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 ('-to-) phase transition, while the sample annealed at temperature (Ta: 90-110°C) shows a double melting behavior considered as the '- phase transition. At Ta>110°C, the ′ -form of PLA was found to transform into the  one during the annealing process. The '-phase transition around 100°C (Ta: 90-110°C), determined from the two melting peaks (Tm1 and Tm2) as function of Ta at different times ta, shows a good correlation with the substantial increase in the crystallinity rate (Xc) with a maximum of 32% at Ta=120°C. 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 nucleation density extremely high in low PLA materials crystallization. After a sharp drop to 3 600 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, temperature region where PLA is usually molded in the industrial melt processing. This interesting effect (improvement of Eg in range Ta=100-120°C) can be correlated with the grow of crystallinity in the same domain of Ta, and the '- phase transition Ta (Ta: 90-110 °C) determinate by the double Tm melting DSC peak, which is confirmed by the increase of Tg for Ta=90-110°C.

Effect of Plasticization/Annealing on Thermal, Dynamic Mechanical, and Rheological Properties of Poly(Lactic Acid)

“…The study found that neat PLA forms a mixture of α ′ and α phases during annealing within a temperature range of 90 • C to 110 • C, as indicated by the double melting behavior. This result is consistent with the findings of Zennaki et al [54]. Additionally, Zhang et al [53] demonstrated that PLA with a molecular weight of M w = 150,000 g/mol crystallizes isothermally from the melt and forms a mixture of α ′ and α phases when the temperature is between 100 • C and 120 • C. Tábi et al [46] investigated the crystalline structure of annealed PLA as a function of annealing temperature.…”

“…This is because in amorphous polymers, there are no restrictions on the motion of the polymer chains. In semi-crystalline polymers, dispersed crystalline regions hinder chain mobility in the amorphous regions, resulting in a reduction of the sharpness and height of the tan δ peak [54]. The tan δ peak was significantly reduced for a-PLA (T a = 80 • C; t a = 1 h) with a maximum peak value of 0.9 due to its low crystallinity (X ′ c = 7.9%) and incomplete crystallization during annealing.…”

“…These properties improved as the α to α ′ ratio increased. Zennaki et al [54] investigated the impact of annealing temperature (80-140 • C) and time (3-30 h) on the crystalline structure of PLA.…”