Strain-induced crystallization of polyethylene terephthalate

Strain-induced crystallization of polyethylene terephthalate occurs by rotation, alignment, and perfection of internal order of paracrystalline ball-like structures originally present in the amorphous material. Subsequent heat-setting of the strained material produces more complete alignment of the ball-like structures, regularization of the previously irregular folds, and more perfection of order within and between the ball-like crystallites. The ball size does not increase on annealing to any significant extent until a temperature very close to the melting point of the polymer is reached. Then the size increases only from 75 to about 100 Å. The four-point small-angle X-ray pattern is a result of an inclined arrangement of the rows of ball-like crystallites, whereas the two-point pattern is due to balls arranged in rows more or less perpendicular to the stretch direction.