Lubrication Mechanism of Surface‐Attached Hydrogel Layers in Sliding Contact

Swollen, surface-attached hydrogel layers in sliding contact exhibit unusual friction properties. To study this phenomenon, hydrogel layers covalently bonded to solid substrates are generated by C-H insertion crosslinking. Brief UV irradiation of thin layers of photoreactive prepolymers leads to simultaneous crosslinking of the polymer chains and attachment of the network to the substrate. As the surface-attached hydrogel layers can only swell perpendicular to the surface, and the polymer subchains on the two sides of the friction pair cannot interpenetrate for entropic reasons, adhesion in such friction pairs is negligible. In this work, the tribological behavior of surface-attached hydrogel pairs is investigated as a function of load and sliding velocity. It is observed that for very thin films, the indentation and accordingly the friction is a strong function of the film thickness and the role of the chain confinement on the lubrication of surface-attached hydrogels is discussed. A model for the friction mechanism of such systems is developed.