Friction coefficient and viscosity of polymer brushes with and without free polymers as slip agents

There is ample evidence that polymer brushes reduce friction between surfaces. Several industrial applications take advantage of this fact, such as those in plastic bag production, where the brushes act as slip agents; however, the complex mechanisms that give rise to such reduction of friction are not yet fully understood. In this work we report coarse grained, dissipative particle dynamics simulations carried out for surfaces functionalized with erukamide brushes, a polymer commonly used in the plastics industry as a slip agent between surfaces. We calculate their rheological properties, such as the coefficient of friction (COF) and the viscosity, η, as functions of the number of chains grafted on the surfaces under the influence of stationary, Couette flow. Moreover, we consider also the case when a fraction of the erukamide chains is not adsorbed and moves freely between the surfaces. We show that the COF reaches an equilibrium value of about 0.29 in these two cases, in agreement with experimental results. On other hand, the viscosity grows monotonically, as a result of the increasing collisions when the erukamide content is increased. The force between brushes is found to be in agreement with predictions from scaling theories. We find that the addition to free chains helps stabilize the film formed by the brushes and the solvent, as others have found experimentally. The mechanisms that give rise to these phenomena are studied in detail.