Radiation-induced cross-linking reactions

Radiation-Induced Cross-Linking in the Presence of CTFE/Butadiene Mixture. On the basis of the results mentioned in the previous section, it is concluded that in the irradiation of polyethylenes in the presence of CTFE the polyethylenes are mainly cross-linked through the addition reaction of the unsaturated groups contained in the main and the side chains of the polymers to the propagating graft chain radical of CTFE. Therefore, the radiation-induced cross-linking of polyethylene is expected to be accelerated by the presence of the mixture of CTFE and a diene monomer effectively than the presence of pure CTFE. 

The formation of a polymer networks starts with an increase in molecular weight and formation of branched structures. At a typical extent of reaction, the gelation point is that point at which a network is first formed. The extent of reaction as well as the cross-linking density in radiation-induced cross-linking processes is determined by the radiation dose. The term dose means the quantity of radiation applied to or absorbed accidentally by a given volume or mass of sample. The absorbed dose is measured in Gray (Gy), 1 Gy = 1J kg-1. Therefore, the formation of a polymeric network needs a certain dose, the gelation dose Dg, which can be determined by sol-gel analysis. ... 

Subsequent reactions are strongly dependent on the chemical nature of the polymer. Recombination of radicals to form a new chemical bond is often observed and is the key process in radiation-induced cross-linking. Examples of polymers in which cross-linking is favored include polyolefins such as polyethylene (PE), natural rubber, or poly-dimethylsiloxane (PDMS). In other polymers, including most fluori-nated polymers, poly(methyl methacrylate) (PMMA), and natural polymers such as DNA and cellulose, chain scission is favored, leading to degradation of the polymer (for a more comprehensive list, see Drobny [2, p. 21]). 

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