3.2.1. Tensile Strength

Figure 2a–c shows the graphs of the trend of tensile strength, and a comparison of all the samples. Referring to all three graphs, the PUSX films had better tensile strength than the nanofibers, because of the porosity and the fiber orientation of the nanofibers. Due to the porosity of the nanofibers, the cross-sectional area was apparent, compared to the cross-sectional area of the films, which made the tensile strength supposedly higher than the results obtained. Pure PU nanofibers had the highest strength because of the high mechanical properties of PU. Figure 2a,b show that for block-type PUSX nanofibers, the tensile strength increased with an increase of silicone chain length, and Si04 samples with the longest silicone chain length (n = 50) showed the highest tensile strength of 5.9 MPa. Samples with longer silicone chain lengths showed smaller diameters and a more improved orientation of the molecular chains while being prepared under the optimized parameters, which contributed to the higher tensile strength. Meanwhile, the tensile strength decreased with the increase of silicone concentration, because of the low cohesive force of the silicone structure, and the decreasing concentration of PU.

When the silicone concentration increased, the low cohesive force changed the tensile strength of the material. On the other hand, the increase of silicone concentration caused the decrease of the ratio of PU in the polymer, and the weight percent of PU as well, which means there was a lack of the higher mechanical structure (PU). The tensile strengths of the films did not show much statistical significance because of the random orientation of the molecular chains.

In the graph of graft-type PUSX nanofibers and films (Figure 2c), Si08 nanofibers showed the highest tensile strength, at 6.8 MPa, with the Si05 sample showing the lowest result of 6.1 MPa. There was almost no difference in the observed trends, because the results were in the same range.

In this case, the silicone groups on the side chain were not able to influence the properties much, because the tensile strength was mainly determined by the high mechanical properties of PU in the main chain.

**Figure 2.** Comparison of tensile strength (MPa). "\*" was statistically significant (*p* < 0.05) between each pair of samples. (**a**) Block-type PUSX nanofibers and films with various chain lengths, (**b**) Block-type PUSX nanofibers and films with various silicone concentrations, (**c**) Graft-type PUSX nanofibers and films with various chain lengths.

The tensile stress–strain curves of the electrospun PUSX nanofibers and the PUSX films are shown in Figure 3. Typical curves, each from a different structure of PUSX, were plotted for an obvious comparison. From Figure 3a, the differences before and after silicone modification could be easily observed. Si01-59 samples, with the highest silicone concentration in the block structure, show the lowest mechanical performances out of the fiber membranes, because of the low cohesive force of

silicone. Instead, the tensile stress–strain curves of PUSX films showed very similar trends and much better mechanical performances, because of the random orientation of the molecular chains.

**Figure 3.** Comparison of stress–strain curves. (**a**) Stress–strain curves of the PUSX nanofibers, (**b**) Stress–strain curves of the PUSX films.

### 3.2.2. Elongation at Break

Referring to Figure 4, the elongation at break decreased with the increase of both silicone chain length and silicone concentration in block-type PUSX nanofibers. For the PUSX nanofibers with different silicone chain length, the increase of silicone chain length caused a decrease in fiber diameters under optimized conditions, which lead to the increase of entanglement and frictional resistance in the nanofibers. For PUSX nanofibers with different silicone concentrations, the increase of silicone concentration resulted in the decrease of polyurethane concentration in the molecular chains, and the high elongation property of polyurethane became difficult to observe. Meanwhile, the elongation at break of the graft-type PUSX nanofibers showed very similar results to each other for the same reason as with the tensile strength. As for the graft type, the elongation at break did not show an obvious trend, as the silicone groups on the side chains are not able to influence the nanofiber properties because the ratio of polyurethane and silicone in the molecular chain do not change with an increase of chain length. PUSX films have better tensile strengths than nanofibers, because of the porosity and fiber orientation of the nanofibers.

**Figure 4.** Comparison of elongation at break (%). "\*" was statistically significant (*p* < 0.05) between each pair of samples. (**a**) Block-type PUSX nanofibers and films with various chain lengths, (**b**) Block-type PUSX nanofibers and films with various silicone concentrations, (**c**) Graft-type PUSX nanofibers and films with various chain lengths.

#### 3.2.3. Young's Modulus

Referring to Figure 5, the Young's modulus increased with the increase of both the chain length and the silicone concentration in block-type PUSX nanofibers. As the silicone chain lengths and concentrations increased, the concentration of PU became lower and lower, and the characteristics of PU (with a high elongation at break) became difficult to observe, which meant that the samples were more elastic. Moreover, the existence of silicone also made it more difficult to change the shapes of the samples.

For both block- and graft-type PUSX nanofibers, all of the tensile test results showed that PUSX films have higher tensile strengths than nanofibers. This phenomenon can be explained by the orientation of the polymer chain, and the different structures of the nanofibers and films (porosity and density).

**Figure 5.** Comparison of Young's modulus (MPa). "\*" was statistically significant (*p* < 0.05) between each pair of samples. (**a**) Block-type PUSX nanofibers and films with various chain lengths, (**b**) Block-type PUSX nanofibers and films with various silicone concentrations, (**c**) Graft-type PUSX nanofibers and films with various chain lengths.
