Effects of gamma-ray irradiation on free volume in polymers by positron annihilation lifetime spectroscopy

The positron annihilation lifetime （PAL） technique has been applied to study the effects of gamma-ray irradiation on polymer materials, such as ethylene propylene diene monomer （EPDM）, nitrile butadiene rubber （NBR）, and chloroprene rubber （CR）, used in various industries. PAL measurements were performed with a conventional fast-fast coincidence system. Polymer materials were irradiated with 25 Gy/h in three conditions （50, 100, 500 Gy） and 5 kGy/h in six conditions （5, 50, 200, 500, 800, and 1 000 kGy） at room temperature with a ^60Co gamma-ray source. Afterwards, the irradiated samples were measured by PAL spectroscopy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters indicate the free-volume size and concentration. The longest lifetime component represents the free volume size in polymer. Free volume and intensity were multiplied as a measure of relative fraction of free-volume hole. Cross-linking and degradation could occur when a polymer is irradiated. The free volumes of polymers were decreased accordingly. These results were verified through PAL measurement. From the Tao-Eldrup model, the mean radius and free volume fraction of CR, EPDM, and NBR were calculated. We certified the decrease of the fractional free volume. From these results, this study verified the feasibility of PA application on polymers.