Double Spherulite Formation via Two-Step Crystallization in PTT/PET Blends

We investigated the crystallization kinetics and morphology evolution of miscible crystalline/crystalline blends of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET) during isothermal melt crystallization. The integrated light-scattering intensity and the spherulite size increased gradually and then steeply as crystallization progressed in 70/30 PTT/PET at 215 °C, indicating the two-step crystallization behavior. The compact PET spherulite grew in the first step, and the dendritic PTT spherulite grew in the second step, forming the double spherulite consisting of a PET component in the inner region and a PTT one in the outer region. The spherulite size of PET increased nonlinearly with time, suggesting the exclusion of PTT from the crystal growth front. Atomic force microscopy (AFM) observation revealed that the PTT fibrils were interfiled within the PET spherulite in the inner region and continued outward to the outer region consisting of the PTT spherulite. These results suggest that the excluded PTT crystallizes into fibrils by interfiling crystallization within the inner PET spherulite, and then the interfiled PTT fibrils continue to grow outward to form the outer dendritic PTT spherulite after the spherulite growth of PET stops due to the excluded PTT at the growth front.