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 (ΔH _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 δ- to α-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 δ- to α-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.

A phase diagram of PLLA crystal structures as a function of crystallization temperature ( T _c ) and pressure ( P _c ).