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

Abstract

Effects of annealing temperature (Ta: 80–140°C) and time (ta: 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 Ta = 80°C with time interval (ta: 10–30 h) demonstrates a peculiarly small exothermal peak (Texo) around 130°C, just prior to the melting point, corresponding to the disorder-to-order (α’-to-α) phase transition, while the sample annealed at temperature (Ta: 90–110°C) shows a double melting behavior considered as the α’-α phase transition. Towards low temperatures, the glass modulus Eg reported by DMA thermograms, shows an important increase (~30,000 MPa) at Ta = 80°C for ta = 3 h, due to the extremely high self-nucleation density in low-crystallized PLA materials. After a sharp drop to 3600 MPa at Ta = 110°C, a marked improvement of Eg (15,900 MPa) is observed around Ta = 120°C for all samples, regardless of time ta. This interesting effect (improvement of Eg in the range Ta = 100–120°C) can be correlated with the grow of crystallinity in the same domain of Ta, and the α’-α phase transition Ta (Ta: 90–110°C) determined by the double Tm melting DSC peak, which is confirmed by the increase of Tg for Ta = 90–110°C.