Humidity sensor using cross-linked copolymers containing viologen moiety

1-[3-(2-Methacryloxyethyl dimethyl ammono)propyl] 1′-propyl-4,4′-bipyridinium tribromide (MDAPBT) was selected as a humidity-sensing monomer. n-Butyl acrylate (n-BA) and 2-(N,N′-dimethylaminoethyl)methacrylate (DAEMA) were used as a comonomer. The major ingredient of the humid membrane is the copolymers of MDAPBT with n-BA and DAEMA. The cross-linked copolymer of MDAPBT/n-BA/DAEMA=40/30/30 showed average resistance of 1000, 52 and 3.1 KΩ at 30, 60 and 90% relative humidity (RH), respectively. Temperature dependence, frequency dependence and response time were also measured. The temperature dependence at 15–40°C is −0.42 to −0.45% RH °C−1 and the hysteresis falls on the range of ±2% RH. The humidity-sensitive characteristics of the sensor did not change even after 20 min when the sensor was soaked in water.