Carborane-Siloxane-Diacetylene Thermosets and Ceramic Precursors
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Figure 1: Chemical structures of repeat unit for DaSCS and DaS. 90/10 is a random copolymer of 90 % DaS and 10 % DaSCS. heating to 100IC, weighed into mold, degassed for 2 hours under vacuum at 110°C and then cured in a tube furnace under argon sequentially for 2 hours at 150'C, 5 hours at 200'C, 5 hours at 250'C, and 12 hours at 300'C. Some samples were postcured/pyrolyzed 6 hours in argon at 400°C, 500°C, 600'C, and 800'C. After initial cure samples were sanded to remove menicus with 280, 340, 400, and 600 grit papers. Samples were stored under desiccant. Cure was followed with infrared spectra obtained using a Nicolet Magna 750 FTIR spectrophotometer. Thermosets cured to 300'C were characterized using dynamic mechanical measurements on a Bohlin VOR rheometer using rectangular specimens in torsion. Measurements were made at 10 hertz and a heating rate of 4°C/minute. Glass transtions were obtained from the maximum of the loss spectra. Moduli, strengths and failure strains were measured in flexure on an Instron 1135 tensile tester according to ASTM D790 with a 16:1 spanto-thickness ratio. Further annealing to 800NC was followed with density, modulus, and char yield measurements. Densities were measured by a bouyancy technique (ASTM D792). Moduli were measured at room temperature in a flexure geometry using a Dynastat mechanical tester at 10 hertz. Thermogravimetric analyses were carried out on roughly 10 mg solid samples under dry argon or air using a Perkin Elmer TGA7. A Perkin Elmer DSC7 was used for calorimetric measurements. Scan rates were I00 C/min under a nitrogen purge. Thermooxidative aging was carried out in a Blue M circulating oven for 100 hour dwells at 316'C and 343°C. Wide angle xray measurements were obtained using a Rigaku RU200 12kW rotating anode generator with a Cu target (CuKa radiation). Solid samples were used. Data was collected in the 0/20 geometry on scans from 10 to 90 degrees.
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RESULTS IR Characterization of Cure DSC scans indicate a well defined broad cure exotherm beginning around 170'C and with a maximum of 295°C for DaS. Addition of the carborane to the backbone shifts the cure exotherm to higher temperatures by about 10C for 90/10 and by about 50'C for DaSCS. The cure cycle was designed so that reaction would occur slowly at the 200NC and 250'C dwells and be completed during the dwell at 300°C. IR spectra clearly show the acetylene stretching bands (2071 cm') in the uncured polymer. After 5 hours at 250'C these bands have mostly disappeared in all polymers. After the 300'C dwell the bands are absent indicating complete reaction. Weak bands at 2142 and 1886 cm' appear as the acetylene groups react, indicative of the ene-yne and 0 butatriene structures, respectively.6 After a six hour dwell at 400 C under argon these peaks disappear suggesting a loss of this structure. In analogous poly[(dimethylsilylene)-diacetylenes], Corriu et al through 13C NMR studies see the loss of the ene-yne and butatriene structures 6 at 2 400 0C and note the shift from sp to sp carbon or from polyene to
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