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.
Liu Guoqiang,Feng Yang,Gao Xiaohua,Chen Zhuo,Zhao Nan,Zhou Feng,Liu Weimin
Synovial fluid is made up of various biomacromolecules, including hyaluronic acid, aggrecans, lubricins, and phosphatidylcholine lipid, which are assembled onto the surface of articular cartilage in a gel state. Among them, brush-like biomacromolecules or assemblies have a vital effect on human joint lubrication. Inspired by this, the combination of brush-like molecular structures and gel-like assembly may be an efficient approach for the synthesis of biomimetic lubricating matters. Learning from the lubrication system of human joints, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes grafted poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-co-AA)) microgels, abbreviated as MBs-g-MGs, were synthesized as one kind of biomimetic lubricating additives. It is worth noting that this bionic strategy considered both molecular structure and assembled form, which enabled this hairy microgel to achieve low friction in aqueous medium. Meanwhile, the effective lubrication was still achieved when using MBs-g-MGs at high temperature, indicating that this microgel maintains a good lubricating effect over a wide range of temperature. In addition, this kind of microgel possessed good biocompatibility, which laid the foundation for potential biomedical applications. Looking beyond, these biomimetic microgels may provide an effective lubricating effect for water-based sliding interfaces, especially in biomedical systems.
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