Dynamics of Branched Polymer Melts in Complex Kinematics Flows: A Computational/Experimental Study.

In this study large scale numerical simulations are coupled with flow visualization and flow induced birefringence measurements to evaluate the performances of the multi-mode Giesekus model in predicting the flow behavior of a rheologically well characterized branched polymer melt in a prototypical processing geometry. Specifically, we have performed extensive finite element flow simulations of a low density polyethylene (LDPE) melt in a cross-slot channel. The simulation results have in turn been compared with experimental measurements of the kinematics and flow induced birefringence in a novel cross-slot channel rheometer, whose front and back viewing windows are lubricated to eliminate end effects as well as undesirable temperature gradients. A detailed comparison of computational and experimental results has revealed that the multimode Giesekus model can accurately capture the flow characteristics at low to moderate Weissenberg numbers (Wi). However, simulation results at high Wi are at best qualitative. The failure of the simulations in quantitatively predicting the flow behavior close to the stagnation point at high Wi is partly attributed to the inaccuracy of the experimental data resulting from multiple orders of retardation occurring within the measurement volume as well as lack of precise control over the lubrication film dynamics. Based on these observations various avenues for improving the lubricated cross-slot channel rheometer are outlined. [ABSTRACT FROM AUTHOR]