Dynamic simulation of freely-draining, flexible bead-rod chains: Start-up of extensional and shear flow

We present a study of the rheology and optical properties during the start-up of uniaxial extensional and shear flow for freely-draining, Kramers bead-rod chains using Brownian dynamics simulations. The viscous and elastic contributions to the polymer stress are unambiguously determined via methods developed in our previous publication [1]. The elastic contribution to the polymer stress is much larger than the viscous contribution beyond a time of 5.3 lambda/N-2 where N is the number of beads in the chain and lambda(1) is the longest relaxation time of the chain. For small Wi (at arbitrary strains) and for small strains (at arbitrary Wi) the stress-optic law is found to be valid. The stress-optic coefficients based on the shear stress and first normal stress difference are equal for all Wi (even when the stress-optic law is not valid) suggesting the stress-optic coefficient is in general a scalar quantity rather than its most general form as a fourth order tensor. We show that a multimode FENE-PM or Rouse model describes the rheology of the bead-rod chains at small strains, while the FENE dumbbell is an accurate model at larger strains. We compare the FENE-PM and FENE model to experimental extensional stress data of dilute polystyrene solutions and find that a multimode FENE-PM with a Zimm relaxation spectrum describes the data well at small strains while a FENE dumbbell with a conformation dependent drag is in quantitative agreement at larger strains. (C) 1998 Elsevier Science B.V. All rights reserved.