Some aspects of the Rheo-NMR behavior of the lyotropic liquid crystal poly(γ-benzyl-L-glutamate) in m-cresol

We report the use of H-2 NMR spectroscopy to study the director orientation in two nematic solutions of poly(gamma-benzyl-L-glutamate) (PBLG) in m-cresol during simple shear flow and after its sudden cessation. In the range of applied shear rates (gamma = 5... 150 s(-1)), a steady-state NMR line shape is reached after a deformation of a few hundred units. Two different line shapes occur that can be related to distinct shear responses in mechanical rheology: the tumbling/wagging and the flow-aligning regimes. The relaxation after cessation of the shear is similar for both cases and essentially follows the "inhomogeneous reorientation'' regime known from sudden-rotation NMR experiments. A fit of the relaxation curves indicates that the tumbling parameter lambda is ( slightly) less than one, as expected for a tumbling system.