Humidity sensor using cross-linked polyelectrolyte prepared from mutually reactive copolymers containing phosphonium salt

Two mutually reactive two-component copolymers were prepared to use as humidity-sensitive membranes. The major component of humid membrane is cross-linked polyelectrolyte prepared from cross-linked copolymers of 4-vinylpyridine (VP)/vinylbenzyl tributyl phosphonium chloride (VTBPC): 3/1, 4/1 and 5/1; and 2-chloroethylvinyl ether (CEVE)/isobutyl vinyl ether (IVE): 3/1, 4/1 and 5/1. The cross-linked copolymers prepared from the reaction of VP/VTBPC: 4/1 with CEVE/IVE: 4/1 showed average resistance of 725, 43 and 3.1 kΩ at 30, 60 and 90% relative humidity (RH), respectively. Temperature dependence, frequency dependence and response time were measured. The temperature dependence between 15 and 35 °C was from −0.82 to −0.54% RH/°C and the hysteresis existed in the range of ±2% RH. The humidity sensors using polyelectrolyte cross-linked were quite resistant to water. The humidity-sensing characteristics of the sensor changed within 0.2% RH when the sensor was soaked in water after 4 h.