Biosynthesis and Characteristics of Aromatic Polyhydroxyalkanoates

Abstract

Polyhydroxyalkanoates (PHAs) are polyesters synthesized by bacteria as a carbon and energy storage material. PHAs are characterized by thermoplasticity, biodegradability, and biocompatibility, and thus have attracted considerable attention for use in medical, agricultural, and marine applications. The properties of PHAs depend on the monomer composition and many types of PHA monomers have been reported. This review focuses on biosynthesized PHAs bearing aromatic groups as side chains. Aromatic PHAs show characteristics different from those of aliphatic PHAs. This review summarizes the types of aromatic PHAs and their characteristics, including their thermal and mechanical properties and degradation behavior. Furthermore, the effect of the introduction of an aromatic monomer on the glass transition temperature (T_g) of PHAs is discussed. The introduction of aromatic monomers into PHA chains is a promising method for improving the properties of PHAs, as the characteristics of aromatic PHAs differ from those of aliphatic PHAs.

Keywords: aromatic polymer; biodegradable polyester; glass transition temperature (Tg); mechanical property; polyhydroxyalkanoate (PHA); thermal property.

Figure 1

Structures of aromatic monomer units introduced into biosynthetic polyhydroxyalkanoates (PHAs). A monomer containing a nitrophenoxy group was also reported, but the detailed structure, including the carbon number, was not presented [24].

Figure 2

Correlation between T_g and average number of methylene units in aromatic side chain. Red circles: T_g of aromatic PHAs containing phenyl group; blue triangles: T_g of aromatic PHAs containing phenoxy, methylphenyl, methylphenoxy, or nitrophenyl group; green circles: T_g predicted from Fox equation. Values are from References [9,14,15,19,26,30,33,38,40,42,44,52,64].