A simple constitutive equation for entangled polymers with chain stretch

We propose a simple way of including chain stretch effects in convective co nstraint release theories for entangled polymers. The main idea is that the characteristic time of orientational relaxation depends in a series-parall el way on all three relevant mechanisms, i.e., reptation, constraint releas e (thermal and convective), and Rouse relaxation. As usual, a separate equa tion describes chain stretch, which however is assumed not to be affected b y constraint release. The model is further simplified by writing the orient ational equation in differential form. For step strains, the successful dam ping function of the Doi-Edwards theory is exactly preserved. Predictions i n steady shear also favorably compare with typical data of nearly monodispe rse polymers. (C) 2001 The Society of Rheology.