## 3 Results and Discussion
### Mechanical Performance of WPUs  
Previous research has shown that the crystallization behavior of semicrystalline polymer plays an essential role in regulating its mechanical properties, and the addition of second materials can influence its crystallinity [42]. The tensile stress-strain curves of WPU films were displayed in Figure 7, and the mechanical properties were presented in Table 2. The tensile strength of CO\({}_{0\%}\)-WPU reached 24.1 MPa without any elongation at break (Table 3), which showed a typical brittle fracture. The tensile strength of CO\({}_{2.50\%}\)-WPU reached 18.7 MPa, which was lower than that of CO\({}_{0\%}\)-WPU. In addition, the elongation at break of CO\({}_{2.50\%}\)-WPU was 110%. As shown in Figure 7, with increasing CO content, the tensile strength of CO-WPUs decreased while the elongation at break increased. The addition of castor oil could lead to a marked improvement of elongation at break due to its  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Sample** & **CO\({}_{0\%}\)-WPU** & **CO\({}_{2.50\%}\)-WPU** & **CO\({}_{5.04\%}\)-WPU** & **CO\({}_{7.40\%}\)-WPU** & **CO\({}_{8.65\%}\)-WPU** \\ \hline Tm (\({}^{\circ}\)C) & 48.8 & 48.2 & 47.5 & 46.8 & 45.2 \\ AH (\({}^{\circ}\)f) & 47.7 & 44.6 & 42.2 & 39.1 & 38.0 \\ \hline \end{tabular}
\end{table}
Table 2: Thermal properties of WPU films.  
Figure 5: XRD-Ray diffraction pattern of WPU films.  
Figure 6: DSC thermograms of WPU films.  
crosslinking effect and a decrease in hard segment portion. For the castor oil/PCL-based WPUs, increasing the content of castor oil could remarkably enhance the cross-link density of WPU films. It can be described that more CO group may increase the toughness of the CO-WPU films and lead to a decrease in the rigidity.  
A previous study demonstrated that the mechanical properties of WPUs were significantly associated with their morphology [44]. Thus, controlling the morphology is essential to obtain the desired properties in the WPUs. As far as we know, the crack of the materials may extend rapidly when a brittle failure occurs. Besides, the fracture surface appears to be quite smooth [45]. However, the surface of a ductile fracture may be misty and intricate due to the complex fracture mechanism and higher energy consumption. To understand the toughening effect of CO on PCL-WPU, the morphologies of the tensile fracture surfaces of various WPU samples were analyzed. As can be seen from Figure 8a, CO\({}_{0\%}\)-WPU demonstrated a common brittle fracture surface with shape of ridges, and the white crack of CO\({}_{0\%}\)-WPU appeared to be large-scale ridge-like features. This suggests that the large crystalline dimension of PCL phase could result in the brittle failure at the macroscopic level, leading to a low elastic modulus. Upon addition of CO to WPU, with a decrease in crystalline dimension, the fracture surface presented increasing roughness. As CO increased, the fracture surfaces appeared more rugged with silk-like appearance, thus, indicating enhanced toughness. Figure 8c-e showed very coarse fracture surfaces with the appearance of many rugged filaments called \"dimples\" [46], a characteristic of ductile rupture. From the above results, the addition of CO overcame brittleness effectively. In summary, on decreasing the crystalline dimension, the fracture mechanism in the WPU film transformed gradually from the brittle failure to the ductile fracture.