Crystallization and melting behaviors of poly(trimethylene terephthalate)

The crystallization and melting behaviors of poly(trimethylene terephthalat e) (PTT) have been studied by differential scanning calorimetry (DSC), wide -angle X-ray diffraction (WAXD), and solid-state NMR. At certain crystalliz ation temperatures (T-c) for a given time, the isothermally crystallized PT T exhibits two melting endotherms, which is similar to that of PBT and PBT. At higher crystallization temperature (T-c = 210 degrees C), the low-tempe rature endotherm is related to the melting of the original crystals, while the high-temperature endotherm is associated with the melting of crystals r ecrystallized during the hearing. The peak temperatures of these double-mel ting endotherms depend on crystallization temperature, crystallization time , and cooling rate from the melt as well as the subsequent heating rate. At a low cooling rate (0.2 degrees C/min) or a high heating rate (40 degrees C/min), these two endotherms tend to coalesce into a single endotherm, whic h is considered as complete melting without reorganization. WAXD results co nfirm that only one crystal structure exists in the PTT sample regardless o f the crystallization conditions even with the appearance of double melting endotherms. The results of NMR reveal that the annealing treatment increas es proton spin lattice relaxation time in the rotation frame, T-1 rho(H), o f the PTT. This phenomenon suggests that the mobility of the PTT molecules decreases after the annealing process. The equilibrium melting temperature (T(m)degrees) determined by the Hoffman-Weeks plot is 248 degrees C.