Polymer radiation-induced degradation

Radiation-induced degradation reactions are in direct opposition to cross-linking or curing processes, in that the average molecular weight of the preformed polymer decreases because of chain scission and without any subsequent... 

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. 

Polystyrene and related polymers have most extensively been studied in organic solutions in connection with the radiation-induced degradation [57, 58, 64]. In... 

Infrared studies of the radiation-induced degradation of PMPS by Bowden et al. (19) supports the oligomerization process and also shows that the oligomers can be removed by post-exposure baking. These effects have not been seen for other poly (olefin sulfone)s (2.3). Figure 8 and Figure 9 show the yield versus dose curves for irradiation of poly(l-butene sulfone) and poly (cyclohexene sulfone) respectively (20). No comparable shift of the S02/olefin ratio towards unity is observed in the radiolysis of these polymers. 

However conventional positive electron beam resists like PMMA(4) or PBS(5.) do not have excellent dry etching resistance. The electron beam sensitivities of these positive resists primarily result from radiation-induced degradation of polymer main chains. If the main chain bonding force of these polymers is weakened in order to improve sensitivities, the dry etching resistances of these polymers will decrease. In such cases, sensitivity to electron beam exposure and dry etching resistance are in a trade-off relationship. 

It is pointed out in the thermal degradation of polymer that as the radical concentration increases with passing the aging time, crosslinking reaction increases by the recombination of the radicals. A certain type of radiation induced degradation of elastomer may have the same tendency. Figure 9 shows the other types of modulus-ultimate elongation relationship. Irradiation raised... 

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]. 

Torikai et al. [1994] compared the effects of gamma irradiation of films of PS/PMMA blends and PS-PMMA copolymer (co-PS-PMMA) (Table 11.9). Polymer films were cast from methylene chloride solutions and were dried under vacuum. Based on the UV and FTIR spectroscopy, and viscosity measurements, Torikai et al. [1994] concluded that whereas the presence of PS in the copolymer provided protection against radiation-induced degradation to the PMMA units, similar... 

Radiation-induced Degradation.—There have been several reports on radiation effects in polymers,288 including single crystals,287 fluoropolymers,288 polyamides,289 polysiloxanes,270 polyethylene and its copolymers,271 polypropylene,272 polyolefins,273 polystyrene and its copolymers,274 poly(vinyl chloride) and related polymers,275 rubbers,278 polysulphones and other sulphur-containing polymers,277 polycarbonate,278 nylon,279 poly(vinylpyridines),280 and wool.281... 

Nagasawa, N., Mitomo, H., Yoshii, F. et al. 2000. Radiation-induced degradation of sodium alginate. Polym Degrad Stabil. 69 279-285. 

In principle, high-energy radiation causes aging when the polymer material can absorb the energy and the absorbed energy is sufficient to split chemical bonds (radiation-induced degradation) [17]. 

FIGURE 31.13 Radiation-induced degradation and cross-linking of DHP-chitosan in various conditions (irradiated at lOOkGy). (From Zhao, L. and Mitomo, H., Carbohydr. Polym., 76, 314, 2009. With permission.)... 

Huang, L., Zhai, M. L., Peng, J., Li, J. Q., Wei, G. S. 2007b. Radiation-induced degradation of carboxymethylated chitosan in aqueous solution. Carbohydrate Polymers 67 305-312. 

Katsuma, Y. Radiation induced degradation of polymers-an approach by using Uquid paraffins as a model system. Angew. MakromoL Chem. 252, 89-101 (1997)... 

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. 

The production of formaldehyde under hydrolytic conditions has been documented. The rate of hydrolysis at pH 8 is two orders of magnitude higher than at pH 7, suggesting that the initial attack on the polymer is by hydroxide ion. Polycyanoacrylates are also susceptible to radiation-induced degradation. ... 

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]. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

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

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. 

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