  We report on the frictional properties of a single contact between a glassy
polymer lens and a flat silica substrate covered either by a disordered or by a
self-assembled alkylsilane monolayer. We find that, in contrast to common
belief, the Amontons proportionality between frictional and normal stresses
does not hold. Besides, we observe that the velocity dependence of the sliding
stress is strongly sensitive to the structure of the silane layer. Analysis of
the frictional rheology observed on both disordered and self-assembled
monolayers suggests that dissipation is controlled by the plasticity of a
glass-like interfacial layer in the former case, and by pinning of polymer
chains on the substrate in the latter one.
