Strain-free

Cakes are dried in low temperature, high humidity conditions for a long time in order to minimize strain and absorbency variabiUty. The continuing usage of the cake system almost a century after its invention owes much to the desirable strain-free yam arising from its washing and drying operations. 

When metals are deformed plastically at room temperature the dislocation density goes up enormously (to =10 m ). Each dislocation has a strain energy of about Gb /2 per unit length and the total dislocation strain energy in a cubic metre of deformed metal is about 2 MJ, equiva-lent to 15 J mol k When cold worked metals are heated to about 0.6T new strain-free grains nucleate and grow to consume all the cold-worked metal. This is called - for obvious reasons - recrystallisation. Metals are much softer when they have been recrystallised (or "annealed"). And provided metals are annealed often enough they can be deformed almost indefinitely. 

Estimate the time that it would take for recrystallisation to be completed at an annealing temperature of 700°C. Because the new strain-free grains grow by diffusion, you may assume that the rate of recrystallisation follows Arrhenius law, i.e. the time for recrystal-lisation, f is given by f, =... 

Fig. 14.11. Typical data for recrystallised grain size as a function of prior plastic deformation. Note that, below a critical deformation, there is not enough strain energy to nucleate the new strain-free grains. This is just like the critical undercooling needed to nucleate a solid from its liquid (see Fig. 7.4).

Compression moulding is used only occasionally with polyethylene. In this process the polymer is heated in a mould at about 150°C, compressed to shape and cooled. The process is slow since heating and cooling of the mould must be carried out in each cycle and it is employed only for the manufacture of large blocks and sheets, for relatively strain-free objects such as test-pieces and where alternative processes cannot be used because of lack of equipment. 

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm. ... 

The process is attractive for a number of reasons. Firstly, since it is a low pressure process the moulds are generally simple and relatively inexpensive. Also the moulded articles can have a very uniform thickness, can contain reinforcement, are virtually strain free and their surface can be textured if desired. The use of this moulding method is growing steadily because although the cycle times are slow compared with injection or blow moulding, it can produce very large, thick walled articles which could not be produced economically by any other technique. Wall thicknesses of 10 mm are not a problem for rotationally moulded articles. 

What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

Because the heat of combustion of a cycloalkane depends on size, we need to looJk at heats of combustion per CH2 unit. Subtracting a reference value derived from a strain-free acyclic alkane and then multiplying by the number of CH2 units in the ring gives the overall strain energy7. Figure 4.3 shows the results. 

The data in Figure 4.3 show that Baeyer s theory is only partially correct. Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted, and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of 14 carbons or more are strain-free. Why is Baeyer s theory wrong ... 

Cyclohexane adopts a strain-free, three-dimensional shape, called a chair conformation because of its similarity to a lounge chair, with a back, a seat, and a footrest (Figure 4.7). Chair cyclohexane has neither angle strain nor torsional strain—all C—C-C bond angles are near 109°, and all neighboring C-H bonds are staggered. 

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C—H bonds are staggered. Chair cyclohexane has two kinds of positions axial and equatorial. Axial positions are oriented up and down, parallel to the ring axis, whereas equatorial positions lie in a belt around the equator of the ring. Each carbon atom has one axial and one equatorial position. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

The strain-free coil of pure platinum used in the resistance thermometer must meet one of the following specifications 1 (302.9146 K) > 1.11807 or 1 (234.3156 K) < 0.844235. If the resistance thermometer is to be used over the entire range 13.8033 to 1234.93 K, it must also meet the requirement that 1 (1234.93 K) > 4.2844. Temperatures 790 are calculated from the equation... 

Materials compatibility. We use scrupulously clean and scratch-free FEP (FEP is the acronym for the copolymer of tetrafluoroethylene and hexafluoropropylene) tubing for handling our mixtures of fluorine and nitrogen at ambient temperature. Corrugated FEP tubing is convenient for making strain-free assemblies. We have found that Monel is excellent for use with dry molten KF-2HF. Mild steel corrodes slowly and stainless steels corrode rapidly. Kel-F polychlorotrifluoroethylene is satisfactory for use with HF and with KF-2HF polypropylene and polymethylpentene are not satisfactory. 

The amount of strain in cycloalkanes is shown in Table 4.6, which lists heats of combustion per CH2 group. As can be seen, cycloalkanes larger than 13 membered are as strain-free as cyclohexane. 

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

Later, the name diamondoids was chosen for all the higher cage hydrocarbon compounds of this series because they have the same structure as the diamond lattice highly symmetrical and strain-free so that their carbon atom structure can be superimposed on a diamond lattice, as shown in Fig. 5 for adamantane, diamantane, and triamantane. These compounds are also known as adamanto-logs and polymantanes. 

These compounds are chemically and thermally stable and strain-free. These characteristics cause high melting points (m.p.) in comparison to other hydrocarbons. For instance, the m.p. of adamantane is estimated to be 269 °C, yet it sublimes easily, even at atmospheric pressure and room temperature. The melting point of diamantane is about 236.5 °C and the melting point of triamantane is estimated to be 221.5 °C. The available melting point data for diamondoids are reported in Table I. 

This is another borosilicate glass, and was developed to make strain-free seals with timgsten. It is made by the General Electric Co., and often described as GEC W.l, or simply as W.l. The linear expansion coefficient (20-350°C) is 3-75 x 10 . 

---