Electromechanical properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method

The piezoelectric resonance was observed in free vibrators of uniaxially oriented and biaxially oriented polyvinylidene fluoride films polarized at high electric fields. Using the piezoelectric resonance method, piezoelectric and elastic constants, and electromechanical coupling factors of such films were determined at high frequencies (20 kHz–30 MHz) in the temperature range −170–100 °C. For the uniaxially oriented films, the coupling factors k33 and k32 are independent of temperature, while k31 increases noticeably above the primary dispersion temperature. The value of k33 is about 0.2, one of the largest values ever reported for piezoelectric polymers. Similar results were obtained for the biaxially oriented films. The temperature dependence of elastic and piezoelectric constants of the uniaxially oriented films is interpreted by a model in which the crystalline and amorphous phases are combined in series along the stretching direction and in parallel along the perpendicular directions. The origin of the strong piezoelectric effect in polyvinylidene fluoride is discussed on the basis that the spontaneous polarization depends on applied stress.