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

Abstract

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-γ-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 ≈75–79° 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.

Elucidations of Structure and Molecular Dynamics of Complex Polymers by State-of-the-Art Solid-State NMR Spectroscopy

“…Sugase et al built mechanical shear unit into conventional cryogenic probe and investigated the aggregation process of proteins, amyotrophic lateral sclerosis-related superoxide dismutase 1 (SOD1). − High-resolution Rheo-NMR can in situ capture the transient events of fibrillation at the molecular level, revealing the coexistence of folded and unfolded SOD1 . Luy et al recently revealed the influence of temperature, concentration, and shear rate on the phase transition behaviors of liquid crystalline poly-γ-benzyl- l -glutamate (PBLG) under shear by 2 H Rheo-NMR . All the above results clearly show the power of Rheo-NMR for studying complicated phase transition under flow conditions and tensile deformation mechanism of various materials at the molecular level.…”