Thermally stimulated current spectra of polypropylene thin films irradiated with pulsed CO2 laser light

There have been many reports on the laser light irradiation effects of polymers. Especially, ablation effect of polymers by irradiation with excimer laser light has been extensively studied by Srinivasan et al. Recently, interest in laser light irradiation to polymers has grown more and more in relation to the laser-induced pressure pulse method for measuring charge profiles in polymer thin films. However, irradiation effect of laser light on electrical properties of polymers has been hardly investigated yet. The electrical properties of insulating polymers strongly depend on charge traps in these polymers and the charge traps are best examined by thermally stimulated current (TSC) spectroscopy. For positively corona-charged polypropylene (PP), their TSC spectra were reported to have two main bands above room temperature: a low temperature band B_L and a high temperature band B_H, which peak around 60-70 °C and 130-140 °C, respectively. The charge traps responsible for the B_L and B_H bands were also reported to exist in the strained amorphous regions and the crystalline defects, respectively. Propagation of the laser-induced pressure pulse or thermal pulse through semi-crystalline polymer films is expected to induce defects in the crystalline regions and some of these defects can act as deep charge traps. Therefore, we tried in this experiment to introduce crystalline defects into PP by irradiation with pulsed CO₂ laser light and detect these defects by TSC method