Multiscale deformations lead to high toughness and circularly polarized emission in helical nacre-like fibres

ARTICLE Received 2 Oct 2015 | Accepted 13 Jan 2016 | Published 24 Feb 2016 DOI: 10.1038/ncomms10701 OPEN Multiscale deformations lead to high toughness and circularly polarized emission in helical nacre-like fibres Jia Zhang1,*, Wenchun Feng2,*, Huangxi Zhang1, Zhenlong Wang1, Heather A. Calcaterra2, Bongjun Yeom2, Ping An Hu1 & Nicholas A. Kotov2 Nacre-like composites have been investigated typically in the form of coatings or freestanding sheets. They demonstrated remarkable mechanical properties and are used as ultrastrong materials but macroscale fibres with nacre-like organization can improve mechanical properties even further. The fiber form or nacre can, simplify manufacturing and offer new functional properties unknown yet for other forms of biomimetic materials. Here we demonstrate that nacre-like fibres can be produced by shear-induced self-assembly of nanoplatelets. The synergy between two structural motifs—nanoscale brick-and-mortar stacking of platelets and microscale twisting of the fibres—gives rise to high stretchability (4400%) and gravimetric toughness (640 J g  1). These unique mechanical properties originate from the multiscale deformation regime involving solid-state self-organization processes that lead to efficient energy dissipation. Incorporating luminescent CdTe nanowires into these fibres imparts the new property of mechanically tunable circularly polarized luminescence. The nacre-like fibres open a novel technological space for optomechanics of biomimetic composites, while their continuous spinning methodology makes scalable production realistic. 1 Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150080, China. 2 Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to P.A.H. (email: ) or to N.A.K. (email: ). NATURE COMMUNICATIONS | 7:10701 | DOI: 10.1038/ncomms10701 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10701 R ealization of materials with high toughness combined with other properties is one of the key challenges for both loadbearing and functional materials1. This materials science challenge can often be addressed using biomimetic design taking naturally occurring composites that have been optimized over long evolutionary periods as inspiration. The ‘brick–and–mortar’ layered design of nacre, with alternating layers of inorganic platelets and biopolymers, inspired biomimetic research for several decades2,3. The materials architecture with alternating layers of hard inorganic components and soft organic polymers effectively arrests the propagation of cracks. The process has been replicated using a large variety of inorganic components, including clay4–7, Al2O3 (refs 1,8) and layered double hydroxides5, combined with various organic polymers including poly(vinyl alcohol) (PVA)5, polyelectrolytes6 and chitosan7. The layered biomimetic nanomaterials are much tougher than their inorganic and organic components alone, and often reveal exceptionally high strength and stiffness1,9. Further improvement of toughness in biomimetic nanocomposites is restricted, however, by the low strains (e) of composite materials, especially when the volume fraction of the stiff inorganic phase is high. The nacre-like composites typically show strains o5% (ref. 10). In fact, the problem of low stretchability is quite general and observed for a variety of nanocomposites including graphene ribbons (e ¼ 6%)11. The combination of two structural motifs at different scales, specifically nanoscale and microscale in this case, is designed to simultaneously increase both the stretchability and toughness of a composite12,13. Indeed, a stretchable graphene film combined with ripples and yarns exhibited improved tensile strain of 30% (ref. 12) and 76% (ref. 13), respectively. This inspired our search for methods to create nacre-like composites with multiscale structural motifs and evaluate their mechanical properties, which we expected to be quite unique as well as technologically valuable. Here we demonstrate that it is possible to transform flat nacre films into fibres that combine layered nanoscale and spiral microscale structural motifs. The resulting fibres can sustain longitudinal strains as high as 414%. This is 10–1,000 times higher than typical biomimeticaly designed layered composites and other fibre-like nanocomposites. The nacre-like fibres display an unusually high gravimetric toughness of B640 J g  1, which significantly exceeds those of natural nacre (B1 J g  1)9, dragline silk (165 J g  1)14, graphene (17 J m  3)13, Kevlar(KM2) (78 J g  1)14 and some of the best examples of composited singlewall carbon nanotube (SWNT) fibres (570–970 J g  1)15–17. Such unusual mechanical properties are attributed to multiscale deformation combining both the sliding of nanoscale platelets and unravelling of microscale spiral curls. Moreover, the described process of fibre spinning and strain-induced particle self-organization enables continuous scalable production of this material18,19. Furthermore, the helical patterns of the multiscale deformations causes circularly polarized luminescence (CPL) to be emitted at B575 nm when cadmium telluride (CdTe) nanowires are incorporated into PVA/CaCO3 fibres. The high stretchability of the fibres allows the wide-range modulation of the luminescence dissymmetry ratio (glum) and the same is expected for many other optically active materials and different wavelengths. This novel optomechanical property of the fibres highlights the emergence of novel possibilities for engineering chiral nanomaterials that may be useful for remote monitoring of materials’ strains. Results Preparation of CaCO3 and graphene nanoplatelets. To prepare the nacre-like fibres we used two types of inorganic ‘building 2 blocks’: one is platelets of CaCO3 (vaterite) synthesized from calcium chloride and ethylene glycol by a hydrothermal method, and the other is graphene-based nanosheets (G) made by electrochemical exfoliation of highly oriented pyrolitic graphite. The vaterite plateletes had diameters and thicknesses of 4–20 mm and 100–500 nm (Supplementary Fig. 1), respectively; these dimensions are very similar to the microplatelets of the aragonite inorganic phase in seashell nacre9. G nanosheets displayed diameters and thicknesses of 5–45 mm and 1–5 nm, respectively (Supplementary Fig. 2a–c). CaCO3 and G were chosen a pair of building blocks for the nacre-like composites because of their low density of defects18, (Supplementary Fig. 2d,e) and they are known to display tensile strength and stiffness higher than the two-dimensional (2D) nanocarbon materials prepared from other oxidation methods, such as Hummers’ method. For the polymeric component in both types of nanocomposites we used (...truncated)