Structure noncrystalline

Structure and Bonding in Noncrystalline Solids G. E. Walrafen, A. G. Revesz, Eds., Plenum, New York (1986). 

Raman spectroscopy is particularly useful for investigating the structure of noncrystalline solids. The vibrational spectra of noncrystalline solids exhibit broad bands centered at wavenumbers corresponding to the vibrational modes of the corresponding crystals (Figure 5). In silicate glasses shifts in the high-wavenumber bands... 

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

In the case of the polycrystalline polyester thermoplastic rubbers the simple domain theory does not seem to apply. With these rubbers it would appear that they contain spherulitic structures consisting of 4GT radial lamellae with inter-radial amorphous regions that are mixtures of PTMEG soft segments and noncrystalline hard segments. 

The development of the internal orientation in formation in the fiber of a specific directional system, arranged relative to the fiber axis, of structural elements takes place as a result of fiber stretching in the production process. The orientation system of structural elements being formed is characterized by a rotational symmetry of the spatial location of structural elements in relation to the fiber axis. Depending on the type of structural elements being taken into account, we can speak of crystalline, amorphous, or overall orientation. The first case has to do with the orientation of crystallites, the second—with the orientation of segments of molecules occurring in the noncrystalline material, and the third—with all kinds of structural constitutive elements. 

Overall orientation is understood as the joint arrangement of all the structural elements of the crystalline phase and noncrystalline part of the fiber in relation to the geometrical axis of the fiber. In its essence, the overall orientation of PET fibers, as a result of the crystalline and amorphous orientation, will be characterized by smaller values of the quantitative index of orientation than for the crystalline phase and by greater ones for the amorphous phase. 

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. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

In order to gain information on the environments of certain atoms in dissolved species, in melts or in solids (crystalline or noncrystalline), which are not accessible to diffraction studies for one reason or another, X-ray absorption spectrometry (XAS) can be applied, with the analysis of the X-ray absorption near-edge structure (XANES) and/or the extended X-ray absorption fine structure (EXAFS). Surveys of these methods are available 39,40 a representative study of the solvation of some mercury species, ElgX2, in water and dimethylsulfoxide (DMSO) by EXAFS and XANES, combined with quantum-chemical calculations, has been published.41... 

A similar comparison can be made with cis-poly(isoprene), natural rubber, by taking advantage of the fact that the polymer is very slow to crystallize [164], Consequently, the comparison can be made between the supercooled, noncrystalline polymers at 0°C and the semi-crystalline polymer (31% crystalline) at the same temperature. The Tlc values for each of the five carbons involved were again found to be the same for the completely disordered polymer and the semicrystalline one, so that a similar conclusion can be made with regard to their chain structure. 

In fluorescence spectroscopy, the orientation distribution of the guest probe is not necessarily identical to the actual orientation of the polymer chains, even if it is added at very small concentrations (i.e., a probe with high fluorescence efficiency). As a matter of fact, it is generally assumed that long linear probes are parallel to the polymer main chain, but this is not necessarily the case. Nevertheless, if the relation between the distribution of the probe axes and those of the polymer axes is known, the ODF of the structural units can be calculated from that of the probe thanks to the Legendre s addition theorem. Finally, the added probe seems to be mainly located in the amorphous domains of the polymer [69] so that fluorescence spectroscopy provides information relative to the noncrystalline regions of the polymeric samples. 

The properties of the polycarbonate of bisphenol A are directly related to the structure of the polymer. The molecular stiffness associated with this polycarbonate arises from the presence of the rigid phenyl groups on the molecular chain or backbone of the polymer and the additional presence of two methyl side groups. The transparency of the material arises from the amorphous (noncrystalline) nature of the polymer. A significant crystalline structure is not observed in the polycarbonate of bisphenol A because intermolecular attractions between phenyl groups of neighboring polymer chains in the melt lead to a lack of flexibility of the chains that deters the development of a crystalline structure. 

Improve the tools for imaging and determining structure so that detailed chemical structures can be determined with tiny amounts of noncrystalline material. 

Determining the structural arrangements of atoms within noncrystalline chemical substances, and resolving how they change as a function of time, on any time scale. 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

The use of X-ray diffraction from crystalline samples can result in a complete three-dimensional crystal structure of a molecule, but requires a single crystal suitable for proper diffraction (see Section 3.3). X-ray absorption spectroscopy (XAS) can yield limited molecular structural information on noncrystalline (amorphous) solid... 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

The notion of a common core structure has been further supported by synchrotron X-ray fiber diffraction patterns of several amyloid fibrils the patterns show common reflections in addition to those at 4.7 and 10 A (Sunde et al., 1997). Although these data give some insight into the arrangement of the amyloid fibril core, the exact molecular structure and organization of the proteins making up this common core have yet to be uniquely defined. The inherently noncrystalline, insoluble nature of the fibrils makes their structures difficult to study via traditional techniques of X-ray crystallography and solution NMR. An impressive breadth of biochemical and biophysical techniques has therefore been employed to illuminate additional features of amyloid fibril structure. 

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