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.