Page 168 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 145

different animal characteristic (Pisauridae, Araneidae, Theridiidae, Tetragnathidae).
These authors do not consider any statistical approach, and it is expected that an
approach taking into account the history of each tested fiber would give rather similar
results to those recorded for B. mori and N. madagascariensis fibers (Dinh et al.,
2008). Moreover, they note “there seems to be significant variability in silk properties
even for the same silk produced by the same spider on different days or even on the
same day under different environmental conditions, spinning conditions, variability
in the amino acid composition, etc., some of which might be affected by diet.” It is
well known by silk producers and users that the art of silk farming (nursing, diet,
selection, etc.) is determinant for the quality of the silk. Table 5.1 gives a representative
view for the main properties of different varieties of silk. Examples of relaxation (strain
rate 4 mm/min) are shown in Fig. 5.3(b). Actually, complementary relaxation takes
place over much longer times (days) and depends on the air moisture level.

5.2 Silk

5.2.1 Silk from the Bombyx mori and other moths
5.2.1.1 Composition

Silk is described using its amino acid sequences (Table 5.2) because much of the
studies of fibrous proteins/peptides have been performed on dissolved/cut polymers,
i.e., by destruction of the solid materials and the loss of its specificity from its structural
point of view. As sketched in Fig. 5.1(b), the stacking of the different bricks, namely
the various amino acid residues, form a polyamide chain, a backbone common to
keratin and synthetic polyamides. This backbone is grafted by functional lateral chains
called residuesdspecific for each amino aciddleading to a specific local geometry.
Obviously, the amino acid grafts play an important role in determining chemical
properties. Various sequences have been proposed (Hayashi et al., 1999; Nirmala

et al., 2001; Fedi c et al., 2003; Zurovec and Sehnal, 2002; Sehnal and Zurovec,
2004) and their modification will change the detail of the shape of the macromolecule
and its chemical properties/reactivity. Some authors assume that the amino acid graft
also determine the mechanical properties and consequently search to add specific
genes (see further).
As for many protein-/peptide-based polymers, the local conformations of the chains
are categorized as regular a-helix, irregular helicoidal fragments/turns, and untwisted
ribbons remain, that could be associated to form b-sheets (Fig. 5.4)(Harrington et al.,
2012). Furthermore, intermediate conformations between the above three categories
also exist and contribute to the disorder. The large size of some “rigid” or “long” amino
acid residues of the silk macromolecule (e.g., arginine and phenylalanine amino acids)
hinders regular configuration of the helix and hence can explain the formation of kinks
as sketched in Fig. 5.4(a and a’). In the case of silk from the B. mori the presence of
tyrosine impedes crystallization and prevents the formation of a regular spacing
between adjacent macromolecules, hence promoting the orientation disorder at the