Radiation chemistry of polymers

Radiatsionnaya khimiya polimerov (radiation chemistry of polymers) Moscow Nauka,... 

Development of new applications of radiation modifications of the properties of polymers in high technology industries such as electronics and the exposure of polymer materials to radiation environments as diverse as medical sterilization and the Van Allen belts of space have resulted in a renewed interest in fundamental radiation chemistry of polymers. 

The radiation chemistry of polymers is therefore the chemistry of neutral, cation and anion radicals, cations and anions, and excited species. 

Chapiro, A., Radiation Chemistry of Polymer Systems, Chap. IV, Wiley-Interscience, New York, 1962. 

Barlow, A., Briggs, J.W. and Meeks, L.A., Radiat. Phys. Chem. 18, p. 267 (1981). Chapiro, A., Radiation Chemistry of Polymer Systems, Wiley-Interscience, New York, (1962). 

Work along these lines could give a better understanding of the role played by ions in the radiation chemistry of polymers. The fact that ions can induce chemical reactions in irradiated organic solids is clearly demonstrated by the polymerization of certain crystalline monomers which are known to polymerize only when treated with ionic catalysts (3). 

II. RADIATION CHEMISTRY OF POLYMERS WITH CARBON-HALOGEN BONDS... 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

The history of the radiation chemistry of polymers is the subject of another lecture in this symposium by Professor Chaplro. However, since the main theme of the symposium is aimed at the use of electron beams In lithography for the manufacture of electronic devices, it is appropriate to refer briefly to the radiation chemistry of polymeric materials. 

The industrial applications in the radiation chemistry of polymers fall into several categories ... 

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

Relations Between Photochemistry and Radiation Chemistry of Polymers... 

It is important to recognize that polypropylene, which is the major constituent of TPO, is a typical degrading-type polymer in the radiation chemistry of polymers, i.e., once a free radical is formed on a polymer chain, the free radical unzips the chain rather than cross-links. CASING effect was first found with polyethylene [24], which is a typical cross-linking-type polymer. The same CASING effect, however, could not be anticipated with the treatment of the degrading-type polymers because the degradation of substrate polymer enhances the extent of weak boundary layer. 

This study provides a method of characterization that can be usefully applied by others in studies of irradiated polyethylenes and other polymers. Use of the powerful NMR technique will undoubtedly yield further significant information about the radiation chemistry of polymers. 

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