Ionic Response Mechanism of Lubricating Properties of Zwitterionic Polymer Brushes through Molecular Dynamics

Zwitterionic materials are an important class of lubricating biomaterials for various applications. Despite such desirable lubricating properties, the molecular-level understanding of the lubrication mechanism of zwitterionic polymer brushes in salt solutions remains to be elucidated. In this work, we computationally studied the surface hydration, the effect of cations, and the lubricating property of three zwitterionic polymer brushes of poly(carboxybetaine methacrylate), poly(sulfobetaine methacrylate), and poly((2(methacryloyloxy)ethyl)phosporylcoline) brushes using a combination of molecular dynamics (MD) and steered MD (SMD) simulations. We studied the structure, dynamics, and orientation of the hydrated layer on the three zwitterionic moieties, while the effect of cations on the hydration. Next, SMD simulations were used to study the friction behavior of the polymer brush surface. The results showed that salt ions would increase the friction resistance of polymer brush surfaces mainly by decreasing the diffusion rate of water molecules. However, at low concentrations, the change in the diffusion rate of water molecules is insignificant, and the salt ions change the friction resistance by affecting the polymer brushes, which is related to the nature of the polymer brushes themselves. Hopefully, this work will provide some structural insights into designing zwitterionic lubricating materials.