Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities

VISAKH P. M.

1.1 Polyhydroxyalkanoates: Structure, Properties and Sources

Polyhydroxyalkanoates are biopolyesters with various side chains and fatty acids with hydroxyl groups at the 4- or 5-position. They consist of (R)-3-hydroxy fatty acids. There are three types of polyhydroxyalkanoates: (a) short chain length hydroxyalkanoic acids (PHASCL) with an alkyl side chain, which are produced by Ralstonia eutropha and many other bacteria. PHASCL contain 3–5 carbon atoms, for example poly-3-hydroxybutyrate (P3HB), poly-4-hydroxybutyrate (P4HB); (b) medium chain length hydroxyalkanoic acids (PHAMCL) with alkyl side chains that are produced by Pseudomonas oleovorans and other Pseudomonas sensu stricto. PHAMCL contain 6–14 carbon atoms and (c) long chain length (PHALCL) obtained from long chain fatty acids, which contain more than 14 carbon atoms The monomer composition, macromolecular structure and physical chemical properties of PHAs vary, depending on the producer organism as well as on the carbon source used for the growth. PHAs containing double bonds can also be produced by recombinant Methylobacterium extorquens strains when fed unsaturated fatty acids. These PHAs comprise PHASCL and PHAMCL. One reason for choosing amethylotrophic microorganism for such a purpose was that an important portion of the production process would use methanol. In spite of PHB being considered an environmentally friendly polymer with similar material properties to polypropylene (PP), it has not been used on a large scale to replace conventional polymers because it presents some drawbacks in its mechanical properties. Considering polymer mechanical properties, it is important to consider three basic properties when comparing the usefulness of a polymer for a given commodity application. It is hard to process PHB due to its high melting temperature of approximately 170 1C, which is very near to its degradation temperature. Therefore, a solution to these drawbacks could be the copolymerization of 3-HB with other monomers that confer less stiffness and tougher properties (which bestow greater flexibility and lessen breakage) and to reduce the melting point.

The monomer composition of PHA has considerable effects on its physical properties.

The PHA structure can effectively be controlled by adjusting the carbon substrates to achieve desired monomer contents, by engineering metabolic pathways in the hosts or by feeding the culture with carbon substrates containing functional side chains that in a second step can suffer chemical modifications. PHAs, such as PHB and poly(3-hydroxybutyrate-co-3 -hydroxyvalerate (PHBV), are brittle, which is related to their high crystalline degree and they may lack the superior mechanical properties required for biomedical and packaging applications. These properties are a consequence of PHA's chemical structure. Therefore, since these different types of PHA have various structural and physical chemical properties, they should be classified according to their properties and modified in order to be easy to use for target applications. Different approaches were explored for the production of PHASCL by the Cupriavidus genus. The main studies were: the utilization of noble versus waste carbon sources and the utilization of a limiting factor to trigger PHA production versus the operation under "nutrient-sufficient" conditions. Thus, Chen et al. (2011) observed that a smaller C–N ratio was more favorable for PHA accumulation in a culture of Gamma proteobacterium in 72 hours, whereas a higher C–N ratio was more favorable for PHA accumulation in longer cultures of up to 150 hours of cultivation.9 Concerning activated sludge systems, Moralejo-Garate et al. (2013) have shown that the presence of ammonia during the PHA accumulation step was not damaging for PHA production.

4-Hydroxybutyrate (4-HB) was produced by Aeromonas hydrophila 4AK4, Escherichia coli S17-1, or Pseudomonas putida KT2442 harboring 1,3 -propanediol dehydrogenase gene dhaT and aldehyde dehydrogenase gene aldD from P. putida KT2442, which are capable of transforming 1,4 -butanediol (1,4-BD) to 4HB. 4HB containing fermentation broth was used for the production of homopolymer poly-4-hydroxybutyrate [P(4HB)] and copolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-4HB)]. In this respect, attention has been given to producing PHAs bearing terminal double bonds in their side chains. The reason behind the preference for unsaturated PHAs can be found in the possibility to establish high-yield production processes. Alkylic substrates containing double bonds are cheaper and generally exhibit less toxicity compared to substrates with reactive functional groups. At the same time, the unsaturated side chains of PHAs are susceptible to chemical modifications. Starting from alkene function, PHAs have been modified to exhibit functionalities en route towards new PHA-based biomaterials. The mechanical properties of PHAs are directly correlated with their structure and crystallinity. An increase in the variety of side chains within one polymer chain of PHAMCL can modify its ability to crystallize and as a consequence there are some distinct differences in the crystallinity of PHAMCL. Obtaining a low crystallinity is possibly done once the polymers have large and irregular pendant side groups attached. These groups inhibit the close packing of the polymeric chains in a regular three-dimensional fashion to form a crystalline array.

The physical and material properties of PHAs are greatly influenced by their monomer composition and chemical structure i.e. the length of the pendant groups that extend from the polymer backbone, the chemical nature of the pendant groups and the distance between the ester linkages in the polymer. The variety of bacterial PHA that can be directly produced by fermentation is extraordinary large...