temperature, and poling time. It was evident that for PVF2 film a significant amount of oriented phase I crystalline material is required for high degrees of piezoelectric activity. The Appendix gives the apparent rate dependence observed for the piezoelectric effect when signal is measured with a voltage sensor of relatively low input impedance.