Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing

Cold-drawing of multimaterial fibres consisting of a brittle core embedded in a ductile polymer cladding results in controllable fragmentation of the core to produce uniformly sized rods parallel to the drawing direction for cylindrical geometries and narrow, parallel strips perpendicular to the drawing direction for flat geometries. Polymer fibres such as nylon and polyester are often formed by cold-drawing, whereby the raw, brittle plastic is put under tensile stress and pulled — or 'drawn' — into thinner fibres. In a study of cold-drawing in the context of multimaterial structures consisting of a brittle core clad in a polymeric fibre or film, Ayman Abouraddy and colleagues have observed a surprising phenomenon that could be exploited to produce novel nanomaterials. They show that as the 'shoulder' between the thicker intact fibre and the thinner 'neck' region of the fibre propagates, the brittle core fragments evenly and predictably to form a train of equally spaced fragments within the polymer fibre, which can, if desired, be dissolved to leave only the core material. The phenomenon occurs regardless of core cross-section or material — silicon, germanium, gold, silk, polystyrene and even ice behave in this way — and both fibres and sheets can be cold-drawn to the same effect. 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.