Rheology in Region

Finally, we come to one of the most interesting, and perplexing, phenomena in LCPs. This is the existence in some LCPs of Region I shear thinning at low shear rates (see Fig. 11-6 and the discussion in Section 11.3.2). Region I cannot be interpreted as classical yield (as described in Section 1.5.3), since the scaling of viscosity with shear rate y is a power law rj xy , with 1 typically n 0.5. 

In addition, measurements of orientation after cessation of shearing show that flow-induced orientation decreases after cessation of shearing toward an isotropic state of orientation, and the dynamic moduli increase with time (Hongladarom et al. 1994). This behavior is seen in PBLG solutions only under conditions where a pronounced Region I is seen for lower concentrations where only Region II is present, the orientation increases after cessation of shearing (Walker et al. 1995). 

These results suggest that for high-viscosity nematics, defects can be packed together so tightly under even modest shearing that a defect-saturated tight texture is achieved, from 

Under steady shearing, these trapped disclinations should play the role of an anchoring condition, much like the role solid walls play in the flow properties of small-molecule nematics. A scaling analysis of this problem in Section 10.2.5 gives an equation, (10-28), for the steady-state shear viscosity for flow between surfaces with strong, homeotropic anchoring  

Asada T, Tanaka T, Onogi S (1985). J Appl Polym Sci Appl Polym Symp 41 229. 

---

Center of Excellence in Extrusion and Pol3uner Rheology, Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, W3mdmoor, Pennsylvania, USA Conesponding author Charles I. Onwulata, E-mail address Charles.Onwulata ars.usda.gov... 

Center of Excellence in Extrusion and Polymer Rheology, Eastern Regional Research Center, Agricultural Research Service, U.S. 

Walker, L. Wagner, N. Rheology of region I flow in a lyotropic liquid-crystal polymer the effects of defect texture. J. Rheol. 1994, 38 (5), 1525-1547. 

The thixotropic rheological behavior of structured disperse system to a significant extent depends on the direction towards which the equilibrium between rupture and re-establishment of interparticle contacts is shifted. Because of the finite rate of contact formation due to Brownian motion, a certain amount of time is needed for the equilibrium to establish. Consequently, some time is required for a spontaneous thixotropic restoration of structure destroyed by mechanical action. Due to a complete structure disintegration taking place in region IV (Fig. IX-24), the strength, i.e. the... 

What are the mechanics of a textured liquid crystal How does the steady state size distribution of domains (or density of defects) change with shear in Region I, and what is the effect on transient and steady state rheology ... 

---