Impact of decoupling approximation between stretch and orientation in rheometrical and complex flow of entangled linear polymers

We study the rheometrical and complex flow response of the coupled version of the double-convection-reptation model with chain stretch. This model for monodisperse entangled linear polymers has recently been proposed by Marrucci and Ianniruberto [G. Marrucci, G. Ianniruberto, Flow-induced orientation and stretching of entangled polymers, Philos. Trans. R. Soc. A 361 (2003) 677-688] to overcome the anomalous shear thickening that was present in an earlier version of the theory. It avoids the decoupling approximation between orientation and stretch. Except for the shear thickening, both coupled and decoupled models show very similar results that are in qualitative agreement with available rheometrical data for two nearly monodisperse polymer solutions. In contraction/expansion flow simulations, however, higher Weissenberg numbers can be attained with the coupled model. Simulations in the stretch-dominated regime predict a dramatic growth of the upstream vortex activity and an increase of the pressure correction. (C) 2004 Elsevier B.V. All rights reserved