Liquid crystals dielectric anisotropy

Cupelli Daniela, Fiore Nicoletta R, De Filpo Giovanni, et al. Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal. J. Polym. Sci. B Polym. Phys. 49 no. 4 (2011) 257-262. 

Comblike electrodes were fabricated on a transparent electrode, separated by a dielectric layer in the so-called finger-on-plate (FOP) configuration [107]. A positive dielectric anisotropy liquid crystal material, as for the original IPS mode, was applied and it was commercialized as the IPSpro LCD [74]. 

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

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... 

There has been an increasing interest in liquid crystals containing fluorine atoms because fluorination produces important changes in the melting temperatures, viscosity, birefringence, dielectric anisotropy and mesophase stability. This why structure-mesomorphism relationship studies were carried out in mono- and... 

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. 

Fig. 35. Modulation function of the polyimide modulator with thickness of the liquid crystal in pm 20 (/), 15 (2), 10 (3), 3 (4). Anisotropy of the dielectric permittivity of 12 (2,2,3) 0.05 (4). Polyimide thickness - 2 pm. Pulse regime [14]...

Table 3.1 Transition temperatures CC), dielectric anisotropy (As) and conductivity (Or cm ) for nematic liquid crystals (1-11) and a commercial mixture (Merck) designed for DSM-LCDs...

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

Table 3.3 Molecular structures, transition temperatures (°C) and dielectric anisotropy (Ae measured at 20°C) for nematic liquid crystals (15-27)...

A standard TN-LCD consists of a nematic liquid crystal mixture of positive dielectric anisotropy contained in a cell with an alignment layer on both substrate surfaces, usually rubbed polyimide, crossed polarisers and a cell gap of 5- 0fim, see Figure 3.7. The nematic director is aligned parallel to the direction of rubbing in the azimuthal plane of the device. The alignment layer induces a small pretilt angle (6 1-3°) of the director in the zenithal plane. The... 

Aromatic liquid crystals, such as compounds 106-109 collated in Table 3.10, have been investigated to a much lesser extent since they were perceived to be intrinsically more viscous than the analogous cyclohexane derivatives. This is indeed often the case. It was hoped that the incorporation of a fluorine atom in a lateral position of a polar compound, such as the ester (106), would create a nematic material, such as the esters (107 and 108), of positive dielectric anisotropy and low ratio of and Ae/sj.. Although this was partially... 

The requirement of a very high and constant resistivity over time of nematic mixtures for TN-LCDs with active matrix addressing meant that new liquid crystals which met this and the other specifications, such as low viscosity and high positive dielectric anisotropy, were needed. The nitriles used in nematic mixtures for TN-LCDs with direct or multiplex addressing were soon found to be unsuitable for TN-LCDs with active matrix addressing. The polar nature of the cyano group leads to the solvation of ions from some of the layers on the... 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

The application of an electric field above the threshold value results in a reorientation of the nematic liquid crystal mixture, if the nematic phase is of negative dielectric anisotropy. The optically active dopant then applies a torque to the nematic phase and causes a helical structure to be formed in the plane of the display. The guest dye molecules are also reoriented and, therefore, the display appears coloured in the activated pixels. Thus, a positive contrast display is produced of coloured information against a white background. The threshold voltage is dependent upon the elastic constants, the magnitude of the dielectric anisotropy, and the ratio of the cell gap to the chiral nematic pitch ... 

Nematic Liquid Crystal Hosts of Positive Dielectric Anisotropy... 

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