Page 337 - Biodegradable Polyesters

12.6 Properties of PLA/PGA Polymer–Polymer Nanofibrillar Composites 315

20

15
Stress (MPa)

10

PLLA/PGA MFC (70/30 by wt)
PLLA/PGA MFC (80/20 by wt)
PLLA
5
0 200 400 600 800 1000 1200
Time (s)
Figure 12.11 Stress relaxation curves of PLLA, MFC based on PLLA/PGA (80/20 by wt%),
∘
and MFC based on PLLA/PGA (70/30 by wt%) at 37 C [47].

stresses of PLLA specimens decreased only to 13–14 MPa. This indicates that
the creep resistances of PLLA/PGA MFCs are lower than that of neat PLLA. The
∘
results are somewhat surprising initially but considering that 37 Cisclose to
∘
the T of PGA (40 C), the reason may be that the amorphous portion of PGA is
g
becoming mobile. All specimens were tested shortly after being manufactured,
so physical aging which may have occurred between manufacturing and test-
ing is not thought to have contributed to the greater creep resistance of neat
PLLA.
In order to verify the assertion that the onset of glass transition of the
amorphous phase of PGA is the cause of the creep propensity of PLLA/PGA
MFCs, their viscoelastic behaviors were compared to that of PLLA via dynamic-
mechanical thermal analysis (DMTA). The storage modulus of PLLA begins to
∘
drop off at 50–55 C while those of the MFCs start to show this decline at much
∘
lower temperatures near 35 C, especially clear in the case of PLLA/PGA MFC
(70/30 by wt%), Figure 12.12a. The loss moduli, which indicate energy dissipated
by viscoelastic effects, reflect these trends (Figure 12.12b). The more rapid
∘
increases in loss moduli of the MFCs implies that from ∼35 C the amorphous
phase of PGA becomes mobile, resulting in more energy dissipation and less
energy storage (as indicated by the declines in storage moduli). These results
are an important consideration for load-bearing implants where creep failure
is a concern. Nevertheless, other applications may benefit from the presence of
PGA fibrils – note that the storage modulus of PLLA is practically negligible
∘
above 65 C, while that of PLLA/PGA MFC (70/30 by wt%) is ∼700 MPa because
of the crystalline phase of PGA. Applications such as biodegradable cups for
hot beverages could take advantage of this. Additional drawing to increase the