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Mécanismes de frottement aux interfaces polymères liquides / solide : propriétés de glissement et structure interfaciale

Abstract : The aim of this work is to identify the molecular mechanisms driving the friction of liquid polymers on a solid substrate.First, we studied the effect of the temperature T on the slippage properties of PDMS melts. Using a velocimetry technique based on photobleaching, we measured the temperature dependence of the slip length of PDMS and checked the validity of Navier’s hypothesis, which defines the friction coefficient k as the ratio between the bulk viscosity to the slip length. Far above the glass transition temperature, k(T) increases exponentially with 1/T, a result that we confirmed with an independent measurement of k for the friction of crosslinked PDMS lenses. The friction mechanism of melts is a thermally activated process. The comparison of the activation energies for the friction and for the viscosity allows one to predict if the slip length increases or deacreases with temperature.We then focused our work on the identification of the friction mechanisms of semi diluted polymer solutions. Based on slip experiments of PS in DEP solutions, we evidenced a transient onset of slippage regime. Considering the viscoelasticity of the fluid and its friction properties, we developed a mechanical model of friction of a Maxwell-like fluid that well describes our experimental data.In the stationary slippage regime, we studied the influence of the polymer volume fraction φ on the slippage of solutions of polymer. In the Newtonian regime, the measured scaling law for k φ as a function of the volume fraction highlights the dependence of the friction coefficient on the distance to the glass transition temperature of the solution. Thus, the friction of the PS in DEP solutions can be attributed to the friction of blobs on the surface, rather than to the existence of a depletion layer.We directly observed the solid / polymer solution interface thanks to neutron reflectivity: these experiments reveal the formation of an adsorbed polymer layer and we could measure its concentration profile close to the solid wall. The interdigitation between volume and surface polymer chains plays a key role in the slippage of the solutions.
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Marion Grzelka. Mécanismes de frottement aux interfaces polymères liquides / solide : propriétés de glissement et structure interfaciale. Matière Molle [cond-mat.soft]. Université Paris Saclay (COmUE), 2019. Français. ⟨NNT : 2019SACLS551⟩. ⟨tel-02898880⟩

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