Polymer-Brush Lubrication in Oil: Sliding Beyond the Stribeck Curve

We report a highly effective macroscopic, non-aqueous lubrication strategy, employing polymer-brush-based surface modification. Poly(alkyl methacrylates) with hexyl, dodecyl and octadecyl side chains have been prepared on both silicon and iron substrates by means of surface-initiated atom-transfer radical polymerisation. Frictional properties of the three polymers have been evaluated in hexadecane, ethanol and toluene. For the case of the poly(dodecyl methacrylate), the study additionally involved nine base lubricating fluids of different chemical natures, with viscosities ranging from 4 to 2,200 cSt at 20 °C. By constructing a set of Stribeck curves at low values of speed × viscosity (at constant load), it was possible to identify an extended hydrodynamic lubrication regime and, for P12MA-based coatings, a low-friction, brush-lubrication regime, reinforced by time-dependant effects, which appeared to replace the mixed and boundary regimes observed for conventional lubricants. Self-mated poly(dodecyl methacrylate) brushes maintained low coefficient-of-friction values (e.g. 0.012 in hexadecane) over thousands of reciprocating cycles (>100 m total distance) under low speed × viscosity conditions (0.1 cm/s and 4 cSt).