Liquid-crystalline order, definitions

The model is divided into four parts (1) the definition of the surface to be interfaced with blood, (2) the mode of the plasma protein(s) and/or electrolyte adsorption, (3) relaxation motion of the blood interfacing side chain groups, and (4) protein denaturation and/or liquid crystalline order. 

The second type of impurity, substitution of a lattice atom with an impurity atom, allows us to enter the world of alloys and intermetallics. Let us diverge slightly for a moment to discuss how control of substitutional impurities can lead to some useful materials, and then we will conclude our description of point defects. An alloy, by definition, is a metallic solid or liquid formed from an intimate combination of two or more elements. By intimate combination, we mean either a liquid or solid solution. In the instance where the solid is crystalline, some of the impurity atoms, usually defined as the minority constituent, occupy sites in the lattice that would normally be occupied by the majority constituent. Alloys need not be crystalline, however. If a liquid alloy is quenched rapidly enough, an amorphous metal can result. The solid material is still an alloy, since the elements are in intimate combination, but there is no crystalline order and hence no substitutional impurities. To aid in our description of substitutional impurities, we will limit the current description to crystalline alloys, but keep in mind that amorphous alloys exist as well. 

A crystalline solid is a solid in which the atoms, ions, or molecules lie in an orderly array called a lattice (Fig. 5.19). An amorphous solid is one in which the atoms, ions, or molecules lie in a random jumble, as in butter, rubber, and glass (Fig. 5.20). They have a structure like that of a frozen instant in the life of a liquid. Crystalline solids typically have flat, well-defined planar surfaces called crystal faces, which lie at definite angles to one another. These faces are formed by orderly layers of atoms (Box 5.1). Amorphous solids do not have well-defined faces unless they have been molded or cut. 

Because nematic liquid-crystalline polymers by definition are both anisotropic and polymeric, they show elastic effects of at least two different kinds. They have director gradient elasticity because they are nematic, and they have molecular elasticity because they are polymeric. As discussed in Section 10.2.2, Frank gradient elastic forces are produced when flow creates inhomogeneities or gradients in the continuum director field. Molecular elasticity, on the other hand, is generated when the flow is strong enough to shift the molecular order parameter S = S2 from its equilibrium value 5 . (Microcrystallites, if present, can produce a third type of elasticity see Section 11.3.6.)... 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

It should be pointed out that along with this definition which describes the liquid ciystalline state as a phase state, use is often made of the term "liquid crystalline structure" which is indicative only of a certain orientation ordering in a system. Despite the narrower meaning of the latter term, the notion of a liquid crystalline structure (or ordering) is widely used in the literature on structural polymer studies, therefore, in some instances, we shall use this term as well. 

The capacity for hydration of a substance in the crystalline state is a measure of its hydration capacity in solution. Sulphuric acid e. g., does form crystalline hydrates. In liquid mixtures of sulphuric acid and water, these hydrates, of course, are present as well. They are then dissolved in an excess of either water or sulphuric acid. In inorganic chemistry there are also numerous examples of solid substances forming chemically definite hydrates, though these cannot be detected by the common methods (X-ray diagram, isothermal or isobaric decomposition) which imply the existence of phases with crystalline order. According to R. Fricke (Walden, Handbuch d. allgemeinen Chemie, 9 (1937), 520), a typical example is chromium (3) hydroxide. In such cases the vapour pressure isotherms do not represent the typical step-ladder type but resemble those of solutions (R. Fricke, Naturwiss., 31 (1943) 469). 

At the beginning of the Gap the water content of the system is not enough to allow a definite structural liquid-crystalline configuration, but the structured entities, within the system, may be indeed oriented and spatially ordered by means of an impressed electric field. Therefore a Kerr-like effect can be observed, due to the optical anisotropy induced by the field. 

The solute order parameters ( 0 0(02)) and (Iq 2 O2) cos2 ip2) are calculated like those for a single component system [Eqs. (3.42) and (3.43)], the difference lies only in the definitions of the a and b coefficients. Now it may reasonably be assumed for the liquid crystalline molecules that C2,o) is much greater than 2,2), and for the expansion coefficients il200 > 202 — 220 > 222- Thus... 

A distinction between a solid and liquid is often made in terms of the presence of a crystalline or noncrystalline state. Crystals have definite lines of cleavage and an orderly geometric structure. Thus, diamond is crystalline and solid, while glass is not. The hardness of the substance does not determine the physical state. Soft crystals such as sodium metal, naphthalene, and ice are solid while supercooled glycerine or supercooled quartz are not crystalline and are better considered to be supercooled liquids. Intermediate between the solid and liquid are liquid crystals, which have orderly structures in one or two dimensions,4 but not all three. These demonstrate that science is never as simple as we try to make it through our classification schemes. We will see that thermodynamics handles such exceptions with ease. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

At the macroscopic level, a solid is a substance that has both a definite volume and a definite shape. At the microscopic level, solids may be one of two types amorphous or crystalline. Amorphous solids lack extensive ordering of the particles. There is a lack of regularity of the structure. There may be small regions of order separated by large areas of disordered particles. They resemble liquids more than solids in this characteristic. Amorphous solids have no distinct melting point. They simply become softer and softer as the temperature rises. Glass, rubber, and charcoal are examples of amorphous solids. 

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