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 [Co(SCN)4]2–, and tetrachlorocobaltate [CoCl4]2−) and imidazolium cation chain lengths (1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium) have been developed for humidity-sensing applications. The influence of the different ILs on the morphology of the IL/PVDF-HFP composites has been evaluated, with a porous structure observed independently of the IL anion type and cation chain length. The inclusion of the ILs into PVDF-HFP induces polymer crystallization into the electroactive β phase, with 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 and 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 yield strength is observed in the hybrid samples with respect to neat PVDF-HFP, with this decrease being 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 analyzing 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.