Superlubricity Achieved with Zwitterionic Brushes in Diverse Conditions Induced by Shear Actions

Polymer brushes are known to have excellent lubricity when grafted on surfaces in water. In this study, poly(2-(methacryloyloxy)-ethyl phosphorylcholine) (PMPC) has been successfully grafted on a silica wafer through a surface-initiated atom transfer radical polymerization (SI-ATRP) method. The microscale friction properties between a silica probe and PMPC brushes were studied by atomic force microscopy (AFM). The PMPC molecules are gradually transferred from the substrate to the silica probe induced by shear actions in the initial friction process, causing the frictional force to gradually reduce with increasing sliding distance. The robust superlubricity with a friction coefficient of 0.006 was finally achieved after enough PMPC molecules had transferred onto the probe, which was attributed to the formation of a uniform hydration layer after the running-in process. The effects of PMPC brush thickness and ion types on friction and normal forces were also examined. This work may help us develop various surface modifications for lubrication enhancement by shear actions.