NMR Field-Cycling Study of Rotational Flow and Viscosity in Nematic Liquid Crystals

Abstract

Abstract A proton NMR method is described which enables rotational flow and viscosity measurements of low molecular weight ṉematic ḻiquid c̱rystals (NLC's). This is achieved by an extension of the common NMR field-cycling technique, namely by fast electronic switching of both the external magnetic field direction and strength. In thermodynamic equilibrium, the director of an NLC with positive diamagnetic anisotropy (Δχ > 0) orients parallel to the external magnetic field. Thus a change of this direction causes a reorientation process of the molecules to align to the new equilib­rium, which in low viscous systems in contrast to polymer liquid crystals is rather fast and hence requires fast field switching. We have studied systematically this response as a function of the initial field rotation angle 90 relative to the director for some homologous NLC's of the n-alkyl-cyano-biphenyl (nCB) series. It is shown that there exists a critical angle θcr in such a way that for θo < θcr, however (e.g. in the case of 5 CB θcr is ≈ 85 degrees) the viscoelastic behaviour is more complicated because of a coupling between flow and director gradients. The analysis of the alignment process by the changes of the proton line spectra also allows to determine the Leslie viscosities α1, α2, α3, α4 + α5, and the average Frank elastic constant K̄ by a single measurement.