The stress jump of a semirigid macromolecule after shear: Steady‐state results

The stress jump after steady shear for a semidilute, semirigid macromolecule, xanthan gum, in a viscous solvent, was measured and are the first such data reported. The amount of stress instantaneously dissipated, σ^v, depends on the shear rate and xanthan gum concentration. It is less dependent on the concentration than the amount of stress retained on cessation, σ^e, yet, the concentration scaling does not follow that for existing theories possibly due to the polydispersity and finite flexibility of the macromolecule. It was found that the stress jump ratio (i.e., the amount of stress retained divided by the total stress due to the polymer prior to cessation) does not initially deviate from the dilute solution prediction which suggests that existing theories may need modification. The stress jump was calculated for a variety of rheological equations of state, including recent theories for liquid crystal polymers, demonstrating that this phenomenon is not without precedent.