# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 4 Oligomers and Low-Molecular Weightaddiitives (Plasticizers)
The elongation at breakup for the PLA-based material with adipate esters is higher than for tributyl citrate (both ranging from 0.7 to 1.0%): 126-148 and 72-103%, respectively. When glycerol was used, it was impossible even to form the pellets since the material was not homogeneous. When cooled, it became brittle and sticky due to excess of glycerol. It is not a suitable plasticizer for PLA, but it is good for starch. That is the reason why there are numerous works dedicated to PLA-based materials plasticized by thermoplastic starch with the same low-molecular weight plasticizers, glycerol, adipate etc. [84; 85; 86; 87; 88]. [PERSON] et al. [87] plasticized PLA with thermoplastic starch containing 36 and 47 wt% of glycerol. It was demonstrated that glycerol partitioned into two phases: starch and PLA. The major part of glycerol was in thermoplastic starch, while it did not diffuse into the PLA. The authors used thermodynamic modeling and predicted some dissolution of PLA in starch but not vice versa. It was concluded that starch led to significant deterioration of the PLA properties. In addition, the oligomers of lactic acid are used to plasticize PLA. It was demonstrated that such plasticizers can be effective up to 25 wt% content (elongation at break up to 300%) [89]: there was a significant decrease in the glass transition point. The authors noted that no phase separation took place, and the material ductility increased. However, with higher plasticizer content, the components are immiscible. [PERSON] et al. [90] added two oligomers of lactic acid, carboxyl or hydroxyl, end-capped as plasticizers. It was found that only 20 and 25% content of plasticizers resulted in ductile behavior, with the rise in elongation at breakup at 480%. Thus, the application of the oligomers of lactic acid as a potential plasticizer was proven. One more interesting and gaining rising attention plasticizers for PLA are essential oils, such as sunflower [91; 92; 93; 94; 95], linesed [96; 97; 98; 99; 100], cottonseed [101; 102], soybean [103; 104; 105; 106; 107; 108; 109; 110], castor [111; 112] oils, etc. [PERSON] and [PERSON] [95] demonstrated the application of epoxidized sunflower oil as a plasticizer for PLA using the solution casting method. Varying the component ratio, it was established that a 20% content of oil led to maximal values of elongation at breakup along with elevated thermal stability. Analysis of SEM micrographs proved that epoxidized sunflower oil is miscible with PLA. In turn, an innovative approach is realized in the utilization of fried sunflower oil (as a byproduct of fast food) as a plasticizer for starch to partially replace glycerol and mixed with PLA [109]. The application of maleinized linseed oil for plasticization of PLA with almond shell flour [104] and hazelnut shell flour [96] allowed one to improve the ductility of the materials, even at relatively low contents of oil. [PERSON] et al. [106] demonstrated the application of the PLA-based systems obtained by solvent casting with modified soybean oil in medicine. As was shown above, the maximal elongation at breakup was observed at 20% oil content. [PERSON] et al. [103] revealed that the epoxidized soybean oil accumulated mainly on the microcrystalline cellulose particle surface, and the elongation was improved to 38.5%. As a promising plasticizer and synthesized at first, epoxidized Brazil nut oil was used to improve the PLA ductility [113]. Even at 2.5 wt% of oil, one observed an improvement in the PLA brittleness. The maximal 70% elongation at breakup was achieved at 7.5 wt% of oil. Another compound used to improve the mechanical properties of PLA is clay and its modifications (for example, [114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124]). [PERSON] et al. [122] used modified montmorillonite to improve the characteristics of the PLA-based material. It resulted in a rise in elongation at breakup at 300% and acceleration of biodegradability. In [120], the elongation at breakup was achieved at 43% with a 5 wt% content of montmorillonite. [PERSON] et al.