Evaluating PET and PEN under gamma irradiation: insights into material stability

Thermoplastic polyesters such as Poly(ethylene terephthalate) –PET- and Poly(ethylene naphthalate) –PEN- are commonly used as capacitors but can find other high-end applications such as scintillators. The resistance of such materials to ageing under thermal, hydrolytic or UV ageing has often been compared notably from the standpoint of physicochemical and optical properties. PEN in general has better performance in resisting to stresses. The purpose of this work is to evaluate and compare the resistance of the materials to ionizing radiation. Films of both materials were subjected to gamma-ray irradiation at several doses in an air environment. Electrical properties were probed by DC conductivity measurements at electric fields of up to 200 kV/mm. Photo-physical properties were investigated by Infrared spectroscopy, UV–visible transmission spectroscopy and photoluminescence experiments. While PEN shows only moderate changes with the irradiation dose, PET evolves substantially, with an absorption band in the region 310–360 nm going with a reduction of the optical band gap, and a strong fluorescence emission appearing at 368 nm. The behaviour has been interpreted as arising mainly from chain scission along the Norrish type mechanisms and the subsequent production of hydroxylated benzoic acids. The high field conductivity of both materials is impacted by irradiation and the behavior cannot easily be explained by chemical evolution alone.