Configuration and hydrodynamic properties of fully acetylated guaran

Abstract

AbstractFully acetylated guaran, (guaran triacetate, GTA), was fractionated and the fractions in the weight‐average degree of polymerization range of 171 to 12,400 were characterized by light scattering and viscometry. At high molecular weights, non‐Newtonian flow was observed and after appropriate correction the intrinsic viscosity was found to be independent of shear gradient. Analysis of viscosity results according to the theory of Kurata and Yamakawa indicated that at high molecular weights the molecule is nondraining and that excluded volume effects are absent. Polymolecularity was estimated from ultracentrifuge and osmotic pressure measurements on certain key fractions and in the case of one fraction it was possible to determine the distribution of molecular weights and to demonstrate the validity of the Zimm‐Schulz distribution. The ratio of the mean square radius of gyration to the molecular weight was found to decrease with increasing molecular weight indicating a non‐Gaussian behavior that cannot be accounted for by the Porod‐Kratky chain model. The dependence of radius of gyration on chain length is identical to that observed for hydroxyethyl cellulose and cellulose tricaproate indicating that the β‐1,4‐linked D‐glucose chain of cellulose and the β‐1,4‐linked chain of GTA have similar configurations, in spite of chemical and structural differences between the anhydro sugar units. The α‐1,6‐linked D‐galactose side groups on every other mannose unit of GTA do not appear to affect main chain configurations to any great extent. Equivalent bond lengths calculated from intrinsic viscosity and from light scattering indicate that it is necessary to distinguish between the hydrodynamic and light‐scattering radius of gyration The Flory coefficient calculated from the light‐scattering radius of gyration was found to increase with increasing molecular weight. When the hydrodynamic radius of gyration was used in the calculation, agreement with the theoretical Flory constant was within experimental error.