Luminescence studies in poly(ethylene terephthalate) films at 77 K. Investigation of emission centres in polymers

Optical luminescence of amorphous bright poly(ethylene terephthalate) (PET) films and the changes in luminescence due to certain morphological changes have been studies at 77 K to identify the luminescent centres in this polymer. The emission spectrum consists of both fluorescence and phosphorescence with distinct band peaks at 304, 326, 365 nm corresponding to fluorescence and at 426, 453, 476 nm corresponding to phosphorescence. Comparison of the PET emission spectrum with that one from dimethyl terephthalate microcrystals indicates that the emitter in the polymer is most probably the basic terephthalate unit. Polarized absorption and emission studies in oriented and unoriented PET films also support this observation. Change in the relative intensity distribution on changing the crystallinity of the film suggests that each of the 304, 326 and 365 nm fluorescence bands arises from different environments of the emitting centre. On γ‐irradiation, a large increase in emission intensity is observed. This is explained on the basis of formation of a radical which in turn causes a large inductive effect on the benzene rings.