Polymers of the degrading type

Decrease in tensile strength and elongation at break is observed for reactor-irradiated PMMA [424]. Conversion of the neutron dose to equivalent 7-irradiation and variation with molecular weight was reported by Charlesby [424] it is reproduced in Fig. 49. It can be seen that the mechanical properties begin to deteriorate rapidly when the molecular weight is lower than 5 x 104. 

Elongation and rupture as a function of temperature has been studied by Chapiro [440] for 7-irradiation. The temperature at break decreases with radiation dose and can be related to the molecular weight. The observed data are, however, complicated by thermal degradation at temperatures higher than 200°C. 

The earlier data on the mechanical properties of PTFE were collected by Chapiro [425] (Fig. 50). 

Polytetrafluoroethylene exhibits exceptionally low radiation stability. The elongation decreases to 25% of its original value at about 5 x 106 rad in vacuum. In air, a lower dose (3 x 10s rad) is sufficient to produce the same damage. 

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