Spider silk morphology for responsive materials
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Spider silk morphology for responsive materials Juan Guan, David Porter*, and Fritz Vollrath Department of Zoology, South Parks Road, University of Oxford, Oxford, OX1 3PS, UK *Correspondence: [email protected] ABSTRACT This study reveals that an “old” mechanism for shape memory in oriented polymers is in fact just one separate contribution for “supercontraction” in Nephila spider major ampulate silks. When Nephila spider silks are in contact with liquid water, they “super”-contract up to 28% of the original stretched length. However, we discovered that under glass transition conditions these silks only relax with a maximum shrinkage of 13%, and this phenomenon is defined as Tg-contraction. Structural components permanent order (PO), permanent disorder (PD), meta order (MO) and meta disorder (MD) were proposed from the primary amino-acid sequence of the silk protein to explain morphological changes in the two contraction phenomena: MD contributes 13% of the full supercontraction and contributes to Tg-contraction; whereas MO (the prolinecontaining motifs) contributes the rest for the full super-contraction and does not contribute to Tg-contraction. The morphology in Nephila spider silk structure suggests two separate mechanisms to generate the shape memory effect in synthetic polymers. INTRODUCTION Spider silk is a natural protein fibre with outstanding mechanical properties, and its high strength combined with large extensibility outshines many synthetic materials. Recently, the thermal conductivity [1], shape memory [2], and humidity-driven changes in mechanical performance [3] have raised new interest in this promising future material. Another remarkable property of spider dragline silk is “super-contraction” [4]: when dragline silk is in contact with liquid water, it contracts instantaneously and often substantially depending on the species of the spider and silk composition and morphology. More interestingly, when the contracted silk is restretched back to its original length, it retains its ability to super-contract. The contraction-stretching cycles can be reproduced many times [3]. Supercontraction was first discussed and defined by Robert Work [5] and now this phenomenon provides new potential for spider silk applications; for example, as tunable shape memory polymers, which are of great technological interest [6]. On supercontraction, the phenomenon itself, there have been substantial observations. However, in respect to the morphology of spider silk and structural contributions to supercontraction, there is not yet a complete picture. Based on a thorough literature study, we proposed two structural relaxations towards supercontraction correlated with two morphologies: (i) the oriented disordered domains
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and (ii) the proline-rich ordered domains. The oriented disordered domains, so-called meta-disorder (MD) are able to relax under glass transition conditions. This partial contraction is compared with full supercontraction, so-called Tg-contraction, in this paper. EXPERIMENT A medium size Neph
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