Liquid Crystalline Characteristics of Natural Silk Secretions
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LIQUID CRYSTALLINE CHARACTERISTICS OF NATURAL SILK SECRETIONS KEVEN KERKAM,* DAVID KAPLAN,** STEPHEN LOMBARDI** AND CHRISTOPHER VINEY* *Department of Materials Science and Engineering FB-10, and the Advanced Materials Technology Program, University of Washington, Seattle, WA 98195 **US Army Research, Development and Engineering Center, Natick, MA 01760
ABSTRACT We have used transmitted polarized light microscopy to examine the fluid contents of silk glands taken from Bombyx mori silkworms and Nephila clavipes orb-weaving spiders. In the absence of shear, the secretions are optically isotropic. As the concentration is allowed to increase by evaporation, microstructures typical of the nematic liquid crystalline state are observed. Thus it appears that naturally spun silk becomes liquid crystalline en route to solidifying into fiber which is advantageous to introducing and retaining global molecular alignment. This will facilitate the formation of strong, stiff fibers without the need for a significant post-spinning draw. We have also found that natural silk does not exhibit the defects in molecular alignment that are typical of synthetic polymer fibers spun from liquid crystalline solutions or melts. INTRODUCTION The breaking strength (up to 2 GN.m-2) and elastic modulus (up to2 30 GN.m-2 ) of spider 2 drag line silk [1,2] exceed the values for highly drawn nylon (0.7 GN.m- and 2.4 GN.m- , respectively [3]). Thus, an industrial product that is spun at elevated temperatures, and that requires a subsequent draw ratio of 8-10 for optimum tensile properties, is matched by a natural process that yields fibers from solution under ambient conditions without the need for a post-spinning draw. High strength and stiffness indicate a microstructure in which there is significant molecular extension and alignment. In conventional polymer fiber processing, both the extension and alignment introduced at the spinnerets are lost during the time taken for the fiber to solidify. The post-spinning draw is required to recover these properties. Liquid crystalline polymers have become increasingly attractive to industry [4] because the molecules are tailored to have an extended conformation in the processing environment, and they align spontaneously into domains that are several microns across. Thus it is only necessary to align whole domains, and not individual molecules, to generate global alignment in a fiber. Still, elevated temperatures and / or hazardous solvents are required to enable processing. The tensile properties of natural silk, given the unextreme processing conditions by which these are realized, suggest that the protein solution secreted by the silk glands might form a liquid crystalline phase. Further indications are: "* The fluid contents of some silk glands are optically birefringent [5-8]. Birefringence is a distinguishing, though not unique, characteristic of undisturbed liquid crystalline fluids. "* Natural silk secretions are fluid and easily spun despite their high concentration (approx. 30% by weight for silkworm
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