**3. Results and discussion**

### **3.1 The effect of molecular structure on the surface charge dynamics of PI films**

The ATR-FTIR spectrum of the PI films before and after fluorination is shown in **Figure 2**. In **Figure 2(a)**, the infrared spectrum of the original polyimide film was described. From the graph, we can find the typical characteristics of PI films, with the absorption peaks at 1780 and 1720 cm�<sup>1</sup> . Furthermore, the absorption peaks of the C]C double bond of the benzene ring are at 1500 and 1100 cm�<sup>1</sup> , the vibrational absorption peaks of the CdN bond are at 1373 cm�<sup>1</sup> , and the absorption peaks at 810 cm�<sup>1</sup> are the vibration of benzene-H bond. The absorption peak at 1237 and 3200 cm�<sup>1</sup> indicates that a small amount of ODA remained during the molecular polymerization reaction. The infrared spectrum of the sample after fluorination is shown in **Figure 2(b)**. As can be seen that the PI film after the surface molecular structure has the obvious CdF, CdF2 and CdF3 absorption peak is in the range of 950–1340 cm�<sup>1</sup> . **Figure 2** shows that the CdF bond is the strongest bond in the polymer, higher a great than the CdH bond. So, the surface fluorination will make the fluorine element replace the hydrogen element on the PI films surface, which results in the reduction and even disappearance of CdH bonds in the molecular structure of the surface layer and accompanied by the formation of CdF, CdF2, and CdF3. There is a dense CdF layer on the PI film surface layer.

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

**Figure 2.** *The infrared spectrum of (a) original film and (b) fluorinated film.*

### **Figure 3.**

*The SEM of (a) original film and (b) fluorinated film of PI.*

**Figure 3** describes the microstructure of original PI film without fluorination and the fluorination film for 60 min by the SEM, respectively. As can be seen from the figure, a dense fluorinated layer is formed on the surface of the sample. The thickness of the sample is about 2.6 μm when the reaction time is 60 min. It is obvious that the surface molecular modification can change the chemical structure of the surface of the sample and form a C-F layer on the surface when combining infrared spectral analysis with the SEM results comprehensively.

The surface charge density of the PI sample was measured using the test system of **Figure 1(a)**. **Figure 4** shows the change of the surface charge density of the original sample and the sample fluorinated for 45 min under different corona times. It is noticeable that the surface charge density dissipates rapidly at the beginning, and then the dissipation speed gradually slows down and remains a steady trend. The positive and negative charges have similar trends. There are three main ways for surface charges to dissipate: (1) migrating to the ground electrode on the back, (2) migrating along the surface by the tangential electric field and entering the earth

**Figure 4.**

*The surface charge density of sample without fluorination under the (a) positive voltage, (b) negative voltage and the surface charge density of sample with fluorination under the (c) positive voltage, (d) negative voltage.*

through the ground electrode, and (3) neutralizing with heterogeneous charges in the air. Which dissipating path dominates depends on various factors such as the surface characteristics of the solid medium, the gas atmosphere, and the electrode structure [17, 18]. Since the normal electric field on the film surface is much larger than the tangential electric field in this experiment. The most possible way for the surface charge to dissipate is to migrate to the ground electrode on the back and neutralize with heterogeneous charges in the air.

**Figure 4(a)** and **(b)** is the surface charge density of the sample without surface molecular modification. It is clear that the surface charge density gradually increases from 2900 to 3200 pC/mm<sup>2</sup> with the corona time rising from 10 to 30 min. During the dissipation process, the surface charge density tends to stabilize as the corona time increases, which is due to the energy required to restrain the injected charge increases with the corona time growing. The electric field formed by the charge which has been injected suppresses the large amount of the original charge transfer, alleviating the charge dissipation process [19].

According to the graph, the initial surface charge density increases with the increase of the corona time, but the dynamic of the charge is different from the dynamic of the original sample. The surface charge density of the sample under the voltage for 20 min is larger than those for 10 min and the attenuation curve is flatter, while the surface charge density of the sample under voltage for 30 min decays faster than the previous two samples. Referring to **Figure 4(c)** and **(d)**, the surface charges of the samples have the similar trend after 8 min for the samples that are under voltage for 30 and 20 min. The reason may be that the surface layer after fluorination has the fluorinated layer on the surface of the sample, which can effectively suppress the injection of charge. As the corona time increases, a large amount of charge accumulates in the fluorinated layer on the surface. When the power is turned off, the charge neutralizes and dissipates into the sample body and

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

the air, and most of change accumulated on the surface charge neutralizes with opposite charge in the air. Therefore, although the initial surface charge density increases when the voltage time is 30 min, most of change accumulates on the surface fluorinated layer and does not enter the deep traps inside sample, which lead to the similar trend after 8 min.

The influence of different fluorination times on the surface charge dissipation of the sample was evaluated by the dissipation time. The dissipation time is regarded as the time from the charge starting dissipates to the remaining 10% of the premier surface charge. The longer time stands for the more slowly charge dissipation. **Figure 5** shows the surface charge dissipation time under different conditions. **Figure 5(a)** is the dissipation time of the sample fluorinated 30 min under different corona times, and **Figure 5(b)** is the dissipation time of sample with fluorination time when the corona time is 10 min.

From **Figure 5(a)**, the dissipation time gradually increases as the corona time increases, and the dissipation time of the negative charge is shorter than that of the positive charge.

According to **Figure 5(b)**, the positive charge dissipation time of original sample is 80 min, whereas the negative charge is 100 min, both which are more than 1 h. After fluorination, the total dissipation time is less than 18 min, and the charge dissipation time of the sample fluorinated for 45 min is the shortest. For the original sample, the dissipation time of the negative charge is longer than the positive charge. However, the dissipation time of the negative charge of the sample after fluorination is lower than the positive charge, due to the strong electronegativity of fluorine element which can absorb electrons to form a shielding layer on the surface layer under the negative corona, thereby improving the dissipating speed.

This section uses polymer trap theory to further study the surface charge transport mechanism of polyimide materials. According to the trap theory, the trap energy level and density in the polymer can be calculated by the following formula (2) and (3).

$$
\Delta E = kT \ln \left( \nu t \right) \tag{2}
$$

$$N(E) = \frac{4\varepsilon\_0 \varepsilon\_r}{qLkTL} \frac{t dV}{dt} \tag{3}$$

*ΔE* is the trap energy level,*T* is the absolute temperature, *k* is the Boltzmann constant, *t* is the time, *N (E)* is the trap density, *L* is the sample thickness, *ν* is the escape frequency, *q* is the basic charge, and *V* is the surface potential.

**Figure 6.** *The trap distributions of sample under various fluorination times.*

The trap parameters of the polyimide film under different fluorination times are calculated. As shown in **Figure 6**, the energy level represents the depth of the trap. For the sample without fluorination, the trap level is most distributed at 0.84 eV, while the samples fluorinated for 45 and 60 min are most distributed at 0.76 and 0.79 eV. The results indicate that the trap level of sample with fluorination was shallower than original sample. The polyimide after fluorination makes up for some defects on the surface of the sample, thereby reducing the trap depth of the sample and causing surface charge difficult to accumulate, and dissipates faster, which is in line with the former conclusion.
