Liquid crystal smectic

The word smectic comes form the Greek word anriyua, meaning soap and reflects the mechanical properties displayed by many of these materials. All smectics are layered structures having well-defined interlayer distances. Smectic materials are therefore more ordered than nematic ones and occur at a temperature below that for the nematic phase. A number of smectic phases have been identified.  

Smectic E Se- Although the smectic E phase may occur on its own or in conjunction with a higher temperature nematic phase, it is most commonly encountered in the sequence of phases N Sa Sb Se, where the arrows indicate decreasing temperature. 

Other smectic phases. Other smectic phases have been observed, where each phase relates to a particular type of crystal structure as defined in Chapter 2. For instance, smectic D is associated with a disordered cubic structure, smectic FI appears to be a tilted smectic B phase and the smectic G phase has close similarities with the smectic E phase but is tilted. A list of smectic phases is presented in Table 3.1. 

2 Influence of Molecular Structure on the Formation of Liquid Crystalline Phases 

A number of molecular factors influence whether or not liquid crystalline behaviour is observed within a specific molecular structure. These factors are also relevant when considering the behaviour of polymeric materials. 

FIGURE 1.4. Some examples of smectogens 4-n-pentylbenzene-thio-4 -n- decyl-oxybenzoate (10S5), p, p -dihetyloxyazoxybenzene (HOAB), and 4-n-pentyloxy-benzylidene-4 -n-heptylaniline (50.7). Transition temperatures are in °C. 

Under the second column of Table 1.1, different orthogonal smectics are classified according to the structures within the layers. Smectic A has layers that can be considered as simple two-dimensional liquids. There is only short-range correlation between the molecular positions within the layers over a distance corresponding to a few molecules. In Fig. 1.5a, two of these short-range clusters of size are shown. These clusters possess a 

FIGURE 1.5. Local two-dimensional lattice for (a) a liquid and (b) for a hexatic phase with bond-orientational order. The molecular centers of mass are denoted by dots. 

When the layer thickness d is less than the length I of the molecule in a smectic A phase, the molecules are either assumed to be symmetric units or their molecular asymmetry is nullified at least at a local level. When molecules possess a terminal parorCN or -NO2 group, the dipole moment can now distinguish a head and a tail in the molecule. For such asymmetric molecules, antiparallel dipole correlation causes pairing of molecules to form a variety of A phases with a layer thickness / d 2Z [1.18]. The dipole-dipole interaction has a profound effect on many physical properties [1.19]. This has been used to account for the re-entrant phenomenon in liquid crystals. Two types of smectic A phases may be visualized when polarized 

FIGURE 1.6. Schematic diagram of polar ordering in smectic A phases. 

---

Smith D P E, Hdrber H, Gerber Ch and Binnig G 1989 Smectic liquid crystal monolayers on graphite observed by scanning tunnelling microscopy Science 245 43... 

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. 

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). 

Keywords Smectic liquid crystals, Molecular conformations, polar and steric frustrations, Polyphilic and perfluorinated mesogens. X-ray diffraction... 

Bruce, D.W., Lalinde, E., Styring, P., Dunmur, D.A. and Maitlis, P.M. (1986) Novel transition metal-containing nematic and smectic liquid crystals. Journal of the Chemical Society, Chemical Communications, (8), 581-582. 

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. 

Niori, T. Sekine, T. Watanabe, J. Furukawa, T. Takezoe, H. Distinct Ferroelectric Smectic Liquid Crystals Consisting of Achiral Molecules with Banana Shape, Abstracts of the 16th International Liquid Crystal Conference, Kent State University, Kent, OH, 1996, p. 126. 

This volume of Topics in Stereochemistry could not be complete without hearing about ferroelectric liquid crystals, where chirality is the essential element behind the wide interest in this mesogenic state. In Chapter 8, Walba, a pioneering contributor to this area, provides a historical overview of the earlier key developments in this field and leads us to the discovery of the unique banana phases. This discussion is followed by a view of the most recent results, which involve, among others, the directed design of chiral ferroelectric banana phases, which display spontaneous polar symmetry breaking in a smectic liquid crystal. 

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

Fig. 2-20 Molecular ordering in nematic and smectic liquid crystals compared to liquid. After Brown and Crooker [1983] (by permission of American Chemical Society, Washington, DC).

Multilamellar bilayers in the fluid phase are also ordered in the sense that they are smectic liquid crystals. Of great interest is the range of molecular order this is long-range in the sense that the molecules are confined to two dimensions there is also some kind of short-range order in molecular orientations and conformations, but the range of this latter ordering is not known at present. 

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. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

Smith, D.P.E. et al. Smectic Liquid Crystal Monolayers on Graphite Observed by Scanning Tunneling Microscopy/ Science, 43 (July 7, 1989)... 

Phase structure Equivalent smectic liquid crystal phase... 

---