Steric Forces of Tethered Polymer Chains as a Function of Grafting Density: Studies with a Single Diblock Molecular Weight

The normal force to compress opposing polymer brushes as a function of grafting density was measured using a surface force apparatus. In contrast to conventional methods where grafting density has been varied by changing polymer molecular weight, in this work high grafting densities ranging from 1.4 to 7.6 mg/m(2) were obtained by spin-coating different concentrations of a single diblock copolymer. Measured force profiles at low grafting density, sigma < 4 mg/m(2), were analogous to previous studies and consistent with Milner-Witten-Cates (MWC) theoretical predictions once chain polydispersity was accounted for. At higher grafting densities the experimentally measured repulsive interaction energy or force showed scaling behavior where the nondimensionalized force curves fall on top of each other; however, the behavior is distinctly different from the low grafting density regime and cannot be modeled using theory developed for the dilute/semiclilute regime.