Multiple melting behavior in isothermally cold-crystallized isotactic polystyrene

The multiple melting behavior and lamellar morphologies of isotactic polystyrene (iPS) isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The influences of different experimental variables (such as crystallization temperature and time, heating rate) on the melting behavior of iPS are studied. The combination of thermal analysis and morphological results indicates that: (1) the ‘annealing peak’ is not associated with the melting of subsidiary crystals formed by secondary crystallization as often suggested in the literature; (2) two lamellar populations with different degrees of perfection within a single lamella are responsible for the so-called double melting behavior in iPS. The partial melting and repairing (via recrystallization) of iPS crystals are observed on lamellar level by TEM observations. From the obtained calorimetric and morphological results, the applicability of several crystallization and (subsequent) melting models is discussed.