Page 219 - Handbook of Properties of Textile and Technical Fibres

194 Handbook of Properties of Textile and Technical Fibres

but less extensible due to the increasingly orientated molecules and liquid crystals of
the silk precursor proteins (Vollrath et al., 2001). Spider dragline silk spun under water
displays greater stiffness and higher resilience compared to silk spun “naturally” into
air (Liu et al., 2005b). Absorbed water leads to a process called super-contraction in an
unrestrained frame silk and dragline fiber and reversibly converts the material into a
rubber (Gosline et al., 1986; Work, 1977; Shao et al., 1999a; Shao and Vollrath,
1999). When immersed in water, the fiber swells, increasing the total volume by a
factor of more than two, and if unrestrained the swollen fiber will shrink to about
half its initial length, causing the fiber diameter to double from roughly 2e4 mm.
This swelling phenomenon dramatically changes the mechanical properties of the fi-
ber. The initial stiffness falls about one thousand fold from 10 GPa to about l0 MPa
and the extensibility increases dramatically (Gosline et al., 1986). Super-contraction
is thought to be controlled by specific motifs in the silk proteins and to be induced
by the entropy-driven recoiling of molecular chains (Liu et al., 2005a; Gosline
et al., 1986; Jelinski et al., 1999; Shao et al., 1999b; Yang et al., 2000; Eles and Michal,
2004). The soft glycine-rich links in amino acid motifs become motionally active
plasticized by water and therefore, amino acid chains collapse induces local phase tran-
sitions, which randomizes the orientation of the b-sheet.

6.3.2 Viscoelastic properties

The mechanical properties of spider silk are time dependent. Under constant strain, the
fiber experiences stress relaxation, that is to say, the stress required to maintain the
deformation of the fiber decreases with time. Under constant stress, the fiber tends
to creep or the strain increases with time. When the fiber is subjected to dynamic
stresses and strains, stress and strain responses are not in-phase (elastic) nor out-of-
phase (viscous). On the basis of the experimental observation of the cyclic tension,
stress relaxation, creep, and dynamic tension behavior under simple elongation a
constitution law for spider silk was established (Ko et al., 2004a; Ko, 1976).
In order to generate the time-dependent material properties, dragline silks were
collected by forcible silking from an A. aurantia spider collected by the author on
the campus of the Georgia Institute of Technology. The spider was anesthetized
with carbon dioxide gas before being taped on a platform with the abdomen of the
spider facing a microscope. The dragline was picked up from the anterior spinneret
and carefully pulled up to a mandrel 7.6 cm above the spinneret of the spider. After
the silk was taped on the mandrel, a black cardboard with a 1.3 cm hole at the center
was placed behind the silk, which was suspended between the mandrel and the spin-
neret of the spider. A schematic drawing of the cardboard is shown in Fig. 6.6. The
cardboard, with the silk adhering to it, was raised vertically at a rate of 2.5 cm/s to
a position above the mandrel and the silk was taped to the mandrel before collecting
another sample. More than 10 m of silk was obtained from the same spider; this silk
was used for all experiments.
Before testing, each specimen was examined under the microscope to ensure that
only single fibers were used. The diameter of the dragline measured by scanning elec-
tron microscopy was 3.1 mm corresponding to 0.085 denier, assuming 1.25 gm/cc as