# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 4 Oligomers and Low-Molecular Weightaddiitives (Plasticizers)  
The introduction of oligomeric and low-molecular-weight plasticizers effectively improve the mobility of the PLA matrix chains. Thanks to the additional application of plasticizers, one can achieve optimal technological properties and flexibility of the materials. In particular, it is expanding the temperature range of the PLA processing. As mentioned above, thanks to its specific characteristics, PEG is well known and often used as a plasticizer for PLA, especially its oligomers [60; 61; 62; 63; 64; 65; 66; 67]. The calculations of theoretical compatibility made in the paper [68] suggest that the PLA and PEG blends  
Figure 4: Schematic diagram of PLA blended with flexible polymers with and without the use of a compatibilizer. are likely miscible at low PEG concentrations (10-30 wt%), but they become apparently immiscible at higher PEG content (>50 wt%). The comparison of the effect of the PEG molecular weight on the material characteristics showed that the lower molecular weight PEG promoted the improvement in the compatibility and ductility of PLA to a significant degree [61; 69]. It should be noted that the introduction of PEG improves plasticity and affects biodegradability since it is a water-soluble substance. With regard to its thermal properties, it was established that the strongest decrease in glass transition point was for the lowest molecular weight PEG. However, the mechanical characteristics deteriorated in time due to the PEG migration (sweating out). This effect can be avoided by the introduction of some substances, such as carbon black, calcium carbonate, zeolite, bran and chitin/cellulose fibrils (for example, [70; 71; 72] etc.). However, the attempt to stop migration is leading to deterioration of the plasticity. There are works devoted to the use of PEG not only as a plasticizer for PLA but also as a technological additive improving adhesion between the layers in the case of producing products using the FDM technique. The heat treatment during FDM influences the penetration of PEG between the PLA chains and decreases the intermolecular forces along the chain, making the PLA more flexible [73]. An alternative to PEG is polypropylene glycol (PPG) [74]. Among the most studied low-molecular plasticizers for PLA are derivatives of hydro-carboxylic acids; first of all, esters, for example, of citric acid, since they are nontoxic and miscible with PLA. However, some studies consider esters as little-explored acids, namely tartaric acid [75], cinnamic acid [76], etc. Tributyl citrate [77; 78; 79], as well as acetyl tributyl citrate [70; 78; 79; 80; 81; 82], have demonstrated a good effect on elongation at the breakup parameter--increasing the plasticizer content led to a rise in ductility. [PERSON] et al. [78] showed that the rise in plasticizer content resulted in a decrease in the glass transition point; there was no low-temperature antibacterial effect. An amount of 10 wt% content of plasticizers led to a decrease in elastic modulus and tensile strength; the further rise in the content of plasticizers deteriorated the mechanical characteristics. This can also be connected with the migration of plasticizers at high concentrations in the composites. However, [PERSON] et al. [83] compared two plasticizers for PLA, namely glycerol and tributyl citrate. The presence of the single glass transition point for the latter additive, in contrast to two glass transition points in the case of glycerol, indicates that tributyl citrate is more compatible with PLA. With the use of mesoscopic dynamic simulations, it was confirmed that there is a phase separation between PLA and glycerol. The comparative study of adipate esters, citrate esters and glycerol for plasticization of PLA-based materials was performed in [84]. The most efficient plasticization was achieved with adipate esters, thanks to their linear structure.