# Shape-Memory Property Acting as a Switch to Change the Surface Property of the Film
## 4 Discussion  
The CA measurements confirmed that the surfaces of P24 and P48 films became hydrophilic in water at 15 \({}^{\circ}\)C, preventing bubbles from coming into contact. The improved hydrophilicity of the film surface supports the successful grafting of PNIPAAm. Interestingly, the sample treated with poor solvent allows for bubble contact by mechanically stretching the film and then submerging it in water at 15 \({}^{\circ}\)C again. That is, the surface of the film has become hydrophobic. The increased hydrophobicity of the film surface can be attributed to the reduction in PNIPAAm per surface area caused by the stretching of the film, allowing more contact with air bubbles. Thus, we have successfully demonstrated a method to reversibly alter the surface density of molecules on the film through macroscopic mechanical manipulation. However, G3 did not show a hydrophilic surface like P24 and P48 by the CA measurements in water at 15 \({}^{\circ}\)C. The lack of a hydrophilic surface was likely due to the roughness of the film. The CA measurement was performed when the PNIPAAm-grafted film was sufficiently swollen with water. The G3 film clearly swelled more in size compared to P24 and P48. Swollen by water affected not only the size of the film but also its surface. Figure 9 shows images of P24 (under tension) and G3 in water at 15 \({}^{\circ}\)C. The boundary between the air bubble and the surface of the P24 film is clear. In contrast, the surface of the G3 film appears rough. The reduced hydrophilicity of the G3 surface compared to P24 may be attributed to the increased surface roughness caused by water swelling. The 3b-PCL-Cl film and P48 showed no significant difference in the maximum strain values. The shape-memory property was repeatedly observed, indicating that the polymer was successfully grafted without interfering with the bulk properties of the film from using a poor solvent. This result is consistent with the DSC, XRD, and TMA results, suggesting that it is the result of the selective surface-modification method. The rapid elongation of the film on the 1 st cycle at about 30 min during the shape-memory property evaluation shown in Figure 8 was also observed in other PCL-based shape-memory materials, and the cause remains unclear [40; 41; 42]. One possible cause is that the film stretches as a result of polymer molecule rearrangement during the cooling process. No crystallinity was observed in the G3 film based on the DSC and XRD results. Additionally, the film's softening point was not detected by TMA in the range of 0 \({}^{\circ}\)C to 100 \({}^{\circ}\)C. However, contrary to expectations, the shape-memory properties of the film exhibited shape changes as observed by DMA. The shape change in G3 with temperature is attributed to the thermophysical properties of PNIPAAm. PNIPAAm has a glass transition temperature of 130 \({}^{\circ}\)C [43]. However, the PNIPAAm within the G3 film is a graft polymer and, unlike free polymers, exhibits a lower glass transition and melting temperature [44]. Thus, the film stretching during shape-memory evaluation occurred because the graft polymer exceeded its glass transition temperature. Upon reheating, the deformation was eliminated, resulting in elastic recovery. The P48 film was deformed rapidly by the shape-memory property compared to the elastic deformation of the G3 film. This indicates that controlling the graft-polymerization density affects the physical properties of the grafted substrate. Figure 9: Contact angle of samples prepared with different solvents from a side view in water at 15 \({}^{\circ}\)C.  
The results of this research are expected to contribute to cell-sheet engineering. Recently, several types of cell sheets have been produced using temperature-responsive cell culture dishes and are being applied to clinical trials. It is known that cell adhesiveness differs depending on the type of cells.