  We investigated the frictional properties of sodium polystyrene sulfonate
(NaPSS) brushes in water by frictional force microscopy (FFM). Polyelectrolyte
brushes were prepared on silicon wafers by the grafting-to method. The brushes
considerably reduce the frictional force and coefficient of kinetic friction
compared to hydrodynamic lubrication on a smooth Si wafer. Frictional force is
independent of sliding speed, but is lower for lower degrees of NaPSS
polymerization. Nanoindentation tests indicate that the polymer chains in a
brush are stretched strongly away from the substrate. These results suggest
that polymer chains point support the FFM probe tip in water and reduced
contact area and friction.
