Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 191

100 mms -1

1500

1200

25 mms -1
10 mms -1

900

600

2.5 mms -1

Yield point

1 mms -1

300

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Strain

Figure 6.7 Comparison of silk fibers drawn at different speeds from spider N. pilipes .
(Reprinted with permission from Ref. [17]. Copyright (2006) Biophysical Society.)

inter-crystallite distance can be determined by XRD. The detailed results on the
structural characterization are outlined below.

6.3.2.1 Crystallite Size
The crystallite size can be obtained from the wide-angle X-ray scattering (WAXS)
pattern of a silk fiber. Radial integration along the equator and meridian on
the WAXS gives the radial peak width on the (200), (120), and (002) reflections.
According to Scherrer's formula [85], the crystal size is

λ
FWHM cos

0.9

L = (6.1)

θ

where FWHM is full width at half-maximum of the peak at the diffraction angle
θ and λ = 1.5418 ˚ A. The equatorial data can be deconvoluted into three crystalline
peaks corresponding to (100), (200), and (120) Bragg reflections and an amorphous
halo. Similarly, the meridian data show a crystalline peak corresponding to the
(002) reflection and a broad amorphous halo. The position and FWHM of these
peaks can be used to determine the crystallite size along the a , b ,and c axes.
It was found that the crystallites become slightly smaller or the crystalline order
becomes poorer with an increase in reeling speed [17].
The effect of reeling speed on the crystallite size can be explained by the
nucleation kinetics. The formation of -sheet crystallites is controlled by nucleation

and growth processes [86]. Some portion of the protein chains undergoes nucleation
when extruded from the spinneret orifice and crystallizes as the solvent evaporates.
The study of the silk secretion of Bombyx mori and Nephila clavipes when the silk
secretion is put on a glass slide indicates that a liquid crystalline phase occurs
quickly followed by a slow formation of the crystalline phase [27]. This suggests
that the amorphous-to-crystalline phase transition much depends on the initial

β