Page 269 - Biodegradable Polyesters

10.2 Polylactide Strengthening and Strategies 247

Build-up of hydrostatic tension Relief of hydrostatic tension

U strain U strain + U surface <0
Initial situation Situation at a relative Internal rubber cavitation
particle size : d 0 volume strain : Δ Size of the cavity : d i
Figure 10.9 Schematic representation of rubber cavitation. Reproduced with permission
from Ref. [64] © 1994, Elsevier.
toughened materials or by decreasing the cross-linking density of the matrix
(which can suppress cavitation) (Figure 10.9).
• The debonding mechanism is the energy dissipation due to interfacial failure.
The interface between the phases influences the final blend properties by effi-
cient stress transfer between the two phases. However, interfacial debonding
can be thought of as being a secondary toughening mechanism more impor-
tant than other rupture mechanisms such as shear yielding. A low interfacial
adhesion readily results from premature interfacial failure, followed by rapid
and catastrophic crack propagation. Otherwise, a strong adhesion is not favor-
able for debonding and it also delays the occurrence of matrix yielding, involving
the matrix–particle interface as an important factor that needs to be controlled
for optimum energy dissipation (Figure 10.10).

Matrix

Fiber
Debond

5 μm

Figure 10.10 Debonded morphology and schematic representation of debonding growth.
Reproduced with permission from Ref. [69] © 1997, Elsevier.
Toughening mechanisms and competition between both modes of frac-
ture are mainly governed by a variety of well-controlled factors, especially
from the polymeric matrix itself. In this respect, the next section will focus
on rubber-toughened PLA, its combination with nanoparticles or even