Analysis of multiple melting behavior of spherulites comprising ring-band shell/ringless core in polymorphic poly(butylene adipate)

In-depth interpretation of ring-banded spherulitic morphology, crystals, polymorphism, and complex melting behavior in poly(1,4-butylene adipate) (PBA) were analyzed via a procedure of designing composite core-shell spherulites, in which two lamellar patterns (ring-band vs. ringless) were packed by subjecting to crystallization at two-step temperature schemes with specific temperatures and times. By heating to 52 °C and holding at that temperature for 30 min annealing, the core can be stripped off by melting, and analysis specifically on the ring-shell portion (with the ringless core stripped by controlled melting) proves that the highest melting peak (P₄ at 55-57 °C) is likely associated with melting of the ring-band lamellae. Furthermore, the unusually complex multiple melting in PBA can be attributed to all three widely proposed mechanisms: (1) multiple types of lamellae preexisting in crystallized PBA, (2) scan/heating induced remelting/reorganization, and (3) polymorphism of dual crystal cells. In addition, this study evidently shows that the extinction rings within the ring-banded shell, regardless of alternate edge-on and flat-on mechanism or alternative origins, can be of all singly α-crystal form, either initially or upon post-heating temperature-induced transformation. Thus, the type of crystal forms (α or β) in polymorphic PBA is mainly associated with temperature of crystallization (T_c = 28 or 35 °C), and not likely with lamellar orientation (flat-on or edge-on).