Lyotropic chromonic liquid crystals: neutron scattering studies of shear-induced orientation and reorientation

Neutron scattering has been carried out on two dye lyotropic liquid crystalline phases (Scheme 1), one an azo dye (A) which aggregates in solution to form rods and a hexagonal phase and the second a cyanine dye (B) which aggregates in sheets forming a layer phase. The dyes were placed in a couette shear cell so that the influence of shear on the aggregate orientation could be determined. Measurements were carried out with the beam passing through the cell centre and the cell side, so that orientations along and at right angles to the how direction were monitored. Both dyes show some unusual reorientation behaviour. The aggregates of the hexagonal phase are highly ordered along the direction of flow at low shear rate (2.78 s(-1)), with the degree of order being reduced at the highest shear rate (1002 s(-1)). Quite remarkably, on cessation of shear, the rods appear to be at right angles to the orientation direction observed for low shear. For the dye B mesophase, the layers are aligned preferentially parallel to the cell walls at low and high shear, although in the latter case the separation between the layers appears to be reduced by ca. 10%. On cessation of shear again we observe that the orientation of the aggregates is very different, with much of the material having layers perpendicular to the cell walls. This is the first report of chromonic mesophase reorientation under shear, and the first time that a shear-induced change in the separation between aggregates has been observed. (C) 1997 Elsevier Science B.V.