Stratification in bidisperse polymer brushes from neutron reflectivity

Two-component (bidisperse) polystyrene brushes, consisting of long and short chain populations end-tethered at a solid/liquid interface, were studied in toluene and cyclohexane solvents using neutron reflection methods. A stratification of the long and short chain components was observed under all solvent conditions, with a maximum in the long chain volume fraction profile occurring at the outer edge of the brush. The short chains resided in the inner brush region nearer to the substrate. Comparison of experimentally derived volume fraction profiles to analytical self-consistent mean-field theory predictions revealed qualitative agreement; however, the experimentally observed stratification is less pronounced. Furthermore, under the poorer, near-Θ conditions in cyclohexane, the total volume fraction profile was rather featureless and did not exhibit the characteristic bidisperse shape present in toluene.