COMPUTER-SIMULATION OF THE RHEOLOGY OF GRAFTED CHAINS UNDER SHEAR .2.DEPLETION OF CHAINS AT THE WALL

We report nonequilibrium molecular dynamics simulations of grafted and free chains surrounded by solvent molecules and sheared between two a tomic walls. Each wall is covered by a layer of amphiphilic molecules, which are twenty units long and chemically bounded to the surface. Si mulations were performed at surface coverages ranging from 1/3 to 0, w here 1/3 corresponds to a system with only grafted chains. Coverages l ower than 1/3 were obtained by randomly detaching chains from the wall . The particles interact through the Weeks-Chandler-Andersen repulsive potential. Bond interactions and the stiffness of the chains are mode led using harmonic potentials. Heat is removed from the system through the walls by applying a Nose-Hoover thermostat. At coverages larger t han 0, the chains behave like a wall resulting in steep velocity gradi ents. With decreasing coverages, the tilt of the amphiphiles is increa sed, and at the same time solvent molecules diffuse into the chain reg ion. This effect of solvent molecules entering into the chain region i s most pronounced at a coverage of 0.22. Frictional forces are higher for the intermediate coverages. This is probably due to entanglements between free and grafted chains. Decreasing the flexibility of the amp hiphilic molecules creates a more dramatic response to the imposed she ar field resulting in a larger chain tilt, higher frictional forces, a nd a higher solvent density at the wall.