Sulfonated polystyrene polymer humidity sensor: Synthesis and characterization

Commercial polystyrene was sulfonated with sulfonic acid groups to a degree of 22 mol%. Polystyrene and sulfonated polystyrene were analyzed with respect to its chemical and physical properties trough titration, FT-IR, microprobe analysis and SEM. These techniques allowed us to determine the effectiveness of the sulfonation reaction and the changes of the physical properties it caused. Humidity sensors were obtained by dip coating. Impedance spectroscopy was used to investigate humidity-sensing properties at controlled relative humidity (RH%) environments. In the range of 33–90 RH% the sensor impedance varied from 450 to 6.4 kΩ. The response time for absorption and desorption measurements were considered between 11 and 90 RH%. Furthermore, the sensor exhibited a quick response (fast absorption: less than 30 s and desorption: 5 min). The experimental results turn the sulfonated polystyrene into a very promising material for the fabrication of inexpensive humidity sensors with similar characteristics to commercially available sensors.