Page 266 - Biodegradable Polyesters

244 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

failure, passing from brittle fracture to ductile fracture [54]. The importance of
this transition zone depends mainly on both strain rate and temperature gradient.
For instance, the same material can exhibit higher brittleness at low temperature
and/or high testing rates. Impact resistance of polymers may be evaluated in
terms of the energy absorbed by the specimen during the impact process via
various test methods including [55] the following:

• Tensile testing: The area under the stress–strain curve is often used to quan-
tify toughness (ASTM D638). However, even with different stress–strain curves
that can be obtained, the mechanical responses to the impact loading may dis-
sipate the same impact energy.
• Impact testing: The energy required to break the sample, which is usually struck
by a hammer, is measured. The related impact strength is expressed in terms
of the difference between the potential energy of the hammer before and after
−2
the impact. It is generally given in units of either J m −1 or kJ m , expressing the
energy required to break the sample to its width or cross-sectional area, respec-
tively. For impact testing, three different tests are typically performed such as
Izod (ASTM D256, samples are clamped as a cantilever vertically at the lower
end), Charpy (ASTM D6110, unclamped samples are supported horizontally at
both ends), and Dynstat (DIN 53453, samples are unclamped at the lower end),
which can be eventually notched.
• Falling weight testing: A projectile thrown into the specimen or dropped on it
under the force of gravity is used to measure the impact energy. Gardner impact
tester is a well-known equipment dedicated for this type of tests. It offers the
advantage over impact testing method that the fracture shape can be also ana-
lyzed.
• Video-controlled mechanical testing:Onthe basisofvideoanalysisfromaseries
of seven markers printed on the specimen, this technique gives access to the
stress–strain behavior at constant true strain rate with simultaneous determi-
nation of the volume strain. Following the local evolution of volume strain dur-
ing the test, the toughening mechanisms related to polymeric materials can be
deduced. In particular, a volume increase is related to the occurrence of either
crazing or cavitation, while isovolumetric deformation mechanism refers to the
activation of shear-banding mechanisms.

Many strategies have been developed in the literature to improve the tough-
ness of several thermoplastic materials including the incorporation of a variety
of soft polymers or rubbers, addition of rigid fillers, and modification of crys-
talline morphology [41, 45]. Because of their impact-absorbing ability, rubbery
microdomains of convenient size distribution act as stress concentrators at many
sites throughout the material. Therefore, they impart great ductility and impact
strength to the material, resulting from dissipative micromechanisms initiated by
the rubbery microdomains. In addition, a change of crystalline morphology within
PLA materials can modify the energy dissipation on impact. All of these phe-
nomena are dependent on the plastic deformation and toughening mechanisms,