Relation between molecular orientation and rheology in lyotropic hydroxypropylcellulose solutions

Molecular orientation in aqueous lyotropic solutions of hydroxypropylcellulose has been studied in steady and transient flows using the technique of flow birefringence. Birefringence is an increasing function of steady shear rate in the range from 0.01–100 s−1. Upon flow cessation, the orientation is seen to decrease towards a globally isotropic condition. It is hypothesized that this decrease in orientation may reflect a transition from a flow‐induced nematic back to a cholesteric phase. In this case, low orientation and ‘‘region I’’ shear thinning at low rates may be consequences of persistence of cholestericity in slow flows. The decrease in orientation appears to be well correlated with gradual increases in the complex modulus of the solution during relaxation. However, while birefringence reveals that the final state is optically isotropic for all previous shear rates, the long‐time value of the modulus depends strongly on previous shear rate. To further investigate structural differences in the rela...