Effect of radiation on polymers

The energy given to the excited electron allows it to leave its atom. The charged defect left behind in organic materials is a positive molecular ion in an excited state, which may dissociate into free radicals [82]. If the ejected electron has enough kinetic energy, it will collide with other electrons near the site and ionize 

This general description of the behavior of organic compounds applies to organic polymers when they are irradiated. The radiation chemistry of polymers is described in detail in several texts [82, 86, 89-91]. 

The specimen in the electron microscope receives very large doses of radiation, so that degradation does not stop with a polymer of reduced molecular weight. It continues until the fragments are small enough to volatilize in the vacuum of the microscope, and leave the specimen. In an extreme case there will be a loss of all the material irradiated in a thin specimen, leaving a hole. 

The energy given to the excited electron allows it to leave its atom. The charged defect left 

When organic molecules are irradiated, specific bonds or types of bonds are likely to be disrupted. These are not always the least stable energetically for example, in n-paraffins, the C-H bonds break much more often than the C-C bonds. Table 3.6 indicates which bonds break in typical polymers, with approximate G values for the specific products of the reaction. G(P) is the number of units of the product P formed for every 100 eV of radiation absorbed by the specimen. It depends on the target, its physical state, and its temperature. It may also depend on the type of radiation and the dose rate. If radiation breaks a bond in a polymer that is part of the main chain, it will undergo degradation or scission at a rate G(S) to pro- 

Polymer Formula unit Scission Crosslinking Products Hydrogen Other 

polyfmethyl methacrylate) PTFE, polytetrafluoroethylene PDMS, polyfdimethyl siloxane). 

Aromatic compounds are much less sensitive to radiation than aliphatic ones. A phenyl group in a compound can reduce the sensitivity of other chemical groups over 1 nm away [205], so even a few aromatic groups can make a material more radiation resistant. The presence of small amounts of oxygen can also have a large effect on the radiolytic yields, due to the formation of peroxides. In cryomicroscopy, water can be a significant source of oxygen. 

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There are miscellaneous uses of low-LET irradiation other than what we have discussed so far. However, for lack of space, we have not treated the vast field of radiation-induced polymerization and the effect of radiation on polymers. Some useful references have been cited in the beginning of this chapter. Here we will only describe briefly two topics—grafting and curing. 

This book emphasizes the technological significance of the effects of radiation on polymers and draws attention to the major interactions between fundamental science and advanced technology. Although the field of polymer radiation chemistry is not exhaustively covered, a sampling of the ongoing basic and applied research in this area is presented. Review chapters have been included that cover fundamental radiation chemistry, spectroscopic methods, materials for microlithography, and radiation-durable materials. 

T he unique properties of polymers make them desirable for use in A space vehicles and apparatus, as well as in nuclear reactor components and auxiliaries. In both applications intense radiation fields can be encountered routinely or occasionally. Several books have been written about the effects of radiation on polymers (I, 2, 4, 5) in general, the effects of high intensity radiation have been measured by exposing the polymer to a given amount of radiation followed by testing of properties later, outside the radiation field. 

The first systematic study of the irradiation of polymers was undertaken by Dole and Rose during the period 1947-1949. These workers discovered that irradiation of polyethylene caused some degradation to low molecular weight products and the introduction of unsaturation in the polymer chains, but by far the most exciting discovery was that cross links were formed between polymer chains and this had a profound effect on the stress-strain curves and the colddrawing properties of polyethylene(57-59). In 1952 Charlesby(60) published the first of many papers on the effects of radiation on polymers. The rapid development of the field upto 1960 is reviewed in books by Charlesby (61) and Chapiro (62). 

Chapiro, A., Radiation Chemistry of Polymeric Systems , Interscience, New York, 1962. A survey of the polymerization processes that are initiated by radiation, the effects of radiation on polymers both in solid form and in solution, and the preparation of graft copolymers by radiation techniques High Polym., vol. 15). 

Various studies have been made on the effects of radiation on lactide/glycolide polymers (24,38,58). Gilding and Reed (24) reported the effect of y rays on Dexon sutures. Those results confirmed that deterioration of the sutures occurs but that random chain scission is not the primary mechanism. Number average-molecular weight Mn showed a dramatic decrease at doses above 1.0 Mrad. Thus, unzipping of the polymer chain appeared to be the more dominant process, at least in the case of polyglycolide. 

Pruitt, L. A. The Effects of Radiation on the Structural andMechanical Properties of Medical Polymers. Vol. 162, pp. 65-95. 

In The Effects of Radiation on High-Technology Polymers Reichmanis, E., et al. ACS Symposium Series American Chemical Society Washington, DC, 1989. 

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. 

Consideration of the relationship between the effects of radiation on homopolymers and copolymers raises the question of the variation from homopolymer behaviour with sequence length. Every copolymer has a distribution of sequence lengths for each comonomer. At what minimum sequence length does methyl methacrylate not show the high scission of PMMA The future will probably see the development of processes for making polymers with controlled mini-block sequences to maximize a number of properties such as scission yield, adhesion, flexural strength, Tg.. 

The properties of polymer materials can e greatly extended by blending two or more homopolymers together. Blends may be classified as compatible or incompatible - although this does depend on the dimensions being considered. Compatibility is influenced by the molecular weight of the homopolymers and is enhanced in practice by incorporation of block copolymers and other compatibilizers. The effects of radiation on blends depend on the degree of compatibility and the extent of inter-molecular interaction (physically and chemically) between the different types of homopolymers. 

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