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Investigation of the properties of liquid crystals applying noble gas and 2H NMR
Noble gas NMR
Thermotropic liquid crystals, that display liquid crystalline phases within certain temperature ranges, have become very important materials particularly in display technology. On the other hand, studies of lyotropic systems is experiencing renaissance since a few years ago it was found out that in certain circumstances water can be replaced by other hydrogen bonding liquids, such as formamide and glycerol, and this may result in new properties and possibilities in applying the systems, for instance, to display technology. A third class of mesogens, ferroelectric liquid crystals, have appeared as most suitable substances for display technology as they possess fast response speed and high bistability.
The mechanisms responsible for the ordering phenomena in liquid crystals are not well understood. An important area of research in this field is the study of small probe molecules or atoms dissolved in liquid crystals. It circumvents the difficulties resulting from the structural complexity (low symmetry and flexibility) of the constituent molecules of typical liquid crystals. The primary experimental technique in this approach is NMR spectroscopy, which has proved to yield quite detailed information on the ordering and interactions in mesophases. NMR active noble gases, 3He (spin ½), 21Ne (3/2), 83Kr (9/2), 129Xe (1/2) and 131Xe (3/2), are very sensitive spies. In particular the 129Xe chemical shift probes sensitively changes in the LC orientational order, phase transitions, density changes, etc. whereas the quadrupolar nuclei can be applied to the detection of electric field gradients created by the LC molecules. We have developed models to explain the behavior of noble gas chemical shifts and quadrupole couplings in the nematic and smectic A phases. The models make feasible the determination of the conventional Saupe LC orientational order parameters and the translational and mixed translational-positional order parameters for smectic A phases.
Further information on the LC properties is gained through translational self-diffusion experiments of 129Xe. A carefully performed experiment reveals phase transitions, which are not clearly visible in the chemical shift data, and activation energies. Moreover, the asymmetry of the self-diffusion tensor casts light on the phase structure and bending of chiral LC molecules. Experiments were performed for a commercial ferroelectric LC, FELIX-R&D (Hoechst) and a so-called critical mixture of two thermotropic LCs with opposite anisotropy of diamagnetic susceptibility.