Page 286 - Biodegradable Polyesters

264 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

1 cm (a) (b)

Figure 10.21 Effect of annealing on fracture crackle on PLA sheets after drop-impact test-
ing without annealing (a) and with annealing (b). Reproduced with permission from Ref.
[188] © 2008, John Wiley and sons.
of PLA-based materials was investigated by some researchers. Most of the
studies demonstrated that the increase of PLA crystallinity usually leads to an
improvement of its overall mechanical and heat-resistant behaviors. Others
focused on the crystallization of the PLA phase by annealing to strengthen the
microstructure of these PLA-based blends, resulting in the increase of their
properties. For instance, the effect of annealing treatment (which improved the
PLA crystallinity) on the fracture behavior of PLA was analyzed by Nascimento
et al. [189]. From this work, different trends of fracture behavior were observed
depending on the testing speed and annealing conditions of PLA, showing a great
improvement in the fracture toughness upon the crystalline fraction. Further
investigation by Park et al. highlighted a unique dependence of the fracture
toughness on the crystallinity. They concluded that any change of fracture
toughness is closely related to the changes of microstructure and crack-growth
behavior [190]. In an attempt to understand the fundamental effect of annealing
and loading rate on the fracture behavior of PLA, Park et al. also annealed
PLA under different conditions to control the PLA crystallinity and reach
different microstructures with varying spherulite size and density. As a result,
the amorphous PLA gave higher fracture toughness due to extensive multiple
craze formation under quasi-static loading. On the contrary, the crystallized
PLA gave higher fracture toughness under impact loading due to formation of
ductile fibrils created by deformation of spherulites at the impact rate (Figure
10.22). Analogous trends were observed by Renouf-Glauser et al. [191], showing
that crystalline materials deform through crystal-mediated deformation with
contributions from both cavitation and fibrillated shear. Accordingly, an effective
energy dissipation regarding two possible structures is reached within crystalline
zones, which are the inter-spherulitic crack growth and the crack growth through
spherulites. However, semicrystalline polymers usually exhibit a typical structure
consisting of closely packed crystalline lamellae separated by amorphous regions.
When the blend is subjected to an external impact force, the fracture energy
is dissipated through shear yielding in the crystalline zones and crazing in the
amorphous zones. According to these deformation mechanisms within highly