Elastomeric Conductive Composites Based on Carbon Nanotube Forests

Electrically conductive materials capable of substantial elastic stretch and bending are needed for such applications as smart clothing, flexible displays, stretchable circuits, strain gauges, implantable devices, high-stroke microelectromechanical systems, and dielectric elastomer actuators. A variety of approaches involving carbon nanotubes (CNTs) and elastic polymers have been suggested for the fabrication of conductive elastic composites. In particular, diverse active and passive electronic components have been embedded in rubber sheet by several research groups to obtain stretchable electronic devices. Sekitani et al. developed rubber-like conductive composites by mixing millimeter-long single-walled carbon nanotubes (SWNTs), an ionic liquid, and a fluorinated copolymer. The stretchability of the resulting composite was enhanced by creating perforated films with a net-shaped structure using a mechanical punching system. Cao et al. fabricated flexible electrodes by incorporating SWNTnetworks in plastics consisting of polyimide, polyurethane, and polyamic acid films. Although quite successful, these studies indicated that high loading of CNTs (or other conductive additive) was necessary to obtain a highly conducting composite. On the other hand, incorporation of high concentrations of CNTs into an elastic polymer increases the stiffness of the resulting composite and decreases its stretchability. In other words, the significant difference in the Young’s modulus of extremely rigid CNTs and the elastic polymer filler makes the creation of a highly stretchable conductive composites a challenging task. It is known that CNTs can be fabricated into macroscopic assemblies, such as mats (bucky paper), yarns, and fibers that possess useful electrical properties, and that these assemblies can be used for the fabrication of conductive polymer composites. While these assemblies are often more elastic than the individual CNTs, the achievable elastic strain range is still quite limited, normally less than 10%. We found that a combination of high stretchability and high electrical conductivity can be obtained for composites prepared from three-dimensional CNT structures, such as CNT forests (vertically aligned arrays of CNTs). Unlike previous methods involving casting CNT/ polymer dispersions as a film, our composites were prepared by the direct infiltration of multiwalled carbon nanotube (MWNT) forests with a polyurethane (PU) solution. Using this procedure, we obtained rubber-like forest/PU composites that combined high stretchability with high electrical conductivity. These composites provide highly reversible stress–strain behavior and little degradation of mechanical and electrical properties even when stretched over a wide strain range. The developed preparation procedure appears scalable for material fabrication on an industrial scale, though transition from present batchbased forest growth processes to continuous forest growth processes would be needed for applications that are price sensitive and depend on sheet weight, rather than the area of elastomeric sheet. The aligned arrays of MWNTs (MWNT forests) used in this study were grown on iron-catalyst-coated silicon wafers using a conventional chemical vapor deposition (CVD) method. Nanotubes in the forests typically had a diameter of about 10 nm; their length could be controlled across a wide range by changing the growth time and other fabrication conditions. The forest-covered area on the substrate used for the preparation of the composites typically had dimensions of about 50 100mm; the height of nanotubes in the forest was about 50mm as determined by the conventional optical microscopy. Since the nanotubes in the forests formed a three-dimensionally interconnected network, the forests were electrically conductive in all directions. The MWNT forests were infiltrated with a PU solution in N,N-dimethylformamide (DMF) using a simple drop-casting procedure, as shown in Figure 1a. The PU used was poly[4,40methylene-bis(phenyl isocyanate)-alt-1,4-butanediol/poly(butylene adipate)]. After evaporation of the solvent, we obtained about 250mm thick forest/PU composite sheets that could be peeled off the underlying Si wafer. Figure 1b shows a photograph of the MWNT/PU composite sheet taken at low magnification. One side of the prepared film facing the substrate (forest side) was black and conductive, and the other side (PU side) was white and insulating. The material was soft, flexible, and highly stretchable in the sheet plane. Figure 1c shows a SEM image of a cross-section of the composite sheet with the top ( 50mm in thickness) being the forest side and the bottom ( 200mm in thickness) being the PU side. A highmagnification image of the forest side is shown in the