Degradation of Polymeric Materials by Ionizing Radiation

Ionizing radiation is not absorbed selectively like the ultraviolet. Instead, the absorption of gamma rays is a function of electron density in the path of the radiation. After a random ray strikes an atom, however, and energy transfer occurs, free-radicals may form and subsequent reactions, like oxidation and chain reactions, may proceed in the same manner as after UV light irradiation. In addition to creating free-radicals, ionizing radiation also gives rise to ions, electrons, and excited molecules in the medium. This is followed by reactions, like, chain scission, formation of unsaturation, and cross linking that is accompanied by formation of volatile products. Due to the high energy of ionizing radiation, if the irradiation dose is sufficiently high, all polymers will degrade to form low molecular weight fragments. The difference is in the rates of degradation and in the amounts of cross-linking. A mongvinyl polymers, those that tend to unzip and yield large quantities of monomers in thermal degradation are also likely to loose molecular weight rapidly upon irradiation. On the other hand, those vinyl polymers that yield low quantities of monomers during pyrolysis tend to cross-link instead. Also, it was observed that polymeric materials that exhibit high heats of polymerization during their formation tend to cross-link, while those that do not tend to degrade [575]. It was reported back in 1952 that polyethylene could be cross-linked by irradiation in a controlled manner [577]. The reaction is accompanied by evolution of considerable quantities of volatile gases that consist mainly of hydrogen. The portion of hydrogen that is released, however, is lower and the amount of volatile hydrocarbons is greater from polyethylene that is branched [578]. There appears to be an inverse relationship between the amount of branching and the quantity of hydrogen gas that is released [578]. Polypropylene was reported to cross-link under irradiation [579]. At the same time, there is a large evolution of gas. If the irradiation is carried out in air, there is marked degradation even at fairly low doses. When polyisobutylene is irradiated, the molecular weight decreases rapidly until a viscous, low molecular weight liquid remains. At the same time, a mixture of hydrogen and methane (95% of total gas) is released [582]. Generally, aromatic structures are more stable to ionizing radiation than are the aliphatic ones. Accordingly, polystyrene is more resistant than polyethylene [575]. In addition, cross-linking predominates over chain scission. The radiolysis of poly (methyl methacrylate) results in a rapid loss of molecular weight. This degradation increases with the intensity of the radiation [579]. The volatile products that form are hydrogen, carbon dioxide, carbon monoxide, methane, propane, and methyl methacrylate monomer. This varies with the temperature and the type of ionizing radiation that the polymer is exposed to. Polytetrafluoroethylene is extremely sensitive to radiation and exhibits marked damage to its mechanical properties. It was shown that fluoride ions evolve from the polymer not only during irradiation, but also for long periods afterward [134]. The molecular weight of the polymer drops steadily with each dose. Many double bonds form in the polymer [580].

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مواضيع ذات صلة

Oxidation of Chain-Growth Polymers

Oxidative Degradation of Polymers

Thermal Degradation of Cellulosic Materials

Thermal Degradation of Polyimides, Polyoxidiazoles, and Polyquinoxalines

Thermal Degradation of Aromatic Polysulfones

Thermal Degradation of Epoxy Resins

Thermal Degradation of Polyamides

Thermal Degradation of Polyesters

Thermal Degradation of Poly (Vinyl Acetate)

Thermal Degradation of Chlorocarbon and Fluorocarbon Polymers

Thermal Degradation of Methacrylic and Acrylic Polymers

Thermal Degradation of Polyolefins and of Polymers from Conjugated Dienes