Structure of the chiral nematic phase

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]... 

The laterally appended dendrimer, 32, shown in Fig. 27, as expected exhibits a chiral nematic phase, with smectic mesophase formation being suppressed. The clearing point is almost 50 °C lower, whereas the melting point is only 25 °C lower in comparison to the terminally appended system. This demonstrates that lateral appendages of the mesogens causes disruption to the intermolecular packing, thereby destabilizing mesophase formation. The local structure of the chiral nematic phase is thus shown in Fig. 28. 

Fig. 44 Schematic representation of the local nematic structure and the helical structure of the chiral nematic phase of dendrimer R...

Figure 6.1. The structure of the chiral nematic phase. The views represent imaginary slices through the stracture and do not imply any type of layered stracture.

When the helical structure of the chiral nematic phase is unwound by the influence of limiting walls, we can observe a linear-in-field light modulation which is caused by a small molecular tilt [85]. The effect is analogous to the electroclinic effect observed in the smectic A phase as the pretransitional phenomenon in the vicinity of the transition. 

Figure 12. Helical structure of the chiral nematic phase.

Fig. 14, then it is found that the number of atoms (n) that the chiral center is removed from the rigid central core determines the handedness of the helical structure of the chiral nematic phase. As the atom count by which the chiral center is removed from the core (n) switches from odd (o) to even (e) (parity), so the handedness of the helix alternates from left to right or vice versa. Sim-... 

Consider first a uniaxial phase that is composed of chiral rod-like molecules. In the simplest situation, a helix can form in a direction perpendicular to the long axis of an object molecule. This example is analogous to the structure of the chiral nematic phase. In the direction parallel to the long axes of the molecules no twist can be effected. Now consider a similar situation, but this time the twist in the orientational order can occur in more than one direction in the plane perpen-... 

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.

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... 

Fig. 5. Helical Structures of the chiral nematic and chiral smectic phases...

Under just the right conditions, a mixture of a highly polar liquid, a slightly polar liquid, and an amphiphilic molecule form micelles that are not spherical. They can be rodlike, disc-like, or biaxial (all three axes of the micelles are different). These anisotropic micelles sometimes order in the solvent just as liquid crystal molecules order in thermotropic phases. There is a nematic phase of rod-shaped micelles, another nematic phase of disc-shaped micelles, and even a biaxial nematic phase, in which the molecular axes transverse to the long molecular axis partially order. Chiral versions of these phases with the same structure as the chiral nematic phase also form. 

As noted earlier, when plane-polarized light traverses the helical structure of a chiral nematic phase, its plane is rotated in the... 

At a normal chiral nematic to smectic A transition, the helical ordering of the chiral nematic phase collapses to give the layered structure of the smectic A phase. However, for a transition mediated by a TGB phase, there is a competition between the need for the molecules to form a helical structure due to their chiral packing requirements and the need for the phase to form a layered structure. Consequently, the molecules relieve this frustration by trying to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the chiral nematic phase), yet at the same time they also try to form a lamellar structure, as shown in Fig. 21. These two... 

Here, ry is the separation between the molecules resolved along the helix axis and is the angle between an appropriate molecular axis in the two chiral molecules. For this system the C axis closest to the symmetry axes of the constituent Gay-Berne molecules is used. In the chiral nematic phase G2(r ) is periodic with a periodicity equal to half the pitch of the helix. For this system, like that with a point chiral centre, the pitch of the helix is approximately twice the dimensions of the simulation box. This clearly shows the influence of the periodic boundary conditions on the structure of the phase formed [74]. As we would expect simulations using the atropisomer with the opposite helicity simply reverses the sense of the helix. 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

The development of storage and loss moduli after flow cessation is a useful tool to analyze structural relaxations on LCPs. Upon flow cessation, the flow-induced orientation is lost. The evolution of the moduli with time is because of the reformation of a chiral nematic phase that had become nematic under flow. In Fig. 10, the relaxation in complex modulus with time for various AEC samples is shown. 

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