Structural formation of poly(ethylene terephthalate) during the induction period of crystallization: 2. Kinetic analysis based on the theories of phase separation

Crystallization processes of poly(ethylene terephthalate) (PET) when annealed at 80°C, or 5°C above the glass transition temperature, T_g, have been investigated by a small-angle X-ray scattering (SAXS) technique. At a very early stage of annealing, a scattering maximum appears at around Q = 0.04 A ̊^-1, where Q is the length of the scattering vector (Q = 4π sin θ/λ). During the so-called induction period of crystallization, this maximum intensity increases with annealing time and the maximum position shifts towards the low Q side. These results confirm the previously reported new finding that the long-range ordered structure is formed in the induction period before crystallization. These ordering processes can be divided into two stages: an early and a late stage. The scattering behaviour in the early stage is in accordance with the prediction of Cahn's linearized theory for spinodal decomposition. The scattering profiles in the late stage can be described in terms of Furukawa's scaling theory for the cluster growth regime. From these experimental results, we conclude that the growth process of the density fluctuation occurs in the induction period and this process is very similar to the spinodal decomposition type of phase separation process. It is considered that after this dense domain grows to a certain size, crystallization begins.