Polymeric humidity sensors with nonlinear response: Properties and mechanism investigation

A class of humidity sensors with switching property based on several hydrophilic polymers [poly(N-vinyl-2-pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), and hydroxyethyl cellulose (HEC)] were researched. These polymers were selected as the sensing materials because the polar groups in the molecules (amide, hydroxyl, and ether bond) could interact with water molecules. The sensors all show nonlinear response to relative humidity (RH) under AC voltage. The impedances of the sensors remain almost unchanged at low RH and decrease sharply at certain humidity (about 75, 65, and 55% RH for PVP, PVA, and HEC sensors, respectively). The switching points and sensitivities of the sensors could be adjusted by changing the operating frequency, polymer blending, or doping with hydrophilic materials. The complex impedances of the sensors demonstrate that the electronic contribution is dominant at low RH, and the ions make a significant contribution for increasing RH levels. The different sensing properties of the polymers are attributed to their different hydrophilic properties and ionic contributions at high RH. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2056-2061, 2013