Nematics material requirements

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Examples of these formulations are systems based on a difunctional LC epoxy monomer (diglycidyl ether of 4-4 -dihydroxy-Q -methylstilbene), cured with methylene dianiline (Ortiz et al., 1997). The generation of liquid-crystalline microdomains (smectic or nematic) in the final material required their phase-separation before polymerization or at low conversions. This could be controlled through the initial cure temperature. Values of GIc, (kJm-2) were 0.68 (isotropic), 0.75 (nematic), and 1.62 (smectic). The large improvement produced by the smectic microdomains was attributed to an extensive plastic deformation. 

For each particular application and for each display mode a nematic material with custom-tailored physical properties [23] is required. The most application-relevant of these properties are the temperature range of the nematic phase (i. e. the tern-... 

Sage, I. Materials Requirements for Nematic and Chiral Nematic Electrooptical Displays. In Thermotropic Liquid Crystals-, Gray, G. W., Ed. Wiley Chichester, 1987 Chapter 3, pp 64-98. 

Modified forms of these displays have been developed - e.g., the dye display which has a pleochroic dye dissolved in the nematic material and requires the use of just one polarizer - but we shall not be discussing them here. Analytical expressions have been derived which simplify the computational effort involved in optimizing the material and device parameters, but one has to rely on numerical modelling to give a complete description of the dynamical characteristics of these devices. Certain unusual dynamical effects observed in the TN device, e.g., the reverse-... 

This set-up allows a pixel to be addressed at each intersection of a row and a column. This works line for nematic LCs in modest sized displays, i.e. up to 120 000 pixels, but beyond this size there is an increase in switching times and cross-talk between adjacent pixel elements leading to a loss in contrast. This problem can be overcome by using STN LCs, which are materials where the hehcal twist is increased to between 180° and 270°. These super twist LCs give a much sharper image than the 90° materials. This system is ideal for monochrome displays but even with these materials the response times start to get very slow with the several million pixels that are required for high contrast, full-colour displays. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

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