Evolution of the absorption edge of PET films irradiated with Kr ions after thermal annealing and ageing

This paper presents the results of a study of the ageing and annealing of films of polyethylene terephthalate (PET) after irradiation with Kr ions. For the studies of ageing, PET film samples were irradiated normally to the surface with Kr15+ ions of energy of 1.75 MeV/a.u. and various fluences between (1-10)x1010 cm-2 and then kept for 7.5 months (5400 h) at room temperature in the dark in sealed cases with air removal. The studies of annealing were made on a PET sample irradiated with Kr13+ ions of energy 1.2 MeV/a.u. at an angle of 40 degrees to the normal and a fluence of 5 x 1010 cm-2. Annealing was carried out in the dark in sealed cases with air removal for 160 and 500 h at a temperature lower than the glass transition. UV/vis/near IR differential spectra were obtained for each sample in the wavelength range 308-1050 nm (photon energy 4.03-1.18 eV). We used an established technique to obtain interference-free transmission curves T(lambda) for both pristine and irradiated PET film samples and a differential spectroscopy technique to obtain differential transmission spectra AT(A) of the changes in transmission after ageing and annealing. For all samples, a region of increased transmission at short (UV) wavelengths and a region of reduced transmission at longer (red/near IR) wavelengths were observed. These changes result from, respectively, the decay of the smallest irradiation-induced chromophores that are responsible for the absorption of short-wave radiation and the growth in the numbers and size of chromophores with larger conjugation values. They confirm our previous experiments showing that swift heavy ion irradiation of PET films creates reversible non-covalent x-x conjugation bonds between neighbouring chain molecules in polymer films. These results also confirm our hypothesis that under annealing and ageing the smallest, least stable irradiationinduced conjugations decay while larger, more stable, conjugations grow. Our results also indicate that oblique irradiation, for which the electric field vector has a significant component perpendicular to the film texture, can enhance the polarization effects of the interaction of neighbouring polymer chain molecules leading to electrooptical amplitude enhancement.

Automated summary

This study focuses on the ageing and annealing of PET films after irradiation with Kr ions, revealing the impact of aging and annealing on the structure of PET films and uncovering the patterns of change within them.