Crystal structure polyamides

Thermoplastic polyester rubbers are also block copolymers of polyethers and polyesters. The polyester groups are capable of crystallisation and the crystal structures act like cross-links. These materials have good hydrocarbon resistance. Similar thermoplastic polyamide rubbers are also now available. 

Tab. 3.1 DNA-binding proteins that have been inhibited by polyamides. The known DNA binding motifs from NMR or crystal structure data are shown. Significant groove contacts and proposed mechanism of polyamide inhibition are also shown for each protein...

Fig. 3.18 Nucleosome core particle (NCP)-polyamide co-crystal structures (PDB codes 1M18 and 1M19). (Top) Partial structure, viewed down the superhelical axis. Base pairs 58-145 (shown in white) and associated proteins (H3, blue H4, green H2A, yellow H2B, red) are shown for each complex. Superhelix locations (SHLs) are labeled as each major...

Metal salts of organic acids (for example, sodium benzoate) are used as nucleating agents, while many other materials will function similarly they are added in order to increase the crystallization in certain materials, notably polyamides, and have the effects of reducing the size of crystal structures but inducing the formation of more of them within a shorter period of time. More... 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

Polyamides are an important example of polymers which do not contain pseu-doasymmetric atoms in their main chains. The chain conformation and crystal structure of such polymers is influenced by the hydrogen bonds between the carbonyls and NH groups of neighboring chains. Polyamides crystallize in the form of sheets, with the macromolecules themselves packed in planar zigzag conformations. 

Figure 2 Designer DNA binding domains, (a) Crystal structure of Zif268 in complex with DNA (PDB accession number 1 aay)(S4). Arg 74, Glu 77, and Arg 80 in positions -1, 3, and 6, respectively, of the recognition helix of finger 3 are shown projected into the major groove, (b) Crystal structure of polyamide (lmHpPyPy)2 in complex with DNA (PDB accession number 407 d).

On the other hand, there are data suggesting that amides have a rather high value for association as pure liquids. Trifan and Terenzi measured IR spectra of several polyamides and polyurethanes from 25°C to 300°G. They deduce an average —AH of 8.5 1.2 kcal/mole for the polymers. Until more data are available we must draw the provisional conclusion that —AH for amide N—bonds in crystals (and possibly in pure liquids) may be much larger than in solutions. Under any circumstances, however, we arc reminded that the data of Davies establish empirically the substantial contribution of the amide groups to the stability of the amide crystal structures. 

Figure 2.11 (a) Hydrogen bonds between neighboring chains of polyamide, (b) Arrangement of chains in hydrogen-bonded sheets in the crystal structure of nylon-6,6. 

It is known that nylon 6 takes two types of a- and y-forms as polymorphs. Weeding et al. [12] and Okada et al. [13] observed the CP/MAS NMR spectra of nylon 6 samples with the a- and y-forms as shown in Fig. 12.7. Table 12.4 shows that there is a significant chemical shift difference between these forms. It is demonstrated that solid-state NMR provides useful information about the crystal structure of polyamides. This leads to solid-state NMR studies on nylon 7 [14] and 11 [15] with polymorphs. 

In addition, the crystal structures of both the racemic copolyamidc 11.7 and the cquimolccular mixture of the two configurationally homogeneous d- and i.-polyamides were studied and compared with that of optically pure 110.93 This study combined X-ray. electron microscopy, and 13C CP-MAS NMR measurements with computational methods. The two optically compensated and the optically pure polymers were shown to be highly crystalline systems the melting point of the racemic mixture was 250°C, considerably higher than those of the homopolymer (232°C) and the racemic polymer (226°C). The crystal structure of the racemic mixture could be... 

Polyamides 2 and 3, which preserve the Im-)S-Im unit, bound DNA sites of similar size, 10- and 11-base pairs, but remarkably different sequence compositions. In the original lexitropsin model based on 1 1 binding of polyamides to DNA, Im was proposed to bind GC/CG > AT/TA [1]. We now know this not to be the case. In a study to be published elsewhere, Im, in the sequence context of polyamide 2, binds all four base pairs (within a factor of 2) [19]. From the crystal structure of the 1 1 netropsin DNA complex, we understand the molecular mechanism by which Py specifies AT/TA > GC/CG [1]. However, this 1 1 recognition code of Py selecting AT/TA > GC/CG must now be modified to include the judicious placement of jS for AT/TA recognition to reset the curvature in the 1 1 motif [9]. This is substantiated by our observation that Im- -Im binds 5 -GAG-3 with higher affinity than Im-Py-Im (Table). 

Bowman-James and co-workers have designed polyamide cryptand-type systems based on triamines, such as tren (e.g. 14) and trpn (e.g. 15), and shown that they bind anions [23]. The crystal structure of the hydrochloric acid and fluoride complexes of 14 reveal that the anions are encapsulated within the cavity of the amidocryptand and boxmd to the six-amide NH groups. In contrast the hydrochloric acid structure of the expanded trpn-based amidocryptand 15 shows the encapsulation of two chloride anions within the cryptand, bridged by a water molecule. Each chloride is boimd to the water molecule as well as a protonated bridgehead amine and two hydrogen bonds from the amides groups. 

The fully extended planar zigzag trcms conformation) is the minimum energy conformation for an isolated section of polyethylene or paraffin hydrocarbon. The energy of the trans conformation is about 800 cal/mol less than that of the gauche form. Consequently, the trans form is favored in polymer crystal structures. Typical polymers that exhibit this trans form include polyethylene, poly(vinyl alcohol), syndiotactic forms of poly(vinyl chloride) and poly(l, 2-butadiene), most polyamides, and cellulose. Note that trans conformation is different from the trans configuration discussed in Section IV.A. 

A few polymers have lower resistivities. Polyamides, with hydrogen bonds that lie in parallel planes in the crystal structure, have resistivities a factor of 100 smaller than non-H-bonded polymers. At temperatures above 120 °C, at least half of the conduction in nylon 6,6 is due to protonic carriers, with hydrogen being liberated at one electrode. Nevertheless, the resistivity is still high. 

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