Effect of the graft architecture of polymer-grafted nanoparticles on tribological behavior in polymer-brushed nanochannels

Abstract

Although polymer brushes reduce friction by suppressing solid contact, performance degradation can occur under high shear and load. To address this, polymer-grafted nanoparticles (PGNPs) have been proposed as nanoscale spacers between wall brushes to stabilize the lubricating film and modulate the frictional response. In this study, dissipative particle dynamics were used to study PGNP solutions confined between polymer-brushed walls and relate self-assembly to the friction coefficient, μ, and shear viscosity, η. AB and BA diblock PGNPs (A: hydrophilic and B: hydrophobic) are compared with Janus architectures while varying the wall brush affinity from hydrophilic to hydrophobic. For hydrophilic wall brushes, AB-type PGNPs dimerized at equilibrium, whereas BA- and Janus-type PGNPs remained dispersed. Under shear, the viscosity exhibited shear-thinning, and architectural differences were minimal due to the dominance of wall brush alignment. Consequently, the tribological properties were similar across all PGNPs. For hydrophobic wall brushes, the PGNP architecture more strongly influenced self-assembly. At rest, AB-type PGNPs formed network-like aggregates, while BA-type and Janus PGNPs remained dispersed. Janus-type PGNPs preserved a solvent-rich core, yielding the lowest μ$ and η. At intermediate shear, BA-type PGNPs became less viscous than the Janus-type due to their low abundance in the channel center, reopening a low-resistance core pathway. The Janus-type PGNPs appeared in the channel center, which narrowed the solvent-rich core. At high shear, both architectures reconcentrated toward the center, and the differences in μ and η diminished. This work reveals how nanoscale self-assembly governs macroscopic tribological responses, offering design principles for next-generation PGNP-based lubricants.