摘要：

In part because of its anticipated application for faster and lower power-consuming electro-optic devices, the biaxial phase of nematic liquid crystals, characterized by two optic axes, has long been sought after. We have investigated the existence of thermotropic nematic biaxiality in a relatively new system, liquid crystalline tetrapodes, through dynamic light scattering. Liquid crystalline tetrapodes are formed by the attachment of four mesogenic molecules to a single silicon or germanium atom through flexible siloxane chains. Our results, obtained in various scattering geometries and tested against available theory, strongly support the existence of a biaxial nematic phase. The temperature dependent slowing down of the biaxial order parameter fluctuations indicates that the uni – biaxial transition is weakly first order in a 4-ring tetrapode and second order in a 3-ring homolog. The temperature dependence of the relaxation rates of the biaxial order parameter mode and of the scattered intensity associated with biaxial optic axis fluctuations is explained by a Landau-deGennes model of the free energy. In particular, we have confirmed that the intensity exhibits the expected scaling with the uniaxial and biaxial order parameter magnitudes, for several distinct geometries, in the biaxial phase. Another relatively new class of nematics is the lyotropic chromonic liquid crystals (LCLCs) formed from concentrated mixtures of disc-like dye molecules in water. Because of their potential applications in dye based polarizing and compensating films and in biological sensing, and because of recent analogies drawn between LCLCs and the liquid crystalline phases of DNA solutions, LCLCs are interesting systems to study. Using dynamic light scattering on well aligned samples, we have explored particularly the temperature dependence of the elastic constants and orientational viscosities of nematic Disodium cromoglycate LCLCs. These parameters show a significant anisotropy. In particular, the bend and splay moduli K33 and K11 are an order of magnitude higher than the twist modulus K22, and the ratio K33/K11 shows an anomalous increase as temperature increases, which we attribute to the shortening of the aggregates. The bend viscosity is three orders of magnitude smaller than the splay and twist viscosities; all viscosity coefficients exhibit a strong temperature dependence.

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