Page 186 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 163

or less sensitive to the different conformations. Nevertheless, by comparison with
common materials, silk fibers are highly amorphous and the irregular helix structure
seems to be dominant.
1
The huge intensity, c. 3450 cm , of the band measured on fresh glands just after
dissection (Fig. 5.13(c1))(Dinh et al., 2008; Percot et al., 2014) indicates that the
liquid in the gland is mainly water and that most of the water is eliminated before
the formation of the silk fiber at the spinneret. Cooling to a low temperature (10K,
Fig. 5.13(c3)) induces a strong narrowing of the water stretching component
1
(w3425 cm ) related to the disappearance of the dynamic disorder of mobile water
molecules (Colomban and Dinh, 2012). On the other hand, no changes are observed
1
for the NeH mode (3268 cm ) due to the very weak hydrogen bonding.
Amide I mode, in other words the stretching mode of the C]O bond coupled with
adjacent CeNand CeC bonds (see Fig. 5.1(b)) is used to discriminate the different
1
chain conformations. The assignment of the main peak (1659e1668 cm )ismuch
debated and may correspond to intermediate structures with some helical character-
istics (Monti et al., 2001; Rousseau et al., 2004; Rousseau et al., 2006; Colomban
et al., 2008a). The decreasing intensity of the “a-helix” and “b-sheet” components
as well as the broadening of the bands for films (Fig. 5.13(c)) indicates that the crys-
tallinity of the fiber decreases with time and that the adsorbed water also decreases
the crystallinity.

5.3.3 Mechanical properties of regenerated silk, films, and
composites

In the literature, the majority of studies on fibroin are aimed at biomedical applications
(such as tissue or bone repair), and mechanical tests are performed when hydrated at

37 C(Ayutsede et al., 2005; Lawrence et al., 2010). Dry fibroin films have been also
been studied (Colomban et al., 2008b), the Young’s modulus varies between 1 and
6.5 GPa, and the ultimate tensile strength between 7 and 100 MPa (to be compared
with 200e800 MPa for the pristine fibers) with a corresponding ultimate strain of
0.6%e1% on average (to be compared to 15%e20% for the pristine fibers). The
mechanical properties of the regenerated silk obtained from the solution are thus
weaker than the initial fibers. For the “best” films (type D, films exhibiting a large
Amide I Raman band, characteristic of heavy orientational disorder of the fibroin
macromolecules), the value of the stress reached is about 10% of that of natural fibers
only. The Young’s modulus is halved, and the ultimate strain is divided by 20
(Wojciesak, 2014; Wojcieszak et al., 2017). Moreover, the dispersion is even higher
than that for the fibers (lower Weibull moduli). The stressestrain curves of such regen-
erated silk exhibit nonlinear behavior, but the ultimate strength and strain to failure is
far from those of the natural fibers (Jelinski et al., 1999; Dinh et al., 2008; Dinh, 2010).
The reinforcement of a film through the addition of fibers (degummed or as spun)
was used to increase its mechanical strength. However, it has been shown that in
B. mori raw silk, the sericin coating played a significant role in the mechanical
behavior of fibers (Jauzein and Bunsell, 2012). This configuration confers to silk