PETER ALEXANDER AND ARTHUR CHARLESBY 



with an equally bulky group, cyclo-decalyl, but one which we knew from 

 experiments with polymethylmethacrylate to be a very feeble energy 

 transfer protector. Table I shows that the energy required to crosslink 

 cyclo-decalyl-dodecane is only slightly greater than that of the pure hydro- 

 carbon and we conclude that steric factors played at most a minor part in 

 the protection of the substituted dodecanes. 



Table I. — Influence of an aromatic group substituted in different positions along the chain of the straight- 

 cliain hydrocarbon dodecane on the energy from ionising radiations which has to be absorbed to 

 produce one crosslink^^. 



Substance 

 CH3-(CH2)io-CH3 

 CH2-lCH2)io-CH3 



Energy per crosslink (eV) 



dodecane 20 



naph+hyl-1- dodecane 32 



CHj-iCHgjg-CH •(CH2)7CH3 naphthyl-4-dodecane 



46 



Cha-iCHaJ 



na' 



phthy 



1-6- dodecane 



49 



CM, 



cyclo-decalyl-e-dodecane 27 



* The absolute values are open to some uncertainty but the relative values are reliable. 



Similar results have been obtained when co-polymers of isobutylene and 

 styrene {see Figure 1) were irradiated. Polyisobutylene is degraded^' and 

 behaves very similarly to polymethylmethacrylate for which detailed results 

 have been published i^. Polystyrene, on the other hand, is extremely 

 radiation resistant but will crosslink, though the energy required is approxi- 

 mately one hundred times that for the straight-chain hydrocarbons |. The 

 behaviour of the co-polymers is complicated and changes as the radiation 

 dose is increased 1^ At first they degrade in a regular manner {i.e. the 



t This is a case of internal protection by the phenyl side chains to vk^hich energy absorbed 

 by the rest of the molecule is transferred. The benzene nucleus is known to be very resistant 

 to radiation^i and can, because of its aromatic structure, dissipate a great deal of energy 

 without chemical change. 



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