STRUCTURE OF BIMODAL POLYMER BRUSHES IN A GOOD SOLVENT BY NEUTRON REFLECTIVITY

Neutron reflection is used to examine the concentration profile of bim odal tethered layers in a good solvent. The tethered layers are Langmu ir monolayers of highly asymmetric poly(dimethyl-siloxane)-polystyrene (PDMS-PS) diblock copolymers at the air surface of ethyl benzoate. Th e 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 bl ock. The presence of the smaller PS blocks (DOP = N-1) leads to additi onal stretching of the larger PS blocks (DOP = N-2) 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 addition al stretching for the larger chains is greatest (up to 30%) for N-2 co ngruent to 3N(1), smaller for N-2 congruent to 5N(1), and negligible f or N-2 congruent to 11N(1). These trends are in agreement with self-co nsistent field calculations reported in the literature. In addition, i t is observed that the chains with shorter dangling blocks add prefere ntially to an existing copolymer monolayer when the chains in the mono layer are strongly interacting. This is attributed to a greater potent ial barrier for the longer chains to incorporate into the monolayer du e to osmotic and steric interactions.