Blends of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 68

The wide substrate range of PHA synthase has resulted in the versatility of
the monomer compositions, which is a clear advantage because the mono-
mer variation provides PHAs an extended spectrum of associated properties.
PHAs are classified into three classes according to their monomer com-
positions: Short-chain-length (SCL)-PHAs, medium-chain-length (MCL)-
PHAs and short-chain-length-medium-chain-length (SCL-MCL)-PHAs. 11,12
Homopolymer poly(3-hydroxybutyrate) [P(3HB)] is the most well-known
SCL-PHA produced by wide ranges of microorganisms and has comparable
material properties with polypropylene. 13 Manipulating the side chains and
compositions of the P(3HB) homopolymer through incorporation of other
monomers can generate different types of polymers with favourable material
properties as the polymers will confer less stiffness and tougher properties.
Among the SCL-PHAs that have been studied with such material pro-
perties are poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] and
poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] copolymers. 14
MCL-PHAs are more elastomer in nature compared to SCL-PHAs, which
are often stiff and brittle. Incorporating both monomers will result in SCL-
MCL-PHA copolymers exhibiting properties between the two states which
will depend on the different proportions of SCL and MCL monomers. 15
Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] copolymer
is one of the successful SCL-MCL-PHAs that is produced on an industrial
scale. 12
Various PHA blends have been developed to improve the performance and
to offset the high price of PHAs. The blending of PHAs will offer more scope to
expand their range of applications. In this chapter, some features of blends of
SCL-PHAs with PLA, starch, PCL, cellulose and chitosan will be discussed.

d n 2 r 4 n g | 6

. 4.2 Production of Blends of PHAs

4.2.1 PHAs/PLA Blends
The P(3HB)/PLA blend is one of the most studied blends, which exhibits
mechanical properties that are intermediate between the individual com-
ponents. Although PLA and P(3HB) are biodegradable polymers synthesized
from renewable resources, their potential applications are hampered due to
their brittleness and the formation of very large spherulities. 16,17 P(3HB)/PLA
blends were studied as early as 1996 to explore their miscibility, crystal-
lisation, morphology, mechanical properties and biodegradation behaviour.
P(3HB)/PLA blends with different compositions (100/0, 80/20, 60/40, 40/60,
20/80 and 0/100, wt%) were prepared by casting a film from a common
solvent, chloroform, at room temperature. 18
A melt-crystallisation method had been employed by Ohkoshi and co-
workers 19 to prepare blends of atacticP(3HB) with PLA. The atacticP(3HB)
[ataP(3HB)] is synthesized by the polymerisation of racemic b-butyrolactone
and is a completely amorphous polymer. The solvent-casted film of
ataP(3HB)/PLA blends were inserted between two Teflon sheets (0.2 mm)