The energy distribution of localized states in polystyrene, based on isothermal discharge measurements

The energy distribution of localized states in monodisperse polystyrene (37000 MWt) has been studied by observing the detrapping of electrons from the polymer. The states are populated by means of an electron beam. A 2.2 kV beam is used to inject a short pulse of charge into the free surface of a thin film of the polymer and a second electron beam monitors the surface potential of the film. The surface potential is proportional to trapped charge density, and the time derivative of the potential is a measure of the current flow, which is related to the release kinetics of the electrons trapped in the polymer. The release of electrons from traps is analyzed in terms of a demarcation energy, E m . Following Simmons and Tam's analysis of the isothermal discharge this energy is so defined that the release of the excess electrons from traps of depth E m occurs in a time equal to the elapsed time from the injection event. The energy E m is then related to elapsed time through the equation E m =kT ln ( nu 0 t), and the density of occupied states at E m is proportioned to tdV/dt. From the temperature dependence of the appearance time of peaks in the density of localized states, the attempt frequency nu 0 can be calculated. Based on such measurements, at temperatures from 30 to 90 degrees C, the author estimates the attempt frequency to be about 10 11 s -1 .