Polyelectrolyte humid membranes anchored to the gold surface on flexible polyimide substrate and their water durability

A novel flexible resistive-type humidity sensor was fabricated by printing a pair of comb-like gold electrodes on a polyimide (PI) film, followed by printing with copolymers made of [2-[(methacryloyloxy)ethyl] dimethyl]propylammonium bromide (MEPAB), methyl methacrylate (MMA), and 2-(cinnamyloxy)ethyl methacryate (CEMA) as a humidity-sensing film. The self-crosslinkable polyelelctrolyte copolymers contained different contents of MEPAB/MMA/CEMA = 70/27/3, 70/25/5, 70/23/7, and 70/20/10. A gold electrode was pretreated with 2-(mercaptoethyl) cinnamamide (MEC) containing a thiol-coupling agent for the purpose of anchoring the humidity-sensitive polyelectrolyte to the gold electrode on the flexible PI substrate. UV irradiation on the humidity-sensitive film stimulated a crosslinking reaction between the cinnamate and cinnamamide groups via [2π+2π] addition. After soaking in water for 60 min, the resulting sensor using MEC showed water durability and increased resistance of 42–65%, corresponding to a reduction in relative humidity of 2.25–2.98%RH. The flexible humidity sensor also showed good hysteresis (−1.1 to −0.8%RH), an acceptable response time (97 s), and good stability at high temperature and high humidity.