Selective light reflection

As is seen from Table 13, cholesteric copolymers display a maximum of selective light reflection ( w) in an IR- or a visible part of the spectrum. By varying the composition of a copolymer, it is possible to vary Xmax, in accordance with the stipulation max = nP, is proportional to the pitch P of the helical structure of a LC polymer (n — is the refractive index). The pitch of the helix in cholesteric copolymers is usually decreased, when the temperature is raised 105) (at temperatures above Tg), which is equally common for low-molecular cholesterics142) (Fig. 23a). The observed fact that the helix pitch for LC copolymers 2.1-2.3 (Table 13, Fig. 23b) is increased, is rather unusual but explicable within the theoretical views regarding vibrational movement of macromolecular fragments and their conformational mobility 60). 

Fig. 13a Salt addition effect on the selective light-reflection phenomenon of aqueous HPC liquid crystals b variation in the cholesteric reflection color with time, observed for an HPC/Lil/water system under the action of an electric field E = 4.5 V/15 mm. HPC cone., 62.5 wt% salt concentration, 0.5 M temp., 20 °C...

Fig. 14 UV-VIS spectra of a cholesteric elastomer, based on a mixture of derivative 2 (44 wt %), monomer 1 (52wt%), and monomer 2 (3wt%), crosslinked at 32 °C. Arrows indicate the maximum position of the selective light-reflection band. (Reproduced from [233])...

D. Medvedeva, A. Bobrovsky, N. Boiko, V. Shibaev, I. Zavarzin, M. Kalik, M. Krayushkin, A combination of selective light reflection and fluorescence modulation in a cholesteric polymer matrix. Macromol. Rapid Commun. 26, 177-182 (2005)... 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

A specific feature of ch selective light reflection an trophotometric study of the f homopolymers in the 20-150 C ing temperatures of homopolym the existence of a broad refl 300 nm region. Circular dich same wavelength region (Fig. homopolymers produce a choles sic selective UV-light reflec molecular cholesterics, chole play a tendency to layer pack the structuring effect of the... 

For cholesteric liquid crystals, the wavelength of selective light reflection Xj is related to the pitch of the cholesteric helix P in the following manner Xj =nP, where n is the refractive index. It is evident that for a cholesteric mesophase with selective reflection of the... 

Fig. 2. CD-spectrum (1) and the curve of selective light reflection (2) for PChA-5.

Fig. 3. Selective light reflection curves for copolymer 1 at room temperature (1), for copolymer 6 at T 102 C (2) and for copolymer 6 at room temperature (3). ...

At the same time the films are characterized by selective reflection of light. It is seen from Fig. 4 and Table 2 that for all copolymers the wavelength of selective light reflection decreases when the fraction of chiral units is increased but for copolymers ChA-5/AM-5, the selective light reflection is observed at lower values of X than for copolymers ChA-5/AC-5 of analogous compositions. Apparently, the helix is more difficult to form from the nematic structure formed by more polar molecules (AC-5), than from less polar molecules (AM-5). 

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. 

The early work of Robinson et al. [31] was also done in this period. This involved solutions of poly-y-benzyl-L-glutamate in organic solvents. These solutions exhibited the selective light reflecting properties of thermotropic cholesteric liquid crystals. 

The characteristic behavior of the selective light reflection (e.g. of a cholesteric phase near a smectic A phase) can be used to observe the transition of phases. In some cases a distinction between a discontinuous and a continuous smectic A-cholesteric phase transition is possible [16],... 

---