Page 182 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 159

5.3.2 Vibrational signatures and conformations

Vibrational (micro)spectroscopy offers an alternative route to understand the structure
of amorphous or low crystalline materials (Gouadec and Colomban, 2007). However,
the vibrational probe is the chemical bond itself, a subnanometric probe: stretching and
bending modes probe the chemical bonds and their environment, the information also
obtained from e.g., NMR technique, but librational and lattice modes probe long
distance correlations in the same manner as X-ray diffraction. Furthermore, the spatial
resolution delivered by long working distance microscope objectives is less than
3
1 mm , permitting the analyze of the fiber spatial anisotropy, e.g., across its fiber diam-
eter (Colomban et al., 2006; Colomban, 2013). For silk, the most pertinent probes
(Fig. 5.12) are those related to the chain backbone (amide group modes), to the inter-
action between the NeH group and neighboring O or N atoms of the grafted amino
acids of the same or adjacent chains (NeH stretching mode) and to the motion of
the macromolecule chains, the “lattice” modes. Amide groups have very characteristic
vibrational signatures: (1) the Amide I mode, originating from the stretching of the
nC]O bond coupled with the CeN bond, shows a well-defined peak, both in IR
1
and Raman spectroscopy at w1630e1670 cm ; (2) the Amide III mode originating
from the nCeN bond, which is also both IR and Raman active and results in peaks
1
between 1200 and 1300 cm . The isolated nNeH stretching vibration mode at
w3285 cm 1 offers a very sensitive tool to probe the short-range environment of
the NeH bond and associated hydrogen bonds. However, a direct relation exists
between the wave number and the hydrogen bond length (Novak, 1974). Finally the
1
low wave number range (<200 cm , librational and lattice modes) is the most perti-
nent region to obtain structural information at medium to long range, including giving
information on the degree of crystallinity (Colomban et al., 2006; Colomban, 2013;
Gouadec and Colomban, 2007). Such studies require high-resolution instruments
and require a solid state physics approach, only recently developed for polymers
(Colomban et al., 2006; Herrera Ramirez et al., 2004; Herrera Ramirez et al., 2006).
Although information on the orientation of molecular bonds is difficult to obtain
from internal modes, the polarization of the low wave number region reflects the fiber
anisotropy very well, as shown in Fig. 5.12. In contrast to other, semicrystalline poly-
mers, such as polyamides or polyethylene terephthalate (Colomban et al., 2006) for
which a separation between “narrow” low wave number lattice modes of the crystal-
line domains merges with the broad contribution of the amorphous matrix, the analo-
gous signature for silk remains broad. Only in some spots (Fig. 5.5) an intense narrow,
1
polarized 143 cm peak can be observed, both for B. mori and N. madagascariensis
fibers (Wojcieszak et al., 2014). This indicates that ordered/crystalline domains are
dispersed as small islands in an amorphous matrix.
Fig. 5.13 compares the Raman signature in the NeH and Amide I regions recorded
at different steps of the production of the silk fiber (Colomban et al., 2008a; Colomban
and Dinh, 2012): on the liquid present in the gland (quasi in vivo study), on fresh silk,
freshly spun from the animal, as well as those of fibers with Type I, II, and III signa-
tures as well as silk in the gland (dried). Similar quasi in vivo IR analysis was made by