Surface interaction forces of well-defined, high-density polymer brushes studied by atomic force microscopy. 2. Effect of graft density

Poly(methyl methacrylate) (PMMA) brushes with the same chain length but different graft densities were prepared on a silicon substrate by the surface-initiated atom transfer radical polymerization technique. The graft density sigma was changed in a wide range extending to an exceptionally high value (0.07 < sigma (chains/nm(2)) < 0.7) by photodecomposing the surface-fixed initiator, 2-(4-cholorosulfonylphenyl)-ethyltrichlorosilane (CTCS). The surface density sigma(i) of CTCS was determined by the grazing-angle reflection-absorption FT-IR measurement. The comparison of sigma and sigma(i) suggests that there is a limiting value in a that cannot be exceeded even if sigma(i) is increased. The direct force measurements were made at the brush surface swollen in toluene by an atomic force microscope with a silica particle-attached cantilever. The analysis of the obtained force-distance profiles revealed that the equilibrium thickness L-e scaled by the full chain length L-c varies such as LeLc proportional to sigma(n) with n increasing from about 0.3 to 0.5 with increasing sigma and that the brush with a higher sigma is more resistant to compression. These results are poorly explained by the scaling theory, which was derived for "moderately dense" brushes. This suggests that the studied sigma range is in the "high-density" regime, in which higher-order interactions among graft. chains are important.