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

This study presented a novel polymeric humidity sensor based on CaCl 2 -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 CaCl 2 , 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 CaCl 2 concentration and relative humidity. Comparing with the pristine polymer, CaCl 2 -doped SPEEK showed improved humidity sensing properties and the 10   wt%- CaCl 2 /SPEEK sensor showed the best sensing properties for high sensitivity, little hysteresis (1.14%), fast response (2   s)/recovery time (65   s) 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, CaCl 2 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.