Blends of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 75

4.3 Characterisation of Blends of PHAs

4.3.1 PHAs/PLA Blends
Zhang et al. 18 studied the miscibility, crystallisation, mechanical properties
and hydrolytic degradation of P(3HB)/PLA blends. The results showed that
the blends prepared through the solvent-casting method were immiscible in
the amorphous state over the range of compositions studied. However,
blends prepared through the melt-compounding method showed some
evidence of greater miscibility, which was assumed to be due to the trans-
esterification of P(3HB) and PLA at high temperatures that had resulted in
the in situ formation of a P(3HB)/PLA block. Addition of PLA was found to
have a greater effect on the crystallisation of P(3HB), especially when its
content was relatively high. The growth of P(3HB) spherulites decreased with
the addition of amorphous PLA and the blend of P(3HB)/ PLA: 20/80 wt% did
not crystallise at all. The mechanical properties of P(3HB)/PLA were
improved as indicated by a reduction in modulus and stress at break and an
enhancement of the elongation at break. Faster hydrolytic degradation could
be observed as the P(3HB)/PLA blend films were more hydrophilic than the
P(3HB) films and the PLA component was quickly degraded.
According to Koyama and Doi, 59 the miscibility of the P(3HB)/P[(S)-LA]
blend was strongly influenced by the molecular weight of the P[(S)-LA]
component. Two phases in the melt at 200 1C were observed for the P(3HB)/
P[(S)-LA] blend with P[(S)-LA] of molecular weight values above 20 000. The
P(3HB)/P[(S)-LA] blend with the P[(S)-LA] of molecular weight values below
18 000 exhibited greater miscibility. However, the T m data did not correlate
with the miscibility of the blend studied. Ohkoshi et al. 19 reported that the
X-ray crystallinities of the PLA components in the melt-crystallised P(3HB)/
PLA blend films increased with the addition of a small amount of ataP(3HB).
This was a contradictory finding as in most cases, the crystal growth rate of
crystallisable component was depressed by mixing amorphous components.
It could be suggested that the addition of the completely amorphous
ataP(3HB) component had accelerated the growth rate of PLA spherulites by
increasing the chain mobility of the crystallisable PLA component.
Vogel et al. 60 studied the phase separation in blends of P(3HB)/PLA as a
function of the blend composition by Fourier transform infrared (FT-IR)
imaging spectroscopy (Figure 4.2), which illustrated that the P(3HB)/PLA: 50/
50 wt% blend was phase separated while the P(3HB/PLA): 30/70 wt% blend
was a compatible one-phase system. The elongation at break of P(3HB)/PLA:
100/0 wt% and P(3HB)/PLA: 0/100 wt% was 2% and 150%, respectively. The
blend of P(3HB)/PLA: 20/80 wt% exhibited an elongation at break of 125%
because it was dominated by a PLA with much better mechanical deform-
ation properties.
It was reported by Nanda and co-workers 22 that the addition of PLA to
P(3HB-co-3HV) copolymer caused the tensile strength and modulus of
the blend to gradually increase. An increase in tensile strength and

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