A Novel Negative Dielectric Constant Material Based on Phosphoric Acid Doped Poly(benzimidazole) Keith L. Gordon, 1 Jin Ho Kang, 2 Cheol Park, 2,3 Peter T. Lillehei, 1 Joycelyn S. Harrison 1 1 Advanced Materials and Processing Branch, NASA-Langley Research Center, Hampton, Virginia 23681-2199 2 National Institute of Aerospace, Hampton, Virginia 23666 3 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904-4746 Received 23 February 2011; accepted 5 May 2011 DOI 10.1002/app.36248 Published online 1 February 2012 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Metamaterials or artificial negative index materials (NIMs) have generated great attention because of their unique electromagnetic properties. The main challenge in current NIM development is creating a homogenous NIM without the need of complex geometric architectures consisting of capacitors and inductors or aggregated fillers, but possessing a tunable plasma frequency. A natural material that can exhibit negative values for permittivity and permeability simultaneously has not been found, or discovered. If one can design a negative dielectric constant material with a tunable plasma frequency of interest, imple- menting negative permeability into the material or system would be much more readily achievable to create a metama- terial. In this regard, a novel negative dielectric constant ma- terial, which is an essential key to creating the NIMs, was developed by doping ions into a polymer, a protonated pol- y(benzimidazole) (PBI). The doped PBI showed a negative dielectric constant at frequencies of kHz to MHz because of its reduced plasma frequency and an induction effect. As temperature increased, the dielectric spectrum changed from a relaxation to a resonance behavior and exhibited a larger magnitude of negative dielectric constant at a lower frequency. The conductivity of the doped PBI measured as a function of both temperature and frequency followed the same trend as the dielectric constant. With respect to the dielectric constant and the conductivity data, it can be assumed that the origin of the negative dielectric constant is attributed to the resonance behavior of the highly mobile ions at elevated temperatures and high frequencies. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: 2977–2985, 2012 Key words: high performance polymers; polymer synthesis and characterization; dielectric properties; metamaterials; resonance frequency INTRODUCTION Metamaterials or artificial Negative Index Materials (NIMs) are a new class of electromagnetic (EM) materials that have generated great attention over the last 10 years due to their unique electromagnetic (EM) properties. 1–5 Through the confirmed existence of negative refraction in resonant radio frequency (RF) structures with negative permittivity and per- meability yield, researchers with DOD and quite recently NASA are exploring many exciting applica- tions for these materials, such as lightweight, com- pact RF or microwave structures, as improved optics for imaging systems, wireless antenna suits, reduc- tion of energy/power beam dispersion, and also as EM cloaking devices, in which the material is used to render a volume effectively invisible to incident radiation. 6,7 NIMs are constructed with specially designed architecture and inclusions and exhibit a negative index of refraction, which is a property not found in any known naturally occurring material. The property of a metamaterial can be described by two effective parameters: electric permittivity (e eff ) and magnetic permeability (l eff ). Most dielectrics only have positive permittivities, e > 0. Metals will exhibit negative permittivity, e < 0 at optical fre- quencies, and plasmas exhibit negative permittivity values in certain frequency bands. 8 A natural mate- rial that can exhibit negative values for permittivity and permeability simultaneously has not been found, nor discovered. If one can design a negative dielectric constant material with a tunable plasma frequency of interest, implementing negative perme- ability into the material or system would be much more readily achievable to create a metamaterial. To date, to achieve a negative dielectric constant, two main approaches have been employed. 4,9 One approach involves the use of either a periodic struc- ture, whose frequency spectrum simulates the response of high pass filter, 10–12 or a waveguiding structure such as a hollow metallic waveguide. 13 Under this condition, EM waves are evanescent at low frequencies and this evanescence in the small fre- quency gap is described in terms of negative permit- tivity values below some specific frequency: the corner (or cutoff) frequency. The second approach involves the use of a composite containing metal Correspondence to: J. H. Kang (jin.h.kang@nasa.gov). Journal of Applied Polymer Science, Vol. 125, 2977–2985 (2012) V C 2012 Wiley Periodicals, Inc.