Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions

Abstract

The concentration (c) and shear rate (γ) dependence of viscosity (η) has been studied for a wide range of random coil polysaccharide solutions, and the following striking generalities are observed: 1. 1. The transition from dilute to concentrated solution behaviour occurs at a critical concentration c ∗ ≈ 4 [η] , when ‘zero shear’ specific viscosity ( η sp) ≈ 10. η sp varies as c 1.4 for dilute solutions, and as c 3.3 for concentrated solutions. 2. 2. The shear rate dependence of viscosity, and frequency dependence of dynamic (oscillatory) viscosity are closely superimposable. 3. 3. Double logarithmic plots of η η 0 against γ ̇ γ ̇ 0.1 (where η 0 is ‘zero shear’ viscosity, and γ ̇ 0.1 is the shear rate at which η = η 0 10 ) are essentially identical for all concentrated solutions studied, and thus the two parameters η 0 and γ ̇ 0.1 completely define the viscosity at all shear rates of practical importance. Departures from points 1 and 2, but not 3, are observed for concentrated solutions of locust bean gum, guar gum, and hyaluronate at low pH and high ionic strength and are attributed to specific intermolecular associations (‘hyperentanglements’) of longer timescale than non-specific physical entanglements.