Cross-linking effect radiolysis

Maleimides Alkyl and aryl maleimides in small concentrations, e.g., 5-10 wt% significantly enhance yield of cross-link for y-irradiated (in vacuo) NR, cw-l,4-polyisoprene, poly(styrene-co-butadiene) rubber, and polychloroprene rubber. A-phenyhnaleimide and m-phenylene dimaleimide have been found to be most effective. The solubihty of the maleimides in the polymer matrix, reactivity of the double bond and the influence of substituent groups also affect the cross-fink promoting ability of these promoters [82]. The mechanism for the cross-link promotion of maleimides is considered to be the copolymerization of the rubber via its unsaturations with the maleimide molecules initiated by radicals and, in particular, by allyfic radicals produced during the radiolysis of the elastomer. Maleimides have also been found to increase the rate of cross-linking in saturated polymers like PE and poly vinylacetate [33]. 

Temperature dependence (related to the temperature dependence of the conformational structure and the morphology of polymers) of the radiation effect on various fluoropolymers e.g., poly (tetrafluoroethylene-co-hexafluoropropylene), poly(tetrafluoroethylene-co-perfluoroalkylvinylether), and poly(tetrafluoroethylene-co-ethylene) copolymers has been reported by Tabata [419]. Hill et al. [420] have investigated the effect of environment and temperature on the radiolysis of FEP. While the irradiation is carried out at temperatures above the glass transition temperature of FEP, cross-linking reactions predominate over chain scission or degradation. Forsythe et al. [421]... 

In addition to main chain scission or cross-linking, gas formation is also observed as a result of irradiation. The gas in the hydrocarbon-based polymers mostly consists of hydrogen. The amount of gas produced depends on the nature of the polymer and also on dose, temperature, type of radiation, etc. In the case of polyethylene the G-value of gas production is high, G 0.32 pmol comparable to the gas yields observed in the radiolysis of hquid n-alkanes G 0.5-0.6 pmol In the radiolysis of polystyrene and polymethylstyrene the yield of gaseous products is only G 0.01 pmol 1 , that value is typical of aromatic compounds. The benzene rings attached to the main chain exert a protective effect against both the C-H and the C-C decompositions in the chain. 

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. 

Since these early reports, a number of further studies have appeared (6,231,339) and these have been summarized in 1973 by Parkinson and Keyser (353). Since that date, efforts have been made to understand the relatively wide discrepancies in reported yields of radiochemical reactions, and also the effect of temperature on the radiolysis products. For example, values of the yield of radicals, G(R), measured at 77 K after irradiation at that temperature range from 0.04 to 0.50 (22,100,354-357). O Donnell and co-workers (358) have proposed that the variation in reported yields of radical formation, and of chain scission and cross-linking, may be in part due to the presence of trace amoimts of residual oxygen. 

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