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No description available.

Polysiloxane networks have excellent oxidative and good UV environmental stability, flexibility at low temperatures, and thermal stability at higher temperatures. This wide service temperature range makes these materials a candidate class of materials for high performance adhesives and sealants, and in particular for applications on high speed aircraft. Polar polysiloxane networks were prepared with cyanopropyl substituents to lower any propensity for the materials to swell in hydrocarbon fuels and to improve adhesion to metal substrates. 1,3,5,7-tetramethyl,1,3,5,7-tetrahydrocyclotetrasiloxane (D4H) was hydrosilated with allyl cyanide to yield the corresponding 3-cyanopropylmethylcyclotetrasiloxane monomer (D4CN). Controlled molecular weight oligomers with vinyl termination were prepared in equilibrium reactions using a basic catalyst. These oligomers were then crosslinked with various hydride functional crosslinking reagents to yield model networks for mechanical and adhesion studies. The network properties of nonpolar polydimethylsiloxane (PDMS), polar poly[methyl(3,3,3-trifluoropropyl)siloxane] (PMTFPS), and the novel polar poly(3-cyanopropylmethyl-siloxane) (PCPMS) were investigated as a function of sidechain chemical structures. Effects of increasing crosslink density were investigated for the PDMS networks by adding a difunctional siloxane dimer with vinyl groups. Moduli and tensile strengths increased while percent elongation decreased as the crosslink density was increased. All networks were thermally stable above 300 °C in both air and N2 (when heated at a rate of 10 °C/min.) and exhibited Tgs lower than -55 °C. The polar networks swelled to a much lesser extent (at least one order of magnitude) than the nonpolar networks in hydrocarbons and Jet fuel. Cohesive failure was observed for the polar networks via metal to metal (Al foil substrate to Al and Ti adherends) 180° peel test. The PCPMS elastomers had average load values twice those of the PDMS networks independent of crosslink density. / Master of Science

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No description available.

No description available.

No description available.

No description available.

The scope of this research entailed the synthesis of novel polyorganosiloxanes with pendent phosphine, phosphine oxide, nitrile and carboxylic acid moieties. Such polysiloxanes were prepared with controlled concentrations of both the polar moieties and hydrido or vinyl pendent crosslinkable sites to afford precursor materials for well-defined networks. The intention was to generate stable microcomposite dispersions with very high concentrations of polar thermally conductive fillers. Lightly crosslinked elastomeric networks with controlled amounts of polar moieties were prepared via a hydrosilation curing mechanism. High concentrations of thermally conductive micro-fillers were dispersed throughout the resins and the microcomposites were investigated as thermally conductive adhesives. Random polysiloxane copolymers containing controlled number average molecular weights (Mns) and compositions with systematically varied concentrations of hydridomethylsiloxy- or vinylmethylsiloxy- units were prepared via ring-opening equilibrations of cyclosiloxane tetramers. These precursors were functionalized with precise concentrations of polar pendent moieties via hydrosilation (nitrile) or free radical addition reactions (phosphine and carboxylic acids). Valuable additions to the family of polysiloxanes were prepared by oxidizing the phosphine moieties to form phosphine oxide containing polysiloxanes. Defined concentrations of residual hydrido- or vinyl- reactive sites were crosslinked via hydrosilation to yield elastomeric adhesives. Specific interactions between the nitrile and phosphine oxide substituted polysiloxanes and the acidic proton of chloroform were shown using 1H NMR. The magnitude of the shift for the deshielded chloroform proton increased with the degree of hydrogen bonding, and was larger for the phosphine oxide species. The polar polysiloxane resins were filled with high concentrations of thermally conductive fillers including silica-coated AlN, Al spheres, BN and Ag flake, then hydrosilated to form microcomposite networks. Microcomposite adhesive strengths, thermal properties (glass transition temperature (Tg) and high temperature stability), and thermal conductivities were studied. An unfilled polysiloxane network containing only 15 mole percent phosphine oxide exhibited a dramatic improvement (46 N/m) in adhesive strength to Al adherends relative to a control polydimethylsiloxane network (2.5 N/m). Importantly, stable polysiloxane micro-dispersions were obtained with up to 67 volume percent (86 weight percent) silica-coated AlN. TEM data confirmed the dispersion homogeneity and XPS demonstrated that the particle surfaces were well-coated with the functionalized polysiloxanes. A microcomposite comprised of 67 volume percent silica-coated AlN and a polysiloxane containing only 9 molar percent nitrile groups had a thermal conductivity of 1.42 W/mK. The glass transition temperatures of the microcomposites were controlled by the amounts of polar functional moieties on the resins and the network crosslink densities. All of the microcomposites exhibited Tgs lower than -44°C and the materials remained stable in dynamic TGA measurements to approximately 400°C in both air and nitrogen. / Ph. D.

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