Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 33

2.4.2 Chemical Modification of PHA Structure
Some PHA SCL may be too rigid and brittle and may lack the superior
mechanical properties required for biomedical and packaging film appli-
cations. In contrast, as described before, PHA MCL may be elastomeric but
have very low mechanical strength. It is reported that the stress-strain curve
for P(3-hydroxy-octanoate) containing minor quantities of other MCL
monomers was typical of those observed for elastomers with no yield stress
appearing in the stress-strain curve. A Young's modulus of 17 MPa and an
elongation to break of 250-350% were also determined. 83
Therefore, for packaging materials, biomedical applications, tissue en-
gineering, and other specific applications, the physical and mechanical
properties of microbial polyesters need to be diversified and improved. 5,10
The mechanical properties of PHA are directly correlated with its 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 differences in the crystallinity of PHA MCL . 4 Obtaining
a low crystallinity could occur once the polymers have attached large and
irregular pendant side groups. These groups inhibit the close packing of
the polymeric chains in a regular three-dimensional fashion to form a
crystalline array.
Once the axial geometry in a polymeric chain is the main factor in deter-
mining the ability of a chain to form crystallites, the crystalline contribution is
probably due to isotactic and syndiotactic structure sequences. 4,84,85 In fact,
saturated PHA MCL , which are able to crystallise due to their isotactic con-
figuration, are also seen to crystallise with alkyl side chains in an extended
conformation to form ordered sheets, but they still show a reduced degree of
crystallinity when compared to P(3HB) or P(3HB-co-3HV), owing to low crys-
tallization rates. 4
The chemical modification of PHA can be done via block copolymerization
and grafting reactions, chlorination, cross-linking, epoxidation, hydroxyl
and carboxylic acid functionalization and so on. With the help of chemical
modification it is possible to prepare a wide range of polymers, from
hydrophobic to hydrophilic with high stability by adjusting the nature,
structure and percentage of grafted side groups. 86
A common route that can be applied for obtaining polyhydroxyalkanoates
with desirable functionalities is to produce PHAs with terminal double
bonds followed by chemical modification steps. Carbon double bonds are
comparatively inert but can be easily transformed into reactive functional
groups under mild reaction conditions. Non-functionalized PHAs can also
be activated by surface modification techniques. The resulting tailor-made
structural and material properties have positioned polyhydroxyalkanoates
well to contribute to the manufacturing of second and third generation
biomaterials. 43
Unsaturated PHA MCL , such as polyhydroxyundecenoate (PHU), have very
low crystallinity and cross link when irradiated with UV light, even in the

d n 2 r 4 n g | 8

.