Page 268 - Biodegradable Polyesters

246 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

• The shear-yielding mechanism is a highly localized plastic deformation charac-
terized by appearance of oriented shear bands under uniaxial tension around
∘
45 to the direction of the applied stress. Shear yielding occurs approximately
at constant volume, while initiation of shear bands is affected by the hydro-
static tension (mean stress). In ductile polymers such as polycarbonate (PC),
poly(vinyl chloride) (PVC), and polyamides (PAs), shear yielding is usually
the major energy-absorbing mechanism. There are also a few polymers such
as acrylonitrile-butadiene-styrene (ABS) and rubber-toughened PMMA that
exhibit both shear yielding and crazing mechanisms. When the craze initiation
stress of the matrix is lower than the yield stress, a polymer will tend to craze.
However, if the yield stress is lower than the craze initiation stress, the matrix
will fail by shear yielding. Mixed crazing and shear yielding tends to occur
when the craze initiation stress and the yield stress are comparable or when
interactions occur between crazes and shear bands (Figure 10.8).

NOTCH
SHEAR BANDS

TENSION

Figure 10.8 Shear bands morphology. Reproduced with permission from Ref. [62] © 1993,
John Wiley and sons.

• The cavitation mechanism is avoidexpansion whichcan occurinthe matrix
(generally coupled with crazing) or be initiated inside the rubber particles. Cav-
itation is generally characterized by stress-whitening transitions. The essen-
tial conditions for void growth is an energy balance between the strain energy
relieved by cavitation and the surface energy associated with the generation of a
new surface. Cavitation is a precursor to other toughening mechanisms, thereby
relieving the hydrostatic strain energy and initiating shear yielding of the matrix.
It is assumed that internal rubber cavitation is an instantaneous process, which
cannot occur for very small particles (less than 200 nm). In other words, rubber-
cavitation mechanism is favored by increasing the rubber particle size within