Differential scanning calorimetry study on thermal behaviors of freeze-dried poly(L-lactide) from dilute solutions

Glass transition, cold crystallization, and melting of freeze-dried poly(L-lactide) (PLLA) prepared from dilute 1,4-dioxane solutions were investigated by differential scanning calorimetry (DSC). Conventional DSC measurements of heating scans revealed that freeze-dried PLLA prepared from a 0.07 wt % solution undergoes a two-step cold crystallization (or reorganization) with a lower exotherm appearing at about 78 degreesC and with a higher broad exotherm. between 110-155 degreesC. The peak temperature of the former exotherm is about 50 K lower than that observed for a reference bulk sample. Step-scan mode DSC, which provides information essentially equivalent to that obtained from the temperature-modulated DSC, revealed that the glass-transition temperature is about 6 K lower than that of the reference bulk. These findings suggest enhanced chain mobility for freeze-dried PLLA. Freeze-dried PLLA that crystallized at 80 degreesC for 40 min was revealed to contain a rather large amount of rigid amorphous material (42%). (C) 2004 Wiley Periodicals, Inc.