Birefringence chiral nematics

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

The major difference between the configuration of the OMI sandwich cell and other STN-LCDs is that the optical path difference (5 = And 1 pm) is much lower. There is no requirement for a significant pretilt (0 < 0 < 5°), the twist angle of the chiral nematic layer is lower (180°), the front polariser is parallel to the nematic director (a = 0°) and the polariser and analyser are crossed (P = 90°). The 180° twist gives rise to strong interference between the two elliptically polarised rays. If the optical path difference is small, e.g. 0.4 m, a bright, white, non-dispersive off-state is produced. The chiral nematic mixture should be of positive dielectric anisotropy, low birefringence and exhibit a low cell gap to pitch ratio dip 0.3). 

POM shows that solid films cast from 10 to 12% wtHPC are isotropic (Fig. 15.2a) (dark field under crossed polars). In contrast, solid films cast from a chiral nematic solution of HPC (60% HPC/40% DMAc, w/w) are clearly birefringent (anisotropic) and present a characteristic banded texture perpendicular to the shear direction, observed for thermotropic and lyotropic liquid crystalline phases of HPC after shear. 

S. Kang, Y. Saito, N. Watanabe, M. Tokita, Y. Takanishi, H. Takezoe, J. Watanabe, Low-birefringent, chiral banana phase below calamitic nematic and/or smectic C phases in oxadiazole derivatives, J. Phys. Chem. B 110 (2006) 5205. 

The effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

Light valves were first produced on the basis of the classical semiconductors, ZnS, CdS, ZnSe, CdTe, and GaAs, in contact with nematic or chiral nematic liquid crystal [18]. The basic effects in liquid crystals included electrically controlled birefringence, dynamic scattering, and the cholesteric-nematic phase transition with the frequency response limited to a few Hertz. 

The assembly of the chiral nematic structure happens in two steps. As the solvent evaporates from the suspension, the critical concentration of the lyotropic liquid crystal phase is reached. The CNCs form discrete localised clusters of chiral nematic phase, called tactoids, as predicted by the phase separation of liquid crystals. These tactoids can be observed between cross-polarisers under a microscope. The first repotted image of tactoids in a cellulose suspension is reproduced in Figure 17.10. The tactoids are visible from the birefringent stripes, which represent axes of crystallite arrangement parallel to the stripes. ... 

The dramatic variation of liquid crystalline properties with respect to temperature has resulted in the widespread use of cholesteric (chiral nematic) liquid crystals for thermography. The property that has been exploited most in liquid crystal thermography is the critical temperature dependence of the selective reflection from cholesteric liquid crystals, though other temperature dependent properties of mesophases have been utilized (e.g. the birefringence of nematic systems and selective reflection from other chiral phases). The helicoidal structure of cholesteric materials results in the selective reflection of visible light within a band of wavelengths of width AX, centered at a wavelength Xq, such that ... 

Figure 1. The temperature dependence of the birefringence of several commercial chiral nematic materials (redrawn from [2]). The symbols refer to mixtures of CB15 in different nematic hosts (all material names are as defined by Merck Ltd.) A is 37.7 wt% CB15+ 62.3 wt% ZLI1695 (a cyclohexylcyclohexane material based mixture) B is 39 wt% CB15 + 60.3 wt% ZLIl 132 (a phenylcyclohexane material based mixture) C is 49.2 wt% CB15 in E63 (a biphenyl material based mixture) D is a mixture of two short chain Osman esters and a chiral Demus esters (see [2] for exact composition).

As described earlier, the spectacular optical properties of chiral nematics are determined by the helicoidal pitch, the birefringence (and refractive index), the direction and polarization of the incident light, and the arrangement of the helix axis. For normally incident light the direction of the helix axis gives rise to the three classical textures depicted in Fig. 2, and typical photomicrographs taken with crossed polarizers are shown in Fig. 6 these are ... 

As the temperature is decreased the chiral nematic structure transforms to a higher order phase. The phase may go through a first order phase transition and crystallize in which case the optical properties are of little interest herein. It may transform to a glass, in which case the optical properties, such as birefringence, pitch, etc., are frozen and may be used in static, or time and environment-independent devices or applications (as discussed in Sec. 2.5 of this Chapter), or it may go through a second order or second order plus a weak first order phase transition to a higher order liquid crystalline phase. Here, for simplicity, we are not considering the so-called re-entrant phases [ 14]... 

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