It has been found that, dependent on the crystallization temperature (T c ), the disorder (R′) and order (R) phases of poly(L-lactide) (PLLA) are formed at low (T c < 100°C) and high (T c g 120°C) temperatures, respectively. In the DSC curves, the sample with R′ phase demonstrates a peculiar small exothermal peak around 160°C just prior to the melting point, while the sample crystallized at temperatures around 120°C (between 110 and 130°C) shows a double melting behavior. These distinct thermal behaviors of various PLLA samples were investigated in detail by simultaneous measurements of WAXD and DSC. It is confirmed that the small exothermal peak corresponds to the disorder-to-order (R′-to-R) phase transition, in which the chain packing of the crystal lattice becomes more compacted. In the process of the R′-to-R phase transition, the isosbestic points were found in the temperature-dependent WAXD profiles. So far, the R′-to-R transition was considered to occur apparently continuously as long as the main 200/110 peak was measured, but detailed investigation of higher 2θ peaks has revealed for the first time that the R′ phase transforms discretely to the R phase in the first-order transition mode. On the basis of the X-ray diffraction and DSC data, a kind of phase diagram concerning the R′ and R forms has been constructed reasonably.

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