Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 2

acids, which contain more than 14 carbon atoms The monomer com-
position, 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. 1-3 PHAs containing double bonds can also be
produced by recombinant Methylobacterium extorquens strains when fed
unsaturated fatty acids. These PHAs comprise PHA SCL and PHA MCL . One
reason for choosing a methylotrophic 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 com-
paring 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 so-
lution 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. 4-8 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 pro-
duction of PHA SCL 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-sucient'' 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 accumu-
lation step was not damaging for PHA production. 10
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

d n 2 r 4 n g | 1

.