Multicritical phenomena in flow of viscoelastic liquids. 3. Liquid-crystalline polymers. The Akay-Leslie anisotropic liquid. Viscometric functions

The advances in determination of the rheological material functions of liquid-crystalline polymers (LCP) in a wide range of concentrations or temperatures are briefly examined. Special attention is focused on a discussion of the concentration region of the existence of liquid-crystalline order and textures initiated by shear flow. The phenomenological rheological equation for an anisotropic liquid proposed by Akay and Leslie (A-L model) was selected for describing the flow resistance of LCP, and the problem of simple shear flow of a liquid was solved. It was shown that the A-L model can describe all of the basic characteristics of the viscometric functions of LCP, including the phenomenon of negative values of the first normal stress difference for some range of concentrations and shear rates. The available data on the initial viscosity of lyotropic LCP were classified based on the types of cohesion (dimensionality) of the system, characterized by the exponent of the concentration dependence of the viscosity. It was found that each type of cohesion has its own kind of viscometric functions and that the A-L rheological model can reflect their basic characteristics. It was shown that by varying the initial ordering parameter of the A-L model, linear (or almost linear) functions N₁(q) and ₁₂(q) can be described in the region of low shear rates. It was hypothesized that region 1 of the flow curve (Onogi and Asada) inevitably exists in the continuous liquid crystalline phase, which could be outside of the region of measurements accessible to rheometry.See 23] for Communication 2.Institute of Polymer Mechanics. Riga. LV-1047 Latvia. Translated from Mekhanika Kompozitnykh Materialov. No. 6. pp. 821–839. November–December, 1997.