Surface interaction forces of well-defined, high-density polymer brushes studied by atomic force microscopy. 1. Effect of chain length

We made direct force measurements by atomic force microscopy (AFM) at surfaces of polymer brushes comprised of low-polydispersity poly(methyl methacrylate) (PMMA) chains densely end-grafted on a silicon substrate by living radical polymerization. These brushes are characterized by an exceptionally high, nearly constant graft density (approximately 0.4 chains/nm(2)) and a wide range of molecular weights of the graft chains. This graft density is at least 10 times larger than those of the previously studied polymer brushes prepared by the adsorption of block copolymers. Force measurements were made in toluene with a silica particle-attached cantilever. The hysteresis behavior of a piezo actuator used in the AFM was corrected by simultaneously measuring its piezo current. The true distance D between the substrate surface and the silica probe, which usually is difficult to define in AFM experiments, was successfully determined by AFM imaging across the boundary of a scratched and an unscratched region on the sample surface. In this way, we could obtain quantitative force-distance profiles. The repulsive force was observed to rapidly increase with decreasing separation. The equilibrium thickness L-e of the brushes, i.e., the critical distance at which a repulsive force was detectable, was found to be proportional to the weight-average chain contour length L-c,L-w, giving L-e/L-c,L-w, = 0.6. This indicates formation of a homogeneous polymer layer with highly stretched graft chains. Unlike the previously reported results for lower density polymer brushes, the force-distance profiles for different graft chain lengths were not scaled by the reduced distance D/L-e. Longer brushes were more resistant to compression: for example, the longest studied brush was compressible only to D/L-e = 0.8, which was about 3 times as large as the scaled dry thickness L-d/L-e.