INVESTIGATION OF XANTHAN GUM SOLUTION BEHAVIOR UNDER SHEAR-FLOW USING RHEOOPTICAL TECHNIQUES

The complementary rheooptical techniques of birefringence and dichroism are used to investigate the dynamics of xanthan gum in dilute/semidilute solution. Because xanthan self-aggregates, there are a range of very different length scales of molecules in solution which the optical methods probe. Birefringence results give information about segmental orientation in flow while scattering dichroism allows interrogation of the much larger aggregate orientation. Two xanthan samples are studied and the results are compared with previous studies. Birefringence and dichroism measurements show very different orientational and relaxational responses in shear flow, indicative of the differences between the structures examined by the two techniques. Dichroism steady-state orientation angle and birefringence/dichroism relaxation results also indicate a critical concentration at approximately 2000 ppm which coincides with that found in the literature by several other investigators and explained by some to be a critical concentration of aggregation. Dichroism measurements detect the presence of aggregates though at concentrations as low as 50 ppm, far below this critical concentration. Overshoots in dichroism, birefringence, and stress signals at approximately 2 strain units are also discussed in light of this aggregation phenomenon. The addition of salt was found to greatly enhance the dichroism signal, providing further evidence that the dichroism signal monitors aggregate dynamics.