Tailored processing methods for cellular polycarbonate and polyetherimide films - new potentials for electret and piezoelectric applications

Polyetherimide (PEI) and Polycarbonate (PC) are known to be a suitable electret material that features a sufficient thermal and mechanical resistance. However, there are still no elaborate processing methods for creation of cellular PEI and PC films with exact controllable anisotropic lens-shaped cell morphologies available. Foaming of compact PEI and PC with carbon dioxide and subsequent stretching of these foams is a "conventional" approach to obtain anisotropic cells, however, this method is limited through the low thermal stability of PEI and PC foams. This report introduces new refined tools regarding the processing of anisotropic cellular polyetherimide and polycarbonate films. A new convenient method is the phase extraction of Polyethylenglycole (PEG) from a biaxially stretched film consisting of an immiscible PEI/PEG or PC/PEG blend. The main advantage offered by this extraction method in the easy processability, furthermore it convinces by its exact morphology control and its possible applicability to other polymers. Besides in case of PEI, double layer cell structures can be adjusted in a "one step" process. Furthermore, the focus of the report is also on biaxial stretching of particle filled PEI films. The application of glass hollow spheres is an ingenious method for creation of large cavities in PEI, resulting in enhanced surface potentials compared to compact PEI films.