Friction induced by grafted polymeric chains

We present experimental results on the friction force exerted by a network of poly(dimethylsiloxane) (PDMS) moving on a solid substrate coated with th e same polymer. Two different coatings are compared: a surface, densely gra fted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) can be gradual ly added and adsorbed PDMS layers. Increasing the sliding velocity between 10(-5) and 10(-1) m/s suggests a transition between "liquidlike" and "solid like" frictional behavior. Increasing the molecular weight of adsorbed or g rafted chains shifts this transition to higher sliding velocities. Increasi ng systematically the areal density (Sigma) of connectors yields two opposi te trends (i) At our lowest velocities, an increase of Sigma results in hig her friction. This increase of friction is due to the pull-out process velo cities, the same connectors lead to a reduction without connectors. of the grafted chains from the network. (ii) At higher of friction (lubrication ef fect!) with respect to a surface without connectors.