The chromic and electrochemical response of CoCl2 − filled polyimide materials for sensing applications

Abstract

Hybrid fluorinated polyimide materials with different chromic responses to a particular event in its environment have been achieved. The chromic materials were obtained by introducing various quantities of cobalt(II) chloride (CoCl2), a well-known moisture sensing metal salt, either to polyamidic acid solutions which are further thermally treated in solid state as to achieve the corresponding imide structure, or into fully imidized polymer solutions that were processed afterwards in thin films. The pristine polyimide was prepared by conventional methods starting from 4,4′-diamino-3′3′-dimethyldiphenyl methane and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. The FTIR and UV–vis spectroscopy were employed as the main techniques to solve the issues concerning the mechanism responsible for the color change as a function of surroundings which occurs when CoCl2 is embedded into polyimide matrix. The properties of the CoCl2/polyimide hybrid materials both in fluid and solid state were evaluated in terms of structural morphology, optical properties, and electrochemical behavior. Cyclic voltammetry (CV) was involved in order to evidence the redox processes regarding both the polyimide matrix and the metal center. The CoCl2/polyimide hybrid materials underwent a visible color change when exposed to moisture and reverted immediately to the original color by heating, under current flow, or after a longer period of time without drying in normal atmosphere. This mechanism responsible for the color change as a function of humidity exposure was investigated in detail, revealing new insights into the sensing ability of CoCl2-modified materials.