## 3 Results and Discussion
### Parameters Optimization in Fabricating PES Electrospun Nanofibers
#### 3.1.4 Effects of the Electrospinning Parameters on the Average Fiber Diameter  
The effects of the electrospinning parameters on the average fiber diameter were considered physicochemically based on the response surface function for the average fiber diameter obtained. Figures 3 and 4 show the morphology and the average diameters of the nanofibers electrospun in 30% (_w/v_) PES solutions with different LiCl concentrations. The PES nanofibers were fabricated at each optimal voltage, the feed rate of 0.5 mL/h, and the tip-to-collector distance of 20 cm. The average fiber diameter decreased considerably with an increasing LiCl concentration in a range of 0-0.10% (_w/v_). The observations coincide with our statistical result that the linear coefficient of the LiCl concentration (\(-\)51.67) was negative in Equation (8) and had the highest absolute value. The addition of LiCl increases the electrical conductivity of the solution, bringing about a higher surface charge density of the polymer jet. When more charges move in the polymer jet, a greater elongation of the jet occurs due to the electrostatic repulsion among charges [11, 22]. This elongation effect increases the period or the nanofibers to exist in the instability region, causing a reduction in the diameter of the resultant nanofibers.  
Figure 2: Contour plot for the average fiber diameter (Y\({}_{2}\), nm) as a function of feed rate and PES concentration at an LiCl concentration of 0.01% (_w/v_) and a tip-to-collector distance of 20 cm (**a**); as a function of the tip-to-collector distance and the PES concentration at an LiCl concentration of 0.01% (_w/v_) and a feed rate of 0.3 mL/h (**b**).  
Our previous statistical analysis indicates that the PES concentration and the feed rate have secondary impacts on the average fiber diameter compared with the LiCl concentration. Since the linear coefficients of the PES concentration (20.92) and the feed rate (34.33) were positive in Equation (8), the average fiber diameter increases with increasing PES concentration and feed rate. The increased PES concentration is closely associated with the increased viscosity of the solution [1; 5]. This is because of the higher entanglement among the increased polymer chains in the solution [25]. The viscoelastic effect increases the diameter of the resultant fibers. The feed rate is related to the mass flow of the polymer. When the feed rate increases, more polymers transfer from the needle tip to the collector in a faster stream of the polymer jet [12; 26]. This results in an increment of resultant fibers.  
Figure 4: The LiCl concentration effects on the average diameter of 30% (_w/v_) PES nanofibers at each optimal voltage, the feed rate of 0.5 mL/h, and the tip-to-collector distance of 20 cm.  
Figure 3: FE-SEM images (5000\(\times\)) of 30% (_w/v_) PES nanofibers with different LiCl concentrations at each optimal voltage, the feed rate of 0.5 mL/h, and the tip-to-collector distance of 20 cm. (**a**) 0% (_w/v_)/13.0 kV, (**b**) 0.01% (_w/v_)/15.5 kV, (**c**) 0.03% (_w/v_)/17.5 kV, (**d**) 0.05% (_w/v_)/19.5 kV, (**e**) 0.10% (_w/v_)/22.0 kV.