Photochemical attachment of polyelectrolyte membrane to electrode substrate and their humidity-sensitive properties

New cinnamate group-containing copolymers for a humidity-sensitive polyelectrolyte were prepared by copolymerization of [2-[(methacryloyloxy)ethyl]dimethyl]propyl ammonium bromide (MEPAB), methyl methacrylate (MMA) and 2-(cinnamyloxy)ethyl methacrylate (CEMA). They were self-crosslinkable terpolymers composed of different contents of MEPAB/MMA/CEMA = 70/27/3, 70/25/5, 70/23/7 and 70/20/10. Pretreatment of the alumina substrate with 3-(triethoxysilyl)propyl cinnamate (TESPC) containing silane-coupling agent was performed to attach the humidity-sensitive membrane to the substrate through covalent bonds. The sensors were irradiated with UV light, causing the electronically excited cinnamate molecules to form a crosslinking reaction via the [2π + 2π] addition. The impedance ranged from 107 to 103 Ω between 20 and 95%RH, which was required for a humidity sensor operating at ambient humidity. Water durability, hysteresis, response time and long-term stability at high temperature and humidity were also measured and estimated.