Melting behavior of poly(L-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(L-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T-c) of 113 degrees C (= T-b) and formation of two crystal modifications for the ICSs obtained in the temperature range T-c <= T-b and T-c >= T-b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (alpha'-form) to the high-temperature one (alpha-form) occurred in a range 155-165 degrees C for the ICSs(T-c <= T-b), and that the crystal structure for the ICSs(T-c >= T-b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve. (c) 2008 Elsevier Ltd. All rights reserved.