The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brush-covered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect of surfactant on lubrication.