Polyhydroxyalkanoates (PHAs), their Blends, Composites and Nanocomposites: State of the Art, New Challenges and Opportunities - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 10

In another study, Arkin and Hazer 91 modified PHA-Cl into quaternary
ammonium salts, thiosulfate moieties and phenyl derivatives. In addition,
they cross-linked the modified PHA-Cl with benzene by electrophilic
aromatic substitution using a Friedel-Crafts reaction. Mihara et al. 92 and
Imamura et al. 93 filed an embodiment that detailed the procedures for PHA
chemical modification by sulfanyl halogenation and the potential appli-
cation of the modified PHA as a toner electrostatic charge controller in
electrophotographic imaging. Another method to modify PHA is by graft
copolymerization, which results in the formation of modified segmented
copolymers with improved properties such as increased wettability and
thermo-mechanical strength. Grafting reactions can be induced by either
chemical, radiation or plasma discharge methods. 94,95 Chemical modifi-
cation methods are sometimes aggressive and lead to reduced polymer
molecular weight, unwanted side reaction(s) and toxic impurities. In some
instances, a mild surface modification process is required without which the
polymer may fail in its intended application(s). Irradiation of polymeric
materials required no addition of polymer contaminants. Irradiations such
as gamma-irradiation normally result in three-dimensional network struc-
tures with improved tensile strength. Several studies have demonstrated the
cross-linking of unsaturated mcl-PHAs by gamma-irradiation. 96-98 The
presence of olefinic bonds in PHA side chains provides an avenue for
polymer modification by several irradiation processes. A highly cross-linked
modified polymer was produced by irradiating unsaturated PHA obtained
from tallow-grown P. resinovorans with 25-50 kilogray (kGy) of g-irradiation.
Ion implantation is another physical method employed in polymer surface
modification. Its advantage over polymer modification methods is that
it only modifies the polymer surface layer, without upsetting the bulk
polymer's properties. Ion implantation has been successfully applied in
several polymer modifications thereby expanding its applications. 99-103
Mirmohammadi et al. 104 compared the biocompatibility of a PHB surface
upon treatment with O 2 and CO 2 plasma at 50 W discharge for 3 min, and
found that O 2 plasma treated PHB showed much improvement. Ying et al. 105
evaluated the biocompatibility and biosorption characteristics of an elec-
trospun scaffold of P3HB4HB through subcutaneous implantation of the
fibers in rats. The researchers found a highly increased tissue response with
increasing content of 4HB monomer.

d n 2 r 4 n g | 1

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1.7 Polyhydroxyalkanoates as Packaging Materials:
Current Applications and Future Prospects
The most common PHA packaging resins are polyhydroxybutyrate (PHB) and
its copolymer with polyhydroxyvalerate (P(HB-co-HV). The potential of PHAs
as biodegradable replacements for conventional bulk commodity plastic
packaging while promoting sustainable development has long been recog-
nised. 106 The potential of PHAs for truly biodegradable packaging was