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Cristallisation des caoutchoucs chargés et non chargés sous contrainte : Effet sur les chaines amorphes

Abstract : We studied the properties of stress-induced crystallization of the natural rubber, as a function of temperature,
cross-link density and percentage of added carbon black. The crystallinity was measured by X-ray
diffraction and the orientation of the polymeric chains in the amorphous phase by 2H NMR. We could
establish that the mechanical hysteresis observed during a cycle of traction was primarily due to the
phenomenon of crystallization. In the case of unfilled samples, the crystallization onset results in a stress
softening; this is observed until a crystallinity of approximately 15% is reached, beyond which one observes
on the contrary a hardening. That cross-over occurs at a critical extension which varies according to material
and additives contained in rubber. During recovery, one frequently observes a geometrical instability
corresponding to the macroscopic separation of an amorphous phase and a semi-crystalline phase (“inverse
yielding”); this effect was studied in detail. A similar phenomenon of micro-yielding probably occurs at a
microscopic level in the other materials which do not display any visible yielding. In the case of filled
rubbers, only hardening is visible. The dissipated energy H, related to the mechanical hysteresis, is function
only of the rate of crystallinity Cmax reached at the highest extension during the cycle. We showed that the
viscoelastic component of this hysteresis is weak (Hvisco<10% when there is crystallization): the breaking
strength of these materials is thus primarily due to deformation-induced crystallization. Moreover we could
establish that in filled rubbers, the effect of hardening due to crystallization was strongly amplified by the
presence of carbon black. In the case of unfilled rubbers, NMR experiments made it possible to highlight the
relaxation of the chains due to crystallization: this phenomenon is observed even during the hardening phase.
We studied other elastomers like polybutadiene, butyl and polychloroprene. The same relation between
hardening and crystallisation has been observed.
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Submitted on : Wednesday, October 11, 2006 - 5:39:20 PM
Last modification on : Wednesday, September 16, 2020 - 4:31:52 PM
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Jeanne Marchal. Cristallisation des caoutchoucs chargés et non chargés sous contrainte : Effet sur les chaines amorphes. Analyse de données, Statistiques et Probabilités [physics.data-an]. Université Paris Sud - Paris XI, 2006. Français. ⟨tel-00105646⟩

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