Dynamics of hairy-rod polymer solutions in simple shear flow: Aging effects

We investigated the dynamics of solutions of a model hairy-rod polyester us ing optical rheometry and find that; in the concentrated regime significant time effects dominate its response in simple shear flow: They are characte rized by an enhanced viscosity and shear thinning behavior, accompanied by a corresponding increase of the absolute value of the induced birefringence and the relevant relaxation time upon cessation of shear. However, the mos t dramatic effect is the change of sign and large increase of the absolute value of the birefringence, signifying the dominance of form effects, assoc iated with the formation of large anisotropic aggregates at longer times, w hich are oriented in the vorticity direction. This observation of anomalous birefringence is in qualitative agreement with the theoretical approach of Cates (J. Phys. II (Fr) 1992, 2, 1109) who considered a presmectic local ( anisotropic) ordering resulting from the interplay of particle interactions and the external field.