Biomedical Engineering Reference
In-Depth Information
additional polymerization with hydroxyl and carboxyl groups lying in the ends of PLA or
PCL molecules. This polymerization results in the improvement of miscibility of PLA and
PCL as shown in Fig.19(c). On the other hand,
M
w
of PLA/PCL decreases slightly due to
annealing, indicating progression of thermal degradation in this blend. On the contrary,
M
w
of PLA/PCL/LTI slightly increases, suggesting that additional polymerization take place
during annealing process.
(a)Elastic modulus (b)Strength
Fig. 20. Effects of annealing on bending mechanical properties.
Blend
X
c,PLA
(%)
M
w
(g/mol)
Quenched PLA/PCL
11.4
1.03
x
10
5
Annealed PLA/PCL
45.6
9.08
x
10
4
Quenched PLA/PCL/LTI
4.8
1.13
x
10
5
Annealed PLA/PCL/LTI
36.6
1.52
x
10
5
Table 1. Effects of annealing on the crystallinity and molecular weight.
Effects of annealing on the critical J-integral at crack initiation,
J
in
, are shown in Fig.21. It is
clearly seen that
J
in
of PLA/PCL/LTI effectively increases due to annealing; on the contrary,
PLA/PCL exhibites decrease of
J
in
.
FE-SEM micrographs of the fracture surfaces of the mode I fracture specimens are shown in
Fig.22. By comparing Figs.22(a) and (b), it is clearly seen that ductile deformation of
spherical PCL phase is suppressed by annealing. Cavities are also observed on the surface of
PLA/PCL, as a result of removal of the spherical PCL phases. FE-SEM micrographs at
higher magnification show that elongated structures of the spherical PCL phases are
observed in the quenched PLA/PCL, while ruptured PLA fibrils and undeformed PCL
spherulites are observed in the annealed PLA/PCL. It is also interesting to see in Fig.22(a)
that some PCL fibrils are penetrated into the PLA phase and seem to be entangled with PLA
fibrils. It is thought that the PLA phase creates a firm structure due to crystallization by
annealing and therefore, entangled PCL fibrils with PLA fibrils are firmly trapped in the
PLA phase. The PCL spherulites are thus surrounded by such firm crystallized PLA phase
with entanglement of PLA and PCL fibrils, resulting in the rupture of the PCL fibrils and the
