Mesophases crystallization

The interference pattern depends both on the symmetry of the liquid crystal mesophase and on the arrangement of the molecules between the glass cover slips. Three examples are given in Fig. 8. 

First, it is important to appreciate that all liquid crystal mesophases exist due to non-covalent interactions between molecules, namely the anisotropic dispersion forces mentioned earlier. However, this section will address more specific non-covalent interactions that have been used either to induce liquid-crystalline behaviour or to generate a new species that is liquid crystalline. 

For example, Marder and co-workers [10] (among others) have studied liquid crystal mesophases induced by the presence of quadrupolar interactions between non-mesomorphic phenyl- and perfluorophenyl-containing moieties (e.g. 1). 

The members of Class II in Table 1 present very small enthalpies of the mesophase-liquid transition [ AHml < 0.5 kJ/(mol of chain bonds)], suggesting that their mesophase is hardly stabilized by specific interatomic interactions. By contrast, we point out that in all cases the crystal-mesophase transition has a significant enthalpy value, mostly AHqm > 1 kJ/(mol of chain bonds). Consistent with their relatively flexible character, the polymers listed in the Tables have their glass transition below ambient temperature. 

Grint, A. Swietlik, U. Marsh, H. Carbonization and Liquid-crystal (Mesophase) Development-9. The Co-carbonization of Vitralns with Ashland A200 Petroleum Pitch. Fuel, 1979, M,... 

Note 1 The tilde is used to indicate a property of a liquid-crystal mesophase. 

Liquid crystal Mesophase Temp. Pitch and Md Optical Ref. 

There are again five pure phases possible (fully ordered crystal, mesophase glass, amorphous glass, mesophase, and isotropic melt). In addition, there is the possibility of seven two-phase materials (fully ordered crystal and mesophase glass, fully ordered crystal and amorphous glass, mesophase glass and amorphous glass, mesophase... 

The crystal-mesophase transitions at Td are first order transitions. In the case of plastic crystals, they show usually the major heat of transition. In the condis crystals the magnitude of the transitions depends on the number of bonds which gain con-... 

Keywords Crown compounds Liquid crystals Mesophases Metallomesogens Polymers Salt effects Self-assembly Substituent effects Supramolecular chemistry Phthalocyanines Cyclophanes... 

Figure 5.11 Proposed arrangement of polysilane molecules in the hexagonal liquid-crystal mesophase.

Filler coke is formed by the same general mechanism as that already described for binder coke. However, the feedstocks used are various petroleum residual fractions, instead of coal tar. Temperatures of 400-500°C convert these resids into green coke within a day. A complex series of endothermic pyrolysis reactions produce liquid-crystal mesophase which is transformed to a carbon polymer of generally graphitic structure. However, there are varying amounts of... 

The greatest amount of work we have carried out with alkoxystil-bazoles is in the field of liquid crystals indeed, this is where our interest in stilbazoles started. After a brief and rather general introduction to liquid crystals, we will consider various types of complex that form liquid-crystal mesophases when complexed to stilbazoles, emphasizing patterns of behavior without delving into the subtleties. A more detailed discussion of the silver systems may be found elsewhere 24). Finally, although this article appears in a series that concentrates on inorganic chemistry, we offer an overview of some of our work with stilbazoles in hydrogen-bonded liquid crystals. 

Although the cubic phase is interesting in several regards, it is particularly worth noting that while liquid crystal mesophases are fluid, cubic phases are very viscous. Furthermore, because of their high symmetry, cubic phases possess isotropic physical properties, setting them apart from other liquid-crystal mesophases. Transitions into, and out of, cubic phases tend to be rather slow. Exactly why cubic phases form in calamitic systems is tmclear at the present time. 

Extension of the molecular sieves to the mesoporosity range is possible nsing lyotropic liquid crystal mesophases (Figine 25.20) as removable templates. These mesophases result from the self-assembly of surfactants or amphiphilic molecules and can be thermally or chemically eliminated after the formation of the inorganic network. This approach enables the preparation of materials exhibiting an ordered... 

Liquid crystals are classified into two groups known as thermotropics and lyotropics. Thermotropics are those that are formed in the melting of crystalline solids, and they can remain in the liquid crystal mesophase without decomposition, passing to the isotropic liquid state when subsequently heated. As their mesophases are turbid, the temperature at which the transition to the isotropic liquid phase takes place is called the clearing temperature. Lyotropic LCs form mesophases in concentrated solution when the concentration exceeds a critical value. 

Ordered mesoporous structures from lyotropic liquid crystal mesophases... 

Figure lb. Schematic plot of free energies vs. temperature for the system In Figure 1 but with G, raised (to G ) so as to "uncover" the mesophase. an< are the crystal-mesophase transition and the lsotroplzatlon transition temperatures. 

This subblock of the Heat Capacity Data Bank contains empirical as well as theoretical prediction calculations of the heat capacity which are continuously updated. Based on the chemical structure (back-bone, side-chain) as well as on the physical state (glass, crystal, mesophase, liquid, semicrystalline, equilibrium, history), a heat capacity is retrieved. The Prediction Scheme subblock can also be searched for its base, i.e., the precise assumptions which go into the prediction, with documentation to the 1iterature. 

The formation of lyotropic liquid-crystal mesophase depends on the structure and properties of surfactant, solvent, and reaction conditions. Although studies on lyotropical liquid crystals have been carried out for many years, the structure and properties of some mesophases are still not very clear. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is difficult to analyse their structures and properties, the boundary in the phase diagram may be not accurate and the minor phase may be missed. 

The common structure models for Iniim symmetry are shown in Figure 8.28. Among the well known lyotropic liquid-crystal mesophases, these are at least two mesostructures with Im3m symmetry one locates near the Ii region in the phase diagram, with a possible spherical micelle packed structure. Another one is close to the Vi region, and its most probable structure can be described by a P surface. 

S.A. El-Safty and J. Evans, Formation of Highly Ordered Mesoporous Silica Materials adopting Lyotropic Liquid Crystal Mesophases. J. Mater. Chem. 2002, 12, 117-123. 

Yoon, S.-H., Korai, Y., and Mochida, I. (1996). Axial nano-scale microstructures in graphitized fibers inherited from liquid crystal mesophase pitch. Carbon, 34, 941-56. 

Figure 103 Schematic diagrams of some common lyotropic liquid crystal mesophases A = lamellar, = hexagonal, Hi = cubic, I. ...

While there are several examples of metal complexes which are amphiphilic in nature, it is in very few cases that lyotropic liquid crystals mesophases have been characterized. Although numerous and strictly classifiable as metallomesogens, in this article we exclude discussion of the amphiphiles with a simple metal ion as the cation (e. g. sodium salts of carboxylic acids), rather concentrating on amphiphiles in which the metal cation is an integral part of the amphiphile. 

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