Crystalline structures

The specific gravities of fully crystalline and fully amorphous polypropylene are 0.9363 and 0.8576, respectively [86], and those of the partially crystalline resin and fiber are usually about 0.905. The bulk density of pellets is approximately 34 Ib/ft.  

If all the lattice points are positioned in the eight corners of a unit cell, then the unit cell is called a primitive unit cell. However, often, for convenience, larger unit cells, which are not primitives, are selected for the description of a particular lattice, as will be explained later. 

A concrete building procedure in three dimensions of the Wigner-Seitz cell can be achieved by representing lines from a lattice point to others in the lattice and then drawing planes that cut in half each of the represented lines, and finally taking the minimum polyhedron enclosing the lattice point surrounded by the constructed planes. 

Till now, we have only considered a mathematical set of points. However, a material, in reality, is not merely an array of points, but the group of points is a lattice. A real crystalline material is constituted of atoms periodically arranged in the structure, where the condition of periodicity implies a translational invariance with respect to a translation operation, and where a lattice translation operation, T, is defined as a vector connecting two lattice points, given by Equation 1.1 as 

The Physical Chemistry of Materials Energy and Environmental Applications 

Until now, we have considered an infinite lattice, but a real material has limited dimensions, that is, n n2, n3 has boundaries. However, an infinite array of unit cells is a good approximation for regions relatively far from the surface, which constitutes the major part of the whole material [5], At this point, it is necessary to recognize that a real crystal has imperfections, such as vacancies, dislocations, and grain boundaries. 

It is known that C-nitropyrazoles are more stable than /V-nitropyrazolcs. Nevertheless it is quite strange that 1-nitropyrazole was among the first of nitropyrazoles, which has been studied using X-ray technique. Bond lengths, valence (O-N-O), and torsion (N2-N1-N-0) angles for the 1-nitropyrazole molecule as well as the same parameters for some other A-nitroazoles are given in Table 3.1. 

Larina and V. Lopyrev, Nitroazoles Synthesis, Structure and Applications, 

Compound N1-N2 N2-C3 C3-C4 C4-C5 C5-N1 C4-NO C5N1N2 N1N2C3 N2C3C4 C3C4C5 C4C5N1 Refs 

Catena-((4-nitropyrazole-3,5-dicarboxylic acid)-bis(dioxane)-sodium) 1.347 1.341 1.386 1.363 1.339 

5-Methyl-4-nitropyrazole (R,R)-trans-4,5-bis(hydroxydiphenyl-methyl)-2,2-dimethyl-1,3-dioxolane toluene clathrate 1.358 1.316 1.390 1.378 1.338 

As shown in the previous section, the ratio of the atomic radii of the el ental components (Rc/Rm) determines the suitability of a system to 

---

Microcrystalline waxes, produced from heavy lubricating oil residues, have a micro-crystalline structure and consist largely of iso-and cycloalkanes with some aromatics. 

There is a fair amount of work reported with films at the mercury-air interface. Rice and co-workers [107] used grazing incidence x-ray diffraction to determine that a crystalline stearic acid monolayer induces order in the Hg substrate. Quinone derivatives spread at the mercury-n-hexane interface form crystalline structures governed primarily by hydrogen bonding interactions [108]. 

The refractory industry has found chromite useful for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure. 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Ordinary tin is composed of nine stable isotopes 18 unstable isotopes are also known. Ordinary tin is a silver-white metal, is malleable, somewhat ductile, and has a highly crystalline structure. Due to the breaking of these crystals, a "tin cry" is heard when a bar is bent. 

The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cms. 

Liquid Crystalline Structures. In certain ceUular organeUes, deoxyribonucleic acid (DNA) occurs in a concentrated form. Striking similarities between the optical properties derived from the underlying supramolecular organization of the concentrated DNA phases and those observed in chiral nematic textures have been described (36). Concentrated aqueous solutions of nucleic acids exhibit a chiral nematic texture in vitro (29,37). 

Crystalline Structures. Crystal shape of amino acids varies widely, for example, monoclinic prisms in glycine and orthorhombic needles in L-alanine. X-ray crystallographic analyses of 23 amino acids have been described (31). L-Glutamic acid crystallizes in two polymorphic forms (a and P) (32), and the a-form is mote facdely handled in industrial processes. The crystal stmeture has been determined (33) and is shown in Figure 1. 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Loss of catalyst activity. The higher regenerator temperature combined with the formation of steam in the regenerator reduces catalyst activity by destroying the catalyst s crystalline structure. 

Figure S.14 The eight P strands in one domain of the crystallin structure in this idealized diagram are drawn along the surface of a barrel. From this diagram it is obvious that the p strands are arranged in two Greek key motifs, one (red) formed by strands 1-4 and the other (green) by strands 5-8. Notice that the p strands that form one motif contribute to both P sheets as shown in Figure 5.12.

If a rubbery polymer of regular structure (e.g. natural rubber) is stretched, the chain segments will be aligned and crystallisation is induced by orientation. This crystallisation causes a pronounced stiffening in natural rubber on extension. The crystalline structures are metastable and on retraction of the sample they disappear. 

Polymers can exist in a number of states. They may be amorphous resins, rubbers or fluids or they can be crystalline structures. TTie molecular and the crystal structures can be monoaxially or biaxially oriented. Heterogeneous blends of polymers in different states of aggregation enable materials to be produced with combinations of properties not shown by single polymers. 

In the case of an amorphous polymer the glass transition temperature will define whether or not a material is glass-like or rubbery at a given temperature. If, however, the polymer will crystallise, rubbery behaviour may be limited since the orderly arrangement of molecules in the crystalline structure by necessity limits the chain mobility. In these circumstances the transition temperature is of less consequence in assessing the physical properties of the polymer. 

These different forms Figure 4.7) take up different crystalline structures and consequently the bulk properties of the polymer differ. At room temperature gutta percha is a stiff leathery material. 

In the case of commercial crystalline polymers wider differences are to be noted. Many polyethylenes have a yield strength below 20001bf/in (14 MPa) whilst the nylons may have a value of 12 000 Ibf/in (83 MPa). In these polymers the intermolecular attraction, the molecular weight and the type and amount of crystalline structure all influence the mechanical properties. 

The conformation adopted by a molecule in the crystalline structure will also affect the density. Whereas polyethylene adopts a planar zigzag conformation, because of steric factors a polypropylene molecule adopts a helical conformation in the crystalline zone. This requires somewhat more space and isotactic polypropylene has a lower density than polyethylene. 

Crystalline structures have a much greater degree of molecular packing and the individual lamellae can be considered as almost impermeable so that diffusion can occur only in amorphous zones or through zones of imperfection. Hence crystalline polymers will tend to resist diffusion more than either rubbers or glassy polymers. 

Polymer compounds vary considerably in the amount of heat required to bring them up to processing temperatures. These differences arise not so much as a result of differing processing temperatures but because of different specific heats. Crystalline polymers additionally have a latent heat of fusion of the crystalline structure which has to be taken into account. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

In the case of crystalline polymers such as types E and F the situation is somewhat more complicated. There is some change in modulus around the which decreases with increasing crystallinity and a catastrophic change around the. Furthermore there are many polymers that soften progressively between the Tg and the due to the wide melting range of the crystalline structures, and the value determined for the softening point can depend very considerably on the test method used. 

The suppliers of nylon 46 have laid particular emphasis on the fact that this polymer, with its highly symmetrical chain structure, leads to both a high level of crystallinity and a high rate of nucleation. In turn the high nucleation rate leads to a fine crystalline structure which in this case is claimed to lead to a higher impact strength (dry as moulded) than with nylons 6 and 66. 

The crystalline structure of bis-phenol A polymers has been thoroughly studied by Prietschk and some of the data he obtained on the crystal structure are summarised in Table 20.1. 

At the turn of the century it was still widely believed that, while a metal in its nalural slate is crystalline, after bending backwards and forwards (i.e., the process of fatigue damage), metal loctilly becomes amorphous (devoid of crystalline structure). Isolated observations (e.g., Percy 1864) showed that evidence of... 

---