Self-Assembly and Tribological Behavior of Polymer-Grafted Nanoparticles Confined between Polymer-Brushed Walls

Polymer-grafted nanoparticles (PGNPs) confined between polymer-brushed walls represent a promising polymer-nanoparticle (NP) hybrid system for achieving friction control and mechanical stability under nanoscale confinement. However, previous studies largely neglected the cooperative assembly and dynamics that emerge from multiparticle interactions under shear, particularly within confined brush-PGNP systems. In this study, we employ dissipative particle dynamics simulations to systematically investigate the self-assembly and tribological response of PGNPs under confinement, focusing on the effects of solvent quality and grafting density. Under good solvent conditions, the PGNPs remained well-dispersed and showed typical shear-thinning behavior, largely independent of grafting density. In contrast, poor solvent conditions promoted PGNP aggregation and led to nonmonotonic frictional behavior characterized by three distinct shear regimes: shear-induced aggregate growth, the start of aggregate breakup, and complete dispersion. Additionally, the extent of shear-induced structural changes strongly depended on the grafting density of the PGNPs. In particular, at high grafting densities, the formation of two-layered aggregates reduced the degree of shear-induced alignment of the grafted chains in the low-to-intermediate shear regimes, in contrast to the more pronounced alignment observed at low grafting densities. Based on structural analysis, shear-thinning behavior at intermediate shear rates is primarily governed by the dynamic reorganization of PGNP aggregates and the reduced alignment tendency of their grafted chains, rather than by the response of the wall brushes. These findings provide molecular-level insights into the cooperative dynamics of polymer-NP hybrid systems under confinement and shear, offering a theoretical basis for the rational design of next-generation NP-based lubricants with tunable rheological properties.