Roles of Physical Aging on Crystallization Kinetics and Induction Period of Poly(L-lactide)

Effects of physical aging temperatures (T-a 40 and 50 degrees C) and times (t(a) 0-840 h) on the crystallization kinetics as well as induction periods of poly(L-lactide) (PLLA) have been studied by differential scanning calorimetry (DSC) and time-resolved Fourier transform infrared (FTIR) spectroscopy. The crystallization rate of PLLA increases considerably after the physical aging process. According to the DSC analysis, the crystallization temperature (T-c) of the nonisothermal crystallization and the crystallization half-time (t(1/2)) in the isothermal crystallization decrease nearly logarithmically with ta in the investigated ta region. With increasing T-a, both the T, and t(1/2) values decrease. Polarized optical microscopy (POM) observation revealed that the nucleation of PLLA becomes faster and the nucleation density increases significantly with physical aging. Besides, the crystallization induction period of PLLA shortens with aging. It was found that the FTIR spectral intensity changes of the conformation-sensitive and the pure crystalline bands are not synchronous in the crystallization process. FTIR results suggest that the conformational rearrangement from gg to gt conformers likely occurs during the crystallization induction period of PLLA. It was concluded that the aging effect on the crystallization behavior of PLLA could be correlated with the chain conformational and microstructural rearrangements that occurred during the aging process.