A water durable resistive humidity sensor based on rigid sulfonated polybenzimidazole and their properties

- Rigid sulfonated polybenzimidazoles (SPBIs) were prepared for the humidity-sensing film.
- SPBI film was attached to aminosilane-treated alumina substrate by epoxy cross-linker.
- Tetramethylammonium salt of SPBI showed good linear response and sensitivity.
- SPBI films showed water durability and stability at a high humidity and high temperature.

A water-durable, polymeric resistive humidity sensor was designed using rigid sulfonated polybenzimidazole (SPBI) with tetramethylammonium (Me4N+) as the counter cation. The Me4N-SPBI structure based on the imidazole group was used to obtain a cross-linkable moiety to attach the humidity-sensitive polyelectrolyte film to an alumina substrate pretreated with an aminosilane coupling agent using an epoxy cross-linker. The SPBI in Me4N+ structure is derived from 3,3′-diaminobenzidine (DAB), sodium 6,6′-oxybis(3-carboxybenzenesulfonate) (SOCBS) and 4,4′-oxybisbenzoic acid (OBBA), followed by quaternization, which allows for easy fabrication, higher sensitivity and better adhesion between the film and the substrate. The effect that cross-linking and anchoring the Me4N-SPBI films had on the humidity-sensing properties and water durability of sensors were investigated. The SPBI films with Me4N-DAB/SOCBS/OBBA = 100/100/0 and 100/70/30 exhibited a linear response with a resistivity of 6.11-1.19 kΩ and 7.60 MΩ-2.51 kΩ between 20 and 95% RH, respectively. Consequently, cross-linking and anchoring the film to the substrates with a coupling reagent significantly improves the water durability and stability at a high humidity and high temperature.