Strip-type flexible capacitive humidity sensor based on composite of polyimide and poly(glycidyl methacrylate): Fabrication, humidity sensitive performance and potential for detecting water content in liquids

Flexible humidity sensors show great potentials in the fields of smart apparels, biomedicine, human-machine interaction, etc., and they have attracted much attention in recent years. Herein, strip-type flexible capacitive humidity sensors (SFCHS) were facilely prepared by intertwining copper wires covered with soluble polyimide (PI) and poly(glycidyl methacrylate) (PGMA). The “pine nut” microstructure on copper wire surface and network structure between PGMA and PI strengthened the adhesion between the copper wire electrode and the polymeric sensitive materials, leading to better stability and sensing performance. The SFCHS with a double helix configuration showed good sensitivity (capacitance change of 111.5% between relative humidity (RH) of 11–98%), moderate hysteresis (∼6.6% RH) and fast response (tadsorption/tdesorption: 10.1 s/5.2 s) at room temperature. Furthermore, the self-standing SFCHS revealed stable capacitance under bending and good long-term stability. In addition, the flexible sensor could monitor real-time change of water content in various liquids (transformer oil, hydraulic oil, ethanol and n-dodecane), suggesting potentials for detecting water content in liquids. The work provides references for developing advanced flexible capacitive humidity sensors for measuring moisture in both air and liquids.