Cholesteric-to-nematic transition

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.  

The phase transition, which is illustrated in Fig. 7, only occurs in an electric field with a cholesteric fluid of positive dielectric anisotro- 

Meyer performed optical measurements of the magnetically induced pitch dilation. By mixing nematic and cholesteric materials, Durand et al were able to vary the pitch and to verify that the threshold magnetic field was inversely proportional to the undeformed helix pitch. 

In the original experiment of Wysocki et al, the threshold field was about 10 V/cm. Heilmeier and Goldmacher reduced the threshold field to 2 X 10 V/cm by using a mixture of cholesteric and high positive dielectric anisotropy nematic materials. More recently, the threshold field has been reduced to 5 X 10 V/cm by use of the positive biphenyl compounds. 

The model presented so far is accurate, but incomplete. In actuality, the scattering texture with the helical axes parallel to the cell walls is not a stable state without an applied field. Rather, it is a metastable state that has a lifetime of from minutes to months, depending upon the surface alignment, fluid thickness, and pitch. The stable state is the planar or Grandjean texture. Kahn made the initial measurements of the various steps in the texture changes. Additional experiments with both electric and magnetic fields have added more detail to the model. 

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Thin films of the solutions between microscope slides were sheared by applying even pressure on a coverslip while sliding it approximately one cm. The anisotropy appeared to increase as measured by increases in the birefiingence. Solutions containing 10-16% (w/w) cellulose developed a threaded texture and the mesophases were stable with time and oriented in the direction of shear. These observations, while not definitive, suggested a cholesteric to nematic transition occurred on shearing. 

Grainy textures were obtained with 22% (w/w) and higher solution concentrations, but no clear evidence of a cholesteric to nematic transition was observed. 

The elastic coefficient K22 could be measured according to (2.34) either from the threshold of the twist distortion of the homogeneous alignment induced by a magnetic field, or from the threshold of the initially twisted director alignment [58]. It is also possible to measure the unwinding voltage t unw of the cholesteric to nematic transition... 

In this context, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

An electric field induced cholesteric-to-nematic transition is demonstrated in Fig. 4 for a partially compensated cholesteric mixture of cholesteryl chloride and cholesteryl nonanoate. The sample has been sandwiched between two glass plates in such a way that the helix axis is parallel to the glass surface. The distance between two adjacent dark (or bright) lines is therefore a measure for the pitch fo the cholesteric phase. It is clearly seen that the pitch increases with increasing field strength. At a critical field strength of = 10,000 V/cm the sample has become nematic with the director oriented parallel to the electric field. 

Figure 4. Electric field-induced cholesteric-to-nematic transition of a 1.8 1 by weight mixture of cholesteryl chloride and cholesteryl nonanoate. Viewed through crossed polarizers (magnification X 200). The helix axis was parallel and the electric field was perpendicular to the glass plates (separated by 20 jum). The mixture was nematic 5°C below the temperature of observation which was T = 40° C. a) o V/cm b) 3000 V/cm c) 6000 V/cm d) 10,000 V/cm.

Greubel has recently analyzed the cholesteric-to-nematic transition for an applied voltage. With perpendicular alignment of the liquid crystal molecules at the cell surfaces, he found that the field transition for the cholesteric-to-nematic transition was higher than the opposite nematic-to-cholesteric transition. With one mixture, the ratio of field thresholds was approximately 2.5. This work points out the importance of the proper surface orientation and cell cleanliness in achieving the best bistability. 

White and Taylor have described a new device that combines both the guest-host effect and the cholesteric-to-nematic transition. The pleochroic dye is added to a cholesteric material and the cell transmission changes when the cholesteric undergoes the cholesteric-to-nematic transition. Because of the rotational symmetry of the long axes of the dye molecules about the cholesteric helical axes, contrast ratios of greater than 4 to 1 were obtained without the use of a polarizer. 

In order to study the mechanism of the transition of cholesteric to nematic structure, several physical properties were measured as a function of temperature from room temperature through this transition region. 

Cholesteric- (a) nematic transition + - HT + Yes No Spiral structure transforms to homeo-tropic. Smooth TVC with hysteresis [151]... 

Figures 3 a, b, c show the temperature dependences of viscosity for the solutions under study. The above dependences are described by curves with well-pronounced sharp maxima. This behavior is typical of the solutions with LC transitions (Kulichikhin Golova, 1985, Vshivkov Rusinova, 2008, Gray, 1962). According to Gray (1962), this profile of the temperature dependences of viscosity corresponds to the (isotropic liquid)-(nematic liquid crystal) phase transition. Therefore, upon cooling of HPC, CEC and PBG solutions under deformation conditions, no cholesteric crystals are formed in other words, under dynamic conditions, a liquid crystal changes its type from cholesteric to nematic. The results obtained are in good agreement with the data of other authors (Volkova et al., 1986), who showed that the shear deformation of CEC solutions (c= 30%) in trifluoroacetic acid and a 2 1...

The interpretation of the double-layer structure of the electrospxm APC fibers is based on the fact that APC solutions subjected to shear show a transition from cholesteric to nematic [104]. Calculation of the Reynolds number shows that the flow inside the needle is laminar. The velocity profile is parabolic and, since the solution becomes nematic, the director should align parallel to walls in the center of the flow, away from the walls of needle, there is some reorientation due to the flow and the director makes an angle with the flow direction. As the solution exits the needle, a rapid evaporation of the solvent freezes the orientation of the director in the different areas of the cross-section of the fiber. 

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 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 introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

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. 

In cholesterics, the structure is similar to nematics, but the director rotates in a corkscrewlike fashion along n. Electric-field-induced transitions between the cholesteric and nematic phases are used in the dye phase change display discussed below. 

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. 

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