Humidity sensor using mutually reactive copolymers containing quaternary ammonium salt and reactive function

Two mutually reactive copolymers were prepared to use as humidity sensitive membranes. The major ingredient of humid membrane is a cross-linked polyelectrolyte obtained from mutually reactive copolymers of dimethyl 2-methecryloxyethyl 3-bromopropyl ammonium bromide (DMBAB)/2-ethylhexyl acrylate (2-EHA) and 2-(N,N-Dimethylamino)ethyl methacrylate (DMEA)/2-EHA=3/1, 4/1, 5/1. The cross-linked copolymers prepared from the reaction of DMBAB/2-EHA= 3/1 with DAEMA/2-EHA= 3/1 showed an average impedance of 585, 36.1 and 2.9 kΩ at 30, 60 and 90% RH, respectively. Temperature dependence, frequency dependence and response time were measured. The temperature dependence coefficient between 15 and 35 °C was from −0.59 to −0.65% RH/°C and little hysteresis was observed. The humidity sensors using cross-linked polyelectrolyte were quite resistant to water. The humidity-sensing characteristics of the sensor changed within ±0.2% RH even after soaking in water for 10 h.