Low friction and high load bearing capacity layers formed by cationic-block-non-ionic bottle-brush copolymers in aqueous media

Xiaoyan Liu,Ricardas Makuska,Per M Claesson,Esben Thormann,Ceslav Visnevskij,Andra Dedinaite,Mark Rutland

doi:10.1039/c3sm27862j

Xiaoyan Liu, Ricardas Makuska + Show 5 more

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https://doi.org/10.1039/c3sm27862j

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Abstract

Efficient lubricants should be able to build surface layers that result in low friction and high load bearing capacity. In this work we show how this can be achieved in aqueous media by means of adsorption of a diblock copolymer consisting of a cationic anchor block without side chains and an uncharged and hydrophilic bottle-brush block that protrudes into solution. Surface and friction forces were measured between negatively charged silica surfaces coated with adsorbed layers of the cationic diblock copolymer, utilizing the atomic force microscope colloidal probe technique. The interactions between the surfaces coated with this copolymer in water are purely repulsive, due to a combination of steric and electrostatic double-layer forces, and no hysteresis is observed between forces measured on approach and separation. Friction forces between the diblock copolymer layers are characterized by a low friction coefficient, μ ≈ 0.03–0.04. The layers remain intact under high load and shear due to the strong electrostatic anchoring, and no destruction of the layer was noted even under the highest pressure employed (about 50 MPa). Addition of NaCl to a concentration of 155 mM weakens the anchoring of the copolymer to the substrate surface, and as a result the friction force increases.

Highlights

IntroductionNature has developed efficient aqueous lubrication systems based on phospholipids combined with biopolymers.[1,2,3] Notably, bottle-brush structures in molecules such as lubricin[3,4] and hyaluronan–aggrecan association structures play important roles in, for example, joint lubrication.[5,6,7] These ndings have inspired the synthesis of polymers with similar architectures, and some of these have been shown to provide favorable lubrication properties.[8,9,10]Dense brush layers that experience good solvency conditions are another alternative to achieve low friction forces.[9,10,11] For instance, recently, polyzwitterionic brush layers were shown to provide a very low coefficient of friction (m z 0.0004) up to a mean pressure of 7.5 MPa.[12]
We have explored the boundary lubrication properties of (METAC)m-b-(PEO45MEMA)n layers adsorbed to silica surfaces
The friction force increases in high ionic strength solutions, which is suggested to be a consequence of the decreased electrostatic affinity to the surface

Summary

Introduction

Nature has developed efficient aqueous lubrication systems based on phospholipids combined with biopolymers.[1,2,3] Notably, bottle-brush structures in molecules such as lubricin[3,4] and hyaluronan–aggrecan association structures play important roles in, for example, joint lubrication.[5,6,7] These ndings have inspired the synthesis of polymers with similar architectures, and some of these have been shown to provide favorable lubrication properties.[8,9,10]Dense brush layers that experience good solvency conditions are another alternative to achieve low friction forces.[9,10,11] For instance, recently, polyzwitterionic brush layers were shown to provide a very low coefficient of friction (m z 0.0004) up to a mean pressure of 7.5 MPa.[12].

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