The PMFB apparatus which will allow time-dependent deformation and orientation of macromolecules in solutions to be deduced in a single experiment, was in our laboratory. As a sample, xanthan gum was used in this study.
Steady state results are as follows. The birefringence exhibits a sharp increase at low velocity gradients and level off at higher shear rates. The measured orientation angle, on the other hand, drops quickly and then level off over the majority of the velocity gradient range. It was observed that the absolute value of the flow-induced birefringence is higher for the salted solution at low velocity gradients (SI). This may be an indication that the chains are more rigid in the presence of NaCl.
Transient behaviors are as follows. At lower value of shear rate ($\gamma{'}=3 sec^{-1}$) Δn increases monotonically with time while at higher shear rates ($\gamma{'}=15sec^{-1}$ and up) an overshoot in Δn is detected. The overshoot increases with shear rate. When shear rate is $100sec^{-1}$, the %overshoot have a value of about 150. Such large overshoots can be explained by the fact that xanthan chains are not completely rigid.
Using Haake viscometer, I measured viscosity and shear stress of xanthan gum solution of 400 ppm concentration. From these results, a stress-optical costant C was calculated. Its value was about $2.00\times 10^{-7}Pa^{-1}$.
