Volatile products, radiolysis

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

In dibenzothiophene-S,S-dioxide the S atom is in a ring, and hence more constrained. The yield of SOz in the radiolysis is linear with the dose to about 13 Mrad after which it levels off as in p,p -ditolyI sulfone. However, the yield of S02 in this case is much lower (a factor of 25) than in the case of p,p -ditolyl sulfone (G = 0.002 compared to G = 0.05). This stability of the dibenzothiophene sulfone could be partially due to back reaction to reform the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product was not detected due to limitations of the analytical method. Bowmer and O Donnell70 studied the volatile products in y-radiolysis of dialkyl, alkyl aryl and diaryl sulfones. Table 2 gives the radiolytic yields of S02 and of the hydrocarbon products of the alkyl or aryl radicals. The hydrocarbon products are those obtained either by H atom abstraction or by radical combination. The authors69 suggested the mechanism... 

Table I. Yields of volatile products for gamma radiolysis of isobutyric acid at 273 K...

The range of products formed on gamma radiolysis of N-acetylglycine was similar to that formed on radiolysis of the aliphatic carboxylic acids, but there are some noticeable differences in the yields of products. Carbon dioxide is by far the major volatile product of radiolysis and the corresponding product of the decarboxylation reaction, N-methyl acetamide, is also present in large yield, but the yield of this product was not quantitatively determined. By contrast, carbon monoxide is found in very small yield. The yield of acetamide, the product of N-Ca bond scission, is found in much greater yield. 

Table IV. G-values for the volatile products formed on gamma radiolysis of N-acetylglycine at 303 K...

As with the aliphatic carboxylic acid model compounds, the major volatile product observed on gamma radiolysis of the poly acids is carbon dioxide. However, the carbon dioxide yields are somewhat larger than those observed for the model compounds. 

Volatile products (hydrogen and ethylene) were also produced after radiolysis of pure crown ethers. Their formation yields were measured T/fl I2) ranged from... 

The volatile products from the radiolysis of PMPS are reported in this paper. The product distribution and its dose dependence are used to decide whether PMPS degrades like other poly(olefin sulfone)s or whether a novel mechanism is required to explain its degradation. 

The volatile products formed by the radiolysis were analyzed by gas chromatography (18). A sealed ampoule was placed in an injection system attached to a Hewlett Packard-5734A Gas Chromatograph (GC). The ampoule was broken in line by depression of a plunger and the products were injected into... 

A modified ampoule preparation vessel was used to study the effect of gaseous products trapped in the polymer. A side arm with glass break-seal was added through which a solvent was introduced after irradiation (1.0 Mrad) of the polymer. Chlorobenzene (—0.3 cm3) was used as the solvent since its chromatographic retention time was significantly longer than the retention times for the radiolysis products. After dissolution, the irradiated polymer, volatile products and solvent were sealed into an ampoule and analysed as outlined above. The addition of solvent, dissolution and ampoule preparation were all performed in a closed system, i.e., the irradiated sample was never exposed to air. 

The main-chain scission yield was recently compared at room and liquid nitrogen temperatures [415] in the presence of a large number of additives known as radical, cation or electron scavengers. The results are given in Table 31. The protection index, in this case defined as 100(7V0 — N)/N where N0 and N are the number of scissions per chain in the absence and in the presence of additive, respectively, is nearly independent of the irradiation temperature marked protection is observed for all the additives studied with the exception of nitrous oxide. It must therefore be concluded that the mechanism of main-chain scission is identical at room and liquid nitrogen temperatures and that ions and radicals are involved in the radiolysis. A detailed study of the effect of ethyl mercaptan on main-chain scission and volatile formation was then undertaken [395]. About 75% protection of main-chain scission was obtained at 313 and at 77°K when the polymer contained 1.49 wt. % of ethyl mercaptan the protection index increases to 90% for concentrations of the order of 10 wt. %. The yield of volatile products was, however, unaltered by the presence of 1.5 wt. % ethyl mercaptan. 

The radiolysis of poly(methyl methacrylate) results in a rapid loss of molecular weight. This degradation increases with the intensity of the radiation [579]. The volatile products that form are hydrogen, carbon dioxide, carbon monoxide, methane, propane, and methyl methacrylate monomer. This varies with the temperature and the type of ionizing radiation that the polymer is exposed to. 

The results of Babanalbandi and co-workers (221), in which new aliphatic chains ends formed by cleavage of the main chain at the ester unit are observed, are in support of a mechanism in which chain scission dominates cross-linking. These authors reported G-values for the formation of chain end structures comparable with earlier study. Furthermore, the main volatile products of radiolysis of PLA and poly(glycolic acid) (GPA) are CO2 and CO, consistent with chain scission being the most important reaction. In addition, small amounts of hydrogen and ethane gas were observed on the radiolysis of PLA. Finally, Montanari and co-workers (226) have examined the effects of radiation sterilization on the stability of PLGA microparticles used for drug delivery. 

By contrast with PMMA, poly(methyl acrylate), PMA, and several other aliphatic polyacrylates were found by Shultz and Bovey (240) to imdergo cross-linking and gel formation on irradiation with 1-MeV electron beams. They reported G(S) = 0.15 and G(X) = 0.52 for PMA. Graham (241,242) reported that the phenyl, benzyl, and 2-phenyl ethyl acrylate polymers also undergo cross-linking on y radiolysis imder vacuum. There was evidence of side-chain scission also, and according to Fox and co-workers (243,244) the major volatile products were similar to those observed for PMMA. 

By comparison with PMMA, the radiolysis of PMA has received much less recent attention. Busfield and co-workers (247) have measured the G-values for gaseous product formation at room temperature and 423 K, and they have also measured the dose to gel. The major volatile products were the same as those observed for the radiolysis of PMMA, but the total gas yield was much smaller G(gas) = 1.6 compared with 3.9 for PMMA. However, the gas yield increased with... 

Radiolysis and u.v.-photolysis studies of S-(cis-prop-l-enyl)-L-cysteine reveal products of both cis- and trans-prop-1-enylthiyl radicals, volatile products from y-radiolysis being the various possible sulphides, while u.v.-irradiation gives only ca. 2% yield of sulphides, the main products being alanine and prop-l-enethiol, with substantial amounts of 2,4-, 3,4-, and 2,5-dimethylthiophens and 3-methylthiophen. 

The irradiation of polymers is widespread in many industries. For example, microlithography is an essential process in the fabrication of integrated circuits that involves the modification of the solubility or volatility of thin polymer resist films by radiation. The sterilization by radiation of medical and pharmaceutical items, many of which are manufactured from polymeric materials, is increasing. This trend arises from both the convenience of the process and the concern about the toxicity of chemical sterilants. Information about the radiolysis products of natural and synthetic polymers used in the medical industry is required for the evaluation of the safety of the process. 

Except in one case [393], the sum of the G values for the —C=0 containing radiolysis product is, however, roughly equivalent to the number of main-chain scissions. Todd [347] has shown, by the use of 14C labelling, that all the methyl groups evolved are formed by decomposition of the —COOCH3 side chain. At 77°K, the same volatiles are formed with lower yields than at room temperature, but the sum of the G values for the —C=0 containing volatiles is also similar to the number of main-chain sicssions [395]. 

To provide further information on this point, we have investigated the y-radiolysis of benzene, toluene, ethylbenzene, and the xylenes in the vapor phase and have determined yields of the gaseous products, 4 polymer, and some products of intermediate volatility. These results are compared with those of parallel irradiations of liquid toluene and o-xylene and with published (2, 12) data for the other hydrocarbons in the liquid phase. 

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