Modified reptation model

We consider a modification of the Doi-Edwards [The Theory of Polymer Dynamics (Clarendon, Oxford, 1986)] and Curtiss-Bird [Dynamics of Polymeric Liquids, Vol. 2, Kinetic Theory (Wiley-Interscience, New York, 1987), 2nd. ed.] models for polymer melts in which the effect of constraint release or partially anisotropic Brownian motion is introduced through a stochastic force in the time-evolution equation for the orientation of inner chain segments. A thermodynamically consistent expression for the stress tensor is proposed. In the linear viscoelastic regime, the results for the moduli are very similar to those predicted by double reptation. In the nonlinear viscoelastic regime, the model describes the rheological behavior in an objective manner. The shear-rate dependence of the viscosity obtained from the modified reptation model is more realistic than the Doi-Edwards prediction