Cellular polymers have recently attracted attention for their property of exhibiting a piezoelectric constant when they are electrically charged. The electrostatic charge generated in the voids by the internal discharges creates and internal macrodipole which is responsible for the piezoelectric effect. Charging by corona discharge is the most used method for cellular polymers. Many works has been published on polypropylene and polyethylene films mainly focused on the required expansion process or on the results obtained for raw cellular materials electrically activated. Our work is based on commercial polyethylene cellular films which have been physically characterized and electrically activated. The effect of thermal treatment, physical uniaxial or biaxial stretching and corona charging was investigated. The new method of corona charging improved the piezoelectric constant under other activation conditions.

Piezoelectric polymer cellular films with silver electrodes are made from a PP film activated by corona discharge. This cellular polymer is characterized by its high piezoelectric d33 coefficients in comparison with piezoelectric ceramics and other polymers like PVDF. One of the advantages is their malleability and shape ability in different kinds of forms. Our work presents the optimization of the activation process and the results of films characterization. All the samples are made from available PP film modified by a thermal biaxial stretching. d33 constant was measured by a quasi-static method and laser interferometry giving similar results. Transducer coefficients of about 700 pC/N were obtained. An approximation of the surface charge density in the polymer voids was obtained with a mathematical model. Thermal stability was checked by thermally stimulated depolarization currents (TSDC) showing a relationship between TSDC and d33 constant. Time stability was determined with more than one year old samples. Finally, the resonance frequency was obtained from interferometry measurement.

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