Fast response and highly sensitive humidity sensors based on CaCl2-doped sulfonated poly (ether ether ketone)s

Abstract

This study presented a novel polymeric humidity sensor based on CaCl2-doped sulfonated poly (ether ether ketone) (SPEEK). For the first time, we developed a crosslinking procedure to modify SPEEK for humidity sensors application. SPEEKs with fixed value of sulfonation degree were synthesized by a direct aromatic nucleophilic substitution polymerization of sulfonated monomers. The obtained polymer was used to prepare humidity sensitive composites by doping CaCl2, and then they were fabricated by spin coating on a ceramic substrate to use as a novel polymeric humidity sensor. Humidity-sensing performances were investigated by varying both CaCl2 concentration and relative humidity. Comparing with the pristine polymer, CaCl2-doped SPEEK showed improved humidity sensing properties and the 10wt%- CaCl2/SPEEK sensor showed the best sensing properties for high sensitivity, little hysteresis (1.14%), fast response (2s)/recovery time (65s) and outstanding stability. Additionally, the morphology and thermal properties of composites and the humidity sensing mechanism of the sensors were discussed in detail. Most importantly, CaCl2 was employed not only as dopant to significantly improve the sensitivity, but also as cross-linker to reduce the sorption hysteresis. This study demonstrated that these humidity sensors could offer a great potential for applications.