Page 272 - Biodegradable Polyesters

250 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

Other parameters explaining the origin of the impact modification through
toughening mechanisms in the existing literature can be obtained in the case
of PLA-based blends made of other (partially) immiscible impact modifiers.
Most of these studies showed that multiple crazing initiated from the dispersed
rubber phase is recognized to be one of the main dissipative mechanisms, which
increases the toughness of PLA-based materials [59, 62, 95]. In this context, Noda
et al. [96] suggested that the addition of fine droplets of Nodax TM (a polyhydrox-
yalkanoate copolymer)-toughened PLA by craze initiation. Additionally, Meng
et al. [97] investigated the addition of poly(butyl acrylate) to promote larger
plastic deformation within PLA upon the emergence of cavities and extensive
fibrils/crazes by increasing the rubber content. The same conclusions were made
by Todo et al. [94] regarding PLA/PCL blends, highlighting that the crazes are
likely derived from void formation. This results in the interfacial debonding
between PCL droplets and PLA matrix. The toughening efficiency of the ultrafine
acrylate rubber particles was also reflected through numerous crazes as the
mechanism for energy absorption provides a significant fourfold increase in
the impact strength of PLA [98]. PLA-based blends can also follow one or a
combination of the most known toughening mechanisms (see Section 10.2.1)
[99]. For instance, Kowalczyk and Piorkowska [100] reported that rubbery PI
microdomains within PLA-based materials initiated crazing at the early stages of
deformation, immediately followed by cavitation phenomena inside the rubbery
microdomains, the latter promoting further shear yielding for the PLA matrix.
Such a combination of crazing and shear yielding within PLA-based blends has
also been reported by Bartczak et al. [101] as well as Ma et al. [102], promot-
ing great plastic deformation and high toughness for the resulting materials
(Figure 10.12).
In contrast, Li and Shimizu [103] ascribed the toughening behavior of the
PLA-based blends to debonding at the rubber/matrix interface during deforma-
tion, which released the hydrostatic stress and facilitated the occurrence of shear
yielding. When the hydrostatic stress is released within a PLA/polyurethane
elastomer (PU) blend, debonding is easily induced at the interface between the
(PU) domains and PLA matrix. This results in voids around the rubber, which
allows shear yielding and improved the toughness of the materials, as shown by

(a) (b) (c)
Debonding

Cavitation
Debonding
Fibril

Figure 10.12 Morphologies of stretched PLA/poly[β-hydroxybutyrate-co-β-hydroxyvalerate]
blends with weight ratio of 90/10 wt/wt% (a), 80/20 wt/wt% (b), and 70/30 wt/wt% (c) taken
after necking. Reproduced with permission from Ref. [102] © 2013, Elsevier.