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

Abstract

Abstract Effects of annealing temperature ( T a : 80–140°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°C with time interval ( t a : 10–30 h) demonstrates a peculiarly small exothermal peak ( T exo ) 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 ( T a : 90–110°C) shows a double melting behavior considered as the α ’‐ α phase transition. Towards low temperatures, the glass modulus E g reported by DMA thermograms, shows an important increase (~30,000 MPa) at T a = 80°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°C, a marked improvement of E g (15,900 MPa) is observed around T a = 120°C for all samples, regardless of time t a . This interesting effect (improvement of E g in the range T a = 100–120°C) can be correlated with the grow of crystallinity in the same domain of T a , and the α ’‐ α phase transition T a ( T a : 90–110°C) determined by the double T m melting DSC peak, which is confirmed by the increase of T g for T a = 90–110°C.