Modification of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 150

A study by Bear et al. 15 reported the chemical epoxidation of mcl-PHA
obtained from P. oleovorans and Rhodospirillum rubrum using m-chloroper-
oxybenzoic acid (m-CPBA). Proton NMR analysis revealed about 36.7%
epoxidation that was calculated upon comparing the a,b-oxirane proton
signals (2.75 and 2.9 ppm) with the signals of methylene protons
(2.6-2.5 ppm) in the PHA backbone. Since Bear et al.'s report on the
chemical epoxidation of PHA, there have been several reports on a similar
epoxidation process. Park et al. 48 epoxidized PHOU with a controlled amount
of olefinic bonds using m-CPBA. 48 Regardless of the number of polymeric
olefinic groups, the researchers observed the process to follow second-order
reaction kinetics with an observed initial reaction rate (v)of1.1 10 3 L
mol 1 s 1 at 20 1C. However, both the melting temperature (T m ) and melting
enthalpy were observed to decrease with increasing conversion of the ole-
finic bonds to epoxy groups. Interestingly, the authors reported an increase
in glass transition temperature (T g ) by about 0.25 1C for each 1 mol% of
epoxide group, irrespective of the PHOU composition used. In similar
studies, the researchers cross-linked the epoxidized PHOU with succinic
anhydride in a reaction initiated by 2-ethyl-4-methylimidazole and carried
out at 90 1C for a period of 0.5 to 4 h, which resulted in a highly elastic cross-
linked PHA. 49 In their study, they found that by carrying out the reaction in
mild acidic conditions, the reported polymer degradation was inhibited. The
cross-linking kinetic parameters were evaluated using Kissinger 50 and
Ozawa 51 models, and they calculated a cross-linking activation energy of
15.6-16.0 kcal mol 1 in all the reactions. 49 When evaluating the thermal
stability of the epoxidized PHA, Park et al. 52 observed that the polymer
thermal stability increases with increasing epoxy group. The observed in-
crease in the thermal stability was attributed to intermolecular thermal
cross-linking reactions between the pendant epoxy groups and the carboxylic
acid groups generated from the polymer random chain scission by b-elim-
ination (Figure 7.6). This interpretation was derived from the appearance of
thermal exothermic peak ''b'' (375 1C) normally associated with a cross-
linked reaction, followed by the endothermic melting temperature peak ''a''
(299 1C) in the differential scanning calorimetric (DSC) thermogram of the
epoxidized polymer (Figure 7.7).
Mcl-PHA obtained from linseed oil was reported to possess a high number
of olefinic side-chains, making the polymer consistently viscous and sticky at
room temperature. 53 The polymer has limited potential applications, except
as a bio-adhesive, but the range of applications can be expanded by im-
proving the rigidity and stiffness of the polymer. Ashby et al. 53 used m-CPBA
(as illustrated in Figure 7.5(a)) to convert about 37% of olefinic bonds in
linseed oil derived mcl-PHA side-chains to epoxy groups in order to enhance
the mcl-PHA cross-linking ability.
Comparing the 13 C NMR spectra of both the neat mcl-PHA (Figure 7.8(a))
and the epoxidized mcl-PHA (Figure 7.8b), the researchers confirmed the
polymer epoxidation by the appearance of an epoxide chemical shift at
58 ppm as shown in Figure 7.8(b). They explained that steric hindrance

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