Relaxation of dilute polymer solutions following extensional flow

The relaxation of dilute polymer solutions following stretch in uniaxial extensional flow is investigated via Brownian dynamic simulations of a flexible freely-draining bead-rod chain. The bead-rod chain simulations are compared to Brownian dynamic simulations of a FENE dumbbell and numerical calculations of a FENE-PM chain. A universal relaxation curve for the stress decay from steady-state is found by shifting the results to lie on the curve described by the relaxation of an initially straight chain. For all the models investigated, the initial rapid decay of the polymer stress decreases at a rate which scales for large Weissenberg number, Wi as Wi(2). Our universal curve is in good qualitative and in some cases quantitative agreement with the available experimental data: it is particularly good in predicting decay after stretch at the largest strains. We find hysteresis in comparing the stress versus birefringence during the startup of flow and subsequent relaxation for the bead-rod chain and FENE dumbbell, but not for the FENE-PM chain. The hysteresis in the latter model is lost in the preaveraging of the nonlinear terms. The bead-rod model also displays a configuration hysteresis. The hysteresis observed in these models is in qualitative agreement with recent experiments involving polystyrene-based Boger fluids. (C) 1998 Elsevier Science B.V. All rights reserved.