Structural formation of poly (ethylene terephthalate) during the induction period of crystallization: 1. Ordered structure appearing before crystal nucleation

A new finding is reported concerning some structural change in the amorphous state of the polymer which occurs during the so-called induction period before the start of crystallization. Annealing of poly(ethylene terephthalate) (PET) at 115°C, or 40°C above the glass transition temperature, Tg, has been investigated by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. At a very early stage of the induction period the SAXS intensity starts to increase in the range of Q = 0.02–0.04 A −1 , where Q is the length of the scattering vector (Q = 4π sin θ/λ), and continues to increase until the beginning of crystallization. Corresponding to the initation of crystallization, another strong peak, caused by the usual long-period structure, appears in the vicinity of Q = 0.05 A −1 . It is confirmed from the WAXS measurements that no local ordering takes place during the induction period. To clarify the crystallization process, distance distribution analysis is also carried out.