**2.2 Testing method**

### *2.2.1 Surface charge dynamic characteristics experiment*

The surface charge measurement system is shown in **Figure 1(a)**. The entire corona measuring device is in a closed transparent container to ensure the same temperature and humidity. The needle plate electrode was used, the plate electrode is below the needle electrode, the distance between the needle tip and the plate electrode is 5 mm, and the distance between the plate and the surface of sample is

**Figure 1.** *(a) Surface charge measurement system and (b) space charge measurement system.* also 5 mm. The sample is placed on the ground electrode. Turn on the voltage power supply, and the corona time is 10, 20, and 30 min. When the corona time is reached, turn off the high voltage power supply, quickly transfer the sample to the electrostatic potentiometer and record its surface potential for 4 hours.

According to the measured results, the surface charge density can be calculated, and the calculation formula is

$$
\sigma = \frac{\varepsilon\_0 \varepsilon\_r}{\mathcal{Q}} \bigvee\_{\{t\_0\}} \tag{1}
$$

Among them, *σ* is the required surface charge density, *d* is the thickness of the sample, *V* is the sample surface potential, *ε*<sup>0</sup> and *ε<sup>r</sup>* are the vacuum dielectric constant and the relative dielectric constant [16].

## *2.2.2 Space charge dynamic characteristics experiment*

This chapter uses the pulsed electro-acoustic (PEA) method to test and study the space charge dynamic distribution in the PI sample. The measurement device is shown in **Figure 1(b)**, which mainly is made up of high voltage DC power supply, pulse generator, electrode system, preamplifier, computer, and oscilloscope. The measuring principle is that a pulse voltage is applied across the sample, and this pulse will generate an electric field inside the sample, causing space charge vibration in the sample. This vibration is propagated outward by means of sound waves. The amplitude of sound waves stands for the amount of charge, and the time when the acoustic wave reaches the piezoelectric sensor reflects the position of the space charge in the sample. During the test, the applied DC voltage was 5000 V, the measuring time was 3600 s, and data were recorded per 600 s. In addition, the ambient temperature was 25°C, and the relative humidity was �40%.
