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

Abstract

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.