THERMAL-ANALYSIS OF MAJOR AMPULLATE (DRAG LINE) SPIDER SILK - THE EFFECT OF SPINNING RATE ON TENSILE MODULUS

Spiders use major ampullate silk (MAS) in web building and as a 'safet y line' when escaping predators. In the former case, the fibre is spun at approximately 1 cm s(-1); in the latter case the rate is an order of magnitude faster. The different spinning rates do not result in ide ntical microstructures, implying that the mechanical properties should not be identical either. However, neither conventional tensile testin g, nor the characterisation of storage modulus with DMA yields reprodu cible data that could be used to correlate stiffness to the spinning r ate. Any correlation is masked by the consequences of MAS having a cro ss-section that is non-circular, difficult to measure and highly Varia ble along the fibre length. Also, DSC shows that fibres contain bound moisture even after drying in a desiccator; fibre thickness affects th e ability of samples to lose this moisture and gain stiffness on heati ng in the DMA. Thermal expansion data obtained with the DMA are reprod ucible below the MAS glass transition (approximately 160 degrees C): t hese results do not depend on the accurate measurement of the sample c ross section, they are insensitive to the moisture content of samples and they are not affected significantly by energy loss arising from fr iction between individual fibres in the specimens tested. The thermal expansion data suggest that the highest intrinsic stiffness of MAS is realised at spinning rates used in web construction. When the spider m ust elude a predator, it spins MAS that is more compliant. (C) 1998 El sevier Science B.V.