another critical strain, the drawn molecules will re-crystallize at low temperatures,
making thinner but highly oriented small crystalline blocks to assemble into nano-
fibrils. Therefore, the overall crystallinity can be maintained during stretching.
With the breaking of lamellar crystals, the dead entanglements locked between
lamellar crystals upon initial crystallization will be released, and the entanglement
network of amorphous polymers locating between fibrils can resist the external
stress at the later stage of deformation, inducing the strain-hardening phenomenon
till to the final breaking of the entanglement network. Therefore, the fracture
strength is highly dependent upon the molecular weight of polymers.
The fracture strength after strain-hardening determines whether the necking can
develop over the whole sample (the success ofcold-drawing). It must be higher
than the shear yielding strength, as illustrated in Fig.6.20. Thus the molecular
weight should be large enough to make the disentanglement between polymer
chains more difficult. Such a performance is called thedrawability.
The shear yielding mostly happens in a homogeneous material. If the material is
inhomogeneous, such as containing impurity or structural defects, stress will be
concentrated on these defects. The local concentrating of stress leads to cracks that
eventually cause the sample to fracture. The fast inhomogeneous breaking often
occurs in thebrittle fracture. If the local stress-concentrating area exhibits shear
yielding and necking, the microfibrils will form with their spacing comparable to
the wavelengths of visible lights, as illustrated in Fig.6.21a, raising thecrazing
phenomenon. The crazing will absorb a part of impact energy, and the huge amount
of crazing will absorb a significant amount of energy prior to fracture, turning the
materials into ductile failure.
The process for adhesives peeling from a solid substrate is quite similar to the
crazing. Both develop from a foam structure rich with air bubbles to an unstable
fingering structure upon splitting fibrils, before eventually breaking. This process
will absorb a great amount of energy, as illustrated in Fig.6.21b. Such tackiness is
the main reason for polymers to be the good adhesive materials.
Fig. 6.20(a) When the breaking strength is higher than the shear yielding strength, the necking of
semi-crystalline polymers can be developed over the whole sample; (b) otherwise, the sample will
immediately be broken at the narrow necking area
6.4 Conventional Mechanical Analysis 121