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 s tart-up of uniaxial extensional and shear flow for freely-draining, Kr amers bead-rod chains using Brownian dynamics simulations. The viscous and elastic contributions to the polymer stress are unambiguously det ermined via methods developed in our previous publication [1]. The ela stic contribution to the polymer stress is much larger than the viscou s contribution beyond a time of 5.3 lambda/N-2 where N is the number o f beads in the chain and lambda(1) is the longest relaxation time of t he chain. For small Wi (at arbitrary strains) and for small strains (a t arbitrary Wi) the stress-optic law is found to be valid. The stress- optic coefficients based on the shear stress and first normal stress d ifference 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. W e 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 mod el to experimental extensional stress data of dilute polystyrene solut ions 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 st rains. (C) 1998 Elsevier Science B.V. All rights reserved.