Publications

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Sandia's scientific and engineering expertise in the fields of computational biology, high-performance prosthetic limbs, biodetection, and bioinformatics has been applied to specific problems at the forefront of cancer research. Molecular modeling was employed to design stable mutations of the enzyme L-asparaginase with improved selectivity for asparagine over other amino acids with the potential for improved cancer chemotherapy. New electrospun polymer composites with improved electrical conductivity and mechanical compliance have been demonstrated with the promise of direct interfacing between the peripheral nervous system and the control electronics of advanced prosthetics. The capture of rare circulating tumor cells has been demonstrated on a microfluidic chip produced with a versatile fabrication processes capable of integration with existing lab-on-a-chip and biosensor technology. And software tools have been developed to increase the calculation speed of clustered heat maps for the display of relationships in large arrays of protein data. All these projects were carried out in collaboration with researchers at the University of Texas M. D. Anderson Cancer Center in Houston, TX.

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We are interested in utilizing the thermo-switchable properties of precursor poly(p-phenylene 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.

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Conjugated polymers such as poly(p-phenylenevinylene) (PPV) have attracted a great deal of attention due to their optoelectronic properties. The ability to control the lateral spatial resolution of conjugated polymers will allow for improved integration into electronic devices. Here, we present a method for photo-patterning xanthate precursor polymers leading to micron scale spatial control of conjugated poly(p-phenylenevinylene). Our photolithographic process is simple and direct, and should be amenable to a range of other xanthate or dithiocarbamate precursor PPV polymers.

The development of functionalized polymer dielectrics based on poly(norbornene) and poly(PhONDI) (PhONDI = N-phenyl-7-oxanorbornene-5,6-dicarboximide) is presented. Functionalization of the polymer backbones by the thiol-ene reaction was examined to determine if thiol addition improved dielectric properties. Poly(norbornene) was not amenable to functionalization due to the propensity to crosslink under the reaction conditions studied. Poly(PhONDI) could be successfully functionalized, and the functionalized polymer was found to have increased breakdown strength as well as improved solution stability. Initial studies on the development of thiol-functionalized silica/poly(PhONDI) nanocomposites and their dielectric properties will also be discussed.

Novel low loss photopatternable matrix materials for IR metamaterial applications were synthesized using the ring opening metathesis polymerization reaction (ROMP) of norbornene followed by a partial hydrogenation to remove most of the IR absorbing olefin groups which absorb in the 8-12 {micro}m range. Photopatterning was achieved via crosslinking of the remaining olefin groups with alpha, omega-dithiols via the thiol-ene coupling reaction. Since ROMP is a living polymerization the molecular weight of the polymer can be controlled simply by varying the ratio of catalyst to monomer. In order to determine the optimum photopattenable IR matrix material we varied the amount of olefin remaining after the partial hydrogenation. Hydrogenation was accomplished using tosyl hydrazide. The degree of hydrogenation can be controlled by altering the reaction time or reaction stoichiometry and the by-products can be easily removed during workup by precipitation into ethanol. Several polymers have been prepared using this reduction scheme including two polymers which had 54% and 68% olefin remaining. Free standing films (approx. 12 {micro}m) were prepared from the 68% olefin material using draw-down technique and subsequently irradiated with a UV lamp (365 nm) for thirty minutes to induce crosslinking via thiol-ene reaction. After crosslinking, the olefin IR-absorption band disappeared and the Tg of the matrix material increased; both desirable properties for IR metamaterial applications. The polymer system has inherent photopatternable behavior primarily because of solubility differences between the pre-polymer and cross-linked matrix. Photopatterned structures using the 54% as well as the 68% olefin material were easily obtained. The synthesis, processing, and IR absorption data and the ramifications to dielectric metamaterials will be discussed.

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