Abstract
We show that a surface-grafted polymer brush, 1-n-butyl-3-vinyl imidazolium bromide-based poly(ionic liquids), is able to reduce the interfacial friction by up to 66% and 42% in dodecane and water, respectively. AFM-based force spectroscopy reveals that the polymer brush adopts distinctively different interfacial conformations: swollen in water but collapsed in dodecane. Minimal surface adhesion was observed with both polymer conformations, which can be attributed to steric repulsion as the result of a swollen conformation in water or surface solvation when the hydrophobic fraction of the polymer was exposed to the dodecane. The work brings additional insight on the polymer lubrication mechanism, which expands the possible design of the polymer architecture for interfacial lubrication and modification.
Highlights
We show that a surface-grafted polymer brush, 1-n-butyl-3-vinyl imidazolium bromide-based poly(ionic liquids), is able to reduce the interfacial friction by up to 66% and 42% in dodecane and water, respectively
It is explained that the effective reduction in the sliding friction between two surfaces is due to a synergistic effect of resistance to penetration of the one surface into the polymers at the other contacting surface and the fluidity of these polymer brushes, which is a characteristic gained from the solvation layers surrounding the polymer brushes.[8−11] For example, poly[2(methacryloyloxy)ethyl phosphorylcholine] showed an unparalleled lubrication performance, with a Coefficient of Friction (CoF, μ) as low as 0.00004 at pressures as high as 7.5 MPa, which is attributed primarily to the strong hydration of the phosphorylcholine-like monomer units known to bind around 15−25 or more water molecules.[12]
Polymer brushes tend to demonstrate an exceptional lubrication in one type of solvent, either polar or nonpolar.[14−16] The effect of preferential solvent intake on polymer lubrication was shown by Mathis and colleagues,[17] whereby poly(dodecyl methacrylate) was exposed to ethanol and an ethanol/toluene mixture at different temperatures
Summary
The PIL brushes form a swollen and lubricating layer in water, but a compact layer in dodecane that inhibits any attractive interactions with the borosilicate probe. The tribological characteristics of surface-grafted imidazolium-based poly(ionic liquid)s were quantitatively measured in both water and dodecane. It was found that the PIL brush can deliver an excellent lubrication behavior when it is either swollen or collapsed, as examined by a ball-on-plate. We thank the anonymous reviewers for the valuable suggestions
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