16:3/ Molecular Action of Ionizing Radiations 



303 



NH 3 is often eliminated. This third effect of small molecule elimination 

 causes negligible changes in the molecular weight or physical properties 

 of the polymers as compared to the changes due to scission and cross- 

 linking. However, such small changes can completely alter the physio- 

 logical actions of a protein. 



Probably all high polymers undergo both scission and crosslinking 

 when irradiated. However, in some, such as polyethylene, illustrated in 

 Figure 3, the crosslinking is the predominant effect. Its physical 



Figure 3. Crosslinking in polyethylene. Crosslinking results 

 in many changes in physical properties. One of the more 

 pronounced changes is the increased density; this change is 

 most pronounced above the melting point at 105°C. After 

 A. Charlesby and M. Ross, "Effect of Crosslinking on the 

 Density and Melting of Polythene," Proc. Roy. Soc. A2I7: 122 

 (1953). 



characteristics are changed in a fashion that indicates only the cross- 

 linking. By contrast, polyisobutylene, illustrated in Figure 4, shows 

 only the effects associated with scission when it is exposed to ionizing 

 radiation. 



The effects of ionizing radiations on all polymers are usually enhanced 

 if oxygen is present. For example, if polyethylene is bombarded in the 

 absence of oxygen, crosslinking occurs. If oxygen is present during the 

 bombardment, considerably more crosslinking takes place. Likewise, 

 the scission in polyisobutylene is greater if it is exposed to radiation 

 damage in the presence of oxygen rather than in the absence of oxygen. 

 The enhancement of the effects of bombardment is manyfold larger than 

 expected from the ionization of the oxygen molecule by the radiation. 

 The increased effects must result from the reactions of oxygen with the 

 free radicals and other less stable forms produced by the ionizations. 



