Nematic liquid crystal mixtures

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. 

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... 

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 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... 

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. 

All nematic liquid crystal mixtures have good lubricating properties at room temperature. However, those having an azomethine central link (Schiff bases) as EBBA, MBBA or ROTN 200 are unstable in humid atmosphere because they hydrolyse. 

A 10% solution of 2-naphthaldehyde in the nematic liquid-crystal mixture 4-cyano-4 -alkylbicyclohexyls (ZLI-1695, Merck) (Figure 2-19) was prepared for the measurements. This nematic phase is very convenient as an anisotropic solvent because of its weak IR spectrum, allowing one to obtain polarization spectra in practically the whole mid-IR region. 

To avoid the occurrence of domains with reverse tilt and domains with reverse twist in a TN-LCD, the use of an alignment layer with a pretilt angle more than about 5° and the addition of cholesteric dopants to the nematic liquid crystal mixture are preferable (see Section 2.1.1). 

Figure 4, The structures and composition of the nematic liquid crystal mixture Imown as E7.

The monomer BMBB-6 (4 4-bis- 4-[6-(methacryloyloxy)-hexyloxy] benzoate - ,I biphenylene) and very small amounts of photoinitiator BME were mixed with 5CB to perform a study of the polymer network-induced birefringence in the isotropic phase of a nematic liquid crystal. Mixtures of 5CB in the nematic phase with 1, 2, 2.5, 3, and 4 wt.% of monomer were filled into planar cells and irradiated by UV light (4 W/cm ) at constant ambient temperature for 1 hour. 

The technique has also been used to study the dynamic behaviour of a polymer-dispersed liquid crystal subjected to an electric field [18]. The liquid polymer used was the commercially available nematic liquid crystal mixture E7, which contains four nitrile and ethyl substituted bi- and tri-phenyls. It was blended with a polymer precursor consisting of a mixture of an acrylate monomer, an acrylate oligomer and a UV curing agent. The 2D correlation analysis showed that the rigid core of the liquid crystal molecules re-orients as a unit, and suggests that the polymer side chains existing in the interface between the polymer and the liquid crystals may re-orient in phase with the liquid crystal re-orientation by interaction with the liquid cryst molecules. 

A twist angle 0>9O° can be achieved by doping the nematic liquid crystal mixture with a cholesteric liquid crystal. For a cell with zero pretilt the twist angle

cell-thickness d to the pitch p of the doped liquid-crystal mixture ... 

Merck E7TM is the liquid crystal host used in the LCPDI device. E7 is a nematic liquid crystal mixture.with a mesophase between -10°C and 60.5°C. The R-group substituents on the nickel dithiolene dye need to be selected for maximum solubility in the E7 host without compromising other important properties (absorption wavelength, optical density or thermal stability). Merck E7 is a room temperature eutectic mixture of cyanobiphenyl and cyanoterphenyl liquid crystal compounds its components are shown in Figure 4. 

C. J. Alder and E. P. Raynes, Room Temperature Nematic Liquid Crystal Mixtures with Positive Dielectric Anisotropy, J. Phys., D6, p. L33 (1973). 

Inorganic organic hybrid polymer dispersed liquid crystalline nanocomposites are reported as suitable candidate for fahricatirHi of photorefractive medium. Winiarz et al. reported one such photorefractive medium which consists of hole transporting polymer composite matrix, electro-opticaUy active nanodroplets of liquid crystal and Cadmium sulphide quantum dots as photosentizers (Winiarz and Prasad 2002). They employed poly(methylmethacrylate) (PMMA) as polymer matrix, N-ethylcarbazole (ECZ) as hole transporting medium and commercial nematic liquid crystal mixture TL 202 for fabrication of the photorefractive medium with Cadmium sulphide as photosentizer. The reported medium exhibits more than 90 % internal diffraction efficiency which is relatively high compared with other PDLC photorefractive medium. 

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