Degradation of a polycarbonate by ionizing radiation

Abstract

The degradation of a polycarbonate by electron irradiation in oxygen and in vacuum has been examined. It was found that the polymer undergoes chain scission and that crosslinking apparently did not occur, the molecular weights of irradiated specimens were therefore determined from the intrinsic viscosity of methylene chloride solutions. From the relationship of molecular weight to radiation dose the value ofG(scissions per 100 e.v. of energy absorbed) was found to be 0.14 in oxygen and 0.09 in vacuum. The effect of both molecular weight and radiation dose on the mechanical properties of polycarbonate irradiated in vacuum was studied. Conventional tensile properties were examined, and fracture energies were also obtained by use of a tensile impact apparatus based on the flywheel principle which was used to strain the speciments at 340 in./sec. With increase in radiation dose the maximum yield strength and extension fall slowly at first and then rapidly at a dose of approximately 100 Mrad. The elastic modulus is, however, virtually unaffected by irradiation and the attendant changes in molecular weight. The fracture energy falls steadily with radiation dose, and from the results a linear relationship,F= 0.0044M_v− 59, between fracture energyFand molecular weightM_vhas been obtained over the range examined, the energy, becoming immeasurably small at molecular weight below 13,500. The results suggested that in this case fracture energy was a better measure of residual strength after irradiation than other tensile properties. The chemical changes caused by radiation lead to greatly enhanced ultraviolet light absorption with a maximum at 305 mμ and a shoulder at 320 mμ. The absorption (E; 1%, 1 cm.), in oxygen and in vacuum was shown to be a linear function,E= 0.020R+ 0.32 of the radiation doseR. The changes in the infrared spectrum, in particular the carbonyl absorption, of irradiated specimens have been discussed. Mass spectrographic measurements showed that degradation proceeded with evolution of H₂, CO, and CO₂, both in oxygen and in vacuum.