  We report on the frictional properties of thin ($\approx \mu m$)
poly(dimethylacrylamide) hydrogel films within contacts with spherical silica
probes. In order to focus on the contribution to friction of interfacial
dissipation, a dedicated rotational setup is designed which allows to suppress
poroelastic flows while ensuring an uniform velocity field at the sliding
interface. The physical-chemistry of the interface is varied from the grafting
of various silanes on the silica probes. Remarkably, we identify a velocity
range in which the average frictional stress systematically varies with the
logarithm of the sliding velocity. This dependency is found to be sensitive to
the physical-chemistry of the silica surfaces. Experimental observations are
discussed in the light of a molecular model where friction arises from
thermally activated adsorption of polymer chains at the sliding interface,
their elastic stretching and subsequent desorption. From this theoretical
description, our experimental data provide us with adhesion energies and
characteristic times for molecular adsorption that are found consistent with
the physico-chemistry of the chemically-modified silica surfaces.
