# Poly(lactic acid)-Based Blends: A Comprehensive Review
## 5 Composites of Poly(lactic acid) with Poly(ethylene-co-vinyl acetate)  
The copolymer of poly(ethylene-co-vinyl acetate) (EVA) belongs to the class of poly(olefins) and is obtained as a result of copolymerization of poly(ethylene-co-vinyl acetate) monomer (Figure 10). The properties of EVA vary in a wide range depending on the number of vinyl acetate groups in the chain. The vinyl acetate content has two fundamental effects affecting the properties of EVA copolymers. The first effect is the destruction of crystalline regions formed by poly(ethylene) segments of the copolymer. Low- and medium-density poly(ethylenes) obtained by volumetric method under high pressure usually have a degree of crystallinity in the range of  
Figure 10: The chemical structure of poly(ethylene-co-vinyl acetate). Figure 9: TG curves of PLA/PBAT composite films (**a**) before and (**b**) after 24 months of degradation [76]. 40-65%. The degree of crystallinity of poly(ethylene-co-vinyl acetate) gradually decreases as the content of VA units increases, the copolymer with 40-50 wt.% of VA units becomes completely amorphous. The second most important effect of the vinyl acetate content is related with their polar nature. Thus, with an increase in the vinyl acetate content, a polarity of the copolymer changes. Although an increase in polarity is not as important as a decrease in crystallinity, it also leads to certain patterns of behaviour of the material [78]. The molecular structure of EVA determines a wide variety of applications of this copolymer. [PERSON] et al. [79] have studied the blends based on PLA and EVA with a scanning electron microscopy (SEM). It has been shown that the blend of PLA/EVA is characterized by a two-phase morphology with EVA domains dispersed in the PLA matrix (Figure 11) [80; 81]. Droplet-matrix morphology of the blends were typical for all the blends with lower content of EVA with signs of phase adhesion [82] and the blends with a higher content of EVA (more 30 wt.%) are characterized by the structure of interpenetrating phases of polymers [83; 84; 85; 86]. By compounding PLA and EVA, the possibility of improving the mechanical properties is realised [87]. It has been found that the compatibility of PLA and EVA increases with an increase in EVA content in the blend [81]. Due to a high elasticity and low crystallinity of EVA, it is possible to fill PLA/EVA blends with dispersed particles. [PERSON] et al. have used crushed pine needles as a reinforcing filler for the PLA/EVA blends. The applied treatment with sodium hydroxide of pine needles increases thermal stability and reduces water absorption, compared to the initial materials [88]. [PERSON] et al. have found that adding EVA reduces the crystallinity and glass transition temperature of PLA. Lower crystallinity leads to an increase in the rate of water absorption and accelerates biodegradation in the amorphous phase. An increase in the EVA content during the continuous PLA phase increases a number of EVA droplets in the material, which, in turn, enhances biodegradation due to facilitated water diffusion [79]. Some scientists have investigated PLA/EVA mixtures as a coating of the stents for drug elution. It became known that a rate and an amount of drug release can be adjusted by varying PLA/EVA ratio. The study by thermogravimetric analysis has showed a two-stage degradation of the blends PLA/EVA. The intensity of the second degradation peak increases symbiotically to increase in the EVA content, and, consequently, the thermal stability of the materials drops. A dynamic modulus of elasticity of the PLA/EVA blends gradually decreases with the addition of EVA component. The rheology data results have shown a monotonous increase in the dynamic modulus of elasticity, loss modulus, and complex viscosity with an increase in EVA content in the blend. This increase has been explained by an enhanced phase interaction between polymers [79]. Figure 11: TEM images of the blends of PLA/EVA: 90/10 (a), 70/30 (b) [81]. [PERSON] et al.