Polyaromatic structures

Although some important details of the polyaromatic structure shown in Figure 5.11a are not clear [6] (e.g., the nature of sites marked with an asterisk), the main feature of this proposal is the role of pyridinic N, and not of the presumably substitutional N. It is expected to facilitate the creation of... 

Sephton M. A., Pillinger C. T., and Gilmour 1. (1998) S C of free and macromolecular polyaromatic structures in the Murchison meteorite. Geochim. Cosmochim. Acta 62, 1821-1828. 

A few years ago, a novel molecular structure of carbon was detected. Globular C o molecules (or C70 and other sizes) form a molecular lattice. The Ceo buckyballs have a diameter of 1 nm. Within the moieties, the polyaromatic structure sp is fully retained in annealed five- and six-membered rings. The fullerene molecule resembles a football in design. The close analogy to graphite, with sp -structures of 100% C, justifies a treatment immediately following Section 3 on graphite. 

Coke accumulated in the process of partial coking of medium-pore zeolites by olefins is of a condensed, polyaromatic structure and is localized on the surface of the zeolite crystals. 

ACRONYM. TRADE NAME PPX, Parylene N (Union Carbide) CLASS Polyaromatics STRUCTURE... 

Zuev and co-workers [23] conclude that the thermodestruction of poly(4- -alkyl styrenes) is mainly similar to the thermodestruction of PS itself, i.e., formation of monomers. The process is not complicated by the formation of any cyclical polyaromatic structure. 

To ascertain which of the earlier-mentioned mechanisms is responsible for the appearance of signal 2, we studied adsorption of benzene and methane molecules on the dehydrated surfaces of carbonized silicas (Figure 4.20). This figure also shows the spectra for benzene and methane adsorbed on the surface of the initial silica gel. It was assumed that these adsorbates in contrast to water molecules could adsorb on the surface fragments with the polyaromatic structure but they could not form the hydrogen-bonded complexes with electron-donor and proton-donor surface groups. 

Sephton, M.A., Pillinger, C.T., Gilmour, I. (1998) 6 C of free and macromolecnlar polyaromatic structures in the Murchison meteorite. Geochimica et Cosmochimica Acta, 62,1821-1828. 

In a fire, zinc borate on its own forms a vitreous mass on the polymer surface providing a barrier between the flame and the source of support. In antimony- or halogen-based systems it helps formation of Sb-O-Cl groups, extinguishing the flame and suppressing fumes while promoting formation of polyaromatic structures. The Borax material has until now been used mainly for total (or more recently, partial (40-75%)) replacement of the more expensive antimony oxide in PVC, polyolefins, nylons, and elastomers. 

Polyaromatic structures such as polyquinolines 12, that have the biphenylene units incorporated into the main chain, undergo crosslinking reac t ions. ... 

It is important to keep in mind that the hydrogenation of aromatics is thermodynamically limited it is favoured by high hydrogen pressure and low temperature. Moreover, the heavier the molecule, the more unfavourable is the thermodynamic equilibrium constant. The kinetics of hydrogenation depends on the type of molecule. Polyaromatic structures are more easily hydrogenated than light ones, if the Aermodynamic conditions are satisfied. 

The density of the a-polymorph is 1.98 g cm amorphous PVDF has a density of 1.68 g cm . Thus, commercial samples with a density of 1.75-1.78 g cm have 45% crystaUinity. The a-polymorph melts at 170 °C however, the processed polymer, because of its polymorphism, displays no sharp melting point but melts between 150 and 190 °C. The thermal decomposition becomes significant at T > 300°C. Pyrolysis of PVDF yields hydrogen fluoride, the monomer C2H2F2 and C4F3H3 [12]. Up to 600 °C, pyrolysis also yields polyaromatic structures by cyclization of polyenic intermediates formed through HF ehmination [16]. This is a particular advantage over PTFE, which is less likely to yield carbonaceous products. Thus in obscurant applications, PVDF is preferred over PTFE as a fluorine source (see Chapter 11). 

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