Lubrication of surfaces covered by surface-attached hydrogel layers

Surface-attached hydrogels can only swell in one dimension and exclude all large molecules due to size exclusion effects and for entropic reasons. Such swollen hydrogel layers are impenetrable by contacting polymer chains. In this work, the frictional properties of two chemically identical surfaces covered with a layer of a surface-attached hydrogel are investigated. The hydrogel layers are generated by coating the substrate with a copolymer containing benzophenone groups and a subsequent UV irradiation. During illumination the system crosslinks and binds simultaneously to the substrate. Adhesion measurements show that such layers exhibit only weak interactions. Under certain conditions extremely low coefficients of friction (COF = 0.002) are observed. We propose a friction model that describes the sliding behavior of these surfaces.