Universal Relaxation Behavior of Entangled Star Poly(hydroxybutyrate) Melts in the Transient Shear Startup

Taghipour, Hamid [UCL] Ebrahimi, Tannaz [UBC] Mehrkhodavandi, Parisa [UBC] G. Hatzikiriakos, Savvas [UBC] Van Ruymbeke, Evelyne [UCL]

Nonlinear viscoelastic stress response for Poly(hydroxybutyrate) star polymers are compared to typical constitutive equations, such as the Graham, Likhtman, McLeish and Milner (GLaMM)1 and the Mead, Larson, Doi (MLD)2 models. This requires modifying these models, which were described for entangled solutions and melts of linear chains, to systems which cannot relax by reptation. At significantly low shear rates (smaller than the inverse reptation time) a reasonable agreement is found between the data of the startup and the linear viscoelastic envelope measured in a cone partitioned plate geometry. However, at moderately high Weissenberg number, the shear stress growth coefficient doesn’t increase monotonically but pass through a maximum indicates a full alignment of the arms along the shear direction. At significantly high shear rates (larger than the inverse Rouse time) the arms undergo stretch and fast retraction process following by local convection of the total constraints around the arm’s extremities which eliminate the deep fluctuation effects on the relaxation process. We compare the predictions of the two mentioned tube models and comment on their performance; we also highlight similarities and/or differences, which we explain in terms of the underlying molecular physics and the way the latter have been implemented in the models.