Fluorescent sensor as physical amplifier of chemiluminescence: Application to the study of poly(ethylene terephthalate)

Intramolecular charge-transfer fluorescent probes have been used as physical enhancers of chemiluminescence. PET exhibits low chemiluminescence emission; however, in the presence of Coumarin 337, the energy transfer from thermally generated excited carbonyl to ground-state probe molecule takes place, giving rise to the fluorescence emission from coumarin. Employing this method, the chemiluminescence emissions of several poly(ethylene terephthalate)s, initial and annealed at different temperatures, were studied. The observed emission has been proved to be sensitive to the morphology of polymer sample. The CL intensity increased in the temperature range 75-130 degreesC, and at higher temperature a decrease was detected. It was associated with exothermic recrystallization during heating, which would restrain the peroxyl radicals recombination, which originates the chemiluminescence emission. A lower chemiluminescence emission for annealed samples was observed. It was due to their enhanced crystallinity, which would affect both the mobility of hydroperoxides to recombine themselves and the energy transfer from excited donor to the acceptor fluorescent probe which occurs through a diffusion-controlled process. The CL results correlate with those obtained by means of other techniques also employed in this work: differential scanning calorimetry, dynamic mechanical analysis, and fluorescence spectroscopy. This result confirms that chemiluminescence emission studies can be used to sense temperature-dependent morphological changes, i.e., annealing processes, and to determine relaxation temperatures and exothermic recrystallization peaks. The use of fluorescent sensor to amplify the chemiluminescence emission allows to apply the innovative method, developed in this work, to all types of polymers, including those with low intensity emission.