Cholesteric-nematic phase transition

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. ... 

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]. 

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

T. Ohtsuka and M. Tsukamoto, AC Electric-Field-Induced Cholesteric-Nematic Phase Transition in Mixed Liquid Crystal Films, Jap. J. Appl. Rhys., 12, p. 22 (1973). 

We do not give a detailed classification of the types of liquid crystals, since the systems under consideration mainly form nematic or in some cases cholesteric phase. The latter phase belongs, in principle, to the same range of liquid crystals as the nematic phase, since there is no phase transition between them (unlike smectic-nematic phase transition). 

The handedness inversion behaviors upon UV irradiation were observed in both wedge cells and homeotropic cells with the cholesteric (N )—nematic (N)— cholesteric (N ) phase transition sequence as confirmed by the Cano s lines and fingerprint texmres (Fig. 5.12). The mechanism of the helix inversion is proposed... 

Chiral dithienylcyclopentene compounds can also be used to induce the cholesteric-isotropic phase transition [36]. A cholesteric polygonal fingerprint texture was exhibited by 10 wt% of 2 as a mesogenic dopant in a conventional achiral nematic 5CB as shown in Figure 5.6a. The cholesteric phase to isotropic transition temperature for the doped 5CB was 42 C. With UV irradiation at 310 nm (30 mW cm ) for 30 s, the sample went into the isotropic phase (Fig. 5.6b) whereas upon visible fight irradiation at 670 nm a reverse process was reached within 30 min (Fig. 5.6c). 

The expression (6.3) for Ecn was calculated for infinitely thick films without taking into account the boundary conditions. However, the cholesteric to nematic phase transition was investigated for different thickness [67], [68]. The influence of the surface orientation was taken into account [68] by introducing a surface free energy per unit area F which leads to the following expression for Vcn -... 

G. Durand, L. Leger, F. Rondelez, and M. Veyssie, "Magnetically Induced Cholesteric-to-Nematic Phase Transition in Liquid Crystals, Phys. Rev. Lett., 22, p. 227 (1969). 

The electric-field-induced cholesteric-to-nematic phase transition was observed by Wysocki et al. The magnetic analog had been previously measured by Sackmann et al, and the theoretical magnetic and electric field dependence has been calculated by deGennes and Meyer." ... 

In the cholesteric-nematic phase change with L/Pq > 1, the wave-vector is given by tt/Pq not tt/L. The experimental rise and decay times are consistent with the theory for the field-induced phase transition. ... 

The cholesteric-to-nematic phase transition effects have also been utilized in matrix-addressed displays. Up to 28 lines have been scanned in the V V/S mode with a bias voltage of 35 Vrms and a contrast ratio of 15 1. The relatively large multiplexing capability is due to the long decay time produced by the bias voltage. ... 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

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