Irradiation with Ionizing Radiation

Polymer radicals having free radical centers on the backbone chain (to initiate graft copolymerization) can also be produced by irradiation of a polymer-monomer mixture with ionizing radiation. For example, poly(ethylene-grq/i-styrene) can be produced by the irradiation of a monomer-swollen polymer and the initiation reactions can be represented (Odian, 1991) by 

A mixture of graft copolymer, parent polymer (polyethylene), and homopolymer (polystyrene) results from the operation. 

Plasma being a mixture of electrons, ions, excited molecules, radicals and energetic photons (UV light) is able to initiate chemical reactions on polymer sur- 

Allcock, H. R. and Lampe, F. W., Contemporary Polymer Chemistry , Prentice Hall, Englewood Cliffs, New Jersey, 1990. 

Brandrup, J. and Immergut, E. H., Eds., Polymer Handbook , 2nd ed., Wiley Interscience, New York, 1975. 

Styrene Acrylonitrile Methyl methacrylate Vinyl acetate Vinyl chloride 

Poly(methyl methacrylate) Poly(vinyl acetate) Poly(vinyl chloride) 

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Flexible polyurethane foam pre-irradiated with ionizing radiation in the presence of air can be readily grafted with vinyl monomers. Relatively low radiation doses (3 to 10 megarads) are required to graft various polar vinyl monomers. Grafting of as little as 2% by weight of acrylamide, methacrylamide, or acrylic acid converts the normally hydro-phobic polyurethane foam into a water-wettable sjjonge. 

The ESR spectra of polyisobutylene after irradiation with ultraviolet light (6) are different from those obtained after irradiation with ionizing radiation. The spectra consists mainly of two components one, a sharp quartet which has a half life of 1% hours at liquid nitrogen temperature, has been attributed to free methyl radicals (XI), in analogy with ultraviolet-irradiated polypropylene (51). The broad component is composed of many superimposed lines and was interpreted as caused by three different radicals, all stable at liquid nitrogen temperature. One of these radicals (XV) is the counterpart to the methyl radical (XI) while the others are the two radicals (XIII and XVI) which can both be formed by hydrogen abstraction. 

A more reliable means of providing a reference of -OH in a biological system maybe by means of irradiation with ionizing radiation (von Sonntag et al. 2000). The action of ionizing radiation on an aqueous medium gives rise to OH whose yield/dose relationship (G value) is known (Chap. 2). Apart from this, since biological media are concentrated solutions the formation of the indicator product, e.g., a phenol (ArOH), via the direct effect [expressions (69) and (70)] must in principle be taken into account as well. It can be shown that with k4i [probe]/ k42 [cellular components] above 10 4 the direct effect contributes less than 10%... 

Free radicals are formed when water is irradiated with ionizing radiation... 

Polyethylene is known as one of the typical polymers which crosslink under the influence of ionizing radiation. The radiation-induced crosslinking (the formation of C-C covalent bonds between polymer chains) of polyethylene has long attracted the interest of a large number of research workers, because this polymer has the most simple chemical structure for the fundamental study of radiation effects on polymers, and also because the irradiation with ionizing radiation is a practically important means of modifying the mechanical and thermal properties of polyethylene. 

Tsuji et al. (77) observed that the component other than methyl radicals observed in the spectrum immediately after irradiation is mainly a four-line one. This spectrum changed into an apparent eightline spectrum after standing in the dark for several days at —196° C, and reverted to the original four-line spectrum on UV irradiation. These changes of spectra are identical to those reported by Iwasaki et al. (80) for polypropylene irradiated with ionizing radiation, and are attributed to radical conversions as follows. 

These free radicals produced in polystyrene irradiated with UV light should be compared with cyclohexadienyl type radicals produced after irradiation with ionizing radiation (89). 

Charlesby et al. (90) initially observed a well-known 5 +4-line spectrum after UV irradiation of the polymer at room temperature, as shown in Fig. 13. This spectrum has been observed by various methods of radical formation, irradiation with ionizing radiation (95), bulk polymerization (93), and mechanical fracture of the polymer (96). Although there has been much discussion of this spectrum over several years, recently Iwasaki et al (97) satisfactorily identified it as the propagating-type free radicals... 

When this polymer was irradiated at 25° C with the light of 313 nm, a poorly resolved, apparent asymmetric five-line spectrum was observed (122), and was tentatively attributed to O—CH2—CH2—(singlet) and p—C6H3- (1 2 1 1 2 1). The quantum yield for radical formation with 313 nm was 1.5 x 10 4. It was also found that the free radicals —O—CH—CH2—O— which could be produced by irradiation with ionizing radiation decayed out under UV irradiation. 

Sterilization can be achieved by moist or dry heat, by ethylene oxide (or other suitable gaseous sterilizing agent), by filtration with subsequent aseptic filling of sterile final containers, or by irradiation with ionizing radiation (but not with ultraviolet radiation unless the process is thoroughly validated). Each method has its particular applications and limitations. Where possible and practicable, heat sterilization is the method of choice. 

Radiation induced reactions in dilute aqueous solutions have been studied over fifty years and, from a practical view point, almost enough knowledge has been accumulated. When the aqueous solution is irradiated with ionizing radiation, ionization and excitation take place. As a result, water decomposition products such as e q", OH and H are formed and will react... 

When the solution contains thiols (RSH) in addition to other requisite metal salts and is irradiated with ionizing radiation, solvated electrons react with the thiol group (Equation 23.7) yielding HS", which reacts with metal ions producing metal sulfide (Equation 23.8) followed by nanoclusters ... 

In the early stages of ESR application to polymer research, many studies on the identification of free radicals produced by irradiation with ionizing radiation, x-ray, and ultraviolet light were made. Some of the irradiation effects in polymeric materials were considered to originate from the radical processes and, therefore, clear identification of the radicals trapped in irradiated polymers was one of the most important problems at that stage. In this meaning, ESR application was considered to be a very convenient technique for this purpose, because detection and identification of the free radicals bearing unpaired electrons in principle can be done easily by the ESR method without any chemical modification of the materials. 

In conclusion, color changes of medical-grade Y-TZP induced by irradiation with ionizing radiation such as y- and X-rays can be explained by two-electron transitions at local lattice defects ... 

Radiation erosslinking is an interesting alternative for thermal cross-linking [2-4] or its complement [5-7]. One of extensively studied polymers in respect to its radiation erosslinking, is acrylonitrile-butadiene mbber (NBR) [8-12], which belongs to group of polymers, which effectively crosslink on irradiation with ionizing radiation. 

ESR is the only technique that is particularly suitable, not only for direct detection, but also for quantitative analyses of free radicals in a solid or fluid. In polymers, free radicals are generally formed as a result of the unpairing of electrons from a particular molecular site (chain, side, or ring) by mechanical or chemical means, or by irradiation with ionizing radiation. The possession of an unpaired electron makes the free radical a good candidate for ESR study. Soon after the ESR instruments became available in mid-1950, free radical measurements in polymers, including polyethylene (PE), began. Most of the early works on... 

The fate of the tryptophan residue when proteins are irradiated with ionizing radiation is discussed subsequently. 

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