Competitive sorption of water vapor and CO2 in photocrosslinked PVCA film for a capacitive-type humidity sensor

The sorption behavior of water vapor and CO2 gas in photocrosslinked poly(v inyl cinnamate) (PVCA) film was examined at 30 degreesC under atmospheric p ressure. Both the water sorption isotherm and the CO2 sorption isotherm obt ained with quartz crystal microbalance (QCM) method obeyed the simple Langm uir's equation. Water vapor/CO2 mixed-gas sorption isotherms were also obta ined. Total amount of sorbed mixed gases was clearly influenced by the part ial pressure of water vapor (p(w)) and CO2 gas (p(c)) in the atmosphere. A modified Langmuir's equation based on a dual-site model was employed for pr edicting the competitive adsorption isotherm, and the isotherm was clearly described by the equation. The theoretically estimated amount of adsorbed w ater at the constant p(w) decreased slightly with increasing p(c). The effe ct of this phenomenon on the sensitivity of the capacitive-type relative hu midity sensor was examined. As expected, the electrical capacitance of the sensor at the constant relative humidity decreased because of the coexisten ce of CO2 gas. However, the influence was quite small in the CO2 concentrat ion range in the ordinary environment. (C) 2002 John Wiley & Sons, Inc.