Dynamic surface decoupling in a sheared polymer melt

– We propose that several mechanisms contribute to friction in a polymer melt adsorbed at a structured surface. The first one is the well known disentanglement of bulk polymer chains from the surface layer. However, if the surface is ideal at the atomic scale, the adsorbed parts of polymer chains can move along the equipotential lines of the surface potential. This gives rise to a strong slippage of the melt. For high shear rates chains partially desorb. However, the friction force on adsorbed chains increases, resulting in quasi-stick boundary conditions. We propose that the adsorbed layers can be efficiently used to adjust the friction force between the polymer melt and the surface. Introduction. – Frictional forces between solid substrates can be dramatically reduced by coating them with thin polymer layers, which are able to sustain large normal loads while remaining fluid [1]. This effect has obvious practical applications, ranging from biolubrication to hard disk drives [2]. The molecular mechanism of friction, however, is still poorly understood [3]. It is believed that the striking reduction of shear forces in systems with endadsorbed and grafted polymers immersed in liquids is due to limited mutual interpenetration of the brushes in good solvent, even under compression, and a fluid interfacial layer where most of the shear occurs. In polymer melts the situation is very different, because of the dominance of entanglement effects (if the chains are long enough), associated high viscosities, and screening of the excluded volume interactions [4, 5]. In spite of these differences, the key parameter affecting the lubrication and shear forces is still the degree of interdigitation between the moving surface layer and the bulk polymer system [6]. Regarding the structure of the adsorbed polymer layers two systems were thoroughly examined, both theoretically and experimentally: (i) Polymer brushes with the chains tethered by one end to a solid surface. At high enough coverage, the chains stretch away from the surface and form a brush-like structure. (ii) Melt layers which are formed when a strongly attractive surface is exposed to a polymer melt and a certain amount of polymer becomes permanently bound to the surface. The resulting layer is made of loops, with a large polydispersity of loop sizes, reflecting the statistics of the chains in the melt [7,8]. As to the rheological properties of brushes, it is now well accepted that the polymer anchoring on solid surfaces plays a key role on the flow behavior of polymer melts, in particular on the appearance of flow with slip at the wall [9,10]. The onset of wall slip is related to the strength of the interactions between the solid surface and