Mechanochemical destruction

This point of view concerning the mechanism of mechanochemical destruction of macromolecular compounds is based on two factors. One is the gradual character of this process, as reported by us earlier (JO). The other factor is the increase in potential energy and, consequently, the chemical reactivity of polymer systems that are placed in a stressed state mechanically. 

Scission of chemical bonds results from the concentration of mechanical energy of the structural inhomogeneities this represents mechanocracking or mechanochemical destruction. 

The idea of this synthesis was suggested by a phenomenon observed during the mechanochemical destruction of polyamides. In addition to the main reaction of homolytic fragmentation of macromolecular backbones, a polycondensation occurred, involving the interaction of the amino and carboxylic end groups. These were condensed until they disappeared from the reaction medium ... 

The mechanochemical polycondensation reaction has been studied using heterochain polymer systems—polyethylene terephthalate poly-(e-caprolactam), cellulose, etc.—characterized by end groups that can be activated to increase their own number by mechanochemical destruction of corresponding polymers. The mechanochemical destruction was done in the presence of some suitable condensing agents, such as aliphatic and aromatic diamines and fatty acid dichlorides. 

An attempt to verify the two hypotheses and to obtain information regarding the nature of the active centers formed by mechanochemical destruction of the support polymer consisted of the selection of three kinds of polymers for study ... 

Those containing end groups whose number can be increased by mechanochemical destruction—polyethylene terephthalate, poly (e-caprolactam), and cellulose. 

The results so far suggest that to achieve this reaction, it is important to use polymers containing functional groups susceptible to mechanochemical destruction along the main chain. It remains to be proved by additional experiments whether the active centers obtained react directly with the condensing agents, or whether there first occurs a stabilization of the end groups, followed by condensation with the partner. 

Furthermore, Yu et al. developed combined process for mechanochemical destruction of pentachloronitrobenzene (PCNB) in solid waste, coupled with the... 

The anisotropy of the physico-mechanical properties constitutes an important factor, which controls the mechanochemical destruction. Thus, the polymers characterized by isotropic properties are milled, forming particles with a reduced degree of asymmetry, so that a determined direction of the destruction process could not be noticed. This fact evidently comes out from the increase of the polymer anisotropy, implicitly characterized, as well by the increase of the asymmetry degree of the produced particles. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

The fundamental task, in our opinion, is to correlate the principles and methods of the proposed synthesis with those of mechanochemical synthesis. Thus, besides the destruction processes and mechanochemical synthesis discussed in the literature, other lands of transformations sometimes occur as side reactions, or even as major processes. These include chemical fixation of small molecules (methyl chloride or butyl alcohol) on mechanically activated macromolecular backbones grafting of inorganic surfaces (quartz, metals, metallic oxides, inorganic salts, etc.) dispersed by vibratory milling on polymerized fragments synthesized from monomers present in the reaction medium, and activated by centers on the inorganic surface (14) and the possibility of some reactions (such as nitration), achieved so far on macromolecular supports and only as side reactions. 

Stability of nanocrystalline powder samples, synthesized in Sr-Ba-Fe-Co-0 system by the mechanochemical activation route in Ha atmosphere is investigated. Samples of various compositions were used to establish the main reasons of samples destruction. XRD, EPR and XPS methods were employed to characterize the samples before and after treatment in the Ha atmosphere (600 °C, 1 h). Essential content of cobalt in a sample was found to degrade its stability, whereas an insertion of additional amount of FeaOj at the end of the mechanochemical activation enhances stability. 

Essential content of cobalt in the mixed-oxide Sr-Ba-Fe-Co-0 system degrades its stability in reducing atmosphere, giving rise to sample destruction because of Co" reduction (metallic cobalt may be a product of this process). Insertion of additional amount of Fc203 at the end of the mechanochemical activation enhances stability of the sample, because it favors formation of amorphous layers preventing destruction. 

In practice, the choice of WI is based on the analysis of service, design, economic and other factors. An efficient means of improving wear resistance of the polyamide-steel 45 friction pair turns out to be inhibition of the thermally oxidative and destructive processes in the polymer surface layers to avoid the formation of corrosion-active oxide compounds. It is possible to break this unfavorable cycle by the introduction of antioxidants into the polymer composition, thus disabling macroradicals through the reactions of mechanochemical synthesis, polyamide alloying by functional additives forming separating layers or more thermally stable products, and so on [108]. Application of WI with this aim abates undesirable thermally oxidative processes in polyamide... 

In the majority of mechanical applications of materials, their surfaces experience contact with another material and take the external load before the bulk of the material is influenced. In some cases, surface interactions influence the bulk (e.g., propagation of cracks dislocations or point defects from the surface in depth). In many cases, only the outermost surface layer is affected by the surface contact with no detectable changes in the bulk of the material. This is like a storm that is frightening and destructive on the ocean surface, but does not have any influence on deep-water life. We are primarily concerned in this review with that kind of interaction. The surface layer thickness affected by external mechanical forces ranges from nanometers to microns. Thus, in our case, the definition of surface is different from the one used by surface scientists, that is, physicists and chemists. We introduce here an engineering definition of surface the outermost layer of the material that can be influenced by physical and/or chemical interaction with other surfaces and/or the environment. In this chapter, we consider only mechanical effects, but both mechanical and chemical interactions are possible and their synergy can lead to mechanochemical alteration of a material surface. 

Ramirez ALB, Kean ZS, Orlicki JA, Champhekar M, Elsakr SM, Krause WE, Craig SL (2013) Mechanochemical strengthening of a synthetic polymer in response to typically destructive shear forces. Nat Chem 5 757-761... 

We should not confuse chemical flow, based on mechanochemical reactions, with chemical relaxation and analogous phenomena [33], which are based on purely chemical decomposition processes (for example, oxidative destruction) of the stressed three-dimensionally structured highly elastic polymers. 

The molecular weight decrease and the possibility of obtaining some reactions of the PVC with the aromatic diamine-type radical acceptors are conclusive arguments for a radical destructive mechanism of the polymers under vibratory milling conditions. Consequently, mechanodispersion is a mechanochemical-type process, developing itself according to the general laws of polymer fracture under stress. 

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