Extended Chains

Annis B K, Noid D W, Sumpter B G, Reffner J R and Wunderlich B 1992 Application of atomic force microscopy (AFM) to a block copolymer and an extended chain polyethylene Makromol. Chem., Rapid. Commun. 13 169 Annis B K, Schwark D W, Reffner J R, Thomas E L and Wunderlich B 1992 Determination of surface morphology of diblock copolymers of styrene and butadiene by atomic force microscopy Makromol. Chem. 193 2589... 

We have so far described the structure of DNA as an extended double helix The crys tallographic evidence that gave rise to this picture was obtained on a sample of DNA removed from the cell that contained it Within a cell—its native state—DNA almost always adopts some shape other than an extended chain We can understand why by doing a little arithmetic Each helix of B DNA makes a complete turn every 3 4 X 10 m and there are about 10 base parrs per turn A typical human DNA contains 10 base parrs Therefore... 

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

Isotactic. All substituents lie on the same side of the extended chain. Alternatively, the stereoconfiguration at the asymmetric centers is the same, say, -DDDDDDDDD-. 

Syndiotactic. Substituents on the fully extended chain lie on alternating sides of the backbone. This alternation of configuration can be represented as -DLDLDLDLDLDL-. 

PBO andPBZT. PBZ, a family of/ -phenylene-heterocycHc rigid-rod and extended chain polymers includes poly(/)-phenylene-2,6-benzobisthiazole) [69794-31-6] trans-V 27V) and poly(/)-phenylene-2,6-benzobisoxazole) [60871-72-9] (ot-PBO). PBZT and PBO were initially prepared at the Air Force Materials Laboratory at Wright-Patterson Air Force Base, Dayton, Ohio. PBZT was prepared by the reaction of... 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

A variety of aromatic and extended-chain polyamides that spontaneously form a mesophase ia coaceatrated solutioas also have beea syathesized (30). Polybeozamide [24991-08-0] with the foUowiag repeat uoit, is an example. 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

The macroscopic orientation of their extended-chain crystals depends on the orientation imparted by flow during mol ding. Because of the fibrous nature of the extended-chain crystals, these materials behave as self-reinforcing composites, with excellent mechanical properties. 

There is only one sequence-specific interaction with an amino acid side chain, which is provided by Lys 18 (red). The linker region is an extended chain that follows one strand of the DNA and provides several nonspecific contacts (blue) to the DNA. The numbering of the base pairs starts from the center of the DNA fragment. (Adapted from R. Matmorstein et al., Nature 356 408 14, 1992.)... 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

There is no unanimity in regard to the exact mechanism of ECC formation under high pressure. Wunderlich et al. [11-18] suggested that when a flexible polymer molecule crystallizes from the melt under high pressure, it does not grow in the form of a stable extended chain, rather it deposits as a metastable folded chain. 

A hexagonal phase appears at atmospheric pressure in ultra-drawn PE fibers containing highly extended chains when the fiber is heated above the normal temperature while the ends are held fixed to prevent retraction [89-92]. 

Sawada et al. [110] and the authors of this Chapter [104,111] have proposed another theory, the bundle-like nucleation theory, for the mechanism of ECC formation. Both groups of workers suggested that crystallization under high pressure starts from partially extended-chain nucleation rather than from the folded-chain nucleation as proposed by Hikosaka [103,104]. This theory was established on the basis of the following facts ... 

Extended- and folded-chain crystallization are mutually independent processes, and extended-chain crystallization can take place prior to folded-chain crystallization [6,19-25]. 

Our results [104,117] have shown that the extended chains of ethyl cellulose liquid crystal at high pressures can act as the nucleus of PE ECC and induce the formation of ECC. (The details will be introduced in Section IX.)... 

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