Sensing carbon dioxide through a solution transparency change in gas-responsive polymers

Abstract

The use of thermosensitive polymers exhibiting an upper critical solution temperature (UCST) for sensing and detecting the presence of CO₂ in aqueous solution was demonstrated. By synthesizing random copolymers of acrylamide (AAm), acrylonitrile (AN) and N,N-diethylaminoethyl methacrylate (DEAEMA), which have different relative contents of AAm and AN but a similar and small amount of CO₂-responsive DEAEMA (below 3%), two means for monitoring the concentration of CO₂ through the transparency change of a polymer sensor solution were investigated. On the one hand, using a single P(AAm-co-AN-co-DEAEMA) copolymer whose phase transition-induced transmittance change upon cooling is completed within a narrow temperature range and whose cloud point (CP) is sensitively decreased upon increasing the amount of CO₂, the polymer solution at a given temperature (25 °C) allows a threshold concentration of CO₂ to be detected by showing an abrupt change in transmittance. On the other hand, by mixing two copolymers with largely different CPs (over 30 K), the sharp UCST-type phase transition is suppressed; instead the change in the solution transmittance upon cooling proceeds slowly and gradually over a wide temperature region (>35 K). The mixed polymer solution can be used to monitor the increase in the amount of CO₂ through gradual and mostly linear changes in transmittance. Both abrupt and gradual transparency changes can be observed by the naked eye.