Dynamic behavior of polymer liquid crystal solutions in transient shear flows

Liquid crystalline solutions of rodlike polymers exhibit a wide range of unusual rheological behavior. This complexity reflects the multiple levels of structure in these materials that respond to applied flow fields. In this work we focus on phenomena that occur at low shear rates. Of particular concern are tumbling of the liquid crystal director in shear flow, and the role of elasticity associated with director field distortions in determining the rheological response.;Rheological studies have been performed on liquid crystalline solutions of poly(benzyl glutamate) (PBG) in m-cresol. Director tumbling is confirmed in this system using a simple flow experiment on a uniformly oriented monodomain. Interpretation of experimental results for textured solutions is complicated by the high density of defects in the director field. We hypothesize that the relevant length scale controlling the director field response is determined by the texture. Comparisons between flow calculations and experimental observations using this assumption further suggest the texture length scale is refined in response to increasing shear rates as a mechanism for limiting the distortional free energy in shear flow. Rheo-optical evidence for such texture refinement is discussed. These considerations lead to several predictions that agree favorably with experimental observations, including a widely observed relaxation scaling law.;Transient shear flow calculations are performed using the Leslie-Ericksen constitutive model. For tumbling nematics, hydrodynamic torques that rotate the director are balanced by elastic torques arising from the resulting director field distortion. The transient response exhibits oscillations reflecting variation in viscous properties as the director rotates, in qualitative agreement with experimental observations. The distortional elastic free energy stored at steady state supports relaxation phenomena, such as constrained recoil upon removal of a shear stress. Strain recovery may be appreciable, again in agreement with experiments. Director field relaxation is addressed through calculations of oscillatory shear flow that highlight the characteristic relaxation time for the director field under the influence of distortional elasticity. The macroscopic response exhibits viscoelastic character despite the neglect of molecular viscoelasticity in the calculations.