Fluorinated aromatic polyimides as high-performance electret materials.

In view of the increasing significance of technology-driven devices such as microelectromechanical systems, energy-harvesting devices, and organic field effect transistors, polymer electret materials with durable electret performance at elevated temperatures become more and more important. However, typical polymer electret materials lose their performance at elevated temperatures. To provide polymer materials with improved electret performance over a broad temperature range, a series of aromatic polyimides with different degree of fluorosubstitution is presented. Isothermal surface potential decay measurements at elevated temperatures reveal that minor differences in the chemical structure have a major influence on the electret behavior. The best performance is found for the polyimide containing a hexafluoroisopropylidene moiety in both the bisanhydride- and the diamine-based unit. Excellent long-term charge storage stability at 120 °C is observed. From the initial surface charge 94% remains after 24 h. This polyimide even tolerates short-term exposure of 30 s at 300 °C with almost no loss of performance. These findings demonstrate that this particular polyimide is suitable for device applications at elevated temperatures during fabrication and use.