Page 270 - Biodegradable Polyesters

248 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

interpenetrated networks, and the crystallization-induced toughness of PLA-
based materials with special emphasis on the literature highlighting key factors
affecting their final toughness as well as involved toughening mechanisms within
the as-produced PLA-based materials, giving some key examples in this field
without being exhaustive.
10.2.2
Rubber-Toughened Polylactide

Intensive studies have been carried out on toughened PLA-based melt blends with
biodegradable/renewable or nonbiodegradable polymers as impact modifiers.
However, these may result in poor final performances when the morphologies
and the interfaces of the blend are not well controlled. In this respect, many
researchers have investigated different strategies to impart the desired change
in toughness of PLA, including the addition of aliphatic polyesters (e.g., Poly(ϵ-
caprolactone) (PCL)), polyhydroxyalkanoates (e.g., poly(3-hydroxybutyrate)
(PHB)), aromatic copolyesters (e.g., poly(butylene adipate-co-terephthalate)
(PBAT)), elastomers and rubbers (e.g., poly(1,4-cis-isoprene) (PI)), oil derivatives
(e.g., soybean oil), and hydrocarbon derivatives (e.g., PE). As of now, three
main reviews report the most representative polymeric additives used in the
literature as impact modifiers for PLA, which are entitled “Toughening poly-
lactide” by Anderson et al. [45], “Research progress in toughening modification
of poly(lactic acid)” by Liu and Zhang [41], and “Recent advances in high
performance poly(lactide): From green plasticization to super-tough materials
via (reactive) compounding,” our newest contribution [47]. However, except for
our recent review, only few studies have investigated the origin of the impact
modification within rubber-toughened PLA [47]. Therefore, this section will
be strictly limited to the literature with the specific purpose of highlighting
toughness-dependent factors, underlying toughening mechanisms and origins of
toughness improvement in the field of rubber-toughened PLA.
Over the past years, PCL-based additives were revealed to be ideal candidates
for toughening PLA. Nowadays, the related literature represents the most
detailed examples to understand the origin of PLA rubber-toughening as well
as toughness-dependent parameters and mechanisms involved. For instance,
the existing literature tells us that a simple melt blending between PLA and
PCL usually leads to poor improvement in toughness due to the lack of com-
patibility between the blend components and related poor interfacial adhesion
(Figure 10.11) [70–76]. Broz et al. found that PLA and PCL are not miscible
except for some adhesion at the PLA/PCL interface when the major phase is
PCL. As a result, no significant improvement in elongation at break is recorded
below 60 wt% of PCL [70]. Therefore, the compatibility at the rubber–matrix
interface should be properly controlled to address impact modification. In this
realm, P[CL-co-LA] [77–80], PCL-b-PLA [78, 81], or PLA-b-PCL-b-PLA [81–83]
copolymers as well as other PLA-b-PEO (poly(ethylene oxide)) [83], poly(ethylene
glycol) (PEG)-b-poly(propylene glycol) (PPG) [84], or PLA-b-PEG [85] are among