DYNAMICS OF POLYMER MELTS IN REVERSING SHEAR FLOWS

Reversing double-step strain deformations provide a severe test for co nstitutive equations. The rheology of reversible deformations is also of considerable importance for polymer processing simulations, specifi cally simulations of extrudate swell. We report double-step shear-stra in experiments on two commercial (polydisperse) polymer melts, a (line ar) PIE and a (long-chain branched) LDPE. Experimental data are compar ed to predictions of (1) a separable single integral constitutive equa tion with a damping function; (2) an irreversible temporary network mo del with a damping functional; and (3) the tube model of Doi and Edwar ds. None of the models is found to give a quantitative description of all of the data. However, irreversible network disentanglement can be explained quantitatively on the basis of the tube model if not only or ientation but also stretch of molecular chains is taken into account. This is done by use of the molecular stress function introduced by Wag ner and Schaeffer, and Doi's calculation for double-step shear-strain experiments is extended to include chain stretching. It is found that linear polymer melts like PIE show a limited amount of chain stretchin g and strong irreversible disentanglement, while enhanced chain stretc hing of long-chain branched LDPE melts prevents irreversible disentang lement at least up to shear strains of ten shear units. (C) 1998 Elsev ier Science B.V. All rights reserved.