Abstract
We report mesoscopic simulations of polymer brushes in the presence of free polymer chains. We have used the dissipative particle dynamics method to model entangled polymer chains in a good solvent. We have studied the structure of the grafted and free chains under shear, as well as the dynamics of the detechment of polymer chains from the grafted surface. A comparison between entangled and nonentangled systems is presented. Entanglements are modeled using a bond repulsion potential. The rheology has been studied at different polymer grafting densities, in the presence of a varying number of free chains. We have shown that the detachment of grafted chains creates a central zone of free chains which exhibits a bulk like behaviour. The viscosity of this region is significantly increased compared to that of a polymer brush with the same grafting density and no detached chains. The presence of free polymer chains dramatically changes the friction coefficient. We have established that a fraction of detached chains of only 15% induces a decrease in the friction of 50%. This behaviour is correlated to the decrease of the interpenetration between the two opposing brushes. The detachment of chains from the surface, that may well occur at high sliding velocity in the surface force apparatus, is likely to have a significant effect on the observed experimental friction.
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