Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(l-lactic acid) with a low optical purity

Abstract

The polymorphous crystallization and multiple melting behavior of poly(L-lactic acid) (PLLA) with an optical purity of 92 % were investigated after isothermally crystallized from the melt state by wide-angle X-ray diffraction and differential scanning calorimetry. Owing to the low optical purity, it was found that the disordered (a 0 ) and ordered (a) crystalline phases of PLLA were formed in the samples crystallized at lower (\95°C) and higher (C95°C) temperatures, respectively. The melting behavior of PLLA is different in three regions of crystallization temperature (T c ) divided into Region I (T c \ 95°C), Region II (95°C B T c \ 120°C), and Region III (T c C 120°C). In Region I, an exothermic peak was observed between the low-temperature and high-temperature endothermic peaks, which results from the solid-solid phase transition of a 0 -form crystal to a one. In Region II, the double-melting peaks can be mainly ascribed to the melting-recrystallization-remelting of less stable a crystals. In Region III, the single endotherm shows that the a crystals formed at higher temperatures are stable enough and melt directly without the recrystallization process during heating.

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“…The cooling cycle gives insufficient time to generate crystallinity; thus, it crystallizes during the second heating cycle resulting in a double melting peak. This double melting behavior is a common phenomenon in semicrystalline polymers, such as polyesters [27][28][29][30]. Possible causes of multiple melting behavior include different types of crystals having different morphologies (lamellar thickness and perfection), recrystallization, and re-melting processes during the subsequent heating [29,30].…”

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“…However, another possible interpretation in the case of PLLA is due to the phase transition from α′-form crystals to α-crystals and the melting of the astransformed α crystals. 14,[44][45][46] To date no consensus has been reached as for the exact origin of the multiple melting behaviour of PLLA. Fig.…”

Isothermal crystallization kinetics and melting behavior of poly(l-lactic acid)/WS2 inorganic nanotube nanocomposites

“…7b) due to the fact that the strong nucleation effect of INT-WS 2 accelerates the crystallization of PLLA in the nanocomposites and in agreement with the aforementioned shift of the crystallization ranges of PLLA along the crystallization temperature axis. Another possible interpretation of these peculiarties in DSC heating scans of PLLA is due to the phase transition from a 0 -form crystals to acrystals and the melting of the as-transformed a crystals [5,[35][36][37]. In particular, it was reported that the disorder a 0 and order a phases are formed at low (T c \ 100°C) and high (T c [ 120°C) temperatures, respectively.…”