# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 3 Compounding with Flexible Polymers (Blends)  
When creating blends of the above polymers, there is a problem with their thermodynamic incompatibility. Thus, one of the tasks of researchers is to establish optimal ratios of polymers in terms of solubility parameters, as well as increase compatibility through the use of compatibilizers. The blending of two incompatible blends with varied melting temperatures is an additional factor promoting delamination of the blend. When cooled from the melt (in the case of melt compounding), the crystallization of each of the polymers occurs in its own temperature range. With a significant difference in temperature ranges, separation at the interface between the phases of blends and the formation of shrinkage cavities can occur [38]. It is important to note that the PLA and flexible polyesters used as modifying additives crystallize in different temperature ranges, which also promotes the delamination of the compositions. Numerous studies have shown that the change in entropy when mixing high-molecular compounds is negligible; therefore polymer blends are thermodynamically incompatible, so the interaction of components is limited by interphase boundaries. However, creating polymer blends is widely practiced as the easiest way to create new materials. Even if there is no synergy compared to additive values, the blend combines the properties of both components. As a result of segmental solubility or mutual diffusion of segments of incompatible polymers, an interfacial layer is formed. Segmental solubility manifests itself in macroscopically uniform physical properties, primarily determined by the structure of the interfacial layer. Therefore, it is preferable to create the maximum possible phase interface, that is, reducing the size of the domains of the dispersed phase with increasing the surface area of the domains per unit volume of the blend. Compounding flexible polyesters with PLA allows one to obtain materials with complementary properties, namely, increased elastic modulus and rigidity on the one hand, as well as flexibility and crack resistance; on the other, bypassing the expensive synthesis stage [39]. Among the most popular polymer additives, PCL, PBAT and PBSA also belong to the class of biodegradable polyesters, with physical and mechanical properties similar to thermoplastic elastomers [2]. The mixing with PBATis also affects the thermal characteristics, namely, the glass transition point of the blends decreases with the rise in the PBAT content [39]. The presence of adipic acid units in copolymers of PBAT and PBSA determines the high flexibility of the polymers and their increased ability to hydrolyze under the action of microbiological enzymes [40]. PCL has excellent macrochain flexibility, and its mechanical behavior is similar to rubbers. Using the theoretical calculations based on the Flory-Huggins model, it is possible to evaluate the possibility of intermolecular interaction of polymers in the melt, which, in turn, determines the structure of the interphase layer, primarily its density and defectiveness [41]. In general, polymers are more compatible if they are of a similar nature and have similar Hildebrant solubility parameters. The higher the molecular weights of the blended polymers, the lower the predicted probability of the blend miscibility. Another tendency is that the higher the temperature, the higher the probability of the blend's miscibility. The study described in [42] provided a theoretical prediction of the miscibility for PBAT/PLA blends. It has been shown that a higher affinity would be possible between the two polymers when the BA and BT monomers reach a molar ratio exceeding 1 to 1. A pair of PLA and poly(ethylene oxide) (PEO), according to the Flory-Huggins interaction parameter, are miscible at nearly all temperatures [43]. The interaction parameters calculated from the Flory-Huggins equation in [44] for the blend of PLA with PBS showed a thermodynamically acceptable immiscible level.