Birefringence measurements on polymer melts in an axisymmetric flow cell

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. Because polarimetry measurements provide the cumulative influence of the material's optical properties along the light's integrated path, this makes determination of local birefringence in the melt impossible. One can nevertheless use experimentally determined birefringence measurements to compare with predictions from computer simulations as a means of evaluating the ma! terials' governing constitutive stress equations. More specifically, in this investigation we compare the light intensity transmitted through the experimental set-up versus 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 flowcell that necessarily consists of materials of different refractive indices. This introduces refraction and reflection effects that must be modelled 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 flowcell glass. We describe how these thermal-stresses are also incorporated ! nin the simulations. Finally, we present a few preliminary results and evaluate the success of the overall method