Thermotropic liquid crystals smectics

The family of primary silver thiolate compounds AgSC H2n+1 ( = 4, 6, 8, 10, 12, 16, or 18), which in the solid state consists of 2x,[AgSR] layers, behaves as thermotropic liquid crystals. On heating, they display successively lamellar (smectic A), cubic, and micellar mesophases.969... 

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. 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). 

The smectic mesophases of the thermotropic liquid crystals show a variety of textures but resemble mainly the fan-shape texture of the lyotropic hexagonal meso-phase. For further reading more comprehensive literature is recommended [11]. 

The viscosity of thermotropic liquid crystals increases following the sequenee nematic< smectic A < smectic C. 

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. 

When they are heated, mesogenic compounds do not melt directly from the highly ordered crystalline state to an isotropic liquid. They form instead, intermediate phases in which the molecules are orientated in a parallel direction and referred to as smectic (centers of the molecules organized in layers) or nematic (centers of the molecules distributed at random). Smectic and nematic mesophases are in turn divided into a variety of subgroups of thermotropic liquid crystals which will not be dealt with in detail in the present article. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

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). 

Fig. 3.3 Phase diagram of diacylphosphatidylcholines as suggested by X-ray, optical birefringence, and electron microscopy. Lj corresponds to the smectic A state and tp, Lf, Pp to the smectic B phase of thermotropic liquid crystals. The shaded area is a region of coexistence. (Reprinted from Fig. 2 of ref. 15 with permission from Wiley-VCH.)...

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 Ref F and G Adapted from Ref. l)...

The smectic mesophases of the thermotropic liquid crystals show a variety of textures but resemble the... 

Liquid crystals can be classified into lyotropic and thermotropic systems. The rheology of thermotropic liquid crystals is less documented, but in general, nematic liquid crystals were found to show Newtonian flow, whereas smectic and cholesteric liquid crystals demonstrated more or less pronounced plug flow. Plug flow is typical for non-Newtonian, shear thinning liquids. ... 

Figure 6.32 Diagrammatic representation of forms of thermotropic liquid crystals (a) smectic, (b) nematic, and (c) cholesteric liquid crystals.

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

N. Boden, Y. K. Levine, D. Lightowlers, and R. T. Squires, "NMR dipolar echoes in liquid crystals," Chem. Phys. Letters 31, 511 (1975) "Internal molecular disorder in the nematic and smectic phases of thermotropic liquid crystals studied by NMR SPDE experiments," ibid 34, 63-68 (1975). 

There are two broad classes of so-called thermotropic liquid crystals which are termed nematic and smectic liquids. These two classes can be subdivided into some... 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

To learn the effect of shear on the spectra, the zero shear rate spectra and the spectra under shear were plotted together for the thermotropic liquid crystal (LC2) in Fig. 14. There were no significant spectral changes with shear. The major peaks did become a little sharper. The spectra corresponding to the smectic A (55 C) and the nematic phase (70"C), shown in Figs. 14 and b respectively, bear this out. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

The molecular organization in thermotropic liquid-crystal line phases is associated predominantly with a rigid anisometric architecture of the constituent single molecules. The triazines 13 were the first examples of electron donors that fotm columnar phases, which give rise to the induction of smectic liquid crystalline structures through donor-acceptor interactions (Figure 11). 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

Thermotropic Liquid Crystals. - Organic molecules, having aromatic rings or unsaturations thus producing elongated shapes, and also polymeric molecules often show thermotropic phase behavior. Mesomorphism comprises typically nematic, smectic A and B, and cholesteric thermotropic phases, but in several cases columnar, discotic and rod-disk self-assembly shapes of thermosensitive mesogens have been observed. 

To our knowledge, this is the first example of the coexistence of both twisted smectic and cholesteric phases in thermotropic liquid crystal polymers. Previous preparations of thermotropic polymers by the use of chiral derivatives both incorporated in the macromolecular backbone and pendant to it as side chain substituents (comb-like polymers) resulted in either cholesteric or smectic " polymeric products. 

There are two principal categories of mesophases, thermotropic and lyotropic. Thermotropic liquid crystals are formed within a particular range of temperature in a molten material, with no solvent present, whereas lyotropic liquid crystals are formed by some substances when they are dissolved in a solvent. Within each of these categories there are three distinct classes of mesophases, which were first identified by Friedel in 1922. The simplest of these to describe are the nematic and smectic classes, illustrated schematically in fig. 12.16. These phases are formed by long thin rigid molecules which tend to line up parallel to each other. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

A thermotropic liquid crystal is capable of polymorphism, the situation in which a material exhibits more than one liquid-crystalline modification between the solid and liquid phases. For instance, when a calamitic liquid crystal melts it can do so via transitions between a number of smectic phases and the nematic phase. For the purposes of phase... 

Figure 4 Schlieren textures in calamitic thermotropic liquid crystals (A) two brushed singularities seen exclusively in the nematic phase, and (B) four bmshed singularities seen for both smectic and nematic phases. The dark lines show the orientation of the director. (Data from Gray and Goodby, 1984.)...

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