Morphological Control of Multifunctional Melting Gel Coatings via Electrospray Deposition

The ability to alter surface interactions with controlled coatings is a fundamental area in materials processing. Oligomeric silsesquioxanes (OSS), or melting gels (MG), are a class of hybrid gels comprised of an inorganic 3D network with grafted functional organic components. They are synthesized via the sol-gel process, which refers to the initiation of an inorganic network from a precursor solution. Melting gels are typically produced through the hydrolysis and polymerization of their alkoxide type precursors such as tetraethyl orthosilicate (TEOS) and mono- or di-substituted siloxanes. The final product is a hybrid gel with nanoscale mixing whose bulk properties are determined by the initial mixing ratios of its components. The reversible “melting” at around 110℃ is a result of the incomplete hydrolyzing/condensation of the substitute siloxane; it is a softening process that occurs by sufficiently exceeding the gel’s glass transition temperature. When gels are heat treated at or above their consolidation point (130℃ - 170℃), they undergo further cross-linking to form organically modified silica glasses. Owing to their hybrid nature, melting gels leverage the benefits of both their components: their coatings are dense, crack-free, and hermetic with low temperature processing. By appropriately tuning surface properties, these glass sprays can be used as protective coatings in electronics and anti-corrosion coatings in metals. Electrospray deposition (ESD) presents itself as a useful MG coating technique because it potentially offers dynamic control over the consolidated gel structure down to the micro- and nanoscale. ESD uses high voltages to produce charged, monodisperse droplets from a solution, and when using low solid contents, it uniformly delivers small amounts of melting gel at a continuous rate. Recent work has also demonstrated ESD’s ability to coat 3D objects due to its self-limiting nature when spraying insulating materials. In order to gain a fundamental understanding of this melting gel deposition technique, the interaction between charged electrospray droplets and simultaneous consolidation reactions must be investigated. ESD was used to spray dilute solutions of 1 wt% methyl- and phenyl-substituted melting gels in 2-butanone onto silicon. Parameters such as the pH of melting gel synthesis, solution viscosity, and spray polarity can be varied to alter and study the effects of charge injection on the consolidation of melting gels into hybrid glasses. Furthermore, the precise effects of charge injection can be isolated by comparing spun and sprayed films. Optical images, film thickness measurements, nanoindentation, FT-IR, and goniometry were used to evaluate and demonstrate the effects of these parameters on both the physical morphology along with the chemical structure of final coatings.