Selective reflection, cholesteric film

Figure 5. Selective reflection by a left-handed cholesteric film (32).

Jig is selectively reflected, while the RH component is conq>letely transmitted. The selective reflection bandwidth is determined by optical birefringence. An = - R A sufficiently thick, single-handed cholesteric film is capable of... 

Figure 2.33 also shows an idealized view of a cross section of the temperature-sensitive film. A variety of different temperature-sensitive products (Figure 2.34), such as paints, coatings, and films, make use of selective cholesteric reflection for temperature measurement. 

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. 

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... 

As prescribed in Sect. 6.4, a constant-pitch cholesteric LC film typically gives rise to a selective reflection bandwidth around 100 nm in the visible region as determined by the An value. The bandwidth can be broadened with a gradient pitch through film thickness, according to the Hajdo-Erigen theory [40]. Photoracemizable (/ )-di-naphtho[2, ID l, 2 -/] [1,3] dioxepin was mixed with a nematic and cholesteric GLC at a molar ratio of (27) (28) (29) = 1.00 0.23 0.13 as depicted in Fig. 6.7a. With a and Tc at 65 and 156 °C, respectively, the ternary mixture was prepared into a... 

The pitch of a cholesteric may vary quite rapidly with temperature, particularly if the phase diagram of the material contains a cholesteric to smectic A transition just below the temperature range of interest since the pitch in this case diverges to infinity as the smectic A phase is approached. Thus, the color of the selective reflection can be used a s a sensitive indicator of temperature. Material for this purpose is incorporated into plastic films with a black backing to absorb the unreflected polarization, or it is encapsulated to apply as a paint and used for a variety of medical, engineering, and consumer applications. 

Optical ProportiGS. All of the optical applications of LCTs have considered the cholesteric phase. The cholesteric phase is interesting because thin films of a cholesteric liquid crystal oriented such that the helical axis is perpendicular to the plane of the film will selectively reflect circularly polarized light with a wavelength determined by the pitch of the helix. For low molar mass liquid crystals. 

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

Fig. 7.7. Phase diagram of the HPC-water system [34] Is isotropic phase 1 suspension of polymer, II gel 1) two-phase solution 2-4) LC cholesteric phase which selectively reflects red (2), green (3), and violet (4) light 5) transparent polymer film.

The cholesteric mesophase is thus very widespread both in natural and in synthetic polymer systems. The capacity of synthetic polymers to retain the properties of cholesteric liquid crystals, primarily the selective reflection of circularly polarized light, in the glassy and highly elastic state, makes them promising materials for creating different optical film elements (selective filters, polaroids, reflectors, heat indicators, etc.). 

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