Cholesteric dynamic scattering

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... 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

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. 

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

Nakano and Hirata found that the cisoid/transoid ratio of 1 increased from ca. 0.3 to 0.7 with an increase of [ACN] from ca. 7 x 10 to 7 x 10 Min cholesteric LC (cholesteryl oleyl carbonate, cholesteryl hnolate, and cholesteryl oleate), whereas nematic LC or benzene as the reaction medium did not affect the ratio. These results suggest that the cholesteric LC structure makes the ACN molecules align in a phase-to-phase manner. For dimerization of ACN in the nematic phase, Oh-hashi et al. reported briefly the effect of electric fields on the cisoid/transoid ratio. Upon irradiation (X, > 300 nm) of ACN in nematic LC (4-hexyloxyphenyl 4 -butylbenzoate, LCl), application of an electric field up to 5 kV cm decreased the cisoid/transoid ratio from 0.33 to 0.15. They attributed this effect to the preferred formation of the Ti state of ACN by molecular stirring under dynamic scattering of the nematic phase. 

While the first paper on liquid crystalline elastomers [30] already reports the detection of a cholesteric-isotropic transition using differential calorimetry and polarizing microscopy, comparatively little work has been done to characterize thy physical properties in the vicinity of this phase transition (compare, however, also the discussion of electromechanical effects in the next section) [9, 30, 31]. Combined liquid crystalline elastomers have been synthesized and various of these materials show a cholesteric-isotropic transition using X-ray scattering, polarizing microscopy and differential scanning calorimetry [31]. Dynamic mechanical investigations have been carried... 

Despite all the drama at the cellular interface when e < 3.5 (Figure 15.2), q is linear in e q/qo = 0.19fi -I- 0.29, independent of the dynamics. On the other hand, Vx has two responses. When <2, Vx/Vei = 0.32e + 0.3 the cholesteric texture left by the traveling phase boundary is unstructured and there are no director relaxation processes observed when v 0. When e> 2, xhei = 0.60e + 0.03 Ex increases twice as fast as when < 2. The texture left behind the traveling interface is a uniform array of line defects (one line per cell) that slowly disappears by defect coarsening when y 0 [1]. We note that taking into account the scatter in the data, the intercept of Vx s e > 2 line is zero and that it intersects its < 2 line at 1 where the breathing mode starts. 

C. C. Yang, Light Scattering Study of the Dynamical Behavior of Ordering Just Above the Phase Transition to a Cholesteric Liquid Crystal, Phys. Rev. Lett., 28, p. 955 (1972). 

Fig. 6.24 The electro-thenno-optic effect of nematic-cholesteric LC mixture MBBAiEBBA CN = 45 45 10. The material here is same with the thermo-optic effect [90]. The transmission variations are enhanced by the electric field application. The scattering centers in the nematic phase are turned to a clear state with the electric field application and then the irradiated portion becomes clear. Thus, the dynamic figures can be displayed...

Fig. 6.25 The electro-thermo-optic effect of smectic-cholesteric mixture COB CN = 90 10. The light transmission is greatly decreased by the electric field application [91]. The light scattering centers are created in the smectic phase by the irradiation on to a part of the LC cell, and they are erased by the electric field application and become clear. This characteristic can be applied to a dynamic display. M mesophase, I isotropic phase...

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