I have seen in the literature that short chain poly-lactic acid (polylactide; PLA) oligomers are water soluble. But I am unable to find a commercial source of this type of oligomers.
Typically, oligomers are hard to find as 'off-the-shelf' products. However, if you perform mild depolymerization of PLA using any method (e.g. pyrolysis, hydrothermal/solvent liquefaction, solvolysis), you will end up with some fraction of PLA getting converted to oligomers. This is because depolymerization rarely leads to 100% of monomeric products. The unreacted PLA will go to the solid char fraction. You can perform solvent-extraction of monomers and then filter them out, and use the remaining liquid fraction (which is nothing but oligomers) for your research.
By considering your objectives, maybe this information could help! Condensia (Spain) is an industrial supplier for LACTIC ACID OLIGOMERS (https://www.condensia.com/en/products/additives-for-bioplastics/lactic-acid-oligomers).
I believe that they can help you, don’t hesitate to contact Condensia with your specific questions/requests.
Condensia was my first option. Unfortunately, I could not get small amounts of free samples of Condensia, as I only need a few tens of mg for proof of concept and I am not willing to buy per kilograms at the moment.
By the way, I saw that you are an engineer in biopolymer (nano)composites. Do you have any experience in producing nanoparticles by spray drying or a similar technique? We are investigating the usefulness of this technology for our needs.