Polymorphous Crystallization and Multiple Melting Behavior of Poly(l-lactide):  Molecular Weight Dependence

Abstract

The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior of poly(l-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR) spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the α‘- and α-form crystals are produced at low and high crystallization temperature (T c), respectively, regardless of the MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly with MW increasing. Moreover, the T c- and MW-dependent melting behavior of PLLA was clarified with combining the DSC and FTIR results. It was found that the α‘- to α-crystalline phase transition occurs prior to the dominant melting in both the low- and high-MW PLLA crystallized at low T c. Unlike the high-MW PLLA, in low-MW PLLA crystallized at low T c, the α‘-form crystals only partially transform into the α-one, and some amounts of α‘-form crystals melt directly without transition during the heating process. With increasing T c, the melting of PLLA with various MWs changes from the phase transition + melting mechanism to the usual melt−recrystallization mechanism.

Structure-properties relationships in melt reprocessed PLA/hydrotalcites nanocomposites

“…The double peak is explained in different ways [33]: (a) formation of a disordered alpha phase of PLA owing to the low crystallization temperature, (b) nucleation of more than one crystal structure and (c) different morphology of the lamellae formed prior to the heating process. Increasing the number of extrusions, the shape of the endotherms changes from a bimodal to an unimodal distribution, which might be attributed to the homogenization of crystals type and size dependant on chain shortening of the PLA [44,45]. In PLA + OM-HT samples bimodal distribution disappears after the first recycle.…”

Development of novel melt-processable biopolymer nanocomposites based on poly(l-lactic acid) and WS2 inorganic nanotubes

“…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.…”

Thermally Grafting Aminosilane onto Kenaf-Derived Cellulose and Its Influence on the Thermal Properties of Poly(Lactic Acid) Composites

“…An in-depth explanation with illustrations of α' and α crystals was offered by Wasanasuk and Tashiro (2011). According to Pan et al (2007), the first melting peak is due to the α' to α phase transition, in addition to the melting of the α crystals, while the second melting peak is a result of the α crystals formed during the phase transition and melt-recrystallisation process.…”