The Nonlinear Response of a Polydisperse Polymer Solution to Step Strain Deformations

The rheological behavior of a concentrated solution of well-characterized, broad molecular weight distribution polystyrene in step strain flows is examined at large deformations. Mechanical and optical techniques are used to obtain the full stress tensor in terms of the shear stress, first normal stress difference, and second normal stress difference. Stress relaxation data on this novel system in single- and double-step strain flows over a wide range of shear strains are presented. Direct comparisons to data on a similar system having a narrow molecular weight distribution are made illustrating the effects of polydispersity on nonlinear rheological behavior. The new data set is also used to evaluate the reptation model and K-BKZ theory.