Molecular-Scale Wear Process of Swollen Polymer Brushes at the Sliding Interface: Coarse-Grained Molecular Dynamics Simulation with Explicit Solvent

Polymer brushes (PBs) swollen in good solvents are promising lubricating materials, and their wear resistance is critical for long-term functionality. However, the wear mechanisms of PBs remain poorly understood. In this study, coarse-grained molecular dynamics-based sliding simulations were conducted on PBs swollen in an explicit solvent to elucidate their wear behavior. The sliding behavior of PBs was categorized into three types depending on the normal load and sliding speed: (1) no penetration of the slider at a low normal load; (2) penetration of the slider at a high normal load and low sliding speed; and (3) penetration accompanied by detachment of the PB surface from the rear of the slider at a high normal load and high sliding speed. Significant wear was observed only in the third type. In this case, large forces are exerted on the polymer chains because they cannot respond quickly enough to follow the rapid motion of the slider, leading to intensive wear at the front part of the contact area. In addition to this intensive wear, sporadic wear was observed in the surface and subsurface regions away from the contact area. Correlation analysis revealed that this sporadic wear is associated with regions of high momentum density of solvent, suggesting a solvent-induced scission mechanism, in which polymer chains are stretched and broken by the solvent flow. Together, these findings clarify the dual origins of wear in polymer brush systems: direct mechanical scission owing to slider contact and solvent-induced scission driven by flow. Our results also demonstrate that explicit solvent treatment is indispensable for revealing the PB wear mechanisms that are overlooked in implicit-solvent models. This understanding provides a unified framework for describing both intensive and sporadic wear phenomena in the swollen PBs.