Smectic liquid crystals: structural

C. Nobile, L. Carbone, A. Fiore, R. Cingolani, L. Manna, R. Krahne, Self-assembly of highly fluorescent semiconductor nanorods into large scale smectic liquid crystal structures by coffee stain evaporation dynamics. J. Phys. Condens. Matter 21, 264013 (2009)... 

The tilting of molecules in the B2 phase is clearly confirmed from the observation that the spherulites emerging from the isotropic phase show an electric field dependence of the position of the optical extinction lines (Fig. 9.26). Because of the tilting of banana molecules to the layer, chirality is spontaneously generated in addition to the polarity this fact sounds shocking but is so simple to be understood [132, 133]. If the molecule is rotated around their polar axis (the orientation of the bent in the molecules), which is akin to tilting the molecules in the layer, the rotation operation cannot be achieved by a simple translation (see Fig. 9.27). That is, these two states are in a mirror relation with left-handed and right-handed chirality. This is called the layer chirality. When the chirality couples with the polarity of the molecules, one would consider various smectic liquid crystal structures. There are two homochiral phases in which either (—) or (-I-) chiral molecules stack in the layers and a racemic phase in which layers are alternately stacked with layers of (—) and (-I-) chiral molecules. Each of those phases can be either ferroelectric or antiferroelectric, so that in total six different phases are present... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

S. H. J. Idziak, I. Koltover, J. N. Israelachvili, C. R. Safinya. Structure in a confined smectic liquid crystal with competing surface and sample elasticities. Phys Rev Lett 76 1477-1480, 1996. 

Similar lamellar structures are formed for l-alkyl-3-methylimida2olium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

Gray GW, Goodby JWG (1984) Smectic Liquid Crystals, Textures and Structures. Blackie, Glasgow... 

Obviously, the model is crude and does not take into account many of the factors operating in a real molecular stack. Lack of symmetry with respect to the polar axis and the fact that dipoles may not necessarily be situated in one plane represent additional complications. The angle a could also be field dependent which is ignored in the model. The model also requires that interactions between molecules in adjacent stacks be very weak in order for fields of 10 to 20KV/cm to overcome barriers for field induced reorientation. The cores are then presumably composed of a more or less ordered assembly of stacks with a structure similar to smectic liquid crystals. 

Phase structure Equivalent smectic liquid crystal phase... 

Figure 7.1. Categories of smectic liquid crystals. From Smectic Liquid Crystals, Textures and Structures, Gray, G.W. and Goodby, J.W.G. 1984 (Leonard Hill, Glasgow). (Reproduced by kind permission of the authors and publishers.) Side elevation and plan representation of molecular ordering in each of the smectic modifications. Triangles or arrows are used to represent tilt direction.

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... 

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. 

G. W. Gray and J. W. G. Goodby, Smectic Liquid Crystals—Textures and Structures, Leonard Hill, Glasgow and London, 1984. 

Gray GW, Goodby JW (1984) Smectic liquid crystals - textures and structures. Leonard Hill,... 

Smectic liquid crystals have both positional and orientational order. The molecules are grouped into layers. In smectic A structures, the molecules tend... 

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. 

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. 

For comprehensive descriptions of the structure and properties of liquid crystals and leading references, see (a) F. D. Saeva (Ed.), Liquid Crystals. The Fourth State of Matter, Marcel Dekker, Inc., New York, 1979 (b) H. Kelker and R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980 (c) G. W. Gray and J. W. Goodby, Smectic Liquid Crystals. Textures and Structures, Leonard Hill, Glasgow, 1984. 

As the concentration increases, the amphiphilic molecules form micelles and then form columns. The columns are arranged into a hexatic array. As the concentration further increases, the system forms a laminar structure, i.e., a smectic liquid crystal phase. Sometimes, a cubic phase may appear between the micelle and hexatic phases. In fact, the micelles are packed to form a cubic phase. The three phases are all liquid crystal phases hexatic phase, laminar phase and cubic phase. As seen in the figure, the phase diagram of amphiphilic molecules actually depends on the temperature as well. Tk in Figure 1.15 is the Kraft temperature, below which the system is phase separated into crystal and water. 

The same was observed for the following two polymers (3.44) and (3.45) with the mesogenic units of a schiff base structure (Frosini et al, 1981). The polyacrylate is a smectic liquid crystal from 93 °C to 258 °C. The polymethacrylate does not form any liquid crystalline phase. 

Polarizing optical microscopy is often the method of first choice. It was with POM that the textures of liquid crystals were observed and the classification of liquid crystals was first made according to these observations. In this field, two books are recommended The Textures of Liquid Crystals by Demus and Richter (1978) and Smectic Liquid Crystals — Textures and Structures by Gray and Goodby (1984). While the latter provides readers with a practical and useful experimental guideline to the textures and classification of smectic liquid crystals of different polymorphic types together with as many as 124 reference photographs of typical textures, the former... 

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