## 3 Results and Discussion
### Parameters Optimization in Fabricating PES Electrospun Nanofibers
#### 3.1.1 Response Surface Model Analysis for Applied Voltage  
The model adequacy checks indicated that the applied voltage was best described with the quadratic polynomial model (Table S1). The quadratic polynomial model involves linear, quadratic, and interaction terms. The significance probability (\(p\)-value) for the quadratic model (\(p\) = 0.000) was less than 0.05, and the adjusted determination coefficient (\(\text{R}_{\text{adj}}\)2) for the quadratic model was the closest to 1 (\(\text{R}_{\text{adj}}\)2 = 0.9627) of the considered models. The results suggest that the quadratic model fits the responses for applied voltage well. The p-value for lack of fit was not calculated because the pure error was 0. The analysis of the variance (ANOVA) in the quadratic model for applied voltage is shown in Table S2. The regression model for applied voltage reacted to linear and quadratic terms significantly at \(p<0.05\), whereas there were no interactive effects among the electrospinningnning parameters on the response of applied voltage at \(p>0.05\). The measured voltages from the experiments and the predicted voltages from the model are distributed in Figure 1a. The good accordance between the measured values and the predicted values demonstrated that the estimated model was suitable. The regression coefficients in the quadratic model for the applied voltage are shown in Table 4. We found that all linear coefficients of the LiCl concentration (\(\text{X}_{1}\)), PES concentration (\(\text{X}_{2}\)), feed rate (\(\text{X}_{3}\)) and tip-to-collector distance (\(\text{X}_{4}\)) affected the applied voltage significantly at \(p<0.05\). One of the quadratic coefficients, \(\text{X}_{1}\)2, also affected the applied voltage significantly. The response surface function for the applied voltage was obtained as follows, eliminating insignificant terms:  
\[\text{Y}_{1}=19.5+2\text{X}_{1}-0.292\text{X}_{2}+\text{X}_{3}+2.125\text{X}_ {4}-0.813\text{X}_{1}\text{}^{2} \tag{6}\]  
Equation (6) indicates the independent correlations between the applied voltage and each of the considered electrospinningnning parameters. The LiCl concentration and the tip-to-collector distance in particular have more noticeable effects on the applied voltage because their predicted coefficients are relatively high. These results suggest that the applied voltage indirectly affects the average fiber diameter included in the other electrospinningnning parameters [16; 17]. Therefore, we propose that the applied voltage may be excluded from the independent variables regarding the average nanofiber diameter in this study. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Coefficient** & **Standard Error** & **T-Value** & \(p\)**-Value** & \\ \hline Constant & 19.500 & 0.242 & 80.66 & 0.000 & Significant \\ X\({}_{1}\) (LiCl concentration) & 2.000 & 0.121 & 16.55 & 0.000 & Significant \\ X\({}_{2}\) (PES concentration) & \(-\)0.292 & 0.121 & \(-\)2.41 & 0.033 & Significant \\ X\({}_{3}\) (Feed rate) & 1.000 & 0.121 & 8.27 & 0.000 & Significant \\ X\({}_{4}\) (Tip-to-collector distance) & 2.125 & 0.121 & 17.58 & 0.000 & Significant \\ X\({}_{1}\)\({}^{2}\) & \(-\)0.813 & 0.181 & \(-\)4.48 & 0.001 & Significant \\ X\({}_{2}\)\({}^{2}\) & \(-\)0.125 & 0.181 & \(-\)0.69 & 0.504 & \\ X\({}_{3}\)\({}^{2}\) & \(-\)0.313 & 0.181 & \(-\)1.72 & 0.110 & \\ X\({}_{4}\)\({}^{2}\) & \(-\)0.375 & 0.181 & \(-\)2.07 & 0.061 & \\ X\({}_{1}\)\({}_{2}\) & 0.250 & 0.209 & 1.19 & 0.256 & \\ X\({}_{1}\)\({}_{3}\) & \(-\)0.250 & 0.209 & \(-\)1.19 & 0.256 & \\ X\({}_{1}\)\({}_{4}\) & 0.250 & 0.209 & 1.19 & 0.256 & \\ X\({}_{2}\)\({}_{3}\) & 0.250 & 0.209 & 1.19 & 0.256 & \\ X\({}_{2}\)\({}_{4}\) & \(-\)0.125 & 0.209 & \(-\)0.60 & 0.562 & \\ X\({}_{3}\)\({}_{4}\) & 0.000 & 0.209 & 0.00 & 1.000 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The regression analysis in the fitted quadratic polynomial model for applied voltage based on the t-statistic. \(\rm{R^{2}=0.9828,R_{adj}^{2}=0.9627}\).