March 21, 2017
In the past couple of months, we have continued to investigate the reaction of silica (SiO2) wafers and hexamethylcyclotrisiloxane (D3) through the vapor phase. To do this, silica wafers were exposed to D3 vapor at 100°C and 150°C and then analyzed by ellipsometry (thickness) and contact angles. To better understand the reaction, we have performed kinetics at both of these temperatures by reacting them at different time intervals ranging from 15 minutes up to a week. The results were not completely surprising, as thickness grows rapidly in the first few hours (See Figure 1) and settled to thicknesses of approximately 11 nm after 72 hours. Additionally, the contact angles reach a minimum hysteresis (difference in values) around the same time with advancing contact angles of 105° and receding contact angles of 100°. This suggests that reaction is near completion after 72 hours under these conditions. While the two graphs below do not show it, each data point is the average of at least 3 samples, or 21 measurements, and so have statistical significance.
Figure 1. Kinetics of the reaction between D3 vapor and silica interfaces at 100 °C. The right graph shows the increase in thickness measured by ellipsometry while the left graph shows changes in the advancing and receding contact angles.
The above results were important in helping to understand the mechanism of this reaction. First, the low contact angle hysteresis tells us that the surface is almost completely covered and very smooth, since hysteresis increases with partial coverage and roughness. The ellipsometry results were at first puzzling. Why should the surface grow to about 17 nm and then suddenly get smaller again? How could the surface made be larger than the starting molecule? However, this would make sense if the layer was actually the ring-opening polymerization (ROP) of D3. Therefore, it would keep growing straight up until it falls over and can’t react anymore. Figure 2 shows a proposed mechanism, which is consistent with our experimental results.
Figure 2. Proposed mechanism for the reaction between D3 vapor and silica surfaces
We decided to also test the kinetics of this reaction at 150 °C. It was expected that the reaction would reach completion much faster, since reactions have faster rates with increasing temperature. From the data in Figure 3, thickness and contact angles reach the same values as they did at 100 °C, but they do so much more quickly. When the reaction was run at 150 °C, completion was reached after 6 hours with an average thickness of 10.5-10.7 nm, proving what I had anticipated. The average contact angles also reflected that of the 100°C reaction, average advancing contact angle being 101.2° and average receding contact angle being 96.6°. What is interesting about these data is that there is no “bump” in the thickness. This means the reaction occurred too quickly for the chains to grow as high, and likely fall over very early in the reaction.
Figure 3. Kinetics of the reaction between D3 vapor and silica interfaces at 150 °C. The right graph shows the increase in thickness measured by ellipsometry while the left graph shows changes in the advancing and receding contact angles.
The work done so far was able to result in the determination of this mechanism, which I see as a huge success, and the only challenge I have faced recently is time management. In the future, I would like to investigate this reaction using different cyclic siloxanes. It is expected that the ring-strain (how much it wants to open) will have a direct effect on the surfaces prepared.
Additionally, I was able to participate in two conferences over the last few months. The first was the Annual Dyson Society of Fellows Meeting. The second was the national PittCon conference in Chicago. Both were very rewarding experiences. I love the feeling of being a part of a larger community that epitomizes hard work and dedication that both of these events portrayed. At the Dyson Society meeting, I was able to see other students work and it was really impressive to see what students can accomplish with the help of funding from this program. PittCon was the first event I ever attended that encompassed not just work from fellow students, but also in the professional field. At PittCon, I also got to talk to people who were genuinely interested in my reactions and how they could use them. The following is an image of the board I presented at PittCon- seeing my work all come together the way that it did was such a rewarding experience.