VISCOELASTICITY OF A FLUID OF DYNAMICALLY DISORDERED HARMONIC MACROMOLECULES

We develop a generalization of the Rouse model for the dynamics of a f lexible, linear macromolecule. This dynamically disordered Rouse (DDR) model is based on a Smoluchowski equation for bead coordinates, in wh ich the bead mobilities are stochastic variables which fluctuate betwe en zero and a finite value. The DDR model may be regarded as a general ization of previous extensions of the Rouse model with nonuniform but time-independent bead mobilities to the case in which the mobilities o f the beads are allowed to fluctuate. We focus on the contribution of intrachain relaxation processes to the viscoelastic shear modulus, G(t ), of a macromolecular fluid. In the limit of rapid medium fluctuation s, we recover for G(t) the prediction of the conventional Rouse model. For a slowly relaxing medium, G(t) is characterized by an initial dec ay, followed by a plateau, and a terminal decay regime exhibiting reno rmalized Rouse behavior, in qualitative agreement with the shear modul us of dense polymer fluids at short and intermediate times. The center -of-mass diffusion constant displays a crossover from the Rouse result to behavior controlled by obstacle relaxation as the lifetime of medi um fluctuations is increased. (C) 1995 American Institute of Physics.