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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
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                    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
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