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The melting and crystallization behavior of poly(L‐lactic acid) (PLLA; weight‐average molecular weight = 3 × 105) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt‐crystallized samples were obtained at various heating rates (HRs). The double‐melting behavior was confirmed by the double endothermic peaks, a high‐temperature peak (H) and a low‐temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [Tm(L) and Tm(H)] decreased linearly with log(HR). The appearance region of the double‐melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. Tm(L) and Tm(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double‐melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004

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.

The multiple melting behavior of poly(butylene succinate) (PBSu) was studied with differential scanning calorimetry (DSC). Three different PBSu resins, with molecular weights (MWs) of 1.1 × 105, 1.8 × 105, and 2.5 × 105, were isothermally crystallized at various crystallization temperatures (Tc) ranging from 70 to 97.5 °C. The Tc dependence of crystallization half‐time (τ) was obtained. DSC melting curves for the isothermally crystallized samples were obtained at a heating rate of 10 K min−1. Three endothermic peaks, an annealing peak, a low‐temperature peak L, and a high‐temperature peak H, and an exothermic peak located between peaks L and H clearly appeared in the DSC curve. In addition, an endothermic small peak S appeared at a lower temperature of peak H. Peak L increased with increasing Tc, whereas peak H decreased. The Tc dependence of the peak melting temperatures [Tm(L) and Tm(H)], recrystallization temperature (Tre), and heat of fusion (ΔH) was obtained. Their fitting curves were obtained as functions of Tc. Tm(L), Tre, and ΔH increased almost linearly with Tc, whereas Tm(H) was almost constant. The maximum rate of recrystallization occurred immediately after the melting. The mechanism of the multiple melting behavior is explained by the melt‐recrystallization model. The high MW samples showed similar Tc dependence of τ, and τ for the lowest MW sample was longer than that for the others. Peak L increased with MW, whereas peak H decreased. In spite of the difference of MW, Tm(L), Tm(H), and Tre almost coincided with each other at the same Tc. The ΔH values, that is crystallinity, for the highest MW sample were smaller than those for the other samples at the same Tc. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2039–2047, 2005

ABSTRACT:The hexagonal phase of a constrained ultra-drawn polyethylene (PE) fiber (UDPF) was studied in detail. The melting process of the constrained UDPF was examined at atmospheric pressure by DSC and X-ray measurement. The hexagonal phase appears prior to melting in the constrained UDPF. The phase diagram showing the pressure change of the hexagonal phase was determined by high pressure DT A up to about 600 MPa. The temperature range of the hexagonal phase increases with pressure and coincides with that of so-called "high pressure phase" (HPP) of it above about 300 MPa. That is, the hexagonal phase of the constrained UDPF is the same as HPP. The phase diagrams of three kinds of PE samples, two constrained UDPFs and a bulk sample composed of extended-chain crystals, determined by high pressure DT A, were compared with one another. The constraining force applied to these PE samples considerably raises the melting temperature of HPP, while it has almost no effect on the transition temperature from orthorhombic phase to HPP. By constraining force, HPP of UDPF appears even in the low pressure region that includes atmospheric pressure.