Free Radicals in Irradiated Polyethylene

Fig. 11. ESR spectra of allyl-type free radicals (dotted curve) and of mixed alkyl and allyl free radicals in irradiated polyethylene after heating to room temperature (solid curve). Decay of alkyl free radicals was calculated from height of peaks marked A [64],...

The Ultraviolet and Infrared Spectra of Free Radicals in Irradiated Polyethylene... 

Sauer and Ward (17), Table I, observed that the cyclodienyl free radical gave absorption maxima at 302 and 311 m/, depending on the medium. These values are somewhat higher than the 285 m/ peak which we attribute to the linear dienyl free radical in irradiated polyethylene. On the other hand the benzyl free radicals observed by Porter and Strachan (16) all have absorption maxima in the range of the bands at 323 mfi in irradiated polyethylene which we attribute to the linear trienyl free radical. (The band at 323 mp is clearly seen after the broad 310 peak has disappeared on warming to room temperature (2).)... 

Dole M. Free radicals in irradiated polyethylene. In Dole M, editor. The radiation chemistry of macromolecules. New York Academic Press 1972. p. 335 8. 

We present ESR applications to radiation effects of polymer materials in this section. The ESR spectral analyses of the polymer radicals produced are described in relation to the geometrical structure of the free radicals and the polymer structure of the crystalline and non-crystalline regions. The decay reaction, radical migration and pair-wise trappings of the polymer radicals are discussed. Eurthermore, inhomogeneous spur-like trapping of polymer radicals by irradiation are presented. Eor simplicity, the study of the free radicals in solid polyethylene is the primary example given. 

Negative Ions and Trapped Electrons. At the present time we can only speculate on the nature of electron traps in irradiated polyethylene. Partridge (33) suggests that electrons are trapped between molecular chains because luminescence in irradiated polyethylene occurs in the temperature intervals where mechanical losses occur. For large doses, free radicals, R , are abundant enough to be significant in trapping electrons. The reaction ... 

Alkyl Free Radicals. The primary alkyl free radical, RCH./, has been postulated to exist in gamma-irradiated polyethylene (23), but its presence has never been unambiguously demonstrated. If all the a and p protons interacted equally with the unpaired electron, the primary alkyl free radical ESR spectrum should consist of five lines, in contrast to those of the secondary and tertiary alkyl radicals whose ESR spectra should consist of six and eight lines, respectively (the latter calculated for the tertiary free radical in polypropylene). The existence of the secondary... 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

Cracco, F., A. J. Arwia, and M. Dole ESR studies of free radical decay in irradiated polyethylene. Chem. Phys. 37, 2449 (1962). 

Free radicals have been detected in irradiated polyethylene by ultraviolet and infrared absorption [60—64]. The Am ax for allyl and polyenyl free radicals are given in Table 6 and compared with the corresponding... 

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. 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

The presence of cross-links in irradiated polyethylene gives the product enhanced temperature resistance. For example, irradiated articles retain their shape at 140°C if no strain is put op. them, whereas-untreated articles have megligible strength at this temperature. Polyethylene may also be cross-linked by heating with peroxides such as dicumyl peroxide. The peroxide decomposes to give free radicals which abstract hydrogen from the polymer the resulting polymeric radicals then combine to form a network. Cross-linked polyethylene finds some application as cable insulation which is subjected to elevated temperatures. 

ESR is the only technique that is particularly suitable, not only for direct detection, but also for quantitative analyses of free radicals in a solid or fluid. In polymers, free radicals are generally formed as a result of the unpairing of electrons from a particular molecular site (chain, side, or ring) by mechanical or chemical means, or by irradiation with ionizing radiation. The possession of an unpaired electron makes the free radical a good candidate for ESR study. Soon after the ESR instruments became available in mid-1950, free radical measurements in polymers, including polyethylene (PE), began. Most of the early works on... 

O Neill P, Birkinshaw C, Leahy J, Barklie R. The role of long lived free radicals in the ageing of irradiated ultra high molecular weight polyethylene. Polym Degrad Stab 1999 63 31-9. 

Jahan MS, Stovall JC, King MC. Observation of a non-radical intermediate in the oxidation pathway of free radicals in gamma-irradiated medical grade polyethylene. Nucl Instr Meth Phys Res 2001 B 185 323-7. 

Ridley MD, Jahan MS. Measurements of free radical in vitamin E-doped ultra-high molecular weight polyethylene dependence of materials processing and irradiation environments. Nucl Instr Meth Phys Res 2007 B 265 62-6. 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

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