Polymers, radiation chemistry

Polymer chemistry and polymer radiation chemistry in particular are key elements of the electronics industry. Polymer materials that undergo radiation induced changes in solubility are used to define the individual elements of integrated circuits. As the demands placed on these materials increases due to increased circuit density and complexity, new materials and chemistry will be required. Many of the new chemistries that are being developed are described in this article. 

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. 

Fifty years of research and development works in polymer radiation Chemistry has led to a number of commercial applications as mentioned very briefly in the Introduction section. Application of ionizing radiation to polymeric materials still remains to be a very active area and the polymer and plastics industry is constantly benefiting from the innovations and fruitful results obtained from the R D works of researchers from all over the world. In the remaining part of this report a modest effort will be made to provide a survey of current developments in applied radiation chemistry of polymers and emerging new applications. 

J. E. Wilson, Radiation Chemistry of Monomers, Polymers, andPlastics, Marcel Dekker, Inc., New York, 1974. 

The radiation chemistry has been mainly discussed in terms of degradation reactions (as above) involving the loss of gaseous products and the irreversible change of the stoichiometry [203]. However, more recent results showed that polymers irradiated with radiation deposit-... 

Detailed studies on radiation chemistry of PEO have been performed [74-77]. Upon y-irradiation, the gel-dose drops abruptly along with an increase in the concentration and molecular weight of the polymer, thus reaching values of 0.15-0.25 Mrad in the range of practical interest [75]. Oxygen is a strong inhibitor and when it is carefully removed from the solution, crosslinking of PEO occurs at doses as low as 0.01 Mrad [76]. 

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

From the earliest days of radiation chemistry it has been known that acetylene polymerizes to a cuprene-like ( alprene ) solid (5, 6,25,28). The characteristics of the polymerization—e.g., lack of effect of temperature, doso rate, and pressure on polymer yield and negligible effect of radical scavengers—led Lind (24) to postulate an ion cluster mechanism. 

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980 s he invented a new class of photochromic molecules - diaryl-ethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives. 

Radiation chemistry in polymer research, 168-169 Reactive macroalkyl radicals, formation, 409 Reactive modifiers addition of reactive antioxidants on rubbers, 417 adhesion, 420,422 demanding applications, 414,416 improving additive performance during melt processing, 412 polymer bound antioxidant, 418-419/ Reduced poly(vinyl chloride),... 

Carlson et al. (1999b) compared laboratory spectra and Galileo data and suggested that hydrated sulphuric acid is the main component in the dark surface material, which probably also contains sulphur polymers modified by radiation chemistry. A sulphur cycle involving three sulphur species is suggested sulphuric acid (H2SO4), sulphur dioxide (SO2) and sulphur polymers (S ). 

In this brief section, we have not touched the vast field of radiation-induced polymerization and radiation effects on polymers. Fortunately, the field has been surveyed very well in international conference proceedings published in Radiation Chemistry and Physics referred in the beginning of this section. The earlier books by Charlesby (1960) and by Dole (1973) provide adequate background information. 

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. 

This comprises mainly neutrons and gamma rays, and large ionized particles (fission products) close to the fuel elements. The neutrons largely produce protons in hydrocarbon polymers by "knock-on" reactions, so that the radiation chemistry of neutrons is similar to that of proton beams, which may alternatively be produced using positive-ion accelerators. 

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

However, the chemical changes observed in low molecular weight compounds can be quite misleading as models for polymers. Difficulties include the high concentration of end groups, e.g. COOH in N-acetyl amino acids, which can dominate the radiation chemistry of the models. Low molecular weight compounds are usually crystalline in the solid state and reactions such as crosslinking may be inhibited or severely retarded. 

Chapiro, A. Radiation Chemistry of Polymeric Systems High Polymers, Vol.15 Interscience New York, 1962. 

Dole, M. Radiation Chemistry of Substituted Vinvl Polymers The Radiation Chemistry of Macromolecules, Vol.2 Dole, M., Ed. Academic ... 

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