Blends of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) with various contents (0, 5, 10, 15, 20 and 30 weight %) and with different molecular weights (M̅_w = 1000, 4000 and 6000 g/mol), called respectively PEG1, PEG2, and PEG3 were prepared by melt blending. Since glass transition temperature (Tg), Tα and loss factor (tan δ) are relevant indicators of polymer chain mobility, plasticisation has been studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Low molecular weight PEG enable increased miscibility with PLA and more efficient reduction of glass transition temperature (Tg) for concentrations of PEG less than 20%. This effect is not only enhanced by the low molecular weight but also by increasing its content up to 20%. As expected, both Tα and Tg decrease when increasing PEG molar mass and content up to 20%, which demonstrates the effectiveness of PEG to act as a plasticizer of PLA.The plasticisation of PLA with PEG effectively lowers Tg and cold crystallization temperatures. The use of low molecular weight PEG decreases the intermolecular forces and increases the mobility of the polymer chains, by improving the flexibility of PLA. The effect of the use of various molecular weights of PEG is confirmed by thermogravimetric analysis (TGA).

Polyethylene glycols (PEG), also known as macrogels, are liquid or solid polymers of general formula H(OCH₂CH₂)_nOH. Low molecular weight PEG is in the liquid form, whereas higher molecular weight PEG are in the solid form at room temperature. PEG are suitable plasticizers for PLA because of their miscibility, biodegradability, and food contact applications. As most of the researchers use high molecular weight PEG as plasticizers, there is about no literature examining the plasticizing effect of low molecular weight PEG on PLA [15-16].

The addition of PEG plasticizer to lower the glass transition temperature (Tg) and increase the crystallinity of the blends is attributed to the enhanced segmental motion of the PLA molecular chains.

PEGs blended with PLA decrease the glass transition temperature and modify the melting and crystallization characteristics. The PEGs are the most efficient for both Tα and Tg reduction and it clearly appears that for compositions up to 20 w% of plasticizer, all the blends present a limit of miscibility as the glass transition temperature reaches a plateau value.

References

1

N. Stoehr, B; Baudrit, E. Haberstroh, M. Nase, P. Heidemeyer, and M. Bastian, (2014). Journal of Applied Polymer Science, 12, 40394–40403.

2

F. J. Li, J. Z. Liang, S. D. Zhang, and B. Zhu, (2015) , Journal of Polymers and the Environment, 3, 407–415.