Layers chiral nematics

In this analysis it must be emphasized that the TGB phase, is not simply a layered chiral nematic phase, and should not be confused with this concept. A layered chiral nematic phase simply cannot exist on a macroscopic scale, and it is a requirement that defects must be formed. 

Fig. 7. The chiral nematic Hquid crystal stmcture. The director (arrow) traces out a heHcal path within the medium. Siace the rotation of the director is continuous, the figure does not mean to imply the existence of layers perpendicular to the heHcal axis.

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Note 3 Two TGBA structures are possible in one, the number of blocks corresponding to a rotation of the layer normal by 2ti is an integer, while in the other, it is a non-integer. Note 4 A TGBA is found in a phase diagram between smectic A and chiral nematic mesophases or between a smectic A mesophase and an isotropic phase. 

The chiral nematic assembly is formed by materials that have chiral centers and that form a nematic phase. Here, a chiral-imposed twist is imparted to the linear chains composing each layer, resulting in a three-dimensional helical arrangement. 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

A cholesteric, or chiral nematic (N ) phase. This is a positionally disordered fluid in which the constituent molecules align on average their axes along a common direction called the nematic director. Being the DNA helices chiral, the orientational order develops an additional macro-helical superstructure with the twist axis perpendicular to the local director. The phase thus consists of local nematic layers continuously twisted with respect to each other, with periodicity p/2 (where p is the cholesteric pitch see Fig. 8a) [27,28]. For 150-bp helices, the N phase appears at a concentration around 150 mg/mL in 100 mM monovalent salt conditions. This LC phase is easily observed in polarized optical microscopy. Since the N pitch extends to tens of micrometers (that is, across... 

Fig. 8 (a) Schematic representation of the structure of the chiral nematic phase of DNA, showing continuously twisting nematic layers, giving rise to a p/2 periodicity easily observable in the side view on the left, (b) N droplets observed in polarized microscopy. The bright and dark stripes correspond to p/2 (size bar is 10 pm). Adapted with permission from [27]... 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

Figure 2.3 Schematic representation of the periodical helical structures of the chiral nematic (cholesteric) phase. The pitch of the helix corresponds to the rotation of the director through 360°. There is no layered structure in a chiral nematic. N. phase.

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). 

The electrode surfaces of a normal LCD sandwich cell d 8-10 pm) are coated with an alignment layer in order to induce a planar alignment of a host (chiral) nematic mixture containing the dichroic dye of positive dichroism and a chiral dopant. Due to the absence of polarisers a very thin mirror can be incorporated within the cell on top of the rear glass plate electrode in direct contact with the guest-host mixture, see Figure 3.15. 

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

As this compound was one of the higher homologues in the series, and because we knew that the earlier homologues did not exhibit a chiral nematic phase, it was clear that the new phase also could not be a chiral nematic phase. In addition, it was clear from the formation of the defect structures seen in the microscope that the phase first formed from the isotropic liquid possessed a helix, see Plate 1, which had its heli-axis at right angles to the heli-axis in the lower temperature chiral ferroelectric smectic phase. This simple observation meant that if the phase was a lamellar smectic phase then the helix would have to be formed, inconceivably, in a direction parallel to the layers. Synthesis of the achiral variant confirmed that the phase formed first on cooling from the isotropic liquid was indeed a smectic A phase, and thus we immediately knew that we had found a smectic A phase where the helical macro structure formed in the planes of the layers, and thus the helix must... 

Plate 4. An oblique fracture through the TGBA phase. Here in addition to the gentle undulating fracture surface, which is similar to that of a chiral nematic phase, discrete layering is visible (shown by the arrows)... 

Looking back, the earliest written report concerning the possibility of having a smectic phase with twisted layers comes from the work of Wolfgang Ullrich Muller at the Technischen Universitat Berlin in 1974 [40], In his thesis entitled Verhalten cholesterischer Mesophasen unter dem Einfluss von Phasenum-wandlungen , Muller examined the mesophase behavior at the chiral nematic to smectic transition for various mixtures of cholesteryl oleoyl carbonate (COG), 5, and cholesteryl chloride (CC), 6. Muller came to the conclusion from his work that the smectic phase must have a helical structure and that the heli-axis must lie in the plane of the layers, and the structure must also have... 

Van de Witte P, Brehmer M, Lub J. 1999. LCD components obtained by patterning of chiral nematic polymer layers. J Mater Chem 9(9) 2087 2094. 

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