From the analysis of the S/N data, (Table 9) it is observed that Nylon concentration (78.7% contribution) play significant role in determining the average fiber diameter and FA/DCM (7% contribution) significant but can’t neglect it because DCM increases the polarity due to improvement of collected fiber without beads as shown in Table 7 with average fiber diameter less than DCM percent increased as shown in Table 7. Also, when the DCM percent decreases less than 20% the average fiber diameter increases as shown in Table 7. Thus the percent (80/20) is optimum value from experimental work. From Table 9 it can be seen that the optimal solvent system performance for the S/N (maximum signal to noise ratio value was obtained at Nylon concentation 105 (level 1) and DCM (80/20) wt. % (level 2).
Table 9: Analysis of average fiber diameter for (S/N)
4.5. Electrospinning parameters optimization
The average values of means and S/N ratios of different parameters at various levels are given in the response table (Table 9), which demonstrates the average of the selected mean S/N ratio for each level of the factors. According to range analysis from S/N, the effects of each parameter were ranked based on delta statistics as follows: factor NY % and factor FA/DCM (Table 10). Figure 5 shows the level average response (main effect) plots of orthogonal experiment average value, and the change of average fiber diameter and mean of S/N ratio with various factors. By these plots, one can figure out the relative consequence of each effect. Besides the effect on fiber morphology, polymer concentration has significant impact on the average fiber diameter from the results obtained by Taguchi method. The fiber diameter increased with increasing polymer concentration because the higher viscosity resisted the extension of the jet. From the results and analysis above, uniform Nylon 6,6 nanofibers can be obtained from solution with concentration of 8 wt.% and 10 wt.% regardless of applied FA/DCM ratio in our study. Accordingly, for the experiments to be conducted, the optimum electrospinning conditions should be determined under the same conditions, which is experiment no. 20 at10 wt% of Nylon 6,6 concentration and FA/DCM ratio of 80/20 because of no beads and the fiber diameter distribution became gradually broader with increasing concentration.
Fig.5: Main effects plots for a) Average fiber diameter and b) signal-to-noise (S/N) ratios.
4.6. Mechanical properties
Three tensile samples of 40 mm gauge length and 10 mm width were trimmed from the ultra-fine fibril fabric of optimized electrospun Nylon 6,6 nanofibers (10wt% of Nylon 6,6 concentration and FA/DCM ratio of 80/20). Instron (model 2519-107) at NRC, Egypt, with 5000N maximum load cell capacity with strain rate of 0.2 mm/min was used to investigate the tensile strength of Nylon 6,6 fiber. Figure 6 shows the stress- strain curves of the three tensile samples Nylon 6,6 nanofibers. The mechanical properties such as Young’s modulus, Ultimate tensile strength and elongation-at-break are calculated and reported as shown in Table 10. These results proved an improvement in Nylon 6,6 mechanical properties. In another study for measuring the mechanical properties of Nylon6,6 by Durmu? and Ekrem 39, the tensile strength, modulus of elasticity and elongation found to be 0.813 MPa, 10 MPa and 19% respectively. Such improvement in the mechanical properties will be discussed later as we are going to discover a remarkable improvement in the thermal properties.
Table 10: Mechanical properties of the electrospun 10wt% Nylon 6,6 nanofibers.
Fig. 6: Stress-strain curves for three samples of 10wt% Nylon 6,6 fiber.
4.7. Glass transition temperature (Tg)
When the melt of a polymer is cooled, it becomes more viscous and flows less readily. If the polymer is not able to crystallize and the temperature is reduced low enough, it becomes rubbery and then as the temperature is reduced further, it becomes a relatively hard and elastic polymer glass is formed. The temperature at which the polymer undergoes the transformation from a rubber to a glass is known as the “glass transition temperature” (Tg) 40. Thermo-mechanical Analysis (TMA) was performed using TMA Q400 (Scientific and Technology Center of Excellence, STCE Egypt), with temperature range from ambient to150 ?C and sensitivity of 15 nm to study the thermal deformation and glass transition temperature (Tg). Tg is one of the most useful thermal parameters for polymer characterization. Tg for pure Nylon 6,6 polymer was reported as 55°C in literature 41. Figure 7 declares TMA for sample (1×1 cm) was trimmed from the ultra-fine fibril fabric (10wt% Nylon 6,6 fiber and (80/20)wt% (FA/DCM ratio). Tg for electrospun Nylon 6,6 fiber has found to be 124°C. Hence, the increase of Tg discloses good solubility of Nylon 6,6 in the FA/DCM solvent ratio (has been proved due to free of beads samples) results in more relaxation and confinement for the molecular chain of Nylon 6,6, which leads to more hindering of the molecular chain’s mobility and consequently a higher Tg value. More over an increase in Tg occurred due to stretching in the molecular chain of the polymer subsequently enhancement in its crystallinity as nano fiber has been produced during electrospinning process. Tg results are in a good agreement with the mechanical testing results for the electrospun Nylon 6,6 samples.