Summer 2019 Final Report
Hydosilation of (Poly)hydridomethylsiloxanes as an Approach to Mixed Surfaces
Silicon is arguably one of, if not the most influential elements of our generation; the myriad of applications associated with silicon chemistry and engineering lends credence to its versatility. If you need further proof of this, look down at your computer or smartphone. The defining feature of these electronic devices is their complex circuitry and processers, which are made possible by their silicon semiconductors.
Chemically speaking, silicon is unique in that it acts in a similar manner to carbon, the hallmark element of organic matter. However, silicon differs from carbon in that its dipole moment, or a measure of the element’s ability to pull electrons towards it in a molecule through a bond, is less than carbon. Because of this fact, hydrogen-bonded to silicon can act as an anion, or a negatively charged atom, and dissociate from silicon. This detail was the inspiration for our experiment. We wanted to see whether or not the siloxane polymer, hydridomethylsiloxane (HMS), could undergo hydrosilation and be used as a base for subsequent reactions.
The reaction mechanism for hydrosilation is rather straight forward. In the presence of heat, the hydrogen atoms in HMS can dissociate from the silicon atom and form diatomic hydrogen gas. At the same time as the dissociation, a vinyl group may donate its two electrons and attack the silicon atom. This, in turn, forms a new bond with the vinyl compound and our siloxane polymer. We first reacted our HMS with a silica substrate. Upon characterization with a goniometer and ellipsometer, we ran a second reaction between our new HMS surfaces and the desired vinyl compound. By comparing ellipsometric thicknesses and contact angles, we were able to discern whether or not a reaction had occurred. We also ran a specific type of spectroscopy called X-ray Photoelectric Spectroscopy (XPS) in order to visualize and quantify the relative abundances of elements on the surface of our samples.
With the exception of two reactions, all of our experiments yielded desirable results that supported our initial hypotheses. The silica samples all show an increase in surface thickness after each reaction. Our contact angle measurements also showed changes that reflected the indicative surface chemistry of the added compounds. For example, upon reacting styrene with our surfaces, we saw a clear increase in the surface thickness, as well as hydrophilic contact angles and an increased hysteresis. XPS also demonstrated a clear increase in the carbon to silicon ratios, thereby supporting our previous characterization.
Out of the twelve reactions we ran, three, in particular, gave us the most trouble: 4-vinyl pyridine, allylamine, and perflourooctene. We were unable to detect any nitrogen atoms on the surfaces of our samples with XPS. This leads us to believe that their high basicity may have denatured our surfaces. Additionally, the XPS analysis of our perflourooctene samples yielded fluorine atoms on some surfaces, but not others. Contact angles for these surfaces were also heterogeneous among the sample set.
One interesting aspect of our surfaces is that we can perform several reactions in order to fine-tune the chemistry to our liking. This would yield samples with the characteristics of both compounds, or a mixed surface. In order to demonstrate this, we reacted to our perflourooctene samples with ethyl vinyl ether. Another advantage of this reaction is that we can run experiments with mixed reaction vessels (i.e, a reaction between two different types of vinyl compounds). with ethyl vinyl ether and styrene. Analysis of these reactions appears to indicate a successful experiment due to a clear shift in contact angles, as well as an increase in ellipsometric thicknesses.
Although our work for this summer has concluded, we would like to further expand upon our research by refining the process of hydrosilation via mixed reaction vessels. At the current moment, our research has applications in a myriad of fields ranging from mechanical engineering and medical implant technology. As I reflect on my research experience this summer, I have come to realize that the most important skill I learned is patience. Research can be extremely rewarding, but also terribly monotonous. My newfound patience helped me to persevere through the repetitive analysis and stay motivated throughout the summer. Moreover, I have really enjoyed my summer project and hope that I may continue to research with other professors in the near future.