Vortrag (eingeladen), 8th Technical Symposium on Polyimides & High Performance Functional Polymers (STEPI 8), Montpellier, France: 09.06.2008 - 11.06.2008
Several commercial available high temperature polymer films, for instance PEI, PEEK, PSU, PES, PI were assessed regarding their charge storage performance as function of their chemical structure. Therefore the films were charged by a corona to 400 V, annealed at 90°C for a total of 24 h, and the remaining surface potential was measured after defined periods of time. This isothermal potential decay (ITPD) is an accelerated test for the charge storage capability simulating an elongated period of time in the order of years. It was found that commercial PEI films exhibited the best charge storage behaviour among all materials tested. To further improve the electret performance of PEI we studied charge storage performance of fluorinated PEIs. The introduction of fluorine into the PEI polymer backbone might change the charge storage behaviour in a favourable fashion. Our synthetic efforts by choosing several fluorinated monomers resulted in two series of fluorinated PEIs (F-PEI) with varying fluorine contents from 0 to 25 wt.%. These F-PEI were synthesized in a one-pot procedure in molten benzoic acid and showed glass transition temperatures higher than 220°C. From the purified F-PEI films were compression molded, corona charged, and ITPD measurements were performed. This is to date the first systematic study of the influence of fluorine in PEI on their electret performance. It was found that the highest fluorine content does not necessarily result in the best electret performance. The best F-PEI exhibited a surface potential after annealing at 90°C for 24 h of 87 % of the initially applied surface potential, whereas the commercial PEI showed only 71 %.