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Influence of pre-shearing on the crystallisation of conventional and metallocene polyethylenes

Abstract : The influence of pre-shearing on the crystallisation of a conventional Ziegler-Natta catalysed and a metallocene catalysed linear low-density polyethylene (LLDPE) samples was studied by a rheo-optical method, coupling light scattering and steady shear flow. Experiments were performed on a high-temperature shearing cell, with a pair of parallel quartz plate windows. The polymers were sheared at a melt temperature Ts (ranging from 190 to 150 °C) at a constant shear rate (γ̇) for a duration time ts. After cessation of shear (at a given deformation of γ̇ts unit), the sample was maintained at the selected melt temperature for a waiting time tw (ranging from 0 to 10 min) before it was cooled down to 90 °C. Small-angle light scattering (SALS) patterns were recorded during the whole process. It was shown that the pre-shear history thus investigated has no measurable influence on crystallisation of the linear metallocene LLDPE. For the Ziegler-Natta sample, the influence of pre-shear on its crystallisation is larger when the shear rate (γ̇) and strain (γ̇ts) are large, the waiting time tw is smaller or the temperature Ts is lower. The consequence of shear is giving ellipsoidally deformed spherulites, elongated in the flow gradient direction, which originate from oriented nuclei. Crystallisation, following flow in particular, is shown to be a very good probe of chain orientation and it turns out to be more sensitive than classical linear rheological methods. It can detect if the sheared sample is in a steady state or if all chains have relaxed after a flow. For example, it is surprisingly shown that chain orientation for the Ziegler-Natta sample requires a large amount of shear deformation, in the order of 1000 strain units, to reach a steady state.
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Submitted on : Tuesday, March 8, 2011 - 4:34:26 PM
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Choon K Chai, Quentin Auzoux, Hery Randrianatoandro, Patrick Navard, Jean-Marc Haudin. Influence of pre-shearing on the crystallisation of conventional and metallocene polyethylenes. Polymer, Elsevier, 2002, 44 (3), pp.Pages 773-782. ⟨10.1016/S0032-3861(02)00773-5⟩. ⟨hal-00574744⟩



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