Design of a Bulk Conductive Polymer Using Embedded Macroscopic Copper Cells

This paper introduces a technique of inducing bulk conductivity in a polymer. The technique uses coiled copper ‘cells’ embedded into a polymer during fabrication which can subsequently create highly redundant series-parallel networks. The preceding body of work aimed to improve the conductivity of non-conducting polymers by embedding particulates (of metal, carbon, etc.) into the polymer, or by altering the polymerization chemistry to incorporate conductive elements. The technique described here keeps the process independent of the specific polymer chosen by not relying on the polymerization chemistry to aid in the incorporation of the cells. The embedding drastically lowers the resistivity of the polymer, from 10 Ω-cm (approx.) for pure silicone rubber to less than 50 Ω-cm for the composite at room temperature: a drop of 12 orders of magnitude. A secondary consideration of this paper is the mechanical stiffness changes brought about by the embedding of metal inside a flexible polymer. Although the connected network of copper cells allows the rubber to be highly conductive in bulk, the cells are themselves compliant and thus have minimal effect on the stiffness of the cured silicone rubber. INTRODUCTION Conductive polymer composites attempt to merge the useful conductive properties of metals with mechanical properties of polymers. Polymers traditionally have high corrosion resistance, elasticity, and tensile strength. Polymer composites attempt to incorporate the high conductivity of metals into a polymer base matrix while retaining these useful properties. Also, polymers are relatively cheap and accessible; hence they are used in a wide variety of applications. Conductive polymer composites find uses in static discharge membranes covering device surfaces, electrical shielding barriers, and absorbers of electromagnetic radiation. Such polymers with especially high conductivity and sensitivity to loading forces can act as transducers, converting mechanical deflections to electrical signals through a change in conductivity. This paper develops the design of a polymerindependent internal skeleton (Fig. 1) to be embedded into a polymer (Fig. 2) in order to improve lower its resistivity. A. Chemical Doping Although most polymers are excellent dielectrics and find use as insulators, intrinsically conductive polymers do exist. A well-known example is Polyaniline, which has been the subject of considerable research since the 1980’s when MacDiarmid demonstrated that chemical redox reactions to doped parts of the polymer chain (with Emeraldine base) decreases the resistivity of the polymer significantly (from the 10 Ω-cm to 1 Ω-cm with 20% dopant)[1]. Fig. 1 A random packing of 300 cells Fig. 2 A polymer sample with 300 embedded cells