Liquid crystals : order, symmetry and dynamics

Abstract

The first orientational order study involves a new class of solute known as iptycenes, and dramatically whose orientational behaviour in a nematic host is essentially solvent-dependent. While there is no appreciable pattern to link the values of the order parameter to the molecular structure of the liquid crystal host, the deuterium NMR results show very dramatic behaviour of the probe.  One particularly significant phenomenon is the behaviour of two iptycenes with D_3h symmetry behaving like a biaxial molecule in the liquid crystal hosts.

The next orientational order study is to observe the effect of a gelator on the macroscopic orientational order of the liquid crystal aligned by a magnetic field.  The NMR results suggest that the effect of gelation on the director order parameter increase as the concentration of the gelator increases which suggests that the spacing between the fibres decreases.  In addition, the deformation of the director in the gelled liquid crystal causes the nematic – isotropic transition temperature to decrease.

One of the latest debates in liquid crystal science concerns the existence of the thermotropic biaxial nematic phase.  Our phase symmetry study is our contribution to this hot discussion to show how the phase symmetry can be unambiguously identified using NMR spectroscopy.  For this, we have chosen the crystal E phase as our model because its biaxiality is not in doubt.  Our results show that the phase symmetry is very sensitive to the probe molecule used but that the biaxiality is found to be high.

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