Investigation on discoloration mechanism of cyclic olefin copolymer under ionizing irradiation sterilization

Cyclic olefin copolymer (COC) is a material used in various aspects of medical and health fields due to its excellent comprehensive properties. When sterilized by ionizing irradiation, COC is typically exposed to an absorbed dose <25 kGy, yet noticeable discoloration still occurs. In this study, the COC with norbornene content of 52 mol% was taken as the research subject to investigate the discoloration mechanism of the COC upon irradiation. After γ-ray irradiation, the transparent and colorless COC changed to varying degrees of yellowish-green. However, after 24 days of storage at room temperature, this color significantly diminished. The color change in COC after gamma irradiation primarily results from the presence of free radicals, which gradually fade through recombination. By employing time-dependent density functional theory simulations to analyze the UV-vis absorption spectrum of free radicals, the cause for the color change was illustrated. Additionally, it is worth noting that the faded sample can be recolored through re-irradiation, indicating a "reversible change" in coloration caused by gamma irradiation.