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

摘要

The interactions between two mica surfaces bearing a fourth-generation carbosilane 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 indicate 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 experience a longer-ranged van der Waals attraction followed by strong short-range adhesion (probably of dipolar origin) as the adsorbed dendrimers come into contact. Within the range of our parameters, the dendrimer layers were incompressible normal to the surfaces. Friction versus load profiles were measured at different shear velocities, revealing marked stick-slip sliding, whereas the magnitude of the yield stress increased with longer times of contact and with normal pressure. This suggests that over time scales comparable with the experimental times the interacting layers rearrange to optimize their interfacial shear strength. The behavior of these -OH-exposing carbosilane dendrimers differs qualitatively from that of CH3-exposing poly(propyleneimine) dendrimers studied earlier, a difference attributable to the much more polar nature of the hydroxyl groups. [References: 34]