Convective orientational renewal in entangled polymers

The predictions of tube models in fast flows have recently been improved by incorporating convective constraint release (CCR). In this paper, we revis it CCR showing that, whenever chains are not stretched, an additional effec t named convective conformation renewal (CCR2 to distinguish it from CCR) s hould also be considered. CCR2 consists in the loss of orientation of the u nstretched chain due to flow-induced lengthening of tube segments. A simple quantitative model including both CCR and CCR2 is then developed, which re duces to double reptation in the linear limit. Although the model ignores f luctuations, the predicted flow curve is monotonic, and compares favourably with data for nearly monodisperse entangled polymers. For the case of a si ngle relaxation time, typical of polymer-like micelles, a maximum in the fl ow curve still remains. (C) 2000 Elsevier Science B.V. All rights reserved.