Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 19

O

C

OCH CH 2

d n 2 r 4 n g | 8

R

(PHA)

R = methyl or ethyl (short chain length)

R = propyl, butyl, pentyl, hexyl or heptyl (medium chain length)

R = more than 14 carbons per repeating unit (long chain length)

Figure 2.1 General classification of bacterial polyesters according to their chemical
structure.

The large variability in the chemical structure and material properties
of PHAs is due to the low substrate specificity of PHA synthases and the
subsequent modifications by chemical reactions. The monomer com-
position, macromolecular structure and physical chemical properties of PHA
vary depending on the producer organism as well as the carbon source used
for the growth. 3-5 Their properties span a wide range, including materials
that resemble polypropylene and others that are elastomeric.
The monomer composition of PHA has a considerable effect on its phys-
ical properties. For instance, in the case of PHA MCL , the combination of T g
values below room temperature and a low degree of crystallinity imparts
elastomeric behavior to these polymers. The PHA structure can effectively be
controlled by adjusting the carbon substrates to achieve the desired mono-
mer content, 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 undergo chemical modifications. 6-10 Their different
properties arise chemically, either from the length of the pendant groups
which extend from the polymer backbones, or from the distance between the
ester linkages in the polymer backbones. Typically, PHAs with short pendant
groups are hard crystalline materials, whereas PHAs with longer pendant
groups are elastomeric. 11,12
PHAs such as PHB and 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. The physical properties
of P(3HB-co-4HB) and P(3HB-co-3HP) show some improved flexibility as
biodegradable thermoplastics compared to those of PHB, as the 4HB
and 3HP monomer fractions in the copolymer increase, respectively. 13,14
On the other hand, PHA MCL may be elastomeric but have very low mech-
anical strength. 10 These properties are a consequence of PHAs' chemical
structure. Therefore, since these different types of PHAs have various
structural and physical chemical properties, they should be classified ac-
cording to their properties and modified in order to be easy to use for target
applications.

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