RHEOLOGY OF POLYMER BRUSHES - A BROWNIAN DYNAMICS STUDY

We present results of Brownian dynamics simulations of polymer brushes under steady and oscillatory shear. The brush is sheared by a bare su rface and the resulting solvent velocity and polymer dynamics are solv ed self-consistently. Under steady shear the deformation of the blush proceeds in two steps: chains tilt in the now direction followed by a physical thinning of the brush. The brush-effective viscosity increase s upon compression to near 60% and decreases thereafter. We develop a scaling based on the Brinkman equation to explain the unusual trends i n the viscosity. Upon introducing oscillatory shear now in the brush, we observe large increases in the normal stress and bead density near the upward surface. Shear-induced collisions of beads in the brush inc rease the osmotic pressure and thus give rise to these normal forces. The strain amplitude determines the dynamics during oscillatory now, a nd we develop scalings for the range of strain amplitude over which th e normal stress increases occur. The simulation results for a single g rafted layer are compared to the experiments performed by Klein et al. for the shearing of two grafted layers.