Investigation of the time-dependent friction behavior of polyacrylamide hydrogels

Hydrogels have attracted enormous frictional investigation owing to their great potential in hydration lubrication. Researchers have revealed that the lubrication behavior of hydrogel depends on a number of parameters such as contact area, sliding velocity, and solution condition. However, less effort has been devoted to understanding the time-dependent friction behavior of polymeric hydrogels. In this work, the time-dependent behavior of hydrogel friction was systematically investigated through three typical polyacrylamide hydrogels (PAM, PAM-PEI, and PAM-PAA) on a rheometer. Interestingly, the friction coefficients (μ) of the hydrogels varied with time, and two distinct regimes (viz., the friction-decrease regime and friction-constant/increase regime) were observed. In the friction-decrease regime, all the friction coefficients for the three PAM-based hydrogels dramatically decreased (e.g. μ decreased from ∼ 0.107 to ∼ 0.034 for PAM, μ decreased from ∼ 0.059 to ∼ 0.021 for PAM-PAA). The decrease of friction coefficients can be explained by frictional relaxation mechanism, where the deformation of polymers in the shear direction and the desorption of polymer chains from solid surface play dominant roles. Tribological results show that the increase in friction is determined by the charge behavior and polymer network of the PAM-based hydrogels. Besides, shear velocity and pH of the solution also significantly affected the friction behavior of hydrogels on the glass substrate. The understanding of the time-dependent friction behavior of PAM hydrogels provides insights to develop water-based lubricants with engineering applications.