| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Biomaterial Surfaces & Interfaces | Wednesday Sessions |
| Session BI-WeM |
| Session: | Soft Surfaces and Biofunctional Coatings |
| Presenter: | Shuichi Uehara, Tohoku University, Japan |
| Authors: | S. Uehara, Tohoku University, Japan Z. Liu, Tohoku University, Japan N. Miyazaki, Tohoku University, Japan Y. Ootani, Tohoku University, Japan N. Ozawa, Tohoku University, Japan M. Kubo, Tohoku University, Japan |
| Correspondent: | Click to Email |
In recent years, concentrated polymer brush (CPB), which is constructed by grafting polymers onto a substrate at high density, has been developed [1]. Especially, zwitterionic CPB produces ultra-low-friction surface and has biocompatibility in aqueous environment. Thus, zwitterionic CPB has attracted much attention for application to a low friction material as artificial joints. Recently, experiment showed the friction force of zwitterionic CPB decreases with increasing ionic strength for salts [2]. However, the details of this mechanism are still unknown because the in-situ observation is difficult. For enhancing the performance of zwitterionic CPB as a low friction material in biological applications, it is important to understand the effect of salt existing in biological environment. Thus, computational simulation is required.
In the present study, we performed molecular dynamics friction simulation between CPB and Au tip to elucidate the effect of salts on friction force of zwitterionic CPB. In the CPB model, 9 zwitterionic polymer chains of 10 monomers were grafted to a silicon (111) substrate (area, 5.75nm✕5.98nm) via covalent bonds. For comparison, we prepared two systems: with salts (80 KCl) and without salts. Both system of CPB solvated in 6000 water molecules.
At a low load (up to 10 MPa), zwitterionic CPB with salt showed lower friction force than system without salts. This result is qualitatively consistent with experimental data [2]. The mean square displacement of water in the system with salts is lower than that in system without salts. This result suggests that waters in the system with salts have higher viscosity. Whereas, we find that zwitterionic chain with salts is harder to move in the sliding direction than system without salts. Therefore, we revealed that the binding of salts to polymer chain made polymer chains hard to collapse in spite of increasing viscosity of waters. Thus, zwitterionic CPB with salt reduced contact area between Au tip and polymer chains. Our previous study showed that the reduction of contact area of CPB in friction interface leads to low friction [3]. On the other hand, at a high load (20 MPa), the friction force of zwitterionic CPB with salts and system without salts were comparable. This is because salts desorb from inner layer of zwitterionic CPB due to the severe load. Therefore, to enhance performance of zwitterionic CPB as a low friction material, it is necessary to design of CPB so as to hold salts which make CPB hard to collapse in the severe load.
[1] H. Sakata et al., Polymer Journal, 2015, 767.
[2] Z. Zhang et al., Langumir, 2016, 32, 5048.
[3] S. Uehara et al., Chem. Lett, 2018, 47, 784.