Projet ANR-06-BLAN-0315 – SPBRUSH : Influence of long-range interactions on sheared polymer brushes
Programme : Programme blanc (BLANC) 2006
Année de financement : 2006
Coordinateur du projet : CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ALSACE (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ALSACE)
Partenaires : Centre national de la recherche scientifique
Montant : 163000.0 euros
Date de début : 2006
Durée : 36 mois
Résumé : 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. With this project we want to contribute to the research of ultrathin polymer layers (some tens of nanometers) by studying aspects of the nanotribology of polymer brushes via computer simulation.  -    -  Polymers grafted on surfaces by one chain end ("polymer brushes") display spectacular tribological properties. If two surfaces covered by polymer brushes come in contact, they strongly repel one another and interpenetrate only weakly. Thus, opposing polymer brushes can carry very high normal loads, whereas simultaneously lateral resistance to sliding may be extremely small. The resulting friction coefficients may be orders of magnitude smaller than those found in solid friction. Thus, polymer brushes have important applications as lubricants. 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 we feel that a detailed understanding of the rheological properties of these layers is essential. -      Although the first experiments on polymer brushes under shear were carried out in the early 1990s, the observed phenomena remain hard to rationalize theoretically. This 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 could 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. This may account for much of the activity in this field. However, the vast majority of these simulations treat the effects of solvent only implicitly. The impact of this uncontrolled approximation on the observed rheological properties is not well understood and could be dramatic. -  -  We wish to explore this impact by the present project. We plan to perform Molecular Dynamics simulations of a commonly employed bead-spring model for polymer brushes, where we now include explicitly solvent molecules. In a first step, we want to simulate electrically neutral systems under shear. In a second step, we plan to extend our simulations to polyelectrolyte brushes, in which we will explicitly account for charge carriers. -      Our investigations will 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. By comparing our results to previous work (without explicit solvent) we expect to gain detailed insight into the effects of hydrodynamics and electrostatics on these grafted polymer layers at contact. Our results should thus contribute to obtaining a better understanding of these systems and of their potential for possible applications as lubricants.