The multiple melting behavior of poly(butylene succinate) (PBSu) was studied with differential scanning calorimetry (DSC). Three different PBSu resins, with molecular weights of 1.1 ϫ 10 5 , 1.8 ϫ 10 5 , and 2.5 ϫ 10 5 , were cooled from the melt (150°C ) at various cooling rates (CRs) ranging from 0.2 to 50 K min Ϫ1 . The peak crystallization temperature (T c ) of the DSC curve in the cooling process decreased almost linearly with the logarithm of the CR. DSC melting curves for the melt-crystallized samples were obtained at 10 K min Ϫ1 . Double endothermic peaks, a high-temperature peak H and a low-temperature peak L, and an exothermic peak located between them appeared. Peak L decreased with increasing CR, whereas peak H increased. An endothermic shoulder peak appeared at the lower temperature of peak H. The CR dependence of the peak melting temperatures [T m (L) and T m (H)], recrystallization temperature (T re ), and heat of fusion (⌬H) was obtained. Their fitting curves were obtained as functions of log(CR). T m (L), T re , and ⌬H decreased almost linearly with log(CR), whereas T m (H) was almost constant. Peak H decreased with the molecular weight, whereas peak L increased. It was suggested that the rate of the recrystallization decreased with the molecular weight. T m (L), T m (H), T re , and T c for the lowest molecular weight sample were lower than those for the others. In contrast, ⌬H for the highest molecular weight sample was lower than that for the others. If the molecular weight dependence of the melting temperature for PBSu is similar to that for polyethylene, the results for the molecular weight dependence of PBSu can be explained.
The double melting behavior of poly(butylene terephthalate) (PBT) was studied with differential scanning calorimetry (DSC) and wide-angle X-ray analysis. DSC melting curves of melt-crystallized PBT samples, which we prepared by cooling from the melt (250°C) at various cooling rates, showed two endothermic peaks and an exothermic peak located between these melting peaks. The cooling rate effect on these peaks was investigated. The melt-crystallized PBT sample cooled at 24 K min Ϫ1 was heated at a rate of 1 K min Ϫ1 , and its diffraction patterns were obtained successively at a rate of one pattern per minute with an X-ray measurement system equipped with a position-sensitive proportional counter. The diffraction pattern did not change in the melting process, except for the change in its peak height. This suggests that the double melting behavior does not originate from a change in the crystal structure. The temperature dependence of the diffraction intensity was obtained from the diffraction patterns. With increasing temperature, the intensity decreased gradually in the lowtemperature region and then increased distinctly before a steep decrease due to the final melting. In other words, the temperature-dependence curve of the diffraction intensity showed a peak that is interpreted as proof of the recrystallization in the melting process. The peak temperature was 216°C. The temperature-dependence curve of the enthalpy change obtained by the integration of the DSC curve almost coincided with that of the diffraction intensity. The double melting behavior in the heating process of PBT is concluded to originate from the increase of crystallinity, that is, recrystallization.
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