Page 194 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 171

2014) due to an increase of the preferential orientation of fibroin chains. The plateau
arise from the untwisting of regular and irregular helix. This means that the third
regime (linear hardening requiring higher energy) could be attributed to the opening
of b-sheet crystals and other “dense” highly bonded entities. It is important to note
that comparisons between fibers of different diameters should be made with caution.
As an example, see Fig. 5.11, the ultimate tensile stress and strength are not only
material-dependent properties, but also object dimensions driven properties: measured
values increase when the fiber diameter decreases (Colomban et al., 2012b).
Two-dimensional correlation spectra (2DCOS) were introduced by Noda (1993,
2014) as a way of studying the correlation between variable parameters at different
times. The technique reveals any perturbation influence on spectral parameters and
is applicable to any series of perturbed spectra, particular for stressed single fibers
in situ/operando analyzed by vibrational spectroscopy (Gouadec et al., 2002). From
the Raman spectra recorded under controlled strain, the 2DCOS are derived. As an
example, Fig. 5.19 shows the synchronous and asynchronous 2DCOS in the Amide
I region for fresh Bombyx mory fiber, which has type I mechanical behavior (Fig. 5.3).
The synchronous correlation spectrum shows apparent auto correlation peak around
1668 cm 1 (at C point marked on Fig. 5.19). Combining observations of the synchro-
nous and asynchronous, showed that the 1668 cm 1 component is shifted to the lower
wave number and increased the full width at half maximum due to the increase of the
short-range disorder. After Raman spectra analysis, the Amide I region of fresh
1
Bombyx mory fiber has three components: 1645, 1668, and 1687 cm (Fig. 5.13).
1
The 2DCOS show that under applied stress, the 1668 cm component is changed

(a) (b)

1720 1720
1700 1700
Wavenumber / (cm –1 ) 1660 1660
1680
1680

1640
1640
1620
1600 1620
1600
1580 1580
1580 1600 1620 1640 1660 1680 1700 1720 1580 1600 1620 1640 1660 1680 1700 1720
–1
–1
Wavenumber / (cm ) Wavenumber / (cm )
Figure 5.19 Synchronous (a) and asynchronous (b) two-dimensional correlation spectra
(2DCOS) in the Amide I region, derived from strain-dependent Raman spectra for fresh
Bombyx mory fiber (Type I).
After Dinh HM: Strain-induced structure change of silk fibres e 2D correlation study, 2017
(Submitted).