Thermal Properties of Polyhydroxyalkanoates
Polyhydroxyalkanoates (PHAs), optically active polyesters with (R) absolute configuration and chiral centers, can be synthesized by variety of Gram-positive and Gram-negative microorganisms. They serve as energy reserve in the cytoplasmic fluid of bacteria, or as a sink for excess reducing equivalents for microorganisms (Doi, 1990). Poly(3-hydroxybutyrate) (PHB) is an example of homopolymer PHAs, which was first isolated by a French microbiologist in 1925 from Bacillus megaterium using chloroform extraction (Lemoigne, 1925; 1926). Following the discovery of PHB, copolymer containing 3-hydroxyvalerate (PHBV) (Doi et al., 1988), 4-hydroxybutyrate (PHBB) (Doi et al., 1988) and 3-hydroxyhexanoate (PHBHHx) (Doi et al., 1995) have been subsequently discovered. The synthesis of PHAs, in general, starts from production of hydroxyacyl-CoAs from carbon substrate and precursors, followed by the polymerization of hydroxyacyl-CoAs by PHAs syntheses into PHAs. To date, there are at least 140 types of PHAs monomers successfully identified (Steinbüchel and Valendin, 1995). Depending on the carbon substrates used and biochemical pathways of the microorganisms, the molecular structure and copolymer composition of PHAs can be judiciously altered to create polymeric materials with specific physical properties ranging from semicrystalline plastic to elastic materials, which resemble rubber. The PHAs are known to exhibit biodegradability (Jendrossek and Handrick, 2002; Mukai and Doi, 1995; Numata et al., 2009) in soil (Mergaert et al., 1993; Sang et al., 2002), sludge (Briese et al., 1994), seawater (Gonda et al., 2000) and tropical mangrove ecosystem (Sridewi et al., 2006). It has been long known that PHAs degrading microorganisms tend to attack the amorphous region of PHAs leading to a reduction of molecular weight (Doi et al., 1989), and a recent study of PHAs soil degradation by Boyandin and co-workers also shows the same tendency (2012).