Molecular Dynamics Simulation Of Polymer Brushes

First, static and dynamic properties of polymer brushes of moderate and high grafting densities were studied by 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, by 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. It was found that an anisotropic fluctuation dynamics is related with two characteristic dynamic length scales in the both directions. The pulling forces acting on the polymer bonds were further investigated. 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.Later polymer brushes were studied via molecular-dynamics simulations at very high grafting densities, where the crossover between the brush regime and the polymer-crystal regime is taking place. This crossover is directly observed with the 3D density distribution,structure factor and pair-correlation function. With increasing grafting density, this crystallization is progressing from the core layer of the brush towards the surface layer. The same process is analyzed using the lateral fluctuations of the monomers as a signature of their diminishing mobility. Additionally, bond forces and the chain excess free energy indicate a transition from the brush regime to the overstretched regime, which is in agreement with predictions of a modified self-consistent field theory. Finally, we studied the properties of the two-component polymer brushes at different temperatures and found, with decreasing the temperature within a wide range, the binary polymer brushes change from disorder to order and then cross theθtemperature. To better understand it, the 3D density distribution (and composition density),structure factor,pair-correlation function,time-auto-correlation function,order parameter and so on were investigated.