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 tec hnological treatments were studied by X-ray diffraction and by the spe ctroscopic and mechanical techniques. The PET films were studied in th e 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 tec hnological conditions were established (T = 180 degrees C; t(a) = 1 h) which provided the maximum strength of the samples of series 3; all o ther samples exhibited a lower strength. It was found that changes in the properties of PET samples were caused by competition of three proc esses: 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 gro up accumulation under the conditions of photo- and photomechanical deg radation in the same samples showed that only the samples of series 3 exhibit a decrease in the rate of the Norrish photochemical reaction o f type II related to the appearance of a more ordered structure with a lower content of gauche conformation. The results indicate that the l ight stability of PETE under these conditions depends not only on the molecular structure (the presence of chromophore and other active grou ps, coiled chain conformations, etc.), but on the features of supermol ecular structure of the polymer as well.