One of the first control experiments I did for this project was to evaluate the swelling of polyamide thin film in DI water and other background solutions. One easy way to do so is the AFM scratch test. Here we basically assume that the swelling in x-y plane is negligible compared to that in the z-direction. A scratch is made on the thin film with the blunt end of a tweezer and the height of the step if measured over time.
In the picture, the grey mud-like thing that you see surrounding the silica wafer is hardened epoxy glue. I used epoxy since a double-sided adhesive tape failed to keep the silica wafer in place over time (in aqeuous solution). This was an initial experiment...hence the overflow of epoxy!
In the picture, the grey mud-like thing that you see surrounding the silica wafer is hardened epoxy glue. I used epoxy since a double-sided adhesive tape failed to keep the silica wafer in place over time (in aqeuous solution). This was an initial experiment...hence the overflow of epoxy!
I adopted a polyamide synthesis method originally developed by Dr. Stafford's group at NIST (Johnson et. al. 2012). This involves molecular layer-by-layer (mLBL) deposition of the monomers trimesoyl chloride (TMC) and m-phenylenediamine (MPD). The generated mLBL film is ultrathin and ultra smooth that allows resolving chemical interactions of polyamide without the confounding effects of surface roughness.
The protocol requires spin-coating the monomer solutions on QCM sensor, which NIST does using an automated system. I of course had to improvise the antiquated spin-coater that lay in an unassuming corner of a Chemical Engineering Lab (I am from Civil and Environmental Engg. btw!). As it was not automated, synthesizing a polyamide film of desired thickness took about an hour and I was synthesizing 3-4 QCM sensors at one go. After following the protocol to the tee, I still wasn't able to replicate the roughness and thickness reported in literature. ..So I did a diagnostic
I identified potential causes for such deviation:
1. The monomers (TMC and MPD) are hygroscopic, meaning they absorb water from atmosphere. Even though the reagents/solutions were stored/prepared in a glove box, the spin-coating (which took several hours) was done in ambient conditions - possible route of moisture exposure!
2. COVID 19 hit and the labs were shut down for almost 6 months. When I finally got back to lab in July 2020, I discovered the glove box had been shut down/stopped in the interim so my monomers might have degraded.
I optimized my operational protocol using capped vials and using solvent-compatible syringes to draw the solutions out of the vials (shown in picture). We ordered fresh chemicals, used molecular sieves for absorbing any dissolved water, etc.
The result was the polymer of my dreams : thin and smooth :D
The protocol requires spin-coating the monomer solutions on QCM sensor, which NIST does using an automated system. I of course had to improvise the antiquated spin-coater that lay in an unassuming corner of a Chemical Engineering Lab (I am from Civil and Environmental Engg. btw!). As it was not automated, synthesizing a polyamide film of desired thickness took about an hour and I was synthesizing 3-4 QCM sensors at one go. After following the protocol to the tee, I still wasn't able to replicate the roughness and thickness reported in literature. ..So I did a diagnostic
I identified potential causes for such deviation:
1. The monomers (TMC and MPD) are hygroscopic, meaning they absorb water from atmosphere. Even though the reagents/solutions were stored/prepared in a glove box, the spin-coating (which took several hours) was done in ambient conditions - possible route of moisture exposure!
2. COVID 19 hit and the labs were shut down for almost 6 months. When I finally got back to lab in July 2020, I discovered the glove box had been shut down/stopped in the interim so my monomers might have degraded.
I optimized my operational protocol using capped vials and using solvent-compatible syringes to draw the solutions out of the vials (shown in picture). We ordered fresh chemicals, used molecular sieves for absorbing any dissolved water, etc.
The result was the polymer of my dreams : thin and smooth :D
