Comparative Research on Lubricating Mechanism of Surfaces Grafted with Hydrophilic and Hydrophobic Polymer Brushes

A couple of polymer brushes were grafted on the subsurface of ceramic ball and glass disk by surface-initiated radical polymerization technique, which were poly(propyl methacrylate-3-sulfonate) potassium salt with hydrophilic functional groups poly(propyl methacrylate-3-sulfonate) potassium salt with hydrophobic functional groups, hereinafter abbreviated as PSPMA and PLMA respectively, and thus two counterpart surfaces with different wettability were successfully constructed, the film thickness and friction were measured by homemade film measurement device based on interference technology. The measurement results demonstrated that the two surfaces showed different antifriction effects. For PLMA, the friction coefficient was very low and stable, it was speculated that it was related to the lower surface free energy and the lower glass transition temperature. The friction coefficient of PSPMA fluctuates greatly and was highly affected by the slip-roll ratio and the entrainment speed, which may be related to the formation of a hydrated layer with normal bearing capacity on the contact surface due to high hydration. The thickness of water film formed on the surface of grafted PSPMA was much higher than that of PLMA, which can be attributed to their different degree of hydration, and it was found that PSPMA showed a lubrication enhancement effect under the coupling action of hydrodynamic pressure and hydration, thus obtaining a higher film thickness. The thickness of the water film formed on the surface of grafted PSPMA was much higher than that of PLMA, which can be attributed to the fact that the hydration degree of PSPMA brush was much higher than that of PLMA. Under the coupling effect of hydrodynamic and hydration, PSPMA presented a lubrication enhancement effect, thus obtaining a higher film thickness. The research showed that the hydration effect of polymer brush was crucial for obtaining excellent lubrication performance, which was of great significance for understanding the mechanism of water-based lubrication mechanism and guiding the design of water-lubricated materials.