Highly drawn poly (ethylene terephthalate) (PET) films were crystallized in air at T_a=130-180°C for one hour. These films (named O. S.) shrunk freely by heating in air at T_c=200, 220 and 230°C for 30min. and the changes in x (degree of crystallinity), _??_ (Scherrer's size of crystallite), L (long period), f (X-ray orientation degree), l (length of film) and S (thermal shrinkage) were traced. Plotting these parameters against ΔT_r=T_c-T_a, changes in temperature causing films to contract, the following results were obtained:
(1) Relation between S(T_c) and ΔT_r, was approximately linear (Fig. 5).
(2) The rate of increment of x, _??_, L was nearly proportional to ΔT_r (Fig. 4, Fig. 7-9 and Fig. 13).
(3) The rate of decrement of n=1/_??_ and the rate of increment of r=(L-_??_) was also nearly proportional to ΔT_r, (Fig. 11 and Fig. 18).
The results of (1) and (2) showed that the higher the degree of crystallinity of O. S., the lower the amount of shrinkage decreased. Assuming the structure of the annealed PET film as the two-phase model of alternating crystalline and non-crystalline regions, the results of (2) and (3) indicated that thicknesses of each lamellar and inter-lamellar regions (_??_ and r) increased on contraction thermally, whereas the numbers of layers of lamellae (n) in the drawn direction of film became smaller. These contractive behaviors seemed to be reasonable compared with the temperature ranges of crystallite-recrystallization into lamellae of PET films²⁾.