Dynamics of nondilute hairy-rod polymer solutions in simple shear flow

We investigate the shear response of nondilute solutions of a hairy-rod pol yester using insitu optical rheometry. The transition to the concentrated r egime, c**, is characterized by a strong concentration dependence of the ze ro shear viscosity, a change in the sign of birefringence, and the emergenc e of linear conservative dichroism. These features indicate the presence of clusters which dominate the dynamics in this concentration regime, and as such they represent their identifying signature. The stress-optical rule is found to hold only in the concentration regime below c**, yielding stress- optical coefficients closer to those of flexible chains rather than rods, c onsistent with the wormlike character of the polyesters. At the highest con centrations these solutions can be viewed as nondilute solutions of flexibl e ellipsoidal clusters. The dynamics as well as the anomalous birefringence (change of sign) can be rationalized using the theoretical analysis of Cat es, who considered a presmectic local ordering resulting from the interplay of sterically interacting particles and the external field.