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

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.