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IDENTIFICATION D'UN COMPORTEMENT TERMINAL ELASTIQUE DANS LES FONDUS DE POLYMERES, DE POLYMERES CRISTAUX LIQUIDES ET ANALYSE DE LA TRANSITION DE PHASE INDUITE PAR CISAILLEMENT

Abstract : SUMMARY: When the shear flow couples to the fluid dynamic, the system enters into a non-linear regime.
The shear-induced birefringence within the isotropic phase of side-chain liquid crystalline polymers (SCLPs) is a non-linear phenomenon associated to long time scales, and although spectacular, it can be explained neither by a flow coupling with phase pretransitional dynamics nor with the conventional viscoelastic relaxation times. A classical approach is not suitable to elucidate the origin of the shear induced birefringence.
By controlling the interfacial properties, we show that the rheological response of SCLP, particularly in the isotropic phase, is fundamentally different from the expected low frequency flow behaviour: the melt displays a predominant elastic response (gel-like or solid-like). This elasticity, observed far away from any transition, does not originate from mesomorphic or pretransitional effect.
The analysis of the dynamic properties of amorphous ordinary polymers entangled or not, confirms that this terminal elasticity is generic, contrasting with the commonly reported flow behaviour. We show that the increase of the strain and the sample thickness leads to a lowering of the elasticity strength. The thickness effect has been interpreted in terms of non-conservative parameters which provide mobility to the system. At low thickness and at low strain rate, the measurement of the elasticity is free from these non-conservative contributions: the intrinsic elasticity is measurable. We show that the intrinsic elasticity increases with the molecular weight up to a critical molecular weight which coincides with the onset of the bulk entanglements and which corresponds to the magnitude of the conventional rubbery plateau. The intermolecular forces of the elasticity are equivalent to the entanglement elasticity. We show that the elastic response strongly depends on the nature of these intermolecular interactions.
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Submitted on : Thursday, January 31, 2008 - 4:55:11 PM
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Hakima Mendil. IDENTIFICATION D'UN COMPORTEMENT TERMINAL ELASTIQUE DANS LES FONDUS DE POLYMERES, DE POLYMERES CRISTAUX LIQUIDES ET ANALYSE DE LA TRANSITION DE PHASE INDUITE PAR CISAILLEMENT. Physique [physics]. Université Paris Sud - Orsay, 2006. Français. ⟨tel-00230247⟩

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