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 the same polymer. Two different coatings are compared: a surface, densely grafted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) can be gradually added and adsorbed PDMS layers. Increasing the sliding velocity between 10-5 and 10-1 m/s suggests a transition between “liquidlike” and “solidlike” frictional behavior. Increasing the molecular weight of adsorbed or grafted chains shifts this transition to higher sliding velocities. Increasing systematically the areal density (Σ) of connectors yields two opposite trends: (i) At our lowest velocities, an increase of Σ results in higher friction. This increase of friction is due to the pull-out process of the grafted chains from the network. (ii) At higher velocities, the same connectors lead to a reduction of friction (lubrication effect!) with respect to a surface without connectors.