Thermodynamically admissible reptation models with anisotropic tube cross sections and convective constraint release

With the restrictions and guidance provided by nonequilibrium thermodynamic s in mind, we develop a thermodynamically admissible single-segment reptati on model that unifies all the effects identified as relevant to the flow be havior of polymer melts on the reptation time scale. Starting from a formul ation of a reptation model without independent alignment, we incorporate Ma rrucci's recent ideas of convective constraint release and anisotropic tube cross sections, and in particular, we provide consistent time-evolution eq uations for the anisotropic tube cross section in flow. Important features such as irreversibility in double-step shear strain experiments with flow r eversal and a non-decaying shear stress at high shear rates are appropriate ly seized by the proposed model. The ratio of normal-stress differences is determined in terms of the change of the mean-square curvature of the tube cross section in shear flow. A model simplification ideal for efficient com puter simulations and for matching the experimentally observed linear visco elastic behavior is proposed. (C) 2000 Elsevier Science B.V. All rights res erved.