Mechanism of Radiation-Induced Degradation

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. 

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... 

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

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. 

The data on radiation resistance -have been compiled mainly for practical polymer materials that have been classified into four groups thermoplastics, elastomers, all aromatic polymers, and composite materials. The values of radiation resistance were determined by the change in mechanical properties. Radiation-induced degradation of polymer materials depends greatly on the irradiation conditions, as indicated in the tables hence, the reader is advised to use these data with caution, considering all the factors. 

Characterization of HTPBs chemical-, thermal-, mechanical- and radiation-induced degradation... 

Cheng et al. [116] reported that the structure of primaquine phosphate irradiated with 0.7—10 Mrad remained unchanged. The energy transfer action of the quinolyl group was considerable due to its resonance stabilization. Radiation-induced degradation of polyl(vinyl alcohol) decreased in the presence of primaquine phosphate but the degradation mechanism was unaffected. The content of primaquine phosphate showed linear relationship with degradation parameters of poly(vinyl alcohol). 

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. 

Stillman MJ, Shaw CF III, Suzuki KT (1992) Metallothioneins. Synthesis, structure, and properties of metallothioneins, phytochelatins and metal-thiolate complexes. VCH Publishers, New York Stratford IJ, Hoe S, Adams GE, Hardy C, Williamson C (1983) Abnormal radiosensitizing and cytotoxic properties of ortho-substituted nitroimidazoles. Int J Radiat Biol 43 31-43 Stubbe J, Kozarich JW (1987) Mechanisms of bleomycin-induced DNA degradation. Chem Rev 87 1107-1136... 

Contrary to the case of most polymers, the radiation-induced degradation of PMMA is inhibited by O, though the mechanism of this inhibition has not been established [Chapiro, 1962]. 

The radiation-induced degradation mechanism of chitosan in the aqueous HjOj solution is more complicated than that in solid state. Besides the reactions in solid state, the primary reactions might occur as follows (reactions (31.11) through (31.14)) ... 

Polytetrafluoroethylene (Teflon) belongs to the group of degrading polymers when irradiated. It is relatively radiation resistant in the absence of oxygen, but rapidly deteriorates in air or oxygen atmosphere. The decomposition takes place with chain mechanism with participation of alkoxy radicals. In practice, radiation-induced degradation is used to produce powdered Teflon. The process requires several hundred kGy dose and the powder is used as lubricant after blending with other materials. 

As mentioned above, the mechanism of degradation of PTFE is unclear, particularly the location of radiation-induced chemical reactions. However, ESR spectroscopy (290-293) has confirmed that the stable free-radical species at low temperatures are the chain-end radicals IX (shown below) and that at higher temperatures the secondary radicals X dominate the spectrum. Hedvig (291) has suggested that chain-end radicals terminate by abstraction to form the more stable secondary radical. These radicals persist for very long times, and this leads some authors (58) to suggest the suitability of PTFE as a radiation dosimeter. 

FTIR techniques in combination with or as complement to other measurement techniques have been used in a wide range of photochemistry studies on polymers. These include bisphenol-A polycarbonate [173], polycarbonate coatings on mirrors [174], PMMA [175], poly( -butyl acrylate) [176] and polypropylene [177]. DSC and FTIR studies have been used in conjunction to investigate the nature of y-radiation-induced degradation and its effect on the 19°C and 30°C phase transitions in PTFE [178]. IR studies of the hydrolysis of melamine-formaldehyde crosslinked acrylic copolymer films have shown that copolymer-melamine formaldehyde crosslinks are broken and that crosslinks between melamine molecules are formed [179]. The thermal and photo-degradation mechanisms in an IR study of cured epoxy resins were found to be related to the autoxidative degradation processes for aliphatic hydrocarbons [180]. 

As has been already mentioned in Chapter 16, the radicals are involved in the chain initiation, propagation, branching, and termination steps of thermally, catalytically, mechanically or radiation induced processes. Two kinds of free radicals are of key importance carbon centered macroradical R, and oxygen centered alkyl peroxyl POO , acyl RO and acylperoxyl RC(0)00 . Suitable stabilizers or stabilizer systems can inhibit or delay degradation [26]. 

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