Temperature selective reflection

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Cholesteric LCs can act as hosts for dyes to produce coloured displays (see section 5.2.2.1 below) " their temperature dependent colour change has found applications in thermochromic inks, °" and as pigments and copy safe colours their selective reflecting capabilities have been applied in colours and filters for reflective displays and projection systems, " reflective polarisers and their electrical field induced switching in displays and smart reflectors, in colour patterning for full-colour recording. ... 

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

Fig. 4 Selective reflection wavelength (optical pitch) as a function of temperature for three samples host material showing SmC and SmCA without dopant, with 1% and 3% dopant The chemical structures of the rod-shaped host and the bent-shaped dopant molecules are also shown. As shown in inset, inverse pitch increases almost linearly with dopant content [4]...

Liquid-crystalline solutions and melts of cellulosic polymers are often colored due to the selective reflection of visible fight, originating from the cholesteric helical periodicity. As a typical example, hydroxypropyl cellulose (HPC) is known to exhibit this optical property in aqueous solutions at polymer concentrations of 50-70 wt%. The aqueous solution system is also known to show an LCST-type of phase diagram and therefore becomes turbid at an elevated temperature [184]. 

For example, the selective reflection of a cholesteric mixture of derivative 1 ( 45 wt %)/monomer 2 (diethylene glycol dimethacrylate) covered the whole visible spectrum in a narrow temperature range i.e., the mixture was bluish at 20 °C, greenish at 30 °C, and reddish at 50 °C. Therefore, by successive photo-crosslinking of the liquid-crystalline sample at different temperatures through a mask, a colored picture or pattern could be fixed onto the resulting composite film. 

A number of problems arise in connection with the use of emission IR spectroscopy (IRES). One of them arises from the existence of temperature gradients, which can cause self-absorption of the emitted radiation by the colder outer parts of the sample itself another is concerned with the selective reflection that occurs in the vicinity of strong absorption bands. This reduces the absorptance and hence the emittance. Moreover, perturbations can be created by reflections and emission by the cell elements. These problems, however, can in part be overcome so that IR emission spectra can be successfully recorded and are widely used, for example, in the fields of polymer and corrosion science and mineralogy. Some uses of IRES... 

Three types of copoly(Y-n-alkyl L-glutamate)s with the combination of methyl-hexyl (MH), methyl-octyl (MO) and propyl-octyl (PO) were prepared by alcoholysis of PMLG in ethylene dichloride using p-toluenesulfonic acid as the catalyst at 60 °C. The thermotropic mesophase was detected by the iridescent cholesteric color which appeared by annealing at temperatures in the region 110 190 C. Also the circular dichroism due to the selective reflection was measured at room temperature for the quenched films. Only the films with the copolymer composition of about 50/50... 

The pitch of the helix for compound 42 was found to be approximately 0.2-0.3 xm, thus the material selectively reflects visible light over a wide temperature range. Moreover, the pitch is relatively temperature insensitive thus the material can be used in large area non-absorbing polarizers, or in optical notch filters or reflectors. In addition, in the glassy state the helical macrostructure of the chiral nematic phases is retained, thus similar applications are possible. 

A sheet of cholesteric liquid crystals is sandwiched between two glass plates separated by a gap of tens microns. The cholesteric liquid crystals on two glass plates are homogeneously aligned to form the planar texture. The cell displays a bright color. The color varies according to the view angle and temperature. This is an important characteristic of cholesteric liquid crystals—selective reflection. 

The pitch P is the most important parameter of cholesteric liquid crystals. The physical properties of cholesteric liquid crystals are associated with P, such as selective reflection, optical rotation dispersion, circular dichroism, etc. The helical pitch is sensitive to the temperature and external field, for example, electric and magnetic field, chemical environment, pressure or radiation, etc. 

The time course of selected ambient temperature and core temperature in the rat monitored by telemetry exemplifies the regulated hypothermic response induced by administration of chlorpyrifos (Fig. 6). When dosed with a control vehicle (corn oil), there was a transient decrease in selected temperature that reflects a heat dissipatory respon.se from the stress of handling and injection. When dosed with chlorpyrifos, selected ambient temperature decreased from 30 to 25 °C. The behavioral response to select a cooler temperature preceded a 2.5 C decrease in core temperature. At the nadir of the decrease in core temperature,. selected temperature increased rapidly,... 

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