Switching of friction by binary polymer brushes.

Surface force studies on polystyrene-poly(2-vinylpyridine) (1 : 1) mixed polymer brushes and corresponding monobrushes were carried out in dried state under a controlled environment. The aim was to explore possibilities to control adhesion and friction between inorganic or polymeric surfaces by use of polymer brushes. The effect of switching of chemical composition of binary brush surfaces (on treatment with suitable solvents) on wettability, surface roughness, and hence the adhesion and friction properties of the surfaces were determined. Atomic force microscopy (AFM) with silicon tips, silicon nitride tips, and colloidal probes with silica particles were employed to investigate the interactions between inorganic surfaces and polymer brushes. To study the interactions between different polymer brush surfaces colloidal probes were covered with polystyrene and poly(acrylic acid) brushes on the surface. For all the dry polymer brushes samples, surface roughness values were in the range of 0.35-1.0 nm only. Adhesion and friction forces of polymer brush samples were reduced in comparison to the silicon wafer and were correlated with each other (except for the silicon tip). Switching in adhesion and friction forces up to a factor of 4.5 was possible by switching of the conformation of mixed brushes on treatment with selective solvents. The friction force on a PS + P2VP gradient polymer brush layer varied laterally and increased with increase in the P2VP content. Friction depends on wettability, scan velocity of the AFM tip, grafting density, and composition gradient of polymer brushes. Moreover, for PS and P2VP monobrushes, friction forces were shown to increase with increasing grafting density. Polymer brush layers thus may be used to control the adhesion and friction behavior of solid surfaces in different ways.