Ordered fluid mesophases

Figure 5.1 illustrates example molecular primary structures, associated schemahc secondary structures, and idealized shapes for representative thermotropic MLCs -organic molecules that melt into ordered, fluid, mesophases. The so-called... 

Two basically different types of mesophases have been observed. First, there are those that retain a 3-dimensional crystal lattice, but are characterized by substantial rotational disorder (i.e., disordered crystal mesophases), and second, there are those with no lattice, which are therefore fluid, but nevertheless exhibit considerable rotational order (i.e., ordered fluid mesophases). Molecular structure is in fact important and, generally speaking, molecules comprising one of these two types of mesophase are distinctly different in shape from molecules comprising the other. Indeed, with the possible exception of some polymorphous smectic materials, there are no known substances that show both disordered crystal and ordered fluid mesophases. 

Ordered fluid mesophases are commonly called "liquid crystals and are most often composed of elongated molecules. In these mesophases, the molecules show some degree of rotational order (and in some cases... 

In what follows, the various ordered fluid mesophases are described, with particular attention being given to the nature of the molecular ordering in each case. Also, some of the consequences of simultaneous liquid-like and solid-like behavior in a single phase are discussed qualitatively. Later chapters will deal at length with many subjects we can consider only briefly here. 

It may be desirable to define certain basic physical processes afresh, when we are dealing with systems essentially subject to two-dimensional conformations and hence two-dimensional constraints. This is the case for membranes, and also for a number of alkali salts of alkali -alkane carboxylates. These melt to give mesophases, in which the anions and cations are arranged in layerlike structures. At considerably higher temperatures the mesophases pass into isotropic ionic melts, but in the intervening temperature range they exhibit marked anisotropy of optical and physical properties. In these mesophases, which are ordered fluid... 

Liquid crystal is a term that is now commonly used to describe materials that exhibit partially ordered fluid phases that are intermediate between the three dimensionally ordered crystalline state and the disordered or isotropic fluid state. Phases with positional and/or orientational long-range order in one or two dimensions are termed mesophases. As a consequence of the molecular order, liquid crystal phases are anisotropic, i.e., their properties are a function of direction. 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

The nature and number of any mesophases that are formed by a given material, and the temperatures at which they exist must be determined experimentally. This requires recourse to a selection of physical techniques, some of which are described below. However, it is helpful to know that, in most cases, the thermodynamic ordering of the various (fluid) mesophases maintains a fairly constant order that is given in Scheme 1 (although note that out-of-sequence or re-entrant phases are known).It is rather unlikely that a single material will show all of these phases and it is found empirically that certain combinations are more likely than others. ... 

The thermal behavior of an analogous series of octakis(alkoxyethyl)porphyrins was also investigated for comparison ((22) M = 2H, Zn, Cu, Pd, Cd, n = 4, 6, 8, 10), and in order to decrease the viscosity, which was still high in the octaester systems. Thus, while only the free-base derivative with octyl chains was mesomorphic (Cr 84 Col 891), all the metal complexes were mesomorphic (Figure 28), the mesophase decreasing in stability as the chains were elongated. " In all cases, the fluid mesophase was identified by optical microscopy as columnar. 

The simplest mesophase is the nematic phase. It is very fluid and involves highly disordered molecules having only short-range positional order, but with the molecules preferentially aligned on average in a particular direction (the director). If the constituent compound is racemic then it is possible to form a phase from the enantiomerically pure compound which is a chiral nematic phase. 

The mesophases of calamitic mesogens are classified in two groups nematic and smectic. The nematic mesophase (N) is characterized by an orientational order of the molecules that are aligned along a preferred direction (defined by a director n) (Figure 8.2). The molecules can slide and move in the nematic mesophase (while roughly keeping their molecular orientation) and rotate around their main axis. This is the less ordered mesophase and it is usually very fluid. 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

Nematic phase this is the simplest structure. It is the most disordered mesophase and therefore very fluid. It is called N. In the nematic phase, the molecules are ordered mainly in one dimension with their long axes parallel, and they are free to move parallel to this axis (there is no long-range order). Nematic liquid crystal mixtures, containing various amounts of different liquid crystal compounds, are used in electro-optic display systems such as flat-panel displays. 

Heat treatment of the S-type fluoride in a fluorine atmosphere Based on the results above mentioned, Fujimoto et al. developed a new fluorination procedure in order to prepare the perfluorinated pitch, and obtained two types of other fluorinated pitches [23,24], The new process is by the heat treatment at 200-400°C of S-type of fluorinated pitch prepared at relatively low temperature in a fluorine atmosphere. They firstly fluorinated the mesophase pitch at 70°C for 10 h (first step for the preparation of S-type fluorinated pitch) and then heated up to a selected temperature between 200°C and 400°C, and maintained this temperature for 12 h (second step for the heat-treatment of fluorinated pitch). Thus, they obtained two kinds of fluorocarbons, a transparent resin (R-type) and a liquid (L-type). L-type is a viscous fluid containing some volatile materials and the viscosity gradually becomes higher when it is kept for a few weeks in an air atmosphere even at ambient temperature. They reported that the R-type was obtained in the nickel boat in the heating zone and L-type at the bottom of the vertical reaction vessel which was cooled down by the water. Therefore, it is likely that the liquid fluorocarbon is formed by the vaporization of some component contained in the S-type fluoride or decomposition reaction during the heat treatment of the S-type fluoride. The yields of these compounds depends on the heat treatment temperature. In Fig. 3, the yields of the R-type and L-type fluorocarbons are plotted as a function of the heat treatment temperature of the S-type fluoride. The yield of the former decreases with increase of the heat treatment temperature and finally, at 400 C, it can not be obtained at all. On the other hand, the yield of the latter increases with increase of temperature and it is selectively obtained at 400°C. 

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