Effect of annealing on thermal and dynamic mechanical properties of poly(lactic acid)

Effects of annealing temperature (T-a: 80-140 degrees C) and time (t(a): 3-30 h) on the crystalline phase transition in poly(lactic acid) (PLA) were studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the DSC curves, the sample annealed at T-a = 80 degrees C with time interval (t(a): 10-30 h) demonstrates a peculiarly small exothermal peak (T-exo) around 130 degrees C, just . prior to the melting point, corresponding to the disorder-to-order (alpha'-to-alpha) phase transition, while the sample annealed at temperature (T-a: 90-110 degrees C) shows a double melting behavior considered as the alpha'-alpha phase transition. Towards low temperatures, the glass modulus E-g reported by DMA thermograms, shows an important increase (similar to 30,000 MPa) at T-a = 80 degrees C for t(a) = 3 h, due to the extremely high self-nucleation density in low-crystallized PLA materials. After a sharp drop to 3600 MPa at T-a =110 degrees C, a marked improvement of E-g (15,900 MPa) is observed around T-a =120 degrees C for all samples, regardless of time t(a). This interesting effect (improvement of E-g in the range T-a = 100-120 degrees C) can be correlated with the grow of crystallinity in the same domain of T-a, and the alpha'-alpha phase transition T-a (T-a: 90-110 degrees C) determined by the double T-m melting DSC peak, which is confirmed by the increase of T-g for T-a =90-110 degrees C.