Abstract
The emergence of order from disorder is a topic of vital interest. We here propose that long-range order can arise from a randomly arranged two-phase material under mechanical load. Using Small-Angle Neutron Scattering (SANS) experiments and Molecular Dynamics based finite element (FE) models we show evidence for stress-induced ordering in spider dragline silk. Both methods show striking quantitative agreement of the position, shift and intensity increase of the long period upon stretching. We demonstrate that mesoscopic ordering does not originate from silk-specific processes such as strain-induced crystallization on the atomistic scale or the alignment of tilted crystallites. It instead is a general phenomenon arising from a non-affine deformation that enhances density fluctuations of the stiff and soft phases along the direction of stress. Our results suggest long-range ordering, analogously to the coalescence of defects in materials, as a wide-spread phenomenon to be exploited for tuning the mechanical properties of many hybrid stiff and soft materials.
Highlights
A material’s mechanical behavior is defined by the underlying structure
To analyze potential stress-induced ordering in silk, we here resorted to a simplified two-phase finite element (FE) model of silk fiber comprising of the stiffer crystalline and the softer amorphous phase
We here present a novel refined FE model, which describes the amorphous phase as a viscoelastic and the crystalline phase as a plastoelastic material, of which the viscosity and yield stress again obtained from MD simulations[28–30]
Summary
A material’s mechanical behavior is defined by the underlying structure. Enormous progress has been made in the last decades in the knowledge and design of the molecular to macroscopic structure for even high-complexity materials, in order to tailor their mechanics and mechanical properties. Amorphous chains straighten and align, and thereby self-assemble into more ordered structures under force This leads to a reduction in the average tilt angle of the backbones within crystallites relative to the fiber axis[1,2,5,6]. Such materials feature different facets of stress-induced ordering on the scale of the individual chains and crystals. They observed the same effect in regular 2D-lattices of macroscopic bubbles, corroborating their conclusion that mechanical stress and not thermal energy induced the order. These seminal experiments nearly two decades ago have not yet been attempted with other systems. Stress-induced order as a general principle has yet to be uncovered
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