Macroscale superlubricity achieved between zwitterionic copolymer hydrogel and sapphire in water

Abstract

Achievement of macroscale superlubricity from the hydration effect could provide a potential application for artificial articular cartilage. In this work, two zwitterionic polymers, 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine methacrylate (SBMA), were used to synthesize a P(MPC-co-SBMA) copolymer hydrogel through the additional polymerization of the alkane carbon-carbon double bond. The incorporation of MPC promoted the water-binding property of copolymer hydrogels as well as improved their anti-compression capability. A superlubricity state with a friction coefficient of approximately 0.002 was achieved when a P(MPC-co-SBMA) copolymer hydrogel hemisphere slid on a sapphire in water, which demonstrated its dependence on sliding velocity, load, and aqueous lubricants. The superlubricity mechanism was mainly attributed to the hydration effect from zwitterionic MPC and SBMA polymer chains, which led to the formation of a uniform hydration layer on the hydrogel surface and the strong adsorption of water molecules on the sapphire surface, which provided additional stabilized hydration layers. These findings might provide insight into the superlubricity mechanism of zwitterionic hydrogels based on the hydration effect, thereby broadening their extensive applications.