Polymers radiation degradation

Changes in the properties of polymer materials caused by absorption of high-energy radiation result from a variety of chemical reactions subsequent to the initial ionization and excitation. A number of experimental procedures may be used to measure, directly or indirectly, the radiation chemical yields for these reactions. The chemical structure of the polymer molecule is the main determinant of the nature and extent of the radiation degradation, but there are many other parameters which influence the behaviour of any polymer material when subjected to high-energy radiation. 

The range of properties of polymers can be greatly extended and varied by copolymerization of two or more monomers. The effects of radiation on copolymers would be expected to show similarities to the homopolymers, but major differences from linear relationships are often experienced. Aromatic groups in one monomer frequently show an intramolecular protective effect so that the influence of that monomer may be much greater than its mole fraction. The Tg of a copolymer is normally intermediate between the homopolymers, except for block copolymers, and this can cause a discontinuity in radiation degradation at a fixed temperature. 

The polymers used in this study were prepared by a nucleophilic activated aromatic substitution reaction of a bisphenate and dihalo diphenyl sulfone ( ). The reaction was carried out in an aprotic dipolar solvent (NMP) at 170°C in the presence of potassium carbonate (Scheme 1) (5,6). The polymers were purified by repeated precipitation into methanol/water, followed by drying to constant weight. The bisphenols used were bisphenol-A (Bis-A), hydroquinone (Hq) and biphenol (Bp). Thus, the aliphatic character of Bis-A could be removed while retaining a similar aromatic content and structure. The use of biphenol allows an investigation of the possible effect of extended conjugation on the radiation degradation. 

The relatively minor role of the isopropylidene group in the radiation degradation of Bis-A PSF was further demonstrated by the small difference in GISO ) for Bis-A PSF (0.146) and Hq PSF (0.136), a wholly aromatic polymer. The small reduction in G(S02) may be due to a slightly higher aromatic content in Hq PSF compared with Bis-A PSF. 

Although antioxidants trap the free radicals produced by the degradation of polymers, radiation stabilizers absorb radiation prior to molecular bond breakage. 

Nitrous oxide reduces the amount of radiation degradation of polyisobutylene (5), though this is a typical polymer which degrades under radiation. 

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. 

The morphology of solid polymers is also an important parameter. Thus, radiation-induced changes can be expected to differ in crystalline and amorphous regions — but in what way and to what extent "Crystallinity" and "amorphous" are not absolute terms and as more becomes known about the solid-state structure of polymers this should be related to radiation degradation. 

Temperature is an important parameter in radiation degradation, yet remarkably little experimental work has been reported for irradiation at other than ambient temperature. The transition temperatures, Tg and Tm, of the polymer are as important in the radiolysis as they are for the properties. The irradiation temperature can be deliberately varied to enhance the radiation-induced modifications of polymers and should not be regarded as just an environmental property. Similarly, once the sensitization to, or protection from, radiation degradation by small molecule additives is understood — as a function... 

Areas of application of radiation degradation of polymers can be usefully classified into (1) modification of polymers, e.g. molar mass or structure, (2) radiation-sensitive polymers, and (3) radiation-resistant polymers. Within the constraints of the required material properties of the polymers, either maximization or minimization of the response to radiation is usually the objective. 

The role of specific chemical groups in or on a polymer chain in radiation degradation can be usefully investigated using model compounds of low molar mass. Much valuable information can be deduced in this way, but the unique properties of polymers are a consequence of their high molar mass and this can also apply to radiation degradation, thereby limiting conclusions drawn from radiation studies on model compounds of low molar mass. 

Thus, there are three possible pathways for the radiation degradation of polymer molecules neutral radical, cation-radical and/or anion-radical intermediates. Interest in the formation of these three types of reaction intermediates has fluctuated over the years with the utilization of different techniques and with the particular interests of different investigators. It is likely that all three species will be produced, but their relative importance in the degradation mechanism will depend on the chemical structure of the polymer. Evidence for their involvement will depend on the experimental methods used and the temperature and time scale of observation. In this paper we illustrate our investigations of many of the fundamental aspects of the radiation degradation of polymers through studies of series of polymers and copolymers. 

When the ESR spectrum has been analyzed into spectral components assigned to particular radicals, the individual spectra are simulated, taking into consideration (1) g value, (2) hyperfine splitting, (3) line shape and (4) line width. These simulated spectra are then summed, corresponding to the percentages of individual radicals, to produce a spectrum similar to that obtained experimentally. The application of ESR to the study of radiation degradation of polymers is extremely valuable and the techniques described above have been used extensively in the present work. 

Hexamethylphosphoramide has been produced commercially in relatively small quantities in several countries of Europe, in Japan and in the United States. It is used as a solvent for polymers, a selective solvent for gases and as a thermal and ultraviolet radiation degradation stabilizer in various polymers (lARC, 1977). 

Somewhat similar conclusions were drawn by Ayrey, and Turner (104) from their work with the effect of scavengers and other additives on the radiation degradation of polyisobutylene. When the polymer is irradiated in monomer the side chain macroradicals can terminate by... 

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