Amorphous Solid
Amorphous solids are materials that lack a regular, crystalline structure. Instead, their atoms or molecules are arranged in a disordered fashion. This results in properties such as a lack of a distinct melting point and the ability to flow over time, distinguishing them from crystalline solids. Examples of amorphous solids include glass and certain plastics.
5 Key excerpts on "Amorphous Solid"
eBook - ePubX-Ray Diffraction Imaging
Technology and Applications
- Joel Greenberg, Joel Greenberg(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...In this discussion, it is important to first acknowledge a distinction among materials types for Amorphous Solids. Based on thermodynamic criteria, two types of Amorphous Solids are definable, those that are non-crystallizable and those that are crystallizable [ 25 ]. The former defines ideal Amorphous Solids that are unable to crystallize under certain time-temperature-transformation conditions. The latter involves metastable materials that have large defect populations and a corresponding lack of crystallinity. It is noted that meta-stable phases may exist, a consequence of materials processing and chemical kinetics considerations. In fact, many times metastable materials are sought based on their desired properties. Nevertheless, diffraction phenomena from these various forms of Amorphous Solids and liquids are fundamentally the same. Were it not for short-range order in amorphous materials with particle (a generalized term representing atoms or molecules in amorphous materials) spacing on the order of X-ray wavelengths, structural characterization, and substance classification in X-ray diffraction imaging systems would not be possible. The reproducible XRD features from Amorphous Solids and liquids give way to the means of determining spatial distributions of particles and, for purposes of threat detection, materials identification. Since short-range order dictates the structure of liquids and glasses, probabilistic atomic distribution functions are used to describe particle configuration [ 24 ]. The association of particles in liquids and Amorphous Solids is defined by the radial pair distribution function (RDF or, pair distribution function). RDF is used to determine bonding topology and the population of atoms in coordination spheres [ 26 ]—whereby a central particle is surrounded by an array of particles in the configuration of a sphere. Thus, pertinent data related to structure may be obtained from diffraction spectra...
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
...This is primarily because many Amorphous Solids are composed of different types of bonding between the atoms or molecules, which require different amounts of energy to break the bonds. But it can also be because the solid is composed of molecules with the same structure but varying molecular weights, as with the polymers discussed in Chapter 13. Fig. 11.15 Some examples of Amorphous Solids. From H. Raab, Takis Iazos, Materialscientist, and Petra Klawikowski; Wikimedia Commons. Amorphous Solids are divided into two groups: microcrystalline solids and pseudo solids. As with crystalline solids, the atoms or molecules in microcrystalline solids are close together with little freedom to move. They have bonding similar to crystalline solids, but the crystalline structure contains many defects that disrupt any formation of an extended regular lattice structure. These defects take the form of lattice points that have missing atoms or atoms that are located at sites other than their regular positions. Because of these many defects, microcrystalline solids have an irregular bonding pattern and do not exhibit any kind of long range regular structure. But they do show a regular lattice arrangement over small regions, known as a short range or microcrystalline order. Any kind of crystalline solid can also exist as a microcrystalline Amorphous Solid. Because the formation of the highly ordered crystal lattice from an unordered liquid solution cannot occur rapidly, the crystallization process must be allowed to proceed slowly for the crystalline solid to form. If the conditions change too rapidly, crystalline order may begin to form, but it will become disrupted before the ordered form continues beyond a microscopic range. Microcrystalline solids also include the ceramics and cements. Hardened Portland Cement (calcium silica hydrate) has a short range ordered geometry consisting of layers of very long chains of silica molecules combined with layers of calcium oxide...
eBook - ePubDeveloping Solid Oral Dosage Forms
Pharmaceutical Theory and Practice
- Yihong Qiu, Yisheng Chen, Geoff G.Z. Zhang, Lawrence Yu, Rao V. Mantri(Authors)
- 2016(Publication Date)
- Academic Press(Publisher)
...Amorphous phases are those solids that do not exhibit long-range order in any of the three physical dimensions. However, short-range order could exist for Amorphous Solids. Because of the importance of this class of solids to pharmaceutical development, it is discussed in detail in Section 2.7 of this chapter. If materials have long-range order in only one or two dimensions, they are liquid crystalline in nature. Liquid crystalline materials can be further categorized based on the number of components contained therein, as is the case for crystalline solids. Since liquid crystals, with properties intermediate to conventional liquids and three-dimensional solids, are not frequently encountered, they will not be discussed in detail. The vast majority of pharmaceutical solids fall into the category of crystalline solids because they exhibit long-range order in all three dimensions. Crystalline solids can be further categorized into various subtypes based on the number of components that make up the solid internally, in a homogeneous fashion. The solid could be composed of the drug alone, or as adducts with one (binary), two (ternary), three (quaternary), other chemical species. Although the number of other chemical species, apart from the drug itself, can increase without limit, it usually is a relatively low integer. When the overall chemical composition of solids is the same, they can be different in internal structures. The ability of a substance to exist as two or more crystalline phases that have different arrangements or conformations of the molecules in a crystalline lattice is called polymorphism, and these different solids are termed polymorphs. One point to emphasize is that, according to the strict definition of this term, different polymorphs are only different physically, not chemically. When these solids are melted or dissolved in solutions, they are exactly the same, both physically and chemically...
eBook - ePub- Leslie H. Sperling(Author)
- 2015(Publication Date)
- Wiley-Interscience(Publisher)
...CHAPTER 5 THE AMORPHOUS STATE The bulk state, sometimes called the condensed or solid state, includes both amorphous and crystalline polymers. As opposed to polymer solutions, generally there is no solvent present. This state comprises polymers as ordinarily observed, such as plastics, elastomers, fibers, adhesives, and coatings. While amorphous polymers do not contain any crystalline regions, “crystalline” polymers generally are only semicrystalline, containing appreciable amounts of amorphous material. When a crystalline polymer is melted, the melt is amorphous. In treating the kinetics and thermodynamics of crystallization, the transformation from the amorphous state to the crystalline state and back again is constantly being considered. The subjects of amorphous and crystalline polymers are treated in the next two chapters. This will be followed by a discussion of liquid crystalline polymers, Chapter 7. Although polymers in the bulk state may contain plasticizers, fillers, and other components, this chapter emphasizes the polymer molecular organization itself. A few definitions are in order. Depending on temperature and structure, amorphous polymers exhibit widely different physical and mechanical behavior patterns. At low temperatures, amorphous polymers are glassy, hard, and brittle. As the temperature is raised, they go through the glass–rubber transition. The glass transition temperature (T g) is defined as the temperature at which the polymer softens because of the onset of long-range coordinated molecular motion. This is the subject of Chapter 8. Above T g, cross-linked amorphous polymers exhibit rubber elasticity. An example is styrene–butadiene rubber (SBR), widely used in materials ranging from rubber bands to automotive tires. Rubber elasticity is treated in Chapter 9. Linear amorphous polymers flow above T g. Polymers that cannot crystallize usually have some irregularity in their structure...
eBook - ePub- Ron Liu, Ron Liu(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...Noncrystalline solids may have some short-range order but lack the long-range periodicity and regular intermolecular bonding of crystalline solids. Their synthesis and properties differ markedly from their crystalline forms, as will be described in detail in the section on amorphous drugs. In general, amorphous forms are high-energy, low-density solids that can yield transient dissolution rates considerably greater than crystalline solids. Liquid crystals are an intermediate state in which the molecules in a crystal can undergo a secondary phase transition to a mesophase, which gives them mobility in 1–2 directions. They are birefringent, but possess flow properties like a liquid phase. Lyotropic liquid crystals form on uptake of water into a system that increases its mobility, and thermotropic liquid crystals can be disrupted by heating above a transition temperature. Cromolyn sodium (Cox et al., 1971), the HMG-CoA reductase inhibitor SQ33600 (Brittain et al., 1995), and the leukotriene D4 antagonist L-660,711 (Vadas et al., 1991) are examples of pharmaceuticals that can form liquid crystals. The crystal habit or external shape may differ when drugs are recrystallized from different solvent systems without changing the internal structure. The presence of additives, the rate of cooling, degree of agitation, and the degree of saturation can all affect crystal habit (Byrn, 1982; Byrn et al., 1999). Habit can affect bulk properties such as density and flowability (Carstensen, 1993) or influence the ability to filter crystals during purification. Chow and Grant (1988) have shown that the dissolution rate of acetaminophen can be altered two to three times by modifying the length to width ratio through incorporation of additives...
