Friction Control via Tunable Interpenetration of Spherical Polyelectrolyte Brushes

Polyelectrolyte brushes are promising materials for achieving superlubricity, but conventional approaches requiring chemical surface grafting limit their broad applicability. Here, we demonstrate a simple, yet effective and readily scalable friction-control strategy using spherical polyelectrolyte brushes as lubricant additives. By tuning particle concentration, we precisely control the degree of polymer chain interpenetration, a key factor determining lubrication performance. Combining scattering, viscosity, and friction measurements, we identify three distinct lubrication regimes defined by brush interpenetration. Enhanced lubrication occurs at the concentration threshold separating the semidilute partly interpenetrated regime from the semidilute fully interpenetrated regime, which aligns closely with the transition between mixed and hydrodynamic lubrication. Our results point to a direct correlation between nanoscale structural transitions and macroscale friction behavior, offering a predictive framework for enhanced lubrication in diverse industrial applications.