## 3 Results and Discussion
### Parameters Optimization in Fabricating PES Electrospun Nanofibers
#### 3.1.3 Response Surface Model Analysis for Average Fiber Diameter  
The model adequacy checks indicated that the average fiber diameter was also best described in the quadratic polynomial model (Table S3). The \(p\)-value for the quadratic model (\(p\) = 0.000) was less than 0.05, and the \(\mathrm{R_{adj}}^{2}\) for the quadratic model was the closest to 1 (\(\mathrm{R_{adj}}^{2}\) = 0.9106) of the considered models. In addition, the \(p\)-value for lack of fit (\(p\) = 0.496) was insignificant at \(p\) > 0.05. The results suggest that the quadratic model fits the responses for the average fiber diameter appropriately. The analysis of the variance (ANOVA) in the quadratic model for the average fiber diameter is shown in Table S4. The regression model for the average fiber diameter reacted to linear, quadratic, and interaction terms significantly at \(p\) < 0.05. The measured fiber diameters from the experiments and predicted fiber diameters from the model are distributed in Figure 1b. 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 average fiber diameter are shown in Table 5. We found that three linear coefficients of the LiCl concentration (X\({}_{1}\)), PES concentration (X\({}_{2}\)), and feed rate (X\({}_{3}\)) affected the average fiber diameter significantly at \(p\) < 0.05. One quadratic coefficient of X\({}_{1}\)\({}^{2}\) and two interaction coefficients of X\({}_{2}\)X\({}_{3}\) and X\({}_{2}\)X\({}_{4}\) also affected the average fiber diameter significantly. The response surface function for the average fiber diameter was obtained as follows, eliminating insignificant terms:  
\[\text{Y}_{2}=301.33-51.67\text{X}_{1}+20.92\text{X}_{2}+34.33\text{X}_{3}-18.71 \text{X}_{1}{}^{2}-26.5\text{X}_{2}\text{X}_{3}-19.75\text{X}_{2}\text{X}_{4} \tag{8}\]  
The LiCl concentration, PES concentration, and feed rate have a direct relevance to the average fiber diameter, as shown in Equation (8). The LiCl concentration in particular is the most decisive parameter in controlling the average fiber diameter because the absolute value of the predicted coefficient is the highest. This observation is consistent with the previous studies which found that the addition of ionic salt has a stronger influence on the fiber diameter than other electrospinning parameters [22; 23]. In addition, the feed rate makes a larger contribution to the average fiber diameter than the PES concentration because the coefficient of the feed rate is higher than that of the PES concentration. This result is not consistent with other studies in which the polymer concentration affected the average fiber diameter significantly compared with the feed rate [1; 24]. The interactive effect of the PES concentration and the feed rate on the average fiber diameter is presented in Figure 2a. To minimize their influences on the average fiber diameter, the LiCl concentration and tip-to-collector distance were fixed at low levels of 0.01% (_w/v_) and 20 cm, respectively. This contour plot suggests that the responses in the average fiber diameter rely on the feed rate at a lower PES concentration. When the feed rate increased at a low PES concentration of 28-30% (_w/v_), the average fiber diameter increased in a wide range of \(<\)330 to \(>\)430 nm; however, the feed rate exerted little influence on the average fiber diameter at a higher PES concentration. This is because the increased viscoelastic effect of the PES concentration counteracted the mass flow effect of the feed rate. The interactive effect of the PES concentration and the tip-to-collector distance on the average fiber diameter is presented in Figure 2b. To minimize their influences on the average fiber diameter, the LiCl concentration and feed rate were fixed at low levels of 0.01% (_w/v_) and 0.3 mL/h, respectively.