  We propose a novel class of responsive polymer brushes, where the effective
grafting density can be controlled by external stimuli. This is achieved by
using end-grafted polymer chains that have an affinity to the substrate. For
sufficiently strong surface interactions, a fraction of chains condenses into a
near-surface layer, while the remaining ones form the outer brush. The dense
layer and the more tenuous outer brush can be seen as coexisting microphases.
The effective grafting density of the outer brush is controlled by the
adsorption strength and can be changed reversibly and in a controlled way as a
response to changes in environmental parameters. The effect is demonstrated by
numerical SCF calculations and analyzed by scaling arguments. Since the
thickness of the denser layer is about a few monomer sizes, its capacity to
form a microphase is limited by the product of the brush chain length and the
grafting density. We explore the range of chain lengths and grafting densities
where the effect is most pronounced. In this range, the SCF studies suggest
that individual chains inside the brush show large rapid fluctuations between
two states that are separated by only a small free energy barrier. The behavior
of the brush as a whole, however, does not reflect these large fluctuations,
and the effective grafting density varies smoothly as a function of the control
parameters.
