Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 24

2.3 Properties of PHAs

2.3.1 Thermo-mechanical Properties
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 which extend from the polymer backbone, the chemical nature of
these pendant groups and the distance between the ester linkages in the
polymer. 4,30
As mentioned before, the PHA can be primarily divided according to the
size of the pendant group (the number of carbon atoms in the monomer
units) into a short chain length, medium chain length and long chain length
(Figure 2.1). 31
Besides its general classification (regarding the length of the side chain),
there are several different types of side chain in the same family of PHA; they
can also have functional groups attached. Of special interest are the
3-hydroxyl group and the carboxylic group of the functionalized groups in
the side chain that allow further chemical modification, e.g., halogens,
carboxyl, hydroxyl, epoxy, phenoxy, cyanophenoxy, nitrophenoxy, thiophen-
oxy, and methylester groups. 9
There are some strategies to target the final PHA structures to achieve
tailor-made compositions. It can be relatively well controlled in three
ways: (i) by choosing the correct microorganism host; (ii) by choosing
appropriate cultivation conditions and carbon sources and/or (iii) by
performing chemical modification after their production and purification.
Combining these three strategies can bring success in achieving the desired
composition. 32
A comparison of the physical properties of some PHAs with polypropylene
is given in Table 2.3.

d n 2 r 4 n g | 8

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2.3.2 Biocompatibility Properties
An important potential for PHA usage in the field of biocompatible materials
has been foreseen for a long time. Their potential applications lie in wound
management, the vascular system, orthopaedics, drug delivery, urology,
etc. 33,34
PHA MCL are usually more biocompatible than PHA SCL . Indeed, recent
studies on PHA degradation products have shown that the cytotoxicity of the
oligo-hydroxyalkanoates (OHA) decreased with the increasing OHA side
chain length. 57 However, the most abundant studies have concerned P4HA
and the PHA SCL -co-PHA MCL due to their larger accessibility and specific
properties.
Studies of the PHB-co-PHHx copolymer have shown that a moderate
content of 3-HHx (12%) co-monomers induces better adhesion of smooth
muscle cells from rabbit aorta (RaSMCs), whereas a higher content of 3-HHx
(20%) is found to be more suitable for the proliferation of these cells. 35