Linear and nonlinear piezoelectric response of charged cellular polypropylene

Piezoelectricity in a charged cellular polypropylene, called EMFi, is investigated with respect to nonlinearities to explain the strong differences in longitudinal piezoelectric constants published in the literature and ranging from 90 to 250 pC/N. The inverse constant was measured interferometrically to be 90 pm/V. Quasistatic and dynamic measurements with small loads yielded the same value for the direct constant. The direct constant was also investigated with respect to large-signal behavior becoming noticeable at static and dynamic loads higher than 10 kPa. Both the quasistatic and the dynamic constant increase up to 130 pC/N at such loads. Furthermore, an additional resonance appears under strong loading in the range of about 10 Hz shifting down with increasing load. In addition, the piezoelectric constant increases also with increasing dynamic load under constant static load. The nonlinearity also results in the generation of harmonics. Finally, boundary effects can be detected if just a small area of the sample is loaded. This effect appearing mainly at frequencies below 20 Hz is attributed to airflow between the air bubbles. A load-dependent Young’s modulus, mainly responsible for the nonlinear behavior, is calculated from the experiments. It diminishes from 2 to 1.5 MPa at a load of 60 kPa.