High temperature hybrid elastomers

Conventional high temperature elastomers are produced by chain polymerization of olefinic or fluorinated olefinic monomers. Ultimate thermal stabilities are limited bybackbone bond strengths, lower thermal stability of cross-link sites relative to backbone bonds, and depolymerization or "unzipping" at high temperatures. In order to develop elastomers with enhanced thermal stability, hybrid thermally cross-linkable polymers that consisted only of organic-inorganic and aromatic bonds were synthesized and evaluated. The addition of phenylethynyl or phenylacetylinic functional groups to these polymers resulted in conversion of the polymers into high temperature elastomers when crosslinked by thermal curing. Polyphenyoxydiphenylsilanes were synthesized via several different condensation reactions. Results of these synthetic reactions, which utilized both hydroquinone and biphenol as monomers, were systematically evaluated to determine the optimal synthetic conditions for subsequent endcapping reactions. It was determined thatdichlorodiphenylsilane condensations with biphenol in toluene or THF were best suited for this work. Use of excess dichlorodiphenylsilane yielded polymers of appropriate molecular weights with terminal reactive chlorosilane groups that could be utilized for coupling with phenylethynyl reagents in a subsequent reaction. Two new synthetic routes were developed to endcap biphenoxysilanes with ethynyl containing substituents, to yield polymers with cross-linkable end groups. Endcapping by lithiumphenylacetylide and 4[(4-fluorophenylethynyl)]phenol yielded two new polymers that could be thermally cross-linked on heating above 300 °C. Successful endcapping was verified chemically by 13C NMR, FTIR and Raman analysis. Exothermic peaks consistent with ethynyl curing reactions were observed in endcapped polymers by DSC. A new diacetylinic polymer was prepared through reaction of 4,4'-buta-1,3-diyne-1,4-diyldiphenol and dichlorodiphenylsilane. This aromatically substituted siloxanepolymer contained thermally cross-linkable diacetylene links in the backbone. FTIR, Raman, and 13C NMR analysis confirmed the diethynyl group was present in thepolymer. DSC analysis showed the polymer had a Tg of 130 °C, and a strong exothermic cure peak at 260 °C. TGA analysis in nitrogen showed a 5% weight loss temperature of 541 °C and a pyrolysis yield of 82% at 800°C. Parallel plate rheological testing confirmed the polymer cross-linked through monitoring of changes in viscosity during heating. After curing above 260 °C, the polymer vitrified, with no detectable Tg observed on subsequent DSC analyses. Curing at 210 °C for 30 minutes in nitrogen resulted in a partially cross-linked material that exhibited elastomeric properties above Tg. Curing under these conditions resulted in an estimated 25% degree of cross linking, and an increase in Tg to 146 °C. The activation energy of thermally initiated curing of the diacetylene groups was estimated to be 100 kJ/mol from DSC data using the Ozawa method.