Chemistry Reference
In-Depth Information
previously reported studies on synthetic polymer-based nanocomposites.
Further attention was paid to the PHAs' degradation in nanocomposite
systems since these polymers are very temperature sensitive.
73-75
Thus, scientists were interested in other PHA-based nanocomposites filled
with e.g. layered double hydroxides (LDH),
76,77
cellulose whiskers
78-80
and
hydroxyapatite (HA),
81
the latter being used in particular for biomedical
and tissue engineering applications. LDH structures are similar to layered
silicate clays.
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1.6 Modification of Polyhydroxyalkanoates
Chemical, physical and enzymatic approaches have been explored for
polymer modifications, resulting in a uniquely transformed PHA endowed
with a functionalized reactive group and/or enhanced properties such as
thermal stability, elasticity, improved hydrophilicity and degradability.
While chemical modification processes provide a large degree of freedom
in controlling and designing the modified PHA in bulk quantities to suit a
particular function, most often they have to contend with the drawback of
toxic impurities that require dicult downstream processing. The struc-
ture of a PHA can be altered chemically to produce a modified polymer with
predictable variation in molecular weight and functionality. For example,
the hydrolytic rate of PHA to give an activated macromer that can accept a
reactive functional group is said to depend on several factors such as the
chemical nature or reactivity of the ester linkages between the mono-
mers.
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PHA modification by carboxylation is the addition of a carboxylic
functional group to the polymeric macromer. Carboxylic groups incorpor-
ated into the polymer usually serve as functional binding sites for bio-
active moieties such as probes for targeting proteins and hydrophilic
components.
83
Condensation reactions between carboxylic acids and amine groups were
exploited to graft a modified PHA and linoleic acid onto chitosan.
84
Recently,
Babinot et al.
85
used click ligation to esterify the pendant -COOH of car-
boxylated PHA with propargyl alcohol resulting in a clickable-alkyne group
that was subsequently used to copolymerize a poly(ethylene glycol) (PEG)
macromer onto the modified PHA. The properties of PHA and its copolymers
have been reported to be modified by hydroxylation.
86-89
Normally, acid- or
base-catalyzed reactions are used in the modification of PHA by hydroxyla-
tion in the presence of low molecular weight mono or diol compounds.
Hydroxy-terminated PHA is important in block copolymerization. Metha-
nolysis of PHA resulted in PHA methyl esters bearing monohydroxy-
terminated groups. Halogenation of PHA is considered as an excellent
method to diversify the polymer's functions and applications. Halogen
atoms such as chlorine, bromine and fluorine were added to the olefinic
bonds of unsaturated PHA through an addition reaction,
90
.
and to the
saturated PHA via substitution reactions.
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