Figures of Merit for Electrically Conductive Polymer Composites
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Figures of Merit for Electrically Conductive Polymer Composites
Jaime C. Grunlan, William W. Gerberich, and Lorraine F. Francis Department of Chemical Engineering and Materials Science, University of Minnesota 151 Amundson Hall, 421 Washington Ave SE, Minneapolis, MN 55455, U.S.A. ABSTRACT In an effort to determine the optimal balance of electrical and mechanical performance for electrically conductive polymer composites, three figures of merit were evaluated. All three figures of merit displayed peaks and/or discontinuities at a particular filler loading. These loadings appear to correspond to the critical pigment volume concentration for a given system. Composite systems based upon latex as the matrix starting material showed peaks in the figures of merit at very low carbon black concentrations (10 vol%), while composites prepared with polymer solutions or melts had peaks above 20 vol% carbon black. These differences in behavior are attributed to differences in microstructural evolution that occur with filler loading. INTRODUCTION A figure of merit provides a measure of several properties, important to a given material, in one convenient number. This approach is especially useful for materials that need to balance two or more properties in order to be useful. Examples of materials using figures of merit include transparent conductive oxides [1], polymer-based light-emitting diodes [2], piezoelectric [3,4] and pyroelectric [5,6] sensors (or actuators). Another class of materials where property balancing is very important is electrically conductive polymer composites [7]. These materials are typically comprised of electrically conductive filler dispersed within an insulating polymer matrix through either melt [8] or solution [9] processing. Despite the need to balance various mechanical and electrical properties, no figures of merit have yet been proposed for these materials. In the following discussion, several figures of merit are proposed and examples are provided to evaluate their effectiveness. EXPERIMENTAL METHODS Carbon black (tradename Conductex 975 Ultra) was the conductive filler used in this study and was supplied by Columbian Chemicals Co. Latex, a polydisperse poly(vinyl acetate) (PVAc) (tradename Vinac XX210) supplied by Air Products Inc., and poly(N-vinylpyrrolidone) (PVP), a water-soluble polymer purchased from Aldrich provided the matrix starting materials for two of the composites examined here. Composites were prepared by mixing all of the ingredients in a high-speed impeller at 3600 rpm for 15 minutes. The composite preparation process in described more thoroughly elsewhere [10]. Other composite systems presented here were taken from the literature and will be referenced as they are presented. Electrical conductivity was evaluated using a Veeco FPP-5000 four-point probe system with a sensitivity range of 5.85x10-4 – 2.39x104 S/cm. Composite microstructures were imaged using Hitachi S-800 and S-900 field-emission gun, scanning electron microscopes (SEM’s). Break strengths (σbreak) and storage mod
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