Cross crystals

The silver acetylide-silver nitrate forms fine needle and cross crystals [28] as shown on Fig. 12.1 with density 5.36 g cm [6] or 5.369 (X-ray) [17]. The density is superior to MF and LA making Ag2C2-AgN03 a primary explosive with one of the highest known densities [29]. It decomposes by action of acids liberating acetylene ... 

Secondly, we examined an effect of seed crystals. For example, when the solution involving 2-butanol or 2-methyl-2-propanol was seeded with the bilayered crystals, only the crossing crystals were obtained. On the other hand, when the solution involving 1-butanol and 2-methyl-1-propanol was seeded with the crossing crystals, a mixture of the crystals was obtained. These results indi-... 

Thirdly, we studied temperature effects. As shown in Table 1, the assembly modes of 1 with acrylonitrile depended on the temperature of recrystallization in the presence of the same solvent. In the cases of 1-butanol and 2-methyl-1-propanol, the borderline for separation of the two assembly modes lies in about 30°C. On the other hand, in the cases of 2-butanol and 2-methyl-2-propanol, the only crossing crystals were obtained at any temperatures. 

Another interesting phenomenon, which can be observed in blends with miscibility gap, is the occurrence of interface crossing crystallization [46]. In conditions of phase separation, two phases with different concentrations of the crystallizable component are present and, due to differences in the number density of heterogeneous nuclei, the phase with a higher content... 

Figure B3.3.13. Intersecting stacking faults in a fee crystal at the impact plane induced by collision with a momentum mirror for a square cross section of side 100 unit cells. The shock wave has advanced half way to the rear ( 250 planes). Atom shading indicates potential energy. Thanks are due to B Holian for tliis figure.

D, H W Hoeffken, D Crosse, J Stuerzebecher, P D Martin, B F P Edwards and W Bode 1992. Refined 2.3 Angstroms X-Ray Crystal Structure of Bovine Thrombin Complexes Formed witli he 3 Benzamidine and Arginine-Based Thrombin Inhibitors NAPAP, 4-TAPAP and MQPA A Starting Point for Improving Antithrombotics. Journal of Molecular Biology 226 1085-1099. 

In order to reach a crystalline state, polymers must have sufficient freedom of motion. Polymer crystals nearly always consist of many strands with a parallel packing. Simply putting strands in parallel does not ensure that they will have the freedom of movement necessary to then find the low-energy con-former. The researcher can check this by examining the cross-sectional profile of the polymer (viewed end on). If the profile is roughly circular, it is likely that the chain will be able to change conformation as necessary. 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

The disks are assumed to lie in the same plane. While this picture is implausible for bulk crystallization, it makes sense for crystals grown in ultrathin films, adjacent to surfaces, and in stretched samples. A similar mathematical formalism can be developed for spherical growth and the disk can be regarded as a cross section of this. 

These three systems describe a set of crystallization curves that cross at 6 = 0.5 and t = 10 sec. For the case where m = 2, the time span over which the change occurs is widest (1430 sec from 9 = 0.1-0.9) and the maximum slope is flattest... 

A larger number of smaller spherulites are produced at larger undercoolings, a situation suggesting nucleation control. Various details of the Maltese cross pattern, such as the presence or absence of banding, may also depend on the temperature of crystallization. 

Twisting of the lamellar ribbons along the radial direction is responsible for the banding superimposed on the Maltese cross in Fig. 4.12. From the spacing of the bands, the period of the twist can be calculated and is found to depend on crystallization conditions. 

Figure 7.14a illustrates the insertion of a propylene monomer into an edge vacancy in a crystal adjacent to an alkylated titanium atom. In Fig. 7.14b a cross-sectional view of the same site shows how the preferential orientation of the coordinated monomer is dictated by constraints imposed by the protuberances on the crystal surface. 

Channels in crystals of thiourea [62-56-6] (87) are comparable but, as a consequence of the larger size of the sulfur atom, have larger cross-sectional areas (0.7 nm) and can trap branched-chain, aUcychc, and other molecules of similar dimensions including polychlorinated hydrocarbons. But they do not include the straight-chain hydrocarbons that work so well with urea. 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

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