Modification of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 148

copolymers at 80-91% yield. 42 It was found that the modified telechelic
PHO-diol showed lower glass transition (T g ) and melting (T m ) temperatures
than neat PHO. However, this observation was found to contradict the re-
ported increase in the thermal stability of di-hydroxy terminated P3HB
synthesized by microbial fermentation. 23 Using 1,4-butanediol and PTSA,
Chen et al. 44 produced modified P3HB4HB and poly-3-hydroxybutyrate-co-3-
hydroxyhexanoate (PHBHHx) diols that were grafted with poly(ester-
urethanes) via Michael addition. Recently, Kwiecie ´ et al. 40 described a new
method for P3HB4HB hydroxylation by selective partial degradation of the
polymer ester bonds using lithium borohydride. The reaction was carried
out by drop-wise addition of 2 M LiBH 4 at a temperature o 20 1C after the
polymer was completely solubilized in tetrahydrofuran (THF). After a series
of purification steps of chloroform and water washing followed by evapor-
ation of the organic phase, about 97% of purified modified PHA oligodiols
were obtained. As revealed by NMR and electrospray ionization-mass spec-
trometry (ESI-MS) analyses, the applied reduction process of the PHA oli-
godiols was highly selective. 40 The researchers claimed the method to be
versatile and applicable to the production of any PHA oligodiol. Block co-
polymers of PHO and poly(ester-urethanes) were synthesized by a reaction
between the polymer hydroxyl group and the -NCO group of aliphatic
L-lysine methyl ester diisocyanate (LDI) that was employed as a junction
unit. 45 The aliphatic diisocyanate (LDI) was chosen over the aromatic dii-
socyanate due to the absence of the toxic, mutagenic, and carcinogenic
aromatic amine degradation byproducts of the aromatic diisocyanate
derived polyurethane. Similarly, Chen et al. 46 used facile melt polymerization
to synthesize a block poly(ester-urethane) based on diols of P3HB4HB and
poly-3-hydroxyhexanoate-co-3-hydroxyoctanoate (P3HHx3HO) using 1,6-
hexamethylene diisocyanate (HDI) as the coupling agent. Lactate dehy-
drogenase (LDH) assay and platelet adhesion determination revealed
that the synthesized material showed much higher platelet adhesion than
the neat polymers, and even higher than that of polylactic acid (PLA) and
poly(3-hydroxybutyrate) (PHB). 46 In fact, this type of poly(ester-urethane) was
reported to possess the properties required for use in medical applications
including enhanced wound healing activity. 46 A large group of PHA-based
materials were produced from hydroxylated PHA, and have been reviewed
extensively. 31

d n 2 r 4 n g | 1

.

7.2.3 Epoxidation
The high reactivity of epoxide groups under mild conditions was described
as an important factor in medium-chain-length (mcl) PHA modification
(Figure 7.5(a)). The epoxide group can participate in diverse reactions such
as cross-linking (Figure 7.5(b)) to attach copolymers, bioactive moieties, or
an ionizable group without unwanted polymer degradation. 31,47 Several
studies have reported successful chemical modification of PHA by
epoxidation. 48,49