On the relative magnitudes of viscous, elastic and texture stresses in liquid crystalline PBG solutions

Abstract

The relative contributions of the viscous, elastic, and texture stresses in flows of modestly concentrated liquid crystalline solutions of poly(γ-benzyl glutamate) or PBG, at low shear rates are assessed by comparing published experimental rheological and light-scattering data to theoretical predictions. The data considered include ratios of Leslie-Ericksen viscosities, the shear viscosity jump at the transition from the isotropic to the liquid-crystalline state, the dependence of the shear viscosity on concentration, and the fraction of slowly relaxing stress after cessation of shearing flow. We quantify the relative contributions of viscous and elastic stresses by estimating from these data a value for the parameter ß^* _V≡ζ _str D ^* _r /kT, where ζ _str , is the solvent-polymer friction coefficient and D ^* _r is the rotary diffusivity in the isotropic phase at the concentration where the isotropic phase becomes unstable to liquid crystalline order. The experimental evidence suggests a value \^* _v ≈ 0.03, five times lower than the value, 0.15, for dilute solutions. The relative contributions of elastic and texture stresses can be quantified by a parameter ε in the mesoscopic theory for polydomain nematics; a value of ε≈0.03 gives reasonable agreement with transient shearing experiments, and predicts that the shear viscosity will show an upturn at high polymer concentrations, in agreement with experiments.