Bioinspired bottle-brush polymer exhibits low friction and Amontons-like behavior

We report the design of a bottle-brush polymer whose architecture closely mimics the lubricating protein lubricin. Interaction forces, assessed using a Surface Forces Apparatus (SFA), between two mica surfaces fully covered by the polymer demonstrate that the polymer adopts a loop conformation giving rise to a weak and long-range repulsive interaction force between the surfaces. Under high compression, stronger repulsive forces appear due to the strong compression of the grafted pendant chains of the polymer. When submitted to shear, the system shows extremely low frictional forces dependent on the salinity of the medium. Friction coefficients measured for this system were as low as ~10(-3). Interestingly, the confined lubricating fluid obeys all three Amontons' laws. We explain this peculiar observation by the strong shear thinning of the confined fluid and the osmotic repulsive forces that dominate the overall (dynamic and equilibrium) surface interactions.