Origins of thermally stimulated current in polyethersulfone

In order to understand the dominant carrier species in electrical conduction in polyethersulfone (PES), thermally stimulated current (TSC) measurements were carried out under various conditions. It was found that PES has two TSC peaks in the temperature range from 20 to 220°C, consisting of an α peak at ∼ 210°C and a β peak whose peak temperature moves towards a higher temperature with an increasing poling temperature. Even when the sample was not poled, PES shows a spontaneous current with its peak at ∼ 210°C. Both the spontaneous current peak and the α peak were found to disappear when the sample had been heated to 230°C, keeping the external circuit closed. From such similarity of appearance and disappearance between the two peaks, it is concluded that they have the same origin. From the change in the spatial distribution of space charges inside the sample measured simultaneously with the TSC measurements, positive charges, probably due to K⁺ ions, existed nonuniformly in the sample from the beginning, and are thought to be responsible for the two peaks. From a similar study, the βpeak is considered to be due to polarization of ionic space charges during the poling process.