Smooth twist

Fig. 2 Dielectric a process in polyethylene showing initial and final states. The combined 180 ° rotation and c/2 translation of the central chain is accomplished by a smooth twist, consisting of 12 carbon atoms out of register, that propagates through the crystal...

Figure 9.12 Propagation of a localized smooth twist along the chain. As the twist starts (1) it leaves behind it a translational mismatch. As the twist proceeds (2) the mismatch becomes attenuated at large distances from the twist by elastic distortion of the stem valence angles and bond length. (Reproduced with permission from Mansfield and Boyd, J. Polymer Sci., Phys. Ed., 13, 1407 (1975))...

Figure 7.50 Dielectric ot relaxation process in linear polyethylene. The smooth twist consisting of 12 carbon atoms out of register propagates through the crystal in the direction of the arrow. Drawn after Boyd (1985b).

Poly(vinylidcne fluoride), PVFj, is another polymer in which banded spherulites are observed. A recent study of the lamellar structure of these objects reveals that, as in polyethylene, lamellae do not adopt a smoothly twisted conformation but rather undergo major re-orientations at regular... 

Figure 1. Representative conformation defects in polyethylene chains, (a) Pech-hold kink, (b) Reneker twist, and (c) smooth twist.

The crux of the present method is how well the relaxation portion converges. Some experience has now been gained with it (11,12,13). Figure 4 shows results of calculations made in our laboratory (11,13) on the energies of three conformational defects in polyethylene crystals. These are a kin)c (15, ), a Reneker twist (Ig) and a smooth twist (12) (see Figure 1). The first two have been proposed as stable point defects cind the last one as a transition state for the motion that accomplishes... 

Cones contain 0.6—4.0 kg of yam. The tip of the cone tube must have a smooth finish to prevent damage to the yam, which is drawn over the top. Again, a magazine wrap may be provided for automatic package transfer. Both compacted and twisted yams are packaged on cones. 

The selection of twist level is important not only in estabUshing the surface characteristics of the yam, eg, low twist for soft, fuzzy yams and high twist for compact, smooth yams, but also in determining yam strength. Yarn strength as a function of yam twist level is shown in Figure 1. 

These are required to make threads, i.e. the final drawing, twisting and winding of cotton. Such motors must possess very smooth acceleration to eliminate breakage of threads. They are recommended to have a starting torque of 150-200% of T, and a pull-out torque of 200-275% of with a mean acceleration torque ol 150-175%. A normal acceleration time of 5-10 seconds is recommended. Faster acceleration may cause more breakages, while a slower acceleration may result in snarls and knots in the yarn as a result of insufficient tension. 

Therefore, for a rough surface, although it is smooth macro-scopically, the friction coefficient is greatly affected by its surface morphology. For a smooth surface on atomic scale, 0 can be neglected and the friction coefficient is only related to the twist angle of the cantilever. 

Enhancement of CHF subcooled water flow boiling was sought to improve the thermal hydraulic design of thermonuclear fusion reactor components. Experimental study was carried out by Celata et al. (1994b), who used two SS-304 test sections of inside diameters 0.6 and 0.8 cm (0.24 and 0.31 in.). Compared with smooth channels, an increase of the CHF up to 50% was reported. Weisman et al. (1994) suggested a phenomenological model for CHF in tubes containing twisted tapes. 

The Claisen reaction can now proceed smoothly, but nature introduces another little twist. The carboxyl group introduced into malonyl-CoA is simultaneously lost by a decarboxylation reaction during the Claisen condensation. Accordingly, we now see that the carboxylation step helps to activate the a-carbon and facilitate Claisen condensation, and the carboxyl is immediately removed on completion of this task. An alternative rationalization is that decarboxylation of the malonyl ester is used to generate the acetyl enolate anion without any requirement for a strong base (see Box 10.17). 

In addition to the chair conformations of six-membered rings the less stable boat (B) conformations are also possible. Tire six boat forms are smoothly interconvertible through intermediate twist (T) forms, which are also called skew (S) forms.3 5 Since the... 

The furanose ring of ribose or deoxyribose is flexible and can be interconverted smoothly among an infinite number of envelope (E) and skew or twist (T) conformations. See Chapter 4, Section A,2. However, there are limits set by steric and anomeric effects.27 62-64 Conformations are often described as in Fig. 5-10 by stating which atom in an envelope conformation lies mostly out of the plane of the other four atoms. If this atom lies above the ring, i.e., toward the base, the ring... 

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