Thermal Stability Enhancement of Rubbery Ormosils
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(carboranes) for methyl side-groups of PDMS or directly incorporating these species within the Si-O-Si backbone [17-28]. A couple of patents refer to stabilization effects of certain diphenylamine antioxidants [29]. However, some of the most effective, commonly used additives are iron and iron-containing compounds, especially Fe 203 [29-34]. In this study one of each of the above polydimethylsiloxane stabilization methods was enlisted in syntheses of rubbery ORMOSILs. In separate ORMOSIL systems, polydimethyldiphenyl-siloxane (PDMDPS) was substituted for PDMS, a diphenyl amine antioxidant was added, and an iron addition was made. Qualitative evaluations of these modified materials as compared to the original ORMOSIL and a commercially available organic rubber (Styrenebutadiene rubber, SBR, Goodyear Rubber #7712) were made through resilience (bounce height) measurements after various heat treatments. Thermal analyses were also performed on all materials. EXPERIMENTAL
Starting Materials Silanol-terminal silicone polymers polydimethylsiloxane (PDMS; MWavg = 1750) and polydimethyl(0.84)-diphenyl(.0.16)siloxane (PDMDPS; q = 500 cs) were obtained from Huls America. Aldrich Chemical supplied tetraethoxysilane (TEOS) of purity 98%. Hydrochloric acid (HCl; 12.1 M) of reagent grade, water of HPLC grade, and isopropanol (iproh) and tetrahydrofuran (THF) of reagent grade were obtained from Fisher Scientific, while Fe20 3 (300 mesh) and FeC13 came from Johnson Matthey. Uniroyal Chemical kindly donated the antioxidant Naugard 445 (4,4'-Bis(a,a-dimethylbenzyl)diphenylamine). All chemicals were used as received with no further purification. Synthesis The production of rubbery ORMOSILs was based upon work by Chung and Hu [18]. In an Erlenmeyer flask, TEOS and polymer (PDMS or PDMDPS) were stirred with solvents of THF and isopropanol in respective molar ratios to TEOS of 1 : 0.082 : 0.80 : 2.27. In a separate flask, water, HCI, and isopropanol were mixed in molar ratios to TEOS of 3.00, 0.30, and 1.36, respectively. This corresponded, in a typical reaction batch, to 30 g TEOS, 20 g PDMS, 15 ml i-proh, and 10 ml THF in the first container and 25 ml i-proh, 4.89 ml H20 and 3.6 ml HC1 (38%) in the second. These two mixtures were then poured together into a boiling flask equipped with a thermometer and an 18" reflux column, then refluxed under moderate stirring at 80°C for 25 min. In the case of PDMS-containing sol, division into four equal weight portions was made. One was left alone while the other three received 1 wt% of Naugard 445, or iron oxide or iron chloride under vigorous stirring. All solutions were cast into polyethylene, snap-cap vials and allowed to gel. PDMScontaining solutions gelled in a matter of hours The PDMDPS-containing sol, however, required heating in an oven at 60'C for 12 hrs. to obtain gelation in a reasonable amount of time. In these samples, pinhole ventilation was necessary to prevent dangerous pressure build-up. After allowing the gels to age and undergo syneresis (shrinkage with subsequent expulsion of
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