Professor Sissi de Beers of the University of Twente

"Controlling friction and adhesion using stimulus responsive polymer coatings"

Polymer coatings, and in particular, polymer brushes in good solvents are typically poor adhesives and well known to lubricate high-pressure contacts, while they can keep a low-viscosity solvent in the contact. In contrast, polymer coatings in poor solvent will show a much higher adhesion and friction upon sliding. This principle has been exploited extensively to reversibly control friction and adhesion using stimulus responsive polymers for which the effective solvent-quality can be altered using e.g. light, temperature or electric field. In this presentation, I will show that this generic picture can fail for several types of stimulus polymer brushes. For these types of brushes, the relation between adhesion / friction and brush swelling can be much more complex and depend on specific solvent-mediated interactions in the contact. In fact, I will show that, when the exact relation between brush swelling and friction / adhesion is known, it can employed to achieve much more dramatic changes in the tribo-mechanical properties of polymer brushes compared to just switching between swollen and collapsed brushes.