Investigations on the lubrication mechanism of surface-attached hydrogels

Covalently bonded hydrogel layers are generated by C-H insertion crosslinking in which subsequent UV irradiation leads to simultaneous crosslinking of the polymer chains and attachment of the substrate to the substrate. In this work, the tribological behavior of surface-attached hydrogel pairs has been investigated under relatively high pressures and sliding velocities. Recent work on tribology of surface-attached hydrogels revealed that adhesion is a negligible term in such layers, as interpenetration between these layers is avoided due to entropic reasons. We proposed a model that gives insight into the influential parameters in the determination of the frictional behavior by considering the contribution of the different force components in the friction force. Furthermore, we studied the role of confinement imposed by the stiff substrate on the lubrication of surface-attached hydrogels, as the extremely low coefficient of friction of these layers appears to be the consequence of high confinement. Finally, the influence of surface micro-structuring and surfactant addition on the friction of surface-attached hydrogels was investigated.