Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 1, 2, 3). Furthermore, implantable devices or stretchable displays4 need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals5. This conductivity–stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent6,7,8,9,10,11. Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition12, or with specific alignment (the way the fillers are arranged in the matrix)13,14. However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment15. Pre-strained substrates16,17, buckled microwires18 and three-dimensional microfluidic polymer networks19 have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.
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We thank the STX foundation, Seoul, Korea, for partial funding of this research by providing a stipend to Y.K. The LBL/VAF preparation and low-temperature-conductivity studies were in part supported by the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center (EFRC) funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0000957. The in-depth study of the self-assembly processes was funded by the Center for Photonic and Multiscale Nanomaterials (C-PHOM) as a part of the National Science Foundation Materials Research Science and Engineering Center programme DMR 1120923. We are also grateful to AFOSR, project FA9550-08-1-0382 and Game Changer programme for funding used for M.D.P.'s salary. The DARPA MATLOG project made possible the measurements of damping coefficients. We thank the University of Michigan’s Electron Microscopy and Analysis Laboratory (EMAL) for its assistance with electron microscopy, and for NSF grants (numbers DMR-0320740 and DMR-9871177), for funding the FEI Nova Nanolab Dualbeam Focused Ion Beam Workstation and Scanning Electron Microscope and the JEOL 2010F analytical electron microscope used in this work. We also thank EMAL and the College of Engineering for assistance with the Bruker NanoStar Small-Angle X-ray Scattering System. We also thank E. Arruda, J. Shaw and S. Daly for the use of mechanical facilities. We are grateful to Y. S. Huh for help with the high-voltage electron microscopy image.
This file contains Supplementary Figures 1-13, Supplementary Table 1 and Supplementary References.
About this article
Journal of Inorganic and Organometallic Polymers and Materials (2018)