Summary

We report experimental measurements of the centrehne stress build up and relaxation as molten polyethelene flows mto a slit die. The time-depen-dent extensional stress distribution is obtamed using flow birefringence techmques and these observations complement the corresponding velocity field measurements already reported. Experimental measurements of the linear viscoelasttc storage and loss modulus are obtamed and, from these results, the polymers are characterized m terms of a modulus spectrum Usmg thrs modulus spectrum together with a Maxwell-type constituttve equation and the experimental centrelme kmemattcs, we fmd that it is possible to simulate successfully the expertmentally observed stress dutribu-ttons. Our results indicate that it is essential to include the polymers ’ broad spectrum of relaxation times when considermg time dependent flow prob-lems. 1. Inlmduction In the companion paper [l] we have measured the centreline velocity profile of molten polyethylene flowing into a two-dimensional slit. The obJective of thrs paper IS to describe the associated stress field as determmed from flow birefringence measurements and relate this stress field to the l Present address * B