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Mechanical and optical analyses provide a network model for spiral silk from the orb web of the spider Araneus diadematus

University of British Columbia

Glue-coated spiral silk from Araneus diadematus orb webs is stretchy compared to the stiff and inextensible frame silk. This thesis examines whether part of the difference between properties may be due to molecular arrangements, not just the glue. Dry and wet spiral samples had average diameters of 2.2 and 3.1 microns and a non-circular cross-section with an average ellipticity of 1.5. The lengthwise contraction of wetted samples is a two-phase process, with an average ratio of wet to dry slack length of 0.6. Stress-extension behavior of samples was analyzed according to theories of rubber elasticity. Dry and wet spiral samples have, on average, network chains with 4.8 and 6.6 random segments between crosslinks; frame silk has two. The average shear moduli of wet and dry spiral samples are 0.67 and 0.32 MN/m² (the modulus of wet frame silk is 0.8 MN/m²). Spiral samples' average volume increase from the dry to the wet state is 1.6 compared to frame's volume change of 2.1, possibly because "dry" spiral silk is already partially plasticized. Samples that are stiffer than average in the dry state require higher than average work of extension in both dry or wet states, but stiffer dry samples have proportionately higher reductions in work of extension when wetted, perhaps because most of the extra stiffness is caused by water-labile structures rather than stable crystals. A solution of guanidine hydrochloride rapidly causes silk to lose its mechanical integrity, supporting hypotheses that the major crosslinking mechanism of Araneus silk is non-covalent. Optical analyses yielded a residual birefringence of approximately 1.5x10⁻³ (half that of frame silk, at 3.8xl0⁻³) and a stress-optical coefficient of 2x10⁻⁹ N/m². Both parameters are significantly higher than expected for wholly amorphous rubbers and provide evidence for crystals or aligned glassy regions. Thus, the hypothesis that the network structure of spiral silk is different than frame silk was supported by the significantly different values for chain densities, segment counts, and birefringences.

Text

2010-12-02

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http://hdl.handle.net/2429/30274

Zoology

University of British Columbia

Graduate