Electron spin resonance study on free radicals in cyclic olefin copolymers irradiated by gamma rays at cryogenic and room temperatures

Cyclic olefin copolymers (COCs) are amorphous thermoplastic polymers that require radiation sterilizations when used in prefilled syringes and drug packaging. In the radiation sterilization process, COCs are ionized, and thus, they produce free radicals, which lead to free radical reactions that affect their performance and biosafety. In this study, species of free radicals generated in COCs by γ-ray irradiation and their decay behaviors under different conditions were investigated using electron spin resonance spectroscopy. Results indicated that many alkyl radicals were produced by the irradiation of COCs in liquid nitrogen, which were transformed to peroxy free radicals upon exposure to air. It was confirmed that free radicals centered on secondary and tertiary carbons were predominant after being irradiated at room temperature by an isotropic simulation, and their concentrations depended on the absorbed dose. After 576 h of storage at room temperature, free radicals were nearly annihilated, or almost decayed, near the glass transition temperature. In addition, this study proposes a free radical reaction mechanism for COCs under different post-processing conditions. 1. Radicals trapped in irradiated EN copolymers are secondary and tertiary C-centered. 2. Radicals in EN copolymers annihilated nearly after a heating of 20 min at the T g . 3. The half-life of radicals in irradiated EN copolymers is 75 h at room temperature.