# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 5 Structural Rearrangement of PLA Chains  
In the case of glassy and partially crystalline polymers, crazing is often one of the main failure mechanisms [125; 126; 127]. However, an application of adsorption-active media can facilitate significantly regulating the process of crazing [128]. Structural-mechanical modification by the mechanism of crazing in an adsorption-active medium is a process of structural rearrangement of PLA during drawing with a formation of fibrillar structure. As a result, the formation of a fibrillar structure occurs, which is a system of oriented and separated fibrils with a diameter of 5-20 nm in the polymer volume [129]. At the same time, the capture, dispersion and fixation of a substance dissolved in the medium in the pores of the polymer matrix can occur. Thus, this method allows for the plasticization of PLA but also its functionalization. Figure 5 presents a schematic illustration of the structural modification of a polymer with simultaneous impregnation of an additive. [PERSON] et al. have carried out a lot of investigations in the field of exploration of PLA crazing [130; 131; 132; 133]. In particular, the effect of different media (air and adsorption-active liquids) during uniaxial stretching on the formation of crazes in PLA was studied in [131]. The crazing mechanism was investigated in hydrocarbons, organosilicon liquids, aliphatic alcohols, and water-alcohol solutions. The maximal elongation at breakup was detected in n-pentanol (570%). Thanks to this investigation, one can predict the critical mechanical stress value when the crazing is initiated in PLA during application. In [134; 135], the authors developed the functional fibrous material of PLA for application in medical practice using the crazing mechanism. [PERSON] et al. [136] investigated PLA with low crystallinity (only 3%), which was subjected to physical aging at room temperature. The authors used post-stretching that improved not only the toughness of PLA but also increased the aging resistance. Investigation of the physical aging of PLA (observation of changes in properties of the injection molded samples in time) showed that large internal stresses led to crazing and, as a result, to brittle failure [137]. With the aim of understanding the plastic deformation mechanism of PLA upon cold-drawing, the authors in [138] used SAXS and AFM. It was revealed that crazing mechanisms are responsible for the brittle character of PLA. [PERSON] et al., using an in situ SAXS method, illustrated that there are complex interactions between shear banding and crazing in PLA during plastic deformation. [PERSON] et al. [139] also used SAXS and some other methods to investigate the mechanical and crystalline structure development during the uniaxial deformation of PLLA while varying strain rates and draw temperatures. As a result of thermal analysis, it was revealed that the drawing occurred due to the orientation of PLA molecular chains. It was shown that the rise in draw temperature resulted in crazing, microvoiding and cavitation. The authors concluded that regulating the parameters (temperature, rate) of the processes above can allow one to control and modify the PLLA-based material properties. [PERSON] et al. [140] demonstrated that specific processing of PLA, namely extrusion casting under melt-stretching flow and cast roll temperature filed, resulted in a rise in elongation at breakup at 270%. The PLA molecular chains could be fully extended by varying the melt-draw ratios. In order to initiate the crazing resulting in improvement of ductility and toughness, the authors [141] mixed PLA with poly(1,4-cis-isoprene), a major component of natural rubber. It should be noted that three sequential mechanisms (including crazing) described in the work resulted in sufficiently efficient toughening of PLA at a small amount of poly(1,4-cis-isoprene) (5%). Other works also explore the changes in structure correlated with stresses, drawing etc. (for example [142; 143; 144; 145]). [PERSON] et al.