Excess reversing Heat Capacity during Quasi-isothermal Crystallization in Poly(L-lactide) Melt

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Excess reversing heat capacity C_ex of poly(L-lactide)(PLLA) during isothermal crystallization from the melt has been investigated by quasi-isothermal temperature modulated differential scanning calorimetry. Temporal evolution C_ex(t) was evaluated on the basis of the two-phase model (crystalline and amorphous phases). At temperatures above 120℃, C_ex(t) exhibits a maximum, and the onset of increase in C_ex coincides with that of crystallization. On the other hand, at temperatures below 115℃, C_ex decreases monotonously from the beginning. These results suggest that the reversible crystallization and melting process is facilitated by the formation of the α phase of PLLA crystal rather than that of the less ordered α' phase. In addition, non-zero excess reversing heat capacity was observed even before the onset of crystallization, indicating the occurrence of reversible crystallization and melting process that is not associated with the crystal growth. On the basis of the three-phase model in which rigid amorphous phase is taken into account, C_ex has been revealed to reach a greater value than the initial non-zero value far after the completion of the non-reversible crystallization. Longterm increase in the rigid amorphous fraction was observed after the completion of the crystallization, which suggests formation of secondary crystals.
2009-03-31