PETER ALEXANDER AND ARTHUR CHARLESB Y 



computed ^^ from the irradiation data that one styrene unit provides com- 

 plete protection to two isobutylene units either side. This is, of course, an 

 oversimphfication since the probabihty of protection will decrease gradually 

 with the distance from the phenyl residue. However, this calculation 

 gives a value for the distance over which energy transfer can occur which is 

 very similar to that obtained from the substitution of naphthyl groups in 

 different positions along the dodecyl chain. 



REPAIR OF DAMAGED MOLECULES 



As already mentioned on page 49 radiations can produce chemical reactions 

 which involve the complete disruption of the molecule and are clearly not 

 reversible. In many cases, however, the first chemical change is the loss 

 of a hydrogen atom from an organic molecule either by direct action or as 

 the result of abstraction by a free radical (e.g. >CH2 + OH*->>CH'4- 

 HgO). This type of damage can be restored by what may be called a 

 transfer reaction and this was found to occur when aqueous solutions of 

 polyvinyl alcohol were irradiated ^''. In the absence of oxygen the molecule 

 does not degrade but crosslinks to give a stiff gel. Some added substances, 

 notably — SH compounds, protect by repairing the activated molecules 

 before these can interact to give a polymer network. 

 The general reaction may be illustrated as follows: 



I loss of I 



CHa > CH* 



I hydrogen I 



part of polymer polymer radicals which combine 



molecule with one another to form a gel 



The protective agent (PH) transfers a hydrogen atom to reconstitute 

 the polymer before the polymer radical has had time to undergo further 

 reactions such as crosslinking. 



PH + GH* -> GHa + P* 



1 I 



reconstituted radical from the protective 



polymer molecule agent which is of low 



chemical reactivity. 



In the presence of oxygen the polymer radical will be converted to an 

 unstable peroxy radical 



I I 



GH* + O2 -> GHOO* 



I I 



which will undergo further changes and may in some cases decompose in 

 such a way as to bring about main-chain breakdown. This degradation 

 can in principle be prevented by the protector as follows : 



GHOO- -f PH ^ GHOOH + P* 



I I 



In this way the polymer is changed but decomposition is prevented by the 

 formation of a stable compound. 



55 



