We study static and dynamic properties of polymer brushes of moderate and high grafting densities using molecular dynamics simulations. The self-consistent-field theory is able to reproduce the observed density profiles once finite extensibility of the chains is included. A similar approach, using a Langevin-type model to account for a coupling between the vertical and lateral chain fluctuations, is successfully implemented to explain the lateral chain fluctuations at moderate-to-high grafting densities. The corresponding fluctuation dynamics in the direction lateral to the surface is well described by Rouse scaling. The dependence of the dynamics on grafting density (concentration) is very well described by the dynamic behavior of thermal blobs for the fluctuations in the direction both perpendicular and parallel to the surface. We find an anisotropic fluctuation dynamics related with two characteristic dynamic length scales in the both directions. We have further investigated the pulling forces acting on the polymer bonds. The force acting on the first bond directly connected to the surface is reduced at higher grafting densities, which is explained with an entropic contribution to the bond tension. A nonvanishing end-monomer tension is observed at high grafting densities. The integrated force (excess free energy) follows the prediction of the thermal blob model of the brush.

Via computer simulations, we demonstrate how a densely grafted layer of polymers, a brush, could be turned into an efficient switch through chemical modification of some of its end monomers. In this way, a surface coating with reversibly switchable properties can be constructed. We analyze the fundamental physical principle behind its function, a recently discovered surface instability, and demonstrate that the combination of a high grafting density, an inflated end-group size, and a high degree of monodispersity is a condition for an optimal functionality of the switch.

Binary polymer brushes with hydrophobic/hydrophilic behavior are forming a two-layer system with a collapsed hydrophobic and a swollen hydrophilic phase. The process of switching upon sudden change of the solvent quality is analyzed in detail for various solvent selectivities, chain lengths, and grafting densities. This process is highly reversible since after a microphase separation the chains are moving collectively inside their phase domains so that the interactions between chains of different species are diminished. The thickness of the collapsed layer does not scale linearly with the chain length N, an unexpected result which is discussed in the paper. The switching relaxation times display a scaling of N 2 like Rouse relaxation and not of N 3 like vertical relaxation times in equilibrium.

Surface effects on polymer brushes are of increasing interest, since recent experimental progress has nourished the hope that functional surfaces could facilitate the creation of novel materials, affecting almost every facet of modern life. [1][2][3] The creation of switchable brushes which react on variations of the external conditions are of particular interest in applications as diverse as textile industry, environmental technology or biotechnology. 4,5 At the same time, novel experimental techniques have allowed to create polymer brushes of very high grafting densities, reaching a chain stretch of 0.7 of maximum chain length. 6 The properties of brushes at low grafting densities are well described by self-consistent field (SCF) approaches 7,8 and scaling arguments. 9 Systems of high grafting densities have been simulated in rather recent studies, in which strong deviations of the density profiles from SCF predictions have been reported. [10][11][12] These were, at least partially, related to the finite extensibility of the chains. 11,13,14 Modifications of brush properties, for instance due to polydispersity effects, have been studied essentially for moderate grafting densities within the framework of SCF approaches. 15 Already Dan et al. 16 and Skvortsov et al. 17 have found a sensitive behavior of the end-monomer distribution with respect to relatively small polydispersity effects using SCF methods. Altogether, existing SCF models were found to be less than satisfactory to describe polydisperse systems, even at low grafting densities. 18 In this work, we show that polymer brushes at high grafting densities display a highly selective behavior even with respect to changes of the size of the end-monomers (functionalized end-group), and we discuss implications for the grafting-from technique and the creation of functionalized surfaces.