# Shape-Memory Property Acting as a Switch to Change the Surface Property of the Film
## 3 Results
### Contact Angle of PNIPAAm-grafted Polyester Composite  
The CAs were measured by bringing air bubbles into contact with the film in water. The grafted film prepared using a good solvent (G3 in Figure 4), intended as a comparison sample, absorbed water over time when the contact angle was measured with a drop of water in the atmosphere. Therefore, the grafted film was used as a control sample by performing contact-angle measurements in water after allowing sufficient water absorption to eliminate the effects of further water absorption.  
Figure 3: FTIR spectra and assigned absorption peaks of PNIPAAm grafted films.  
As shown in Figure 4, the film before graft polymerization showed no change in CA with varying water temperatures. In contrast, the contact angle of the G3 film was 150.4 \(\pm\) 1.57\({}^{\circ}\) at 15 \({}^{\circ}\)C and 141.6 \(\pm\) 5.3\({}^{\circ}\) at 40 \({}^{\circ}\)C. Thus, the unmodified film did not undergo temperature-dependent changes, confirming that the graft polymerization of PNIPAAm imparts the LCST and thermo-responsive behavior of PNIPAAm on the film surface of G3. Interestingly, the P24 and P48 films failed to allow hydrophobic air bubbles to contact the film surface at a water temperature of 15 \({}^{\circ}\)C. It means that the surface is so hydrophilic to prevent contact with air at this temperature. However, when the water temperature was raised to 40 \({}^{\circ}\)C and the air bubbles were reintroduced, the film surface became hydrophobic, allowing the air bubbles to adhere. The contact angles were 151.7 \(\pm\) 1.1\({}^{\circ}\) in 40 \({}^{\circ}\)C water for P24 and 153.7 \(\pm\) 1.0\({}^{\circ}\) for P48. The films grafted with a poor solvent exhibited improved surface hydrophilicity, regardless of the water temperature changes, compared to those grafted with a good solvent. Furthermore, results supporting the surface-selective response of graft polymerization in a poor solvent were obtained with the film stretched in water at 15 \({}^{\circ}\)C. The stretched film showed a contact angle of 143.4 \(\pm\) 1.8\({}^{\circ}\) for P24 and 146.4 \(\pm\) 1.8\({}^{\circ}\) for P48, confirming a hydrophobic surface compared to the original shape, as bubbles were observed adhering to the film. As shown in Figure 5, no bubbles adhered to the film before deformation, whereas bubbles adhered to the film after deformation as the graft-polymer density decreased and the surface became hydrophobic. The contact angle values of deformation samples at 15 \({}^{\circ}\)C were equivalent to those of the G3 film at a water temperature of 40 \({}^{\circ}\)C. The reason for the hydrophobicity at 15 \({}^{\circ}\)C, below the LCST of PNIPAAm, is that stretching the film reduces the graft-polymer density per unit area, exposing the hydrophobic film surface. However, the presence of even a small amount of PNIPAAm under poor solvent conditions rendered the contact angle more hydrophilic than that of the unmodified 3b-PCL-Cl film. The stretched film, after being returned to its original shape, did not allow bubbles to contact the surface during the subsequent CA measurement in water at 15 \({}^{\circ}\)C. P24 and P48 confirmed that the thermo-responsive functionality of the PNIPAAm on the film surface could be repeatedly activated.  
Figure 4: Contact angle of air bubble underneath PNIPAAm-grafted films in water. The designation “n.d.” indicates there was no contact with the air bubble, and there was no detectable.