LC materials

Among spatial reorientation techniques, one can find MAS, OMAS, variable angle spinning (VAS) and switched angle spinning (SAS) techniques. Mechanical spinning of LC materials, which has dated back to the early seventies,63,64 can control the averaging of anisotropic spin interactions... 

Figure 16.10 Conformational changes of polypropylene imine) dendrimers in LC materials from spherical to (a) cylindrical, in hexagonal columnar mesophase [130], and (b) ellipsoid, in smectic A mesophase [132]...

In contrast, diols 2a-2g and 3a-3g are not LC materials. Diols 2a-2g were shown to be crystalline by microscopy and by the existence of single first order transitions in the DSC. Diols 3a-3g appeared amorphous in the micrographs and had no first order transitions in the DSC. 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

LC materials have also been employed as films, plastics, and resins. Poly(l,4-benzoate) has been marketed under the name Ekonol. It decomposes before it melts, hence it does not form LC melts. Copolymerization with 4,4 -biphenol and terephthalic acid gives Ekkcel,... 

Their low melt viscosity permits molding of thin sections and complex shapes. However, their tendency to form ordered structures causes LC materials to be particularly susceptible to molecular orientation effects during processing. 

Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low a temperature, most LC materials will form a conventional (though anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as the temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally isotropic) phases as temperature is increased. 

Thermotropic chiral LCs whose pitch vary strongly with temperature can be used as crude thermometers since the color of the material will change as the pitch is changed. LC color transitions are used on many aquarium and pool thermometers. Other LC materials change color when stretched or stressed. Thus, LC sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, etc. The LC in fluid form is used to detect electrically generated hot spots for failure analysis in the semiconductor industry. LC memory units with extensive capacity were used in Space Shuttle navigation equipment. 

In the case of those LC materials which exhibit negative dielectric anisotropy, cells can be constracted which align vertically and twist on applying a field, exactly the converse to the twisted nematic effect from positive anisotropic LCs. Cells of this type are of interest because they can form a superior black state. 

Whichever system is used, in order to produce the full colour gamut it is necessary to display one-third of the pixels as red, one-third as green and one-third as blue. This is achieved by having a colour hlter layer as one of the substrates within the display panel. A schematic of this arrangement is shown in Figure 5.6. The role of the LC material in this system is to act as a light switch. 

Cholesteric LC materials are able to reflect visible light and also respond to temperature changes causing variations in the shade of the reflected colour. The... 

Approaches used to the development of LC materials with valuable luminescent properties include the synthesis of rod-like mesogens from fluorescent moieties, e.g. benzopyrans, " and discotic LCs with a fluorescent polyaromatic core. ... 

Photorefractive PDLCs are designed to combine the high efficiency at low fields of LC materials with the high resolution of photorefractive polymers. The polymer... 

During the studies of phase behaviour two types of liquid crystalline phases were identified. LC material was viscous and exhibited intense "white" birefingence. material was apparently homogeneous but of low viscosity and exhibited "multi-coloured" birefringence. The liquid crystalline phases observed in the equilibrium studies of surfactant concentrations up to 25 are unlikely to take part in the self-emulsification process due to the presence of two-phase regions between L2 and liquid crystalline phases however, LC material may account for the improved stability of emulsions formed by 25 surfactant systems (Table II). Figure 4c indicates that by increasing the surfactant concentration to 30 the... 

Fluorination of LC provides a powerful tool for the design of new LC materials with unusual and practically important properties. The specific effects of F in organic molecules result from a unique combination of high polarity and low polarizability, as well as steric and conformational effects (see next section). 

Examples of rod-like molecules with a branched fluorinated chain at one end (compounds 90-93) are collated in Fig. 26 [166]. It is interesting that smectic phases are retained despite the significant size of these chains. This is mainly a result of the intercalation of the aromatic cores and aliphatic spacers of these molecules, which can compensate this steric distortion. These branched chains remove the B and E phases with crystalline layers and replace them by fluid smectic phases, including SmC phases. The comparison in this figure also shows that the bulky and flexible bis(perfluoropropylene oxide) derived chains (compounds 92, 93) can provide LC materials with especially low melting points and broad mesophase ranges due to the higher conformational flexibility of perfluoroethers compared to linear perfluoroalkyl chains [99, 176]. 

In general, the photoluminescent properties of molecular assemblies depend on their assembled structures [21], Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Recently, some photoluminescent materials showing stimuli-responsive properties have been reported [11, 21-34]. However, the number of stimuli-responsive photoluminescent liquid crystals is still limited [11, 21, 28-34], Here we focus on mechano- and thermochromic photoluminescent mesomorphic materials. As the first topic, we focus on LC materials that change their photoluminescent colors by mechanical and/or thermal stimuli. We then describe another... 

The design and properties of stimuli-responsive photoluminescent LC materials are described. It is essential to control the interactions between luminescent cores to induce stimuli-responsive luminescent properties. These classes of anisotropic materials would be important for future photonic materials. 

The most studied erasable LC materials for holography are LC phases that contain a dye, most often an azobenzene, either dissolved in the LC or as part of the LC molecule [46], In these phases, light can cause disordering or... 

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