Parametric Control of Melting Gel Morphology and Chemistry via Electrospray Deposition

Melting gels are a class of hybrid organic-inorganic, silica-based sol-gels which are solid below their glass transition temperatures, near room temperature, but show thermoplastic behavior when heated. While this phase change can be repeated multiple times, heating the gel past its consolidation temperature, typically above 130 °C, initiates an irreversible reaction that produces highly crosslinked glassy organic/inorganic materials via hydrolysis and polycondensation. This ability makes melting gels uniquely compatible with processing techniques inaccessible to other sol-gels. By properly tuning their properties, it should be possible to create protective coatings for electronics and anti-corrosive coatings for metals that are highly hydrophobic and insulating. However, melting gel consolidation reactions are highly dependent on charge interactions, raising the question of how these materials will respond to a processing technique, like electrospray deposition (ESD), which is dependent on charge delivery. In this study, we focus on the role that substrate temperature and charge polarity play on film morphology, consolidation chemistry, and surface properties when processing via ESD. Optical images, film thickness measurements, and FTIR were used to characterize the sprayed melting gel with the goal of developing a robust processing space for producing highly cross linked, hydrophobic, dielectric coatings.

29Si NMR and SAXS investigation of the hybrid organic–inorganic glasses obtained by consolidation of the melting gels

This study is focused on structural characterization of hybrid glasses obtained by consolidation of melting gels.

Surfaces

Because of the great importance of the surface properties of the polysiloxanes, this topic is treated separately in this chapter. Hydrophobic polysiloxanes having simple aliphatic or aromatic side groups have surfaces that show essentially no attraction to water. In fact, polysiloxanes can serve as water repellants. This property is very useful for applications such as protective coatings on historical monuments and for controlling the surfaces of other polymers, sensors, and quantum dots. Hydrophobic surfaces can be readily regenerated if the surface becomes damaged. Regeneration occurs by rearrangements of the polysiloxane chains so that the hydrophobic methyl groups are once again covering the surface. The flexibility of the siloxane chain backbone facilitates this process. It is also possible to prepare hydrophobic films using methyl-modified siloxane melting gels. Glass surfaces or wool fibers can be coated with polydimethylsiloxane (PDMS) to make them more hydrophobic. In some cases, it is necessary to modify a polysiloxane surface to make it hydrophilic or hydrophobic. Hydrophobization is one aspect of the general topic of modifying and managing the properties of polymer surfaces. An important example involves soft contact lenses that contain PDMS, which is often used because of its very high permeability to oxygen, which is required for metabolic processes within the eye. Such lenses do not feel comfortable however because they do not float properly on the aqueous tears that coat the eye. There are a number of ways to modify the surfaces. There is even a way to make “unreactive” silicones react with inorganic surfaces. In some applications it is useful to have hydrophilicity in the bulk of the polymer instead of just at the surface. One way of doing this is by simultaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains and hydrophobic PDMS chains. Another way is to make a PDMS network with a trifunctional organosilane R’Si(OR) end linker that contains a hydrophilic R’ side chain, such as a polyoxide. Treating only the surfaces is another possibility, for example, by adding hydrophilic brushes by vapor deposition/hydrolysis cycles. Such hydrophilic polysiloxanes can also serve as surfactants.

Silica-Containing Hybrid Nanocomposite "Melting Gels"

Hybrid organic-inorganic nanocomposites can be prepared by the sol-gel process. An attribute of the sol-gel process is that organic and inorganic components can be linked together on the nanoscale through chemical reactions at room temperature. Functionalized siloxanes can be mixed and reacted to form nanocomposites with a wide range of mechanical, optical and dielectric properties.

Melting Gel Films for Low Temperature Seals

ABSTRACTMelting gels are silica-based hybrid gels with the curious behavior that they are rigid at room temperature, but soften around 110°C. 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, G″(t,ω0) that is larger than the elastic modulus, G′(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.