The application of the multi-pass rheometer for precise rheo-optic characterisation of polyethylene melts

This paper presents an overview of the application of the Multi-Pass Rheome ter (MPR) for the study of polymer processing with particular reference to How birefringence measurements. The MPR is a two-piston capillary rheometer recently developed at the University of Cambridge. We review previous expe riments that have been carried out on molten polymers using the MPR and we report on recent results of flow birefringence experiments for a polyethyle ne (PE) melt. Using a specialty designed optic flow cell. molten PE was pas sed within a Double Cavity Die. A field-wise flow-induced birefringence tec hnique was used to obtain the stress field in a complex flow of a flowing m olten polymer. In general., for the PE tested, the entry stress field in th e second cavity was found to be different from that in the first. The diffe rences observed in the overall entry stress fields in the two slit configur ation could be explained as resulting from the extensional and shear strain history experienced by the polymer. We have carried out matching K-B.K.Z.- Wagner numerical simulations and found that reasonable consistency can be a chieved for certain polymers. As a consequence of these experiments. we hav e found that the MPR is a very good platform for experimental flow birefrin gence studies that can be matched with numerical simulation and in addition experiments can be carried out with small 20 g quantities of material. (C) 2001 Elsevier Science Ltd. All rights reserved.