Blends of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 89

Addition of glycerol as a plasticizer to the blend of P(3HB-co-3HV) polymer
and starch produced the same trend where the elongation at break de-
creased as the amount of starch added increased reflecting the rigid nature
of this polymer. 49 In another study by Reis et al, 52 the addition of starch
content decreased the Young's modulus of P(3HB-co-3HV) polymer resulting
in more flexible materials. The same trend was reported by Godbole et al. 67
and Thire et al. 68 Contradicting findings were reported by Koller and Owen 69
whereby the addition of starch increased the Young's modulus.
As the content of starch increased, the tensile strength decreased. 52 The
same trend was observed by Koller and Owen. 69 Reis et al. 52 reported that the
decrease in mechanical properties of the blend could be due to the low
interfacial interaction between the components of the blend, which could
lead to mechanical rupture. In another study by Thire et al, 68 it was a general
trend that a better dispersed starch on the matrix would give better mech-
anical properties. This explained that the mechanical properties of the
P(3HB-co-3HV) copolymer and starch blend were mainly contributed by the
compatibility between hydrophobic P(3HB-co-3HV) copolymer and hydro-
philic starch. As for the degradation, Ramsay et al. 48 showed an increased
rate of degradation as the starch content increased. Loss of starch might be
due to its solubility in water or enzymatic hydrolysis. Not only was it lost
more rapidly than P(3HB-co-3HV) copolymer, but also its removal resulted in
a larger exposed surface for the degradation of the P(3HB-co-3HV) co-
polymer. Since PHA is biodegradable, a decrease in starch content limited
the rate rather than the extent of degradation.
The T m of the blends depends on many factors. The T m of P(3HB-co-3HV)
copolymer in the blend was lower than that of the pure copolymer. 52
Matzinos et al. 70 reported that a small decrease in the T m indicated that a
phase separation had occurred. The melting enthalpy of the blend decreased
with the increase of starch. Godbole et al. 67 reported an increase in the T g
value with increasing starch content. The same trend was observed by Seves
et al. 51 using dynamic mechanical thermal behaviour (DMTA). Reis et al. 52
also reported a decrease in crystallisation enthalpy and T c as the starch
content increased. This indicated that blends had a lower degree of crys-
tallinity compared to the pure P(3HB-co-3HV) copolymer. Any solid polymer
can have amorphous, crystalline or both properties. In order to determine
this, XRD was used by Reis et al. 52 There were no crystallinity value from the
broad hump for the P(3HB-co-3HV)/starch blends. It was also observed that
the crystallinity decreased as the starch content in the blend increased.
Zhang et al. 53 studied the thermal and mechanical properties of P(3HB-co-
7%4HB)/corn starch blends. Besides that, the water absorption capability of
the resulting blend films was also examined. DSC results showed that an
increment of the corn starch content in the blend films had led to a decrease
of crystallinity and also the T m in comparison to pure P(3HB-co-7%4HB). The
range of T m was enlarged with the presence of double peaks. In terms of
mechanical strength, an increase in the corn starch content led to a gradual
decrease of the tensile and bending strength of the P(3HB-co-7%4HB)/corn

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