Phenyl and Halophenyl Substituted Poly( p -phenylene vinylene)s for Capacitor Dielectrics
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Phenyl and Halophenyl Substituted Poly(p-phenylene vinylene)s for Capacitor 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 Conducting polymers have attracted attention in many areas of materials chemistry due to their tunability and wide range of applications [1,2]. We are interested in utilizing the thermoswitchable properties of precursor PPV polymers to develop capacitor dielectrics that will fail at specific temperatures due to the material irreversibly converting from an insulating to a conducting state [3,4]. Here, we report the synthesis and characterization of a new halophenyl substituted PPV polymer. We show that the precursor polymer has good dielectric properties over a range of temperatures, but will fail at high temperatures due to the polymer backbone conjugating. By utilizing thermo-switchable dielectrics in capacitors, the unintentional discharge of electricity in the event of a fire or overheating could be averted, providing a fundamental safety mechanism for high-voltage electronics. INTRODUCTION We have previously reported that halogen precursor polymers to poly[(2,3-diphenyl-pphenylene)vinylene] (DP-PPV) can be utilized as advanced capacitor dielectrics [3,4]. These materials were demonstrated to have good dielectric properties until they reached their thermoconversion temperature (dictated by the stability of the leaving group), at which point conjugation of the polymer backbone caused capacitor failure. Here, we wanted to determine what affect incorporating fluoro substitutents on the diphenyl rings would have on the dielectric properties as well as the thermo-conversion process. Due to the high electronegativity of fluorine, electron density should be withdrawn from the phenyl substituents and to a lesser extent the PPV core, potentially affecting the thermo-conversion temperature. Also, introducing a permanent dipole moment into the polymer structure could result in an increase in the dielectric constant [5-7], which would be advantageous for minimizing capacitor dimensions. EXPERIMENT The synthetic route utilized for obtaining the fluorophenyl-substituted precursor polymer (F-DP-PPV) was based on previous reports [8]; full details will be published elsewhere. A condensation reaction with 4,4’-difluorobenzil and diethyl 1,3-acetonedicarboxylate afforded the cyclopentanone derivative 1 in 81% yield. A Diels-Alder reaction was carried out with norbornadiene to produce the terephthalate derivative 2 in 90% yield. The diester was reduced by refluxing in LiAlH4 for 7 h, affording the diol 3 in 81% yield. The monomer 4 was obtained in quantitative yield after stirring 3 in thionyl chloride for 16 h. Polymerization was achieved by adding 1 equivalent of t-BuOK to a stirred solution of the monomer, producing the precursor
polymer 5 as a pale yellow solid in good yield. The conjugated polymer 6 could be obtained after heating the precursor polymer to high tem
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