Polymer cold-drawing1,2,3,4 is a process in which tensile stress reduces the diameter of a drawn fibre (or thickness of a drawn film) and orients the polymeric chains. Cold-drawing has long been used in industrial applications5,6,7, including the production of flexible fibres with high tensile strength such as polyester and nylon8,9. However, cold-drawing of a composite structure has been less studied. Here we show that in a multimaterial fibre10,11 composed of a brittle core embedded in a ductile polymer cladding, cold-drawing results in a surprising phenomenon: controllable and sequential fragmentation of the core to produce uniformly sized rods along metres of fibre, rather than the expected random or chaotic fragmentation. These embedded structures arise from mechanical–geometric instabilities associated with ‘neck’ propagation2,3. Embedded, structured multimaterial threads with complex transverse geometry are thus fragmented into a periodic train of rods held stationary in the polymer cladding. These rods can then be easily extracted via selective dissolution of the cladding, or can self-heal by thermal restoration to re-form the brittle thread. Our method is also applicable to composites with flat rather than cylindrical geometries, in which case cold-drawing leads to the break-up of an embedded or coated brittle film into narrow parallel strips that are aligned normally to the drawing axis. A range of materials was explored to establish the universality of this effect, including silicon, germanium, gold, glasses, silk, polystyrene, biodegradable polymers and ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal break-up period. These results may lead to the development of dynamical and thermoreversible camouflaging via a nanoscale Venetian-blind effect, and the fabrication of large-area structured surfaces that facilitate high-sensitivity bio-detection.
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We thank E.-H. Banaei, L. N. Pye, F. Tan, A. Schülzgen, C. Jollivet, C. Cariker, A. Schulte, M. Lodge, M. Ishigami, E. Duenas, C. Burchett, M. Finke, Y. Xu, S. Dai, H. Ren and X. Wang for technical assistance. We also thank M. Rein, F. Sorin, M. Kolle, A. Dogariu, D. N. Christodoulides and B. E. A. Saleh for discussions. The authors acknowledge the University of Central Florida Stokes Advanced Research Computing Center for providing computational resources and support that have contributed to results reported here. We also thank Simulia, Inc. for providing the license of the ABAQUS software package. This work was supported by the US Air Force Office of Scientific Research (AFOSR) under contract FA-9550-12-1-0148 and AFOSR MURI contract FA9550-14-1-0037, and the US National Science Foundation (CMMI-1300773). This work was supported in part by the MIT MRSEC through the MRSEC Program of the National Science Foundation under award number DMR-1419807.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Schematic contrasting controlled (sequential) and uncontrolled (random) thread fragmentation.
a, The designed fragmentation process that takes place during cold-drawing of a fibre consisting of a brittle core embedded in a ductile cladding. The overall length of the sample increases considerably when fully drawn. b, The random fragmentation that takes place during stress transfer in a composite sample consisting of a fibre embedded in a matrix. Thick purple arrows indicate externally applied stress.
Extended Data Figure 2 Stress–strain models of the materials used in the finite-element computational model.
a, Axisymmetric structure used in the computational model. P, polymer (PES); I, interfacial layer; C, core. The same polymer and interfacial layer are used in the cylindrical and flat fibre simulations. Various core materials are used. b, Stress–strain model for the PES cladding (‘P’ in a). c, Stress–strain model (including both elastic range and post-failure softening) for the PES interfacial layer (‘I’ in a). d, Stress–strain model for an As2Se3 core material (‘C’ in a). e, Stress–strain model for a silicon (Si) core; see Supplementary Figs 14 and 15.
The results of nonlinear finite-element simulations showing contour plots of the evolving von Mises stress distribution with increasing stretch, using the same (isotropic) materials (PES and As2Se3) as in the cylindrical case (Fig. 2e and Extended Data Fig. 2). The five steps (i)–(v) correspond to increasing stretch values. Top panels depict the full fibre; bottoms panels show the regions corresponding to that highlighted by the rectangle in (i). P, polymer (PES); G, glass (As2Se3).
Each row corresponds to a different thickness of gold (20 nm, 30 nm, 40 nm and 70 nm) sputtered onto a 75-μm-thick PES film. The columns show SEM micrographs of the gold films after cold-drawing at two different scales to highlight the dependence of the average fragment size on the thickness of the gold layer. P, PES film; Au, sputtered gold.
Extended Data Figure 5 Spectral diffraction measurements from a gold film fragmented by cold-drawing.
a, Optical set-up used to measure the spectrum of light diffracted at an angle θ from a thin gold film of thickness 70 nm on a 75-μm-thick PES film after fragmentation via cold-drawing (Extended Data Fig. 4, first row). OSA, optical spectrum analyser; FC, fibre coupler; θ is the angle with respect to normal incidence on the film. b, Measured diffracted spectra on a vertical logarithmic scale. The spectra are normalized with respect to the input optical spectrum. Each spectrum is then normalized to its maximum value.
a, b, Photographs depicting the cold-drawing procedure. a, A line is drawn on a 75-μm-thick PES film (5 mm × 10 cm) using a dry-erase marker pen. b, Using two pliers, the two ends of the strip are pulled symmetrically by hand until cold-drawing is complete. c, After cold-drawing, the optical appearance of the strip changes and coloured diffracted bands are apparent to the naked eye (the marker pen is used to write across the whole film surface). d, SEM micrograph of the drawn line reveals that a crust is formed at the PES surface that fragments into strips that are orthogonal to the cold-drawing axis (similarly to in Figs 3 and 4f), which are behind the new optical properties of the strip seen in c. e, SEM micrograph of the edge of the drawn line, showing a tapering of the thickness of the ink crust, and concomitant drop in fragmentation period. I, ink-polymer crust; P, PES polymer film.
This file contains Supplementary Text, Supplementary Figures 1-17 and Supplementary References. (PDF 2158 kb)
Real time videos of the cold-drawing of a 20 μm diameter As2Se3 glass fibre contained in PES cladding of 1 mm outer diameter. 7 different experiments are shown in this video, where the dimensions of each fibre are the same, but the cold-drawing speed and fibre pre-stress are varied. (MP4 22163 kb)
Animation of cold-drawing-induced fragmentation process for cylindrical fibres; details of the simulations can be found in Methods. (MP4 24403 kb)
Real time videos of the cold-drawing of a 300 nm thick layer of As2Se3 glass embedded in a 1mm wide, 350 μm thick PES cladding. 6 different experiments are shown in this video, where the dimensions of each fibre are the same, but the cold-drawing speed and fibre pre-stress are varied. This video was replaced on 10 June 2016 as the original video was corrupted. (MP4 9195 kb)
Animation of cold-drawing-induced fragmentation process for flat fibres; details of the simulations can be found in Methods. (MP4 27589 kb)
A 1-mm-wide flat PES fibre containing six cylindrical 20-mm-diameter As2Se3 cores is cold-drawn in real time, with a pre-stress of 30 g and at a speed of 20 mm/min. Two segments of the video show two different zooms of the fibre. (MP4 9080 kb)
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Shabahang, S., Tao, G., Kaufman, J. et al. Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing. Nature 534, 529–533 (2016). https://doi.org/10.1038/nature17980
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