Emergent Tribological Behavior from Concentration-Driven Conformational Transitions in Adsorbed Polymer Films

Adsorbed polymer films on surfaces from lubricant solutions play important roles in preventing direct contacts of solid surfaces during rubbing and govern tribological behavior in nanosliding gaps. Using neutron reflectometry (NR) and self-developed vertical-objective-type ellipsometric microscopy (VEM), we report a concentration-driven emergent transition in the structure and tribological behavior of adsorbed polymer films at solid-liquid interfaces. At low concentrations, polymer chains adopt flattened conformations that show low friction under sliding. Above a critical concentration threshold, competitive adsorption induces a transition to double-layer structures rich in loops and tails, enabling thick lubricating layers to form during sliding but to exhibit increased friction.