Photophysical studies of amorphous orientation in poly(ethylene terephthalate) films

The effects of uniaxial and biaxial extension on the intrinsic fluorescence of poly(ethylene terephthalate) (PET) films have been investigated. A power law relationship, valid for both uniaxially and biaxially deformed samples, was found between the fluorescence emission at 368 nm and the planar extension, which is defined as the product of the extension ratios in the transverse and machine directions. Dimethyl terephthalate (DMT) model compound studies indicate that the fluorescent species in the PET films is not the monomeric unit. The dependence of emission intensity at 368 nm on orientation was greater when the films were excited with 300 nm light than with 340 nm. However, the absolute emission intensity was an order of magnitude larger when the excitation wavelength was 340 nm as compared to 300 nm. Transient fluorescence measurements show that the 368 nm emission has an average lifetime of 1.3 ns when using 340 nm excitation, and an average lifetime of 3.8 ns when exciting at 300 nm. A model incorporating energy migration in the non-crystalline region has been proposed to explain these results.