The rheology of dilute solutions of flexible polymers: Progress and problems

Recent progress toward understanding the rheology of dilute solutions of flexible polymers is reviewed, emphasizing experimental results from flows imaging single deoxyribonucleic acid (DNA) molecules and filament-stretching rheometry of dilute polystyrene Boger fluids, as well as Brownian dynamics (BD) simulations of these flows. The bead-spring and bead-rod models are presented, the range of their applicability discussed, and methods presented for inclusion of hydrodynamics interactions, excluded volume, and other physical effects within BD simulations. After reviewing and updating work in the linear viscoelastic regime, the primary focus shifts to the more complex nonlinear regime. While BD predictions of the conformations of 20 to 100 micron long DNA molecules in strong shear and extensional flows has been in good to excellent agreement with the corresponding experiments, predictions of the polystyrene dilute solution rheometry data have been hit or miss, with poorer results obtained for the higher molecular weights. This may be due, in part, to the more important roles of hydrodynamic interactions and excluded volume interactions in the more flexible, and therefore more condensed, polystyrene coils. Inclusion of these effects in BD simulations has led to improved predictions, but does not lead to the accurate prediction of the plateau Trouton viscosity for higher molecular weight samples, nor alleviate the complete failure of simulations to predict measurements of coil distortion by light scattering. Thus, despite enormous progress in the past decade, some significant gaps in understanding remain. (C) 2005 The Society of Rheology.