Deformation Mechanisms in Natural Polymer Fibers and Composites

Abstract

The molecular deformation of both silkworm (Bombyx mori) and spider dragline (Nephila edulis) silks has been studied using a combination of mechanical testing and Raman spectroscopy. It was found that both materials have well-defined Raman spectra and that some of the bands in the spectra shift to lower wavenumber under the action of tensile stress or strain. The band shift was linearly dependent upon stress for both types of silk fiber for the 1085/1095 cm-1 band. This observation provides a unique insight into the effect of tensile deformation upon molecular structure and the relationship between structure and mechanical properties. The measurement of micromechanical deformation within samples of wood, flax and hemp fibers using Raman spectroscopy is also reported. Upon tensile deformation of the samples it was found that the characteristic Raman peak for cellulose, located at 1095 cm-1, shifted towards a lower wavenumber, indicating that the polymer chains within the cellulose were also being deformed. The magnitude of the shift with strain was found to be similar for all samples. No shift occurred of the peak that is characteristic of the non-load-bearing lignin (1600 cm-1) in the wood samples due to its amorphous structure. The similarities between the Raman band shifts in silk and cellulose are discussed.