Mesophase system

The synthesis of MCM-41 structure in fluoride medium has been reported by Silva and Pastore [10], who used sodium silicate as silica source and carried out crystallization at 150 °C. They suggested the behaviour of Si02-CTMA+-F system was significantly different from that reported previously on the mesophase system. It is known that fluoride ions do influence the nature, activity and polymerizing capacity of silica precursors, and a fluorinated silica surface is much more hydrophobic and more resistant to the attack of water molecules than a silanol silica surface [11]. 

Chanzy, H. Chaunis, S. Monzie, P. Oriented cellulose films and fibers from a mesophase system. J. Polym. Sci. Polym. Phys. Ed. 1980, 18 (5), 1137-1144. 

During surfactant dissolution the two diffusion processes can be identified. On the molecular scale a molecule undergoes self diffusion where the diffusion coefficient is determined from its mean squared displacement. Various NMR techniques have been used to study quantitatively self diffusion processes (21-24), It is important to note that each component in the system will have a self diffusion coefficient. Diffusion coefficients for a number of mesophase systems have been collected where values of order 10 - 10 m s" were reported (25). The self diffusion coefficients of the solvent are typically reduced no more than an order of magnitude in the presence of mesophases which essentially act as obstacles to the solvent (25, 26). 

In this way the old Vorlander effect of directing mesophase systems by interaction possibilities of small amounts of inducers, elucidated and used in theoretical and practical aspects by Stegemeyer and his group mainly in the field of classical thermotropics,... 

M. Chanzy, A. Peguy, S. Chaunis, and P. Monzie, "Oriented Cellulose Films and Fibers from a Mesophase System," J. Polym. Phys. Ed., 18, 1137-1144 (1980). 

As is usually found for mesophase systems, the boundaries on the phase diagram are not all equally easy to detect. All of the known chromonic mesophases are birefrin-gent and the clearing points can therefore be readily seen by optical microscopy. The crystalline solid-M phase boundary is often harder to detect optically, but it does show up readily in X-ray difiraction studies. Note also that some early chromonic phase diagrams determined by NMR did not show... 

The history of the structural studies of the DSCG/water mesophase system is unusually complex. In retrospect it can be seen that the interpretation of the early X-ray diffraction results was complicated by three factors ... 

Concerning the term chromonic, this word was coined to replace phrases such as lyotropic mesophases of the types formed by soluble aromatic mesogens . It was intended to echo the bis-chromone structure of DSCG and to carry connotations of dyes and chromosomes (and hence nucleic acids). It was also intended to mark Hart-shorne s studies of DSCG (which although by no means the first such mesophase system to be explored, was one of the most extensively examined), and it is to Norman Hartshome, who taught me and many others a love of optical microscopy, that I respectfully dedicate this chapter. 

The number of examples of Uquid crystalline systems is limited. A simple discotic system, hexapentyloxytriphenylene (17) (Fig. 4), has been studied for its hole mobUity (24). These molecules show a crystalline to mesophase transition at 69°C and a mesophase to isotropic phase transition at 122°C (25). 

Other examples include ditholium salts, shown in Figure 4.1-8 [30]. The scattering data show that a range of mesophase behavior is present, dependent - as with the metal-containing systems - on alkyl chain length. 

In order to solve the system of the above-described equations, and which are derived by applying the self-consistent model, applied for composites by Budiansky 7), it is necessary to evaluate experimentally the moduli of elasticity (tension, shear, bulk) and Poisson s ratios of the constituent phases and the composite. Thus, the only unknown are the radius r of the mesophase layer and its mechanical properties and thermal expansion coefficient, which are then derived. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

Similar behavior can occur when a crystalline network is disassembled by adding a solvent rather than by heating. These mesogens are called lyotropic liquid crystals and the mesophase formation shows temperature and concentration dependence. They are very important in biological systems, but have been much less studied in materials science. 

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