Nematic orientation

Figure 2 shows the increase in the rigidity (1 - f) of macromolecules induced by the field as a function of the parameter x = e/kT + Fl/kT. As soon as the flexibility decreases to f < 0.63, a system of molecules flexible in the state of rest will undergo a spontaneous transition into a nematic oriented state upon the action of the stretching field, just as it occurs for rigid molecules at rest. 

First, we will consider the design of polyphosphazenes as side-chain liquid-crystalline materials.241 248 Side-chain liquid-crystalline polymers are a subclass of species described earlier as structure 3.72. Liquid crystallinity occurs when the rigid side groups become organized, usually in the semi-liquid state. The organization may be nematic (oriented but unlayered) or smectic (layered) as illustrated in Figure 3.25. 

Supramolecular assemblies assume smectic, cholesteric or nematic orientations, whereby the planar axis may be perpendicular (smectic) or parallel (cholesteric) to the molecular axis. In a nematic orientation, the assemblies are not planar, although the molecular axes are parallel (Elias, 1979). 

In certain cases, nematic (orientational but not positional) order may be observed in phases of (relatively small) micellar rods (canonic) or disks (discotic) [177], which are associated with lower temperature hexagonal and lamellar phases, respectively [178]. (See Fig. 17.) The nematic isotropic phases were thought to be built up of discrete aggregates of different shapes, but the presence of continuous aggregates has also been recently suggested in this case [178]. Some systems (e.g., potassium laurate-decanol-water) form biaxial nematic phases. In this case, the micelles are believed to be neither rods nor disks but rather to... 

As discussed abfeve, ellipsometry is directly sensitive only to the interfacial variations of the nematic order parameter, which is connected to the optical refraction indices. The interfacial smectic order, which has no direct influence on the optical properties, can only be observed due to its coupling to the nematic (orientational) order. The same experimental setup as described in Sect. 4.1.3 has been used to study the interface between smectic liquid crystal dodecylcyanobiphenyl (12CB) in the isotropic phase and the silanated glass. Although only orientational order is observed, the temperature dependence of pb is in this case quite different from the case with the nematic liquid crystal, as evident from Fig. 4.4. 

The smectic layering transition [45] is a phenomenon, where smectic layers grow one by one, parallel to the surface, when the temperature approaches the isotropic-smectic phase transition from above. Ellipsometric observation of these steps is possible due to the coupling between the smectic order and the nematic (orientational) ordering. Consequently, the orientational order in... 

M. Monkade, M. Boix and G. Durand, Order electricity and oblique nematic orientation on rough solid surface, Europhys. Lett. 5(8), 697-702, (1988). doi 10.1209/0295-5075/5/8/006... 

We keep equality of nematic orientational order parameters in both phases 5n = Sp, and take only the amplitude of the second harmonic p2 of the density wave as an additional SmA order parameter... 

The orientation interaction is considered for light waves and liquid crystals with a uniform initial director orientation (such crystals include twisted nematics and cholesterics, and crystals with hybrid or hyperhybrid structures). Conditions are found for critical behavior to occur for a nonthreshold interaction. A light-induced Frederiks transition is predicted for cells with a hybrid nematic orientation, in which radiation absorption may reorient the director. 

This modulates the nematic orientation at spatial frequency A = 2Tt/q, producing a phase grating. Using the thin phase grating approximation (/ (V/) = / /4), the equilibrium diffraction efficiency has the form. ... 

CB, MBBA Clean or Sn02 covered glass + CDC Homeoplanar nematic orientation Kff- 3-5 X 10" [53]... 

In the current approach, the term 3 cos cj) — 1 is called the nematic orientation parameter. 

The parameter controlling the instabilities is N=XYo)/D where D is the diffusivity for the nematic orientation. The experiments are performed for increasing values of N keeping the ellipticity E =Xq / Y constant. At low shear rates, the director moves at frequency o) on an elliptic cone the axis of which is perpendicular to the plates but the system is homogeneous in a... 

At extremely high fields the isotropic phase should be indistinguishable from the nematic one, even well above the zero field transition temperature, since the uniaxial order induced by a magnetic or ac electric field in the isotropic phase will be comparable with the nematic orientational order. However, such fields are hardly accessible, even with the pulse technique. Much stronger changes in order parameter may be achieved with ferroelectric transitions (see below). 

The flexoelectric polarization Pf depends on the curvature of the director field (div n and curln) at constant modulus of the order parameter S. For uniform director orientation ( =constant), Pf=0. A simplified molecular picture of the phenomenon is shown in Fig. 15 a, b. Equation (39) defines the so-called ordoelectric polarization Pq, which depends on the gradient of the nematic orientational order parameter S (quadrupo-lar in nature) and does not vanish for a uniform director distribution. For instance. 

Figure 4. Diagram of a cell with domains in cross-section (1) Ultrasound, (2) system of oscillatory vortex flow with the velocity v =-cosqz, (3) stationary flows, (4) profile of distortion of the nematic orientation, (5) nematics molecule, (6) acoustically rigid boundary, (7) acoustically soft boundary.

In addition Pd(II) complexes of related ligands PCH and CCH were made. All complexes with PCH ligands show monotropic nematic phase while with the CCH ligands, CCH-3 (n = 3) is not liquid crystal, CCH-5 ( = 5) is monotropic nematic and CCH-7 ( = 7) is enantiotropic nematic. Orientational order of the dimer [PtCl2(5-CB)2] formed by linking of the two cyanobiphenyl via platinum dichloride bridge, dissolved in a nematic solvent was studied using deuterium NMR spectroscopy [13]. 

Fig. 2—Diffraction by domains. A nematic orientation that is originally planar usually results in a well-ordered domain structure that behaves like a phase grating, and a diffraction pattern consisting of a series of spots is obtained (top). When the orientation is originally perpendicular, clusters of domains may give rise to diffraction spots or rings depending on the relative diameter of the probe (w) and the dimensions of the domain clusters.

Fig. 9—Electro-optic transfer coefficient along surface orientation for twisted structure. With the source (S) in a plane perpendicular to the electrodes and parallel to the optic axis of the nematic orientation at that electrode, the optical transmission is a monotonic function of angle and voltage (calculated curves). ...

The textures of the brushes observed in the polarizing microscope show domains that are much larger than the film thickness. Even for brushes as thin as 38 nm one finds a liquid crystal texture with a lateral length scale on the order of micrometers (Fig. 18). Apparently, the film thickness is not the natural length scale of the lateral pattern of nematic orientation. 

External fields of force such as mechanical, hydrodynamic, magnetic and surface forces can enhance order and increase size and orientation of the domains. Fields of force can also induce a nematic ordering in non-mesogenic polymers, but this nematic orientation is metastable and disappears as soon as the orienting field is removed and the polymer allowed to relax (heating, anneal-ing). ... 

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