**3. Results**

### **3.1 Dielectric spectroscopy**

To check the dielectric characteristics during each aging stage, the dielectric spectroscopy test was employed. **Figures 5** and **6** show the real and imaginary parts of the permittivity (ε' and ε"). It can be found that both ε' and ε" change to some extent in stage 1. More notable increases can be found in aging stage 2, particularly for frequencies below 100 Hz. ε' reaches a value of ~7.0 at low frequency. During aging stage 2, the outer layer (nanocomposites) was exposed in high temperature and humidity, where hydrolysis and water absorption will occur as a result [19, 20]. The polar production after hydrolysis and absorbed water (with ε' of ~80) contributes to the increased ε' and ε". After, in stage 3, the sample was kept in oven at 320°C. The acid and amine groups formed by hydrolysis can be recombined, and the absorbed water will be removed in a short time [21]. As a result, the dielectric response was reverted to be near to that of PI without aging.

To get more details of the changing dielectric properties, ε' and ε" of some selected frequencies are shown in **Figures 7** and **8**. As shown in **Figure 7**, the increasing ε' can be found in the first 576 h in stage 1, while it decreases till the end of this stage. A peak value of ε' at each frequency can be confirmed in stage 2, which indicates that the aging parameters of stage 2 affect the dielectric properties

**Figure 5.** *Real permittivity at the end of each aging stage.*

*Electrical Endurance of Corona-Resistant Polyimide for Electrical Traction DOI: http://dx.doi.org/10.5772/intechopen.93253*

**Figure 6.** *Imaginary permittivity at the end of each aging stage.*

**Figure 7.**

*Trend of ε' at different frequencies and different aging stages.*

obviously. In contrast, ε' at each stage is stable in the whole stage 3. As shown in **Figure 8**, a similar tendency can be found in ε".
