Humidity sensors based on magnetic ionic liquids blended in poly(vinylidene fluoride-co-hexafluoropropylene)

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films incorporating 40 %wt. of different types of imidazolium ionic liquids (ILs) comprising different magnetic anions (tetrachloroferrate [FeCl4] - , tetrathiocyanatocobaltate [(SCN)4Co]2 - and tetrachlorocobaltate [CoCl4] - ) and imidazolium cation chain lengths 1-butyl-3-methyl imidazolium and 1-ethyl-3-methyl imidazolium, have been developed for humidity sensing applications. The influence of the different ILs into the morphology of the IL/PVDF-HFP composites has been evaluated, being observed a porous structure independently of the IL anion type and cation chain length. The inclusion of the ILs into PVDF-HFP induces the polymer crystallization into the electroactive β phase, being the increase more noticeable for PVDF-HFP/[Bmim][FeCl4], which reaches a β phase content of 85%. The degree of crystallinity slightly decreases with the incorporation of the filler, the incorporation of [Bmim]2[CoCl4], [Bmim]2[Co(SCN)4], and [Emim]2[Co(SCN)4] leading to a degree of crystallinity between 11 and 13%. A decrease in the thermal stability and the yield strength is observed in the hybrid samples with respect to neat PVDF-HFP, being this decrease more noticeable for the PVDF-HFP/[Bmim]2[Co(SCN)4] samples. Further, the highest magnetization has been obtained for the PVDF-HFP/[Bmim]2[Co(SCN)4] and PVDF-HFP/[Bmim][FeCl4] composites with similar filler concentrations. The suitability of the developed materials for humidity sensing has been evaluated by analysing the impedance variation with varying relative humidity from 30% to 85%. [Bmim]2[CoCl4]/PVDF-HFP displays the highest relative humidity (RH) sensing response of -2245 ±240 Ω/RH. Finally, the applicability of the developed hybrid IL/polymer composites has been demonstrated by the development of a real time breathing monitoring device.This work was supported by the Portuguese Foundation for Science and Technology (FCT) under strategic funding UIDB/04650/2020, UID/FIS/04650/2020, UID/QUI/0686/2020, UIDB/50006/2020, UIDP/50006/2020 and LA/P/0008/2020UIDB/50006/2020 and UIDP/50006/2020, project PTDC/FIS-MAC/28157/2017, and grants 2021.08158.BD (J. S.), SFRH/BD/145345/2019 (L.C.F.), SFRH/BD/131729/2017 (N.P), and Investigator FCT Contract 2020.02915.CEECIND (D.M.C) and 2020.04028.CEECIND (C.M.C.). The authors acknowledge funding by the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RBC43/AEI/10.13039/501100011033. The authors also acknowledge funding from the Basque Government Industry Department under the ELKARTEK program. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF and ESF) is gratefully acknowledged