Friction of an elastomer sliding on polymeric model surfaces
Prof. Dr. Günter Reiter Institute of Physics and Freiburg Materials Research Center, University of Freiburg, 79104, Freiburg, Germany
We investigated experimentally the frictional behavior of compatible polymers sliding past each other at model interfaces of controlled molecular parameters. In particular, a crosslinked polydimethylsiloxane (PDMS) drop was moved on solid substrates coated with grafted or adsorbed PDMS "monolayers". We compared two different coatings, layers of and adsorbed PDMS and model surfaces with densely grafted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) were gradually added. Increasing the sliding velocity between 10-5 m/s 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.