Ferroelectrets: highly anisotropic electrically charged polymer foams for electromechanical transducer applications

Charged polymer foams show ferroelectret behavior. The anisotropy of the foamed structure can be controlled by gas-diffusion expansion. By means of well-designed void-inflation processes, the resulting dipole size is varied over a broad range. This also changes the mechanical properties, and ferroelectret foams with elastic moduli between 1 and 15 MPa can be prepared. Cellular polypropylene foams with transducer coefficients from 15 to around 500 pC/N, electromechanical coupling factors between 0.02 and 0.1 and thickness-extension resonance frequencies from 500 kHz to 2 MHz were achieved. Therefore, inflation is a suitable tool for the optimization of electromechanical transducer properties. We discuss the preparation and characterization of cellular ferroelectretic polypropylene foams as well as first steps for modeling the mechanical properties of anisotropic foams.