## 3 Results and Discussion
### The Surface Morphology and Wettability Analysis  
The 3D perspectives of the surface topography of the prepared membranes were characterized by AFM at low tapping forces, indicating that the content of the hard segment in the PCUs had decisive influence on the surface morphology. As shown in Figure 3, the increment in the hard segment content had an obvious influence on the surface roughness of the films, resulting in a somewhat consequent decrease to make the processed surface much smoother. The mean roughness parameters ranged between 50 nm and 100 nm. By our knowledge of prior studies, a direct-viewing impression on the distribution of the microphase morphology could also be presented by the AFM images, not showing a clear phase contrast but merely indicating the hard domains appearing as extended lighter areas while the smaller darker areas represent the soft domains in the phase images [33]. The analysis of the AFM images in the probed regions clearly demonstrated that the microphase separation structures existed and that as the fraction of the hard segment composition increases, there were more hard segments involving in the lighter regions, exhibiting a greater degree of the discernible microphase separation with the average size of the hard domains between 5 and 10 nm. This result was also in agreement with the mechanical behavior of polyurethane. The wettability of the polyurethane membranes was determined by the static contact angle measurement at room temperatures and pressures (101 kpa). As mentioned in the initial literature and as promoted by Cassie-Baxter's law, the static contact angle on a specific solid/liquid surface greatly depended on the fraction of the trait of the solid [34]. Generally speaking, the relatively smaller contact angle (<90\({}^{\circ}\)) indicated a higher hydrophilic property of the surface while the relative larger contact angle (>90\({}^{\circ}\)) indicated a more hydrophobic property [35]. Figure 4 shows the static contact angles of polyurethane films with different contents of hard segments of the polyurethane. It is noticeable that the contact angle increases gradually with increasing contents of the hard segment, which was also consistent with the analysis of the AFM images regarding the surface relief. As we know, the contact angle is related to the surface structure of the film and is controlled by fluid dynamics. Also, from the comparison of Table 1 and Figure 4, we can see that the angles changed with the molecular weight where the value of the contact angles increased with decreasing molecular weight, indicating the rising surface hydrophobic property. It has been proven that the surface roughness of the membrane surface has an inseparable relationship with the adhesion of protein in blood vessels, and affects the final applications of biomedical materials, especially for blood-compatible materials, indicating that with larger surface roughness, the lower the probability is to form blood clots [36]. When the contact angle increases, it goes against the adhesion procedure, making medical operations relatively easier to carry out. Additionally, for smaller molecular weights of polyurethane, the existence of more side chains may be another vital impact that leads to much better hydrophilicity with strong hydrogen bonding interactions. Biomaterials with high surface free energy would have a relative small contact angle value indicating good wettability. Additionally, the decrease of work adhesion could restrict the capability of the polyurethane surface to generate less hydrogen bonds with water molecules. The work of adhesion values (\(W_{a}\)) were calculated based on the following equation [37]:  
\[W_{a}=\gamma\ (1+\cos\theta) \tag{1}\]  
where \(\gamma\) and \(\theta\) represent the constants for surface tension of the fluid (water = 72.81 mN/m) and the static contact angle, respectively. From the analysis of the obtained data and calculated results, we can gain some insight towards enhancing the blood compatibility of organisms by modifications in the future.