PHYSICO-CHEMICAL METHODS OF PROTECTION AGAINST IONIZING RADIATIONS 



dimeta-tolyl-thiourea, aniline or allylthiourea raised the energy which 

 had to be taken up by the polymer per main-chain break to 227, 152 or 

 143eV respectively. Other substances such as long-chain paraffins and 

 ethyl urea do not protect. 



We can see two possible interpretations : (1) the energy absorbed by the 

 polymer is not immediately utilized to bring about a chemical change and 

 in the interval the energy is transferred (see footnote on page 51) from the 

 polymer to the additive. (2) The protection is brought about by repair 

 of the polymer by the added substance which increases, for example by 

 crosslinking, the molecular weight and thereby hides some of the main- 

 chain breaks. We believe that the energy transfer mechanism occurs for 

 the following reasons, (a) A detailed quantitative study by Toms (un- 

 published) relating the protection to the concentration of the protector and 

 to the radiation dose indicates energy transfer, [b) The list of substances 

 which protect is so diverse that it is highly improbable that they share a 

 chemical property such as ability to produce crosslinks, {c) Preliminary 

 experiments indicate that the same substances which protect polymethyl 

 methacrylate against degradation protect other polymers against crosslinking. 



The decisive test is to see if the energy required to modify chemically a 

 molecule of the additive is less when incorporated in a polymer than when 

 irradiated by itself This experiment is difficult to carry out since even 

 when extremely large doses of radiation are used, only a very small fraction 

 of the low molecular weight additive is changed. Preliminary experiments 

 showed that a considerably greater proportion of aniline was changed when 

 this was irradiated in a film with polymethylmethacrylate than by itself 



Instead of treating this effect as one of pi'otection the polymer can be 

 considered to enhance the decomposition of the additive by handing on to 

 it some of the energy absorbed {i.e. the polymer then fulfils the same function 

 as the solvent in the case of indirect action). In this way the energy which 

 was initially absorbed uniformly in the system* concentrate at certain points. 



Energy transfer within inolecides. — Two series of experiments indicate that 

 energy transfer can occur over considerable distances within a macromolecule. 

 The energy required to form a crosslink by irradiation with gamma rays 

 in a straight chain hydrocarbon was independent of its molecular weight^^. 

 We examined the energy to form a crosslink in a hydrocarbon to which an 

 aromatic group (naphthyl) was attached to see if energy transfer to this 

 group occurred [i.e. if some of the energy absorbed in the hydrocarbon 

 chain was transferred to the aromatic group) . As can be seen from Table I 

 the energy required to produce a crosslink is greater in the substituted 

 dodecane derivatives and the protection is most marked when the naphthyl 

 group is in the centre of the chain. We believe that a reasonable interpre- 

 tation is that energy originally absorbed by the hydrocarbon chain is trans- 

 ferred to the aromatic group and that the transfer is not efficient over a 

 distance of more than a few carbon-carbon bonds. To eliminate the possi- 

 bility that the decrease in crosslinking found was due to steric interference 

 by the naphthyl group, a hydrocarbon was examined which was substituted 



* The absorption coefficients of different organic materials to hard X- and gamma rays 

 and to particulate radiations other than neutrons do not vary by more than a few per cent. 



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