Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 3

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. 11 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 12,13 of
PHAs are directly correlated with their structure and crystallinity. An increase
in the variety of side chains within one polymer chain of PHA MCL can modify
its ability to crystallize and as a consequence there are some distinct differ-
ences in the crystallinity of PHA MCL . 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 with more than 150 different hydro-
xyalkanoic acids, and even mercaptoalkanoic acid, that are known con-
stituents of these PHAs. 14 Depending on the subunit compositions and
substrate specificities of the enzymes, PHA synthases are generally classified
into four groups: class I, II, III, and IV. 15

d n 2 r 4 n g | 1

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1.2 Recovery of Polyhydroxyalkanoates
The recovery system may affect the amount of product recovered, the con-
venience of the subsequent purification steps and the quality of the final
product. Cell separation from the fermentation broth is the preliminary step
of the recovery method. In order to recover the PHA granules, it is necessary
to rupture the bacterial cell and remove the protein layer that coats the PHA
granules. Alternatively, the PHA has to be selectively dissolved in a suitable
solvent. Generally, two methods are usually utilized for the recovery and
purification of PHAs from cell biomass, which include PHA solubilization or
non-polymer cellular material (NPCM) dissolution. The majority of the PHA
recovery method is performed using a solvent extraction process mainly
by chloroform and methanol. Modifying the cell wall's permeability and
then PHA dissolution in the solvent are the mechanisms for PHA extraction.