Friction Studies of Polymer Lubricated Surfaces

Abstract

The shear properties of thin films of polymer melts confined between two molecularly smooth solid surfaces have been measured using a surface forces apparatus (SFA). Poly(dimethylsiloxane) (PDMS), polybutadiene (PBD), and perfluoropolyether (PFPE) of various molecular masses were studied, including both entangled and nonentangled systems. In all cases, the systems exhibited “smooth” sliding, with no intermittent or stick−slip motion at moderate shear rates (50−500 s-1). The coefficients of friction ranged from approximately 0.5 to 0.006, generally decreasing with increasing molecular mass and bulk viscosity for each polymer. The measured friction coefficients were smaller than values measured previously for lower molecular mass molecules such as n-alkanes, which is consistent with the above trend. During sliding, the surfaces remain separated by a thin tenacious polymer film, even at high loads where the minimum surface separation was found to be proportional to the radius of gyration, Rg, for each polymer. The friction coefficients for systems that are not entangled in the bulk are higher than for entangled systems. Shear for entangled systems appears to occur at the wall rather than within the polymer film, resulting in very low friction forces. For each type of polymer studied, one finds a decreasing friction or “effective” viscosity of the confined fluids with increasing (i) molecular mass, (ii) bulk viscosity, and (iii) degree of entanglement. These opposite trends (compared to those of the bulk fluids, including simple hydrocarbon liquids) are due to the effects of the atomically (or molecularly) structured walls on the molecular configurations of the confined fluids, and the increased film thicknesses with increasing molecular mass.