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 us ing the surface forces apparatus-FECO optics technique. For a solid surface of mica sliding across a polymer surface of poly-II-butyl methacrylate (Pn BMA) at low sliding velocities and at temperatures close to the glass trans ition 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 r elatively constant, attaining its equilibrium steady-state value immediatel y 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 depen dencies and cannot be properly described in terms of a single parameter suc h as a "friction coefficient" or "shear stress". These, and other tribologi cal characteristics of this type of system, appear to be very different fro m those of hard surfaces or simple liquid-lubricated surfaces. Where compar ison with literature data is possible, the tribological results on these mo lecularly smooth "model" surfaces are similar to those for "engineering" su rfaces sliding on bulk polymer. The molecular mechanisms and relaxation pro cesses responsible for the observed tribological behavior and adhesion hyst eresis of this type of system (solid surface sliding on polymer) are discus sed at the end of this and the accompanying paper on the complementary syst em of polymer sliding on a solid surface of mica.(1)