Polyzwitterionic brushes: Extreme lubrication by design |
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Authors: | Meng Chen Wuge H. Briscoe Steven P. Armes Hagai Cohen Jacob Klein[Author vitae] |
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Affiliation: | aDepartment of Chemistry, University of Oxford, Oxford OX1 3QZ, UK;bDepartment of Chemistry, University of Sheffield, Sheffield S3 7HF, UK;cWeizmann Institute of Science, Rehovot 76100, Israel |
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Abstract: | Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature’s benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems. |
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Keywords: | Polymer brush Polyzwitterionic brush ATRP (atom transfer radical polymerisation) Surface forces Boundary lubrication Hydration lubrication |
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