RELATION BETWEEN MOLECULAR-ORIENTATION AND RHEOLOGY IN LYOTROPIC HYDROXYPROPYLCELLULOSE SOLUTIONS

Molecular orientation in aqueous lyotropic solutions of hydroxypropylc ellulose has been studied in steady and transient flows using the tech nique of flow birefringence. Birefringence is an increasing function o f steady shear rate in the range from 0.01-100 s-1. Upon flow cessatio n, the orientation is seen to decrease towards a globally isotropic co ndition. It is hypothesized that this decrease in orientation may refl ect a transition from a flow-induced nematic back to a cholesteric pha se. In this case, low orientation and ''region I'' shear thinning at l ow 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 relaxa tion. However, while birefringence reveals that the final state is opt ically isotropic for all previous shear rates, the long-time value of the modulus depends strongly on previous shear rate. To further invest igate structural differences in the relaxed state, shear stress and bi refringence were measured upon flow resumption. If the relaxed state r esulted from a high previous shear rate, a very large overshoot is obs erved in stress, while if the previous shear rate is low, a much weake r oscillatory pattern is observed. Birefringence results are quantitat ively similar. Finally, birefringence and stress were studied upon flo w reversal. Both exhibit damped oscillatory responses, but the qualita tive shape of the profiles change as a function of shear rate within t he linear regime. These results are compared and contrasted with the b ehavior of liquid-crystalline solutions of poly(benzyl glutamate).