Effect of thermal prehistory on the structure and photomechanical degradation kinetics of poly(ethylene terephthalate)

Changes in the structure and properties of PET films after various technological treatments were studied by X-ray diffraction and by the spectroscopic and mechanical techniques. The PET films were studied in the initial unannealed state (series 1) and after annealing in the free (series 2) and fixed (series 3) states. By varying the annealing time (t(a) = 0-5 h) and temperature T = 120-240 degrees C), the optimal technological conditions were established (T = 180 degrees C; t(a) = 1 h) which provided the maximum strength of the samples of series 3; all other samples exhibited a lower strength. It was found that changes in the properties of PET samples were caused by competition of three processes: orientation of the structural elements (crystallites and chain molecules) in the amorphous regions, crystallization, and relaxation. Investigation of the kinetics of submicrocracks formation and COOH group accumulation under the conditions of photo- and photomechanical degradation in the same samples showed that only the samples of series 3 exhibit a decrease in the rate of the Norrish photochemical reaction of type II related to the appearance of a more ordered structure with a lower content of gauche conformation. The results indicate that the light stability of PETE under these conditions depends not only on the molecular structure (the presence of chromophore and other active groups, coiled chain conformations, etc.), but on the features of supermolecular structure of the polymer as well.