Orthogonal and parallel superposition measurements on lyotropic liquid crystalline polymers

Mechanical spectroscopy is used to probe the structure of lyotropic liquid crystalline polymers during flow and after the cessation of flow. The oscil latory flow is either parallel or perpendicular to the steady-state flow. T he resulting moduli provide information about the time- and shear-dependent microstructure, including anisotropy. Two different concentrations of poly (benzylglutamate) (PBG) in m-cresol and a concentrated hydroxypropylcellulo se (HPC) solution, also in m-cresol, are investigated. In all cases. the or thogonal superposition moduli evolve differently from the parallel ones. Th e former are less sensitive to the flow-induced changes in structure than t he latter ones. Together with the lack of sensitivity of the superposition moduli to texture refinement during flow, this suggests a strong relation b etween director orientation and superposition moduli. After the cessation o f flow the parallel moduli decrease for the PBG solutions, whereas the oppo site is observed in the HPC solutions. A comparison with the orthogonal mod uli provides a direct measure of anisotropy. At rest, the PBG solutions ten d toward a higher degree of anisotropy while the HPC solutions become more isotropic. In the latter systems, all moduli are much larger, reflecting a larger contribution from the texture.