Lubrication mechanism of concentrated polymer brushes in solvents: effect of solvent viscosity

We investigated the frictional properties of concentrated polymer brushes (CPBs) of poly(methyl methacrylate) (PMMA) immersed in two highly viscous ionic liquids (ILs) that are good solvents for PMMA: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). These ILs could swell the CPB of PMMA to the same degree as toluene, a more commonly used good solvent of much lower viscosity. The CPB of PMMA effectively reduced the interaction between surfaces in these ILs and hence developed otherwise hidden hydrodynamic lubrication, which was characterized by the dependence of the frictional coefficient μ on shear velocity v. In the hydrodynamic-lubrication regime, the linear relationship was obtained between them in the double logarithmic plot, giving a slope close to unity, which is expected for a Newtonian fluid, in the solvents studied even though the viscosity differed by nearly two orders of magnitude. More interestingly, the μ values in this regime of lubrication were found to be scaled by the viscosity of the solvent. These results not only enable us to discuss the details of the effect of solvent viscosity on lubrication properties of the CPB systems but also demonstrated their ability to act as an excellent lubrication system in combination with ILs as lubricating fluids.