Role of Director Tumbling in the Rheology of Polymer Liquid Crystal Solutions

The role of director tumbling in the dynamic response of polymer liquid crystals is investigated. A flow experiment on an oriented monodomain confirms director tumbling in liquid crystalline poly(benzyl glutamate) solutions. Mechanical and optical rheometric techniques are used to investigate the behavior of textured liquid crystal solutions in slow shear flows. Director tumbling in this system is shown to play a major role in determining the rheological behavior, particularly in that it results in large distortions in the director profile, leading to significant distortional elastic effects. The rheological data are interpreted in terms of general scaling arguments based on the assumption that the director field responds over a length scale associated with the texture rather than the macroscopic flow dimension. Comparison of our results with model calculations using the Leslie-Ericksen continuum model further suggests that the texture length scale is refined in response to increased shear rates, as a mechanism for limiting distortional free energy in the shear flow, rheooptical evidence for this texture refinement is described. A broad range of experimental data may be explained by these arguments, including a widely observed relaxation scaling law.