Dynamic Dielectric Properties of Nematics

In this section, we describe the dynamic features of EOM effects. In particular, we focus on the response times to field-on and field-off these are the rise and decay times, respectively. In the case of the EO effects of nematic liquids, the rise and decay times exhibit the characteristic dependencies on voltage, and these characteristic times reflect the elastic, viscous, and dielectric properties of the materials. They have fully characterized the dynamics of the electric-field responses of LMM-LCs in experiments and established the theoretical background [6]. The dynamic features of the EOM effects in nematic gels are expected to differ from those of the EO effects in LMM-LCs, because the gels possess rubber elasticity and also have a different origin of the memory of the initial director. In addition, the dynamic properties give important information about the applicability of EOM effects in practical applications. 

W. H. DeJeu and T. W. Lathouwers, "Dielectric Properties of Some Nematic Liquid Crystals for Dynamic Scattering Displays. Chem. Phys. Lett., Vol. 28. p. 239 (1974) and Ref. 5 therein. 

The mononuclear, o/ r o-palladated 4,4 -bishexyloxyazoxybenzene acetylacetonate, (255), showed a nematic phase between 90 °C and 105 C, with temperatures very comparable to that of the free azoxy ligand (Cr SON 126 I). This complex, dubbed Azpac, has been subjected to a vast number of studies in order to analyze some specific properties such as electric and dielectric " properties, conductivity, viscoelasticity, and dynamics. ... 

In order to understand better the application potential of this electro-optic effect and its fundamental differences from nematic electro-optics based on dielectric anisotropy, and in particular to appreciate the many remaining challenges in its practical application in a device, we will give a derivation of the dynamical properties. The formahsm is slightly different from that used by some authors " and more similar to other descriptions. In the presence of an electric field E the free energy density is written, if we neglect the influence fi om the surfaces,... 

Dielectric anisotropy is an additive molar property. Thus, a small amount of PEBAB [Ae 10] (about 10-15 mol %) dissolved in MBBA [Ac —0.2] will provide a material suitable for twisted nematic devices. The threshold will, of course, be higher for this mixture than for a pure positive one such as 4-pentyl-4 -cyanobiphenyl, where the dielectric anisotropy is much higher. There are other influences on the threshold voltage for liquid crystal cells, principally the materials elastic constants and, in the case of dynamic scattering, material viscosity. The response times also are dependent upon elastic constants, viscosity, and dielectric anisotropy. These factors are discussed at length in a review by Goodman." ... 

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