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 T s (ranging from 190 to 150 °C) at a constant shear rate ( γ ̇ ) for a duration time t s. After cessation of shear (at a given deformation of γ ̇ t s unit), the sample was maintained at the selected melt temperature for a waiting time t w (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 ( γ ̇ ) and strain ( γ ̇ t s ) are large, the waiting time t w is smaller or the temperature T s 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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