Effects of Wetting and Desiccation on the Creep Properties of Spider Dragline Silk
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Effects of Wetting and Desiccation on the Creep Properties of Spider Dragline Silk Joanne Ritchie2, Christopher Smith2, Fraser I. Bell2, Iain J. McEwen2, and Christopher Viney1, 2 1
School of Engineering, University of California at Merced, P.O. Box 2039, Merced, CA 95344, USA 2 Chemistry, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Midlothian, Scotland
ABSTRACT We have previously devised a simple method for quantifying the creep resistance of silk fibres – i.e. the ability of silk to maintain dimensional stability while supporting a constant load. We demonstrated (C. Smith et al., J. Arachnol. 31:421-424, 2003) that creep of spider dragline is significant at stresses that are small compared to the conventional yield strength. In addition, the existence of a limiting creep stress was revealed: if samples are loaded smoothly and quickly to a constant stress lying above the limiting creep stress, they break within a few seconds of the stress being applied. The magnitude of the limiting creep stress is equal to approximately one fifth of the fracture stress recorded in conventional constant strain rate tests (in which the stress is applied more gradually). Here we develop the method and broaden the study of creep to investigate the effects of immersion in water and in ethanol. Experiments are conducted by attaching a small weight (an appropriate number of office staples) to an approximately 20 cm length of dragline from Nephila clavipes spiders, and monitoring extension as a function of time while the samples are suspended vertically in the liquid. The silk is previously conditioned (immersed without imposing any geometrical constraint) in the liquid, to allow it to supercontract or relax if the liquid promotes such a response. Allowance for buoyancy is made when calculating the net force applied to samples by the weight of the staples. Water is found to have a plasticizing effect; it exacerbates the rate of creep in response to a given small load, and it decreases the limiting creep stress. Ethanol, a desiccant, delays the transition from creep dominated by changes in chain conformation to creep dominated by chain slip, and increases the limiting creep stress. These results draw attention to an important limitation that must be overcome if biomimetic silk is to be used in applications where dimensional stability is required while loads are being supported for long times, especially in a wet environment. The results also point to the molecular origins of the creep sensitivity, and thus to ways of making silk and silk analogues less susceptible to creep.
INTRODUCTION Spider and silkworm silks have been studied extensively during the past two decades [1-3], prompted by the high mean values of breaking strength, initial stiffness, and toughness reported for some of these materials. It is necessary to recognize, however, that the impressive tensile properties are deduced from experiments conducted at constant strain rate, on a timescale where the test is completed (t
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