Sorption Kinetics of Poly(ethyleneimine)–Poly(ethylene Oxide) Blends and the Implication for Low-Cost, Small-Scale CO2 Sensors

Public health depends on reliable, low-cost methods of measuring indoor air quality (IAQ), and carbon dioxide (CO2) is often used as a surrogate measure for IAQ. Thus, there is an increasing interest in developing inexpensive CO2 sensors that can be seamlessly integrated into existing ventilation systems for smart and connected healthy buildings. To this aim, microelectromechanical system-based (MEMS-based) resonant mass sensors functionalized with specific surface chemistries are a promising sensing platform because of their compact size, low cost, and fast response times. Furthermore, poly(ethyleneimine)-based (PEI-based) materials capture CO2 selectively and reversibly. Here, we report the temperature dependencies of a polymer blend composed of PEI and poly(ethylene oxide) (PEO), as this synergetically blended material platform is useful as a CO2 sensing system when coated atop a MEMS-based resonant mass sensor. Importantly, we report how temperature can impact polymer characteristics, which ultimately dictate sensor performance. To achieve this aim, adsorption rate constants and thermodynamic parameters were calculated using a Langmuir model for a series of polymer blends with different compositions. Throughout a range of temperatures relevant to indoor sensing systems, these polymer blends adsorbed less and desorbed more CO2 with increasing temperature. This was due, in part, to the melting of the polymer materials and a decrease in the availability of PEI amines to capture CO2. Ultimately, these data provide a deeper understanding of the selection criteria and boundaries to consider when using polymer-based selective recognition layers in indoor environments and inform the development of low-cost and small-scale IAQ sensors.