304 



Molecular Action of Ionizing Radiations /I6 



In a molecule such as polymethylene, which is the same as poly- 

 ethylene except that it has no side chains, there is no reason to suppose 

 one carbon bond to be weaker than another. In other molecules, such 

 as polyisobutylene, the carbon atoms attached to four other carbon 

 atoms seem to be the weak links in the chain. In more complex poly- 

 mers, there is usually one weakest bond. It appears that the extra 

 energy imparted by the ionizing radiation is often carried to this spot. 

 Thus, iso-octane breaks preferentially at one particular bond. 



Another example of the transport 

 of the ionization energy can be found 

 in solutions. Some molecules which 

 fluoresce because of the formation 

 of free radicals will do so when the 

 solution is irradiated, even if the 

 ionization occurs in the solvent at a 

 distance of 50 A or more away. 

 Similarly, most aromatic-ring comp- 

 ounds tend to stabilize free radicals 

 and can protect polymers. This 

 occurs either if the aromatic com- 

 pounds are placed in solution with the 

 polymer or if they are incorporated 

 into the polymer, as in polystyrene. 

 The extra energy imparted by ioni- 

 zation may be transferred within 

 molecules or between molecules. In 

 some cases, this involves charge 

 transfer, but in others it does not. 

 Exactly how the latter happens is not 

 always understood. 



There are situations in which 

 the extra energy imparted by the 

 ionizing radiation is not transferred. 

 For instance, color changes and 

 electrical-resistance changes in polyethylene, both of which are due to 

 the presence of free radicals, may remain for weeks or even months after 

 irradiation, provided oxygen is excluded. In this case, there is no doubt 

 that the extra energy is not transferred through the polymer. No 

 general rule exists to predict why the energy is transferred in some cases 

 and not in others. 



In the more complex natural polymers, all of the effects discussed 

 above occur. The phenomena which should be considered in natural 

 polymers are: direct and indirect action; crosslinking and scission; the 



0-5 



2 3 5 10 

 R (megarep) 



20 30 50 



Figure 4. Scission in polyisobutylene. 

 Decrease in molecular weight of 

 polyisobutylene as a result of irradia- 

 tion. The average molecular weight 

 M was determined by viscosity 

 measurements. After P. M. Alex- 

 ander, R. M. Black, and A. Charlesby, 

 "Radiation Induced Changes in the 

 Structure of Polyisobutylene," Proc. 

 Roy. Soc. A232: 31 (1955). 



