Thermo-switchable Polymer Dielectrics
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Thermo-switchable Polymer Dielectrics Ross S. Johnson, Fenil M. Kholwadwala, Shawn M. Dirk Organic Materials Department Sandia National Laboratories P.O. Box 5800, Albuquerque, New Mexico 87185 ABSTRACT We are interested in utilizing the thermo-switchable properties of precursor poly(pphenylene vinylene) (PPV) polymers to develop capacitor dielectrics that will fail at specific temperatures due to the material irreversibly switching from an insulator to a conducting polymer. By utilizing different leaving groups on the polymer main chain, the temperature at which the polymer transforms into a conductor can be varied over a range of temperatures. Electrical characterization of thin-film capacitors prepared from several precursor PPV polymers indicates that these materials have good dielectric properties until they reach elevated temperatures, at which point conjugation of the polymer backbone effectively disables the device. Here, we present the synthesis, dielectric processing, and electrical characterization of a new thermo-switchable polymer dielectric. INTRODUCTION Capacitors that have a built in safety mechanism may be useful for a variety of applications including hybrid and all electric vehicles. In the event of a crash of such a vehicle, a charged capacitor may represent a significant electrical hazard to first responders as a typical capacitor bank uses 1000 mF of capacitance at a voltage of up to 2000 V. We have previously reported that precursor polymers to poly[(2,3-diphenyl-p-phenylene)vinylene] (DP-PPV) can be utilized as capacitor dielectrics [1-3]. At high temperatures (> 200 °C), conjugation of the polymer backbone greatly increases electron flow through the capacitor causing a large (~90%) reduction in capacitance and a 75-fold increase in dissipation factor, effectively disabling the device from storing charge [1]. To gain a better understanding of how the chemical structure affects the dielectric properties of these types of polymers, a new chlorinated variant was synthesized, characterized, and its electrical properties were evaluated. EXPERIMENT Our synthetic route (scheme 1) was based upon previous literature reports [4-6] and full details will be published elsewhere. Commercially available benzimidazole was reacted with two equivalents of octyl bromide to produce the benzimidazolium salt 2 in 46% yield, which was used as a condensation catalyst in the following reaction. Coupling 4-chlorobenzaldehyde 3 in the presence of the condensation catalyst 2 under basic aqueous conditions followed by oxidation with iron trichloride produced 4,4’-dichlorobenzil 4 in 69% yield. The benzil derivative was then condensed with diethyl l,3-acetonedicarboxylate to afford the cyclopentanone derivative 5 in 64% yield. A Diels-Alder reaction with norbornadiene produced the terephthalate derivative 6 in 89% yield. Reduction of hindered diesters is typically carried out using LiAlH4; however, we obtained a complex mixture of products as a result of the partial reduction of the phenyl
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chlorines. Better results
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