PHYSICO-CHEMICAL METHODS OF PROTECTION AGAINST IONIZING RADIATIONS 



number of main-chain bonds broken is proportional to the radiation dose) . 

 Then fairly suddenly the average molecular weight of the material (from 

 viscosity measurements) is no longer changed by irradiation and the main- 

 chain breakdown in the polyisobutylene units is counterbalanced by the 



— CH 



Polysty 



rene 



CH3 CH3 CH3 



C CH2 C CH2 C CHg- 



CH3 CH3 CH3 



Pol y/sobutylene 



CH — 



Co -Polymer 



* The components are assumed to be distributed at random in the 

 proportion in which they are present. 



Figure 1. — Composition of Polymers 



crosslinking of the polystyrene units. At still higher doses crosslinking pre- 

 dominates and part of the material is converted to an insoluble gel. The 

 energy necessary to break a main-chain band in the polyisobutylene part of 

 the molecule can be obtained from experiments with relatively low doses 

 and from Table II it can be seen that the polystyrene exerts a very definite 



Table II. — Degradation of co-polymers by ionizing radiations 



Polymer Energy deposited in the polymer 



per one main-chain break 



Polyisobutylene 17eV 



Co-polymer of 20 per cent styrene + 80 per cent isobutylene 32 eV 



,50 „ „ +50 „ „ 55eV 



,80 „ „ + 20 „ „ ~ lOOeV 



Polystyrene ~ 2,000 eV t 



t Polystyrene is not degraded but becomes crosslinked and the energy quoted is that required to produce one 

 crosslink. 



protective effect. By making reasonable assumptions concerning the distri- 

 bution of the isobutylene and styrene units in the macromolecule we 



54 



5> 



