Amorphous network

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

The structure of a-C H DLC consists of an essentially amorphous network with isolated clusters dominated by the sp configuration (graphite) with some sp (diamond). Hydrogen is believed to play an essential role in determining the bonding configuration by helping to form the sp bond, probably in a manner similar to the formation of CVD diamond. 

This broad band at 1500 cm was ascribed by Kaufman. Metin, and Saper-stein [10], to an IR observation of the amorphous carbon Raman D and G bands. This is forbidden by the selection rules, and has been attributed to the symmetry breaking introduced by the presence of CN bonds in the amorphous network. As carbon and nitrogen have different electronegativities, the formation of CN bonds gives the necessary charge polarity to allow the IR observation of the collective C=C vibrations in the IR spectrum. This conclusion was stated by the comparison of spectra taken from films deposited from N2 and N2. In the N2-film spectrum, no shift was observed for the 1500-cm band, whereas all other bands shifted as expected from the mass difference of the isotopes. Figure 25 compares... 

Both appear to be acceptable. And indeed they lead to quite acceptable solutions for amorphous networks. But neither of these constraints appear satisfactory for semi-crystalline networks, at least a proper solution has not yet been found. Our choice of vectois then is not a choice at all but rather an Imposition. 

We can assert outright that a semi-crystalline network in a continuous state of equilibrium might be satisfactorily described by a representative chain of average contour length and average crystallization. This is, after all, the same assumption that is invariably applied to amorphous networks. The results in this instance are exactly the same as those presented herein. 

Figure 2.3 In the first step, the mixture is emulsified by stirring, and in the second step, an amorphous network of glassy material is prepared at room temperature by the hydrolysis of suitable monomers. The reaction proceeds to a condensation polymerization reaction, followed by subsequent formation of the sol to the gel and xerogel stages. (Adopted from Merck.com)...

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

According to the statistical-mechanical theory of rubber elasticity, it is possible to obtain the temperature coefficient of the unperturbed dimensions, d InsjdT, from measurements of elastic moduli as a function of temperature for lightly cross-linked amorphous networks [Volken-stein and Ptitsyn (258 ) Flory, Hoeve and Ciferri (103a)]. This possibility, which rests on the reasonable assumption that the chains in undiluted amorphous polymer have essentially their unperturbed mean dimensions [see Flory (5)j, has been realized experimentally for polyethylene, polyisobutylene, natural rubber and poly(dimethylsiloxane) [Ciferri, Hoeve and Flory (66") and Ciferri (66 )] and the results have been confirmed by observations of intrinsic viscosities in athermal (but not theta ) solvents for polyethylene and poly(dimethylsiloxane). In all these cases, the derivative d In sjdT is no greater than about 10-3 per degree, and is actually positive for natural rubber and for the siloxane polymer. 

Cross-linked amorphous networks of flexible chains Rigid chains... 

Fig. 1.8. The Anderson model of the potential wells for (a) a crystalline lattice and (b) an amorphous network. is the disorder potential.

The removal of hydrogen from the surface is more complicated than in the above model, because many dangling bonds which result from the release of hydrogen reconstruct into Si—Si bonds which form the amorphous network. The final hydrogen content is set by a delicate... 

The first question to address is the definition of a defect in an amorphous material. In a crystal any departure from the perfect crystalline lattice is a defect, which could be a point defect, such as a vacancy or interstitial, an extended defect, such as a dislocation or stacking fault, or an impurity. A different definition is required in an amorphous material because there is no perfect lattice. The inevitable disorder of the random network is an integral part of the amorphous material and it is not helpful to think of this as a collection of many defects. By analogy with the crystal one can define a defect as a departure from the ideal amorphous network which is a continuous... 

---