Modification of surface interactions and friction by adsorbed dendrimers: 1. Low surface-energy fifth-generation amino-acid-modified poly(propylene-imine) dendrimers

The interactions between two mica surfaces bearing fifth-generation amino acid-modified poly(propyleneimine) dendrimers (the dendritic box that exposes methyl groups at their 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 indicated that dendrimers adsorb from dilute toluene solution (ca. 5 × 10-5-3 × 10-4 w/w) as a monolayer on the surfaces. Two interacting dendrimer monolayer-bearing surfaces experience a van der Waals attraction followed by steric repulsion on compression. The dendrimer bilayer could be compressed reversibly, yielding a measure of the compressibility of the molecules. Frictional force versus normal load profiles were measured at different shear velocities, and reveal both solidlike and liquidlike behavior of the confined dendrimers, consistent with NMR measurements on the dendritic box. The results show that the yield stress increases with compression of the layers. Observation of the relaxation behavior of sheared dendrimer layersfor adsorption from dilute solutionsuggest that, within the parameters of our experiments, the relaxation times are insensitive to the compression. For the case of surface interactions after incubation in more concentrated dendrimer/toluene solutions (ca. 10-3 w/w), the results of both normal and shear force measurements suggested aggregation of much thicker, loose dendrimer layers on the mica surfaces. These layers resulted in hysteretic and longer-ranged monotonic repulsion, and much weaker frictional forcesat comparable loads and shear velocitiesthan in the case of the monolayers adsorbed from dilute solutions