Birefringence measurements on polymer melts in an axisymmetric flow cell |
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Authors: | Philip Harrison Luc J P Janssen Vincent P Navez Gerrit W M Peters Frank P T Baaijens |
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Institution: | (1) Section of Materials Technology Department of Mechanical Engineering Eindhoven University of Technology PO Box 513, 5600 MB Eindhoven The Netherlands e-mail: g.w.m.peters@tue.nl, NL;(2) NMC sa, Rovert 10, B-4731 Raeren Belgium, BE |
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Abstract: | The stress-optical rule relates birefringence to stress. Consequently, measurement of flow birefringence provides a non-intrusive
technique of measuring stresses in complex flows. In this investigation we explore the use of an axisymmetric geometry to
create a uniaxial elongational flow in polymer melts. In axisymmetric flows both birefringence and orientation angle change
continuously along the path of the propagating light. The cumulative influence of the material's optical properties along
the light's integrated path makes determination of local birefringence in the melt impossible. One can nevertheless use birefringence
measurements to compare with predictions from computer simulations as a means of evaluating the constitutive equations for
the stress. More specifically, in this investigation we compare the light intensity transmitted through the experimental set-up
vs entry position, with the theoretically calculated transmitted intensity distribution as a means of comparing experiment
and simulation. The main complication in our experiments is the use of a flow cell that necessarily consists of materials
of different refractive indices. This introduces refraction and reflection effects that must be modeled before experimental
results can be correctly interpreted. We describe how these effects are taken into account and test the accuracy of predictions
against experiments. In addition, the high temperatures required to investigate polymer melts mean that a further complication
is introduced by thermal stresses present in the flow cell glass. We describe how these thermal-stresses are also incorporated
in the simulations. Finally, we present some preliminary results and evaluate the success of the overall method.
Received: 2 April 2001 Accepted: 27 August 2001 |
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Keywords: | Viscoelastic Axisymmetric flow Birefringence Polymer melt |
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