Liquid crystals dielectric properties

The order parameter S is a very important quantity in a partially ordered system. It is the measure of the extent of the anisotropy of the liquid crystal physical properties, e.g., elastic constants, viscosity coefficients, dielectric anisotropy, birefringence, and so on. S is temperature dependent and decreases as the temperature increases. The typical temperature dependence of S is shown in Figure 1.16. 

Let us point out a specific property of FLCs which makes its dielectric behavior very different, in comparison with nematic liquid crystals. Dielectric susceptibility, defined as x = dPfdE strongly depends on the field amplitude and for E < (J5u is the helix unwinding field, Fig. 7.4) can reach values of 10 — 10 and even higher. This should be taken into account in applications, for instance, in photosensitive cells, where impedance matching between photoconductor and liquid crystal layers is needed [57]. 

Parameters (ii)-(vii) depend on the dielectric, mechanical and optical properties of the mesogens. To optimize a dis compromise between different molecular characteristics is often required and mixtures of liquid crystals are usually commercial displays. 

In most devices the liquid crystal molecules are confined between two thin walls which act as capacitor plates. This allows the determination of the dielectric properties of the liquid crystalline material through the simple relations... 

A new class of liquid crystals with strongly negative dielectric anisotropy was explored by employing the ambivalent characteristics of the 1,3-dioxane moiety <2006EJ04819> due to both the polarity of 1,3-dioxane and axial fluorination, compounds 238-240 proved to have very useful mesogenic and electrooptical properties. 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

PFMB can be used to prepare aromatic polyimides that display solubility in ketone, ether, and polar aprotic solvents. This unusual solubility can be utilized in die facile preparation of thin films that display anisotropy in their structures and properties. The anisotropy in the optical properties of the films makes them promising candidates for use as compensation layers in liquid-crystal displays. Their low dielectric constants and CTEs in combination with their outstanding thennal and thermooxidative stabilities make diem candidates for dielectric layers in microelectronics applications. 

In a majority of works on LC polymers, the main attention was paid to the synthesis and structural studies of such polymers. Significantly less information is available on physical properties of LC polymers, especially, when compared to low-molecular liquid crystals. In this chapter some rheological and dielectric properties of polymeric liquid crystals, characteristics of their dynamic properties and intramolecular mobility, are considered. 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

The extension of continuum models to complex environments is further analyzed by Ferrarini and Corni Frediani, respectively. In the first contribution the use of PCM models in anisotropic dielectric media such as liquid crystals is presented in relation to the calculation of response properties and spectroscopies. In the second contribution, PCM formulations to account for gas-liquid or liquid-liquid interfaces, as well for the presence of a meso- or nano-scopic metal body, are presented. In the case of molecular systems close to metal bodies, particular attention is devoted to the description of the surface enhanced effects on their spectroscopic properties. 

Significant variations in the magnitude of the physical properties, such as the liquid crystal transition temperatures, viscosity, birefringence and the elastic and dielectric constants, of alkenyl-substituted compounds with a carbon-... 

Dielectric properties differ between ice samples grown from the vapor phase and the liquid phase the relaxation time and activation energy of ice grown from the vapor phase have lower values than of liquid growth ice. This difference suggests that vapor-phase growth introduces a crystal imperfection (such as vacancies and inclusion of gas) with increasing Bjerrum defects. 

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