Effects of molecular weight on the crystallization and melting behaviors of poly(L-lactide)

In this study, a series of monodispersed poly(L-lactide) (PLLA) were synthesized by the ring-opening polymerization with Schiff base aluminum catalyst, and the effects of the number-average molecular weight (M (n)) on the crystallization and melting behaviors of PLLA were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The total crystallization rate of PLLA was M (n)-dependent, which reached the maximum value for PLLA with M (n) of 18.6 kg/mol. In addition, when Mn of PLLA was 18.6 kg/mol, the melting enthalpy (Delta H (m)) showed a maximum value (87.1 J/g), which was the highest reported value till now. The critical temperature for change of crystal formation from delta- to alpha-form crystals increased in the isothermal crystallization process with M (n) increasing. In the reheating procedure, high-M (n) PLLA demonstrated a small exothermal peak prior to the dominant melting peak, corresponding to crystal transition from delta- to alpha-form, but low-M (n) PLLA didn't show the peak of crystal transition. These different crystallization and melting behaviors were attributed to the different chain mobility of PLLA with different M (n).