Evolution of charge distributions in polymers during annealing

Polymer films of polyfluoroethylenepropylene (FEP) and polyimide (PI) of 25 μm thickness are charged with monoenergetic electron beams of a range smaller than the sample thickness. Thereafter, the charge distribution in the thickness direction and its change with time at an annealing temperature of 120°C are measured with the laser-induced pressure-pulse (LIPP) method. The experimental data are analyzed with a model of charge buildup during electron-beam irradiation and charge transport during annealing. The model takes into consideration the charge deposition and dose profiles, the radiation-induced conductivity, the carrier mobility, trapping time and maximum trap density. Evaluation of the data yields values of the mobility-lifetime product and shows that retrapping is fast in FEP and slow in PI in the temperature range considered