Macromolecules, Vol.34, No.10, 3396-3411, 2001
Rouse chains with excluded volume interactions: Linear viscoelasticity
Linear viscoelastic properties for a dilute polymer solution are predicted by modeling the solution as a suspension of noninteracting bead-spring chains. The present model, unlike the Rouse model, can describe the solution's theological behavior even when the solvent quality is good, since excluded volume effects are explicitly taken into account through a narrow Gaussian repulsive potential between pairs of beads in a bead-spring chain. The use of the narrow Gaussian potential, which tends to the more commonly used delta -function repulsive potential in the limit of a width parameter d going to zero, enables the performance of Brownian dynamics simulations. The simulation results, which describe the exact behavior of the model, indicate that for chains of arbitrary but finite length a delta -function potential leads to equilibrium and zero shear rate properties which are identical to the predictions of the Rouse model. On the other hand, a nonzero value of d gives rise to a prediction of swelling at equilibrium and an increase in zero shear rate properties relative to their Rouse model values. The use of a delta -function potential appears to be justified in the limit of infinite chain length. The exact simulation results are compared with those obtained with an approximate solution which is based on the assumption that the nonequilibrium configurational distribution function is Gaussian. The Gaussian approximation is shown to be exact to first order in the strength of excluded volume interaction and is found to be accurate above a threshold value of d, for given values of chain length and strength of excluded volume interaction.