Page 188 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 165

of the composite. An optimal composite requires a fiber/matrix bond strength neither
too strong nor too weak. In this case, it will display a pseudoductile behavior coming
from a progressive multicracking of the matrix, and the load transfer from broken
zones to more rigid areas. The rigidity of the material decreases gradually during
the loading. Such behavior allows the composite to have a higher work of fracture
than when the fiberematrix interaction is too high or too low. Ductile failure is pref-
erable to a brittle failure for a significant dissipative effect.

5.3.4 Mechanical properties of spider silk

The diversity of spiders and of the polymers they produce, depending on the gland
employed, leads to a large range of mechanical properties (Table 5.1). A typical
stressestrain curve of major ampullate gland silk fiber of N. madagascariensis can
be seen in Fig. 5.3(b) (Colomban et al., 2008a).
The mechanical properties of fibers are directly linked to their microstructures
(Colomban, 2002, 2013; Colomban et al., 2006). Hence, the smaller the crystallites
and the better they are orientated the stronger will be the fiber. Ultimate mechanical
properties, such as stiffness, can then be improved artificially. Indeed, even if the
orientation of natural fibers is already almost maximized, the reduction of the crystal-
lite size can still be improved by forced silking at higher reeled rates than that which
occurs naturally. Optimization of the mechanical properties of synthetic polymer fibers
is achieved by a combination of thermal treatments and tensile stress. Crystallite size
reduces with elongation showing that crystals are involved in the mechanical behavior
from the beginning of the tensile test. Their role has been shown also through simula-
tions that can explain the elastic phase of the stressestrain curves (Vehoff et al., 2007;
Termonia, 1994).
Mechanical properties can also be modified by the environment. For example,
immersion in water or methanol, of spider silk induces supercontraction, which is a
specific state when fibers shrink by approximately 40%e50%. The initial stiffness
drops by three orders of magnitude, and the material becomes rubber-like in its
behavior (Gosline et al., 1999). The nature of the solvent has great importance in
determining behavior, indeed, for example, ethanol does not induce any supercontrac-
tion. Silk is also more or less sensitive to supercontraction depending on its composi-
tion, hence minor and major ampullate silks do not react in the same way. It has been
shown by NMR that plasticizing by water occurs with the glycine, tyrosine, and
leucine but not with the alanine, explaining the different reaction depending on amino
acid composition (Jelinski et al., 1999; Vollrath and Porter, 2006).

5.3.5 Structures of silks

5.3.5.1 Crystallinity
As for many complex polymers, the structure of silk remains a source of debate. At a
first approach, silk structures of silkworms and spiders are rather similar and are
commonly described as having a hierarchical organization that reaches from the