Smectic form crystals

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. 

Tbe purpose of tbe bydroxyl group is to acbieve some hydrogen bonding with the nearby carbonyl group and therefore hinder the motion of the chiral center. Another way to achieve the chiral smectic Cphase is to add a chiral dopant to a smectic Chquid crystal. In order to achieve a material with fast switching times, a chiral compound with high spontaneous polarization is sometimes added to a mixture of low viscosity achiral smectic C compounds. These dopants sometimes possess Hquid crystal phases in pure form and sometimes do not. 

Brosse et al. [41] modified isotactic polypropylene and other polyolefins by a cold plasma. In isotactic polypropylene, plasma treatment results in a polypropylene crystallization of paracrystalline or smectic form into a a-crystalline form. Further, the active films are susceptible to react with monomers in a postgrafting reaction. 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

Smectic. Smectic liquid crystals are distinguished from nematics by Ihe presencc ol some positional order (a tendency to form layers in addition to orientational order. The direction of preferred orientational order is perpendicular to the layers in a smectic, 4 liquid crystal and al an angle wilh the layer normal in a smectie C liquid crystal. 

In order to avoid this section becoming too abstract, a selection of molecules which can form nematic or smectic liquid crystals is illustrated in Figure 7.2. For a discussion of how particular molecular structures lead to formation of particular mesophases, reference should be made to the work by Gray and Goodby [402] already cited or to Chapters 1 and 12 of Molecular Physics of Liquid Crystals edited by Luckhurst and Gray [28]. 

Chiral molecules which form smectic liquid crystals are often capable of forming structures in which the electric dipoles associated with the molecules all point approximately in the same direction in a particular region but in which this direction rotates as one moves in a direction normal to the smectic planes. Such materials are rather misleadingly referred to as ferroelectric liquid crystals. The mechanism responsible for this effect is illustrated in Figure 7.3. The molecules tilt into a smectic-C phase due to their structure as illustrated. Dipoles associated with the molecules are supposed to point in a direction normal to the page. Thus, if the molecules all have the same handedness the dipoles all point in he same direction. This description is an oversimplification as the molecules rotate about their long axes but point preferentially in the manner indicated. This phenomenon has been successfully applied to... 

Some of the phenomena discussed in earlier sections of this book involve forms of matter which might well be described as smectic liquid crystals but are better dealt with under other headings. However, there are three particular processes which do not fit into these other categories but which certainly make use of liquid crystalline behaviour and will thus be examined here. 

Decher et al. [420] have developed the idea of using freely suspended smectic liquid crystals and have used it to form thin hlms on solid substrates. A thermotropic smectic liquid crystal is drawn across an aperture in a solid support and is capable of bridging the aperture (which can be up to 15 mm in diameter) with a him which can be between two and several hundred layers thick. The him consists of a smectic structure with the layers lying in the plane of the him. The him is formed a short distance above a solid substrate and the apparatus is constructed so that a difference of pressure between the two sides of the him can be used to force the him down in contact with the substrate. These authors have thus formed good quality hlms up to an area of about 1 cm2. In the work described the material used was ethyl-4 -n-octyloxybiphenyl-4-carboxylate. 

For good Y layers to be formed by evaporation in vacuo one requires conditions such that the bulk material is stable but the surface region is sufficiently liquid that rearrangement of the molecules into a layer structure is possible. It thus seems likely that, in the temperature range of interest, the surface region behaves as a smectic liquid crystal. It is not yet clear how far the bulk regions of such multilayers should be thought of as liquid crystal-like in nature. 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

A common procedure to determine the values of the interaction parameters between bilayers is to fit the experimental data with a functional form predicted by theory. It was shown1,19 that the equilibrium separation distance between bilayers as a function of the osmotic pressure applied can be well fitted by expression (18). A recent improvement of Caille theory20,21 ofX-ray scattering by smectic liquid crystals allowed the simultaneous measurement of the average separation distance and the... 

If p-methacrylylhydroxybenzoic acid is mixed with p-cetylhydroxy-benzoic acid, a smectic form of liquid crystals results, but if it is mixed with p-nonylhydroxybenzoic acid, the resulting form is nematic above 104°C. and is smectic below this. This makes it possible to compare the polymerization behavior of the same monomer in solution, where the mutual ordering of its molecules is minimal, in a liquid crystal state with only orientation order (nematic form), and in the liquid-crystal state involving both orientation and coordination order (smectic form). 

The rotational crystalline phase has conceptually the same meaning as that of the smectic liquid crystal [8]. Molecules of substances forming liquid crystals have, as a rule, an anisometrically elongated shape and low symmetry. The main structural feature of the liquid-crystalline state is a parallel array of molecules with very light contacts between them. 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

Cholesteric liquid crystals are similar to smectic liquid crystals in that mesogenic molecules form layers. However, in the latter case molecules lie in two-dimensional layers with the long axes parallel to one another and perpendicular or at a uniform tilt angle to the plane of the layer. In the former molecules lie in a layer with one-dimensional nematic order and the direction of orientation of the molecules rotates by a small constant angle from one layer to the next. The displacement occurs about an axis of torsion, Z, which is normal to the planes. The distance between the two layers with molecular orientation differing by 360° is called the cholesteric pitch or simply the pitch. This model for the supermolecular structure in cholesteric liquid crystals was proposed by de Vries in 1951 long after cholesteric liquid crystals had been discovered. All of the optical features of the cholesteric liquid crystals can be explained with the structure proposed by de Vries and are described below. 

Figure 10.1 Molecules that form nematic and smectic liquid crystals A -(p methoxybenzylidene)-/j-butylani-...

The molecules in smectic liquid crystals are more ordered than the nematic since, not only are they arranged with their long axes parallel, but they are also arranged into distinct layers. As a result of this two-dimensional order the smectic liquid crystals are viscous and are not orientated by magnetic fields. Examples of compounds forming smectic liquid crystals are octyl p-azoxycinnamate and ethyl p-azoxy-benzoate. 

Solid-state NMR spectroscopy has been available for the analysis of polymorphs for isotactic polypropylene (i-PP) for approximately 15 years [1-3]. By different sample preparation (e.g., the method of crystallization), i-PP forms a, /3 and smectic forms. Figure 11.1 shows the CP/MAS NMR spectra of these three forms of i-PP at 20°C. 

Two-dimensional ion conduction was also achieved for the smectic liquid crystals consisting of the mixture of ionic liquids and hydroxyl-functionalized rod molecules [49-51] and smectic LC imidazolium salts having a long alkyl chain [52-55]. One-dimensional ion conduction was performed for aligned columnar LC phases forming ion-channels of fan-shaped imidazolium salts having a tris(alkoxy)phenyl moiety [56]. 

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