Origin of the variability of the mechanical properties of silk fibers: 4. Order/crystallinity along silkworm and spider fibers
Résumé
The poor crystallinity of proteic fibers has fuelled an ongoing debate over their exact organization. We present a full-range Raman comparison of Nephila madagascariensis spider and Bombyx mori silkworm silks that sheds some new light on that matter. On the one hand, a large variability is observed along the fibers in the -200 to 200cm(-1) spectral window, which is sensitive to the long-range order signature of polyamide chains. This questions the validity of previous literature data considering silk fiber as a homogeneous material. On the other hand, the amide I' band is almost independent of the targeted point, which sets a limit to this widely used structure probe. In-line mapping of the fibers showed that the extension of the ordered zones ranges between 1 and 3 mu m. The correlation between the macromechanical behavior (the stress-strain curves) and the nanomechanics (Raman low wavenumber signatures) under controlled tensile strain demonstrates a Prevorsek's type microstructure: the macromolecular chains belong to both ordered and amorphous regions.