## 4 Conclusions  
The influence of the electrospinning parameters on the diameter of the polyethersulfone (PES) nanofibers was demonstrated using response surface methodology (RSM). The preliminary factors consisted of a lithium chloride (LiCl) concentration, PES concentration, applied voltage, feed rate, and tip-to-collector distance. The applied voltage was left out because it was found that the applied voltage responded to other parameters sensitively in the conductive polymer solution. A quadratic model on the average fiber diameter was fitted through the analysis of the variance (ANOVA). The responses in the model indicated that the LiCl concentration was the most important factor in determining the average fiber diameter and that the tip-to-collector distance had no connection with the average fiber diameter. This work shows the possibility of predicting the fiber diameter in specific conditions and producing PES nanofibers with the desired diameter. The properties of the PES solutions with LiCl loadings also need to be observed for proving the effects of the LiCl concentration in a chemical aspect.  
Based on the parametrization, three PES electrospun nanofibrous membranes (ENMs) with different fiber diameters were fabricated. The synthesized membranes were used for the liquid filtration application. It was observed that the pore sizes and the pore size distribution also decreased as the fiber diameter decreased. The pure water flux test and the retention test were performed to identify the relationship between fiber diameter and filtration performance. When the fiber diameter decreased, the initial pure water flux decreased, and the removal rate of the micro-particles increased due to a denser membrane structure. This study demonstrates the possibility of producing various size selective membranes through the adjustment of the fiber diameter; however, further research is required to thoroughly understand the liquid filtration mechanism and to identify the structural and mechanical integrity of PES ENMs in membrane processes for water treatment.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/20/7295/s1](http://www.mdpi.com/2076-3417/10/20/7295/s1)]([http://www.mdpi.com/2076-3417/10/20/7295/s1](http://www.mdpi.com/2076-3417/10/20/7295/s1)), Figure S1: ATR-FTIR spectra of (a) ENM 30, (b) ENM 30-0.01, and (c) ENM 30-0.10, Table S1: Analysis of variance (ANOVA) in various mathematical models for the applied voltage, Table S2: Analysis of the variance (ANOVA) in the fitted quadratic polynomial model for applied voltage, Table S3: Analysis of variance (ANOVA) in various mathematical models for the average fiber diameter. Table S4: Analysis of the variance (ANOVA) in the fitted quadratic polynomial model for the average fiber diameter, Table S5: Fiber diameter and membrane thickness of the PES ENMs, Table S6: Pore properties of the PES ENMs.  
Conceptualization, J.B. and H.K.; methodology, H.K.; software, H.K; formal analysis, H.K.; data curation, S.P.; writing--original draft preparation, H.K.; writing--review and editing, J.B.; supervision, H.C.; project administration, K.S.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Korea Institute of Civil Engineering and Building Technology (KICT), grant number 20200451-001.  
Authors are very grateful for the funds [Project # 20200451-001] provided by \"Korea Institute of Civil Engineering and Building Technology\" (KICT).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.