**3.2 The effect of molecular structure on surface charge dynamics of PI nanocomposites**

**Figure 7** is the SEM of the PI nanocomposite film doping with 3 and 5 wt% of Al2O3. The nanoparticles are dispersed in the PI matrix uniform when the mass fraction is 3 wt%, but large-size agglomerates appear with a particle size above

**Figure 7.** *The SEM images of PI films doping with (a) 3 wt% and (b) 5 wt% of Al2O3.*

*Effect of Molecular Structure Modification and Nano-Doping on Charge Transportation… DOI: http://dx.doi.org/10.5772/intechopen.92024*

500 nm when the mass fraction is 5 wt%. Nano-composite PI films with different mass amounts are divided into two groups, one of which is subjected to fluorinate for 30 min and the other is original films.

The experiment used the surface charge dynamic measurement system to study surface charge dynamics. The conditions were to maintain relative humidity at 40%, room temperature, grid voltages to 3 kV, and corona times to 5, 10, and 15 min. The result is shown in **Figure 8**.

According to **Figure 8(a)**, the surface charge density gradually increases as the corona time increases, and the initial surface charge density of the negative charge is higher than the positive charge. Comparing **Figure 8(a)** and **(c)**, it is found that the surface charge density of the sample after fluorination decreased more and dissipate faster. Referring to **Figure 8**, it can be found that the fluorination has stronger effect on the PI film than the nanocomposite PI film. Comparing **Figure 8 (c)** and **(d)** with **Figure 4(c)** and **(d)**, the surface charge density of the sample with nanoparticles is higher than sample without nanoparticles and the charge dissipation is slower. The surface charge density of fluorinated PI film with Al2O3 decays below 200 pC/mm2 in a short time. But after the Al2O3 are added, the sample charge density is 300 pC/mm<sup>2</sup> after 35 min, which indicates that the sample with nanoparticles has a stronger ability to capture surface charges. These studies show that nanoparticles and surface molecular modification have opposite effect on the surface charge dynamics of polyimide films. The latter can make sample traps shallower and the former makes the surface charge agglomerate and dissipate slowly.

**Figure 9** describes the surface charge dissipation of the nano-composite PI film. It is clear that the surface charge dissipating time of the fluorinated sample is

### **Figure 8.**

*The surface charge density of original sample doping with 3 wt% Al2O3 under the (a) positive voltage, (b) negative voltage and the surface charge density of fluorination sample doping with 3 wt% Al2O3 under the (c) positive voltage, (d) negative voltage.*

**Figure 9.**

*The surface charge dissipation time of the nano-composite PI film (a) with fluorination (wt%) and (b) without fluorination (wt%).*

shorter than the sample without fluorination, indicating that fluorination improves the surface charge dissipation. The dissipation time increases firstly and then decreases with the increase of the Al2O3 content for all samples. The surface charge dissipates slowly when Al2O3 content is 3 wt%.

In order to further study the effect of nanoparticles on the transport mechanism of the surface charge of the PI film, the trap properties are analyzed. It can be seen from **Figure 10(a)** that the nano-composite film with 3 wt% Al2O3 has the highest trap energy level and the most trap density, the trap energy level of the film without nanoparticles is the lowest. The trap energy level is lower when the Al2O3 content is 7 wt% than 3 wt%. This indicates that the interface which formed between the nanoparticles and matrix makes the trap level of nanocomposite PI deeper, and the deep trap inhibits the injection of the charge. As the content of Al2O3 changes, the distribution and quantity of the interface will change. When the content is high, the agglomeration will occur and affect trap depth of the sample and the accumulation as well as dissipation characteristics. This is also the reason why the dissipation time is longest when the content is 3 wt%. This chapter finds that the nanoparticles will deepen the traps of the polyimide film, increase the ability of surface charges to accumulate on the surface, and suppress the dissipation.

**Figure 10.**

*The trap distributions of sample doping with various mass contents of Al2O3 (a) without fluorination, (b) with fluorination.*

*Effect of Molecular Structure Modification and Nano-Doping on Charge Transportation… DOI: http://dx.doi.org/10.5772/intechopen.92024*

Combining the conclusion of fluorination with nanocomposites, it is clear that comprehensive application of surface molecular structure modification and nanoparticles can improve the electrical resistance while improving its surface charge dissipation, optimizing the trap distribution of the sample, and reducing the surface energy.
