Melting behavior of isotactic polystyrene revealed by differential scanning calorimetry and transmission electron microscopy

The triple melting behavior and lamellar morphologies of isotactic polystyrene isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC), temperature-modulated DSC and transmission electron microscopy (TEM). The combination of thermal analysis measurements and morphological observations indicates that: (1) The lowest endothermic peak, the so-called “annealing peak” (Ta), is not associated with the melting of the subsidiary crystals formed by secondary crystallization as often suggested in the literature, but probably with a constrained interphase between the amorphous and crystalline regions; (2) Within spherulites two lamellar populations with different degrees of perfection (or thermal stability) are confirmed by direct TEM observations following partial melting experiments, which are responsible for the so-called double melting peaks (Tm,1 and Tm,2) at higher temperatures observed in DSC curves; (3) The highest endothermic peak (Tm,2) is partially originated from the melting of the recrystallized lamellae formed during heating process in DSC.