Deformation Mechanisms in Natural Polymer Fibers and Composites
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Deformation Mechanisms in Natural Polymer Fibers and Composites Robert J. Young, Stephen J. Eichhorn, J. Sirichaisit and Victoria L. Brookes Manchester Materials Science Centre UMIST/University of Manchester, Manchester M1 7HS, United Kingdom 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.
INTRODUCTION In recent years the use of Raman spectroscopy as a tool to study the deformation of materials has increased considerably. This is due to the development of high performance equipment, whereby spectra can be obtained in a relatively short period allowing for real-time deformation studies. The first studies of polymer deformation concentrated on single crystal polydiacetylene fibers [1] and later followed by studies of high performance fibers, as summarized by Young [2]. The technique relies on measuring accurately the position of a structurally characteristic Raman peak during deformation. When tensile deformation is applied to most polymeric fibers these peaks shift towards a lower wavenumber, and this is thought to be due to direct stressing/straining of the molecular backbone [3]. Silk Fibers Over recent years considerable research has focused on spider silks [4,5] because of their unique properties – good extensibility combined with high levels of tensile strength. Spiders produce a wide variety of silks for many particular purposes coupled with a wide variety of mechanical properties [6] and it is thought that a better understanding of structure/property relationships in silk could be used to develop superior fibrous materials, be they man-made or of natural origin. At present the commercial silk industry is almost entirely based upon material FF7.1.1
produced by an insect - the silk worm, Bombyx mori. Thus, among the wide variety of silks av
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