Transition properties of PET observed by CL and PALS

PALS and CL measurements were performed for PET as a function of temperature. The combination of PALS and CL exhibits the character of the transition properties of PET. In PALS, the glass transition of PET was clearly observed as a change in the intensity and lifetime of the o-Ps component. However, no singularity was observed in the CL emission properties at the glass-transition temperature. These differences would illustrate the nature of the glass transition considered to be produced by a cooperative interaction between the molecules. On the other hand, the luminescence from the cascade transition of molecules was illustrated in time-resolved CL spectra. For the first emission peak, the lifetime of this state was estimated to be 23.8 ns. From the temperature dependence of the intensities of the luminescence, a decrease in the yield was found at 390 K. This result suggests that a degeneration of the band structure occurred for the molecules to move to optically inactive states