Computer simulation of the rheology of grafted chains under shear

We report nonequilibrium molecular dynamics simulations of grafted chains surrounded by solvent molecules and sheared between two atomic walls. Each wall is covered by a layer of amphiphilic molecules 20 units long. The chains are firmly bound at their ends to the wall at a surface coverage of 33%. The particles interact through the Weeks-Chandler-Andersen repulsive potential. Bond interactions and the stiffness of the chains are modeled using harmonic potentials. The heat produced by shearing is removed from the system by conduction through the boundaries, which leads to characteristic gradients in temperature, density, and shear rate. At low shear rates, these effects are small, but structural reordering of the solvent molecules and chains is observed. Solvent molecules are expelled from the chain region close to the boundary wall during shearing. Decreasing the flexibility of the amphiphilic molecules creates a more dramatic response to the imposed shear field, resulting in a larger chain tilt and fewer solvent molecules close to the wall.