Microtribological and nanomechanical properties of switchable Y-shaped amphiphilic polymer brushes

We have characterized the morphology and nanomechanical properties of surface-grafted nanoscale layers consisting of Y-shaped binary molecules with one polystyrene (PS)arm and one poly(acrylic acid) (PAA) arm. We examined these amphilic brushes in fluids (in-situ visualization), and measured their microtribological characteristics as a function of chemical composition. Atomic force microscopy (AFM) based nonmechanical testing has shown that nanoscale reorganization greatly influences the adhesion and elastic properties of the naoscale brush layer. In water, a bimodal distribution of the elastic modulus, arising from the mixed chemical composition of the topmost layer, is observed. In contrast, the top layer is completely dominated by PS in toluene. As a result of this reorganization, the Y-shaped-brush layer exhibits a dramatic variation in the friction and wear properties after exposure to different solvents. Unexpectedly, the tribological properties are enhanced for the hydrophilic and polar. PAA-dominated, surface, which shows a lower friction coefficient and higher wear stability, despite higher adhesion and heterogeneous surface composition. We suggest that this unusual behavior is cased by the combination of the PAA chains.