ROLE OF ASPERITY CREEP IN TIME-DEPENDENT AND VELOCITY-DEPENDENT FRICTION OF A POLYMER GLASS

We study time-strengthening and velocity-weakening friction of poly(me thyl methacrylate) from room temperature up to the glass transition. T he role of creep of asperities under normal load is systematically inv estigated. It is confirmed to be a primary cause of ageing of the stat ic friction force. Velocity weakening is also strongly enhanced on app roaching the glass transition, whence promoting stick-slip oscillation s. The quantitative analysis of sliding friction, based on a widely us ed rule of ''time and velocity equivalence'', is found to be inadequat e in the transition region.