Elastomers filled with exfoliated graphite as compliant electrodes M. Kujawski a , J.D. Pearse b , E. Smela b, * a Department of Materials Science and Engineering, University of Maryland, College Park, United States b Department of Mechanical Engineering, University of Maryland, College Park, United States ARTICLE INFO Article history: Received 26 January 2010 Accepted 25 February 2010 Available online 3 March 2010 ABSTRACT A compliant electrode material has been realized by blending an insulating poly- dimethylsiloxane (PDMS) elastomer with a conductive exfoliated graphite filler, which was produced by microwave irradiation. The conductivity and stiffness of the electrodes were determined as a function of filler concentration. These materials exhibited a low per- colation threshold: above 3 wt.% loading they became conductive, with conductivities reaching as high as 0.4 S/cm. They remained elastomeric upon loading up to 25 wt.%, hav- ing a Young’s modulus of only 1.4 MPa. This modulus (corresponding to a 220% increase compared to the unloaded PDMS) is the lowest reported for loaded elastomers above the percolation threshold. Scanning electron microscopy showed that the composites con- tained small voids, unlike unloaded PDMS, which might be responsible for the low modu- lus. The performance of these electrodes is comparable to that of PDMS loaded with carbon nanotubes, but the exfoliated graphite material can be produced at a fraction of the cost. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Compliant electrodes that can undergo significant stretching while maintaining good conductivity are needed in applica- tions such as strain gauges, flexible displays, wearable elec- tronics, and dielectric elastomer actuators. However, the development of a really good compliant electrode material has proven to be challenging because of the numerous, some- times incompatible, requirements on it. As implied by the name, the two most important requirements are mechanical compliance, which equates to a low Young’s modulus, and high electrical conductivity. Other characteristics required by particular applications may include extended strain cycling, high ultimate tensile strain, negligible creep, ease of fabrication, low material costs, and resistance to wear, rubbing off, and delamination. Much of the compliant electrode research over the past decade has focused on two-phase composites composed of conductive filler particles within an insulating polymer matrix [1–16]. Conductive filler materials include carbon black (CB) [8,16], carbon fibers [8], multi-wall carbon nanotubes (MWCNTs) [3,7,9–11,17], and single wall carbon nanotubes (SWCNTs) [14,15,18–21]. Unfortunately, there is normally a dramatic increase in stiffness with loading of the conductive filler [1,11,15]. Stiffness increases can be minimized by using a lower percentage of the filler, but if the loading is too low then the material remains insulating. The loading at which the material becomes conductive is known as the percolation threshold, at which a conductive pathway through the host matrix is formed by the filler particles. Graphite, which can be easily obtained and is relatively inexpensive, has also been used as a filler [4,5,12,13,22]. 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.02.040 * Corresponding author: Fax: +1 301 314 9477. E-mail address: smela@umd.edu (E. Smela). CARBON 48 (2010) 2409 – 2417 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon