Abstract
We report experimental measurements fo the centreline stress build up and relaxation as molten polyethelene flows into a slit die. The time-dependent extensional stress distribution is obtained using flow birefringence techniques and these observations complement the corresponding velocity field measurements already reported. Experimental measurements of the linear viscoelastic storage and loss modulus are obtained and, from these results, the polymers are characterized in terms of a modulus spectrum. Using this modulus spectrum together with a Maxwell-type constitutive equation and the experimental centreline kinematics, we find that it is possible to simulate successfully the experimentally observed stress distributions. Our results indicate that it is essential to include the polymers' broad spectrum of relaxation times when considering time dependent flow problems.
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