Polymer Brushes under Shear: Molecular Dynamics Simulations Compared to Experiments

MK Singh and P Ilg and RM Espinosa-Marzal and M Kroger and ND Spencer, LANGMUIR, 31, 4798-4805 (2015).

DOI: 10.1021/acs.langmuir.5b00641

Surfaces coated with polymer brushes in a good solvent are known to exhibit excellent tribological properties. We have performed coarse- grained equilibrium and nonequilibrium molecular dynamics (MD) simulations to investigate dextran polymer brushes in an aqueous environment in molecular detail. In a first step, we determined simulation parameters and units by matching experimental results for a single dextran chain. Analyzing this model when applied to a multichain system, density profiles of end-tethered polymer brushes obtained from equilibrium MD simulations compare very well with expectations based on self-consistent field theory. Simulation results were further validated against and correlated with available experimental results. The simulated compression curves (normal force as a function of surface separation) compare successfully with results obtained with a surface forces apparatus. Shear stress (friction) obtained via nonequilibrium MD is contrasted with nanoscale friction studies employing colloidal-probe lateral force microscopy. We find good agreement in the hydrodynamic regime and explain the observed leveling-off of the friction forces in the boundary regime by means of an effective polymer-wall attraction.

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