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

The interactions between two mica surfaces bearing fifth-generation amino a cid-modified poly(propyleneimine) dendrimers (the dendritic box that expose s methyl groups at their outer surface) were studied across a toluene mediu m; using a surface force balance capable of measuring shear as well as norm al forces. Normal force measurements indicated that dendrimers adsorb from dilute toluene son (ca. 5 x 10(-5)-3 x 10(-4) w/w) as a monolayer on the su rfaces. Two interacting dendrimer monolayer-bearing surfaces experience a v an der Waals attraction followed by steric repulsion on compression. The de ndrimer bilayer could be compressed reversibly, yielding a measure of the c ompressibility of the molecules. Frictional force versus normal load profil es were measured at different sheer velocities, and reveal bath solidlike a nd liquidlike behavior of the confined dendrimers, consistent with NMR meas urements on the dendritic box. The results show hat the yield stress increa ses with compression of the layers. Observation of the relaxation behavior of sheared dendrimer layers-for adsorption from dilute solution-suggest tha t, within the parameters of our experiments, the relaxation times are insen sitive to the compression. For the case of surface interactions after incub ation in more concentrated dendrimer/toluene solutions (ca. 10(-3) w/w), th e 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 wea ker frictional forces-at. comparable loads and shear velocities-than in the case of the monolayers adsorbed from dilute solutions.