Page 267 - Biodegradable Polyesters

10.2 Polylactide Strengthening and Strategies 245

namely, crazing, shear yielding, cavitation, or debonding as mostly reported in the
literature [53, 54, 56–68]:
• The crazing mechanism can be initiated in a material when the stress or hydro-
static tension is locally concentrated at a defect, which can be a notch, voids,
inhomogeneities, or rubber particles. Therefore, interpenetrating micro-voids
and microfibrils are formed, giving rise to macroscopic localized zones of
ultrafine cracks, namely, crazes. They are visible in the material perpendicular
to the direction of the maximum principal stress. The crazing mechanism is
dilatational in nature and consumes the predominant part of fracture energy
in many thermoplastics by micro-void formation and growth of craze fibrils.
However, if the local stress exceeds a critical value, the microfibrils elongate
until breaking and cause the micro-void growth and coalescence, turning into
micro-cracks. Crazing is therefore viewed as a damaging mechanism in the case
of brittle polymers when the craze evolution into a macroscopic micro-crack
cannot be refrained. However, when blended with the brittle matrix, the
rubbery impact modifier particles can have two important effects as a response
to loading application. They first concentrate locally the stress where craze
initiation takes place. The crazes then grow perpendicular to the maximum
applied stress-direction. In a second step, the surrounding rubber particles
play the role of “craze terminators,” preventing the generation of micro-cracks.
The result is that a large number of small crazes are formed, in contrast with
the small number of large crazes (micro-cracks) within the same polymer
matrix in the absence of rubbery microdomains. This multiple crazing occurs
throughout a comparatively large volume of the rubbery modified material.
It is responsible for the high energy absorption during fracture tests and the
extensive stress whitening that accompanies the deformation and failure. Some
matrices tend to craze because of low entanglement density while high molec-
ular weight is required to stabilize crazes. For example, in brittle polymers such
as high-impact polystyrene (HIPS), poly[styrene-co-acrylonitrile] (SAN), and
rubber-toughened poly(methyl methacrylate) (PMMA), the rubber particles
promote crazing in the matrix (Figure 10.7).

1 mm

σ B
NOTCH x
Craze boundary stress, σ
CR

Creep of the drawn fibrils Drawing in of polymer
Y from the bulk–fibril
interface
CRAZE X σ B

Z
Figure 10.7 Craze morphology and schematic representation of crazing growth. Repro-
duced with permission from Ref. [61, 62] © (2002,1993), John Wiley and sons.