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      Cryogenic toughness of natural silk and a proposed structure–function relationship

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          Abstract

          A highly aligned and relatively independent nanofibril structure contributes to the cryogenic toughness of natural silk.

          Abstract

          Natural spider and worm silks can provide key insights into bio-polymer technology. No-one would have thought that ductility and toughness at cryogenic temperatures would be among their properties. Here we examine the behavior and function of several animal silks by focusing on the multi-fibrillar fibres of Antheraea pernyi silkworm cooled down to −196 °C. In essence, on the micro- and nanoscale, the extrinsic toughening mechanism of the aligned nanofibrils of silk-protein blunts the crack tip and deviates the fracture path. At the molecular level, an intrinsic toughening mechanism within each nanofibril can be attributed to high degrees of orientation of both ordered and disordered chain-domains. We propose that the highly aligned yet relatively independent nanofibrillar structure allows the partly frozen molecular chain at cryogenic temperature to be activated to induce crack blunting, to allow fibril slipping, and to facilitate the effective unfolding of silk fibroin molecular chains thus preventing or delaying brittle failure of the whole fibre. The spider and mulberry silks examined diplayed comparable functional mechanisms. We envision that our study will lead to the design and fabrication of new families of tough structural composites using natural silk or silk-inspired filaments for testing applications even at arctic or indeed outer-space conditions.

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          Most cited references42

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          A fracture-resistant high-entropy alloy for cryogenic applications.

          High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening. Copyright © 2014, American Association for the Advancement of Science.
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            Tough, bio-inspired hybrid materials.

            The notion of mimicking natural structures in the synthesis of new structural materials has generated enormous interest but has yielded few practical advances. Natural composites achieve strength and toughness through complex hierarchical designs that are extremely difficult to replicate synthetically. We emulate nature's toughening mechanisms by combining two ordinary compounds, aluminum oxide and polymethyl methacrylate, into ice-templated structures whose toughness can be more than 300 times (in energy terms) that of their constituents. The final product is a bulk hybrid ceramic-based material whose high yield strength and fracture toughness [ approximately 200 megapascals (MPa) and approximately 30 MPa.m(1/2)] represent specific properties comparable to those of aluminum alloys. These model materials can be used to identify the key microstructural features that should guide the synthesis of bio-inspired ceramic-based composites with unique strength and toughness.
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              New opportunities for an ancient material.

              Spiders and silkworms generate silk protein fibers that embody strength and beauty. Orb webs are fascinating feats of bioengineering in nature, displaying magnificent architectures while providing essential survival utility for spiders. The unusual combination of high strength and extensibility is a characteristic unavailable to date in synthetic materials yet is attained in nature with a relatively simple protein processed from water. This biological template suggests new directions to emulate in the pursuit of new high-performance, multifunctional materials generated with a green chemistry and processing approach. These bio-inspired and high-technology materials can lead to multifunctional material platforms that integrate with living systems for medical materials and a host of other applications.
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                Author and article information

                Journal
                MCFAC5
                Materials Chemistry Frontiers
                Mater. Chem. Front.
                Royal Society of Chemistry (RSC)
                2052-1537
                2019
                2019
                Affiliations
                [1 ]State Key Laboratory of Molecular Engineering of Polymers
                [2 ]Advanced Materials Laboratory
                [3 ]Department of Macromolecular Science
                [4 ]Fudan University
                [5 ]Shanghai 200433
                [6 ]School of Materials Science and Engineering
                [7 ]Beijing Innovation Center of Biomedical Engineering
                [8 ]Beihang University
                [9 ]Beijing 100191
                [10 ]People's Republic of China
                [11 ]Department of Zoology
                [12 ]University of Oxford
                [13 ]Oxford OX1 3PS
                [14 ]UK
                Article
                10.1039/C9QM00282K
                f3037d53-349b-4558-bd59-e665bc912240
                © 2019

                http://creativecommons.org/licenses/by/3.0/

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