Humidity sensor based on PMMA simultaneously doped with two different salts

The humidity-sensing properties of poly(methyl methacrylate) (PMMA) were enhanced by doping with two alkali salts (KOH and K2CO3), whose dissociation constants were distinctly different from each other. The electrical properties of PMMA doped with different amounts of KOH or a mixture of KOH and K2CO3 were examined in detail as a function of relative humidity (RH), to elucidate the contribution of the salts to the sensing properties (linearity and sensitivity). The PMMA doped only with KOH had a high sensitivity but low linearity. The poor linearity of the PMMA/KOH complex was improved by further doping of K2CO3. The PMMA doped with a mixture of KOH (0.6%, w/w) and K2CO3 (0.6%, w/w) was optimal in both sensitivity and linearity in the range of 30–90%RH.