Atomic force microscopy measurements of adsorbed polyelectrolyte layers. 1. Dynamics of forces and friction

The measurement of surface forces and friction between silica substrates bearing adsorbed layers acrylamide-1% [3-(2-methylpropionamide)propyl]trimethyl ammonium chloride (AM-MAPTAC) was examined using atomic force microscopy. The cationic polymer had a large molecular weight (900 000) and a very low charge density (1% MAPTAC units). The force curves in the presence of adsorbed polyelectrolyte showed a very long range repulsive interaction and large deformation on compression typical of electrosteric interactions and consistent with the expected adsorption profile. There was also a strong dependence on the scan rate with increased repulsion during approach and increased attraction during separation as the scan rate increased. The hysteresis was attributed entirely to a hydrodynamic interaction induced by the polyelectrolyte. At slow enough scan rates, the hysteresis between approach and retract curves was absent. The friction force measurements were sensitive to both applied load and scan rate. With increasing applied load, a critical load was apparent, above which the friction force increased. Upon decrease of the load, the friction force remained higher than that before the yield point, suggesting a metastable deformation of the polymer layer. In addition, the friction coefficient increased with increasing scan rate. At low scan rates the friction coefficient was lower than the bare surfaces, whereas at high scan rates the friction coefficient was significantly larger that obtained than for the bare surfaces. Subsequent force curves taken after friction measurements confirm that a permanent change in the polyelectrolyte layer had occurred. The magnitude of the repulsive interaction was reduced to about half of its original force.