Friction of Zwitterionic Hydrogel by Dynamic Polymer Adsorption

A simplified model describing the sliding friction of hydrogel on solid surface by dynamic adsorption of the polymer chains is proposed on the basis of polymer adsorption-repulsion theory. This dynamic adsorption model is used to analyze the friction results of zwitterionic hydrogels sliding over glass substrates with different substrate wettability, hydrogel swelling degree, ionic strength, and pH of bath solution. The adsorption time tau(b) of polymer strands is found to decrease with the increase in sliding velocity or the Weissenberg number as a result of stretching. The adsorption time tau(0)(b), and the adsorption energy U-ads at stress-free condition, which are characteristic for each friction system, are also estimated. Roughly, a master curve is observed for the normalized adsorption lifetime tau(b)/tau(0)(b) and the Weissenberg number, with less dependence on the adsorption energy and the bulk properties of the gels in the observed experimental conditions. Thus, the dynamic adsorption model successfully correlates the frictional behavior of hydrogels with the adsorption dynamics of polymer strands, which gives insight into the molecular design of hydrogels with predefined frictional properties for biomedical applications.