Absorption and Humidity Sensing

Humidity sensors (like the one in your dryer!) are often made of polymer thin films that can absorb water. In this lab, we use what’s called a Quartz Crystal Microbalance (QCM) to determine how much water a particular polymer can absorb at a given film thickness and relative humidity level. As water adsorbs/absorbs into the polymer film, the mass of the polymer on the quartz crystal changes. This in turn changes the resonance frequency that quartz crystal vibrates at. By measuring the change in frequency, the students are able to calculate just how much water has been added. The absorption behavior can then be compared to various theories (Henry’s Law, the Langmuir equation, and Flory-Huggins) using an isotherm plot. Students also get to practice spin coating polymer films on to the quartz crystals, and try to determine how film thickness varies with rotation speed in the spin coater along the way.



Equipment Used

  • SRS Quartz Crystal Microbalance with SRSQCM software
  • Quartz crystals with plated electrodes
  • Flow cell with both wet and dry gas lines
  • Nitrogen tank and water bubbler
  • Spin coater

Materials Tested

  • Poly(vinylpyrrolidone) in acetonitrile solution (aka “PVP”)
  • Poly(ethylene oxide) in acetonitrile solution (aka “PEO”)