Constraint release effects in monodisperse and bidisperse polystyrenes in fast transient shearing flows

Predictions of the Mead-Larson -Doi (MLD) and the Doi-Edwards-Marrucci-Grizzuti (DEMG) models(1) are compared with the rheological data in start up of steady shearing of concentrated bidisperse polystyrene solutions. Both the MLD and DEMG models are tube theories of flow with reptation, but the MLD theory includes both reptative and convective constraint-release effects, which are neglected in the DEMG theory. The adjustable parameters for the models are the reptation times (tau (d,i)) of each molecular weight component, and the plateau modulus (G(N)(0)). A procedure is given for obtaining the stretch times (tau (s,i)) from the reptation times. The convective constraint release included in the MLD model remedies the extreme shear thinning predicted by the DEMG model, and the reptative constraint release extends the good predictions found for monodisperse polymers to bidisperse systems, especially at steady state. In the transient start up of shear for monodisperse polymers, both models provide good predictions in both the viscosities and the first normal stress differences at low shear rates. At high shear rates, the MLD model, as well as the DEMG model, can predict the strain at the peak in stress accurately; however, the magnitude of the overshoots predicted by the MLD model are not as high as those of the DEMG model, with the latter predicting overshoots that are closer to the observed results.