Packing, and crystal lattices

In Chapter 2, you learned how individual atoms pack in crystal lattices. Moreover, the nature of metallic bonding was described, which is responsible for characteristic physical properties of these materials. This chapter will continue this discussion, focusing on the relationship between specific metallic lattice structures and their impact on overall physical properties. 

An ionic bond forms when anions and cations close to each other attract, forming a tightly packed geometric crystal lattice. 

No serious attempt has as yet been made to solve, by modern methods, the crystal structure of any alginate, although x-ray diffraction data have been published122 and have been discussed elsewhere in this Series.27 It is sufficient to note that alginic acid and calcium alginate, both of which are known to form gels, are insoluble in water, and seem to pack in crystal lattices in the solid state. The conformation in salt solution is disordered, but very extended.1228... 

TABLE 18.1. Unit cells, close-packed structures, and crystal lattices. A. Simple Cubic Unit Cell... 

The macrocyclic structure of cyclodextrins resembles a truncated cone with the wide side occupied by 02, 03 hydroxyls and the narrow side by 06 hydroxyls. This polarity gives the cyclodextrins properties which are essential if packing in crystal lattices is discussed. 

Chain Packing and Crystal Structures. The chain packing and the suhmolecular arrangement of repeat units and pendant side groups of macromolecules in crystalline domains of polymers can be visualized using contact mode SFM. The resolution is in most cases not true resolution, since the area of the contact area (1 — few nm ) exceeds the molecular scale and must be considered lattice resolution instead. The first example of molecularly resolved structures of a polymer dates back to 1988, when Marti and co-workers reported on an SFM study on a polydiacetylene film (128). Examples for resolved chain packing and polymer crystal structure determination at the surface of semicrystalline polymers include poly(tetrafiuoroethylene) (PTFE) (129,130), polyethylene (PE) (131-133), polypropylene (PP) (134,135), poly(ethylene oxide) (PEO) (136), aramids (137,138), and poly(oxy methylene) (POM) (139). 

Packing of the cyclodexthn molecules (a, P, P) within the crystal lattice of iaclusion compounds (58,59) occurs in one of two modes, described as cage and channel stmctures (Fig. 7). In channel-type inclusions, cyclodextrin molecules are stacked on top of one another like coins in a roU producing endless channels in which guest molecules are embedded (Fig. 7a). In crystal stmctures of the cage type, the cavity of one cyclodextrin molecule is blocked off on both sides by neighboring cyclodextrin molecules packed crosswise in herringbone fashion (Fig. 7b), or in a motif reminiscent of bricks in a wall (Fig. 7c). 

Another method for the determination of the structure of the crystal lattice is SAXS [30,31]. Figure 6 shows the specific SAXS profiles of microsphere film (MC2). The cubic packing values (dl/di) are listed in Table 3. Three clear peaks appeared at 0.35, 0.42, and 0.66 degrees in Fig. 6. The dl/di values of the second and third peaks are >/4/3 and >/U/3, respectively. These values are peculiar to the FC(T structure. Thus, the lattice structure of the microspheres is an estimated FCC. As both... 

Certain monomers crystallize in a conformation such that they can be zipped together without changing the symmetry of the crystal lattice. In the crystalline state, the arrangement of monomers is strictly determined by crystal packing. Polymerization is usually initialed by irradiation with UV, X- or y-rays and is assumed to proceed by a radical mechanism. For example, muconic acid esters (25, 27, 29) and ammonium salts (26,28, 30) can be stcrcospccifically polymerized in the crystalline state to high conversion.224-227228 This form of... 

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