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Company USA
The company have been collaborating on projects for nanomaterial synthesis and applications, particularly using layer-by-layer deposition.
Low energy, or ultrahydrophobic, surfaces require both (1) micrometer-scale surface roughness, creating cavities that trap air between the surface and the water and (2) hydrophobic properties on the nanoscale.
Depositing silica nanoparticles at the surface provides the required roughness. Ideally, several layers of silica nanoparticles would be deposited alternating with polyelectrolytes that will improve adhesion. In this layer-by-layer deposition, the charged polyelectrolyte is attracted to the surface and then the silica nanoparticles are attracted to the polyelectrolyte. By depositing these materials in alternating layers, a film is built up of the desired thickness. The company have applied such films by immersion or by spraying of aqueous solutions. Hydrophobic properties are provided by exposing the coatings to hydrophobic alkylsilane molecules.
The ultrahydrophobic coatings are an early stage research concept. They have had several discussions about layer-by-layer films for other purposes and are working with a commercial partner for a specific application of LBL films, but have not explored commercialization of hydrophobic films.
These films are applied from aqueous solution (generally detergent-free) and provide a low-energy surface. The LBL method has been used to form films on various hard surfaces, including silica, metal oxide, and some plastics.
To realize ultrahydrophobicity requires a surface with both (1) micrometer-scale surface roughness, creating cavities that trap air between the surface and the water and (2) hydrophobic properties on the nanoscale. In the company’s concept, the micrometer-scale surface roughness will be provided by silica nanoparticles while the hydrophobicity is provided by alkylsilane molecules bonded to the nanoparticles.
Layer-by-layer (LBL) deposition has been used to prepare coatings derived from a variety of oppositely charged polymers (polyelectrolytes) and other species on various substrate materials. This assembly technique involves the adsorption of charged species from solution onto an oppositely charged substrate. For example, the assembly of an LBL film on a negatively charged substrate is a cyclic repetition of immersing the substrate in a solution of positively charged species, rinsing, immersion in a solution of negatively charged species, and rinsing. In each immersion, a (mono) layer of the species adsorbs to the substrate, while the rinse step removes the excess. Subsequent repetitive immersions of the growing film into alternating charged solutions allow the build-up of a thicker LBL assembled multilayer coating system.
LBL assembly is a flexible technique. The deposited species can be polyelectrolytes, nanometre size sheets of clay, nanoparticles, proteins, dyes, or DNA. The substrate can be glass slides, mica sheets, cellulose acetate, aluminium, or polymers such as polyethylene terephthalate, polyalkene, siloxanes, polyacrylonitrile, and teflon. The substrate can be exposed to the charged species by immersion in a solution, spraying the solution onto the substrate, or sequentially filling and draining a tank, which may be the substrate or hold the substrate. The LBL method is not limited to flat substrates for successful deposition. The company have applied this method to the manufacture of composite structures on both flat surfaces and surfaces with complex structures.
The LBL process also provides opportunities to produce ultra thin coatings with multiple functions. By changing the materials deposited in different layers, films with graded properties are created. For example, initial layers can be chosen for high adhesion or mechanical strength followed by the hydrophobic portion of the film.
The company have developed standard strategies to prepare multi-component thin films on substrates via the layer-by-layer method. For the initial tests, LBL films with the desired thickness will be formed on solid substrates (glass slides or cellulose acetate film) by sequentially depositing oppositely charged polyelectrolytes, i.e., poly(diallyldimethylammonium chloride) (PDDA) or poly(ally amine hydrochloride) (PAH), and spherical silica nanoparticles.
The silica nanoparticles increase both the roughness and asperities of the film, as needed for an ultrahydrophobic surface. The size of the silica nanoparticles (5-5000 nm), concentration of SiO2 nanoparticles, deposition time, and salt concentration will be varied to adjust the density of the asperities. Finally, the films will be immersed in a solution of long-chain alkysilane, e.g., octadecyltriethoxysilane. This will form a self-assembled monolayer on the SiO2 nanoparticles, providing the film with hydrophobicity.
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