The spinning process parameters including solution concentration, applied voltage and collection distance had a direct influence on the nanofiber diameter.

Figure 4 shows the experiment results.

Figure 4A presents the effects of the different solution concentrations (10, 11 and 12 wt %) on the nanofiber diameter. In order to investigate the nanofiber diameter more accurately, three different applied voltages (55, 60 and 65 kV) were used to fabricate nanofiber membranes for each solution concentration. As shown in

Figure 4B, the average nanofiber diameter and average error could be calculated according to three different nanofiber membranes by using three different applied voltages for each solution concentration. The average nanofiber diameters were 108 ± 26, 170 ± 39 and 210 ± 55 nm, and the average errors were 24%, 23% and 26% for solution concentrations of 10, 11 and 12 wt %, respectively. We found that the average nanofiber diameter gradually increased with the increasing of solution concentration. A high solution concentration led to more macromolecular chain entanglement, so the nanofiber diameter increased. Here, the average error was an important index to evaluate the dispersion degree of nanofibers, which could reflect the quality of nanofibers. Low error represented a small dispersion degree, which demonstrated a high quality of nanofibers. The results showed that the average error increased with the higher solution concentrations. Therefore, it was very important for improving the quality of nanofibers to adjust to an appropriate solution concentration.

Figure 4C shows the effect of applied voltage on the nanofiber diameter under the condition of three different solution concentrations. As for each applied voltage, we found that the nanofiber diameter gradually increased with the increasing of solution concentration.

Figure 4D presents the results of average nanofiber diameter and average error with the increasing of applied voltages. The average nanofiber diameters were 164 ± 37, 171 ± 45 and 153 ± 26 nm, and the average errors were 23%, 26% and 17% for applied voltages of 55, 60 and 65 kV, respectively. We found that the average nanofiber diameter first increased and then decreased with the increasing of applied voltage. This may be explained by the interaction between electrostatic force and viscous resistance. The results indicated that the applied voltage had a different impact on the nanofiber diameter compared with the solution concentration. It should be noted that the average error first increased and then decreased with the increasing of applied voltage. The error was only 17% when the applied voltage was 65 kV. This result proved that a feasible method to improve the quality of nanofibers was to use a higher voltage to fabricate nanofibers. The effect of collection distance on nanofiber diameter under the condition of three different applied voltages is showed in

Figure 4E. For each collection distance, the nanofiber diameter first increased and then decreased with the increasing of collection distance.

Figure 4F exhibits the results of average nanofiber diameter and average error with the increasing of collection distances. The average nanofiber diameters were 108 ± 26, 152 ± 35 and 167 ± 41 nm, and the average errors were 24%, 23% and 25% for collection distances of 15, 20 and 25 cm, respectively. It was found that the average nanofiber diameter gradually increased with the increasing of collection distance. The reason was that increasing the collection distance weakened the electric field intensity between the linear flume spinneret and collector. Those jets were not adequately stretched, therefore, the nanofiber diameter became thicker. The average error for nanofiber diameter showed little change with the increasing of collection distances. The error was only 23% when the collection distance was 20 cm. Except for solution concentration and applied voltage, collection distance was also an important parameter and it should be considered during needleless electrospinning.