Thermal Degradation of Guar Gum

Information about the thermal degradation of guar gum has been obtained from the time dependence of the viscosity at high temperatures measured using a slit viscometer. The viscosity is related to the molecular weight using previously published relations between the zero shear specific viscosity and the coil overlap parameter in conjunction with the appropriate Mark-Houwink equation. Unlike anionic polysaccharides it is found that the kinetics determined by this method depend on solution concentration. This was found to be caused by thixotropic effects at the higher concentrations and degradation in flow. At the lowest concentration studied these two effects were absent and the kinetic equation obtained was K r = 0"51 exp[-56330 (KJ/mol)/RT] s-i, where K r is the rate constant. The viscosity results were supported at least in a semi-quantitative fashion by low speed sedimentation equilibrium measurements in the analytical ultracentrifuge. The relationship between the intrinsic viscosity and the measured molecular weight for the series of native/ degraded polymers could not be represented by a Mark-Houwink equation, suggesting that the molecular weight measurements were complicated by self-association. Gel permeation chromatography confirmed that self-association does occur and the elution profiles revealed a significant quantity of very low molecular weight material in the heat degraded material suggesting that thermal degradation is not a random process.