Post‐irradiation free‐radical reactions in poly(ethylene terephthalate)

Exposure of poly(ethylene terephthalate) to γ‐rays results in the formation of radical I, radical II (tentatively), and to an unassigned radical (III) which is responsible for a magnified image central peak in the ESR spectrum. It is believed that completely amorphous samples of polymer contain radicals II and III. On heating, the radicals decay, and the relative proportion of radical III increases. The kinetics of the overall decay process were followed by measuring the decrease in peak height with time. After an initially rapid reaction the decay of the radical population conformed to second order kinetics. An Arrhenius plot of the logarithm of specific rate versus 1/ T indicated two lines which intersected at 72°C, which is close to the glass transition temperature. The activation energies were 112 kcal/mole above 72° and roughly 25 kcal/mole below 72°C. Reference to reports in the literature suggests that this decay can be explained by long‐range movement of the polymer molecules, even in the glassy solid. The decay of radical I in the crystalline regions of an oriented sample was shown to follow first‐order kinetics. As the decay occurs at temperatures as low as 100°C (the melting point is about 260°C), it seems that decay by normal physical movement is unlikely. The results might be explained by invoking the hypothesis of chemical migration of free radical sites by hydrogen atom hopping.