Transition cholesteric-nematic

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. 

Fig. 32a, b. Volt-contrast (a) and current-voltage (b) characteristics for polyimide modulator with cholesteric-nematic phase transition without (/) and with switch (2) on the recording light [254]... 

Cholesteric liquid crystals show many electro-optic effects. Among them the cholesteric-nematic phase transition effect is the most interesting one which was addressed in the early part of this chapter. Others are the square grid effect, storage effects and color effects, etc. 

Square grid effect Before the electric field becomes great enough to cause the cholesteric-nematic phase transition, a periodic deformation may appear in cholesteric liquid crystals. The layer undulation occurs in two orthogonal directions so that a square pattern is observed. This effect is more likely to happen for cholesteric liquid crystals of large pitch (about microns). 

Cholesteric-nematic phase transition change from negative uniaxiality to positive uniaxiality The electric field is parallel to the helix axis. ... 

It is interesting to note that this threshold field is lower than that for a conical distortion or that for cholesteric-nematic (unwinding) transition. For a conical distortion, the theory is closely similar to that discussed in 3.4.2 and has been treated by Leslie the critical field is given by... 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

It is known that the trans isomer of azobenzene has elongated rod shape that is favorable for the stabilization of the LC phases, while the cis isomer in bent form is unfavorable and tends to destabilize the LC phases. The trans-cis photoisomerization will decrease the order parameters, if significantly enough, which could lead to the destruction of the ordered LC phase and formation of isotropic state. The reverse transition can be achieved with either visible light irradiation or thermal relaxation as a result of cis-trans isomerization. This principle was initially used to induce the phase transition from nematic to isotropic in azobenzene LCs [39]. Cholesteric mesophase is intrinsically similar to nematic with additional helical arrangement of nematic layers, thus the photoisomerization of azobenzenes can also bring out the phase transition from the cholesteric state to the photoin-duced isotropic (PHI) state. 

C. Mioskowski, J. Bourguignon, S. Candau, G. Solladie, Photochemically induced cholesteric-nematic transition in liquid-crystals. Chem. Phys. Lett. 38, 456-459 (1976)... 

In both the cases considered, an optical contrast of the patterns observed in isotropic liquids is very small. Certainly, the anisotropy of Uquid crystals brings new features in. For instance, the anisotropy of (helectric or diamagnetic susceptibility causes the Fredericks transition in nematics and wave like instabilities in cholesterics (see next Section), and the flexoelectric polarizaticm results in the field-controllable domain patterns. In turn, the anisotropy of electric conductivity is responsible for instability in the form of rolls to be discussed below. All these instabilities are not observed in the isotropic liquids and have an electric field threshold controlled by the corresponding parameters of anisotropy. In addition, due to the optical anisotropy, the contrast of the patterns that are driven by isotropic mechanisms , i.e. only indirectly dependent on anisotropy parameters, increases dramatically. Thanks to this, one can easily study specific features and mechanisms of different instability modes, both isotropic and anisotropic. The characteristic pattern formation is a special branch of physics dealing with a nonlinear response of dissipative media to external fields, and liquid crystals are suitable model objects for investigation of the relevant phenomena [39]. 

De Gennes-Meyer Model for Field Induced Cholesteric-Nematic Transition... 

The racemic axially chiral bicyclic ketone 4 reported by Schuster and Suarez underwent partial photoresolution with /-CPL irradiation and a PSS was achieved in 6.7 h with 0.4% ee (Fig. 5.8), which is in a good agreement with the calculated ee value from the anisotropy factor (g = 0.0105 at 305 nm) [38]. However, when this compound was employed as a dopant, the enantiomeric enrichment was not sufficient to induce the phase transition from nematic to cholesteric state probably due to the low HTP. The Schuster s group further designed and synthesized a series of these... 

With an increasing external field a series of the field-induced phase transitions BP I — cholesteric, BP II cholesteric, and then cholesteric —> nematic are observed. This is illustrated by Fig. 6.27 [91] where the voltage-temperature phase diagram is presented for a mixture (47-53 mol.%) of... 

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. 

In CLCs with positive dielectric anisotropy, an electric field-induced cholesteric-nematic phase transition was theoretically predicted [45], [46] and experimentally observed [47], [48]. If the electric field E is applied perpendicular to the helix axis hot a CLC, the helix unwinds like in a magnetic field (Chapter 2). At sufficiently high field strengths, the homeotropic nematic structure is stabilized (Figure 6.3). The critical field strength E = Ecn depends on the pitch P, the dielectric anisotropy As, and the twist elastic constant K22 ... 

Figure 6.3. Schematic representation of the cholesteric-nematic phase transition.

The bistability discovered in [61] was also used for practical applications. The strain texture, which possesses the effect of storage was applied in image converters [73]. A color projection display has been developed based on the bistability of the cholesteric-nematic phase transition [74], [75]. 

When electric fields are applied to liquid crystals, the molecules tend to align—either parallel to the field (for Sa > 0) or perpendicular (for < 0). For the case of nematics, which already have a preferred direction, the director is simply reoriented without breaking the symmetry. However, the helical phase has two nonequivalent directions the twist axis, and the director, which rotates spatially about the twist axis. If > 0, such a helical director is clearly incompatible with a uniform field. For this case, an increasing field first distorts the helix, then stretches out the pitch, and finally causes the well-known cholesteric-nematic transition [1], If <0, the helical director is only compatible with a uniform field if the twist axis and field are parallel. 

We made C15 8CB cholesteric nematic mixtures where the concentration, Cqo, of C15 in 8CB lowered the cholesteric-isotropic transition temperature by Tch (°C) = 40.36 0.22cqo (%). The pitch decreased with Coo as qo... 

Cholesteric-Nematic Phase Transition (Hehcal Unwinding)... 

Field-induced cholesteric - nematic transitions were first observed by Sackmann et al [26]. in a magnetic field and by Wysocki et al. [29] in an electric field. An exact theory of the field-induced unwinding of a helical structure was first given by Meyer [30] and by de Gennes [31]. The first quantitative check of these theories by Bassler and Labes [32] showed good agreement between theoretical and experimental results. 

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