Bent unit

If, however, the chain contains inherently bent units, are ultraproperties precluded It seems probable to the authors that, in due course, research will show that such structures can be coaxed into a high modulus form. Maybe some of the stiff polymers with less linear chemical structures can, with sufficient attention to purity, spinning conditions. 

Bent-core mesogens are a very special type of mesogen and possess quite distinct properties from the known rod-shaped mesogens. Compared to a rod-shaped LC molecule including one rigid aromatic core and two flexible tails, a bent-core molecule consists of one central bent unit (BU), two rod-like aromatic wings (RWs), and two flexible tails (Fig. 6.2). Hence, bent-core molecules are also called banana or bow-shaped molecules due to their distinct bent shape (Fig. 6.1c). It is known that rod-shaped molecules can rotate freely around their long molecular axes. In contrast. 

The third common strategy for lowering in MCLCPs is to introduce non-linear units into the polymer backbone. These can take the form of met a- or ortho-substituted phenyls, 1,6- or 2,5-linked naphthy-lenes " or the incorporation of kinked bonds within the monomer unit such as —C(CH3)2—, —CH2—, —O—, —S—, —SO2—, —CO—This method of disruption is especially effective at lowering Tm, but the introduction of large amounts of such bent units will lead ultimately to loss of liquid crystallinity. In general, the stereochemistry of the bisphenol containing a kinked bond is more important than the polar... 

U-Tube U-Bundle Only one tube sheet required. Tubes bent in U-shape. Bundle is removable. High temperature differentials which might require provision for expansion in fixed tube units. Clean service or easily cleaned conditions on both tube side and shell side. Horizontal or vertical. Bends must be carefully made or mechanical damage and danger of rupture can result. Tube side velocities can cause erosion of inside of bends. Fluid should be free of suspended particles. 1.08... 

As he gi ew older, Helmholtz became more and more interested in the mathematical side of physics and made noteworthy theoretical contributions to classical mechanics, fluid mechanics, thermodynamics and electrodynamics. He devoted the last decade of his life to an attempt to unify all of physics under one fundamental principle, the principle of least action. This attempt, while evidence of Helmholtz s philosphical bent, was no more successtul than was Albert Einstein s later quest for a unified field theory. Helmholtz died m 1894 as the result of a fall suffered on board ship while on his way back to Germany from the United States, after representing Germany at the Electrical Congress m Chicago in August, 1893. 

Note that for U-bent bundles the reverse end of the bent tube acts like a head on a normal fixed tubesheet unit. Therefore, the number of tubes and the number of tube holes are not to be confused in determining velocities. 

The differing malleabilities of metals can be traced to their crystal structures. The crystal structure of a metal typically has slip planes, which are planes of atoms that under stress may slip or slide relative to one another. The slip planes of a ccp structure are the close-packed planes, and careful inspection of a unit cell shows that there are eight sets of slip planes in different directions. As a result, metals with cubic close-packed structures, such as copper, are malleable they can be easily bent, flattened, or pounded into shape. In contrast, a hexagonal close-packed structure has only one set of slip planes, and metals with hexagonal close packing, such as zinc or cadmium, tend to be relatively brittle. 

If the BF2 groups in Ni(dmg-BF2)2 are substituted by BPh2 units (122), the complex also adopts the saddle-shaped conformation of type D (Fig. 32), in which the two dimethylglyoxime fragments are bent down from the N4 plane with a dihedral angle of about 27° between the two least-squares planes of the dioxime units. The coordination geometry around the nickel ion is distorted square-pyramidal, but there are no intermolecular Ni Ni interactions [167]. 

In late 1995, a team led by Vollhardt and Youngs reported their work on the strained PAM/PDM hybrid 80 [55]. Whereas the synthesis of 80 was not remarkable [Eq. (2)1, the solid-state behavior of the molecule was. X-ray crystallography revealed that the macrocycle was moderately strained, with the monoynes bent inward toward the center of the macrocycle by 3.9 -11.5° and the diyne unit bent outward by 8.6-11.2°. More importantly, crystal packing revealed that the diyne moieties were aligned in the prerequisite fashion for topochemical diacetylene polymerization to occur. Indeed, irradiation of crystals of 80 produced a violet... 

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. 

The complex [Au2(PPh3)2] has been prepared by reduction of [AuI(PPh3)] with sodium napht-halide. Its core unit is isoelectronic with mercurous halides Hg2X2 but, curiously enough, it has a dinuclear trans bent structure with d(Au—An) = 2.76 A and a Au—Au—P angle of 129°.3325 There is no confirmation as yet of the original preliminary report. 

From CdCl2, K[Ag(CN)2] (ratio 1 2), and NH3 at pH 10 (citrate buffer), colorless crystals of [Cd(NH3)2 Ag(CN)2 2] (P42/mbc, Z= 4) are formed. Therein, /nmv-Cd(NI I3)2 entities (2/m — C2h) are linked by slightly bent NC—Ag—CN units to form a puckered 2-D network with large, 24-membered meshes. Two of these networks (transformed into each other by the 42 operation, i.e., they are perpendicular) interpenetrate.1 3 Similar corrugated sheets occur in [trans-CA (4-Mepy)2 Ag(CN)2 2] (4-Mepy) (4-Mepy = 4-methylpyridin) (Ibca, Z= 16 all atoms in general positions) but here, they are pairwise interwoven in the same layer. The intercalated 4-Mepy molecules are located in the meshes of this layer.219 From the same reaction medium, [Cd(4-Mepy)4 Ag2(CN)3 ][Ag(CN)2] (C2/m, Z = 2) crystallizes after some days. Here, the Cd(4-Mepy)4 units (2jm — C2h) are linked by almost linear NC—Ag—(C,N)—Ag CN rods to form chains, which in turn are arranged in layers. Inserted between the chains or into the layers are linear [Ag(CN)2] anions.219... 

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