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We numerically and analytically analyze the startup continuous shear rheologyof heavily entangled rigid rod polymer fluids based on our self-consistent,force-level theory of anharmonic tube confinement. The approach is simplifiedby neglecting stress-assisted transverse barrier hopping, and irreversiblerelaxation proceeds solely via deformation-perturbed reptation. This process isself-consistently coupled to tube dilation and macroscopic rheologicalresponse. We predict that with increasing strain the tube strongly dilates,entanglements are lost, and reptation speeds up. As a consequence, a stressovershoot emerges not due to affine over-orientation, but rather to strongweakening of the entanglement network. Just beyond the stress overshoot thelongest relaxation time is predicted to scale as the inverse shear rate,corresponding to the emergence of a generic form of convective constraintrelease (CCR). Its origin is a stress-induced local force thatself-consistently couples the mesoscopic tube-scale physics with macroscopicmechanical response. Tube weakening at the overshoot occurs via the samequalitative mechanism that leads to microscopic absolute yielding in thenonlinear elastic scenario analyzed in the preceding paper. The stressovershoot and emergent CCR thus correspond to an elastic-viscous crossover dueto deformation-induced disentanglement, which at long times and high shearrates results in a quantitatively sensible flow stress plateau and shearthinning behavior. We suggest that the predicted behavior for needles isrelevant to flexible chain melts at low Rouse Weissenberg numbers if contourlength equilibration is fast. Quantitative predictions for tube dilation in thesteady state flow of chains melts are favorably compared to a recentsimulation.

A Force-Level Theory of the Rheology of Entangled Rod and Chain Polymer  Liquids. II. Perturbed Reptation, Stress Overshoot, Emergent Convective  Constraint Release and Steady State Flow