BONCEL ET AL. VOL. 5 ’ NO. 12 ’ 9339–9344 ’ 2011 www.acsnano.org 9339 November 21, 2011 C 2011 American Chemical Society Enhancement of the Mechanical Properties of Directly Spun CNT Fibers by Chemical Treatment Slawomir Boncel, Rajyashree M. Sundaram, Alan H. Windle, and Krzysztof K. K. Koziol * Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom F undamental research on carbon nano- tubes (CNTs) and CNT-based materials is eliciting intense interest from the scientific research community. Due to their excellent mechanical properties, as well as superior electrical and thermal conduc- tivities, 1,2 carbon nanotubes show promise for multifunctional materials for a range of applications. Nevertheless, the transfer of the science into a realistic technology has yet to be completed, and key challenges re- main, especially in terms of self-assembling the nanotubes to form useful materials on a macroscopic scale which express the excit- ing properties of their nanotube building blocks. 3 Substantial attention is required toward the control of interaction between nanotubes, which plays a major role in load transfer 4 and electron 5 and phonon trans- port. 6 The direct assembly of CNTs into fibers involving continuous spinning from a che- mical vapor deposition (CVD) reactor 7,8 is a significant step toward the scalable produc- tion of macroscopic entities of CNTs. This process, at the laboratory scale, can gen- erate kilometers of ∼10 μm diameter fiber, consisting of millimeter-long collapsed double-walled nanotubes (d =510 nm), arranged into bundles (Figure 1). The CNT fiber shows, on a macroscale, attractive mechanical properties 810 but only a frac- tion of those reported for individual nano- tubes. Nevertheless, the CNT fiber is already in the high-performance materials range. 1114 The sliding of bundles (as shown in Figure 1C,D) of CNTs past each other (graphite is a lubricant) during stress transfer is the major limiting factor of the mechanical properties of the fiber under tensile loading. 15,16 Of the various methods for making carbon nanotube fibers reported, 1719 those pro- duced by the coagulation method, 17 where the nanotube fiber is spun in a polymeric matrix, point to several advantages resulting from the presence of the polymer, especially with regard to strain to failure and energy absorbed before tensile failure. The preferentially aligned macroscopic as- semblies of carbon nanotubes were synthe- sized using the direct spinning CVD method. 7 The synthesis process operates in the tem- perature range of 11001400 °C. Ferrocene is used as the source of iron catalyst, thio- phene is used as the source of sulfur for catalyst activation, and di fferent carbon sup- ply is used for supplying the main building element of the nanotubes. CNTs are rapidly synthesized in the hot zone with residence time not exceeding 10 s. The network of * Address correspondence to kk292@cam.ac.uk. Received for review August 28, 2010 and accepted November 20, 2011. Published online 10.1021/nn202685x ABSTRACT Translating the remarkable mechanical properties of individual carbon nanotubes to macro- scopic assemblies presents a unique challenge in maximizing the potential of these remarkable entities for new materials. Infinitely long individual nanotubes would represent the ideal molecular building blocks; however, in the case of length-limited nanotubes, typically in the range of micro- and millimeters, an alternative strategy could be based on the improvement of the mechanical coherency between bundles assembling the macroscopic materials, like fibers or films. Here, we present a method to enhance the mechanical performance of fibers continuously spun from a CVD reactor, by a postproduction processing methodology utilizing a chemical agent aided by UV irradiation. The treatment results in an increase of 100% in specific strength and 300% in toughness of the fibers with strength values rocketing to as high as 3.5 GPa SG 1 . An attempt has been made to explore the nature of the chemical modifications introduced in the fiber and the consequential effects on its properties. KEYWORDS: carbon nanotube fibers . carbon nanotube yarns . mechanical properties . chemical cross-linking ARTICLE