Surface Interaction Forces of Well-Defined, High-Density Polymer Brushes Studied by Atomic Force Microscopy. 2. Effect of Graft Density

Abstract

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 σ was changed in a wide range extending to an exceptionally high value (0.07 < σ (chains/nm 2 ) < 0.7) by photodecomposing the surface-fixed initiator, 2-(4-cholorosulfonylphenyl)ethyltrichlorosilane (CTCS). The surface density σ i of CTCS was determined by the grazing-angle reflection−absorption FT-IR measurement. The comparison of σ and σ i suggests that there is a limiting value in σ that cannot be exceeded even if σ 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 L e / L c ∝ σ n with n increasing from about 0.3 to 0.5 with increasing σ and that the brush with a higher σ 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 σ range is in the “high-density” regime, in which higher-order interactions among graft chains are important.