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Polymer Brushes Dynamics in Melt, Sheared and Confined Situations
End tethered polymer chains have been recognized to be excellent adhesion promoters at polymer interfaces provided that they can penetrate and interdigitate into the bulk polymer. As a consequence, tethered chain dynamics is of major importance to understand the mechanical answer of a polymer close to a surface. Indeed the dynamics of the chains, the way they can entangle or not with the bulk, have been shown to modify either the adhesion with a glassy layer at low temperature or the transmission of shear stresses to the solid, through the appearance of large slip length for the polymer flow at higher temperatures. During this talk we will present 3 different situations where neutron reflectivity has been a unique tool to characterize the degree of interdigitation of a polymer brush with a polymer melt and its dynamics. In a first part, we have determined the density profiles and their evolution with annealing time, when the system is heated above the glass transition temperature, and then rapidly quenched below the glass transition. The molecular parameters (grafting densities and molecular weights of the grafted chains) have been carefully controlled in the experiments. The results will be compared to a model which takes into account the long relaxation time of the grafted chains associated to the necessary retraction of the long grafted arm down to the grafting point in order to recover equilibrium conformation, along with the reptation of the melt chains which accelerate the arm retraction process. If the bulk (non grafted) layer is of nanometric thickness, the grafted chains cannot expand completely to their equilibrium shape and remain confined after a partial relaxation inside the available melt slab. We have performed some systematic experiments with this confined geometry and we will show that the interdigitation dynamics is affected by the confinement in a way which is reminiscent of the change of glass transition temperature in nanometric films. Finally, when the upper d-PS layer is sheared above Tg, the polymer flows and slip can appear at the solid-fluid interface. A transition from a weak slip to a large slip boundary condition for the flow velocity at the interface is related to the disentanglement between surface and bulk chains (reference) associated to the fact that for large enough shear rate, the surface grafted chains become sufficiently elongated by the friction force to be fully disentangled from the melt chains. We will show how Neutron reflectivity allows for a direct characterization of the deformation of the grafted layer resulting from both the applied shear forces and the molecular organization inside the grafted layer.