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

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ABSTRACT:The effects of mixing poly(ethylene oxide) (PEO) on the melting and crystallization process of poly(L-lactic acid) (PLLA) were studied by DSC, polarized microscope and wide angle X-ray diffraction (WAXD). Melting point (Tm) depression of PLLA in the mixtures crystallized from the melt occurred with increasing PEO content but it did not occur in the mixtures crystallized from chloroform solution. Tm of PEO also decreased with increasing PLLA content. The ringed spherulite is formed in the binary mixture although non-ringed spherulite is formed for pure PLLA and PEO crystallized from the melt. In the observation of the ringed spherulite during heating, the field of vision under the microscope only changed to be slightly dark above Tm of PEO without changing the pattern of the spherulite. The result of W AXD leads to an idea that co-crystallization of PLLA and PEO molecule does not occur in the melt-crystallization and the ringed spherulite is made of PLLA and PEO lamella.KEY WORDS Poly(L-lactic acid) / Poly(ethylene oxide) / Binary Mixture / Melting / DSC / Crystallization / Ringed Spherulite / Miscibility / X-Ray Diffraction / It is known that poly(L-lactic acid) (PLLA) is a biodegradable crystalline polymer. The crystal structure of PLLA is pseudo-orthorhombic1 and the molecular comformation is 10 3 helix. The equilibrium melting temperature is 215°C and the glass transition temperature (Tg) is 55°C, respectively. 2Miscibility and phase separation of PLLA and poly(ethylene oxide) (PEO) were studied by Younes and Cohn. 3 According to them, melting point depression of both PLLA and PEO occurred with decreasing the component, especially at lower content below the weight fraction 0.2. They also suggested that the inclusion of two polymer molecules occurs in the amorphous state from IR study.There are many miscible polymer pairs including amorphous-amorphous polymer pairs and crystalline-amorphous polymer pairs, but there are not so many miscible crystallinecrystalline polymer pairs. In some of the crystalline-crystalline polymer pairs, co-crystallization occurs. These crystalline-crystalline polymer pairs have similar molecular structure, for example poly(ethylene) and its copolymers and copolymers of vinylidene fluoride and trifluoroethylene. 4 • 5 However, there is no report on the crystallization of crystallinecrystalline polymer pairs with different molecular structure such as PLLA and PEO. In the paper of Younes and Cohn, the melting behavior of the sample of solution cast blend film was described but it is not clear on the melt-crystallized sample. Furthermore, morphological observation of the blend was not performed in relation with crystallization condition.It is the purpose of this paper, to elucidate the relationship between crystallization condition and melting and crystallization behavior of PLLA and PEO in the mixture with the change of the weight fraction. DSC measure-909

The pressure dependence of the melting and crystallization temperatures of poly(vinylidene fluoride) is determined by high-pressure differential thermal analysis (DTA). These results are used in the investigation of the polymorphic crystal form obtained by pressure quenching molten poly(vinylidene fluoride): The resulting crystal form depends on both the initial and final pressures. The pressure-quenching experiments were performed in a high-pressure piston-cylinder system and in a high-pressure DTA system; a comparison is made of the results obtained by both methods.