Melting behavior of poly(l-lactic acid): Effects of crystallization temperature and time

Effects of crystallization temperature and time on the melting behavior of poly(l-lactic acid) were studied with differential scanning calorimetry (DSC). The isothermal crystallization was performed at various temperatures (Tcs), and DSC melting curves for the isothermally crystallized samples were obtained at 10 K min−1. When Tc was lower than Td (∼135 °C), the double melting peaks appeared. The melting behavior, especially Tc dependence of the melting temperature (Tm), discretely changed at Tb (=113 °C), in accordance with the discrete change of the crystallization behavior at Tb, which was previously reported. When Tc was higher than Td, a single melting peak appeared. In addition, Tc dependence of dTm/dTc discretely changed at Td. That is, the melting behavior, especially Tc dependence of Tm and dTm/dTc, are different in three temperature regions of Tc divided by Tb and Td: Regions I (Tc ≤ Tb), II (Tb ≤ Tc ≤ Td), and III (Td ≤ Tc). The effect of crystallization time on the melting behavior, melting temperature and heat of fusion in each temperature region of Tc is also discussed.