Order amorphous

Anhydrous alumina can have a variety of structures, from an entirely disordered one (truly amorphous), through short-range ordered amorphous, to highly ordered in a tetrahedral or octahedral arrangement (y,y -or a-alumina) [cf. Section IV(3)]. 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

A simple phenomenological method can be used to describe changing crystallinity from WAXS data of isotropic materials. It is based on the computation of areas in Fig. 8.2. First we search the border between first-order and second-order amorphous halo. For PET this is at 29 37° (vertical line in the plot). Then we integrate the area between the amorphous halo and the machine background. Let us call the area Iam. Finally we integrate the area between the crystalline reflections and the amorphous halo, call it Icr, and compute a crystallinity index... 

Most polymers contain ordered crystalline regions and less ordered amorphous regions (30.7). The greater the crystallinity, the harder the polymer. 

According to the nucleation mechanism, the rate of appearance of a new phase must decrease in the following order amorphous phase > liquid crystalline phase > crystalline phase. This can be explained by the fact that for nuclei of the amorphous phase to appear, any fluctuation of particles, whose dimension exceed a certain critical value, is sufficient. At the same time, the nuclei in the liquid crystalline phase appear only at a more strict ordering of molecules, and for the appearance of nuclei of the crystalline phase a still more strict mutual arrangement of molecules is required. 

Ivanov, M, Naydenova, I., Todorov, T., Nikolova, L., Petrova, X, 7 omova, N., Dragostinova, V. (2000). Light-induced optical activity in optically ordered amorphous side-chain azobenzene containing polymer./. Mod. Opt. 47, 861-867. 

Inorganic fibers can currently be produced from a wide range of element combinations and further fiber-types are in development (see Section 5.2.7), so that a classification according to chemical composition, as favored by preparative chemists, is not reasonable. Other possible classification criteria are e.g. the production process, the source of the fibers (natural or synthetic), their degree of order (amorphous or crystalline), their thermal stability (27 - 2227°C) or physical properties (tensile strength, elasticity modulus). The boundaries between the individual fiber types are, however, often fluid. 

XRD may be very common in catalysis, but it does have disadvantages. Because XRD is based on interference between reflecting X-rays from lattice planes, the technique requires samples which possess sufficient long range order. Amorphous phases and small particles give either broad and weak diffraction lines or no diffraction at all, with the consequence that if catalysts contain particles with a size distribution, XRD may only detect the larger ones. In unfortunate cases, the diffraction lines from the metal may overlap with those from the support. Finally, the surface region is where catalytic activity resides, but this part of the catalyst is virtually invisible for XRD. 

In this model, well-ordered crystalline regions alternate with less-ordered amorphous domains. Since single chains pass from several crystalline regions... 

The bulk structures of silicas are classified as crystalline and amorphous polymorphs. More than 35 well-defined crystalline silicas are known, which are well-characterized by the Si-O length, the Si-O-Si bond angle, and the Si-O bond topology and coordination (10). Some of the crystalline polymorphs are collected in Table I. Because of the lack of sufficiently precise methods to assess the long-range structural order, amorphous silicas remain poorly characterized. They can be loosely discriminated according to their dispersity, bulk density, and type of pore structure. 

Ivanov M, et al. 2000. Light induced optical activity in optically ordered amorphous side chain azobenzene containing polymer. J Mod Opt 47(5) 861 867. 

True liquids, in contrast, do not show an extensive order. Even with very slight stresses applied for a short time, they deform so completely that they very quickly adopt the form of the surrounding container. Low-molar-mass liquids thus behave in a purely viscous way under normal conditions. When stress is applied, the molecules are displaced irreversibly in relation to one another. In high-molar-mass substances above the glass transition temperatures, flow can be produced relatively easily. Deformations are much more difficult below the glass transition temperature of amorphous polymers. For this reason, and because of their lack of order, amorphous substances below their glass-transition temperatures are often termed supercooled liquids. 

The orientation of the crystalline region is measured by x-ray diffraction techniques. The orientation in the less-ordered amorphous region is determined by x-ray, infrared measurements, sonic modulus technique [183], or by separating the crystalline and amorphous contributions to the fiber birefringence [79]. The average orientation of the crystalline regions is specified by the Hermans Stein orientation functions ... 

In order to obtain rather well-ordered amorphous mesostructures, thin PBh-PEO(21-4), PS-PEO(3-3), PBh-PEO(3.7-2.9), or PBh-PEO(3.7-3.6) films were annealed in the molten state at temperatures up to 150 °C before crystallization experiments were performed. It is important to mention that in contrast to PS-PEO(3-3), PBh-PEO(3.7-2.9), or PBh-PEO(3.7-3.6) samples annealed PBh-PEO(21-4) samples did not crystallize at ambient conditions, even after storage at room temperature for many months. Only after coohng the samples to temperatures below about - 20 °C, was crystallization observed. Melting, on the other hand, occurred at temperatures above about + 40 °C. Consequently, at room temperature the samples neither crystallized nor melted. This fact enabled AFM measurements of partially crystallized samples at room temperature, preserving the structure obtained by crystallization at low temperatures. 

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