302 Molecular Action of ionizing Radiations / 1 6 : 3 



seven electrons, as shown in Figure 2. Those atoms with unpaired 



HH HH HHHH 



*C::C + C::C - C:C:C:G- 



HH HH HHHH 



ethylene ethylene free-radical dimer 



CH 2 =CH 2 CH 2 =CH 2 CH 2 — CH 2 — CH 2 — CH 2 - 



Figure 2. Free-radical formation during polymerization. 



electrons are called free radicals. Many free radicals are extremely 

 reactive; they are believed to be responsible for continuing a chain-type 

 reaction once polymerization is started. 



Free radicals are also responsible for some of the effects of ionizing 

 radiations on high polymers. In fact, the primary action of the ioniza- 

 tion is to knock an electron or proton away from the polymer leaving the 

 latter as a free radical. The extra energy imparted to the polymer in 

 this fashion may result in a number of different changes, a few of which 

 are discussed in the next section. 



3. Radiation Damage to Synthetic High Polymers 



When high polymers are irradiated in solution, two different types of 

 damage may occur. The first is that the polymer molecule itself may be 

 irreversibly altered owing to the direct action of the radiation in pro- 

 ducing ionizations within the polymer molecule. The second type of 

 damage possible is due to indirect effects ; these result from the reactions 

 of the polymer with the free radicals formed in the solvent by the 

 ionizing radiation. In dilute solutions, it is very likely that the indirect 

 effects may be the more important ones. On the other hand, in the 

 solid state, the direct actions of the radiation on the polymers are the 

 only important type. 



As a result of both direct and indirect damage, two major changes are 

 found in synthetic high polymers. The first is called crosslinking, which 

 means forming bonds between chains. It results in increased molecular 

 weight, increased elastic moduli, increased transparency, and decreased 

 solubility. The other type of effect, called scission, consists of breaking 

 bonds along chains. It is characterized by the exact opposite of the 

 effects described for crosslinking. The effects of crosslinking and scission 

 are illustrated in Figures 3 and 4, respectively. 



With both crosslinking and scission, a small molecule such as H 2 or 



