Mechanism of Radiation Degradation

TABLE 24.2 Absorption Wavelength and Absorption Coefficient for Some Chemical Bonds in Low Molecular Weight Compounds and Their Transition Types 

---

Understanding of the mechanism of radiation degradation of polymer molecules is essential for development of improved and new industrial processes, for radiation-induced modification of polymer properties, and for selection of polymers for use in radiation environments. This means that the detailed chemical reactions resulting from absorption of radiation must be known. This fundamental understanding must enable us to relate the chemical structure of a polymer to changes in its chemical, physical and material properties. Such structure-property relationships require a great deal of research work, but they are the key to further advancement on a scientific basis. 

Similar mechanism of radiation degradation of poly(tetrafluoroethylene-co-perfluoropropylene) (FEP) was reported based on the NMR spectra [OlGl]. As it is shown in Fig. 44, some of segments disappear, especially the units centered on tertiary carbons, but others are reattached onto molecules building up highly branched structures. 

The capabilities of the ESR technique for providing fundamental information about the mechanism of radiation degradation of polymers are shown in observations on gamma-irradiated poly(methyl methacrylate), polystyrene and their random copolymers. 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

It is from these perspectives that we have reviewed the pulse radiolysis experiments on polymers and polymerization in this article. The examples chosen for discussion have wide spread interest not only in polymer science but also in chemistry in general. This review is presented in six sections. Section 2 interprets the experimental techniques as well as the principle of pulse radiolysis the description is confined to the systems using optical detection methods. However, the purpose of this section is not to survey detail techniques of pulse radiolysis but to outline them concisely. In Sect. 3, the pulse radiolysis studies of radiation-induced polymerizations are discussed with special reference to the initiation mechanisms. Section 4 deals with applications of pulse radiolysis to the polymer reactions in solution including the systems related to biology. In Sect. 5 reaction intermediates produced in irradiated solid and molten polymers are discussed. Most studies are aimed at elucidating the mechanism of radiation-induced degradation, but, in some cases, polymers are used just as a medium for short-lived species of chemical interest We conclude, in Sect. 6, by summarizing the contribution of pulse radiolysis experiments to the field of polymer science. 

The protection of the polymers against low doses occurring in the food irradiation may seem as a simpler task. The packaging films of irradiated food should tolerate much less dose, in the range of 1 kGy or less, but the extract-free stability of those materials in food-contact is even more important than the possible mechanical consequences of radiation degradation. 

One usually observes chain scission type radicals, —CpHp2 (Ca ) R(CHs) in y-irradiated PMMA. Is the chain end radical the primary species of y-irradiation Ichikawa et al. [12] studied the mechanism of radiation-induced degradation of PMMA by ESR and electron spin echo (ESE) methods. Figure 7.12 shows ESR spectra of y-irradiated PMMA observed at 77 K. They assigned spectrum (a) to three kinds of radicals, (C )HO, (C )H3, and —(C )(0 )(OCH3), which were a doublet... 

Figure 12.21 Mechanisms of the degradation of paint fihns (a) degradation of a polymer due to ultraviolet radiation or chemical attack (b) the formation of blisters by osmosis and (c) cathodic delamination due to the formation of corrosion cells.

Mechanism of radiation-induced degradation of poly(methyl methacrylate) as studied by ESR and electron spin echo methods./. Polym. Sci. Part A Polym. 

For the medical device industry, radiation has traditionally been associated with sterilization and the inevitable degradation to material property it causes. However, while destructive to bio-burden and labile materials, radiation can also create novel properties from ordinary polymers. To gain a better understanding of this seemingly contradictory behavior, one needs to explore deeper into the mechanisms of radiation interaction with matter. 

Simulation programs for the ESR line shapes of peroxy radicals for specific models of dynamics have been developed for the study of oxidative degradation of polymers due to ionizing radiation [66]. The motional mechanism of the peroxy radicals, ROO, was deduced by simulation of the temperature dependence of the spectra, and a correlation between dynamics and reactivity has been established. In general, peroxy radicals at the chain ends are less stable and more reactive. This approach has been extended to protiated polymers, for instance polyethylene and polypropylene (PP) [67],... 

---