Breakdown-induced light emission and poling dynamics of porous fluoropolymers

To date the charging mechanism of porous electret materials is not completely understood. The present study is focused on the investigation of light emission during corona poling of individual films of expanded polytetrafluoroethylene (ePTFE) with nominal porosity of 91% and of the same film sandwiched between two solid poly-fluroethylene propylene copolymer (FEP) films. One observes that as soon as the applied electric field exceeds a certain threshold value (V-th), the individual film and sandwiched structure start to emit light. For the sandwich structure the light intensity decays with time to zero, whereas for an individual ePTFE film a continuous light emission at V>V-th is observed. The emitted light is a result of electrical breakdown events in air developing in the pores of the media. For the sandwich sample the positive and negative charges generated during breakdown are trapped at opposite ePTFE/solid film interfaces, thus resulting in a macroscopic dipole formation (polarization). In the case of individual ePTFE films the continuous light emission indicates that these charges are not trapped permanently at the fibers in the open-pore media. The present results explain the drastic difference in piezoactivity between individual porous films and sandwiched structures. (c) 2007 American Institute of Physics.