Modification of surface interactions and friction by adsorbed dendrimers: 2. High surface-energy -OH-terminated carbosilane dendrimers

The interactions between two mica surfaces bearing a fourth-generation carb osilane dendrimer (modified to expose -OH groups on its outer surface) were studied across a toluene medium, using a surface force balance capable of measuring shear as well as normal forces. Normal force measurements indicat e that the dendrimers adsorb from dilute toluene solution (ca. 5 x 10(-4) w /w) as a monolayer on each surface. Two such interacting surfaces experienc e a longer-ranged van der Waals attraction followed by strong short-range a dhesion (probably of dipolar origin) as the adsorbed dendrimers come into c ontact. Within the range of our parameters, the dendrimer layers were incom pressible normal to the surfaces. Friction versus load profiles were measur ed at different shear velocities, revealing marked stick-slip sliding, wher eas the magnitude of the yield stress increased with longer times of contac t and with normal pressure. This suggests that over time scales comparable with the experimental times the interacting layers rearrange to optimize th eir interfacial shear strength. The behavior of these -OH-exposing carbosil ane dendrimers differs qualitatively from that of CH3-exposing poly(propyle neimine) dendrimers studied earlier, a difference attributable to the much more polar nature of the hydroxyl groups.