Stimuli-Responsive Brushes with Active Minority Components: Monte Carlo Study and Analytical Theory

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_bare inside the brush. The properties of the brushes can be reproduced by self-consistent field theory if one replaces v_bare by an effective parameter v_eff, where the ratio of second virial coefficients B_eff/B_bare depends on the range of monomer interactions but not on the grafting density, the chain length, and v_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.