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Fondus de polymère en mouillage pseudo-partiel sur la silice : morphologie, structure et dynamique des films précurseurs

Abstract : Oxidized silicon wafers are high energy surfaces : most liquids spontaneously spread on the entire surface. When a dropletis deposited, a nanometric film called a precursor film first spreads ahead of the droplet. The liquids that we considerin this study are polymer melts (polybutadiene, polyisoprene, polystyrene) in pseudo-partial wetting condition on these surfaces : a sessile droplet coexists with a film. By taking advantage of ellipsometric microscopy, we study the morphologyand dynamics of such films, and quantitatively probe the interactions at stake between the polymer segments and thesurface. Two different parts can be distinguished in the precursor films : the primary film of subnanometric thickness, and the secondary film, which is dense. In the primary film, polymer chains are in a 2D semi-dilute state : they do not cover theentire substrate and do not interact with each other. From the spatio-temporal thickness profiles, we measure the diffusion coefficient of the chains on the surface. We show that they diffuse with a thermally activated Rouse motion, that can be described by the sole friction of the chains on the surface, with an activation energy that reveals the interactions at stake.We measure that the polymer/surface interactions largely dominate the polymer/polymer interactions. We generalize thismodel for polymers with specific terminal groups, and to the case of chains with monomer conformational mobility that depends on the monomer position along the chain. The secondary film connects the primary film to the droplet, and is comprised of chains in a dense state. At long times, its thickness profile is a step of uniform thickness, which is proportionalto the square root of the chain length. Remarkably, this equilibrium thickness does not depend on the polymer chemistrynor on the surface state – temperature, water adsorbed, oxide layer thickness, etc. The evolution toward this equilibriumstate can be modeled by taking into account both the polymer/surface friction and the polymer/polymer friction. In the literature, only few studies deal with precursor films when the liquid is in pseudo-partial wetting condition. In addition tothe robust measurement of the interactions at stake at the scale of the polymer chains, our work highlights the necessityto re-think the theoretical existing framework for precursor films, and opens many perspectives.
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Submitted on : Tuesday, February 16, 2021 - 10:17:16 AM
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Claire Schune. Fondus de polymère en mouillage pseudo-partiel sur la silice : morphologie, structure et dynamique des films précurseurs. Matériaux. Université Paris sciences et lettres, 2020. Français. ⟨NNT : 2020UPSLS017⟩. ⟨tel-03142537⟩



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