Natural materials are renowned for their exquisite designs that optimize function, as illustrated by the elasticity of blood vessels, the toughness of bone and the protection offered by nacre 1,2,3,4,5 . Particularly intriguing are spider silks, with studies having explored properties ranging from their protein sequence 6 to the geometry of a web 7 . This highly adapted material system 8 , which is wellknown to meet a spider's many needs, exhibits exemplary mechanical properties 9,10,11,12,13,14,15 . It thus comes as no surprise that there has been much interest in the molecular design underpinning the outstanding performance of silk fibres 1,6,10,13,19,20 , and in the mechanical characteristics of web-like structures 16,17,18,21 . Yet it remains unknown how the mechanical characteristics of spider silk contribute to the integrity and performance of a spider web. Here we report web deformation experiments and simulations that identify the nonlinear response of silk threads to stress-involving softening at a yield point and dramatic stiffening at large strain until failure-as crucial for localizing the load-induced deformation and hence for endowing spider webs with robustness. Control simulations confirm that a nonlinear stress response results in superior resistance to defects compared to linear elastic or elastic-plastic (softening) material behaviour. We further show that under distributed loads, such as exerted by wind, the behaviour of silk under small-deformation is essential in maintaining the web's structural integrity. The superior performance of silk in webs is therefore not merely due to its exceptional ultimate strength and strain, but more importantly arises from the nonlinear response of silk threads to strain and their geometrical arrangement in a web.While spider silk is employed in a myriad of functions from wrapping prey to lining retreats 22,23 , here we focus on silk's structural role in aerial webs and on how silk's material properties relate to web function. The mechanical behaviour of silk, like that of other biological materials, is determined by the nature of its constituent molecules and their hierarchical assembly into fibres 13,19,20,24,25,26 (Fig. S1). Spider webs themselves are characterized by a highly organized geometry that optimizes their function 7,8,16,17,18 . To explore the contribution of the material characteristics to web function, we develop a web model with spiral and radial threads based on the geometry commonly found in orb webs 1 . The silk material behaviour is parameterized from atomistic simulations of dragline silk from the species Nephila clavipes (Model A) 19,20 ( Fig. 1a-b) and validated against experiments 10 (Methods Summary). As properties of silk can vary across evolutionary lineages by over 100% 9,27,28 (SI Section S1), we avoid species-specific silk properties and use instead a representative model to reflect the characteristic nonlinear stress-strain behaviour of silk found in a web. The mechanical performance of individual silk threads has been...

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing).

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One of the newest carbon allotropes synthesized are diamond nanothreads. Using molecular dynamics, we determine the stiffness (850 GPa), strength (26.4 nN), extension (14.9%), and bending rigidity (5.35 × 10(-28) N·m(2)). The 1D nature of the nanothread results in a tenacity of 4.1 × 10(7) N·m/kg, exceeding nanotubes and graphene. As the thread consists of repeating Stone-Wales defects, through steered molecular dynamics (SMD), we explore the effect of defect density on the strength, stiffness, and extension of the system.

Graphdiyne, a recently synthesized one-atom-thick carbon allotrope, is atomistically porous - characterized by a regular "nanomesh"- and suggests application as a separation membrane for hydrogen purification. Here we report a full atomistic reactive molecular dynamics investigation to determine the selective diffusion properties of hydrogen (H(2)) amongst carbon monoxide (CO) and methane (CH(4)), a mixture otherwise known as syngas, a product of the gasification of renewable biomass (such as animal wastes). Under constant temperature simulations, we find the mass flux of hydrogen molecules through a graphdiyne membrane to be on the order of 7 to 10 g cm(-2) s(-1) (between 300 K and 500 K), with carbon monoxide and methane remaining isolated. Using a simple Arrhenius relation, we determine the energy required for permeation on the order of 0.11 ± 0.03 eV for single H(2) molecules. We find that addition of marginal applied force (approximately 1 to 2 pN per molecule, representing a controlled pressure gradient, ΔP, on the order of 100 to 500 kPa) can successfully enhance the separation of hydrogen gas. Addition of larger driving forces (50 to 100 pN per molecule) is required to selectively filter carbon monoxide or methane, suggesting that, under near-atmospheric conditions, only hydrogen gas will pass such a membrane. Graphdiyne provides a unique, chemically inert and mechanically stable platform facilitating selective gas separation at nominal pressures using a homogeneous material system, without a need for chemical functionalization or the explicit introduction of molecular pores.