Elastic properties and electromechanical coupling factor of inflated polypropylene ferroelectrets

Closed-cell polymer-foam electrets with internal bipolar charge exhibit strong longitudinal piezoelectricity. The size and the shape of their internal voids can be controlled by means of an inflation process that consists of a high-pressure treatment followed by a heat-setting procedure. Experimental data reveal a maximum of the piezoelectric coefficient and of the electromechanical coupling factor as functions of the polymer-foam density. These and related experimental results are discussed within a simplified model for polymer foams. The piezoelectric d₃₃ coefficient and the respective coupling factor k₃₃ are calculated by means of a series-connection model for cellular polymers. In the model, the piezoelectric coefficient and the coupling factor can be expressed in terms of the charge density on the respective void surface, the relative dielectric permittivity, the effective Young´s modulus and the relative density of the foam. The effective Young´s modulus is in turn determined by means of a truss-like mechanical model of the polymer foam. The model yields qualitative agreement with the experimentally observed data.