Rheological properties of concentrated solutions of galactomannans in an ionic liquid

Abstract

The rheological behavior of galactomannans in concentrated solutions was examined by using dynamic viscoelasticity measurements. Concentrated solutions of three galactomannans, guar gum, tara gum, and locust bean gum were prepared with an ionic liquid 1-butyl-3-methylimidazolium chloride as the solvent. Each galactomannan solution showed angular frequency dependence curves of the storage modulus and the loss modulus which were characteristic of a solution of entangled polymer chains. The molecular weight between entanglements (Me) was obtained from the plateau modulus and the concentration dependence of Me showed Me in the molten state (Me,melt) to be 4.6×103, 3.2×103, and 2.7×103 for guar gum, tara gum, and locust bean gum, respectively. It was found that the material constant Me,melt depends on the mannose/galactose ratio of the galactomannans. The number of monosaccharide units between entanglements in the molten state for the galactomannans varied within the range found for other polysaccharides such as cellulose and agarose in ionic liquids, suggesting that all the galactomannans take a random-coil conformation in ionic liquid solutions.