Tribological Properties of Poly(methyl methacrylate) Brushes Prepared by Surface-Initiated Atom Transfer Radical Polymerization

Surface-initiated living radical polymerization of methyl methacrylate (MMA) using copper/bipyridyl complex was carried out from silicon wafer immobilized with a 2-bromoisobutylate moiety, resulting in the assembly of polymer chains tethered by one end to a surface, a so-called “polymer brush”. The thickness of the PMMA brush, which was estimated by ellipsometry and atomic force microscopy, increased linearly with the molecular weight of the chains, and was controlled by the free initiator concentration. The graft density of the PMMA brushes was estimated to be as high as 0.56 chains/nm2. The frictional properties of the high density PMMA brush were characterized by sliding the stainless ball probe on its surface across a width of 20 mm at a rate of 90 mm/min under loading of 0.49 N in air at room temperature. The PMMA brush was found to have a lower frictional coefficient and much better wear resistance than the corresponding spin-coated PMMA film because of the anchoring of the chain ends in the brush. In addition, the friction coefficient of the polymer brush significantly decreased in response to soaking in acetone and toluene, and increased in response to immersion in hexane. The tribological properties depended on the solvent quality.