Tribology of shearing polymer surfaces. 1. Mica sliding on polymer (PnBMA)

The energy-dissipating processes associated with shearing polymer junctions were investigated at the molecular, microscopic, and macroscopic levels using the surface forces apparatus-FECO optics technique. For a solid surface of mica sliding across a polymer surface of poly-II-butyl methacrylate (PnBMA) at low sliding velocities and at temperatures close to the glass transition temperature, T-g = 25 degrees C, the friction was mainly of the stick-slip variety; the static friction force F-s was always high and remained relatively constant, attaining its equilibrium steady-state value immediately on commencement of sliding, but the kinetic friction force F-k decreased from an initially high value to avery low value as sliding progressed. The friction forces exhibit complex time? temperature, load, and velocity dependencies and cannot be properly described in terms of a single parameter such as a "friction coefficient" or "shear stress". These, and other tribological characteristics of this type of system, appear to be very different from those of hard surfaces or simple liquid-lubricated surfaces. Where comparison with literature data is possible, the tribological results on these molecularly smooth "model" surfaces are similar to those for "engineering" surfaces sliding on bulk polymer. The molecular mechanisms and relaxation processes responsible for the observed tribological behavior and adhesion hysteresis of this type of system (solid surface sliding on polymer) are discussed at the end of this and the accompanying paper on the complementary system of polymer sliding on a solid surface of mica.(1)