Liquid crystal reflection band

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

The very small residual increase (less than 0.2 nm/K) can be attributed to thermal expansion of the cured polymer. This was verified by measuring the coefficient of thermal expansion via the change of density by heating pieces of cured solid film from 20°C to 50°C. A value of A7JAT = (0.1 l+i.0.04) nm/K was calculated from this measurement, which is in good agreement with the observed wavelength shift. Preservation of the reflection band was observed down to -196°C. This sounds trivial for a polymer but should be mentioned for comparison with monomeric liquid crystalline materials, which tend to crystallize at low temperature. 

The natural pitch P of cholesteric liquid crystals is in general in the order of 102 nm, comparable with the visible band of light. The Bragg reflection from cholesteric liquid crystal occurs at the wavelength... 

Fig. 12.5 Comparison of the non-polarized light transmission by a stack of dielectric layers and a cholesteric liquid crystal (CLC). The two materials have the same Bragg reflection frequency (numerical calculations, for parameters see the text), (a) Transmission spectra on the frequency scale showing the absence of high harmonics in the case of CLC (b) blown transmission spectra at the wavelength scale showing the flat form of the CLC Bragg band and oscillations of transmission at the edges of the band...

Basically, a thin layer of liquid crystalline material is deposited by evaporation in a high vacuum on a fused silica window. Its molecular orientation is determined by attenuated total reflection (ATR) spectroscopy. Use is made of the fact that most liquid crystal molecules are highly absorbing in the near ultra-violet, and strongly pleochroic, i.e., the transition moment for the first strong absorption band in the UV is pretty well oriented along the long axis of the molecule. Thus, a... 

The spectral width of this reflection band is proportional to the optical anisotropy for cholesteric liquid crystals... 

Figure 9 shows that increase in the excited state quencher concentration gives rise to an increased width of the reflection band. The fact that intensity plays a very important role indicates the competition between the phase separation process which causes the increase as opposed to the polymerisation speed which tends to freeze in the structure and the pitch present within the system. Such a pitch gradient has been observed in polymerized acrylate networks (9). In the case of studies of other gels, the band broadening occurred as the system phase separated into two regions containing liquid crystal and the liquid crystal swollen network. 

The self-organizing nature of liquid crystal polymers is reflected in their complex flow behavior. The relaxation phenomenon of lyotropic polymer solution after shear cessation leads to band texture morphology that can be further induced to isotropic materials. Future research should focus on solvents influence on band size implicitly related to the induced pattern in polymers with different structures. Another aspect that could be explored is the imidization of patterned polyimide precursors and those conditions in which the texture is still maintained. 

Outside the reflection band, cholesteric liquid crystals exhibit an extremely strong optical rotatory power which can amount to values as large as 20.000° or 50 complete rotations per millimeter. The sign of the rotatory power is different below and above the reflection band. 

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. 

---