Deuterium NMR spectra of the cholesteric and blue phases: A study of biaxiality

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Abstract
Deuterium NMR spectra from a set of cholesteric materials differing in pitch length have been interpreted in terms of a biaxial ordering of the molecules. It is found that, in these materials, the biaxiality in the cholesteric phase is a universal function of the reduced temperature (independent of pitch length) only when there is a blue phase present in the material. In contrast, when the cholesteric phase transforms directly into the isotropic phase without the presence of a blue phase the biaxiality on a reduced-temperature plot is strongly pitch-length dependent. By the study of different deuterated sites of the same molecule, the mechanism for biaxiality is examined. Deuterium NMR spectra are also used to measure the pitch dependence of the self-diffusion constant. Finally, the NMR technique of measuring biaxiality in the cholesteric phase is discussed and a critique made. The materials used in the study were selectively deuterated 4-methoxybenzylidene-4′-butylaniline (MBBA) twisted by the addition of chiral 4-methoxybenzylidene-4′-[(+)-2-methylbutyl]aniline (MBMBA) in various weight percentages to obtain samples of different pitch lengths.