EPR Study of the Temperature Dependence of Molecular Rotation in Nematic Liquid Crystals

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Abstract
The EPR spectrum of vanadyl acetylacetonate dissolved in viscous nematic liquid crystals has been studied as a function of temperature. A plot of the effective order parameter versus temperature shows a definite discontinuity in slope as the molecular rotation passes from weakly to strongly hindered motion. It is shown that viscosity effects do not completely account for this discontinuity in slope. These results are supported by nuclear spin lattice relaxation measurements on the pure liquid crystals which indicate a change in the dominant relaxation mechanism at the same temperature. In slightly less viscous nematics an approximate correction for the viscosity brings the experimental data into fair agreement with a universal curve calculated by Luckhurst.