Study on physical and chemical structure changes of polyimide caused by corona ageing

Modified isothermal discharge current method (MIDC) was introduced to investigate the trap level distribution in Kapton 100HN and 100CR (nanodielectrics) PI films before and after corona ageing. The physical and chemical changes were studied by AFM and FT-IR respectively. Experiments showed that the quantity of traps was increased evidently after corona ageing and lots of deeper traps were produced in 100HN film, whereas for 100CR, changes were not very significant. Further IR spectra analysis showed that -COOH group was introduced in both 100HN and 100CR films while the general chemical structures were kept unchanged. AFM graph also found that surface morphology of 100HN PI films changed obviously by corona ageing while that of 100CR has no significant changes. Thus conclusions can be drawn that, corona ageing is a complicated process leading to surface degradation of PI film and addition of nanometer inorganic particles in polyimide can improve corona-resistance ability remarkably. In addition, since both physical and chemical degradation can introduce lots of defects and change the trap properties, so trap energy level and quantity changes can be used as an indicator maker for determining the ageing extent of polymer dielectrics.