## 3 Results and Discussion  
### Surface Voltage  
The purpose of adding TiO\({}_{2}\) is to strengthen the surface potential of electret melt-blown fabrics. Figure 3 shows that the fabrics have comparable surface potentials when the electric field intensities are 1.5 and 2.0 kV/cm because the electric field intensity fails to provide enough energy in one minute. When the electric field intensity is 2.5 kV/cm, there is a significant rise in the surface voltage (Figure 3c). Increasing the ratio of TiO\({}_{2}\) improves the surface voltage, especially for the case of 3 wt% of TiO\({}_{2}\), which helps to reach the maximal surface voltage. Nonetheless, 4 wt% of TiO\({}_{2}\) adversely affects the surface voltage due to the agglomeration of TiO\({}_{2}\).  
Table 2 indicates that the crystallinity increases when TiO\({}_{2}\) increases from 1 to 3 wt%, but the crystallinity descends with 4 wt% of TiO\({}_{2}\) and is similar to that with 2 wt% of TiO\({}_{2}\). The descending crystallinity may be ascribed to the agglomeration of TiO\({}_{2}\). Moreover, surface voltage is also dependent on the crystallinity. The interface between crystallization and amorphous regions becomes a trap against charge storage [14; 28; 29]. A lower crystallinity diminishes the region available to store charges, which in turn reduces the surface voltage. In Figure 3c, the melt-blown fabrics made of 1 and 4 wt% of TiO\({}_{2}\) exhibit comparable surface voltages in terms of the trend and the value. The electret electric charges of TiO\({}_{2}\) are mainly stored over its particles [20], so 4 wt% of TiO\({}_{2}\) does not acquire the surface area as expected, which is attributed to the agglomeration. Moreover, the space charges from crystalline and amorphous regions are also decreased. Both factors render the melt-blown fabrics made of 1 and  
Figure 3: Surface voltage of electret melt-blown fabrics as related to the ratio of TiO\({}_{2}\) (1, 2, 3, and 4 wt%) and electric field intensities of (**a**) 1.5, (**b**) 2.0, and (**c**) 2.5 kV/cm.  
4 wt% of TiO\({}_{2}\) with similar surface voltages. The variation in the crystallinity changes the apparent enthalpy of crystallization, which subsequently shifts the temperature of Tc and Tm. The stability of electrostatic charges in melt-blown fabrics is pertinent to the surface potential, which can be computed with the following equation:  
\[\text{Surface Potential}=\frac{V}{V_{0}} \tag{3}\]  
where V is the surface voltage that changes with time, and V\({}_{0}\) is the initial surface voltage.  
Based on Figure 4a-c, the initial surface voltage appears relatively lower at 1.5 and 2.0 kV/cm, which in turn makes the surface potential higher than the surface potential of 2.5 kV/cm. In particular, 3 wt% of TiO\({}_{2}\) provides the fabrics with an optimal surface potential, which corresponds to the surface voltage. TiO\({}_{2}\) is a material with a high dielectric constant and a low conductivity, and thus can effectively strengthen the charge storage and stability [19]. At the same time, it also serves as an initial point for heterogeneous nucleation, which facilitates the crystallinity of polymer and effectively enlarges the contact area between crystalline and amorphous regions, which is a trap for charge storage. Hence, the surface potential can be significantly improved [14; 15; 17].  
\begin{table}
\begin{tabular}{c c c c} \hline
**TiO\({}_{2}\) Ratio (wt%)** & **T\({}_{c}\) (\({}^{\circ}\)C)** & **T\({}_{\text{m}}\) (\({}^{\circ}\)C)** & **Crystallinity (\%)** \\ \hline Pure PP & 121.0 & 146.6 & 6.9 \\
1 & 120.0 & 159.6 & 15.3 \\
2 & 122.2 & 149.9 & 15.1 \\
3 & 118.3 & 147.7 & 22.3 \\
4 & 118.2 & 144.5 & 16.2 \\ \hline \end{tabular}
\end{table}
Table 2: Thermal properties of samples as related to the ratio of TiO\({}_{2}\) being 1, 2, 3, and 4 wt%.  
Figure 4: Surface potential of electret melt-blown fabrics as related to the ratio of TiO\({}_{2}\) (1, 2, 3, and 4 wt%) and electric field intensities of (**a**) 1.5, (**b**) 2.0, and (**c**) 2.5 kV/cm.