Techniques, high-energy radiation

Table S.9 Applications of high-energy radiation techniques in nanotechnology [118].

In comparison with most other analytical techniques, radiochemical methods are usually more expensive and require more time to complete an analysis. Radiochemical methods also are subject to significant safety concerns due to the analyst s potential exposure to high-energy radiation and the need to safely dispose of radioactive waste. 

Polytetrafluorethylene (p.t.f.e.) This polymer does not absorb water, has no solvents and is almost completely inert to chemical attack molten alkali metals and sodium in liquid ammonia are the rare exceptions. Furthermore it does not soften below 320°C, is electrically inert and has a very low coefficient of friction. It is more expensive than general purpose plastics, requires special fabrication techniques, is degraded by high energy radiation, and has a low creep resistance. 

A major complication in applying radiation chemical techniques to ion-molecule reaction studies is the formation of nonionic initial species by high energy radiation. Another difficulty arises from the neutralization of ions, which may also result in the formation of free radicals and stable products. The chemical effects arising from the formation of ions and their reactions with molecules are therefore superimposed on those of the neutral species resulting from excitation and neutralization. To derive information of ion-molecule reactions, it is necessary to identify unequivocally products typical of such reactions. Progress beyond a speculative rationalization of results is possible only when concrete evidence that ionic species participate in the mechanism of product formation can be presented. This evidence is the first subject of this discussion. 

When a polymeric system is exposed to high-energy radiation, the system undergoes main-chain scission and the creation of cross-links, end-links, double bonds, free radicals, etc. The neutral radicals are the key promoters of the reactions above, and the structure, reactivity (stability), migration, etc., have been extensively investigated by many techniques including ESR. Details of the techniques and their results are described in other chapters or reviews (for a book, see Ref. 29), and the eifects of the above reactions on polymeric systems are mainly discussed in the present section. 

Low energy initiation techniques [179, 180, 181] (near infrared, ultrasonic radiation and line tuneable pulse laser) have lately emerged to be better alternatives to the high-energy radiations (y-irradiation and e-beam). Laser-induced polymerisation of monomers have attracted significant attention in recent years generating a considerable literature published on both pulsed... 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

Rather than approach the answer to this question by reviewing the possible intermediates formed either directly or indirectly by the absorption of high energy radiation in an organic liquid, let us look at some of the experimental data obtained by different techniques in various mono-... 

Radiolysis Bond cleavage induced by high-energy radiation. The term is also more loosely used for any chemical process brought about by high-energy radiation. The term has also been used to refer to the irradiation technique itself ( pulse radiolysis ). 

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