Structure of monodisperse and bimodal brushes

The structure of monodisperse and bimodal brushes consisting of poly(ethyle ne oxide) (PEO) at the air/water interface is investigated with neutron ref lectivity, and the results are compared with the structure predicted by the Scheutjens-Fleer self-consistent-field lattice model. The monomer density profile of a monodisperse PEO brush at low and intermediate grafting densit ies is block-like with an extended tail region in which the density smoothl y decays to zero. At high grafting densities, however, the profile is predo minantly parabolic, as predicted by analytical self-consistent-field models . Quantitative agreement is found between the experimentally measured profi les and those predicted by the Scheutjens-Fleer model. Bimodal brushes are investigated for three chain length ratios and mixing ratios at various gra fting densities. It is concluded that at a given grafting density, the long chains are more extended in bimodal brushes than in monodisperse brushes a t the same grafting density. This additional stretching increases with incr easing length of the smaller block or increasing fraction of smaller blocks . The agreement between the Scheutjens-Fleer and measured density profiles is also good in the case of bimodal brushes.