Smectic thermotropic

Aldonamides 5 display smectic thermotropic as well as lyotropic mesophases [70,71] they form helical tubes with diameters between 40 and 500 A and lengths of several pm and have gelation properties [72-76]. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

V. Percec, C. Peihwei, G. Ungar, and J. Zhou, calamitic nematic and smectic thermotropic liquid crystalline phases. J. Am. Chem. Soc., 117 11441, 1995. 

Percec V, Kawasumi M (1992) Synthesis and characterization of a thermotropic nematic liquid crystalline dendrimeric polymer. Macromolecules 25 3843-3850 Percec V, Chu P, Ungar G, Zhou J (1995) Rational design of the first nonspherical dendrimer which displays calamitic nematic and smectic thermotropic liquid crystalline phases. J Am Chem Soc 117 11441-11454... 

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

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... 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

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... 

With increasing temperature, phase transitions occur, including ciystalline to smectic C to smectic A to nematic to isotropic, or crystalline to nematic to isotropic. These examples demonstrate that not all possible transitions neeessarily oceur. Depending on the number of mesophases occurring, thermotropic mono-, di-, tri-, or tetramorphism may be distinguished. 

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 so ealled a phase of the fatty alcohols is a thermotropic type smectic B liquid crystal with a hexagonal arrangement of the moleeules within the double layers. It is initially formed from the melt during the manufaeturing proeess and normally transformed into a erystalline modifleation on eooling. However, the erystallization of the gel matrix ean be avoided if the a phase ean be kept stable as it eools to room temperature. This ean be achieved by eombining appropriate surfaetants, such as myristyl or lauryl alcohol and cholesterol, a mixture of whieh forms a lamellar liquid... 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

Note 3 A focal-conic domain built around an ellipse and an hyperbola is the most common type of defect in thermotropic smectic A phases. The hyperbola passes through a focus of the ellipse and the ellipse passes through the focus of the hyperbola (see Fig. 24). Note 4 In a particular limiting case of an ellipse-hyperbola focal-conic domain, the ellipse becomes a straight line passing through the center of a circle. 

Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low a temperature, most LC materials will form a conventional (though anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as the temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally isotropic) phases as temperature is increased. 

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. 

Liquid crystals are classified into lyotropic and thermotropic crystals depending on the way in which the mesomorphic phase is generated. Lyotropic liquid-crystalline solvents are formed by addition of controlled amounts of polar solvents to certain amphiphilic compounds. Thermotropic liquid-crystalline solvents, simply obtained by temperature variations, can be further classified into nematic, smectic, and cholesteric solvents depending on the type of molecular order present. Liquid crystals are usually excellent solvents for other organic compounds. Nonmesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix (Michl and Thulstrup, 1986). Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions (Nakano and Hirata, 1982). 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

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