Hydrophobic Qualities of Cyclic Siloxanes

In this project, I am reacting cyclic siloxanes with inorganic oxide surfaces via ring opening polymerizations at solid-vapor interfaces. A siloxane is composed of silicon, oxygen, carbon, and hydrogen atoms. The skeleton of siloxane is made up of silicon and oxygen, and the other two bonds that silicon can make are each with a methyl group (CH3). Depending on the number of siloxane groups present in the cyclic molecule, it will have either high or low ring strain. Ring strain, or instability of the molecule, will exist when the angles in ring are less than 112°. The higher the ring strain, the more easily the ring will open and therefore be able to polymerize. Hexamethylcyclotrisiloxane (D3) is the cyclic siloxane I am using in this project, and it has highest ring strain of all cyclic siloxanes. As a result, I expect it to open easily and bond with the silicon surface. My intention is to investigate the effects of time and temperature on the reactions of D3 with silicon surfaces to create the most hydrophobic (water repelling) surfaces.

There are many surfaces that we come across in daily life that would be most effective if they were hydrophobic, such as windshields, surfaces of electronics, and cooking materials. In this reaction, I am using silicon as the solid that the siloxanes will bond to, but in principle, the polymerized siloxanes can be used to coat any inorganic oxide-containing surface. Some examples of this would be aluminum, tin, and nickel. An advantage of this technique is that it can create conformal films on textured surfaces. In other words, it bonds to the surface evenly and will maintain the texture of the surface. I’m initially hoping to find the best time and temperature to run the reaction at to create the most hydrophobic siloxane coating through polymerization of hexamethylcyclotrisiloxane. I then intend to see how using different cyclic siloxanes would change these variables and results. I expect to see significant effects from the time and temperature of the reaction on the quality of the hydrophobic surfaces I prepare.

In order to find the best parameters to run the reaction at, I am performing many trials of the same reaction, adjusting one variable at a time. In all of them, a silicon disc will be the source of the inorganic oxide surface and the siloxane will be in vapor phase during the reaction. I’m starting off with hexamethylcyclotrisiloxane as the source of the siloxane polymer and 100 °C as the temperature to run the reaction, as well as keeping the size of he silicon discs and quantity of the cyclic siloxane uniform. The variable first being altered is the length of time the reaction will run for, starting with 15 minutes and ending at 1 week. In order to measure the results, I’m using two different techniques: ellipsometry and dynamic contact angle analysis. Through ellipsometry, I can measure how thick a siloxane coating is, and through contact angle analysis I’m able to measure what the contact angles of a droplet of water are on the surface, both advancing and receding, to determine its hydrophobicity. Through these two techniques, I can gain much information about the nature of the chemistry of the surfaces I am analyzing.

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