SHEAR-FLOW OF NEMATIC POLYMERS .2. STATIONARY REGIMES AND START-UP DYNAMICS

The previously introduced macroscopic model of Farhoudi and Rey for uniaxial, rodlike, spatially homogeneous and monodomain nematics was used to describe the transient microstructural and rheological responses of the material during start-up of simple shear flow and to determine the nature of the final steady state. A full characterization of the nature of the transients and of the final steady states was obtained by the complementary approaches of dynamic simulation and eigenvalue computation. The magnitudes of the shear rate and of the nematic potential, and the functional dependence of the tumbling function on the scalar order parameter were found to determine the existence and the nature of the steady state. It was found that even the present simplified form of the model has the ability to predict well-known rheological features of liquid crystalline polymers such as overshoots, undershoots, and damped oscillations observed during the start-up of shear flow. The model is shown to be system identifiable. Three identifiable simplified models of practical utility for several operating and material conditions are presented.