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

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 theo-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 degreesC) at a constant shear rate ((gamma)over dot) for a duration time t(s). After cessation of shear (at a given deformation of (gamma)over dot 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 degreesC. 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 ((gamma) over dot) and strain ((gamma) over dot 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 theological 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. (C) 2002 Elsevier Science Ltd. All rights reserved.