Voided cyclo-olefin polymer films: ferroelectrets with high thermal stability

Recently, new ferroelectrets were developed from foams of polyethylene terephthalate and of cyclo-olefin polymers and copolymers. Here, we focus on ferroelectret films based on compounds of cyclo-olefin polymers. Suitable compounding and preparation led to cyclo-olefin polymer ferroelectrets which show an electromechanical activity of around 15 pC/N that is thermally stable at least up to 110/spl deg/C. We discuss the preparation, the electrical charging and the electromechanical activity, the short-and long-term thermal stability as well as the electro-acoustical properties of an eight-layer cyclo-olefin film stack. The results obtained on the cyclo-olefin foams represent a significant improvement of the service temperature of ferroelectret transducer materials.