Novel polymeric humidity sensors based on sulfonated poly (ether ether ketone)s: Influence of sulfonation degree on sensing properties

Abstract

Sulfonated poly (ether ether ketone)s (SPEEKs) with different values of sulfonation degree were successfully synthesized by a direct aromatic nucleophilic substitution polymerization of sulfonated monomers. By the direct-synthesis method, the number of sulfonic acid groups can be precisely controlled. They were then fabricated by spin coating on a ceramic substrate to be used as a novel polymeric humidity sensor. The morphology of the ionic polymers especially the microstructure of ionic clusters, water sorption and the complex impedance spectra of the sensors were investigated to explain the humidity sensing characteristics and the sensing mechanism under different relative humidity. In addition, SPEEK-6 polymer with sulfonation degree (Ds=1.18) was chosen as the optimal humidity sensing materials, which possessed high sensitivity and satisfactory linearity ranging from 11% RH to 97% RH. Furthermore, it exhibited a small hysteresis (less than 3% RH) during the sorption processes, an acceptable response/recovery time (100s response and 105s of recovery time) and an excellent repeatability. These results indicated SPEEKs could be promising for applications in humidity sensor.