2.2.3. XRD

The tertiary structure of the Col-TPU composite nanofiber membranes is shown in Figure 3D. Each Col-TPU composite nanofiber membrane has only one characteristic peak at about 20◦ in the XRD pattern. This represents the distance between collagen frameworks associated with the diffuse scattering of collagen fibers [30]. These data indicate that the

collagen phase does not change upon the addition of TPU. TPU is an amorphous polymer, and there is a wide diffraction peak from 16◦ to 26◦ caused by the polyether chain segmen<sup>t</sup> in its amorphous structure [31].

Figure 3D shows that the wide peak of the Col-TPU composite nanofiber membranes increased to a TPU diffraction peak with an increasing TPU ratio. No obvious characteristic TPU peaks were observed in the pattern. These results indicate that TPU and collagen have high compatibility, and the mixed-membrane matrix can accommodate TPU without affecting its crystal shape. However, 2θ was at 28.72◦ when the ratio of TPU shifted to almost 40◦ while maintaining the wide amorphous peak. This suggests that when the ratio of collagen and TPU was lower than 6:4, the tertiary structure of the Col-TPU nanofibrous membrane resulted in a wider diffraction peak. This result was caused by the strong hydrogen bond action between groups of collagen and the secondary amine groups of TPU, which is beneficial in improving the toughness of Col-TPU composite nanofiber membranes [32].

### *2.3. Differential Scanning Calorimetry (DSC)*

The DSC curves of the Col-TPU composite nanofiber membranes are shown in Figure 4. The changes in disappearance of the endothermic peak, the emergence of new peaks, and the change in the enthalpy potentially indicate the components are incompatible [33]. From Figure 4, the endothermic peak of Col100 is 70 ◦C. There was no obvious peak in TPU because it belongs to the elastomer. Therefore, it does not contain boundaries between soft and hard phases. According to the peaks of collagen-based composites, the addition of TPU in collagen did not generate a new crest, indicating the two had good compatibility. Due to the uniformity of compatibility, the summits of the endothermic peak will be in close proximity to each other if the two phases are compatible [34]. The endothermic peaks of Col95, Col90, and Col80 were 72.1 ◦C, 74.4 ◦C, and 73.7 ◦C, respectively. When the ratio of TPU to composite nanofiber membranes increased to 20, the endothermic peak began to decrease. When the ratio of TPU doubled (60:40), the peak appeared at 65.5 ◦C. Thus, at the appropriate TPU proportion, the compatibility could be maintained. In addition, the melting enthalpy ( Δ Hf) of Col100, Col95, Col90, Col80, and Col60 was 49.5 J/g, 47.5 J/g, 46.4 J/g, 41.3 J/g, and 31.8 J/g, respectively. The decrease in enthalpy, in association with the changes in the temperature transition, may indicate there is a limited consistency in the boundary of the two phases [35].

**Figure 4.** The DSC curves of Col-based composite nanofiber membranes.
