Tilting or Twisting? Using Anisotropic Solvent Diffusion in Polymeric Thermo-Responsive LLC Phases for the Distinction of Spatial Reorientation versus Inversion of the Helical Polymer Backbone

We have recently investigated lyotropic liquid crystalline (LLC) phases from the class of helically chiral polypeptides, which exhibit temperature-dependent changes in their LLC behavior. Both poly-gamma-p-biphenylmethyl-L-glutamate (PBPMLG) and p oly-beta-(b enzyl )-co-(beta-phenethyl)-L-asp art ate (PBLA-co-PPLA) exhibit a transition at which the polymer rods rearrange from a perpendicular orientation to a parallel orientation with respect to the magnetic field. A characteristic jump of the quadrupolar splittings Delta nu Q of the solvent's 2H signal is observed in the deuterium spectra. In this work, we present NMR diffusometry in three spatial dimensions as a promising method to obtain information about the spatial arrangement of polymers in LLC phases by making use of the anisotropy of solvent translational diffusion. By correlating Delta nu Q with the diffusion coefficients measured parallelly (D parallel to) and perpendicularly (D perpendicular to) to the magnetic field, we find a significant change of the D parallel to/D perpendicular to ratio at the transition temperature of the LLC phases. This confirms previous results from experiments on deuterated polymers, which showed that a flipping of the alignment director with respect to the magnetic field is the process responsible for the observed transition.

Automated summary

We studied the temperature-dependent LLC behavior of helically chiral polypeptides, specifically PBPMLG and PBLA-co-PPLA. We observed a characteristic jump in the quadrupolar splittings Delta nu Q of the solvent's 2H signal during the transition of polymer rods from perpendicular to parallel orientation with respect to the magnetic field. By correlating Delta nu Q with the diffusion coefficients measured parallelly (D parallel to) and perpendicularly (D perpendicular to) to the magnetic field, we found a significant change in the D parallel to/D perpendicular to ratio at the transition temperature of the LLC phases.