# Poly(lactic acid)-Based Blends: A Comprehensive Review
## 6 Composites of Poly(lactic acid) with Poly(3-hydroxybutyrate)  
Poly(3-hydroxybutyrate) (PHB) was the first isolated and characterized among poly(hydroxyalkanoates) [94]. It is formed as a carbon reserve in a wide range of producing bacterial strains and is produced industrially by bacterial fermentation [95]. Figure 13 shows the typical chemical structure of poly(3-hydroxybutyrate). PHB is characterized by the presence of a methyl functional group (-CH\({}_{3}\)) and a group of ester bonds (-COOR); these functional groups are responsible for the thermoplasticity of materials, hydrophobicity, high crystallinity, and brittleness. PHB is semi-crystalline polymer due to its linear-chain structure containing both amorphous and crystalline phases. Typical values of the degree of crystallinity range from 50 to 80% [96]. A higher the degree of crystallinity induces more rigidity and durability. The degree of crystallinity can also influence hardness, the modulus of elasticity, density, transparency and a pattern of cold drawing or ductile flows. One of the problems associated with PHB is its narrow temperature range of processing, particularly a small difference  
Figure 12: Physical properties of the EVA and EVA/PLA blends Young’s modulus [92]. between malting and decomposition temperatures, as a result of which this material is subjected to thermal decomposition at temperatures around the melting point. The addition of PLA to PHB provides an increase in the thermal stability of the latter. PHB has better barrier properties compared to both polyethylene terephthalate (PET) and polyvinyl chloride (PVC); however, it is more rigid and less flexible than traditional synthetic polymers. PHB is characterised by a high ability to biodegrade when the material is in contact with decomposing microorganisms in biologically active media, such as soils, fresh water, and industrial composting conditions, which make it an environmentally friendly alternative to synthetic polymers. The creation of blends based on PLA and PHB makes it possible to level the disadvantages of each of the polymers. [PERSON] et al. [97] have described the effect of PHB on barrier properties of PLA in the blends. The materials were obtained from PLA/PHB blends with glycerin triacetate (GTA) as a plasticizer and chitin nanocrystals (CNA) as a structural modifier (increasing the crystallinity). The blend of 75 PLA/25 PHB showed the formation of many small spherulites of PHB with the highest crystallinity of PHB among the studied composites and was chosen as a matrix for nanocomposites with CAN (Figure 14). Figure 14: Polarized optical micrographs of melted films on the hot stage showing the crystallization of (**a**) pure PLA, (**b**) 90:10 with no visible crystallization, (**c**) 75:25 small crystallites well-dispersed in the PLA matrix, (**d**) 50:50 with large spherulites and (**e**) pure PHB showing very large spherulite size compared to the blends [97]. Figure 13: The chemical structure of poly(3-hydroxybutyrate). GTA has a negative effect on thermal properties. CNA acts as a nucleating agent for the PHB phase and lead to the formation of well-dispersed PHB spherulites in the PLA phase and a significant increase in the crystallization rate and degree of crystallinity. The nanocomposite with the highest crystallinity is characterized by significantly reduced permeability to oxygen and carbon dioxide and increased mechanical properties compared to the non-filled blend. A significant improvement in crystallinity and the formation of homogeneous spherulites was reflected in the excellent barrier characteristics of nanocomposites. [PERSON] et al. [98] have studied a surface morphology of the PLA/PHB blends by SEM (Figure 15). Neat PLA is cauterized by a smooth and uniform surface containing several holes, while neat PHB is characterized by an uneven surface due to its crystalline structure. Roughness has been observed on the surface of the 75 PLA/25 PHB, and cracks mainly develops in one direction, indicating that the resistance to crack propagation is rather low.