Liquid crystals, anisotropy smectic

The second issue concerns the anisotropy of the membrane. The models presented in this section all assume that the membrane has the symmetry of a chiral smectic-C liquid crystal, so that the only anisotropy in the membrane plane comes from the direction of the molecular tilt. With this assumption, the membrane has a twofold rotational symmetry about an axis in the membrane plane, perpendicular to the tilt direction. It is possible that a membrane... 

It is also possible that a membrane might have an even lower symmetry than a chiral smectic-C liquid crystal in particular, it might lose the twofold rotational symmetry. This would occur if the molecular tilt defines one orientation in the membrane plane and the direction of one-dimensional chains defines another orientation. In that case, the free energy would take a form similar to Eq. (5) but with additional elastic constants favoring curvature. The argument for tubule formation presented above would still apply, but it would become more mathematically complex because of the extra elastic constants. As an approximation, we can suppose that there is one principal direction of elastic anisotropy, with some slight perturbations about the ideal twofold symmetry. In that approximation, we can use the results presented above, with 4) representing the orientation of the principal elastic anisotropy. 

The Tfj-symmetrical hexaaddition pattern also represents an attractive core tecton for dendrimer chemistry [26, 31, 63-67]. Examples for such dendrimers, involving a core branching multiplicity of 12, are 38 and 39 [63, 64], Addition of six mesotropic cyanobiphenyl malonate addends produced the spherical thermotropic liquid crystal 40 [65], DSC and POM investigations revealed a smectic A phase between 80 and 133 °C. Interestingly, this spherical and highly symmetrical compound gives rise to liquid crystallinity despite the absence of molecular anisotropy. 

Just because a molecule is long, narrow, and meets the requirement of geometric anisotropy docs not ensure that it will have a liquid crystal phase. The particular phase structure that occurs in a compound, i.e.. smectic, nematic, or chiral nematic, not only dc >cnds on the molecular shape hut is intimately connected with the strength and position of the polar or polarizable groups within the molecule, the overall polarizability of the molecule, and the presence ol chiral centers. 

Various other instances of hydrodynamic and electrohydrodynamic instabilities in nematic and, to a lesser extent, smectic liquid crystals have been investigated. No attempt is made here to review this work. For the present discussion, it is sufficient to note that (a) most of the work has dealt with oriented layers having anisotropic properties, and (b) some interesting instabilities arise in oriented layers which do not occur for isotropic materials. An example of the latter is cellular convection in a fluid layer confined between horizontal plates maintained at different temperatures. With an isotropic fluid, convection can arise only if the lower plate is hotter than the upper plate. Then, fluid near the lower plate is less dense and tends to rise while fluid near the upper plate is denser and tends to sink. With an oriented layer, however, convection can arise even when the upper plate is hotter if the anisotropy of thermal conduction properties is of a particular type (8). 

Blinov, L. M., Barberi, R., Kozlovsky, M. V., Lazarev, V. V., and de Santo, M. P. Optical anisotropy and four possible orientations of a nematic liquid crystal on the same film of a photochromic chiral smectic polymer. / Nonlinear Opt. Phys. Mat. 9, 1 (2000). 

Smectic liquid crystal(S) One -dimensional long-range order, High viscosity, Optically positive uni/bi-exiality Optical axis rotation change in the molecular axis by the anisotropy of the dielectric contants dielectric constants i< electric conductivity [Pg.168]

Due to a pronounced optical anisotropy of ordered liquid crystals, reflection elhpsometry is a powerful experimental tool for the study of surface-modified liquid crystalline order. The sensitivity of the technique enables quantitative measurements of the nematic order parameter profile at the interfaces. Prom observation of the nematic wetting of the solid-liquid crystal interface, we have determined the values of the coupling energies between the surface and the nematic order. Even the pretransitional effects at solid-smectic liquid crystals can be studied with elhpsometry due to the strong coupling between the nematic and the smectic order. The time evolution of the complex structure of the liquid crystal adsorbate on a solid substi ate has also been successfully monitored using BAE. 

This section will concentrate on rod-like complexes which typically form nematic and smectic phases. While the approach does not concentrate on design aspects, it may become apparent that in rod-like systems, design is not a simple matter and there are many factors to be taken into account to do with anisotropy, disposition of ligands and functional groups associated with the metal and introduced on complexation. Thus, while it has been said (albeit with tongue slightly in cheek) that for discotic systems it is possible to realize liquid crystal systems by taking a... 

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

CD samples must be isotropic, at least in the direction perpendicular to the light path, since linear dichroism (LD, birefringence) causes large distortion of CD spectra. Therefore, samples of solid dispersions, membranes, films, gels, and liquid crystals should be prepared with great care to minimize anisotropy. Furthermore, the measurement of nonsolution samples should be repeated several times after successive rotation of the cell around the light beam axis to check the independency from linear dichroic interference. Some reliable studies show LD data with CD curve in order to examine artifacts. The linear dichroic interference may be too large for solid-state samples, and hence their measurement may require specialized instruments. Chiral liquid crystal (nematic or smectic C phase) exhibits a characteristic circular dichroic phenomenon irrelevant to... 

The texture transition can also be observed for smectic A liquid crystals with negative dielectric anisotropy [112]. In that case, the transition from a homeotropic into a planar texture occurs. The threshold of this, dielectric transition, can be modified (lowered) at the low frequencies of an applied field by the anisotropy of the electrical conductivity of a substance. 

Extensive measurements of the Prederiks transition in various smectic C liquid crystals have been carried out by the Halle group [117, 118]. In experiments, the conventional sandwich cells with optically transparent electrodes were used. The director was oriented uniaxially by rubbing the electrodes and the smectic layers were tilted with respect to the cell plane (a bookshelf geometry). An electric field applied along the cell normal ( -direction), due to positive dielectric anisotropy, induces the director rotation around the normal to the smectic layers. Fig. 6.32. Two optical effects are observed ... 

If the amplitude of the complex order parameter ij/ is constant, (10.8) describes changes of the free energy which are due to compression or dilation of the layers (i.e., deviations of Q from Qo ) or due to phase shifts of ij/, i.e., displacements of the smectic layers. In contrast to the gradient term appearing in the Ginzburg-Landau ansatz [44], two coefficients, C and C , appear because of the anisotropy of the liquid crystal. In a one-constant approximation, C = C = C, (10.8) is reduced to... 

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