Friction of an elastomer sliding on polymeric model surfaces

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". Several regimes exhibiting different dependences on sliding velocity have been observed. Systematic variation of the grafting density (Sigma) of long connector molecules which tend to penetrate into the network yielded two opposing trends. At our lowest velocities, an increase of Sigma resulted in higher friction, whereas at higher velocities, the same connectors led to a reduction of friction (lubrication effect!) with respect to a surface without connectors.