Page 285 - Biodegradable Polyesters

10.3 Crystallization-Induced Toughness and Morphological Control 263

OH
OH Crosslinked HBP
OH particles in PLA matrix
+ O O
O
HO OH
Polyanhydride (PA)
OH
Hyperbranched polymer
(HBP)
Reaction site of Neat PLA
HBP with PA O PLA
OH
O
O
HO
O
HO OH Reactive blending
O
OH O
O
O OH
O O O HBP based
OH crosslinked
O OH A
OH particles
(a) (b) (c)
20 μm 20 μm 20 μm
Figure 10.20 Schematic illustrations of in fracture surfaces of neat PLA (a), PLA/HBP
®
situ crosslinking of HBP (Boltorn H2004 ) 92/8 wt/wt% (b) and PLA/HBP/PA-18 (LV)
in the PLA melt with the help of a polyan- 92/5.4/2.6 wt/wt/wt% (c). Reproduced with
hydride (PA-18 (LV)) and related morpholo- permission from Ref. [181] © 2007, American
gies (up) as well as morphologies of tensile chemical society.

segments, has been synthesized by Peponi et al. [186]. Surprisingly, an excellent
shape memory behavior was observed in terms of both strain fixity and strain
recovery due to their synergetic combination. Its characteristic features, in
terms of physicochemical properties as well as mechanical behavior, have been
investigated and correlated with the polymer structure.

10.3
Crystallization-Induced Toughness and Morphological Control

In general, progression of crystallization in a semicrystalline polymer results in
embrittlement of the polymer and hence decrease of fracture toughness [187].
Crystallites act likely as stress concentrators and reduce energy-dissipative mech-
anisms (i.e., multiple crazing and shear yielding) of polymer matrices. However,
energy dissipation and toughness can be found in the literature through the crys-
tallization of PLA (Figure 10.21). As a result, the crystallinity-to-toughness rela-
tionship becomes the subject of increasing number of investigations controlling
the PLA crystallinity through thermal treatments (i.e., annealing or quenching) or
the addition of nucleating agents.
Interestingly, annealing is perhaps the most effective treatment to get highly
crystalline PLA. Accordingly, the effect of annealing treatment on the toughening