Structure of bimodal polymer brushes in a good solvent by neutron reflectivity

Neutron reflection is used to examine the concentration profile of bimodal tethered layers in a good solvent. The tethered layers are Langmuir monolayers of highly asymmetric poly(dimethyl-siloxane) - polystyrene (PDMS - PS) diblock copolymers at the air surface of ethyl benzoate. The PS blocks dangle into the good solvent, while the PDMS blocks anchor the copolymers to the surface. Bimodal layers are prepared using two block copolymers which vary in the molecular weight of the dangling block. The presence of the smaller PS blocks (DOP = N1) leads to additional stretching of the larger PS blocks (DOP = N2) relative to their dimension in a single-component monolayer, while the dimension of the smaller PS blocks is largely unaffected by the presence of the larger blocks. For the three bimodal brushes examined, the effect of additional stretching for the larger chains is greatest (up to 30%) for N2 ≅ 3N1, smaller for N2 ≅ 5N1, and negligible for N2 ≅ 11N1. These trends are in agreement with self-consistent field calculations reported in the literature. In addition, it is observed that the chains with shorter dangling blocks add preferentially to an existing copolymer monolayer when the chains in the monolayer are strongly interacting. This is attributed to a greater potential barrier for the longer chains to incorporate into the monolayer due to osmotic and steric interactions.