Melting Gel Films for Low Temperature Seals
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Melting Gel Films for Low Temperature Seals Mihaela Jitianu1, Andrei Jitianu2*, Michael Stamper1, Doreen Aboagye2, Lisa C. Klein3 1
Department of Chemistry, William Paterson University, 300 Pompton Road, Wayne, New Jersey 07470 2 Department of Chemistry, Lehman College, CUNY, Davis Hall, 250 Bedford Park Boulevard West, Bronx, New York 10468 3 Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854 ABSTRACT Melting gels are silica-based hybrid gels with the curious behavior that they are rigid at room temperature, but soften around 110ÛC. A typical melting gel is prepared by mixing methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DMDES). MTES has one methyl group substituted for an ethoxy, and DMDES has two substitutions. The methyl groups do not hydrolyze, which limits the network-forming capability of the precursors. To gain insight into the molecular structure of the melting gels, differential scanning calorimetry and oscillatory rheometry studies were performed on melting gels before consolidation. According to oscillatory rheometry, at room temperature, the gels behave as viscous fluids, with a viscous modulus, GƎ(t,ω0) that is larger than the elastic modulus, Gƍ(t,ω0). As the temperature is decreased, gels continue to behave as viscous fluids, with both moduli increasing with decreasing temperature. At some point, the moduli cross over, and this temperature is recorded as the glass transition temperature Tg. The Tg values obtained from both methods are in excellent agreement. The Tg decreases from -0.3oC to -56oC with an increase in the amount of di-substituted siloxane (DMDES) from 30 to 50 mole %. A decrease of the Tg follows an increase of the number of hydrolytically stable groups, meaning a decrease in the number of oxygen bridges between siloxane chains. INTRODUCTION Hybrid materials are studied primarily for their ability to combine the properties of a high temperature inorganic material with low temperature processing. Silica-containing hybrid organic-inorganic gels can be used, for example, for films and coatings, due to the fact that the inorganic silica backbone provides hardness and stability, while the organic components provide flexibility, low temperature of processing and hydrophobicity [1,2]. The phenomenon of softening in hybrid silica-based gels first was reported by Matsuda et al. [3], who observed this behavior in the poly(benzylsilsesquioxane) system. He named these gels “melting gels”. These hybrid gels differ from classical hybrids, or “ormosils”, in that hybrid melting gels are solid at room temperature, become fluid at a modest temperature T1 (~ 110oC), return to the rigid condition at room temperature, and can be cycled through softening and becoming rigid many
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times. However, after consolidation at a temperature T2 (T2>T1), the gels no longer soften. The consolidation temperature T2 corresponds to cross-linking of the silica chains [4, 5]. Our previous studies investigated the preparation and properties of mel
