Radiation-induced reaction

The possibility of initiating chemical reactions by means of the ionizing radiation from radionuclides has been extensively investigated, but it has not found the broad application in industry that had been expected, because of possible radiation hazards. 

As radiation sources y-ray emitters, such as Co or Cs, and electron accelerators are applied. Nuclear reactors and spent fuel elements have also been discussed as radiation sources. 

Because of the similarities, photochemistiy can be considered to be the low-energy branch of radiation chemistry. In both fields, the primary reactions are followed by secondary and subsequent reactions. Chain reactions are of special interest. 

The yield of a photochemical reaction is characterized by the quantum yield, which is the number of molecules formed or decomposed by one photon. In radiation chemistry, the C-value is used as a measure of the chemical yield. It is defined by the number of molecules formed or decomposed per lOOeV energy absorbed in the system. For example, C(H2) = 3 means that 3 molecules of H2 are formed per 100 eV absorbed and C(—H20) = ll means that 11 molecules H2O are decomposed per lOOeV absorbed. 

Some examples of radiation-induced reactions are listed in Table 20.2. Chain reactions exhibiting C-values 10 are of greatest interest for technical applications. 

Of the relaxation methods only the temperature-jump and pressure-jump methods have been adapted for high pressure application, and of these two only the former (hptj) has been used in many systems for volume of activation determinations. Despite the flurry of activity in developing hptj,84 88 the method has not found application in organometallic chemistry, although in principle it could be employed if the system properties and solvent were suitable. 

Details of a standard pressure-jump instrument89 and high-pressure pressure-jump cells can be obtained from appropriate literature.90 93 The method has found very limited application and not at all in organometallic chemistry. Commercial units or modules for high pressure relaxation methods are not available. 

In principle reactions that are induced by a light signal could be in the conventional time range, or rapid, requiring rapid data acquisition. The latter class of reactions became a reality in the history of this research area upon the development of fast 

In this method of inducing reaction, usually in aqueous medium, an electron beam forms radicals from the radiolysis of water in a few microseconds. Subsequent radical reactions with a variety of substrates can be monitored. Authoritative reviews that include results from high pressure kinetics studies have appeared recently.108,109 A specially designed pill-box cell has been developed to enable certain radical reactions to be studied at high pressures. An initial difficulty to construct a cell that is sufficiently robust to withstand high pressures, but allows a required level of electron flux to enter the cell, has been overcome by design of a special window (see Fig. 6).110 

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A thorough consideration of mechanisms of formation of the organometallic products led to the conclusion " that the j5-decay itself must be the cause of the molecule formation. Neither purely mechanical collisiona substitution, nor thermal chemical reactions, nor radical reactions, nor radiation-induced reactions seem to be responsible for the synthesis reactions. 

Chapiro, A. "Radiation Induced Reactions", in Encyclopedia of Polymer Science and Technology, Vol. 11, Interscience, New York, 1969. 

While the presence of methane indicates a reducing atmosphere, that of nitrogen fits better into a (weakly) oxidising environment. It is believed that the present composition of Titan s atmosphere is the result of chemical or radiation-induced reactions. 

The electron itself is frequently used as a primary source of radiation, various kinds of accelerators being available for that purpose. Particularly important are pulsed electron sources, such as the nanosecond and picosecond pulse radiolysis machines, which allow very fast radiation-induced reactions to be studied (Tabata et al, 1991). Note that secondary electron radiation always constitutes a significant part of energy transferred by heavy charged particles. For these reasons, the electron occupies a central role in radiation chemistry. 

In the preceding ten chapters of this book, we have described various important chemical and physical changes brought about by the absorption of ionizing radiation in gaseous and condensed media. Wherever possible, we have tried to elucidate the underlying mechanism with a discussion of the properties and reactivities of the intermediate species. However, the book would remain incomplete without discussion of some of the various uses that have been found for radiation-induced reactions in science and industry. 

Radiation synthesis has been reviewed by Wagner (1969), Vershchinskii (1972), Wilson (1972), and Chutny and Kucera (1974). A good summary is also available in Spinks and Woods (1990). On the theoretical side, radiation-induced reactions of importance to industry can be classified as addition reactions, sub-sitution reactions, and other reactions including polymerization, cross-linking,... 

Various other radiation-induced reactions have been studied for potential use in the industry on a pilot-plant scale. Among these may be mentioned hydrocarbon cracking (i.e., production of lower-molecular-weight hydrocarbons from higher-molecular-weight material), isomerization of organic molecules, and synthesis of labeled compounds with radioactive nuclei. When organic compounds are irradiated in the pure state or in aqueous solution, dimeric... 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

RADIATION-SENSITIVE GROUPS. Although the absorption of radiation energy is dependent only on the electron density of the substrate and therefore occurs spatially at random on a molecular scale, the subsequent chemical changes are not random. Some chemical bonds and groups are particularly sensitive to radiation-induced reactions. They include COOH, C-Hal, -SO2-, NHz, C=C. Spatial specificity of chemical reaction may result from intramolecular or intermolecular migration of energy or of reactive species -free radicals or ions. 

Radiation Induced Reactions. Graft polymers have been prepared from poly(vinyl alcohol) by the irradiation of the polymer-monomer system and some other methods. The grafted side chains reported include acrylamide, acrylic acid, acrylonitrile, ethyl acrylate, ethylene, ethyl methacrylate, methyl methacrylate, styrene, vinyl acetate, vinyl chloride, vinyl pyridine and vinyl pyrrolidone (13). Poly(vinyl alcohols) with grafted methyl methacrylate and sometimes methyl acrylate have been studied as membranes for hemodialysis (14). Graft polymers consisting of 50% poly(vinyl alcohol), 25% poly(vinyl acetate) and 25% grafted ethylene oxide units can be used to prepare capsule cases for drugs which do not require any additional plasticizers (15). 

RADIATION-INDUCED REACTIONS IN POLYMERS 3.1. Fundamentals of Radiation Effects on Polymers... 

It is known that more than 30 reactions are needed to reproduce the radiation-induced reactions occurring in pure water. Intensive measurements with a pulse radiolysis method have been done at elevated temperature up to 300°C [25 2], and the temperature dependence of some reactions does not exhibit a straight line but a curved one in Arrhenius plot. These examples are the reactions of the hydrated electron with N2O, NOJ, NO2, phenol, Se04, 8203 , and Mn [33,35], and two examples, egq + NOJ and ejq -i- NOJ, are shown in Fig. 2. The rate constant for the reaction of hydrated electron with NOJ is near diffusion-controlled reaction at room temperature and is increasing with increasing temperature. Above 100°C, the rate does not increase and reaches the maximum at 150°C, and then decreases. Therefore the curve is concave upward in Arrhenius plot. 

Tabie 4 Rate Constants of Radiation-Induced Reactions in Sodium (Bi)carbonate Aqueous Solution... 

Efficiencies of radiation-induced reactions of selected elastomers are in Table 5.7. Tensile stiength data from another group are in Table 5.8. 

The scheme of radiation-induced reactions of CMS, negative electron resist, is proposed as follows on the basis of the present pulse radiolysis data. 

Radiation-induced Reactions in EAP. To elucidate the reason no vacuum curing effect is observed in EAP, we measured the infrared spectra of EAP films under various conditions. The results are summerized in Figures 3 to 5. The infrared spectrum of a standard EAP film is shown in Figure 3. Absorbance at 910 cm-1 (Aep) caused by the epoxy group and that at 2110 cm-1 (Aaz) caused by the azide group were chosen as measures for concentrations of these groups in the films. 

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