olarized IR has been used to follow the changes in the size

conformation and orientation of oriented amorphous poly (ethylene terephthalate) films during increasing temperature. The results showed that relaxation of the oriented segments took place in lower temperature as compared with the relaxation of the on'ented molecular chains in global sense. The considerable thermal contraction of the oriented PET films is due to the relaxation of global molecular chains. Spontaneous extension is related to crystallization of oriented PET films. The degree of orienta-tion of chains has an important effect on magnitude of spontaneous extension of film.