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Polymers adsorbed on surfaces play an important role in many phenomena and applications, such as stabilization of colloidal dispersions, prevention against corrosion, flotation of minerals or wetting and spreading phenomena. This project addresses specifically the nanotribology of (neutral and charged) polymers grafted on surfaces by one chain end ("polymer brushes") via computer simulation.
If two surfaces covered by polymer brushes come in contact, they strongly repel one another and interpenetrate only weakly. (See left figure.) Thus, opposing polymer brushes can carry very high normal loads, whereas simultaneously lateral resistance to sliding may be extremely small. (See figure on the right.) Pioneering experiment of J. Klein[1,2] demonstrate friction coefficients (defined as the ratio between shear stress and normal pressure) which are orders of magnitude smaller than those of the bare, untreated surfaces. Basically, the small friction coefficients are due to the fact that polymer brushes under shear can deform (under a certain cost of entropy) and thus reduce the lateral shear forces.[3] However, the mechanisms, which are responsible for the extremely good lubrication properties of polymer brushes, are not yet well understood. There are different theories[4,5,6] which partially contradict each other, as they might possibly describe different regimes of shear velocity. At higher shear velocities, the chains incline and stretch, which leads to a reduction of overlap within the brush-brush interface. Without solvent, the rheological change in the grafted layer coincides with the shear-thinning regime. The energy dissipation in the interface is balanced by the entropy loss. However, the role of the solvent is unclear and is one of the key questions of this research focus. As illustrated in the figure on the right, it seems likely that the solvent will be expelled by the brushes, such that a fluid layer forms in between. If so, one might find a shear induced change of the hydrodynamic boundary conditions, as the solvent could perfectly slide between the brushes, which would thus dynamically decouple. From the viewpoint of applications it would be desirable to improve the resistance of the polymer layers against damage, in particular, against chain extraction. To achieve this a detailed understanding of the rheological properties of these layers are essential.
Although the first experiments on polymer brushes under shear were carried out already in the early 1990s, the observed phenomena remain hard to rationalize theoretically. The difficulty is partly related to the fact that, in experiments, it is almost impossible to provide sufficiently detailed information about the molecular factors causing the rheological response of the grafted layers to the external stimuli. In this respect, computer simulations provide a promising avenue because the simulation may be employed as a "high-resolution microscope" to explore structure-property relations of the studied (model) systems.
Our molecular dynamics simulations[7,8,9] cover steady state sliding processes and also address non-stationary behavior, such as the onset of motion, the relaxation from steady state towards static equilibrium, the inversion of the shear direction, or oscillatory shear at finite frequency. At present, our simulations treat the effects of solvent only implicitly. The impact of the uncontrolled approximation on the observed rheological properties is not understood and could be dramatic. We wish to explore this impact in the future.