Liquid crystal mixtures

This study of Gay-Berne mixtures indicates that the generalised Gay-Berne potential should provide a promising route with which to investigate liquid crystal mixtures. However, more work is necessary both to locate and understand the structure of the liquid crystalline regions exhibited by these models. 

Figure 8.29 Compounds used to prepare liquid crystal mixtures containing gold nanoparticles.

The amorphous diacrylate monomers chosen for study were two commercially available monomers, p-phenylene diacrylate (PPDA) and 1,6-hexanediol diacrylate (HDDA) (Polysciences, Inc., Warrington, PA). The liquid crystalline diacrylate studied was 1,4-di-(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene (C6M) (13). Chemical structures of these monomers as well as pertinent physical and LC properties are given in Figure 1. All monomers were used without further purification. The ferroelectric liquid crystal mixture consisted of a 1 1 mixture of W7 and W82 (1) (Displaytech, Boulder, CO). This mixture exhibits isotropic (I), smectic A... 

Nematic phase this is the simplest structure. It is the most disordered mesophase and therefore very fluid. It is called N. In the nematic phase, the molecules are ordered mainly in one dimension with their long axes parallel, and they are free to move parallel to this axis (there is no long-range order). Nematic liquid crystal mixtures, containing various amounts of different liquid crystal compounds, are used in electro-optic display systems such as flat-panel displays. 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

Chen et al. reported on a general approach by which the polarization of the emission from semiconductor nanorods can be manipulated by an external bias. In their device, the composite of a nematic liquid crystal mixture (E7, Merck) and nanorods (CdS) filled into an ITO-coated cell with an optimized concentration of one CdS nanorod per 1010 LC molecules was used to achieve the highest polarization ratio of the suspended nanorods [447, 448]. The nematic liquid crystal in this system acts as a solvent and media whose direction of alignment can be tuned by an applied electric field. Hence, the orientation of the CdS nanorods can be fine-tuned by an external bias because of the anchoring force between the liquid crystal... 

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. 

Another example of a dispersion of SWCNTs in a multi-component antiferro-electric smectic-C liquid crystal mixture was shown by Lagerwall and Dabrowski et al. [497]. In this study, SWCNTs caused the appearance of a single-layer SmC phase between the SmA phase and the crystalline state in comparison to the non-doped sample exhibiting an SmA and two specific intermediate phases, an SmC p and an SmC Y phase. 

Merck s LC division has a particular challenge as it has to operate in the context of today s fast moving electronics industry. Displays and related technologies are being developed ever faster, which means that more efficient liquid crystal mixtures are needed ever more rapidly. 

MeOAn-ANI-3 NI in the nematic liquid crystal mixture E-7 (Merck) at two orientations of the liquid crystal director, L, taken 700 ns after a 420 nm laser pulse at 150 K. The narrow signal is an expansion of the radical pair signal, (b) Numerical differentiation of the B L L spectrum. 

The basic modes of construction of the various commercial types LCD described in detail in Chapter 3 share many common elements. They consist of a very thin layer of a nematic liquid crystal mixture enclosed between two transparent parallel glass substrates held apart by solid spacers and glued together around the edges, see Figure 2.11 for a schematic representation of the optical elements, some or all of which can be combined to construct an LCD. The cell gap (d ss 2-10 pm) should be as uniform as possible in order to minimise variations in the optics of the display. Thick cells are turbid, due to... 

A chiral nematic (cholesteric) liquid crystal mixture is contained in a liquid crystal cell as shown in Figure 3.3. 

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... 

In order to produce black-and-white as well as full-colour STN-LCDs, the monochrome interference colours must first be eliminated. This was achieved initially by using two STN-LCDs in a combined double-layer (DSTN) LCD configuration. This involves the use of another non-addressed, passive STN cell in addition to the active display STN-LCD. However, the non-addressed cell has an opposite sense of twist of the nematic director in the cell to that of the addressed STN-LCD. The second STN-LCD, which is identical to the first, but not addressed at all, acts as a retardation compensation layer. The use of an identical second STN-LCD in combination with the active STN-LCD has the advantage that both displays exhibit exactly the same temperature dependence of the birefringence with the same dispersion, assuming that both cells are filled with the same liquid crystal mixture. The second STN-LCD is not addressed and, therefore, there is no increase in power consumption. However, the use of two identical STN-LCDs instead of only one clearly increases the cost and weight of the final product significantly. 

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 between the electrodes results in a realignment of the nematic liquid crystal mixture and the dichroic dye molecules parallel to the electric field resulting in a lower optical density (absorption) and, theoretically, the disappearance of colour assuming an ideal order parameter (S = 1) of the nematic liquid crystal director and the dye molecules. A residual absorption in this state gives rise to a display with a strongly coloured background and weakly coloured information. 

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 ... 

The homogeneously aligned nematic liquid crystal mixture does not rotate the plane polarised light generated by the first polariser. Therefore, the light is absorbed by the analyser. Very efficient absorption of light takes place. 

Y. Onogi, T. Hayashi, and M. Yamamoto, Liquid crystal formation control of azobenzene-liquid crystal mixtures by photoisomerization of azobenzene, Nippon Kagaku Kaishi 1990, 250-254. 

Van Deun et al. were the first to observe near-infrared luminescence from lanthanide-doped liquid crystal mixtures They studied the spectroscopic properties of the lanthanide(III) / -diketonate complexes [L (dbm)3(phen)], where Ln = neodymium, erbium, ytterbium, and dbm is dibenzoylmethane, in the liquid crystal MBBA. By incorporation of an erbium(III)-doped nematic liquid crystal (ErCls dissolved in E7) in the pores of microporous silicon, narrowing of the erbium(III) emission band in the near-infrared was observedJ Luminescent optically active liquid crystals were obtained by doping [Eu(tta)3-3H20] into a mixture of cho-lesteryl nonanoate, cholesteryl tetradecanoate and the ternary liquid crystal mixture ZLI1083 from MerckJ ... 

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