Morphology and electret behaviour of microcellular high glass temperature films

Here we report the processing of cellular, high-temperature polymers for electret applications based on polyetherimide (PEI) and polyetheretherketone (PEEK). The two approaches were utilized to improve the electret properties by insertion of cavities are the physical batch foaming process using carbon dioxide as the blowing agent and the stretching of polymeric plates containing suitable particles, which allows the manufacturing of porous films. The electret properties of foamed PEI and PEEK as well as stretched PEI containing NA11 particles are characterized by isothermal charge decay measurements at 90 degrees C after corona charging to surface potentials of about 400 V. Investigation of the cellular morphologies was achieved by scanning electron microscopy (SEM) and micro-computertomography (mu CT). As result, the cellular films show a clear improved charge storage behaviour compared to the compact films. The best results were achieved with porous PEI films, prepared by stretching and after twofold charging.