RHEOLOGY OF AN LDPE MELT IN REVERSING MULTISTEP SHEAR AND ELONGATIONAL FLOWS

The hypotheses of separability and irreversibility, which have greatly improved the accuracy of constitutive models of the KBKZ type are tested in reversing step shear and elongational deformations. A reversing double-step deformation is a severe test for constitutive models and a new rheometer has been developed to perform this type of deformation, both in shear and elongation, in a single machine. Test results of a commercial low-density polyethylene melt are reported here with model predictions. In particular, the performance of the KBKZ-type integral model and a differential analogue is considered. Wagner's idea of irreversibility and the KBKZ-equivalent ideas of Larson are tested with these data. While these versions show a moderate agreement with experiment, the irreversible integral model gives an accurate fit and emerges clearly as the best description of polymer melts in these flows. © 1990 Steinkopff.