Biomedical Engineering Reference
In-Depth Information
keep unchanged up to 1phr of LTI content, and then rapidly increase up to 2phr, suggesting
that the chemical reaction between the hydroxyl groups of PLA and PCL and the isocyanate
groups of LTI was promoted by addition of LTI more than 1 phr. This microstructural
change in molecular level due to LTI addition strongly support the macroscopic
improvement of the fracture energy.
Dependence of LTI content on the crystallinity of PLA,
x
c,PLA
, is shownIn Fig.15.
．
x
c,PLA
of
PLA/PCL and PLA/PCL/LTI with 0.5phr of LTI are higher than that of PLA.
．
It is
considered that the crystallization of PLA in the blends was progressed actively more than
in neat PLA. With increase of LTI content,
x
c,PLA
decreases rapidly at 1 phr and this value is
slightly lower than that of neat PLA. These results support that the phase separation
between PLA and PCL is improved dramatically at 1 phr. It is presumed that the chemical
reation between LTI and PLA/PCL during melt-mixing process results in the improvement
of miscibility, and therefore the mobility of PLA and PCL molecules during solidification in
cooling process is reduced, resulting in the reduction of crystallization.
15
10
PLA
5
0
0
0.5
1
1.5
2
LTI content [phr]
Fig. 15. Dependence of LTI content on crystallinity of PLA in PLA/PCL and PLA/PCL/LTI.
Dependence of LTI content on
J
in
is shown in Fig. 16. It is seen that
J
in
of PLA/PCL is a little
larger than that of PLA, indicating the effectiveness of PCL blend on
J
in
is very low.
J
in
of
PLA/PCL is effectively improved by LTI addition, and
J
in
increases with increase of LTI
content up to 1.5 phr. There is no difference of
J
in
between 2 phr and 1 phr of LTI addition,
suggesting that the improvement of
J
in
is saturated with about 1.5 phr of LTI.
Poralized micrographs of crack growth behaviors in PLA/PCL/LTI are shown in Fig.17. In
PLA/PCL/LTI with 0.5 phr of LTI, craze-like features are still seen in the crack-tip region as
also seen in neat PLA (Fig.7(a)) and PLA/PCL blend (Fig.10). The number of the craze-lines
is obviously decreased due to LTI addition. With higher content of LTI, such craze-like
feature is no longer generated, and instead, the crack-tip region is plastically deformed, very
similar to the crack-tip deformation in ductile plastics and metal. It is known that this kind
of plastic deformation dissipates more energy than the craze-like damage, resulting in the
greater fracture energy. It is therefore thought that LTI addition to PLA/PCL dramatically
changes the crack-tip deformation mechanism; as a result,
J
in
is greatly improved.
