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

摘要

There is ample evidence that polymer brushes reduce friction betweensurfaces. Several industrial applications take advantage of this fact, such asthose 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 yetfully understood. In this work we report coarse grained, dissipative particledynamics simulations carried out for surfaces functionalized with erukamidebrushes, a polymer commonly used in the plastics industry as a slip agentbetween surfaces. We calculate their rheological properties, such as thecoefficient of friction (COF) and the viscosity, {\eta}, as functions of thenumber of chains grafted on the surfaces under the influence of stationary,Couette flow. Moreover, we consider also the case when a fraction of theerukamide chains is not adsorbed and moves freely between the surfaces. We showthat the COF reaches an equilibrium value of about 0.29 in these two cases, inagreement with experimental results. On other hand, the viscosity growsmonotonically, as a result of the increasing collisions when the erukamidecontent is increased. The force between brushes is found to be in agreementwith predictions from scaling theories. We find that the addition to freechains helps stabilize the film formed by the brushes and the solvent, asothers have found experimentally. The mechanisms that give rise to thesephenomena are studied in detail.