# Poly(lactic acid)-Based Blends: A Comprehensive Review
## 4 Composites of Poly(lactic acid) with Poly(butylene adipate-co-terephthalalate)
The signs of interfacial separation in the blends observed by SEM have indicated poor interfacial adhesion between PLA and PBAT in the blends (Figure 8) [70; 71]. Despite the limited compatibility of these polymers, numerous researchers have shown synergistic effects when mixing PLA and PBAT. Studying the patterns of changes in the phase structure of the PLA/PBAT blends depending on the composition allows us to control the formation of two continuous phases and create an interpenetrating structure with an appropriate unique combination of properties. As a rule, the methods of capillary rheometry, scanning electron microscopy, atomic force microscopy, dynamic mechanical and thermomechanical analysis are used for the structure investigation. To improve the compatibility of PLA and PBAT, special mixing conditions and compatibilizers can be applied. It has been shown that temperature or pre-treatment conditions (removal of moisture and oxygen during the mixing process) influence developing PLA oligomers with high diffusion and reactivity, which easily migrate to the PLA/PBAT interface region, with the formation of mixed chains (copolymers) that, in turn, determine the materials' degradability [72]. [PERSON] et al. [73] have calculated the threshold value for the formation of a co-continuous phase structure for the blend of \(\varphi\)PBAT/\(\varphi\)PLA, equal to 0.235, which corresponds to a PBAT concentration of 19 wt.%. This result has been confirmed by the mechanical properties study: an elongation at break sharply increases from about 10% to 300% in the composition range from 10 to 20 wt.% PBAT in the blend. [PERSON] et al. [74] have stated that a modulus of elasticity of the blends of PLA/PBAT can be improved if the PLA content is at least 40 vol.%. When the content of PLA in the blend is from 10 to 40 vol.%, there is no significant effect on the tensile strength of the materials. [PERSON] et al. [70] have shown that with an increase in the PBAT content (5-20 wt.%) a decrease in tensile strength and modulus of elasticity occurs; however, elongation and toughness increase suddenly. With the addition of PBAT, a type of deformation under tension changes from brittle fracture for pure PLA to plastic flow for the blend with PBAT content more 20 wt.%. [PERSON] et al. [72] have shown that the introduction of PBAT significantly contributes to the cold crystallization of PLA using the method of differential scanning calorimetry. The degree of PLA crystallinity has increased from 2 to 19% and the PLA crystallization temperature has increased from 121.9 \({}^{\circ}\)C to 99.1 \({}^{\circ}\)C in the blend PLA/PBAT = 70/30 wt.% compared to pure PLA. An ability to degrade the PLA/PBAT blends has been studied under the influence of various media [75; 76]. The degradability of such blends was influenced by a composition of the mixture, hydrophilicity, and the mechanisms of biodegradation of the components. It  
Figure 8: SEM image of the blend of PLA/PBAT 90/10 wt.% [71]. has been shown that a presence of butylene adipate units in the PBAT copolymer causes an increased ability to hydrolyse under the action of microbiological enzymes [57,77]. According to these studies, in the process of biodegradation, the molecular weight of polymers decreases; however, the crystallinity increases, which is due to the greater availability of amorphous regions for microorganisms. Hydrolysis during biodegradation also affects the thermal stability and thermal decomposition of PLA /PBAT composites. The TGA curves of composite films before and after 24 months of biodegradation are shown in Figure 9. The degradation temperature of PBAT after degradation migrates to a lower temperature, though not as significantly as PLA, which can be attributed to the faster molecular chain fracture of PLA [76].