Polyhydroxyalkanoate-based Multiphase Materials - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 124

carbons in the repeating units. The term mCL was coined because the
number of carbons in the monomers roughly corresponds to those of
medium-chain-length carboxylic acids. PHA nomenclature may still be in a
state of flux as new structures continue to be discovered. The main polymer
of the polyhydroxyalkanoates family is the polyhydroxybutyrate homo-
polymer (PHB), but different poly(hydroxybutyrate-co-hydroxyalkanoates)
copolyesters exist, such as poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV),
or poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHx), poly(hydroxybutyrate-
co-hydroxyoctanoate) (PHBO) or poly(hydroxybutyrate-co-hydroxyoctadecanoate)
(PHBOd) (Figure 6.2 and Table 6.1).
Figure 6.2 shows the generic formula for PHAs where x is 1 or higher, and
R can be either hydrogen or hydrocarbon chains of up to around C16 in
length. The main members of the PHA homopolymers family are presented
in Table 6.1.
A wide range of PHA homopolymers and copolymers have been produced,
in most cases at the laboratory scale. PHAs are probably the largest family of
thermoplastics with over 125 different types prepared from various hydro-
xyalkanoate monomers. The properties of the different PHAs depend on the
comonomer structure (length of pendant groups and length of carbon chain
in the repeating unit of the polymer backbone) along with their mol% in the
polymer chain. Varying the PHA comonomer and its mol% content is quite
well pursued as it allows, to a certain degree, the tailoring of the PHA
properties. Therefore, while a lot of emphasis is placed on the chemical
modification, degradation characteristics, and material properties with re-
gard to industrial applications of PHAs, a great deal of importance is also
placed on the bioengineering of the bacterial genome to produce different
types of PHAs.
PHB is a highly crystalline polymer and due to this, its applications are
quite limited. However, PHB can be modified by incorporating comonomers
in the polymer chain that allow a certain degree of flexibility in the chain and

d n 2 r 4 n g | 9

.

Figure 6.2 Chemical structure of polyhydroxyalkanoates.

Table 6.1 Some PHA homopolymer structures based on Figure 6.2.

Chemical name Abbreviation x value R group

Poly(3-hydroxypropionate) P(3HP) 1 Hydrogen

Poly(3-hydroxybutyrate) P(3HB) 1 Methyl

Poly(3-hydroxyvalerate) P(3HV) 1 Ethyl

Poly(4-hydroxybutyrate) P(4HB) 2 Hydrogen

Poly(5-hydroxybutyrate) P(5HB) 2 Methyl

Poly(5-hydroxyvalerate) P(5HV) 3 Hydrogen