Understanding the role of viscous solvent confinement in the tribological behavior of polymer brushes: A bioinspired approach

Polymer brushes consisting of micrometer-long poly(dodecyl methacrylate) (P12MA) chains can impart very low coefficients of friction to surfaces sliding in oil. Under harsh tribological conditions, however, this behavior can only be maintained for long periods of time in the presence of highly viscous fluids, which appear to protect the brushes from wear. This behaviour is analogous to the lubricating mechanism in synovial joints, inasmuch as the confinement of pressurized synovial fluid in cartilage is partially responsible for the low wear rate of the system. Sliding induces solvent confinement within the brushes, provided that the viscosity is sufficiently high, and confinement can also be observed upon indentation at sufficient high rates. Such indentation experiments have been performed to characterize the mechanical (viscoelastic) behavior of the polymer brushes in solvents. By applying Hertzian contact theory and considering the stiffening of the polymer brushes upon compression, an effective elastic contact modulus of the P12MA brushes is obtained that is much higher than the equilibrium value. The proportion of the load carried by the fluid can also be estimated as a function of speed and viscosity. Thus, high solvent viscosity, as well as substantial brush height, can lead to a shielding of the polymer from the effects of the load, and the enhancement of the effective brush elastic modulus. This explains the observed low wear rates observed in tribological experiments in viscous liquids.