Dear Teresa Cecchi, most studied are PEGs and glycerol. Please have a look at the following links and the attached file. My Regards

https://www.researchgate.net/post/What_are_the_suitable_plasticizer_for_Polylactic_acid_PLA_composite_and_what_are_the_criteria_to_choose_the_right_plasticizer

https://www.italianfoodtech.com/eco-friendly-plasticizers-for-pla-films/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5832258/

https://link.springer.com/article/10.1007/s10973-020-10403-9

https://doi.org/10.1016/j.compositesb.2018.07.011

https://doi.org/10.1002/app.43223

Here oligomeric lactic acid was used

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