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

Abstract

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-γ-p-biphenylmethyl-l-glutamate (PBPMLG) and poly-β-(benzyl)-co-(β-phenethyl)-l-aspartate (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 ΔνQ 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 ΔνQ with the diffusion coefficients measured parallelly (D∥) and perpendicularly (D⊥) to the magnetic field, we find a significant change of the D∥/D⊥ 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