FLOW-INDUCED ANISOTROPY AND ITS DECAY IN POLYMERIC LIQUID-CRYSTALS

Flow induces anisotropy in polymeric liquid crystals. This can be demonstrated by comparing the stress transients during a sudden increase in shear rate with those during flow reversal: the damped oscillations of the shear stress resulting from these two experiments are shifted by nearly 180-degrees. Flow-induced anisotropy decays after the flow stops. Its evolution in time is followed by stress growth experiments in the flow direction and in the opposite one, with systematic changes in the rest period. The phase shift only disappears after several thousands of seconds. This time is much longer than the time necessary for the shear stress to relax. Other rheological characteristics, such as the variation of the dynamic moduli after stopping the flow, occur on the same time scale as the anisotropy decay. The anisotropy decay is not affected by temperature and consequently viscosity can be ruled out as a governing factor. The results agree in part with recent polydomain models.