Discussion of the Mechanism

Thermal deconsolidation can be regarded in a certain sense as an inverse process of consolidation. To investigate the mechanisms of thermal deconsolida- 

Fiber reinforcements are compacted in a consolidation procedure in which external loads are applied to compress fibers, to squeeze air and resin out, to suppress voids, and to increase the fiber volume fraction. Before compaction, the fiber reinforcement networks are unable to carry traction stresses at/or below a certain initial critical fiber volume fraction,. As the fiber volume fraction, pj, increases under compression, the network can carry a rapidly increasing load. Eventually, the fiber volume fraction of the network approaches a theoretical maximum based on the relevant close-packed geometry, and cannot increase without an enormous increase in load. The compressibility of the fiber reinforcement network is dependent not only on the elastic properties of fibers, but also on the configuration of the fiber reinforcement network as well, that is 

p compaction the pressure required to compress the fiber reinforcement network to a specified Pf. 

After compaction of the fiber reinforcement network is completed and the matrix melt becomes a solid after cooling down (achieving consolidation), the elastic energy of the fiber reinforcement network is stored in the composite in the form of residual stresses. When the consolidated composite undergoes a reheating operation and its matrix commences to remelt, the elastic energy stored in the fiber reinforcement network tends to release, which is referred to as decompaction of fiber reinforcement. In this way, the decompaction of the fiber reinforcement network, denoted by should be proportional iop, , namely. 

is a factor relevant to the material and structural properties of the fiber reinforcement network, ranging from 0 to 1. The limiting case is that k = 1 for the compressibility of the fiber reinforcement network being fully elastic and k = 0 for it being fully plastic . Since in most practical consolidation procedures the fiber reinforcement networks are not severely damaged, k = 1 can be taken as a reasonable first order approximation. 

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Other aspects of stabilization of acetal resins are briefly discussed under processing and fabrication. Reference 15 provides a more detailed discussion of the mechanism of polymer degradation. 

Stacking faults thereby providing barriers to sHp. If carbides are allowed to precipitate to the point of becoming continuous along the grain boundaries, they often initiate fracture (see Fracture mechanics). A thorough discussion of the mechanical properties of cobalt alloys is given in References 29 and 30 (see also Refractories). 

CRV737) A critical discussion of the mechanisms of ring-opening of epoxides by nucleophiles. 

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

Interestingly enough, both protons at C-11 are exchanged quite readily in 12-keto steroids. In these compounds C-11 is the only possible enolization site where the axial (/3) proton is probably expelled first. During ketonization, the deuteron attack is more likely to occur from the less hindered a-side. By this sequence the proton which was originally at the lla-equato-rial position becomes axial and readily available for expulsion in the next enolization step. Thus, isomerization of the C-11 hydrogens may be an important reason for the facile exchange at this position. (For a more detailed discussion of the mechanism of enolization and ketonization reactions, see ref 114.)... 

Alkynes can be prepared by the elimination of HX from alkyl halides in much the same manner as alkenes (Section 7.1). Treatment of a 1,2-dihaloaJkane (a vicinal dihalide) with excess strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, we ll defer a discussion of the mechanism until Chapter 11. 

We ll defer a detailed discussion of the mechanisms of these reductions until Chapter 19. For the moment, we ll simply note that they involve the addition of a nucleophilic hydride ion ( H ) to the positively polarized, electrophilic carbon atom of the carbonyl group. The initial product is an afkoxide ion, which is protonated by addition of H 0+ in a second step to yield the alcohol product. 

The paper is oiganized to describe, first, the materials that have been used in OLEDs, then the device structures that have been evaluated. After a description of the methods used to characterize and evaluate materials and devices, we summarize the current stale of understanding of the physics of device operation, followed by a discussion of the mechanisms which lead to degradation and failure. Finally, we present the issues that must be addressed to develop a viable flat-panel display technology using OLEDs. Space and schedule prevent a comprehensive review of the vast literature in this rapidly moving field. We have tried to present... 

A review of the rich and varied chemistry of this compound has appeared.287 The reader is referred to this article for a discussion of the mechanism of the various transformations, a selection of which follows. 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

As described more fully in Sections 3.1-3.3, with increasing pH the reactive forms of the diazotizing agent are converted into ineffective ones, namely free nitrous acid, HN02, and the nitrite ion, N02. From the discussion of the mechanism of diazotization it will also become apparent why the reaction proceeds better, that is faster, in dilute hydrochloric than in dilute sulfuric acid. With very slow diazotizations for instance, because of high dilution as in nitrite titrations, the use... 

The position is complicated by several factors. The significance of the equilibria in which nitrous acid takes part will be detailed in the discussion of the mechanism... 

In Sections 3.1 to 3.3 the discussion of the mechanism of diazotization concentrated on the rate-determining part of the reaction which, in most cases, does not include steps occurring after the formation of the A-nitrosoamine. The various pathways of nitrosation are summarized in Scheme 3-35. The transformation of the N-nitrosoamine will be discussed in Section 3.4. 

It is apparent from DTA studies [1021] of the decompositions of Group IA formates in inert or oxidizing atmospheres that reaction is either preceded by or accompanied by melting. Anion breakdown leading to carbonate production may involve formation of the oxalate, through dimerization [1022] of the postulated intermediate, C02, especially during reaction of the Na and K salts in an inert atmosphere and under isothermal conditions. Oxalate production is negligible in reactions of the Li and Cs formates. Reference to oxalate formation is included here since this possibility has seldom been considered [1014] in discussions of the mechanisms of decompositions of solid formates. 

Decompositions of oxalates containing the strongly electropositive metals yield an oxide product but the more noble elements yield the metal. Discussion of the mechanisms of these reactions and, in particular, whether metal formation necessarily involves the intermediate production of oxide which is subsequently reduced by CO has been extended to consideration of the kinetics of pyrolysis of the mixed oxalates [32]. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

What impressed me particularly was the wealth of high standard theoretical electrochemistry in discussions of the mechanism of NEMCA, for one seldom sees publications showing so much erudition in the theory of electrified surfaces. On the other hand, the book contains a very full treatment, rich in examples, of the practical and experimental side of NEMCA and thus will be attractive to the chemists and chemical engineers who serve in corporate research laboratories. It is likely to lead to advances in industrial... 

The foregoing considerations provide the basis for a discussion of the mechanism of nuclear fission. 

The hydrolysis of epoxides is a eonvenient method for the preparation of vic-diols. The reaction is catalyzed by acids or bases (see discussion of the mechanism on p. 462). Among acid catalysts the reagent of choice is perchloric acid, since side... 

For a discussion of the mechanism of hydrolysis of alkyl nitrites, see Williams, D.L.H. Nitrosation Cambridge University Press Cambridge, 1988, p. 162. 

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