Radiation exerts two opposing cffccts on polymers. On the one hand, it breaks up the polymer molcculcs into smaller picccs. On the other, it causes liberation of a hydrogen atom from cach of the two adjoining molcculcs with formation of a link between the two molcculcs (cross- linking). The cxistcncc of cross-links in a polymer makes the material tougher and higher melting and is very desirable for ccrtain applications.
The cross-linking of polymers by radiation has been much studied. The irradiation of any organic compounds results in breaking of CH bonds, leaving free bonds on the carbon atoms while the hydrogen atoms go off together in pairs to form hydrogen gas. In a liquid the resulting free radicals can diffuse as a whole through the solution and eventually meet together and combine. In a solid polymer it is not clear how these ccntcrs get together. One proposed mechanism is that a hydrogen atom from a neighbouring carbon will pop into the vacatcd hydrogen space, producing a new free bond on the atom adjaccnt to the original free bond position. This proccss will continue, with the free bond flowing up and down the chain, until the free bond happens to find itself next to a free bond formed on the adjaccnt molcculc which is likewise travelling up and down. Another mechanism, possible perhaps only with amorphous polymer, is that the long-chain molcculcs as a whole may move with rcspcct to one another until the free bonds find themselves in proximity.
Whatever the mechanism of cross-linking may be, the result is of commercial value.
Text 8