Deuterium NMR study of liquid crystals

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Cheng, Mowei
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Deuterium nuclear magnetic resonance (NMR) spectroscopy was used to explore molecular motions in the mesophases of S-4-(2-methylbutyloxy)carbonylphenyl1 4-(10-undecenyloxy)-benzoate(MBPUB-d2), and a mixture of 4-n-octyloxy-4 '-cyanobiphenyl (80CB-d17) and 4-n-hexyloxy-4 '-cyanobiphenyl (6OCB). The deuteron quadrupolar and Zeeman spin-lattice relaxation times were measured as a function of temperature in the nematic, smectic A and reentrant nematic phases of the mixture and in the smectic A phase of MBPUB at two different Larmor frequencies. For MBPUB, the quadrupolar and proton-deuteron dipole splittings of the ring were also measured. The derived spectral densities of motion at different temperatures were analyzed simultaneously using a small-step rotational diffusion model. Internal ring rotations were superimposed onto the overall motion. For this particular chiral molecule, we found an anomalous behaviour (' D'? > 'D'∥) which is different from non-chiral rodlike liquid crystals. For the 8OCB/6OCB mixture, the additive potential method is employed to model the quadrupolar splittings, from which the potential of mean torque is parametrized, and the order parameter tensor for an "average" conformer is determined. A decoupled model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model, while the former motion is described using a master rate equation. In addition, the order director fluctuations were also taken into account in order to fit experimental results in the nematic phase.