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