This paper reviews the authors' investigation into polymer crystallization, especially involving a spinodal decomposition (SD) type phase separation due to the orientation fluctuation of stiff segments prior to crystal nucleation. Evidences for SD obtained from small-angle X-ray and neutron scattering (SAXS and SANS), depolarized light scattering (DPLS), Fourier-transform infrared spectroscopy (FT-IR) are discussed in detail in the case of the glass crystallization of poly(ethylene terephthalate) (PET) just above Tg. SD-like optical micrographs are also shown as a function of crystallization temperature for the melt crystallization of PET; their characteristic wavelengths Λ, which are of the order of μm above 120 °C, follow a van Aartsen equation derived from the Cahn-Hilliard theory for SD. By fitting the equation to the observed characteristic wavelengths the spinodal temperature Ts was determined to be Ts = 213±5 °C for the PET melt, above which the SD pattern suddenly changed to the usual spherulite pattern. On the basis of a theory by Olmsted et al. , the general mechanisms of polymer crystallization are also discussed; the crystallization from the metastable melt causes the nucleation and growth (N&G) of dense (nematic) domains while that from the unstable melt causes SD into the dense (nematic) and less dense (isotropic) domains. Furthermore, the secondary phase separation of the SD-type phase separation into smectic and amorphous domains subsequently occurs inside the nematic domain for both these cases.
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