Page 271 - Biodegradable Polyesters

10.2 Polylactide Strengthening and Strategies 249

2 μm

Figure 10.11 Microstructures of PLA/PCL (85/15 wt/wt%) blend. Reproduced with permis-
sion from Ref. [94] © 2007, Elsevier.

the compatibilizers used to promote acceptable adhesion within immiscible
PLA/PCL blends. The use of reactive additives during compounding has also
been investigated as an effective way to improve the compatibility between PLA
and PCL. Wang et al. [86] used triphenyl phosphite (TPP) as a coupling agent
to promote transesterification and in situ formation of block-like copolymers
at the interface, which give rise to significant mechanical improvements for
the reactively compatibilized blends. Dicumyl peroxide (DCP), a free-radical
initiator, was used by Semba et al. to promote cross-linking and lead to great
tensile and impact improvements [87, 88]. On the basis of the high reactivity of
isocyanate, Harada et al. [89] improved the compatibility of PLA/PCL blends
using lysine triisocyanate (LTI) to induce tough materials. Other PCL-based addi-
tives were also investigated as PLA impact modifiers, including P[ε-caprolactone
(CL)-co-LA] [72], P[CL-co-1,3-trimethylene carbonate (TMC)] [90], or P[CL-
co-δ-valerolactone (VL)] [91] copolymers, affording an elegant way to modulate
the toughness upon the characteristics of the dispersed phase (softness, viscosity,
miscibility, etc.). These reported studies highlight the intrinsic characteristics of
the dispersed phase like other prevalent toughness-dependent parameters. For
instance, our own contribution showed the effect of comonomer content and
molecular weight of an amorphous P[CL-co-VL] random aliphatic copolyester
on the overall material toughness [91]. According to this study, the impact
modification depends on the blend morphology. Directly related to the phase
morphology, the control of the mean size and size distribution of the dispersed
phase throughout the PLA matrix appears significant to achieve high toughness.
On the basis of the toughening mechanisms, it is usually recognized that spherical
microdomains act as stress reservoirs and initiate crazing upon the microdomain
size. Accordingly, an optimum range of particle sizes and size distributions is
required to nucleate crazing mechanism, enhance fracture energy absorption,
and lead to the best toughness improvement [92, 93]. This suggested blend mor-
phology and further rubber particle size and size distribution as likely the main
key parameters influencing the toughness of the resulting PLA-based materials.