PETER ALEXANDER AND ARTHUR CHARLESBY 



with a-particles, and that the passage of one a-particle anywhere through 

 the molecule was sufficient to produce this change. This indicated that 

 energy absorl^ed in one part of the molecule could be transferred to those 

 bonds responsible for holding the two halves together. If energy transfer 

 of this kind can occur, then protection against direct action is theoretically 

 possible. The term ' energy transfer ' does not imply any particular mechan- 

 ism but is used as a general description of processes we do not at present 

 understand. Thus before it was established that indirect action in aqueous 

 solution was brought about by the intervention of free radicals the process 

 could have been referred to as energy transfer from the solvent to the solute. 

 Recently Milton Burton and his colleagues ^^ have found evidence for 

 energy transport from one molecule to another when a mixture of two 

 liquids such as benzene and cyclohexane is irradiated. In this study, transfer 

 of charge by collision between an ionized and un-ionized molecule plays an 

 important part. How far such a process can occur in our solid systems 

 cannot at the moment be assessed. 



Protection of Polymers. — We decided to test the possibility of protecting 

 against the direct action of ionizing radiations by admixing different 

 chemicals with synthetic polymers and irradiating these as solids. Some 

 polymers are crosslinked while others are degraded^- when irradiated with 

 gamma rays * 



Polymethylmethacrylate, 



CH3 



I 

 -C1H2 — Ci — 



COOCH, 



n 



is degraded on irradiation and the number of breaks produced is directly 

 proportional to the radiation dose. For every 61 eV of energy from gamma 

 rays which is absorbed by the polymer one main-chain bond is broken. This 

 does not mean that all the 61 eV is used to break the bond, and other 

 chemical reactions, notably side-chain breakdown, also occur, f Films of 

 this polymer were prepared which contained a small quantity of a low 

 molecular weight additive and the energy required to produce one main 

 chain bond in the polymer was again measured. Some substances exerted 

 a truly remarkable protective action and the presence of 10 per cent of 



* This difference in behaviour has been explained ^^' ^^ as follows : Degradation can only 

 occur when a chemical rearrangement of the main chain is possible to give two non- 

 interacting products. Direct rupture of a bond into two radicals will not produce a per- 

 manent break since the two parts, being in close proximity (cage effect), will immediately 

 recombine. When such a rearrangement of the main chain cannot occur the predominant 

 chemical reaction will be confined to the side chains with the result that acti\e centres 

 capable of forming inter-chain bonds (crosslinking) will be produced. Crosslinking will not 

 occur exclusively and some rupture of inain chains is usually also found ; this is to be ex- 

 pected on our hypothesis since a proportion of main-chain radicals are bound to escape from 

 the cage and result in breaks. Escape from the cage is more likely to occur with radiations 

 of high specific ionization and these should be less effective for producing crosslinked polymers. 



t Much of the energy does not bring about any chemical change but is wasted in heating 

 up the polymer by a minute amount. 



51 



