Rheo-NMR at the Phase Transition of Liquid Crystalline Poly--benzyl-l-glutamate: Phase Kinetics and a Valuable Tool for the Measurement of Residual Dipolar Couplings

Using a home-built rheo-NMR apparatus with Couette geometry that allows pulse program-controlled switching of spinning speeds, the formation of liquid crystalline phases of poly-gamma-benzyl-l-glutamate (PBLG) in CDCl3 is studied at concentrations up to 15% (w/v) using deuterium NMR of the solvent under static and shear force conditions. A tilt of director alignment from 0 to approximate to 75-79(degrees) is observed. While the well-known first-order phase transition from isotropic to the liquid crystalline phase is detected in the static case, the transition vanishes under applied shear with freely scalable partial alignment down to the lowest concentrations measured. Close to the static phase transition concentration, a coexistence of isotropic and aligned phases is observed when a cosolute is added, which disappears upon application of shear forces. Temperature, concentration, and shear-rate-dependent alignment properties are characterized. In addition, kinetic studies involving the switching of shear forces reveal buildup rates for the phases in the range of milliseconds to minutes, which allowed the acquisition of an unaligned/aligned 2D correlation experiment in a favorable case. Finally, the usefulness of very low, rheo-induced alignment for the measurement of residual dipolar couplings is demonstrated.