Radiation chemistry of polyethylene

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

T wo aspects of the radiation chemistry of polyethylene terephthalate (PET) are reviewed here the dependence of product yields on radiation dose and on dose rate. The review is limited to work with thin films from which air and water were pumped prior to irradiation. Moreover, it is judged that in the experiments described postirradiation effects were negligible. 

Budzol,M., Dole,M. The radiation chemistry of polyethylene. XI. The molten state. J. Phys. Chem. 75,1671-1676 (1971). 

Dole M (1974) Radiation chemistry of polyethylene. In Burton M, Magee JL (eds) Advances in radiation chemistry, vol 4. Wiley, New York, p 307... 

Both Odian and Silverman models satisfactorily explain most of the observed results in all solvents (Tables I-III, VI) used in the present study, however there are some exceptions especially when solvents other than the alcohols are used (Table VI). Thus the Odian mechanism is not consistent with the DMF data nor can the Silverman model account for the acetone results. In addition, in further preliminary studies with grafting of styrene to polyethylene (10) in solvents other than those reported here both Odian and Silverman mechanisms are deficient. The problem is that possible contributions from the radiation chemistry of the components in the grafting reaction need to be considered in formulating a complete mechanism for the overall process. 

Mandelkern,L. Radiation chemistry of linear polyethylene, Vol. I. pp. 287-334. In Dole,M. (Ed.) The radiation chemistry of macromolecules. New York Academic Press 1972. 

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. 

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. 

In the following we shall first give some general features of the radiation chemistry of polymers and then discuss polyethylene. 

Grishina, A. D., Larin, V. A., and Bach, N. A., Structure and reactivity changes in polyethylene under high-dose irradiation, in Radiation Chemistry of Polymers (in Russian), Nauka, Moscow, 1966, pp. 257-262. 

Free-Radical Intermediates. From an early time the participation of free radicals in the radiation chemistry of polymers has been well understood. Charlesby (125) in 1952 invoked a free-radical mechanism of cross-linking of polyethylene, although for other materials the participation of ionic species has also been suggested (126-137). The primary radicals observed at 77 K, at which temperature a significant proportion of the radicals are assumed to be trapped and prevented from further reaction, are the secondary alkyl radicals I (49,138-148). 

Another application in macromolecular chemistry is radiation-induced graft polymerization, by which favourable properties of two polymers can be combined. In this process, copolymers of A and B are produced by irradiation of the polymer A in the presence of the monomer B. Examples are graft polymers of polyethylene and acrylic acid or of polyvinyl chloride and styrene. The properties of textiles (cellulose, wool, natural silk, polyamides, polyesters) can also be modified by graft polymerization, for example for the production of weatherproof products. 

Time resolved spectroscopy has developed assignments of intermediate species in radiation chemistry as revealed in the other sections. However, because solid polymers are less transparent, the works obtained so far seem to be limited mainly to polymer solution systems or liquid model-compounds. The lifetime of intermediates depends on LET the fluorescence lifetime of n-dodecane is shorter for higher LET radiation [83], which was studied as liquid model compounds for polyethylene. The observation is attributed to scavenging upon encountering of intermediates. Light emission from excimers of solid polystyrene has constant lifetime irrespective to LET [84], whereas polystyrene... 

Przybytniak, G. K., Zagorski, Z. R, Zuchowska, D., Free radicals in electron beam irradiated blends of polyethylene and butadiene-styrene block copolymer. Radiation Physics and Chemistry 1999,55(5-6), 655-658. 

Zhang, W. X., Liu, Y. T., Sun, J. Z., The relationship between sol fraction and radiation-dose in radiation crosslinking of low-density polyethylene (LDPE) ethylen-evinylacetate copolymer (EVA) blend. Radiation Physics and Chemistry 1990,35(1-3), 163-166. 

Martinez-Pardo, E., Vera-Graziano, R., Gamma-radiation induced cross-linking of polyethylene ethylene vinylacetate blends. Radiation Physics and Chemistry 1995,45(1), 93-102. 

Oonishi H., Y. Takayama, and E. Tsuji. 1992. Improvement of polyethylene by irradiation in artificial joints. Radiation Physics and Chemistry 39 495-504. 

Zhang, L., Zhang, W., Zhang, Z., et al. (1992) Radiation effects on crystalline polymers - I. Gamma-radiation-induced crosshnking and structural characterization of polyethylene oxide. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry, 40 (6), 501-505. 

On the other hand, if precautions are not taken to prevent the presence of molecular oxygen at the time of irradiation, polyethylene will undergo a dominant scission mechanism. This has lead to numerous embrittlement problems associated with oxidation and chain scission [9-12]. This radiation chemistry plagued the orthopedics community for years when many medical device companies utilized gamma radiation in air as the primary sterilization method [55]. 

Hsu, F.H., Choi, Y.J., Hadley Jr, J.H. (2000) Temperature dependence of positron annihilation lifetime spectra for polyethylene positron irradiation effects . Radiation Physics and Chemistry. 58,473. 

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