Mechanochemical phenomena

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

II. MECHANOCHEMICAL PHENOMENA IN THE CASE OF REPEATED DEFORMATION OF HIGHLY ELASTIC POLYMERS... 

Therefore, the mechanochemical phenomena are also present in the case of highly elastic state polymers processing even if their intensity is lower in comparison with vitreous state polymers. 

The mechanochemical phenomena connected with the polymer fluid at inlet are determined to the greatest extent by elongation and to a lesser extent by shearing. 

A catastrophic failure of stressed metals in corrosive solutions in comparison with their stable behavior in air can be explained by the synergetic interaction between mechanical and chemical processes described as mechanochemical phenomena [53]. Chemical (electrochemical) reactions proceeding on the metal surface and causing additional dislocation flux and localized enhanced plasticity, affect the fine microstructure and creep properties of a solid. 

Apparently all the cases of breakdown of polymer bodies under mechanical influences are related to one degree or another to the development of mechanochemical reactions in these bodies. As was shown above, this is unquestionably observed in the breakdown of elastic polymer bodies under repeated mechanical influences. We can scarcely doubt that mechanochemical phenomenon also lie at the basis of the phenomena of mechanical wear of elastic polymer materials operating under conditions of friction. 

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 ... 

It has been established that mechanochemical treatment increases the specific surface area of V2O5 and produces V" ions. The latter phenomenon is indicated by an appearance of the low-energy contribution in the XPS spectrum of V 2p electrons. The STM study [13,14] showed that after the mechanochemical treatment in ethanol the change of V2O5 texture took place due to an anisotropic plastic shift deformation along the planes parallel to (001). This leads to an increase of the relative exposure... 

A simple concentration consideration alone is difficult to explain the above phenomenon. It is possible that the associated, liquid-like water can build a strong osmotic pressure at the interface that will stress the oxane bonds. The oxane bonds at the interface will be subjected to stress. When a tensile stress is applied to the bond, the mechanochemical effect lowers the apparent activation energy of the hydrolysis of the oxane bonds, making it easier to be hydrolytically cleaved. The aforementioned differences in the hydrothermal stability of various oxane bonds may become especially important under such circumstances, ily cleaved, non-siloxane bonds may become the source of water collection and exert pressure on the surrounding siloxane bonds, which will be cleaved easier than the siloxane bonds with pure silica surface. 

The Gaussian scission probability distribution function, with a preference for mid-chain scission, is frequently encountered in shear-induced mechanochemical degradation. The parabolic distribution, on the contrary, indicates a preference for chain-end degradation. This phenomenon has been reported in the hydrolysis of dextran, as a result of chain branching. 

Other techniques have found specific use in evaluation of mechanochem-istry. For example, chemiluminescence is observed in certain polymer reactions involving oxidation. Thus, luminescence has been proposed for following the extent of mechanochemistry. The method can be 10-100 times as sensitive as ESR. It has been observed that, in cyclic stretching and contraction, polymers begin to luminesce with the maximum corresponding to the moment of rupture. The same phenomenon appears on grinding and fatigue failure [43, 44]. 

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