A RHEOOPTICAL STUDY OF FLOW-INDUCED PRETRANSITIONAL ORDERING IN SOLUTIONS OF LYOTROPIC SEMIRIGID MACROMOLECULES

We report on a rheo-optical study of flow-induced structuring in the l yotropic system, hydroxypropylcellulose (HPC) in acetic acid, using mo dulated polarimetry. Anomalies in the shear rate dependence of the ste ady-state and transient linear dichroism suggest the appearance of rot ary optical activity in the solutions at concentrations well below the quiescent, liquid-crystalline transition point. With the exception of the lowest concentration, these anomalies appear at a critical shear rate for each solution. Below this point, the system dynamics are larg ely those one would expect of an isotropic solution of semirigid rods. The linear birefringence shows a well-behaved, monotonic pattern over the full range of shear rates and concentrations suggesting the overa ll orientation of the molecular chains is unaffected by the structurin g process. In situ stress measurements show the viscosity is also unaf fected, exhibiting in most cases simple power-law behavior with shear rate. A modification of the modulated polarimetry experiment allows me asurement of the circular as well as linear optical properties. Result s show the development of circular birefringence is occurring under th e conditions of our experiments and is most likely due to a pretransit ional chiral ordering.