Nematic calamitic

Molecules of a disc-like shape form either nematic or columnar phases. Structurally the most simple mesophase is the discotic nematic phase (Nd). Just as in the case of the nematic phase formed by calamitic molecules, the molecules possess orientational order, but no positional order (Figure 2.8). The columnar nematic phase (Nc) consists of short columns of a few molecules that act like the rod-like molecules in the nematic calamitic phase (Figure 2.9). There is no organisation of the columns in a two-dimensional lattice. 

Figure 27b displays the results of flow reversals obtained at two different shear rates (y= 0.5 and y = 1 s ), where, to obey Eq.6, the stress has been divided by the stationary value, and the time replaced by the deformation. The data at 0.5 and 1 s are found to be superimposed, demonstrating the scaling of the stress responses. Scaling laws were also obtained with nematic calamitic and discotic phases of SDS/Dec, although, with more complex patterns [284,290,309]. It is important to realize that the mechanical responses in Fig. 27 are very different from those of the isotropic phases, for which the scaling with deformation or the dependence on the shear history have not been observed [138]. 

Fig. 28 (a) 3D-plot of the neutron scattering cross-section measured in the (qv,qvv)-vorticity plane for the SDS-Dec nematic calamitic phase. The SDS/Dec solution was prepared in D2O for contrast reasons at total concentration c = 29.5 wt. % and molar ratio [Dec]/[SDS] = 0.33 [317], The shear rate of the experiment was 7 = 50 s. (b) Iso-intensity contours obtained for the same conditions as in (a). Along the qv- and qvv-axis, the wave-vector ranges from —0.16 to +0.16 A . The dashed circles set the limits of the domain of integration for the data treatment (see text)... 

Fig. 31 (a) Transient behavior of the NMR splitting for an SDS/Dec nematic calamitic solution at concentration c = 29.5 wt. % and molar ratio [Dec]/[SDS] = 0.33 and sheared at 7= 0.32 s [284]. (b) Evolution of the orientation angle of the nematic director with respect to the magnetic field as function of the strain. The symbols correspond to the spectra in (a), and the continuous line is determined from Eq. 7, yielding an asymptotic angle of = 78 2°. Inset Orientation of the nematic director in presence of the flow and of the magnetic field... 

The constitutive hydrodynamic equations for uniaxial nematic calamitic and nematic discotic liquid crystals are identical. In comparison to nematic phases the hydro-dynamic theory of smectic phases and its experimental verification is by far less elaborated. Martin et al. [17] have developed a hydrodynamic theory (MPP theory) covering all smectic phases but only for small deformations of the director and the smectic layers, respectively. The theories of Schiller [18] and Leslie et al. [19, 20] for SmC-phases are direct continuations of the theory of Leslie and Ericksen for nematic phases. The Leslie theory is still valid in the case of deformations of the smectic layers and the director alignment whereas the theory of Schiller assumes undeformed layers. The discussion of smectic phases will be restricted to some flow phenomena observed in SmA, SmC, and SmC phases. 

Figare 12. Flow alignment of nematic discotic and a nematic calamitic liquid crystals. The director alignment is shown schematically by the alignment of one molecule. 

Referring to the nomenclature defined in Table 1.1, we shall encounter mainly one-comb, double-comb, parallel (and biparallel) and network LCPs that exhibit calamitic phases (such as smectic and nematic). Calamitic phases result from mesogens that are lath-shaped. We shall... 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

The mesophases of calamitic mesogens are classified in two groups nematic and smectic. The nematic mesophase (N) is characterized by an orientational order of the molecules that are aligned along a preferred direction (defined by a director n) (Figure 8.2). The molecules can slide and move in the nematic mesophase (while roughly keeping their molecular orientation) and rotate around their main axis. This is the less ordered mesophase and it is usually very fluid. 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

FIG. 2. Schematic representation of different calamitic and discotic thermotropic liquid crystals (a) nematic, (b) cholesteric, (c-e) smectic, (f) columnar hexagonal, (g) columnar hexagonal tilted a-e adapted from Demus, D., and Richter, L., Textures of Liquid Crystals, Verlag Chemie, Weinheim, Germany, 1978 f,g adapted from Eidenschenk, R., Flussige Kristalle, Chem. Unserer Zeit, 18, 168-176 (1978). 

Mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director n precesses around a single axis. 

Note 5 If the mesogenic side-groups are rod-like (calamitic) in nature, the resulting polymer may, depending upon its detailed structure, exhibit any of the common types of calamitic mesophases nematic, chiral nematic or smectic. Side-on fixed SGPLC, however, are predominantly nematic or chiral nematic in character. Similarly, disc-shaped side-groups tend to promote discotic nematic or columnar mesophases while amphiphilic side-chains tend to promote amphiphilic or lyotropic mesophases. 

Liquid crystals are materials that exist in a state that is intermediate between a liquid state and a solid. They are formed by anisotropic molecules, known as calamitic molecules, which are long and narrow, i.e. have a large length-to-breadth ratio. These rod shaped molecules orientate themselves in different ways as they change from the crystalline to the liquid state at different temperatures, existing in the smectic and nematic phases, as shown in Figure 5.1. 

Upon addition of a terminal crown ether to the calamitic core unit, nematic phases were observed for 2, 3 in agreement with previous studies on substituted cyano biphenyls [25, 26]. 

Chiral monophosphorus ligands, in hydrogenation, 10, 16 Chiral nematic phase, calamitic mesogens, 12, 200 Chiral N,P ligands, in hydrogenation, 10, 17 Chiral phosphines... 

Thermotropic liquid crystals come in two types calamitic and discotic. Calamitic phases (from the Greek for tube ) are all those that are caused by rod-like mesogens. The more recently characterised discotic phases are caused by disc-like species. Calamitic phases may be either nematic (from the Greek for thread ), smectic (from the Greek for soap ) or cholesteric (named after the cholesterol derivatives such as 13.4, which exhibit this behaviour). 

Figure 13.13 Schematic representation of calamitic liquid crystalline phases (a) nematic, (b) smectic A, (c) smectic B, (d) smectic C, (e) smectic E and (f) cholesteric (reproduced with permission from [7]).

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