Theory of the structure of adsorbed block copolymers: Detailed comparison with experiment

Abstract

We present a numerical, self-consistent field study of adsorbed diblock copolymers in thermal solvents, with a detailed and quantitative comparison with recent experiments performed on poly(dimethylsiloxane-block-styrene) copolymer spread as a monolayer at the free surface of ethyl benzoate [M. S. Kent, L. T. Lee, B. Farnoux, and F. Rondelez, Macromolecules 25, 6240 (1992); M. S. Kent, L. T. Lee, B. J. Factor, F. Rondelez, and G. S. Smith, J. Chem. Phys. 103, 2320 (1995)]. These neutron reflectivity experiments, for the first time, independently varied both the molecular weight and surface density, and probed the size and nature of the depletion layer at the surface. In the calculations, the polymer and solvent are characterized by realistic values of the pure component densities, the Kuhn length and the Flory interaction parameter. We examine the properties of the dangling block, specifically the depletion layer, the thickness of the brush, the maximum polymer concentration and its location, and the dependence of these properties on surface density and molecular weight. We obtain very good agreement with the experiments, especially for the functional dependences, which indicate that these systems are not in the asymptotic brush limit. We also argue that this conclusion applies to many experiments, some of which were previously thought to be in the asymptotic limit.