Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

The nanoscopic adhesive and frictional behaviour of end-grafted poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) films (brushes) in contact with gold- or PDMAEMA-coated atomic force microscope tips in potassium halide solutions with different concentrations up to 300 mM is a strong function of salt concentration. The conformation of the polymers in the brush layer is sensitive to salt concentration, which leads to large changes in adhesive forces and the contact mechanics at the tip–sample contact, with swollen brushes (which occur at low salt concentrations) yielding large areas of contact and friction–load plots that fit JKR behaviour, while collapsed brushes (which occur at high salt concentrations) yield sliding dominated by ploughing, with conformations in between fitting DMT mechanics. The relative effect of the different anions follows the Hofmeister series, with I − collapsing the brushes more than Br − and Cl − for the same salt concentration

Friction measurements on phase-separated thin films with a modified atomic force microscope

THE study of chemical phase separation in multicomponent thin organic films typically involves the addition of a dye which is selectively more soluble in one of the phases, thereby making it possible to probe the domain structures by fluorescence microscopy1-4. The resolution of this approach is generally limited to tens of micrometres. The atomic force microscope, on the other hand, has recently proved useful for imaging organic thin films down to the atomic scale5-9, but this technique provides details of the overall film topography, rather than the chemical composition. Here we show that the recently developed friction force microscope10-13, which simultaneously measures both the normal and lateral forces on the scanning tip, can be used to image and identify compositional domains with a resolution of approximately 5 angstrom. Although the topography of the individual domains can be imaged with a standard atomic force microscope, it is the additional information provided by the friction measurement that allows them to be chemically differentiated