Dynamics of Nondilute Hairy-Rod Polymer Solutions in Simple Shear Flow

Abstract

We investigate the shear response of nondilute solutions of a hairy-rod polyester using in-situ optical rheometry. The transition to the concentrated regime, c**, is characterized by a strong concentration dependence of the zero shear viscosity, a change in the sign of birefringence, and the emergence 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, consistent with the wormlike character of the polyesters. At the highest concentrations these solutions can be viewed as nondilute solutions of flexible ellipsoidal clusters. The dynamics as well as the anomalous birefringence (change of sign) can be rationalized using the theoretical analysis of Cates, who considered a presmectic local orderi...