A polymer-brush-based, surface-modification strategy for friction and wear reduction in hard contact under boundary-lubrication conditions is proposed, specifically for a non-aqueous environment. Surface-initiated atom-transfer radical polymerization was employed for the synthesis of three different oil-compatible, hydrophobic polymer brushes based on alkyl methacrylates. This study presents polymerization kinetics, chemical characterization by means of Fourier-transform infrared spectroscopy and surface morphologies observed in atomic force microscopy. The lubrication properties of the anchored polymers were evaluated macroscopically by means of ball-on-disk methods and on the nanonewton scale by lateral force microscopy and showed significant reduction in friction up to contact pressures as high as 460 MPa. The frictional response of surface-grafted polymers is shown to depend strongly on the compatibility of the polymer with the chosen lubricating fluid.

Poly(methacrylic acid) (PMAA) brushes were grafted from Si/SiO2 substrates by means of immobilized-photoiniferter-mediated controlled radical polymerization. The employed UV setup was based on ultraviolet light-emitting diodes (UV-LEDs), which allowed for a precise control of the brush height with irradiation time, as observed by in situ quartz crystal microbalance experiments with dissipation monitoring (QCM-D). In contrast to many alternative approaches, it was shown that the novel UV source in combination with a photoiniferter renders lengthy postcleaning steps of the synthesized brushes unnecessary. Following characterization of the polymer layers by means of variable-angle spectroscopic ellipsometry (VASE) and static contact angle measurements, the lubrication properties of the PMAA brushes were investigated in macroscopic tribological experiments under low-contact-pressure, aqueous conditions. Results indicated that PMAA brushes have the potential to dramatically reduce sliding friction in an aqueous environment.

We report a highly effective macroscopic, nonaqueous lubrication strategy, employing polymer-brushbased surface modification. Poly(alkyl methacrylates) with hexyl, dodecyl and octadecyl side chains have been prepared on both silicon and iron substrates by means of surface-initiated atom-transfer radical polymerisation. Frictional properties of the three polymers have been evaluated in hexadecane, ethanol and toluene. For the case of the poly(dodecyl methacrylate), the study additionally involved nine base lubricating fluids of different chemical natures, with viscosities ranging from 4 to 2,200 cSt at 20°C. By constructing a set of Stribeck curves at low values of speed 9 viscosity (at constant load), it was possible to identify an extended hydrodynamic lubrication regime and, for P12MA-based coatings, a low-friction, brush-lubrication regime, reinforced by time-dependant effects, which appeared to replace the mixed and boundary regimes observed for conventional lubricants. Self-mated poly(dodecyl methacrylate) brushes maintained low coefficient-of-friction values (e.g. 0.012 in hexadecane) over thousands of reciprocating cycles ([100 m total distance) under low speed 9 viscosity conditions (0.1 cm/s and 4 cSt).

Polymer brushes consisting of micrometer-long poly(dodecyl methacrylate) (P12MA) chains can impart very low coefficients of friction to surfaces sliding in oil. Under harsh tribological conditions, however, this behavior can only be maintained for long periods of time in the presence of highly viscous fluids, which appear to protect the brushes from wear. This behaviour is analogous to the lubricating mechanism in synovial joints, inasmuch as the confinement of pressurized synovial fluid in cartilage is partially responsible for the low wear rate of the system. Sliding induces solvent confinement within the brushes, provided that the viscosity is sufficiently high, and confinement can also be observed upon indentation at sufficient high rates. Such indentation experiments have been performed to characterize the mechanical (viscoelastic) behavior of the polymer brushes in solvents. By applying Hertzian contact theory and considering the stiffening of the polymer brushes upon compression, an effective elastic contact modulus of the P12MA brushes is obtained that is much higher than the equilibrium value. The proportion of the load carried by the fluid can also be estimated as a function of speed and viscosity. Thus, high solvent viscosity, as well as substantial brush height, can lead to a shielding of the polymer from the effects of the load, and the enhancement of the effective brush elastic modulus. This explains the observed low wear rates observed in tribological experiments in viscous liquids.

A grafting-to approach is described as a means of fabricating ultrathin, oil-compatible, friction-reducing coatings. The tribological properties of grafted-to coatings of poly(dodecyl methacrylate) (P12MA), prepared by means of a photoactivated perfluorophenylazide adhesion promoter, were compared with those of grafted-from coatings of the same polymer of comparable molecular weight, prepared via surface-initiated, atom-transfer radical polymerisation. It was shown that in a sub-hydrodynamic regime (i.e. boundary/brush lubrication), both coating types can be equivalently effective, exhibiting very low friction coefficients (0.02) against a bare silica countersurface with no detectable wear over the duration of the test (around 1,000 cycles, and maximal Hertzian contact pressure ca 170 MPa as calculated for a bare-bare configuration), providing that sufficiently viscous base lubricants are employed.