Mechanisms of reorganization of lamellae in syndiotactic polystyrene

The multiple melting behavior of syndiotactic polystyrene (sPS) and its possible mechanisms via preexisting lamella types and/or scanning-induced lamellar reorganization were investigated by using X-ray diffraction, DSC, and scanning electron microscopy. Melt-crystallized sPS samples, upon DSC scanning, exhibited three melting peaks (I, II, III). A morphological analysis showed that flat-on lamellae develop first, which yield P-I and P-II melting, and during scanning recrystallize to thickened edge-on lamellae with a P-III melting peak. Upon scanning, melting of P-I (crystal of the lowest melting peak) is followed by repacking into thickened P-III crystal, the lamella of which also reoriented to a perpendicular orientation. The P-II crystal, however, melts at temperatures too close to the melting temperature of P-III; thus, during scanning up, the P-II crystal simply melts without sufficient time to repack into the thickened P-III crystal. In addition to the P-III crystal species that can be added by melting P-I and repacking to P-III, it is believed that preexistence of different lamella crystals was jointly responsible for the multiple melting.