Electrical characterization of gamma-irradiated polypropylene

The aim of this work is to understand the role of gamma radiation in trap creation and/or trapping and detrapping space charge mechanisms in isotactic polypropylene (iPP). The measurements are performed on 25 μm thick films: virgin polypropylene and one irradiated with gamma rays at a dose of 100 kGy. The experimental study is focused on measuring final thermostimulated discharge currents. At the end of the isothermal discharge process, obtained after charging under a DC field of 4 MV/m, the temperature was increased from 40 °C at a heating rate of 3 °C/mn. As a result, the discharging current of the irradiated film exhibits two peaks, the first one at 75 °C and the second one, of lower amplitude, at 122 °C. It was noticed that no peak was obtained in virgin iPP even at other charging temperatures. Indeed, gamma irradiation creates defects in the polymer that constitute traps of various depths, depending on the nature of the defect. The transient charging current exhibits oscillations (or multiple peaks) associated with charge packets and the current-voltage characteristic shows that the current is controlled by the space charge (IαV²). The differential scanning calorimetry DSC analysis shows a glass transition temperature of 9.4 °C and a crystallization temperature of 114 °C.