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262 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques
provide a good option to significantly increase the toughness of PLA-based
materials. Accordingly, numerous studies have been conducted to investigate
the effects of blending HBP as reactive modifiers within PLA, highlighting
the generation of new nanostructures inside a polymer matrix ranging from
core–shell to highly networked morphologies. For instance, Zhang et al. have
studied the effects of a hydroxyl group-ended dendritic hyperbranched poly-
mer (DHP) from Perstorp on the mechanical and crystallization behaviors
of PLA, highlighting significant improved PLA elongation and crystallization
rate. These are attributed to strong hydrogen bonds between DHP and PLA
[178]. Further evidence for interfacial adhesion via hydrogen bonding within
PLA/hyperbranched poly(ester amide) blends was then reported by Lin et al.
[179]. According to these interactions, they found that the material changed from
brittle to ductile failure with the addition of such HBPs, which acted as stress
concentrators undergoing cavitation via debonding. Ternary PLA/hyperbranched
poly(ester amide)/silica nanocomposites were even investigated by Wen et al.
[166], displaying dramatically improved mechanical properties including excel-
lent toughness and fairly high stiffness according to the compatibilization effect
of HBP and enhanced nanoparticle mobility in the nanocomposites. Pilla et al.
[180] have then studied the effects of addition of HBPs (Boltorn H20 ® and
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Boltorn H2004 from Perstorp) and nanoclay on the PLA material properties.
Interestingly, a nanostructure-controlled PLA created by in situ cross-linking
®
of Boltorn H2004 with a linear polyanhydride, trade name PA-18 (LV) having
a 1 : 1 mol ratio of 1-octadecene and maleic anhydride, in the PLA matrix was
reported by Bhardwaj and Mohanty [181] through reactive extrusion blending.
In this approach, the generation of new HBP-based nanostructures in the PLA
matrix improved the toughness and elongation at break, which were related to
stress whitening and multiple crazing initiated in the presence of cross-linked
HBP particles (Figure 10.20).
Recently, the combination of hydroxyl-terminated hyperbranched poly(ester
amide) and isocyanate-terminated prepolymer of butadiene (ITPB) has been
investigated by Nyambo et al. [182] with the aim to make tough PLA blends upon
in situ cross-linking. Resulting from physical and chemical interactions, a change
in fracture behavior from brittle to ductile nature in the PLA-based blends arose
after chemical modification. Similarly, Yuan and Ruckenstein [183] toughened
PLA by forming a PU thermoset in the PLA matrix. This study indicates that
the toughening effect might be influenced by the extent of cross-linking of the
PU, while optimum toughness was thought to result from a balance between
the compatibility of the semi-interpenetrating PU–PLA network with PLA and
the stiffness of this network. Another urethane-type structure was prepared by
Chen et al. [184], reacting PLA with a small amount of methacryloyloxyethyl
isocyanate (MOI) to obtain a ductile PLA with markedly improved mechanical
properties. Cross-linking structures were even introduced by Quynh et al. [185]
in PLA stereocomplexes, improving thermal and mechanical properties of the
resulting materials. Further, a bio-based polyurethane structure based on a
PCL diol, that constitutes the soft segments, and PLA, that constitutes the hard