  Using a combination of analytical theory, Monte Carlo simulations, and three
dimensional self-consistent field calculations, we study the equilibrium
properties and the switching behavior of adsorption-active polymer chains
included in a homopolymer brush. The switching transition is driven by a
conformational change of a small fraction of minority chains, which are
attracted by the substrate. Depending on the strength of the attractive
interaction, the minority chains assume one of two states: An exposed state
characterized by a stem-crown-like conformation, and an adsorbed state
characterized by a flat two-dimensional structure. Comparing the Monte Carlo
simulations, which use an Edwards-type Hamiltonian with density dependent
interactions, with the predictions from self-consistent-field theory based on
the same Hamiltonian, we find that thermal density fluctuations affect the
system in two different ways. First, they renormalize the excluded volume
interaction parameter $v_\mathrm{\tiny bare}$ inside the brush. The properties
of the brushes can be reproduced by self-consistent field theory if one
replaces $v_\mathrm{\tiny bare}$ by an effective parameter $v_{\mathrm{\tiny
eff}}$, where the ratio of second virial coefficients $B_{\mathrm{\tiny
eff}}/B_\mathrm{\tiny bare}$ depends on the range of monomer interactions, but
not on the grafting density, the chain length, and $v_\mathrm{\tiny bare}$.
Second, density fluctuations affect the conformations of chains at the brush
surface and have a favorable effect on the characteristics of the switching
transition: In the interesting regime where the transition is sharp, they
reduce the free energy barrier between the two states significantly. The
scaling behavior of various quantities is also analyzed and compared with
analytical predictions.
