Ross Stefan Johnson

Thermally Induced Failure of Polymer Dielectrics

2010 WILEY-VCH Verlag Gmb Poly(p-phenylene vinylenes) (PPVs) are a class of conjugated polymers with applications ranging from light-emitting diodes to photovoltaics. To overcome the limited solubility of many PPV polymers, films are cast from the soluble precursor polymers; thermal treatment of the films then converts the material into its final conjugated state. We are interested in utilizing precursor PPV polymers for developing dielectric materials that can fail as a short by conjugating at specific temperatures. By employing a capacitor dielectric that can convert to a conductive state, the build-up and discharge of electricity in the event of a fire or overheating would be averted, providing a fundamental safety mechanism for high-voltage electrical devices (Fig. 1). We anticipated that the high dipole density of a halogen precursor polymer to poly[(2,3diphenyl-p-phenylene)vinylene] (DP-PPV) would allow the material to function as a good dielectric. At high temperatures, however, elimination of the halogen has been shown to irreversibly convert the polymer to a conjugated state. We suspected that after conjugation, the delocalized p-system of the polymer backbone would be sufficiently conductive to short out a capacitor. Here, we present the synthesis and characterization of two new DP-PPV precursor polymers that utilize different halogen leaving groups, which effectively thermo-switch the polymers to a conjugated state over a range of temperatures. Electrical characterization of the chloro precursor polymer indicates the material has good dielectric properties; however, once a preset temperature is reached, conjugation of the polymer backbone causes capacitor failure. Hsieh and co-workers have previously reported the synthesis and electroluminescent properties of DP-PPV converted from a chloro precursor polymer. Synthesis of intermediates 1, 2, and the chloro precursor polymer (Scheme 1) are based on these reports. The bromo monomer 3 was obtained by reaction of the diol 1 with thionyl bromide. The iodo monomer 4 was afforded utilizing a Finkelstein reaction with the chloro monomer 2. Polymerization of the three halogenated monomers was achieved by adding one equivalent of t-BuOK to a stirred solution of the monomer in anhydrous THF at 0 8C. The reactions were allowed to warm to rt over a 1 h time period. The bromo and iodo polymers were isolated by precipitation with methanol followed by centrifugation, which was found to limit material losses compared to filtering. It was noted that when stored under ambient light, the bromo polymer turned a deeper shade of yellow while the iodo polymer turned orange in color. A H NMR of the iodo polymer exposed to ambient light for 21 days indicated the halogen had eliminated (data not shown). Subsequent samples were stored in the absence of light, which was found to prevent elimination of the halogen. Thermogravimetric analysis (TGA) was performed to determine the temperatures at which the halogens eliminated. It was found that the onset of elimination occurred at 180 8C for the chloro polymer, 137 8C for the bromo polymer, and 90 8C for the iodo polymer, indicating the relative decrease in carbon-halogen bond strengths (Fig. 2a). It was estimated that the chloro polymer underwent a 12.3% mass loss (12.5% expected theoretically), the bromo polymer underwent a 21.4% mass loss (24.1% expected theoretically), and the iodo polymer underwent a 30.1%mass loss (33.5% expected theoretically), consistent with the loss of the corresponding halide (HX). The TGA results indicate the thermo-conversion temperature can be modulated by utilizing different stability leaving groups. To confirm the halogen elimination resulted in the polymers irreversible conversion to a conjugated state, UV-vis spectroscopy was performed on the three precursor polymers. Solutions were prepared in chloroform (10mg/mL) and deposited onto quartz slides. Excess sample was removed by tilting the slides to a vertical position while the sample was in contact with filter paper. The samples were air dried and UV–vis spectra were recorded

Photolithographic patterning of alkoxy substituted poly(p-phenylenevinylene)s from xanthate precursors

Conducting polymers are seeing ever-increased use in electronic and optoelectronic applications. While a variety of techniques are available to pattern conducting polymers, the demand for low cost, high throughput, and good spatial resolution continues to drive research efforts in this area. We have previously developed a method to pattern poly(p-phenylenevinylene) (PPV) using contact photolithography. Here, the synthesis, characterization, and photopatterning of alkoxy substituted PPVs (a much more commonly utilized derivative) is presented. Utilizing a photoacid generator, the polymer systems are demonstrated to pattern to one micron spatial resolution. The patterning process is demonstrated to have little effect on the polymers properties as the materials retain good optical characteristics and high conductivities upon doping.

Conjugated Polymer Patterning through Photooxidative Backbone Cleavage

Photolithographic patterning of a xanthate precursor to poly(3,4-diphenyl-2,5-thienylene vinylene) is described. Unlike xanthate precursors to poly(p-phenylene vinylene), the thienylene vinylene analogue patterns as a positive tone resist. Characterization of irradiated films reveals photooxidative cleavage of the vinylene linker decreases the molecular weight of the polymer (increasing the solubility of the UV-exposed areas). As a result of the mechanism, the developed pattern sees no UV light exposure, which is a significant advantage compared with negative-tone-conjugated polymer resists. Single micron resolution of a low-bandgap polymer is achieved in an efficient and scalable process.

Thermally-activated pentanol delivery from precursor poly(p-phenylenevinylene)s for MEMS lubrication.

The synthesis of two new polyphenylene vinylene (PPV) precursor polymers which can be thermally induced to eliminate pentanol is presented. Pentanol has recently been discovered to be a very useful lubricant in MicroElectroMechanical Systems. The utilization of the elimination reaction of precursor polymers to PPV as a small molecule delivery platform has, to the best of our knowledge, not been previously reported. The elimination reactions were examined using thermal gravimetric analysis, gas chromatography, and UV-Vis spectroscopy. Using PPV precursors allows for (1) a high loading of lubricant (one molecule per monomeric unit), (2) a platform that requires relatively high temperatures (>145 °C) to eliminate the lubricant, and (3) a non-volatile, mechanically and chemically stable by-product of the elimination reaction (PPV).