Molecular drag-strain coupling in branched polymer melts

The "pom-pom" model of McLeish and Larson [J. Rheol. 42, 81-110 (1998)] pro vides a simple molecular theory for the nonlinear rheology of long chain br anched polymer melts. A feature of this model is a maximum stretch for the branched molecules. Sharp transitions were predicted in the extensional vis cosity at this maximum stretch. We introduce a simple treatment of the coup ling between relaxed and unrelaxed polymer segments at branch points. The b ranch point is allowed to move in a quadratic localizing potential of unkno wn strength. Taking account of this effect smoothes the sharp transitions o f the model and accounts for the extensional viscosity of "pom-pom" model p olymers at their maximum stretch. The result is an improved multimode pom-p om fit for low-density polyethylene rheology. By fitting the nonlinear exte nsional viscosity, quantitative predictions are made for the nonlinear stea dy shear viscosity and transient first normal stress difference in shear. ( C) 2000 The Society of Rheology. [S0148-6055(00)00701-X].